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UBC Theses and Dissertations

Corrosion and passivation behaviour of noble metal coated anodes in copper electrowinning applications Wensley, Donald Arthur 1977

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CORROSION AND PASSIVATION BEHAVIOUR OF NOBLE METAL COATED ANODES IN COPPER ELECTRONINNING APPLICATIONS  by DONALD ARTHUR WENSLEY B . A . S c , The U n i v e r s i t y M . A . S c , The U n i v e r s i t y  A T H E S I S SUBMITTED  of Br i t i sh "Col umbi a, of B r i t i s h Columbia,  IN PARTIAL FULFILMENT OF  THE REQUIREMENTS  FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY i n The  Faculty  of  Graduate  Studies  in the Department of METALLURGY  We a c c e p t required  this  thesis  as c o n f o r m i n g  to the  standard  THE UNIVERSITY  1970 1973  OF B R I T I S H COLUMBIA  January,  1977  © Donald A r t h u r Wensley,  1977  In p r e s e n t i n g t h i s  thesis  in p a r t i a l  fulfilment of  an advanced degree at the U n i v e r s i t y of B r i t i s h the L i b r a r y s h a l l I  f u r t h e r agree  for scholarly by h i s of  this  written  make i t  that permission  for  Columbia,  I agree  r e f e r e n c e and  f o r e x t e n s i v e copying o f  this  It  for financial  is understood that gain s h a l l  permission.  Depa rtment The U n i v e r s i t y o f B r i t i s h 2075 Wesbrook P l a c e V a n c o u v e r , Canada V6T 1 W 5  Columbia  >  not  copying or  for  that  study. thesis  purposes may be granted by the Head of my Department  representatives. thesis  freely available  the requirements  or  publication  be allowed without my  ABSTRACT E v a l u a t i o n of i r i d i u m a l l o y coated  the changes i n l o a d i n g and c o m p o s i t i o n  l i q u o r s , was  platinum/  t i t a n i u m anodes employed i n c o n d i t i o n s comparable to  those encountered i n e l e c t r o w i n n i n g of l y t e s , t y p i c a l of  of  copper from h i g h l y a c i d i c  electro-  those produced from s o l v e n t - e x t r a c t i o n t r e a t m e n t  a c c o m p l i s h e d by means o f an X-ray f l u o r e s c e n c e  of leach  spectroscopic  technique. Under continuous 2M HzSCU or i n 2M H S 0 2  4  o p e r a t i o n a t a c o n s t a n t anode c u r r e n t i n  + 0.5M  CuSO*.. E l e c t r o l y t e s , the l o s s of  coating  metal from the p a r t i c u l a r anode m a t e r i a l chosen f o r t h i s study i s predominantly attack.  e l e c t r o c h e m i c a l i n n a t u r e , w i t h p l a t i n u m showing p r e f e r e n t i a l  Anode m a t e r i a l s from o t h e r s o u r c e s , o f n o m i n a l l y s i m i l a r composi-  t i o n and m a n u f a c t u r e , show v a r y i n g degrees of chemical  corrosion.  Operation  mixed mechanical and  electro-  with pulsed c u r r e n t or a d d i t i o n of t h i o u r e a  i s found to promote a c c e l e r a t e d d i s s o l u t i o n o f the c o a t i n g m e t a l s .  On  imminent anode f a l u r e the mechanism of c o a t i n g metal l o s s becomes predominantly  mechanical i n n a t u r e . The  development o f s u r f a c e oxygen coverage on the a l l o y  coatings  d u r i n g a n o d i c p o l a r i z a t i o n i n s u l f u r i c a c i d s o l u t i o n s , which i n t u r n determines the n a t u r e o f the c o r r o s i o n and  p a s s i v a t i o n ( d e f i n e d as  i n c r e a s e i n anode p o t e n t i a l w i t h t i m e ) i s analogous to the case of  the unalloyed  p l a t i n u m m e t a l , showing growth o f oxygen coverage to the e q u i v a l e n t o f  ii  o n l y 2-3 monolayers w i t h p r o l o n g e d a n o d i z a t i o n .  In p a r t i c u l a r , the a l l o y  c o a t i n g s behave i n a s i m i l a r manner t o p l a t i n u m metal w i t h r e s p e c t t o the f o r m a t i o n  and removal o f s u r f a c e oxygen c o v e r a g e , thus p e r m i t t i n g the  determination  o f e l e c t r o c h e m i c a l l y a c t i v e s u r f a c e a r e a s by means o f forma-  t i o n and s t r i p p i n g  o f oxygen  monolayers.  iii  TABLE OF CONTENTS  Page ABSTRACT  i i  LIST OF TABLES  ix  LIST OF FIGURES  x i i  ACKNOWLEDGMENTS  xvii  Chapter 1  2  INTRODUCTION  1  1.1  General  2  1.2  Anodes i n Copper E l e c t r o w i n n i n g  5  LITERATURE SURVEY:  ANODIC CORROSION OF PLATINUM  AND IRIDIUM 2.1  2.2  14  Platinum  14  2.1.1  Chloride Electrolytes  14  2.1.2  Inert E l e c t r o l y t e s  15  2.1.3  Organic-Containing E l e c t r o l y t e s  16  2.1.4  Alternating  Current Corrosion  17  2.1.5  State of Platinum i n Solution  18  Iridium  18  2.2.1  Chloride Electrolytes  19  2.2.2  Inert Electrolytes .  19  iv  Chapter  Page Organic-Containing  2.2.4  A l t e r n a t i n g Current Corrosion  19  2.2.5  State of I r i d i u m i n S o l u t i o n  20 20  2.4  TSA  21  Corrosion  2.4.1  Anodic Corrosion  22  2.4.2  A l t e r n a t i n g Current Corrosion  29  2.4.3  Anodic Breakdown  31  2.4.4  Noble Metal Coated TSA's i n E l e c t r o w i n n i n g . . .  33  Summary and R e l a t i o n t o t h i s Work OXYGEN FILMS ON  3.3  37  Platinum  37  3.1.1  Nature o f the S u r f a c e Oxygen Coverage  39  3.1.2  Strengthening  53  3.1.3  A c t i v e Oxygen  53  3.1.4  Type I I Oxide  54  3.1.5  Non-Electrochemical  o f the Oxygen Bond  57  Review  58  Iridium  60  3.2.1  Degree o f O x i d a t i o n  61  3.2.2  Nature o f the S u r f a c e Oxygen Coverage  63  Summary and R e l a t i o n t o t h i s Work  EXPERIMENTAL 4.1  Techniques f o r  Oxygen F i l m E v a l u a t i o n 3.1.6 3.2  34  PLATINUM  IRIDIUM ANODES 3.1'  4  19  Platinum/Iridiuni A l l o y s  LITERATURE SURVEY: AND  Electrolytes  2.3  2.5 3  2.2.3  64 70  M a t e r i a l s and Apparatus v  70  Chapter  5  Page Anode M a t e r i a l s and C o n s t r u c t i o n  70  4.1.2  E l e c t r o l y t e s and Gases  73  4.1.3  Cells  75  4.1.4  Reference E l e c t r o d e s  77  4.1.5  Electrochemical  78  4.1.6  Other Apparatus  Instrumentation  80  PROCEDURE 5.1  6  4.1.1  81  Measurement o f Anode Loading and  Surface  Composition  81  5.2  S u r f a c e Charge and S u r f a c e Area S t u d i e s  82  5.3  P o l a r i z a t i o n Curves  84  5.3.1  Noble Metal  Electrodes  5.3.2  Titanium Electrodes.  5.4  Long-Term E l e c t r o l y s i s  5.5  E l e c t r o d e C h a r a c t e r i z a t i o n between R e p e t i t i v e  84 85 85  Runs  87  5.6  Pulsed E l e c t r o l y s i s  88  5.7  Anode P o t e n t i a l C o r r e c t i o n s  88  RESULTS 6.1  6.2  90 D e s c r i p t i o n o f New E l e c t r o d e s  90  6.1.1  S u r f a c e Areas  90  6.1.2  Loadings  92  6.1.3  Diffractometry  92  6.1.4  Morphology  96  C o r r o s i o n o f Noble Metal 6.2.1  Coated Anodes  Changes i n Loading and C o m p o s i t i o n vi  99 99  Chapter  Page  6.3  6.4  6.5  6.2.2  S u r f a c e Area Changes  110  6.2.3  C o r r o s i o n Rates  113  6.2.4  Pulsed E l e c t r o l y s i s  130  6.2.5  A d d i t i v e and Contaminant E f f e c t s  138  6.2.6  Morphology Changes  140  Passivation  146  6.3.1  Potential  v s . Time Behavour  146  6.3.2  S u r f a c e Charge S t u d i e s  162  6.3.3  Polarization  172  6.3.4  Titanium . . . .  Curves  175  Anode D e p o s i t s  180  6.4.1  Growth  180  6.4.2  Identification  183  6.4.3  Morphology  186  Complete D e g r a d a t i o n  186  6.5.1  Anode P o t e n t i a l  Behaviour  186  6.5.2  C o r r o s i o n Rates  189  6.5.3  Morphology  194  6.5.4  Re-Coating  194  6.5.5  I d e n t i f i c a t i o n o f t h e S u r f a c e Degra- . dation Product  7  194  DISCUSSION 7.1  197  Corrosion 7.1.1 During R e v e r s i b l e P o t e n t i a l Time O p e r a t i o n 7.1.2  vs.  S u r f a c e Oxygen Coverage and C o r r o s i o n  vii  198 ^ 198 202  Chapter  Page  7.2  7.1.3  Pulsed E l e c t r o l y s i s  204  7.1.4  Complete D e g r a d a t i o n  206  7.1.5  Thiourea A d d i t i o n s  207  Passivation  208  7.2.1  S u r f a c e Area S t u d i e s  208  7.2.2  Surface  209  7.2.3  Time-dependence o f O v e r p o t e n t i a l : R e v e r s i b l e Behaviour  210  Time-dependence o f O v e r p o t e n t i a l : I r r e v e r s i b l e Behaviour  210  Polarization Composition  213  7.2.4 7.2.5 7.2.6 8  Oxygen Coverage  Curves:  Effect of A l l o y  Pulsed Current Operation  CONCLUSIONS  214 215  8.1  Corrosion  215  8.2  Passivation  8.3  S u i t a b i l i t y o f Pt/30 I r - T i Anodes f o r  . 216  Copper E l e c t r o w i n n i n g REFERENCES  218 220  APPENDICES Al  THE COPPER/COPPER SULFATE ELECTRODE  238  A2  X-RAY FLUORESCENCE SPECTROSCOPY  240  A3  IR-DROP CALCULATIONS  261  A4  ESTIMATION OF INDIVIDUAL ION ACTIVITIES  261  A5  SURFACE AREA CALCULATIONS  271  vi i i  LIST OF TABLES Table 1.1  1.2 1.3 1.4 2.1 3.1 6.1 6.2  Page Comparative E l e c t r o l y s i s C h a r a c t e r i s t i c s f o r C o n v e n t i o n a l Copper E l e c t r o m e t a l l u r g i c a l Processes  4  Summary o f O p e r a t i o n a l Data f o r S o l v e n t E x t r a c t i o n / E l e c t r o w i n n i n g (SX/EW) P l a n t s  6  Techniques P e r m i t t i n g High C u r r e n t O p e r a t i o n i n Copper E l e c t r o w i n n i n g  7  Density  P r o p e r t i e s o f S u b s t r a t e and C o a t i n g M e t a l s and the Oxides  11  Summary"of C o r r o s i o n Rates Reported f o r Noble Metal Coated Anodes  23  Probable Stages o f O x i d a t i o n o f t h e S u r f a c e o f P l a t i n u m w i t h I n c r e a s i n g Anode P o t e n t i a l  59  I d e n t i f i c a t i o n o f X-ray D i f f r a c t i o n Peaks Observed w i t h New Pt/30 I r - T i Anodes  93  L a t t i c e Parameters f o r I r and P t , and f o r Pt/30 I r a l l o y s  95  6.3  Electron D i f f r a c t i o n Results  6.4  E f f e c t o f E l e c t r o l y s i s Time on t h e S u r f a c e Areas o f Pt/30 I r - T i Anodes. P l a t i n u m C o r r o s i o n R e s u l t s from C u m u l a t i v e E l e c t r o l y s i s Runs i n 2M H S 0 + 0.5M CuSO^  118  I r i d i u m C o r r o s i o n R e s u l t s from C u m u l a t i v e Runs i n 2M H S 0 + 0.5M CuS0 .  119  6.5  2  6.6  2  6.7  . 97  4  4  Electrolysis  4  P l a t i n u m C o r r o s i o n R e s u l t s from C u m u l a t i v e E l e c t r o l y s i s runs i n 2M H S 0 2  4  ix  112  120  Table 6.8  Page I r i d i u m C o r r o s i o n R e s u l t s from C u m u l a t i v e Runs i n 2M H S 0 2  6.9 6.10 6.11 6.12 6.13 6.14  6.15 6.16 6.17  Electrolysis 121  4  I n d i v i d u a l and C u m u l a t i v e C o r r o s i o n Data f o r a Pt/30 Anode which E x h i b i t s C o a t i n g Loss by S p a l l i n g  Ir-Ti 122  P l a t i n u m and I r i d i u m C o r r o s i o n R e s u l t s i n S u l f u r i c Acid Solutions of Various Strengths. .  129  C u m u l a t i v e P t and I r C o r r o s i o n Rates f o r I n d i v i d u a l E l e c t r o d e s from D i f f e r e n t M a n u f a c t u r i n g L o t s  131  Noble Metal Loading Data f o r E l e c t r o d e s S u b j e c t e d to Pulsed E l e c t r o l y s i s  134  C o r r o s i o n Rate Data f o r E l e c t r o d e s S u b j e c t to Pulsed E l e c t r o l y s i s  136  Loading and C o r r o s i o n Rate Data f o r an Anode, P r e v i o u s l y Subjected to Pulsed Current Operation, S u b s e q u e n t l y Operated Under Continuous Anodic C u r r e n t  137  Loading and C o r r o s i o n Rate Data f o r an Anode Operated w i t h .05 gpl T h i o u r e a A d d i t i o n  139  E x p r e s s i o n s f o r the P o t e n t i a l v s . Time B e h a v i o u r o f New Anodes  148  E f f e c t o f Anodic P o l a r i z a t i o n Times on S u r f a c e Oxygen Coverage.  .'.  170  6.18  I d e n t i f i c a t i o n o f L e a d - C o n t a i n i n g Anode D e p o s i t s  184  6.19  C o n v e r s i o n o f $-Pb0  185  6.20  C o r r o s i o n Data f o r Pt/30 I r - T i Anodes O p e r a t i n g Under I r r e v e r s i b l e Anode P o t e n t i a l C o n d i t i o n s C o r r o s i o n Data f o r Pt/30 I r - T i Anodes which have S u f f e r e d Complete D e g r a d a t i o n  192  E f f e c t s o f V a r i a t i o n s i n the E l e c t r o l y t e C o m p o s i t i o n on the P o t e n t i a l o f the Copper/Copper S u l f a t e E l e c t r o d e  239  P r e c i s i o n o f Measured Count Values ( P t L a i I n t e n s i t y and Background) on Prepared P t - T i Standards  253  6.21  Al.l.  A2.1 A2.2  2  D e p o s i t s t o PbSO.,  Dead-time and Background C o r r e c t i o n s f o r the Prepared P t - T i S t a n d a r d s , w i t h the R e s u l t a n t True Peak I n t e n s i t i e s .  x  190  . . . 255  Table  Page  A3  IR-Drop C a l c u l a t i o n s  260  A4.1  I n d i v i d u a l Ion M o l a l i t i e s , E q u i l i b r i u m C o n s t a n t s and Ionic A c t i v i t y C o e f f i c i e n t Ratios Calculated f o r Sulfuric Acid Solutions  262  S t o i c h i o m e t r i c Mean A c t i v i t y C o e f f i c i e n t s f o r S u l f u r i c Acid Solutions  265  E s t i m a t e d Hydrogen and S u l f a t e Ion A c t i v i t y C o e f f i c i e n t s and Hydrogen Ion A c t i v i t i e s i n Aqueous S u l f u r i c A c i d Solutions  266  Hydrogen and Copper Ion A c t i v i t i e s i n Mixed H S0 /CuS0 E l e c t r o l y t e . . . .  269  C a l c u l a t e d Reference E l e c t r o d e Respect t o the SHE  270  A4.2 A4.3  A4.4  2  A4.5  4  4  xi  P o t e n t i a l s with  LIST OF FIGURES Figure  Page  4.1  Working e l e c t r o d e c o n s t r u c t i o n .  6.1  E l e c t r o c h e m i c a l l y a c t i v e surface areas, expressed as roughness  factors,  72  o f new Pt/30 I r - T i e l e c t r o d e s  6.2  S.E.M. view o f t h e t i t a n i u m s u b s t r a t e s u r f a c e  6.3  S.E.M. view o f a t y p i c a l new "4.33 g/m " metal l o a d i n g anode s u r f a c e S.E.M. view o f a t y p i c a l new "4.33 g/m " metal l o a d i n g anode s u r f a c e  6.4 6.5 6.6  S.E.M. view o f a t y p i c a l new "20 g/m " metal l o a d i n g anode s u r f a c e 2  98  2  noble  2  noble  98 100  noble 100  V a r i a t i o n i n platinum loading f o r i n d i v i d u a l anodes w i t h e l e c t r o l y s i s time i n 2M H S 0 + 0.5M CuSO,, 22°.. 2  6.7  91  4  102  V a r i a t i o n i n p l a t i n u m l o a d i n g f o r i n d i v i d u a l anodes w i t h e l e c t r o l y s i s time i n 2M H^SO* + 0.5M C u S 0 , 40°  103  V a r i a t i o n i n platinum loading f o r i n d i v i d u a l w i t h e l e c t r o l y s i s time i n 2M HzSO*, 22°  anodes 104  V a r i a t i o n i n platinum loading f o r individual w i t h e l e c t r o l y s i s time i n 2M H S 0 , 40°  anodes  4  6.8  6.9  2  6.10  105  4  V a r i a t i o n o f the platinum weight f r a c t i o n f o r i n d i v i d u a l anodes w i t h e l e c t r o l y s i s time i n 2M H2SO4 + 0.5 C u S 0 , 22°. . . 106 4  6.11  V a r i a t i o n o f the platinum weight f r a c t i o n i n d i v i d u a l anodes w i t h e l e c t r o l y s i s time i n 2M H S 0 0.5M CuSO,*, 40° f  o  r  2  6.12  4  +  V a r i a t i o n o f t h e p l a t i n u m weight f r a c t i o n f o r i n d i v i d u a l anodes w i t h e l e c t r o l y s i s time i n 2M H S 0 , 22° 2  xi i  4  107  .108  Figure 6.13  Page V a r i a t i o n o f the p l a t i n u m w e i g h t f r a c t i o n f o r i n d i v i d u a l anodes w i t h e l e c t r o l y s i s time i n 2M H S ( K , 40° 2  6.14  6.15  Loading change and p l a t i n u m w e i g h t f r a c t i o n change w i t h e l e c t r o l y s i s time f o r an anode which e x h i b i t s c o a t i n g l o s s by s p a l l i n g Change i n roughness f a c t o r w i t h time w i t h r e p e t i t i v e e l e c t r o l y s i s a t 52.1 mA/cm (.geometric a r e a ) i n 2M H S 0 , 22° '. . . , 2  6.16 6.17  6.20 6.21  6.22  1  1  4  Roughness f a c t o r s f o r Pt/30 I r - T i anodes as a f u n c t i o n o f r e m a i n i n g noble metal l o a d i n g  .115  Platinum corrosion e f f i c i e n c y vs. applied current d e n s i t y from the c u m u l a t i v e c o a t i n g l o s s r e s u l t s o f r e p e t i t i v e e l e c t r o l y s i s runs i n 2M H S0n + 0.5M CuSO^  123  Iridium corrosion e f f i c i e n c y vs. applied current d e n s i t y from the c u m u l a t i v e c o a t i n g l o s s r e s u l t s o f r e p e t i t i v e e l e c t r o l y s i s runs i n 2M H S0i,. + 0.5M CuSO.*  124  P l a t i n u m c o r r o s i o n r a t e per m o f a c t u a l p l a t i n u m s u r f a c e v s . mean a c t u a l a p p l i e d c u r r e n t d e n s i t y  126  2  6.19  I l l  4  2  6.18  109  2  I r i d i u m c o r r o s i o n r a t e per m o f a c t u a l i r i d i u m s u r f a c e v s . mean a c t u a l a p p l i e d c u r r e n t d e n s i t y 2  j.27  C o r r o s i o n r a t e measurements on an i n d i v i d u a l anode w i t h r e s p e c t t o : (a) t h e g e o m e t r i c area o f p l a t i n u m o n l y ; (b) t h e a c t u a l (mean) a r e a o f p l a t i n u m c a l c u l a t e d f o r each run  128  S.E.M. view o f the s u r f a c e o f an anode a f t e r o p e r a t i o n a t 104 mA/cm (geometric a r e a ) i n 2M I^SO*, 22°, f o r a t o t a l o f 957 hours  141  S.E.M. view o f the s u r f a c e o f an anode a f t e r o p e r a t i o n a t 52.1 mA/cm ( g e o m e t r i c a r e a ) i n 2M H S 0 + 0.5M C u S 0 , 22°, f o r a t o t a l o f 1848 hours .  141  S.E.M. view o f t h e s u r f a c e o f an anode o p e r a t e d a t 260 mA/cm (geometric a r e a ) i n 2M H S 0 , 22°, f o r a t o t a l o f 455 hours •  142  S.E.M. view o f t h e s u r f a c e o f an anode o p e r a t e d a t 52.1 mA/cm ( g e o m e t r i c a r e a ) i n 2M H S 0 , 22°, f o r a t o t a l o f 385 hours  142  2  6.23  2  2  4  4  6.24  2  2  6.25  4  2  2  xi i i  4  Figure 6.26  Page S.E.M. view o f t h e s u r f a c e o f an anode s u b j e c t e d to p u l s e d e l e c t r o l y s i s a t 52.1 mA/cm (geometric a r e a ) i n 2M H S 0 + 0.5M C u S 0 , 22° f o r 194 hours  144  S.E.M. view o f t h e s u r f a c e o f t h e anode d e s c r i b e d i n F i g u r e 6.26, a f t e r f u r t h e r o p e r a t i o n f o r 258 hours a t a c o n t i n u o u s anodic c u r r e n t o f 52.1 mA/cm (geometric area)  144  2  2  6.27  4  4  2  6.28  S.E.M. view o f t h e s u r f a c e o f an anode s u b j e c t e d t o p u l s e d e l e c t r o l y s i s a t 52.1 mA/cm (geometric a r e a ) i n 2M H SO + 0.5M CuS0^,.22°, f o r 47.6 hours 2  2  6.29  k  S.E.M. view o f t h e s u r f a c e o f an anode o p e r a t e d a t 52.1 mA/cm i n 2M H S 0 + 0.5M C u S 0 , 22°, f o r 92 hours . . . . 2  2  6.30  4  5  1  4  5  1  4  7  4  V a r i a t i o n o f anode p o t e n t i a l w i t h time f o r i n d i v i d u a l anodes i n 2M H S 0 + 0.5M CuSOi*. 22° 2  6.31  4  1  4  I n i t i a l p o t e n t i a l v s . time b e h a v i o u r f o r new anodes operated i n 2M H S 0 + 0.5M C u S 0 , 22°  149  Change i n anode p o t e n t i a l w i t h time f o r a new anode o p e r a t e d a t 15.6 mA/cm (geometric a r e a ) i n 2M H S 0 + 0.5M CuSO^, 22°  150  Anode p o t e n t i a l behaviour f o r i n d i v i d u a l anodes subjected to r e p e t i t i v e e l e c t r o l y s i s a t constant applied current density  152  P o t e n t i a l change w i t h time f o r an anode s u b j e c t e d t o r e p e t i t i v e e l e c t r o l y s i s a t high c u r r e n t d e n s i t y  154  P o t e n t i a l change w i t h time f o r an anode s u b j e c t e d t o r e p e t i t i v e e l e c t r o l y s i s a t high current density  155  P o t e n t i a l v s . time b e h a v i o u r f o r an anode, a f t e r p r i o r p o l a r i z a t i o n t o imminent f a i l u r e  156  P o t e n t i a l v s . time b e h a v i o u r f o r an e l e c t r o d e subjected to pulsed e l e c t r o l y s i s  157  P o t e n t i a l v s . time b e h a v i o u r f o r an e l e c t r o d e subjected t o pulsed e l e c t r o l y s i s  158  6.39  E f f e c t o f 0.05 g p l t h i o u r e a a d d i t i o n  161  6.40  T y p i c a l charge curves f o r a Pt/25 I r a l l o y w i r e electrode . . •  163  2  6.32  4  4  2  2  6.33  6.34  6.35  6.36  6.37  6.38  4  xi v  Figure 6.41  Page T y p i c a l charge c u r v e s f o r an i r i d i u m w i r e electrode  164  6.42  T y p i c a l charge c u r v e s f o r a Pt/30 I r - T i e l e c t r o d e  165  6.43  S u r f a c e oxygen coverage vs. p o t e n t i a l r e l a t i o n s f o r s e v e r a l anodes C a t h o d i c s u r f a c e oxygen s t r i p p i n g c u r v e s f o r a Pt/25 I r a l l o y s w i r e e l e c t r o d e  169  P o l a r i z a t i o n c u r v e s f o r oxygen e v o l u t i o n on Pt/30 I r - T i anodes. ( C u r r e n t d e n s i t i e s based on geometric area.)  173  A n o d i c p o l a r i z a t i o n c u r v e s f o r oxygen e v o l u t i o n on Pt and P t / I r a l l o y s  174  Anodic p o l a r i z a t i o n c u r v e s f o r oxygen e v o l u t i o n on Pt and P t / I r a l l o y s ,  176  P o l a r i z a t i o n c u r v e s f o r P t and I r w i r e anodes i n computer-generated curves f o r v a r i o u s P t / I r a l l o y s  177  Anodic p o l a r i z a t i o n Pt/30 I r - T i anode  179  6.44 6.45  6.46 6.47; 6.48 6.49 6.50 6.51  6.52  c u r v e f o r the T i base o f a !•  I n c r e a s e i n amount o f s u r f a c e l e a d d e p o s i t s w i t h e l e c t r o l y s i s time  181  8-Pb0 d e p o s i t formed on an anode o p e r a t e d f o r 96 hours  187  2  8-Pb0  2  d e p o s i t formed on an anode o p e r a t e d f o r  192 hours  187  6.53  Mixed 3-Pb0  6.54  P r e d o m i n a n t l y PbS0  6.55  S.E.M. view o f t h e s u r f a c e o f an anode which has undergone  2  and PbS0 4  4  desposit  188  deposit  188  complete d e g r a d a t i o n  S.E.M. view o f bent, c o n i c a l  6.57  S.E.M. view o f t h e s u r f a c e o f an anode which has undergone complete d e g r a d a t i o n S.E.M. view o f t h e s u r f a c e o f an anode which has undergone complete d e g r a d a t i o n  xv  s u r f a c e growth  195  6.56  6.58  167  ...  195  196 196  Figure A2.1  A2.2 A4.1  Page R e l a t i v e P t L a j i n t e n s i t y vs. various P t / I r alloys  loading r e l a t i o n  R e l a t i v e I r L a i n t e n s i t y vs. various P t / I r alloys  loading r e l a t i o n  x  256  pH of c o n c e n t r a t e d s u l f u r i c a c i d s o l u t i o n s various calculations  xv i  for  for 257  by 267  ACKNOWLEDGEMENTS  The a u t h o r wishes t o e x p r e s s h i s g r a t i t u d e t o Dr. I.H. Warren f o r h i s encouragement throughout t h e c o u r s e o f t h i s s t u d y , t o members o f the f a c u l t y and f e l l o w graduate s t u d e n t s f o r h e l p f u l d i s c u s s i o n s and a d v i c e , and t o members o f t h e t e c h n i c a l  s t a f f - i n p a r t i c u l a r , J i m Walker.  F u r t h e r , I thank my w i f e , D a r l e n e , f o r her e n d u r i n g p a t i e n c e over t h e past s e v e r a l y e a r s .  I a l s o wish t o acknowledge t h e i n e s t i m a b l e c o n t r i -  b u t i o n o f o u r dog, S a i n t , who made o u r l i v e s j u s t a l i t t l e more e n j o y a b l e d u r i n g t h i s time. Financial  s u p p o r t i n t h e form o f N a t i o n a l Research C o u n c i l  o f Canada S c h o l a r s h i p s i s a l s o g r e a t l y a p p r e c i a t e d .  xvi i  Chapter 1  INTRODUCTION  Recent developments i n t h e t e c h n o l o g y  o f copper e l e c t r o w i n n i n g ,  p a r t i c u l a r l y s i n c e t h e advent o f s o l v e n t e x t r a c t i o n p r o c e s s e s  f o r the  r e c o v e r y o f copper from l e a c h l i q u o r s , have l e d t o renewed i n t e r e s t i n a l t e r n a t i v e anode m a t e r i a l s t o t h e c o n v e n t i o n a l  l e a d - b a s e d a l l o y s which  were developed f o r use i n e l e c t r o l y t e s o f r e l a t i v e l y low a c i d i t y (15-90gplH S0\) and a t low c u r r e n t d e n s i t i e s (8-19 2  of conventional  anode m a t e r i a l s t o w i t h s t a n d  mA/cm ). 2  The a b i l i t y  the c o n d i t i o n s o f higher  a c i d i t y i n e l e c t r o l y t e s produced by s o l v e n t e x t r a c t i o n  processes  (150-320 g p l H S 0 i J and t o s u s t a i n h i g h e r c u r r e n t d e n s i t i e s (25-300 mA/cm ) 2  2  p e r m i t t e d w i t h t h e development o f improved e l e c t r o l y t e a g i t a t i o n t e c h n i q u e s , such as r e c i r c u l a t i o n and gas s p a r g i n g , and p u l s e d c u r r e n t o p e r a t i o n , i s questionable. with regard  In p a r t i c u l a r , concern has been e x p r e s s e d i n t h e l i t e r a t u r e  t o enhanced anode c o r r o s i o n i n t h e h i g h l y a c i d i c e l e c t r o l y t e s  t y p i c a l o f solvent e x t r a t i o n processes, lead contamination factory  which l e a d s i n t u r n t o l e v e l s o f  (>10 ppm) i n t h e cathode copper which r e n d e r i t u n s a t i s -  f o r a p p l i c a t i o n s such as  wire-drawing.  P o s s i b l e a l t e r n a t i v e anode m a t e r i a l s f o r use i n copper e l e c t r o w i n n ing c e l l s can be chosen from t h e v a r i o u s t i t a n i u m s u b s t r a t e anodes (TSA) which have been developed w i t h i n t h e p a s t two decades.  In p a r t i c u l a r , an anode  m a t e r i a l o f t h i s type c o n s i s t i n g o f an e l e c t r o c a t a l y t i c a l l y a c t i v e c o a t i n g 1  2  of an a l l o y of p l a t i n u m and  i r i d i u m on a t i t a n i u m s u b s t r a t e has  acceptance i n the c h l o r - a l k a l i  i n d u s t r y , where i t i s c h a r a c t e r i z e d by a  low c o r r o s i o n e f f i c i e n c y ( l e s s than 0.1-1 potential operation. copper-containing  seen w i d e s p r e a d  ug/A«hr) and s t a b l e , low  over-  A l t h o u g h the r e p o r t e d c o r r o s i o n r a t e s f o r TSA's i n  s u l f u r i c and  s o l u t i o n s have been f a r from p r o m i s i n g ,  the  c o r r o s i o n b e h a v i o u r o f t h e s e anodes under c o n d i t i o n s s i m i l a r t o t h o s e encountered i n copper e l e c t r o w i n n i n g i n p a r t i c u l a r the nature l o s s occurs  i s f a r from being c h a r a c t e r i z e d  o f the c o a t i n g l o s s mechanism (whether c o a t i n g  by a mechanical " s p a l l i n g " mechanism o r by an  mechanism) has not been determined.  electrochemical  Indeed, based on the c o r r o s i o n b e h a v i o u r  r e p o r t e d f o r TSA's i n c h l o r i d e e l e c t r o l y t e s , t h i s q u e s t i o n unresolved  —  although  —  remains l a r g e l y  s e v e r a l s p e c i f i c cases o f the mechanical mode o f  d i s s o l u t i o n have been mentioned i n the l i t e r a t u r e .  The  greatest  difficulty  i n o b t a i n i n g r e l i a b l e c o r r o s i o n data f o r such anodes i s a consequence of t h e i r profound i n e r t n e s s towards a n o d i c d i s s o l u t i o n . On c o n s i d e r a t i o n o f corrosion rates reported  i n the l i t e r a t u r e , i t i s found t h a t noble metal  l o s s e s o c c u r a t e f f e c t i v e " p a r t i a l c u r r e n t d e n s i t i e s " o f the o r d e r nanoamps per cm . 2  Such low r a t e s o f c o a t i n g metal l o s s p l a c e  demands on the p r e c i s i o n of the measurement t e c h n i q u e s  of  considerable  employed to determine  these l o s s e s . In the p r e s e n t work, anode m a t e r i a l s o f the t y p e P t / 3 0 I r - T i evaluated  w i t h r e s p e c t to t h e i r c o r r o s i o n and  are  p a s s i v a t i o n behaviour i n a  v a r i e t y o f o p e r a t i n g c o n d i t i o n s which encompass those encountered i n , or proposed f o r , the e l e c t r o w i n n i n g o f copper f r o m s o l v e n t electrolytes.  An X-ray f l u o r e s c e n c e  measurement of metal f o i l  thicknesses  spectrometric  extraction-produced  technique,  i n o t h e r f i e l d s , was  used f o r the  developed t o  3  enable the a c c u r a t e d e t e r m i n a t i o n  o f changes i n l o a d i n g and c o m p o s i t i o n  of  the anode c o a t i n g s . With r e s p e c t t o the l o s s e s o f both c o a t i n g metals d u r i n g c o n d i t i o n s o f p o l a r i z a t i o n , e x p e r i m e n t s were performed from which c a l l y r e l i a b l e changes i n t h e i r l o a d i n g s c o u l d be produced, and  various statisti-  the  results  were i n t e r p r e t e d w i t h r e s p e c t t o t h e i r c o n f o r m i t y t o a c o a t i n g l o s s  process  which was chemical  e i t h e r predominantly  due  to mechanical detachment or to e l e c t r o -  d i s s o l u t i o n , or t o a combination P a s s i v a t i o n behaviour  was  of  both.  examined w i t h r e s p e c t t o  determination  o f the degrees o f s u r f a c e oxygen coverage under v a r i o u s c o n d i t i o n s o f p o l a r i z a t i o n i n s u l f u r i c a c i d s o l u t i o n s , and was the e x i s t i n g l i t e r a t u r e c o n c e r n i n g  anodic  i n t e r p r e t e d i n terms o f  oxygen f i l m f o r m a t i o n on p l a t i n u m  and  iridium.  1.1  General Recent developments i n copper e l e c t r o w i n n i n g t e c h n o l o g y  noted by the commercial r e a l i z a t i o n o f processes from l e a c h l i q u o r s by s o l v e n t e x t r a c t i o n (SX) and p r o d u c t i v i t y , without  by attempts to i n c r e a s e current  d e n s i t y o p e r a t i o n which i s f e a s i b l e i n the m o r e - c o n c e n t r a t e d and  gpl copper, 15-90  The  gpl i r o n  v a r i o u s copper e l e c t r o m e t a l 1 u r g i c a l pro-  cesses are compared i n T a b l e 1.1. necessary  from s o l u t i o n s c o n t a i n i n g  gpl s u l f u r i c a c i d , and c o n t a i n i n g 2-20  the major i m p u r i t y [ 1 - 1 0 ] .  highly-  processing.  C o n v e n t i o n a l l y , copper i s e l e c t r o w o n 15-60  key-  f o r the r e c o v e r y o f copper  s a c r i f i c i n g cathode q u a l i t y , through high  p u r i f i e d e l e c t r o l y t e s t y p i c a l o f SX  are  C u r r e n t d e n s i t i e s below 20 mA/cm  i n o r d e r to p r o v i d e both smooth and  2  l e v e l cathode d e p o s i t s  are and  as  4  T a b l e 1.1 Comparative  E l e c t r o l y s i s C h a r a c t e r i s t i c s f o r Conventional Electrometal1urgical  Processes  Electrowinning  Electrorefining  Electroplating  Electrolytes Cu, gpl H S0 , gpl 2  15-60 15-90  4  30-60 125-240  75-250 40-200  Impurities Fe,  gpl  2-20 0.2-6  Fe , gpl Others  0.5 0.5  0 0  26-55  50-65  20-50  Pb/6-15 Sb/0-1 Ac al l o y  99.7 Cu  99.99 Cu  Pb,Sb,Ag  Au,Ag,Pt"metals, As,Sb,Ni,Fe  As,Sb  99.95+Cu Au,Ag,S,As,Sb  99.9+Cu  CI,Mg,Mo,Co, C1,N0 3  Temperature, °C Anodes Impurities of anode source Cathodes  99.8-99.9 Cu, As,Sb,Ag,S,Pb  C u r r e n t d e n s i t i e s , mA/cm  8-19  12-27  30T55  Current e f f i c i e n c i e s  75-90  90-98  99+  2  P r i m a r y anode r e a c t i o n  2rl 0+4H +0 +4e  Primary cathode  Cu  reaction  T h e o r e t i c a l standard c e l l voltage, volts  +  2  + +  2  + 2e -»• Cu 0.892  Cu+Cu +2e  Cu+Cu  ++  Cu  + +  + 2e 0  Cu  Cu  + +  ++  + 2e  + 2e -> Cu 0  5  to m a i n t a i n  high c u r r e n t e f f i c i e n c y .  (Increased current density  operation  would r e s u l t i n t h e u n d e s i r a b l e f o r m a t i o n o f d e n d r i t i c d e p o s i t s due t o the non-uniformity  o f a g i t a t i o n by a n o d i c a l l y - g e n e r a t e d oxygen, and t o de-  c r e a s e d energy e f f i c i e n c y due t o enhanced l i m i t i n g c u r r e n t s f o r i m p u r i t y s p e c i e s and t o c o - d e p o s i t i o n o f hydrogen.) S o l v e n t e x t r a c t i o n / e l e c t r o w i n n i n g (SX/EW) p r o c e s s i n g , on t h e o t h e r hand, i s a b l e t o p r o v i d e e l e c t r o l y t e s w i t h a u n i f o r m l y high copper (34-40 g p l ) , h i g h e r s u l f u r i c a c i d c o n t e n t l e v e l s (< 2.6 gpl i r o n ) [11-17].  impurity  The a v a i l a b l e o p e r a t i o n a l data f o r SX/EW  p l a n t s a r e summarized i n Table 1.2. i n g process  (132-200 g p l ) and lower  content  The energy e f f i c i e n c y i n t h e e l e c t r o w i n n -  i s improved both as a r e s u l t o f t h e lower  o f the h i g h e r e l e c t r o l y t e c o n d u c t i v i t y .  i m p u r i t y l e v e l s and  Techniques which p e r m i t c e l l  opera-  t i o n a t h i g h e r c u r r e n t d e n s i t i e s (see Table 1.3) i n v o l v e r e d u c t i o n o f the t h i c k n e s s o f t h e d i f f u s i o n l a y e r f o r copper d e p o s i t i o n by e i t h e r improved electrolyte agitation (recirculation  [1822], gas o r a i r s p a r g i n g  or by u l t r a s o n i c a g i t a t i o n [21,23,33] o r p u l s e d c u r r e n t o p e r a t i o n (The  l a t t e r processes  [22,34-36].  were i n i t i a l l y developed f o r e l e c t r o r e f i n i n g a p p l i c a -  t i o n s , where e l e c t r o l y t e m i x i n g  1.2  [23-32])  i s undesirable.)  Anodes i n Copper E l e c t r o w i n n i n g I n s o l u b l e anodes used i n copper e l e c t r o w i n n i n g a r e lead-based  a l l o y s c o n t a i n i n g 6-15 p e r cent antimony and, i n some c a s e s , 0-1 per c e n t s i l v e r [38]. and  Other anode m a t e r i a l s , such as s i l i c i d e s  " L u i l u " [37] anodes), magnetite,  have i n f e r i o r m e c h a n i c a l ,  (the " C h i l e x " [1,8]  massive l e a d d i o x i d e , and g r a p h i t e  e l e c t r i c a l , or electrochemical p r o p e r t i e s .  The  e l e c t r o c a t a l y t i c p r o p e r t i e s o f l e a d a r e due t o t h e f o r m a t i o n o f a s t a b l e ,  Table 1.2 Summary o f O p e r a t i o n a l  Plant Duval plant  Feed  Reagent  4  4  10%LIX64 i n kerosene  (4.56 g p l Cu)  Ranchers Bluebird Mine  3.02 g p l Cu 2.4 gpl Fe 4.5 g p l H SO  7% LIX64N i n kerosene  Bagdad Copper Corp.  (1 gpl Cu)  LIX64 i n kerosene  SEC Corp., El Paso  Evaporated and LIX64N d i l u t e d spent refinery electrolyte (Cu + N i )  Inco, Thompson (proposed)  58.5 g p l Ni .52 g p l Cu 49.3 g p l CI 126 g p l S 0 " pH 1.8  4  3/1  3  2  37.gpl Cu 2 gpl Fe 132 g p l H SO  3  4/1  2  k  1/4  16  93.9  18.3  80-89  [13]  2  34.2 g p l Cu 2.6 g p l Fe 142.5 gpl H S 0  99.9  2  2  (165 gpl H S 0 J  54  1  50 gpl Cu 200 g p l H S 0  60  99.9+  2  2  [12,14]  4  3  10/1  [11]  h  4  3  99.99  H S0L,  gpl  2  10% LIX64N i n kerosene  20.5  38 g p l Cu 165  2.5/1  (SX/EW) P l a n t s  Current Current Cathode Density References Efficiency Purity (mA/cm )  Electrolyte  2  Ranchers pilot plant  2  Extraction Stripping 0/A Stages 0/A Stages  7% LIX64 i n kerosene  p i l o t 1.5 g p l Cu .45 g p l Fe pH 2.5  Data f o r S o l v e n t E x t r a c t i o n / E l e c t r o w i n n i n g  [15]  [16]  >90  (99.9+)  [17]  4  4  CD  7  Table  1.3  Techniques P e r m i t t i n g High C u r r e n t D e n s i t y  Operation  i n Copper E l e c t r o w i n n i n g  E l e c t r o l y t e Composition (gpi)  Technique  Cu Conventional  electrowinning  Electrolyte recirculation (.1-...2 g p m / f t cathode) i n bench-scale c e l l  15-60  H S0 2  4  15-90  40  160  35-50  50-70  Fe  °C  2-20  26-55 60  Current Densi t i e s (mA/cm )  Reference  2  8-19 38-43  18  2  Electrolyte  circulation  Periodic current reversal (9 sec on/0.5 sec r e v e r s e ) in i n d u s t r i a l eel 1 .  31  60  P u l s e d c u r r e n t (.07-30 us pulse length)  up t o 97 43-65 optimum 1.8  63  48  up t o 250  -  20  30  36  Gas s p a r g i n g (6-8 i/min ... per cathode f a c e ) i n h a l f scale c e l l  45  150  2  100-200  25  Gas s p a r g i n g (33 £/min per cathode f a c e ) i n f u l l h e i g h t model c e l l  60  140  5  125-300  25  Electrolyte recirculation (12 £/m «min) i n f u l l h e i g h t model e e l 1  60  150  40  25  22  Air agitation trial  32  225  63  26  24  A i r a g i t a t i o n (12-15 S f t / h between s t a n d a r d - s i z e e l e c . )  32  171  63  Series bipolar  46  156  2  i n plant  3  cell  150-287  31  23-92  30  8  c o n d u c t i n g f i l m o f l e a d d i o x i d e upon which oxygen i s e v o l v e d i n a c i d i c sulfate electrolytes.  As pure l e a d i s t o o s o f t t o p e r m i t i t t o w i t h s t a n d  the r i g o r s o f h a n d l i n g o r t o m a i n t a i n i t s " d i m e n s i o n a l s t a b i l i t y " o p e r a t i o n , antimony i s added t o i m p a r t improved  during  mechanical p r o p e r t i e s  w i t h o u t a f f e c t i n g the f o r m a t i o n o f t h e l e a d d i o x i d e s u r f a c e l a y e r . a d d i t i o n s a r e made t o i m p a r t enhanced anode c o r r o s i o n r e s i s t a n c e . based anodes cannot  be used i n c h l o r i d e o r n i t r a t e - c o n t a i n i n g  due t o t h e i r tendency mechanical  improved  n e c e s s a r y t o employ anodes h a v i n g  nesses o f t h e o r d e r o f 1 cm i n o r d e r t o p r e v e n t b u c k l i n g [ 6 ] .  thick-  The l i f e t i m e  o f an 84.5 Pb/14.5 Sb/0.6 Ag anode o p e r a t e d a t 7.5-18.3 mA/cm  2  t o be 6-8 y e a r s [ 8 ] .  (Lead-  electrolytes  t o d i s s o l v e . ) A l t h o u g h a l l o y i n g produces  qualities, i t is s t i l l  Silver  i s reported  The o p e r a t i n g p o t e n t i a l o f a l e a d anode a l s o  i n c r e a s e s p r o g r e s s i v e l y w i t h time owing t o t h e c o n t i n u o u s growth o f t h e o x i d e f i l m w i t h time [ 3 9 ] . In SX/EW p r o c e s s e s , where the pregnant e l e c t r o l y t e generated i n the s t r i p p i n g stage may c o n t a i n 130-320 g p l s u l f u r i c a c i d , depending on t h e p a r t i c u l a r SX reagent i n q u e s t i o n [ 4 0 - 5 1 ] , enhanced c o r r o s i o n o f conventional  l e a d a l l o y anodes w i t h subsequent c o n t a m i n a t i o n o f t h e cathode  p r o d u c t has been r e p o r t e d [16,21,31,47,52-55]. render copper u n s u i t a b l e f o r w i r e b a r [ 1 6 , 2 1 ] .  copper  Lead c o n t e n t s o f 10-50 ppm S e v e r a l methods f o r l o w e r i n g  cathode  l e a d l e v e l s t o a c c e p t a b l e v a l u e s f o r t h i s purpose have been pro-  posed.  These a r e :  gas s p a r g i n g , e l e c t r o l y t e c i r c u l a t i o n and f i l t r a t i o n  [24-26,28,30-32], the use o f diaphragms [ 5 5 ] , a d d i t i o n s o f i n h i b i t o r s f o r +2 l e a d c o r r o s i o n (such as Co  ) [ 5 3 - 5 7 ] , anode p r e c o n d i t i o n i n g [ 5 5 , 5 8 ] , and  the use o f a l t e r n a t i v e lead-based a l l o y s ( p a r t i c u l a r l y Pb/Ca and Pb/Sn/Ca) [53-55].  9  A logical  s o l u t i o n t o the l e a d c o n t a m i n a t i o n problem i s the  replacement o f l e a d a l l o y anodes w i t h an anode m a t e r i a l which i s l e a d - f r e e . F u r t h e r , i t i s thus p o s s i b l e to c o n s i d e r s o l u t i o n o f o t h e r problems  which  are c h a r a c t e r i s t i c o f lead-based a l l o y anodes such as d i m e n s i o n a l i n s t a b i l i t y (which i s caused by bending or b u c k l i n g o f anodes d u r i n g h a n d l i n g o r i n s e r v i c e , and i n t u r n , causes m i s a l i g n m e n t o f e l e c t r o d e s and o p e r a t i o n a t greater-than-optimum e l e c t r o d e s p a c i n g ) and h a n d l i n g problems  (the t h i c k n e s s  o f the anodes r e q u i r e d to p r o v i d e adequate mechanical s t r e n g t h r e s u l t s i n i n e f f i c i e n t use o f c e l l a d i f f i c u l t task).  space, and the w e i g h t o f the anodes makes h a n d l i n g  With the development  of i n d u s t r i a l  anodes c o n s i s t i n g  o f an e l e c t r o c a t a l y t i c a l l y a c t i v e c o a t i n g on a v a l v e metal s u b s t r a t e such as t i t a n i u m — TSA's — a n d  g e n e r a l l y r e f e r r e d t o as " t i t a n i u m s u b s t r a t e anodes,"  or  t h e i r subsequent widespread acceptance i n the c h l o r - a l k a l i  i n d u s t r y (where they have r e p l a c e d the p r e v i o u s l y - c o n v e n t i o n a l  graphite  anodes) i t i s r e a s o n a b l e t h a t anodes o f t h i s type s h o u l d be c o n s i d e r e d f o r a p p l i c a t i o n i n o t h e r i n d u s t r i a l e l e c t r o l y t i c p r o c e s s e s , such as copper electrowinning. The r e a l i z a t i o n o f the a b i l i t y o f g a l v a n i c c o u p l e s o f a noble metal such as p l a t i n u m and a v a l v e metal such as t i t a n i u m t o f u n c t i o n as " i n s o l u b l e " anodes w i t h no d e t e r i o r a t i o n o f the exposed t i t a n i u m stems p r i m a r i l y from the e a r l y work o f C o t t o n [ 5 9 , 6 0 ] .  He found t h a t c o u p l i n g  (or d i s c o n t i n u o u s c o a t i n g ) o f p l a t i n u m on t i t a n i u m a f f o r d e d a n o d i c p r o t e c t i o n o f the base metal i n systems where i t would n o r m a l l y e x p e r i e n c e a c t i v e corrosion.  F u r t h e r , on a p p l i c a t i o n o f an a n o d i c c u r r e n t the e l e c t r o n  t r a n s f e r r e a c t i o n s t a k e p l a c e p r e d o m i n a n t l y a t the noble metal  surface/  e l e c t r o l y t e i n t e r f a c e , r e s u l t i n g i n b e h a v i o u r e s s e n t i a l l y comparable  to  10  t h a t o f the massive noble m e t a l , but a t o n l y a f r a c t i o n o f the c o s t . p r o l i f e r a t i o n of patents the p r e p a r a t i o n  and  l i t e r a t u r e has  subsequently a r i s e n d e s c r i b i n g  of t i t a n i u m s u b s t r a t e anodes (TSA's) w i t h o t h e r  noble  metals o r a l l o y s , noble metal o x i d e s , base metal o x i d e s , o r o t h e r c a l l y c o n d u c t i n g compounds as the a c t i v e c o a t i n g m a t e r i a l . o f s e v e r a l m e t a l s and  t h e i r oxides  c o n s t i t u e n t s are g i v e n  i n Table  The  electroni-  properties  which are s u i t a b l e s u b s t r a t e s  or  i n impressed c u r r e n t c a t h o d i c  include electrochemical [38,39,61-71].  coating  1.4.  TSA's have been a p p l i e d p r e d o m i n a n t l y i n the c h l o r - a l k a l i and  A  industry,  p r o t e c t i o n , although other a p p l i c a t i o n s  peroxidation  r e a c t i o n s and e l e c t r o o r g a n i c  synthesis  Noble metal coated TSA's have f u r t h e r seen s a t i s f a c t o r y  s e r v i c e i n the e l e c t r o p o l a t i n g i n d u s t r i e s , which i n c l u d e the  electrodeposi-  t i o n o f chromium, g o l d , p l a t i n u m  t i n [38,64,  67,  72-76].  Several  been c o n s i d e r e d operational chlor-alkali  m e t a l s , n i c k e l , c o p p e r , and  noble metal ( o r n o b l e metal o x i d e ) c o a t e d TSA's have  f o r use as i n s o l u b l e anodes i n e l e c t r o w i n n i n g ,  data i s scarce. i n d u s t r y and  Based on t h e i r e x c e l l e n t performance i n the  on t h e i r s e r v i c e i n many e l e c t r o p l a t i n g a p p l i c a -  t i o n s , i t i s worthwhile to consider  the d e s i r a b l e or advantageous p r o p e r t i e s  of such anodes i n the p a r t i c u l a r f i e l d o f copper 1.  Dimensional  although  stability.  electrowinning:  The e l i m i n a t i o n o f  bending  or b u c k l i n g problems e n a b l e s o p e r a t i o n a t optimum e l e c t r o d e s p a c i n g and p r o v i d e s f o r a more u n i f o r m c u r r e n t d i s t r i b u t i o n b e t w e e n e l e c t r o d e p a i r s due to b e t t e r alignment. 2.  Thinness. N o b l e m e t a l c o a t e d t i t a n i u m a n o d e s need o n l y be 0 . 2 - 0 . 3 cm t h i c k i n o r d e r t o p r o v i d e d i m e n s i o n a l s t a b i l i t y , thus p e r m i t t i n g the i n s t a l l a t i o n o f more e l e c t r o d e s ' p e r c e l l . The t h i n n e s s o f t h e a n o d e s , c o m b i n e d w i t h t h e low d e n s i t y and e x c e l l e n t m e c h a n i c a l s t r e n g t h o f t i t a n i u m makes f o r e a s i e r hand 1 i ng .  Table  1.4  P r o p e r t i e s o f S u b s t r a t e and C o a t i n g M e t a l s and t h e i r Ti (Ti02) Atomic weight M o l e c u l a r weight Density,  g/cm 3  Meltping °C  point,  Resistivity, microohm-cm Crystal structure  Tensile strength, 1000 p s i Metal i o n / m e t a l std. potential Metal c x i d e / m e t a l std. potential Cost  Ta (Ta205)  27.90 (79.90) 4.507 (4.24 r u t i l e ) (3.84 anatase) 1658 (2128)  34 95 ( 9 9 . 0  Ir (Ir02) 192.2 (224.2)  92.91 (265.81)  195.09 (227.09)  16.6 (8.73)  8.57 (4.95)  21 .45 (10.2)3  2468 (1783)  Rh (Rh203)  22.5 (11 . 6 6 ) 2454 (1373)  1769 (723)  Ru (Ru02)  102.91 (253.81)  101.1 (133.1)  12.44  12.2  62o  1966 (1388)  2500 (1400)  10.6 „ (3.6xl0'°)a  5.3 (0.49)  4.51  7.6 (0.35)  BCC (Rhombic)  BCC (Rhombic)  FCC (Hexagonal)  FCC (Tetragonal)  FCC (Rhombic)  CPH (Tetragonal)  50  40  20  90 160  Ti)  (Pt/30Ir;  138  78  Mn (Mn0 2 )  Pb (Pb02) 207.21 (239.19)  54.94 (86.94) 7.43 (5.026)  11 . 3 6 (9.375)  6.37  12.5  12.45 (109)  CPH (Tetragonal)  Pt (Pt02)  180.95 (441.89)  2996 (2150)  42 (10")  Nb (Nb205)  Oxides  327 (563) 20.65 (0.908) FCC rhombic tl tetragonal]  fa: [g: 2-3 2-7  1245 (847) 185  fg: [a:  Cubic tetragonal] rhombic J 72  (Pb/6Sb)  -1.63  -1.12  •1.2b  1.16b  Ti++/Ti  Ta3+/Ta  Nb3+/Nb  Pt++/Pt  Ir3+/Ir  Rh3+/Rh  Ru + + , /Ru  Pb++/Pb  Mn + + /Mn  -0.86 Ti02/Ti  -0.31 Ta20s/Ta  -0.65 Nb20»/Nb  Pt02/Pt  0.94b Ir02/Ir  0.9b Rh203/Rh  0.79 Ru0 2 /Ru  0.666e BPb0 2 /Pb  0.05f MnOj/Mn  $170-190/ troy oz.  $300-310/ troy oz.  $400-410/ troy oz.  $60-65/ troy oz.  $2.70/1b sponge  $35-48/lb powder  -1.1  0A0  Sources f o r d a t a , unless otherwise n o t e d , were: Metals Handbook ( 8 t h e d . ) , V o l . 1 ( 1 9 6 1 ) , CRC HandbooHiSth e d . ) ( 1 9 7 5 ) , Tiie Oxide Handbook, e d . G.V. Samsonov, I F I / P l e n u m ( 1 9 7 3 ) , Stability Constants, S p e c . P u b l . 17, The Chem. S o c . ( 1 9 6 4 ) , Enaineering and hiinina Journal, YT]_ ( 1 1 ) , (19-76).  A  Other a) b) c) d) e) f)  0.7b  0.46c  -.126  $.257/lb  -1.20f  $.58/lb  sources: B a i e j , J . and 0 . S p a l e k , Coll. Cz. Chem. Com., 37 ( 1 9 7 2 ) , p. 4 9 9 , G o l d b e r g , R.N. and L . G . H e p l e r , Chem. Rev., 68TT968), p. 2 2 9 , Atlas of Electrochem. Equilibria, ed. M Pourbaix (1966), S u k h o t i n , A . M . et al., Zh. Prikl. Khim, 45 ( 1 9 7 2 ) , p . 1478, C a r r , J . P . and N.A. Hampson, Chem. Rev., 72 ( 1 9 7 2 ) , p. 679. Z o r d a n , T . A . and L . G . H e p l e r , Chem. Rev., 68 ( 1 9 6 8 ) , p . 737.  12  3.  Res-istanoe to abrasion. A c c i d e n t a l damage d u r i n g h a n d l i n g o r o p e r a t i o n i s m i n i m i z e d by t h e n o n - f r i a b l e n a t u r e o f t h e c o a t i n g s and t h e " s e l f - h e a l i n g " a c t i o n of the s u b s t r a t e whereby a p r o t e c t i v e o x i d e f i l m grows where the s u r f a c e i s s c r a t c h e d o r o t h e r w i s e damaged s u c h t h a t t h e s u b s t r a t e becomes e x p o s e d .  k.  Available forms. TSA's a r e r e a d i l y f a b r i c a t e d i n d e s i r e d s h a p e s and s i z e s , and a r e a l s o a v a i l a b l e i n e i t h e r s h e e t o r e x p a n d e d mesh f o r m . Further, varying degrees of a c t i v e c o a t i n g metal l o a d i n g are o b t a i n a b l e , d e p e n d i n g on t h e o p t i m u m r e l a t i o n b e t w e e n i n i t i a l c o s t and p r o j e c t e d l i f e t i m e .  5.  Low oxygen overvoltage. The p l a t i n u m m e t a l s and t h e i r o x i d e s - p a r t i c u l a r l y p l a t i n u m and r u t h e n i u m - a r e among t h e most e f f i c i e n t e l e c t r o c a t a l y s t s f o r t h e oxygen e v o l u t i o n r e a c t i o n .  6.  Low mechanical corrosion rates. Problems of s p a l l i n g of the a c t i v e c o a t i n g , which t y p i f i e d the e a r l y anodes whose c o a t i n g s w e r e a p p l i e d by e l e c t r o d e p o s i t i o n m e t h o d s , h a v e been l a r g e l y o v e r c o m e w i t h t h e d e v e l o p ment.of a l t e r n a t i v e c o a t i n g techniques - i n p a r t i c u l a r , thermal decomposition of noble m e t a 1 - c o n t a i n i n g o r g a n i c solutions.  7.  Low, anodic corrosion rates. The p l a t i n u m m e t a l s dissolve a n o d i c a l l y in conventional electrolytes ( c h l o r i d e o r s u l f a t e ) o n l y a t v e r y low r a t e s . For systems where s u f f i c i e n t d a t a e x i s t s , i r i d i u m c h a r a c t e r i s t i c a l l y shows h i g h e r c o r r o s i o n r e s i s t a n c e t h a n platinum. A l t h o u g h no d a t a e x i s t s f o r i r i d i u m i n s u l f u r i c a c i d s o l u t i o n s , p l a t i n u m shows a c u r r e n t e f f i c i e n c y f o r d i s s o l u t i o n o f t h e o r d e r o f 0.0001 p e r c e n t o v e r a w i d e r a g n e o f p o t e n t i a l s and a c i d s t r e n g t h s . F u r t h e r , the anode c o r r o s i o n p r o d u c t s , u n l i k e l e a d , w o u l d n o t be e x p e c t e d t o be d e l e t e r i o u s t o c a t h o d e quali ty.  8.  High current density operation. The p r o v e n s t a b i l i t y o f TSA's i n t h e c h l o r - a l k a l i i n d u s t r i e s a t c u r r e n t d e n s i t i e s f i f t y times g r e a t e r than those e n c o u n t e r e d i n c o n v e n t i o n a l c o p p e r e l e c t r o w i n n i n g recommend them for proposed higher c u r r e n t d e n s i t y o p e r a t i o n s in t h i s field.  9-  Resistance  to open-circuit  conditions.  No d e g r a d a t i o n  o r d e a c t i v a t i o n o f t h e n o b l e m e t a l c o a t e d TSA's o c c u r on h o l d i n g a t open c i r c u i t ; i n d e e d s u c h t r e a t m e n t may e v e n be c o n s i d e r e d t o be b e n e f i c i a l i n a s m u c h as i t w o u l d r e s u l t i n l o w e r o v e r p o t e n t i a l b e h a v i o u r on s t a r t - u p .  13  10.  Resistance to overrent reversals. A l t h o u g h TSA's a r e not s u i t a b l e f o r a p p l i c a t i o n where p r o l o n g e d c a t h o d i c c u r r e n t s a r e passed (the "anodic p r o t e c t i o n " o f t h e s u b s t r a t e no l o n g e r e x i s t s ) , t h e o n l y e f f e c t o f s u c h t r e a t m e n t on t h e n o b l e m e t a l c o a t i n g i s t o r e d u c e a n y p r e v i o u s l y formed s u r f a c e oxygen - which would have an " a c t i v a t i n g " e f f e c t on t h e e l e c t r o d e .  In p a r t i c u l a r , anodes h a v i n g a Pt/30 I r a l l o y c o a t i n g operating  overpotentials  and low c o a t i n g  metal  loss rates  e l e c t r o l y t e even a f t e r thousands o f hours o f o p e r a t i o n reasons, i t i s d e s i r a b l e  t o have n o b l e metal  loadings  ness i s below a m i c r o n - o r r o u g h l y t o l o a d i n g s 20 g/m  2  (mass  o f n o b l e metal  A l t h o u g h Pt/30 I r - T i  i n chloride  [77,78].  F o r economic  whose average t h i c k -  o f l e s s than t h e o r d e r o f  per u n i t o f geometric area o f the s u b s t r a t e ) .  anodes o f f e r t h e advantages d e s c r i b e d  important questions require  show low  above,  several  clarification:  1.  What i s t h e r a t e o f l o s s o f c o a t i n g m e t a l u n d e r c o n d i t i o n s s i m i l a r to those encountered i n a c t u a l copper electrowinning practice? Does t h e c o a t i n g l o s s r a t e depend o n d i f f e r e n c e s i n m a n u f a c t u r e f o r n o m i n a l l y i d e n t i c a l ( l o a d i n g and c o m p o s i t i o n ) anodes? Does t h e c o a t i n g l o s s r a t e d e p e n d on n o b l e m e t a l l o a d i n g ? What i s t h e n a t u r e o f t h e m e c h a n i s m o f c o a t i n g l o s s ?  2.  What i s t h e p o t e n t i a l v s . t i m e ( " p a s s i v a t i o n " ) b e h a v i o u r o f t h o s e a n o d e s i n t h e same c o n d i t i o n s ? Is the p a s s i v a t i o n r e v e r s i b l e o r i r r e v e r s i b l e (as d e f i n e d by r e p r o d u c i b i l i t y o f p o t e n t i a l v s . t i m e b e h a v i o u r ) ? What i s t h e c a u s e o f t h e " p a s s i v a t i o n " phenomenon?  3.  What a r e t h e e f f e c t s o f e l e c t r o l y t e a d d i t i v e s a n d p u l s e d c u r r e n t o p e r a t i o n on t h e c o r r o s i o n a n d p a s s i vation behaviour?  Chapter 2  LITERATURE SURVEY: ANODIC CORROSION OF PLATINUM AND IRIDIUM 2.1 2.1.1  Platinum Chloride Electrolytes P l a t i n u m shows a c t i v e d i s s o l u t i o n w i t h p r a c t i c a l l y 100 p e r c e n t  c u r r e n t e f f i c i e n c y i n c h l o r i d e e l e c t r o l y t e s a t p o t e n t i a l s below about 1.1 v o l t s (SHE), and undergoes p a s s i v a t i o n ( d e f i n e d as a d e c l i n e i n d i s s o l u t i o n e f f i c i e n c y , n o t n e c e s s a r i l y a d e c r e a s e i n c o r r o s i o n r a t e ) above t h i s potential  [79-98].  A c t i v e d i s s o l u t i o n rates are increased with higher a c i d  strengths  ( r e a c h i n g a maximum o f 1.6 mA/cm  2  i n 10.2N HC1 [ 9 1 ] ) , c h l o r i d e  c o n c e n t r a t i o n s , and t e m p e r a t u r e s , and obey a l i n e a r p o t e n t i a l v s . l o g i r e l a t i o n w i t h a s l o p e o f 102-103 mv, s u g g e s t i n g i s the discharge o f a u n i v a l e n t i o n .  the rate-determining  P a s s i v a t i o n of platinum  step  dissolution  i s a t t r i b u t e d t o t h e c o m p e t i t i v e a d s o r p t i o n o f oxygen and o f c h l o r i d e i o n s [79-82].  P l a t i n u m d i s s o l u t i o n i n t h e " p a s s i v e " r e g i o n appears t o be  a s s o c i a t e d w i t h t h e p a r t i a l oxygen e v o l u t i o n p r o c e s s , as i n d i c a t e d by t h e p a r a l l e l i s m o f the p a r t i a l  p o l a r i z a t i o n curves  f o r oxygen e v o l u t i o n and  p l a t i n u m d i s s o l u t i o n , and the l a c k o f p a r a l l e l i s m w i t h t h e c h l o r i n e evol u t i o n process  [ 9 7 ] . T r a c e r s t u d i e s have r e v e a l e d a c o n s i d e r a b l e  14  15  time-dependence o f the p a r t i a l d i s s o l u t i o n c u r r e n t , state conditions  requiring  several  " p u l s e " o f metal d i s s o l u t i o n  2.1.2  Inert  days i n some c a s e s , from an i n i t i a l l y  the p a s s i v e s t a t e .  [88,89].  and s u l f u r i c a c i d s p l a t i n u m d i s s o l u t i o n o n l y o c c u r s i n  No p o t e n t i a l - d e p e n d e n c e o f the c o r r o s i o n  t h e anode p o t e n t i a l  time-dependence o f the c o r r o s i o n  platinum dissolves  A pronounced  r a t e a l s o e x i s t s on s w i t c h i n g on o r on changing  r e s u l t s o f the  tracer studies  by Chemodanov, assuming  i n the +4 s t a t e , the c u r r e n t e f f i c i e n c y f o r p l a t i n u m d i s s o l u -  tion i n s u l f u r i c acid solutions r a t e o f 1.8 ug/A-hr [ 9 9 ] . corrosion  [94,99-102].  i s c o n s t a n t over wide  o r c u r r e n t d e n s i t y and a c i d c o n c e n t r a t i o n . )  From the  i s observed  process a t higher p o t e n t i a l s  (That i s , t h e c u r r e n t e f f i c i e n c y o f p l a t i n u m d i s s o l u t i o n ranges o f p o t e n t i a l  rate  exceeds about 1.8 v o l t s , and t h e d i s s o l u t i o n process i s  found t o p a r a l l e l the oxygen e v o l u t i o n  the p o t e n t i a l .  high  Electrolytes  In p e r c h l o r i c  until  w i t h a t t a i n m e n t o f steady  o f .0001 p e r c e n t can be r e l a t e d t o a metal  Other s t u d i e s  by Baboian [39]  and Ives [103]  o f p l a t i n u m i n 160 and 150 gpl s u l f u r i c a c i d s o l u t i o n s ,  show h i g h e r l o s s r a t e s  (or more c o r r e c t l y , c o r r o s i o n  data o f Baboian, a c o r r o s i o n  on t h e  respectively,  efficiencies).  From t h e  e f f i c i e n c y o f 6.4 ug/A-hr can be c a l c u l a t e d  ing o p e r a t i o n f o r 45 days a t 108 mA/cm  2  i n 30°C s o l u t i o n .  loss  follow-  Ives found t h e  c u r r e n t e f f i c i e n c y f o r p l a t i n u m d i s s o l u t i o n was r e l a t i v e l y independent o f c u r r e n t d e n s i t y over the range 20-200 mA/cm , but t h a t i t i n c r e a s e d 2  w i t h temperature. 24,  C o r r o s i o n e f f i c i e n c i e s were found t o be a p p r o x i m a t e l y 6, 10,  and 43 ug/A-hr i n 20, 4 0 , 60, and 80°C s o l u t i o n s ,  found t h a t i n c r e a s e d a c i d c o n c e n t r a t i o n i n an i n c r e a s e i n c o r r o s i o n chlorate  strongly  respectively.  Ives  also  (from 150 t o 220 gpl a t 70°C) r e s u l t e d  e f f i c i e n c y from 30 t o 60 ug/A'hr a t 193 mA/cm . In  e l e c t r o l y t e s , the platinum d i s s o l u t i o n rate i s s t i l l  2  observed t o p a r a l l e l  16  the oxygen e v o l u t i o n  r a t e , even a t high p o t e n t i a l s  (above 2.2  chlorate  p r o d u c t i o n becomes the p r i m a r y e l e c t r o d e  [91]  noted t h a t the commencement of the p o t e n t i a l - d e p e n d e n c e of the  has  reaction  v o l t s ) where  [97].  per-  Chemodanov platinum  d i s s o l u t i o n p r o c e s s o n l y occurs a t p o t e n t i a l s where monomolecular oxygen c o v e r age  exists.  Further,  and  oxygen e v o l u t i o n  the p a r a l l e l i s m between the r a t e s o f p l a t i n u m d i s s o l u t i o n suggest a s i m i l a r r a t e - d e t e r m i n i n g s t e p f o r t h e s e p r o c e s s e s .  At anode p o t e n t i a l s above 1.8 amounts of NCI,  NaCl, or C l  d e c r e a s e i n the c o r r o s i o n  2.1.3  2  v o l t s , the a d d i t i o n of  increasing  t o an i n e r t e l e c t r o l y t e r e s u l t s i n a p r o g r e s s i v e  r a t e of p l a t i n u m [94,99,100,104-106].  Organic-Containing E l e c t r o l y t e s The  a n o d i c d i s s o l u t i o n o f p l a t i n u m i n nonaqueous e l e c t r o l y t i c s o l u -  t i o n s such as e t h y l e n e g l y c o l and o f v a r i o u s o r g a n i c compounds has the use  of aqueous s o l v e n t s ,  studies  again reveal  methanol used f o r the been s t u d i e d  [107-1121.  Kolbe  Addition  o f w a t e r , or  r e s u l t s i n enhanced d i s s o l u t i o n l o s s e s .  an e x t r e m e l y slow s t a b i l i z a t i o n of the  platinum d i s s o l u t i o n rate.  electrosynthesis  Tanaka [113]  Tracer  (initially  high)  found t h a t the presence o f EDTA i n  a c e t a t e b u f f e r s o l u t i o n promoted h i g h e r a n o d i c d i s s o l u t i o n r a t e s f o r p l a t i n u m . P l a t i n u m anode l o s s e s  i n the p r e p a r a t i o n  of p e r h y d r o l have a l s o been r e p o r t e d  [114]. T h i o u r e a i n <.1M  c o n c e n t r a t i o n i s an e f f e c t i v e i n h i b i t o r f o r the  d i s s o l u t i o n of p l a t i n u m i n h y d r o c h l o r i c c h l o r i c acid solutions, thiourea  has  acid solutions  an i n h i b i t i n g a c t i o n above 1.95  promotes d i s s o l u t i o n below t h i s p o t e n t i a l s u l f o a c i d and solutions  [95,115,116].  [194,112,116-118].  In pervolts,  acid  In s u l f u r i c a c i d s o l u t i o n , on the o t h e r hand, t h i o u r e a  a s l i g h t promoting e f f e c t ( a t 2.3  v o l t s i n 3MH S0 ,50°).[T16]. 2  lt  but  Naphthalene  sodium s u l f i d e a l s o e x h i b i t i n h i b i t i n g a c t i o n i n p e r c h l o r i c  [104-116].  anodic  has  Under the same  17  o n d i t i o n s , a d d i t i o n o f naphthalene s u l f o a c i d s o r a r o m a t i c benzene cause an a c c e l e r a t i o n o f t h e d i s s o l u t i o n r a t e .  compounds such as  Enhanced c o r r o s i o n o f  p l a t i n u m anodes w i t h 0.1 g p l t h i o u r e a a d d i t i o n i n g o l d - and s i l v e r - p l a t i n g a p p l i c a t i o n s has been r e p o r t e d [ 1 1 9 ] .  Thiourea  a d d i t i o n s have no e f f e c t on  the c o r r o s i o n o f p l a t i n u m anodes i n aqueous a c e t a t e e l e c t r o l y t e s  2.1.4  A l t e r n a t i n g Current  [116].  Corrosion  P l a t i n u m d i s s o l v e s a t a high r a t e under t h e i n f l u e n c e o f A.C. i n h y d r o c h l o r i c o r s u l f u r i c a c i d s o l u t i o n s , w i t h enhanced d i s s o l u t i o n o c c u r r i n g a t h i g h e r c u r r e n t d e n s i t i e s , h i g h e r a c i d s t r e n g t h s , h i g h e r temperature and lower frequencies  [120-128].  T y p i c a l c o r r o s i o n e f f i c i e n c i e s , based on t h e t o t a l  time  o f o p e r a t i o n , range from 11000 t o 48000 ug/A-hr i n 3 t o 12M HC1 w i t h 60 Hz [122] A.C.  Superimposed A.C. on D.C. i s a l s o known t o i n c r e a s e the d i s s o l u t i o n r a t e  o f p l a t i n u m , w i t h i n t e n s e c o r r o s i o n o c c u r r i n g when t h e magnitude o f t h e superimposed A.C. i s such t h a t b i p o l a r c o n d i t i o n s a t t a i n  [87,129-134].  The d i s s o l u t i o n o f p l a t i n u m d u r i n g c y c l i c voltammetry i n i n e r t e l e c t r o l y t e s , where t h e p o t e n t i a l i s v a r i e d between t h e p o t e n t i a l s f o r e v o l u t i o n o f hydrogen and oxygen, has been c o n f i r m e d 143].  i n r e c e n t y e a r s [136-  P l a t i n u m l o s s r a t e s depend on t h e p o t e n t i a l scan r a t e s and t h e a n o d i c  potential  limit. Square-wave c u r r e n t o r p o t e n t i a l c y c l i n g [87,140,144] and o f f / o n  s w i t c h i n g [88,89,99] a r e a l s o known t o cause enhanced p l a t i n u m c o r r o s i o n . Chemodanov [101] has observed a time-dependent " p u l s e " o f d i s s o l u t i o n on commencement o f c a t h o d i c p o l a r i z a t i o n , l i k e l y due t o t h e d i s s o l u t i o n o f surface  oxides. A.C. " r i p p l e " i s n o t l i k e l y t o be d e l e t e r i o u s under c o n d i t i o n s  o f t h e high frequency  (> 100 Hz) and low ( o f t h e o r d e r o f a few per c e n t )  18  r a t i o o f the r.m.s. A.C.  c u r r e n t to the D.C.  s u p p l i e d to i n d u s t r i a l e l e c t r o l y t i c c e l l s  2.1.5  c u r r e n t t y p i c a l o f the  D.C.  [130].  State of Platinum i n S o l u t i o n Aquo-ions o f p l a t i n u m do not appear to be formed [ 1 4 5 ] .  p l a t i n u m tends t o form s t a b l e complexes i n the ( I I ) and s t a t e s whereby h y b r i d d s p [146].  (Indeed,  2  and d s p 2  However,  (IV) o x i d a t i o n  ("inner o r b i t a l " ) bonds a r e  3  created  the s t a b i l i t y o f such complexes i s so high t h a t e l e c t r o -  d e p o s i t i o n o f p l a t i n u m would not be p o s s i b l e i f the m e t a l l i c s t a t e d i d not have a h i g h s t a b i l i t y [ 1 4 7 ] . ) chloric acid solutions, P t C l " 6  a l s o been r e p o r t e d [ 9 6 ] .  2  itself  During a c t i v e c o r r o s i o n i n hydro-  i s formed [ 9 5 , 9 8 ] , a l t h o u g h  PtCU"  2  has  Hawkins [98] a l s o suggests t h a t P t C l ( H 0 ) ~ i s 5  2  formed a t lower c h l o r i d e c o n c e n t r a t i o n s . Both P t ( I I ) [137,139,143] and  P t ( I V ) [139,140] have been d e t e c t e d  i n s u l f u r i c a c i d s o l u t i o n s a f t e r c y c l i c voltammetry. has d e s c r i b e d the p r e p a r a t i o n o f two t h a t may  e x i s t i n both P t ( I I ) and  e x i s t s concerning  Ginzburg  [148,149]  s o l u b l e p l a t i n u m s u l f a t e compounds  P t ( I V ) forms.  No f u r t h e r i n f o r m a t i o n  the nature o f p l a t i n u m s u l f u r i c a c i d s o l u t i o n s , a l t h o u g h  Kukushkin [150] r e p o r t e d t h a t the s o l u t i o n s p e c i e s o f p l a t i n u m a n o d i c a l l y d i s s o l v e d from Cu/Pt a l l o y s were s i m i l a r to the "brown" s u l f a t e o f  2.2 2.2.1  Ginzburg.  Iridium Chloride Electrolytes I r i d i u m o n l y shows a c t i v e d i s s o l u t i o n i n s t r o n g  t i o n s , and m a n i f e s t s  p a s s i v e behaviour  otherwise  (>4N)  - indicating  HC1  solu-  superior  19  corrosion r e s i s t a n c e t o platinum  under s i m i l a r c o n d i t i o n s  [95,151-153].  In 12N HC1, 50°, a maximum a n o d i c d i s s o l u t i o n r a t e o f 1.4 x 10 * mA/cm _1  c a l c u l a t e d from t h e t r a c e r - s t u d y d a t a o f L l o p i s [ 1 5 3 ] . of 1.3 x 1 0 " mA/cm 5  2  2  A corrosion  current  i s c a l c u l a t e d f o r the p a s s i v e d i s s o l u t i o n o f i r i d i u m ;  i n s a t u r a t e d N a C l , 50°, a t 1.64 v o l t s (SHE) [ 1 5 3 ] , which i s comparable t o the d i s s o l u t i o n c u r r e n t f o r p l a t i n u m  2.2.2  i n 5N NaCl e l e c t r o l y t e [ 9 7 ] .  Inert Electrolytes No s t u d i e s have been made c o n c e r n i n g t h e a n o d i c d i s s o l u t i o n o f  i r i d i u m i n i n e r t e l e c t r o l y t e s such as p e r c h l o r i c o r s u l f u r i c a c i d s .  If i t  i s assumed t h a t t h e c o r r o s i o n b e h a v i o u r i s s i m i l a r i n n a t u r e t o t h a t o f platinum,  then d i s s o l u t i o n may be c o n s i d e r e d  t o o c c u r on a p a s s i v e  electrode  surface.  2.2.3  Organic-Containing  Electrolytes  Only a s i n g l e r e p o r t has been made w i t h r e s p e c t t o the b e h a v i o u r of i r i d i u m i n organic-containing  solutions.  Kovsman [107]  i r i d i u m was i n f e r i o r t o p l a t i n u m  i n c o r r o s i o n r e s i s t a n c e when used f o r  the Kolbe e l e c t r o o r g a n i c s y n t h e s i s i n e t h y l e n e  2.2.4  A l t e r n a t i n g Current  found t h a t  g l y c o l and methanol  solvents.  Corrosion  I r i d i u m d i s s o l v e s under t h e a c t i o n o f pure A.C. i n s o l u t i o n s o f h y d r o c h l o r i c [95,123,125,151,153-157], hydrobromic [ 1 2 5 ] , s u l f u r i c and  sulfamic  [159]  acids.  Box [123]  [156,158],  found t h a t i r i d i u m u n l i k e o t h e r  metals,  20  showed a d e c r e a s i n g (3-12N HC1)  rate of d i s s o l u t i o n with i n c r e a s i n g a c i d  under 60 Hz A.C.  c o r r o s i o n r a t e to p l a t i n u m  of 460 mA/cm . 2  Few imposed A.C.  Hoare [160]  a n o d i c p o t e n t i a l s of 0.75 Rand [142]  and  2.05  t o swing between  v o l t s produced a s o l u b l e p u r p l e conditions  oxide (scan  i n 1MH S0 , i r i d i u m shows a 2  4  higher  platinum.  aquo-ions of i r i d i u m are unknown [ 1 4 5 ] .  forms s t a b l e complexes i n the bonds [146].  3  i n d i l u t e (<6N) formed. exist.  HC1  ( I I I ) and  (IV) o x i d a t i o n s t a t e s w i t h  In c h l o r i d e e l e c t r o l y t e s , L l o p i s has determined  both I r ( I I I ) and  hybrid that  I r ( I V ) c h l o r o complexes  In s u l f u r i c a c i d s o l u t i o n s , a n i o n i c I r ( I I I ) and e x i s t i n s o l u t i o n [158,161,162].  the anode p o t e n t i a l exceeds 1.5  Platinum/Iridium The  Iridium  s o l u t i o n s , time-dependent a q u o c h l o r o complexes are  At h i g h e r c o n c e n t r a t i o n s ,  complexes may  2.3  super-  S t a t e of I r i d i u m i n S o l u t i o n As w i t h p l a t i n u m  2  biased  r e p o r t s t h a t , under i d e n t i c a l  metal l o s s r a t e per c y c l e than  d sp  considerably  solutions.)  found t h a t an A.C.  r a t e , a n o d i c l i m i t ) of c y c l i c voltamrnetry  2.2.5  and was  data e x i s t c o n c e r n i n g the d i s s o l u t i o n o f i r i d i u m w i t h on U.C.  on i r i d i u m .  ( I r i d i u m showed a s i m i l a r  a t the lower c o n c e n t r a t i o n s ,  more c o r r o s i o n - r e s i s t a n t i n the c o n c e n t r a t e d  strength  Ir(IV) sulfate  I r ( I V ) i s found o n l y when  v o l t s [161],162].  Alloys  few c o r r o s i o n data t h a t e x i s t f o r P t / I r a l l o y s i n v a r i o u s  e l e c t r o l y t e systems are somewhat c o n f l i c t i n g - w i t h a l l o y s showing both lowered and  improved c o r r o s i o n r e s i s t a n c e over pure p l a t i n u m .  Anderson  [163]  21  found t h a t P t / 2 5 I r a l l o y was l e s s c o r r o s i o n - r e s i s t a n t i n f l o w i n g sea w a t e r , operating  a t a c u r r e n t o f 538 mA/cm  2  (1.25 v s . 0.68 yg/A«hr f o r p l a t i n u m ) .  Cook [164,165] measured r e l a t i v e l y high c o r r o s i o n e f f i c i e n c i e s f o r P t / l O I r , operating  a t 250 mA/cm i n 6.2M NaC10 , 44° (16 ug/A-'hr) and even 2  3  higher  l o s s e s i n 6.2M NaClOi, (130-220 u g / A - h r ) , where oxygen e v o l u t i o n i s t h e predominant anode p r o c e s s .  Baboian [39] i n v e s t i g a t e d t h e c o r r o s i o n o f  Pt/10,20,30 I r a l l o y s i n 160 gpl H S 0 , 30°, over 45 days' o p e r a t i o n a t 2  108 mA/cm . 2  4  C a l c u l a t i n g c o r r o s i o n e f f i c i e n c i e s from h i s d a t a , t h e a l l o y s  Pt/10 I r and Pt/20 I r show e s s e n t i a l l y t h e same c o r r o s i o n b e h a v i o u r as platinum  (6.4 ug/A«hr), and Pt/30 I r i s found t o be s u p e r i o r  (4.5 ug/A«hr).  Yufa [154] found t h a t Pt/25 I r a l l o y d i s s o l v e d q u a n t i t a t i v e l y under t h e i n f l u e n c e o f A.C.  i n 20% HC1 e l e c t r o l y t e , w i t h t h e c o n c e n t r a t i o n s  of the  d i s s o l v e d m e t a l s h a v i n g t h e same r a t i o as i n t h e a l l o y .  2.4  TSA C o r r o s i o n C o r r o s i o n o f TSA's must be c o n s i d e r e d  the pure noble m e t a l s o r a l l o y s d e s c r i b e d  s e p a r a t e l y from t h a t o f  above because o f the p o s s i b i l i t y  t h a t the l o s s o f c o a t i n g metal may be due t o mechanical detachment, o r s p a l l i n g , as a r e s u l t o f poor c o a t i n g / s u b s t r a t e  adhesion, residual  stresses  i n t h e c o a t i n g , undermining o f t h e c o a t i n g due t o a t t a c k o f the exposed substrate, generation  o f gas bubbles w i t h i n p o r e s , o r t h e c o r r o s i o n o f con-  tact points i n highly dispersed  coatings.  F u r t h e r , the a n o d i c breakdown  o f TSA's may o c c u r i n s o l u t i o n s w i t h s p e c i e s a g g r e s s i v e a f t e r l o s s o f most o f the a c t i v e c o a t i n g m a t e r i a l .  to titanium, or  22  2.4.1  Anodic  Corrosion  Much o f t h e c o r r o s i o n  d a t a f o r TSA's i s found i n t h e p a t e n t  l i t e r a t u r e , which i n t u r n d e a l s predominantly such anodes i n t h e c h l o r - a l k a l i  with the a p p l i c a t i o n s of  i n d u s t r y - hence much i n f o r m a t i o n  regarding corrosion e f f i c i e n c i e s i n c h l o r i d e e l e c t r o l y t e s . procedure i s seldom t r e a t e d a d e q u a t e l y  exists  Experimental  i n such s o u r c e s , however.  The  c o r r o s i o n e f f i c i e n c i e s f o r v a r i o u s noble metal c o a t i n g s which c o n t a i n e i t h e r p l a t i n u m o r i r i d i u m m e t a l s , manufactured by d i f f e r e n t and  techniques,  s u b j e c t e d t o anodic e l e c t r o l y s i s i n v a r i o u s e l e c t r o l y t e s i n both  l a b o r a t o r y and i n d u s t r i a l  c e l l s a r e summarized i n T a b l e 2.1.  The a c c u r a c y  o f much o f these c o r r o s i o n r e s u l t s i s s u b j e c t t o q u e s t i o n as a r e s u l t o f the high l e v e l s o f u n c e r t a i n t y a s s o c i a t e d w i t h many o f t h e c o a t i n g l o s s measurement t e c h n i q u e s ,  i n c l u d i n g t i m e - t o - f a i l u r e measurements, w e i g h t  l o s s d e t e r m i n a t i o n s , and s o l u t i o n a n a l y s i s .  (These methods may n e g l e c t  r e s i d u a l c o a t i n g l e v e l s a f t e r f a i l u r e , weight i n c r e a s e s due t o o x i d e growth o r the presence o f anode s u r f a c e d e p o s i t s , and l o s s e s o f d i s s o l v e d metal t o the cathode, r e s p e c t i v e l y . s e n s i t i v i t y (approaching  10  - 1 1  Tracer s t u d i e s permit  the highest  p e r c e n t ) [ 1 1 2 ] , but a r e l i m i t e d by t h e  h a l f - l i v e s o f the isotopes of i n t e r e s t .  Two X-ray s p e c t r o s c o p i c  niques e x i s t , which r e l y e i t h e r on the r a d i o a c t i v e decay o f a  tech-  convenient  n u c l i d e t o e x c i t e X - r a d i a t i o n ("Betascope") o r t h e i r r a d i a t i o n o f t h e specimen w i t h c o n t i n u o u s  r a d i a t i o n from an X-ray tube.  The former measure-  ments can be performed w i t h p o r t a b l e u n i t s , whereas t h e l a t t e r r e q u i r e s a s t a t i o n a r y X-ray s p e c t r o m e t e r .  Betascope measurements a r e , on t h e o t h e r  hand, not s e l e c t i v e as t o the element o f i n t e r e s t , u n l i k e wavelength- o r energy-dispersive  X-ray  spectrometers.  Table  2.1  Summary o f C o r r o s i o n Rates Reported f o r Noble M e t a l Coated Anodes  Coating  (P) *  patent c a l c u l a t e d from g/Ton C l  type:  2  Year Type  Loading  Current D e n s i t y (mA/cm 2 )  F W X B S T  time to f a i l u r e weight l o s s X-ray spectroscopy Betascope Solution analysis Tracer  Electrolytes  1958  ED  0.127u  Pt-Ti  65  sea w a t e r ;  167 167  1959 1959  Ed ED  17.5 17.5u 1.75,2.5u 0.2p  Pt-Ti Pt-Ti  23 23  sea water sea water  F F  168  1960  ED  2.5,3p  Pt-Ti  650  250 g p l H a C l , p H - 3 . 3 5 ; 7 5 °  W  169  1952  ED  7-10u  Pt-Ti  100  300 g p l NaCl  170  1964  ED  7f0  brine  (1 hour e l e c t r o l y s i s )  W  171(P)  1965  ED  Pt/50Rh-Ti Pt-Ti Rh-Ti Pt/50Rh-Ti Pt-Ti Rh-Ti  260  satd.  N a C l ; 50°  X X X X X X  Pt-Ti Pt-Ti Pt-Ti Pt-Ti Pt-Ti Pt-Ti Pt-Ti  50 50 117 300 200 500 100  f l o w i n g sea w a t e r ( c a t h o d i c p r o t e c t i o n ) sea w a t e r ( c a t h o d i c p r o t e c t i o n ) 2% NaCl sea w a t e r ( h y p o c h l o r i t e p r o d u c t i o n ) 22% NaCl nickel sulfate plating solution 27% NaCl  B B S B W B W  Pt-Ti  250  s y n t h e t i c sea water  B  Pt-Ti 12.7u " "  " 1966  ED ED ED ED ED ED  97 172(P) 72  1966  ED  1967  ED  1967  ED ED  lp  Pt-Ti  2.5u "  Pt-Ti Pt-Ti Pt-Ti  .  " "  520  R.T.  Measurement Procedure  166  129  '  Measurement p r o c e d u r e :  (assuming l g / T o n C I 2 = 1 . 4 5 8 u g / A - h r )  Coating Reference  ED e l e c t r o d e p o s i t e d TO thermal d e c o m p o s i t i o n CL c l a d  N a C l ; 80°  "  78-310 97 431 431  290 g p l NaCl  JA^h?| <16.8-2805 ( d e p . on h e a t treatment) 660 66 230 5-10  1000 .17* .75* .15* .14* 1.22* .38* . 1 (6 y e a r s ) 1.85 (100 d a y s ) 1.2-2.7 .05 .28 1.79 32.6 (.49 hr) 4 . 8 (49 and 143 h r ) .1-3.5 3-7  (Chlorate production)  (anodes w i t h edges) (no edges) .71  75-100 g p l H 2 S 0 „ , 5 0 - 5 7 g p l C u S 0 „ ; 4 0 ° nickel plating solutions + additives n i c k e l p l a t i n g s o l u t i o n s (no a d d i t i v e s )  W W W  3  B B B  1.46* .23-.32* .52*  173  1967  ED  50-200 g/m 2 P t - T i  550-750  satd.  174(P)  1967  TD  30 g/m 2 P t - T i Pt-Ti 40 g/m 2 P t - T i  400 600 290-540  mercury c e l l diaphragm e e l 1; 7 0 ° mercury e e l 1; 60°  "  Efficiency  1.2  " satd.  F  Corrosion  NaCl,  pH 3 ; 60°  200 13-200 5-27  CONTINUED  T a b l e 2.1 ( C o n t i n u e d ) Coating Reference  Year Type  175(P)  1958  176  1969  177  1959  178(P)  1970  TD  20 g/m  2  Pt-Ti TD TD  1970  ED  38  1971  TD TD  1972  ED  15-18 g/m 16 g/m  2  2  181(P)  1972  182  1972  183(P)  1972  TD  184(P)  1973  TD TD  185(P)  1973  186(P)  TD  Pt-Pt-Ti Pt/Rh-Pt-Ti Pt-Ti  10 g/m  Pt/30 I r - T i Pt/30 I r - T i 12.5-15u P t - T i 2  1.5u 1.5u  ED  4.8 g/m  2  Current Densi t y (mA/cm )  Corrosion  brine 30- 490 155 1300 1300  .016  3% N a C l ; 25°C ( h y p o c h l o r i t e chlorate  c e l l ; pH 7; 105-123°  3X'NaCl + s a c c h a r o s e diaphragm c e l l chlorate c e l l peroxidation  Ir/Li-Ti  1800  Pt-Ti  S W  3.1-5 .2 1.06 .26- 4.3  w  >100 .(157 .1 .4- . 5  20 g p l c i t r i c a c i d , 25 q p l NaOH.pH 12,60° "  1800 800000  mercury c e l l synthetic  <.06 ( a f t e r 330 d a y s )  seawater ( c a t h o d i c p r o t e c t i o n )  w  .7  400  300 g p l N a C l ; 80°  .07  65Ir/21Rh/14Ru-Ti Ir/Rh-Ti  400 "  300 g p l N a C l ; 30°  .09 .14  TD TD  45Pt/45Rh/10Ru-Ti Pt/60Rh-Ti  400  300 g p l N a C l ; 80° "  .08 .14  1973  TD TD  43Ir/42Pt/15Ru-Ti Pt/40Ir-Ti  400 "  300 gpl N a C l ; 80°  .10 .19  187(P)  1973  TD  iu  27Pt/63Rh/10Pd-Ti  800  300 g p l N a C l ; 80"  .06  188(P)  . 1973  TD TD  lu  27/Ir/64Rh/9Ru-Ti• Ir/30Rh-Ti  800 "  300 g p l NaOH; 80°  .09 .13  189(P)  1973  TO  lp  2OIr/60Pt/20Rh-Ti  800  300 g p l N a C l ; 80°  .07  190(P)  1973  TD  lp  59Ir/31Pt/10Pd-Ti  800  300 g p l N a C l ; 80°  .08  1 91 CP)  1973  TD  lp  55I;736Rh/9Pd-Ti  800  300 g p l N a C l ; 80°  .05  192(P)  1973  TD TD  1.5p 10Ru/60Ir/30Rh-Ti Pt-Ti  600 "  270 gpl N a C l ; 90°  .02 2.5 .05 2.7  M  193(P)  1973  TD TD  1.5p 2 0 I r / 3 0 R h / 5 0 P t - T i Rh-Ti  600 "  270 g p l N a C l ; 90°  194  1973  TD TD  1.3u 8.9p  300  chlorate  78  1973  TD TD  39  1974  CL  Pt/40Ir-Ti Pt/40Ir-Ti  5-10 g/m  2  lp  Pt/30Ir-Ti Pt-Nb  "  p r o d u c t i o n ; 60°C  industrial industrial 108  chlorate chlorate  160 gpl H S0»; 30° 2  Efficiency  A-hr J *  0-1.11  2  100  50 50  prodn.)  250 g p l N a C l , 15 g p l N a S 0 u , pH 2.5; 70°  200  Ru/Rh-Ti Pt-Ti  Ir/Rh-Ti  Measurement Procedure  Electrolytes  2  Pt/Ir-Ti Pt-Ti  179  180(P)  Loading  cell cell  (Klong,Malaysia) (Cornwall,Ont.)  T T  .037 ( I r ) .102 ( I r )  X X  .004 (330 d a y s ) .16 (374 d a y s ) 1.5 CONTINUED  T a b l e 2.1  Reference  Coating Year Type  195  1974  ED EO ED ED ED  195  1975  ED ED  197  1975  TD TD TO  Loading l-8p  Pt-Ti Pt-Ti Pt-Ti 4p P t - T i 3p P t - T i Pt-Ti Pt-Ti  1 . 3 g/m 2 4 . 5 g/m 2 8 . 9 g/m 2  Pt/40Ir-Ti Pt/40Ir-Ti Pt/40Ir-Ti  Current D e n s i t y (mA/cm 2 ) 120-200 100 100 130 130 30-500 30-500 300 300 300  (Continued)  Electrolytes  i n d u s t r i a l diaphragm e e l Is 300 g p l N a C l ; 80° 270 g p l N a C l ; 80° i n d u s t r i a l diaphragm c e l l  Measurement Procedure B T W B B  sea w a t e r ( c a t h o d i c p r o t e c t i o n ) d i l u t e d sea w a t e r ( c a t h o d i c p r o t e c t i o n ) 5M N a C l , pH 6 ; 60°  Corrosion E f f i c i e n c y (A^F) .20 - .85 80 ( a f t e r 20 ( a f t e r .29 (after .44 ( a f t e r  (averages) 20-40 h r . ) 300 h r . ) 2000 days 1500 d a y s )  .4-1.5 1-3 T T T  .021 ( a f t e r 400 d a y s ) .063 .052  26  E a r l y i n v e s t i g a t i o n s showed l o s s r a t e s , due  i n c o n s i s t e n t and  high noble metal  to imperfections i n coating technology.  improved d e p o s i t i o n t e c h n i q u e s  has  subsequently  Development of  l e d t o d e c r e a s e s o f the  o r d e r o f two o r d e r s of magnitude i n the c o r r o s i o n e f f i c i e n c i e s of noble metal c o a t e d anodes. behaviour  Because t h e r e are marked d i f f e r e n c e s i n c o r r o s i o n  of TSA's produced by d i f f e r e n t m a n u f a c t u r i n g  processes,  c o r r o s i o n r e s u l t s are d i s c u s s e d s e p a r a t e l y f o r each c o a t i n g  the ..  preparation  method.  2.4.1.1  Coatings  Prepared  by E l e c t r o d e p o s i t i o n  Noble metal l o s s r a t e s f o r P t - T i e l e c t r o d e s whose c o a t i n g s were prepared  by e l e c t r o d e p o s i t i o n were found i n many cases to be  considerably  h i g h e r than the d i s s o l u t i o n r a t e o f smooth p l a t i n u m under the same c o n d i t i o n s , i n d i c a t i n g t h a t s p a l l i n g of the c o a t i n g was c o a t i n g l o s s mechanism.  likely  the predominant  For the e a r l i e s t e l e c t o d e s d e v e l o p e d , t h i s problem  appears to be r e l a t e d to poor s u b s t r a t e p r e t r e a t m e n t  p r i o r to e l e c t r o -  d e p o s i t i o n i n c o n j u n c t i o n w i t h h i g h l y s t r e s s e d d e p o s i t s , which would e x p l a i n the s h o r t e r anode l i f e t i m e s observed w i t h t h i c k c o a t i n g s [ 1 6 7 ] . coating techniques  subsequently  Improved  r e s u l t e d i n d e c r e a s e s i n n o b l e metal  l o s s r a t e s to l e v e l s where they became i n d u s t r i a l l y a c c e p t a b l e .  Effort  next appears to have been d i r e c t e d towards the development o f c o a t i n g s decreased p o r o s i t y i n o r d e r t o f u r t h e r reduce l o s s e s due substrate attack during  to progressive  operation.  A n t l e r [198] found t h a t the d e g r a d a t i o n operated  of  o f P t - T i and Rh-Ti anodes  i n c h l o r i d e / c h l o r a t e e l e c t r o l y t e took the form o f severe  c u t t i n g o f the c o a t i n g , w i t h e x t e n s i v e l a t e r a l  spreading  under-  o f the c o r r o s i o n  27  films.  Piontelli  noted i n d i s c u s s i o n t h a t c o n s i d e r a b l y s u p e r i o r performance  c o u l d be a t t a i n e d under s i m i l a r c o n d i t i o n s w i t h b e t t e r s u b s t r a t e p r e t r e a t ment p r i o r t o e l e c t r o p l a t i n g .  Van L a e r [ 9 3 ] showed t h a t p r e - o x i d a t i o n o f  the t i t a n i u m s u b s t r a t e i n c r e a s e d c o r r o s i o n r a t e s .  Warne and H a y f i e l d [ 7 2 ]  a l s o emphasized t h e importance o f s u b s t r a t e p r e t r e a t m e n t and p o s t - p l a t i n g heat t r e a t m e n t s on the adhesion o f the c o a t i n g t o the s u b s t r a t e .  Indeed,  optimum heat t r e a t m e n t s can r e s u l t i n the i n t e r f a c e having  a mechanical  s t r e n g t h i n excess o f e i t h e r o f t h e components.  Cotton [ 1 6 6 ]  (Although  found t h a t heat t r e a t m e n t s above 500°C caused e x c e s s i v e  d i f f i s u o n of the  c o a t i n g i n t o t h e s u b s t r a t e w i t h a consequent d e l e t e r i o u s e f f e c t on c o a t i n g lifetime.)  A l t e r n a t e s u b s t r a t e s , such as Nb o r Ti/15Mo a l l o y were found  to g i v e improved c o a t i n g adhesion c h a r a c t e r i s t i c s [ 7 2 ] ,  Veselovskaya  [195,199] determined t h a t c o a t i n g s , prepared from baths which produced l e s s - s t r e s s e d d e p o s i t s , gave l o n g e r s e r v i c e l i v e s i n a n o d i c o p e r a t i o n . The  "leakage c u r r e n t " on exposed t i t a n i u m has been determined  t o be e x t r e m e l y small  ( o f the o r d e r o f 1 uA/cm  i s 100 mA/cm , f o r a T i : P t s u r f a c e r a t i o 2  300  gpl N a C l , pH 4-4.5, a t 80°) [ 2 0 0 ] .  2  when t h e t o t a l  current  o f 15:1, o p e r a t i n g a t 1.85v i n Khodkevich [201,202] found the  leakage c u r r e n t t o be h i g h e r i n a l k a l i n e than i n a c i d i c e l e c t r o l y t e s . pH 13 c o n s i d e r a b l e  s u b s t r a t e a t t a c k was observed.  Coating  At  porosity ( i n  g e n e r a l , f o r a given p l a t i n g procedure) decreases with i n c r e a s i n g l o a d i n g , r e s u l t i n g i n improved anode l i f e t i m e s .  Haley [ 7 3 ] showed t h a t anodes w i t h  s i m i l a r l o a d i n g s showed l o n g e r l i f e t i m e s when prepared under c o n d i t i o n s which gave l e s s - p o r o u s having  deposits.  Khodkevich [ 2 0 3 ] found t h a t  coatings  g r e a t e r than 5 u t h i c k n e s s were s t a b l e under o p e n - c i r c u i t c o n d i t i o n s  i n s t r o n g h y d r o c h l o r i c a c i d s o l u t i o n s , whereas t h i n n e r coated  specimens  28  had  r e s t - p o t e n t i a l s which tended t o become a c t i v e ( t i t a n i u m d i s s o l u t i o n )  w i t h time.  G i n z b u r g [204]  notes an i n c r e a s e d tendency towards c r a c k i n g  c o r r o s i o n o f t i t a n i u m d u r i n g a n o d i c p o l a r i z a t i o n , w i t h anodes having s e c t i o n s having  slotted  increased p r o b a b i l i t y f o r corrosion.  A predominantly electrochemical  c o a t i n g l o s s mechanism may  be  i n d i c a t e d f o r e l e c t r o d e p o s i t e d n o b l e metal c o a t i n g s , f o r those cases where the c o r r o s i o n r a t e i s observed t o decrease w i t h time [129,195,205,206] o r where no dependence on c o a t i n g t h i c k n e s s i s o b s e r v e d . [ 1 7 3 ] . Examples of enhanced a n o d i c c o r r o s i o n o f e l e c t r o d e p o s i t e d  coatings  i n the presence o f c e r t a i n n i c k e l e l e c t r o p l a t i n g a d d i t i v e s [ 7 2 ] , p o l y saccharides  [ 1 7 9 ] , and  i n c i t r i c a c i d - c o n t a i n i n g e l e c t r o l y t e s [72,180]  suggest t h a t n o b l e metal d i s s o l u t i o n i s due  t o the f o r m a t i o n  of s o l u b l e  complexes o f the noble m e t a l s w i t h t h e o r g a n i c s or t h e i r a n o d i c decomposition  products.  2.4.1.2  Coatings  P r e p a r e d by Thermal D e c o m p o s i t i o n  As can be seen i n T a b l e 2.1, by thermal  anodes having  coatings  prepared  d e c o m p o s i t i o n show c o n s i d e r a b l y b e t t e r c o r r o s i o n r e s i s t a n c e  than t h o s e h a v i n g e l e c t r o d e p o s i t e d c o a t i n g s .  A l l o y i n g w i t h i r i d i u m t o improve  o v e r p o t e n t i a l c h a r a c t e r i s t i c s a l s o r e s u l t s i n improved c o r r o s i o n r e s i s t a n c e over platinum  coatings alone.  Other a l l o y s , e s p e c i a l l y the b i n a r y  t e r n a r y noble metal a l l o y s developed by Suzuki  and  [183-193], a l s o show e x t r e m e l y  low l o s s r a t e s . Whereas the c o r r o s i o n performance of e l e c t r o d e p o s i t e d b e g i n s t o d e t e r i o r a t e f o r l o a d i n g s under 50 g/m  2  P t ) , c o a t i n g s of l e s s than 10 g/m  2  coatings  ( e q u i v a l e n t t o about  ( e q u i v a l e n t to about 0.5u  2.5u  Pt) produced  29  by thermal d e c o m p o s i t i o n do not show i m p a i r e d Indeed, A t a n a s o s k i  [197]  corrosion c h a r a c t e r i s t i c s .  found t h a t Pt/40 I r c o a t i n g s  of o n l y 1.3  l o a d i n g showed s u p e r i o r c o r r o s i o n r e s i s t a n c e (expressed l o s s of noble m e t a l ) to s i m i l a r but h i g h e r - l o a d i n g coatings  a f t e r 400  days' o p e r a t i o n  2.4.1.3  C o a t i n g s Prepared by  in chlorate  g/m  2  i n terms of  (4.5 and  8.9  absolute  g/m ) 2  production.  Cladding  C l a d metal anodes l a c k i n g p o r o s i t y would i n t u i t i v e l y be expected t o show s i m i l a r c o r r o s i o n b e h a v i o u r t o the s o l i d noble m e t a l , w i t h s u b s t r a t e e f f e c t , or t h i c k n e s s - d e p e n d e n c e o f p o l a r i z a t i o n or behaviour.  invariably  ( P t - T i ) anodes i n numerous e l e c t r o l y t e s ,  the c l a d anodes showed s u p e r i o r c o r r o s i o n r e s i s t a n c e , e s p e c i a l l y  i n e l e c t r o l y t e s aggressive  to t i t a n i u m .  anodes showed c o n s i d e r a b l y  lower c o r r o s i o n r a t e s than s o l i d  ( e l e c t r o l y s i s i n 160 due  corrosion  Skomoroski [75] i n v e s t i g a t e d the c o r r o s i o n b e h a v i o u r o f c l a d  (Pt-Ta) anodes vs. e l e c t r o d e p o s i t e d  was  gpl  H2SO4,  30°,  Baboian [39] found t h a t c l a d ( l y )  a t 108 mA/cm ), which he 2  to d i f f e r e n c e s i n m i c r o s t r u c t u r e .  F u r t h e r , t h e r e was  platinum suggested a small  improvement i n c o r r o s i o n r e s i s t a n c e i n going from T i t o Nb t o Ta  2.4.2  no  A l t e r n a t i n g Current  substrates.  Corrosion  A l t e r n a t i n g c u r r e n t e l e c t r o l y s i s leads t o r a p i d d e g r a d a t i o n s o l i d noble metal e l e c t r o d e s .  - ,  The  e f f e c t o f an  of  A.C.  component superimposed on an anodic c u r r e n t , however, i s not n e c e s s a r i l y deleterious.  J u c h n i e w i c z [129-133] has  imposed 50 Hz A.C.  i n v e s t i g a t e d the e f f e c t of super-  on P t - T i i n 3% NaCl s o l u t i o n s .  His e a r l y work [133-134]  30  showed t h a t as l i t t l e as 1% A.C. siderable deterioration.  In d i s c u s s i o n , however, Walkiden suggested  such r e s u l t s were the consequence deposited  coatings.  component ( 1 ^ ^ / I ^ ^ ) c o u l d cause conthat  o f the poor q u a l i t y o f t h e e l e c t r o -  The l a t e r work o f J u c h n i e w i c z [129,135,136] and a l s o  Royuela [207] showed t h a t 50 Hz A.C.  was not d e s t r u c t i v e u n t i l  i t reached  70%, where the P t - T i e l e c t r o d e a c h i e v e d b i p o l a r c h a r a c t e r i s t i c s .  Below  t h i s p e r c e n t a g e , A.C.  rate  ( f o r example,  30% A.C.  produced o n l y a small on 115 mA/cm D.C. 2  increase  caused the c o r r o s i o n r a t e t o n e a r l y  double (4.8 vs. 2.6 ug/A«hr) from the pure D.C. (>100  cps) A.C.  in corrosion  case.  Higher-frequency  components (which are l i k e l y t o be encountered i n i n d u s t r i a l  e l e c t r o l y t i c a p p l i c a t i o n s where the e l e c t r o l y s i s c u r r e n t i s o b t a i n e d  from  a t r a n s f o r m e r / r e c t i f i e r i n s t a l l a t i o n ) produce l e s s c o r r o s i o n o f P t - T i anodes [130,179].  Yukhevich [208] showed t h a t i n c r e a s i n g A.C.  f r e q u e n c y from 20  t o 100 cps decreased P t - T i c o r r o s i o n r a t e s under both monopolar and b i p o l a r operating  conditions. On/off s w i t c h i n g i s known t o produce a temporary " p u l s e "  of  p l a t i n u m d i s s o l u t i o n and i t i s l i k e l y t h a t such t r e a t m e n t , i f repeated many t i m e s , may  c o n t r i b u t e to enhanced c o r r o s i o n r a t e s .  A t a n a s o s k i [194,  197] noted t h a t the c o a t i n g l o s s vs. time r e l a t i o n f o s e v e r a l Pt/40  Ir-Ti  anodes i n c h l o r a t e c e l l s showed a s t e p - w i s e c h a r a c t e r , w h i c h , he suggested, may  be due to i n t e r r u p t i o n o f the e l e c t r o l y s i s . In cases where an e l e c t r o d e i s c y c l e d a n o d i c a l l y and then  c a t h o d i c a l l y ("low-frequency A . C " ) , enhanced c o r r o s i o n may by s o r p t i o n o f hydrogen  i n t o the t i t a n i u m .  be  (Prolonged cathodic  r e s u l t s i n hydrogen e m b r i t t l e m e n t o f t h e t i t a n i u m and consequent o f the c o a t i n g ) .  produced operation spalling  In c y c l i n g however, c o a t i n g l o s s o c c u r s by the r a p i d  31  diffusion  o f hydrogen o u t o f t h e t i t a n i u m on the anodic c y c l e  immediately  f o l l o w i n g c a t h o d i c o p e r a t i o n , where t h e p r e s s u r e due t o t h e b u i l d - u p o f hydrogen gas beneath the c o a t i n g causes t h e c o a t i n g t o l i f t and s p a l l [ 7 3 ] . In such c a s e s , porous c o a t i n g s g i v e s u p e r i o r performance as they a l l o w t h e hydrogen t o escape t o t h e s u r f a c e r a t h e r than t o t h e s u b s t r a t e / c o a t i n g i n t e r f a c e as on l e s s - p o r o u s e l e c t r o d e s . anomolous o b s e r v a t i o n o f V e s e l o v s k a y a  T h i s may e x p l a i n t h e a p p a r e n t l y  [199] who found t h a t h i g h l y - s t r e s s e d  e l e c t r o d e p o s i t s showed g r e a t e r s t a b i l i t y w i t h r e s p e c t t o c u r r e n t r e v e r s a l s than l e s s - s t r e s s e d d e p o s i t s , inasmuch as t h e " p e e l i n g " tendency o f t h e h i g h l y - s t r e s s e d d e p o s i t would l i k e l y r e s u l t i n g r e a t e r s u b s t r a t e e x p o s u r e , p e r m i t t i n g the e a s i e r escape o f absorbed hydrogen.  2.4.3  Anodic Breakdown As the l o a d i n g s o f noble metals o r o x i d e s on v a l u e metal sub-  s t r a t e s a r e f i n i t e , t h e i r c o r r o s i o n must e v e n t u a l l y l e a d t o anodic down when i n s u f f i c i e n t c o a t i n g remains i n e l e c t r i c a l s t r a t e metal.  break-  c o n t a c t w i t h the sub-  " A c c e l e r a t e d " t e s t s i n v o l v e the use o f h i g h e r  d e n s i t i e s than would n o r m a l l y be e n c o u t e r e d i n i n d u s t r i a l  current  practice to  e f f e c t complete f a i l u r e i n a r e l a t i v e l y s h o r t t i m e , as i n d i c a t e d by the r i s e i n anode p o t e n t i a l t o e x c e s s i v e l y h i g h v a l u e s .  Such t e s t s may be  performed w i t h the a p p l i c a t i o n o f a c o n s t a n t c u r r e n t , w i t h time t o f a i l u r e being measured; o r may perform as t h e e x t e n s i o n o f an upward o f a p o l a r i z a t i o n curve current.  determination  t o e x c e s s i v e v a l u e s o f the a p p l i e d p o t e n t i a l o r  The breakdown c h a r a c t e r i s t i c i n e i t h e r case i s a r a p i d l y a c c e l e r -  a t i n g r a t e o f voltage increase which, i f allowed to continue, r e s u l t s i n anodic  breakdown o f the t i t a n i u m s u b s t r a t e .  32  Cotton  [166] noted t h a t the f a i l u r e o f P t - T i anodes, w i t h T i w i r e  l e a d s , i n sea water o c c u r r e d a t the w a t e r l i n e on the l e a d s . [ 2 0 9 ] , who  Mikhailova  used rod-shaped anodes i n sea w a t e r , w i t h a p l a t i n u m  o n l y a t the end  coating  ( T i : P t a r e a r a t i o 5 0 : 1 ) , a l s o found t h a t breakdown o c c u r r e d  on the bare T i - but always w i t h i n 1-3  mm  o f the edge o f the Pt c o a t i n g .  F u r t h e r , he observed c o n s i d e r a b l e t h i c k e n i n g o f the o x i d e f i l m on t i t a n i u m i n the r e g i o n o f the a c c e l e r a t i n g r a t e o f i n c r e a s e o f anode v o l t a g e , i n a s s o c i a t i o n w i t h a p p r e c i a b l e gas e v o l u t i o n , i n d i c a t i n g a s u b s t a n t i a l change i n the nature o f the f i l m p r i o r to breakdown. rosion products  A t breakdown, c o r -  o f t i t a n i u m appeared i n s o l u t i o n i n the form o f  white  "floes." Veselovskaya  [135] and  Khodkevich [210] observed t h a t the mor-  phology o f s u b s t r a t e a t t a c k changed when P t - T i anodes were s u b j e c t e d  to  c u r r e n t s which produced a r a p i d r i s e i n p o t e n t i a l d u r i n g o p e r a t i o n i n c h l o r i d e e l e c t r o l y t e , which r e s u l t e d i n the appearance o f l o c a l i z e d ("burst  bubble") f e a t u r e s .  The  "critical  puncture  c u r r e n t d e n s i t y " above which  the  anode p o t e n t i a l r i s e s r a p i d l y t o h i g h v a l u e s and the amount o f charge passed to f a i l u r e i n c r e a s e w i t h l o a d i n g on e l e c t r o d e p o s i t e d P t - T i anodes i n seawater a t 55°  [210].  The f a i l u r e o f anodes o p e r a t e d the " c r i t i c a l  a t c u r r e n t d e n s i t i e s w e l l below  v a l u e " has been a t t r i b u t e d to the growth o f an  o x i d e l a y e r a t the t i t a n i u m / c o a t i n g i n t e r f a c e [ 2 1 1 ] .  insulating  Such a phenomenon  has been used t o e x p l a i n the i n c r e a s e i n p o t e n t i a l o f TSA's w i t h t i m e [ 2 1 2 ] , the decrease i n a c t i v i t y o f e l e c t r o d e p o s i t e d P t - T i w i t h c y c l i n g [213] and d e v i a t i o n s from T a f e l behaviour coated anodes [ 2 1 4 ] .  voltammetric  found w i t h mixed  oxide  B y s t r o v [215] has suggested t h a t an o x i d e l a y e r always  33  e x i s t s between the a c t i v e c o a t i n g and s u b s t r a t e , but t h a t i t i s e l e c t r o n i c a l l y conductive  as a r e s u l t o f the i n t r o d u c t i o n o f a s u b s t a n t i a l i m p u r i t y con-  tent into i t during  the m a n u f a c t u r i n g p r o c e s s .  Growth o f t h i s f i l m due t o  i o n m i g r a t i o n , on the o t h e r hand, would r e s u l t i n a measurable  increase  i n anode p o t e n t i a l .  2.4.4  Noble Metal On  Coated TSA's i n E l e c t r o w i n n i n g  t h e b a s i s o f the known c o r r o s i o n r e s i s t a n c e o f p l a t i n u m i n  s u l f u r i c a c i d s o l u t i o n s , platinum-coated in electrowinning  cells [39,103].  Few s p e c i f i c c o r r o s i o n data  e v e r , and t h e a v a i l a b l e i n f o r m a t i o n Hayfield [ 7 2 ] reported  TSA's have been proposed f o r use  i s f a r from p r o m i s i n g .  e x i s t , how-  Warne and  t h a t TSA's w i t h e l e c t r o p l a t e d p l a t i n u m  coatings  c o r r o d e d a t 200 yg/A«hr under an a p p l i e d c u r r e n t d e n s i t y o f 97 mA/cm i n 2  a copper p l a t i n g bath c o n t a i n i n g 7 5 - 1 0 0 gpl H S 0 i t , 5 0 - 5 7 gpl C u S 0 , a t 2  40°.  (Such a " r a t e " i m p l i e s t h a t an anode having a t y p i c a l  5 g/m  2  [21]  would f a i l  found t h a t ruthenium o x i d e c o a t e d anodes ( w i t h l o a d i n g s  electrowinning 4  a t 1 9 . 4 mA/cm  2  up t o 15 g Ru/m ) 2  i n a small-scale  E g g e t t [ 5 4 ] has r e p o r t e d  favourable  r e s u l t s w i t h an  u n s p e c i f i e d noble metal c o a t e d anode i n u n s p e c i f i e d l a b o r a t o r y tests.  De Nora [ 3 7 ] has d e s c r i b e d  c o m b i n a t i o n s o f the o x i d e s o r Sn, which a r e c l a i m e d has  reported  Hopkins  c e l l w i t h an e l e c t r o l y t e c o n t a i n i n g 35 g p l copper and 1 3 5 -  gpl H S 0 , 4 5 ° . 2  loading of  w i t h i n 25 hours due t o l o s s o f c o a t i n g m e t a l . )  f a i l e d w i t h i n 8 0 0 hours o f o p e r a t i o n  150  4  electrowinning  s e v e r a l mixed o x i d e e l e c t r o d e s i n v o l v i n g  o f Ru, I r , o r Pd w i t h those o f S i , T i , T a , Nb, t o be s u i t a b l e f o r e l e c t r o w i n n i n g .  t h a t an anode having a P b 0  2  c o a t i n g upon a Pd-,  coated T i s u b s t r a t e i s s u i t a b l e f o r z i n c e l e c t r o w i n n i n g .  Antonov [ 2 1 6 ] Rh-, o r P t -  34  A l t h o u g h i t was mentioned  p r e v i o u s l y t h a t TSA's had seen s u c c e s s f u l  a p p l i c a t i o n i n many e l e c t r o p l a t i n g o p e r a t i o n s ,  such i s not always -Hv . n J e .  Warne and H a y f i e l d [72] found t h a t enhanced a n o d i c c o r r o s i o n o c c u r r e d  with  P t - T i anodes employed i n n i c k e l - p l a t i n g baths i n the presence o f c e r t a i n proprietary additives. polysaccharides P t - T i anodes.  Juchnewicz [179] a l s o found t h a t the a d d i t i o n o f  t o 3% NaCl e l e c t r o l y t e promoted enhanced d i s s o l u t i o n o f High d i s s o l u t i o n r a t e s a r e a l s o observed w i t h P t - T i anodes  i n c i t r i c a c i d - c o n t a i n i n g e l e c t r o l y t e s [72,180]  (See Table 2.1).  The  s u s c e p t i b i l i t y o f TSA's t o a c c e l e r a t e d c o r r o s i o n i n t h e p r e s e n c e o f c e r t a i n organics  would r e q u i r e t h a t the e f f e c t s o f t r a c e o r g a n i c  contaminants  o r o f e l e c t r o l y t e a d d i t i v e s be determined p r i o r to any d e c l a r a t i o n o f t h e i r s u i t a b i l i t y as i n s o l u b l e anodes i n copper  2.5  electrowinning.  Summary and R e l a t i o n to t h i s Work On the b a s i s o f the most r e l i a b l e c o r r o s i o n r a t e measurements -  the r a d i o c h e m i c a l  work o f Chemodanov [94,99-102] - a c o r r o s i o n e f f i c i e n c y  o f 1.8 ug/A*hr ( c a l c u l a t e d from h i s s t a t e d c u r r e n t e f f i c i e n c y value .0001  per c e n t ) would be expected f o r the case o f e l e c t r o c h e m i c a l  t i o n of platinum  in sulfuric acid solutions.  Higher c o r r o s i o n  f o r TSA's would tend to i n d i c a t e a c o n t r i b u t i o n from mechanical  dissolu-  efficiencies detachment,  i n the absence o f e l e c t r o l y t e a d d i t i v e s o r o t h e r f a c t o r s which may accelerated platinum  dissolution.  Unfortunately,  of  promote  no c o r r o s i o n d a t a e x i s t  f o r i r i d i u m i n s u l f a t e systems, a l t h o u g h g r e a t e r c o r r o s i o n r e s i s t a n c e would be t e n t a t i v e l y p r e d i c t e d f o r t h i s metal on the b a s i s o f i t s b e h a v i o u r i n o t h e r e l e c t r o l y t e s and the known s u p e r i o r i t y o f P t / I r a l l o y c o a t e d anodes to Pt-Ti i n t h i s regard.  The p o s s i b i l i t y t h a t both c o a t i n g metals d i s s o l v e  35  e l e c t r o c h e m i c a l l y a t d i f f e r e n t r a t e s a l s o a f f o r d s a f u r t h e r check as  to  the n a t u r e o f the d i s s o l u t i o n p r o c e s s inasmuch as a s p a l l i n g mechanism ( f o r a homogeneous a l l o y ) would not be expected to l e a d to enrichment of remaining  c o a t i n g metal w i t h the more c o r r o s i o n - r e s i s t a n t component. What few data e x i s t c o n c e r n i n g  the two  the  the r e l a t i v e  c o r r o s i o n b e h a v i o u r of  c o a t i n g metals i n o r g a n i c - c o n t a i n i n g e l e c t r o l y t e s suggest t h a t  i r i d i u m may  be somewhat more s u s c e p t i b l e to a c c e l e r a t e d c o r r o s i o n i n such  systems [ 1 0 7 ] .  As t h i o u r e a i s r e p o r t e d  d i s s o l u t i o n at high  t o be an i n h i b i t o r f o r  platinum  (> 1.95v) anode p o t e n t i a l s i n p e r c h l o r i c e l e c t r o l y t e s  [104,112,116-118], and  because i t i s a l s o a common a d d i t i v e i n copper  e l e c t r o d e p o s i t i o n systems [217,218], i t i s of i n t e r e s t t o f u r t h e r e x p l o r e the e f f e c t s of t h i s a d d i t i v e on the b e h a v i o u r o f Pt/30 I r - T i anodes i n simulated  copper e l e c t r o w i n n i n g The A.C.  experiments.  c o r r o s i o n data are a l s o i n s u f f i c i e n t to p e r m i t an  e s t i m a t i o n of the r e l a t i v e c o r r o s i o n b e h a v i o u r of p l a t i n u m and i r i d i u m from a l l o y c o a t i n g s under c o n d i t i o n s o f p u l s e d e l e c t r o l y s i s which have been proposed f o r copper e l e c t r o w i n n i n g .  On the b a s i s of t h e i r b e h a v i o u r  under comparable c o n d i t i o n s w i t h the a p p l i c a t i o n o f 60 Hz A.C.  i n hydro-  c h l o r i c a c i d s o l u t i o n s [ 1 2 3 ] , i r i d i u m shows s u p e r i o r c o r r o s i o n r e s i s t a n c e in concentrated  solutions.  No such comparable data e x i s t s f o r s u l f u r i c  a c i d systems, however, a l t h o u g h  Rand [142] r e p o r t e d  t h a t i r i d i u m was  less  c o r r o s i o n r e s i s t a n t than p l a t i n u m under c o n d i t i o n s o f c y c l i c voltammetry i n 1M H S 0 . 2  4  Yufa [154]  reported  t h a t a Pt/25 I r a l l o y d i s s o l v e d q u a n t i -  t a t i v e l y i n h y d r o c h l o r i c a c i d s o l u t i o n under Platinum  A.C.  i s known t o y i e l d a h i g h , time-dependent " p u l s e " o f metal  d i s s o l u t i o n w i t h o f f / o n s w i t c h i n g [88.89,99], which suggests t h a t such  36  t r e a t m e n t - even w i t h a f r e q u e n c y o f s e v e r a l days - may mean d i s s o l u t i o n r a t e s .  lead to  increased  No d a t a e x i s t w i t h r e s p e c t t o the e f f e c t o f  such t r e a t m e n t on i r i d i u m . I t i s l i k e l y t h a t under the c u r r e n t r e v e r s a l c o n d i t i o n s employed i n copper e l e c t r o w i n n i n g  c e l l s a t G6camines [34]  s h o r t e d ) , h i g h e r noble metal l o s s r a t e s may r e l a t i v e c o r r o s i o n rates of platinum from p r e s e n t l y a v a i l a b l e data.  and  (9 seconds on/0.5 second  be e n c o u n t e r e d , a l t h o u g h  i r i d i u m cannot be  estimated  the  Chapter 3  LITERATURE SURVEY: OXYGEN FILMS ON PLATINUM AND IRIDIUM ANODES 3.1  PIatinum On a p p l i c a t i o n o f an anodic  current to a platinum electrode i n  an e l e c t r o l y t e where oxygen e v o l u t i o n can o c c u r , t h e anode s u r f a c e  acquires  an oxygen coverage whose n a t u r e changes depending on t h e d u r a t i o n o f anodic p o l a r i z a t i o n under a g i v e n s e t o f c o n d i t i o n s .  Although  there are  s e v e r a l fundamental reasons which have made t h e study o f t h e oxygen coverage on p l a t i n u m one o f t h e most a c t i v e areas o f e l e c t r o c h e m i c a l r e s e a r c h , a very p r a c t i c a l authors  r e s u l t which can be e x t r a p o l a t e d from t h e work o f many  i s t h e a b i l i t y t o determine e l e c t r o c h e m i c a l l y a c t i v e s u r f a c e areas  by e l e c t r o c h e m i c a l f o r m a t i o n and/or s t r i p p i n g o f s u r f a c e oxygen l a y e r s . Of concern i n the p r e s e n t work i s t h e r e l a t i o n s h i p between t h e s t a t e o f the anode s u r f a c e and t h e c o r r o s i o n r a t e , and a l s o t h e i n c r e a s e i n anode potential  ( " p a s s i v a t i o n " ) w i t h t i m e on e l e c t r o l y s i s i n " i n e r t "  electro-  l y t e s such as s u l f u r i c a c i d s o l u t i o n s . Host o f t h e i n f o r m a t i o n c o n c e r n i n g coverage on p l a t i n u m techniques  t h e nature o f t h e oxygen  has come from t h e n o n s t a t i o n a r y  o f chronopotentiometry  electrochemical  and voltammetry, as t h e degree o f coverage  o f p l a t i n u m anodes w i t h oxygen r a r e l y exceeds t h e e q u i v a l e n t o f a few 37  38  monolayers, and i s thus i n a c c e s s i b l e t o study by such c o n v e n t i o n a l niques as X-ray o r e l e c t r o n d i f f r a c t i o n , o r chemical a n a l y s i s . a platinum  e l e c t r o d e i n i t i a l l y h e l d a t 0 v o l t s (RHE) w i l l  tech-  Typically,  show s e v e r a l  c h a r a c t e r i s t i c f e a t u r e s i n t h e r e s u l t i n g charge c u r v e o r voltammogram measured on a p p l i c a t i o n o f an a n o d i c c u r r e n t p u l s e o r a l i n e a r p o t e n t i a l "sweep."  The F a r a d a i c  p r o c e s s e s which can be d i s t i n g u i s h e d , p r o v i d e d  i s chosen as t o t h e r a t e o f c h a r g i n g  care  (such t h a t t h e non-steady p r o c e s s e s  are n o t obscured by t h e n o n - F a r a d a i c p r o c e s s e s o r by i m p u r i t y  reactions)  are t h e o x i d a t i o n o f t h e adsorbed hydrogen, t h e d e p o s i t i o n o f oxygen s p e c i e s , and u l t i m a t e l y , t h e e v o l u t i o n o f oxygen. oxygen coverage c o n t i n u e s  simultaneously  Development o f t h e  w i t h oxygen e v o l u t i o n , a l t h o u g h  t h i s stage i s c h a r a c t e r i z e d by a r e l a t i v e l y slow i n c r e a s e i n anode p o t e n t i a l w i t h time.  On r e v e r s a l o f t h e a p p l i e d c u r r e n t o r d i r e c t i o n o f t h e p o t e n t i a l  sweep, f e a t u r e s c h a r a c t e r i s t i c o f t h e r e d u c t i o n o f t h e oxygen coverage (and o f d i s s o l v e d oxygen gas,  u n l e s s attempts a r e made t o remove i t p r i o r  to a p p l i c a t i o n o f t h e c a t h o d i c - g o i n g  p u l s e ) and o f t h e d e p o s i t i o n o f  hydrogen s p e c i e s , f o l l o w e d by t h e e v o l u t i o n o f hydrogen gas, a r e apparent on t h e charge curve o r voltammogram. When care i s taken t o e l i m i n a t e any charge-consuming  reac-  t i o n s , t h e degree o f o x i d a t i o n can be d i r e c t l y measured from t h e t r a n s i t i o n times c o r r e s p o n d i n g t o t h e oxygen r e g i o n s  on t h e charge c u r v e s , o r  from t h e wave o r peak areas on t h e voltammograms. the imminent e v o l u t i o n o f oxygen gas,  At potentials p r i o r to  t h e oxygen coverage does not exceed  a monolayer.  T h i s can be i n f e r r e d from t h e r e s u l t s o f many w o r k e r s , such  as S c h u l d i n e r  [219] who found t h a t t h e t o t a l q u a n t i t y o f charge passed i n  a n o d i c g a l v a n o s t a t i c p u l s i n g t o the p o t e n t i a l o f t h e imminent e v o l u t i o n  39  o f oxygen gas, Q , 0  was  double the v a l u e f o r the d e p o s i t i o n o r removal  of  a f i l m o f hydrogen s p e c i e s p r i o r to immient hydrogen e v o l u t i o n , Q^.  The  value Qo/2Q^ = 1 can be r e a d i l y e x p l a i n e d i f i t i s p o s t u l a t e d t h a t  every  s u r f a c e p l a t i n u m atom takes p a r t i n a 2 - e l e c t r o n t r a n s f e r r e a c t i o n w i t h oxygen and a 1 - e l e c t r o n t r a n s f e r r e a c t i o n w i t h hydrogen.  Surface  area  e s t i m a t i o n thus r e q u i r e s o n l y an assumption as to the s u r f a c e metal atom density. The  charge f o r f o r m a t i o n o f s u r f a c e oxygen coverage cannot  be s i m u l t a n e o u s l y measured when oxygen e v o l u t i o n commences, as the c u r r e n t overwhelms t h a t f o r s u r f a c e o x i d a t i o n . employ c a t h o d i c s t r i p p i n g t e c h n i q u e s  latter  I t i s thus n e c e s s a r y  to  which i n v o l v e , i n a d d i t i o n , the  s t e p p i n g o f the anode p o t e n t i a l to a v a l u e where oxygen e v o l u t i o n does not occur to a measurable degree (which  i s most r e a d i l y a c c o m p l i s h e d  s i m p l y by p u t t i n g the e l e c t r o d e a t o p e n - c i r c u i t ) , and a l s o , p o s s i b l y , removal t o a n o t h e r c e l l  t o ensure minimal c o n t a m i n a t i o n  generated anode p r o d u c t s . of approximately  with previously-  Oxygen coverages extend to a l i m i t i n g  the e q u i v a l e n t o f two monolayers, a l t h o u g h  value  under c e r t a i n  c o n d i t i o n s the f o r m a t i o n o f a second k i n d o f s u r f a c e oxygen coverage ("type I I o x i d e " ) becomes p o s s i b l e , m a n i f e s t e d  by the appearance o f a  second r e d u c t i o n f e a t u r e on the c a t h o d i c charge curve or voltammogram, and  3.1.1  showing no l i m i t i n g coverage v a l u e .  Nature o f the S u r f a c e Oxygen Coverage Attempts to c o r r e l a t e the f e a t u r e s found on anodic o r  cathodic  charge curves o r voltammograms w i t h the p o t e n t i a l s c a l c u l a t e d f o r p l a t i n u m / oxygen s p e c i e s from thermochemical data have been u n s u c c e s s f u l .  Although  40  t h e r e i s no l a c k o f data c o n c e r n i n g the s t a n d a r d r e d u c t i o n p o t e n t i a l s f o r a v a r i e t y of metal/oxide  o r o x i d e / o x i d e c o u p l e s f o r many s p e c i e s -  both r e a l and imaginary - t h e themochemistry i s i n most cases speculative. twenty-nine  merely  In t h i s r e g a r d , Novak [220] has p r e s e n t e d no fewer than such p o t e n t i a l s f o r p l a t i n u m .  o f t h e thermochemistry  o f t h e o x i d e s o f p l a t i n u m i s based can be regarded  as s u s p e c t due t o t h e probable may have obscured  The e a r l y work on which much  i n t e r f e r e n c e o f competing r e a c t i o n s which  the reported metal/oxide  p o t e n t i a l s f o r prepared  h e l d i n p e r f o r a t e d p l a t i n u m c o n t a i n e r s i n 2N h^SO^.  oxides  Further, the exact  n a t u r e o f t h e " o x i d e s " was not a c c u r a t e l y known, nor was system p u r i t y considered. Measurements o f c a t h o d i c d i s c h a r g e " l a g p o i n t s " p r o v i d e d the metal/oxide  potential  v a l u e r e p o r t e d by Lorenz  Pt0 «3H 0 + 4 H  +  Pt0 -2H 0 + 4H  +  Pt0-2H 0  +  2  2  2  + 2H  2  + 4e - P t + 5H 0  E - 0.98v  (1)  + 4e ^ P t + 4H 0  E = 0.96v  (2)  + 2e ^ P t + 3H 0  E = 0.95v  (3)  2  2  [221-223] i n 1909:  2  2  Grube [ 2 2 4 ] , i n 1910, measured r a t h e r u n s t a b l e r e s t p o t e n t i a l values:  Pt0 -4H 0 + 4H  +  Pt0 -2H 0 + 4H  +  2  2  Pt0 PtO  3  + 4e ^ P t + 6H 0  E = 1.06v  (4)  + 4e ^ P t + 4 H 0  E = 1.04v  (5)  + 6e - P t + 3H 0 2  E = 1.5v  (6)  + 2e - P t + H 0  E = 0.9v  (7)  2  2  + 6H  +  + 2H  +  2  2  2  The f o l l o w i n g r e a c t i o n and s t a n d a r d r e d u c t i o n p o t e n t i a l  41  Pt(OH)  + 2H  2  +  + 2e - P t + 2H 0  E = 0.98v  2  (8)  o f t e n quoted i n t a b l e s o f e l e c t r o d e p o t e n t i a l s and used t o e x p l a i n t h e c o n s i s t e n c y o f the e a r l y work w i t h modern thermochemical r e a l i t y i t s e l f based on thermochemical  data, i s i n  data d e r i v e d from r e a c t i o n (3) [ 2 2 5 ] .  L a t e r , Nagel and D i e t z [226,227] p u b l i s h e d s t a n d a r d r e d u c t i o n p o t e n t i a l s f o r t h e o x i d e s P t C \ and Pt0 «nH 0. 3  Pt 0 a  4  + 8H  + 8e  +  Pt0 -nH 0 + 4H 2  The  +  2  2  2  ^ 3Pt + 4H 0 2  + 4e - P t + (n+2)H 0 2  p o t e n t i a l o f (9) was determined  (9)  E° = 0.80v  (10)  from an assumed v a l u e f o r t h e s t a n d a r d  f r e e energy o f f o r m a t i o n and thus possesses p o t e n t i a l f o r (10) was determined  E° = l . l l v  no i n t r i n s i c a c c u r a c y .  The  from the r e s t p o t e n t i a l o f an e l e c t r o d e  on which i t was assumed t h a t t h i c k Pt0 «nH 0 l a y e r s were generated 2  2  by A.C.  polarization. As a consequence o f t h e high degrees o f u n c e r t a i n t y o f t h e above p o t e n t i a l s the complete l i s t o f s t a n d a r d r e d u c t i o n p o t e n t i a l s f o r p l a t i n u m o x i d e s computed and c o m p i l e d by Novak [220] must be c o n s i d e r e d to be i n e r r o r , because h i s data were d e r i v e d from r e a c t i o n s ( 1 ) - ( 1 0 ) whose s t a n d a r d r e d u c t i o n p o t e n t i a l s a r e s u s p e c t . Hoare [228] o b t a i n e d a p o t e n t i a l  P t 0  hyd  +  2  H  +  +  2 e  *  P t  +  H 2  °  value f o r the couple:  E  °  0  -  8  8  v  from r e s t p o t e n t i a l measurements on a p r e - a n o d i z e d purged s o l u t i o n The  electrode i n nitrogen-  (which e l i m i n a t e d t h e O^/H^O c o u p l e from t h e mixed p o t e n t i a l ) .  species " P t O  U w J  " i s t h e "hydrated s u r f a c e oxygen l a y e r . "  42  R e c e n t l y t h e e l e c t r o c h e m i c a l behaviour  o f the o x i d e s P t 0  P t C \ has been i n v e s t i g a t e d , u s i n g c h e m i c a l l y prepared 3  The  oxides  2  and  [229-231].  p o t e n t i a l s o f s t a b i l i t y w i t h r e s p e c t t o c a t h o d i c r e d u c t i o n were d e t e r -  mined i n both a c i d and a l k a l i n e s o l u t i o n s , w i t h t h e l o w e s t p o t e n t i a l s a t which the o x i d e s were s t a b l e w i t h r e s p e c t t o r e d u c t i o n t o m e t a l l i c p l a t i n u m considered  t o be the s t a n d a r d  P t  ° hyd 2  +  4  H  PtaCK + 8 H  The on p l a t i n u m ages.  +  +  4 e  + 8e  ^  P t  +  2 H 2  °  E  °  =  0  ^ P t + 4H 0  ,  4  0  ( ^  v  1 2  E° = 0.27v  2  (13)  a p p l i c a b i l i t y o f b u l k o x i d e data t o the anodic  f i l m formed  i s q u e s t i o n a b l e , however, e s p e c i a l l y a t sub-monolayer c o v e r -  Even a t monolayer c o v e r a g e , t h e o x i d e , i f i t e x i s t s , i s o n l y two-  dimensional. and  +  reduction p o t e n t i a l s :  The "PtO" s p e c i e s r e p o r t e d by Hoare [ 2 2 8 ] i n r e a c t i o n (11)  i n f e r r e d by most o t h e r i n v e s t i g a t o r s t o r e p r e s e n t t h e s t o i c h i o m e t r y  o f the oxygen l a y e r a t p o t e n t i a l s c o r r e s p o n d i n g t i o n does n o t appear t o e x i s t i n bulk  t o imminent oxygen e v o l u -  form.[232].  Attempts t o c o r r e l a t e f e a t u r e s on the charge curves o r v o l tarnm e t r i c curves w i t h b u l k o x i d e p o t e n t i a l s show o n l y a rough correspondence of the anodic-going  curves with the ( i n a c c u r a t e ) metal/oxide  potentials  r e p o r t e d i n r e a c t i o n s (1) t o ( 1 0 ) . The  q u e s t i o n as t o whether t h e o x y g e n - f i l m  on p l a t i n u m  i s an  adsorbed l a y e r o r a phase o x i d e has always been the major source o f cont r o v e r s y i n t h e study o f p l a t i n u m o x i d a t i o n .  Convincing  arguments have  been proposed by both t h e "adsorbed oxygen" and "phase o x i d e " The  f e a t u r e s on the a n o d i c / c a t h o d i c  schools.  charge curves o r voltammo-  grams r e v e a l an " i n t r i n s i c " h y s t e r e s i s , p r o v i d e d  t h e anodic  l i m i t i s above.  43  0.9 v o l t s .  I f o n l y an adsorbed  l a y e r was formed, t h e r e d u c t i o n o f oxygen  d e p o s i t e d a t h i g h e r p o t e n t i a l s d u r i n g t h e a n o d i c sweep would be expected to o c c u r a t the same p o t e n t i a l s d u r i n g the c a t h o d i c sweep - t h a t i s , the shapes o f t h e a n o d i c - and c a t h o d i c - g o i n g c u r v e s s h o u l d m i r r o r one a n o t h e r , indicating reversibility.  On t h e o t h e r hand, i f an o x i d e was p r e s e n t the  c u r r e n t on a c a t h o d i c - g o i n g p o t e n t i a l  sweep s h o u l d n o t be expected t o  become n e g a t i v e u n t i l the p o t e n t i a l was depressed the r e v e r s i b l e p o t e n t i a l f o r the phase o x i d e .  to values cathodic to  The v a r i a b i l i t y o f the  p o t e n t i a l a t which t h e c a t h o d i c - g o i n g t r a c e i n voltammetry a t t a i n s v a l u e s c o u l d perhaps be accounted  negative  f o r by assuming t h a t t h e thermodynamic  p r o p e r t i e s o f t h i n oxygen f i l m s vary w i t h t h i c k n e s s , and o n l y approach those o f the b u l k o x i d e a t h i g h e r t h i c k n e s s e s than a r e encountered oxidation of platinum surfaces. satisfactorily  with  A t present, n e i t h e r theory i s able to  e x p l a i n the oxygen d e p o s i t i o n and removal f e a t u r e s  observed  on charge c u r v e s o r voltammograms.  3.1.1.1  Surface Species Deposited The  P r i o r t o Oxygen E v o l u t i o n  p o t e n t i a l r e g i o n between about 0 . 8 and 1.6 v o l t s i s p a r t i c u l a r l y  amenable t o study o f t h e oxygen-species  coverage  due t o t h e l a c k o f  s i g n i f i c a n t i n t e r f e r i n g steady s t a t e current-consuming  processes.  to a v e r y good a p p r o x i m a t i o n , a l l t h e charge passed w i t h i n t h i s r e g i o n corresponds vs.  to deposition of surface species.  p o t e n t i a l r e l a t i o n observed  potential  The l i n e a r  f o r p r o g r e s s i v e oxygen-species  Thus  c o v e r a g e  deposition  under i m p o s i t i o n o f a c o n s t a n t a n o d i c c u r r e n t can be e x p l a i n e d i n two ways: (1)  Ternkin a d s o r p t i o n ( E l o v i c h  (2)  H i g h - f i e l d o x i d e growth  kinetics)  44  F u r t h e r , i t i s not l i k e l y t h a t the d e p o s i t i o n o f a s i n g l e s p e c i e s i s i n v o l v e d , but r a t h e r t h a t s e v e r a l steps are i n v o l v e d i n the o v e r a l l t i o n p r o c e s s , w i t h the k i n e t i c b e h a v i o u r c o n t r o l l e d by the "slow" I t i s l i k e l y , t h a t the s p e c i e s i n i t i a l l y d e p o s i t e d to an OH  radical.  Burke [233]  oxida-  step.  corresponds  suggests t h i s i s the r e s u l t of the o r i e n t a -  t i o n o f water m o l e c u l e s on a p o s i t i v e l y charged s u r f a c e , whereby the remaining  lone-pair sp  3  o r b i t a l s are c l o s e s t t o the e l e c t r o d e  surface.  Water o x i d a t i o n ( l o s s of an e l e c t r o n ) i s most l i k e l y t o o c c u r from of these o r b i t a l s due  t o the h i g h e r t u n n e l i n g p r o b a b i l i t y .  a c a t i o n i c intermediate become an adsorbed OH The  one  Consequently  i s formed which s u b s e q u e n t l y l o s e s a proton  to  radical.  pH-dependence of the d e p o s i t i o n  (and removal) of oxygen  s p e c i e s suggests t h a t the o v e r a l l p r o c e s s e s i n v o l v e equal  numbers o f  hydrogen i o n s and e l e c t r o n s i n the s t o i c h i o m e t r i c e q u a t i o n s are s h i f t e d c a t h o d i c a l l y 59 mv tion).  two  f o r each u n i t of pH i n the a l k a l i n e d i r e c -  F u r t h e r , s i n c e the charge c o n s i d e r a t i o n s  o x y g e n - s p e c i e s on p l a t i n u m  (the f e a t u r e s  imply a 1:1  coverage o f  i m m e d i a t e l y p r i o r t o oxygen e v o l u t i o n , t h e  f o l l o w i n g r e a c t i o n sequence f o r e l e c t r o d e o x i d a t i o n i s i n d i c a t e d :  Pt + H 0 2  + PtOH + H  PtOH + PtO + H  where r e d u c t i o n o c c u r s  i n the o p p o s i t e  + e  (14a)  + e  +  (14b)  sense [225,234-238].  Other r e a c t i o n mechanisms have been proposed f o r  fi1m-formation  [239] 2Pt + 2H 0 + 2PtOH + 2H 2  2PtOH + PtO + Pt +  H0 2  +  2e  (15a) (15b)  45  and  f o r f i l m reduction  [239,240]:  2Pt0 + 2H  + 2e + 2Pt0H  (16a)  2Pt0H + PtO + P t + H 0  (16b)  2  " S t r u c t u r e " i s observed i n the oxygen d e p o s i t i o n r e g i o n s o f anodic  charge curves  [241,242] o r voltammograms [243-249] under r i g o r o u s  c o n d i t i o n s o f system p u r i t y .  Conway et at. [243-248] have a t t r i b u t e d the  s t r u c t u r e t o the s u c c e s s i v e d e p o s i t i o n o f PtOH on v a r i o u s The  sub-lattices.  s u b - l a t t i c e s t o i c h i o m e t r y can be i n f e r r e d from t h e f r a c t i o n a l  s p e c i e s coverages a t the p o t e n t i a l s c o r r e s p o n d i n g  oxygen-  to the various s t r u c t u r e s  observed i n t h e voltammogram.  For the (100) plane these a r e i d e n t i f i e d  as f o u r s t a g e s o f d e p o s i t i o n :  0 ^ , O^'  ^ 3 ' 0^: Maximum F r a c t i o n a l  Stage  J  Sub-lattice  A1  7\2 J  Stoichiometry  0 A4  4  0.89v  0.25  Pt 0H  0.94v  0.33  PtOH  1.05v  0.50  PtO  Such b e h a v i o u r  - .-. Coverager  Pt 0H 2  A3  Potential  suggests  broad r e g i o n  [2QHj •  1.00  d i f f e r e n t r e l a t i v e adsorption energies e x i s t f o r  the main t y p e s o f s u r f a c e  sites.  Those i n v e s t i g a t o r s who conform t o t h e " o x i d e " t h e o r y  consider  t h a t . t h e oxygen coverage i s merely t h e p r o g r e s s i v e t h i c k e n i n g o f a PtO o x i d e l a y e r [250-264]. Perhaps t h e major problem i n d e f i n i t i o n o f t h e n a t u r e o f t h e oxygen coverage on p l a t i n u m i s t h e i n t r i n s i c i r r e v e r s i b i l i t y o r h y s t e r e s i s observed  46  i n the shapes o f the anodic and c a t h o d i c charge curves o r voltammograms, which i n d i c a t e s t h a t e i t h e r a profound change i n the n a t u r e o f the oxygen f i l m occurs  immediately  and r e d u c t i o n are  a f t e r f o r m a t i o n o r t h a t the mechanisms o f o x i d a t i o n  different.  Such behaviour  has been accounted f o r by c o n s i d e r i n g t h a t the  d e p o s i t i o n o f oxygen s p e c i e s obeys Ternkin k i n e t i c s , w i t h the  rate-determining  step s u b j e c t e d t o a p r o g r e s s i v e l y i n c r e a s i n g a c t i v a t i o n energy b a r r i e r . [236,265-268].  For the r e a c t i o n  + ° ads H  +  H  k  +  e  ^  i H z 0  k  ( 1 7 )  2  the r a t e o f i n c r e a s e o f coverage w i t h time d u r i n g anodic be expressed  de  dT  =  polarization  can  as:  k  2 H 0 S  ( 1 _ 9 )  2  expl(1-3)(^f- - me)  - k,C eexp  +  H+  (18)  me)  where the Ternkin c o n s t a n t m r e p r e s e n t s the l i n e a r change o f the heat o f adsorption with coverage, 0 i -  The  d i f f e r e n c e between'.'therformation  o f the adsorbed f i l m can be e x p l a i n e d by a d e c r e a s e i n the Ternkin m,  due  and  removal  constant,  to m i n i m i z a t i o n o f r e p u l s i v e i n t e r a c t i o n s between adsorbed s p e c i e s .  ( R e a c t i o n s w i t h a l a r g e m o c c u r over a wide range o f p o t e n t i a l s , whereas those w i t h a small m occur i n a narrow p o t e n t i a l range.)  Such an argument  can e x p l a i n the d i f f e r e n c e between the shapes o f the o x i d a t i o n and f e a t u r e s found on c y c l i c charge c u r v e s  o r voltammograms.  Hysteresis,  however, must be accounted f o r by a p r o g r e s s i v e d e c r e a s e i n the redox p o t e n t i a l .  The  reduction  surface  Ternkin k i n e t i c argument breaks down as coverage  47  approaches and s u r p a s s e s monolayer v a l u e s , i n d i c a t i n g t h a t t h e r e must be a s u b s t a n t i a l d i f f e r e n c e between the n a t u r e o f the o x y g e n - s p e c i e s d e p o s i t e d a t p o t e n t i a l s p r i o r to oxygen e v o l u t i o n and t h a t a t h i g h e r p o t e n t i a l s t h a t i s , p r i o r t o and a f t e r monolayer coverage. with 0  1 8  i t i s found t h a t the oxygen d e p o s i t e d  on the e l e c t r o d e d u r i n g e v o l u t i o n [269-272].  In r a d i o t r a c e r s t u d i e s  i n the f i r s t  l a y e r remains  subsequent p o l a r i z a t i o n w i t h c o n c u r r e n t oxygen  Hence the n a t u r e o f the oxygen f i l m does indeed change  a f t e r growth beyond monolayer c o v e r a g e , and Temkin k i n e t i c s can a t l e a s t be used to e x p l a i n t h e i n i t i a l f i l l i n g o f t h e p l a t i n u m  surface with  oxygen  species. I t i s u n l i k e l y t h a t the adsorbed oxygen f i l m remains i n i t s i n i t a l s t a t e , however, Pool  [ 2 7 ] , Reddy [274] and Conway et al. [243-248] suggest  t h a t t h e o x i d a t i o n / r e d u c t i o n h y s t e r e s i s i s due t o a " p l a c e exchange" process whereby some s u r f a c e oxygen s p e c i e s  and u n d e r l y i n g metal atoms  rearrange:  PtOH + OHPt PtO  (19)  + OPt  as a consequence o f mutual r e p u l s i o n e f f e c t s .  The r e s u l t i n g s u r f a c e  would thus c o n s i s t o f an o r d e r e d arrangement when Qg/2Q = 1. H  top l a y e r  •••  second l a y e r metal s u b s t r a t e  Pt  0  Pt  0  Pt  0  •••0  Pt  0  Pt  0  Pt  •••  •••  Pt  Pt  Pt  Pt  Pt  •••  Pt  For example  48  Consequently i t i s the s t a b i l i t y of t h i s  "place-exchanged" s t r u c t u r e over  the one where a l l the oxygen coverage i s c o n f i n e d  to the top l a y e r t h a t i s  said to e x p l a i n the h y s t e r e s i s i n the p o t e n t i a l s f o r formation tion of the oxygen-film.  Conway et at.  m a n i f e s t e d a t coverages above  QQ/2Q  H  =  [243-248]  0.25  and reduc-  note t h a t h y s t e r e s i s i s  - t h a t i s f o r s t a g e s above  0^-j - which i n d i c a t e s t h a t place-exchange may be promoted as s u r f a c e filling  V e t t e r [ 2 5 0 ] has  (and t h e mutual r e p u l s i o n e f f e c t ) i n c r e a s e s .  a l s o suggested p l a c e exchange as t h e mechanism o f growth o f an o x i d e f i l m on t h e p l a t i n u m The  surface.  "oxide"  t h e o r y f o r t h e oxygen coverage  on p l a t i n u m a c c o u n t s  f o r the increase  i n potential a t constant current  i n t h e pre-oxygen e v o l u t i o n  potential region  by assuming t h e f i l m grows by a h i g h - f i e l d  mechanism  whereby t h e p r o g r e s s i v e l y i n c r e a s i n g p o t e n t i a l drop a c r o s s a p r o g r e s s i v e l y t h i c k e n i n g o x i d e f i l m r e s u l t s i n h i g h e r measured p o t e n t i a l s . and  Ward  [260,263]  found t h a t t h e growth o f t h e o x i d e f i l m on p l a t i n u m  under g a l v a n o s t a t i c c h a r g i n g region  conditions  c o u l d be f i t t e d t o a c l a s s i c  i n the pre-oxygen e v o l u t i o n p o t e n t i a l  oxidation rate  c  i = 'i o exp  where i o and <* a r e c o n s t a n t s , the f i l m t h i c k n e s s .  Damjanovic  equation.  Av  (21)  Av i s t h e p o t e n t i a l a c r o s s t h e f i l m , and d i s  I t i s d i f f i c u l t , however, t o c o n c e i v e o f t h e u n i f o r m  growth o f an o x i d e f i l m by such a mechanism a t .coverages below  QQ/2Q^  =  High f i e l d growth i n d i s c r e t e patches i s a l s o d i f f i c u l t t o r e c o n c i l e w i t h the u n i t y evolution.  ©averages observed a t p o t e n t i a l s i m m e d i a t e l y p r i o r t o oxygen The o x i d e t h e o r y can,  however, e x p l a i n t h e h y s t e r e s i s between  1.  49  the p o t e n t i a l s o f f o r m a t i o n and  r e d u c t i o n o f the oxygen coverage - growth  occurs by a u n i f o r m advance over the e n t i r e s u r f a c e , whereas r e d u c t i o n occurs a l o n g the edges o f o x i d e "islands', [250].  As mentioned p r e v i o u s l y ,  1  the shape o f the r e d u c t i o n f e a t u r e i n e i t h e r c h r o n o p o t e n t i o m e t r i c voltammetric  curves  i s s u g g e s t i v e o f an o x i d e .  The  o b s e r v a t i o n t h a t the  r e d u c t i o n p l a t e a u or peak i s s h i f t e d to lower p o t e n t i a l s , w i t h h o l d i n g time a t anodic  p o t e n t i a l s where the o x y g e n - f i l m  bulk oxide i n character i t w i l l  increased  i s formed, i s i n  conformance w i t h the p r o g r e s s i v e c o n v e r s i o n o f the f i l m t o one thermodynamic p r o p e r t i e s o f a b u l k o x i d e .  or  having  the  As the f i l m approaches the  tend t o be reduced a t p r o g r e s s i v e l y  lower  p o t e n t i a l s as the r e v e r s i b l e p o t e n t i a l o f the f i l m approaches t h a t f o r the o x i d e , f o r c i n g r e d u c t i o n t o lower p o t e n t i a l s .  3.1.1.2  Surface Species The  Deposited  During Oxygen E v o l t u i o n  n a t u r e o f the s u r f a c e o f a p l a t i n u m e l e c t r o d e which i s e v o l v -  ing oxygen gas  is still  f a r from c h a r a c t e r i z e d .  Charge s t u d i e s r e v e a l t h a t  h i g h e r than monolayer coverages e x i s t d u r i n g oxygen e v o l u t i o n .  The  higher  coverage v a l u e s i n d i c a t e d by the charge s t u d i e s have been i n t e r p r e t e d as: 1)  f i l l i n g of the metal s u r f a c e s t o i c h i o m e t r y of Pt:0,  beyond a  2)  continued  film,  3)  absorption  g r o w t h o f an of oxygen  Case (1) i s supported  oxide  into  the  metal.  by the r e s u l t s o f many i n v e s t i g a t o r s who  found t h a t the coverage vs. p o t e n t i a l r e l a t i o n was 2.0  1:1  l i n e a r up t o about  v o l t s (RHE), whereupon the degree o f s u r f a c e o x i d a t i o n d i d not  thereafter.  The  n e a r l y 1:2  s t o i c h i o m e t r y o f the " l i m i t i n g "  increase  coverage  50  (1Pt:20)  r e p o r t e d by s e v e r a l i n v e s t i g a t o r s [ 1 4 1 , 2 4 9 , 2 7 6 , 2 7 7 ] does not  n e c e s s a r i l y r e f e r t o the s t r u c t u r e " P t 0 " nor does i t i n d i c a t e t h a t the 2  (1:1)  coverage i n e x c e s s o f "monolayer" second l a y e r .  B i e g l e r [249]  values n e c e s s a r i l y c o n s t i t u t e s a  has noted t h a t t h e r e are no s t e r i c h i n d r a n c e s  t o i n c r e a s e d s u r f a c e f i l l i n g w i t h oxygen.  Indeed, the f i n d i n g o f a " P t 0 " 2  l i m i t i n g s t o i c h i o m e t r y appears t o be f o r t u i t o u s under t h e measurement cond i t i o n s commonly employed  (1-2N  H S 0 i , 20°C). 2  t  Under c o n d i t i o n s o f h i g h e r  a c i d c o n c e n t r a t i o n [ 2 7 8 - 2 8 0 ] and l o w e r temperatures [ 2 8 1 ] coverage s t o i c h i o m e t r y i s found t o d e c r e a s e .  the l i m i t i n g  Such dependences are l i k e l y  a s s o c i a t e d w i t h the s t r o n g a d s o r p t i o n o f s u l f a t e a n i o n s which i s known t o o c c u r on p l a t i n u m above 1.6  v o l t s [ 2 8 2 , 2 8 3 ] , where a d s o r p t i v e d i s p l a c e m e n t  o f oxygen would r e s u l t i n lower measured c o v e r a g e s . F u r t h e r , t h e l i m i t i n g coverage r e p o r t e d by B i e g l e r [ 2 7 6 ]  appears  t o be dependent on t h e d u r a t i o n o f p r e a n o d i z a t i o n where t h e anode was h e l d a t p o t e n t i a l s i n the l i m i t i n g coverage r e g i o n (> 2.2 [284J,  volts).  Gilman  whose procedure was adopted by B i e g l e r f o r h i s measurements, d i d  not f i n d t h a t the growth o f oxygen coverage ceased a t any g i v e n potential  ( a l t h o u g h h i s work d i d not extend t o the l i m i t i n g  potential  region).  anodization  coverage  V o l o d i n [ 2 7 5 ] , on t h e o t h e r hand, found the coverage  o f an anode o p e r a t i n g a t 2.40  v o l t s a t t a i n e d a time-independent l i m i t i n g  v a l u e a f t e r 50 seconds' p o l a r i z a t i o n which d i d n o t v a r y even a f t e r 36 at t h i s  days  potential. The appearance o f a l i m i t i n g v a l u e o f the coverage i n the coverage  vs. p o t e n t i a l  r e a l t i o n may  n a t u r e o f the oxygen  be s a i d t o be i n d i c a t i v e o f the chemisorbed  f i l m on p l a t i n u m e l e c t r o d e s .  In the " o x i d e "  where f i l m growth i s c o n t r o l l e d by the a p p l i e d p o t e n t i a l , no potential  value should  exist.  theory,  limiting  51  Proponents o f t h e " o x i d e " t h e o r y do not f i n d e v i d e n c e p o t e n t i a l - l i m i t e d maximum coverage [ 2 5 4 ] ,  for a  F u r t h e r , o x i d e growth can account  f o r t h e slow r a t e o f i n c r e a s e o f t h e oxygen coverage w i t h time t o v a l u e s beyond " P t 0 " s t o i c h i o m e t r y . 2  V e t t e r [254] has determined  t h a t t h e oxygen  coverage i n c r e a s e s a t a g i v e n p o t e n t i a l a c c o r d i n g t o :  Qo = a l o g t + b  where a and b a r e c o n s t a n t s f o r the g i v e n p o t e n t i a l . that "Pt02  n  (22)  F u r t h e r , he c l a i m s  s t o i c h i o m e t r y i s exceeded by h o l d i n g f o r 1000 seconds o r l e s s  a t p o t e n t i a l s o f 1.5 v o l t s and above. i n t e r p r e t e d as e v i d e n c e  High coverage v a l u e s were i n t u r n  f o r t h e e x i s t e n c e o f s u r f a c e oxygen i n o x i d e  form.  The a b s o r p t i o n o f oxygen i n t o p l a t i n u m has been p o s t u l a t e d t o o c c u r a t p o t e n t i a l s both p r i o r t o and subsequent t o oxygen e v o l u t i o n . a b s o r p t i o n o f oxygen i n t o the metal several  has been proposed t o account f o r  phenomena i n a d d i t i o n t o t h e i n c r e a s e i n oxygen coverage beyond  v a l u e s p r e d i c t e d from c h e m i s o r p t i o n  theory:  1.  The s m a l l potential [235].  residual current observed a t constant ( a t v a l u e s p r i o r t o oxygen e v o l u t i o n )  2.  The c h a r g e i m b a l a n c e o f t e n m e a s u r e d b e t w e e n t h e d e p o s i t i o n and removal o f oxygen s p e c i e s [236,241, 2 4 2 , 2 8 5 , 2 8 6 ] . The i m b a l a n c e o f c h a r g e ( w i t h t h e anodic charge exceeding the c a t h o d i c charge) i s found to decrease w i t h c o n t i n u e d c y c l i n g which s u g g e s t s s a t u r a t i o n o f t h e metal s u r f a c e w i t h absorbed oxygen. The i m b a l a n c e c a n a l s o be r e d u c e d w i t h r a p i d c y c l i n g , w h e r e t h e a b s o r p t i o n does n o t h a v e s u f f i c i e n t t i m e t o o c c u r , o r by p r e a n o d i z a t i o n t o p r e - s a t u r a t e the s u r f a c e l a y e r s w i t h oxygen.  3.  S t a b i l i z a t i o n o f a c o m p l e t e o x y g e n m o n o l a y e r on t h e surface enabling the establishment of the r e v e r s i b l e o x y g e n p o t e n t i a l [287,288].  The  52  k.  I n c r e a s e o f t h e h y d r o g e n o v e r p o t e n t i a l on a platinum f o i l b i e l e c t r o d e [289].  5.  I n c r e a s e o f t h e r e s t p o t e n t i a l on t h e s u r f a c e o f a p l a t i n u m f o i l w h i c h i s a c t i v e l y e v o l v i n g oxygen on t h e o t h e r s i d e [ 2 9 0 , 2 9 1 , 2 9 2 ] .  6.  The a p p e a r a n c e o f two a r r e s t s i n t h e g a l v a n o s t a t i c s t r i p p i n g curve a f t e r p r i o r anodization a t potent i a l s a b o v e 1.25 v o l t s (R.H.E.) [ 2 8 8 ] .  7.  The r e - a p p e a r a n c e o f p a r t i a l o x y g e n c o v e r a g e on an e l e c t r o d e t h a t h a s been c a t h o d i c a l l y s t r i p p e d and a l l o w e d t o r e c o v e r u n d e r o p e n - c i r c u i t t o anodic values [288]. (Absorbed oxygen can d i f f u s e outward onto the surface.)  8.  The i n a b i l i t y t o d e t e r m i n e a r e a c t i o n o r d e r f o r the s t r i p p i n g o f s u r f a c e oxygen w i t h hydrogen gas [293].  9.  Enhancement o f t h e e l e c t r o c a t a l y t i c p l a t i n u m towards t h e f e r r i c / f e r r o u s [29*0.  a c t i v i t y of redox r e a c t i o n  10.  The i n a b i l i t y o f i n f r a r e d s p e c t r o s c o p y platinum oxidation [295].  11.  The d e c r e a s e o f t h e oxygen r e d u c t i o n o v e r p o t e n t i a l on t h e b a c k o f a p l a t i n u m f o i l e l e c t r o d e whose o t h e r s i d e was s u b j e c t e d t o s t r o n g a n o d i z a t i o n [ 2 9 6 ] .  12.  The t i m e - d e p e n d e n c e  Thacker and Hoare [ 2 8 8 ]  o f t h e oxygen  to detect  overpotential [297]-  found t h a t s u r f a c e adsorbed oxygen c o u l d  be d i s t i n g u i s h e d from "dermasorbed" oxygen i n g a l v a n o s t a t i c charge f o r the s t r i p p i n g o f smooth p l a t i n u m e l e c t r o d e s .  curves  D e r m a s o r p t i o n was found  t o be pronounced f o r a n o d i z a t i o n s a t p o t e n t i a l s  above 1 . 8  which c o u l d r e s u l t i n e r r o n e o u s coverage v a l u e s  i f t h e subsequent charge  measurements were a t t r i b u t e d  to s u r f a c e coverage o n l y .  o f t h e s u r f a c e c o v e r a g e and d e r m a s o r p t i o n c h a r g e s , was found t o i n c r e a s e w i t h t i m e , under c o n s t a n t limiting  value o f  QQ/2Q^  -  2.  volts  With  (RHE),  separation  the s u r f a c e coverage  current conditions, to a  53  3.1.2  Strengthening The  charge curves  o f the Oxygen Bond  p o t e n t i a l o f the r e d u c t i o n p l a t e a u or peak i n c a t h o d i c  or voltammograms does not remain c o n s t a n t , but r a t h e r i s  s h i f t e d t o more c a t h o d i c v a l u e s w i t h i n c r e a s e d anodic anodic/cathodic  potential l i m i t in  c h a r g i n g or w i t h i n c r e a s e d h o l d i n g time a t a g i v e n  p o t e n t i a l or current density.  Such "ageing"  anodic  i s tantamount t o the s t r e n g t h -  ening o f the Pt-0 bond w i t h t i m e , p o s s i b l y due  to a change i n the  nature  r a t h e r than the degree o f the s u r f a c e coverage.  3.1.3  A c t i v e Oxygen T r a c e r s t u d i e s [269-272] have r e v e a l e d t h a t the oxygen  coverage formed p r i o r to oxygen e v o l u t i o n i s i n a c t i v e w i t h r e s p e c t t o subsequent oxygen e v o l u t i o n .  Oxygen d e p o s i t e d on the s u r f a c e a t  higher  p o t e n t i a l s , however, i s found to be more l a b i l e , and to p a r t i c i p a t e i n the oxygen e v o l u t i o n r e a c t i o n - and i n the oxygen e v o l u t i o n p r o c e s s .  thus may  be c o n s i d e r e d  to be  Typical chronopotentiometric  or  intermediates voltam-  m e t r i c s t u d i e s , however, do not r e v e a l the e x i s t e n c e o f two d i s t i n c t of s u r f a c e coverage ( t h e "type  I I " oxide discussed  forms  below i s not under  c o n s i d e r a t i o n here) as o n l y a s i n g l e r e d u c t i o n p l a t e a u o r peak i s observed ( a l t h o u g h the Temkin a d s o r p t i o n c o n s i d e r a t i o n s d i s c u s s e d above suggest t h a t the n a t u r e o f the f i l m changes past monolayer coverage. C e r t a i n i n v e s t i g a t o r s , however, u s i n g h i g h g a l v a n o s t a t i c charge r a t e s [298] or very f a s t p o t e n t i a l sweep r a t e s (e.g. > 30 v/sec) [239,275,279,281,289,298,299] have d e t e c t e d a s p l i t t i n g o f the r e d u c t i o n p l a t e a u o r peak t h a t becomes i n c r e a s i n g l y apparent a t h i g h e r sweep r a t e s .  54  I t has been argued  t h a t the second  peak corresponds  t o i n t e r m e d i a t e or  " a c t i v e " p a r t i c l e s f o r oxygen e v o l u t i o n whose presence  can o n l y be d e t e c t e d  a t h i g h sweep r a t e s due t o t h e i r r a p i d k i n e t i c s o f d e c o m p o s i t i o n form c o r r e s p o n d i n g t o the peak n o r m a l l y observed. these two coverages observed  3.1.4  Thus i t i s t h e sum  which r e p r e s e n t s the approximate  as the l i m i t i n g coverage  i n t o the of  "Pt0 " stoichiometry 2  v a l u e i n many c a s e s .  Type I I Oxide "Type I I " o x i d e f o r m a t i o n o n l y o c c u r s under p a r t i c u l a r c o n d i -  t i o n s of high c u r r e n t d e n s i t y g a l v a n o s t a t i c o p e r a t i o n or high p o t e n t i a l p o t e n t i o s t a t i c o p e r a t i o n , and i s m a n i f e s t e d by the appearance o f  two  a r r e s t s or peaks i n the subsequent c a t h o d i c charge curve o r voltammogram [276,300-320].  The  "type I I " o x i d e i s reduced a t a c o n s i d e r a b l y lower  p o t e n t i a l than "type I " and i t s coverage t i o n time t o v a l u e s w e l l  i s found t o i n c r e a s e w i t h a n o d i z a -  i n excess o f monolayer s t o i c h i o m e t r y - hence i t i s  assumed t o c o r r e s p o n d to a phase o x i d e .  Bulk o x i d e c h a r a c t e r i s t i c s  are  a l s o i n d i c a t e d by the v i s u a l o b s e r v a t i o n o f a red [ 3 0 4 ] , orange-brown [ 3 1 0 ] , o r y e l l o w [300,307,313] f i l m on the e l e c t r o d e .  X-ray  diffraction  [307] i s unable to d e t e c t c r y s t a l l i n e s t r u c t u r e , but e l e c t r o n [313] suggests the f i l m i s p o o r l y - c r y s t a l l i n e P t 0 . 2  f o r "type I I " o x i d e drops from 0.40 anodization.  v o l t s t o 0.26  The  reduction potential  v o l t s w i t h prolonged  S u k h o t i n [231] has p o i n t e d o u t t h a t t h e s e p o t e n t i a l s  e x a c t l y to the r e d u c t i o n p o t e n t i a l s f o r bulk h y d r a t e d P t 0 respectively  diffraction  (see r e a c t i o n s (12) and  2  and  correspond  Pt 0 , 3  4  ( 1 3 ) ) , i n f e r r i n g t h a t t h e type I I o x i d e  i s l i k e l y a v a r y i n g m i x t u r e of these two phases.  Inoue [321] found  both  t h e s e o x i d e s p r e s e n t on an anode used i n p e r s u l f a t e manufacture f o r t h r e e y e a r s a t 1050-1250 mA/cm . 2  55  Type I I o x i d e has some r a t h e r p e c u l i a r c h a r a c t e r i s t i c s : 1)  i t does n o t a f f e c t t h e p o s i t i o n o f t h e type I r e d u c t i o n p l a t e a u o r p o t e n t i a l , n o r does i t a l t e r t h e coverage w i t h type I (which i s t h e l i m i t i n g v a l u e o f Qq/IQ = 2 ) , e v e n t h o u g h i t grows c o n s i d e r a b l y beyond monolayer c o v e r a g e ,  2)  i t o n l y forms o v e r a narrow range o f e l e c t r o d e potent i a l s ,  3)  i t s formation  4)  i n p e r c h l o r i c a c i d s o l u t i o n s , t h e t y p e II o x i d a t i o n h a s a maximum g r o w t h r a t e a t 2N concentration,  5)  i t a c t i v a t e s the e l e c t r o d e w i t h r e s p e c t t o oxygen e v o l u t i o n ( t h e c u r r e n t d e n s i t y does n o t c o n t i n u e to decrease w i t h constant p o t e n t i a l o p e r a t i o n , but r a t h e r i n c r e a s e s a f t e r a c e r t a i n t i m e ) ,  6)  p l a t i n u m d i s s o l u t i o n i s enhanced under the same c o n d i t i o n s a s r e q u i r e d f o r f o r m a t i o n o f t y p e II o x i d e ,  7)  a n n e a l i n g o f an e l e c t r o d e h i n d e r s i t s subsequent formation.  i s prevented  a t lower  temperatures,  or  prevents  C h a r a c t e r i s t i c (1) has been e x p l a i n e d by S h i b a t a [ 3 1 1 ] and V i n n i k o v [ 3 1 7 ] as due t o o x i d a t i o n i n t o the metal beneath t h e type  I film.  Hence no p h y s i c a l i n h i b i t i o n o f t h e r e d u c t i o n o f t h e type I f i l m by t h e type I I o x i d e o c c u r s , nor does t h e type I I o x i d e prevent coverage f o r type I t o be a t t i n e d . t h a t the type  the l i m i t i n g  C h a r a c t e r i s t i c s (2) and (3) suggest  I f i l m i s p r o t e c t i v e with respect to penetration of oxidation  to t h e u n d e r l y i n g l a t t i c under o t h e r c o n d i t i o n s o f p o t e n t i a l and temperature.  There appears t o be some d i s c r e p a n c y  i n t h e p o t e n t i a l range o v e r  which type I I o x i d e can form: Shibata [311] Balej [315] Vinnikov [317]  2.10-2.17 v o l t s (RHE) i n IN H S 0 2.1 -2.5 v o l t s (RHE) i n IN H S0^ 2.1 -2.4 v o l t s (RHE) i n IN H S 0 2  4  2  2  4  56  The e x t e n t o f the f o r m a t i o n range appears to be a f u n c t i o n o f the degree o f d i s r u p t i o n o f the metal annealed  electrodes.  surface.  S h i b a t a [ 3 1 1 ] , f o r example, used o n l y  Indeed, l a t e r work by S h i b a t a [312] showed t h a t the  f o r m a t i o n o f type I I o x i d e can be c o m p l e t e l y supressed 1000°C, but t h a t type I I may  by a n n e a l i n g above  a l s o grow on such an e l e c t r o d e t h a t has been  s u b j e c t e d to e l e c t r o c h e m i c a l a n o d i c / c a t h o d i c a c t i v a t i o n which promotes surface dissruption.  The e l e c t r o l y t e c o n c e n t r a t i o n dependence o f type I I  f o r m a t i o n o r growth has been a t t r i b u t e d to i n c r e a s e d s o l u t i o n a n i o n t i o n which p r o g r e s s i v e l y i n h i b i t s oxygen l a y e r growth i n s u l f u r i c [315] o r o n l y becomes m a n i f e s t e d beyond a c e r t a i n c r i t i c a l the case o f p e r c h l o r i c a c i d  adsorp-  acid  potential in  [300].  The n a t u r e o f the type I I o x i d e i s f a r from c h a r a c t e r i z e d , however, and s e v e r a l e x p e r i m e n t a l e x p l a n a t i o n o f the apparent  anomalies  decrease  f i l m over a narrow p o t e n t i a l range.  still  e x i s t - l e t alone  the  i n p a s s i v a t i n g a b i l i t y o f the type I For example, S h i b a t a [311] r e p o r t s  t h a t an anode on which type I I o x i d e has been p e r m i t t e d to grow and i s subsequently  p o t e n t i o s t a t t e d to a p o t e n t i a l where o n l y the p a s s i v e f i l m  (type I) i s known t o o c c u r , the type I I o x i d e w i l l  c o n t i n u e t o grow.  On  the o t h e r hand, B a l e j [315] f i n d s t h a t under s i m i l a r c o n d i t i o n s t h e type I I oxide  disappears. The apparent  above 2.2-2.5 v o l t s (RHE)  i n a b i l i t y o f p l a t i n u m t o grow a type II coverage p o t e n t i a l may  be r e l a t e d to i t s d i s s o l u t i o n .  Kravchenko [300] has shown t h a t under c o n d i t i o n s where t h i c k o x i d e l a y e r s form on p l a t i n u m i n p e r c h l o r i c a c i d s o l u t i o n s the d i s s o l u t i o n r a t e i s h i g h e r than a t a s i m i l a r p o t e n t i a l form.  i n s o l u t i o n s where t h i c k o x i d e l a y e r s do  As the r a t e o f p l a t i n u m d i s s o l u t i o n i s known t o i n c r e a s e w i t h  not  57  p o t e n t i a l above 1.8-2.0 v o l t s (RHE), p a r a l l e l i n g the oxygen e v o l u t i o n c u r r e n t , i t i s p o s s i b l e t h a t the appearance o f type I I o x i d a t i o n may occur when the b a l a n c e between the two p r o c e s s e s o f o x i d e growth d i s s o l u t i o n i s i n f a v o u r o f the former p r o c e s s .  then a maximum w i l l type I I o x i d e .  be observed  i n the coverage  vs. p o t e n t i a l  At h i g h p o t e n t i a l s (above 2.2-2.5 v o l t s  h i g h e r r a t e o f d i s s o l u t i o n does not p e r m i t the growth  3.1.5  N o n - E l e c t r o c h e m i c a l Techniques  and  I f the o x i d e growth  i s l e s s - s e n s i t i v e to changing p o t e n t i a l than i s the d i s s o l u t i o n  only  rate  rate,  relation for  (RHE)) t h u s , the  o f type I I o x i d e .  f o r Oxygen F i l m E v a l u a t i o n  C o n s i d e r a b l e e l l i p s o m e t r i c work has been done i n c o n j u n c t i o n w i t h e l e c t r o c h e m i c a l charge s t u d i e s .  The t e c h n i q u e i s l e s s - s e n s i t i v e  than  the e l e c t r o c h e m i c a l t e c h n i q u e s , a l t h o u g h changes i n the e l l i p s o m e t r i c parameters  which a r e r e l a t e d to f i l m t h i c k n e s s i n d i c a t e a l i n e a r i n c r e a s e  in surface oxidation with potential determined  i n an analogous  manner t o the  coverage  by charge curve s t u d i e s [ 2 4 6 ] , w i t h a change i n the n a t u r e of  the s u r f a c e f i l m i n d i c a t e d above about 1.5  v o l t s [320].  Low energy e l e c t r o n d i f f r a c t i o n  (LEED) a p p l i e d t o gas-phase  a d s o r p t i o n s t u d i e s w i t h p l a t i n u m s i n g l e c r y s t a l s i n d i c a t e t h a t oxygen adsorption occurs at s p e c i f i c s i t e s with c r y s t a l l o g r a p h i c r e g u l a r i t y t h a t i s , c e r t a i n s u r f a c e " s t r u c t u r e s " are formed.  There i s a s i m i l a r i t y  between these " s t r u c t u r e s " and the s u b l a t t i c e f i l l i n g et al.  i n f e r r e d by Conway  [243-248] from v o l t a m m e t r i c e x p e r i m e n t s . E l e c t r o n s p e c t r o s c o p y f o r chemical a n a l y s i s (ESCA) p e r m i t s  d e t e r m i n a t i o n o f the b i n d i n g energy o f v a r i o u s compounds, o r a l l o w s the d e t e c t i o n o f compound f o r m a t i o n on a metal  s u b s t r a t e through the measurement  58  of s h i f t s i n the core e l e c t r o n b i n d i n g e n e r g i e s . a b l e t o determine the d i f f e r e n c e between Pt and  ESCA was Pt0  2  found t o  [ 3 2 4 ] , and  i t s d e t e c t i o n has been r e p o r t e d a f t e r s h o r t a n o d i z a t i o n a t 2.2 a l s o a f t e r v e r y long a n o d i z a t i o n a t 4 v o l t s r e p o r t e d the e x i s t e n c e o f PtO j treatment,  [325]  but A l l e n  and  PtO  2  Kim  volts  On  volts.  and  [302,324] anodic  the o t h e r hand, A l l e n  o x i d a t i o n s t a t e than PtO was  s u r f a c e s p e c i e s o c c u r r i n g between 1-2.4 to Pt0.-H 0 or  [324].  subsequently  species a f t e r less-severe  d i d not d e t e c t t h e s e .  r e p o r t e d a s p e c i e s w i t h a lower formal  correspond  (RHE)  be  the o n l y  He suggested t h a t t h i s  may  Pt(0H) . 2  Auger s p e c t r o s c o p y  r e l i e s on d e t e c t i o n o f e n e r g i e s of "Auger  e l e c t r o n s " e m i t t e d as a r e s u l t o f r e a b s o r p t i o n o f c h a r a c t e r i s t i c X-rays i n t e r n a l l y w i t h i n the atom i n which they were c r e a t e d . X-rays which escape are those d e t e c t e d  (The  characteristic  i n X-ray f l u o r e s c e n c e  spectroscopy.)  Of p a r t i c u l a r advantage i s the f a c t t h a t Auger e l e c t r o n s r a r e l y exceed 0.5  kev  i n energy and  f i r s t few  thus are reabsorbed  i f they emanate from below the  s u r f a c e l a y e r s - hence the t e c h n i q u e  Johnson [ 3 2 6 ]  presence o f the u n d e r l y i n g metal as i n ESCA. the s u r f a c e o f a p l a t i n u m anode o p e r a t e d  a t 77.5  showed both P t and 0 peaks, w i t h a p r o b a b l e  3.1.6  i s not hampered by  "PtO"  the  found t h a t  mA/cm f o r 16 hr i n IN H S0i» 2  2  stoichiometry.  Review On the b a s i s of the degrees and  n a t u r e o f the s u r f a c e oxygen  coverage on p l a t i n u m as r e p o r t e d i n the l i t e r a t u r e , i t i s f e l t t h a t the p r o g r e s s i v e o x i d a t i o n o f the p l a t i n u m s u r f a c e can be d e s c r i b e d by stages  l i s t e d i n Table  3.1.  sub-monolayer s u r f a c e f i l l i n g  the  S t a r t i n g w i t h a bare p l a t i n u m s u r f a c e , the occupies  successive sub-lattices  until  monolayer  Table P r o b a b l e Stages o f O x i d a t i o n Anode P o t e n t i a l ( v o l t s v s . RHE)  Major  Charge-Consuming Processes  C h a r g i n g the d o u b l e  layer  3.1  o f t h e S u r f a c e o f P l a t i n u m w i t h I n c r e a s i n g Anode P o t e n t i a l  Surface  Stoichiometry  Pt  Comments  Coverage  An o r i e n t e d water d i p o l e l a y e r e x i s t s to the metal s u r f a c e .  next  (0C/2QH)  0  Surface  oxidation  Pt_0H (100  plane)  Reversible adsorption in a sublattice structure.  0.25  Surface  oxidation  P t 2 0 H (100  plane)  P l a c e - e x c h a n g e between s u r f a c e OH and underl y i n g Pt g i v e s r i s e to i r r e v e r s i b i l i t y .  0.33  Surface  oxidation  PtOH  Surface  oxidation  PtO  Monolayer coverage p r i o r t o oxygen  PtO(0)n(0<n<l)  - C o n t i n u e d f i l l i n g o r coverage o f the s u r f a c e t o an e x t e n t which depends on the n a t u r e o f the e l e c t r o l y t e and t e m p e r a t u r e . - L i m i t i n g coverage o c c u r s above 1 . 8 - 2 . 2 v o l t s .  1.0-2.0  -Growth o f phase o x i d e s  1 . 0 - 2 . 0 (type I) Unlimited (type II)  1. 2. 3.  Oxygen e v o l u t i o n Surface oxidation Platinum d i s s o l u t i o n  1. 2.  Oxygen e v o l u t i o n Type II s u r f a c e o x i dation Platinum d i s s o l u t i o n  3. 1. 2. 3.  Oxygen e v o l u t i o n Ozone and p e r o x y s u l f a t e production Platinum d i s s o l u t i o n  0.5  P t 0 <°>n,lim. +Pt02 and/or  PtO(O)  n,lim.  Pt30»  evolution.  begins  - R a t e o f d i s s o l u t i o n exceeds r a t e o f t i o n o f type II o x i d e s  forma-  1.0  1 . 0 - 2 . 0 (type 0 (type  I) II)  CO  60  coverage i s a c h i e v e d immediately  p r i o r to a n o d i c e v o l u t i o n o f oxygen gas.  S u r f a c e coverage c o n t i n u e s t o i n c r e a s e , but a t a much slower r a t e ,  after  commencement o f oxygen e v o l u t i o n , e v e n t u a l l y r e a c h i n g a l i m i t i n g  value  which depends on the e l e c t r o l y t e c o n c e n t r a t i o n and temperature.  Only the  oxygen d e p o s i t e d a f t e r f o r m a t i o n o f the i n i t i a l  monolayer t a k e s p l a c e i n  the oxygen e v o l u t i o n p r o c e s s , i n d i c a t i n g t h a t the n a t u r e s of these  two  types o f s u r f a c e oxygen are d i f f e r e n t , even though no d i f f e r e n c e i s d e t e c t e d by e l e c t r o c h e m i c a l measurement t e c h n i q u e s .  Only under s p e c i a l c o n d i t i o n s ,  p r o b a b l y as a r e s u l t of the balance between the r a t e o f growth and d i s s o l u t i o n o f "type I I " o x i d e , does the growth o f phase o x i d e s ever o c c u r  on  platinum.  3.2  Iridium The  same e l e c t r o c h e m i c a l t e c h n i q u e s  (chronopotentiometry  and  voltammetry) as were d i s c u s s e d w i t h r e f e r e n c e t o p l a t i n u m can a l s o be employed to s t u d y the oxygen coverage on i r i d i u m e l e c t r o d e s . festures corresponding  Similar  to the f o r m a t i o n and removal o f s u r f a c e oxygen  s p e c i e s can be i d e n t i f i e d on the r e s u l t a n t charge c u r v e s and a l t h o u g h i r i d i u m shows marked d i f f e r e n c e s from p l a t i n u m .  voltammograms,  Primarily,  iridium  i s d i s t i n g u i s h e d by a poor s e p a r a t i o n o f the hydrogen and oxygen r e g i o n s and by r e v e r s i b i l i t y of the shapes o f the a n o d i c - and  cathodic-going  charge curves or voltammograms. Much o f the e a r l y work w i t h i r i d i u m i m p l i e s a s i m i l a r i t y w i t h p l a t i n u m w i t h r e g a r d t o the development o f monolayer coverage w i t h s u r f a c e oxygen s p e c i e s immediately  p r i o r t o oxygen e v o l u t i o n , a l t h o u g h , as w i l l  be  61  seen below, i r i d i u m i s a b l e t o have the e q u i v a l e n t o f s e v e r a l l a y e r s o f oxygen a t such p o t e n t i a l s .  3.2.1  Degree o f O x i d a t i o n The slowness o f t h e d e p o s i t i o n and r e d u c t i o n k i n e t i c s , which  have not been taken i n t o account by most i n v e s t i g a t o r s , e x p l a i n s t h e v a s t d i s c r e p a n c i e s among t h e coverage r e s u l t s r e p o r t e d of the l i t e r a t u r e .  The e a r l y work w i t h r e g a r d  f o r t h i s metal i n much  t o t h e degree o f coverage o f  i r i d i u m w i t h oxygen s p e c i e s was performed w i t h h i g h c h a r g i n g potential  sweep r a t e s t h a t l e d t o t h e o f t e n - r e p e a t e d  rates or  conclusion that i r i d i u m  a t t a i n s o n l y monolayer coverage a t p o t e n t i a l s below about 1.4 v o l t s [327330].  A t h i g h e r p o t e n t i a l s growth w e l l beyond monolayer coverage was i n d i c a t e d  by B r e i t e r [ 3 2 7 ] , Damjanovic  [ 3 3 1 ] , and Hoare [ 3 3 2 ] .  the coverage v s . p o t e n t i a l r e l a t i o n  Danjamovic found t h a t  (up t o 1.6v) had two l i n e a r  sections  which i n d i c a t e d t h a t t h e r a t e o f coverage w i t h p o t e n t i a l was g r e a t e r a t p o t e n t i a l s above about 1.35v than below and t h a t monolayer coverage was a t t a i n e d above 1.0v. I t was Kurnikov  [333-370], however, who determined the k i n e t i c s  of the oxygen d e p o s i t i o n p r o c e s s on i r i d i u m , by u s i n g t h e " p o t e n t i a l s t e p " technique  he determined t h a t t h e oxygen coverage a t any p o t e n t i a l v a r i e d  w i t h l o g t i n 3 l i n e a r s t a g e s , and t h a t t h e growth o f coverage was s u f f i c i e n t l y slow t h a t the f a s t sweep speeds o r h i g h - c h a r g i n g  c u r r e n t s used by the e a r l i e r  i n v e s t i g a t o r s d i d n o t p e r m i t the b u i l d - u p o f g r e a t e r - t h a n - m o n o l a y e r coverages a t low anode p o t e n t i a l s . Kurnikov  d e t e r m i n e d the coverage v s . p o t e n t i a l r e l a t i o n f o r  a v a r i e t y o f sweep r a t e s and found t h a t monolayer coverage c o u l d e x i s t a t  62  potentials  as low as 0.6 v o l t s  (RHE) and t h a t t h e e q u i v a l e n t o f tens o f  monolayers c o u l d e x i s t p r i o r t o oxygen e v o l u t i o n i f a slow sweep r a t e o f 0.05  v/sec was employed.  [328]  Higher sweep r a t e s such as those employed by W i l l  (1 v/sec) gave c o n s i d e r a b l y s m a l l e r coverage vs. p o t e n t i a l There i s c o n s i d e r a b l e o t h e r e v i d e n c e t o s u p p o r t  t h a t h i g h oxygen coverages e x i s t on i r i d i u m anodes.  results.  the c o n t e n t i o n  Chodos [338]  studied  the s u r f a c e s o f Pt/5 I r and Pt/20 I r a l l o y s w i t h an e l e c t r o n probe a f t e r a n o d i z a t i o n , f o l l o w e d by h o l d i n g f o r 15 h r a t Ov (SCE), and found t h a t c o n s i d e r a b l e amounts o f oxygen had remained on and beneath t h e s u r f a c e . B r e i t e r [330] and Capon [339] noted t h a t t h e c a t h o d i c voltammogram f o r a b r o a d I<OA«/A b e " l i A > * « p >  i r i d i u m i s c h a r a c t e r i z e d by!l0.3 v o l t s reversible detected  surface oxidation.  0.7 c*J>d  (RHE), i n d i c a t i n g t h e presence o f l e s s -  Schubert [340], using f i e l d - i o n microscopy,  the presence o f t h i c k o x i d e f i l m s on i r i d i u m anodized  a t 1.5 v o l t s -  w i t h p e n e t r a t i o n r e a c h i n g 100 angstroms i n t o t h e metal a t l o c a l i z e d Kuhn [341] found t h a t e x t e n s i o n o f f a s t (10 v/sec) c y c l i c sweeps t o 1.95 v o l t s as t h e anodic new  r e d u c t i o n peak a t 0.0 v o l t s  l i m i t resulted  sites.  voltammetric  i n t h e appearance o f a  (RHE) on t h e subsequent c a t h o d i c sweep.  F u r t h e r , he found t h a t i f t h e c a t h o d i c l i m i t was s e t a t 0.4 v o l t s , n o t a l l o f t h e a n o d i c a l l y d e p o s i t e d o x i d e was removed on t h e c a t h o d i c Otten  [342,,343] and Rand [142] found t h a t , w i t h c o n t i n u e d  between 0 and 1.4 v o l t s and above, t h e oxygen charge the anodic  potential  Kim [302]  e l e c t r o d e c y c l e d f i v e times  identified Ir0  2  between 0 and 1.6v.  cycling  determined under  o r c a t h o d i c oxygen peaks i n c r e a s e d p r o g r e s s i v e l y t o v a l u e s  beyond monolayer v a l u e s . foil  cycle.  well  (by ESCA) on an i r i d i u m  63  3.2.2  Nature o f the S u r f a c e Oxygen Coverage  3.2.2.1  Adsorbed Oxygen The  monolayer or sub-monolayer coverages r e p o r t e d i n e a r l y  s t u d i e s were a t t r i b u t e d to the f o r m a t i o n o f adsorbed oxygen. concluded  Breiter  [330]  t h a t a s t a b l e adsorbed l a y e r i s i n d i c a t e d by the r e v e r s i b i l i t y  the f e a t u r e s on the c y c l i c voltammograms (the anodic  "peak" i s m i r r o r e d  by a c a t h o d i c "peak" a t v i r t u a l l y the same p o t e n t i a l ) , and  the  t h a t the c u r r e n t d u r i n g the p o t e n t i a l sweep drops immediately v a l u e s on the c a t h o d i c - g o i n g  cycle.  of  observation to c a t h o d i c  (In c o n t r a s t , p l a t i n u m shows a profound  h y s t e r e s i s i n both shape and p o t e n t i a l s o f the f e a t u r e s f o r o x y g e n - s p e c i e s f o r m a t i o n and  r e m o v a l , and  the c u r r e n t does not drop immediately  v a l u e s a f t e r r e v e r s a l o f the v o l t a m m e t r i c  sweep from the anodic  Conway [244] a t t r i b u t e d the r e v e r s i b i l i t y o f the o x i d a t i o n and  to c a t h o d i c limit.) reduction  f e a t u r e s on i r i d i u m to the i n a b i l i t y o f s u r f a c e oxygen t o undergo " p l a c e exchange" - hence the adsorbed l a y e r does not undergo f u r t h e r s t a b i l i z a t i o n and  i s reduced a t a s i m i l a r p o t e n t i a l to t h a t a t which i t was  B r i e t e r [330] d e r i v e d k i n e t i c e x p r e s s i o n s  which p r e d i c t e d the shape o f  oxygen-peaks i n the voltammograms by c o n s i d e r i n g two o f Langmuirian behaviour  the  p o t e n t i a l regions  f o r the r e a c t i o n s :  Ir + H 0 2  I r - OH  The  l a i d down.  o v e r l a p o f the p a r t i a l  -y I r - 0 H ads  I r - 0  + H  a d s  a  d  s  +  H  + e +  (23)  e  (24)  c u r r e n t d e n s i t y vs. p o t e n t i a l peaks p r e d i c t e d  f o r these r e a c t i o n s gave the observed shape o f the voltammogram.  Hoare  [334] found t h a t hydrogen gas b u b b l i n g r a p i d l y reduced the s u r f a c e o f an  64  i r i d i u m anode p r e v i o u s l y covered  w i t h oxygen, which he a t t r i b u t e d to  the  easy r e d u c t i o n o f adsorbed oxygen l a y e r s . In the l i g h t o f h i s f i n d i n g s t h a t the e q u i v a l e n t o f m u l t i l a y e r oxygen coverage may  develop at p o t e n t i a l s well-below  o f oxygen e v o l u t i o n , Kurnikov  t h a t f o r commencement  [337] suggested t h a t a t high sweep r a t e s o n l y  p a r t s o f the oxygen f i l m would be reduced, as oxygen deep w i t h i n o x i d e or w i t h i n the metal i s unable to d i f f u s e out to the s u r f a c e i n o r d e r r e a c t and  hence be d e t e c t e d d u r i n g c a t h o d i c sweeping.  Rand [142]  layers  to  stated  t h a t the o x i d e phase formed on i r i d i u m remains on the s u r f a c e d u r i n g p o t e n t i a l c y c l i n g and t h a t oxygen a d s o r p t i o n and d e s o r p t i o n o c c u r on an o x i d i z e d surface.  On the o t h e r hand, the o x i d e l a y e r was  as l i t t l e  i n h i b i t i o n o f the hydrogen a d s o r p t i o n r e a c t i o n (which must o c c u r  on the bare m e t a l ) was  observed.  hydrogen c h a r g i n g , where growth, whereas Q  0  was  (Indeed  considered  to be porous,  the s u r f a c e area c a l c u l a t e d from  measured, remained c o n s t a n t w i t h  increased without  l i m i t and was  oxide  not i n d i c a t i v e o f the  s u r f a c e a r e a . ) „'  3.2.2.2  Oxide Formation I t i s d i f f i c u l t to r e c o n c i l e the o b s e r v a t i o n s  by  Kurnikov  [333-337] t h a t l a r g e t h i c k n e s s e s o f o x i d e l a y e r s are formed w i t h : 1)  t h e r e v e r s i b i l i t y o f t h e a n o d i c and p o t e n t i a l sweeps o r c h a r g e c u r v e s ,  cathodic  2)  the a b i l i t y o f hydrogen a d s o r p t i o n t o reach the metal s u r f a c e even i n the p r e s e n c e o f t h i c k o x i d e fi1ms.  I f a phase o x i d e i s formed i t s r e d u c t i o n s h o u l d o n l y be thermodynamically  p o s s i b l e a t p o t e n t i a l s below i t s r e d u c t i o n p o t e n t i a l .  The  65  growth o f the a r e a under the oxygen peak i n the voltammograms produced w i t h r e p e a t e d c y c l i n g [142,342] t o p o t e n t i a l s o f 1.4v  (RHE) and above  c l e a r l y shows t h e f o r m a t i o n o f the e q u i v a l e n t o f m u l t i l a y e r f i l m s which a r e a p p a r e n t l y formed and reduced w i t h o u t h y s t e r e s i s , w i t h no i n d i c a t i o n whatsoever t h a t an o x i d e  i s i n v o l v e d , as i n such a case a c o n s t a n t  reduction  p o t e n t i a l would be e x p e c t e d , w i t h r e d u c t i o n o c c u r r i n g o n l y a t more c a t h o d i c values.  B r i e t e r [330] a l l u d e s t o t h i s anomaly, s u g g e s t i n g  energy of f o r m a t i o n o f t h i n phase o x i d e s may  t h a t the f r e e  vary w i t h t h i c k n e s s .  Unfor-.  t u n a t e l y as t h i c k e r oxygen m u l t i l a y e r s would be e x p e c t e d t o p r o g r e s s i v e l y approach the thermodynamic  p r o p e r t i e s o f b u l k o x i d e s , h y s t e r e s i s would  be  e x p e c t e d t o become more profound as the a n o d i c l i m i t on p o t e n t i a l sweeping i s extended. The second "anomaly"  mentioned above can be e x p l a i n e d i f the  " p i t " model o f o x i d a t i o n proposed by O t t e n [342,343] i s c o n s i d e r e d . i s , o x i d a t i o n o c c u r s by p e n e t r a t i o n o f oxygen l o c a l i z e d s i t e s on the s u r f a c e .  That  i n t o the metal o n l y a t c e r t a i n  Such a model would l e a v e the s u r f a c e f i l m e d  w i t h o n l y a t h i n adsorbed l a y e r which c o u l d be reduced r e v e r s i b l y d u r i n g anodic/cathodic  sweeping, and which c o u l d account f o r the a b i l i t y of hydrogen  a d s o r p t i o n t o i n d i c a t e a bare metal s u r f a c e .  With f a s t p o t e n t i a l sweeping,  the oxygen w i t h i n the " p i t s " would be unable t o d i f f u s e outward and hence would not be d e t e c t e d , and the QQ v a l u e s measured would o n l y r e l a t e t o the s u r f a c e coverage which would e f f e c t i v e l y be covered w i t h oxygen t o the e x t e n t of a monolayer o r l e s s .  O t t e n proposed the " p i t " model t o e x p l a i n  the apparent d i f f e r e n c e s between e l l i p s o m e t r i c and c o u l o m e t r i c  layer thick-  nesses found i n h i s s t u d i e s , where e l l i p s o m e t r y i n d i c a t e d a r a t h e r t h i c k l a y e r of varying o p t i c a l coverage w i t h oxygen.  p r o p e r t i e s when charge s t u d i e s r e v e a l e d o n l y small  The p e n e t r a t i o n o f o x i d a t i o n i n t o the; metal a t p i t s  66  was  thus suggested  to account f o r d i s r u p t i o n o f the metal  s u r f a c e and  sequent change i n the o p t i c a l p r o p e r t i e s o f the i r i d i u m s u r f a c e . t u l a t i o n o f deep p e n e t r a t i o n o f oxygen i n t o i r i d i u m i s supported f i e l d - i o n microscopy  The by  posboth  [340] and e l e c t r o n - p r o b e [338] s t u d i e s .  I r i d i u m i s known to have two s o l i d o x i d e phases [ 1 4 5 ] , and  lr 0 . 2  Van Muylder and P o u r b a i x [345,346] s t a t e t h a t l r 0  3  con-  2  d y n a m i c a l l y u n s t a b l e w i t h r e s p e c t t o I r and hydrated form o f l r 0 2  3  2  i s thermo-  I r 0 , and f u r t h e r t h a t the 2  i s readily soluble in acids.  t h a t i r i d i u m o n l y possesses  3  Ir0  Thus they  conclude  one s t a b l e o x i d e phase i n aqueous s o l u t i o n ,  namely I r 0 . 2  Hoare [334] has determined  the s t a n d a r d r e d u c t i o n p o t e n t i a l s  of iridium/oxygen species:  Ir-0 IrO  Potential  + 2H  a d s  h y d  + 4H  +  +  + 2e - I r + H 0  E° = 0.87v  (25)  + 4e " I r + 2H 0  E° = 0.935v  (26)  2  2  (25) was measured by p r e - a n o d i z i n g an i r i d i u m w i r e f o l l o w e d by  determination of i t s r e s t p o t e n t i a l . o f a pre-heated  (26) was measured by immersion  ( i n a i r ) w i r e i n t o a s i m i l a r e l e c t r o l y t e , f o l l o w e d by  potential determination. found  Potential  Potential  (26) was  rest  not c o m p l e t e l y s t a b l e , and  was  to tend t o d r i f t to the v a l u e f o r (25) w i t h time. Kurnikov  [337] suggests  t h a t the f o r m a t i o n o f an o x i d e  film  l i k e l y o c c u r s v i a the f o l l o w i n g sequence:  Ir + H 0 2  -> IrOH + H  IrOH + IrO + H  +  + e  +  + e  a t E - 0.6v  (27)  a t E = 0.95v  (28)  67  IrO + H 0  IrOOH + H  2  IrOOH  Reactions  Ir0  2  (28)-(30) c o r r e s p o n d  corresponding  + H  +  +  + e  + e  at E = l . l v  (29)  a t E = 1.4v  (30)  to the s t o i c h i o m e t r i c s a t the  t o s t r u c t u r e observed  potentials  i n the anodic o r c a t h o d i c voltammograms,  w i t h the s e q u e n t i a l o c c u r r e n c e o f (27-)-(30) i n d i c a t e d by the p o t e n t i a l s o f these peaks o r waves.  These p o t e n t i a l s were found  to v a r y w i t h sweep r a t e ,  but appeared to l e v e l o f f a t low ( v i r t u a l l y e q u i l i b r i u m ) sweep r a t e s . The  p o t e n t i a l s f o r the peaks a t a sweep r a t e o f 0.05  given f o r r e a c t i o n s (27)-(30).  v/sec  (slow) are  those  The c o i n c i d e n c e o f t h e s e p o t e n t i a l s measured  w i t h e i t h e r a n o d i c o r c a t h o d i c sweeping may  i n d i c a t e t h a t the r e p o r t e d  v a l u e s are c l o s e t o the e q u i l i b r i u m p o t e n t i a l s f o r the v a r i o u s r e a c t i o n s .  3.3  Summary and R e l a t i o n to t h i s Work D e s p i t e the e x t e n s i v e l i t e r a t u r e on the e l e c t r o c h e m i s t r y o f  oxygen f i l m s on p l a t i n u m and  i r i d i u m , no work has been p u b l i s h e d  concerning  P t / I r a l l o y s , save the v e r y l i m i t e d work o f Chodos [338] w i t h r e s p e c t t o the d e t e r m i n a t i o n o f r e s i d u a l oxygen c o n t e n t i n the metal cathodic treatment.  I t i s o f i n t e r e s t t o determine  after  anodic/  the nature of the  oxygen coverage on such a l l o y s to see whether i t conforms t o t h a t o f p l a t i n u m or t h a t o f i r i d i u m , which i n t u r n determines  the a p p l i c a b i l i t y o f e l e c t r o -  chemical  i n v o l v i n g the s t r i p p i n g and/or  s u r f a c e area measurement t e c h n i q u e s  removal o f oxygen monolayers.  F u r t h e r , the e l e c t r o c h e m i c a l d i s s o l u t i o n  the p a s s i v a t i o n ( d e f i n e d as the change i n o v e r p o t e n t i a l w i t h time)  processes  are d i r e c t l y r e l a t e d to the degree o f s u r f a c e oxygen coverage under the anodic p o l a r i z a t i o n c o n d i t i o n s employed i n the p r e s e n t work.  and  68  On c o n s i d e r a t i o n o f t h e e x i s t i n g l i t e r a t u r e , p l a t i n u m and i r i d i u m a r e found t o d i f f e r w i t h r e s p e c t t o the nature o f t h e i r e l e c t r o chemical l y - formed oxygen f i l m s : 1.  T h e r e i s a c l e a r l y - d e f i n e d " d o u b l e 1 a y e r " . r e g i on., s e p a r a t i n g t h e hydrogen and oxygen r e g i o n s i n t h e c h a r g e c u r v e s o r vo1tammograms on p l a t i n u m . On i r i d i u m , t h e s e r e g i o n s t e n d t o o v e r l a p .  2.  P l a t i n u m a t t a i n s monolayer oxygen coverage o n l y on i m m i n e n t o x y g e n e v o l u t i o n . W h i l e t h i s has a l s o been o b s e r v e d w i t h i r i d i u m , i t a p p e a r s t o be a f o r t u i t o u s r e s u l t o f t h e n a t u r e o f t h e measurement p r o c e d u r e . Under- s l o w e r c h a r g i n g p r o c e d u r e s , i r i d i u m may f o r m t h e e q u i v a l e n t o f m u l t i l a y e r oxygen coverage.  3.  D u r i n g o x y g e n e v o l u t i o n t h e o x y g e n f i l m on p l a t i n u m does n o t grow beyond a v a l u e e q u i v a l e n t t o a b o u t two m o n o l a y e r s , e x c e p t u n d e r p a r t i c u l a r c o n d i t i o n s o f " s e v e r e " a n o d i z a t i o n where m u l t i l a y e r o x i d e f i l m s c a n d e v e l o p when t h e p o t e n t i a l f a l l s w i t h i n the r a t h e r narrow range f o r i t s growth. The g r o w t h o f o x y g e n f i l m s on i r i d i u m a n o d e s u n d e r e q u i v a l e n t c o n d i t i o n s i s much l e s s c h a r a c t e r i z e d , although the growth o f m u l t i l a y e r f i l m s and even t h e f o r m a t i o n o f phase o x i d e i s i nd i c a t e d .  k.  The a n o d i c - g o i n g and c a t h o d i c - g o i n g c h a r g e c u r v e s o r voltammograms f o r p l a t i n u m a r e c h a r a c t e r i z e d by a m a r k e d h y s t e r e s i s i n t h e s h a p e s o f t h e c o r r e s p o n d i n g o x i d a t i o n and r e d u c t i o n f e a t u r e s . T h i s h y s t e r e s i s i s much l e s s a p p a r e n t f o r i r i d i u m . On t h e o t h e r h a n d , n o t a l l o f t h e t h e o x y g e n c o v e r a g e i s removed o n i r i d i u m w i t h a n o d i c / c a t h o d i c c y c l i n g t o a c a t h o d i c l i m i t w h e r e t h e o x y g e n c o v e r a g e on p l a t i n u m i s c e r t a i n l y removed. The r e v e r s i b i l i t y o f t h e shapes o f t h e c h a r g e c u r v e s o r voltammograms on i r i d i u m i s l i k e l y due t o t h e l a c k o f " p l a c e e x c h a n g e " b e t w e e n some o f t h e s u r f a c e o x y g e n a n d t h e i r a s s o c i a t e d i n d i v i d u a l i r i d i u m atoms.  5.  P l a t i n u m i s c a p a b l e o f a b s o r b i n g c o n s i d e r a b l e oxygen i n t o i t s i n t e r i o r , a s h a s been d e m o n s t r a t e d by many experiments with thin f o i l s . No s i m i l a r w o r k h a s been done w i t h i r i d i u m , a l t h o u g h t h e r e i s e v i d e n c e f o r a " p i t " model o f o x i d a t i o n o f t h i s m e t a l , w h i c h i n t u r n i m p l i e s t h e p e n e t r a t i o n o f oxygen deep wi t h i n t h i s m e t a 1 .  69  S p e c u l a t i o n as t o the nature o f the s u r f a c e oxygen coverage on P t / I r a l l o y s r e q u i r e s f u r t h e r s p e c u l a t i o n as to the n a t u r e o f the surface.  I f the a l l o y s are homogeneous and  alloy  p r e s e n t s u r f a c e s which e x h i b i t  s i m i l a r homogeneous d i s t r i b u t i o n s o f the c o n s t i t u e n t atoms, then the format i o n o f an adsorbed oxygen l a y e r w i l l  necessarily involve individual  s i t e s which d i f f e r c o n s i d e r a b l y i n t h e i r a d s o r p t i o n b e h a v i o u r .  The  o f h y s t e r e s i s between the shapes o f the anodic and c a t h o d i c charge would suggest t h a t " p l a c e exchange" between the oxygen atom and l y i n g noble metal atom c o u l d o c c u r . w i t h prolonged  surface finding curves  the under-  Whether the oxygen coverage i n c r e a s e s  a n o d i z a t i o n to the e q u i v a l e n t of o n l y a few monolayers or  to m u l t i l a y e r c o v e r a g e , cannot be p r e d i c t e d from the p r e s e n t  literature.  Chapter 4  EXPERIMENTAL  4.1 4.1.1  M a t e r i a l s and Apparatus Anode M a t e r i a l s and C o n s t r u c t i o n P t / I r - T i anodes having a nominal c o a t i n g c o m p o s i t i o n  o f 30  per c e n t i r i d i u m by w e i g h t , and l o a d i n g s v a r y i n g from 2.3 t o 20 g/m  2  obtained  i n t h e form o f sheets from I m p e r i a l Metal  The c o a t i n g s were a p p l i e d by t h e thermal Experimental 1.86-1.92 cm  2  i n diameter.  were  Industries Limited.  decomposition  method [174,347].  anodes were g e n e r a l l y c u t i n t h e form o f d i s c s No e f f e c t s o f t h e d i s r u p t i o n o f t h e c o a t i n g a t  the d i s c edge due t o c u t t i n g were ever observed.  (Indeed,  the substrate  was exposed randomly over t h e s u r f a c e s o f t h e d i s c s , as S.E.M. o b s e r v a t i o n revealed.)  I t was d e s i r e d t o p r e s e n t a s i n g l e d i s c f a c e t o t h e e l e c t r o l y t i c  solutions, with e l e c t r i c a l  connections  was i s o l a t e d from t h e s o l u t i o n . disassembly  being made t o t h e back s i d e which  T h i s would permit repeated  assembly and  t o be made w i t h o u t damage t o t h e o p e r a t i n g s u r f a c e caused by  making and b r e a k i n g o f t h e e l e c t r i c a l  contact.  Besides  assuring better  c u r r e n t d i s t r i b u t i o n when a s i n g l e f l a t a u x i l i a r y e l e c t r o d e was used, t h e electrical  i s o l a t i o n o f one f a c e o f t h e d i s c e l i m i n a t e d any e f f e c t s t h e  l e a d m a t e r i a l may have had on t h e behaviour  o f t h e working e l e c t r o d e .  E l e c t r i c a l c o n t a c t was made by s p o t - w e l d i n g  two s h o r t p l a t i n u m w i r e s t o t h e  70  71  disc.  Copper w i r e s , s o l d e r e d t o the p l a t i n u m w i r e s , served as t h e l e a d s  to t h e e x t e r n a l e l e c t r i c a l F i g u r e 4.1.  circuit.  E l e c t r o d e c o n s t r u c t i o n i s shown i n  The d i s c s were mounted i n T e f l o n h o l d e r s , i n t o which a Pyrex  tube was i n s e r t e d f o r t h e l e a d s f o l l o w e d by c a s t i n g o f t h e i n s i d e w i t h epoxy t o p r e v e n t  breaking o f the e l e c t r i c a l  c o n t a c t s a t t h e d i s c under  mechanical s t r e s s and t o f u r t h e r ensure t h e i s o l a t i o n o f t h e back f a c e o f the d i s c and t h e l e a d s .  In runs a t 60 and 80°, l a r g e r e x p e r i m e n t a l  (3 x 3.7,cm) were used where mounting i n T e f l o n / P y r e x i m p r a c t i c a l due t o a d v e r s e e x p a n s i o n e f f e c t s .  assemblies  anodes  was  I n such c a s e s a d u a l - a u x i l i a r y  e l e c t r o d e arrangement was used w i t h t h e working e l e c t r o d e h e l d i n p o s i t i o n between them, w i t h both f a c e s o p e r a t i n g e l e c t r o l y t i c a l l y . was  made by s p o t - w e l d i n g  Electrical  a t i t a n i u m w i r e t o a " t a b " which was l e f t  contact  during  c u t t i n g o f t h e anode. As t h e range o f c u r r e n t d e n s i t i e s encountered i n i n d u s t r i a l e l e c t r o l y t i c processes  may extend t o s e v e r a l hundred mA/cm , and as t h e power 2  ranges o f many o f t h e power s u p p l i e s used i n e l e c t r o c h e m i c a l r e s e a r c h a r e l i m i t e d , i t i s d e s i r a b l e t o have working e l e c t r o d e s as small as p o s s i b l e i n area to permit operation a t higher current d e n s i t i e s . the d i s c e l e c t r o d e s were used whenever c o n d i t i o n s  from Johnson Matthey M e t a l s  reason,  permitted.  Wires o f P t , P t / 5 I r , P t / l O I r , P t / 2 0 I r , P t / 2 5 I r obtained  For t h i s  composition,  L t d . , were a l s o employed as e l e c t r o d e s .  C o n v e n t i o n a l l y w i r e e l e c t r o d e s a r e made by i n s e r t i n g t h e w i r e through a g l a s s tube f o l l o w e d by f u s i o n o f t h e g l a s s t o the w i r e .  Such heat t r e a t -  ment e f f e c t s , however, can cause i r r e p r o d u c i b l e a l t e r a t i o n o f t h e e l e c t r o d e a c t i v i t y - p a r t i c u l a r l y when an a l l o y i s i n v o l v e d - hence an a l t e r n a t i v e procedure was employed f o r e l e c t r o d e c o n s t r u c t i o n ( F i g u r e 4.1).  Lengths  72  Figure 4 . 1 .  Working e l e c t r o d e c o n s t r u c t i o n (a) f o r d i s c specimens c u t from s h e e t s ; (b) f o r w i r e e l e c t r o d e s .  73  of t h e r e s p e c t i v e noble metal w i r e s were s o l d e r e d the e x t e r n a l e l e c t r i c a l  contact.  f o r c e d through a small hole  t o copper w i r e s  to p r o v i d e  A p o r t i o n o f the noble metal w i r e was then  ( s m a l l e r than the w i r e d i a m e t e r s - t y p i c a l l y  .020 i n c h ) i n a T e f l o n cap which was i n t u r n i n s e r t e d i n t o , a Pyrex tube t o p r e v e n t t h e s o l u t i o n from r e a c h i n g  the wires  i n s i d e t h e tube.  C o m m e r c i a l l y pure t i t a n i u m s h e e t s and w i r e s were a l s o f a b r i c a t e d i n t o e l e c t r o d e s , w i t h the sheet specimens being in Teflon/Pyrex  a s s e m b l i e s as d e s c r i b e d  the w i r e e l e c t r o d e s metal w i r e  4.1.2  being  constructed  c u t i n t o d i s c s and mounted  f o r the coated e l e c t r o d e s , and  i n a s i m i l a r manner t o the n o b l e  electrodes.  E l e c t r o l y t e s and Gases All  s o l u t i o n s were prepared from r e a g e n t grade c h e m i c a l s and  d o u b l e - d i s t i l l e d water.  A base e l e c t r o l y t e o f 2M  f o r most o f the work as t h i s c o n c e n t r a t i o n  H2SO4  (196  g p l ) was chosen  approximates t h e h i g h l y a c i d  c o n d i t i o n s t h a t a r e encountered i n e l e c t r o w i n n i n g  o f s o l u t i o n s t h a t have  been produced by s t r i p p i n g o f v a r i o u s commercial s o l v e n t e x t r a c t i o n S o l u t i o n s o f 4M (392 g p l ) and 8M (784 g p l ) H S 0 2  All  4  reagents.  were a l s o used i n a few.  s u r f a c e a r e a and charge s t u d i e s were performed i n t h e base e l e c t r o l y t e . C o p p e r - c o n t a i n i n g s o l u t i o n s were prepared w i t h t h e n e c e s s a r y  amount o f C u S 0 i * 5 H 0 r e q u i r e d to g i v e a s o l u t i o n 0.5M (32 g p l ) i n copper. t  2  The r e a g e n t grade C u S 0 4 * 5 H 0 was s u b s e q u e n t l y found t o have 50 ppm o f l e a d 2  as i m p u r i t y on s p e c t r o g r a p h i c  analysis.  L e a d - f r e e s o l u t i o n s were prepared  by t h e d i s s o l u t i o n o f CuO ( i n s o l u t i o n s c o n t a i n i n g an excess o f a c i d such t h a t the f i n a l  acid concentration  was 2M).  Copper and a c i d l e v e l s were  m a i n t a i n e d t h r o u g h o u t batch e x p e r i m e n t s w i t h d a i l y a d d i t i o n s o f CuO.  74  Reagent grade t h i o u r e a was used i n experiments where t h e use o f t h i s common e l e c t r o p l a t i n g a d d i t i v e was i n v e s t i g a t e d .  In one e x p e r i m e n t ,  d e l i b e r a t e c o n t a m i n a t i o n w i t h kerosene was employed t o a s s e s s t h e e f f e c t s o f t h i s i m p u r i t y , which i s p r e s e n t i n SX/EW systems. The p u r i t y o f t h e base e l e c t r o l y t e s o l u t i o n to be adequate  (2M H S0 ) was found 2  1+  f o r s u r f a c e a r e a , s u r f a c e charge and p o l a r i z a t i o n  s t u d i e s where c u r r e n t s were w e l l  curve  i n excess o f t h e l i m i t i n g c u r r e n t s p o s s i b l e  from i m p u r i t y o x i d a t i o n o r r e d u c t i o n .  A t s m a l l e r c u r r e n t s however, such  as those used d u r i n g t r a c i n g o f oxygen o v e r p o t e n t i a l  c u r v e s t o v e r y low  p o t e n t i a l s , t r a c e i m p u r i t y o x i d a t i o n obscured t h e anode p r o c e s s u n l e s s a d d i t i o n a l p u r i f i c a t i o n was c a r r i e d out.  In such cases t h e procedure o f  B o c k r i s [348] was adopted, where a 24 h r . c a t h o d i c p r e - e l e c t r o l y s i s f o l l o w e d by a 48 h r . a n o d i c p r e - e l e c t r o l y s i s a t 10 mA/cm was employed t o lower 2  solution impurity  levels.  Gases used f o r "sweeping" ( f o r hydrogen  o f e l e c t r o l y t e s i n c l u d e d tank  hydrogen  r e f e r e n c e e l e c t r o d e s and n o n - e l e c t r o l y t i c r e d u c t i o n s t u d i e s ) ,  h e l i u m ( f o r s u r f a c e a r e a and s u r f a c e charge s t u d i e s ) , and oxygen ( f o r p o l a r i z a t i o n curve s t u d i e s ) .  Both hydrogen  and h e l i u m were passed  through  heated copper o x i d e c a t a l y s t columns i n o r d e r t o lower t h e l e v e l s o f r e d u c i b l e contaminants  (such as oxygen).  Oxygen gas was passed through a  s i m i l a r c a t a l y s t i n o r d e r t o lower t h e l e v e l s o f o x i d i z a b l e All  contaminants.  gases were s u b s e q u e n t l y bubbled through water p r i o r t o i n t r o d u c t i o n t o  the c e l l s t o m i n i m i z e e l e c t r o l y t e l o s s e s through v a p o r i z a t i o n .  A l l gas  t r a i n s , from t h e c a t a l y s t columns t o t h e c e l l s , were c o n s t r u c t e d o f e i t h e r Pyrex o r T e f l o n i n o r d e r t o p r e v e n t p o s s i b l e c o n t a m i n a t i o n o f e i t h e r t h e gas o r the e l e c t r o l y t e d u r i n g a c c i d e n t a l c o n t a c t w i t h t h e gas t r a i n .  No grease  o r l u b r i c a n t s o f any k i n d were used i n t h e gas t r a i n s o r o t h e r a p p a r a t u s .  75  4.1.3  Cells Two b a s i c c e l l d e s i g n s were employed i n t h e p r e s e n t work.  For  l o n g - t e r m runs i n s i m u l a t e d c o p p e r - e l e c t r o w i n n i n g e x p e r i m e n t s , s i n g l e compartment c e l l s were used.  A l l such c e l l s were c o n s t r u c t e d such t h a t o n l y  P y r e x , T e f l o n and the e l e c t r o d e s themselves were i n d i r e c t c o n t a c t w i t h the electrolyte. closed c e l l sphere.  E s c a p i n g g a s ( e s ) were vented through w a t e r " b u b b l e r s " on tops a l t h o u g h no r i g o r o u s e f f o r t was made t o e x c l u d e t h e atmo-  The e l e c t r o d e s , i n c l u d i n g the Luggin c a p i l l a r y , were supported  from the T e f l o n c e l l  top whose c o n s t r u c t i o n p e r m i t t e d independent  o f t h e p o s i t i o n s o f the e l e c t r o d e s .  variation  During o p e r a t i o n t h e Luggin c a p i l l a r y  was f i x e d a t about 1 mm d i r e c t l y i n f r o n t o f the c e n t r e o f the w o r k i n g e l e c t r o d e , whose p l a n a r f a c e was p a r a l l e l  to that of the a u x i l i a r y  which was u s u a l l y p o s i t i o n e d , f o r c o n v e n i e n c e , about 3 cm away. c o n t a i n i n g e l e c t r o l y t e s the o b v i o u s c h o i c e f o r an a u x i l i a r y material  i s copper.  electrode  In copper-  electrode  In s u l f u r i c a c i d s o l u t i o n s w i t h o u t copper, both p l a t i n u m  and copper a u x i l i a r y e l e c t r o d e s were employed i n v a r i o u s c e l l s . A l l s t i r r i n g was performed w i t h magnetic s t i r r i n g b a r s .  Cell  volumes v a r i e d  from 1.8 t o 4 I . P r i o r to each run the c e l l s were d i s a s s e m b l e d , c l e a n e d , r i n s e d i n double- d i s t i l l e d water and re-assembled.  The c l e a n i n g procedure g e n e r a l l y  i n v o l v e d s o a k i n g i n c h r o m i c / s u l f u r i c a c i d , f o l l o w e d by r i n s i n g i n d i l u t e n i t r i c and s u l f u r i c a c i d s . in d i l u t e n i t r i c acid previous runs).  The a u x i l i a r y e l e c t r o d e s were c l e a n e d by r i n s i n g  (which d i s s o l v e d much o f t h e copper d e p o s i t e d i n  Such procedures were adopted t o a v o i d u n c e r t a i n t i e s w i t h  r e s p e c t t o the p o i s o n i n g o f t h e w o r k i n g e l e c t r o d e s by unknown  contaminants  76  and  to remove any  r e a c t i o n products  been p r e s e n t on the c e l l  or spent e l e c t r o l y t e which may  components from the p r e v i o u s  run.  For s h o r t term r u n s , such as s u r f a c e area and experiments,  have  s u r f a c e charge  the s e p a r a t i o n o f the working e l e c t r o d e and a u x i l i a r y e l e c t r o d e  r e a c t i o n products  becomes p r a c t i c a b l e .  as hydrogen or oxygen gas  Indeed, such s e p a r a t i o n i s  necessary,  produced on the a u x i l i a r y e l e c t r o d e c o u l d  s e r i o u s l y a f f e c t the charge measurements by t r a n s i e n t g a l v a n o s t a t i c p u l s e techniques.  (The d e p o l a r i z i n g e f f e c t s o f these gases on the oxygen or  hydrogen e v o l u t i o n r e a c t i o n s i s o n l y s i g n i f i c a n t a t v e r y low c u r r e n t d e n s i t i e s as the r e s p e c t i v e o x i d a t i o n and is diffusion-limited.) such measurements.  r e d u c t i o n o f t r a c e hydrogen o r oxygen gases  Hence, a two-compartment c e l l  A g a i n , c o n s t r u c t i o n was  or the e l e c t r o d e m a t e r i a l s c o n t a c t e d used was  was  such t h a t o n l y T e f l o n , Pyrex  the e l e c t r o l y t e .  The  a Pt/Rh gauze o f l a r g e g e o m e t r i c s u r f a c e a r e a .  Teflon c e l l  a u x i l i a r y electrode  The  d e s i g n o f the  top f o r the working e l e c t r o d e compartment p e r m i t t e d  o f both types o f e l e c t r o d e s d e s c r i b e d which c o u l d be p o s i t i o n e d  i n S e c t i o n 4.1.1.  independently,  was  The  the  use  Luggin  f i x e d a t about 1 mm  c e n t r e o f the l e n g t h o f w i r e working e l e c t r o d e s . the Luggin  employed f o r  capillary,  from the  For p l a n a r e l e c t r o d e s  c a p i l l a r y s h a f t c o u l d be p o s i t i o n e d to the s i d e o f the w o r k i n g  e l e c t r o d e such t h a t the t i p touched the e l e c t r o d e s u r f a c e , thus f i x i n g  the  d i s t a n c e between the c a p i l l a r y opening and the e l e c t r o d e s u r f a c e a t about 0.5  mm.  The The  volume o f each compartment was  approximately  1 I.  two-compartment c e l l s a l s o had porous Pyrex gas d i s p e r s i o n  tubes to enable sweeping o f the e l e c t r o l y t e w i t h v a r i o u s tank gases, as r e q u i r e d i n the charge and  s u r f a c e area s t u d i e s .  Temperature-control with conventional  f o r the s i n g l e compartment c e l l s was  w a t e r - b a t h s i n which the c e l l s were immersed.  achieved  For  the  77  two-compartment c e l l  used i n the s h o r t e r - t e r m r u n s , s t i r r e r - h e a t e r s pro-  v i d e d s a t i s f a c t o r y temperature c o n t r o l .  4.1.4  Reference  Electrodes  Three k i n d s o f r e f e r e n c e e l e c t r o d e s were employed: 1. 2. 3.  R e v e r s i b l e hydrogen e l e c t r o d e s Copper/copper s u l f a t e e l e c t r o d e s (Commercial) mercury/mercurous s u l f a t e e l e c t r o d e s  These e l e c t r o d e s , p a r t i c u l a r l y t h e f i r s t two, were chosen f o r t h e i r c o m p a t a b i l i t y with the experimental  electrolyte solutions.  hydrogen e l e c t r o d e (RHE) i s p a r t i c u l a r l y c o n v e n i e n t  The r e v e r s i b l e  in sulfuric acid solu-  t i o n s f o r s u r f a c e charge o r s u r f a c e area measurement s t u d i e s where e l e c t r o d e p o t e n t i a l s vary t y p i c a l l y between 0 and 2 v o l t s (RHE).  F u r t h e r , when con-  s i d e r i n g r e a c t i o n s which vary w i t h pH i n the same manner as t h e hydrogen e l e c t r o d e , the use o f a RHE p e r m i t s d i r e c t comparison among r e s u l t s f o r s i m i l a r e x p e r i m e n t s i n s o l u t i o n s o f d i f f e r e n t pH. The  RHE was c o n s t r u c t e d from a d i s c o f Pt/30 I r - T i m a t e r i a l ,  with e l e c t r i c a l to  connection  the e l e c t r o l y t e .  made by a l e n g t h o f Pt/25 I r w i r e , a l s o exposed  The s u l f u r i c a c i d s o l u t i o n e l e c t r o l y t e was s a t u r a t e d  w i t h hydrogen g a s , r e s u l t i n g i n t h e a t t a i n m e n t about t h r e e m i n u t e s .  of a stable potential within  C o n n e c t i o n o f the RHE c e l l  with a Teflon top) with the experimental  ( t y p i c a l l y a 1000 ml beaker  c e l l was made by means o f a T e f l o n  tube " b r i d g e " f i l l e d w i t h the same e l e c t r o l y t e .  To m i n i m i z e e l e c t r o l y t e  f l o w , t h e ends o f t h e T e f l o n tube were i n s e r t e d i n t o 6 mm O.D. P y r e x  tubes,  drawn t o a c a p i l l a r y a t one end, which were suspended i n the r e s p e c t i v e cells.  The measured p o t e n t i a l d i f f e r e n c e s between the Pt/30 I r - T i d i s c  78  and a p l a t i n i z e d p l a t i n u m s h e e t o r a P t / I r a l l o y w i r e were o n l y o f the o r d e r of a f r a c t i o n of a r n u l t i v o l t . For copper c o n t a i n i n g e l e c t r o l y t e s , a v e r y s i m p l e , and  convenient,  p r a c t i c a l e l e c t r o d e was c o n s t r u c t e d - a copper w i r e was dipped  the e l e c t r o l y t e s o l u t i o n .  into  Copper/copper s u l f a t e e l e c t r o d e s have been  d e s c r i b e d i n the l i t e r a t u r e , b u t r e f e r o n l y t o s a t u r a t e d s o l u t i o n s o f weak a c i d s t r e n g t h [349,350]. The e l e c t r o d e s were c o n s t r u c t e d from ( a p p r o x i m a t e l y )  30 cm  lengths  o f 0.020 i n c h d i a m e t e r copper w i r e , which were c o i l e d i n o r d e r t o f i t w i t h i n the 6 mm 0.0. Pyrex Luggin acid solution.  C a p i l l a r y tube, and p r e - c l e a n e d  in dilute  nitric  A r e s e r v o i r o f e l e c t r o l y t e , 2M h^SO^ + 0.5M C u S 0 , was 4  employed t o m a i n t a i n a s l i g h t f l o w o f f r e s h e l e c t r o l y t e p a s t t h e w i r e . No r i g o r o u s attempt was made t o e x c l u d e  the atmosphere from t h e e l e c t r o l y t e ,  w i t h the r e s u l t f a i l u r e c o u l d e v e n t u a l l y occur due to w a t e r l i n e c o r r o s i o n o f the copper w i r e . were a s s e s s e d  The e f f e c t s o f v a r i a t i o n s o f the e l e c t r o l y t e  composition  f o r t h e copper/copper s u l f a t e e l e c t r o d e , and a r e g i v e n i n  Appendix A l . All course  r e f e r e n c e e l e c t r o d e s were c o n t i n u o u s l y m o n i t o r e d d u r i n g the  o f the e x p e r i m e n t a l  work t o ensure t h a t t h e i r p o t e n t i a l s d i d not  v a r y and t o d e t e r m i n e the e f f e c t s o f any a c c i d e n t i a l  4.1.5  Electrochemical Several  work.  contamination.  Instrumentation  power s u p p l i e s were u t i l i z e d d u r i n g the course  For long-term  g a l v a n o s t a t i c experiments,  r e g u l a t e d DC power s u p p l i e s  by Anatek (models 50-1S and 50-1D, maximum output: f o r use w i t h the d i s c e l e c t r o d e s .  of t h i s  l A / 5 0 v ) were s u f f i c i e n t  For l a r g e r - a r e a e l e c t r o d e s , which r e q u i r e d  79  l a r g e r c u r r e n t s to p r o v i d e e q u i v a l e n t c u r r e n t d e n s i t i e s , a H e w l e t t model 6256B power s u p p l y  (maximum o u t p u t :  24 A/12v) was employed.  Packard Runs  r e q u i r i n g c u r r e n t s s m a l l e r than 30 mA were performed w i t h a Beckman E l e c t r o s c a n 30 as the o t h e r power s u p p l i e s c o u l d not be c a l i b r a t e d a t low current outputs.  Short-term  g a l v a n o s t a t i c e x p e r i m e n t s were performed w i t h  a Wenking P o t e n t i o s t a t Model 68 FRO.5 o p e r a t e d  i n the g a l v a n o s t a t i c mode  by p l a c i n g a r e s i s t o r i n the w o r k i n g e l e c t r o d e c i r c u i t and c o n t r o l l i n g the p o t e n t i a l a c r o s s the r e s i s t o r ( r a t h e r than between the r e f e r e n c e and working e l e c t r o d e s as i s performed i n p o t e n t i o s t a t i c o p e r a t i o n ) .  A l l surface  charge and s u r f a c e area s t u d i e s were performed w i t h the p o t e n t i o s t a t o p e r a t i n the g a l v a n o s t a t i c mode.  P o t e n t i o s t a t i c s t u d i e s were performed w i t h  the same p o t e n t i o s t a t , operated  n o r m a l l y , w i t h a f o u r - l e a d system.  P o t e n t i a l measurements d u r i n g long-term  g a l v a n o s t a t i c experiments  were made w i t h a K e i t h l e y 630 P o t e n t i o m e t r i c E l e c t r o m e t e r , which had an i n p u t impedance o f 1 0  1If  ohms.  In s h o r t - t e r m  area measurements, p o t e n t i a l s were r e c o r d e d  s u r f a c e charge and s u r f a c e w i t h a Honeywell E l e c t r o n i k  194 r e c o r d e r , which was p a r t i c u l a r l y u s e f u l due t o i t s expanded-range capabilities.  S h i e l d e d , non-current  c a r r y i n g " r e f e r e n c e " and "sense">leads  were used t o m i n i m i z e IR drops i n the l e a d s . C u r r e n t s were measured d u r i n g p o t e n t i o s t a t i c experiments both on the ammeter p r o v i d e d on the Wenking 68 FRO.5 and w i t h a K e i t h l e y 153 M i c r o v o l t Ammeter, which c o u l d be used f o r c u r r e n t s s m a l l e r than 1A and a l s o p e r m i t t e d measurement o f c u r r e n t s lower than a yA.  High c u r r e n t s were  measured w i t h t h e ammeter p r o v i d e d on t h e Wenking 68 FRO.5. C a l i b r a t i o n o f the g a l v a n o s t a t i c power s u p p l i e s was performed p r i o r t o each r u n , w i t h the o u t p u t c u r r e n t measured by the p o t e n t i a l drop  80  across a p r e c i s i o n r e s i s t o r .  The p o t e n t i o s t a t was found t o v a r y i n o u t p u t  from run t o r u n , r e q u i r i n g c a l i b r a t i o n a g a i n s t a p o t e n t i o m e t e r p r i o r t o each e x p e r i m e n t .  4.1.6  Other Apparatus W a v e l e n g t h - d i s p e r s i v e X-ray s p e c t r o m e t r y was  performed w i t h a  N o r e l c o s p e c t r o m e t e r which was o p e r a t e d a t 35 kv w i t h a t u n g s t e n tube. X-ray d i f f r a c t o m e t r y was performed w i t h a c h r o m a t i c CuKa r a d i a t i o n .  P h i l i p s d i f f r a c t o m e t e r and mono-  Morphology s t u d i e s i n v o l v e d t h e use o f an E t e c  Autoscan s c a n n i n g e l e c t r o n microscope (SEM) which was a l s o equipped w i t h an e n e r g y - d i s p e r s i v e X-ray s p e c t r o m e t e r .  Chapter  5  PROCEDURE  5.1  Measurement o f Anode Loading and S u r f a c e Noble metal  l o a d i n g s were determined  s p e c t r o s c o p i c t e c h n i q u e , whereby the i n d i v i d u a l  Composition by an X-ray f l u o r e s c e n c e specimens were i r r a d i a t e d  by c o n t i n u o u s t u n g s t e n X - r a d i a t i o n and t h e i n t e n s i t i e s o f t h e r e s u l t a n t c h a r a c t e r i s t i c X-radiation (PtLqi  and I r L a i )  were measured.  The method  r e l i e s on the c o a t i n g being l e s s than " i n f i n i t e l y t h i c k " - p r a c t i c a l l y about 5-10 microns  t h i c k ( c o r r e s p o n d i n g t o a l o a d i n g o f 100-200 g/m ) 2  the r e s u l t a n t c h a r a c t e r i s t i c X-ray i n t e n s i t i e s a r e c o n s e q u e n t l y t o t h e a b s o l u t e amount o f noble metal  present.  as  related  As t h e r e i s a h i g h l e v e l  o f u n c e r t a i n t y w i t h r e s p e c t to the a c t u a l l o a d i n g s o f P t / I r - T i e l e c t r o d e s , and as the r e l a t i v e amounts o f i r i d i u m and p l a t i n u m may not be c o n s t a n t as a r e s u l t o f a n o d i c o p e r a t i o n , the use o f Pt/30 I r - T i  " s t a n d a r d s " from which  the l o a d i n g s and c o m p o s i t i o n o f unknown specimens c o u l d be determined considered unsuitable.  was  I n s t e a d , i t was d e c i d e d t o approach the problem  from a fundamental parameter c a l c u l a t i o n o f X-ray i n t e n s i t i e s , a p p l i c a b l e t o the e n t i r e range o f P t / I r a l l o y c o m p o s i t i o n s  and l o a d i n g s , and t o a s s e s s  the r e l i a b i l i t y o f t h i s method by comparison w i t h p l a t i n u m coated t i t a n i u m standards prepared  by s p u t t e r r i n g p l a t i n u m on t i t a n i u m , and whose l o a d i n g s 81  82  were d e t e r m i n e d by w e i g h t g a i n measurements. parameter c a l c u l a t i o n , measured i n t e n s i t y  5.2  gas.  S u r f a c e Area  s u r f a c e charge and  fresh  f o r several  the  p r o c e d u r e was  l e a d s t o the  fundamental  conversion  of A2.  s u r f a c e a r e a e x p e r i m e n t s were performed H S0\ e l e c t r o l y t e 2  minutes i n a c h r o m i c / s u l f u r i c a c i d water.  the  Studies  e l e c t r o d e s , a p r e l i m i n a r y c l e a n i n g was  with d o u b l e - d i s t i l l e d t i o n of  the  of  d a t a i s d e s c r i b e d i n Appendix  two-compartment c e l l s c o n t a i n i n g 2M For  details  s t a n d a r d s and  d a t a to a n a l y t i c a l  S u r f a c e Charge and All  in  preparation of  The  After  power s u p p l y the  done by  solution,  immersion i n the  swept w i t h  immersion  f o l l o w e d by  electrolyte  c h a r t r e c o r d e r , the  helium  and  rinsing connec-  following  adopted f o r s u r f a c e a r e a measurement:  1.  A c t i v a t i o n by r e p e t i t i v e a n o d i c / c a t h o d i c c u r r e n t p u l s i n g w i t h s u f f i c i e n t i n t e n s i t y to p r o v i d e about twenty c y c l e s between the p o t e n t i a l s o f hydrogen and o x y g e n e v o l u t i o n i n 100 s e c o n d s , and c e a s i n g on t h e c a t h o d i c c y c l e .  2.  H e l i u m s w e e p i n g and s t i r r i n g w e r e m a i n t a i n e d f o r an a d d i t i o n a l 60 s e c o n d s t o remove d i s s o l v e d g a s e s p r o d u c e d d u r i n g a c t i v a t i o n ; s u b s e q u e n t l y t h e sweepi n g and s t i r r i n g w e r e s t o p p e d a n d t h e s o l u t i o n a l l o w e d t o become q u i e s c e n t . The o p e n - c i r c u i t p o t e n t i a l o f the w o r k i n g e l e c t r o d e rose o n l y s l i g h t l y a b o v e Ov (RHE) in t h i s time.  3)  Anodic/Cathodic current charging at a predetermined c u r r e n t magnitude. The i n c r e a s e i n t h e w o r k i n g e l e c t r o d e p o t e n t i a l was m o n i t o r e d w i t h t h e c h a r t r e c o r d e r (or o s c i l l o s c o p e , i f h i g h e r c h a r g i n g r a t e s w e r e e m p l o y e d ) and t h e c u r r e n t was r e v e r s e d on a t t a i n ment o f t h e p o t e n t i a l o f i m m i n e n t e v o l u t i o n o f o x y g e n gas. The d e c r e a s e i n p o t e n t i a l was t h e n f o l l o w e d u n t i l imminent e v o l u t i o n o f hydrogen gas. The anodic/ c a t h o d i c c y c l i n g was r e p e a t e d s e v e r a l t i m e s , and t h e e 1 e c t r o c h e m i c a 1 l y a c t i v e s u r f a c e a r e a was c a l c u l a t e d from the average o f the t r a n s i t i o n times d e t e r m i n e d g r a p h i c a l l y f o r s u r f a c e oxygen removal d u r i n g the  83  cathodic cycle. (Except f o r the case of pure i r i d i u m e l e c t r o d e s , where the a n o d i c oxygend e p o s i t i o n c h a r g e was u s e d . ) D e t a i l s of the surface area c a l c u l a t i o n are given in Appendix  A5.  For s u r f a c e charge s t u d i e s w i t h new e l e c t r o d e s , two s e t s o f experiments can be d i s t i n g u i s h e d : a)  oxygen c o v e r a g e development o f oxygen e v o l u t i o n ,  prior  b)  oxygen oxygen  simultaneously with  coverage development evolution.  t o commencement  In case ( a ) , the a n o d i c / c a t h o d i c c h a r g i n g was performed i n a s i m i l a r manner as i n s t e p (3) above, e x c e p t t h a t the a n o d i c l i m i t f o r c u r r e n t r e v e r s a l was p r o g r e s s i v e l y v a r i e d between 0.2v evolution  (about 1.55v  and the p o t e n t i a l  f o r imminent  oxygen  f o r platinum) with r e p e t i t i v e anodic/cathodic c y c l i n g .  In case (b) the e l e c t r o d e was p o l a r i z e d a n o d i c a l l y a t a g i v e n c u r r e n t d e n s i t y f o r a predetermined t i m e , which v a r i e d from seconds t o days. During such p o l a r i z a t i o n , h e l i u m sweeping and s t i r r i n g was m a i n t a i n e d t o remove the a n o d i c a l l y g e n e r a t e d oxygen gas.  On t e r m i n a t i o n o f the a n o d i c  p o l a r i z a t i o n the e l e c t r o d e was kept a t o p e n - c i r c u i t f o r 100 seconds w i t h h e l i u m sweeping and s t i r r i n g m a i n t a i n e d t o remove f u r t h e r t r a c e s o f oxygen. The p o t e n t i a l g e n e r a l l y d r i f t e d t o a v a l u e between 1.3 and 1.5v time.  in this  A c a t h o d i c " s t r i p p i n g " charge was then imposed and the charge c u r v e  recorded.  The t r a n s i t i o n time f o r removal o f the oxygen l a y e r p r o v i d e d  the b a s i s f o r the c a l c u l a t i o n o f the s u r f a c e oxygen  charge.  Charge s t u d i e s performed on e l e c t r o d e s which were p r e v i o u s l y o p e r a t e d i n l o n g - t e r m g a l v a n o s t a t i c runs i n s i n g l e - c o m p a r t m e n t c e l l s w i t h s u l f u r i c a c i d o r s u l f u r i c a c i d p l u s copper s u l f a t e e l e c t r o l y t e s r e q u i r e d removal o f the e l e c t r o d e from the single-compartment c e l l on t e r m i n a t i o n  84  o f the run,  f o l l o w e d by r i n s i n g i n d o u b l e - d i s t i l l e d w a t e r , t r a n s f e r to  two-compartment c e l l where h e l i u m sweeping and and  electrical  connection.  case (b) d e s c r i b e d above.  The  the  s t i r r i n g were m a i n t a i n e d ,  procedure was  then i d e n t i c a l  to t h a t  A f t e r s t r i p p i n g o f the oxygen l a y e r  for  formed  d u r i n g the p r o l o n g e d a n o d i c p o l a r i z a t i o n , the s u r f a c e a r e a c o u l d then  be  a s s e s s e d by r e p e t i t i v e a n o d i c / c a t h o d i c c y c l i n g .  5.3  P o l a r i z a t i o n Curves  5.3.1  Noble Metal  Electrodes  Under a p p l i c a t i o n of a c o n s t a n t p o t e n t i a l  or a constant  current,  the r e s u l t a n t c u r r e n t o r p o t e n t i a l o f an o x y g e n - e v o l v i n g noble metal t r o d e i s not  observed to a t t a i n a steady v a l u e - even a f t e r hundreds of  hours of e l e c t r o l y s i s .  C l e a r l y i t i s not  possible  s t a t e " p o l a r i z a t i o n c u r v e f o r oxygen e v o l u t i o n . s t a t e " p o l a r i z a t i o n c u r v e s can  downward p o t e n t i a l  remains c o n s t a n t , and  initial  preliminary  mA/cm  2  For p o t e n t i o s t a t i c p o l a r i z a t i o n c u r v e d e t e r generally  For  galvanostatic  t y p i c a l l y s u b j e c t e d to a 300  (based on  condition  show l i t t l e v a r i a t i o n w i t h  2.0v,  m a i n t a i n e d f o r 300  10,000 seconds, f o l l o w e d by downward s t e p p i n g a t 20 mv/50 sec  e l e c t r o d e was  by  Under these  s u r f a c e formed under the extreme  p o t e n t i a l was  r a t e f o r manual o p e r a t i o n ) .  "non-steady  or c u r r e n t employed, f o l l o w e d  the c u r r e n t o r p o t e n t i a l  lower s e t t i n g s .  m i n a t i o n s the  Satisfactory  or current stepping at a constant r a t e .  the n a t u r e of the  time a t the  to determine a "steady  be o b t a i n e d , however, w i t h  p o l a r i z a t i o n a t the h i g h e s t p o t e n t i a l  conditions  elec-  to  (a c o n v e n i e n t  p o l a r i z a t i o n curves  the  second p r e - a n o d i z a t i o n a t  geometric a r e a ) , f o l l o w e d by downward s t e p p i n g  at  114  85  Alog i / A t = 0.17/50 sec.  A l l p o l a r i z a t i o n curves were determined  i n a two-  compartment e e l 1.  5.3.2  Titanium Electrodes Whereas both g a l v a n o s t a t i c and p o t e n t i o s t a t i c p o l a r i z a t i o n methods  can be used t o determine  the p o t e n t i a l  v s . c u r r e n t r e l a t i o n when o n l y a  s i n g l e e l e c t r o d e process i s i n v o l v e d (such as oxygen e v o l u t i o n on noble metal e l e c t r o d e s ) , the p o t e n t i o s t a t i c method must be employed when t h e primary e l e c t r o d e r e a c t i o n changes (such as i n metals showing an a c t i v e / passive t r a n s i t i o n ) .  In o r d e r f o r t i t a n i u m t o m a n i f e s t an a c t i v e c o r r o s i o n  p o t e n t i a l on immersion  i n t h e e l e c t r o l y t e , the n a t u r a l l y - f o r m e d o x i d e  must be removed and t h e e l e c t r o l y t e must be m a i n t a i n e d o x y g e n - f r e e . was a c c o m p l i s h e d  by chemical p r e t r e a t m e n t i n a h y d r o f l u o r i c  film This  acid/nitric  a c i d / w a t e r s o l u t i o n and by sweeping t h e e l e c t r o l y t e w i t h hydrogen g a s . The  p o l a r i z a t i o n c u r v e was s u b s e q u e n t l y t r a c e d by s t e p p i n g i n an upward  d i r e c t i o n a t a r a t e o f 20 mv/100 sec which gave good s e p a r a t i o n o f the a c t i v e and p a s s i v e r e g i o n s .  The maximum p o t e n t i a l o u t p u t o f t h e p o t e n t i o s t a t  c o u l d be i n c r e a s e d t o 100 v o l t s by b i a s i n g w i t h t h e dual Anatek model 50-1D  power s u p p l y .  A l l p o l a r i z a t i o n curves were determined  i n a two-  compartment e e l 1.  5.4  Long-Term E l e c t r o l y s i s Individual  ( u n i n t e r r u p t e d ) runs o f s e v e r a l hundreds o r even  thousands o f hours' d u r a t i o n were made i n o r d e r t o p r o v i d e i n f o r m a t i o n c o n c e r n i n g the c o r r o s i o n , p a s s i v a t i o n and u l t i m a t e d e g r a d a t i o n o f the  86  Pt/30 I r - T i anodes.  The e l e c t r o d e s were c h e m i c a l l y p r e t r e a t e d i n c h r o m i c /  s u l f u r i c a c i d , f o l l o w e d by r i n s i n g i n d o u b l e - d i s t i l l e d w a t e r p r i o r t o immersion i n t h e working e l e c t r o l y t e i n s i n g l e compartment c e l l s . electrochemical pretreatments  No  were employed, and t h e w o r k i n g e l e c t r o d e was  a l l o w e d t o d r i f t a t o p e n - c i r c u i t p r i o r t o commencement o f t h e r u n . E l e c t r o l y s i s began ( t = 0) when t h e c i r c u i t was completed t o t h e ( p r e - c a l i b r a t e d ) power s u p p l y .  T h i s method was n e c e s s i t a t e d by an u n d e s i r a b l e  characteristic  o f t h e c o n s t a n t c u r r e n t power s u p p l i e s - namely t h a t t h e o u t p u t l e a d s were shorted  when the power s u p p l y was " o f f , " which r e s u l t e d i n the e f f e c t i v e  f o r m a t i o n o f a b a t t e r y w i t h t h e w o r k i n g and a u x i l i a r y e l e c t r o d e s .  In o r d e r  to cope w i t h t h e problem o f power f a i l u r e s , which c o u l d cause t h e anode t o d i s c h a r g e , a diode was p l a c e d i n t h e c i r c u i t t o p r e v e n t t h e r e v e r s e d flow of current.  Anode p o t e n t i a l s were monitored w i t h r e s p e c t t o time a t  10 second i n t e r v a l s f o r t h e f i r s t 100 s e c o n d s , and a t l o n g e r thereafter.  intervals  A f t e r about an hour t h e r a t e o f change o f t h e anode p o t e n t i a l  was so s m a l l t h a t o n l y i n f r e q u e n t m o n i t o r i n g was n e c e s s a r y .  A l l electrolytes  were s t i r r e d because o f t h e tendency f o r d e n s i t y - d i f f e r e n c e s t o develop i n aqueous s u l f u r i c a c i d s o l u t i o n s . sharp s e p a r a t i o n o t h e r w i s e  (In copper-containing  solutions a  o c c u r r e d w i t h a c l e a r s o l u t i o n s i t t i n g above a  blue-coloured s o l u t i o n , w i t h the e l e c t r o d e s i n the copper-depleted Runs were t e r m i n a t e d  "live"  c e l l w h i l e i t was s t i l l d o u b l e - d i s t i l l e d water.  by w i t h d r a w i n g  zone.)  t h e working e l e c t r o d e from t h e  p o l a r i z e d , and immersing i n a beaker c o n t a i n i n g  87  5.5  E l e c t r o d e C h a r a c t e r i z a t i o n between R e p e t i t i v e Runs In o r d e r t o study the p r o g r e s s i v e changes i n such parameters as  noble metal  loading, surface composition, e l e c t r o c h e m i c a l l y a c t i v e surface  a r e a , anodic d e p o s i t s and s u r f a c e morphology i t was n e c e s s a r y Pt/30 I r - T i anodes t o r e p e t i t i v e l o n g - t e r m being made between runs.  to subject  r u n s , w i t h such c h a r a c t e r i z a t i o n  On " l i v e ! t e r m i n a t i o n o f long-term 1  runs t h e  e l e c t r o d e s were r i n s e d i n d o u b l e - d i s t i l l e d w a t e r ( i n some cases t h e s u r f a c e charge and area were measured i n a two-compartment c e l l , f o l l o w e d by removal to storage i n d o u b l e - d i s t i l l e d water). removed and s e p a r a t e d  A f t e r r i n s i n g t h e e l e c t r o d e s were  from t h e i r T e f l o n / P y r e x  h o l d e r s , w i t h c a r e taken n o t  t o touch o r o t h e r w i s e damage t h e e l e c t r o d e s u r f a c e .  Several  procedures  followed: (1)  X - r a y d i f f r a c t i o n . An i n t e n s i t y v s 28 scan r e v e a l e d any c h a n g e s i n t h e c r y s t a l l i n e n a t u r e o f t h e anode s u r f a c e s - i n p a r t i c u l a r , t h e p r e s e n c e o f l e a d - c o n t a i n i n g anode d e p o s i t s . I f no d e p o s i t s w e r e f o u n d , t h e c h a r a c t e r i z a t i o n c o n t i n u e d a t s t e p (5) b e l o w .  (2)  X-ray f l u o r e s c e n c e spectroscopy. Theamount o f l e a d on t h e a n o d e s u r f a c e was e s t i m a t e d f r o m t h e i n t e n s i t y o f t h e PbLai p e a k .  (3)  S c a n n i n g e l e c t r o n m i c r o s c o p y (SEM) t o d e t e r m i n e the n a t u r e and d e g r e e o f c o v e r a g e o f t h e l e a d containing deposits .  (4)  T r e a t m e n t i n d i l u t e HC1 s o l u t i o n i n o r d e r t o remove t h e l e a d - c o n t a i n i n g s u r f a c e d e p o s i t s .  (5)  X-ray f l u o r e s c e n c e spectroscopy a c c o r d i n g p r o c e d u r e s d e s c r i b e d i n A p p e n d i x A2.  (6)  SEM o b s e r v a t i o n o f t h e m o r p h o l o g y o f t h e a n o d e surface.  (7)  R e - c o n s t r u c t i o n o f t h e e l e c t r o d e i n a new T e f l o n / P y r e x h o l d e r , f o l l o w e d by c l e a n i n g i n c h r o m i c / s u l f u r i c a c i d and r i n s i n g i n d o u b l e - d i s t i l l e d w a t e r  to the  88  p r i o r t o commencement o f t h e r e p e t i t i v e r u n , under i d e n t i c a l o p e r a t i n g c o n d i t i o n s , but w i t h a fresh electrolyte.  R e p e t i t i v e runs were c o n t i n u e d  i n many cases t o anode f a i l u r e , where t h e  anode p o t e n t i a l rose t o a high v a l u e , under which c o n d i t i o n s t h e powersupplies switched  from c u r r e n t t o e e l 1 - p o t e n t i a l c o n t r o l w i t h subsequent  o p e r a t i o n a t t h e v o l t a g e l i m i t o f t h e s u p p l i e s (50v i n the p r e s e n t  5.6  Pulsed  case).  Electrolysis  P u l s e d e l e c t r o l y s i s runs were o f long-term  d u r a t i o n and were  performed i n s i n g l e compartment c e l l s as d e s c r i b e d above.  Two types o f  p u l s e d e l e c t r o l y s i s were employed: (1)  Periodic current  (2)  P e r i o d i c open c i r c u i t  reversal  (  t  / 0  t  N  S  H  0  R  T  E  n)  (t ../t . -f-) n  UN  r  I  Urr  In the former case the working and a u x i l i a r y e l e c t r o d e s were s h o r t e d the  "^SHORTED"  P  E R  ''  O C  '>  which r e s u l t e d i n t h e f l o w o f a non-steady  during  reverse  c u r r e n t as a r e s u l t o f " d i s c h a r g e " o f t h e e f f e c t i v e b a t t e r y formed w i t h connection  o f these two e l e c t r o d e s .  was m a i n t a i n e d  5.7  In t h e l a t t e r case the w o r k i n g e l e c t r o d e  a t o p e n - c i r c u i t during the " t  n F F  "  period.  Anode P o t e n t i a l C o r r e c t i o n s In o r d e r t o p r e s e n t t h e r e c o r d e d  form, namely w i t h r e s p e c t t o the s t a n d a r d  anode p o t e n t i a l s i n a s i n g l e hydrogen e l e c t r o d e (SHE), c o r r e c -  t i o n s f o r t h e ohmic c o n t r i b u t i o n (IR-drop) and f o r the d i f f e r e n c e s i n t h e r e f e r e n c e e l e c t r o d e p o t e n t i a l s from t h e v a l u e o f t h e SHE must be made. The f i r s t i s a r e l a t i v e l y s t r a i g h t f o r w a r d p r o c e d u r e , i n v o l v i n g t h e  89  s u b t r a c t i o n o f the c a l c u l a t e d p o t e n t i a l drops a c r o s s known " t h i c k n e s s e s " of e l e c t r o l y t e whose c o n d u c t i v i t y i s known. summarized i n Appendix A3  The  IR-drop  c o r r e c t i o n s are  f o r the common e l e c t r o l y t e s employed i n the  p r e s e n t work. C a l c u l a t i o n s o f the p o t e n t i a l s o f the r e v e r s i b l e hydrogen e l e c t r o d e and o f the copper/copper ionic a c t i v i t i e s .  s u l f a t e e l e c t r o d e r e q u i r e a knowledge o f i n d i v i d u a l  As t h e s e q u a n t i t i e s cannot be measured, they must be  e s t i m a t e d from t a b u l a t e d v a l u e s o f mean a c t i v i t y c o e f f i c i e n t . the methods used t o determine  both the c o n c e n t r a t i o n s and  individual  a c t i v i t y c o e f f i c i e n t s , and hence to e s t i m a t e the i n d i v i d u a l a r e g i v e n i n Appendix A4.  D e t a i l s of ionic  ion a c t i v i t i e s ,  The v a l u e o f the r e f e r e n c e e l e c t r o d e p o t e n t i a l s  c a l c u l a t e d by i n s e r t i n g these v a l u e s i n t o the Nernst e q u a t i o n depends on the method employed i n the e s t i m a t i o n .  For the hydrogen e l e c t r o d e , the  measured e l e c t r o d e p o t e n t i a l data are c o n v e r t e d to v a l u e s v s . SHE of the r e s u l t s o f "method 2" d e s c r i b e d i n Appendix A4.  by means  Chapter 6  RESULTS  6.1 6.1.1  D e s c r i p t i o n o f New  Electrodes  S u r f a c e Areas New  e l e c t r o d e s were c o n s i d e r e d  been s u b j e c t e d  to anodic  treatment  o f any  t o be e l e c t r o d e s which had  not  k i n d , e x c e p t i n the case o f  s u r f a c e area measurement where c e r t a i n anodic o p e r a t i o n s are a p r e r e q u i s i t e for that particular  case.  Samples were taken from sheets of Pt/30 I r - T i t i o n ) m a t e r i a l o f v a r i o u s l o a d i n g s and and  (nominal  composi-  from d i f f e r e n t m a n u f a c t u r i n g  lots,  t h e i r noble metal l o a d i n g s and e l e c t r o c h e m i c a l l y a c t i v e s u r f a c e areas  were determined by the X-ray f l u o r e s c e n c e s p e c t r o s c o p y oxygen l a y e r s t r i p p i n g methods, r e s p e c t i v e l y . as a roughness f a c t o r ) was  The  o f s e v e r a l such d e t e r m i n a t i o n s  (expressed  (Each p o i n t r e p r e s e n t s the average  on a g i v e n e l e c t r o d e m a t e r i a l . )  be seen, no r e g u l a r r e l a t i o n i s obeyed, p r o b a b l y  decomposition  s u r f a c e area  p l o t t e d a g a i n s t the determined l o a d i n g f o r each  e l e c t r o d e t y p e , as shown i n F i g u r e 6.1.  i n manufacturing  and g a l v a n o s t a t i c  As  can  as a r e s u l t o f d i f f e r e n c e  p r o c e d u r e s ( a l t h o u g h a l l were produced by the same thermal  process).  In g e n e r a l , however, h i g h e r - l o a d i n g e l e c t r o d e s  e x h i b i t higher surface areas.  The  m a j o r i t y o f the e l e c t r o d e s employed i n 90  91  F i g u r e 6.1.  E l e c t r o c h e m i c a l l y a c t i v e s u r f a c e a r e a s , e x p r e s s e d as roughness f a c t o r s , of new Pt/30 I r - T i e l e c t r o d e s from d i f f e r e n t manufacturing lots. (The t r i a n g l e corresponds t o the m a t e r i a l used i n the p r e s e n t s t u d y . )  92  the p r e s e n t work were o b t a i n e d from a s i n g l e sheet o f Pt/30 I r - T i m a t e r i a l , denoted by the t r i a n g l e on F i g u r e 6.1,  and  showed roughness f a c t o r s o f  40 ± 3.  6.1.2  Loadings I n d i v i d u a l disc-shaped  the source  e l e c t r o d e s c u t from the sheet which  o f the e l e c t r o d e s i n the p r e s e n t  t o t a l noble metal l o a d i n g and  study showed the f o l l o w i n g mean  platinum weight f r a c t i o n :  L o a d i n g = 4.337 ± .203 w  = 0.687 ±  pt  As can be seen, the l o a d i n g s on new  g/m  2  .003  d i s c e l e c t r o d e s v a r y somewhat from  d i s c to d i s c as a consequence o f the n o n u n i f o r m i t y compositions  was  o f the c o a t i n g , but  are r e l a t i v e l y c o n s t a n t , i n d i c a t i n g good d i s p e r s i o n o f  c o a t i n g c o n s t i t u e n t s , a t l e a s t on the s c a l e o f the d i s c s (1.92  cm  the  the geometric  2  area). Loadings o f o t h e r Pt/30 I r - T i e l e c t r o d e s used i n t h i s work a r e shown i n F i g u r e 6.1,  6.1.3  r a n g i n g from 2.3  to 20.0  as  g/m . 2  Diffractometry C a l c u l a t e d d-spacings from X-ray d i f f r a c t o m e t e r scans (between  10°  and  90°  20 w i t h CuKa r a d i a t i o n ) w i t h new  summarized i n T a b l e 6.1.  Peaks c o r r e s p o n d i n g  Pt/30 I r - T i anode m a t e r i a l t o p l a t i n u m and  titanium  metal are r e a d i l y i d e n t i f i e d , and a s i n g l e weak peak a t d = 3.20 c l o s e l y t o the s t r o n g e s t T i 0  2  ( r u t i l e ) l i n e a t d = 3.24  corresponds  angstroms.  T i t a n i u m peaks appear because the c o a t i n g i s both i m p e r f e c t thin.  The  t i t a n i u m peaks are c o n s i d e r a b l y a t t e n u a t e d  l o a d i n g m a t e r i a l , however.  The  are  w i t h the  and  higher-  t i t a n i u m peaks do not f o l l o w the ASTM Index  93  T a b l e 6.1 Identification  o f X-ray D i f f r a c t i o n Peaks Observed w i t h new Pt/30 I r - T i  Anodes o f Nominal 5 and 20 g/m  noble Metal L o a d i n g s  2  ( D i f f r a c t o m e t e r Scan Range: 10-90° 29 w i t h CuKa R a d i a t i o n ) Peak From DiffractometerChart d(A)  Comments 5 g/m Anode  2  Identification 20 g/m Anode  (ASTM Index)  2  Species  I / I i hkl  d(A)  100  3.24  3.20  weak,broad  weak,broad  Ti0 (rutile)  2.56  medium,broad  weak,broad  Ti  30  01.0 2.56  2.34  intense,sharp  medium,sharp  Ti  26  00.2 2.34  2.24 2.26  i ntense,complex  ; pt  100 100  01.1 2.24 111 2.26  1.95  medium,broad  medium,broad  pt  53  200  1.72  intense,sharp  weak,broad  Ti  19  01.2 1.73  1.47  weak,sharp  (not p r e s e n t )  Ti  17  11.0 1.47  1.38  mediumbroad  mediumbroad  Pt  31  220  1.33  intense,sharp  medium,sharp  Ti  16  10.3 1.33  1.25  weak  (not p r e s e n t )  Ti  16  11.2 1.25  1.23  weak  (not p r e s e n t )  Ti  13  20.1 1.23  1.18  mediumbroad  medium,broad  Pt  33  311  1.18  1.13  weak,broad  weak,broad  Pt  12  222  1.13  i ntense,complex  2  f  -<  T  i  110  1.96  1.39  94  o r d e r o f r e l a t i v e i n t e n s i t i e s due t o p r e f e r r e d o r i e n t a t i o n o f t h e s h e e t . o  From the most i n t e n s e peak (1.33A), assuming most f a v o u r a b l y f o r d i f f r a c t i o n  (parallel  t h a t these planes are o r i e n t e d  to the specimen  s u r f a c e ) , i t can  be e s t i m a t e d t h a t the b a s a l p l a n e s i n the t i t a n i u m s u b s t r a t e a r e o r i e n t e d a t 28° to the s u r f a c e . On the assumption t h a t p l a t i n u m and i r i d i u m from a random s u b s t i tutional crystal  s o l i d s o l u t i o n as a consequence  of the s i m i l a r i t i e s i n t h e i r  s t r u c t u r e s (FCC) and l a t t i c e parameters, the l a t t i c e s p a c i n g s can  be e s t i m a t e d f o r P t / I r a l l o y s by Vegard's  law [352] where the l a t t i c e  i s d i r e c t l y p r o p o r t i o n a l t o the atomic p e r c e n t a g e s o f the a l l o y  parameter  constituents.  The r e s u l t s o f these c a l c u l a t i o n s , t o g e t h e r w i t h a c t u a l measurements on Pt/30 I r - T i c o a t i n g s , a r e shown i n T a b l e 6.2.  As can be seen, the measured  peaks, w h i l e c l o s e enough to those o f pure p l a t i n u m t o p e r m i t t h e i r  iden-  t i f i c a t i o n as p l a t i n u m , a c t u a l l y c o r r e s p o n d more c l o s e l y to those p r e d i c t e d f o r a s o l i d s o l u t i o n o f p l a t i n u m and i r i d i u m . reason why  Vegard's  F u r t h e r , s i n c e t h e r e i s no  law s h o u l d be adhered to (and many metals which form  c o n t i n u o u s s o l i d s o l u t i o n s d e v i a t e both p o s i t i v e l y and n e g a t i v e l y from  it),  i t i s l i k e l y t h a t the measured peaks a r e indeed produced from a random substitutional  s o l i d s o l u t i o n o f the two c o a t i n g metals h a v i n g the l a t t i c e  parameter: a = 3.909 angstroms. The broad n a t u r e o f the P t / I r peaks i s l i k e l y a consequence the t h i n n e s s o f the c o a t i n g s (a 5 g/m  2  of  f i l m has an average t h i c k n e s s o f about  0.4 m i c r o n s ) and the r e l a t i v e l y low temperatures a c h i e v e d ( l e s s than 500°C) d u r i n g manufacture material.  [183] which would tend to produce a v e r y f i n e - g r a i n e d  95  Table  6.2  L a t t i c e Parameters f o r I r i d i u m and P l a t i n u m from the ASTM Index, and Pt/30 I r a l l o y s as C a l c u l a t e d by Vegard's Law and as Measured f o r Pt/30 I r - T i  L a t t i c e Parameters and d-Spacings (Angstroms)  Specimens  Ir  Pt  Calculated Pt/30 I r  Measured Pt/30 I r - T i  a  3. 8389  3 .9237  3.8983  3.909  dm  2. 2170  2 .265  2.251  obscured by T i peak  d oo  1. 9197  1 .962  1.949  1.953  d ii  1. 1574  1 .1826  1.175  1.180  d  1. 3575  1 .387  1 .378  1.382  1. 1082  1 .1325  1.125  1.128  2  3  d  2  2 0  2 2 2  96  The  s i n g l e peak a t t r i b u t e d to r u t i l e i s not d i m i n i s h e d  on going from the 5 g/m  to the 20 g/m  2  t h a t the o x i d e may  2  i n any  way  l o a d i n g m a t e r i a l , which suggests  not o n l y a r i s e from the f i l m e d t i t a n i u m s u r f a c e where  i t i s exposed, but may  be i n c o r p o r a t e d i n the c o a t i n g .  The  source o f the  o x i d e , however, must be the s u b s t r a t e as no t i t a n i u m s p e c i e s a r e i n v o l v e d i n the o r g a n i c s o l u t i o n used i n the thermal process  decomposition  manufacturing  [174]. In o r d e r to d e f i n i t e l y e s t a b l i s h the n a t u r e of the s p e c i e s c o r r e -  sponding t o the s i n g l e broad X-ray d i f f r a c t i o n peak, e l e c t r o n d i f f r a c t i o n was  employed w i t h s u i t a b l e specimens being prepared  by s c r a p i n g the anode  s u r f a c e w i t h a s c a l p e l , and c a t c h i n g t h e m a t e r i a l which was copper/carbon g r i d . probably  removed on a  No noble metal d i f f r a c t i o n p a t t e r n s were o b s e r v e d ,  as a consequence o f the high r e l a t i v e t h i c k n e s s and  c h a r a c t e r i s t i c s o f the " f l a k e s " o f c o a t i n g m e t a l . corresponded c l o s e l y to t h a t o f r u t i l e  The  (Table 6.3),  high  absorption  observed r i n g  thus c o n f i r m i n g  patterns the  e a r l i e r assumption.  6.1.4  Morphology SEM  s u b s t r a t e and  o b s e r v a t i o n s were made w i t h an e t c h e d , s e v e r a l new  uncoated t i t a n i u m  Pt/30 I r - T i anodes i n o r d e r t o p r o v i d e a b a s i s  f o r comparison w i t h anode s u r f a c e s a f t e r a p p r e c i a b l e c o r r o s i o n o r u l t i m a t e degradation  has taken p l a c e .  The  base t i t a n i u m s u r f a c e ( F i g u r e  6.2)  i s very rough and shows hexagonal " g r a i n s " over i t s s u r f a c e i n an o r i e n t a t i o n which agrees w e l l w i t h the p r e f e r r e d o r i e n t a t i o n e s t i m a t e d X-ray d i f f r a c t i o n i n t e n s i t y c o n s i d e r a t i o n .  from  97  Table 6.3 Electron Diffraction  Observed Rings d(A)  Results  Rutile  Intensity  (Ti0 ) 2  d(A)  I/I i  hkl  3.29  weak  3.245  100  no  2.51  strong  2.489  40  101  2.297  7  200  -  -  2.19  medium  2.188  22  111  2.08  weak  2.054  9  210  1.69  weak  1.687  50  211  1.54  strong  1.524  16  220  1.50  weak  1.480  8  002  1.45  weak  1.453  6  310  1.37  weak  1.360  16  301  1.32  medi urn  1.347  7  112  98  F i g u r e 6.2.  S.E.M. view o f the t i t a n i u m s u b s t r a t e s u r f a c e as e t c h e d p r i o r to thermal d e c o m p o s i t i o n o f t h e noble metal c o a t i n g . (lOOOx)  F i g u r e 6.3.  S.E.M. view o f a t y p i c a l new 4.33 g/m surface. (400x)  2  n o b l e metal l o a d i n g anode  99  Coated e l e c t r o d e s r e v e a l c o n s i d e r a b l e h e t e r o g e n e i t y .  Perhaps  the b e s t d e s c r i p t i o n i n v o l v e s an analogy w i t h " d e s e r t mud" where the c o a t i n g material  becomes c r a c k e d  t i o n process.  and f i s s u r e d  as a r e s u l t o f the thermal  decomposi-  The f i s s u r r e d t e x t u r e i s most p r e v a l e n t i n t h e " v a l l e y s "  where g r e a t e r amounts o f t h e c o a t i n g m a t e r i a l would accumulate d u r i n g manufacture, resulting i n l o c a l l y greater thicknesses. t y p i c a l o f new 4.3 g/m  2  l o a d i n g anodes.  F i g u r e s 6.3 and 6.4 a r e  The s u b s t r a t e i s o f t e n exposed  between i n d i v i d u a l c o a t i n g " p l a t e s , " and t o a l e s s e r e x t e n t on the "peaks" where c o a t i n g t h i c k n e s s e s a r e s m a l l e r (as determined by s e l e c t e d - a r e a X-ray energy a n a l y s i s ) .  The r e p e t i t i v e p a i n t i n g / h e a t i n g / c o o l i n g process f o r  s u c c e s s i v e a p p l i c a t i o n o f the noble metal c o a t i n g i s m a n i f e s t e d  by t h e  appearance o f c o a t i n g " p l a t e s " randomly c o v e r i n g o t h e r c o a t i n g " p l a t e s " (whereas o n l y a s i n g l e f i s s u r e d coating application).  l a y e r would be e x p e c t e d f o r a s i n g l e  Greater loadings  ( F i g u r e 6.5) a r e c h a r a c t e r i z e d by  c o n s i d e r a b l e degree o f p o r o s i t y o f t h e c o a t i n g  6.2 6.2.1  C o r r o s i o n o f Noble Metal  itself.  Coated Anodes  Changes i n Loading and C o m p o s i t i o n Treatment o f the c o r r o s i o n data generated from long-term  runs i n  v a r i o u s e l e c t r o l y t e s i s b e s t approached by c o n s i d e r a t i o n o f anode p o t e n t i a l behaviour  which can be d i s t i n g u i s h e d i n t o t h r e e groups.  These a r e :  1.  Reproducible p o t e n t i a l v s . time c h a r a c t e r i s t i c s on r e p e t i t i v e e l e c t r o l y s i s .  2.  I r r e p r o d u c i b l e p o t e n t i a l vs. time c h a r a c t e r i s t i c s on r e p e t i t i v e e l e c t r o l y s i s .  3.  Anodic e l e c t r o l y s i s  to complete  degradation.  F i g u r e 6.5.  S.E.M. view o f a t y p i c a l new 20 g/m surface. (lOOOx)  2  n o b l e metal l o a d i n g anode  101  A g i v e n anode passes through a l l t h r e e o f these stages d u r i n g runs over i t s o p e r a t i n g l i f e t i m e . two stages w i l l  Only behaviour  repetitive  t y p i c a l o f the  first  be t r e a t e d h e r e , w i t h the case o f complete d e g r a d a t i o n  being c o n s i d e r e d i n S e c t i o n P l a t i n u m and  6.5.  i r i d i u m l o a d i n g s , as determined  cence s p e c t r o s c o p i c t e c h n i q u e , were found t o decrease time o f a n o d i c p o l a r i z a t i o n , w i t h g r e a t e r decreases current densities.  by the X-ray  fluores-  progressively with  occurring with higher  Changes i n p l a t i n u m l o a d i n g w i t h time are summarized  i n F i g u r e s 6.6-6.9, which p r e s e n t data f o r i n d i v i d u a l to r e p e t i t i v e e l e c t r o l y s i s runs.  electrodes subjected  In most c a s e s , the r e p e t i t i v e runs were  c o n t i n u e d f o r l o n g e r times than d e p i c t e d on F i g u r e s 6.6-6.9, but are i n c l u d e d because complete d e g r a d a t i o n o c c u r r e d . below 50 mA/cm  2  With the e x c e p t i o n o f runs  (geometric a r e a ) , where the l i f e t i m e s o f i n d i v i d u a l  are very l o n g , the data i n F i g u r e s 6.6  t o 6.9  not  anodes  can be s a i d t o d e p i c t the  complete l o a d i n g v s . time b e h a v i o u r over the " u s e f u l "  ( i . e . p r i o r to  f a i l u r e ) l i f e t i m e s of the g i v e n anodes. From the v a r i a t i o n i n the weight  f r a c t i o n o f p l a t i n u m i n the  P t / I r a l l o y c o a t i n g s , as shown i n F i g u r e 6.10-6.13, the v a r i a t i o n  in iridium  l o a d i n g w i t h time can be deduced from the c o r r e s p o n d i n g p o i n t s on F i g u r e s 6.6-6v9.  The  l o a d i n g v s . e l e c t r o l y s i s time behaviour f o r i r i d i u m i s  c o m p l e t e l y analogous to t h a t f o r p l a t i n u m .  Of more i n t e r e s t , however, i s  the o b s e r v a t i o n ( w i t h a few e x c e p t i o n s ) t h a t the weight decreases  f r a c t i o n of platinum  p r o g r e s s i v e l y w i t h the time o f e l e c t r o l y s i s which i n d i c a t e s  t h a t the p l a t i n u m l o a d i n g decreases F u r t h e r , l a r g e r decreases  more r a p i d l y than the i r i d i u m l o a d i n g .  i n the p l a t i n u m weight f r a c t i o n are found  with higher applied current d e n s i t i e s  (geometric a r e a ) .  to occur  o 7.8 mA/cm of5.6 •299 A 52.1  T I M E , F i g u r e 6.6.  H O U R S  V a r i a t i o n i n p l a t i n u m l o a d i n g i n P t / I r a l l o y c o a t i n g s f o r i n d i v i d u a l anodes w i t h e l e c t r o l y s i s time i n 2M r ^ S C t + 0.5M CuSCU, 22°, a t v a r i o u s (geometric) c u r r e n t d e n s i t i e s .  ro  3  I  f  CM  e> 2 < O  TIME F i g u r e 6.7.  1000  HOURS  2000  V a r i a t i o n i n p l a t i n u m l o a d i n g f o r P t / I r a l l o y c o a t i n g s f o r i n d i v i d u a l anodes w i t h e l e c t r o l y s i s time i n 2M H 2 S O 4 + 0.5M CuSOi*, 40°, a t v a r i o u s (geometric) c u r r e n t d e n s i t i e s .  o 0 0  A 7.8 mA/cm • 15.6 • 52.1 OI04  !  i I  I  I  1  0  1 TIME  i  F i g u r e 6.8.  1  1  0  0  0 HOURS  Variation i n platinum loading i n P t / I r a l l o y coatings f o r individual time i n 2M H S04, 22°, a t v a r i o u s (geometric) c u r r e n t d e n s i t i e s . 2  1  2000 anodes w i t h e l e c t r o l y s i s  o  4:1  F i g u r e 6.9.  Variation i n platinum loading i n P t / I r a l l o y coatings f o r individual time i n 2M H S0\, 40°, a t v a r i o u s (geometric) c u r r e n t d e n s i t i e s . 2  anodes w i t h  electrolysis  ocn  F i g u r e 6.10.  V a r i a t i o n o f the weight f r a c t i o n o f p l a t i n u m i n P t / I r a l l o y c o a t i n g s f o r i n d i v i d u a l anodes w i t h e l e c t r o l y s i s time i n 2M H 2 S O 4 + 0.5M C u S 0 , 22°, a t v a r i o u s ( g e o m e t r i c ) c u r r e n t d e n s i t i e s . 4  o  •7 0  r  o  15.6 m A / c m  a  52.1  2  ^ 52.1  _  VI04 •260  ^  z  {\  O  H.65  < or  x LU  .60  i  i  0  T I M E  1000  i  H O U  R S  i 2 0 0 0  o F i g u r e 6.11. V a r i a t i o n o f t h e weight f r a c t i o n o f P t i n P t / I r a l l o y c o a t i n g s w i t h e l e c t r o l y s i s time i n 2M H 2 S O 4 + 0.5M CuSG\, 40°, a t v a r i o u s (geometric) c u r r e n t d e n s i t i e s . .  T I M E F i g u r e 6.12.  , H O U R S  V a r i a t i o n o f the weight f r a c t i o n of p l a t i n u m i n P t / I r a l l o y c o a t i n g s f o r i n d i v i d u a l e l e c t r o l y s i s time i n 2M H 2 S O 4 , 22°, a t v a r i o u s ( g e o m e t r i c ) c u r r e n t d e n s i t i e s .  anodes w i t h  §  F i g u r e 6.13.  V a r i a t i o n o f the weight f r a c t i o n of p l a t i n u m i n P t / I r a l l o y c o a t i n g s f o r i n d i v i d u a l e l e c t r o l y s i s time i n 2M H S0\, 40°, a t v a r i o u s ( g e o m e t r i c ) c u r r e n t d e n s i t i e s . 2  anodes  withS  no  In one p a r t i c u l a r case w i t h an anode c u t from the "4.33 l o a d i n g s h e e t , the r a t i o o f the c o a t i n g elements unchanged a f t e r r e p e a t e d e l e c t r o l y s i s a t 52.1 2M H S 0 , 22°. 2  T h i s case w i l l  4  remained  mA/cm  2  g/m " 2  practically  ( g e o m e t r i c area) i n  be d e a l t w i t h s e p a r a t e l y below, and i s  hence p r e s e n t e d i n a s e p a r a t e f i g u r e ( F i g u r e 6.14), as i t suggests a predominantly m e c h a n i c a l , or s p a l l i n g , mode o f c o a t i n g l o s s .  6.2.2  S u r f a c e Area Changes The a n o d i c / c a t h o d i c t r e a t m e n t s employed i n the e l e c t r o c h e m i c a l  measurement o f s u r f a c e areas may c o a t i n g metal  themselves  be r e s p o n s i b l e f o r enhanced  l o s s e s and i n changes i n the s u r f a c e a r e a i t s e l f ,  although  the a c c u r a c i e s o f the c o a t i n g measurement t e c h n i q u e ( c o e f f i c i e n t o f v a r i a t i o n i s 1.1%)and o f the s u r f a c e area measurements ( c o e f f i c i e n t o f v a r i a t i o n i s 6.2%)  do not p e r m i t d e t e r m i n a t i o n o f s m a l l changes i n these q u a n t i t i e s .  Several e l e c t r o d e s subjected to t y p i c a l  s u r f a c e a r e a measurement p r o c e d u r e s  were examined, and no s t a t i s t i c a l l y meaningful  changes i n l o a d i n g o r s u r f a c e  area were d e t e c t e d , but t h i s does not p r e c l u d e the p o s s i b i l i t y o f t h e o c c u r r e n c e o f such changes.  For t h i s reason the e l e c t r o d e s used i n the  l o n g - t e r m c o r r o s i o n experiments were not s u b j e c t e d to the s u r f a c e area measurement t e c h n i q u e u n t i l l  a f t e r c o m p l e t i o n o f t h e i r planned s e r i e s o f  r e p e t i t i v e runs. Other e l e c t r o d e s were a n o d i c a l l y p o l a r i z e d i n s i n g l e or i n r e p e t i t i v e experiments  experiments  ( w i t h o u t break-up o f t h e anode assembly  between  runs) i n o r d e r to a s c e r t a i n the e f f e c t o f longT.term e l e c t r o l y s i s on the e l e c t r o c h e m i c a l l y a c t i v e surface area.  The r e s u l t s , summarized i n T a b l e  show t h a t the s u r f a c e area a c t u a l l y i n c r e a s e s s l i g h t l y  6.4,  (up to 10 per c e n t )  F i g u r e 6.14.  Loading changes and p l a t i n u m weight f r a c t i o n change w i t h time f o r an anode o p e r a t e d a t 52.1 mA/cm ( g e o m e t r i c area) i n 2M H S0\, 22°, which e x h i b i t s c o a t i n g l o s s by s p a l l i n g . 2  2  112  T a b l e 6.4 E f f e c t o f E l e c t r o l y s i s Time i n 2M H $0 2  h  A c t i v e S u r f a c e Areas o f I n d i v i d u a l  Current Density (Geometric Area) mA/cm 2  I n i t i a l Noble Metal Loading g(Pt + I r ) / m 2  on t h e E l e c t r o c h e m i c a l l y Pt/30 I r - T i anodes  Run Time A- hr Hours  Initial R.F.  Fi nal R.F.  F i n a l Noble Metal Loading g(Pt+Ir)/m 2  (a)  15.6  4.36  671  20. 1  39  ±3  41 ±3  4.32  (b)  52.1  4.15  645  64. 5  40  ±3  44 ±3  4.03  (c)  52.1  4.27  0. 278  33  ±2  33 ±2  -  64  6.4  33  ±2  35 ±2  -  128  12. 8  35  ±2  35 ±2  -  472  47. 2  35  ±2  22 ±2  -  431  86. 2  42  ±3  46 ±3  (d)  104  4.55  2.78  4.32  113  w i t h t i m e , but u l t i m a t e l y d e c r e a s e s t o a v a l u e w e l l below i t s i n i t i a l l y determined  surface area.  For an i n d i v i d u a l anode s u b j e c t e d t o r e p e t i t i v e  e l e c t r o l y s i s a t 52.1 mA/cm  2  i n 2M h^SCU, t h i s behaviour i s shown i n  F i g u r e 6.15. P l o t t i n g o f the s u r f a c e a r e a v a l u e s o f anodes ( a l l i n i t i a l l y cut  from t h e same s h e e t ) used i n t h e long-term c o r r o s i o n experiments as a  f u n c t i o n o f t h e i r r e m a i n i n g noble metal  l o a d i n g s ( F i g u r e 6.16) r e v e a l s  t h a t t h e r e i s a d e f i n i t e r e l a t i o n s h i p between these q u a n t i t i e s . for  e l e c t r o d e s whose t o t a l n o b l e metal l o a d i n g had dropped  about 3 . 8 g/m , 2  2  t o a v a l u e below  the e l e c t r o c h e m i c a l l y a c t i v e s u r f a c e area was found t o  decrease r a p i d l y t o l e s s than h a l f t h e i n i t i a l 3 g/m  Thus,  area v a l u e .  Below about  l o a d i n g t h e s u r f a c e area c o n t i n u e d t o d e c l i n e , but a t a l e s s  rapid  rate. I t i s r e a d i l y apparent, thus, t h a t f o r e l e c t r o d e s subjected to prolonged a n o d i c p o l a r i z a t i o n under " g a l v a n o s t a t i c " c o n d i t i o n s , t h e e l e c t r o c h e m i c a l l y a c t i v e s u r f a c e area may change and c o n s e q u e n t l y the a c t u a l c u r r e n t d e n s i t y may v a r y .  6.2.3 6.2.3.1  C o r r o s i o n Rates Rate E x p r e s s i o n s The c o r r o s i o n r a t e s f o r i n d i v i d u a l anodes were c a l c u l a t e d  v  . . (change g o a d i n g )  v  . . (change^  loading)  (  _  ^  i  t  m  ( a c t u a  t  r  1  ,  a  c  r  e  a  r  a  )  e  a  )  from:  „,  ( 2 )  Change i n roughness f a c t o r 4.27 g/m ) w i t h r e p e t i t i v e 2  with- time f o r a Pt/30 I r - T i . anode ( i n i t i a l noble metal l o a d i n g e l e c t r o l y s i s a t 52.1 mA/cm [geometric area), i n 2M H SO . 2  2  k  F i g u r e 6.16.  Roughness f a c t o r s f o r P t / 3 0 I r - T i anodes as a f u n c t i o n o f remaining n o b l e metal l o a d i n g a f t e r r e p e t i t i v e p r o l o n g e d e l e c t r o l y s i s runs i n 2M H S0<i and i n 2M H S 0 + 0.5M CuSO,,. (The p o i n t s r e f e r t o i n d i v i d u a l anodes c u t from the same sheet o f anode m a t e r i a l . ) . 2  2  4  116  where t i s the run t i m e , w^ the w e i g h t f r a c t i o n o f c o n s t i t u e n t i assumed to be e q u i v a l e n t t o the area f r a c t i o n , and R.F. i s the roughness f a c t o r . P a r t i a l c u r r e n t d e n s i t i e s a r e c a l c u l a t e d from Faraday's law:  i.  = n Fv i  (3)  i  on the assumption t h a t the c o a t i n g m e t a l s d i s s o l v e e l e c t r o c h e m i c a l l y , according  t o an n - e l e c t r o n  be f o u r f o r both p l a t i n u m  reaction.  For t h i s c a s e , n was c o n s i d e r e d  to  and i r i d i u m .  The c o r r o s i o n e f f i c i e n c y , e x p r e s s e d as yg/A-hr, was o b t a i n e d d i v i d i n g the a b s o l u t e and  amount o f c o a t i n g metal l o s t by t h e a p p l i e d  by  current  the time.  6.2.3.2  R e s u l t s o f Long-Term C o r r o s i o n Considerable  experimental  Experiments  s c a t t e r due t o the l e v e l s o f u n c e r t a i n t y  i n the l o a d i n g measurements was found f o r the c o r r o s i o n r a t e s determined from s i n g l e runs f o r i n d i v i d u a l e l e c t r o d e s . improved by e x t e n s i o n  A l t h o u g h t h e s c a t t e r was  o f runs t o l o n g e r t i m e s , and thus t o g r e a t e r  l o s s e s , d i f f e r e n t i n d i v i d u a l anodes o p e r a t e d under n o m i n a l l y d i t i o n s o f t e n showed l a r g e d i f f e r e n c e s i n t h e i r s t a t i s t i c a l l y corrosion rate values.  coating  s i m i l a r conreliable  Thus, the cumulate c o r r o s i o n r e s u l t s o f r e p e t i t i v e  runs w i t h one o r more anodes a t each a p p l i e d  (geometric) current  were employed f o r c o r r o s i o n r a t e c a l c u l a t i o n s .  density  F u r t h e r , o n l y those  electrodes  which e x h i b i t e d r e v e r s i b l e p o t e n t i a l v s . time b e h a v i o u r on r e p e t i t i v e runs were c o n s i d e r e d .  The r e s u l t s o f the r a t e c a l c u l a t i o n s f o r l o n g - t e r m  e l e c t r o l y s i s i n 2M H S0\ + 0.5M CuS0\ and i n 2M H S0\ a t v a r i o u s 2  2  temperatures  117  are condensed i n T a b l e 6.5-6.8.  The  data a t 22°  measurements w i t h d i s c e l e c t r o d e s (1.92 cm 60°  and 80°  i r i d i u m as d i s c u s s e d above, and  a r e taken  g e o m e t r i c a r e a ) and  from the l a r g e r sheet anodes (22.5  c o r r o s i o n r a t e s r e f e r to the f r a c t i o n a l and  2  and 40°  cm  2  those  geometric a r e a ) .  surface occupations  the mean p a r t i a l  of  from at All  platinum  (corrosion) current  d e n s i t i e s are c a l c u l a t e d w i t h r e s p e c t t o the a c t u a l s u r f a c e areas o f the coating  metals. For the p a r t i c u l a r case o f the anode d e s c r i b e d i n F i g u r e  6.14,  the c o r r o s i o n data a r e g i v e n , f o r each r e p e t i t i v e run w i t h t h i s anode, i n Table  6.9.From the r e s u l t s summarized i n T a b l e 6.5-6.8, s e v e r a l  observa-  t i o n s can be made: 1.  The c o r r o s i o n r a t e s o f b o t h p l a t i n u m and iridium increase with applied current density, although the s c a t t e r of the r e s u l t s i s poor.  2.  Platinum corrodes i r i d i urn.  3.  The c o r r o s i o n e f f i c i e n c i e s f o r p l a t i n u m and iridium do n o t show any d e p e n d e n c e on a p p l i e d c u r r e n t dens i t y .  4.  T h e r e i s no c l e a r t e m p e r a t u r e - d e p e n d e n c e o f t h e c o r r o s i o n r a t e s , although the experimental s c a t t e r could obscure a r e l a t i v e l y small temperaturedependence .  5.  C o r r o s i o n r a t e s a r e , on a v e r a g e , l o w e r i n s u l f u r i c a c i d so 1 u t i o n s . t h e n i n c o p p e r - c o n t a i n i n g electrolyte, a l t h o u g h the d i f f e r e n c e s l i e l a r g e l y w i t h i n the range o f experimental s c a t t e r .  at a greater  r a t e than  does  The c o r r o s i o n e f f i c i e n c y r e s u l t s f o r p l a t i n u m and 2M H SG\ + 0.5M 2  CuSO^ e l e c t r o l y t e a t both 22°  and 40°  iridium in  are p l o t t e d i n Figures  6.17-6.18, w i t h the h o r i z o n t a l l i n e s r e p r e s e n t i n g the weighted means o f the  Table 6.5 P l a t i n u m C o r r o s i o n R e s u l t s from Cumulative E l e c t r o l y s i s Runs i n 2M H S 0 2  Temperature  Current Density ^geometric^  Corrosion  Total Time  Efficiency  C o r r o s i o n Rate (Geometric Area)  4  + 0.5M CuSCU  C o r r o s i o n Rate (Actual Area)  ug  Mean Mean A c t u a l C o r r o s i o n Current Current Density Density mA nA cm cm  °C  mA cm  Hours  A«hr  22  7.8  ;2591  38.9  1.13  .27  129  31  3.5  0.5  0.21  0.19  15.6  1728  51.8  2.09  .18  479  •:43  15.5  1.5  0.51  0.83  2579  148  1.16  .06  5.8  29  18  1  1.04  0.99  4482  448  0.95  .04  735  36  28  1.5  1.97  1.54  117  1613  363  1.12  .05  2000  100  74  4  3.45  4.07  260  474  237  0.97  .04  3730  150  141  6  6.49  7.75  0.83  .80  189  180  5  5  0.39  0.27  2  30.0 52.1  40  60  '+  hr •rrr  hr«m  z  2  2  15.6  387  52.1  2513  251  1.27  .13  985  100  34.5  2.5  1.80  1.90  104  572  114  1 .38  .08  2170  140  72.5  4.5  3.47  3.98  260  98  49  1.35  .22  5180  830  138  22  6.92  7.58  16.1  1210  435  0.53  .28  125  71  3  1.5  0.40  0.16  53.8  194  233  0.59  .54  473  440  12  11  1.35  0.66  98  238  1.35  .55  2170  860  54  22  2.69  2.97  108 80  A«hr  5.8  16.1  194  70.4  3.37  1.93  807  460  20  11.5  0.40  1.10  53.8  260  315  0.60  .41  479  330  12  8  1.35  0.66  99  238  1.11  .55  1770  880  44.5  22  2.69  2.45  108  Table 6.6 I r i d i u m C o r r o s i o n R e s u l t s from C u m u l a t i v e E l e c t r o l y s i s Runs i n 2M H S0^ + 0.5M CuSGs 2  Current Density  Temperature ^  1  geometric^  °C  mA cm  22  40  60  Total Time  Efficiency  +  C o r r o s i o n Rate (Geometric Area) +  C o r r o s i o n Rate (Actual Area)  Mean Mean A c t u a l Corrosion Current Current Density Density mA nA cm cm  Hours  A«hr  7.8  2591  38.9  0.20  .13  50  30  1.5  0.5  0.21  0.08  15.6  1728  51.8  0.67  .09  327  43  10.5  1.5  0.51  0.59  30.0  2579  148  0.34  .03  309  29  11  1  1 .04  0.61  52.1  4482  448  0.34  .02  547  36  23'  2  1.97  1.28  117  1613  363  0.27  .03  884  90  34  3  3.45  1.90  260  474  237  0.33  .02  2610  140  98  8  6.49  5.47  0  .40  0  200  0  5  0.39  0.00  2  A-hr  hr-m  z  hr«m  z  +  2  2  15.6  387  52.1  2513  251  0.41  .06  655  100  23  3  1 .80  1.28  104  572  114  0.38  .04  1170  130  39  4  3.47  2.18  260  98  49  0.37  .10  3000  820  80  22  6.92  4.46  16.1  1210  435  0.30  .14  149  71  3.5  0.40  0.20  53.8  194  233  0.31  .26  498  440  12.5  11  1.35  0.70  98  238  0.56  .27  1850  860  46  22  2.69  2.57  11.5  0.40  1.00  108 80  Corrosion  5.8  1.5  16.1  194  70.4  1 .48  .96  111  460  18  53.8  260  315  0.45  .20  739  330  18.5  8  1.35  1.03  99  238  0.52  .27  1700  880  42.5  22  2.69  2.37  108  Table 6.7 P l a t i n u m C o r r o s i o n R e s u l t s from Cumulative E l e c t r o l y s i s Runs i n 2M H SQ\ 2  Temperature  Current Density ^geometric^ mA  C o r r o s i o n E1" f i c i e n c y A-hr  ^9 A«hr  +  392  5.8  1.34  1.4  15.6  863  25.9  0.59  52.1  1035  104  104  2110  260  °C  cm  Hours  7.8  2  22  40  Total Time  Corrosion Rate (Geometric Area)  hr-m  2  +  C o r r o s i o n Rate (Actual Area)  hr-m  152  200  .44  133  0.35  .11  422  0.75  .188  94  521  49.4  15.6 52.1  +  2  ..Mean Mean A c t u a l Corrosion Current Current Density Density mA _nA cm cm 2  2  5  5  0.21  0.27  99  3.5  2.5  0.39  0.19  264  78  7  2  1.30  0.38  .05  1170  70  49  3  3.60  2.69  0.92  .12  3530  460  88  11  6.51  4.84  49.4  1.92  .15  14800  1200  890  70  1633  49.0  0.67  .23  150  52  13.5  1.5  0.39  0.74  2417  242  0.71  .04  548  30  20  1  1 .88  1.10  104  810  162  0.35  .05  526  120  13  3  2.60  0.71  260  211  106  0.53  .10  2018  400  50  10  6.51  2.75  31.2  48.9  60  53.8  250  302  0.51  .45  413  360  5  5  1.35  0.27  80  53.8  430  521  0.26  .26  208  200  2.5  5  1.35  0.14  Table 6.8 I r i d i u m C o r r o s i o n R e s u l t s from Cumulative E l e c t r o l y s i s Runs i n 2M H S0\ 2  Temperature  Current Density ^geometric^ mA  °C  cm  22  40  2  Total Time Hours  C o r r o s i o n E-' f i c i e n c y A«hr  ug A«hr  +  Corrosion Rate (Geometric- Area) f  hr«m  42  7.8  392  5.8  1.84  .90  463  200  15.6  863  25.9  0.49  .20  248  99  52.1  1035  104  0.30  .05  519  77  104  2110  422  0.17  .04  551  260  188  94  0.23  .06  521  49.4  49.4  0.79  15.6  1633  49.0  52.1  2417  104 260  C o r r o s i on Rate (Actual Area)  hr-m  +  2  1:2  Mean A c t u a l Mean Current Corrosion Density Current Density mA nA cm cm 2  2  6  0.20  0.67  2.5  0.39  0.36  14  2  1.30  0.78  70  24  3  3.60  1 .34  1930  450  48  11  6.51  2.68  .08  12600  1200  756  70  0.34  .10  172  52  242  0.23  .02  359  30  810  162  0.20  .04  695  211  106  0.15  .05  6.5  4.5  31.2  42.2  1.5  0.39  0.25  13  1  1.88  0.73  120  17.5  2.5  2.60  0.98  1200  400  30  10  6.51  1.67  60  53.8  250  302  0.11  .22  178  360  4.5  9  1.35  0.25  80  53.8  430  521  0.06  .12  102  200  2.5  5  1.35  0.14  Table 6.9 Individual  and C u m u l a t i v e C o r r o s i o n Data f o r a Pt/30 I r - T i anode, Operated a t 52.1 mA/cm  2  (Geometric A r e a ) i n 2M h^SO^, 22°, which E x h i b i t s  Run Time  Pt C o r r o s i o n Rate (Geometric Area)  I r C o r r o s i o n Rate (Geometric Area)  Hours  V  v  hr *m  hr*m  -  0  lr Pt  -  C o a t i n g Loss by S p a l l i n g  Pt C o r r o s i o n Efficiency  Ir Corrosion Efficiency  M k'hr  M9  Ir:Pt  Efficiency Ratio  W  Pt  A«hr -  • -  W  lr  w  Pt  -  0 687  0.456  97  2270  ±940  2350  ±940  1.04  3.00  ±1.24  1.41  ±0.57  0.470  0 687  0.456  99  2030  ±870  2820  ±870  1 .39 2.68  ±1.16  1 .68  ±0.52  0.627  0 691  0.447  189  1680  ±430  1920  ±430  1.14  2.24  ±0.56  1.14  ±0.25  0.509  0 693  0.443  961  2530  ± 60  2400  ± 60  0.95  3.35  ±0.07  1 .44  ±0.03  0.430  0 683  0.464  1346  2400  ± 20  2380  ± 20  0.99  3.12  ±0.06  1 .41  ±0.03  0.452  -  -  •  ro ro  F i g u r e 6.17.  Platinum c o r r o s i o n e f f i c i e n c y vs. a p p l i e d current d e n s i t y (with r e s p e c t t o g e o m e t r i c a r e a ) from t h e cumulate c o a t i n g l o s s r e s u l t s o f r e p e t i t i v e e l e c t r o l y s i s runs i n 2M H S 0 i , + 0.5M CuSCV 2  F i g u r e 6.18.  Iridium c o r r o s i o n e f f i c i e n c y vs. applied current d e n s i t y (with r e s p e c t t o g e o m e t r i c a r e a ) from t h e c u m u l a t i v e c o a t i n g l o s s r e s u l t s o f r e p e t i t i v e e l e c t r o l y s i s runs i n 2M H SO + 0.5M CuSG\. z  k  125  c o r r o s i o n e f f i c i e n c i e s of the anodes employed i n t h a t e l e c t r o l y t e .  These  are:  The  platinum  and  Pt:  1.13  ug/A-hr  Ir:  0.35  ug/A-hr  i r i d i u m corrosion rates (with respect  a c t u a l s u r f a c e a r e a s ) i n 2M H S 0 2  p l o t t e d i n Figures with applied  6.19  and  6.20.  4  + 0.5M The  C u S 0 , a t both 22° 4  and  to  their  40°,  are  c o r r o s i o n r a t e s are seen t o  vary  ( a c t u a l ) c u r r e n t d e n s i t y e s s e n t i a l l y i n a l i n e a r manner.  The  c o r r o s i o n r a t e vs. a p p l i e d c u r r e n t d e n s i t y r e l a t i o n s c o u l d  a l s o be p l o t t e d c a l c u l a t e d f o r the g e o m e t r i c , r a t h e r than the a c t u a l , e l e c t r o d e a r e a f r a c t i o n s o c c u p i e d by p l a t i n u m  and  i r i d i u m , and  g e o m e t r i c e l e c t r o d e areas.  However, such p l o t s would o n l y be  t o those i n F i g u r e 6.19  6.20  and  using  the  equivalent  i f the roughness f a c t o r s were i n v a r i a n t .  T h i s i s c e r t a i n l y not the case w i t h the p r e s e n t work.  Anodes s u b j e c t e d  high a p p l i e d c u r r e n t d e n s i t i e s , show s u f f i c i e n t l y l a r g e c o a t i n g l o s s e s s u r f a c e a r e a changes per r e p e t i t i v e run t h a t the general vs. mean a c t u a l a p p l i e d c u r r e n t d e n s i t y r e l a t i o n can an i n d i v i d u a l anode.  F i g u r e 6.21  corrosion  + 0.5M  CuSO^, 22°C.  rate  shows both types o f c o r r o s i o n r a t e 2  H2SO4  vs.  (geometric area)  As can be c l e a r l y seen, the  representation  w i t h r e s p e c t t o a c t u a l s u r f a c e a r e a i s much more m e a n i n g f u l , and  indeed  conforms w e l l w i t h the s l o p e o f the curve determined from F i g u r e  6.19.  The 40°  e f f e c t s of v a r i o u s c o n d i t i o n s o f a c i d i t y , from 0.5  (Table 6.10), d i d not r e v e a l any  t o 8M  s i g n i f i c a n t d i f f e r e n c e s i n the  r a t e s or e f f i c i e n c i e s from those found i n the more e x t e n s i v e work i n 2M H S0^. 2  and  be d i s t i n g u i s h e d f o r  c u r r e n t d e n s i t y p l o t s f o r an anode o p e r a t e d a t 117 mA/cm i n 2M  to  H2SO4,  corrosion  Figure  6.19.  Platinum c o r r o s i o n r a t e percm of actual platinum surface v s . mean a c t u a l a p p l i e d c u r r e n t d e n s i t y . (Each p o i n t c o r r e s p o n d s t o t h e c u m u l a t i v e c o a t i n g l o s s measurements f o r one o r more anodes s u b j e c t e d t o r e p e t i t i v e e l e c t r o l y s i s runs i n 2M H SG\ + 0..5M CuSCV ) 2  2  127  F i g u r e 6.20.  I r i d i u m c o r r o s i o n r a t e p e r c m o f a c t u a l i r i d i u m s u r f a c e v s . mean actual applied current density. (Each p o i n t c o r r e s p o n d s t o the c u m u l a t i v e c o a t i n g l o s s measurements f o r one o r more anodes s u b j e c t e d t o r e p e t i t i v e e l e c t r o l y s i s runs i n 2M H S 0 + 0.5M CuS0 .) 2  2  4  4  128  4 0 0 Q  MEAN  F i g u r e 6.21.  ACTUAL  CD.  ,  mA/cm2  C o r r o s i o n r a t e measurements on an i n d i v i d u a l anode s u b j e c t e d t o r e p e t i t i v e e l e c t r o l y s i s i n 2M H SC% + 0.5M CuSCK, 22°, w i t h respect t o : (a) t h e g e o m e t r i c area o f p l a t i n u m o n l y , (b) t h e a c t u a l (mean) area o f p l a t i n u m c a l c u l a t e d f o r each r u n (the l i n e r e p r e s e n t s t h e curve i n F i g u r e 6 . 1 9 ) . 2  T a b l e 6.10 P l a t i n u m and I r i d i u m C o r r o s i o n Electrodes  Current Electrolyte  Density  ^geometric' mA_ cm  Total Hours  Time  Corrosion  A-hr  2  52.1  ±  hr-m" 3.5 9  19.2  Pt) Ir)  0.08 0.26  .58 .26  57 441  440 440  1 11  379  75.8  Pt) Ir)  0.65 0.12  .15 .08  1010 390  230 230  25 10  957  95.7  Pt) Ir)  0.59 0.13  .14 .07  451 207  105 105  • 11 5  192  8!1 H S 0 „ , 40°  ii q hr-rrr'  61 61  52.1  104  +  130 363  4M H S0.,, 40°  4  C o r r o s i o n Rate C o r r o s i on Rate (Geometri c A r e a ) \ ( A c t u a l Area)  .08 .04  1540  154  Individual  Acidities  0.17 0.22  52.1  2  of Various  Pt) Ir)  0.5H H , S 0 » , 40°  4M K S 0 , 40°  Efficiency  A-hr  ;  2  Rates from Cumulative E l e c t r o l y s i s Runs w i t h  in Sulfuric Acid Solutions  +  Mean A c t u a l Current Density  Mean C o r r o s i o n Current Densities  mA cm'  cm'  1.30  0.19 0.50  11 11  1.30  0.05 0.61  6 6  2.60  1.37 0.56  1.5 1.5  2.5 2.5  1.30  0.60 0.28  N5 XO  130  Anodes from d i f f e r e n t i n i t i a l initial mA/cm  2  s o u r c e s , and having  n o b l e metal l o a d i n g s , were s u b j e c t e d t o a " s t a n d a r d " (geometric  a r e a ) i n 2M  H2SO4  + 0.5M CuSO^, 22°.  different run a t 52.1  The p l a t i n u m and  i r i d i u m c o r r o s i o n d a t a , summarized i n T a b l e 6.11a and 6.11b, shows t h a t wide v a r i a t i o n s i n t h e c o r r o s i o n r a t e and c o r r o s i o n e f f i c i e n c y v a l u e s can o c c u r , and t h a t i n most cases these v a l u e s are much h i g h e r than those  found  for  As  the anodes (average i n i t i a l  l o a d i n g 4.33 g/m ) used i n t h i s work. 2  w i t h t h e s u r f a c e area d a t a , no c o r r e l a t i o n was found t o e x i s t w i t h the new electrode loading.  Further, the i n d i v i d u a l  d i f f e r e n c e s i n s u r f a c e area o f  the v a r i o u s anodes c o u l d not e x p l a i n the d i f f e r e n t observed c o r r o s i o n r a t e s ( i n d e e d , t h e e l e c t r o d e s w i t h t h e h i g h e s t s u r f a c e a r e a s , and hence the s m a l l e s t a c t u a l a p p l i e d c u r r e n t d e n s i t i e s , showed t h e h i g h e s t c o r r o s i o n rates.  Such i n c o n s i s t e n c i e s i n d i c a t e t h a t the c o r r o s i o n behaviour o f  anodes from d i f f e r e n t m a n u f a c t u r i n g  6.2.4  Pulsed  l o t s may vary  Electrolysis  Several  types o f p u l s e d o p e r a t i o n were employed, i n c l u d i n g an  "optimum" procedure found by L i e k e n s  [34] t o e n a b l e h i g h c u r r e n t d e n s i t y  copper e l e c t r o w i n n i n g (48 mA/cm ) i n an i n d u s t r i a l 2  cell  considerably.  copper e l e c t r o w i n n i n g  (9 seconds on; 0.5 seconds s h o r t e d ) and a high f r e q u e n c y  c u r r e n t p u l s i n g method d e s c r i b e d by I b l [36] as being capable  square-wave of permitting  e x c e l l e n t copper d e p o s i t s a t c u r r e n t d e n s i t i e s as high as 250nnA/cm . I n 2  a d d i t i o n t h e e f f e c t o f o p e r a t i o n a t s l o w e r f r e q u e n c i e s and w i t h o u t s h o r t i n g were i n v e s t i g a t e d .  I n a l l , o n l y seven s e p a r a t e e x p e r i m e n t s  electrode (not  r e p e t i t i v e ) were performed as t h e purpose was o n l y t o d e t e r m i n e whether  Table 6.11a Cumulative  P l a t i n u m C o r r o s i o n Rates f o r I n d i v i d u a l E l e c t r o d e s o f Nominal Pt/30 I r - T i C o m p o s i t i o n , but from D i f f e r e n t M a n u f a c t u r i n g L o t s , i n 2M Applied  Initial Loading  Mean A c t u a l Current Density  g(Pt+Ir) m  mA cm  2.31  H2SO4  + 0.5M CuS0\, 22°, a t 52.1 mA/cm  2  (Geometric) C u r r e n t D e n s i t y  T o t a l Time  Corrosion  Efficiency  Corrosion Rate (Geometric A r e a )  C o r r o s i o n Rate (Actual Area)  hours  Arhr  y9 A»hr  +  4.84  191  19.1  1.51  .30  1160  240  109  21  3.31  2.58  191  19.1  0.75  .44  582  340  29  17  3.56  2.47  197  19.7  2.91  .35  2780  340  132  16  4.01*  1 .96  2634  263  0.93  .03  726  26  27.5  1.5  1.97  1848  185  0.98  .04  747  36  28  1 .5  5. 36  0.87  .191  19.1  2.62  .71  2020  540  34  9  11.26  0.63  379  37.9  5.32  .73  4080  560  50  6  19.19  0.40  605  60.5  2.97  .80  2270  610  18  4  2  * 4.36  2  Cut from t h e sheet  hr«m  2  +  hr «m  employed as source o f t h e bulk o f the anodes f o r t h i s work (4.33 g/m ). 2  2  +  Table 6.11b C u m u l a t i v e I r i d i u m C o r r o s i o n Rates f o r I n d i v i d u a l E l e c t r o d e s o f Nominal from D i f f e r e n t M a n u f a c t u r i n g L o t s , i n 2M H S 0 2  Pt/30 I r - T i C o m p o s i t i o n , but  + 0.5M CuSO^, 22°, a t 52.1 mA/cm  2  4  A p p l i e d (Geometric) C u r r e n t D e n s i t y Initial Loading  Mean A c t u a l Current Density  g(Pt+Ir) m  mA cm  2.31  T o t a l Time  Corrosion  Efficiency  Corrosion (Geometric  Rate Area)  C o r r o s i o n Rate (Actual Area) ug hr«m  +  hours  A-hr  4.84  191  19.1  0.62  .14  990  240  93  21  3.31  2.58  191  19.1  0.26  .21  412  340  20  17  3.56  2.47  197  19.7  2.04  .30  2340  340  111  16  *  1.96  2634  263  0.31  .02  487  26  20  1.5  *  185  0.39  .02  632  36  26  1.5  2  4.01  2  hr«m  hr«m  2  2  2  4.36 5.36  1.97  1848  0.87  191  19.1  0.90  .34  1450  540  24  9  11.26  0.63  379  37.9  2.07  .34  3350  560  41  6  19.19  0.40  605  60.5  1 .56  .37  2540  610  19  4  * Cut from t h e sheet employed as source o f t h e bulk o f the anodes f o r t h i s work (4.33 g/m ). 2  CO  133  such t r e a t m e n t s All  had  adverse or b e n e f i c i a l e f f e c t on e l e c t r o d e performance.  r u n s , save one, were c a r r i e d out i n 2M H S 0 i , + 0.5M  CuS0\.  2  Coating reversal  (runs  l o s s v a l u e s are p r e s e n t e d  ( a ) - ( e ) ) was  coatings platinum.  l a r g e l o s s e s being found i n a l l c a s e s .  a t t a c k o f the p l a t i n u m  i s a l s o e v i d e n t , w i t h the  Higher a p p l i e d current  ( d u r i n g t^)  p a r t i a l - c y c l e s enhances the c o a t i n g l o s s e s .  i s e f f e c t i v e i n reducing e i t h e r the  Operation  gave h i g h e r c o a t i n g l o s s e s than an e q u i v a l e n t run e l e c t r o l y t e a t 22°.  Operation  with  ^QN^SHORTED  h i g h e s t c o a t i n g metal l o s s e s o f a l l the p u l s e d Operation  =  i n 2M  2  40°,  4  copper-containing  ^  sec/76 sec gave the investigated.  w i t h o p e n - c i r c u i t c o n d i t i o n s d u r i n g the  "off-partial  not as d e l e t e r i o u s as t h a t w i t h s h o r t - c i r c u i t i n g , but c o a t i n g  durrent operation.  The  high-frequency  operation  direct  (^Q[\J/^OPEN ~ ^'^  ] i s e c  usee) gave the l e a s t - a d v e r s e r e s u l t s o f a l l the p u l s e d c u r r e n t  cedures employed. a b l y l e s s harmful  Run  ( g ) , where tfjN^OPEN  than the e q u i v a l e n t run  F u r t h e r , o n l y i n run  Corrosion  =  ^  sec/76 s e c , was  (e) w i t h  pro-  consider-  short-circuiting.  occur.  r a t e a n a l y s i s was  corrosion efficiencies  approached from s e v e r a l  (ug/A^hr) and  current operation.  viewpoints.  c o r r o s i o n r a t e s (ug/hr*m ) were  c a l c u l a t e d both w i t h r e s p e c t t o the t o t a l o p e r a t i n g t i m e and a p p l i e d anodic  ^  (g) d i d an i n s i g n i f i c a n t v a r i a t i o n i n the w e i g h t  f r a c t i o n s o f the c o a t i n g metals  The  H S0 ,  in  techniques  the  o r tc^Qpjyfrn  metal l o s s e s were a l s o found to be h i g h e r than i n c o n t i n u o u s  1.6  remaining  i n a l l cases showing h i g h e r w e i g h t f r a c t i o n s o f i r i d i u m than o f  c o a t i n g metal l o s s e s somewhat, but l e n g t h e n i n g  c y c l e was  Periodic current  found to be h i g h l y d e l e t e r i o u s towards both  c o a t i n g metals, with extremely Preferential  i n T a b l e 6.12.  2  the t i m e o f  W h i l e the former i s o f more p r a c t i c a l  T a b l e 6.12 Noble Metal L o a d i n g Data f o r E l e c t r o d e s S u b j e c t e d t o P u l s e d E l e c t r o l y s i s i n 2.0M H S 0 2  0.5M CuOo,,, 22° (Except  'ON  'SHORTED  'OPEN  (a)  9 sec  0.5 sec  -  (b)  9 sec  0.5 sec  -  (c)  9 sec  0.5 sec  -  (d)  60 sec  0.5 sec  (e)  76 sec  (f)  4.4 psec  -  (s)  76 sec  -  76 sec  1.6  Current Density (Geometric) mA/cm 2  52.1  Run Time Hours  k  +  ( c ) : 2M H S0<., 40°) 2  Initial Loadings g/m 2  Final Loadings g/m 2  Inital w  Pt  Final w  Pt  Metal L o s s / C y c l e ( A c t u a l Area) ug/m- C y c l e 2  Mean A c t u a l R.F.  194  Pt) Ir)  3.007 1.374  0.291 0.828  0.685  0.260  3.25 1.08  24  192  Pt) Ir)  3.263 1.476  0.421' 1.008  0.689  0.295  3.31 0.53  24  52.1  194  Pt) Ir)  3.171 1.442  0.119 0.323  0.687  0.269  3.78 1.27  23  -  52.1  237  Pt) Ir)  2.755 1.250  0.028 0.235  0.688  0.106  22.14 5.43  22  -  52.1  Pt) Ir)  2.778 1.366  0.C81 0.085  0.670  0.489  104  ysec  76 sec  47.6  187 123  22  26.0  503  Pt) Ir)  2.285 1.091  1.768 0.968  0.677  0.646  1.3(10)6.1(10)"°  20  52.1  204  Pt) Ir)  3:174 1.369  2.673 1.131  0.699  0.703  3.70 4.12  40  7  135  concern f o r e s t i m a t i n g c o r r o s i o n l o s s e s f o r i n d u s t r i a l i s more r e a l i s t i c and  anodes, the l a t t e r  i f an anodic d i s s o l u t i o n mechanism i s being  for calculation of partial  (corrosion) current densities.  data a r e c a l c u l a t e d w i t h r e s p e c t t o the mean f r a c t i o n a l actual  s u r f a c e areas o c c u p i e d  summarized i n T a b l e 6.13.  considered  by t h e c o a t i n g m e t a l s .  A l l rate  g e o m e t r i c and  The r e s u l t s a r e  As can be seen t h e c o r r o s i o n r a t e and e f f i c i e n c y  data i n a l l cases a r e c o n s i d e r a b l y h i g h e r than t h a t found f o r continuous d i r e c t c u r r e n t o p e r a t i o n under o t h e r w i s e  similar conditions.  In a l l c a s e s ,  e x c e p t run ( g ) , the r a t e o f i r i d i u m c o r r o s i o n i s s i g n i f i c a n t l y s m a l l e r than that of platinum.  C o r r o s i o n c u r r e n t e f f i c i e n c e s , d e s p i t e being much h i g h e r  than t h a t observed f o r c o n t i n u o u s l e s s , no g r e a t e r than 2 . 0 5 ( 1 0 ) ~ The e f f e c t o f i n i t i a l c o r r o s i o n behaviour  d i r e c t c u r r e n t o p e r a t i o n were,  neverthe-  per c e n t f o r p l a t i n u m i n the w o r s t  5  case.  p u l s e d c u r r e n t o p e r a t i o n on the subsequent  o f an anode d u r i n g prolonged  o p e r a t i o n was a l s o i n v e s t i g a t e d .  constant applied current  For t h i s p u r p o s e , t h e e l e c t r o d e used i n  p u l s e d c u r r e n t run (a) was s u b j e c t e d t o f u r t h e r o p e r a t i o n a t 52.1 mA/cm  2  (geometric  a r e a ) i n 2M H Su\, 22°, f o r 258 hours. 2  The l o a d i n g and c o r r o s i o n  r a t e data f o r t h i s p a r t i c u l a r anode a r e g i v e n i n T a b l e 6.14.  I t can be  seen t h a t t h e c o r r o s i o n r a t e s , whether r e f e r r e d t o g e o m e t r i c o r a c t u a l p a r t i a l areas o f t h e i n d i v i d u a l  c o a t i n g m e t a l s , a r e much h i g h e r than  encountered w i t h new anodes s u b j e c t e d t o s i m i l a r o p e r a t i o n  (without  those prior  p u l s e d c u r r e n t t r e a t m e n t ) , and these h i g h e r c o r r o s i o n r a t e s a r e r e f l e c t e d in the high c o r r o s i o n e f f i c i e n c y values.  Platinum continued  t o be s e l e c -  t i v e l y l o s t from t h e c o a t i n g , as was t h e case d u r i n g p u l s e d c u r r e n t t i o n , l e a d i n g t o f u r t h e r i r i d i u m "enrichment" o f t h e remaining metal.  opera-  coating  Table  6.13  C o r r o s i o n Rate Data f o r E l e c t r o d e s S u b j e c t e d t o P u l s e d E l e c t r o l y s i s . are A n a l y z e d w i t h R e s p e c t t o the T o t a l Time, t  Run Time, t  t  o  a  l  ,  (Geometric Area) pg/hr-m 2  C o a t i n g L o s s Measurements  the A n o d i c P a r t i a l - C y c l e  , and w i t h R e s p e c t to t h e G e o m e t r i c and A c t u a l  Surface Area Values  Corrosion c.d.  C o r r o s i o n Rate Corrosion E f f i c i e n c y pg/A-hr  t  (Actual Area) pg/hr-m 2  (Geometric A r e a ) nA/cm 2  (Actual Area) nA/cm  Corrosion Current  Efficiency  2  %  (ton>  «J  (a)  Pt) Ir)  26 88 5 40  28 34 5 70  29600 5340  31200 5630  1233 • 223  1300 235  1715 314  71 13  3 6 6 0 29(10)3(10)"  (b)  Pt) Ir)  14 21 2 34  14 99 2 44  30100 4800  31700 5000  1309 198  1320 209  1740 279  73 3.0  1 67(10)" 2 68(10)"  (c)  Pt) Ir)  30 21 9 90  31 85 10 43  32900 9380  34700 10400  1430 430  1509 453  1907 581  83 25  3 66(10)" 1 12(10)"  (d)  Pt) Ir)  22 09 8 22  22 28 8 30  109000 7100  110000 7170  4940 323  4980 326  6020 400  273 18  1 5 7 6S(10)-  (e)  P t ) 109 Ir) 41 7  97800 64100  196000 128000  17900 11600  8890 5820  10700 7150  488 325  2 05(10)" 1 37(1OJ"  (f)  Pt) Ir)  3 95 0 94  5 38 1 28  1550 723  2120 985  77 36  105 49  116 55  5.8 2.8  4 46(1OJ" 2 12(10)"'  (g)  Pt) Ir)  4 72 2 24  9 43 4 48  3500 3900  7010 7800  350 390  175 195  385 435  9.5 10.9  7 39(10) 8 35(10)"  ( t  totai  ('on'  218 103  (  ttotal)  ''on) 7  6 7  6 6  16(10r 7  5 5  7  - 7 7  137  T a b l e 6.14 Loading and C o r r o s i o n Rate Data f o r an Anode, P r e v i o u s l y S u b j e c t e d t o Pulsed Current Operation  Described  i n T a b l e s 6.12 and 6.13,  Case ( a ) , S u b s e q u e n t l y Operated under Continuous Anodic Current Conditions  i n 2M H2SO4,  f o r 258 Hours  Current  d e n s i t y (geometric  area)  Current  d e n s i t y (mean a c t u a l a r e a )  52.1 mA/cm  2  7.6  Ampere«hours Initial Final  25.8  total  total  loading (Pt + I r )  1.12  loading (Pt + Ir)  0.37  Platinum Initial Final  loading  loading  Initial  (g/m ) 2  (g/m ) 2  weight f r a c t i o n  Final weight f r a c t i o n Corrosion e f f i c i e n c y  (ug/A h r )  Corrosion rate with respect to g e o m e t r i c area (ug/hr«m ) 2  C o r r o s i o n r a t e w i t h r e s p e c t t o mean a c t u a l area (ug/hr«m ) 2  P a r t i a l (corrosion) current w i t h r e s p e c t t o mean a c t u a l (nA/cm ) 2  density area  g/m  2  g/m  2  Iridium  0.291  0.828  0.041  0.331  0.260  0.740  0.111  0.889  .1.86  3.70  5220  2470  770  350  42  20  138  6.2.5  A d d i t i v e and Contaminant E f f e c t s T h i o u r e a was chosen f o r an a d d i t i o n agent i n s i m u l a t e d e l e c t r o -  w i n n i n g experiments i n 2M H S 0 2  + 0.5M CuS0\, 22°, w i t h 52.1 mA/cm  2  4  (geometric area) a p p l i e d c u r r e n t d e n s i t y . In  the f i r s t e x p e r i m e n t , the e l e c t r o l y t e was prepared w i t h  0.5 g p l t h i o u r e a a d d i t i o n , f o l l o w i n g by commencement o f the run u s i n g an anode o f i n i t i a l failure  total  noble metal l o a d i n g o f 4.53 g/m . 2  o c c u r r e d a f t e r o n l y 23 hours' o p e r a t i o n .  thiourea addition level l o a d i n g 4.46 g/m ) 2  A second run w i t h a lower  (.05 g p l ) and a n o t h e r new anode ( t o t a l n o b l e metal  a l s o ended i n c a t a s t r o p h i c f a i l u r e w i t h i n 160 h o u r s .  A t h i r d e x p e r i m e n t , w i t h y e t a n o t h e r new anode ( t o t a l 3.18 g/m ) 2  C a t a s t r o p h i c anode  noble metal  loading  from a n o t h e r s o u r c e than the o t h e r s , but whose c o r r o s i o n  behaviour  was found to be s i m i l a r , was commenced w i t h o u t p r i o r a d d i t i o n o f the thiourea.  A f t e r 18 h o u r s , t h i o u r e a was added t o the l e v e l o f .05 g p l and  the e l e c t r o l y s i s c o n t i n u e d f o r an a d d i t i o n a l 74 hours, a t which time the run was s t o p p e d .  A l t h o u g h the anode was o p e r a t i n g a t a v e r y h i g h p o t e n t i a l  on t e r m i n a t i o n (9.05v SHE) f a i l u r e had not o c c u r r e d .  N e v e r t h e l e s s i t was  p r o j e c t e d , from the r a t e o f r i s e o f the anode p o t e n t i a l , t h a t f a i l u r e was imminent. T a b l e 6.15.  Loading and c o r r o s i o n r a t e d a t a f o r t h i s anode a r e g i v e n i n C o r r o s i o n r a t e s were c a l c u l a t e d on the assumption t h a t the  c o r r o s i o n b e h a v i o u r p r i o r t o the t h i o u r e a a d d i t i o n f o l l o w e d t h e r e l a t i o n s in  F i g u r e s 6.19 and 6.20  (which lowered the i n i t i a l  and i r i d i u m by 0.013 and 0.003 g/m , 2  respectively).  loadings of platinum On t h i s b a s i s i t can  be seen t h a t both c o a t i n g metals a r e l o s t a t much g r e a t e r r a t e s than f o r  until  The a n o d e p o t e n t i a l r o s e r a p i d l y a t an a c c e l e r a t i n g t h e v o l t a g e l i m i t (50v) o f t h e power s u p p l y was e x c e e d e d .  rate  139  Table 6.15 Loading and C o r r o s i o n Rate Data f o r an Anode Operated a t 52.1 mA/cm  2  (geometric  area) i n 2 M H S 0 2  + 0.5M C u S 0 , 22°, w i t h  4  4  T h i o u r e a A d d i t i o n a f t e r 18 hours'  .05 g p l  Operation.  E l e c t r o l y s i s was Terminated a f t e r an A d d i t i o n a l 74 Hours Initial  total  loading  Estimated t o t a l a f t e r 18 hours Final  total  (Pt + I r )  loading (Pt + I r )  loading (Pt + I r )  Measured f i n a l  roughness f a c t o r  Mean a c t u a l c u r r e n t  density  F i n a l weight  (g/m ) 2  2  fraction  fraction  Corrosion e f f i c i e n c y  g/m  2.654  g/m  2  2  2.2  (g/m )  I n i t i a l weight  3.164  2  6.2  Loading p r i o r t o t h i o u r e a a d d i t i o n loading  g/m  14.7  Ampere • hours  Final  3.180  (ug/A*hr)  Corrosion r a t e with respect to geometric (yg/hr-m )  area  2  C o r r o s i o n r a t e w i t h r e s p e c t t o mean a c t u a l area (yg/hr*m ) 2  P a r t i a l (corrosion) current density with r e s p e c t t o mean a c t u a l area (nA/cm ) 2  Platinum  Iridium  2.134  1.030  1.843  0.812  0.674  0.326  0.694  0.306  7.55  5.66  5740  9350  675  1100  37  61  140  the case o f e l e c t r o l y s i s under s i m i l a r c o n d i t i o n s but w i t h o u t a d d i t i o n and The  t h a t i r i d i u m , r a t h e r than p l a t i n u m , i s s e l e c t i v e l y  attacked.  p o o r e r c o r r o s i o n r e s i s t a n c e o f i r i d i u m has p r e v i o u s l y been noted i n  organic-containing electrolytes The winning  [107].  e f f e c t o f entrapment o f o r g a n i c s o l v e n t i n the  c i r c u i t , such as may  o p e r a t i o n , was  electro-  occur i n a s o l v e n t e x t r a c t i o n / e l e c t r o w i n n i n g  i n v e s t i g a t e d w i t h the a d d i t i o n o f 3 volume per c e n t  to the e l e c t r o l y t e (2M H S 0 2  two  thiourea  4  + 0.5M  CuS0 ). 4  The  kerosene  i m m i s c i b i 1 i t y of  these  phases, d e s p i t e the use of r a p i d magnetic s t i r r i n g , r e s u l t e d i n the  f o r m a t i o n o f a l a y e r o f kerosene on top o f the aqueous phase.  No  significant  e f f e c t s o f the o r g a n i c s o l v e n t a d d i t i o n were found a f t e r 479 hours'  opera-  t i o n a t 52.1  but  mA/cm  2  (geometric  the i r i d i u m l o s s r a t e was  a r e a ) f o r the c o r r o s i o n o f p l a t i n u m ,  over t h r e e times g r e a t e r than t h a t o f  C o r r o s i o n e f f i c i e n c i e s were 0.82 and  iridium, respectively.  ± 0.18  and  1.17  ± 0.08  platinum.  yg/A-hr f o r  A much g r e a t e r number o f e x p e r i m e n t s would  necessary,  however, t o determine whether t h i s r e s u l t i s t y p i c a l .  Individual  runs i n 2M h^SO^  or i n 2M H S 0 2  4  + 0.5M  CuS0  4  6.2.6  not manifested  i n the c u m u l a t i v e  behaviour  corrosion data).  Morphology Changes S.E.M. o b s e r v a t i o n o f anode s u r f a c e s r e v e a l e d  changes i n morphology o c c u r r e d o n l y a f t e r l o s s o f 10-20 coating metal.  significant  per c e n t o f the  O t h e r w i s e , the anode s u r f a c e s c o u l d not be d i s t i n g u i s h e d  from new, r e g a r d l e s s of the i n t e n s i t y o f the a p p l i e d c u r r e n t . 6.25  be  o f t e n showed s l i g h t l y  g r e a t e r i r i d i u m c o r r o s i o n r a t e s or c o r r o s i o n e f f i c i e n c i e s , but t h i s was  platinum  show the s u r f a c e morphologies o f anodes having  Figures  6.22-  progressively greater  141  F i g u r e 6.22.  SEM v i e w o f the s u r f a c e o f an anode a f t e r o p e r a t i o n a t 104 mA/cm (geometric area) i n 2M H 2 S O 4 , 22°, f o r a t o t a l o f 957 h o u r s . T o t a l remaining l o a d i n g : 2.63 g/m . C o a t i n g l o s s : 32 p e r c e n t . (lOOOx) 2  2  F i g u r e 6.23.  SEM view o f t h e s u r f a c e o f an anode a f t e r o p e r a t i o n a t 52.1 mA/cm (geometric area) i n 2M H S 0 + 0.5M C u S 0 , 22° f o r a t o t a l o f 1848 h o u r s . T o t a l remaining l o a d i n g : 2*70 g/m . Coating l o s s : 33 per c e n t . (lOOOx)  2  2  4  4  2  F i g u r e 6.24.  SEM view o f t h e s u r f a c e o f an anode operated a t 260 mA/cm (geometric a r e a ) i n 2M H 2 S O 4 , 22°C, f o r a t o t a l o f 455 hours up to imminent anode f a i l u r e . F i n a l anode p o t e n t i a l : 47 v o l t s v s . SHE. T o t a l remaining l o a d i n g : 0.62 g/m*. C o a t i n g l o s s : 85 per c e n t . (lOOOx)  F i g u r e 6.25.  SEM view o f t h e s u r f a c e o f an anode (geometric area) i n 2M ti S0 , 22°C, showing evidence o f c o a t i n g l o s s by loading: 3.8 g/m . C o a t i n g l o s s :  2  2  2  k  operated a t 52.1 mA/cm f o r a t o t a l o f 385 hours, s p a l l i n g . T o t a l remaining 17 p e r c e n t . (lOOOx) 2  143  percentage c o a t i n g metal l o s s e s .  F i g u r e 6.24 shows t h e e x t e n s i v e d e t e r i o -  r a t i o n o f an anode under c o n d i t i o n s o f imminent anode f a i l u r e  (operating  p o t e n t i a l was 47 v o l t s vs. SHE). F i g u r e 6.25 i s the s u r f a c e morphology o f t h e anode d e s c r i b e d i n F i g u r e 6.14 and T a b l e 6.9 where t h e c o r r o s i o n r e s u l t s s t r o n g l y suggest a s p a l l i n g mechanism f o r c o a t i n g  loss.  Electrodes subjected t o pulsed e l e c t r o l y s i s c l e a r l y t h e i r e x t e n s i v e c o a t i n g l o s s e s under S.E.M. o b s e r v a t i o n . the s u r f a c e o f an anode o p e r a t e d  a t 52.1 mA/cm  2  F i g u r e 6.26 shows  (geometric  a r e a ) f o r 194  hours under c o n d i t i o n s o f 9 seconds on/0.5 second s h o r t e d (a)).  Although  not m a n i f e s t  revealed  (see Table 6.12;  74 per c e n t o f t h e c o a t i n g has been l o s t , t h e s u r f a c e does  t h e pronounced d e t e r i o r a t i o n o f anodes operated  under  continuous  d i r e c t c u r r e n t c o n d i t i o n s and which have c o n s i d e r a b l y more r e s i d u a l c o a t ing.  F i g u r e 6.27 shows t h e same e l e c t r o d e a f t e r a f u r t h e r 258 hours o f  o p e r a t i o n under a c o n t i n u o u s  a n o d i c c u r r e n t o f 52.1 mA/cm . 2  The r e s i d u a l  c o a t i n g metal i s t h i n l y d i s p e r s e d over t h e s u r f a c e where presumably once much t h i c k e r c o a t i n g " p l a t e s " e x i s t e d .  The most d e l e t e r i o u s p u l s e d  current  o p e r a t i n g c o n d i t i o n - 76 seconds on/76 seconds s h o r t e d - produced t h e l o s s o f 97 p e r c e n t o f t h e o r i g i n a l c o a t i n g metal a f t e r 47.6 hours a t 52.1 mA/cm  2  (geometric  area).  S.E.M. o b s e r v a t i o n r e v e a l e d e s s e n t i a l l y no c o a t -  i n g metal over l a r g e areas o f t h e anode s u r f a c e , w i t h t h e r e m a i n i n g present  i n h i g h l y - d e t e r i o r a t e d "patches" Thiourea  c o a t i n g metal  metal  ( F i g u r e 6.28).  a d d i t i o n caused a r e a d i l y - a p p a r e n t a t t a c k o f t h e  ( F i g u r e 6.29) s e l e c t e d area X-ray energy s p e c t r o s c o p y i n  c o n j u n c t i o n w i t h t h e S.E.M. d i d n o t r e v e a l t h e presence o f s u l f u r on t h e degraded s u r f a c e .  144  F i g u r e 6.26.  SEM view o f the s u r f a c e o f an anode s u b j e c t e d t o p u l s e d e l e c t r o l y s i s (9 sec on/0.5 s e c s h o r t e d ) a t 52.1 mA/cm ( g e o m e t r i c area) i n 2M H S0^ + 0.5M C u S 0 , 22°, f o r 194 h o u r s . T o t a l r e m a i n i n g l o a d i n g : 1.12 g/m . C o a t i n g l o s s : 74 per c e n t . (lOOOx) 2  2  4  2  F i g u r e 6.27.  SEM view o f the anode d e s c r i b e d i n F i g u r e 6.26, a f t e r further o p e r a t i o n a t a c o n t i n u o u s a n o d i c c u r r e n t o f 52.1 mA/cm (geom e t r i c a r e a ) i n 2M H SQ + 0.5M C u S 0 , 22", f o r 258 hours. T o t a l r e m a i n i n g l o a d i n g : 0.37 g/m . C o a t i n g l o s s : 92 p e r cent. (lOOOx) 2  2  k  4  2  F i g u r e 6.28.  SEM view o f t h e s u r f a c e o f an anode s u b j e c t e d t o p u l s e d e l e c t r o l y s i s (76 sec on/76 sec s h o r t e d ) a t 52.1 mA/cm ( g e o m e t r i c a r e a ) i n 2M H S0\ + 0.5M CuS0\, 22°, f o r 47.6 hours. T o t a l r e m a i n i n g l o a d i n g : 0.17 g/m . C o a t i n g l o s s : 97 p e r cent.(IOOOx) 2  2  2  F i g u r e 6.29.  SEM view o f the s u r f a c e o f an anode o p e r a t e d a t 52.1 mA/cm ( g e o m e t r i c area) i n 2M H S 0 + 0.5M CuSO-, 22°, f o r 92 hours, w i t h t h e a d d i t i o n o f 0.5 gpl t h i o u r e a a f t e r 18 hours. T o t a l r e m a i n i n g l o a d i n g : 2.65 g/m . C o a t i n g l o s s : 17 p e r cent.(40O0x) 2  2  4  2  146  6.3  Passivation  6.3.1  P o t e n t i a l v s . Time B e h a v i o u r  6.3.1.1  New Pt/30 I r - T i Anodes A rapid increase  i n anode p o t e n t i a l i s observed on commence-  ment o f e l e c t r o l y s i s a t a g i v e n c u r r e n t s o l u t i o n s , where oxygen e v o l u t i o n 6.30).  The r a t e o f i n c r e a s e  density  in sulfuric  i s t h e p r i m a r y anode p r o c e s s  at a small, nearly  constant rate.  current  observed.  density,  a f u r t h e r rapid increase  time,  i s seen t o  As t h e anode p o t e n t i a l  above about 2.2 v o l t s v s . SHE, a v a l u e which i s a t t a i n e d applied  (Figure  o f the anode p o t e n t i a l d e c r e a s e s w i t h  however, such t h a t a f t e r about one day t h e anode p o t e n t i a l increase  acid-containing  climbs  sooner w i t h  higher  i n anode p o t e n t i a l i s  T h i s phenomenon i n d i c a t e s t h e commencement o f t h e s i m u l t a n e o u s  g e n e r a t i o n o f ozone o r p e r s u l f a t e s . copper e l e c t r o w i n n i n g  applications  The p o t e n t i a l ' r e g i o n  of i n t e r e s t i n  l i e s below t h e 2.2 v o l t p o t e n t i a l  Immediately on a p p l i c a t i o n o f an a n o d i c c u r r e n t Pt/30 I r - T i  anode which was i n i t i a l l y a t i t s r e s t p o t e n t i a l  value.  t o a new (typically  0.8-0.84 v o l t s v s . SHE), and a f t e r commencement o f oxygen e v o l u t i o n , t h e p o t e n t i a l i s o b s e r v e d t o obey.a l i n e a r r e l a t i o n w i t h t h e l o g a r i t h m time ( F i g u r e  6.31).  The s l o p e o f the dependence o f p o t e n t i a l on l o g t  was found t o be, i n most c a s e s , 0.030 ± .005. p o t e n t i a l could  of the  In g e n e r a l ,  hence, the anode  be e x p r e s s e d by:  E = CONSTANT + 0.03 l o g t  (4)  Such dependence does n o t h o l d , however, beyond 1000-10,000 s e c o n d s , where the  anode p o t e n t i a l begins t o d e v i a t e upwards from t h e l o g a r i t h m i c  relation.  F i g u r e 6.30.  V a r i a t i o n o f anode p o t e n t i a l w i t h time f o r i n d i v i d u a l anodes operated a t v a r i o u s c u r r e n t dens i t i e s (geometric a r e a ) i n 2M H S0^ + 0.5M C u S 0 , 22°, (a) 15.6; (b) 30.0; ( c ) 52.1; (d) 117; (e) 260 mA/cm . 2  2  4  -p=»  148  T h i s b e h a v i o u r i s shown i n F i g u r e 6.32, time b e h a v i o u r o f new  which i s t y p i c a l o f the p o t e n t i a l /  electrodes.  Many k i n d s o f e m p i r i c a l mathematical  r e l a t i o n s can be used t o  d e s c r i b e c u r v e s o f t h i s form, i n c l u d i n g " p a r a b o l i c " o r " c u b i c " r a t e laws commonly employed i n o x i d a t i o n s t u d i e s .  The e x p e r i m e n t a l data c o u l d  be  b e s t f i t to an e q u a t i o n o f the form:  E = Ci + c t ' 0  + 0.03  5  2  log t  (5)  which r e p r e s e n t s s i m u l t a n e o u s l o g a r i t h m i c and p a r a b o l i c processes and where C i and c  2  are c o n s t a n t s .  Such an e q u a t i o n a c c u r a t e l y p r e d i c t s the  initial  l i n e a r l o g a r i t h m i c dependence o f the p o t e n t i a l as observed i n F i g u r e and 6.32,  6.31  and a l s o the s t e a d y r i s e i n anode p o t e n t i a l w i t h time which  t i n u e s a f t e r hundreds o f hours of e l e c t r o l y s i s . every achieved.)  Table 6.16  (No "steady s t a t e " i s  summarizes t h e p o t e n t i a l / t i m e b e h a v i o u r o f  Table  6.16  t x p r e s s i o n o f the P o t e n t i a l v s . Time B e h a v i o u r o f New i n 2M HzSO., + 0.5M  con-  Anodes Operated  CuSO,,, 22°, a t V a r i o u s C u r r e n t D e n s i t i e s  A c c o r d i n g t o the E q u a t i o n : E = Ci + c t 0  2  Applied Current Density (Geometric Area) mA  Ci  5  + .03 l o g t  c  P r e d i c t e d Time t o Reach 2 . 2 . v o l t s Days  2  cm  2  15.6  1.610  7.11(10)-  52.1  1.712  1.19(10)  1.892  1.82(10)-  117  5  310  _,t  66  4  5.8  149  F i g u r e 6.31.  I n i t i a l p o t e n t i a l v s . time b e h a v i o u r f o r new anodes o p e r a t e d a t various a p p l i e d current d e n s i t i e s (with respect to geometric a r e a ) i n 2M + 0.5M CuSO*, 22°.  H2SCH  F i g u r e 6.32.  Semi l o g a r i t h m i c p l o t o f t h e change i n anode p o t e n t i a l w i t h time f o r a new anode o p e r a t e d a t 15.6 mA/cm ( g e o m e t r i c a r e a ) i n 2M H S0\ + 0 . 5 M CuSG\, 22°. 2  2  cn o  151  several  new e l e c t r o d e s o p e r a t e d a t d i f f e r e n t a p p l i e d  current d e n i s i t i e s ,  w i t h r e s p e c t t o the parameters i n e q u a t i o n ( 5 ) .  6.3.1.2  Repetitive  Electrolysis  Two types o f r e v e r s i b l e  potential  t i v e e l e c t r o l y s i s can be d i s t i n g u i s h e d , attains during p o l a r i z a t i o n . e l e c t r o d e s whose p o t e n t i a l where the p o t e n t i a l  v s . time b e h a v i o u r on r e p e t i -  depending on t h e v a l u e the p o t e n t i a l  These a r e p r e s e n t e d i n F i g u r e 6.33.  remains below t h e v a l u e o f 2.2 v o l t s  For  (SHE)  "jump" o c c u r s , r e p e t i t i v e e l e c t r o l y s i s ( w i t h  break-up  o f t h e e l e c t r o d e , X-ray s p e c t r o s c o p y , X-ray d i f f r a c t i o n , S.E.M. and e l e c t r o d e re-construction  between r u n s ) g i v e s e s s e n t i a l l y t h e same p o t e n t i a l v s .  time b e h a v i o u r . the  potential  T h i s b e h a v i o u r i s termed " r e v e r s i b l e . "  "jump" o c c u r s , an i n d u c t i o n  included e n t i r e l y within of p o t e n t i a l )  the i n i t i a l  For anodes where  p e r i o d (which may o r may not be  r u n , depending on t h e r a t e o f c l i m b  e x i s t s p r i o r t o the "jump."  S u b s e q u e n t l y , the  induction  p e r i o d i n r e p e t i t i v e runs i s c o n s i d e r a b l y d i m i n i s h e d , a l t h o u g h i t never vanishes.  A g a i n , however, the p o t e n t i a l  essentially reversible duplicates.  v s . time b e h a v i o u r can be termed  f o r t h i s c a s e , as r e p e t i t i v e runs p r a c t i c a l l y a r e  I t i s i n t e r e s t i n g t o note t h a t the r e v e r s i b l e  potential vs.  time b e h a v i o u r f o r a g i v e n anode does not show any dependence on the noble metal l o a d i n g ,  6.3.1.3  and hence the a c t u a l  Irreversible  surface area.  Behaviour  Prolonged anodic e l e c t r o l y s i s w i l l an  i n c r e a s e o f anode p o t e n t i a l  eventually•result  i n not o n l y  above 2.2 v o l t s , where the p o t e n t i a l  "jump"  T  1  1  1  1  1  r  F i g u r e 6.33. Anode p o t e n t i a l b e h a v i o u r f o r i n d i v i d u a l anodes s u b j e c t e d t o r e p e t i t i v e e l e c t r o l y s i s a t c o n s t a n t —• a p p l i e d c u r r e n t d e n s i t y (geometric s u r f a c e a r e a ) . Top: 117 ma/cm i n 2M H S0\ + 0.5M CuS0\, 22°. ro Bottom: 52.1 mA/cm i n 2M ti SQ* + 0.5M C L I S O ^ , 40°. I n i t i a l and f i n a l noble metal l o a d i n g s f o r the r e s p e c t i v e anodes a r e i n d i c a t e d on t h e diagram ( g ( P t + I r ) / m ) . 2  2  2  2  2  153  o c c u r s , but to much h i g h e r v a l u e s not n o r m a l l y encountered i n o p e r a t i o n noble metal anodes ( F i g u r e 6.34). potential  vs. time behaviour  F u r t h e r , on r e p e t i t i v e runs the  of  previous  i s not d u p l i c a t e d , but r a t h e r the anode  p o t e n t i a l on commencement o f the r e p e t i t i v e run i s c l o s e to the p o t e n t i a l o f the p r e v i o u s run.  final  Such an e l e c t r o d e e x h i b i t s " i r r e v e r s i b l e "  behaviour. The  l e n g t h o f time t h a t an anode w i l l c o n t i n u e to support  the  c u r r e n t demanded o f i t depends on the magnitude o f the a p p l i e d c u r r e n t density.  In one  area) operated 2M  H2SO4  p a r t i c u l a r c a s e , an anode o p e r a t e d  a t 52.1  f o r o v e r 700 hours a t p o t e n t i a l s above 4.0  + 0.5M  C u S 0 , 22°, 4  much s h o r t e r times.  (geometric  2  v o l t s (SHE)  p r i o r to complete d e g r a d a t i o n .  c u r r e n t d e n s i t i e s the anode w i l l  mA/cm  At  in  higher  sustain high-potential operation f o r  Continued p o l a r i z a t i o n under i r r e v e r s i b l e c o n d i t i o n s  e v e n t u a l l y r e s u l t s i n a steep r i s e i n p o t e n t i a l w i t h t i m e , which i s i n d i c a t i v e o f imminent anode f a i l u r e behaviour  o f the f i n a l  ( F i g u r e 6.35).  Pulsed The  p o t e n t i a l vs. time  run f o r the anode d e s c r i b e d i n F i g u r e 6.35  on an expanded t i m e - s c a l e i n F i g u r e  6.3.1.4  The  i s shown  6.36.  Electrolysis  adverse e f f e c t s o f p u l s e d e l e c t r o l y s i s on the c o r r o s i o n of  the c o a t i n g metals has a l r e a d y been noted.  The  e f f e c t s on the anode  p o t e n t i a l are s i m i l a r l y d e l e t e r i o u s , f o r the case o f p e r i o d i c c u r r e n t reversal.  Anode p o t e n t i a l vs. time curves  p l o t t e d u s i n g the f i n a l  ( F i g u r e s 6.37  and 6.38)  p o t e n t i a l a t t a i n e d i n each " o n - c y c l e . "  are  For  the  case o f o p e r a t i o n a t 9 sec.on/0.5 sec. s h o r t e d , the p o t e n t i a l "jump" occurs  r e l a t i v e l y soon ( w i t h i n ten hours as opposed to about 2000 hours  F i g u r e 6.34.  P o t e n t i a l change w i t h time f o r an anode s u b j e c t e d t o r e p e t i t i v e e l e c t r o l y s i s a t h i g h c u r r e n t d e n s i t y (260 mA/cm w i t h r e s p e c t t o geometric s u r f a c e area) i n 2M H 2 S O 4 + 0.5M CuSOu, 22°. I n i t i a l and f i n a l noble metal l o a d i n g s a r e g i v e n on the diagram. 2  cn  155  F i g u r e 6.35.  P o t e n t i a l change w i t h time c u r r e n t d e n s i t y (260 mA/cm i n 2M H2SO1H 22°. I n i t i a l in g ( P t + I r ) / m are given  2  2  f o r an anode o p e r a t e d a t high w i t h respect t o geometric area) and f i n a l n o b l e metal,1oadings on the diagram.  F i g u r e 6.36.  P o t e n t i a l v s . time b e h a v i o u r f o r an anode o p e r a t e d a t 260 mA/cm (geometric area) i n 2M H 2 S O 4 , 22°, a f t e r p r i o r p o l a r i z a t i o n to imminent anode f a i l u r e (see F i g u r e 6.35).  2  F i g u r e 6.37.  P o t e n t i a l v s . time b e h a v i o u r  f o r an e l e c t r o d e s u b j e c t e d t o p u l s e d e l e c t r o l y s i s  {t^  = 60 s e c ;  ^SHORTED ^ ^ (geometric area) i n 2M H S 0 + 0.5M CuS0 , 22°. Curve r e p r e s e n t s p o t e n t i a l s measured a t the end o f each o n - c y c l e . I n i t i a l and f i n a l noble metal l o a d i n g s g i v e n on diagram. =  0  ,  5  s e c  a  t  5 2 , 1  [ i  / c m 2  2  4  4  158 i  ~  —  i  1  r  0.17  |  I  i  i  »  i  I  F i g u r e 6.38. P o t e n t i a l v s . time b e h a v i o u r f o r an e l e c t r o d e s u b j e c t e d t o p u l s e d e l e c t r o l y s i s (76 sec on/76 sec s h o r t e d ) a t 52.1 mA/cm ( g e o m e t r i c a r e a ) i n 2M H SO\ + 0.5M CuSO^, 22°. (Curve r e p r e s e n t s t h e p o t e n t i a l s measured a t t h e end o f each o n - c y c l e . ) I n i t i a l and f i n a l noble metal l o a d i n g s in(g)(Pt + I r ) / m a r e g i v e n on t h e d i agram. 2  2  2  159  for  o p e r a t i o n a t 52.1 mA/cm i n 2M H S 0 2  2  tinuous anodic c u r r e n t ) . sec.  4  + 0.5M C u S 0 , 22°, under con4  The most d e l e t e r i o u s t r e a t m e n t (76 sec.on/76  s h o r t e d ) caused a r a p i d r i s e i n anode p o t e n t i a l t o v a l u e s c h a r a c -  t e r i s t i c o f i r r e v e r s i b l e o p e r a t i o n , but w i t h a d e c r e a s i n g t r e n d f o r t h e r a t e o f i n c r e a s e o f anode p o t e n t i a l w i t h t i m e , r a t h e r than t h e a c c e l e r a t i n g i n c r e a s e o f anode p o t e n t i a l w i t h time n o r m a l l y observed w i t h i r r e v e r s i b l e beavhiour. O p e r a t i o n under h i g h - f r e q u e n c y c o n d i t i o n s (4.4 usee, on/1.6 usee, open) d i d n o t produce p o t e n t i a l v s . time b e a v h i o u r d i s t i n g u i s h a b l e from t h a t o f an anode o p e r a t e d w i t h c o n t i n u o u s a n o d i c c u r r e n t under o t h e r wise s i m i l a r c o n d i t i o n s .  A t lower f r e q u e n c i e s (76 s e c . on/76 s e c . open)  the anode p o t e n t i a l v s . time b e a v h i o u r was a l s o s i m i l a r , i n i t i a l l y , of  an e l e c t r o d e s u b j e c t e d t o c o n t i n u o u s a n o d i c c u r r e n t .  t o that  A f t e r about 200  hours ( a t 52.1 mA/cm ) the anode p o t e n t i a l was observed t o begin t o r i s e 2  at  a s t e e p e r r a t e (about .4 mv/hour) than i s encountered i n c o n t i n u o u s  c u r r e n t o p e r a t i o n (about .1 mv/hour). O s c i l l o s c o p i c i n v e s t i g a t i o n o f t h e p o t e n t i a l v s . time b e h a v i o u r d u r i n g t h e p e r i o d when the anode and cathode were s h o r t c i r c u i t e d showed t h a t the p o t e n t i a l dropped  from t h e v a l u e i t a t t a i n e d i m m e d i a t e l y p r i o r t o t h e  p u l s e t o a v a l u e o f 0 v o l t s v s . the copper/copper i n t h e case o f e l e c t r o l y s i s i n 2M  H2SO4+  sulfate reference electrode  0.5M CuSO^, o r t o 0 v o l t s v s . t h e  hydrogen r e f e r e n c e e l e c t r o d e i n t h e case o f 2M H S 0 , w i t h i n 20 m i l l i 2  seconds.  The p o t e n t i a l then remained  4  v i r t u a l l y constant thereafter  until  c o m p l e t i o n o f t h e p u l s e whereupon i t r o s e q u i c k l y ( w i t h i n 125 m i l l i s e c o n d s ) to  t h e p o t e n t i a l o f commencement o f oxygen e v o l u t i o n .  The a n o d i c - g o i n g  160  p u l s e showed a s i m i l a r p o t e n t i a l v s . time r e l a t i o n t o t h a t observed i n g a l v a n o s t a t i c charge s t u d i e s (see S e c t i o n 6.3.2). c y c l e " changed  from t h e i n i t i a l  time t o v i r t u a l l y a square-wave  The shape o f t h e "on-  case o f a . s l o w l y r i s i n g p o t e n t i a l form a f t e r t h r e e days.  with  For anodes sub-  j e c t e d t o o p e n - c i r c u i t p u l s i n g the anode p o t e n t i a l was n o t observed t o change f o r t h e case o f t h e 1.6 usee p u l s e t i m e , but i n the case o f t h e long o f f - p e r i o d  (76 seconds) t h e p o t e n t i a l dropped t o 1.4 v o l t s and main-  tained t h i s value u n t i l  6.3.1.5  t h e next o n - p e r i o d .  Effect of Additives On a d d i t i o n o f .05 g p l t h i o u r e a t o the e l e c t r o l y t e a f t e r e l e c -  trolysis  f o r 18 hours a t 52.1 mA/cm  i n 2M H S 0  2  2  + 0.5M CuSO^, 22°, the  4  p o t e n t i a l was observed t o c l i m b r a p i d l y by almost 0.4 v o l t s S u b s e q u e n t l y , t h e anode p o t e n t i a l  (Figure  6.39).  remained a t a h i g h v a l u e f o r about 40  h o u r s , whereupon an a c c e l e r a t i n g i n c r e a s e i n anode p o t e n t i a l w i t h time ( c h a r a c t e r i s t i c o f imminent complete d e g r a d a t i o n ) commenced. +2  A d d i t i o n o f .01M Co 166 hours a t 52.1 mA/cm  2  to the e l e c t r o l y t e a f t e r operation f o r  i n 2M H S 0 2  + 0.5M C u S 0 , 22° (a procedure which  4  4  has a b e n e f i c i a l e f f e c t on t h e p o t e n t i a l and c o r r o s i o n b e h a v i o u r o f l e a d anodes i n copper e l e c t r o w i n n i n g [53.55]) produced no change i n t h e o p e r a t ing  anode p o t e n t i a l . The presence o f kerosene i n t h e c e l l  d i d n o t produce any s i g n i f i -  c a n t change i n t h e p o t e n t i a l v s . t i m e b e h a v i o u r o f an anode o p e r a t e d a t 52.1 mA/cm  2  ( g e o m e t r i c a r e a ) i n 2M H S 0 2  4  + 0.5M CUSO^, 22°.  F i g u r e 6.39.  E f f e c t o f 0.05 g p l t h i o u r e a a d d i t i o n on the p o t e n t i a l o f an anode operated a t 52.1 mA/cm ( g e o m e t r i c a r e a ) i n 2M H S 0 + 0.5M C u S 0 , 22°. 2  4  4  162  6.3.2  S u r f a c e Charge S t u d i e s  6.3.2.1  Oxygen L a y e r Development P r i o r to Oxygen E v o l u t i o n The  and  anodic/cathodic  g a l v a n o s t a t i c charge curves f o r  platinum  the P t / 5 , 10, 20, 25 a l l o y w i r e e l e c t r o d e s were found t o be  s i m i l a r i n form ( F i g u r e 6.40,  completely  measured f o r the Pt/25 I r a l l o y , i s t y p i c a l  f o r a l l c a s e s ) , i n p a r t i c u l a r , showing a d e f i n i t e r e d u c t i o n " p l a t e a u " on the c a t h o d i c s i d e and c o n s i d e r a b l e h y s t e r e s i s between the shapes o f the a n o d i c and  cathodic curves.  I r i d i u m w i r e e l e c t r o d e s , on the o t h e r hand,  show no r e d u c t i o n " p l a t e a u , " have c o n s i d e r a b l y l e s s h y s t e r e s i s , and show a charge imbalance between the anodic and 6.41).  Charge curves  f o r a t y p i c a l new  c a t h o d i c curves  also  (Figure  Pt/30 I r - T i anode ( F i g u r e  6.42)  show s i m i l a r f e a t u r e s t o p l a t i n u m , e x c e p t f o r the l e s s - s t e e p s l o p e s  of  the " d o u b l e - l a y e r " c h a r g i n g  curves.  r e g i o n s i n both the anodic and c a t h o d i c  A d d i t i o n a l e v i d e n c e f o r the p l a t i n u m - l i k e behaviour provided  by the c l o s e balance  d e p o s i t i o n and  of P t / I r a l l o y s i s  between anodic and c a t h o d i c charges f o r  removal o f oxygen, which i s not observed i n the case o f  iridium. The  development o f oxygen coverage p r i o r to oxygen e v o l u t i o n  s t u d i e d by g a l a n o s t a t i c p o l a r i z a t i o n  t o the p o t e n t i a l  of i n t e r e s t , followed  by c a t h o d i c s t r i p p i n g a t the same magnitude o f c u r r e n t d e n s i t y . p l a t i n u m and  P t / I r a l l o y w i r e e l e c t r o d e s , and  was  For  f o r Pt/30 I r - T i , the  the  partial  oxygen coverage charges c o u l d be compared d i r e c t l y w i t h t h a t measured a t imminent oxygen e v o l u t i o n t o o b t a i n a f r a c t i o n a l degree o f c o v e r a g e v a l u e . For i r i d i u m however, i t i s not p o s s i b l e to measure c a t h o d i c oxygen charges by the g a l v a n o s t a t i c t e c h n i q u e . .  stripping  (Hence i t i s not p o s s i b l e t o  measure the coverage a t imminent oxygen e v o l u t i o n by g a l v a n o s t a t i c  j T  TIME F i g u r e 6.40.  1  ,  1  r  SECONDS  T y p i c a l charge c u r v e s f o r a Pt/25 I r a l l o y w i r e e l e c t r o d e s u b j e c t e d t o a l t e r n a t e anodic and c a t h o d i c c u r r e n t s (0.06 mA/cm o f a c t u a l a r e a ) i n helium-purged 2M H 2 S O 4 , 22°. 2  F i g u r e 6.41.  T y p i c a l charge c u r v e s o f an i r i d i u m w i r e e l e c t r o d e s u b j e c t e d t o a l t e r n a t e a n o d i c and c a t h o d i c c u r r e n t s (0.14 mA/cm o f a c t u a l area) i n helium-purged 2M H2SO1H 22°. Note the imbalance o f the a n o d i c and c a t h o d i c c h a r g e s . 2  F i g u r e 6.42.  T y p i c a l charge c u r v e s f o r a Pt/30 I r - T i e l e c t r o d e s u b j e c t e d to a l t e r n a t e a n o d i c and c u r r e n t s (0.0088 mA/cm o f a c t u a l area) i n helium-purged 2M H S 0 , 22°. 2  2  4  cathodic  166  stripping.  Indeed, the knowledge t h a t i r i d i u m may  form the  equivalent  o f m u l t i l a y e r f i l m s does not make t h i s p a r t i c u l a r t e c h n i q u e anyway.)  satisfactory  S u r f a c e areas f o r i r i d i u m were determined from the charge con-  sumed to i o n i z e the adsorbed hydrogen produced by p o l a r i z i n g to hydrogen e v o l u t i o n and f o l l o w e d by anodic  charging.  T h i s v a l u e was  compute the charge e q u i v a l e n t to an oxygen monolayer.  then used t o  The  oxygen coverages  f o r i r i d i u m were then c a l c u l a t e d r e l a t i v e to t h i s v a l u e , from the times f o r the anodic  transition  d e p o s i t i o n o f the oxygen l a y e r .  F i g u r e 6.43  shows the s u r f a c e oxygen coverage vs. p o t e n t i a l  r e l a t i o n s found f o r the v a r i o u s e l e c t r o d e s i n v e s t i g a t e d .  The  platinum  P t / I r a l l o y w i r e e l e c t r o d e s are v i r t u a l l y i n d i s t i n g u i s h a b l e i n  and  behaviour,  whereas the Pt/30 I r - T i shows h i g h e r coverages a t a l l p o t e n t i a l s (which i s i n c o n f o r m i t y w i t h the o b s e r v a t i o n o f the commencement o f oxygen e v o l u t i o n a t 1.53  v o l t s (RHE)  electrodes).  on the coated anodes v s . 1.58-1.65 v o l t s f o r the  I r i d i u m shows monolayer coverage a t about 1.15  v o l t s under  the p a r t i c u l a r c o n d i t i o n s o f measurement used i n t h i s work, and has the e q u i v a l e n t o f two monolayers when oxygen e v o l u t i o n commences. all  cases the coverage vs. p o t e n t i a l r e l a t i o n s were found to be  above about 1.0  6.3.2.2  wire  nearly In  linear  volt.  Oxygen Layer Development w i t h Simultaneous Oxygen E v o l u t i o n G a l v a n o s t a t i c s t r i p p i n g o f the s u r f a c e oxygen from p l a t i n u m  and  P t / I r a l l o y electrodes a f t e r holding at a given current density f o r a predetermined l e n g t h of time r e v e a l e d t h a t the s u r f a c e oxygen coverage i n c r e a s e s w i t h time i n a l o g a r i t h m i c f a s h i o n , a t t a i n i n g v a l u e s a p p r o a c h i n g two monolayers and beyond.  No l i m i t i n g coverage v a l u e s were a p p a r e n t from  167  F i g u r e 6.43.  S u r f a c e oxygen coverage v s . p o t e n t i a l r e l a t i o n s f o r s e v e r a l anodes, over the p o t e n t i a l r e g i o n p r i o r t o commencement o f oxygen e v o l u t i o n . E l e c t r o l y t e i s 2M H SG\, 22°, helium-purged. P o i n t s o b t a i n e d by g a l v a n o s t a t i c a n o d i c c h a r g i n g t o t h e p o t e n t i a l of i n t e r e s t , f o l l o w e d i m m e d i a t e l y by c a t h o d i c s t r i p p i n g . 2  168  the p r e s e n t work. curves  F i g u r e 6.44  shows the d i f f e r e n c e s i n the c a t h o d i c charge  f o r a Pt/25 I r a l l o y w i r e e l e c t r o d e which are produced by  p r i o r anodic  treatments.  ment t e c h n i q u e  Curve (a) i s t y p i c a l  various  o f the s u r f a c e a r e a s measure-  and r e p r e s e n t s monolayer coverage.  Continued anodic  opera-  t i o n under c o n d i t i o n s o f oxygen e v o l u t i o n causes an i n c r e a s e i n the  length  o f the r e d u c t i o n p l a t e a u and a d e c r e a s e i n the p l a t e a u p o t e n t i a l ( c u r v e s (b) and  (c)).  potential  I f the e l e c t r o d e i s a n o d i c a l l y p o l a r i z e d such t h a t the  reaches about 2.0  v o l t s (RHE)  and above, the development o f  m u l t i l a y e r ("type I I " o x i d e ) f i l m s becomes apparent through the appearance o f a l o w e r - p o t e n t i a l p l a t e a u which may  extend to times e q u i v a l e n t t o the  removal o f s e v e r a l monolayers of s u r f a c e oxygen (curves No data c o u l d be o b t a i n e d  (3) and  by the g a l v a n o s t a t i c t e c h n i q u e  for iridium,  the shape o f the c a t h o d i c curve remained unchanged even a f t e r e l e c t r o l y s i s a t high c u r r e n t d e n s i t i e s . are summarized i n Table  The  (3)). as  prolonged  oxygen l a y e r s t r i p p i n g  studies  6.17.  Pt/30 I r - T i e l e c t r o d e s a l s o show a l o g a r i t h m i c i n c r e a s e o f s u r f a c e oxygen coverage w i t h time.  In a l l cases the oxygen coverage  e v e n t u a l l y reached the e q u i v a l e n t o f 2-3  monolayers.  Only data f o r  e l e c t r o d e s used i n 2M H S0n were used, as c a t h o d i c s t r i p p i n g o f anodes 2  a f t e r e l e c t r o l y s i s . i n copper s u l f a t e c o n t i n u i n g s o l u t i o n s gave a l o n g high-potential plateau reduction of Pb0 cupric oxide).  2  ( a t 1.4-1.5 v o l t s ) , l i k e l y a s s o c i a t e d w i t h  the  t o P-bSOn. (even i n e l e c t r o l y t e s produced from " l e a d - f r e e "  M u l t i l a y e r coverage was  anode whose p o t e n t i a l was  observed o n l y f o r a Pt/30 I r - T i  w i t h i n the range (2.1-2.5 v o l t s ) where t h i s  phenomenon i s known to o c c u r f o r p l a t i n u m . Anodes which had  l o s t most o f t h e i r p l a t i n u m  l o a d i n g due  p u l s e d e l e c t r o l y s i s behaved s i m i l a r l y t o i r i d i u m w i r e and  to  consequently  F i g u r e 6.44.  C a t h o d i c s u r f a c e oxygen s t r i p p i n g curves f o r a Pt/25 I r a l l o y w i r e e l e c t r o d e s u b j e c t e d t o v a r i o u s anodic t r e a t m e n t s i n helium-purged 2M H SO , 22°, (a) p r e - p o l a r i z e d t o imminent oxygen e v o l u t i o n ; (b) 3000 s e c a t 0.06 mA/cm ; ( c ) 19 hours a t 0.06 mA/cm ; (d) 7000 seconds a t 6 mA/cm ; (e) 23 hours a t 6 mA/cm . ( A l l c u r r e n t d e n s i t i e s a r e g i v e n w i t h r e s p e c t t o a c t u a l e l e c t r o d e area. S t r i p p i n g c u r r e n t d e n s i t y was 0.06 mA/cm .) z  h  2  2  2  2  2  170  Table 6.17 E f f e c t o f A n o d i c P o l a r i z a t i o n a t V a r i o u s C u r r e n t D e n s i t i e s and Times on the S u r f a c e Oxygen Coverage as Measured by G a l v a n o s t a t i c Cathodic S t r i p p i n g .  E l e c t r o l y t e : 2M H S 0 , 22° 2  Applied Current Density Anode  (Actual Area) mA/cm  .18  .07  2  Pt w i r e  Pt/20 I r  .23  Pt/25 I r  .24  wire  2T4" 24  Ir-Ti ( 4 . 3 g/m2)  52.1  Seconds  Volts vs. SHE  2  ^monolayer 1.25 1.53 1 .59 1.71 2.33  1.67 1.76 1.80  1 .27 1.59 1.71  100 1000 3000 6.5(10)"  1.67 1.75 1.79  1 .27 1.58 1.76  0.6  2.4(10)  1 .99  6.0  100 1000 7000  .06  .06  1.53  100 1000 3000 6000 2.4(10)  ^measured  1.74 1.81 1.85 1.87 1 .93  wi r e  Pt/30  F i n a l Anode Potential  Time (Geometric Area) mA/cm  4  100 1000 3000 2.4(10)  5  5  5  2.05 2.09 2.08  8.3(10)"  2.06  100 300 10" 7.6(10)" 2.4(10) 6.9(10) 2.2(10)  1.66 1.69 1 .74 1.81 1.90 1.99 2.98  5  5  6  CONTINUED I n d i c a t e s f o r m a t i o n o f type I I o x i d e .  \  ' 2.80 3.33*  1.67 2.46 '(3.20) "(0.80)*  i \  2.17 7.24* 1.27 1 .45 1 .59 1.91 1.96 2.83 3.15  171  Table 6.17 (Continued) A p p l i e d Durrent Density (Geometric Area) mA/cm  Anode  2  Pt/30 I r - T i (20 g/m;)  52.1  F i n a l Anode Potential  Time (Actual Area) mA/cm  Seconds  Volts vs. SHE  2  0.77  2  ^measured ^monolayer  100 300 1000 7.2(10)'* 2.4(10)  1.61 1.63 1.64 1.72 1.93  1.20 1.28 1.34 1.65 2.47  6  Pt/30 I r - T i (4.32 g/m )  15.6  0.39  2.4(10)  s  1.96  3.03  Pt/30 I r - T i 4.03 g/m )  52.1  1.30  2.3(10)  6  1.93  2.02  2.60  1.6(10)  6  2.80  2.83  0.39  5.2(10)  6  2.36  2  2  Pt/30 I r - T i 4.32 g/m )  104  2  Pt/30 I r - T i (4.23 g/m ) (2M H S 0 , 40°) 2  2  4  15.6  -35*  172  i t was n o t p o s s i b l e t o determine t h e s u r f a c e oxygen coverage produced on anodic  p o l a r i z a t i o n w i t h such anodes by t h e g a l v a n o s t a t i s s t r i p p i n g  6.3.3  P o l a r i z a t i o n Curves The  technique.  p o l a r i z a t i o n curves f o r oxygen e v o l u t i o n on Pt/30 I r - T i  show two l i n e a r r e g i o n s s e p a r a t e d  by an i n f l e c t i o n a t about 1.61 v o l t s i n a  potential vs. log (current density) plots.  T a f e l slopes o f these  linear  r e g i o n s a r e .047 and .080 f o r t h e low- and h i g h - o v e r v o l t a g e r e g i o n s , tively.  respec-  (The l a t t e r s l o p e tended t o i n c r e a s e f o r anodes which had been sub-  j e c t e d t o long-term trodes.  anodes  a n o d i z a t i o n , but was e s s e n t i a l l y c o n s t a n t f o r new e l e c -  A l l e l e c t r o d e s - new and used - showed the same lower o v e r p o t e n t i a l  region slope.)  R e p r o d u c i b i l i t y o f i n d i v i d u a l curves was, a t w o r s t , w i t h i n 5  mi 1 1 i v o l t s . The  importance o f c o n s i d e r a t i o n o f t h e t r u e c u r r e n t d e n s i t y i s shown  i n F i g u r e 6.45 and 6.46 which show t h e p o l a r i z a t i o n curves f o r i n d i v i d u a l anodes having d i f f e r e n t measured roughness f a c t o r s , w i t h c u r r e n t d e n s i t i e s c a l c u l a t e d both w i t h r e s p e c t t o t h e geometric The  area and t o t h e a c t u a l a r e a .  c o i n c i d e n c e o f t h e curves p l o t t e d w i t h r e s p e c t t o t h e l a t t e r i s e x c e l l e n t .  E x t r a p o l a t i o n o f t h e h i g h - o v e r p o t e n t i a l T a f e l r e g i o n t o 1.23 v o l t s (SHE) g i v e s an exchange c u r r e n t d e n s i t y , w i t h r e s p e c t t o a c t u a l s u r f a c e a r e a , o f :  i = 9.3(10)"  9  A/cm  2  As v e r y many mechanisms have been proposed f o r t h e oxygen e v o l u t i o n r e a c t i o n , i t i s n o t u s e f u l t o c o n s i d e r t h e T a f e l s l o p e s r e p o r t e d here on a mechanistic basis.  Indeed, t h e v a r i a t i o n o f t h e k i n e t i c parameters on n o m i n a l l y  i d e n t i c a l e l e c t r o d e s remains one o f t h e major problems i n t h e e l e c t r o c h e m i s t r y o f oxygen.  The p r e s e n t v a l u e s do, however, f a l l  the i n d i v i d u a l  noble  metals.  w i t h i n t h e ranges r e p o r t e d f o r  173  F i g u r e 6.45.  P o l a r i z a t i o n c u r v e s f o r oxygen e v o l u t i o n on Pt/30 I r - T i anodes o f d i f f e r e n t roughness f a c t o r s i n 2M h^SO^, 22°. C u r r e n t d e n s i t i e s based on g e o m e t r i c a r e a .  174  F i g u r e 6.46.  P o l a r i z a t i o n c u r v e s f o r oxygen e v o l u t i o n on Pt/30 I r - T i anodes o f d i f f e r e n t roughness f a c t o r s i n 2M H 2 S O 4 , 22°. C u r r e n t d e n s i t i e s based on a c t u a l s u r f a c e a r e a .  175  P o l a r i z a t i o n c u r v e s measured on p l a t i n u m and P t / I r a l l o y w i r e e l e c t r o d e s r e v e a l e d t h a t i n c r e a s i n g i r i d i u m c o n t e n t has a b e n e f i c i a l e f f e c t on the oxygen o v e r p o t e n t i a l .  While a l i n e a r p o t e n t i a l  vs. log i  r e l a t i o n c o u l d be found f o r p l a t i n u m , the p o l a r i z a t i o n c u r v e s f o r the a l l o y w i r e s showed r a t h e r complex b e h a v i o u r ( F i g u r e 6 . 4 7 ) .  For the a l l o y s  P t / 2 0 I r and P t / 2 5 I r the p o l a r i z a t i o n c u r v e s assumed an "S-shape" when plotted  i n the c o n v e n t i o n a l manner. Using the measured p o t e n t i a l  vs. l o g i d a t a o b t a i n e d from both  p l a t i n u m and i r i d i u m o v e r f i v e decades o f l o g i , a computer programme was  w r i t t e n t o generate p o l a r i z a t i o n c u r v e s f o r h y p o t h e t i c a l p l a t i n u m /  i r i d i u m a l l o y s on the assumption  t h a t the two m e t a l s behaved as s e p a r a t e  anodes o f i n d i v i d u a l s u r f a c e a r e a s p r o p o r t i o n a l The  to t h e i r weight  r e s u l t s o f t h i s c a l c u l a t i o n are g i v e n i n F i g u r e 6 . 4 8  w i t h the measured p o l a r i z a t i o n c u r v e f o r the P t / 3 0 I r - T i  fractions.  and are compared anode.  As  can  be seen, the e f f e c t of even as l i t t l e as one per c e n t o f i r i d i u m i n the a l l o y i s p r e d i c t e d t o cause a decrease i n the oxygen o v e r p o t e n t i a l over 0.1  volt.  H i g h e r amounts o f i r i d i u m  by  (> 10 per c e n t ) g i v e r e s u l t a n t  p o l a r i z a t i o n c u r v e s c l o s e t o t h a t o f i r i d i u m and h a v i n g a s i m i l a r shape to t h a t o f i r i d i u m .  6.3.4  Titanium In o r d e r t o o b t a i n a p o l a r i z a t i o n c u r v e f o r t i t a n i u m which showed  an " a c t i v e "  r e g i o n under the c o n d i t i o n s employed f o r measurement, i t was  n e c e s s a r y t o use a h i g h e r a c i d c o n c e n t r a t i o n (4M H S 0 i J and 2  (60°).  temperature  The p o l a r i z a t i o n c u r v e o f the t i t a n i u m base, c u t from a s h e e t of  m a t e r i a l which has been e t c h e d and o t h e r w i s e prepared f o r the thermal  176  r  T  L  F i g u r e 6.47.  0  G ('actual>  •  fJLA/cm  2  A n o d i c p o l a r i z a t i o n curves f o r oxygen e v o l u t i o n on p l a t i n u m and p l a t i n u m / i r i d u m a l l o y s i n 2M H SO , 22°. C u r r e n t d e n s i t i e s r e f e r r e d to actual electrode areas. z  k  F i g u r e 6.48.  P o l a r i z a t i o n curves f o r p l a t i n u m and i r i d i u m w i r e anodes i n 2M H SG\, 22°, w i t h computer-generated curves f o r v a r i o u s P t / I r a l l o y s . A l s o i n c l u d e d are p o i n t s from the measured p o l a r i z a t i o n curve f o r Pt/30 I r - T i . 2  ^ ^  178  d e c o m p o s i t i o n o f a noble metal c o a t i n g onto i t , shows a c h a r a c t e r i s t i c active/passive transition occurred  ( F i g u r e 6.49).  A s l i g h t transpassive  region  a t p o t e n t i a l s above about 2.2 v o l t s (SHE), which i s n o t c h a r a c -  t e r i s t i c of commercially-pure t i t a n i u m . The  c u r r e n t d e n s i t i e s measured i n such non-steady p o l a r i z a t i o n  c u r v e s have no r e l a t i o n t o those which a c t u a l l y o c c u r d u r i n g a n o d i c o p e r a t i o n a t a given p o t e n t i a l . region  the p a s s i v e  Under p o t e n t i o s t a t i c o p e r a t i o n  c u r r e n t d e n s i t y was observed t o drop c o n t i n u o u s l y  t i m e , w i t h o u t ever r e a c h i n g  a steady value.  2  2  22° decayed t o  ( g e o m e t r i c area) w i t h i n two hours.  High-potential  p o t e n t i o s t a t i c operation  t i g a t e d w i t h t h e Anatek 50-1D dual the v o l t a g e - c o n t r o l of t h e p o t e n t i o s t a t . were a t t a i n e d .  with  The c u r r e n t d e n s i t y on a  t i t a n i u m "base" anode h e l d a t 1.8 v o l t s (SHE) i n 2M H S0*, l e s s than 1 uA/cm  i n the p a s s i v e  power s u p p l y  o f t i t a n i u m was i n v e s -  operated i n s e r i e s , i n  mode used t o "buck-out" t h e p o t e n t i a l - s e n s i n g  input  Under these c o n d i t i o n s anode p o t e n t i a l s o f 100 v o l t s  C u r r e n t d e n s i t i e s o f t h e o r d e r o f s e v e r a l mi 11iamps/cm  (geometric a r e a ) were observed d u r i n g h i g h - p o t e n t i a l o p e r a t i o n sequence o f the r a t h e r r a p i d c h a r g i n g  as a con-  r a t e employed (10 v o l t s / 1 0 0  Oxygen e v o l u t i o n was observed a t high p o t e n t i a l s .  Immediate  2  seconds).  switching  t o a p o t e n t i a l o f 1.8v (SHE) w i t h an anode which had n o t been p r e v i o u s l y a n o d i c a l l y t r e a t e d r e s u l t e d i n an i n i t i a l l y v i g o u r o u s oxygen e v o l u t i o n which ceased w i t h i n minutes.  I f a platinum  wire  i s touched t o t h e s u r f a c e  o f a t i t a n i u m anode h e l d p o t e n t o s t a t i c a l l y a t high a n o d i c p o t e n t i a l s an e x t r e m e l y l a r g e c u r r e n t f l o w s and oxygen i s e v o l v e d the noble metal s u r f a c e .  a t a high r a t e on  On removal o f t h e w i r e , oxygen e v o l u t i o n p e r s i s t s  f o r some time on t h a t p a r t o f t h e t i t a n i u m s u r f a c e which was touched by  179  F i g u r e 6.49.  Anodic p o l a r i z a t i o n c u r v e f o r the t i t a n i u m base o f a Pt/30 I r - T i anode, i n 4M H 2 S O 4 , 60°. Curve t r a c e d by s t e p p i n g the p o t e n t i a l upward from the c o r r o s i o n p o t e n t i a l a t 20 mv/50 seconds.  180  the p l a t i n u m , p r o b a b l y f o r breakdown was The  film destruction.  observed i n any o f the a n o d i c e x p e r i m e n t s on  a d d i t i o n o f 0.4  operating  as a r e s u l t of l o c a l  gpl t h i o u r e a t o a c e l l  No  evidence  titanium.  c o n t a i n i n g a t i t a n i u m anode  i n the p a s s i v e r e g i o n caused no change i n c u r r e n t d e n s i t y  whatsoever.  6.4 6.4.1  Anode  Deposits  Growth In 2M h^SO^  + 0.5M  CUSOL  e l e c t r o l y t e s containing trace lead  i m p u r i t y the growth o f l e a d - c o n t a i n i n g s u r f a c e d e p o s i t s was i n c r e a s e w i t h time f o r a l l cases s t u d i e d s u r f a c e l e a d was  estimated  ( F i g u r e 6.50).  found t o  The  amount of  by t a k i n g the i n t e n s i t y of the PbLot! peak  measured on the X-ray s p e c t r o m e t e r w i t h r e s p e c t t o t h a t o b s e r v e d on a pure,lead  sample o f s i m i l a r s i z e to the anodes.  I f the " l o a d i n g " vs.  r e l a t i v e i n t e n s i t y r a t i o i s assumed to be s i m i l a r to t h a t f o r  platinum,  the h i g h e s t l e a d coverages observed c o r r e s p o n d to about 5 m i l l i g r a m s l e a d on the anode s u r f a c e  (1.92  cm  A f t e r about  200  h o u r s ' , o p e r a t i o n , " l o a d i n g s " of about a m i l l i g r a m are i n d i c a t e d .  No  2  geometric area).  o b v i o u s r e l a t i o n between the amount o f l e a d d e p o s i t e d  and  c u r r e n t d e n s i t y was  s c a t t e r of  experimental 6.50  f o u n d , and  p o i n t s above and  t h e r e was  the a p p l i e d the  below the approximate curve drawn on  f o r higher current density An  considerable  of  Figure  operation.  " i n d u c t i o n " p e r i o d of g r e a t e r than 24 hours was  necessary i n  o r d e r t o produce d e t e c t a b l e d e p o s i t s  ( r e l a t i v e i n t e n s i t y o f background-  c o r r e c t e d peaks g r e a t e r than .002).  A f t e r 100  v i s a b l e , showing a b l u e - b l a c k c o l o u r .  On  hours the d e p o s i t became  removal of f i l m e d anodes from  F i g u r e 6.50.  I n c r e a s e i n amount o f s u r f a c e a t e d a t 52.1 mA/cm ( g e o m e t r i c in the i n t e n s i t y of the PbLai and t r i a n g l e a r e d i s c u s s e d i n 2  l e a d d e p o s i t s w i t h e l e c t r o l y s i s time f o r Pt/30 I r - T i anodes oper-c» area) i n 2M H S0 + 0.5M CuSO^, 22°, as i n d i c a t e d by t h e change peak i n X-ray f l u o r e s c e n t s p e c t r o s c o p y . P o i n t s denoted by square the text. z  h  182  the c e l l  the d e p o s i t was found t o be e a s i l y removed by w i p i n g w i t h a ..  tissue.  A f t e r d r y i n g t h e anode d e p o s i t r e t a i n e d i t s c o l o u r and suscep-  t i b i l i t y t o removal by g e n t l e  wiping.  Two s p e c i a l cases a r e i n c l u d e d i n F i g u r e 6.50.  The square data  p o i n t c o r r e s p o n d s t o the case where a s o l u t i o n c o n t a i n i n g d i s s o l v e d l e a d n i t r a t e was added t o the e l e c t r o l y t e t o b r i n g t h e t o t a l to 7 g p l .  lead concentration  ( T h i s c o n c e n t r a t i o n i s n o t r e a l i z e d i n p r a c t i c e , however, as  the s o l u b i l i t y product o f l e a d s u l f a t e i s :  K  Hence i n 2f*1  where the a c t i v i t y o f s u l f a t e i o n s may be e s t i m a t e d by  H2SO4,  a  S0,  = 1.06(10- ) 8  sp  =  Y  S0, ' S0, m  = (.002 t o .05) • (.70) = .001 t o .035  depending on t h e method used t o e s t i m a t e  (6)  the a c t i v i t y c o e f f i c i e n t of  s u l f a t e i o n s (see Appendix A 4 ) .  Hence the maximum amount o f d i s s o l v e d  l e a d i n s o l u t i o n must c o r r e s p o n d  t o an a c t i v i t y o f l e s s than 1 0 ~  5  molal  or 5 ( 1 0 ) " g p l . 8  The t r i a n g l e data p o i n t i n F i g u r e 6.50 c o r r e s p o n d s t o the case o f p u l s e d e l e c t r o l y s i s w i t h 4.4 usee, on/1.6 usee, open, and r e p r e s e n t s the h i g h e s t l e a d - l o a d i n g d e t e c t e d on a l l t h e anodes used i n s o l u t i o n s cont a i n i n g t r a c e amounts o f l e a d .  E l e c t r o d e s employed i n the o t h e r cases  o f p u l s e d e l e c t r o l y s i s , where the f r e q u e n c i e s were much g r e a t e r , d i d not  183  have any s u r f a c e l e a d d e p o s i t s e i t h e r by X-ray s p e c t r o m e t r y  o r S.E.M.  observation. No e f f e c t was found f o r the presence o f s u r f a c e l e a d d e p o s i t s on t h e p o t e n t i a l v s . time b e h a v i o u r , c o n d i t i o n s i n 2M H SO z  The  k  as t h e c u r v e s measured under s i m i l a r  and 2M H S0^ + 0.5M CuSC\ were e s s e n t i a l l y  identical.  z  c a t h o d i c charge curve o f a l e a d - c o v e r e d  anode showed a l o n g  high-  p o t e n t i a l p l a t e a u ( a t 1.4-1.5 v o l t s ) and a r e s t p o t e n t i a l i n 2M H S0\ o f 2  1.60  v o l t s , f e a t u r e s n o t observed w i t h " c l e a n " noble metal  Further, although  surfaces.  hydrogen gas b u b b l i n g c o u l d be used t o complete reduce  the oxygen coverage w i t h i n minutes on c l e a n s u r f a c e s , t h e hydrogen p o t e n t i a l c o u l d n o t be a t t a i n e d i n t h e case o f e l e c t r o d e s covered ing  f i l m s even a f t e r s e v e r a l hours.  with lead-contain-  Removal o f the f i l m s was r e a d i l y  a c c o m p l i s h e d on immersion i n d i l u t e HC1 s o l u t i o n .  6.4.2  Identification X-ray d i f f r a c t o m e t r y w i t h anodes covered  d e p o s i t s , which were removed " l i v e " from t h e c e l l  with  lead-containing  on t e r m i n a t i o n o f t h e  r u n , showed i n t e n s e sharp peaks which c o u l d be i d e n t i f i e d e i t h e r w i t h 3 - P b 0 j t h e anodic 2  c o r r o s i o n product  or w i t h t h e m i n e r a l for  plattnerite  the l a t t e r species.  o f lead i n s u l f u r i c a c i d s o l u t i o n s ,  (Table 6.18).  The f i t was s l i g h t l y b e t t e r  In t h e i r r e c e n t r e v i e w on t h e l e a d d i o x i d e e l e c -  t r o d e , C a r r and Hampson [365] i d e n t i f y t h e d - s p a c i n g s measured f o r p l a t t n e r i t e w i t h those o f 6-Pb0 . 2  184  Table Identification  6.18  o f L e a d - C o n t a i n i n g Anode D e p o s i t s by X-ray f o r 20 up t o 80° w i t h  Observed Peak d(A) 3.50  Diffraction,  CuKaGiRadiation  Data from A.S. T.M.  Index  • I/Ii (from c h a r t )  d  3-Pb0  2  Plattnerite(PbO,) d I/Ii I/I i  hkl  100  3.50  100  3.50  100  no  2.788  98  2.79  80  2.80  100  101  2.475  36  2.46  90  2.48  70  200  -  2.21  10  210  (obscured b^' P t and T i peaks)  -  1.852  40  1.85  50  1.856  100  211  1.751  9  1.75  50  1.754  60  220  1.691  7  1.68  50  1.693  40  002  1.564  11  1.556  30  1.569  60  310  1.523  11  1.519  30  1.527  70  112  1.484  10  1.486  50  1.486  70  301  1.397  4  1.387  30  1.399  50  202  1.273  5  1.273  50  1.276  70  321  1.237  2  1.238  30  1.240  20  400  1.216  3  1.214  30  1.220  50  220  185  I f an o x i d e - c o v e r e d  anode was  a l l o w e d to stand i n the e l e c t r o l y t e  a t o p e n - c i r c u i t f o l l o w i n g c o m p l e t i o n o f a r u n , the c o a t i n g would spont a n e o u s l y decompose to l e a d s u l f a t e w i t h time. appearance and responding  T h i s was  confirmed  i n c r e a s i n g i n t e n s i t y o f the x - r a y d i f f r a c t i o n peaks c o r -  to l e a d s u l f a t e , coupled w i t h the s i m u l t a n e o u s l y - d e c r e a s i n g  i n t e n s i t i e s f o r the (3-Pb0 peaks.  An e s t i m a t e o f the degree o f c o n v e r s i o n  2  t o s u l f a t e was  made by h o l d i n g a g i v e n o x i d e - c o v e r e d  times a t o p e n - c i r c u i t i n 2M  H2SO4  + 0.5M  anode f o r v a r i o u s  CuSC\ e l e c t r o l y t e and measuring  the i n t e n s i t y a t the 20 angle c o r r e s p o n d i n g  t o the s t r o n g e s t PbSC\ peak  o  •o  (3.001 A) r e l a t i v e to t h a t o f the s t r o n g e s t 3-Pb0 experiments  are summarized i n Table 6.19. Table  Conversion 2M H S 0 2  by the  o f 3-Pb0 4  + 0.5M  2  The  2  peak (3.50 A ) .  c o n v e r s i o n o f the  These oxide  6.19  D e p o s i t s to 3-PbSOi* on H o l d i n g a t O p e n - C i r c u i t i n  CuSO^, 22°,  f o r V a r i o u s Lengths o f Time, Measured  by Comparing the I n t e n s i t i e s o f the S t r o n g e s t PbS0  Time a t O p e n - C i r c u i t (minutes)  4  and  g-Pb0  lines  2  R e l a t i v e Peak I n t e n s i t y :  P  b  S  2  1  1  2  0  0  1  0.04  5  0.18  30  0.35  60  0.40  3 days  j M ) 3-Pb0 (110)  (3-Pb0  2  barely  detected)  t o the s u l f a t e begins w i t h i n the f i r s t minute o f h o l d i n g a t o p e n - c i r c u i t and i s v i r t u a l l y complete a f t e r t h r e e days.  H o l d i n g f o r s e v e r a l days  186  i n d o u b l e - d i s t i l l e d w a t e r , on the o t h e r hand, has no e f f e c t on the n a t u r e o f t h e anode d e p o s i t , w i t h 3-Pb0  6.4.3  2  being t h e o n l y s p e c i e s  detected.  Morphology The l e a d - c o n t a i n i n g s p e c i e s were observed t o grow as d i s c r e t e  c l u s t e r s on the anode s u r f a c e s .  I n i t i a l l y , the 6-Pb0  2  was f o u n d t t o  appear  p r e f e r e n t i a l l y on the "smooth" areas o f the Pt/30 I r c o a t i n g , which represent  "hills"  i n the anode s u r f a c e s t r u c t u r e ( F i g u r e 6.51).  the d e p o s i t s appear t o form a l o n g r i d g e s i n these  "hills."  With  Further, increased  time o f o p e r a t i o n , however, coverage becomes more random ( F i g u r e 6.52). F i g u r e s 6.53 and 6.54 show B-Pb0 t o t a l l y converted  6.5  Complete  6.5.1  2  c o a t i n g s t h a t a r e p a r t i a l l y and almost  to PbS0 , r e s p e c t i v e l y . 4  Degradation  Anode P o t e n t i a l Behaviour Complete anode d e g r a d a t i o n  potential operation.  i s preceded by i r r e v e r s i b l e  high-  The r a t e o f change o f p o t e n t i a l on imminent degrada-  t i o n i s c h a r a c t e r i z e d by an e x p o n e n t i a l  i n c r e a s e w i t h t i m e which reaches  a maximum o f 200 mv/second immediately  p r i o r to attainment  l i m i t o f the power s u p p l y  A t the v o l t a g e l i m i t , the anode  c u r r e n t then begins nential  (50 v o l t s ) .  t o decay.  During  the  o f the v o l t a g e  expo-  v o l t a g e r i s e the anode p o t e n t i a l e x h i b i t s an i n c r e a s i n g degree o f  i n s t a b i l i t y , beginning  w i t h p e r i o d i c " s p i k e s " o f p o t e n t i a l which a r e  superimposed on the anode p o t e n t i a l (commencing a t about 5.0 v o l t s , and having  a h e i g h t o f about 25 mv).  The p e r i o d o f these  " s p i k e s " v a r i e d from  187  F i g u r e 6.51.  3 - P b 0 d e p o s i t formed on an anode o p e r a t e d f o r 96 hours a t 52.1 mA/cm i n 2M H S 0 + 0.5M CuS0„, 22°. (lOOOx) 2  2  2  F i g u r e 6.52.  4  3 - P b 0 d e p o s i t formed on an anode o p e r a t e d f o r 192 hours a t 15.6 mA/cm i n 2M HaSO,, + 0.5M C u S 0 , 22°. (lOOOx) 2  2  4  188  F i g u r e 6.53.  Mixed 3-Pb0 and PbSO^ d e p o s i t formed on an anode o p e r a t e d f o r 475 hours a t 30 mA/cm i n 2M H S 0 + 0.5M CuSOi,, 22°, f o l l o w e d by h o l d i n g a t o p e n - c i r c u i t i n t h e e l e c t r o l y t e f o r s e v e r a l hours. (lOOOx) 2  2  2  F i g u r e 6.54.  4  P r e d o m i n a n t l y PbS0 d e p o s i t formed on an anode o p e r a t e d f o r 195 hours a t 15.6 mA/cm i n 2M H S 0 + 0.5M CuSC*. 40°, f o l l o w e d by h o l d i n g a t o p e n - c i r c u i t i n t h e e l e c t r o l y t e f o r t h r e e days. (lOOOx) 4  2  2  4  189  s e v e r a l minutes i n i t i a l l y t o the o r d e r o f a few seconds above about 7.0 v o l t s , whereupon the anode p o t e n t i a l showed i n c r e a s i n g e r r a t i c i t y o f the order of several  hundred m u l t i v o l t s .  Higher-potential  operation  i s also  c h a r a c t e r i z e d by r a p i d p e r i o d i c (about 4/minute_at 8.5 v o l t s ) drops i n potential  by n e a r l y two v o l t s , f o l l o w e d by a slow r e c o v e r y  "steady" higher  6.5.2  Corrosion  to the r e l a t i v e l y  value.  Rates  The c o r r o s i o n r a t e s o f anodes which showed i r r e v e r s i b l e p o t e n t i a l v s . time b e h a v i o u r , o r which s u f f e r e d complete d e g r a d a t i o n , t r e a t e d i n S e c t i o n 6.2.1.  were n o t  C a l c u l a t i o n o f t h e c o r r o s i o n e f f i c i e n c i e s and  r a t e s f o r e l e c t r o d e s w h i c h , as a r e s u l t o f p r o l o n g e d a n o d i c  operation  and/or high c u r r e n t d e n s i t y o p e r a t i o n , showed very high anode p o t e n t i a l s (above 4 v o l t s v s . SHE) y e t were o t h e r w i s e unimparied i n t h e i r a b i l i t y t o s u s t a i n the a n o d i c c u r r e n t , showed no s i g n i f i c a n t d i f f e r e n c e s from the c o r r o s i o n r a t e s o f anodes which o p e r a t e d under l o w e r , r e v e r s i b l e p o t e n t i a l v s . time c o n d i t i o n s individual 6.21.  (Table 6.20).  Corrosion  e f f i c i e n c y and r a t e data f o r  anodes which s u f f e r e d complete d e g r a d a t i o n  C a l c u l a t i o n s were based on the t o t a l  f a i l u r e occurred,  are given  l e n g t h o f the run i n which  w i t h r e s p e c t t o the changes i n noble metal  over t h a t p a r t i c u l a r run.  i n Table  In most c a s e s f a i l u r e o c c u r r e d  loadings  a f t e r several  r e p e t i t i v e runs a t the same ( g e o m e t r i c ) c u r r e n t d e n s i t y , and the t o t a l time t o f a i l u r e i s g i v e n run i n which i t o c c u r r e d .  i n brackets  b e s i d e the time t o f a i l u r e f o r the  Depending on the l e n g t h o f the r u n , two groups  o f c o r r o s i o n e f f i c i e n c i e s can be d i s t i n g u i s h e d .  Table 6.20 C o r r o s i o n Data f o r Pt/30 I r - T i Anodes O p e r a t i n g Under I r r e v e r s i b l e Anode P o t e n t i a l (a) (b) Current Density Electrolyte  (Geometric) mA/cm  2  Run Time Hours  Conditions  2M H S 0 + 0.5M C u S 0 , 22° 2M H S 0 , 22° 2  4  2  4  4  Corrosion E f f i c i e n c y Corrosion"Rate Weight F r a c t i o n P t yg/A hr (Geometric Area)  (Cumulative)  Pt  Ir  Pt  V  Initial  Final  F i n a l Anode Potential v o l t s v s . SHE  (b)  104  194 (766)  0.73  0.44  1200  1300  .639  .641  5.95  (b)  104  191 (957)  0.64  0.29  . 1050  800  .641  .637  6.8  (a)  117  479 (1258)  1.52  0.83  2800  2650  .630  .616  5.26  (b)  260  304  0.41  0.23  1440  1850  .685  .690  4.87  (b)  260  455  0.50  0.13  5800  5400  .688  .675  (a)  260  185 (474)  1.48  0.75  5800  5600  .659  .657  12.5 4.86  191  The  (1)  L o n g - t e r m r u n s - t o - f a i 1 u r e showed c o r r o s i o n e f f i c i e n c e s o n l y s l i g h t l y h i g h e r than those f o r anodes o p e r a t e d under s i m i l a r c o n d i t i o n s but w i t h o u t c o m p l e t e d e g r a d a t i o n .  (2)  S h o r t - t e r m r u n s - t o - f a i 1 u r e gave h i g h c o r r o s i o n e f f i c i e n c i e s , w i t h the h i g h e s t v a l u e s o b t a i n e d for the s h o r t e s t runs.  f i r s t o b s e r v a t i o n can be e x p l a i n e d , i n the l i g h t o f the c o r r o s i o n r a t e s  found i n T a b l e 6.20,  by the low c o r r o s i o n r a t e c o n d i t i o n s t h a t p e r s i s t e d  o v e r most o f the run.  P o t e n t i a l vs. time o b s e r v a t i o n s  have shown t h a t  the f a i l u r e phenomenon i s a r a p i d one, w i t h anode l i f e t i m e b e i n g o n l y about t e n hours on a t t a i n m e n t attainment process  o f 10 v o l t s .  o f 8 v o l t s p o t e n t i a l , and l e s s than an hour on Thus the c o r r o s i o n r a t e s which occur d u r i n g  the  o f f a i l u r e are obscured by the much l o n g e r times over which the  anode c o r r o s i o n r a t e s were t y p i c a l l y low, as the l o a d i n g s can o n l y  be  measured p r i o r t o and f o l l o w i n g a g i v e n run.  one  Thus the i n f o r m a t i o n  may  e x t r a c t from anodes b e l o n g i n g  ing  t o group ( 2 ) , however, c l e a r l y show much h i g h e r c o r r o s i o n e f f i c i e n c i e s  and  r a t e s than a r e encountered w i t h o p e r a t i o n under c o n d i t i o n s where  complete d e g r a d a t i o n may  be c o n s i d e r e d  t o group (1) i s l i m i t e d .  d i d not o c c u r .  Anodes  In p a r t i c u l a r , case (8) i n T a b l e  t o t y p i f y the c o r r o s i o n c o n d i t i o n s d u r i n g the  p r o c e s s , as i t was  the o n l y example where the c o r r o s i o n was  the anode i n i t i a l l y  belong-  at a high, i r r e v e r s i b l e potential  w i t h t e r m i n a t i o n o f the run immediately  on a t t a i n m e n t  (11.5  6.21  failure  measured w i t h v o l t s vs.  o f the v o l t a g e  SHE) limit  o f the power s u p p l y - t h a t i s , over the p o t e n t i a l range where r a p i d f a i l u r e occurs. extremely  The  c o r r o s i o n r a t e s f o r both c o a t i n g m e t a l s were found to be  h i g h - 1.8  ...  grams/m -hr - i n d i c a t i n g t h a t much o f the c o a t i n g 2  metal i s l o s t w i t h i n o n l y a very s h o r t time. r a t e s of i r i d i u m and p l a t i n u m f o r t h i s and  The  e q u a l i t y of the c o r r o s i o n  the o t h e r cases i n group  (2)  T a b l e 6.21 C o r r o s i o n Data f o r P t / 3 0  I r - T i Anodes which have S u f f e r e d Complete  Electrolyte  C u r r e n t De • . i t y (Geometr c ) mA/cm z  Charge Passed to F a i l u r e A-hr  Degradation  2M H2SO,. + 0 . 5 H C u S C , 2 2 ° 2M H2SO,. + 0.5M C u S C , 40" 2M H : S 0 4 , 2 2 °  (a) (b) (c)  Time t o Failure Hours ( PIPIT" i " i t -  Run Time Hours  i \/o 1  * Noble Metal Loadings g(Pt + Ir)/m2  Corrosion Efficiency pg/A-hr  *  Corrosion rate (Geometric A r e a ) pg/hr-m2  Initial  Final  Pt  Ir  Pt  Ir  1.  (a)  52.1  410  2250 (4098)  2278  2 70  0 33  1.46  0.56  1500  590  2.  (b)  52.1  263  880 (2628  885  2 96  2 01  1.51  0.56  1200  800  3.  (b)  104  219  524 (1096)  536  3 26  1 39  2.24  1.20  3500  3450  4.  1 12  10.03  6.11  41000  42500  (b)  260  115  35 (230)  45  3 00  5.  (c)  104  199  37  (994)  46  2 62  0 99  22.04  12.23  36000  34500  6.  (c)  260  311  50 (622)  71  3 59  0 77  10.30  4.93  40000  38500  7.  (c)  260  20  39 5  44  2  0 16  13.91  6.81  53000  29 1 42  **  8.  (c)  260  228  9.  (d)  521  104  104  163  4 24  0 22  3.42  1.32  26000  10.  id)  521  90  90  95  4 45  0 77  5.00  2.24  38000  11.  (a)  +.5 g p l thiourea  52.1  21  23  4 53  2 14  12.  (a)  +.05 g p l thiourea  52.1  160  166  4 46  3 22  * :*  2.1 16  1610 sec(455)  R e f e r r e d t o t h e measured l o a d i n g s p r i o r t o and a f t e r particular run. New,  l o w - l o a d i n g anode.  1610 sec  t h e run i n which d e g r a d a t i o n  0 62  452  138 10.13  228  61 4.26  o c c u r r e d , with respect to the t o t a l  1.8(10)  55500 1.8(10)6  6  1.06(10)  22000 36000 5  770.0  99000 7000  time o f t h a t  CO  —1  193  are f u r t h e r e v i d e n c e f o r t h e d i f f e r e n c e o f t h e n a t u r e o f t h e c o r r o s i o n p r o c e s s e s o c c u r r i n g under c o n d i t i o n s o f f a i l u r e and a t lower p o t e n t i a l s . ( I t i s because o f t h e s p a l l i n g mechanism i n d i c a t e d f o r c o r r o s i o n o f anodes undergoing complete d e g r a d a t i o n t h a t c o r r o s i o n r a t e s were c a l c u l a t e d w i t h r e s p e c t t o g e o m e t r i c , r a t h e r than a c t u a l , s u r f a c e a r e a s . ) The charge passed n e c e s s a r y t o produce f a i l u r e o f t h e anodes i n v e s t i g a t e d i n t h i s work (average l o a d i n g :  4.33 g/m ) 2  can be determined  from t h e t i m e - t o - f a i l u r e o f i n d i v i d u a l anodes i n T a b l e 6.21.  This value  appears t o be more a p r o p e r t y o f a g i v e n anode r a t h e r than i t s o p e r a t i n g c u r r e n t d e n s i t y o r t e m p e r a t u r e , a l t h o u g h anodes employed a t t h e h i g h e s t current densities lifetimes.  (521 rnA/cm g e o m e t r i c a r e a ) showed the l o w e s t charge 2  Charge-to f a i l u r e v a l u e s were found t o v a r y between 90 and 410  ampere*hours f o r o p e r a t i o n i n e i t h e r t e s t e l e c t r o l y t e a t c u r r e n t d e n s i t i e s r a n g i n g from 52.1 t o 521 mA/cm . 2  A new l o w - l o a d i n g anode (case ( 7 ) , T a b l e  6.21) showed a c o n s i d e r a b l y s m a l l e r c h a r g e - t o - f a i l u r e v a l u e (20 A«hr), but otherwise t y p i c a l g/m ) 2  c o r r o s i o n e f f i c i e n c i e s and r a t e t o an ( i n i t i a l l y 4.33  anode,whose l o a d i n g had decreased t o a s i m i l a r v a l u e p r i o r t o  commencement o f t h e run i n which f a i l u r e o c c u r r e d , and which was o p e r a t e d a t t h e same ( g e o m e t r i c ) c u r r e n t d e n s i t y (compare case (4) i n T a b l e  6.21).  T h i o u r e a a d d i t i o n s were found t o produce f a i l u r e a f t e r o n l y 2.1 A-hr i n one case (.5 g p l a d d i t i o n ) and 16 A*hr i n a n o t h e r (0.5 g p l a d d i tion).  While the c o a t i n g l o s s r a t e s d u r i n g o p e r a t i o n where f a i l u r e does  not o c c u r a r e h i g h e r when t h i o u r e a i s added ( T a b l e 6.15) t h e u l t i m a t e complete a n o d i c d e g r a d a t i o n i s not a s s o c i a t e d w i t h removal o f more c o a t i n g metal than i s t h e case f o r f a i l u r e i n t h e cases where no a d d i t i v e s were present.  Indeed, t h e c o n v e r s e i s t r u e , w i t h case (12) i n T a b l e 6.21  a r e s i d u a l l o a d i n g o f 3.22 g/m  2  after  failure.  showing  194  6.5.3  Morphology The  predominant f e a t u r e s on t h e s u r f a c e s o f c o m p l e t e l y  degraded  anodes ( F i g u r e s 6.55-6.58) a r e b e n t , c o n i c a l p r o j e c t i o n s which appear only a f t e r operation a t the voltage l i m i t  (50 v o l t s ) .  With extended  opera-  t i o n a t t h i s p o t e n t i a l (60 hours) such f e a t u r e s a r e t h e o n l y d i s c e r n a b l e a s p e c t o f s u r f a c e morphology w i t h no c o a t i n g metal e v i d e n t by X-ray s p e c t r o s c o p y ) .  (as c o n f i r m e d  H i g h e r m a g n i f i c a t i o n ( F i g u r e 6.56) shows t h a t t h e  b e n t , c o n i c a l p r o j e c t s a r e a c t u a l l y composed o f many f i n e l a y e r s p e r p e n d i c u l a r t o t h e a x i s o f t h e cone.  arranged  Under c o n d i t i o n s o f l e s s - p r o l o n g e d  o p e r a t i o n under v o l t a g e - l i m i t c o n d i t i o n s (20 h o u r s ) , remnants o f c o a t i n g metal a r e s t i l l  visible.  The c o n i c a l p r o j e c t i o n s , however, appear t o  grow next t o and even beneath t h e p i e c e s o f c o a t i n g metal 6.57).  (Figure  The l e a d - c o n t a i n i n g s u r f a c e d e p o s i t s were f o u n d , on f a i l u r e , t o  have changed i n nature from 3-Pb0  2  t o PbSO^ ( F i g u r e 6.58).  Such a t r a n s -  f o r m a t i o n , which occurs o n l y under o p e n - c i r c u i t c o n d i t i o n s , suggests t h a t the d e p o s i t had become e l e c t r i c a l l y i s o l a t e d from t h e s u b s t r a t e .  6.5.4  Re-coating An anode which had s u f f e r e d complete d e g r a d a t i o n  s p u t t e r e d p l a t i n u m c o a t i n g e q u i v a l e n t t o 4 g/m  2  and then employed as an  anode i n 2M H S 0 , 22°. S u r f a c e area d e t e r m i n a t i o n 2  o f 2mA/cm  2  4  (geometric  was g i v e n a  using charging  currents  area) c o u l d be performed w i t h o u t d i f f i c u l t y ( g i v i n g  a roughness f a c t o r o f 3 2 ) , b u t a n o d i c o p e r a t i o n a t 52.1 mA/cm  2  (geometric  a r e a ) caused t h e ; . p o t e n t i a l t o r i s e t o t h e v o l t a g e l i m i t w i t h i n 100 seconds. Recovery o f normal anode behaviour  w i t h r e c o a t i n g , hence, cannot be  F i g u r e 6.55.  SEM view o f the s u r f a c e o f an anode which has undergone comp l e t e d e g r a d a t i o n as a r e s u l t o f o p e r a t i o n f o r 163 hours a t 521 mA/cm (geometric area) i n 2M H SC\, 22°. T o t a l remaini n g l o a d i n g : 0.22 g/m . (lOOOx) 2  2  2  F i g u r e 6.56.  SEM view o f an i n d i v i d u a l bent, c o n i c a l p r o j e c t i o n found a f t e r c o n t i n u e d anodic o p e r a t i o n a t the v o l t a g e l i m i t (50 v o l t s ) . (lOOOx)  F i g u r e 6.57.  SEM view o f the s u r f a c e o f an anode which has undergone comp l e t e d e g r a d a t i o n as a r e s u l t o f o p e r a t i o n f o r a t o t a l o f 622 hours a t 260 mA/cm (geometric a r e a ) i n 2M H S0 , 22°. T o t a l r e m a i n i n g l o a d i n g : 0.77 g/m . (lOOOx) 2  2  k  2  F i g u r e 6.58.  SEM view o f the s u r f a c e o f an anode which has undergone comp l e t e d e g r a d a t i o n as a r e s u l t o f o p e r a t i o n f o r a t o t a l o f 1096 hours a t 104 mA/cm (geometric a r e a ) i n 2M H 2 S O 4 + 0.5M CuSO^, 40°. T o t a l remaining l o a d i n g : 1.39 g/m . (lOOOx) 2  2  197  a c c o m p l i s h e d w i t h o u t p r i o r t r e a t m e n t o f t h e degraded  s u r f a c e t o remove  the c o r r o s i o n p r o d u c t s .  6.5.5  I d e n t i f i c a t i o n of the Surface Degradation Product On d e g r a d a t i o n , s e v e r a l new peaks appear i n t h e X-ray  p a t t e r n , which c o r r e s p o n d w e l l t o t h o s e o f T i 0  2  diffraction  (anatase), the expected  c o r r o s i o n p r o d u c t on bare t i t a n i u m a t high anode p o t e n t i a l s .  Otherwise,  used anodes showed no d i f f e r e n c e from new m a t e r i a l w i t h r e s p e c t t o t h e n a t u r e o f the c r y s t a l l i n e  species.  Chapter 7  DISCUSSION  7.1  Corrosion  7.1.1 7.1.1.1  During R e v e r s i b l e Anode M a t e r i a l  P o t e n t i a l v s . Time O p e r a t i o n from t h e Same Source  The c o r r o s i o n  b e h a v i o u r o f t h e Pt/30 I r - T i  used f o r most o f t h e p r e s e n t work (average l o a d i n g : sidered  t o be p r e d o m i n a n t l y e l e c t r o c h e m i c a l  conditions  investigated  (7.8 - 521 mA/cm  2M H2SO.4 + 0.5M CuSOit, 22-80°).  2  anode m a t e r i a l  4.33  g/m ) 2  i s con-  i n n a t u r e under t h e range o f g e o m e t r i c a r e a i n 2M H2S0,+ and  E v i d e n c e f o r t h i s may be summarized:  1.  t h e p l a t i n u m and i r i d i u m c o r r o s i o n r a t e s w i t h respect t o t h e i r p a r t i a l surface areas (either geometric o r actual) a r enotequal. Under p u r e l y mechanical c o a t i n g detachment, a homogeneous a l l o y must show e q u a l c o r r o s i o n r a t e s f o r each a l l o y c o n s t i t u e n t .  2.  t h e r e m a i n i n g c o a t i n g m e t a l becomes e n r i c h e d i n i r i d i u m due t o t h e p r e f e r e n t i a l d i s s o l u t i o n o f platinum. O p e r a t i o n under p u l s e d c u r r e n t cond i t i o n s , i n p a r t i c u l a r , produces high i r i d i u m l e v e l s i n the remaining coating metal. Under p u r e l y mechanical detachment conditions, a homogeneous a l l o y w o u l d r e m a i n u n c h a n g e d i n compos i t i o n .  3.  t h e ratios of thecorrosion e f f i c i e n c i e s o f the c o a t i n g metals ( l r : P t ) , as a consequence o f 198  199  t h e i r weight f r a c t i o n s i n the a l l o y . The o b s e r v e d r a t i o s t a k e n f r o m t h e w e i g h t e d mean v a l u e s o f a l l r u n s i n 2M r^SOit and i n 2M H 2 S C H + 0.5M CuSC\ w e r e 0 . 3 ^ 8 a n d 0 . 3 1 0 , r e s p e c t i v e l y , compared w i t h t h e v a l u e o f 0 . 4 5 6 w h i c h w o u l d o c c u r a s a r e s u l t o f p u r e l y mecha n i c a l c o r r o s i o n o f t h e a l l o y (w = O.087). h.  5.  t h e r e i s a p a r a l l e l i s m between t h e c o r r o s i o n r a t e s o f b o t h p l a t i n u m and i r i d i u m , and t h e t o t a l a n o d e r e a c t i o n r a t e ( a s g i v e n by t h e applied current d e n s i t y ) . This i s manifested both i n t h e l i n e a r i t y o f t h e c o r r o s i o n c u r r e n t d e n s i t y v s . a p p l i e d c u r r e n t d e n s i t y p l o t s and in t h e l a c k o f dependence o f t h e c o r r o s i o n e f f i c i e n c i e s on a p p l i e d c u r r e n t d e n s i t y . Such behaviour i s c h a r a c t e r i s t i c o f the e l e c t r o chemical d i s s o l u t i o n o f p l a t i n u m [ 9 9 , 1 0 ] ] and r u t h e n i u m [206] i n s u l f u r i c a c i d s o l u t i o n s . (No l i t e r a t u r e e x i s t s concerning the anodic d i s s o l u t i o n o f i r i d i u m metal under s i m i l a r c o n d i t o n s . ) t h e low v a l u e s f o r e f f i c i e n c y . A weighted mean v a l u e o f 1.13 u g / A h r was ( d e t e r m i n e d i n 2M H 2 S O 4 + 0.5M C u S 0 ) c o m p a r e s f a v o u r a b l y w i t h t h e v a l u e t h a t c a n be c a l c u l a t e d f r o m t h e d a t a o f Chemodanov [ 9 9 , 1 0 1 ] ( 1 . 8 u g / A * h r ) f o r t h e a n o d i c d i s s o l u t i o n o f platinum. To be e x a c t , t h e c o r r o s i o n e f f i c i e n c i e s s h o u l d be c a l c u l a t e d w i t h r e s p e c t t o t h e amount o f c h a r g e p a s s e d on t h e p a r t i a l areas corresponding to the i n d i v i d u a l metals. If i t i s assumed t h a t t h e p a r t i a l c u r r e n t d e n s i t i e s a r e equal t o t h e a p p l i e d c u r r e n t d e n s i t y then t h e c o r r o s i o n e f f i c i e n c i e s c a n be c a l c u l a t e d f o r p u r p o s e s o f d i r e c t c o m p a r i s o n w i t h t h a t o f p u r e 1".. p l a t i n u m , by d i v i s i o n w i t h t h e a r e a f r a c t i o n (assumed e q u a l t o t h e w e i g h t f r a c t i o n ) o f p l a t i n u m . U s i n g a mean v a l u e o f 0 . 6 7 f o r t h e a r e a f r a c t i o n o f p l a t i n u m , t h e c o r r o s i o n e f f i c i e n c y i s hence I . 6 9 y g / A ' h r i n 2M H 2 S O 4 + 0.5M C u S 0 electrolyte. The s m a l l d i s c r e p a n c y f o r t h e p l a t i n u m c o r r o s i o n e f f i c i e n c y found i n t h i s work and t h a t r e p o r t e d by Chemodanov may l i k e l y be d u e t o t h e p o s s i b i l i t y t h a t t h e r e s u l t s o f Chemodanov do n o t r e p r e s e n t s t e a d y - s t a t e c o r r o s i o n c o n d i t i o n s , due t o t h e unsuitabi1 i t y of the r a d i o t r a c e r technique f o r measurement p e r i o d s as l o n g as t h o s e e m p l o y e d i n t h i s work. 4  4  The c o r r o s i o n c u r r e n t d e n s i t i e s c a l c u l a t e d f o r platinum, with respect to the actual surface area, a l s o compare w e l l w i t h t h e 10~6 v a l u e f o r t h e c u r r e n t e f f i c i e n c y o f p l a t i n u m c o r r o s i o n d e t e r m i n e d by Chemodanov.  200  In one p a r t i c u l a r case w i t h the "4.33  g/m " 2  anode m a t e r i a l  however, c l e a r e v i d e n c e was found f o r a p r e d o m i n a n t l y mechanical l u t i o n mechanism.  disso-  Thus the n a t u r e o f the c o r r o s i o n b e h a v i o u r o f Pt/30  I r - T i anodes may v a r y o v e r the same anode sheet.  The v a r i a b i l i t y o f t h e  n a t u r e o f the c o r r o s i o n process o v e r the s u r f a c e o f a g i v e n anode sheet can e x p l a i n the d i f f e r e n c e s i n s t a t i s t i c a l l y r e l i a b l e c o r r o s i o n  rates  and e f f i c i e n c i e s c a l c u l a t e d f o r e x p e r i m e n t a l anodes c u t from t h a t s h e e t and o p e r a t e d under s i m i l a r c o n d i t i o n s . The a n o d i c c o r r o s i o n b e h a v i o u r o f i r i d i u m was found t o be analogous t o t h a t o f p l a t i n u m , a l b e i t w i t h s m a l l e r c o r r o s i o n r a t e s and efficiencies.  The p a r a l l e l i s m w i t h the o v e r a l l r e a c t i o n r a t e  (oxygen  e v o l u t i o n ) as g i v e n by the a p p l i e d c u r r e n t d e n s i t y suggests t h a t , as i n the case o f p l a t i n u m , the p a r t i a l  c o r r o s i o n p r o c e s s e s and the p r o c e s s o f  oxygen e v o l u t i o n share common s t e p s . [ 9 9 , 1 0 1 ] .  7.1.1.2  Anode M a t e r i a l From D i f f e r e n t M a n u f a c t u r i n g L o t s ' Anode m a t e r i a l s o f n o m i n a l l y i d e n t i c a l  a l l o y c o m p o s i t i o n and  manufacture, but from d i f f e r e n t m a n u f a c t u r i n g l o t s , have w i d e l y d i f f e r e n t c o r r o s i o n b e h a v i o u r under s i m i l a r o p e r a t i n g  c o n d i t i o n s w i t h no o b v i o u s  dependence on g e o m e t r i c o r a c t u a l s u r f a c e a r e a s , o r t o i n i t i a l noble metal l o a d i n g .  total  Comparison o f p l a t i n u m and i r i d i u m c o r r o s i o n  e f f i c i e n c i e s shows t h a t most o f the o t h e r anode m a t e r i a l s  investigated  showed h i g h e r c o r r o s i o n e f f i c i e n c i e s f o r both c o a t i n g m e t a l s than the anode m a t e r i a l which was used f o r the b u l k o f the p r e s e n t work loading:  4.33  g/m ). 2  (average  As the c o r r o s i o n r a t e r e s u l t s s t r o n g l y i n d i c a t e  a predominantly electrochemical  mechanism f o r metal l o s s f o r the  201  "4.33  g/m " 2  anodes, the h i g h e r v a l u e s o f c o r r o s i o n e f f i c i e n c y r e p o r t e d  f o r o t h e r anode m a t e r i a l s o f n o m i n a l l y i d e n t i c a l c o m p o s i t i o n may be a t t r i b u t e d t o the r e l a t i v e predominance o f mechanical c o r r o s i o n i n these cases. F a c t o r s which c o u l d promote high mechanical d i s s o l u t i o n  rates  o f new anodes i n c l u d e : 1)  p o o r c o a t i n g / s u b s t r a t e a d h e s i o n due t o i m p r o p e r s u b s t r a t e p r e t r e a t m e n t and/or e x c e s s i v e o x i d a tion during the coating procedure,  2)  p o o r a d h e s i o n among s u c c e s s i v e n o b l e m e t a l c o a t i n g s due t o i m p r o p e r h e a t t r e a t m e n t t o p r o m o t e i n t e r d i f f u s i o n bonding,  3)  h i g h c o a t i n g p o r o s i t y due t o t h e a p p l i c a t i o n o f high noble metal l o a d i n g s which would f a c i l i t a t e c o a t i n g m e t a l l o s s by m e c h a n i c a l a c t i o n o f bubbles generated w i t h i n the pore.  A l t h o u g h the p l a t i n u m c o r r o s i o n e f f i c i e n c i e s determined f o r new Pt/30  Ir-Ti  anodes from d i f f e r e n t sources ranged from 0.75 t o 5.32 ug/A«hr, and those f o r i r i d i u m from 0.26 t o 2.07 ug/A-hr, i t i s l i k e l y t h a t o t h e r anodes may e x h i b i t y e t p o o r e r b e h a v i o u r due t o g r e a t e r d e v i a t i o n s from optimum m a n u f a c t u r i n g procedure.  From c o n s i d e r a t i o n o f t h e l i t e r a t u r e i t i s  c l e a r t h a t the method o f manufacture  has a profound e f f e c t on the c o r r o s i o n  o f P t - T i , w i t h anodes h a v i n g e l e c t r o d e p o s i t e d c o a t i n g s t y p i c a l l y  exhibit-  i n g h i g h e r c o r r o s i o n e f f i c i e n c i e s than those h a v i n g c o a t i n g s produced by the thermal d e c o m p o s i t i o n method, due t o the predominance o f mechanical d i s s o l u t i o n i n t h e former case. In o r d e r t o c h a r a c t e r i z e t h e c o r r o s i o n b e h a v i o u r o f Pt/30 I r - T i anode m a t e r i a l from o t h e r s o u r c e s under a p a r t i c u l a r s e t o f o p e r a t i n g c o n d i t i o n s , i t i s thus n e c e s s a r y ( o r a t l e a s t , w i s e ) t o perform a d d i t i o n a l  202  experiments of a "standard"  type i n o r d e r to o b t a i n s t a t i s t i c a l l y  c o r r o s i o n r a t e s and e f f i c i e n c y  7.1.2  values.  S u r f a c e Oxygen Coverage and The  reliable  Corrosion  s u r f a c e oxygen coverage r e s u l t s o b t a i n e d  i n the  present  work show t h a t the p l a t i n u m / i r i d i u r n a l l o y c o a t i n g s behave i n a manner s i m i l a r to p l a t i n u m , showing a l o g a r i t h m i c i n c r e a s e i n coverage from 0 = 1 a t the commencement o f oxygen e v o l u t i o n to 2 < 0 < 3 o v e r the d u r a t i o n o f most of the i n d i v i d u a l  runs employed.  The  corrosion r e s u l t s obtained in  the p r e s e n t work thus r e p r e s e n t the combined e f f e c t s of c o r r o s i o n phenomena i n two r e g i o n s o f oxygen coverage. 1)  2)  As i n d i v i d u a l  1 - 0 5 2 . The f i r s t s e v e r a l h o u r s o f e l e c t r o l y s i s , which r e p r e s e n t the p e r i o d o f g r e a t e s t change o f oxygen coverage w i t h time, are a l s o c h a r a c t e r i z e d by: (a)  a rapid.climb [99,101],  in anode p o t e n t i a l w i t h  (b)  a r a p i d drop in the rate of platinum.  instantaneous  time  corrosion  2 ^ 0 5 3 . A f t e r 1-2 d a y s t h e s u r f a c e o x y g e n c o v e r a g e c h a n g e s o n l y s l i g h t l y as a c o n s e q u e n c e of the l o g a r i t h m i c coverage/time r e l a t i o n . F u r t h e r , the anode p o t e n t i a l i n c r e a s e s o n l y s l o w l y t h e r e after. The c o r r o s i o n r a t e o f p l a t i n u m , f r o m r a d i o c h e m i c a l w o r k , a l s o assumes a r e l a t i v e l y steady va1ue.  run l e n g t h s were t y p i c a l l y o f 8 days' d u r a t i o n , and o v e r  days i n some s p e c i f i c c a s e s , the c o r r o s i o n r e s u l t s may r e p r e s e n t v i r t u a l l y steady the e q u i v a l e n t o f 2-3  be c o n s i d e r e d  to  s t a t e d i s s o l u t i o n from a s u r f a c e c o n t a i n i n g  oxygen monolayers, as the c o n t r i b u t i o n due  high_non-steady d i s s o l u t i o n r a t e o f p l a t i n u m  i s small.  Further,  t o the no  100  203  e f f e c t s o f run d u r a t i o n on c o r r o s i o n r a t e were observed i n the  present  work. I t i s i n t e r e s t i n g t o note t h a t Chemodanov [99,101] r e p o r t s t h a t anodic d i s s o l u t i o n o f p l a t i n u m o n l y o c c u r s a t p o t e n t i a l s above which the e q u i v a l e n t o f a second monolayer o f oxygen i s d e p o s i t e d on and  platinum,  s t a t e s t h a t the r a t e of d i s s o l u t i o n f o r anodes o f lower oxygen  coverage i s s m a l l .  T h i s i s a consequence o f the "steady  h i s experiments (he o n l y c o n s i d e r e d  s t a t e " nature  the d i s s o l u t i o n r a t e o f p l a t i n u m  s e v e r a l h o u r s , where i t no l o n g e r changed r a p i d l y w i t h t i m e ) .  of  after  The p a s s i -  v a t i o n r e s u l t s i n t h i s and o t h e r work, however, c l e a r l y show t h a t the oxygen coverage on p l a t i n u m  i n c r e a s e s from 0 = 1  to 0 = 2 a f t e r commence-  ment o f oxygen e v o l u t i o n , and t h a t t h i s i n c r e a s e i n oxygen coverage t a k e s p l a c e s i m u l t a n e o u s l y w i t h the r a p i d drop i n c o r r o s i o n r a t e w i t h time observed by Chenodanov.  Thus the a t t a i n m e n t  o f the e q u i v a l e n t o f a second  monolayer o f oxygen, f a r from being a p r e r e q u i s i t e f o r the a n o d i c d i s s o l u t i o n of p l a t i n u m  i n s u l f u r i c a c i d s o l u t i o n s , i s indeed  o f a " p a s s i v e " s t a t e w i t h r e s p e c t to d i s s o l u t i o n .  the  attainment  As oxygen coverage  does not grow much beyond t h i s l e v e l w i t h c o n t i n u e d e l e c t r o l y s i s ,  the  i n c r e a s e i n the d i s s o l u t i o n r a t e o f p l a t i n u m w i t h c u r r e n t d e n s i t y i s due t o the g r e a t e r s u r f a c e c o n c e n t r a t i o n o f i n t e r m e d i a t e s  i n the oxygen e v o l u -  t i o n r e a c t i o n , whose i n v o l v e m e n t i n the p l a t i n u m d i s s o l u t i o n r e a c t i o n i s s t r o n g l y suggested by the p a r a l l e l i s m o f the r a t e s o f oxygen e v o l u t i o n and  platinum  dissolution.  204  7.1.3  Pulsed  Electrolysis  Although  no d e f i n i t e c o n c l u s i o n s can be drawn from the  limited  number o f p u l s e d e l e c t r o l y s i s experiments performed, the r e s u l t s o f runs c e r t a i n l y show the e x t r e m e l y  d e l e t e r i o u s nature  these  of p e r i o d i c c u r r e n t  r e v e r s a l , p a r t i c u l a r l y w i t h r e s p e c t t o p l a t i n u m d i s s o l u t i o n from the coatings ing  (as apparent from the high i r i d i u m l e v e l s i n the remaining  metal).  While p e r i o d i c c u r r e n t r e v e r s a l p e r m i t s  operation with residual  noble metal l o a d i n g s w e l l below those encountered on complete o f anodes operated  under continuous  D.C.  (as low as  .16 g/m  2  degradation i n one  the r a t e s o f noble metal l o s s are n e v e r t h e l e s s so high (up t o 109 for  platinum  coat-  i n one case) t h a t the anode l i f e t i m e i s not  case)  ug/A-hr  substantially  improved. The  observed c o r r o s i o n r a t e s can l i k e l y be a t t r i b u t e d t o the  high non-steady d i s s o l u t i o n o f p l a t i n u m occur immediately  (and by a n a l o g y , i r i d i u m ) which  on s w i t c h i n g on the anodic  c u r r e n t d u r i n g each c y c l e , as  the s u r f a c e oxygen coverage i s reduced d u r i n g the p r e c e d i n g a " f r e s h l y reduced" s u r f a c e f o r each subsequent o n - p u l s e . ential  c o r r o s i o n o f p l a t i n u m i s i n f e r r e d from the A.C.  o f the two metals d i s c u s s e d  i n S e c t i o n 2.5.  reversal, leaving The  prefer-  corrosion  behaviour  Longer r e v e r s a l t i m e s , found  to produce more i n t e n s e c o r r o s i o n , l i k e l y i n v o l v e s i g n i f i c a n t amounts o f hydrogen being absorbed i n t o the s u b s t r a t e , which i s r e l e a s e d d u r i n g subsequent on-pulse a t such a r a p i d r a t e t h a t the c o a t i n g i s l i f t e d spalls.  the and  Such a mechanism has a l r e a d y been suggested f o r P t - T i e l e c t r o d e s  subjected to r e p e t i t i v e anodic/cathodic procedure [ 7 3 ] .  e l e c t r o l y s i s i n an  electroplating  205  The  lack of s i m i l a r l y d e l e t e r i o u s e f f e c t s with  electrolysis  w i t h h i g h - f r e q u e n c y o n / o f f p u l s i n g (4.4 usee on/1.6 y s e c o f f ) i s a consequence o f the i n a b i l i t y o f the anode t o become reduced d u r i n g off-cycle.  the  short  D u r i n g the time the e x t e r n a l c u r r e n t i s i n t e r r u p t e d the double  l a y e r capactance w i l l thus m a i n t a i n i n g The  tend t o d i s c h a r g e  through the " f a r a d a i c r e s i s t a n c e "  the a n o d i c c u r r e n t .  high-frequency pulsed-current  operation  t h i c k e s t l e a d d i o x i d e d e p o s i t on the anode t h a t was p r e s e n t work.  Such a r e s u l t may  current density, i ^ ,  produced  determined i n the  be r e a d i l y understood i f the  f o r mass-transfer  the  limiting  c o n t r o l l e d d e p o s i t i o n of lead i s  considered:  \  Any  process tending  discharge  =  [  2 F  D  Pb 2j[ +  to decrease 6 w i l l  o f t r a c e l e a d i o n s and  d i o x i d e on the e l e c t r o d e  5  j  (  r e s u l t i n an i n c r e a s e d  rate of  hence produce h i g h e r amounts o f l e a d  surface.  With l o n g e r o p e n - c i r c u i t " o f f - t i m e " oxygen e v o l u t i o n ceases and  the e l e c t r o d e p o t e n t i a l d r o p s .  however, and  hence the e l e c t r o d e  The  i s not  p r i o r t o the subsequent " o n - p u l s e . "  oxygen coverage i s not a f f e c t e d , i n a " f r e s h l y reduced" s t a t e  Thus the high l e v e l s o f metal d i s s o -  l u t i o n c h a r a c t e r i s t i c o f c u r r e n t - r e v e r s a l o p e r a t i o n would not be although  expected  i t i s apparent t h a t the c o r r o s i o n l o s s e s are h i g h e r than f o r  continuous anodic current operation. anodes which are not p e r m i t t e d  These r e s u l t s a g a i n  suggest t h a t  t o form oxygen coverages up t o and  beyond  206  the e q u i v a l e n t o f two monolayers show h i g h e r c o r r o s i o n l o s s e s due t o the i n a b i l i t y o f the e l e c t r o d e t o reach "steady s t a t e " c o r r o s i o n c o n d i t i o n s c h a r a c t e r i s t i c o f these  7.1.4  coverages.  Complete D e g r a d a t i o n Over the course o f i t s o p e r a t i n g l i f e t i m e a g i v e n anode passes  through stages o f r e v e r s i b l e and i r r e v e r s i b l e p o t e n t i a l v s . time b e h a v i o u r , f o l l o w e d by complete  degradation.  In the i n i t i a l  case ( r e v e r s i b l e ) the  anode behaves as i f i t were a s o l i d noble metal e l e c t r o d e , a b l e to s u s t a i n high a p p l i e d a n o d i c c u r r e n t s a t low o v e r v o l t a g e s whereas on d e g r a d a t i o n t h e anode behaves e s s e n t i a l l y as uncoated  t i t a n i u m , and i s unable t o  c a r r y the anode c u r r e n t w i t h o u t a r a p i d r i s e i n p o t e n t i a l , i n s p i t e o f the presence o f c o n s i d e r a b l e r e s i d u a l c o a t i n g m e t a l .  From the c o r r o s i o n  r e s u l t s o b t a i n e d f o r anodes which s u s t a i n e d f a i l u r e , o r imminent f a i l u r e , a change i n the nature o f the c o r r o s i o n mechanism from e l e c t r o c h e m i c a l to p r e d o m i n a n t l y mechanical of  complete  potential  predominantly  i s i n d i c a t e d w i t h the onset  d e g r a d a t i o n , as m a n i f e s t e d by an e x p o n e n t i a l i n c r e a s e i n anode  t o t h e p o t e n t i a l - l i m i t o f the power s u p p l i e s . Such behaviour can be e x p l a i n e d by the growth o f a l a y e r o f  t i t a n i u m o x i d e between the c o a t i n g and s u b s t r a t e metals whose c o n d u c t i v i t y decreases from i n i t i a l l y e l e c t r o n i c i n n a t u r e (due to a h i g h i m p u r i t y c o n t e n t ) to a much lower v a l u e [ 2 1 5 ] . potential  T h i s would e x p l a i n the  v s . time b e h a v i o u r o f c o a t e d anodes a f t e r p r o l o n g e d a n o d i c  electrolysis,  Complete d e g r a d a t i o n would be a s s o c i a t e d w i t h d i e l e c t r i c  breakdown o f t h i s f i l m when i t s growth causes e l e c t r i c a l coating.  irreversible  Evidence f o r t h i s appears  i s o l a t i o n o f the  i n the p e r i o d i c appearance o f " s p i k e s "  207  i n the p o t e n t i a l v s . time r e l a t i o n as t h e p o t e n t i a l begins t o c l i m b exponentially. across  These may be a s s o c i a t e d w i t h u n s u s t a i n e d  the o x i d e .  Coating  e l e c t r o n avalanches  l o s s e s due t o s p a l l i n g would be a s s o c i a t e d  w i t h such l o c a l d e s t r u c t i o n o f the u n d e r l y i n g o x i d e f i l m  On  attainment  o f the v o l t a g e l i m i t o f t h e power s u p p l i e s , l o c a l i z e d t i t a n i u m d i s s o l u t i o n occurs  a t f i x e d s i t e s s c a t t e r e d o v e r the anode s u r f a c e , f o l l o w e d by  p r e c i p i t a t i o n as a s p a r i n g l y s o l u b l e o x i d e o r h y d r o x i d e , conical  growths observed on S.E.M. o b s e r v a t i o n  i n a c t i v i t y or e l e c t r i c a l completely  p r o d u c i n g the  o f f a i l e d anodes.  The  i s o l a t i o n o f t h e r e s i d u a l c o a t i n g metal on a  degraded anode i s c l e a r l y shown by t h e presence o f the c o n i c a l  growths s u r r o u n d i n g  and even p r o t r u d i n g from beneath the r e m a i n i n g  coating  metal.  7.1.5  Thiourea A d d i t i o n s The  i n c r e a s e d d i s s o l u t i o n o f both c o a t i n g metals w i t h  thiourea  a d d i t i o n , p r i o r t o anode f a i l u r e , may be due e i t h e r t o : 1)  f a c i l i t a t e d s o l u t i o n due t o t h e f o r m a t i o n complexes w i t h t h i o u r e a o r i t s r e a c t i o n in termed i a t e s ,  of  2)  e x t e n s i v e p o i s o n i n g o f t h e a n o d e s u r f a c e due to " d e s t r u c t i v e " adsorption of thiourea (with c l e a v a g e o f t h e C = S bond) [104,1 I 2 , 1 1 6 - 1 1 8 ] , w h i c h i n t u r n f o r c e s t h e oxygen e v o l u t i o n and c o a t i n g d i s s o l u t i o n r e a c t i o n s t o o c c u r on a much s m a l l e r p o r t i o n o f t h e anode s u r f a c e a t o m e f f e c t i v e l y greater current density.  or  Prolonged h i g h - p o t e n t i a l o p e r a t i o n as a r e s u l t o f t h i o u r e a a d d i t i o n t o premature anode f a i l u r e  leads  ( a f t e r passage o f o n l y 2.1 A*hr i n one c a s e ,  compared w i t h 410 A«hr f o r an e l e c t r o d e o p e r a t e d under s i m i l a r c o n d i t i o n s in thiourea -free e l e c t r o l y t e ) .  Such r a p i d f a i l u r e can o n l y be r e c o n c i l e d  208  w i t h the l a r g e f r a c t i o n o f c o a t i n g metal which remains i f the i s o l a t i o n o f the c o a t i n g metal i s a g a i n c o n s i d e r e d . i n anode p o t e n t i a l which occurs  immediately  The  electrical  large increase  after thiourea addition  promote the r a p i d a b s o r p t i o n o f oxygen through the noble metal i n S e c t i o n 7.2.4), thus l e a d i n g t o the m o r e - r a p i d e s t a b l i s h m e n t i n s u l a t i n g f i l m between the c o a t i n g and  may  (discussed o f an  s u b s t r a t e than i n the case of  anodic o p e r a t i o n i n t h i o u r e a - f r e e s o l u t i o n s where the p o t e n t i a l does not r i s e as q u i c k l y .  7.2 7.2.1  Passivation S u r f a c e Area The  Studies  gal v a n o s t a t i c charge c u r v e s  f o r P-f/30 I r r T i  electrodes  resemble those f o r p l a t i n u m and p l a t i n u m / i r i d i u m a l l o y w i r e  electrodes  inasmuch as they show h y s t e r e s i s between the oxygen l a y e r f o r m a t i o n removal r e g i o n s , y e t e x h i b i t charge balance processes.  I t i s p o s s i b l e , however, t h a t the charge c u r v e s may  a mixed s i t u a t i o n w i t h s e p a r a t e and  removal on both p l a t i n u m and  formation  between the anodic and  and  c o n t r i b u t i o n s due  cathodic  represent  t o oxygen l a y e r f o r m a t i o n  i r i d i u m - i n which case the  irreversible  b u i l d - u p o f the e q u i v a l e n t o f m u l t i l a y e r i r i d i u m  c o u l d be p o s t u l a t e d by analogy w i t h the known behaviour such e v e n t ,  and  oxides  of i r i d i u m .  In  the s u r f a c e areas measured i n the p r e s e n t work would r e f e r o n l y  to t h a t p o r t i o n , o f the anode s u r f a c e which c o n s i s t s o f p l a t i n u m . e v i d e n c e f o r m u l t i l a y e r coverages p r i o r to oxygen e v o l u t i o n was  No found i n  t h i s work, however. Observations  o f a small  (up to 10 per c e n t ) i n c r e a s e i n e l e c -  t r o c h e m i c a l l y a c t i v e s u r f a c e a r e a w i t h prolonged  galvanostatic operation,  209  f o l l o w e d b y a decrease i n t h i s v a l u e , i s c o n s i s t e n t w i t h the  competing  p r o c e s s e s o f noble metal s u r f a c e roughening due to a n o d i c d i s s o l u t i o n and the complete l o s s , or r e d u c t i o n i n s i z e , o f i n d i v i d u a l c o a t i n g Such a mechanism i s a l s o c o n s i s t e n t w i t h the roughness  features.  f a c t o r vs. loading  r e l a t i o n observed f o r used Pt/30 I r - T i anodes, where the s u r f a c e a r e a s of e l e c t r o d e s which had l o s t 20-30 per c e n t o f t h e i r l o a d i n g were lower than those o f s i m i l a r e l e c t r o d e s which had h i g h e r l o a d i n g s .  7.2.2  S u r f a c e Oxygen Coverage For p l a t i n u m and p l a t i n u m / i r i d i u m w i r e anodes, the s u r f a c e  oxygen coverage v s . p o t e n t i a l  b e h a v i o u r f o r the p o t e n t i a l  r e g i o n p r i o r to  commencement o f oxygen e v o l u t i o n i s e s s e n t i a l l y i d e n t i c a l .  Pt/30 I r c o a t i n g s  show s i m i l a r b e h a v i o u r , but w i t h somewhat h i g h e r coverages a t e q u i v a l e n t anode p o t e n t i a l s , g i v i n g i n t e r m e d i a t e v a l u e s between the case o f p l a t i n u m (and Pt/5-25 I r w i r e s ) and i r i d i u m . than monolayer  No e v i d e n c e was found f o r g r e a t e r -  coverage under these c o n d i t i o n s f o r e i t h e r the P t / I r w i r e s  o r the c o a t i n g s , however. The analogy o f the oxygen coverage b e h a v i o u r o f P t / I r a l l o y s w i t h p l a t i n u m extended i n t o the coverage v s . time conduct under c o n d i t i o n s o f s i m u l t a n e o u s oxygen e v o l u t i o n , w i t h a maximum oxygen coverage  of  2 < 0 < 3 a c h i e v e d i n the l o n g e s t runs employed i n the p r e s e n t work. Formation o f "type I I " o x i d e was  a c h i e v e d w i t h both a Pt/25 I r w i r e anode  and a c o a t e d e l e c t r o d e (where the f i n a l  electrode potential  put the anode w i t h i n the r a t h e r s m a l l p o t e n t i a l (SHE)  [315] - n e c e s s a r y f o r i t s g r o w t h ) .  m u l t i l a y e r o x i d e was  o f 2.3  volts  range - 2.1-2.5 v o l t s  In g e n e r a l , the f o r m a t i o n o f  not observed e i t h e r because the anode p o t e n t i a l  210  remained below the v a l u e n e c e s s a r y  f o r i t s f o r m a t i o n o r because i t passed  q u i c k l y through the r e g i o n f o r i t s f o r m a t i o n  d u r i n g t h e " p o t e n t i a l jump"  phenomenon encountered f o r anodes p o l a r i z e d above about 2.2 v o l t s  7.2.3  Time-dependence o f O v e r p o t e n t i a l : The  (SHE).  R e v e r s i b l e Behaviour  change i n anode p o t e n t i a l w i t h time observed i n anodic  oxygen e v o l u t i o n o c c u r s o v e r t h e range o f oxygen coverages beyond monolayer values.  F o r the " o x i d e " t h e o r y , t h i s e f f e c t i s e x p l a i n e d by t h e i n c r e a s e  i n o x i d e - l a y e r t h i c k n e s s w i t h time [ 2 5 ] .  S o l e [366] d e r i v e d the time-  dependence o f t h e oxygen e v o l u t i o n r a t e a t c o n s t a n t  p o t e n t i a l from t h e o r e t i c a l  c o n s i d e r a t i o n s o f t h e v a r i a t i o n o f t h e a c t i v a t i o n energy o f oxygen e v o l u t i o n with f r a c t i o n a l  s u r f a c e oxygen coverage.  In the p r e s e n t c a s e ,  however,  monolayer coverage i s a l r e a d y a t t a i n e d p r i o r t o commencement o f oxygen evolution.  T h i s has been p o i n t e d out by G o r o d e t s k i i [ 2 9 7 ] , who suggested  t h a t the i n c r e a s e i n anode p o t e n t i a l w i t h time was due both t o t h e d i f f u s i o n o f oxygen i n t o the metal and t o the t i m e - v a r i a t i o n o f t h e p l a t i n u m oxygen bond energy (as evidenced  by t h e l o w e r i n g o f the p o t e n t i a l o f t h e  r e d u c t i o n p l a t e a u s f o r removal o f s u r f a c e oxygen, i n the g a l v a n o s t a t i c charge curves  taken a f t e r prolonged  periods of a n o d i z a t i o n ) .  A similar  reason f o r t h e tirne-dependence o f o v e r p o t e n t i a l on Pt/30 I r - T i anodes can be p o s t u l a t e d from t h e observed analogous d e p r e s s i o n  o f the r e d u c t i o n ,  pleateau to that of platinum.  7.2.4  Time-dependence o f O v e r p o t e n t i a l : Prolonged  I r r e v e r s i b l e Behaviour  anodic o p e r a t i o n , where the anode p o t e n t i a l i s per-  m i t t e d t o r i s e beyond 3-4 v o l t s  (SHE), r e s u l t s i n a t r a n s i t i o n  from  211  r e v e r s i b l e to i r r e v e r s i b l e p o t e n t i a l v s . time b e h a v i o u r , as c h a r a c t e r i z e d by the i n a b i l i t y o f the anode to s u s t a i n l o w - p o t e n t i a l o p e r a t i o n on sequent r e p e t i t i v e r u n s .  A l t h o u g h such an e f f e c t may  sub-  be p o s t u l a t e d to  be due to a profound decrease noble metal l o a d i n g as a r e s u l t o f c o r r o s i o n , the o n s e t o f i r r e v e r s i b i l i t y has been observed to o c c u r i n one i n s t a n c e a f t e r o n l y a 12% l o s s o f o o a t i n g m e t a l .  specific  F u r t h e r , measurements  o f the s u r f a c e a r e a showed t h a t t h i s v a l u e had decreased o n l y by 20 per c e n t p r i o r to the onset o f i r r e v e r s i b l e o p e r a t i o n . The  i r r e v e r s i b l e p o t e n t i a l v s . time b e h a v i o u r may  be r e c o n c i l e d  w i t h the high r e s i d u a l noble metal l o a d i n g s and s u r f a c e a r e a i f the e l e c t r i c a l i s o l a t i o n o f the c o a t i n g metal  i s a g a i n proposed  to o c c u r by means o f  the development o f an o x i d e f i l m h a v i n g i n s u l a t i n g q u a l i t i e s , between the c o a t i n g and s u b s t r a t e m e t a l s .  B y s t r o v [215] has r e c e n t l y p o s t u l a t e d such  a mechanism f o r the i n c r e a s e i n p o t e n t i a l w i t h time f o r t i t a n i u m s u b s t r a t e anodes. The s o u r c e o f oxygen f o r the growth o f an o x i d e f i l m the c o a t i n g metal may  be oxygen absorbed  i n t o the noble metal  beneath  itself.  Hoare [289-292] has shown t h a t s t r o n g a n o d i z a t i o n o f one s i d e o f a t h i n platinum f o i l  r e s u l t s i n r a p i d d i f f u s i o n o f oxygen to the o t h e r s i d e ,  with D = 10-  1 2  cm /sec 2  f o r the d i f f u s i o n c o e f f i c i e n t o f oxygen i n p l a t i n u m .  The g r a d i e n t f o r  d i f f u s i o n i s c r e a t e d by the s a t u r a t i o n of the s u r f a c e l a y e r s o f the noble metal w i t h oxygen. In o r d e r to c a l c u l a t e the f l u x o f oxygen through a noble metal f i l m i t i s n e c e s s a r y t o know the s o l u b i l i t y o f oxygen i n the m e t a l .  This  212  v a l u e i s not known, a l t h o u g h Hoare [291] suggests bility  t h a t the oxygen s o l u -  i n p l a t i n u m amounts t o one oxygen atom per u n i t c e l l  lattice.  o f the FCC  A b e t t e r e s t i m a t e o f t h i s v a l u e can be made from c o n s i d e r a t i o n  o f h i s data [290] f o r the d i f f u s i o n o f oxygen through  a p l a t i n u m diaphragm.  I f the appearance o f oxygen on the o t h e r s i d e o f the f o i l  i s assumed t o  be due t o the f o r m a t i o n o f a monolayer o f oxygen on t h a t s u r f a c e ,  i -31 n n u s  p  1.31(10]  t  atoms  ^2—  1 0 atom  x  1  p  t  a  t  Q  m  x  1 mole 0  6 ( 1 Q )  o  23  a  t  o  m  _  s  o/  - 2.18(10) 9  n  n n  N_  -  9  moles y  , x  (2)  9  then the s o l u b i l i t y o f oxygen i n p l a t i n u m can be e s t i m a t e d from the known value o f h i s f o i l  t h i c k n e s s , the d i f f u s i o n c o e f f i c i e n t , and the time  r e q u i r e d f o r the passage o f the 2 . 1 8 ( 1 0 ) foil.  - 9  moles o f 0 atoms through t h e  The s o l u b i l i t y i s hence:  = 2.52(10)- M f l 5  S  'i  (  Thus, assuming t h i s v a l u e h o l d s f o r the Pt/30 I r a l l o y  o f average t h i c k n e s s 2 ( 1 0 )  _ 5  cm, the f l u x o f oxygen through  3  )  4  )  films,  the c o a t i n g  can be e s t i m a t e d by F i c k s ' f i r s t law:  J  i _ Ode ~d^ =  m-12  cm ] f 2 . 5 2 ( 1 0 ) - moles/cm — > . " secl l 2  io  5  n  1 0 )  b  1 0  2  enrsec  ( v  the c o a t i n g i s u t i l i z e d  (which c o n t a i n s 1 0 " moles 0/cm 8  2  .i moles_  m  I f the oxygen which d i f f u s e s through i n the f o r m a t i o n o f T i 0  :  2  of surface area),  then a time o f 1 0 seconds i s p r e d i c t e d f o r the f o r m a t i o n o f a s i n g l e mono4  l a y e r o f t h i s o x i d e between the c o a t i n g and s u b s t r a t e .  As a p p r o x i m a t e l y  '  213  300 hours a r e r e q u i r e d f o r the onset o f i r r e v e r s i b l e p o t e n t i a l v s . time b e h a v i o u r , as i n d i c a t e d i n F i g u r e s 6.34  and 6.35,  f o r anodes o p e r a t e d a t  high c u r r e n t d e n s i t i e s where s a t u r a t i o n o f the s u r f a c e l a y e r s o f the metal w i t h oxygen may  be presumed from the work o f Hoare [289-292], the t h i c k -  ness o f the i n s u l a t i n g o x i d e l a y e r i n such cases may 100 monolayers. purity r u t i l e  be o f the o r d e r o f  I f t h i s l a y e r possessed the same r e s i s t i v i t y as h i g h -  (Table 1.4), namely 1 0  1 7  microohnrcm,  be o r i e n t e d w i t h the c - l a t t i c e parameters  and i s presumed t o  (4.583 angstroms) normal  to the  s u r f a c e , then the ohmic drop through such a f i l m would c o r r e s p o n d t o 1 0 v o l t s a t the o p e r a t i n g c u r r e n t d e n s i t y o f 260 mA/cm . 2  5  C l e a r l y the r e s i s -  t i v i t y o f the l a y e r i n the p r e s e n t case i s not so h i g h , o r the l a y e r i s o f d i f f e r i n g t h i c k n e s s (which may  be expected from the  heterogeneous  n a t u r e o f the noble metal c o a t i n g on the t i t a n i u m s u b s t r a t e .  7.2.5  P o l a r i z a t i o n Curves:  E f f e c t of A l l o y Composition  P t / I r a l l o y s were found t o show i n t e r m e d i a t e p o l a r i z a t i o n b e h a v i o u r between t h a t o f p l a t i n u m and i r i d i u m , as may  be expected f o r the  case o f the c o m b i n a t i o n o f two metals which show d i f f e r e n t a c t i v i t i e s f o r a given electrode reaction.  The Pt/30 I r - T i  electrocatalytic electrodes  were found t o be c l o s e to i r i d i u m i n b e h a v i o u r as e v i d e n c e d by the c l o s e n e s s of t h e i r p o l a r i z a t i o n curves.  The P t / I r w i r e a l l o y s , w h i l e more a c t i v e  than p l a t i n u m , were i n f e r i o r to the a l l o y c o a t i n g s .  Similar differences  i n the a c t i v i t y o f o t h e r n o b l e metal a l l o y s have been noted i n the l i t e r a t u r e [367] and are a t t r i b u t e d to the e f f e c t s o f manufacture and p r e t r e a t m e n t s which may bulk v a l u e .  a l t e r the s u r f a c e c o m p o s i t i o n o f the a l l o y from i t s  214  7.2.6  Pulsed Current  Operation  I n i t i a l l y , p u l s e d c u r r e n t o p e r a t i o n i s advantageous from t h e s t a n d p o i n t o f energy-consumption d u r i n g the a n o d i c p r o c e s s , as the anode p o t e n t i a l  i s not p e r m i t t e d to c l i m b t o v a l u e s c h a r a c t e r i s t i c o f  the h i g h e r degrees o f oxygen coverage encountered operation.  w i t h prolonged  D.C.  The c o r r o s i o n l o s s e s w i t h p e r i o d i c c u r r e n t r e v e r a l s are  so  h i g h , however, t h a t the anode s u r f a c e area i s r a p i d l y d e p l e t e d t o the stage where the anode p o t e n t i a l  r i s e s t o high v a l u e s as a r e s u l t o f the  i n c r e a s e . i n the a c t u a l c u r r e n t d e n s i t y .  F a i l u r e due  i n s u l a t i n g f i l m between the c o a t i n g and  s u b s t r a t e was  w i t h r e s i d u a l c o a t i n g l e v e l s as low as 0.06  g/m , 2  to the b u i l d - u p o f an not observed  l i k e l y as a consequence  o f the l a c k of e s t a b l i s h m e n t o f a d i f f u s i o n g r a d i e n t f o r oxygen the noble  metal.  even  through  Chapter 8  CONCLUSIONS  8.1  Corrosion The measurement o f small changes i n both l o a d i n g and  composi-  t i o n o f Pt/30 I r - T i anodes produced as a r e s u l t o f o p e r a t i o n under t i o n s encompassing  those encountered i n the e l e c t r o w i n n i n g o f copper  from h i g h l y a c i d i c e l e c t r o l y t e s was made p o s s i b l e by the X-ray s c o p i c t e c h n i q u e and fundamental Appendix  condi-  spectro-  parameter c a l c u l a t i o n d e s c r i b e d i n  A2. The l o s s o f c o a t i n g metal from such e l e c t r o d e s can, i n g e n e r a l ,  be e x p l a i n e d by the s i m u l t a n e o u s o c c u r r e n c e o f both e l e c t r o c h e m i c a l  disso-  l u t i o n and mechanical detachment,processes, whose r e l a t i v e  predominance  depends on the n a t u r e o f the anode i t s e l f , as a consequence  of differences  i n manufacture.  For the anode m a t e r i a l chosen f o r most o f the p r e s e n t  work - a sheet h a v i n g 4.337 g/m  2  was  average l o a d i n g - the d i s s o l u t i o n p r o c e s s  found to be p r e d o m i n a n t l y e l e c t r o c h e m i c a l  2M H S 0 2  4  i n nature.  + 0.5M CuSO^ a t 22° and 40°C, the mean c o r r o s i o n  f o r the noble metals i n the c o a t i n g a r e : Pt: Ir: Total:  1.13 0.35 1.48 215  ug/A-hr ug/A-hr ug/A-hr  For the e l e c t r o l y t e efficiencies  216  I r i d i u m i s found to have analogous d i s s o l u t i o n b e h a v i o u r to  platinum,  inasmuch as i t p a r a l l e l s the r a t e o f oxygen e v o l u t i o n , but a t a r a t e than p l a t i n u m  smaller  d i s s o l u t i o n , r e s u l t i n g i n p r e f e r e n t i a l enrichment of  the r e s i d u a l c o a t i n g metal w i t h i r i d i u m . P e r i o d i c c u r r e n t r e v e r s a l i s h i g h l y d e l e t e r i o u s as i t promotes accelerated  l o s s of both c o a t i n g m e t a l s , a l t h o u g h i r i d i u m i s more r e s i s t a n t  in t h i s regard,  l e a d i n g to i r i d i u m enrichment of the r e s i d u a l  coating  metal to the e x t e n t of 89 weight per c e n t i n the most extreme case. T h i o u r e a does not a c t as a c o r r o s i o n i n h i b i t o r f o r the electrodes.  On  d i s s o l u t i o n and and  0.5  the c o n t r a r y , t h i s a d d i t i v e a c t s t o promote r a p i d  coating  premature f a i l u r e of the anodes a t c o n c e n t r a t i o n s  of  0.05  gpl. U l t i m a t e anode f a i l u r e i s preceded by an i n c r e a s e  c o a t i n g metal l o s s r a t e s by about two chemical d i s s o l u t i o n r a t e v a l u e s .  anode which may  orders  Coating  i s p r e d o m i n a n t l y mechanical i n n a t u r e .  i n the  of magnitude over t h e i r e l e c t r o -  l o s s under these  conditions  Continued o p e r a t i o n w i t h a f a i l e d  r e t a i n p o r t i o n s o f e l e c t r i c a l l y i s o l a t e d , and  c o a t i n g metal r e s u l t s i n i n t e n s e  8.2  coated  l o c a l i z e d c o r r o s i o n of the  hence i n a c t i v e , substrate.  Passivation Pt/30 I r - T i e l e c t r o d e s  i n g a l v a n o s t a t i c charge s t u d i e s .  showed b e h a v i o u r t y p i c a l o f S u r f a c e areas of new  anodes from d i f f e r e n t  sources d i d not show a r e g u l a r r e l a t i o n s h i p w i t h n o b l e metal a l t h o u g h the h i g h e r  4.337 g/m ) 2  loading,  roughness f a c t o r v a l u e s were observed w i t h anodes  having h i g h e r l o a d i n g s . loading:  platinum  The  s u r f a c e areas of anodes (average  initial  operated i n s u l f u r i c a c i d e l e c t r o l y t e increase  by  217  up t o 10 per cent w i t h t i m e , and  then d e c r e a s e p r o g r e s s i v e l y a f t e r  o f 10-20  loading.  per c e n t o f the i n i t i a l The  Pt/30 I r - T i and  P t / I r a l l o y w i r e anodes were found t o  d e v e l o p oxygen coverage i n an analogous manner t o p l a t i n u m , monolayer coverage w i t h oxygen immediately  simultaneous layer  "type  then growing to the e q u i -  to t h r e e monolayers w i t h prolonged oxygen e v o l u t i o n .  I I " o x i d e was The  anodic o p e r a t i o n  p o l a r i z a t i o n behaviour  anode.  of P t / I r a l l o y s  i s intermediate  i r i d i u m , w i t h Pt/30 I r - T i showing  behaviour  v e r y c l o s e to t h a t f o r i r i d i u m m e t a l , the more e f f i c i e n t oxygen c a t a l y s t o f the two metals The and  potential  i r r e v e r s i b l e behaviour  with  In an i s o l a t e d case t h e growth o f m u l t i -  observed on a Pt/30 I r - T i  between t h a t o f p l a t i n u m and  attaining  p r i o r t o the commencement o f  the e v o l u t i o n o f oxygen on anodic c h a r g i n g , and v a l e n t o f two  loss  electro-  considered. o f Pt/30 I r - T i anodes shows both w i t h e l e c t r o l y s i s time.  t i o n s , which i s the case w i t h new the change i n anode p o t e n t i a l  reversible  Under r e v e r s i b l e  condi-  e l e c t r o d e s subjected to r e p e t i t i v e  i s related  to the nature o f the  runs,  surface  oxygen coverage. P u l s e d c u r r e n t o p e r a t i o n i s not e f f e c t i v e i n m a i n t a i n i n g anode potentials  a t low v a l u e s  ( d e s i r a b l e from an economic v i e w p o i n t ) as  high r a t e s o f c o a t i n g l o s s r e s u l t i n decreased s u r f a c e a r e a , and sequently anode  i n an i n c r e a s e i n the a c t u a l c u r r e n t d e n s i t y and  the  con-  hence h i g h e r  potentials. The  i r r e v e r s i b l y high anode p o t e n t i a l s  observed f o r an anode  p r i o r to f a i l u r e can be e x p l a i n e d i f the development o f an i n s u l a t i n g i s considered  to o c c u r between the c o a t i n g and  substrate metals.  The  film  218  growth o f t h i s f i l m i s l i k e l y due t o t h e d i f f u s i o n o f absorbed oxygen through the n o b l e metal c o a t i n g .  As t h i s f i l m grows, t h e c o a t i n g metal becomes  e l e c t r i c a l l y i s o l a t e d from t h e s u b s t r a t e u n t i l e v e n t u a l l y t h e e l e c t r o d e behaves e s s e n t i a l l y as i f i t was t i t a n i u m m e t a l , l e a d i n g t o an e x p o n e n t i a l i n c r e a s e i n anode p o t e n t i a l t o t h e v o l t a g e l i m i t o f t h e power s u p p l i e s , which i s below t h e breakdown p o t e n t i a l f o r t i t a n i u m .  8.3  S u i t a b i l i t y o f Pt/30 I r - T i Anodes f o r Copper E l e c t r o w i n n i n g In S e c t i o n 1.2, s e v e r a l q u e s t i o n s were r a i s e d c o n c e r n i n g t h e  a p p l i c a b i l i t y o f n o b l e metal coated i n copper e l e c t r o w i n n i n g . material  t i t a n i u m anodes as i n s o l u b l e anodes  The c o r r o s i o n e f f i c i e n c y f o r t h e p a r t i c u l a r  used i n most o f t h i s work was found t o be 1.13 ug/A-hr.  l i f e t i m e o f t h i s p a r t i c u l a r anode c o u l d thus be e s t i m a t e d o p e r a t i o n a t 20 mA/cm  2  (geometric  area).  a t 800 days f o r  Anodes w i t h t h i c k e r c o a t i n g s  would have p r o p o r t i o n a t e l y l o n g e r maximum l i f e t i m e s , p r o v i d e d l o s s r a t e s were s i m i l a r .  The maximum  the c o a t i n g  U n f o r t u n a t e l y , t h e p r e s e n t work has r e v e a l e d a  high degree o f v a r i a t i o n i n t h e c o a t i n g metal l o s s r a t e s f o r n o m i n a l l y s i m i l a r anodes from d i f f e r e n t m a n u f a c t u r i n g  lots.  D i f f e r e n t anode s h e e t s ,  t h u s , may have c o n s i d e r a b l e d i f f e r e n c e s i n t h e i r c o r r o s i o n behaviour and hence i n t h e i r maximum l i f e t i m e s . As a common e l e c t r o d e p o s i t i o n a d d i t i v e , t h i o u r e a , and a l s o p e r i o d i c c u r r e n t r e v e r s a l were found t o enhance c o a t i n g l o s s r a t e s , these would have t o be a v o i d e d  i n any c e l l s where coated  t i t a n i u m anodes were employed.  The i n c r e a s e i n anode p o t e n t i a l w i t h time on t h e coated i s u n d e s i r a b l e from an energy consumption s t a n d p o i n t , a l t h o u g h  anodes  anode  p o t e n t i a l s o f 2.0 v o l t s S.H.E. and above were o n l y a t t a i n e d i n s h o r t  times  219  w i t h c u r r e n t d e n s i t i e s much h i g h e r than those encountered i n copper trowinning practice.  elec-  A l t h o u g h p u l s e d c u r r e n t o p e r a t i o n was found t o be  d e l e t e r i o u s t o corrosion behaviour, i n t e r m i t t e n t p u l s i n g  ("on-periods"  o f the o r d e r o f days) may y e t be found t o be d e s i r a b l e from t h e p o i n t o f view o f m a i n t a i n i n g lower anode p o t e n t i a l s .  Actual  ( p r a c t i c a l ) anode  l i f e t i m e s would be determined by t h e time i t t a k e s f o r t h e o p e r a t i n g potential  t o r i s e above some p r e d e t e r m i n e d maximum (economic) v a l u e , such  as 2.0 v o l t s S.H.E.  The a c t u a l  l i f e t i m e f o r t h e "4.33 g/m " 2  anodes by  such a c r i t e r i o n would o n l y be about 200 days f o r o p e r a t i o n a t 20 mA/cm , 2  a l t h o u g h i n t e r m i t t e n t p u l s i n g may extend t h i s . sumed t h a t h i g h e r - l o a d i n g anodes would  A l t h o u g h i t may be pre-  show l o w e r - o v e r p o t e n t i a l  behaviour  (and thus have a l o n g e r a c t u a l l i f e t i m e as a consequence o f t h e i r  initially  high s u r f a c e area v a l u e s ) , no f i r m r e l a t i o n was found t o e x i s t between s u r f a c e a r e a and noble metal l o a d i n g i n t h i s work.  REFERENCES  [1]  Campbell, T.C., Trans.  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Acta,_ VI,  1171 (1967).  APPENDIX A l  THE COPPER/COPPER SULFATE ELECTRODE  Using a r e f e r e n c e e l e c t r o d e c o n s i s t i n g o f a copper w i r e .dipped i n t o 2M HzSO^ + 0.5M CuS0  4  s o l u t i o n a t 295°K as a " s t a n d a r d " t h e e f f e c t s  o f changes i n t h e e l e c t r o l y t e c o m p o s i t i o n and t h e e f f e c t s o f a d d i t i v e s and gas b u b b l i n g were i n v e s t i g a t e d f o r s i m i l a r copper w i r e s dipped i n t o t h e respective solutions.  The d a t a a r e p r e s e n t e d i n Table A l . 1 and a r e pre-  sented i n t h e form o f d i f f e r e n c e s i n t h e measured e l e c t r o d e p o t e n t i a l ( p l u s t h e l i q u i d j u n c t i o n c o n t r i b u t i o n ) from t h a t o f t h e " s t a n d a r d " copper  sulfate electrode.  electrode potential  copper/  As a r e s u l t i t can be seen t h a t t h e r e f e r e n c e  i s n o t g r e a t l y a f f e c t e d (does n o t change by, say, more  than 10 mv) by r a t h e r l a r g e v a r i a t i o n s i n t h e e l e c t r o l y t e c o m p o s i t i o n a t l e a s t over the ranges o f v a r i a t i o n o f c o m p o s i t i o n o f e x p e r i m e n t a l e l e c t r o l y t e s during simulated e l e c t r o w i n n i n g experiments.  The l a c k o f  s i g n i f i c a n t e f f e c t s o f gas b u b b l i n g o r small p o l a r i z a t i o n s a r e a consequence o f t h e high exchange c u r r e n t o f t h e c o p p e r / c u p r i c i o n couple r e s u l t s i n a n o d i c c o n t r o l o f t h e mixed c o r r o s i o n p o t e n t i a l ) .  238  (which  239  Table A l . l Effects of Variations  i n the E l e c t r o l y t e Composition on t h e C e l l :  Cu(T)/H S0 (M ,T)+CuS04M ,T)/2M 2  4  1  2  (295° )/H ,Pt(295° )  H2SO-  2  A l l AE v a l u e s a r e r e f e r r e d t o the measured p o t e n t i a l s o l u t i o n a t 298°K, (E + E.) = .298 v o l t s v s . S.H.E.  w i t h 2M H SC\ + 0.5M CuSO^ 2  T  Mi  M  295  8 4 2 1 .1  0.5 0.5 0.5 0.5 0.5  -.012 -.010 0 + .018 + .045  295  2 2 2 2 2  satd. 0.5 0.2 0.1 0.01  + .002 0 -.016 -.025 -.034  350  2  0.5  + .023  295  2  0.5  2  0.5  2  0.5  2  0.5  + 0  2  0.5  + He b u b b l i n g  0  2  0.5  + 1 uA p o l a r i z a t i o n + 10 uA + 100 uA  0 + .001 + .003  1 uA p o l a r i z a t i o n 10 uA 100 uA  0 -.007 -.012  295  Other  2  AE ( v o l t s )  +.03M C o S 0 + a few drops  + 1M HN0  -  2  4  EDTA 3  bubbling  -.001 0 -.001 + .002  APPENDIX A2  X-RAY FLUORESCENCE SPECTROSCOPY  A2.1  Fundamental  Parameter C a l c u l a t i o n  of Characteristic  X-ray  Intensities  The P t L ^ i and I r l _ i c h a r a c t e r i s t i c r a d i a t i o n were chosen f o r a  the  purposes o f t h i s c a l c u l a t i o n as they a r e t h e most i n t e n s e peaks  observed d u r i n g X-ray s p e c t r o s c o p y w i t h c o n t i n u o u s t u n g s t e n r a d i a t i o n , and because they a r e n o t c l o s e t o any major t u n g s t e n c h a r a c t e r i s t i c  radia-  t i o n peak which may o t h e r w i s e i n t e r f e r e w i t h t h e i n t e n s i t y measurement. The wavelengths o f t h e c h a r a c t e r i s t i c r a d i a t i o n a r e :  Only t h a t p a r t o f t h e i n c i d e n t l e s s than t h e L J radiation.  T  J  PtLcxi:  1. 313  angstroms  IrLcti:  1.352 angstroms  ( p r i m a r y ) r a d i a t i o n having wavelengths  a d s o r p t i o n edges i s c a p a b l e o f e x c i t i n g L a i c h a r a c t e r i s t i c  Secondary f l u o r e s c e n c e i s a l s o p o s s i b l e when on element has  characteristic radiation  peaks a t wavelengths below t h e L J J J  edge o f t h e  o t h e r element. Bertin  [351] has d e s c r i b e d t h e t h e o r e t i c a l c a l c u l a t i o n o f  c h a r a c t e r i s t i c X-ray i n t e n s i t i e s as a r e s u l t o f p r i m a r y r a d i a t i o n i m p i n g i n g on a metal specimen, which e n a b l e t h e c o n v e r s i o n o f i n t e n s i t y data t o  240  241  analytical  (composition)  data.  He a l s o reviews  the v a r i o u s  f o r measurement o f the t h i c k n e s s e s o f metal f i l m s . the t h e o r e t i c a l  treatment  following  For the p r e s e n t work,  has been extended to the p a r t i c u l a r case o f  P t / I r a l l o y coatings of v a r i a b l e The  techniques  t h i c k n e s s and  composition.  n o t a t i o n i s used throughout t h i s  section:  I (A)  the i n t e n s i t y vs. wavelength d i s t r i b u t i o n o f primary X-ray beam.  r.  the a b s o r p t i o n jump r a t i o f o r element i . For L c h a r a c t e r i s t i c r a d i a t i o n , t h i s i s the r a t i o o f the a b s o r p t i o n c o e f f i c i e n t o f the element on the s h o r t wavelength s i d e t o t h a t on the long-wavel e n g t h s i d e o f the c o r r e s p o n d i n g ( L ) a b s o r p t i o n edge.  0  1  the  T T T 1  U  r..-l r  the f r a c t i o n o f p h o t o i o n i z a t i o n s o c c u r r i n g w i t h i n a given subshel1.  i  the f l u o r e s c e n c e y i e l d , or the p r o b a b i l i t y t h a t vacancy c r e a t e d i n a g i v e n s u b s h e l l w i l l r e s u l t in a radiative t r a n s i t i o n .  y(x).  the p r o b a b i l i t y o f a g i v e n e l e c t r o n t r a n s f e r i n a g i v e n s u b s h e l l , d e f i n e d as the r e l a t i v e i n t e n s i t y o f the l i n e i n q u e s t i o n over the sum o f the r e l a t i v e i n t e n s i t i e s of a l l other l i n e s f o r that p a r t i c u l a r element produced from e l e c t r o n s i n t h a t p a r t i c u l a r subshel1. the mass a b s o r p t i o n c o e f f i c i e n t vs. wavelength r e l a tion. For each element t h i s r e l a t i o n has d i s c o n t i n u i t i e s a t the a b s o r p t i o n edges. For an a l l o y , at a given wavelength, a l 1 oy  y  i p  where W- are the w e i g h t f r a c t i o n s stituents. n  y ( A )  a11oy  P  o f the a l l o y con-  he mass a b s o r p t i o n c o e f f i c i e n t r e l a t i o n f o r the a l l o y . T h i s r e l a t i o n w i l l have d i s c o n t i n u i t i e s a t a l l a b s o r p t i o n edges c o r r e s p o n d i n g t o a l l o f the a l l o y elements.  242  u(A).  the l i n e a r a b s o r p t i o n c o e f f i c i e n t v s . wavelength r e l a t i o n ( o b t a i n e d by m u l t i p l y i n g t h e mass absorpt i o n c o e f f i c i e n t by t h e d e n s i t y o f e l e m e n t . i i .  u ( A ) ,, y  a  I ,1 x  x  t h e l i n e a r a b s o r p t i o n c o e f f i c i e n t v s . wavelength r e l a t i o n f o r t h e a l l o y ( o b t a i n e d by m u l t i p l y i n g the mass a b s o r p t i o n c o e f f i c i e n t f o r t h e a l l o y by the d e n s i t y o f t h e a l l o y ) . t h e i n t e n s i t y o f t h e p r i m a r y r a d i a t i o n a t some depth x, and an i n c r e m e n t a l l y g r e a t e r d e p t h , w i t h i n t h e specimen.  x  01  the angle o f incidence o f the primary  0  t h e angle o f e x i t o f t h e f l u o r e s c e n t the c o l l i m a t o r .  2  dfi  r a d i a t i o n to  the f r a c t i o n o f the f l u o r e s c e n t c h a r a c t e r i s t i c t i o n which passes through t h e c o l l i m a t o r .  4TT  k  radia-  i n t e n s i t y l o s s e s due t o a b s o r p t i o n i n a i t r , i n d i f f r a c t i o n from t h e a n a l y z i n g c r y s t a l , and i n t h e detector.  k, • '  t h e "Lorenz f a c t o r " due t o t h e d i f f e r e n c e i n t h e Bragg c o n d i t i o n f o r t h e d i f f r a c t i o n o f t h e P t L ^ i and IrLcti wavelengths.  n  K, L,i I ,  iV c  a r  i''  '  a r  '  n  t h e i n t e n s i t y o f c h a r a c t e r i s t i c r a d i a t i o n o f element i produced by an i n c r e m e n t a l amount o f p r i m a r y r a d i a t i o n , w i t h i n a l a y e r Ax i n t h e specimen.  .  t h e i n t e n s i t y o f c h a r a c t e r i s t i c r a d i a t i o n o f element i produced by an i n c r e m e n t a l amount o f p r i m a r y r a d i a t i o n , a t t h e s u r f a c e o f t h e specimen.  .  t h e i n t e n s i t y o f c h a r a c t e r i s t i c r a d i a t i o n o f element i produced by an i n c r e m e n t a l amount o f p r i m a r y r a d i a t i o n , a t the d e t e c t o r .  n  1  1 s i n 20 Bragg,i  . '  I  radiation.  n  . . primary,!  the t o t a l t i o n  (a) (b)  Q f  e  l  e  m  i n t e n s i t y o f primary c h a r a c t e r i s t i c ^ detector:  e  n  t  a t  radia-  t h e  f o r an i n f i n i t e l y t h i c k specimen, (1^) p r i m a r y , i , f o r a specimen o f t o t a l t h i c k n e s s x,°°(I ) p r i m a r y , i A  243  I  I  I  I  char/sec, j  the i n t e n s i t y o f o t h e r c h a r a c t e r i s t i c r a d i a t i o n produced by element j produced by an increment amount o f p r i m a r y r a d i a t i o n w i t h i n a l a y e r Ax i n t h e specimen, which i s i t s e l f c a p a b l e o f . . e x c i t i n g (secondary) c h a r a c t e r i s t i c f l u o r e s c e n t r a d i a t i o n o f element i . the amount o f secondary c h a r a c t e r i s t i c f l u o r e s c e n t r a d i a t i o n produced w i t h i n a l a y e r Ax i n t h e specimen.  sec, i  the amount o f secondary c h a r a c t e r i s t i c f l u o r e s c e n t r a d i a t i o n produced w i t h i n a l a y e r Ax i n t h e specimen t h a t reaches t h e d e t e c t o r . ..  sec,i  the t o t a l i n t e n s i t y o f secondary c h a r a c t e r i s t i c f l u o r e s c e n t r a d i a t i o n o f element i a t the d e t e c t o r :  sec, l  (a)  f o r an i n f i n i t e l y t h i c k specimen, (I )  (b)  f o r a specimen o f t o t a l  t h i c k n e s s x, ( I ) X  I  51  SGC  the t o t a l i n t e n s i t y o f ( p r i m a r y + secondary) c h a r a c t e r i s t i c f l u o r e s c e n t r a d i a t i o n o f element i a t the d e t e c t o r :  total, i  (a)  f o r an i n f i n i t e l y t h i c k specimen, (  (b)  f o r a specimen o f t o t a l  The d e r i v a t i o n t e r i s t i c fluorescent  I O 0  )  t 0  ^ ] -j a  t h i c k n e s s x, ( I ^ )  t Q t a  -j ^  o f fundamental e q u a t i o n s f o r the t o t a l c h a r a c -  radiation  i n t e n s i t y f o r element i i n v o l v e s  c u l a t i o n of the v a r i a t i o n o f the i n t e n s i t y of that portion  the c a l -  of the primary  r a d i a t i o n whose wavelengths a r e s u f f i c i e n t t o e x c i t e t h e d e s i r e d  char-  a c t e r i s t i c r a d i a t i o n , w i t h depth i n the specimen metal f i l m , assumed t o be p e r f e c t l y smooth and homogeneous.  C o n s e q u e n t l y , the e f f i c i e n c y o f  conversion o f the primary r a d i a t i o n to the desired t i o n can be c a l c u l a t e d ,  characteristic  radia-  f o l l o w e d by a s i m i l a r c a l c u l a t i o n o f t h e v a r i a t i o n  of the i n t e n s i t y o f t h e e s c a p i n g r a d i a t i o n , which i s o r i e n t e d  c o r r e c t l y to  pass through t h e c o l l i m a t o r , as i t reaches t h e specimen s u r f a c e from v a r i o u s depths.  Intensity  losses  i n t h e c h a r a c t e r i s t i c r a d i a t i o n can then  244  be c o n s i d e r e d t o g i v e a v a l u e f o r t h e i n t e n s i t y o f t h i s r a d i a t i o n a t the d e t e c t o r .  Secondary f l u o r e s c e n c e e f f e c t s a r e e v a l u a t e d by c o n s i d e r i n g  the p r o d u c t i o n o f o t h e r c h a r a c t e r i s t i c r a d i a t i o n o f s u f f i c i e n t energy t o excite  the d e s i r e d c h a r a c t e r i s t i c r a d i a t i o n o f the o t h e r element.  calculation  involves,  This  as above, t h e d e t e r m i n a t i o n o f the i n t e n s i t i e s o f  such o t h e r c h a r a c t e r i s t i c r a d i a t i o n which i s c a p a b l e o f secondary  fluores-  cent e x c i t a t i o n , t h e e f f i c i e n c i e s o f such e x c i t a t i o n , and t h e r e s u l t a n t a t t e n u a t i o n o f t h e secondary f l u o r e s c e n t r a d i a t i o n as i t passes from i n s i d e t h e specimen a t d i f f e r e n t depths towards t h e c o l l i m a t o r mately  and u l t i -  t o the d e t e c t o r . As s e v e r a l terms i n t h e above c a l c u l a t i o n cannot be r e a d i l y  e v a l u a t e d , the approach taken  i n v o l v e s the d e t e r m i n a t i o n o f r e l a t i v e  i n t e n s i t y v a l u e s , where an i n f i n i t e l y t h i c k pure p l a t i n u m specimen i s c o n s i d e r e d f o r t h i s purpose.  By t h i s means, t h e i n d e t e r m i n a t e  terms  C o n s i d e r an i n c r e m e n t a l amount o f primary r a d i a t i o n ,  I AA,  cancel.  i m p i n g i n g on the specimen. incident  0  The wavelength range o f t h i s p o r t i o n o f t h e  beam i s s u f f i c i e n t l y s m a l l t h a t i t can be c o n s i d e r e d monochromatic.  Consequently,  the e q u a t i o n f o r the a b s o r p t i o n o f t h e p r i m a r y  radiation  [ 3 5 2 ] , g i v i n g the i n t e n s i t y o f t h a t p o r t i o n o f the primary r a d i a t i o n a t some depth x w i t h i n  the specimen, can be w r i t t e n :  [  x  =  I AA 0  exp  '  anoy s i n 91  u ( X )  . x  0)  S i m i l a r l y , the e q u a t i o n f o r the i n t e n s i t y o f t h e i n c r e m e n t a l amount o f t h e primary  r a d i a t i o n a t an i n c r e m e n t a l l y g r e a t e r depth x + A x :  245  •y(A) J  x+Ax  =  I o A A  e x  P  alloy s i n 91  Thus t h e amount o f t h e i n c r e m e n t a l p o r t i o n  (x + Ax) (2)  of the primary r a d i a t i o n  which  i s absorbed between depth x and'depth x + Ax i s :  x  l  ' W  ^ =  I o A X  e X  ailoy_ * s i n 01  y ( A )  P  -u(A)  x  Ax  al loy s i n 01  1 - exp  (3)  As Ax approaches z e r o , e q u a t i o n (3) s i m p l i f i e s t o : 'u(A)  f-u(A)  I  -  . W  5  I  o  A  A  e  x  a 11 oy s i n 01  p  alloy s i n 01  (4)  Of t h e amount o f absorbed r a d i a t i o n , t h e amount absorbed by element i depends on t h e a c t u a l  amount o f t h a t element p r e s e n t .  Only t h e f r a c t i o n  (5) P^alloy can absorb t h e i n c i d e n t  photons and thus be r e s p o n s i b l e f o r t h e p r o d u c t i o n  of the d e s i r e d c h a r a c t e r i s t i c r a d i a t i o n of p r o d u c t i o n o f t h i s r a d i a t i o n  f o r element i . The e f f i c i e n c y  i s g i v e n by t h e p r o d u c t o f t h e f r a c t i o n  of the p h o t o i o n i z a t i o n s o c c u r r i n g w i t h i n  the d e s i r e d s u b s h e l l ,  the f l u r o e -  scence y i e l d , and t h e p r o b a b i l i t y f o r the e x c i t a t i o n o f t h e d e s i r e d Thus, the i n t e n s i t y o f c h a r a c t e r i s t i c r a d i a t i o n of p r i m a r y r a d i a t i o n w i t h i n  line.  produced by t h e increment  t h e l a y e r Ax i s g i v e n by:  r.-l I  char, i  w r  i  i  '  g  i  I x  - I , » x+Ax  (6)  W. l  P  U ;  alloy  246  The i n t e n s i t y o f the c h a r a c t e r i s t i c r a d i a t i o n which i s o r i e n t e d to pass through t h e c o l l i m a t o r  i s g i v e n by c o n s i d e r a t i o n  o f t h e absorp-  t i o n o f t h i s r a d i a t i o n as i t passes through t h e specimen. specimen s u r f a c e ,  correctly  Thus, a t t h e  t h e i n t e n s i t y o f the p r i m a r y c h a r a c t e r i s t i c  radiation  which was produced over the depth x to x + Ax by an i n c r e m e n t a l amount o f primary i n c i d e n t  radiation i s :  char,i  dfi  *char,i  *char,i  alloy s i n 62  exp  diT  (7)  F u r t h e r , the i n t e n s i t y o f t h i s r a d i a t i o n a t t h e d e t e c t o r i s :  I  char,i  I v  L,i  (8)  char,i  The i n t e n s i t y o f c h a r a c t e r i s t i c r a d i a t i o n which reaches t h e d e t e c t o r as a r e s u l t o f i r r a d i a t i o n o f the specimen by t h e c o n t i n u o u s range o f wavelengths capable o f t h i s e x c i t a t i o n i n the primary i n c i d e n t a r i s i n g from a l l depths w i t h i n  t h e specimen i s hence:  I I  I  r a d i a t i o n , and  fOO  W . i  primary,l  d x  d x  SWL where i n t e g r a t i o n s primary i n c i d e n t LJJJ  let  are performed over the s h o r t wavelength l i m i t o f t h e  radiation  ( S W L ) t o the wavelength c o r r e s p o n d i n g t o t h e  edge, and over a l l depths i n t h e specimen.  F o r , convenience i f we  (9)  247  V K  i  1  oi  4TT  • g  i  i  • W-  (10)  n  then the i n t e n s i t y f o r p r i m a r y c h a r a c t e r i s t i c r a d i a t i o n produced from an i n f i n i t e l y thick.;specimen  i s [351]:  "III  IoU)  sinOl SWL  P  U ;  alloy  s i n 61  p +  (11)  dX  primary,l char, i sin  _  a l 1 oy  02  For a specimen o f t h i c k n e s s x, on the o t h e r hand:  ^ x I  )  primary,i  -in  char,i sinG 1  Io(X)  1 - exp -  sin..01  P al1o,y s i n 01 u ;  +  p  sin  a Hoy 02  (12)  char, i sin  In o r d e r to account f o r the secondary f l u o r e s c e n t enhancement due  t o those peaks o f element j which are capable o f e x c i t i n g the d e s i r e d  c h a r a c t e r i s t i c r a d i a t i o n of i ,  one must c o n s i d e r an analogous d e r i v a t i o n  f o r the p r o d u c t i o n o f t h i s o t h e r c h a r a c t e r i s t i c r a d i a t i o n o f element j . By comparison w i t h e q u a t i o n  ( 6 ) , and c o n s i d e r i n g a l l n l i n e s capable  c a u s i n g secondary f l u o r e s c e n c e , the r e s u l t a n t sum i s :  of  248  y i L  n  char/sec,j  r.-l  = y  _J  r• J  L  n  co . J  J,n  • q. j,n  x  s  w. J  x+Ax  P  ( X )  anoyJj  That p o r t i o n o f the above r a d i a t i o n which i s c o n v e r t e d t o secondary f l u o r e s c e n t r a d i a t i o n o f element i i s :  i  . = y (I ,  sec,n  L  v  n  , .) char/sec,j'  V  1  c h a r , j ,n  • co. • q. • W.  r.  char,j,n  alloy  The amount o f secondary f l u o r e s c e n t r a d i a t i o n produced w i t h i n t h e l a y e r Ax which reaches t h e d e t e c t o r i s :  [ char ,i J alloy s i n 62 X  I. sec,i  I  sec,i  dfi 4T '  l,i  On c o n s i d e r a t i o n o f the t o t a l  e x  P  i n t e n s i t y o f secondary f l u o r e s c e n t r a d i a t i o n ,  and c o l l e c t i n g the v a r i o u s terms f o r each o f the n c h a r a c t e r i s t i c c a p a b l e o f p r o d u c i n g secondary  f -I * r.-l r. I JJ  lines  fluorescence,  [ char,j,n  s  J,n  3  j,n  y char,j,n  alloy  then i t can be shown t h a t the r e s u l t a n t e q u a t i o n s f o r the secondary f l u o r e s c e n t i n t e n s i t y from a specimen o f t h i c k n e s s x i s :  249  ^x^sec,i  L P -W. n J  edge,j,n f  ^-(x)  p "M, sine! v  T  m  I o ( A )  .  a1loy . s i n 01 v  1 -exp  p 'alloy  p  u  SWL  +  s i n 61  _  y  C  a  r  >  1  sin  char , i  —  h  sin  ^lloy]. 62 j  ) x  (17)  alloy  02  In the p r e s e n t c a s e , t h e i n t e n s i t y o f a pure p l a t i n u m specimen o f i n f i n i t e t h i c k n e s s was e v a l u a t e d a c c o r d i n g t o e q u a t i o n  (11) and a computer  programme was w r i t t e n i n o r d e r t o p r o v i d e v a l u e s o f t h e i n t e n s i t y  ratios  according t o : I  I total, i total,Pt  I primary,i  sec, i  (18)  I primary, Pt  R e l e v a n t data f o r edge jump r a t i o s , f l u o r e s c e n t y i e l d s , tion e f f i c i e n c i e s , absorption c o e f f i c i e n t s , c h a r a c t e r i s t i c l e n g t h s and a b s o r p t i o n edges v/ere o b t a i n e d from s e v e r a l For t h e purposes o f t h e c a l c u l a t i o n  radiation  sources  wave-  [351-365],  t h e i n t e n s i t y d i s t r i b u t i o n i n the  primary beam was e s t i m a t e d by measuring t h e r e l a t i v e i n t e n s i t y of t h e s c a t t e r e d beam produced by i n s e r t i n g specimen chamber o f the X-ray s p e c t r o g r a p h . quently expressed  excita-  as a 5 t h - o r d e r p o l y n o m i a l  a piece of p l a s t i c i n the The  r e l a t i o n was subse-  f o r the c a l c u l a t i o n .  mass a b s o r p t i o n c o e f f i c i e n t v s . wavelength r e l a t i o n s i r i d i u m were f i t t e d t o c u r v e s o f t h e form:  profile  The  f o r p l a t i n u m and  250  e  = a +  bA  3  (19)  between each d i s c o n t i n u i t y ( a b s o r p t i o n edge) over the r e l e v a n t range o f wavelengths (11) and  (up to the I  j j j edge).  The  i n t e g r a t i o n s shown i n e q u a t i o n s  (12) are c o n s e q u e n t l y not p o s s i b l e due to the presence o f s e v e r a l  d i s c o n t i n u i t i e s i n the mass a b s o r p t i o n c o e f f i c i e n t vs wavelength f o r the a l l o y .  T h i s problem was  t i o n s to be the summation o f a l l over the d e s i r e d wavelength  c i r c u m v e n t e d by c o n s i d e r i n g a l l i n t e g r a i n t e g r a l s between a l l d i s c o n t i n u i t i e s  range.  For the p r o d u c t i o n o f I r L c n c h a r a c -  t e r i s t i c r a d i a t i o n , f o r example, e q u a t i o n s (11) and o v e r the  relation  (12) are e v a l u a t e d  sum: IrL  II  PtL,  IrL,  PtL  II  IrL  PtL  IrL  III (20)  swL  swL  Secondary  PtL  I  IrL  I  PtL  II  IrL  PtL  II  III  f l u o r e s c e n c e was o n l y c o n s i d e r e d f o r those o t h e r c h a r -  a c t e r i s t i c r a d i a t i o n s whose i n t e n s i t y was a t l e a s t 10 per c e n t t h a t o f the L a i l i n e .  Under these c o n d i t i o n s , o n l y the I r L y i l i n e was c o n s i d e r e d  to be c a p a b l e o f p r o d u c i n g secondary PtLcti r a d i a t i o n , and the PtL|3 PtLy6 l i n e s were c o n s i d e r e d c a p a b l e o f e x c i t i n g secondary  A2.2  IrLai  2  and  radiation.  P r e p a r a t i o n o f P l a t i n u m - C o a t e d T i t a n i u m Standards A Hummer D.C.  S p u t t e r i n g System was employed to produce p l a t i n u m  l o a d i n g s on f l a t t i t a n i u m sheet s u b s t r a t e s c u t i n t o d i s c s of the same s i z e as the anodes prepared from the commercial S e c t i o n 4.1.1.  Pt/30 I r - T i  sheet d e s c r i b e d i n  The d i s c s were p r e v i o u s l y degreased, t r e a t e d i n s t r o n g  251  n i t r i c a c i d s o l u t i o n , d r i e d i n an oven and c a r e f u l l y weighed. was  performed i n a vacuum o f 150-200 m i l l i t o r r a t 1000  A f t e r s p u t t e r i n g , the specimens were re-weighed. determined  by d i v i d i n g the weight  Sputtering  v o l t s and 10  mA.  P l a t i n u m l o a d i n g s were  g a i n by the exposed area o f the t i t a n i u m  sheets. The  specimens were then p l a c e d i n a P h i l i p s X-ray  Spectrograph  such t h a t they were c e n t r e d w i t h i n the p r i m a r y beam, p r o v i d e d by a X-ray tube o p e r a t e d a t 35 KV and 15 mA.  tungsten  For a L i F a n a l y z i n g c r y s t a l  the  Bragg c o n d i t i o n f o r d i f f r a c t i o n o f P t L a i r a d i a t i o n i s s a t i s f i e d a t 38.05° 20.  Measurements performed on an X-ray s p e c t r o m e t e r  have an i n h e r e n t  l i m i t o f a c c u r a c y which depends on the c o u n t i n g procedure o f the equipment.  As the a r r i v a l  and the  o f photons a t the d e t e c t o r i s p u r e l y  random w i t h t i m e , the arrangement o f a l a r g e number o f counts obey a Gaussian tistical are 68.3,  distribution.  and 99.7%  i s found  The c o e f f i c i e n t o f v a r i a t i o n e i s the  term most commonly a p p l i e d i n s p e c t r o s c o p y . 95.4  stability  (The  to  sta-  probabilities  t h a t the t r u e mean v a l u e l i e s w i t h i n ± e,  2e,  and 3e% o f the measured average v a l u e , r e s p e c t i v e l y . ) In the absence o f s y s t e m i c e r r o r s the a c c u r a c y o f a measurement i s r e l a t e d to the number o f counts N, taken i n a g i v e n i n t e r v a l and the number o f r e p l i c a t e c o u n t i n g p e r i o d s , n:  me =  tri'lOO >¥  The  1% • —  / i o o (m=l,2,3  v^n  p r e c i s i o n o f the measured average v a l u e under i d e a l  hence l i m i t e d o n l y by.the t o t a l  to (2  \ )  number o f counts t a k e n .  conditions i s Actual operating  252  c o n d i t i o n s a r e not i d e a l , however, and the t o t a l  c o e f f i c i e n t of v a r i a t i o n  is:  e = Jz  1  counting s t a t i s t i c s + E e  z  equipment e r r o r s  (22)  Indeed, when c o u n t i n g i s performed on a peak of high i n t e n s i t y (thus pro- d u c i n g a l a r g e number o f t o t a l c o u n t s ) the predominant e r r o r i s due equipment i n s t a b i l i t y .  to  T h i s s i t u a t i o n makes i t i m p o s s i b l e t o s u b s t a n t i a l l y  improve measurement p r e c i s i o n by o b t a i n i n g more t o t a l  counts once the  c o u n t i n g e r r o r i s , say, an o r d e r o f magnitude l e s s than the equipment e r r o r . In the p r e s e n t c a s e , measurements o f the ( P t L a i + background) i n t e n s i t y were made over 17 second count i n t e r v a l s , u s u a l l y w i t h 25 cate determinations.  repli-  The c o u n t i n g e r r o r and the measured t o t a l  coefficient  of v a r i a t i o n f o r the v a r i o u s specimens a r e shown i n T a b l e A2.1.  Instrument  instability  ( l o n g term e r r o r ) can be compensated t o a g r e a t degree by  r e p e a t e d measurements on a s u i t a b l e s t a n d a r d . a d j u s t e d t o p r o v i d e a g i v e n count t o t a l  The  equipment i s i n i t i a l l y  on the s t a n d a r d and a l l subsequent  measurements on the s t a n d a r d are compared w i t h t h i s " s t a n d a r d " v a l u e . That i s , subsequent d e t e r m i n a t i o n s o f the average o f a number o f r e p l i c a t e c o u n t i n g i n t e r v a l s w i t h the s t a n d a r d are d i v i d e d i n t o the a c c e p t e d  standard  v a l u e t o p r o v i d e a c o r r e c t i o n f a c t o r which i s a p p l i e d t o the count t o t a l s measured f o r o t h e r specimens e i t h e r immediately ment w i t h the s t a n d a r d . convenient  The  s t a n d a r d may  p r i o r t o or a f t e r measure-  be a s t a b l e specimen o f  any  kind. The  average t o t a l  count f i g u r e s i n T a b l e A2.1  must be c o r r e c t e d  f o r both the dead-time o f the p r o p o r t i o n a l c o u n t e r , and f o r the background  253  T a b l e A2.1 P r e c i s i o n o f Measured Count V a l u e s ( P t L a i I n t e n s i t y & Background) on Prepared P t - T i S t a n d a r d s , and Comparison w i t h Counting  E r r o r Due t o  Statistics  ^ ^counting e  ^  £  ^measured  Average T o t a l Counts (17 s e c )  No. o f Count Periods  1467  15  2.02%  10.06%  6208  25  0.76  10.42  1.74  11232  25  0.57  5.65  2.74  16592  25  0.47  4.01  3.56  20544  25  0.42  3.14  4.88  26560  25  0.37  4.03  6.69  36032  25  0.32  3.42  8.23  42752  25  0.29  2.63  14.65  66176  25  0.23  3.16  152446  25  0.15  3.47  Specimen  0 gPt/m 0.872  Pt  foil  2  =  300  TuTn-  = 300(f)  254  radiation.  The former  i s r e a d i l y e v a l u a t e d a c c o r d i n g t o the e q u a t i o n :  -r . . True count r a t e =  1  _  Measured count r a t e (  M  e  a  s  u  r  e  d  C  Q  U  n  t  r  a  t  e  )  (  D  e  a  d  / 0 0  t  i  m  e  (23)  )  For a p r o p o r t i o n a l c o u n t e r , t h e dead time i s about 2 microseconds [ 3 5 6 ] . S i n c e i t i s i m p o s s i b l e t o d i r e c t l y measure the i n t e n s i t y o f the background o f p l a t i n u m - c o a t e d  specimens a t t h e P t L a i a n g l e , t h e v a r i a t i o n  o f the background w i t h l o a d i n g was measured a t 35° 20, where no i n t e r f e r e n c e i s encountered  from t i t a n i u m , t u n g s t e n , o r p l a t i n u m l i n e s .  v a r i a t i o n was then added t o t h e background determined (38.05° 20) f o r a bare t i t a n i u m specimen. i n c r e a s e s l i g h t l y w i t h measured P t L a i  a t the PtLai  The angle  The background was found t o  counts.  T a b l e A2.2 summarizes t h e dead-time and background c o r r e c t i o n s f o r the P t L a i  i n t e n s i t i e s measured w i t h t h e prepared  with the f i n a l  r e s u l t s being e x p r e s s e d  thick platinum  standard.  A2.3  Conversion The  o f Measured I n t e n s i t y  Pt-Ti  standards,  as an i n t e n s i t y r e l a t i v e t o a  Data t o A n a l y t i c a l  r e l a t i v e i n t e n s i t y v s . noble metal  Data  l o a d i n g data f o r s e l e c t e d  P t / I r a l l o y s o f v a r i o u s c o m p o s i t i o n a r e g i v e n i n F i g u r e s A2.1 and A2.2 f o r P t L a i and I r L a  x  radiation, respectively.  As can be seen, t h e dead-time  and background c o r r e c t e d r e l a t i v e i n t e n s i t y v a l u e s f o r t h e prepared  Pt-Ti  standards c l o s e l y conform t o the t h e o r e t i c a l l y p r e d i c t e d c u r v e . For P t / I r a l l o y c o a t i n g s o f unknown l o a d i n g and c o m p o s i t i o n , the a n a l y t i c a l data were determined  by means o f another computer programme  which i n c o r p o r a t e d the r e s u l t s o f t h e fundamental parameter  calculation  \  255  Table A2.2 Dead-Time and Background C o r r e c t i o n s f o r PtLoti C h a r a c t e r i s t i c R a d i a t i o n Measured w i t h Prepared  P t - T i Standards,  w i t h t h e R e s u l t a n t True Peak  I n t e n s i t i e s and R a t i o s R e l a t i v e t o a T h i c k P l a t i n u m (17 Second Counting  Measured PtLct! Counts  Specimen 0 gPt/m  Dead-Time Corrected Counts  Calculated Background  Standard  Interval)  True P t L a i Counts ( I ) A  Peak: Background Ratio  I X  I 00  1467  1467  2380  0  0  0.0000  6208  6212  1486  4726  3  0.0310  1.74  11232  11246  1502  9744  6  0.0638  2.74  16592  16623  1521  15102  10  0.0989  3.56  20544  20591  1535  19056  12  0.125  4.88  26560  26639  1558  25081  16  0.164  6.69  36032  36177  1598  34579  22  0.226  8.23  42752  42956  1628  41328  25  0.271  14.65  66176  66666  1747  64919  37  0.425  152446  155070  2380  152690  64  1.000  0.872  Pt  foil  2  256  Noble metal loading ,  F i g u r e A2.1  g(Pt-Hr)/m  2  R e l a t i v e P t L a i i n t e n s i t y vs. l o a d i n g r e l a t i o n f o r v a r i o u s P t / I r a l l o y s , f o r 9 i = 60°, 9 = 35°, W-tube o p e r a t e d a t 35 kv. Data p o i n t s r e f e r t o t h e c o r r e c t e d , P t L a i i n t e n s i t i e s measured f o r t h e prepared P t - T i s t a n d a r d s . 2  257  Noble metal loading,  F i g u r e A2.2.  g(Pt+lr)/m  2  R e l a t i v e IrLcti i n t e n s i t y vs. l o a d i n g r e l a t i o n f o r v a r i o u s P t / I r a l l o y s f o r 9 i = 60°, 6 = 35°, W-tube o p e r a t e d a t 35 kv. 2  258  d e s c r i b e d i n S e c t i o n A2.1.  The raw i n t e n s i t y data were i n i t i a l l y dead-  time c o r r e c t e d and then used t o p r o v i d e an i n i t i a l fractions PtLai  of the a l l o y components.  Background  e s t i m a t e o f the w e i g h t  c o r r e c t i o n s f o r I r L a i and  i n t e n s i t i e s were e s t i m a t e d by u s i n g l i n e a r sums (weighted t o the  e s t i m a t e d wiehgt f r a c t i o n s o f the components) o f the background v s . t o t a l counts r e l a t i o n s e s t i m a t e d or d i r e c t l y measured f o r the I r L a i and wavelengths on P t - T i  and I r - T i specimens.  r a t i o o f the I r L a i and P t L a i  On background c o r r e c t i o n ,  i n t e n s i t i e s was  e s t i m a t e o f the w e i g h t f r a c t i o n s  W  =  f  n  (24)  n  Pt  was g r e a t e r than 0.1  e s t i m a t e , and i f the  per c e n t , the backgrounds were  T h i s i t e r a t i v e procedure was r e p e a t e d u n t i l converged.  calculation:  ^IrLai,total ,I .. ,total [ PtLai  The second e s t i m a t e was compared w i t h the i n i t i a l difference  the  of the a l l o y components, a c c o r d i n g t o a  Ir  H  x  used t o determine a second  r e l a t i o n determined from the fundamental parameter  W  Pta  recalculated.  the weight f r a c t i o n e s t i m a t e s  The next s t e p then i n v o l v e d the l o a d i n g d e t e r m i n a t i o n .  was found t h a t c u r v e s o f the type shown i n F i g u r e A2.1  It  c o u l d be expressed  by e q u a t i o n s of the form:  Pt l o a d i n g = p  x  l n ( l + y) + p  2  In(1 - y)  (25)  where  W  t  P t L a i i n t e n s i t y produced from an a l l o y o f t h i c k n e s s x • P t L a i i n t e n s i t y produced from an i n f i n i t e l y t h i c k P t s t a n d a r d  (26)  259  and  p! and p  2  are functions of W pt  The form o f e q u a t i o n  (25) i s par-  t i c u l a r l y u s e f u l i n t h a t i t p e r m i t s t h e mathematical r e l a t i o n o f t h e e n t i r e f a m i l y o f c u r v e s o f t h e type shown i n F i g u r e A2.1 w i t h o n l y two p a r a m e t e r s , y e t p r o v i d i n g an e x c e l l e n t e m p i r i c a l f i t t o t h e t h e o r e t i c a l equations.  Once the p l a t i n u m  was r e a d i l y p r o v i d e d  l o a d i n g was d e t e r m i n e d , the i r i d i u m l o a d i n g  from t h e weight f r a c t i o n o f t h a t m e t a l .  APPENDIX A.3 IK-DKOP  CALCULATIONS THE  PRESENT BMRNARTT  where  IR  FOR E L E C T R O L Y T E S O L U T I O N S  WuRK  EMPLOYED  IN  A C C O R D I N G TO T H E E Q U A T I O N OF  C357]  FOR PLANAR  "IR  k  "(D  4) 3 ;  ELECTRODES:  =  i6 k  IR drop i n mi 1 1 i v o l t s a p p l i e d ( g e o m e t r i c ) c u r r e n t d e n s i t y i n mA/cm w o r k i n g e l e c t r o d e / L u g g i n c a p i l l a r y t i p s e p a r a t i o n i n cm d i a m e t e r o f t h e Luggin c a p i l l a r y t i p i n cm e l e c t r o l y t e c o n d u c t i v i t y (ohm c n r )  i D d k  2  -1  1  i Temperature k (from r e f . [ 3 5 8 ] ) mA/cm ohm- cm- i °C  Electrolyte  V  [R  ^  2  1  6=0.1 cm 6=0.2 cm 6=1 cm  2M H S(H  22  0.71  7.8 15.6 52.1 104 260 521  1 2 7 15 37 73  2 4 15 29 73 147  11 22 73 146 366 734  2M H S 0  40  0.80  52.1 104 260 521  7 13 33 65  13 26 65 130  65 130 325 561  22  0.62  52.1 104 117 260 521  8 17 19 42 84  17 34 38 84 168  84 168 189 419 840  40  0.68  52.1 104 117 260 521  8 15 17 38 77  15 31 34 76 153  77 153 172 382 766  2  2  4  2M H SC\ + 0.5M CuS0  4  2M H S 0 + 0.5M C u S 0  4  2  2  4  260  APPENDIX A4  ESTIMATION OF INDIVIDUAL ION ACTIVITIES  A4.1  A c t i v i t y o f t h e Hydrogen Ion i n S u l f u r i c A c i d The  problem o f d e t e r m i n i n g i n d i v i d u a l  Solutions  i o n a c t i v i t i e s i s two-  f o l d , as i t i n v o l v e s t h e e s t i m a t i o n o f both t h e c o n c e n t r a t i n g o f t h e i o n i n t h e system o f i n t e r e s t and i t s i n d i v i d u a l For  s u l f u r i c acid solutions,  ionic a c t i v i t y coefficient.  t h e problem i s made more d i f f i c u l t by t h e  v a r i a t i o n o f t h e second c o n c e n t r a t i o n e q u i l i b r i u m  constant, K  with  i o n i c s t r e n g t h , although the a c t i v i t y e q u i l i b r i u m  constant i s , of course,  i n v a r i ant:  „  a  H S0, a  a  m  H SL\ . H S0, m  HSCU  Y  HS0  Y  Y 4  1/  HS0,  C  Q  K  Young [359,360] has determined t h e c o n c e n t r a t i o n s o f the i n d i vidual  i o n i c s p e c i e s i n aqueous s u l f u r i c a c i d s o l u t i o n s .  From h i s data  these c o n c e n t r a t i o n s , e x p r e s s e d as m o l a l i t i e s , can be d e t e r m i n e d . f o r any electrolytes stant, mated.  of interest.  Further, the concentration equilibrium  con-  IO> , and t h e r a t i o o f t h e i o n i c a c t i v i t y c o e f f i c i e n t s can be e s t i These c a l c u l a t i o n s Individual  a r e summarized i n T a b l e A4.1.  i o n i c a c t i v i t y c o e f f i c i e n t s can be e s t i m a t e d from  the mean a c t i v i t y c o e f f i c i e n t r e s u l t s o f Harned [ 3 6 2 ] , who measured t h e EMF o f t h e c e l l : 261  262  T a b l e A4.1 Individual  Ion M o l a l i t i e s , I o n i c S t r e n g t h s ,  Ionic A c t i v i t y C o e f f i c i e n t Ratios  E q u i l i b r i u m C o n s t a n t s and  Calculated  from the Raman  S p e c t r a Data o f Young [ 7 3 6 ] f o r Aqueous S u l f u r i c A c i d Solutions  t  H S0 ?  4  HSOV m  °c  M  25  .1 1 2 4 8  .1 1.04 2.17 4.75 12.2  .07 .73 1.5 3.1 9.5  .1 1 2 4 8  .1 1.04 2.17 4.75 12.2  .1 1 2 4 8 .1 1 2 4 8  40  60  80  m  (K  so;  2  v a l u e s from Robinson  [361])  2  _ H^S0, Y  H  +  I  K  y 2  .03 .31 .70 1.7 2.8  .13 1.35 2.9 6.4 15.0  .16 1 .66 3.60 8.15 17.9  .0106  .075 .78 1.6 3.4 10.0  .025 .26 .54 1 .3 2.3  .125 1.3 2.7 6.1 14.6  .15 1.56 3.23 7.35 16.9  .0064 n  .1 1.04 2.17 4.75 12.2  .08 .83 1.7 3.8 10.6  .02 .21 .43 .95 1.7  .12 1.25 2.6 5.7 14.0  .14 1.46 3.01 6.65 15.7  .0036  .1 1.04 2.17 4.75 12.2  .085 .88 1 .8 4.0 11.1  .015 .16 .32 .71 1.2  .115 1.2 2.5 5.5 13.4  .13 1.36 2.79 6.17 14.7  .002 n n  II II II  n  M II  II  M II II  II  K  2,c  .056 .57 1 .35 3.51 4.42  R  Y  HS0\  .19 .019 .0079 .0030 .0024  .042 .43 .91 2.73 3.36  .15 .015 .0070 .0023 .0012  .030 .32 .66 1.43 2.25  .12 .011 .0055 .0025 .0016  .020 .22 .44 .98 1.45  .10 .0091 .0045 .0020 .0014  263  P t , H |H SC\(m)|PbS0 |Pb0 , P t 2  2  4  (2)  2  The mean a c t i v i t y c o e f f i c i e n t , y±, i s r e l a t e d t o t h e c e l l  E = E° + § - l n ( 4 m  3  ± ) - f i In a , ^ 3  Y  where a l l terms have t h e i r c o n v e n t i o n a l assumes complete d i s s o c a t i o n ions.  EMF by  (3)  2  significance.  Such an e x p r e s s i o n  o f s u l f u r i c a c i d i n t o hydrogen and s u l f a t e  As t h i s i s n o t t r u e f o r s u l f u r i c a c i d s o l u t i o n s , the mean a c t i v i t y  c o e f f i c i e n t v a l u e s o f Harned were r e - c a l c u l a t e d  using the expression:  y±(incomplete d i s s o c i a t i o n ) = m ±(complete d i s s o c i a t i o n ) Y  m  using the i n d i v i d u a l i o n m o l a l i t i e s c a l c u l a t e d  H  2  m  S0,  from Young's d a t a .  mean a c t i v i t y c o e f f i c i e n t s determined by e q u a t i o n  ( 4 )  (4) do not d i f f e r  (The greatly  from those t a b u l a t e d by Harned, and c o i n c i d e f o r the case o f complete d i s sociation.)  The r e - c a l c u l a t e d  mean a c t i v i t y c o e f f i c i e n t s a r e g i v e n i n  T a b l e A4.2. The means.  a c t i v i t y o f t h e hydrogen i o n may be e s t i m a t e d by s e v e r a l  Four methods a r e d e s c r i b e d 1)  from t h e  Assume  = 1.  here:  The a^ v a l u e s a r e hence d i r e c t l y c a l c u l a b l e  v a l u e s g i v e n i n Table A4.1,  u s i n g the mean i o n i c a c t i v i t y  coef-  f i c i e n t v a l u e s i n Table A4.2 and t h e e q u a t i o n : 1/3  V  Y  S0,  (5)  264  2)  Assume  from the r a t i o y  R  YU/Y^Q^  calculated  =  "I-  The r e s u l t a n t  v a l u e determined  Y Q S  i n T a b l e A4.1 can then be used, i n c o n j u n c t i o n  w i t h the mean i o n i c a c t i v i t y c o e f f i c i e n t v a l u e s i n T a b l e A4.2 and e q u a t i o n (5) t o p e r m i t e s t i m a t i o n o f y ^ .  3)  Assume t h a t y < - Q  can be e s t i m a t e d from t h e t a b u l a t e d mean  i o n i c a c t i v i t y c o e f f i c i e n t data f o r o t h e r b i n a r y s u l f a t e s i m i l a r i o n i c s t r e n g t h , on the assumption t h a t y ± - y  salt: solutions  of  .. = y for canon oU c r i  4  that  electrolyte.  4)  Assume t h a t y ^ can be e s t i m a t e d from the t a b u l a t e d mean  i o n i c a c t i v i t y c o e f f i c i e n t data f o r the HC1 s o l u t i o n s s t r e n g t h , on t h e assumption t h a t y ± = y  The  r e s u l t s o f these c a l c u l a t i o n s  H  = y^  of s i m i l a r  f o r that  electrolyte.  a r e summarized i n T a b l e A4.3.  gence o f t h e hydrogen a c t i v i t y v a l u e s f o r s o l u t i o n s  ionic  The d i v e r -  above 0.01M, as c a l c u -  l a t e d by t h e above methods, can be r e a d i l y seen i n F i g u r e A4.1 which shows the  A4.2  variation  of calculated  solution  pH w i t h c o n c e n t r a t i o n .  A c t i v i t i e s i n Copper-containing S u l f u r i c Acid  Solutions  No mean i o n i c a c t i v i t y c o e f f i c i e n t data e x i s t f o r mixed s u l f u r i c acid/cupric sulfate  solutions.  s u l f u r i c a c i d and c u p r i c s u l f a t e  The mean i o n i c a c t i v i t y c o e f f i c i e n t s f o r can be e s t i m a t e d , however, by c o n s i d e r i n g  t h a t they a r e the same as those i n s o l u t i o n s  o f the i n d i v i d u a l  at the same i o n i c s t r e n g t h as t h e mixed e l e c t r o l y t e .  The i o n i c  electrolytes strengths,  265  T a b l e A4.2 Stoichiometric Y±,  Mean A c t i v i t y C o e f f i c i e n t s f o r S u l f u r i c A c i d  from t h e Data o f Harned the  m  Y ± Incomplete Dissociation  .1  .1 1.04 2.17 4.75 12.2  .265 .130 .130 .212 .835  .332 .163 .156 .245 1.19  .0366 .00433 .00380 .0147 1.69  .1  .1 1.04 2.17 4.75 12.2  .227 .111 .105 .160 .535  .311 .152 .144 .209 .829  .0301 .00351 .00299 .00913 .570  .1  .1 1.04 4.75 12.2  .197 .092 .120 .347  .298 .139 .182 .611  .0265 .00269 .00603 .228  .1  .1 1 .04 2.17 4.75 12.2  .185 .080 .075 .100 .275  .317 .136 .129 .171 .560  .0319 .00252 .00215 .00500 .176  2  M 1 2 4 8  40 1 2 4 8 60 1 2 8  80* 1 2 4 8  Incomplete D i s s o c i a t i o n o f S u l f u r i c A c i d  Y ± Incomplete Dissociation  H S0  25  [ 3 6 2 ] , R e c a l c u l a t e d t o Account f o r  Y ± Complete Dissociation  t  °c  Solutions,  4  E s t i m a t e d y± v a l u e s f o r complete d i s s o c i a t i o n .  3  Table A4.3 E s t i m a t e d Hydrogen and S u l f a t e Acid Solutions,  Ion A c t i v i t y C o e f f i c i e n t s and Hydrogen Ion A c t i v i t i e s i n Aqueous S u l f u r i c  Based on t h e Four Methods D e s c r i b e d i n the T e x t , U t i l i z i n g t h e I n d i v i d u a l  Ion M o l o l i t i e s  C a l c u l a t e d i n T a b l e A4.1 and t h e Mean I o n i c A c t i v i t y C o e f f i c i e n t s from Table A4.2 t  °c  M  25  60  1 2 4 8  H  .1  .1 1.04 2.17 4.75 12.2  .15 1.56 3.23 7.35 16.9  1 1 1 1 1  .1  .1 1.04 2.17 4.75 12.2  .14 1.46 3.01 6.65 15.7  .1  .1 1.04 2.17 4.75 12.2  .13 1.36 2.79 6.17 14.7  1 2 4 8 80  Y  .16 1.66 3.60 8.15 17.9  1 2 4 8  Method (3)  Method (4)  I  .1 1.04 2.17 4.75 12.2  .1  40  m  1 1 1 ] 1  1 2 4 8  Method (2)  Method (1)  H2SO4  Y  S0,  a  H  Y  H  Y  S0,  a  H  Y  H  Y  S0,  a  H  Y  H  Y  S0,  a  H  .13 1.35 2.9 6.4 15.0  .44 .48 .69 2.21 26.5  .19 .019 .0079 .0030 .0024  .057 .65 2.00 14.1 398.  .39 .25 .29 .63 6.06  .24 .068 .046 .037 .046  .051 .779 .34 .929 .84 1.567 4.03 6.21 91. 55.  .060 .0050 .0015 .00038 8(10)-  .030 .0035 .0030 .0091 .57  .125 1.3 2.17 6.1 1.4.6  .45 .48 .65 1.99 21.8  .15 .015 .0070 .0023 .0012  .056 .63 1.69 11.3 318.  .35 .22 .25 .50 3.56  .25 .070 .048 .037 .045  .782 .043 .29 .908 .67 1.409 3.02 4.91 52. 45.  .049 .0039 .0015 .00038 .00028  .10 1.18 3.80 30. 657.  1 1 1 1 1  .026 .0027 .0022 .0060 ,.23  .12 1.25 2.6 5.7 14.0  .47 .50 .63 1.55 11.9  .12 .011 .0055 .0025 .0016  .056 .62 1.63 8.9 167.  .32 .19 .21 .40 2.30  .26 .073 .050 .038 .043  .038 .784 .888 .24 .54 1.320 3.97 2.27 32. 39.  .043 .0034 .0012 .00038 .00014  .094 1.11 3.43 22.6 559.  1 1 1 1  .032 .0025 .0022 .0050 ...18  .115 1.2 2.5 5.5 13.4  .56 .53 .69 1.58 11.2  .10 .0091 .0045 .0020 .0014  .065 .63 1.73 8.7 150.  .34 .18 .21 .36 2.10  .27 .076 .048 .039 .040  .787 .040 .22 .869 .53 1.089 3.42 1.97 28. 32.1  .052 .0033 .0018 .00043 .00017  .091 1.04 2.72 18.8 430.  1  .037 .0043 .0038 .015 1.69  7  .10 1.25 4.54 40. 825.  cn cn  267  F i g u r e A4.1.  pH o f c o n c e n t r a t e d  s u l f u r i c acid solutions  by v a r i o u s  calculations.  268  however, cannot be r e a d i l y determined as t h e a d d i t i o n o f s u l f a t e i o n s a f f e c t s the v a l u e s o f t h e c o n c e n t r a t i o n s o f hydrogen and b i s u l f a t e i o n s i n o r d e r t o conform t o the second d i s s o c i a t i o n c o n s t a n t . obtained  The i o n i c s t r e n g t h can be  by an i t e r a t i v e procedure o f e s t i m a t i o n , c a l c u l a t i o n o f i o n i c  e q u i l i b r i a , comparison, and r e f i n e m e n t p a r t i c u l a r e l e c t r o l y t e , 2M H S 0 2  4  o f the e s t i m a t e , e t c e t e r a .  + 0.5M CuSC\, t h e e s t i m a t e d  activities  o f t h e hydrogen i o n and c u p r i c i o n s a r e g i v e n i n T a b l e A4.4. ion a c t i v i t y was c a l c u l a t e d on t h e assumption t h a t ( Y ± )  A4.3  r  c-n  For a  The copper =  ~ Yen '  Reference E l e c t r o d e P o t e n t i a l s The  Nernst e q u a t i o n s  r e f e r e n c e e l e c t r o d e p o t e n t i a l s were c a l c u l a t e d from t h e f o r t h e hydrogen e l e c t r o d e and copper/copper s u l f a t e e l e c t r o d e  using the estimated  individual  i o n i c a c t i v i t i e s , a., determined above.  Temperature c o e f f i c i e n t s o f t h e s t a n d a r d e l e c t r o d e p o t e n t i a l s were o b t a i n e d from De Bethune [ 3 6 3 ] .  These r e s u l t s a r e summarized i n T a b l e A4.5.  In  a d d i t i o n , t h e e l e c t r o d e p o t e n t i a l v a l u e s f o r the commercial mercury/mercurous s u l f a t e e l e c t r o d e as g i v e n by Caton [364] a r e i n c l u d e d f o r c o m p l e t e n e s s .  269  T a b l e A4.4 I o n i c M o l a l i t i e s , I o n i c S t r e n g t h , Second D i s s o c i a t i o n C o n s t a n t , Mean I o n i c A c t i v i t y C o e f f i c i e n t s and Hydrogen and Copper Ion A c t i v i t i e s f o r t h e E l e c t r o l y t e : 2M H S 0 2  4  + 0.5M CuS0  4  Temperature (°C)  ""rlSO., m  so  m  R  2c  ( Y ± )  ( Y ± )  40  60  80  1.51  1.64  1.9  2.05  1.16  1.03  .77  .61  2.85  2.72  2.47  2.32  5.5  5.2  4.7  4.4  2.2  1.7  1.0  .7  4  I  K  25  H S(V  .192  .175  .154  .149  .037  .038  .039  .040  2  CUS0,  a^ (method 1)  2.85  2.72  2.47  2.32  a  H  (method 2)  3.4  3.0  2.2  2.0  a  u  (method 3)  1.2  1 .0  u  (method 4)  7.9  6.7  a  a  n  Cu  .019  .019  .74 5.4 .020  .65 4.6 .020  270  T a b l e A4.5 C a l c u l a t e d R e f e r e n c e E l e c t r o d e P o t e n t i a l s w i t h Respect t o the Hypothetical  Standard Hydrogen E l e c t r o d e  L i q u i d J u n c t i o n s ) f o r the C e l l :  (Neglecting  Electrode(T)/  E l e c t r o l y t e CO/SHE (298° K) Reference  Electrode  Pt,H /2M H S 0 2  2  t°C 25  4  lt  2  1.35 .65 .34 1.25  0  (method (method (method (method  1) 2) 3) 4)  .027 .018 -.004 .039  .013  1.3 .63 .29 1.18  .052 .040 .016 .062  60  .030  1.25 .62 .54 1.11  .088 .074 .042 .096  80  .048  1.2 1.73 .53 2.72  .124 .112 .076 .126  25  .337  .019  .286  40  .350  .019  .297  60  .368  .020  .312  80  .385  .020  .325  25  .612  .656*  40  .613  .657  60  .614  .658  80  .615  .659  4  Hg/HgS0 satd/K S0  E (volts)  40  Cu/2M H S 0 + .5M CuSO,, 2  E° ( v o l t s )  4  Data from Caton [ 3 6 4 ] .  APPENDIX A5  SURFACE AREA CALCULATIONS  The measured t r a n s i t i o n time f o r g a l v a n o s t a t i c s t r i p p i n g o f oxygen p r e v i o u s l y d e p o s i t e d on an anode p o l a r i z e d t o imminent oxygen e v o l u t i o n can be d i r e c t l y r e l a t e d t o t h e amount o f charge consumed i n t h i s  process,  which i n t u r n can be equated w i t h t h e amount o f charge n e c e s s a r y t o remove a monolayer o f oxygen atoms (1:1 s t o i c h i o m e t r y w i t h s u r f a c e noble  metal  atoms) [ 2 1 9 ] . Although  t h e e x a c t atom " d e n s i t y " o f a r e a l cm  2  o f noble  metal  s u r f a c e s i s n o t known, r e l i a b l e e s t i m a t e s o f these v a l u e s were made by Will  [328] and hence on t h e b a s i s o f t h e 2 - e l e c t r o n t r a n s f e r  M + H0  M-0 + 2 H  2  monolayer charge v a l u e s can be c a l c u l a t e d .  monolayer  Q,monolayer]  +  + 2e  reaction:  (1)  These a r e :  = 420 u C / r e a l  cm  2  (2)  = 440 u C / r e a l  cm  2  (3)  Ir  271  272  For a l l o y s , a weighted mean i s c a l c u l a t e d from these v a l u e s .  The e l e c t r o -  c h e m i c a l l y a c t i v e s u r f a c e area i s thus g i v e n by t h e p r o d u c t o f t h e t r a n s i t i o n t i m e , T , and t h e a p p l i e d c u r r e n t , I , d i v i d e d by t h e monolayer charge f o r a real  cm  2  of electrode  surface:  A....... = -rn actual Q monolayer a l l o y Roughness f a c t o r s a r e i n t u r n c a l c u l a t e d by d i v i s i o n o f t h e v a l u e g i v e n b( 4 ) , by t h e g e o m e t r i c e l e c t r o d e  area:  (4)  

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