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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 Un ivers i ty of Br i t i sh "Col umbi a, 1970 M.A . Sc , The Univers i ty of B r i t i s h Columbia, 1973 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n T h e F a c u l t y o f G r a d u a t e S t u d i e s i n t he Depar tment o f METALLURGY We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t he r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA J a n u a r y , 1977 © Dona ld A r t h u r Wen s l e y , 1977 In presenting th i s thes is in pa r t i a l fu l f i lment of the requirements for an advanced degree at the Univers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i l ab le for reference and study. I further agree that permission for extensive copying of th is thes is for scho lar ly purposes may be granted by the Head of my Department or by his representat ives. It is understood that copying or pub l i ca t ion of th is thes is for f inanc ia l gain sha l l not be allowed without my writ ten permission. The Univers i ty of B r i t i s h Columbia> 2075 Wesbrook Place Vancouver, Canada V6T 1 W 5 Depa rtment ABSTRACT Evaluation of the changes i n loading and composition of platinum/ i r i d i u m a l l o y coated 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 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 , t y p i c a l of those produced from sol v e n t - e x t r a c t i o n treatment of leach l i q u o r s , was accomplished by means of an X-ray fluorescence s p e c t r o s c o p i c technique. Under continuous operation at a constant anode cur r e n t i n 2M HzSCU or i n 2M H 2S0 4 + 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 material chosen f o r t h i s study i s pre-dominantly electrochemical i n nature, with platinum showing p r e f e r e n t i a l a ttack. Anode m a t e r i a l s from other sources, of nominally s i m i l a r composi-t i o n and manufacture, show varying degrees of mixed mechanical and e l e c t r o -chemical c o r r o s i o n . Operation 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 ac c e l e r a t e d d i s s o l u t i o n of the c o a t i n g metals. On imminent anode f a l u r e the mechanism of coating metal l o s s becomes pre-dominantly mechanical i n nature. The development of surface oxygen coverage on the a l l o y coatings during anodic 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 turn determines the nature of the c o r r o s i o n and p a s s i v a t i o n (defined as the increase i n anode p o t e n t i a l with time) i s analogous to the case of unalloyed platinum metal, showing growth of oxygen coverage to the e q u i v a l e n t of i i only 2-3 monolayers with prolonged 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 coatings behave i n a s i m i l a r manner to platinum metal with respect to the formation and removal of surface oxygen coverage, thus p e r m i t t i n g the determination of 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 by means of forma-t i o n and s t r i p p i n g of oxygen monolayers. i i i TABLE OF CONTENTS Page ABSTRACT i i LIST OF TABLES i x LIST OF FIGURES x i i ACKNOWLEDGMENTS x v i i Chapter 1 INTRODUCTION 1 1.1 General 2 1.2 Anodes i n Copper Electrowinning 5 2 LITERATURE SURVEY: ANODIC CORROSION OF PLATINUM AND IRIDIUM 14 2.1 Platinum 14 2.1.1 C h l o r i d e E l e c t r o l y t e s 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 A l t e r n a t i n g Current Corrosion 17 2.1.5 State of Platinum i n S o l u t i o n 18 2.2 I r i d i u m 18 2.2.1 C h l o r i d e E l e c t r o l y t e s 19 2.2.2 In e r t E l e c t r o l y t e s . 19 i v Chapter Page 2.2.3 Organic-Containing E l e c t r o l y t e s 19 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 2.3 P l a t i n u m / I r i d i u n i A l l o y s 20 2.4 TSA Corrosion 21 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 Electrowinning . . . 33 2.5 Summary and R e l a t i o n to t h i s Work 34 3 LITERATURE SURVEY: OXYGEN FILMS ON PLATINUM AND IRIDIUM ANODES 37 3.1' Platinum 37 3.1.1 Nature of the Surface Oxygen Coverage 39 3.1.2 Strengthening of the Oxygen Bond 53 3.1.3 A c t i v e Oxygen 53 3.1.4 Type II Oxide 54 3.1.5 Non-Electrochemical Techniques f o r Oxygen Film Evaluation 57 3.1.6 Review 58 3.2 I r i d i u m 60 3.2.1 Degree of Oxidation 61 3.2.2 Nature of the Surface Oxygen Coverage 63 3.3 Summary and R e l a t i o n to t h i s Work 64 4 EXPERIMENTAL 70 4.1 M a t e r i a l s and Apparatus 70 v Chapter Page 4.1.1 Anode M a t e r i a l s and Con 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 C e l l s 75 4.1.4 Reference Electrodes 77 4.1.5 Electrochemical Instrumentation 78 4.1.6 Other Apparatus 80 5 PROCEDURE 81 5.1 Measurement of Anode Loading and Surface Composition 81 5.2 Surface Charge and Surface Area Studies 82 5.3 P o l a r i z a t i o n Curves 84 5.3.1 Noble Metal Electrodes 84 5.3.2 Titanium E l e c t r o d e s . 85 5.4 Long-Term E l e c t r o l y s i s 85 5.5 El 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 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 Corrections 88 6 RESULTS 90 6.1 D e s c r i p t i o n of New Electrodes 90 6.1.1 Surface Areas 90 6.1.2 Loadings 92 6.1.3 D i f f r a c t o m e t r y 92 6.1.4 Morphology 96 6.2 Corrosion of Noble Metal Coated Anodes 99 6.2.1 Changes i n Loading and Composition 99 v i Chapter Page 6.2.2 Surface Area Changes 110 6.2.3 Corrosion 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 6.3 P a s s i v a t i o n 146 6.3.1 P o t e n t i a l vs. Time Behavour 146 6.3.2 Surface Charge Studies 162 6.3.3 P o l a r i z a t i o n Curves 172 6.3.4 Titanium . . . . 175 6.4 Anode Deposits 180 6.4.1 Growth 180 6.4.2 I d e n t i f i c a t i o n 183 6.4.3 Morphology 186 6.5 Complete Degradation 186 6.5.1 Anode P o t e n t i a l Behaviour 186 6.5.2 Corrosion 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 of the Surface Degra- . dation Product 194 7 DISCUSSION 197 7.1 Corrosion 198 7.1.1 During Reve r s i b l e P o t e n t i a l vs. ^ Time Operation 198 7.1.2 Surface Oxygen Coverage and Corrosion 202 v i i Chapter Page 7.1.3 Pulsed E l e c t r o l y s i s 204 7.1.4 Complete Degradation 206 7.1.5 Thiourea A d d i t i o n s 207 7.2 P a s s i v a t i o n 208 7.2.1 Surface Area Studies 208 7.2.2 Surface Oxygen Coverage 209 7.2.3 Time-dependence of 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 7.2.4 Time-dependence of 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 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 213 7.2.6 Pulsed Current Operation 214 8 CONCLUSIONS 215 8.1 Corrosion 215 8.2 P a s s i v a t i o n . 216 8.3 S u i t a b i l i t y of Pt/30 I r - T i Anodes f o r Copper Electrowinning 218 REFERENCES 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 Page 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 Copper E l e c t r o m e t a l l u r g i c a l Processes 4 1.2 Summary of Operational Data f o r Solvent E x t r a c t i o n / Electrowinning (SX/EW) Plants 6 1.3 Techniques P e r m i t t i n g High Current Density Operation i n Copper Electrowinning 7 1.4 P r o p e r t i e s of Substrate and Coating Metals and the Oxides 11 2.1 Summary"of Corrosion Rates Reported f o r Noble Metal Coated Anodes 23 3.1 Probable Stages of Oxidation of the Surface of Platinum with Increasing Anode P o t e n t i a l 59 6.1 I d e n t i f i c a t i o n of X-ray D i f f r a c t i o n Peaks Observed with New Pt/30 I r - T i Anodes 93 6.2 L a t t i c e Parameters f o r I r and Pt, and f o r Pt/30 I r a l l o y s 95 6.3 El e c t r o n D i f f r a c t i o n Results . 97 6.4 E f f e c t of E l e c t r o l y s i s Time on the Surface Areas of Pt/30 I r - T i Anodes. 112 6.5 Platinum Corrosion Results from Cumulative E l e c t r o l y s i s Runs i n 2M H 2S0 4 + 0.5M CuSO^ 118 6.6 I r i d i u m Corrosion Results from Cumulative E l e c t r o l y s i s Runs i n 2M H 2S0 4 + 0.5M CuS0 4. 119 6.7 Platinum Corrosion Results from Cumulative E l e c t r o l y s i s runs i n 2M H 2S0 4 120 i x Table Page 6.8 I r i d i u m Corrosion Results from Cumulative E l e c t r o l y s i s Runs i n 2M H 2S0 4 121 6.9 I n d i v i d u a l and Cumulative Corrosion Data f o r a Pt/30 I r - T i Anode which E x h i b i t s Coating Loss by S p a l l i n g 122 6.10 Platinum and I r i d i u m Corrosion Results i n S u l f u r i c A cid S o l u t i o n s of Various Strengths. . 129 6.11 Cumulative Pt and I r Corrosion Rates f o r I n d i v i d u a l Electrodes from D i f f e r e n t Manufacturing Lots 131 6.12 Noble Metal Loading Data f o r Electrodes Subjected to Pulsed E l e c t r o l y s i s 134 6.13 Corrosion Rate Data f o r Electrodes Subject to Pulsed E l e c t r o l y s i s 136 6.14 Loading and Corrosion Rate Data f o r an Anode, P r e v i o u s l y Subjected to Pulsed Current Operation, Subsequently Operated Under Continuous Anodic Current 137 6.15 Loading and Corrosion Rate Data f o r an Anode Operated with .05 gpl Thiourea A d d i t i o n 139 6.16 Expressions f o r the P o t e n t i a l vs. Time Behaviour of New Anodes 148 6.17 E f f e c t of Anodic P o l a r i z a t i o n Times on Surface Oxygen Coverage. .'. 170 6.18 I d e n t i f i c a t i o n of Lead-Containing Anode Deposits 184 6.19 Conversion of $-Pb0 2 Deposits to PbSO., 185 6.20 Corrosion Data f o r Pt/30 I r - T i Anodes Operating Under I r r e v e r s i b l e Anode P o t e n t i a l Conditions 190 6.21 Corrosion Data f o r Pt/30 I r - T i Anodes which have Suffered Complete Degradation 192 A l . l . E f f e c t s of V a r i a t i o n s i n the E l e c t r o l y t e Composition on the P o t e n t i a l of the Copper/Copper S u l f a t e Electrode 239 A2.1 P r e c i s i o n of Measured Count Values (PtLai I n t e n s i t y and Background) on Prepared P t - T i Standards 253 A2.2 Dead-time and Background Corrections f o r the Prepared Pt - T i Standards, with the Resultant True Peak I n t e n s i t i e s . . . . 255 x 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 Constants and Ionic A c t i v i t y C o e f f i c i e n t Ratios C a l c u l a t e d f o r S u l f u r i c A c i d S o l u t i o n s 262 A4.2 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 A c i d S o l u t i o n s 265 A4.3 Estimated 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 S o l u t i o n s 266 A4.4 Hydrogen and Copper Ion A c t i v i t i e s i n Mixed H 2S0 4/CuS0 4 E l e c t r o l y t e . . . . 269 A4.5 C a l c u l a t e d Reference Electrode P o t e n t i a l s with Respect to the SHE 270 x i LIST OF FIGURES Figure Page 4.1 Working electrode c o n s t r u c t i o n . 72 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 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 91 6.2 S.E.M. view of the t i t a n i u m substrate surface 98 6.3 S.E.M. view of a t y p i c a l new "4.33 g/m2" noble metal loading anode surface 98 6.4 S.E.M. view of a t y p i c a l new "4.33 g/m2" noble metal loading anode surface 100 6.5 S.E.M. view of a t y p i c a l new "20 g/m2" noble metal loading anode surface 100 6.6 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 with e l e c t r o l y s i s time i n 2M H 2S0 4 + 0.5M CuSO,, 22°.. 102 6.7 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 with e l e c t r o l y s i s time i n 2M H^SO* + 0.5M CuS0 4, 40° 103 6.8 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 with e l e c t r o l y s i s time i n 2M HzSO*, 22° 104 6.9 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 with e l e c t r o l y s i s time i n 2M H 2S0 4, 40° 105 6.10 V a r i a t i o n of 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 with e l e c t r o l y s i s time i n 2M H2SO4 + 0.5 CuS0 4, 22°. . . 106 6.11 V a r i a t i o n of 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 with e l e c t r o l y s i s time i n 2M H 2S0 4 + 0.5M CuSO,*, 40° 107 6.12 V a r i a t i o n of 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 with e l e c t r o l y s i s time i n 2M H 2S0 4, 22° .108 x i i Figure Page 6.13 V a r i a t i o n of 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 with e l e c t r o l y s i s time i n 2M H 2S(K, 40° 109 6.14 Loading change and platinum weight f r a c t i o n change with 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 coating l o s s by s p a l l i n g I l l 6.15 Change i n roughness f a c t o r with time with r e p e t i t i v e e l e c t r o l y s i s at 52.1 mA/cm (.geometric area) i n 2M H 2S0 4, 22° '. . . , 1 1 4 6.16 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 of remaining noble metal loading .115 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 from the cumulative coating loss r e s u l t s of 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 H2S0n + 0.5M CuSO^ 123 6.18 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 y vs. ap p l i e d current d e n s i t y from the cumulative coating loss r e s u l t s of 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 H2S0i,. + 0.5M CuSO.* 124 6.19 Platinum c o r r o s i o n r a t e per m2 of actual platinum surface vs. mean actual a p p l i e d current d e n s i t y 126 6.20 I r i d i u m c o r r o s i o n r a t e per m2 of actual i r i d i u m surface vs. mean actu a l a p p l i e d current d e n s i t y j.27 6.21 Corrosion rate measurements on an i n d i v i d u a l anode with respect to: (a) the geometric area of platinum only; (b) the actual (mean) area of platinum c a l c u -l a t e d f o r each run 128 6.22 S.E.M. view of the surface of an anode a f t e r operation at 104 mA/cm2 (geometric area) i n 2M I^SO*, 22°, f o r a t o t a l of 957 hours 141 6.23 S.E.M. view of the surface of an anode a f t e r operation at 52.1 mA/cm2 (geometric area) i n 2M H 2S0 4 + 0.5M CuS0 4, 22°, f o r a t o t a l of 1848 hours . 141 6.24 S.E.M. view of the surface of an anode operated at 260 mA/cm2 (geometric area) i n 2M H 2S0 4, 22°, f o r a t o t a l of 455 hours • 142 6.25 S.E.M. view of the surface of an anode operated at 52.1 mA/cm2 (geometric area) i n 2M H 2S0 4, 22°, f o r a t o t a l of 385 hours 142 x i i i Figure Page 6.26 S.E.M. view of the surface of an anode subjected to pulsed e l e c t r o l y s i s at 52.1 mA/cm2 (geometric area) i n 2M H 2S0 4 + 0.5M CuS0 4, 22° f o r 194 hours 144 6.27 S.E.M. view of the surface of the anode described i n Figure 6.26, a f t e r f u r t h e r operation f o r 258 hours at a continuous anodic current of 52.1 mA/cm2 (geo-metric area) 144 6.28 S.E.M. view of the surface of an anode subjected to pulsed e l e c t r o l y s i s a t 52.1 mA/cm2 (geometric area) i n 2M H2SOk + 0.5M CuS0^,.22°, f o r 47.6 hours 1 4 5 6.29 S.E.M. view of the surface of an anode operated a t 52.1 mA/cm2 i n 2M H 2S0 4 + 0.5M CuS0 4, 22°, f o r 92 hours . . . . 1 4 5 6.30 V a r i a t i o n of 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 2S0 4 + 0.5M CuSOi*. 22° 1 4 7 6.31 I n i t i a l p o t e n t i a l vs. time behaviour f o r new anodes operated i n 2M H 2S0 4 + 0.5M CuS0 4, 22° 149 6.32 Change i n anode p o t e n t i a l with time f o r a new anode operated at 15.6 mA/cm2 (geometric area) i n 2M H 2S0 4 + 0.5M CuSO^, 22° 150 6.33 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 sub-j e c t e d to r e p e t i t i v e e l e c t r o l y s i s at constant a p p l i e d current d e n s i t y 152 6.34 P o t e n t i a l change with time f o r an anode subjected to r e p e t i t i v e e l e c t r o l y s i s at high current d e n s i t y 154 6.35 P o t e n t i a l change with time f o r an anode 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 high current d e n s i t y 155 6.36 P o t e n t i a l vs. time behaviour 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 to imminent f a i l u r e 156 6.37 P o t e n t i a l vs. time behaviour f o r an electrode sub-j e c t e d to pulsed e l e c t r o l y s i s 157 6.38 P o t e n t i a l vs. time behaviour f o r an ele c t r o d e sub-j e c t e d to pulsed e l e c t r o l y s i s 158 6.39 E f f e c t of 0.05 gpl thiourea a d d i t i o n 161 6.40 Typical charge curves f o r a Pt/25 I r a l l o y wire e l e c t r o d e . . • 163 x i v Figure Page 6.41 T y p i c a l charge curves f o r an i r i d i u m wire el e c t r o d e 164 6.42 T y p i c a l charge curves f o r a Pt/30 I r - T i e l e c t r o d e 165 6.43 Surface oxygen coverage vs. p o t e n t i a l r e l a t i o n s f o r several anodes 167 6.44 Cathodic surface oxygen s t r i p p i n g curves f o r a Pt/25 I r a l l o y s wire e l e c t r o d e 169 6.45 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 anodes. (Current d e n s i t i e s based on geometric area.) 173 6.46 Anodic 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 and P t / I r a l l o y s 174 6.47; Anodic 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 and P t / I r a l l o y s , 176 6.48 P o l a r i z a t i o n curves f o r Pt and I r wire anodes i n computer-generated curves f o r various P t / I r a l l o y s 177 6.49 Anodic p o l a r i z a t i o n curve f o r the Ti base of a !• Pt/30 I r - T i anode 179 6.50 Increase i n amount of surface lead deposits w i t h e l e c t r o l y s i s time 181 6.51 8-Pb0 2 deposit formed on an anode operated f o r 96 hours 187 6.52 8-Pb0 2 deposit formed on an anode operated f o r 192 hours 187 6.53 Mixed 3-Pb0 2 and PbS0 4 desposit 188 6.54 Predominantly PbS0 4 deposit 188 6.55 S.E.M. view of the surface of an anode which has undergone complete degradation 195 6.56 S.E.M. view of bent, c o n i c a l surface growth . . . 195 6.57 S.E.M. view of the surface of an anode which has undergone complete degradation 196 6.58 S.E.M. view of the surface of an anode which has undergone complete degradation 196 xv Figure Page A2.1 R e l a t i v e PtLaj i n t e n s i t y vs. loading r e l a t i o n f o r various P t / I r a l l o y s 256 A2.2 R e l a t i v e I r L a x i n t e n s i t y vs. loading r e l a t i o n f o r various P t / I r a l l o y s 257 A4.1 pH of concentrated s u l f u r i c a c i d s o l u t i o n s by various c a l c u l a t i o n s 267 xv i ACKNOWLEDGEMENTS The author wishes to express h i s g r a t i t u d e to Dr. I.H. Warren f o r h i s encouragement throughout the course of t h i s study, to members of the f a c u l t y and f e l l o w graduate students f o r h e l p f u l d i s c u s s i o n s and advice, and to members of the 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 , Jim Walker. Further, I thank my w i f e , Darlene, f o r her enduring patience over the past several years. I al s o wish to acknowledge the inesti m a b l e c o n t r i -bution of our dog, S a i n t , who made our l i v e s j u s t a l i t t l e more enjoyable during t h i s time. F i n a n c i a l support i n the form of National Research Council of Canada Scholarships i s a l s o g r e a t l y appreciated. xvi i Chapter 1 INTRODUCTION Recent developments i n the technology of 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 since the advent of solvent e x t r a c t i o n processes f o r the recovery of copper from leach l i q u o r s , have l e d to 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 to the conventional lead-based 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 of r e l a t i v e l y low a c i d i t y (15-90gplH 2S0\) and at low curr e n t d e n s i t i e s (8-19 mA/cm2). The a b i l i t y of conventional anode m a t e r i a l s to withstand the c o n d i t i o n s of higher a c i d i t y i n e l e c t r o l y t e s produced by sol v e n t e x t r a c t i o n processes (150-320 gpl H 2 S 0 i J and to s u s t a i n higher c u r r e n t d e n s i t i e s (25-300 mA/cm2) permitted with the development of improved e l e c t r o l y t e a g i t a t i o n techniques, such as r e c i r c u l a t i o n and gas sparging, and pulsed c u r r e n t o p e r a t i o n , i s questionable. In p a r t i c u l a r , concern has been expressed i n the l i t e r a t u r e with regard to enhanced anode c o r r o s i o n i n the 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 of solvent e x t r a t i o n processes, which leads i n turn to l e v e l s of lead contamination (>10 ppm) i n the cathode copper which render i t u n s a t i s -f a c t o r y 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 the various t i t a n i u m substrate anodes (TSA) which have been developed w i t h i n the past two decades. In p a r t i c u l a r , an anode mate r i a l of t h i s type c o n s i s t i n g of 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 coating 1 2 of an a l l o y of platinum and i r i d i u m on a t i t a n i u m substrate has seen 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 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 ug/A«hr) and s t a b l e , low over-p o t e n t i a l operation. Although the reported c o r r o s i o n rates f o r TSA's i n copper-containing s u l f u r i c and s o l u t i o n s have been f a r from promising, the c o r r o s i o n behaviour of these anodes under c o n d i t i o n s s i m i l a r to those encountered i n copper e l e c t r o w i n n i n g i s f a r from being c h a r a c t e r i z e d — i n p a r t i c u l a r the nature of the coating loss mechanism (whether coating l o s s occurs by a mechanical " s p a l l i n g " mechanism or by an e l e c t r o c h e m i c a l mechanism) has not been determined. Indeed, based on the c o r r o s i o n behaviour reported 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 question remains l a r g e l y unresolved — although several s p e c i f i c cases of the mechanical mode of 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 g r e a t e s t d i f f i c u l t y 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 anodic 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 of c o r r o s i o n 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 losses occur at 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 " of the order of nanoamps per cm 2. Such low r a t e s of c o a t i n g metal l o s s place c o n s i d e r a b l e demands on the p r e c i s i o n of the measurement techniques employed to determine these l o s s e s . In the present work, anode m a t e r i a l s of the type P t / 3 0 I r - T i are evaluated with respect to t h e i r c o r r o s i o n and p a s s i v a t i o n behaviour i n a v a r i e t y of operating 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 of copper from s o l v e n t extraction-produced e l e c t r o l y t e s . An X-ray fluorescence spectrometric technique, used f o r the measurement of metal f o i l thicknesses i n other f i e l d s , was developed to 3 enable the accurate determination of changes i n loading and composition of the anode coatings. With respect to the losses of both coating metals during various c o n d i t i o n s of p o l a r i z a t i o n , experiments were performed from which s t a t i s t i -c a l l y r e l i a b l e changes i n t h e i r loadings could be produced, and the r e s u l t s were i n t e r p r e t e d with respect to t h e i r conformity to a coating l o s s process which was e i t h e r predominantly due to mechanical detachment or to e l e c t r o -chemical d i s s o l u t i o n , or to a combination of both. P a s s i v a t i o n behaviour was examined with respect to determination of the degrees of surface oxygen coverage under various c o n d i t i o n s of anodic 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 i n t e r p r e t e d i n terms of the e x i s t i n g l i t e r a t u r e concerning oxygen f i l m formation on platinum and i r i d i u m . 1.1 General Recent developments i n copper e l e c t r o w i n n i n g technology are key-noted by the commercial r e a l i z a t i o n of processes f o r the recovery of copper from leach l i q u o r s by solvent e x t r a c t i o n (SX) and by attempts to increase p r o d u c t i v i t y , without s a c r i f i c i n g cathode q u a l i t y , through high c u r r e n t d e n s i t y operation which i s f e a s i b l e i n the more-concentrated and h i g h l y -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 of SX processing. C o n v e n t i o n a l l y , copper i s electrowon from s o l u t i o n s c o n t a i n i n g 15-60 gpl copper, 15-90 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 gpl i r o n as the major impurity [1-10]. The various 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 Table 1.1. Current d e n s i t i e s below 20 mA/cm2 are necessary i n order to provide both smooth and l e v e l cathode deposits and 4 Table 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 E l e c t r o m e t a l 1 u r g i c a l Processes Electrowinning E l e c t r o r e f i n i n g E l e c t r o p l a t i n g E l e c t r o l y t e s Cu, gpl H 2S0 4, gpl 15-60 15-90 30-60 125-240 75-250 40-200 Impurities Fe, gpl Fe , gpl Others 2-20 0.2-6 CI,Mg,Mo,Co, C1,N03 0.5 0.5 0 0 Temperature, °C 26-55 50-65 20-50 Anodes Pb/6-15 Sb/0-1 Ac al l o y 99.7 Cu 99.99 Cu Impurities of anode source Pb,Sb,Ag Au,Ag,Pt"metals, As,Sb,Ni,Fe As,Sb Cathodes 99.8-99.9 Cu, As,Sb,Ag,S,Pb 99.95+Cu Au,Ag,S,As,Sb 99.9+Cu Current d e n s i t i e s , mA/cm2 8-19 12-27 30T55 Current e f f i c i e n c i e s 75-90 90-98 99+ Primary anode r e a c t i o n 2rl 20+4H ++0 2+4e Cu+Cu+ ++2e Cu+Cu + + + 2e Primary cathode r e a c t i o n C u + + + 2e -»• Cu C u + + + 2e Cu C u + + + 2e -> Cu Th e o r e t i c a l standard c e l l v o l t a g e , v o l t s 0.892 0 0 5 to maintain high current e f f i c i e n c y . (Increased current density operation would r e s u l t i n the undesirable formation of d e n d r i t i c deposits due to the non-uniformity of a g i t a t i o n by anodically-generated oxygen, and to de-creased energy e f f i c i e n c y due to enhanced l i m i t i n g c u r rents f o r impurity species and to co- d e p o s i t i o n of hydrogen.) Solvent 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) processing, on the other hand, i s able to provide e l e c t r o l y t e s with a uniformly high copper content (34-40 g p l ) , higher s u l f u r i c a c i d content (132-200 gpl) and lower impurity l e v e l s (< 2.6 gpl i r o n ) [11-17]. 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 plants are summarized i n Table 1.2. The energy e f f i c i e n c y i n the el e c t r o w i n n -ing process i s improved both as a r e s u l t of the lower impurity l e v e l s and of the higher e l e c t r o l y t e c o n d u c t i v i t y . Techniques which permit c e l l opera-t i o n at higher 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 reduction of the thickness of the 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 e l e c t r o l y t e a g i t a t i o n ( r e c i r c u l a t i o n [1822], gas or a i r sparging [23-32]) or by u l t r a s o n i c a g i t a t i o n [21,23,33] or pulsed c u r r e n t operation [22,34-36]. (The l a t t e r processes 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 mixing i s undesirable.) 1.2 Anodes i n Copper Electrowinning Insoluble anodes used i n copper e l e c t r o w i n n i n g are lead-based a l l o y s c o n t a i n i n g 6-15 per cent antimony and, i n some cases, 0-1 per cent s i l v e r [38]. Other anode m a t e r i a l s , such as s i l i c i d e s (the " C h i l e x " [1,8] and " L u i l u " [37] anodes), magnetite, massive lead d i o x i d e , and graphite have i n f e r i o r mechanical, 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 of lead are due to the formation of a s t a b l e , Table 1.2 Summary of Operational Data f o r Solvent 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) Plants Plant Feed Reagent E x t r a c t i o n S t r i p p i n g 0/A Stages 0/A Stages E l e c t r o l y t e Current Density (mA/cm2) Current E f f i c i e n c y Cathode P u r i t y References Duval p i l o t p l a n t 1.5 gpl Cu .45 gpl Fe pH 2.5 7% LIX64 i n kerosene 4 4 37.gpl Cu 2 gpl Fe 132 gpl H2SOh 20.5 99.99 [11] Ranchers p i l o t p l a n t (4.56 gpl Cu) 10%LIX64 i n kerosene 4 3/1 3 38 gpl Cu 165 gpl H 2 S0L, 16 93.9 [13] Ranchers Blu e b i r d Mine 3.02 gpl Cu 2.4 gpl Fe 4.5 gpl H2SOk 7% LIX64N in kerosene 2.5/1 3 4/1 2 34.2 gpl Cu 2.6 gpl Fe 142.5 gpl H 2S0 4 18.3 80-89 99.9 [12,14] Bagdad Copper Corp. (1 gpl Cu) LIX64 i n kerosene 4 3 99.9+ [15] SEC Corp., El Paso Evaporated and d i l u t e d spent r e f i n e r y e l e c t r o l y t e (Cu + Ni) LIX64N 2 2 (165 gpl H 2 S 0 J 54 [16] Inco, Thompson (proposed) 58.5 gpl Ni .52 gpl Cu 49.3 gpl CI 126 gpl S0 4" pH 1.8 10% LIX64N i n kerosene 1/4 3 10/1 1 50 gpl Cu 200 gpl H 2S0 4 60 >90 (99.9+) [17] CD 7 Table 1.3 Techniques P e r m i t t i n g High Current Density Operation i n Copper Electrowinning Technique E l e c t r o l y t e Composition (gpi) Current Densi t i e s (mA/cm2) Reference Cu H 2S0 4 Fe °C Conventional e l e c t r o w i n n i n g 15-60 15-90 2-20 26-55 8-19 E l e c t r o l y t e r e c i r c u l a t i o n (.1-...2 gpm/ft 2 cathode) in bench-scale c e l l 40 160 60 38-43 18 E l e c t r o l y t e c i r c u l a t i o n 35-50 50-70 up to 97 43-65 optimum 20 P e r i o d i c c u r r e n t r e v e r s a l (9 sec on/0.5 sec reverse) in i n d u s t r i a l eel 1 . 31 60 1.8 63 48 30 Pulsed current (.07-30 us pulse length) -up to 250 36 Gas sparging (6-8 i/min ... per cathode face) i n h a l f -s c a l e c e l l 45 150 2 100-200 25 Gas sparging (33 £/min per cathode face) i n f u l l -height model c e l l 60 140 5 125-300 25 E l e c t r o l y t e r e c i r c u l a t i o n (12 £/m2«min) i n f u l l -height model eel 1 60 150 40 25 22 A i r a g i t a t i o n i n p l a n t t r i a l 32 225 63 26 24 A i r a g i t a t i o n (12-15 S f t 3 / h between standard-size e l e c . ) 32 171 63 150-287 31 Ser i e s b i p o l a r c e l l 46 156 23-92 30 8 conducting f i l m of lead d i o x i d e upon which oxygen i s evolved i n a c i d i c s u l f a t e e l e c t r o l y t e s . As pure lead i s too s o f t to permit i t to withstand the r i g o r s of handling or to maintain i t s "dimensional s t a b i l i t y " during o p e r a t i o n , antimony i s added to impart improved mechanical p r o p e r t i e s without a f f e c t i n g the formation of the lead d i o x i d e surface l a y e r . S i l v e r a d d i t i o n s are made to impart enhanced anode c o r r o s i o n r e s i s t a n c e . (Lead-based anodes cannot be used i n c h l o r i d e or n i t r a t e - c o n t a i n i n g e l e c t r o l y t e s due to t h e i r tendency to d i s s o l v e . ) Although a l l o y i n g produces improved mechanical q u a l i t i e s , i t i s s t i l l necessary to employ anodes having t h i c k -nesses of the order of 1 cm i n order to prevent b u c k l i n g [ 6 ] . The l i f e t i m e of an 84.5 Pb/14.5 Sb/0.6 Ag anode operated at 7.5-18.3 mA/cm2 i s reported to be 6-8 years [ 8 ] . The operating p o t e n t i a l of a lead anode a l s o increases p r o g r e s s i v e l y with time owing to the continuous growth of the oxide f i l m with time [39]. In SX/EW processes, 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 contain 130-320 gpl s u l f u r i c a c i d , depending on the p a r t i c u l a r SX reagent i n question [40-51], enhanced c o r r o s i o n of conven-t i o n a l lead a l l o y anodes with subsequent contamination of the cathode copper product has been reported [16,21,31,47,52-55]. Lead contents of 10-50 ppm render copper u n s u i t a b l e f o r wirebar [16,21]. Several methods f o r lowering cathode lead l e v e l s to acceptable values f o r t h i s purpose have been pro-posed. These are: gas sparging, 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 of diaphragms [ 5 5 ] , a d d i t i o n s of i n h i b i t o r s f o r +2 lead c o r r o s i o n (such as Co ) [53-57], anode p r e c o n d i t i o n i n g [55,58], and the use of 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 l o g i c a l s o l u t i o n to the lead contamination problem i s the replacement of lead a l l o y anodes with an anode material which i s l e a d - f r e e . Further, i t i s thus p o s s i b l e to consider s o l u t i o n of other problems which are c h a r a c t e r i s t i c of lead-based a l l o y anodes such as dimensional i n s t a b i l i t y (which i s caused by bending or buckling of anodes during handling or i n s e r v i c e , and i n t u r n , causes misalignment of elec t r o d e s and operation at greater-than-optimum e l e c t r o d e spacing) and handling problems (the thickness of the anodes required to provide adequate mechanical strength r e s u l t s i n i n e f f i c i e n t use of c e l l space, and the weight of the anodes makes handling a d i f f i c u l t t a s k ) . 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 of 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 coating on a valve metal substrate such as t i t a n i u m — g e n e r a l l y r e f e r r e d to as " t i t a n i u m substrate anodes," or TSA's — a n d 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 replaced the pre 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 reasonable that anodes of t h i s type should be considered f o r a p p l i c a t i o n i n other i n d u s t r i a l e l e c t r o l y t i c processes, such as copper e l e c t r o w i n n i n g . The r e a l i z a t i o n of the a b i l i t y of g a l v a n i c couples of a noble metal such as platinum and a valve metal such as t i t a n i u m to f u n c t i o n as " i n s o l u b l e " anodes with no d e t e r i o r a t i o n of 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 of Cotton [59,60]. He found t h a t coupling (or discontinuous coating) of platinum on t i t a n i u m afforded anodic p r o t e c t i o n of the base metal i n systems where i t would normally experience a c t i v e c o r r o s i o n . Further, on a p p l i c a t i o n of an anodic current 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 take place predominantly at 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 behaviour e s s e n t i a l l y comparable to 10 t h a t of the massive noble metal, but at only a f r a c t i o n of the c o s t . A p r o l i f e r a t i o n of patents 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 the preparation of t i t a n i u m substrate anodes (TSA's) with other noble metals or a l l o y s , noble metal oxides, base metal o x i d e s , or other e l e c t r o n i -c a l l y conducting compounds as the a c t i v e coating m a t e r i a l . The p r o p e r t i e s of several metals and t h e i r oxides which are s u i t a b l e substrates or coating c o n s t i t u e n t s are given i n Table 1.4. TSA's have been a p p l i e d predominantly i n the c h l o r - a l k a l i i n d u s t r y , and i n impressed c u r r e n t cathodic p r o t e c t i o n , although other a p p l i c a t i o n s i n c l u d e electrochemical p e r o x i d a t i o n 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 [38,39,61-71]. 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 e l e c t r o d e p o s i -t i o n of chromium, gold , platinum metals, n i c k e l , copper, and t i n [38,64, 67, 72-76]. Several noble metal (or noble metal oxide) coated TSA's have been considered 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 , although operat i o n a l data i s scarce. Based on t h e i r e x c e l l e n t performance i n the c h l o r - a l k a l i i n d u s t r y and 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 of copper e l e c t r o w i n n i n g : 1. Dimensional stability. 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 between e l e c t r o d e p a i r s due t o b e t t e r a l i g n m e n t . 2. Thinness. Noble metal c o a t e d t i t a n i u m anodes 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 dimen-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 ' per c e l l . The t h i n n e s s o f t h e anodes, combined w i t h the 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 of t i t a n i u m makes f o r e a s i e r hand 1 i ng . T a b l e 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 O x i d e s T i ( T i 0 2 ) Ta ( T a 2 0 5 ) Nb ( N b 2 0 5 ) P t ( P t 0 2 ) I r ( I r 0 2 ) Rh ( R h 2 0 3 ) Ru ( R u 0 2 ) Pb ( P b 0 2 ) Mn (Mn0 2 ) A t o m i c w e i g h t M o l e c u l a r w e i g h t 2 7 . 9 0 ( 7 9 . 9 0 ) 1 8 0 . 9 5 ( 4 4 1 . 8 9 ) 92.91 ( 2 6 5 . 8 1 ) 1 9 5 . 0 9 ( 2 2 7 . 0 9 ) 192.2 ( 2 2 4 . 2 ) 102.91 ( 2 5 3 . 8 1 ) 101.1 ( 1 3 3 . 1 ) 207.21 ( 2 3 9 . 1 9 ) 5 4 . 9 4 ( 8 6 . 9 4 ) D e n s i t y , g/cm 3 4 . 5 0 7 ( 4 . 2 4 r u t i l e ) ( 3 . 8 4 a n a t a s e ) 1 6 . 6 ( 8 . 7 3 ) 8 . 5 7 ( 4 . 9 5 ) 21 . 4 5 ( 1 0 . 2 ) 3 2 2 . 5 (11 . 6 6 ) 12.44 62o 12.2 6.37 11 . 3 6 ( 9 . 3 7 5 ) 7 . 4 3 ( 5 . 0 2 6 ) M e l t p i n g p o i n t , °C 1658 (2128) 2996 (2150) 2468 (1783) 1769 (723) 2454 (1373) 1966 (1388) 2500 (1400) 327 (563) 1245 (847) R e s i s t i v i t y , microohm-cm 42 ( 1 0 " ) 1 2 . 4 5 ( 1 0 9 ) 1 2 . 5 1 0 . 6 „ ( 3 . 6 x l 0 ' ° ) a 5 . 3 ( 0 . 4 9 ) 4.51 7 . 6 ( 0 . 3 5 ) 2 0 . 6 5 ( 0 . 9 0 8 ) 185 C r y s t a l s t r u c t u r e CPH ( T e t r a g o n a l ) BCC (Rhombic) BCC (Rhombic) FCC (Hexagonal) FCC ( T e t r a g o n a l ) FCC (Rhombic) CPH ( T e t r a g o n a l ) FCC f a : rhombic tl [g: t e t r a g o n a l ] C u b i c f g : t e t r a g o n a l ] [ a : rhombic J T e n s i l e s t r e n g t h , 1000 p s i 34 95 ( 9 9 . 0 T i ) 50 40 20 90 160 ( P t / 3 0 I r ; 138 78 2 - 3 2-7 (Pb/6Sb) 72 M e t a l i o n / m e t a l s t d . p o t e n t i a l - 1 . 6 3 T i + + / T i - 1 . 1 2 T a 3 + / T a - 1 . 1 N b 3 + / N b • 1 . 2 b P t + + / P t 1 . 1 6 b I r 3 + / I r 0 . 7 b R h 3 + / R h 0 . 4 6 c Ru + + , /Ru - . 1 2 6 P b + + / P b - 1 . 2 0 f Mn + + /Mn M e t a l c x i d e / m e t a l s t d . p o t e n t i a l - 0 . 8 6 T i 0 2 / T i - 0 . 3 1 T a 2 0 s / T a - 0 . 6 5 N b 2 0 » / N b 0A0A P t 0 2 / P t 0 . 9 4 b I r 0 2 / I r 0 . 9 b R h 2 0 3 / R h 0 . 7 9 Ru0 2 /Ru 0 . 6 6 6 e BPb0 2 /Pb 0 . 0 5 f MnOj/Mn C o s t $ 2 . 7 0 / 1 b sponge $ 3 5 - 4 8 / l b powder $170-190/ t r o y o z . $300-310/ t r o y o z . $400-410/ t r o y o z . $ 6 0 - 6 5 / t r o y o z . $ . 2 5 7 / l b $ . 5 8 / l b S o u r c e s f o r d a t a , u n l e s s o t h e r w i s e n o t e d , w e r e : Other s o u r c e s : Metals Handbook ( 8 t h e d . ) , V o l . 1 ( 1 9 6 1 ) , a) 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 , CRC HandbooHiSth e d . ) ( 1 9 7 5 ) , b) 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 , Tiie Oxide Handbook, e d . G.V. Samsonov, I F I / P l e n u m ( 1 9 7 3 ) , c ) Atlas of Electrochem. Equilibria, e d . M P o u r b a i x ( 1 9 6 6 ) , Stability Constants, S p e c . P u b l . 17, The Chem. S o c . ( 1 9 6 4 ) , d) S u k h o t i n , A . M . et al., Zh. Prikl. Khim, 45 ( 1 9 7 2 ) , p . 1478, Enaineering and hiinina Journal, YT]_ ( 1 1 ) , ( 1 9 - 7 6 ) . e) C a r r , J . P . and N.A. Hampson, Chem. Rev., 72 ( 1 9 7 2 ) , p. 679. f ) 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 the n o n - f r i a b l e n a t u r e o f the c o a t i n g s and the " s e l f - h e a l i n g " a c t i o n o f 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 such t h a t t h e s u b s t r a t e becomes exposed. 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 shapes 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 expanded mesh form. F u r t h e r , v a r y i n g degrees o f a c t i v e c o a t i n g metal l o a d i n g a r e o b t a i n a b l e , depending on the optimum r e l a t i o n between 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 uthenium - a r e among th 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 o f 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 were 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 methods, have been l a r g e l y overcome w i t h the 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 t e c h n i q u e s - i n p a r t i c u l a r , thermal d e c o m p o s i t i o n o f n o b l e m e t a 1 - c o n t a i n i n g o r g a n i c s o l u t i o n s . 7. Low, anodic corrosion rates. The p l a t i n u m m e t a l s d i s s o l v e a n o d i c a l l y i n c o n v e n t i o n a l e l e c t r o l y t e s ( 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 than p l a t i n u m . A l t h o u g h no d a t a e x i s t s f o r iridium 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 the o r d e r o f 0.0001 per c e n t o v e r a wide ragne 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 , would not be e x p e c t e d to be d e l e t e r i o u s t o cathode q u a l i t y . 8 . High current density operation. The proven s t a b i l i t y o f TSA's i n the 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 t h o s e 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 copper e l e c t r o w i n n i n g recommend them f o r proposed h i g h e r c u r r e n t d e n s i t y o p e r a t i o n s i n t h i s f i e l d . 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 the n o b l e metal 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 ; indeed such t r e a t m e n t may even be c o n s i d e r e d t o be b e n e f i c i a l inasmuch as i t would r e s u l t i n lower 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 ( t h e " a n o d i c 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 ) , the o n l y e f f e c t o f such t r e a t m e n t on the n o b l e metal c o a t i n g i s t o reduce any 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 the e l e c t r o d e . In p a r t i c u l a r , anodes having a Pt/30 I r a l l o y c o a t i n g show low operating o v e r p o t e n t i a l s and low coating metal l o s s r a t e s i n c h l o r i d e e l e c t r o l y t e even a f t e r thousands of hours of operation [77,78]. For economic reasons, i t i s d e s i r a b l e to have noble metal loadings whose average t h i c k -ness i s below a micron - or roughly to loadings of l e s s than the order of 20 g/m2 (mass of noble metal per u n i t of geometric area of the s u b s t r a t e ) . Although Pt/30 I r - T i anodes o f f e r the advantages described above, several important questions r e q u i r e c l a r i f i c a t i o n : 1. What i s the r a t e o f l o s s o f c o a t i n g metal under con-d i t i o n s s i m i l a r to tho s e e n c o u n t e r e d i n a c t u a l copper e l e c t r o w i n n i n g p r a c t i c e ? Does the c o a t i n g l o s s r a t e depend on 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 the c o a t i n g l o s s r a t e depend on n o b l e metal l o a d i n g ? What i s the n a t u r e o f the mechanism 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 . time ( " 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 anodes i n the 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 . time b e h a v i o u r ) ? What i s the cause o f the " p a s s i v a t i o n " phenomenon? 3. What a r e the 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 and p u l s e d c u r r e n t o p e r a t i o n on the c o r r o s i o n and p a s s i -v a t i o n b e h a v i o u r ? Chapter 2 LITERATURE SURVEY: ANODIC CORROSION OF PLATINUM AND IRIDIUM 2.1 Platinum 2.1.1 C h l o r i d e E l e c t r o l y t e s Platinum shows a c t i v e d i s s o l u t i o n with p r a c t i c a l l y 100 per cent current 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 at 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 (defined 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 , not n e c e s s a r i l y a decrease i n c o r r o s i o n r a t e ) above t h i s p o t e n t i a l [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 (reaching a maximum of 1.6 mA/cm2 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 temperatures, and obey 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 with a slope of 102-103 mv, suggesting the rate-determining step i s the discharge of a un i v a l e n t ion. P a s s i v a t i o n of platinum d i s s o l u t i o n i s a t t r i b u t e d to the competitive adsorption of oxygen and of c h l o r i d e ions [79-82]. Platinum d i s s o l u t i o n i n the "passive" region appears to be asso c i a t e d with the p a r t i a l oxygen e v o l u t i o n process, as i n d i c a t e d by the p a r a l l e l i s m of 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 platinum d i s s o l u t i o n , and the lack of p a r a l l e l i s m with the c h l o r i n e evo-l u t i o n process [97]. Tracer s t u d i e s have revealed a considerable 14 15 time-dependence of 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 , with attainment of steady s t a t e c o n d i t i o n s r e q u i r i n g several days i n some cases, from an i n i t i a l l y high "pulse" of metal d i s s o l u t i o n [88,89]. 2.1.2 I n e r t E l e c t r o l y t e s In p e r c h l o r i c and s u l f u r i c acids platinum d i s s o l u t i o n only occurs i n the passive s t a t e . No potential-dependence of the c o r r o s i o n rate i s observed u n t i l the anode p o t e n t i a l exceeds about 1.8 v o l t s , and the d i s s o l u t i o n process i s found to p a r a l l e l the oxygen e v o l u t i o n process at higher p o t e n t i a l s [94,99-102]. (That i s , the current e f f i c i e n c y of platinum d i s s o l u t i o n i s constant over wide ranges of p o t e n t i a l or current d e n s i t y and a c i d concentration.) A pronounced time-dependence of the c o r r o s i o n r a t e a l s o e x i s t s on switching on or on changing the p o t e n t i a l . From the r e s u l t s of the t r a c e r s t u d i e s by Chemodanov, assuming platinum d i s s o l v e s 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 platinum d i s s o l u -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 of .0001 per cent can be r e l a t e d to a metal l o s s rate of 1.8 ug/A-hr [99]. Other st u d i e s by Baboian [39] and Ives [103] on the co r r o s i o n of platinum 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 , r e s p e c t i v e l y , show higher loss rates (or more c o r r e c t l y , c o r r o s i o n e f f i c i e n c i e s ) . From the data of Baboian, a co r r o s i o n e f f i c i e n c y of 6.4 ug/A-hr can be c a l c u l a t e d f o l l o w -ing operation f o r 45 days at 108 mA/cm2 i n 30°C s o l u t i o n . Ives found the curre n t e f f i c i e n c y f o r platinum d i s s o l u t i o n was r e l a t i v e l y independent of current d e n s i t y over the range 20-200 mA/cm2, but that i t increased s t r o n g l y w i t h temperature. Corrosion e f f i c i e n c i e s were found to be approximately 6, 10, 24, and 43 ug/A-hr i n 20, 40, 60, and 80°C s o l u t i o n s , r e s p e c t i v e l y . Ives a l s o found that increased a c i d concentration (from 150 to 220 gpl at 70°C) r e s u l t e d i n an increase i n c o r r o s i o n e f f i c i e n c y from 30 to 60 ug/A'hr a t 193 mA/cm2. In ch l o r a t e 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 r a t e i s s t i l l observed to 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 at high p o t e n t i a l s (above 2.2 v o l t s ) where per-c h l o r a t e production becomes the primary e l e c t r o d e r e a c t i o n [97]. Chemodanov [91] has noted that the commencement of the potential-dependence of the platinum d i s s o l u t i o n process only occurs at p o t e n t i a l s where monomolecular oxygen cover-age e x i s t s . Further, the p a r a l l e l i s m between the r a t e s of platinum d i s s o l u t i o n and oxygen e v o l u t i o n suggest a s i m i l a r rate-determining step f o r these processes. At anode p o t e n t i a l s above 1.8 v o l t s , the a d d i t i o n of i n c r e a s i n g amounts of NCI, NaCl, or C l 2 to 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 progressive decrease i n the c o r r o s i o n r a t e of platinum [94,99,100,104-106]. 2.1.3 Organic-Containing E l e c t r o l y t e s The anodic d i s s o l u t i o n of platinum 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 ethylene g l y c o l and methanol used f o r the Kolbe e l e c t r o s y n t h e s i s of various organic compounds has been s t u d i e d [107-1121. A d d i t i o n of water, or the use of aqueous s o l v e n t s , 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 . Tracer st u d i e s again reveal an extremely slow s t a b i l i z a t i o n of the ( i n i t i a l l y high) platinum d i s s o l u t i o n r a t e . Tanaka [113] found t h a t the presence of EDTA i n acetate b u f f e r s o l u t i o n promoted higher anodic d i s s o l u t i o n rates f o r platinum. Platinum anode losses i n the preparation of perhydrol have al s o been reported [114]. Thiourea i n <.1M concentration 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 anodic d i s s o l u t i o n of platinum 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 s [95,115,116]. In per-c h l o r i c a c i d s o l u t i o n s , t h i o u r e a has an i n h i b i t i n g a c t i o n above 1.95 v o l t s , but 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 [194,112,116-118]. Naphthalene s u l f o a c i d and 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 a c i d s o l u t i o n s [104-116]. In s u l f u r i c a c i d s o l u t i o n , on the other hand, thiourea has a s l i g h t promoting e f f e c t (at 2.3 v o l t s i n 3MH2S0lt,50°).[T16]. Under the same 17 o n d i t i o n s , a d d i t i o n of naphthalene s u l f o a c i d s or aromatic compounds such as benzene cause an a c c e l e r a t i o n of the d i s s o l u t i o n r a t e . Enhanced c o r r o s i o n of platinum anodes with 0.1 gpl thiourea a d d i t i o n i n gold- 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 reported [119]. 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 of platinum anodes i n aqueous acetate e l e c t r o l y t e s [116]. 2.1.4 A l t e r n a t i n g Current Corrosion Platinum d i s s o l v e s at a high r a t e under the i n f l u e n c e of A.C. i n h y d r o c h l o r i c or 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 at higher c u r r e n t d e n s i t i e s , higher a c i d s t r e n g t h s , higher temperature and lower frequencies [120-128]. Typi 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 the t o t a l time of o p e r a t i o n , range from 11000 to 48000 ug/A-hr i n 3 to 12M HC1 with 60 Hz [122] A.C. Superimposed A.C. on D.C. i s a l s o known to increase the d i s s o l u t i o n rate of platinum, w i t h intense c o r r o s i o n o c c u r r i n g when the magnitude of the super-imposed A.C. i s such that 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 of platinum during c y c l i c voltammetry in i n e r t e l e c t r o l y t e s , where the p o t e n t i a l i s v a r i e d between the p o t e n t i a l s f o r e v o l u t i o n of hydrogen and oxygen, has been confirmed i n recent years [136-143]. Platinum l o s s r a t e s depend on the p o t e n t i a l scan r a t e s and the anodic p o t e n t i a l l i m i t . Square-wave current or p o t e n t i a l c y c l i n g [87,140,144] and of f / o n s w i t c h i n g [88,89,99] are a l s o known to cause enhanced platinum c o r r o s i o n . Chemodanov [101] has observed a time-dependent "pulse" of d i s s o l u t i o n on commencement of cathodic p o l a r i z a t i o n , l i k e l y due to the d i s s o l u t i o n of surface oxides. A.C. " r i p p l e " i s not l i k e l y to be d e l e t e r i o u s under c o n d i t i o n s of the high frequency (> 100 Hz) and low (of the order of a few per cent) 18 r a t i o of the r.m.s. A.C. current to the D.C. current t y p i c a l of the D.C. supplied 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 [130]. 2.1.5 State of Platinum i n S o l u t i o n Aquo-ions of platinum do not appear to be formed [145]. However, platinum tends to form s t a b l e complexes i n the ( I I ) and (IV) o x i d a t i o n s t a t e s whereby hybrid dsp 2 and d 2 s p 3 ("inner o r b i t a l " ) bonds are created [146]. (Indeed, the s t a b i l i t y of such complexes i s so high that e l e c t r o -d e p o s i t i o n of platinum 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 i t s e l f d i d not have a high s t a b i l i t y [147].) During a c t i v e c o r r o s i o n i n hydro-c h l o r i c a c i d s o l u t i o n s , P t C l 6 " 2 i s formed [95,98], although P t C U " 2 has a l s o been reported [96]. Hawkins [98] a l s o suggests t h a t P t C l 5 ( H 2 0 ) ~ i s formed at lower c h l o r i d e concentrations. Both P t ( I I ) [137,139,143] and Pt(IV) [139,140] have been detected 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. Ginzburg [148,149] has described the preparation of two s o l u b l e platinum s u l f a t e compounds tha t may e x i s t i n both P t ( I I ) and Pt(IV) forms. No f u r t h e r information e x i s t s concerning the nature of platinum s u l f u r i c a c i d s o l u t i o n s , although Kukushkin [150] reported that the s o l u t i o n species of platinum 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 of Ginzburg. 2.2 I r i d i u m 2.2.1 C h l o r i d e E l e c t r o l y t e s I r i d i u m only shows a c t i v e d i s s o l u t i o n i n strong (>4N) HC1 s o l u -t i o n s , and manifests passive behaviour otherwise - i n d i c a t i n g s u p e r i o r 19 c o r r o s i o n r e s i s t a n c e to 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 anodic d i s s o l u t i o n r a t e of 1.4 x 10 _ 1* mA/cm2 c a l c u l a t e d from the tra c e r - s t u d y data of L l o p i s [153]. A c o r r o s i o n current of 1.3 x 10" 5 mA/cm2 i s c a l c u l a t e d f o r the passive d i s s o l u t i o n of i r i d i u m ; i n saturated NaCl, 50°, a t 1.64 v o l t s (SHE) [153], which i s comparable to the d i s s o l u t i o n current f o r platinum i n 5N NaCl e l e c t r o l y t e [97]. 2.2.2 I n e r t E l e c t r o l y t e s No studies have been made concerning the anodic d i s s o l u t i o n of 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 or s u l f u r i c a c i d s . I f i t i s assumed t h a t the c o r r o s i o n behaviour i s s i m i l a r i n nature to t h a t of platinum, then d i s s o l u t i o n may be considered to occur on a passive e l e c t r o d e surface. 2.2.3 Organic-Containing E l e c t r o l y t e s Only a s i n g l e r e p o r t has been made with respect to the behaviour of i r i d i u m i n or g a n i c - c o n t a i n i n g s o l u t i o n s . Kovsman [107] found that i r i d i u m was i n f e r i o r to platinum 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 synthesis i n ethylene g l y c o l and methanol s o l v e n t s . 2.2.4 A l t e r n a t i n g Current Corrosion I r i d i u m d i s s o l v e s under the a c t i o n of pure A.C. in s o l u t i o n s of hy d r o c h l o r i c [95,123,125,151,153-157], hydrobromic [125], s u l f u r i c [156,158], and sulfamic [159] a c i d s . Box [123] found that i r i d i u m u n l i k e other metals, 20 showed a decreasing 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 strength (3-12N HC1) under 60 Hz A.C. of 460 mA/cm2. ( I r i d i u m showed a s i m i l a r c o r r o s i o n r a t e to platinum at the lower c o n c e n t r a t i o n s , and was c o n s i d e r a b l y more c o r r o s i o n - r e s i s t a n t i n the concentrated s o l u t i o n s . ) Few data e x i s t concerning the d i s s o l u t i o n of i r i d i u m w i t h super-imposed A.C. on U.C. Hoare [160] found that an A.C. biased to swing between anodic p o t e n t i a l s of 0.75 and 2.05 v o l t s produced a s o l u b l e purple oxide on i r i d i u m . Rand [142] reports t h a t , under i d e n t i c a l c o n d i t i o n s (scan r a t e , anodic l i m i t ) of c y c l i c voltamrnetry i n 1MH 2S0 4, i r i d i u m shows a higher metal l o s s rate per c y c l e than platinum. 2.2.5 State of I r i d i u m i n S o l u t i o n As with platinum aquo-ions of i r i d i u m are unknown [145]. I r i d i u m forms s t a b l e complexes i n the ( I I I ) and (IV) o x i d a t i o n s t a t e s with hybrid d 2 s p 3 bonds [146]. 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 that i n d i l u t e (<6N) HC1 s o l u t i o n s , time-dependent aquochloro complexes are formed. At higher c o n c e n t r a t i o n s , both I r ( I I I ) and I r ( I V ) chloro complexes e x i s t . 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 I r ( I V ) s u l f a t e complexes may e x i s t i n s o l u t i o n [158,161,162]. I r ( I V ) i s found only when the anode p o t e n t i a l exceeds 1.5 v o l t s [161],162]. 2.3 Platinum/Iridium A l l o y s The few c o r r o s i o n data that e x i s t f o r P t / I r a l l o y s i n various 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 - with 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 platinum. Anderson [163] 21 found that Pt/25Ir a l l o y was le s s c o r r o s i o n - r e s i s t a n t i n flow i n g sea water, operating at a curr e n t of 538 mA/cm2 (1.25 vs. 0.68 yg/A«hr f o r platinum). 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 at 250 mA/cm2 i n 6.2M NaC10 3, 44° (16 ug/A-'hr) and even higher losses i n 6.2M NaClOi, (130-220 ug/A-hr), where oxygen e v o l u t i o n i s the predominant anode process. Baboian [39] i n v e s t i g a t e d the c o r r o s i o n of Pt/10,20,30 I r a l l o y s i n 160 gpl H 2S0 4, 30°, over 45 days' operation at 108 mA/cm2. 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 data, the 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 the same c o r r o s i o n behaviour as platinum (6.4 ug/A«hr), and Pt/30 I r i s found to be sup 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 the in f l u e n c e of A.C. i n 20% HC1 e l e c t r o l y t e , with the concentrations of the di s s o l v e d metals having the same r a t i o as i n the a l l o y . 2.4 TSA Corrosion Corrosion of TSA's must be considered s e p a r a t e l y from t h a t of the pure noble metals or a l l o y s described above because of the p o s s i b i l i t y t h a t the l o s s of coa t i n g metal may be due to mechanical detachment, or s p a l l i n g , as a r e s u l t of poor c o a t i n g / s u b s t r a t e adhesion, r e s i d u a l s t r e s s e s i n the c o a t i n g , undermining of the coating due to attack of the exposed s u b s t r a t e , generation of gas bubbles w i t h i n pores, or the c o r r o s i o n of con-t a c t points i n hi g h l y dispersed coatings. Further, the anodic breakdown of TSA's may occur i n s o l u t i o n s with species aggressive to t i t a n i u m , or a f t e r l o s s of most of the a c t i v e coating m a t e r i a l . 22 2.4.1 Anodic Corrosion Much of the c o r r o s i o n data f o r TSA's i s found i n the patent l i t e r a t u r e , which i n turn deals predominantly with the a p p l i c a t i o n s of such anodes i n the c h l o r - a l k a l i i n d u s t r y - hence much information e x i s t s regarding c o r r o s i o n 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 . Experimental procedure i s seldom t r e a t e d adequately i n such sources, 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 various noble metal coatings which c o n t a i n e i t h e r platinum or i r i d i u m metals, manufactured by d i f f e r e n t techniques, and subjected to anodic e l e c t r o l y s i s i n various 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 are summarized i n Table 2.1. The accuracy of much of these c o r r o s i o n r e s u l t s i s subject to question as a r e s u l t of the high l e v e l s of un c e r t a i n t y associated with many of the coating loss measurement techniques, i n c l u d i n g t i m e - t o - f a i l u r e measurements, weight l o s s determinations, and s o l u t i o n a n a l y s i s . (These methods may neglect r e s i d u a l coating l e v e l s a f t e r f a i l u r e , weight increases due to oxide growth or the presence of anode surface d e p o s i t s , and losses of d i s s o l v e d metal to the cathode, r e s p e c t i v e l y . Tracer s t u d i e s permit the highest s e n s i t i v i t y (approaching 1 0 - 1 1 p e r cent) [112], but are l i m i t e d by the h a l f - l i v e s of the isotopes of i n t e r e s t . Two X-ray spectroscopic tech-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 of a convenient n u c l i d e to e x c i t e X - r a d i a t i o n ("Betascope") or the i r r a d i a t i o n of the specimen with continuous r a d i a t i o n from an X-ray tube. The former measure-ments can be performed with portable u n i t s , whereas the 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 spectrometer. Betascope measurements are, on the other hand, not s e l e c t i v e as to the element of i n t e r e s t , u n l i k e wavelength- or energy-dispersive X-ray spectrometers. T a b l e 2.1 Summary o f C o r r o s i o n Rates R e p o r t e d f o r Noble M e t a l Coated Anodes C o a t i n g t y p e : 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 Measurement p r o c e d u r e : (P) p a t e n t * c a l c u l a t e d from g/Ton C l 2 (assuming l g / T o n C I 2 = 1 . 4 5 8 u g / A - h r ) F t i m e t o f a i l u r e W w e i g h t l o s s X X - r a y s p e c t r o s c o p y B B e t a s c o p e S S o l u t i o n a n a l y s i s T T r a c e r R e f e r e n c e Year C o a t i n g C u r r e n t D e n s i t y (mA/cm2) E l e c t r o l y t e s Measurement P r o c e d u r e C o r r o s i o n E f f i c i e n c y J A ^ h ? | Type L o a d i n g 166 1958 ED 0.127u P t - T i 65 sea w a t e r ; R.T. F <16.8-2805 ( d e p . on h e a t -167 1959 Ed 17 .5 P t - T i 23 sea w a t e r F t r e a t m e n t ) 167 1959 ED 17.5u P t - T i 23 sea water F 660 1 . 7 5 , 2 . 5 u 66 0 . 2 p 230 168 1960 ED 2 . 5 , 3 p P t - T i 650 250 gpl H a C l , p H - 3 . 3 5 ; 7 5 ° W 5-10 169 1952 ED 7-10u P t - T i 100 300 g p l NaCl 1 .2 170 1964 ED P t - T i . 7 f 0 b r i n e (1 hour e l e c t r o l y s i s ) W 1000 171(P) 1965 ED 1 2 . 7 u P t / 5 0 R h - T i 260 s a t d . N a C l ; 50° X .17* " P t - T i " " X . 7 5 * " R h - T i " X . 1 5 * P t / 5 0 R h - T i 520 s a t d . N a C l ; 80° X . 1 4 * P t - T i X 1 . 2 2 * " R h - T i " X . 3 8 * 129 1966 ED P t - T i 50 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 ) B . 1 (6 y e a r s ) ED P t - T i 50 sea w a t e r ( c a t h o d i c p r o t e c t i o n ) B 1 .85 (100 d a y s ) ED P t - T i 117 2% NaCl S 1 . 2 - 2 . 7 ED P t - T i 300 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 ) B . 0 5 P t - T i 200 22% NaCl W .28 ED P t - T i 500 n i c k e l s u l f a t e p l a t i n g s o l u t i o n B 1.79 ED P t - T i 100 27% NaCl W 3 2 . 6 ( . 4 9 h r ) 4 . 8 (49 and 143 h r ) 97 1966 ED l p P t - T i 250 s y n t h e t i c sea w a t e r B . 1 - 3 . 5 (anodes w i t h edges) 3-7 (no edges) ' 172(P) 1967 ED P t - T i 78-310 290 g p l NaCl ( C h l o r a t e p r o d u c t i o n ) .71 72 1967 ED ED P t - T i 97 75-100 gpl H 2 S 0 „ , 5 0 - 5 7 g p l C u S 0 „ ; 4 0 ° 200 2 . 5 u P t - T i 431 n i c k e l p l a t i n g s o l u t i o n s + a d d i t i v e s W 13-200 " P t - T i 431 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 5-27 173 1967 ED 50-200 g/m2 P t - T i 550-750 s a t d . N a C l , pH 3 ; 60° W 3 174(P) 1967 TD 30 g/m 2 P t - T i 400 mercury c e l l B 1 . 4 6 * P t - T i 600 diaphragm e e l 1; 7 0 ° B . 2 3 - . 3 2 * " 40 g/m2 P t - T i 2 9 0 - 5 4 0 mercury e e l 1; 60° B . 5 2 * CONTINUED Table 2.1 (Continued) Coating Reference Year Current Measurement Corrosion E f f i c i e n c y E l e c t r o l y t e s Type Loading Densi ty (mA/cm2) Procedure A-hr J 175(P) 1958 TD 20 g/m2 P t / I r - T i b r i n e .016 * 176 1969 P t - T i 30- 490 3% NaCl; 25°C (h y p o c h l o r i t e prodn.) S 0-1.11 177 1959 P t - T i 155 c h l o r a t e c e l l ; pH 7; 105-123° W 3.1-5 .2 178(P) 1970 TD 15-18 g/m2 P t - P t - T i 1300 250 gpl NaCl, 15 gpl Na 2 S0u, pH 2.5; 70° 1.06 TD 16 g/m2 Pt / R h-Pt-Ti 1300 .26- 4.3 179 1970 ED P t - T i 100 3X'NaCl + saccharose w >100 38 1971 TD 10 g/m2 Pt/30 I r - T i 200 diaphragm c e l l .(157 TD Pt/30 I r - T i 12.5-15u P t - T i c h l o r a t e c e l l p e r o x i d a t i o n .1 .4- . 5 180(P) 1972 E D 1.5u Ru/Rh-Ti 50 20 gpl c i t r i c a c i d , 25 qpl NaOH.pH 12,60° 1800 ED 1.5u P t - T i 50 " 800000 181(P) 1972 TD 4.8 g/m2 I r / L i - T i 1800 mercury c e l l <.06 ( a f t e r 330 days) 182 1972 P t - T i s y n t h e t i c seawater (cathodic p r o t e c t i o n ) w .7 183(P) 1972 TD Ir / R h-Ti 400 300 gpl NaCl; 80° .07 184(P) 1973 TD 65Ir/21Rh/14Ru-Ti 400 300 gpl NaCl; 30° .09 TD Ir / R h-Ti " .14 185(P) 1973 TD 45Pt/45Rh/10Ru-Ti 400 300 gpl NaCl; 80° .08 TD Pt/60Rh-Ti " .14 186(P) 1973 TD 43Ir/42Pt/15Ru-Ti 400 300 gpl NaCl; 80° .10 TD P t / 4 0 I r - T i " .19 187(P) 1973 TD i u 27Pt/63Rh/10Pd-Ti 800 300 gpl NaCl; 80" .06 188(P) . 1973 TD l u 27/Ir/64Rh/9Ru-Ti• 800 300 gpl NaOH; 80° .09 TD Ir/30Rh-Ti " .13 189(P) 1973 TO l p 2OIr/60Pt/20Rh-Ti 800 300 gpl NaCl; 80° .07 190(P) 1973 TD lp 59Ir/31Pt/10Pd-Ti 800 300 gpl NaCl; 80° .08 1 91 CP) 1973 TD l p 55I;736Rh/9Pd-Ti 800 300 gpl NaCl; 80° .05 192(P) 1973 TD 1.5p 10Ru/60Ir/30Rh-Ti 600 270 gpl NaCl; 90° .02 TD P t - T i " M 2.5 193(P) 1973 TD 1.5p 20Ir/30Rh/50Pt-Ti 600 270 gpl NaCl; 90° .05 TD Rh-Ti " 2.7 194 1973 TD 1.3u P t / 4 0 I r - T i 300 c h l o r a t e p r oduction; 60°C T .037 ( I r ) ( I r ) TD 8.9p P t / 4 0 I r - T i " T .102 78 1973 TD TD 5-10 g/m2 P t / 3 0 I r - T i i n d u s t r i a l c h l o r a t e c e l l (Klong,Malaysia) i n d u s t r i a l c h l o r a t e c e l l (Cornwall,Ont.) X X .004 .16 (330 days) (374 days) 39 1974 CL l p Pt-Nb 108 160 gpl H 2S0»; 30° 1.5 CONTINUED T a b l e 2.1 ( C o n t i n u e d ) R e f e r e n c e Year C o a t i n g C u r r e n t E l e c t r o l y t e s Measurement C o r r o s i o n E f f i c i e n c y Type L o a d i n g D e n s i t y (mA/cm2) P r o c e d u r e ( A ^ F ) 195 1974 ED EO ED ED ED l - 8 p P t - T i P t - T i P t - T i 4p P t - T i 3p P t - T i 120-200 100 100 130 130 i n d u s t r i a l diaphragm e e l I s 300 g p l N a C l ; 8 0 ° 270 g p l N a C l ; 8 0 ° i n d u s t r i a l diaphragm c e l l B T W B B . 2 0 - . 8 5 ( a v e r a g e s ) 80 ( a f t e r 2 0 - 4 0 h r . ) 20 ( a f t e r 300 h r . ) . 2 9 ( a f t e r 2000 days . 4 4 ( a f t e r 1500 d a y s ) 195 1975 ED ED P t - T i P t - T i 3 0 - 5 0 0 3 0 - 5 0 0 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 ) . 4 - 1 . 5 1-3 197 1975 TD TD TO 1 . 3 g/m2 P t / 4 0 I r - T i 4 . 5 g/m2 P t / 4 0 I r - T i 8 . 9 g/m2 P t / 4 0 I r - T i 300 300 300 5M N a C l , pH 6 ; 60° T T T .021 ( a f t e r 400 d a y s ) . 0 6 3 . 0 5 2 26 E a r l y i n v e s t i g a t i o n s showed i n c o n s i s t e n t and high noble metal l o s s r a t e s , due to imperfections i n coating technology. Development of improved d e p o s i t i o n techniques has subsequently l e d to decreases of the order of two orders 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 coated anodes. Because there are marked d i f f e r e n c e s i n c o r r o s i o n behaviour of TSA's produced by d i f f e r e n t manufacturing processes, the .. c o r r o s i o n r e s u l t s are discussed s e p a r a t e l y f o r each coating 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 rates f o r P t - T i e l e c t r o d e s whose coatings 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 higher than the d i s s o l u t i o n rate of smooth platinum under the same condi-t i o n s , i n d i c a t i n g that s p a l l i n g of the coating was l i k e l y the predominant coating l o s s mechanism. For the e a r l i e s t electodes developed, t h i s problem appears to be r e l a t e d to poor substrate pretreatment 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 conjunction with 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 shorter anode l i f e t i m e s observed with t h i c k coatings [167]. Improved coating techniques subsequently r e s u l t e d i n decreases i n noble metal l o s s rates to l e v e l s where they became i n d u s t r i a l l y acceptable. E f f o r t next appears to have been d i r e c t e d towards the development of coatings of decreased p o r o s i t y i n order to f u r t h e r reduce lo s s e s due to progressive substrate attack during operation. A n t l e r [198] found that the degradation of P t - T i and Rh-Ti anodes operated 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 of severe under-c u t t i n g of the c o a t i n g , with extensive l a t e r a l spreading of the c o r r o s i o n 27 f i l m s . P i o n t e l l i noted i n d i s c u s s i o n that c o n s i d e r a b l y s u p e r i o r performance could 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 with b e t t e r substrate p r e t r e a t -ment p r i o r to e l e c t r o p l a t i n g . Van Laer [93] showed that p r e - o x i d a t i o n of the t i t a n i u m substrate increased c o r r o s i o n r a t e s . Warne and H a y f i e l d [72] also emphasized the importance of substrate pretreatment and p o s t - p l a t i n g heat treatments on the adhesion of the coating to the su b s t r a t e . Indeed, optimum heat treatments can r e s u l t i n the i n t e r f a c e having a mechanical strength i n excess of e i t h e r of the components. (Although Cotton [166] found that heat treatments above 500°C caused excessive d i f f i s u o n of the coating i n t o the substrate with a consequent d e l e t e r i o u s e f f e c t on co a t i n g l i f e t i m e . ) A l t e r n a t e s u b s t r a t e s , such as Nb or Ti/15Mo a l l o y were found to give improved coating adhesion c h a r a c t e r i s t i c s [72], Veselovskaya [195,199] determined that 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 longer s e r v i c e l i v e s i n anodic 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 to be extremely small (of the order of 1 uA/cm2 when the t o t a l current i s 100 mA/cm2, f o r a T i : P t surface r a t i o of 15:1, operating at 1.85v i n 300 gpl NaCl, pH 4-4.5, at 80°) [200]. Khodkevich [201,202] found the leakage current to be higher 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 . At pH 13 considerable substrate attack was observed. Coating p o r o s i t y ( i n gener 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 [73] showed that anodes with s i m i l a r loadings showed longer l i f e t i m e s when prepared under c o n d i t i o n s which gave less-porous d e p o s i t s . Khodkevich [203] found t h a t coatings having greater than 5 u thickness 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 strong 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 to 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 ) with time. Ginzburg [204] notes an increased tendency towards c r a c k i n g c o r r o s i o n of t i t a n i u m during anodic p o l a r i z a t i o n , with anodes having s l o t t e d s ections having increased p r o b a b i l i t y f o r c o r r o s i o n . A predominantly electrochemical coating 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 noble 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 to decrease with time [129,195,205,206] or where no dependence on c o a t i n g thickness i s observed.[173]. Examples of enhanced anodic c o r r o s i o n of e l e c t r o d e p o s i t e d coatings i n the presence of 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 [72], poly-saccharides [179], 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 noble metal d i s s o l u t i o n i s due to the formation of s o l u b l e complexes of the noble metals with the organics or t h e i r anodic decomposi-t i o n products. 2.4.1.2 Coatings Prepared by Thermal Decomposition As can be seen i n Table 2.1, anodes having coatings prepared by thermal decomposition show consider 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 those having e l e c t r o d e p o s i t e d coatings. A l l o y i n g w i t h i r i d i u m to 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 binary and ternary noble metal a l l o y s developed by Suzuki [183-193], a l s o show extremely low loss 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 coatings begins to d e t e r i o r a t e f o r loadings under 50 g/m2 (equivalent to about 2.5u P t ) , coatings of l e s s than 10 g/m2 (equivalent to about 0.5u Pt) produced 29 by thermal decomposition do not show impaired c o r r o s i o n c h a r a c t e r i s t i c s . Indeed, Atanasoski [197] found that Pt/40 I r coatings of only 1.3 g/m2 loading showed su 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 i n terms of absolute l o s s of noble metal) to s i m i l a r but higher-loading (4.5 and 8.9 g/m2) coatings a f t e r 400 days' operation i n c h l o r a t e production. 2.4.1.3 Coatings Prepared by Cladding Clad 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 to show s i m i l a r c o r r o s i o n behaviour to the s o l i d noble metal, with no substrate e f f e c t , or thickness-dependence of p o l a r i z a t i o n or c o r r o s i o n behaviour. Skomoroski [75] i n v e s t i g a t e d the c o r r o s i o n behaviour of c l a d (Pt-Ta) anodes vs. e l e c t r o d e p o s i t e d ( P t - T i ) anodes i n numerous e l e c t r o l y t e s , i n v a r i a b l y the c l a d anodes showed su 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 . Baboian [39] found that c l a d ( l y ) anodes showed considerably lower c o r r o s i o n rates than s o l i d platinum ( e l e c t r o l y s i s i n 160 gpl H 2 S O 4 , 30°, at 108 mA/cm2), which he suggested was due 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 . Further, there was 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 Ti to Nb to Ta s u b s t r a t e s . 2.4.2 A l t e r n a t i n g Current Corrosion A l t e r n a t i n g current e l e c t r o l y s i s leads to r a p i d degradation of s o l i d noble metal electrodes. - , The e f f e c t of an 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 d e l e t e r i o u s . Juchniewicz [129-133] has i n v e s t i g a t e d the e f f e c t of super-imposed 50 Hz A.C. 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 that as l i t t l e as 1% A.C. component ( 1 ^ ^ / I ^ ^  ) could cause con-s i d e r a b l e d e t e r i o r a t i o n . In d i s c u s s i o n , however, Walkiden suggested t h a t such r e s u l t s were the consequence of the poor q u a l i t y of the e l e c t r o -deposited coatings. The l a t e r work of Juchniewicz [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 achieved 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 percentage, A.C. produced only a small increase i n c o r r o s i o n r a t e ( f o r example, 30% A.C. on 115 mA/cm2 D.C. caused the c o r r o s i o n rate to nearly double (4.8 vs. 2.6 ug/A«hr) from the pure D.C. case. Higher-frequency (>100 cps) A.C. components (which are l i k e l y to 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 obtained 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 of P t - T i anodes [130,179]. Yukhevich [208] showed that i n c r e a s i n g A.C. frequency from 20 to 100 cps decreased P t - T i c o r r o s i o n rates under both monopolar and b i p o l a r operating c o n d i t i o n s . On/off switching i s known to produce a temporary "pulse" of platinum d i s s o l u t i o n and i t i s l i k e l y that such treatment, i f repeated many times, 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 . Atanasoski [194, 197] noted t h a t the coating l o s s vs. time r e l a t i o n fo several Pt/40 I r - T i anodes i n c h l o r a t e c e l l s showed a step-wise c h a r a c t e r , which, he suggested, may be due to i n t e r r u p t i o n of 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 be produced by s o r p t i o n of hydrogen i n t o the t i t a n i u m . (Prolonged cathodic operation r e s u l t s i n hydrogen embrittlement of the t i t a n i u m and consequent s p a l l i n g of the c o a t i n g ) . In c y c l i n g however, coating l o s s occurs by the r a p i d 31 d i f f u s i o n of hydrogen out of the 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 cathodic o p e r a t i o n , where the pressure due to the build-up of hydrogen gas beneath the coating causes the coating to l i f t and s p a l l [73]. In such cases, porous coatings give s u p e r i o r performance as they a l l o w the hydrogen to escape to the surface rather than to the 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 less-porous e l e c t r o d e s . This may e x p l a i n the apparently anomolous observation of Veselovskaya [199] who found that 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 greater s t a b i l i t y with respect to cur 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 the "pe e l i n g " tendency of the h i g h l y - s t r e s s e d deposit would l i k e l y r e s u l t i n greater substrate exposure, p e r m i t t i n g the e a s i e r escape of absorbed hydrogen. 2.4.3 Anodic Breakdown As the loadings of noble metals or oxides on value metal sub-s t r a t e s are 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 lead to anodic break-down when i n s u f f i c i e n t c o ating remains i n e l e c t r i c a l contact with the sub-s t r a t e metal. "Accelerated" t e s t s i n v o l v e the use of higher c u r r e n t d e n s i t i e s than would normally be encoutered i n i n d u s t r i a l p r a c t i c e 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 time, 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 to e x c e s s i v e l y high values. Such t e s t s may be performed with the a p p l i c a t i o n of a constant c u r r e n t , w i t h time to f a i l u r e being measured; or may perform as the extension of an upward determination of a p o l a r i z a t i o n curve to excessive values of the a p p l i e d p o t e n t i a l or curr e n t . 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 of voltage increase which, i f allowed to continue, r e s u l t s i n anodic breakdown of the t i t a n i u m substrate. 32 Cotton [166] noted t h a t the f a i l u r e of P t - T i anodes, with Ti wire leads, i n sea water occurred at the w a t e r l i n e on the leads. Mikhailova [209], who used rod-shaped anodes i n sea water, with a platinum c o a t i n g only at the end ( T i : P t area r a t i o 50:1), a l s o found t h a t breakdown occurred on the bare T i - but always w i t h i n 1-3 mm of the edge of the Pt c o a t i n g . Further, he observed considerable t h i c k e n i n g of the oxide f i l m on t i t a n i u m i n the region of the a c c e l e r a t i n g r a t e of increase of anode v o l t a g e , i n a s s o c i a t i o n with appreciable 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 of the f i l m p r i o r to breakdown. At breakdown, cor-rosion products of t i t a n i u m appeared i n s o l u t i o n i n the form of white " f l o e s . " Veselovskaya [135] and Khodkevich [210] observed that the mor-phology of substrate attack changed when P t - T i anodes were subjected to currents which produced a r a p i d r i s e i n p o t e n t i a l during operation 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 of l o c a l i z e d puncture ("burst bubble") f e a t u r e s . The " c r i t i c a l c urrent 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 to high values and the amount of charge passed to f a i l u r e increase with loading 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 at 55° [210]. The f a i l u r e of anodes operated a t current d e n s i t i e s w e l l below the " c r i t i c a l value" has been a t t r i b u t e d to the growth of an i n s u l a t i n g oxide l a y e r at 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 [211]. Such a phenomenon has been used to e x p l a i n the increase i n p o t e n t i a l of TSA's w i t h time [212], the decrease i n a c t i v i t y of e l e c t r o d e p o s i t e d P t - T i with voltammetric c y c l i n g [213] and d e v i a t i o n s from Tafel behaviour found with mixed oxide coated anodes [214]. Bystrov [215] has suggested t h a t an oxide l a y e r always 33 e x i s t s between the a c t i v e coating and s u b s t r a t e , but that 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 of the i n t r o d u c t i o n of a s u b s t a n t i a l impurity con-te n t i n t o i t during the manufacturing process. Growth of t h i s f i l m due to ion m i g r a t i o n , on the other 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 Coated TSA's i n Electrowinning On the basis of the known c o r r o s i o n r e s i s t a n c e of platinum 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 TSA's have been proposed f o r use in e l e c t r o w i n n i n g c e l l s [ 3 9 , 1 0 3 ] . Few s p e c i f i c c o r r o s i o n data e x i s t , how-ever, and the a v a i l a b l e information i s f a r from promising. Warne and H a y f i e l d [ 72 ] reported t h a t TSA's with e l e c t r o p l a t e d platinum coatings corroded at 200 yg/A«hr under an ap p l i e d c u r r e n t d e n s i t y of 97 mA/cm2 i n a copper p l a t i n g bath containing 75-100 gpl H 2 S0it, 50-57 gpl C u S 0 4 , at 4 0 ° . (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 loading of 5 g/m2 would f a i l w i t h i n 25 hours due to l o s s of coa t i n g metal.) Hopkins [ 21 ] found that ruthenium oxide coated anodes (with loadings up to 15 g Ru/m2) f a i l e d w i t h i n 800 hours of operation a t 19.4 mA/cm2 i n a s m a l l - s c a l e e l e c t r o w i n n i n g c e l l with an e l e c t r o l y t e c o n t a i n i n g 35 gpl copper and 1 35 -150 gpl H 2 S 0 4 , 4 5 ° . Eggett [ 5 4 ] has reported favourable r e s u l t s w i t h an un s p e c i f i e d noble metal coated anode i n u n s p e c i f i e d l a b o r a t o r y e l e c t r o w i n n i n g t e s t s . De Nora [ 3 7 ] has described several mixed oxide e l e c t r o d e s i n v o l v i n g combinations of the oxides of Ru, I r , or Pd with those of S i , T i , Ta, Nb, or Sn, which are claimed to 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 . Antonov [ 2 16 ] has reported that an anode having a Pb0 2 coating upon a Pd-, Rh-, or Pt-coated Ti substrate 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 . 34 Although 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 operations, such i s not always -Hv . n J e . Warne and H a y f i e l d [72] found that enhanced anodic c o r r o s i o n occurred 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 of c e r t a i n p r o p r i e t a r y a d d i t i v e s . Juchnewicz [179] a l s o found that the a d d i t i o n of polysaccharides to 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 of Pt - T i anodes. High d i s s o l u t i o n rates are a l s o observed with 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 of TSA's to a c c e l e r a t e d c o r r o s i o n i n the presence of c e r t a i n organics would req u i r e that the e f f e c t s of tr a c e organic contaminants or of 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 of 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 e l e c t r o w i n n i n g . 2.5 Summary and R e l a t i o n to t h i s Work On the basis of 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 radiochemical work of Chemodanov [94,99-102] - a c o r r o s i o n e f f i c i e n c y of 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 current e f f i c i e n c y value of .0001 per cent) would be expected f o r the case of electrochemical d i s s o l u -t i o n of platinum i n s u l f u r i c a c i d s o l u t i o n s . Higher c o r r o s i o n e f f i c i e n c i e s 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 detachment, i n the absence of e l e c t r o l y t e a d d i t i v e s or other f a c t o r s which may promote ac c e l e r a t e d platinum d i s s o l u t i o n . U n f o r t u n a t e l y , no c o r r o s i o n data 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, although greater 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 basis of i t s behaviour i n other 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 of P t / I r a l l o y coated anodes to P t - T i i n t h i s regard. The p o s s i b i l i t y t h a t both coating 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 at 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 nature of the d i s s o l u t i o n process 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 lead to enrichment of the remaining c o a t i n g metal with 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 concerning the r e l a t i v e c o r r o s i o n behaviour of the two coating 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 that 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 [107]. As t h i o u r e a i s reported to be an i n h i b i t o r f o r platinum d i s s o l u t i o n at high (> 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 to f u r t h e r explore the e f f e c t s of t h i s a d d i t i v e on the behaviour of 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 experiments. The A.C. 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 permit an estimation of the r e l a t i v e c o r r o s i o n behaviour of platinum and i r i d i u m from a l l o y coatings under c o n d i t i o n s of pulsed 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 basis of t h e i r behaviour under comparable c o n d i t i o n s with the a p p l i c a t i o n of 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 [123], i r i d i u m shows su 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 i n concentrated s o l u t i o n s . 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, although Rand [142] reported that 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 platinum under c o n d i t i o n s of c y c l i c voltammetry i n 1M H 2S0 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 A.C. Platinum i s known to y i e l d a high, time-dependent "pulse" of metal d i s s o l u t i o n with off/on switching [88.89,99], which suggests t h a t such 36 treatment - even with a frequencyof several days - may lead to increased mean d i s s o l u t i o n r a t e s . No data e x i s t w i t h respect to the e f f e c t of such treatment on i r i d i u m . I t i s l i k e l y that under the current r e v e r s a l c o n d i t i o n s employed i n copper el e c t r o w i n n i n g c e l l s at G6camines [34] (9 seconds on/0.5 second sh o r t e d ) , higher noble metal l o s s rates may be encountered, although the r e l a t i v e c o r r o s i o n rates of platinum and i r i d i u m cannot be estimated from p r e s e n t l y a v a i l a b l e data. 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 of an anodic current to a platinum e l e c t r o d e 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 occur, the anode surface acquires an oxygen coverage whose nature changes depending on the d u r a t i o n of anodic p o l a r i z a t i o n under a given s e t of c o n d i t i o n s . Although there are several fundamental reasons which have made the study o f the oxygen coverage on platinum one of the most a c t i v e areas of electrochemical research, a very p r a c t i c a l r e s u l t which can be ext r a p o l a t e d from the work of many authors i s the a b i l i t y to determine 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 by electrochemical formation and/or s t r i p p i n g of surface oxygen l a y e r s . Of concern i n the present work i s the r e l a t i o n s h i p between the s t a t e of the anode surface and the c o r r o s i o n r a t e , and a l s o the increase i n anode p o t e n t i a l ("passivation") w i t h time on e l e c t r o l y s i s i n " i n e r t " e l e c t r o -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 of the information concerning the nature of the oxygen coverage on platinum has come from the nonstationary electrochemical techniques of chronopotentiometry and voltammetry, as the degree of coverage of platinum anodes with oxygen r a r e l y exceeds the e q u i v a l e n t of a few 37 3 8 monolayers, and i s thus i n a c c e s s i b l e to study by such conventional tech-niques as X-ray or e l e c t r o n d i f f r a c t i o n , or chemical a n a l y s i s . T y p i c a l l y , a platinum e l e c t r o d e i n i t i a l l y held at 0 v o l t s (RHE) w i l l show several c h a r a c t e r i s t i c features i n the r e s u l t i n g charge curve or voltammogram measured on a p p l i c a t i o n of an anodic current pulse or a l i n e a r p o t e n t i a l "sweep." The Faradaic processes which can be d i s t i n g u i s h e d , provided care i s chosen as to the rate of charging (such t h a t the non-steady processes are not obscured by the non-Faradaic processes or by impurity r e a c t i o n s ) are the o x i d a t i o n of the adsorbed hydrogen, the d e p o s i t i o n of oxygen species, and u l t i m a t e l y , the e v o l u t i o n of oxygen. Development of the oxygen coverage continues simultaneously with oxygen e v o l u t i o n , although 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 increase i n anode p o t e n t i a l with time. On r e v e r s a l of the ap p l i e d current or d i r e c t i o n of the p o t e n t i a l sweep, features c h a r a c t e r i s t i c of the reduction of the oxygen coverage (and of d i s s o l v e d oxygen gas, unless attempts are made to remove i t p r i o r to a p p l i c a t i o n of the cathodic-going pulse) and of the d e p o s i t i o n of hydrogen s p e c i e s , followed by the e v o l u t i o n of hydrogen gas, are apparent on the charge curve or voltammogram. When care i s taken to e l i m i n a t e any charge-consuming reac-t i o n s , the degree of o x i d a t i o n can be d i r e c t l y measured from the t r a n s i -t i o n times corresponding to the oxygen regions on the charge curves, or from the wave or peak areas on the voltammograms. At p o t e n t i a l s p r i o r to the imminent e v o l u t i o n of oxygen gas, the oxygen coverage does not exceed a monolayer. This can be i n f e r r e d from the r e s u l t s of many workers, such as Schuldiner [219] who found that the t o t a l q u a n t i t y of charge passed i n anodic g a l v a n o s t a t i c p u l s i n g to the p o t e n t i a l of the imminent e v o l u t i o n 39 of oxygen gas, Q0, was double the value f o r the d e p o s i t i o n or removal of a f i l m of hydrogen species 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 explained i f i t i s postulated that every surface platinum atom takes part i n a 2-electron t r a n s f e r r e a c t i o n with oxygen and a 1-electron t r a n s f e r r e a c t i o n with hydrogen. Surface area e s t i m a t i o n thus requires only an assumption as to the surface metal atom dens i t y . The charge f o r formation of surface oxygen coverage cannot be simultaneously measured when oxygen e v o l u t i o n commences, as the l a t t e r current overwhelms t h a t f o r surface o x i d a t i o n . I t i s thus necessary to employ cathodic s t r i p p i n g techniques which i n v o l v e , i n a d d i t i o n , the stepping of the anode p o t e n t i a l to a value 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 accomplished simply by p u t t i n g the e l e c t r o d e at 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 to another c e l l to ensure minimal contamination w i t h p r e v i o u s l y -generated anode products. Oxygen coverages extend to a l i m i t i n g value of approximately the e q u i v a l e n t of two monolayers, although under c e r t a i n c o n d i t i o n s the formation of a second kind of surface oxygen coverage ("type II oxide") becomes p o s s i b l e , manifested by the appearance of a second reduction feature on the cathodic charge curve or voltammogram, and showing no l i m i t i n g coverage value. 3.1.1 Nature of the Surface Oxygen Coverage Attempts to c o r r e l a t e the features found on anodic or cathodic charge curves or voltammograms with the p o t e n t i a l s c a l c u l a t e d f o r platinum/ oxygen species from thermochemical data have been unsuccessful. Although 40 there i s no lack of data concerning the standard reduction p o t e n t i a l s f o r a v a r i e t y of metal/oxide or oxide/oxide couples f o r many species -both real and imaginary - the themochemistry i s i n most cases merely s p e c u l a t i v e . In t h i s regard, Novak [220] has presented no fewer than twenty-nine such p o t e n t i a l s f o r platinum. The e a r l y work on which much of the thermochemistry of the oxides of platinum i s based can be regarded as suspect due to the probable i n t e r f e r e n c e of competing r e a c t i o n s which may have obscured the reported metal/oxide p o t e n t i a l s f o r prepared oxides held i n perforated platinum containers i n 2N h^SO^. Further, the exact nature of the "oxides" was not a c c u r a t e l y known, nor was system p u r i t y considered. Measurements of cathodic discharge " l a g p o i n t s " provided the metal/oxide p o t e n t i a l value reported by Lorenz [221-223] i n 1909: Pt0 2«3H 20 + 4H + + 4e Pt0 2-2H 20 + 4H + + 4e Pt0-2H 20 + 2H + + 2e - Pt + 5H 20 E -^ Pt + 4H 20 E = ^ Pt + 3H 20 E = 0.98v (1) 0.96v (2) 0.95v (3) values: Grube [224], i n 1910, measured ra t h e r unstable r e s t p o t e n t i a l Pt0 2-4H 20 + 4H + + 4e ^  Pt + 6H 20 E = 1.06v (4) Pt0 2-2H 20 + 4H + + 4e ^  Pt + 4H 20 E = 1.04v (5) P t 0 3 + 6H + + 6e - Pt + 3H 20 E = 1.5v (6) PtO + 2H + + 2e - Pt + H 20 E = 0.9v (7) The f o l l o w i n g r e a c t i o n and standard reduction p o t e n t i a l 41 Pt(OH) 2 + 2H + + 2e - Pt + 2H 20 E = 0.98v (8) often quoted i n t a b l e s of electrode p o t e n t i a l s and used to e x p l a i n the consistency of the e a r l y work with modern thermochemical data, i s i n r e a l i t y i t s e l f based on thermochemical data derived from r e a c t i o n (3) [225]. L a t e r , Nagel and Dietz [226,227] published standard reduction p o t e n t i a l s f o r the oxides P t 3 C \ and Pt0 2«nH 20. P t a 0 4 + 8H + + 8e ^ 3Pt + 4H 20 E° = l . l l v (9) Pt0 2-nH 20 + 4H + + 4e - Pt + (n+2)H 20 E° = 0.80v (10) The p o t e n t i a l of (9) was determined from an assumed value f o r the standard f r e e energy of formation and thus possesses no i n t r i n s i c accuracy. The p o t e n t i a l f o r (10) was determined from the r e s t p o t e n t i a l of an el e c t r o d e on which i t was assumed that t h i c k Pt0 2«nH 20 l a y e r s were generated by A.C. p o l a r i z a t i o n . As a consequence of the high degrees of u n c e r t a i n t y of the above p o t e n t i a l s the complete l i s t of standard reduction p o t e n t i a l s f o r platinum oxides computed and compiled by Novak [220] must be considered to be i n e r r o r , because h i s data were derived from r e a c t i o n s (1)-(10) whose standard reduction p o t e n t i a l s are suspect. Hoare [228] obtained a p o t e n t i a l value f o r the couple: P t 0 h y d + 2 H + + 2 e * P t + H 2 ° E° 0 - 8 8 v from r e s t p o t e n t i a l measurements on a pre-anodized e l e c t r o d e i n n i t r o g e n -purged s o l u t i o n (which e l i m i n a t e d the O^ /H^ O couple from the mixed p o t e n t i a l ) . The species " P t O U w J " i s the "hydrated surface oxygen l a y e r . " 42 Recently the electrochemical behaviour of the oxides P t 0 2 and Pt 3C\ has been i n v e s t i g a t e d , using chemically prepared oxides [229-231]. The p o t e n t i a l s of s t a b i l i t y with respect to cathodic r e d u c t i o n were deter-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 , with the lowest p o t e n t i a l s at which the oxides were s t a b l e with respect to reduction to m e t a l l i c platinum considered to be the standard reduction p o t e n t i a l s : P t° 2hyd + 4 H + + 4 e ^ P t + 2 H 2 ° E ° = 0 , 4 0 v ( 1 2^ PtaCK + 8H + + 8e ^ Pt + 4H 20 E° = 0.27v (13) The a p p l i c a b i l i t y of bulk oxide data to the anodic f i l m formed on platinum i s questionable, however, e s p e c i a l l y at sub-monolayer cover-ages. Even at monolayer coverage, the oxide, i f i t e x i s t s , i s only two-dimensional. The "PtO" species reported by Hoare [228] i n r e a c t i o n (11) and i n f e r r e d by most other i n v e s t i g a t o r s to represent the stoic h i o m e t r y of the oxygen l a y e r a t p o t e n t i a l s corresponding to imminent oxygen evolu-t i o n does not appear to e x i s t i n bulk form.[232]. Attempts to c o r r e l a t e features on the charge curves or vol tarn-metric curves with bulk oxide p o t e n t i a l s show only a rough correspondence of the anodic-going curves with the (i n a c c u r a t e ) metal/oxide p o t e n t i a l s reported i n re a c t i o n s (1) to (10). The question as to whether the oxygen-film on platinum i s an adsorbed l a y e r or a phase oxide has always been the major source of con-t r o v e r s y i n the study of platinum o x i d a t i o n . Convincing arguments have been proposed by both the "adsorbed oxygen" and "phase oxide" schools. The features on the anodic/cathodic charge curves or voltammo-grams reveal an " i n t r i n s i c " h y s t e r e s i s , provided the anodic l i m i t i s above. 43 0.9 v o l t s . I f only an adsorbed l a y e r was formed, the reduction of oxygen deposited at higher p o t e n t i a l s during the anodic sweep would be expected to occur a t the same p o t e n t i a l s during the cathodic sweep - th a t i s , the shapes of the anodic- and cathodic-going curves should m i r r o r one another, i n d i c a t i n g r e v e r s i b i l i t y . On the other hand, i f an oxide was present the cur r e n t on a cathodic-going p o t e n t i a l sweep should not be expected to become negative u n t i l the p o t e n t i a l was depressed to values cathodic to 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 oxide. The v a r i a b i l i t y of the p o t e n t i a l at which the cathodic-going t r a c e i n voltammetry a t t a i n s negative values could perhaps be accounted f o r by assuming that the thermodynamic p r o p e r t i e s of t h i n oxygen f i l m s vary with t h i c k n e s s , and only approach those of the bulk oxide at higher thicknesses than are encountered with o x i d a t i o n of platinum surfaces. At present, n e i t h e r theory i s able to s a t i s f a c t o r i l y e x p l a i n the oxygen d e p o s i t i o n and removal features observed on charge curves or voltammograms. 3.1.1.1 Surface Species Deposited P r i o r to Oxygen E v o l u t i o n The p o t e n t i a l region 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 to study of the oxygen-species coverage due to the lack of 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. Thus to a very good approximation, a l l the charge passed w i t h i n t h i s p o t e n t i a l region corresponds to d e p o s i t i o n of surface species. The l i n e a r c o v e r a g e vs. p o t e n t i a l r e l a t i o n observed f o r progressive oxygen-species d e p o s i t i o n under i m p o s i t i o n of a constant anodic current can be explained i n two ways: (1) Ternkin adsorption ( E l o v i c h k i n e t i c s ) (2) H i g h - f i e l d oxide growth 44 F u r t h e r , i t i s not l i k e l y that the d e p o s i t i o n of a s i n g l e species i s i n v o l v e d , but r a t h e r that several steps are involved i n the o v e r a l l oxida-t i o n process, with the k i n e t i c behaviour c o n t r o l l e d by the "slow" step. to an OH r a d i c a l . Burke [233] 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 of water molecules on a p o s i t i v e l y charged s u r f a c e , whereby the two remaining l o n e - p a i r s p 3 o r b i t a l s are c l o s e s t to 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 to occur from one of these o r b i t a l s due to the higher tunneling p r o b a b i l i t y . Consequently a c a t i o n i c intermediate i s formed which subsequently loses a proton to become an adsorbed OH r a d i c a l . species suggests that the o v e r a l l processes i n v o l v e equal numbers of hydrogen ions 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 equations (the f e a t u r e s are s h i f t e d c a t h o d i c a l l y 59 mv 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 -t i o n ) . Further, since the charge c o n s i d e r a t i o n s imply a 1:1 coverage of oxygen-species on platinum immediately p r i o r to oxygen e v o l u t i o n , the 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 : I t i s l i k e l y , t h a t the species i n i t i a l l y deposited corresponds The pH-dependence of the d e p o s i t i o n (and removal) of oxygen Pt + H 20 + PtOH + H + e PtOH + PtO + H + + e (14a) (14b) where reduction occurs i n the opposite 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 20 + 2PtOH + 2H + 2e (15a) 2PtOH + PtO + Pt + H 20 (15b) 45 and f o r f i l m reduction [239,240]: 2Pt0 + 2H + 2e + 2Pt0H 2Pt0H + PtO + Pt + H 20 (16a) (16b) " 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 regions of anodic charge curves [241,242] or voltammograms [243-249] under rigorous c o n d i t i o n s of 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 to the successive d e p o s i t i o n of PtOH on various s u b - l a t t i c e s . The 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 the f r a c t i o n a l oxygen-species coverages at the p o t e n t i a l s corresponding to the various s t r u c t u r e s observed i n the voltammogram. For the (100) plane these are i d e n t i f i e d as four stages of d e p o s i t i o n : 0 ^ , O^' ^ 3 ' 0^: Stage S u b - l a t t i c e Stoichiometry P o t e n t i a l - .-. Coverager 0.89v 0.25 0.94v 0.33 1.05v 0.50 broad region 1.00 Maximum F r a c t i o n a l [2QHj • JA1 7\2 JA3 0 A4 Pt 40H Pt 20H PtOH PtO Such behaviour suggests 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 types of surface s i t e s . Those i n v e s t i g a t o r s who conform to the "oxide" theory consider that.the oxygen coverage i s merely the progressive t h i c k e n i n g of a PtO oxide l a y e r [250-264]. Perhaps the major problem i n d e f i n i t i o n of the nature of the oxygen coverage on platinum i s the i n t r i n s i c i r r e v e r s i b i l i t y or h y s t e r e s i s observed 46 i n the shapes of the anodic and cathodic charge curves or 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 nature of the oxygen f i l m occurs immediately a f t e r formation or that the mechanisms of o x i d a t i o n and reduction are d i f f e r e n t . Such behaviour has been accounted f o r by c o n s i d e r i n g that the d e p o s i t i o n of oxygen species obeys Ternkin k i n e t i c s , with the rate-determining step subjected to 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 + k i °Hads + H + e ^ H z 0 ( 1 7 ) k 2 the rate of increase of coverage with time during anodic p o l a r i z a t i o n can be expressed as: d e dT = k 2 S H 2 0 ( 1 _ 9 ) expl(1-3)(^f- - m e ) - k,C H +eexp + m e ) (18) where the Ternkin constant m represents the l i n e a r change of the heat of adsorption with coverage, 0 i - The d i f f e r e n c e between'.'therformation and removal of the adsorbed f i l m can be explained by a decrease i n the Ternkin constant, m, due to minimization of r e p u l s i v e i n t e r a c t i o n s between adsorbed species. (Reactions with a large m occur over a wide range of p o t e n t i a l s , whereas those with 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 of the o x i d a t i o n and reduction features found on c y c l i c charge curves or voltammograms. H y s t e r e s i s , however, must be accounted f o r by a progressive decrease i n the surface redox p o t e n t i a l . The Ternkin k i n e t i c argument breaks down as coverage 47 approaches and surpasses monolayer values, i n d i c a t i n g t h a t there 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 nature of the oxygen-species deposited at 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 that at higher p o t e n t i a l s -that i s , p r i o r to and a f t e r monolayer coverage. In r a d i o t r a c e r s t u d i e s with 0 1 8 i t i s found that the oxygen deposited i n the f i r s t l a y e r remains on the e l e c t r o d e during subsequent p o l a r i z a t i o n with concurrent oxygen e v o l u t i o n [269-272]. Hence the nature of the oxygen f i l m does indeed change a f t e r growth beyond monolayer coverage, and Temkin k i n e t i c s can at l e a s t be used to e x p l a i n the i n i t i a l f i l l i n g of the platinum surface with oxygen species. I t i s u n l i k e l y that 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 [27], Reddy [274] and Conway et al. [243-248] suggest t h a t the 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 to a "place exchange" process whereby some surface oxygen species and underlying metal atoms rearrange: PtOH + OHPt (19) PtO + OPt as a consequence of 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 surface would thus c o n s i s t of an ordered arrangement when Qg/2QH = 1. For example top l a y e r ••• Pt 0 Pt 0 Pt 0 second l a y e r • • • 0 Pt 0 Pt 0 Pt ••• metal substrate ••• Pt Pt Pt Pt Pt Pt ••• 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 confined 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 and reduc-t i o n of the oxygen-film. Conway et at. [ 2 4 3 - 2 4 8 ] note that h y s t e r e s i s i s manifested at coverages above Q Q / 2 Q H = 0 . 2 5 - that i s f o r stages above 0^-j - which i n d i c a t e s t h a t place-exchange may be promoted as surface f i l l i n g (and the mutual r e p u l s i o n e f f e c t ) increases. V e t t e r [ 2 5 0 ] has a l s o suggested place exchange as the mechanism of growth of an oxide f i l m on the platinum surface. The "oxide" theory f o r the oxygen coverage on platinum accounts f o r the increase i n p o t e n t i a l a t constant current in the pre-oxygen e v o l u t i o n p o t e n t i a l region by assuming the f i l m grows by a h i g h - f i e l d mechanism whereby the 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 across a p r o g r e s s i v e l y t h i c k e n i n g oxide f i l m r e s u l t s i n higher measured p o t e n t i a l s . Damjanovic and Ward [ 2 6 0 , 2 6 3 ] found t h a t the growth of the oxide f i l m on platinum under g a l v a n o s t a t i c charging c o n d i t i o n s i n the pre-oxygen e v o l u t i o n p o t e n t i a l region could be f i t t e d to a c l a s s i c o x i d a t i o n r a t e equation. i = 'i o exp cAv ( 2 1 ) where io and <* are constants, Av i s the p o t e n t i a l across the f i l m , and d i s the f i l m t h i c k n e s s . I t i s d i f f i c u l t , however, to conceive of the uniform growth of an oxide f i l m by such a mechanism at .coverages below Q Q / 2 Q ^ = 1 . 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 to r e c o n c i l e with the u n i t y ©averages observed at p o t e n t i a l s immediately p r i o r to oxygen e v o l u t i o n . The oxide theory can, however, e x p l a i n the h y s t e r e s i s between 4 9 the p o t e n t i a l s of formation and reduction of the oxygen coverage - growth occurs by a uniform advance over the e n t i r e s u r f a c e , whereas reduction occurs along the edges of oxide "islands', 1 [250]. As mentioned p r e v i o u s l y , the shape of the reduction feature i n e i t h e r chronopotentiometric or voltammetric curves i s suggestive of an oxide. The observation that the reduction plateau 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 increased holding time at anodic p o t e n t i a l s where the oxygen-film i s formed, i s i n conformance with the progressive conversion of the f i l m to one having the thermodynamic p r o p e r t i e s of a bulk oxide. As the f i l m approaches the bulk oxide i n character i t w i l l tend to be reduced at 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 of the f i l m approaches that f o r the oxide, f o r c i n g reduction to lower p o t e n t i a l s . 3.1.1.2 Surface Species Deposited During Oxygen Evol t u i o n The nature of the surface of a platinum e l e c t r o d e which i s evolv-ing oxygen gas i s s t i l l f a r from c h a r a c t e r i z e d . Charge st u d i e s reveal that higher than monolayer coverages e x i s t during oxygen e v o l u t i o n . The higher coverage values i n d i c a t e d by the charge studies have been i n t e r p r e t e d as: 1) f i l l i n g o f the metal s u r f a c e beyond a 1:1 s t o i c h i o m e t r y o f Pt:0, 2 ) c o n t i n u e d growth of an o x i d e f i l m , 3) a b s o r p t i o n o f oxygen i n t o the m e t a l . Case (1) i s supported by the r e s u l t s of many i n v e s t i g a t o r s who found that the coverage vs. p o t e n t i a l r e l a t i o n was l i n e a r up to about 2.0 v o l t s (RHE), whereupon the degree of surface o x i d a t i o n d i d not increase t h e r e a f t e r . The nearly 1:2 stoichiometry of the " l i m i t i n g " coverage 50 ( 1Pt:20 ) reported by several 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 to the s t r u c t u r e " P t 0 2 " nor does i t i n d i c a t e that the coverage i n excess of "monolayer" ( 1 : 1 ) values n e c e s s a r i l y c o n s t i t u t e s a second l a y e r . B i e g l e r [ 249 ] has noted that there are no s t e r i c hindrances to increased surface f i l l i n g with oxygen. Indeed, the f i n d i n g of 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 to be f o r t u i t o u s under the measurement con-d i t i o n s commonly employed (1-2N H 2 S0 i t , 20°C). Under c o n d i t i o n s of higher a c i d concentration [ 2 78 - 280 ] and lower temperatures [ 281 ] the l i m i t i n g coverage sto i c h i o m e t r y i s found to decrease. Such dependences are l i k e l y a s s o c iated with the strong adsorption of s u l f a t e anions which i s known to occur on platinum above 1.6 v o l t s [ 2 8 2 , 2 8 3 ] , where adso r p t i v e displacement of oxygen would r e s u l t i n lower measured coverages. Further, the l i m i t i n g coverage reported by B i e g l e r [ 276 ] appears to be dependent on the d u r a t i o n of preanodization where the anode was held at p o t e n t i a l s i n the l i m i t i n g coverage region (> 2.2 v o l t s ) . Gilman [ 2 8 4 J , whose procedure was adopted by B i e g l e r f o r h i s measurements, di d not f i n d that the growth of oxygen coverage ceased at any given anodization p o t e n t i a l (although h i s work d i d not extend to the l i m i t i n g coverage p o t e n t i a l r e g i o n ) . Volodin [ 2 7 5 ] , on the other hand, found the coverage of an anode operating at 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 value a f t e r 50 seconds' p o l a r i z a t i o n which d i d not vary even a f t e r 36 days at t h i s p o t e n t i a l . The appearance of a l i m i t i n g value of 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 be s a i d to be i n d i c a t i v e of the chemisorbed nature of the oxygen f i l m on platinum e l e c t r o d e s . In the "oxide" theory, 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 l i m i t i n g p o t e n t i a l value should e x i s t . 51 Proponents of the "oxide" theory do not f i n d evidence f o r a p o t e n t i a l - l i m i t e d maximum coverage [254], Further, oxide growth can account f o r the slow r a t e of increase of the oxygen coverage wit h time t o values beyond " P t 0 2 " s t o i c h i o m e t r y . V e t t e r [254] has determined that the oxygen coverage increases a t a given p o t e n t i a l according t o: Qo = a log t + b (22) where a and b are constants f o r the given p o t e n t i a l . Further, he claims t h a t "Pt02 n s t o i c h i o m e t r y i s exceeded by holding f o r 1000 seconds or l e s s a t p o t e n t i a l s of 1.5 v o l t s and above. High coverage values were i n turn i n t e r p r e t e d as evidence f o r the exi s t e n c e of surface oxygen i n oxide form. The absorption of oxygen i n t o platinum has been post u l a t e d to occur at p o t e n t i a l s both p r i o r to and subsequent to oxygen e v o l u t i o n . The absorption of oxygen i n t o the metal has been proposed to account f o r several phenomena i n a d d i t i o n to the increase i n oxygen coverage beyond values p r e d i c t e d from chemisorption theory: 1. The s m a l l r e s i d u a l c u r r e n t o b s e r v e d a t c o n s t a n t p o t e n t i a l ( 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 3 5 ] . 2. The charge imbalance o f t e n measured between the d e p o s i t i o n and removal o f oxygen s p e c i e s [236,241, 242 ,285,286]. The imbalance o f c h a r g e ( w i t h the a n o d i c c h a r g e e x c e e d i n g the c a t h o d i c charge) i s found to d e c r e a s e 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 the metal s u r f a c e w i t h absorbed oxygen. The imbalance can a l s o be reduced w i t h r a p i d c y c l i n g , where the a b s o r p t i o n does not have s u f f i c i e n t time to 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 complete oxygen monolayer on the s u r f a c e e n a b l i n g the e s t a b l i s h m e n t o f the r e v e r s i b l e oxygen p o t e n t i a l [287,288]. 52 k. I n c r e a s e o f the hydrogen o v e r p o t e n t i a l on a p l a t i n u m f o i l b i e l e c t r o d e [ 2 8 9 ] . 5. I n c r e a s e o f the r e s t p o t e n t i a l on the 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 the o t h e r s i d e [ 2 9 0 , 2 9 1 , 2 9 2 ] . 6. The appearance o f two a r r e s t s i n the g a l v a n o s t a t i c s t r i p p i n g c u r v e a f t e r p r i o r a n o d i z a t i o n a t p o t e n -t i a l s above 1.25 v o l t s (R.H.E.) [ 2 8 8 ] . 7. The re- a p p e a r a n c e o f p a r t i a l oxygen c o v e r a g e on an e l e c t r o d e t h a t has 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 to r e c o v e r under o p e n - c i r c u i t t o a n o d i c v a l u e s [ 2 8 8 ] . (Absorbed oxygen can d i f f u s e outward o n t o the s u r f a c e . ) 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 [ 2 9 3 ] . 9. Enhancement o f the e l e c t r o c a t a l y t i c a c t i v i t y o f 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 redox r e a c t i o n [29*0. 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 t o d e t e c t p l a t i n u m o x i d a t i o n [ 2 9 5 ] . 11. The d e c r e a s e o f the oxygen r e d u c t i o n o v e r p o t e n t i a l on the back 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 time-dependence o f the oxygen o v e r p o t e n t i a l [ 2 9 7 ] -Thacker and Hoare [ 2 8 8 ] found that surface adsorbed oxygen could 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 curves f o r the s t r i p p i n g of smooth platinum e l e c t r o d e s . Dermasorption was found to be pronounced f o r anodizations at p o t e n t i a l s above 1 . 8 v o l t s (RHE), which could r e s u l t i n erroneous coverage values i f the subsequent charge measurements were a t t r i b u t e d to surface coverage only. With separation of the surface coverage and dermasorption charges, the surface coverage was found to increase with time, under constant current c o n d i t i o n s , to a l i m i t i n g value of Q Q / 2 Q ^ - 2 . 53 3.1.2 Strengthening of the Oxygen Bond The p o t e n t i a l of the reduction plateau or peak i n cathodic charge curves or voltammograms does not remain constant, but r a t h e r i s s h i f t e d to more cathodic values w i t h increased anodic p o t e n t i a l l i m i t i n anodic/cathodic charging or with increased holding time at a given anodic p o t e n t i a l or c u r r e n t d e n s i t y . Such "ageing" i s tantamount to the s t r e n g t h -ening of the Pt-0 bond with time, p o s s i b l y due to a change i n the nature rather than the degree of the surface coverage. 3.1.3 A c t i v e Oxygen Tracer studies [269-272] have revealed 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 with respect to subsequent oxygen e v o l u t i o n . Oxygen deposited on the surface at 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 thus may be considered to be intermediates i n the oxygen e v o l u t i o n process. T y p i c a l chronopotentiometric or voltam-metric s t u d i e s , however, do not reveal the existence of two d i s t i n c t forms of surface coverage (the "type I I " oxide discussed below i s not under c o n s i d e r a t i o n here) as only a s i n g l e reduction plateau or peak i s observed (although the Temkin adsorption c o n s i d e r a t i o n s discussed above suggest that the nature of 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, using high g a l v a n o s t a t i c charge rates [298] or very f a s t p o t e n t i a l sweep rates (e.g. > 30 v/sec) [239,275,279,281,289,298,299] have detected a s p l i t t i n g of the reduction plateau or peak that becomes i n c r e a s i n g l y apparent at higher sweep r a t e s . 54 I t has been argued that the second peak corresponds to intermediate 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 only be detected at high sweep rates due to t h e i r r a p i d k i n e t i c s of decomposition i n t o the form corresponding t o the peak normally observed. Thus i t i s the sum of these two coverages which represents the approximate " P t 0 2 " s t o i c h i o m e t r y observed as the l i m i t i n g coverage value i n many cases. 3.1.4 Type I I Oxide "Type I I " oxide formation only occurs under p a r t i c u l a r condi-t i o n s of high current d e n s i t y g a l v a n o s t a t i c operation 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 manifested by the appearance of two a r r e s t s or peaks i n the subsequent cathodic charge curve or voltammogram [276,300-320]. The "type I I " oxide i s reduced at 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 i s found to increase with anodiza-t i o n time to values well i n excess of monolayer s t o i c h i o m e t r y - hence i t i s assumed to correspond to a phase oxide. Bulk oxide 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 observation of a red [304], orange-brown [310], or 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 d i f f r a c t i o n [307] i s unable to detect 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 d i f f r a c t i 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 . The reduction p o t e n t i a l f o r "type I I " oxide drops from 0.40 v o l t s to 0.26 v o l t s w i t h prolonged a n o d i z a t i o n . Sukhotin [231] has pointed out t h a t these p o t e n t i a l s correspond e x a c t l y to the reduction p o t e n t i a l s f o r bulk hydrated P t 0 2 and P t 3 0 4 , r e s p e c t i v e l y (see r e a c t i o n s (12) and ( 1 3 ) ) , i n f e r r i n g t h a t the type I I oxide i s l i k e l y a varying mixture of these two phases. Inoue [321] found both these oxides present on an anode used i n p e r s u l f a t e manufacture f o r three years at 1050-1250 mA/cm2. 55 Type II oxide has some rather 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 not a f f e c t t h e p o s i t i o n o f the 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 , nor does i t a l t e r t h e c o v e r a g e w i t h t y p e I (which i s the l i m i t i n g v a l u e o f Qq/IQ = 2) , even though 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 p o t e n t i a l s , 3 ) i t s f o r m a t i o n i s p r e v e n t e d a t lower t e m p e r a t u r e s , 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 , the type II o x i d a t i o n has a maximum growth r a t e a t 2N c o n c e n t r a t i o n , 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 he c u r r e n t d e n s i t y does not c o n t i n u e t o d e c r e a s e w i t h c o n s t a n t 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 as r e q u i r e d f o r f o r m a t i o n o f type II o x i de, 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 o r p r e v e n t s i t s subsequent f o r m a t i o n . C h a r a c t e r i s t i c (1) has been explained by Shibata [311] and Vinnikov [317] as due to o x i d a t i o n i n t o the metal beneath the type I f i l m . Hence no p h y s i c a l i n h i b i t i o n of the red u c t i o n of the type I f i l m by the type II oxide occurs, nor does the type I I oxide prevent the l i m i t i n g coverage f o r type I to be a t t i n e d . C h a r a c t e r i s t i c s (2) and (3) suggest that the type I f i l m i s p r o t e c t i v e with respect to penetration of o x i d a t i o n to the underlying l a t t i c under other c o n d i t i o n s of p o t e n t i a l and tempera-ture . There appears to be some discrepancy i n the p o t e n t i a l range over which type II oxide can form: Shibata [311] Ba l e j [315] Vinnikov [317] 2.10-2.17 v o l t s (RHE) i n IN H 2S0 4 2.1 -2.5 v o l t s (RHE) i n IN H2S0^ 2.1 -2.4 v o l t s (RHE) i n IN H 2S0 4 56 The extent of the formation range appears to be a f u n c t i o n of the degree of d i s r u p t i o n of the metal surface. Shibata [311], f o r example, used only annealed e l e c t r o d e s . Indeed, l a t e r work by Shibata [312] showed that the formation of type II oxide can be completely supressed by annealing above 1000°C, but t h a t type II may a l s o grow on such an e l e c t r o d e that has been subjected to electrochemical anodic/cathodic a c t i v a t i o n which promotes surface d i s s r u p t i o n . The e l e c t r o l y t e concentration dependence of type II formation or growth has been a t t r i b u t e d to increased s o l u t i o n anion adsorp-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 a c i d [315] or only becomes manifested beyond a c e r t a i n c r i t i c a l p o t e n t i a l i n the case of p e r c h l o r i c a c i d [300]. The nature of the type II oxide i s f a r from c h a r a c t e r i z e d , however, and several experimental anomalies s t i l l e x i s t - l e t alone the explanation of the apparent decrease i n p a s s i v a t i n g a b i l i t y of the type I f i l m over a narrow p o t e n t i a l range. For example, Shibata [311] reports that an anode on which type II oxide has been permitted 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 only the passive f i l m (type I) i s known to occur, the type II oxide w i l l continue to grow. On the other 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 the type I I oxide disappears. The apparent i n a b i l i t y of platinum to grow a type II coverage above 2.2-2.5 v o l t s (RHE) 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 that under c o n d i t i o n s where t h i c k oxide l a y e r s form on platinum 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 rate i s higher than at a s i m i l a r p o t e n t i a l i n s o l u t i o n s where t h i c k oxide l a y e r s do not form. As the r a t e of platinum d i s s o l u t i o n i s known to increase with 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 that the appearance of type I I o x i d a t i o n may only occur when the balance between the two processes of oxide growth and d i s s o l u t i o n i s i n favour of the former process. I f the oxide growth r a t e 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 r a t e , then a maximum w i l l be observed i n the coverage vs. p o t e n t i a l r e l a t i o n f o r type I I oxide. At high p o t e n t i a l s (above 2.2-2.5 v o l t s (RHE)) thus, the higher r a t e of d i s s o l u t i o n does not permit the growth of type II oxide. 3.1.5 Non-Electrochemical Techniques f o r Oxygen Film E v a luation Considerable e l l i p s o m e t r i c work has been done i n conjunction with electrochemical charge s t u d i e s . The technique i s l e s s - s e n s i t i v e than the electrochemical techniques, although changes i n the e l l i p s o m e t r i c parameters which are r e l a t e d to f i l m thickness i n d i c a t e a l i n e a r increase i n surface o x i d a t i o n with p o t e n t i a l i n an analogous manner to the coverage determined by charge curve s t u d i e s [246], with a change i n the nature of the surface 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 to gas-phase adsorption studies with platinum s i n g l e c r y s t a l s i n d i c a t e that 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 surface " 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 i n f e r r e d by Conway et al. [243-248] from voltammetric experiments. E l e c t r o n spectroscopy f o r chemical a n a l y s i s (ESCA) permits determination of the binding energy of various compounds, or allows the d e t e c t i o n of compound formation on a metal substrate through the measurement 58 of s h i f t s i n the core e l e c t r o n binding energies. ESCA was found to be able to determine the d i f f e r e n c e between Pt and P t 0 2 [ 3 2 4 ] , and subsequently i t s d e t e c t i o n has been reported a f t e r s h o r t a n o d i z a t i o n at 2 .2 v o l t s and a l s o a f t e r very long anodization at 4 v o l t s (RHE) [ 3 2 4 ] . Kim [ 3 02 , 324 ] reported the existence of PtO j and PtO species a f t e r l e s s - s e v e r e anodic treatment, but A l l e n [ 325 ] did not detect these. On the other hand, A l l e n reported a species with a lower formal o x i d a t i o n s t a t e than PtO was the only surface species o c c u r r i n g between 1-2.4 v o l t s . He suggested that t h i s may correspond to Pt0 . -H 2 0 or P t ( 0 H ) 2 . Auger spectroscopy r e l i e s on d e t e c t i o n of energies of "Auger e l e c t r o n s " emitted as a r e s u l t of reabsorption of 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 created. (The c h a r a c t e r i s t i c X-rays which escape are those detected i n X-ray fluorescence spectroscopy.) Of p a r t i c u l a r advantage i s the f a c t that Auger e l e c t r o n s r a r e l y exceed 0.5 kev i n energy and thus are reabsorbed i f they emanate from below the f i r s t few surface l a y e r s - hence the technique i s not hampered by the presence of the underlying metal as i n ESCA. Johnson [ 326 ] found that the surface of a platinum anode operated at 77.5 mA/cm2 f o r 16 hr i n IN H 2S0i» showed both Pt and 0 peaks, with a probable "PtO" s t o i c h i o m e t r y . 3.1 .6 Review On the basis of the degrees and nature of the surface oxygen coverage on platinum as reported i n the l i t e r a t u r e , i t i s f e l t t hat the progressive o x i d a t i o n of the platinum surface can be described by the stages l i s t e d i n Table 3 . 1 . S t a r t i n g with a bare platinum s u r f a c e , the sub-monolayer surface f i l l i n g occupies successive s u b - l a t t i c e s u n t i l monolayer T a b l e 3.1 P r o b a b l e S t a g e s 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 Anode P o t e n t i a l ( v o l t s v s . RHE) M a j o r C h a r g e - C o n s u m i n g P r o c e s s e s C h a r g i n g t h e d o u b l e l a y e r S u r f a c e o x i d a t i o n S u r f a c e o x i d a t i o n S u r f a c e o x i d a t i o n S u r f a c e o x i d a t i o n 1 . Oxygen e v o l u t i o n 2 . S u r f a c e o x i d a t i o n 3 . P l a t i n u m d i s s o l u t i o n 1 . Oxygen e v o l u t i o n 2. Type II s u r f a c e o x i -d a t i o n 3 . P l a t i n u m d i s s o l u t i o n 1 . Oxygen e v o l u t i o n 2 . Ozone and p e r o x y s u l f a t e p r o d u c t i o n 3 . P l a t i n u m d i s s o l u t i o n S u r f a c e S t o i c h i o m e t r y P t Pt_0H (100 p l a n e ) P t 2 0 H (100 p l a n e ) PtOH PtO P t O ( 0 ) n ( 0 < n < l ) P t 0 < ° > n , l i m . + P t 0 2 a n d / o r P t 3 0 » PtO(O) n , l i m . Comments An o r i e n t e d w a t e r d i p o l e l a y e r e x i s t s next t o the metal s u r f a c e . R e v e r s i b l e a d s o r p t i o n i n a s u b l a t t i c e s t r u c t u r e . P l a c e - e x c h a n g e between s u r f a c e OH and u n d e r -l y i n g Pt g i v e s r i s e t o i r r e v e r s i b i l i t y . M o n o l a y e r c o v e r a g e p r i o r t o oxygen e v o l u t i o n . - C o n t i n u e d f i l l i n g o r c o v e r a g e o f t h e s u r f a c e t o an e x t e n t w h i c h depends on the n a t u r e o f t h e 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 c o v e r a g e o c c u r s above 1 . 8 - 2 . 2 v o l t s . - G r o w t h o f phase o x i d e s b e g i n s - R a t e o f d i s s o l u t i o n exceeds r a t e o f f o r m a -t i o n o f t y p e II o x i d e s Coverage ( 0 C / 2 Q H ) 0 0 . 2 5 0 . 3 3 0 . 5 1 . 0 1 . 0 - 2 . 0 1 . 0 - 2 . 0 ( t y p e I ) U n l i m i t e d ( t y p e I I ) 1 . 0 - 2 . 0 ( t y p e I ) 0 ( t y p e I I ) CO 60 coverage i s achieved immediately p r i o r to anodic e v o l u t i o n of oxygen gas. Surface coverage continues to i n c r e a s e , but at a much slower r a t e , a f t e r commencement of oxygen e v o l u t i o n , e v e n t u a l l y reaching 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 concentration and temperature. Only the oxygen deposited a f t e r formation of the i n i t i a l monolayer takes place i n the oxygen e v o l u t i o n process, i n d i c a t i n g t h a t the natures of these two types of surface 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 detected by e l e c t r o c h e m i c a l measurement techniques. Only under s p e c i a l c o n d i t i o n s , probably as a r e s u l t of the balance between the r a t e of growth and d i s s o -l u t i o n of "type I I " oxide, does the growth of phase oxides ever occur on platinum. 3.2 I r i d i u m The same ele c t r o c h e m i c a l techniques (chronopotentiometry and voltammetry) as were discussed w i t h reference to platinum can a l s o be employed to study the oxygen coverage on i r i d i u m e l e c t r o d e s . S i m i l a r f e s t u r e s corresponding to the formation and removal of surface oxygen species can be i d e n t i f i e d on the r e s u l t a n t charge curves and voltammograms, although i r i d i u m shows marked d i f f e r e n c e s from platinum. P r i m a r i l y , i r i d i u m i s d i s t i n g u i s h e d by a poor separation of the hydrogen and oxygen regions and by r e v e r s i b i l i t y of the shapes of the anodic- and cathodic-going charge curves or voltammograms. Much of the e a r l y work with 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 with platinum with regard to the development of monolayer coverage with surface oxygen species immediately p r i o r to oxygen e v o l u t i o n , although, as w i l l be 61 seen below, i r i d i u m i s able to have the eq u i v a l e n t of several l a y e r s of oxygen at such p o t e n t i a l s . 3.2.1 Degree of Oxidation The slowness of the d e p o s i t i o n and reduction 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 the vast d i s c r e p a n c i e s among the coverage r e s u l t s reported f o r t h i s metal i n much of the l i t e r a t u r e . The e a r l y work with regard to the degree of coverage of i r i d i u m w i t h oxygen species was performed with high charging rates or p o t e n t i a l sweep rates t h a t l e d to the often-repeated c o n c l u s i o n that i r i d i u m a t t a i n s only monolayer coverage a t p o t e n t i a l s below about 1.4 v o l t s [327-330]. At higher p o t e n t i a l s growth well beyond monolayer coverage was i n d i c a t e d by B r e i t e r [327], Damjanovic [331], and Hoare [332]. Danjamovic 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 (up to 1.6v) had two l i n e a r s e c t i o n s which i n d i c a t e d that the rate of coverage with p o t e n t i a l was greater a t p o t e n t i a l s above about 1.35v than below and that 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 process on i r i d i u m , by using the " p o t e n t i a l step" technique he determined that the oxygen coverage at any p o t e n t i a l v a r i e d with log t i n 3 l i n e a r stages, and th a t the growth of coverage was s u f f i c i e n t l y slow that the f a s t sweep speeds or high-charging currents 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 not permit the build-up of greater-than-monolayer coverages at low anode p o t e n t i a l s . Kurnikov determined the coverage vs. 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 of sweep rates and found that monolayer coverage could e x i s t a t 62 p o t e n t i a l s as low as 0.6 v o l t s (RHE) and that the eq u i v a l e n t of tens of monolayers could e x i s t p r i o r to oxygen e v o l u t i o n i f a slow sweep rat e of 0.05 v/sec was employed. Higher sweep rates such as those employed by W i l l [328] (1 v/sec) gave conside r a b l y smaller coverage vs. p o t e n t i a l r e s u l t s . There i s considerable other evidence to support the contention t h a t high oxygen coverages e x i s t on i r i d i u m anodes. Chodos [338] s t u d i e d the surfaces of Pt/5 I r and Pt/20 I r a l l o y s with 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 , followed by holding f o r 15 hr at Ov (SCE), and found that considerable amounts of oxygen had remained on and beneath the surface. B r e i t e r [330] and Capon [339] noted that the cathodic voltammogram f o r a broad I<OA«/A b e " l i A> *«p> 0.7 c*J>d 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 (RHE), i n d i c a t i n g the presence of l e s s -r e v e r s i b l e surface o x i d a t i o n . Schubert [340], using f i e l d - i o n microscopy, detected the presence of t h i c k oxide f i l m s on i r i d i u m anodized at 1.5 v o l t s -with penetration reaching 100 angstroms i n t o the metal a t l o c a l i z e d s i t e s . Kuhn [341] found that extension of f a s t (10 v/sec) c y c l i c voltammetric sweeps to 1.95 v o l t s as the anodic l i m i t r e s u l t e d i n the appearance of a new reduction peak at 0.0 v o l t s (RHE) on the subsequent cathodic sweep. Further, he found t h a t i f the cathodic l i m i t was set at 0.4 v o l t s , not a l l of the a n o d i c a l l y deposited oxide was removed on the cathodic c y c l e . Otten [342,,343] and Rand [142] found t h a t , with continued p o t e n t i a l c y c l i n g between 0 and 1.4 v o l t s and above, the oxygen charge determined under the anodic or cathodic oxygen peaks increased p r o g r e s s i v e l y to values well beyond monolayer values. Kim [302] i d e n t i f i e d I r 0 2 (by ESCA) on an i r i d i u m f o i l e l e c t r o d e c y c l e d f i v e times between 0 and 1.6v. 63 3.2.2 Nature of the Surface Oxygen Coverage 3.2.2.1 Adsorbed Oxygen The monolayer or sub-monolayer coverages reported i n e a r l y studies were a t t r i b u t e d to the formation of adsorbed oxygen. B r e i t e r [330] concluded 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 of the features on the c y c l i c voltammograms (the anodic "peak" i s mirrored by a cathodic "peak" at v i r t u a l l y the same p o t e n t i a l ) , and the observation t h a t the c u r r e n t during the p o t e n t i a l sweep drops immediately to cathodic values on the cathodic-going c y c l e . (In c o n t r a s t , platinum 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 of the features f o r oxygen-species formation and removal, and the current does not drop immediately to cathodic values a f t e r r e v e r s a l of the voltammetric sweep from the anodic l i m i t . ) 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 of the o x i d a t i o n and reduction features on i r i d i u m to the i n a b i l i t y of surface oxygen to undergo "place-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 at a s i m i l a r p o t e n t i a l to that at which i t was l a i d down. B r i e t e r [330] derived k i n e t i c expressions which pr e d i c t e d the shape of the oxygen-peaks i n the voltammograms by c o n s i d e r i n g two p o t e n t i a l regions of Langmuirian behaviour f o r the r e a c t i o n s : I r + H 20 -y I r - 0 H a d s + H + e I r - OH ads I r - 0 a d s + H + e (23) (24) The overlap of the p a r t i a l c urrent 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 of the voltammogram. Hoare [334] found that hydrogen gas bubbling r a p i d l y reduced the surface of an 64 i r i d i u m anode p r e v i o u s l y covered with oxygen, which he a t t r i b u t e d to the easy reduction of adsorbed oxygen l a y e r s . In the l i g h t of 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 of 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 that f o r commencement of oxygen e v o l u t i o n , Kurnikov [337] suggested that at high sweep rates only parts of the oxygen f i l m would be reduced, as oxygen deep w i t h i n oxide l a y e r s or w i t h i n the metal i s unable to d i f f u s e out to the surface i n order to react and hence be detected during cathodic sweeping. Rand [142] s t a t e d t h a t the oxide phase formed on i r i d i u m remains on the surface during p o t e n t i a l c y c l i n g and t h a t oxygen adsorption and desorption occur on an o x i d i z e d surface. On the other hand, the oxide l a y e r was considered to be porous, as l i t t l e i n h i b i t i o n of the hydrogen adsorption r e a c t i o n (which must occur on the bare metal) was observed. (Indeed the surface area c a l c u l a t e d from hydrogen charging, where was measured, remained constant with oxide growth, whereas Q0 increased without l i m i t and was not i n d i c a t i v e of the surface area.) „' 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 observations by Kurnikov [333-337] t h a t large thicknesses of oxide l a y e r s are formed w i t h : 1) the r e v e r s i b i l i t y o f the a n o d i c and c a t h o d i c p o t e n t i a l sweeps or charge c u r v e s , 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 t h e 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 oxide i s formed i t s reduction should only be thermo-dynamically p o s s i b l e at p o t e n t i a l s below i t s reduction p o t e n t i a l . The 6 5 growth of the area under the oxygen peak i n the voltammograms produced with repeated c y c l i n g [142,342] to p o t e n t i a l s of 1.4v (RHE) and above c l e a r l y shows the formation of the e q u i v a l e n t of m u l t i l a y e r f i l m s which are apparently formed and reduced without h y s t e r e s i s , with no i n d i c a t i o n what-soever that an oxide i s i n v o l v e d , as i n such a case a constant reduction p o t e n t i a l would be expected, with reduction o c c u r r i n g only at more cathodic values. B r i e t e r [330] a l l u d e s to t h i s anomaly, suggesting t h a t the free energy of formation of t h i n phase oxides may vary with 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 expected to 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 of bulk oxides, h y s t e r e s i s would be expected to become more profound as the anodic 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 explained i f the " p i t " model of o x i d a t i o n proposed by Otten [342,343] i s considered. That i s , o x i d a t i o n occurs by penetration of oxygen i n t o the metal only at c e r t a i n l o c a l i z e d s i t e s on the surface. Such a model would leave the surface f i l m e d with only a t h i n adsorbed l a y e r which could be reduced r e v e r s i b l y during anodic/cathodic sweeping, and which could account f o r the a b i l i t y of hydrogen adsorption to i n d i c a t e a bare metal surface. 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 to d i f f u s e outward and hence would not be detected, and the QQ values measured would only r e l a t e to the surface coverage which would e f f e c t i v e l y be covered with oxygen to the extent of a monolayer or l e s s . Otten proposed the " p i t " model to 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 coulometric l a y e r t h i c k -nesses found i n his 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 p r o p e r t i e s when charge studies revealed only small coverage with oxygen. The penetration of o x i d a t i o n i n t o the; metal at p i t s 66 was thus suggested to account f o r d i s r u p t i o n of the metal surface and con-sequent change i n the o p t i c a l p r o p e r t i e s of the i r i d i u m surface. The pos-t u l a t i o n of deep penetration of oxygen i n t o i r i d i u m i s supported by both f i e l d - i o n microscopy [340] and electron-probe [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 oxide phases [145], I r 0 2 and l r 2 0 3 . Van Muylder and Pourbaix [345,346] s t a t e that l r 2 0 3 i s thermo-dynamically unstable with respect to I r and I r 0 2 , and f u r t h e r t h a t the hydrated form of l r 2 0 3 i s r e a d i l y s o l u b l e i n a c i d s . Thus they conclude that i r i d i u m only possesses one s t a b l e oxide phase i n aqueous s o l u t i o n , namely I r 0 2 . Hoare [334] has determined the standard reduction p o t e n t i a l s of iridium/oxygen species: I r - 0 a d s + 2H + + 2e - I r + H 20 E° = 0.87v (25) IrO h y d + 4H + + 4e " I r + 2H 20 E° = 0.935v (26) P o t e n t i a l (25) was measured by pre-anodizing an i r i d i u m wire followed by determination of i t s r e s t p o t e n t i a l . P o t e n t i a l (26) was measured by immersion of a pre-heated ( i n a i r ) wire 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 owed by r e s t p o t e n t i a l determination. P o t e n t i a l (26) was not completely s t a b l e , and was found to tend to d r i f t to the value f o r (25) with time. Kurnikov [337] suggests that the formation of an oxide f i l m l i k e l y occurs v i a the f o l l o w i n g sequence: I r + H 20 -> IrOH + H + + e at E - 0.6v (27) IrOH + IrO + H + + e at E = 0.95v (28) 67 IrO + H 20 IrOOH + H + + e at E = l . l v (29) IrOOH I r 0 2 + H + + e at E = 1.4v (30) Reactions (28)-(30) correspond to the s t o i c h i o m e t r i c s at the p o t e n t i a l s corresponding to s t r u c t u r e observed i n the anodic or cathodic voltammograms, with the sequential occurrence of (27-)-(30) i n d i c a t e d by the p o t e n t i a l s of these peaks or waves. These p o t e n t i a l s were found to vary with sweep r a t e , but appeared to l e v e l o f f at 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 at a sweep rat e of 0.05 v/sec (slow) are those given f o r r e a c t i o n s (27)-(30). The coincidence of these p o t e n t i a l s measured with e i t h e r anodic or cathodic sweeping may i n d i c a t e that the reported values are c l o s e to 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 various 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 Despite the extensive 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 of oxygen f i l m s on platinum and i r i d i u m , no work has been published concerning P t / I r a l l o y s , save the very l i m i t e d work of Chodos [338] w i t h respect to the determination of r e s i d u a l oxygen content i n the metal a f t e r anodic/ cathodic treatment. I t i s of i n t e r e s t to determine the nature of the oxygen coverage on such a l l o y s to see whether i t conforms to that of platinum or that of i r i d i u m , which i n turn determines the a p p l i c a b i l i t y of e l e c t r o -chemical surface area measurement techniques i n v o l v i n g the s t r i p p i n g and/or removal of oxygen monolayers. Further, the electrochemical d i s s o l u t i o n and the p a s s i v a t i o n (defined as the change i n o v e r p o t e n t i a l with time) processes are d i r e c t l y r e l a t e d to the degree of surface 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 present work. 6 8 On c o n s i d e r a t i o n of the e x i s t i n g l i t e r a t u r e , platinum and i r i d i u m are found to d i f f e r with respect to the nature of t h e i r e l e c t r o -chemical l y - formed oxygen f i l m s : 1. There 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 " . reg i on., s e p a r a t i n g the hydrogen and oxygen r e g i o n s i n the 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 tend 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 c o v e r a g e o n l y on imminent oxygen 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 appears t o be a f o r t u i t o u s r e s u l t o f the 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 form the e q u i v a l e n t o f m u l t i l a y e r oxygen c o v e r a g e . 3 . D u r i n g oxygen e v o l u t i o n the oxygen f i l m on p l a t i n u m does not grow beyond a v a l u e e q u i v a l e n t t o about two m o n o l a y e r s , e x c e p t under 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 can d e v e l o p when the 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 growth o f oxygen f i l m s on i r i d i u m anodes under 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 , a l t h o u g h the growth o f m u l t i l a y e r f i l m s and even the 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 marked h y s t e r e s i s i n the shapes 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 the o t h e r hand, not a l l o f the t h e oxygen c o v e r a g e i s removed on 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 where t h e oxygen 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 the shapes o f the char 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 to the l a c k o f " p l a c e exchange" between some o f the s u r f a c e oxygen and 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 , as has been demonstrated by many e x p e r i m e n t s w i t h t h i n f o i l s . No s i m i l a r work has 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 , which 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 meta1 . 6 9 Speculation as to the nature of the surface 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 nature of the a l l o y surface. I f the a l l o y s are homogeneous and present surfaces 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 of the c o n s t i t u e n t atoms, then the forma-t i o n of an adsorbed oxygen l a y e r w i l l n e c e s s a r i l y i n v o l v e i n d i v i d u a l surface 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 adsorption behaviour. The f i n d i n g of h y s t e r e s i s between the shapes of the anodic and cathodic charge curves would suggest that "place exchange" between the oxygen atom and the under-l y i n g noble metal atom could occur. Whether the oxygen coverage increases with prolonged anodization to the e q u i v a l e n t of only a few monolayers or to m u l t i l a y e r coverage, cannot be p r e d i c t e d from the present l i t e r a t u r e . Chapter 4 EXPERIMENTAL 4.1 M a t e r i a l s and Apparatus 4.1.1 Anode M a t e r i a l s and Construction P t / I r - T i anodes having a nominal coating composition of 30 per cent i r i d i u m by weight, and loadings varying from 2.3 to 20 g/m2 were obtained i n the form of sheets from Imperial Metal I n d u s t r i e s L i m i t e d . The coatings were a p p l i e d by the thermal decomposition method [174,347]. Experimental anodes were g e n e r a l l y cut i n the form of d i s c s 1.86-1.92 cm 2 i n diameter. No e f f e c t s of the d i s r u p t i o n of the coating at the d i s c edge due to c u t t i n g were ever observed. (Indeed, the substrate was exposed randomly over the surfaces of the d i s c s , as S.E.M. observation revealed.) I t was des i r e d to present a s i n g l e d i s c face to the e l e c t r o l y t i c s o l u t i o n s , with e l e c t r i c a l connections being made to the back side which was i s o l a t e d from the s o l u t i o n . This would permit repeated assembly and disassembly to be made without damage to the operating surface caused by making and breaking of the e l e c t r i c a l contact. Besides a s s u r i n g b e t t e r current 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, the e l e c t r i c a l i s o l a t i o n of one face of the d i s c e l i m i n a t e d any e f f e c t s the lead material may have had on the behaviour of the working e l e c t r o d e . E l e c t r i c a l contact was made by spot-welding two short platinum wires to the 70 71 d i s c . Copper w i r e s , soldered to the platinum w i r e s , served as the leads to the exte r n a l e l e c t r i c a l c i r c u i t . E lectrode c o n s t r u c t i o n i s shown i n Figure 4.1. The d i s c s were mounted i n Teflon holders, i n t o which a Pyrex tube was i n s e r t e d f o r the leads followed by c a s t i n g of the i n s i d e with epoxy to prevent breaking of the e l e c t r i c a l contacts at the d i s c under mechanical s t r e s s and to f u r t h e r ensure the i s o l a t i o n of the back face of the d i s c and the leads. In runs at 60 and 80°, l a r g e r experimental anodes (3 x 3.7,cm) were used where mounting i n Teflon/Pyrex assemblies was i m p r a c t i c a l due to adverse expansion e f f e c t s . In such cases 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 with the working e l e c t r o d e held i n p o s i t i o n between them, with both faces operating e l e c t r o l y t i c a l l y . E l e c t r i c a l contact was made by spot-welding a t i t a n i u m wire to a "tab" which was l e f t during c u t t i n g of the anode. As the range of current 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 to several hundred mA/cm2, and as the power ranges of many of the power sup p l i e s used i n electrochemical research are l i m i t e d , i t i s d e s i r a b l e to 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 c u r r e n t d e n s i t i e s . For t h i s reason, 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 permitted. Wires of P t , P t / 5 I r , P t / l O I r , P t / 2 0 I r , Pt/25Ir composition, obtained from Johnson Matthey Metals L t d . , were a l s o employed as e l e c t r o d e s . Conventionally wire electrodes are made by i n s e r t i n g the wire through a glass tube followed by f u s i o n of the glass to the wire. 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 of the 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 involved - 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 (Figure 4.1). Lengths 7 2 Figure 4 . 1 . Working electrode c o n s t r u c t i o n (a) f o r d i s c specimens cut from sheets; (b) f o r wire e l e c t r o d e s . 73 of the r e s p e c t i v e noble metal wires were soldered to copper wires to provide the external e l e c t r i c a l contact. A po r t i o n of the noble metal wire was then forced through a small hole (smaller than the wire diameters - t y p i c a l l y .020 inch) i n a Teflon cap which was i n turn i n s e r t e d i n t o , a Pyrex tube to prevent the s o l u t i o n from reaching the wires i n s i d e the tube. Commercially pure t i t a n i u m sheets and wires 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 , with the sheet specimens being cut i n t o d i s c s and mounted i n Teflon/Pyrex assemblies as described f o r the coated e l e c t r o d e s , and the wire e l e c t r o d e s being constructed i n a s i m i l a r manner to the noble metal wire e l e c t r o d e s . 4 . 1 . 2 E l e c t r o l y t e s and Gases A l l s o l u t i o n s were prepared from reagent grade chemicals 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 of 2M H 2 S O 4 (196 gpl) was chosen f o r most of the work as t h i s concentration approximates the h i g h l y a c i d c o n d i t i o n s t h a t are encountered i n el e c t r o w i n n i n g of s o l u t i o n s that have been produced by s t r i p p i n g of various commercial so l v e n t e x t r a c t i o n reagents. S o l u t i o n s of 4M (392 gpl) and 8M (784 gpl) H 2 S 0 4 were a l s o used i n a few. A l l surface area and charge st u d i e s were performed i n the base e l e c t r o l y t e . Copper-containing s o l u t i o n s were prepared with the necessary amount of Cu S0 i t * 5H 2 0 required to give a s o l u t i o n 0.5M (32 gpl) i n copper. The reagent grade CuS04 *5H 2 0 was subsequently found to have 50 ppm of lead as impurity on spectrographic a n a l y s i s . Lead-free s o l u t i o n s were prepared by the d i s s o l u t i o n of CuO ( i n s o l u t i o n s c o n t a i n i n g an excess of a c i d such that the f i n a l a c i d concentration was 2M). Copper and a c i d l e v e l s were maintained throughout batch experiments with d a i l y a d d i t i o n s of CuO. 74 Reagent grade thiourea was used i n experiments where the use of 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 experiment, d e l i b e r a t e contamination with kerosene was employed to assess the e f f e c t s of t h i s i m p u r i t y , which i s present i n SX/EW systems. The p u r i t y of the base e l e c t r o l y t e s o l u t i o n (2M H 2S0 1 +) was found to be adequate f o r surface area, surface charge and p o l a r i z a t i o n curve studies where currents were well i n excess of the l i m i t i n g currents p o s s i b l e from impurity o x i d a t i o n or red u c t i o n . At smaller currents however, such as those used during t r a c i n g of oxygen o v e r p o t e n t i a l curves to very low p o t e n t i a l s , trace impurity o x i d a t i o n obscured the anode process unless 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 the procedure of Bockris [348] was adopted, where a 24 hr. cathodic p r e - e l e c t r o l y s i s followed by a 48 hr. anodic p r e - e l e c t r o l y s i s at 10 mA/cm2 was employed to lower s o l u t i o n i m purity l e v e l s . Gases used f o r "sweeping" of e l e c t r o l y t e s included tank hydrogen ( f o r hydrogen reference 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 reduction s t u d i e s ) , helium ( f o r surface area and surface 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 helium were passed through heated copper oxide c a t a l y s t columns i n order to lower the l e v e l s of re 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 order to lower the l e v e l s of o x i d i z a b l e contaminants. A l l gases were subsequently bubbled through water p r i o r to i n t r o d u c t i o n to the c e l l s to minimize e l e c t r o l y t e losses through v a p o r i z a t i o n . A l l gas t r a i n s , from the c a t a l y s t columns to the c e l l s , were constructed of e i t h e r Pyrex or Teflon i n order to prevent p o s s i b l e contamination of e i t h e r the gas or the e l e c t r o l y t e during a c c i d e n t a l contact w i t h the gas t r a i n . No grease or l u b r i c a n t s of any kind were used i n the gas t r a i n s or other apparatus. 75 4 . 1 . 3 C e l l s Two ba s i c c e l l designs were employed i n the present work. For long-term runs i n simulated copper-electrowinning experiments, s i n g l e com-partment c e l l s were used. A l l such c e l l s were constructed such t h a t only Pyrex, Teflon and the elect r o d e s themselves were i n d i r e c t contact with the e l e c t r o l y t e . Escaping gas(es) were vented through water "bubblers" on closed c e l l tops although no rigorous e f f o r t was made to exclude the atmo-sphere. 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 Teflon c e l l top whose c o n s t r u c t i o n permitted independent v a r i a t i o n of the p o s i t i o n s of the e l e c t r o d e s . During operation the Luggin c a p i l l a r y was f i x e d at about 1 mm d i r e c t l y i n f r o n t of the centre of the working e l e c t r o d e , whose planar face was p a r a l l e l to that of the a u x i l i a r y e l e c t r o d e which was u s u a l l y p o s i t i o n e d , f o r convenience, about 3 cm away. In copper-c o n t a i n i n g e l e c t r o l y t e s the obvious choice f o r an a u x i l i a r y e l e c t r o d e material i s copper. In s u l f u r i c a c i d s o l u t i o n s without copper, both platinum 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 various c e l l s . A l l s t i r r i n g was performed with magnetic s t i r r i n g bars. C e l l volumes v a r i e d from 1.8 to 4 I . P r i o r to each run the c e l l s were disassembled, cleaned, 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 involved soaking i n c h r o m i c / s u l f u r i c a c i d , followed 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 . The a u x i l i a r y e l e c t r o d e s were cleaned by r i n s i n g i n d i l u t e n i t r i c a c i d (which d i s s o l v e d much of the copper deposited i n previous runs). Such procedures were adopted to avoid u n c e r t a i n t i e s with respect to the poisoning of the working e l e c t r o d e s by unknown contaminants 7 6 and to remove any r e a c t i o n products or spent e l e c t r o l y t e which may have been present on the c e l l components from the previous run. For short term runs, such as surface area and surface charge experiments, the separation of 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 . Indeed, such separation i s necessary, as hydrogen or oxygen gas produced on the a u x i l i a r y e l e c t r o d e could 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 pulse techniques. (The d e p o l a r i z i n g e f f e c t s of 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 only s i g n i f i c a n t at very 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 reduction of trace hydrogen or oxygen gases i s d i f f u s i o n - l i m i t e d . ) Hence, a two-compartment c e l l was employed f o r such measurements. Again, c o n s t r u c t i o n was such that only T e f l o n , Pyrex or the e l e c t r o d e m a t e r i a l s contacted the e l e c t r o l y t e . The a u x i l i a r y e l e c t r o d e used was a Pt/Rh gauze of l a r g e geometric surface area. The design of the Teflon c e l l top f o r the working e l e c t r o d e compartment permitted the use of both types of e l e c t r o d e s described i n Section 4.1.1. The Luggin c a p i l l a r y , which could be p o s i t i o n e d independently, was f i x e d at about 1 mm from the centre of the length of wire working e l e c t r o d e s . For planar e l e c t r o d e s the Luggin c a p i l l a r y s h a f t could be p o s i t i o n e d to the side of the working el 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 distance between the c a p i l l a r y opening and the e l e c t r o d e surface at about 0.5 mm. The volume of each compartment was approximately 1 I . The 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 of the e l e c t r o l y t e with various tank gases, as required i n the charge and surface area s t u d i e s . Temperature-control f o r the s i n g l e compartment c e l l s was achieved with conventional water-baths i n which the c e l l s were immersed. For the 77 two-compartment c e l l used i n the shorter-term runs, s t i r r e r - h e a t e r s pro-vided 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 kinds of reference e l e c t r o d e s were employed: 1. R e v e r s i b l e hydrogen e l e c t r o d e s 2 . Copper/copper s u l f a t e e l e c t r o d e s 3 . (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 the 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 e l e c t r o l y t e s o l u t i o n s . The r e v e r s i b l e 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 convenient i n s u l f u r i c a c i d s o l u -t i o n s f o r surface charge or surface area measurement st 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). Furt h e r , when con-s i d e r i n g r e a c t i o n s which vary with pH i n the same manner as the hydrogen e l e c t r o d e , the use of a RHE permits 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 experiments i n s o l u t i o n s of d i f f e r e n t pH. The RHE was constructed from a d i s c of 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 connection made by a length of Pt/25 I r wire, a l s o exposed to the e l e c t r o l y t e . 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 saturated with hydrogen gas, r e s u l t i n g i n the attainment of a s t a b l e p o t e n t i a l w i t h i n about three minutes. Connection of the RHE c e l l ( t y p i c a l l y a 1000 ml beaker with a Teflon top) with the experimental c e l l was made by means of a Teflon tube "bridge" f i l l e d with the same e l e c t r o l y t e . To minimize e l e c t r o l y t e f l o w , the ends of the Teflon tube were i n s e r t e d i n t o 6 mm O.D. Pyrex tubes, drawn to a c a p i l l a r y at one end, which were suspended i n the r e s p e c t i v e c e l l s . 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 7 8 and a p l a t i n i z e d platinum sheet or a P t / I r a l l o y wire were only of the order 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 very simple, convenient, and p r a c t i c a l e l e c t r o d e was constructed - a copper wire was dipped i n t o the e l e c t r o l y t e s o l u t i o n . Copper/copper s u l f a t e e l e c t r o d e s have been described i n the l i t e r a t u r e , but r e f e r only to saturated s o l u t i o n s of weak a c i d strength [349,350]. The electrodes were constructed from (approximately) 30 cm lengths of 0.020 inch diameter copper w i r e , which were c o i l e d i n order to f i t w i t h i n the 6 mm 0.0. Pyrex Luggin C a p i l l a r y tube, and pre-cleaned i n d i l u t e n i t r i c a c i d s o l u t i o n . A r e s e r v o i r of e l e c t r o l y t e , 2M h^SO^ + 0.5M CuS0 4, was employed to maintain a s l i g h t flow of f r e s h e l e c t r o l y t e past the wire. No rigorous attempt was made to exclude the atmosphere from the e l e c t r o l y t e , with the r e s u l t f a i l u r e could 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 of the copper wire. The e f f e c t s of v a r i a t i o n s of the e l e c t r o l y t e composition were assessed f o r the copper/copper s u l f a t e e l e c t r o d e , and are given i n Appendix A l . A l l reference e l e c t r o d e s were continuously monitored during the course of the experimental work to ensure that t h e i r p o t e n t i a l s d i d not vary and to determine the e f f e c t s of any a c c i d e n t i a l contamination. 4.1.5 Electrochemical Instrumentation Several power s u p p l i e s were u t i l i z e d during the course of t h i s work. For long-term g a l v a n o s t a t i c experiments, regulated DC power sup p l i e s by Anatek (models 50-1S and 50-1D, maximum output: lA/50v) were s u f f i c i e n t f o r use with the d i s c e l e c t r o d e s . For l a r g e r - a r e a e l e c t r o d e s , which required 79 l a r g e r currents to provide equivalent c u r r e n t d e n s i t i e s , a Hewlett Packard model 6256B power supply (maximum output: 24 A/12v) was employed. Runs r e q u i r i n g c u rrents smaller than 30 mA were performed with a Beckman Electroscan 30 as the other power sup p l i e s could not be c a l i b r a t e d at low c u r r e n t outputs. Short-term g a l v a n o s t a t i c experiments were performed with a Wenking P o t e n t i o s t a t Model 68 FRO.5 operated 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 working 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 across the r e s i s t o r ( r a t h e r than between the reference and working el 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 surface area studies were performed with the p o t e n t i o s t a t operat-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 with the same p o t e n t i o s t a t , operated normally, with a f o u r - l e a d system. P o t e n t i a l measurements during long-term g a l v a n o s t a t i c experiments were made with a K e i t h l e y 630 Potentiometric Electrometer, which had an input impedance of 10 1 I f ohms. In short-term surface charge and surface area measurements, p o t e n t i a l s were recorded with a Honeywell E l e c t r o n i k 194 recorder, which was p a r t i c u l a r l y useful due to i t s expanded-range cap-a b i l i t i e s . S h i e l d e d , non-current c a r r y i n g "reference" and "sense">leads were used to minimize IR drops i n the leads. Currents were measured during p o t e n t i o s t a t i c experiments both on the ammeter provided on the Wenking 68 FRO.5 and with a K e i t h l e y 153 M i c r o v o l t Ammeter, which could be used f o r cu r r e n t s s m a l l e r than 1A and a l s o permitted measurement of currents lower than a yA. High currents were measured with the ammeter provided on the Wenking 68 FRO.5. C a l i b r a t i o n of the g a l v a n o s t a t i c power sup p l i e s was performed p r i o r to each run, with the output 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 to vary i n output from run to run, r e q u i r i n g c a l i b r a t i o n against a potentiometer p r i o r to each experiment. 4.1.6 Other Apparatus Wavelength-dispersive X-ray spectrometry was performed with a Norelco spectrometer which was operated at 35 kv with a tungsten tube. X-ray d i f f r a c t o m e t r y was performed with a P h i l i p s d i f f r a c t o m e t e r and mono-chromatic CuKa r a d i a t i o n . Morphology s t u d i e s i n v o l v e d the use of an Etec Autoscan scanning e l e c t r o n microscope (SEM) which was a l s o equipped with an energy-dispersive X-ray spectrometer. Chapter 5 PROCEDURE 5.1 Measurement of Anode Loading and Surface Composition Noble metal loadings were determined by an X-ray fluorescence spectroscopic technique, whereby the i n d i v i d u a l specimens were i r r a d i a t e d by continuous tungsten X - r a d i a t i o n and the i n t e n s i t i e s of 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 - r a d i a t i o n (PtLqi and I r L a i ) were measured. The method r e l i e s on the coating 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 (corresponding to a loading of 100-200 g/m2) as 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 are consequently r e l a t e d to the absolute amount of noble metal present. As there i s a high l e v e l of u n c e r t a i n t y with respect to the act u a l loadings of 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 of i r i d i u m and platinum may not be constant as a r e s u l t of anodic o p e r a t i o n , the use of Pt/30 I r - T i "standards" from which the loadings and composition of unknown specimens could be determined was considered u n s u i t a b l e . Instead, i t was decided to approach the problem from a fundamental parameter c a l c u l a t i o n of X-ray i n t e n s i t i e s , a p p l i c a b l e to the e n t i r e range of P t / I r a l l o y compositions and loa d i n g s , and to assess the r e l i a b i l i t y of t h i s method by comparison with platinum coated t i t a n i u m standards prepared by s p u t t e r r i n g platinum on t i t a n i u m , and whose loadings 81 82 were determined by weight gain measurements. The d e t a i l s of the fundamental parameter c a l c u l a t i o n , preparation of standards and the conversion of measured i n t e n s i t y data to a n a l y t i c a l data i s described i n Appendix A2. 5.2 Surface Charge and Surface Area Studies A l l surface charge and surface area experiments were performed i n two-compartment c e l l s c o n t a i n i n g 2M H 2S0\ e l e c t r o l y t e swept with helium gas. For f r e s h 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 done by immersion f o r several minutes i n a c h r o m i c / s u l f u r i c a c i d s o l u t i o n , followed by r i n s i n g with d o u b l e - d i s t i l l e d water. A f t e r immersion i n the e l e c t r o l y t e and connec-t i o n of the leads to the power supply the c h a r t recorder, the f o l l o w i n g procedure was adopted f o r surface area 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 t o 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 oxygen 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 the c a t h o d i c c y c l e . 2. H e l i u m sweeping and s t i r r i n g were 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 seconds to remove d i s s o l v e d gases produced 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 the sweep-ing and s t i r r i n g were s t o p p e d and the 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 r o s e o n l y s l i g h t l y above Ov (RHE) i n t h i s t i m e . 3) A n o d i c / C a t h o d i c c u r r e n t c h a r g i n g a t a p r e d e t e r m i n e d c u r r e n t magnitude. The i n c r e a s e i n the 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 the 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 were employed) and the 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 the p o t e n t i a l o f imminent e v o l u t i o n o f oxygen gas. The d e c r e a s e i n p o t e n t i a l was then 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 a n o d i c / 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 the 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 c a t h o d i c c y c l e . ( E x cept f o r the case o f 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 oxygen-d e p o s i t i o n charge was used.) D e t a i l s o f the s u r f a c e a r e a c a l c u l a t i o n a r e g i v e n i n Appendix A5. For surface charge s t u d i e s with new e l e c t r o d e s , two sets of 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 p r i o r to commencement o f oxygen e v o l u t i o n , b) oxygen coverage development s i m u l t a n e o u s l y w i t h oxygen e v o l u t i o n . In case ( a ) , the anodic/cathodic charging was performed i n a s i m i l a r manner as i n step (3) above, except that the anodic l i m i t f o r current 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 and the p o t e n t i a l f o r imminent oxygen e v o l u t i o n (about 1.55v 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 given cur r e n t d e n s i t y f o r a predetermined time, which v a r i e d from seconds to days. During such p o l a r i z a t i o n , helium sweeping and s t i r r i n g was maintained to remove the a n o d i c a l l y generated oxygen gas. On termination of the anodic p o l a r i z a t i o n the e l e c t r o d e was kept at o p e n - c i r c u i t f o r 100 seconds with helium sweeping and s t i r r i n g maintained to remove f u r t h e r traces of 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 to a value between 1.3 and 1.5v i n t h i s time. A cathodic " s t r i p p i n g " charge was then imposed and the charge curve recorded. The t r a n s i t i o n time f o r removal of the oxygen l a y e r provided the basis f o r the c a l c u l a t i o n of the surface 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 operated i n long-term g a l v a n o s t a t i c runs i n single-compartment c e l l s with s u l f u r i c a c i d or s u l f u r i c a c i d plus copper s u l f a t e e l e c t r o l y t e s r e quired removal of the e l e c t r o d e from the single-compartment c e l l on termination 84 of the run, followed 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 water, t r a n s f e r to the two-compartment c e l l where helium sweeping and s t i r r i n g were maintained, and e l e c t r i c a l connection. The procedure was then i d e n t i c a l to t h a t f o r case (b) described above. A f t e r s t r i p p i n g of the oxygen l a y e r formed during the prolonged anodic p o l a r i z a t i o n , the surface area could then be assessed by r e p e t i t i v e anodic/cathodic 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 constant p o t e n t i a l or a constant c u r r e n t , the r e s u l t a n t c u r r e n t or p o t e n t i a l of an oxygen-evolving noble metal e l e c -trode i s not observed to a t t a i n a steady value - 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 p o s s i b l e to determine a "steady s t a t e " p o l a r i z a t i o n curve f o r oxygen e v o l u t i o n . S a t i s f a c t o r y "non-steady s t a t e " p o l a r i z a t i o n curves can be obtained, however, with p r e l i m i n a r y p o l a r i z a t i o n at the highest p o t e n t i a l or c u r r e n t employed, followed by downward p o t e n t i a l or current stepping at a constant r a t e . Under these co n d i t i o n s the nature of the surface formed under the extreme c o n d i t i o n remains constant, and the c u r r e n t or p o t e n t i a l show l i t t l e v a r i a t i o n with time at the lower s e t t i n g s . 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 curve deter-minations the i n i t i a l p o t e n t i a l was g e n e r a l l y 2.0v, maintained f o r 300 to 10,000 seconds, followed by downward stepping at 20 mv/50 sec (a convenient rate f o r manual o p e r a t i o n ) . For g a l v a n o s t a t i c p o l a r i z a t i o n curves the e l e c t r o d e was t y p i c a l l y subjected to a 300 second pre-anodization at 114 mA/cm2 (based on geometric a r e a ) , followed by downward stepping at 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 eel 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 to determine the p o t e n t i a l vs. cur r e n t r e l a t i o n when only a s i n g l e e l e c t r o d e process i s involved (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 the 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 order f o r t i t a n i u m to manifest 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 the e l e c t r o l y t e , the naturally-formed oxide f i l m must be removed and the e l e c t r o l y t e must be maintained oxygen-free. This was accomplished by chemical pretreatment i n a h y d r o f l u o r i c a c i d / n i t r i c acid/water s o l u t i o n and by sweeping the e l e c t r o l y t e w i t h hydrogen gas. The p o l a r i z a t i o n curve was subsequently traced by stepping i n an upward d i r e c t i o n a t a rat e of 20 mv/100 sec which gave good separation of the a c t i v e and passive regions. The maximum p o t e n t i a l output of the p o t e n t i o s t a t could be increased to 100 v o l t s by b i a s i n g with the dual Anatek model 50-1D power supply. A l l p o l a r i z a t i o n curves were determined i n a two-compartment eel 1. 5.4 Long-Term E l e c t r o l y s i s I n d i v i d u a l (uninterrupted) runs of several hundreds or even thousands of hours' duration were made i n order to provide information concerning 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 degradation of the 8 6 Pt/30 I r - T i anodes. The el e c t r o d e s were ch e m i c a l l y pretreated i n chromic/ 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 water p r i o r to immersion i n the 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 . No elec t r o c h e m i c a l pretreatments were employed, and the working e l e c t r o d e was allowed to d r i f t a t o p e n - c i r c u i t p r i o r to commencement of the run. E l e c -t r o l y s i s began ( t = 0) when the c i r c u i t was completed to the ( p r e - c a l i b r a t e d ) power supply. This method was ne c e s s i t a t e d by an undesirable c h a r a c t e r i s t i c of the constant c u r r e n t power s u p p l i e s - namely t h a t the output leads were shorted when the power supply 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 formation of a bat t e r y with the working and a u x i l i a r y e l e c t r o d e s . In order to cope with the problem of power f a i l u r e s , which could cause the anode to discharge, a diode was placed i n the c i r c u i t to prevent the reversed flow of c u r r e n t . Anode p o t e n t i a l s were monitored with respect to time at 10 second i n t e r v a l s f o r the f i r s t 100 seconds, and at longer i n t e r v a l s t h e r e a f t e r . A f t e r about an hour the rate of change of the anode p o t e n t i a l was so small t h a t only i n f r e q u e n t monitoring was necessary. A l l e l e c t r o l y t e s were s t i r r e d because of the tendency f o r d e n s i t y - d i f f e r e n c e s to 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 . (In copper-containing s o l u t i o n s a sharp separation otherwise occurred with 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 zone.) Runs were terminated " l i v e " by withdrawing the working e l e c t r o d e from the c e l l w h i l e i t was s t i l l 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 d o u b l e - d i s t i l l e d water. 8 7 5.5 Electrode 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 order to study the progressive changes i n such parameters as noble metal l o a d i n g , 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 area, anodic deposits and surface morphology i t was necessary to subject Pt/30 I r - T i anodes to r e p e t i t i v e long-term runs, with such c h a r a c t e r i z a t i o n being made between runs. On " l i v e 1 ! termination of long-term runs the ele 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 water ( i n some cases the surface charge and area were measured i n a two-compartment c e l l , f o l l o wed by removal to storage i n d o u b l e - d i s t i l l e d water). A f t e r r i n s i n g the e l e c t r o d e s were removed and separated from t h e i r Teflon/Pyrex holders, with care taken not to touch or otherwise damage the e l e c t r o d e surface. Several procedures followed: (1) X-ray d i f f r a c t i o n . An i n t e n s i t y vs 28 scan r e v e a l e d any changes i n the c r y s t a l l i n e n a t u r e o f the anode s u r f a c e s - i n p a r t i c u l a r , t he pre 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 were f o u n d , the 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) below. (2) 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 . Theamount o f l e a d on the anode s u r f a c e was e s t i m a t e d from the i n t e n s i t y o f t h e PbLai peak. (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 degree o f c o v e r a g e o f the l e a d -c o n t a i n i n g d e p o s i t s . (4) Treatment 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 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 . (5) 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 a c c o r d i n g t o the p r o c e d u r e s d e s c r i b e d i n Appendix A2. ( 6 ) SEM o b s e r v a t i o n o f the morphology o f t h e anode s u r f a c e . ( 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 88 p r i o r t o commencement o f the 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 f r e s h e l e c t r o l y t e . R e p e t i t i v e runs were continued i n many cases to anode f a i l u r e , where the anode p o t e n t i a l rose to a high value, under which c o n d i t i o n s the power-supplies switched from cu r r e n t to eel 1-potential c o n t r o l with subsequent operation at the voltage l i m i t of the su p p l i e s (50v i n the present case). 5.6 Pulsed E l e c t r o l y s i s Pulsed e l e c t r o l y s i s runs were of long-term duration and were performed i n s i n g l e compartment c e l l s as described above. Two types of pulsed e l e c t r o l y s i s were employed: (1) P e r i o d i c c u r r e n t r e v e r s a l ( t 0 N / t S H 0 R T E n ) ( 2 ) P e r i o d i c open c i r c u i t ( t n . . / t r . I - f - ) UN U r r 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 shorted during the " ^ S H O R T E D " P E R ' ' O C ' > which r e s u l t e d i n the flow of a non-steady reverse current as a r e s u l t of "discharge" of the e f f e c t i v e b a t t e r y formed with connection of these two e l e c t r o d e s . In the l a t t e r case the working e l e c t r o d e was maintained a t o p e n - c i r c u i t during the " t n F F " period. 5.7 Anode P o t e n t i a l C o r r e c t i o n s In order to present the recorded anode p o t e n t i a l s i n a s i n g l e form, namely with respect to the standard 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 the 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 the reference e l e c t r o d e p o t e n t i a l s from the value of the 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 procedure, i n v o l v i n g the 8 9 s u b t r a c t i o n of the c a l c u l a t e d p o t e n t i a l drops across known "thicknesses" 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. The IR-drop c o r r e c t i o n s are summarized i n Appendix A3 f o r the common e l e c t r o l y t e s employed i n the present work. C a l c u l a t i o n s of the p o t e n t i a l s of the r e v e r s i b l e hydrogen e l e c t r o d e and of the copper/copper s u l f a t e e l e c t r o d e require a knowledge of i n d i v i d u a l i o n i c a c t i v i t i e s . As these q u a n t i t i e s cannot be measured, they must be estimated from t a b u l a t e d values of mean a c t i v i t y c o e f f i c i e n t . D e t a i l s of the methods used to determine both the concentrations and i n d i v i d u a l 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 hence to estimate the i n d i v i d u a l ion a c t i v i t i e s , are given i n Appendix A4. The value of the reference 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 values i n t o the Nernst equation 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 converted to values vs. SHE by means of the r e s u l t s of "method 2" described i n Appendix A4. Chapter 6 RESULTS 6.1 D e s c r i p t i o n of New Electrodes 6.1.1 Surface Areas New e l e c t r o d e s were considered to be e l e c t r o d e s which had not been subjected to anodic treatment of any k i n d , except i n the case of surface area measurement where c e r t a i n anodic operations are a p r e r e q u i s i t e f o r t h a t p a r t i c u l a r case. Samples were taken from sheets of Pt/30 I r - T i (nominal composi-t i o n ) material of various loadings and from d i f f e r e n t manufacturing l o t s , and t h e i r noble metal loadings and 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 were determined by the X-ray fluorescence spectroscopy and g a l v a n o s t a t i c 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 . The surface area (expressed as a roughness f a c t o r ) was p l o t t e d against the determined lo a d i n g f o r each el e c t r o d e type, as shown i n Figure 6.1. (Each point represents the average of several such determinations on a given electrode m a t e r i a l . ) As can be seen, no r e g u l a r r e l a t i o n i s obeyed, probably as a r e s u l t of d i f f e r e n c e i n manufacturing procedures (although a l l were produced by the same thermal decomposition process). In g e n e r a l , however, higher-loading 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 of the e l e c t r o d e s employed i n 90 91 Figure 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 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 manu-f a c t u r i n g l o t s . (The t r i a n g l e corresponds to the material used i n the present study.) 92 the present work were obtained from a s i n g l e sheet of 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 Figure 6.1, and showed roughness f a c t o r s of 40 ± 3. 6.1.2 Loadings I n d i v i d u a l disc-shaped e l e c t r o d e s cut from the sheet which was the source of the e l e c t r o d e s i n the present study showed the f o l l o w i n g mean t o t a l noble metal loading and platinum weight f r a c t i o n : L o a d i n g = 4.337 ± .203 g/m2 w p t = 0.687 ± .003 As can be seen, the loadings on new d i s c e l e c t r o d e s vary somewhat from d i s c to d i s c as a consequence of the nonuniformity of the c o a t i n g , but the compositions are r e l a t i v e l y constant, i n d i c a t i n g good d i s p e r s i o n of the coating c o n s t i t u e n t s , at l e a s t on the s c a l e of the d i s c s (1.92 cm 2 geometric area). Loadings of other Pt/30 I r - T i e l e c t r o d e s used i n t h i s work are as shown i n Figure 6.1, ranging from 2.3 to 20.0 g/m2. 6.1.3 D i f f r a c t o m e t r y 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 with CuKa r a d i a t i o n ) with new Pt/30 I r - T i anode material are summarized i n Table 6.1. Peaks corresponding to platinum and t i t a n i u m 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 at d = 3.20 corresponds c l o s e l y to the strongest T i 0 2 ( r u t i l e ) l i n e at d = 3.24 angstroms. Titanium peaks appear because the coating i s both imperfect and t h i n . The t i t a n i u m peaks are c o n s i d e r a b l y attenuated with the higher-loading m a t e r i a l , however. The t i t a n i u m peaks do not f o l l o w the ASTM Index 93 Table 6.1 I d e n t i f i c a t i o n of X-ray D i f f r a c t i o n Peaks Observed with new Pt/30 I r - T i Anodes of Nominal 5 and 20 g/m2 noble Metal Loadings (Di f f r a c t o m e t e r Scan Range: 10-90° 29 with CuKa Radiation) Peak From Diffractometer-Chart d(A) Comments I d e n t i f i c a t i o n (ASTM Index) 5 g/m2 Anode 20 g/m2 Anode Species I/I i hkl d(A) 3.20 weak,broad weak,broad T i 0 2 ( r u t i l e ) 100 110 3.24 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 i ntense,complex -< f T i ; pt 100 100 01.1 111 2.24 2.26 1.95 medium,broad medium,broad pt 53 200 1.96 1.72 intense,sharp weak,broad Ti 19 01.2 1.73 1.47 weak,sharp (not present) Ti 17 11.0 1.47 1.38 mediumbroad mediumbroad Pt 31 220 1.39 1.33 intense,sharp medium,sharp Ti 16 10.3 1.33 1.25 weak (not present) Ti 16 11.2 1.25 1.23 weak (not present) 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 94 order of r e l a t i v e i n t e n s i t i e s due to p r e f e r r e d o r i e n t a t i o n of the sheet. o From the most intense peak (1.33A), assuming that these planes are o r i e n t e d most favourably f o r d i f f r a c t i o n ( p a r a l l e l to the specimen s u r f a c e ) , i t can be estimated t h a t the basal planes i n the t i t a n i u m substrate are o r i e n t e d at 28° to the surface. On the assumption that platinum and i r i d i u m from a random s u b s t i -t u t i o n a l 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 c r y s t a l 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 spacings can be estimated 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 parameter i s d i r e c t l y p r o p o r t i o n a l to the atomic percentages of the a l l o y c o n s t i t u e n t s . The r e s u l t s of these c a l c u l a t i o n s , together with actual measurements on Pt/30 I r - T i c o a t i n g s , are shown i n Table 6.2. As can be seen, the measured peaks, while c l o s e enough to those of pure platinum to permit t h e i r iden-t i f i c a t i o n as platinum, a c t u a l l y correspond 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 of platinum and i r i d i u m . F u r t h e r , since there i s no reason why Vegard's law should be adhered to (and many metals which form continuous s o l i d s o l u t i o n s deviate both p o s i t i v e l y and n e g a t i v e l y from i t ) , i t i s l i k e l y t h a t the measured peaks are indeed produced from a random s u b s t i t u t i o n a l s o l i d s o l u t i o n of the two coating metals having the l a t t i c e parameter: a = 3.909 angstroms. The broad nature of the P t / I r peaks i s l i k e l y a consequence of the thinness of the coatings (a 5 g/m2 f i l m has an average thickness of about 0.4 microns) and the r e l a t i v e l y low temperatures achieved ( l e s s than 500°C) during manufacture [183] which would tend to produce a very f i n e - g r a i n e d m a t e r i a l . 95 Table 6.2 L a t t i c e Parameters f o r I r i d i u m and Platinum 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 Specimens L a t t i c e Parameters and d-Spacings (Angstroms) I r Pt C a l c u l a t e d Pt/30 I r Measured Pt/30 I r - T i a 3. 8389 3 .9237 3.8983 3.909 d m 2. 2170 2 .265 2.251 obscured by Ti peak d 2 o o 1. 9197 1 .962 1.949 1.953 d 3 i i 1. 1574 1 .1826 1.175 1.180 d 2 2 0 1. 3575 1 .387 1 .378 1.382 d 2 2 2 1. 1082 1 .1325 1.125 1.128 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 diminished i n any way on going from the 5 g/m2 to the 20 g/m2 loading m a t e r i a l , which suggests that the oxide may not only a r i s e from the f i l m e d t i t a n i u m surface where i t i s exposed, but may be incorporated i n the c o a t i n g . The source of the oxide, however, must be the substrate as no t i t a n i u m species are involved i n the organic s o l u t i o n used i n the thermal decomposition manufacturing process [174]. In order to d e f i n i t e l y e s t a b l i s h the nature of the species c o r r e -sponding to 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 with s u i t a b l e specimens being prepared by scraping the anode surface w i t h a s c a l p e l , and catching the m a t e r i a l which was removed on a copper/carbon g r i d . No noble metal d i f f r a c t i o n patterns were observed, probably as a consequence of the high r e l a t i v e thickness and high absorption c h a r a c t e r i s t i c s of the " f l a k e s " of coating metal. The observed r i n g patterns corresponded c l o s e l y to that of r u t i l e (Table 6.3), thus confirming the e a r l i e r assumption. 6.1.4 Morphology SEM observations were made with an etched, uncoated t i t a n i u m substrate and several new Pt/30 I r - T i anodes i n order to provide a basis f o r comparison with anode surfaces a f t e r a ppreciable c o r r o s i o n or u l t i m a t e degradation has taken place. The base t i t a n i u m surface (Figure 6.2) i s very rough and shows hexagonal " g r a i n s " over i t s surface i n an o r i e n t a t i o n which agrees well with the p r e f e r r e d o r i e n t a t i o n estimated from 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 . 97 Table 6.3 El e c t r o n D i f f r a c t i o n Results Observed Rings R u t i l e ( T i 0 2 ) d(A) I n t e n s i t y d(A) I/I i hkl 3.29 weak 3.245 100 n o 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 Figure 6.2. S.E.M. view of the ti t a n i u m substrate surface as etched p r i o r to thermal decomposition of the noble metal c o a t i n g . (lOOOx) Figure 6.3. S.E.M. view of a t y p i c a l new 4.33 g/m2 noble metal loading anode surface. (400x) 99 Coated e l e c t r o d e s reveal considerable heterogeneity. Perhaps the best 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 "desert mud" where the co a t i n g material becomes cracked and f i s s u r e d as a r e s u l t of the thermal decomposi-t i o n process. The f i s s u r r e d t e x t u re i s most prevalent i n the " v a l l e y s " where gre a t e r amounts of the co a t i n g m a t e r i a l would accumulate during manu-f a c t u r e , r e s u l t i n g i n l o c a l l y greater thicknesses. Figures 6.3 and 6.4 are t y p i c a l of new 4.3 g/m2 loading anodes. The substrate i s often exposed between i n d i v i d u a l coating " p l a t e s , " and to a l e s s e r extent on the "peaks" where coating thicknesses are smaller (as determined by selected-area 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 successive a p p l i c a t i o n of the noble metal coating i s manifested by the appearance of co a t i n g " p l a t e s " randomly covering other coating " p l a t e s " (whereas only a s i n g l e f i s s u r e d l a y e r would be expected f o r a s i n g l e coating a p p l i c a t i o n ) . Greater loadings (Figure 6.5) are c h a r a c t e r i z e d by considerable degree of p o r o s i t y of the co a t i n g i t s e l f . 6.2 Corrosion of Noble Metal Coated Anodes 6.2.1 Changes i n Loading and Composition Treatment of the c o r r o s i o n data generated from long-term runs i n various e l e c t r o l y t e s i s best approached by c o n s i d e r a t i o n of 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 three groups. These are: 1. R e p r o d u c i b l e 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 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 . 3 . A n o d i c e l e c t r o l y s i s t o complete d e g r a d a t i o n . Figure 6.5. S.E.M. view of a t y p i c a l new 20 g/m2 noble metal l o a d i n g anode sur f a c e . (lOOOx) 101 A given anode passes through a l l three of these stages during r e p e t i t i v e runs over i t s operating l i f e t i m e . Only behaviour t y p i c a l of the f i r s t two stages w i l l be t r e a t e d here, with the case of complete degradation being considered i n Section 6.5. Platinum and i r i d i u m l o a d i n g s , as determined by the X-ray f l u o r e s -cence spectroscopic technique, were found to decrease p r o g r e s s i v e l y with time of anodic p o l a r i z a t i o n , with greater decreases o c c u r r i n g with higher c u r r e n t d e n s i t i e s . Changes i n platinum loading with time are summarized i n Figures 6.6-6.9, which present data f o r i n d i v i d u a l e l e c t r o d e s subjected to r e p e t i t i v e e l e c t r o l y s i s runs. In most cases, the r e p e t i t i v e runs were continued f o r longer times than depicted on Figures 6.6-6.9, but are not included because complete degradation occurred. With the exception of runs below 50 mA/cm2 (geometric a r e a ) , where the l i f e t i m e s of i n d i v i d u a l anodes are very long, the data i n Figures 6.6 to 6.9 can be s a i d to d e p i c t the complete loading vs. time behaviour 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 given anodes. From the v a r i a t i o n i n the weight f r a c t i o n of platinum i n the P t / I r a l l o y c o a t i n g s , as shown i n Figure 6.10-6.13, the v a r i a t i o n i n i r i d i u m loading with time can be deduced from the corresponding p o i n t s on Figures 6.6-6v9. The loading vs. 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 completely analogous to that f o r platinum. Of more i n t e r e s t , however, i s the observation (with a few exceptions) that the weight f r a c t i o n of platinum decreases p r o g r e s s i v e l y with the time of e l e c t r o l y s i s which i n d i c a t e s that the platinum loading 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 . Further, l a r g e r decreases i n the platinum weight f r a c t i o n are found to occur with higher a p p l i e d c u r r e n t d e n s i t i e s (geometric area). o 7.8 mA/cm of5.6 •299 A 52.1 T I M E , H O U R S Figure 6.6. V a r i a t i o n i n platinum l o a d i n g i n P t / I r a l l o y coatings f o r i n d i v i d u a l anodes with e l e c t r o l y s i s r o time i n 2M r ^ S C t + 0.5M CuSCU, 22°, at various (geometric) current d e n s i t i e s . I 3 f CM e > 2 < O T I M E 1 0 0 0 H O U R S 2 0 0 0 o Figure 6.7. V a r i a t i o n i n platinum loading f o r P t / I r a l l o y coatings f o r i n d i v i d u a l anodes with e l e c t r o l y s i s 0 0 time i n 2M H2SO4 + 0.5M CuSOi*, 40°, at various (geometric) current d e n s i t i e s . A 7.8 mA/cm • 15.6 • 52.1 OI04 ! I I 1 1 1 1 i 0 1 0 0 0 2 0 0 0 I T I M E H O U R S i o Figure 6 .8 . V a r i a t i o n i n platinum l o a d i n g i n P t / I r a l l o y coatings f o r i n d i v i d u a l anodes with e l e c t r o l y s i s 4 : 1 time i n 2M H 2S04, 22°, at various (geometric) current d e n s i t i e s . Figure 6.9. V a r i a t i o n i n platinum loading i n P t / I r a l l o y coatings f o r i n d i v i d u a l anodes with e l e c t r o l y s i s time i n 2M H 2S0\, 40°, at various (geometric) current d e n s i t i e s . o cn Figure 6.10. V a r i a t i o n of the weight f r a c t i o n of platinum i n P t / I r a l l o y coatings f o r i n d i v i d u a l anodes with e l e c t r o l y s i s time i n 2M H 2 S O 4 + 0.5M CuS0 4, 22°, at various (geometric) c u r r e n t d e n s i t i e s . o • 7 0 r z O H . 6 5 < o r x LU .60 o 15.6 m A / c m 2 a 52.1 _ ^ 52.1 V I 0 4 • 2 6 0 ^ {\ i i i i 0 T I M E 1 0 0 0 H O U R S 2 0 0 0 Figure 6.11. V a r i a t i o n of the weight f r a c t i o n of Pt in P t / I r a l l o y c o a t i n g s with 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°, at various (geometric) c u r r e n t d e n s i t i e s . . o T I M E , H O U R S Figure 6.12. V a r i a t i o n of the weight f r a c t i o n of platinum 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 , 22°, at various (geometric) c u r r e n t d e n s i t i e s . Figure 6.13. V a r i a t i o n of the weight f r a c t i o n of platinum i n P t / I r a l l o y coatings f o r i n d i v i d u a l anodes withS e l e c t r o l y s i s time i n 2M H 2S0\, 40°, at various (geometric) c u r r e n t d e n s i t i e s . n o In one p a r t i c u l a r case with an anode cut from the "4.33 g/m2" loading sheet, the r a t i o of the c o a t i n g elements remained p r a c t i c a l l y unchanged a f t e r repeated e l e c t r o l y s i s at 52.1 mA/cm2 (geometric area) i n 2M H 2S0 4, 22°. This case w i l l be d e a l t with separately below, and i s hence presented i n a separate f i g u r e (Figure 6.14), as i t suggests a pre-dominantly mechanical, or s p a l l i n g , mode of coating l o s s . 6.2.2 Surface Area Changes The anodic/cathodic treatments employed i n the e l e c t r o c h e m i c a l measurement of surface areas may themselves be r e s p o n s i b l e f o r enhanced coating metal losses and i n changes i n the surface area i t s e l f , although the a c c u r a c i e s of the coating measurement technique ( c o e f f i c i e n t of v a r i a t i o n i s 1.1%)and of the surface area measurements ( c o e f f i c i e n t of v a r i a t i o n i s 6.2%) do not permit determination of small 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 surface area measurement procedures were examined, and no s t a t i s t i c a l l y meaningful changes i n loading or surface area were detected, but t h i s does not preclude the p o s s i b i l i t y of the occurrence of such changes. For t h i s reason the e l e c t r o d e s used i n the long-term c o r r o s i o n experiments were not subjected to the surface area measurement technique u n t i l l a f t e r completion of t h e i r planned s e r i e s of 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 experiments or i n r e p e t i t i v e experiments (without break-up of the anode assembly between runs) i n order to a s c e r t a i n the e f f e c t of 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 Table 6.4, show th a t the surface area a c t u a l l y increases s l i g h t l y (up to 10 per cent) Figure 6.14. Loading changes and platinum weight f r a c t i o n change with time f o r an anode operated at 52.1 mA/cm2 (geometric area) i n 2M H 2S0\, 22°, which e x h i b i t s coating loss by s p a l l i n g . 112 Table 6.4 E f f e c t of E l e c t r o l y s i s Time i n 2M H2$0h 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 Areas of I n d i v i d u a l Pt/30 I r - T i anodes Current Density (Geometric Area) mA/cm2 I n i t i a l Noble Metal Loading g(Pt + I r ) / m 2 Run Time Hours A- hr I n i t i a l R.F. Fi nal R.F. Fi 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 2.78 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 -(d) 104 4.55 431 86. 2 42 ±3 46 ±3 4.32 113 with time, but u l t i m a t e l y decreases to a value 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 subjected to r e p e t i t i v e e l e c t r o l y s i s at 52.1 mA/cm2 i n 2M h^SCU, t h i s behaviour i s shown i n Figure 6.15. P l o t t i n g of the surface area values of anodes ( a l l i n i t i a l l y cut from the same sheet) used i n the long-term c o r r o s i o n experiments as a f u n c t i o n of t h e i r remaining noble metal loadings (Figure 6.16) reveals t h a t there 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 . Thus, f o r e l e c t r o d e s whose t o t a l noble metal l o a d i n g had dropped to a value below about 3 . 8 g/m2, 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 was found to decrease r a p i d l y to l e s s than h a l f the i n i t i a l area value. Below about 3 g/m2 loading the surface area continued to d e c l i n e , but at a l e s s r a p i d r a t e . I t i s r e a d i l y apparent, thus, that f o r electr o d e s subjected to prolonged anodic 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 , the e l e c t r o -c hemically a c t i v e surface area may change and consequently the actual current d e n s i t y may vary. 6.2.3 Corrosion Rates 6.2.3.1 Rate Expressions The c o r r o s i o n rates 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 from: v . . (change g o a d i n g ) ( i t m t r 1 c a r e a ) „ , v. . ( c h a n g e ^ loading) _ ^ ( a c t u a , a r e a ) ( 2 ) Change i n roughness f a c t o r with- time f o r a Pt/30 Ir-T i . anode ( i n i t i a l noble metal loading 4.27 g/m2) with r e p e t i t i v e e l e c t r o l y s i s at 52.1 mA/cm2 [geometric area), i n 2M H2SOk. Figure 6.16. Roughness f a c t o r s f o r Pt/30 I r - T i anodes as a fu n c t i o n of remaining noble metal loading a f t e r r e p e t i t i v e prolonged e l e c t r o l y s i s runs in 2M H2S0<i and i n 2M H 2S0 4 + 0.5M CuSO,,. (The po i n t s r e f e r to i n d i v i d u a l anodes cut from the same sheet of anode material.). 116 where t i s the run time, w^  the weight f r a c t i o n of c o n s t i t u e n t i assumed to be equ i v a l e n t to 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 are c a l c u l a t e d from Faraday's law: i . = n i F v i (3) on the assumption t h a t the coating metals 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 to an n-electron r e a c t i o n . For t h i s case, n was considered to be four f o r both platinum 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 , expressed as yg/A-hr, was obtained by d i v i d i n g the absolute amount of co a t i n g metal l o s t by the a p p l i e d current and the time. 6.2.3.2 Results of Long-Term Corrosion Experiments Considerable experimental s c a t t e r due to the l e v e l s of u n c e r t a i n t y i n the loa d i n g measurements was found f o r the c o r r o s i o n rates 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 . Although the s c a t t e r was improved by extension of runs to longer times, and thus to greater coating 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 operated under nominally s i m i l a r con-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 r e l i a b l e c o r r o s i o n r a t e values. Thus, the cumulate c o r r o s i o n r e s u l t s of r e p e t i t i v e runs with one or more anodes at each a p p l i e d (geometric) c u r r e n t d e n s i t y 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 . Further, only those e l e c t r o d e s 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 vs. time behaviour on r e p e t i t i v e runs were considered. The r e s u l t s of the rate c a l c u l a t i o n s f o r long-term e l e c t r o l y s i s i n 2M H 2S0\ + 0.5M CuS0\ and i n 2M H 2S0\ at various temperatures 117 are condensed i n Table 6.5-6.8. The data a t 22° and 40° are taken from measurements with d i s c e l e c t r o d e s (1.92 cm 2 geometric area) and those at 60° and 80° from the l a r g e r sheet anodes (22.5 cm 2 geometric area). A l l c o r r o s i o n rates r e f e r to the f r a c t i o n a l surface occupations of platinum and i r i d i u m as discussed above, and the mean p a r t i a l ( c o r r o s i o n ) current d e n s i t i e s are c a l c u l a t e d with respect to the a c t u a l surface areas of the coating metals. For the p a r t i c u l a r case of the anode described i n Figure 6.14, the c o r r o s i o n data are given, 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 Table 6.5-6.8, several 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 both p l a t i n u m and i r i d i u m i n c r e a s e w i t h a p p l i e d c u r r e n t d e n s i t y , a l t h o u g h the s c a t t e r o f the r e s u l t s i s poor. 2 . P l a t i n u m c o r r o d e s a t a g r e a t e r r a t e than does 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 i r i d i u m do not show any dependence on a p p l i e d c u r r e n t dens i t y . 4. There i s no c l e a r temperature-dependence o f the c o r r o s i o n r a t e s , a l t h o u g h the e x p e r i m e n t a l s c a t t e r c o u l d o b s c u r e a r e l a t i v e l y s m a l l t e m p e r a t u r e -dependence . 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 , lower 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 e l e c t r o l y t e , 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 e x p e r i m e n t a l s c a t t e r . 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 platinum and i r i d i u m i n 2M H2SG\ + 0.5M CuSO^ e l e c t r o l y t e at both 22° and 40° are p l o t t e d i n Figures 6.17-6.18, with the h o r i z o n t a l l i n e s representing the weighted means of the Table 6.5 Platinum Corrosion Results from Cumulative E l e c t r o l y s i s Runs i n 2M H 2S0 4 + 0.5M CuSCU Temperature Current Density ^ g e o m e t r i c ^ mA T o t a l Time Hours Corrosion E f f i c i e n c y Corrosion Rate (Geometric Area) Corrosion Rate (Actual Area) Mean Actual Current Mean Corrosion Current °C A«hr '+ ug Density mA Density nA cm 2 A«hr hr •rrr hr«mz cm 2 cm2 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 30.0 2579 148 1.16 .06 5.8 29 18 1 1.04 0.99 52.1 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 40 15.6 387 5.8 0.83 .80 189 180 5 5 0.39 0.27 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 60 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 0.66 108 98 238 1.35 .55 2170 860 54 22 2.69 2.97 80 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 0.66 108 99 238 1.11 .55 1770 880 44.5 22 2.69 2.45 Table 6.6 Ir i d i u m Corrosion Results from Cumulative E l e c t r o l y s i s Runs i n 2M H2S0^ + 0.5M CuSGs Temperature Current Density ^ 1 geometric^ mA Total Corrosion E f f i c i e n c y Corrosion Rate (Geometric Area) Corrosion Rate (Actual Area) Mean Actual Current Density mA Mean Corrosion Current °C Time Hours A«hr + + + Density nA cm 2 A-hr hr-mz hr«mz cm 2 cm 2 22 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 40 15.6 387 5.8 0 .40 0 200 0 5 0.39 0.00 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 60 16.1 1210 435 0.30 .14 149 71 3.5 1.5 0.40 0.20 53.8 194 233 0.31 .26 498 440 12.5 11 1.35 0.70 108 98 238 0.56 .27 1850 860 46 22 2.69 2.57 80 16.1 194 70.4 1 .48 .96 111 460 18 11.5 0.40 1.00 53.8 260 315 0.45 .20 739 330 18.5 8 1.35 1.03 108 99 238 0.52 .27 1700 880 42.5 22 2.69 2.37 Table 6.7 Platinum Corrosion Results from Cumulative E l e c t r o l y s i s Runs i n 2M H2SQ\ Temperature °C Current Density ^ g e o m e t r i c ^ mA Total Time Hours A-hr Corrosion E1 " f i c i e n c y Corrosion Rate (Geometric Area) Corrosion Rate (Actual Area) Mean Actual Current Density mA ..Mean Corrosion Current Density _nA cm2-^9 + + + cm2 A«hr hr-m2 hr-m2 cm 2 22 7.8 392 5.8 1.34 1.4 152 200 5 5 0.21 0.27 15.6 863 25.9 0.59 .44 133 99 3.5 2.5 0.39 0.19 52.1 1035 104 0.35 .11 264 78 7 2 1.30 0.38 104 2110 422 0.75 .05 1170 70 49 3 3.60 2.69 260 .188 94 0.92 .12 3530 460 88 11 6.51 4.84 521 49.4 49.4 1.92 .15 14800 1200 890 70 31.2 48.9 40 15.6 1633 49.0 0.67 .23 150 52 13.5 1.5 0.39 0.74 52.1 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 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 Ir i d i u m Corrosion Results from Cumulative E l e c t r o l y s i s Runs i n 2M H2S0\ Temperature Current Density ^ g e o m e t r i c ^ mA Total Time Corrosion E-' f i c i e n c y Corrosion (Geometric Rate -f Area) Corrosi (Actual on Rate Area) Mean Actual Current Density Mean Corrosion Current °C Hours A«hr ug + 4- + mA Density nA cm2 A«hr hr«m2 hr-m2 cm 2 cm2 22 7.8 392 5.8 1.84 .90 463 200 1:2 6 0.20 0.67 15.6 863 25.9 0.49 .20 248 99 6.5 2.5 0.39 0.36 52.1 1035 104 0.30 .05 519 77 14 2 1.30 0.78 104 2110 422 0.17 .04 551 70 24 3 3.60 1 .34 260 188 94 0.23 .06 1930 450 48 11 6.51 2.68 521 49.4 49.4 0.79 .08 12600 1200 756 70 31.2 42.2 40 15.6 1633 49.0 0.34 .10 172 52 4.5 1.5 0.39 0.25 52.1 2417 242 0.23 .02 359 30 13 1 1.88 0.73 104 810 162 0.20 .04 695 120 17.5 2.5 2.60 0.98 260 211 106 0.15 .05 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 In d i v i d u a l and Cumulative Corrosion Data f o r a Pt/30 I r - T i anode, Operated at 52.1 mA/cm2 (Geometric Area) i n 2M h^SO^, 22°, which E x h i b i t s Coating Loss by S p a l l i n g Run Time Hours Pt Corrosion Rate (Geometric Area) hr *m I r Corrosion Rate (Geometric Area) hr*m V l r v P t Pt Corrosion E f f i c i e n c y M k'hr I r Corrosion E f f i c i e n c y M9 A«hr I r : P t E f f i c i e n c y Ratio W P t W l r w P t 0 - - • - - - 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 Figure 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 respect to geometric area) from the cumulate c o a t i n g l o s s r e s u l t s of 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 2 S0i, + 0.5M CuSCV Figure 6.18. Ir i d i u m c o r r o s i o n e f f i c i e n c y vs. ap p l i e d current d e n s i t y (with respect to geometric area) from the cumulative coating l o s s r e s u l t s of 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 HzSOk + 0.5M CuSG\. 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 that e l e c t r o l y t e . These are: Pt: 1.13 ug/A-hr I r : 0.35 ug/A-hr The platinum and i r i d i u m c o r r o s i o n rates (with respect to t h e i r actual surface areas) i n 2M H 2S0 4 + 0.5M CuS0 4, at both 22° and 40°, are p l o t t e d i n Figures 6.19 and 6.20. The c o r r o s i o n rates are seen to vary with a p p l i e d ( 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 current d e n s i t y r e l a t i o n s could 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 geometric, r a t h e r than the a c t u a l , electrode area f r a c t i o n s occupied by platinum and i r i d i u m , and using the geometric e l e c t r o d e areas. However, such p l o t s would only be e q u i v a l e n t to those i n Figure 6.19 and 6.20 i f the roughness f a c t o r s were i n v a r i a n t . This i s c e r t a i n l y not the case with the present work. Anodes subjected to 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 ge coating losses and surface area changes per r e p e t i t i v e run that the general c o r r o s i o n r a t e vs. mean actual a p p l i e d current d e n s i t y r e l a t i o n can be d i s t i n g u i s h e d f o r an i n d i v i d u a l anode. Figure 6.21 shows both types of c o r r o s i o n r a t e vs. c u r r e n t d e n s i t y p l o t s f o r an anode operated a t 117 mA/cm2 (geometric area) i n 2M H 2 S O 4 + 0.5M CuSO^, 22°C. As can be c l e a r l y seen, the r e p r e s e n t a t i o n with respect to actual surface area i s much more meaningful, and indeed conforms we l l with the slope of the curve determined from Figure 6.19. The e f f e c t s of various c o n d i t i o n s of a c i d i t y , from 0.5 to 8M H 2 S O 4 , 40° (Table 6.10), did not reveal any 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 c o r r o s i o n rates or e f f i c i e n c i e s from those found i n the more extensive work i n 2M H 2S0^. Figure 6.19. Platinum c o r r o s i o n r a t e percm 2 of actual platinum surface vs. mean actual a p p l i e d current d e n s i t y . (Each po i n t corresponds to the cumulative coating loss measurements f o r one or more 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 runs i n 2M H2SG\ + 0..5M CuSCV ) 127 Figure 6.20. I r i d i u m c o r r o s i o n r a t e percm 2 of actual i r i d i u m surface vs. mean actual a p p l i e d current d e n s i t y . (Each po i n t corresponds to the cumulative coating l o s s measurements f o r one or more 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 runs i n 2M H 2S0 4 + 0.5M CuS0 4.) 128 4 0 0 Q MEAN ACTUAL CD. , mA/cm2 Figure 6.21. Corrosion rate measurements on an i n d i v i d u a l anode subjected to 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 H2SC% + 0.5M CuSCK, 22°, with respect t o : (a) the geometric area of platinum only, (b) the ac t u a l (mean) area of platinum c a l c u l a t e d f o r each run (the l i n e represents the curve i n Figure 6 . 1 9 ) . Table 6.10 P l a t i num and I r i d i u m Co r ro s i on Rates from Cumulative E l e c t r o l y s i s Runs wi th I n d i v i d u a l E l e c t r ode s i n S u l f u r i c A c i d S o l u t i o n s o f Var ious A c i d i t i e s E l e c t r o l y t e Cur rent Dens i ty ^ g e o m e t r i c ' mA_ cm ; Tota l Time A-hr Co r ro s i on E f f i c i e n c y Co r r o s i on (Geometri c Rate Area) \ Cor ro s ion Rate (Actua l Area) Mean Ac tua l Cur rent Dens i t y Mean Co r ro s i on Cur rent D e n s i t i e s cm' Hours A -hr + ii q hr-rrr' ± hr-m" + mA cm' 0.5H H,S0», 40° 52.1 1540 154 P t ) 0.17 I r ) 0.22 .08 .04 130 363 61 61 3.5 9 1.5 1.5 1.30 0.19 0.50 4M H 2S0.,, 40° 52.1 192 19.2 P t ) 0.08 I r ) 0.26 .58 .26 57 441 440 440 1 11 11 11 1.30 0.05 0.61 4M K 2 S 0 4 , 40° 104 379 75.8 P t ) 0.65 I r ) 0.12 .15 .08 1010 390 230 230 25 10 6 6 2.60 1.37 0.56 8!1 H 2 S 0 „ , 40° 52.1 957 95.7 Pt) 0.59 I r ) 0.13 .14 .07 451 207 105 105 • 11 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 sources, and having d i f f e r e n t i n i t i a l noble metal l o a d i n g s , were subjected to a "standard" run a t 52.1 mA/cm2 (geometric area) i n 2M H 2 S O 4 + 0.5M CuSO^, 22°. The platinum and i r i d i u m c o r r o s i o n data, summarized i n Table 6.11a and 6.11b, shows t h a t wide v a r i a t i o n s i n the co 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 values can occur, and that i n most cases these values are much higher than those found f o r the anodes (average i n i t i a l loading 4.33 g/m2) used i n t h i s work. As with the surface area data, no c o r r e l a t i o n was found to e x i s t with the new ele c t r o d e 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 surface area of the various anodes could 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 rates (indeed, the electro d e s with the highest surface areas, and hence the smallest actual a p p l i e d current d e n s i t i e s , showed the highest c o r r o s i o n r a t e s . Such i n c o n s i s t e n c i e s i n d i c a t e that the c o r r o s i o n behaviour of anodes from d i f f e r e n t manufacturing l o t s may vary c o n s i d e r a b l y . 6.2.4 Pulsed E l e c t r o l y s i s Several types of pulsed operation were employed, i n c l u d i n g an "optimum" procedure found by Liekens [34] to enable high 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/cm2) i n an i n d u s t r i a l copper e l e c t r o w i n n i n g c e l l (9 seconds on; 0.5 seconds shorted) and a high frequency square-wave current p u l s i n g method described by Ib l [36] as being capable of pe r m i t t i n g e x c e l l e n t copper deposits a t cu r r e n t d e n s i t i e s as high as 250nnA/cm2. In a d d i t i o n the e f f e c t of operation a t slower frequencies and without e l e c t r o d e s h o r t i n g were i n v e s t i g a t e d . In a l l , only seven separate experiments (not r e p e t i t i v e ) were performed as the purpose was only to determine whether Table 6.11a Cumulative Platinum Corrosion Rates f o r In d i v i d u a l Electrodes of Nominal Pt/30 I r - T i Composition, but from D i f f e r e n t Manufacturing Lots, i n 2M H 2 S O 4 + 0.5M CuS0\, 22°, at 52.1 mA/cm2 A p p l i e d (Geometric) Current Density I n i t i a l Loading g(Pt+Ir) m2 Mean Actual Current Density mA cm 2 Total Time hours Arhr Corrosion E f f i c i e n c y Corrosion Rate (Geometric Area) Corrosion Rate (Actual Area) y9 + + + A»hr hr«m2 hr «m2 2.31 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 * 4.36 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 Cut from the sheet employed as source of the bulk of the anodes f o r t h i s work (4.33 g/m2). Table 6.11b Cumulative I r i d i u m Corrosion Rates f o r I n d i v i d u a l Electrodes of Nominal Pt/30 I r - T i Composition, but from D i f f e r e n t Manufacturing L o t s , i n 2M H 2S0 4 + 0.5M CuSO^, 22°, at 52.1 mA/cm2 A p p l i e d (Geometric) Current Density I n i t i a l Loading Mean Actual Current Density Total Time Corrosion E f f i c i e n c y Corrosion Rate (Geometric Area) Corrosion Rate (Actual Area) g(Pt+Ir) mA hours A-hr + ug m2 cm 2 hr«m2 hr«m2 hr«m2 2.31 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 * 4.01 1.96 2634 263 0.31 .02 487 26 20 1.5 * 4.36 1.97 1848 185 0.39 .02 632 36 26 1.5 5.36 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 the sheet employed as source of the bulk of the anodes f o r t h i s work (4.33 g/m2). CO 133 such treatments 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. A l l runs, save one, were c a r r i e d out i n 2M H 2S0i, + 0.5M Cu S0 \ . Coating l o s s values are presented i n Table 6.12. P e r i o d i c current r e v e r s a l (runs ( a ) - ( e ) ) was found to be h i g h l y d e l e t e r i o u s towards both co a t i n g metals, with extremely large losses being found i n a l l cases. P r e f e r e n t i a l attack of the platinum i s a l s o evident, with the remaining coatings i n a l l cases showing higher weight f r a c t i o n s of i r i d i u m than of platinum. Higher a p p l i e d current (during t^) i s e f f e c t i v e i n reducing the c o a t i n g metal lo s s e s somewhat, but lengthening e i t h e r the or tc^Qpjyfrn p a r t i a l - c y c l e s enhances the coating l o s s e s . Operation i n 2M H 2 S 0 4 , 4 0 ° , gave higher coating l o s s e s than an e q u i v a l e n t run i n copper-containing e l e c t r o l y t e at 22°. Operation with ^ Q N ^ S H O R T E D = ^ sec/76 sec gave the highest c o a t i n g metal losses of a l l the pulsed techniques i n v e s t i g a t e d . Operation with o p e n - c i r c u i t c o n d i t i o n s during the " o f f - p a r t i a l c y c l e was not as d e l e t e r i o u s as that with s h o r t - c i r c u i t i n g , but coating metal l o s s e s were a l s o found to be higher than i n continuous d i r e c t durrent operation. The high-frequency operation (^Q[\J/^OPEN ~ ^'^ ] i s e c ^ 1.6 usee) gave the least-adverse r e s u l t s of a l l the pulsed c u r r e n t pro-cedures employed. Run ( g ) , where tfjN^OPEN = ^ sec/76 sec, was consider-ably l e s s harmful than the e q u i v a l e n t run (e) with s h o r t - c i r c u i t i n g . F urther, only i n run (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 weight f r a c t i o n s of the coating metals occur. Corrosion rate a n a l y s i s was approached from several viewpoints. The c o r r o s i o n e f f i c i e n c i e s (ug/A^hr) and c o r r o s i o n rates (ug/hr*m 2) were c a l c u l a t e d both with respect to the t o t a l operating time and the time of a p p l i e d anodic current operation. While the former i s of more p r a c t i c a l Table 6.12 Noble Metal Loading Data f o r E l e c t r ode s Subjected to Pulsed E l e c t r o l y s i s i n 2.0M H 2 S0 k + 0.5M CuOo,,, 22° (Except ( c ) : 2M H2S0<., 40°) 'ON ' S H O R T E D ' O P E N Cur ren t Dens i t y (Geometr ic) mA/cm2 Run Time Hours I n i t i a l Loadings g/m2 F i na l Loadings g/m2 I n i t a l w P t F i n a l w P t Metal Loss/Cyc le (Actua l Area) ug/m- 2 Cyc le Mean Ac tua l R.F. (a) 9 sec 0.5 sec - 52.1 194 P t ) 3.007 I r ) 1.374 0.291 0.828 0.685 0.260 3.25 1.08 24 (b) 9 sec 0.5 sec - 104 192 Pt ) 3.263 I r ) 1.476 0.421' 1.008 0.689 0.295 3.31 0.53 24 (c) 9 sec 0.5 sec - 52.1 194 P t ) 3.171 I r ) 1.442 0.119 0.323 0.687 0.269 3.78 1.27 23 (d) 60 sec 0.5 sec - 52.1 237 P t ) 2.755 I r ) 1.250 0.028 0.235 0.688 0.106 22.14 5.43 22 (e) 76 sec 76 sec - 52.1 47.6 Pt ) 2.778 I r ) 1.366 0.C81 0.085 0.670 0.489 187 123 22 ( f ) 4.4 psec - 1.6 ysec 26.0 503 Pt) 2.285 I r ) 1.091 1.768 0.968 0.677 0.646 1 .3 (10 ) - 7 6 . 1 ( 1 0 ) " ° 20 (s) 76 sec - 76 sec 52.1 204 P t ) 3:174 I r ) 1.369 2.673 1.131 0.699 0.703 3.70 4.12 40 135 concern f o r estimating c o r r o s i o n losses f o r i n d u s t r i a l anodes, the l a t t e r i s more r e a l i s t i c i f an anodic d i s s o l u t i o n mechanism i s being considered and f o r c a l c u l a t i o n of p a r t i a l ( c o r r o s i o n ) current d e n s i t i e s . A l l rate data are c a l c u l a t e d with respect to the mean f r a c t i o n a l geometric and actual surface areas occupied by the coating metals. The r e s u l t s are summarized i n Table 6.13. As can be seen the 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 are consider a b l y higher than that found f o r continuous d i r e c t c u r r e n t operation under otherwise s i m i l a r c o n d i t i o n s . In a l l cases, except run ( g ) , the rate of 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 smaller than that of platinum. Corrosion current e f f i c i e n c e s , d e s p i t e being much higher than that observed f o r continuous d i r e c t current operation were, neverthe-l e s s , no greater than 2.05(10)~ 5 per cent f o r platinum i n the worst case. The e f f e c t of i n i t i a l pulsed c u r r e n t operation on the subsequent c o r r o s i o n behaviour of an anode during prolonged constant a p p l i e d current operation was a l s o i n v e s t i g a t e d . For t h i s purpose, the e l e c t r o d e used i n pulsed c u r r e n t run (a) was subjected to f u r t h e r operation at 52.1 mA/cm2 (geometric area) i n 2M H 2Su\, 22°, f o r 258 hours. The loading and c o r r o s i o n rate data f o r t h i s p a r t i c u l a r anode are given i n Table 6.14. I t can be seen t h a t the c o r r o s i o n r a t e s , whether r e f e r r e d to geometric or actu a l p a r t i a l areas of the i n d i v i d u a l c oating metals, are much higher than those encountered with new anodes subjected to s i m i l a r operation (without p r i o r pulsed c u r r e n t treatment), and these higher c o r r o s i o n rates are r e f l e c t e d i n the high c o r r o s i o n e f f i c i e n c y values. Platinum continued to be s e l e c -t i v e l y l o s t from the c o a t i n g , as was the case during pulsed c u r r e n t opera-t i o n , leading to f u r t h e r i r i d i u m "enrichment" of the remaining coating metal. Tab le 6.13 C o r r o s i o n Rate Data f o r E l e c t r ode s Subjected to Pu l sed E l e c t r o l y s i s . Coat ing Loss Measurements are Ana lyzed w i t h Respect t o the To t a l Run Time, t t o t a l , the Anodic P a r t i a l - C y c l e Time, t , and w i t h Respect to the Geometric and Actua l Surface Area Values Co r r o s i on E f f i c i e n c y pg/A-hr Co r ro s i on Rate Co r ro s i on c . d . Co r ro s i on Cur rent E f f i c i e n c y % (Geometric Area) pg/hr-m 2 (Actua l Area) pg/hr-m 2 (Geometric Area) nA/cm 2 (Actua l Area) nA/cm 2 ( t t o t a i ( ' o n ' ( t t o t a l ) (ton> «J ' ' o n ) (a) P t ) 26 88 28 34 29600 31200 1233 1300 1715 71 3 29(10)-6 I r ) 5 40 5 70 5340 5630 • 223 235 314 13 6 0 3 1 0 ) " 7 (b) P t ) 14 21 14 99 30100 31700 1309 1320 1740 73 1 6 7 ( 1 0 ) "6 I r ) 2 34 2 44 4800 5000 198 209 279 3.0 2 6 8 ( 1 0 ) "7 (c) P t ) 30 21 31 85 32900 34700 1430 1509 1907 83 3 6 6 ( 1 0 ) "6 I r ) 9 90 10 43 9380 10400 430 453 581 25 1 12 (10 ) "6 (d) P t ) 22 09 22 28 109000 110000 4940 4980 6020 273 1 16(10r5 I r ) 8 22 8 30 7100 7170 323 326 400 18 7 6 S 10 ) - 7 (e) P t ) 109 218 97800 196000 17900 8890 10700 488 2 0 5 ( 1 0 ) "5 I r ) 41 7 103 64100 128000 11600 5820 7150 325 1 37 (1OJ "5 ( f ) P t ) 3 95 5 38 1550 2120 77 105 116 5.8 4 46 (1OJ "7 I r ) 0 94 1 28 723 985 36 49 55 2.8 2 1 2 ( 1 0 ) " ' (g) P t ) 4 72 9 43 3500 7010 350 175 385 9.5 7 3 9 ( 1 0 )- 7 I r ) 2 24 4 48 3900 7800 390 195 435 10.9 8 3 5 ( 1 0 ) "7 137 Table 6.14 Loading and Corrosion Rate Data f o r an Anode, P r e v i o u s l y Subjected to Pulsed Current Operation Described i n Tables 6.12 and 6.13, Case ( a ) , Subsequently Operated under Continuous Anodic Current Conditions i n 2M H2SO4, f o r 258 Hours Current density (geometric area) 52.1 mA/cm2 Current d e n s i t y (mean actu a l area) 7.6 Ampere«hours 25.8 I n i t i a l t o t a l loading (Pt + I r ) 1.12 g/m2 F i n a l t o t a l loading (Pt + I r ) 0.37 g/m2 Platinum I r i d i u m I n i t i a l loading (g/m2) 0.291 0.828 F i n a l loading (g/m2) 0.041 0.331 I n i t i a l weight f r a c t i o n 0.260 0.740 F i n a l weight f r a c t i o n 0.111 0.889 Corrosion e f f i c i e n c y (ug/A hr) .1.86 3.70 Corrosion r a t e with respect to geometric area (ug/hr«m2) 5220 2470 Corrosion r a t e with respect to mean actual area (ug/hr«m2) 770 350 P a r t i a l ( c o r r o s i o n ) current d e n s i t y with respect to mean actual area 42 20 (nA/cm 2) 138 6.2.5 A d d i t i v e and Contaminant E f f e c t s Thiourea was chosen f o r an a d d i t i o n agent i n simulated e l e c t r o -winning experiments i n 2M H 2S0 4 + 0.5M CuS0\, 22°, with 52.1 mA/cm2 (geometric area) a p p l i e d current d e n s i t y . In the f i r s t experiment, the e l e c t r o l y t e was prepared with 0.5 gpl thiou r e a a d d i t i o n , f o l l o w i n g by commencement of the run using an anode of i n i t i a l t o t a l noble metal loading of 4.53 g/m2. Catas t r o p h i c anode f a i l u r e occurred a f t e r only 23 hours' operation. A second run with a lower t h i o u r e a a d d i t i o n l e v e l (.05 gpl) and another new anode ( t o t a l noble metal loading 4.46 g/m2) 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 hours. A t h i r d experiment, with yet another new anode ( t o t a l noble metal loading 3.18 g/m2) from another source than the oth 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 without p r i o r a d d i t i o n of the th i o u r e a . A f t e r 18 hours, thiourea was added to the l e v e l of .05 gpl and the e l e c t r o l y s i s continued f o r an a d d i t i o n a l 74 hours, at which time the run was stopped. Although the anode was operating at a very high p o t e n t i a l on termination (9.05v SHE) f a i l u r e had not occurred. Nevertheless i t was p r o j e c t e d , from the rate of r i s e of the anode p o t e n t i a l , that f a i l u r e was imminent. Loading and c o r r o s i o n r a t e data f o r t h i s anode are given i n Table 6.15. Corrosion rates 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 behaviour p r i o r to the thiourea a d d i t i o n f o l lowed the r e l a t i o n s in Figures 6.19 and 6.20 (which lowered the i n i t i a l loadings of platinum and i r i d i u m by 0.013 and 0.003 g/m2, r e s p e c t i v e l y ) . On t h i s basis i t can be seen that both coating metals are l o s t at much greater r a t e s than f o r The anode p o t e n t i a l rose r a p i d l y a t an a c c e l e r a t i n g r a t e u n t i l the v o l t a g e l i m i t (50v) o f the power s u p p l y was exceeded. 139 Table 6.15 Loading and Corrosion Rate Data f o r an Anode Operated a t 52.1 mA/cm2 (geometric area) i n 2 M H 2S0 4 + 0.5M CuS0 4, 22°, with .05 gpl Thiourea A d d i t i o n a f t e r 18 hours' 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 I n i t i a l t o t a l loading (Pt + I r ) Estimated t o t a l loading (Pt + I r ) a f t e r 18 hours F i n a l t o t a l loading (Pt + I r ) Measured f i n a l roughness f a c t o r Mean actual current d e n s i t y Ampere • hours 3.180 g/m2 3.164 g/m2 2.654 g/m2 14.7 2.2 6.2 Platinum I r i d i u m Loading p r i o r to thiour e a a d d i t i o n (g/m2) 2.134 1.030 F i n a l loading (g/m2) 1.843 0.812 I n i t i a l weight f r a c t i o n 0.674 0.326 F i n a l weight f r a c t i o n 0.694 0.306 Corrosion e f f i c i e n c y (ug/A*hr) 7.55 5.66 Corrosion r a t e with respect to geometric area (yg/hr-m 2) 5740 9350 Corrosion rate with respect to mean actual area (yg/hr*m 2) 675 1100 P a r t i a l ( c o r r o s i o n ) current d e n s i t y with respect to mean actual area (nA/cm 2) 37 61 140 the case of 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 without t h i o u r e a a d d i t i o n and that i r i d i u m , r a t h e r than platinum, i s s e l e c t i v e l y attacked. The poorer c o r r o s i o n r e s i s t a n c e of i r i d i u m has p r e v i o u s l y been noted 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 [107]. The e f f e c t of entrapment of organic solvent i n the e l e c t r o -winning c i r c u i t , such as may occur i n a solvent e x t r a c t i o n / e l e c t r o w i n n i n g o p e r a t i o n , was i n v e s t i g a t e d with the a d d i t i o n of 3 volume per cent kerosene to the e l e c t r o l y t e (2M H 2S0 4 + 0.5M CuS0 4). The i m m i s c i b i 1 i t y of these two phases, desp 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 formation of a l a y e r of kerosene on top of the aqueous phase. No s i g n i f i c a n t e f f e c t s of the organic solvent a d d i t i o n were found a f t e r 479 hours' opera-t i o n at 52.1 mA/cm2 (geometric area) f o r the c o r r o s i o n of platinum, but the i r i d i u m l o s s r a t e was over three times greater than that of platinum. Corrosion e f f i c i e n c i e s were 0.82 ± 0.18 and 1.17 ± 0.08 yg/A-hr f o r platinum and i r i d i u m , r e s p e c t i v e l y . A much greater number of experiments would be necessary, however, to determine whether t h i s r e s u l t i s t y p i c a l . I n d i v i d u a l runs i n 2M h^SO^ or i n 2M H 2S0 4 + 0.5M CuS0 4 often showed s l i g h t l y greater i r i d i u m c o r r o s i o n rates 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 behaviour was not manifested i n the cumulative c o r r o s i o n data). 6.2.6 Morphology Changes S.E.M. observation of anode surfaces revealed s i g n i f i c a n t changes i n morphology occurred only a f t e r l o s s of 10-20 per cent of the c o a t i n g metal. Otherwise, the anode surfaces could 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 of the a p p l i e d current. Figures 6.22-6.25 show the surface morphologies of anodes having p r o g r e s s i v e l y greater 141 Figure 6.22. SEM view of the surface of an anode a f t e r operation at 104 mA/cm2 (geometric area) i n 2M H 2 S O 4 , 22°, f o r a t o t a l o f 957 hours. Total remaining loading: 2.63 g/m2. Coating l o s s : 32 per cent. (lOOOx) Figure 6.23. SEM view of the surface of an anode a f t e r operation at 52.1 mA/cm2 (geometric area) i n 2M H 2S0 4 + 0.5M CuS0 4, 22° f o r a t o t a l of 1848 hours. Total remaining loading: 2*70 g/m2. Coating l o s s : 33 per cent. (lOOOx) Figure 6.24. SEM view of the surface of an anode operated a t 260 mA/cm2 (geometric area) i n 2M H 2 S O 4 , 22°C, f o r a t o t a l of 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 vs. SHE. Total remaining loading: 0.62 g/m*. Coating l o s s : 85 per cent. (lOOOx) Figure 6.25. SEM view of the surface of an anode operated at 52.1 mA/cm2 (geometric area) i n 2M ti2S0k, 22°C, f o r a t o t a l of 385 hours, showing evidence of coating l o s s by s p a l l i n g . T o tal remaining l o a d i n g : 3.8 g/m2. Coating l o s s : 17 per cent. (lOOOx) 143 percentage c o a t i n g metal l o s s e s . Figure 6.24 shows the extensive d e t e r i o -r a t i o n of an anode under c o n d i t i o n s of 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). Figure 6.25 i s the surface morphology of the anode described i n Figure 6.14 and Table 6.9 where the 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 coating l o s s . Electrodes subjected to pulsed e l e c t r o l y s i s c l e a r l y revealed t h e i r extensive coating losses under S.E.M. observation. Figure 6.26 shows the surface of an anode operated at 52.1 mA/cm2 (geometric area) f o r 194 hours under c o n d i t i o n s of 9 seconds on/0.5 second shorted (see Table 6.12; ( a ) ) . Although 74 per cent of the coating has been l o s t , the surface does not manifest the pronounced d e t e r i o r a t i o n of anodes operated under continuous d i r e c t current c o n d i t i o n s and which have co n s i d e r a b l y more r e s i d u a l coat-i n g . Figure 6.27 shows the 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 of operation under a continuous anodic current of 52.1 mA/cm2. The r e s i d u a l coating metal i s t h i n l y dispersed over the surface where presumably once much t h i c k e r coating " 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 pulsed current operating c o n d i t i o n - 76 seconds on/76 seconds shorted - produced the lo s s of 97 per cent of the o r i g i n a l coating metal a f t e r 47.6 hours at 52.1 mA/cm2 (geometric area). S.E.M. observation revealed e s s e n t i a l l y no coat-ing metal over large areas of the anode s u r f a c e , with the remaining metal present i n h i g h l y - d e t e r i o r a t e d "patches" (Figure 6.28). Thiourea a d d i t i o n caused a rea d i l y - a p p a r e n t a t t a c k of the coating metal (Figure 6.29) s e l e c t e d area X-ray energy spectroscopy i n conjunction with the S.E.M. did not reveal the presence of s u l f u r on the degraded surface. 144 Figure 6.26. SEM view of the surface of an anode subjected to pulsed e l e c -t r o l y s i s (9 sec on/0.5 sec shorted) at 52.1 mA/cm2 (geometric area) i n 2M H2S0^ + 0.5M CuS0 4, 22°, f o r 194 hours. Total remaining loading: 1.12 g/m2. Coating l o s s : 74 per cent. (lOOOx) Figure 6.27. SEM view of the anode described i n Figure 6.26, a f t e r further operation a t a continuous anodic current of 52.1 mA/cm2 (geo-metric area) i n 2M H2SQk + 0.5M CuS0 4, 22", f o r 258 hours. Total remaining loading: 0.37 g/m2. Coating l o s s : 92 per cent. (lOOOx) Figure 6.28. Figure 6.29. SEM view of the surface of an anode subjected to pulsed e l e c -t r o l y s i s (76 sec on/76 sec shorted) a t 52.1 mA/cm2 (geometric area) i n 2M H 2S0\ + 0.5M CuS0\, 22°, f o r 47.6 hours. Total remaining loading: 0.17 g/m2. Coating l o s s : 97 per cent.(IOOOx) SEM view of the surface of an anode operated a t 52.1 mA/cm2 (geometric area) i n 2M H 2S0 4 + 0.5M CuSO-, 22°, f o r 92 hours, with the a d d i t i o n of 0.5 gpl th i o u r e a a f t e r 18 hours. Total remaining loading: 2.65 g/m2. Coating l o s s : 17 per cent.(40O0x) 146 6.3 P a s s i v a t i o n 6.3.1 P o t e n t i a l vs. Time Behaviour 6.3.1.1 New Pt/30 I r - T i Anodes A r a p i d increase i n anode p o t e n t i a l i s observed on commence-ment of e l e c t r o l y s i s at a given current d e n s i t y i n s u l f u r i c a c i d - c o n t a i n i n g s o l u t i o n s , where oxygen e v o l u t i o n i s the primary anode process (Figure 6.30). The rate of increase of the anode p o t e n t i a l decreases with time, however, such t h a t a f t e r about one day the anode p o t e n t i a l i s seen to increase at a s m a l l , nearly constant r a t e . As the anode p o t e n t i a l climbs above about 2.2 v o l t s vs. SHE, a value which i s a t t a i n e d sooner with higher a p p l i e d c u r r e n t d e n s i t y , a f u r t h e r r a p i d increase i n anode p o t e n t i a l i s observed. This phenomenon i n d i c a t e s the commencement of the simultaneous generation of ozone or p e r s u l f a t e s . 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 copper e l e c t r o w i n n i n g a p p l i c a t i o n s l i e s below the 2.2 v o l t p o t e n t i a l value. Immediately on a p p l i c a t i o n of an anodic current to a new Pt/30 I r - T i anode which was i n i t i a l l y at i t s r e s t p o t e n t i a l ( t y p i c a l l y 0.8-0.84 v o l t s vs. SHE), and a f t e r commencement of oxygen e v o l u t i o n , the p o t e n t i a l i s observed to obey.a l i n e a r r e l a t i o n w i t h the loga r i t h m of the time (Figure 6.31). The slope of the dependence of p o t e n t i a l on log t was found to be, i n most cases, 0.030 ± .005. In general, hence, the anode p o t e n t i a l could be expressed by: E = CONSTANT + 0.03 log t (4) Such dependence does not hold, however, beyond 1000-10,000 seconds, where the anode p o t e n t i a l begins to deviate upwards from the l o g a r i t h m i c r e l a t i o n . Figure 6.30. V a r i a t i o n of 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 at various current den-s i t i e s (geometric area) i n 2M H 2S0^ + 0.5M CuS0 4, 22°, (a) 15.6; (b) 30.0; (c) 52.1; (d) 117; (e) 260 mA/cm2. -p=» 148 This behaviour i s shown i n Figure 6.32, which i s t y p i c a l of the p o t e n t i a l / time behaviour of new e l e c t r o d e s . Many kinds of e m p i r i c a l mathematical r e l a t i o n s can be used to describe curves of t h i s form, i n c l u d i n g " p a r a b o l i c " or "cubic" 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 experimental data could be best f i t to an equation of the form: E = C i + c 2 t 0 ' 5 + 0.03 l o g t (5) which represents simultaneous 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 constants. Such an equation a c c u r a t e l y p r e d i c t s the i n i t i a l l i n e a r l o g a r i t h m i c dependence of the p o t e n t i a l as observed i n Figure 6.31 and 6.32, and a l s o the steady r i s e i n anode p o t e n t i a l with time which con-tinues a f t e r hundreds of hours of e l e c t r o l y s i s . (No "steady s t a t e " i s every achieved.) Table 6.16 summarizes the p o t e n t i a l / t i m e behaviour of Table 6.16 t x p r e s s i o n of the P o t e n t i a l vs. Time Behaviour of New Anodes Operated i n 2M HzSO., + 0.5M CuSO,,, 22°, at Various Current D e n s i t i e s According to the Equation: E = C i + c 2 t 0 - 5 + .03 log t Applied Current Density (Geometric Area) mA cm2 C i c 2 P r e d i c t e d Time to Reach 2.2.volts Days 15.6 1.610 7.11(10)- 5 310 52.1 1.712 1.19(10) _ , t 66 117 1.892 1.82(10)- 4 5.8 149 Figure 6.31. I n i t i a l p o t e n t i a l vs. time behaviour f o r new anodes operated at various a p p l i e d current d e n s i t i e s (with respect to geometric area) i n 2M H2SCH + 0.5M CuSO*, 22°. Figure 6.32. Semi l o g a r i t h m i c p l o t of the change i n anode p o t e n t i a l with time f o r a new anode operated a t 15.6 mA/cm2 (geometric area) i n 2M H 2S0\ + 0 . 5 M CuSG\, 22°. cn o 151 several new el e c t r o d e s operated at d i f f e r e n t a p p l i e d c u r r e n t d e n i s i t i e s , with respect to the parameters i n equation (5). 6.3.1.2 R e p e t i t i v e E l e c t r o l y s i s Two types of r e v e r s i b l e p o t e n t i a l vs. time behaviour on r e p e t i -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 , depending on the value the p o t e n t i a l a t t a i n s during p o l a r i z a t i o n . These are presented i n Figure 6.33. For el e c t r o d e s whose p o t e n t i a l remains below the value of 2.2 v o l t s (SHE) where the p o t e n t i a l "jump" occurs, r e p e t i t i v e e l e c t r o l y s i s (with break-up of the e l e c t r o d e , X-ray spectroscopy, 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 r e - c o n s t r u c t i o n between runs) gives e s s e n t i a l l y the same p o t e n t i a l vs. time behaviour. This behaviour i s termed " r e v e r s i b l e . " For anodes where the p o t e n t i a l "jump" occurs, an i n d u c t i o n p eriod (which may or may not be included e n t i r e l y w i t h i n the i n i t i a l run, depending on the rate of climb of p o t e n t i a l ) e x i s t s p r i o r to the "jump." Subsequently, the i n d u c t i o n period i n r e p e t i t i v e runs i s conside r a b l y diminished, although i t never vanishes. Again, however, the p o t e n t i a l vs. time behaviour can be termed e s s e n t i a l l y r e v e r s i b l e f o r t h i s case, as r e p e t i t i v e runs p r a c t i c a l l y are d u p l i c a t e s . I t i s i n t e r e s t i n g to note that the r e v e r s i b l e p o t e n t i a l vs. time behaviour f o r a given anode does not show any dependence on the noble metal l o a d i n g , and hence the actual surface area. 6.3.1.3 I r r e v e r s i b l e Behaviour Prolonged anodic e l e c t r o l y s i s w i l l e v e n t u a l l y • r e s u l t i n not only an increase of anode p o t e n t i a l 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 Figure 6.33. 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 at constant —• app l i e d current d e n s i t y (geometric surface area). Top: 117 ma/cm2 i n 2M H 2S0\ + 0.5M CuS0\, 22°. ro Bottom: 52.1 mA/cm2 i n 2M ti2SQ* + 0.5M CLISO^, 40°. I n i t i a l and f i n a l noble metal loadings f o r the r e s p e c t i v e anodes are i n d i c a t e d on the diagram (g(Pt + I r ) / m 2 ) . 153 occurs, but to much higher values not normally encountered i n operation of noble metal anodes (Figure 6.34). Further, on r e p e t i t i v e runs the previous p o t e n t i a l vs. time behaviour 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 of the r e p e t i t i v e run i s c l o s e to the f i n a l p o t e n t i a l of the previous run. 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 length of time that an anode w i l l continue to support the c u r r e n t demanded of i t depends on the magnitude of the a p p l i e d current d e n s i t y . In one p a r t i c u l a r case, an anode operated at 52.1 mA/cm2 (geometric area) operated f o r over 700 hours at p o t e n t i a l s above 4.0 v o l t s (SHE) i n 2M H2SO4 + 0.5M CuS0 4, 22°, p r i o r to complete degradation. At higher current d e n s i t i e s the anode w i l l s u s t a i n h i g h - p o t e n t i a l operation f o r much sh o r t e r times. 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 with time, which i s i n d i c a -t i v e of imminent anode f a i l u r e (Figure 6.35). The p o t e n t i a l vs. time behaviour of the f i n a l run f o r the anode described i n Figure 6.35 i s shown on an expanded time-scale i n Figure 6.36. 6.3.1.4 Pulsed E l e c t r o l y s i s The adverse e f f e c t s of pulsed 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 coating metals has already 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 of p e r i o d i c c u r r e n t r e v e r s a l . Anode p o t e n t i a l vs. time curves (Figures 6.37 and 6.38) are p l o t t e d using the f i n a l p o t e n t i a l a t t a i n e d i n each "on-cycle." For the case of operation at 9 sec.on/0.5 sec. shorted, 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 Figure 6.34. P o t e n t i a l change with time f o r an anode subjected to r e p e t i t i v e e l e c t r o l y s i s at high current d e n s i t y (260 mA/cm2 with respect to geometric surface 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 loadings are given on the diagram. cn 155 Figure 6.35. P o t e n t i a l change with time f o r an anode operated at high current d e n s i t y (260 mA/cm2 with respect to geometric area) i n 2M H2SO1H 22°. I n i t i a l and f i n a l noble metal,1oadings i n g(Pt + Ir)/m 2 are given on the diagram. Figure 6.36. P o t e n t i a l vs. time behaviour f o r an anode operated at 260 mA/cm2 (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 Figure 6.35). Figure 6.37. P o t e n t i a l vs. time behaviour f o r an electrode subjected to pulsed e l e c t r o l y s i s {t^ = 60 sec; ^SHORTED = 0 , 5 s e c ^ a t 5 2 , 1 [ i ^ / c m 2 (geometric area) i n 2M H 2S0 4 + 0.5M CuS0 4, 22°. Curve represents p o t e n t i a l s measured at the end of each on-cycle. I n i t i a l and f i n a l noble metal loadings given on diagram. 158 i ~ — i 1 r 0.17 | I i i i » I Figure 6.38. P o t e n t i a l vs. time behaviour f o r an el e c t r o d e subjected to pulsed e l e c t r o l y s i s (76 sec on/76 sec shorted) at 52.1 mA/cm2 (geometric area) i n 2M H2SO\ + 0.5M CuSO^, 22°. (Curve represents the p o t e n t i a l s measured at the end of each on-cycle.) I n i t i a l and f i n a l noble metal loadings in(g)(Pt + Ir) / m 2 are given on the di agram. 159 f o r operation at 52.1 mA/cm2 i n 2M H 2S0 4 + 0.5M CuS0 4, 22°, under con-tinuous anodic c u r r e n t ) . The most d e l e t e r i o u s treatment (76 sec.on/76 sec. shorted) caused a r a p i d r i s e i n anode p o t e n t i a l to values charac-t e r i s t i c of i r r e v e r s i b l e o p e r a t i o n , but with a decreasing trend f o r the rat e of increase of anode p o t e n t i a l with time, r a t h e r than the a c c e l e r -a t i n g increase of anode p o t e n t i a l with time normally observed with i r r e v e r -s i b l e beavhiour. Operation under high-frequency c o n d i t i o n s (4.4 usee, on/1.6 usee, open) d i d not produce p o t e n t i a l vs. time beavhiour d i s t i n g u i s h a b l e from t h a t of an anode operated with continuous anodic current under other-wise s i m i l a r c o n d i t i o n s . At lower frequencies (76 sec. on/76 sec. open) the anode p o t e n t i a l vs. time beavhiour was a l s o s i m i l a r , i n i t i a l l y , to that of an el e c t r o d e subjected to continuous anodic c u r r e n t . A f t e r about 200 hours (at 52.1 mA/cm2) the anode p o t e n t i a l was observed to begin to r i s e at a steeper ra t e (about .4 mv/hour) than i s encountered i n continuous cu r r e n t operation (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 of the p o t e n t i a l vs. time behaviour during the period when the anode and cathode were short c i r c u i t e d showed that the p o t e n t i a l dropped from the value i t a t t a i n e d immediately p r i o r to the pulse to a value of 0 v o l t s vs. the copper/copper s u l f a t e reference e l e c t r o d e i n the case of e l e c t r o l y s i s i n 2M H 2 S O 4 + 0.5M CuSO^, or to 0 v o l t s vs. the hydrogen reference e l e c t r o d e i n the case of 2M H 2S0 4, w i t h i n 20 m i l l i -seconds. The p o t e n t i a l then remained v i r t u a l l y constant t h e r e a f t e r u n t i l completion of the pulse whereupon i t rose 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 the p o t e n t i a l of commencement of oxygen e v o l u t i o n . The anodic-going 160 pulse showed a s i m i l a r p o t e n t i a l vs. time r e l a t i o n to that 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 Sectio n 6.3.2). The shape of the "on-c y c l e " changed from the i n i t i a l case of a.slowly r i s i n g p o t e n t i a l with time to v i r t u a l l y a square-wave form a f t e r three days. For anodes sub-j e c t e d to 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 not observed to change f o r the case of the 1.6 usee pulse time, but i n the case of the long o f f - p e r i o d (76 seconds) the p o t e n t i a l dropped to 1.4 v o l t s and main-tained t h i s value u n t i l the next on-period. 6.3.1.5 E f f e c t of A d d i t i v e s On a d d i t i o n of .05 gpl thiourea to the e l e c t r o l y t e a f t e r e l e c -t r o l y s i s f o r 18 hours at 52.1 mA/cm2 i n 2M H 2S0 4 + 0.5M CuSO^, 22°, the p o t e n t i a l was observed to climb r a p i d l y by almost 0.4 v o l t s (Figure 6.39). Subsequently, the anode p o t e n t i a l remained a t a high value f o r about 40 hours, whereupon an a c c e l e r a t i n g increase 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 of imminent complete degradation) commenced. +2 A d d i t i o n of .01M Co to the e l e c t r o l y t e a f t e r operation f o r 166 hours at 52.1 mA/cm2 i n 2M H 2S0 4 + 0.5M CuS0 4, 22° (a procedure which has a b e n e f i c i a l e f f e c t on the p o t e n t i a l and c o r r o s i o n behaviour of lead 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 the operat-ing anode p o t e n t i a l . The presence of kerosene i n the c e l l d i d not produce any s i g n i f i -cant change i n the p o t e n t i a l vs. time behaviour of an anode operated a t 52.1 mA/cm2 (geometric area) i n 2M H 2S0 4 + 0.5M CUSO^, 22°. Figure 6.39. E f f e c t of 0.05 gpl thiou r e a a d d i t i o n on the p o t e n t i a l of an anode operated at 52.1 mA/cm (geometric area) i n 2M H 2S0 4 + 0.5M CuS0 4, 22°. 162 6.3.2 Surface Charge Studies 6.3.2.1 Oxygen Layer Development P r i o r to Oxygen Ev o l u t i o n The anodic/cathodic g a l v a n o s t a t i c charge curves f o r platinum and the P t / 5 , 10, 20, 25 a l l o y wire e l e c t r o d e s were found to be completely s i m i l a r i n form (Figure 6.40, 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 reduction "plateau" on the cathodic side and considerable h y s t e r e s i s between the shapes of the anodic and cathodic curves. I r i d i u m wire e l e c t r o d e s , on the other hand, show no reduction "plateau," have conside r a b l y l e s s h y s t e r e s i s , and a l s o show a charge imbalance between the anodic and cathodic curves (Figure 6.41). Charge curves f o r a t y p i c a l new Pt/30 I r - T i anode (Figure 6.42) show s i m i l a r f e a t u r e s to platinum, except f o r the l e s s - s t e e p slopes of the "double-layer" charging regions i n both the anodic and cathodic curves. A d d i t i o n a l evidence f o r the p l a t i n u m - l i k e behaviour of P t / I r a l l o y s i s provided by the c l o s e balance between anodic and cathodic charges f o r d e p o s i t i o n and removal of oxygen, which i s not observed i n the case of i r i d i u m . The development of oxygen coverage p r i o r to oxygen e v o l u t i o n was studied 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 to the p o t e n t i a l of i n t e r e s t , followed by cathodic s t r i p p i n g at the same magnitude of c u r r e n t d e n s i t y . For the platinum and P t / I r a l l o y wire e l e c t r o d e s , and f o r Pt/30 I r - T i , the p a r t i a l oxygen coverage charges could be compared d i r e c t l y w i t h that measured at imminent oxygen e v o l u t i o n to o b t a i n a f r a c t i o n a l degree of coverage value. For i r i d i u m however, i t i s not p o s s i b l e to measure cathodic oxygen s t r i p p i n g charges by the g a l v a n o s t a t i c technique.. (Hence i t i s not p o s s i b l e to measure the coverage at 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 1 1 r TIME , SECONDS Figure 6.40. T y p i c a l charge curves f o r a Pt/25 I r a l l o y wire e l e c t r o d e subjected to a l t e r n a t e anodic and cathodic c u r r e n t s (0.06 mA/cm2 of actual area) i n helium-purged 2M H 2 S O 4 , 22°. Figure 6.41. T y p i c a l charge curves of an i r i d i u m wire electrode subjected to a l t e r n a t e anodic and cathodic currents (0.14 mA/cm2 of actual area) i n helium-purged 2M H2SO1H 22°. Note the imbalance of the anodic and cat h o d i c charges. Figure 6.42. T y p i c a l charge curves f o r a Pt/30 I r - T i e l e c t r o d e subjected to a l t e r n a t e anodic and cathodic currents (0.0088 mA/cm2 of act u a l area) i n helium-purged 2M H 2S0 4, 22°. 166 s t r i p p i n g . Indeed, the knowledge that i r i d i u m may form the e q u i v a l e n t of 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 technique s a t i s f a c t o r y anyway.) Surface 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 followed by anodic charging. This value was then used to compute the charge eq u i v a l e n t to an oxygen monolayer. 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 lue, from the t r a n s i t i o n times f o r the anodic d e p o s i t i o n of the oxygen l a y e r . Figure 6.43 shows the surface 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 various e l e c t r o d e s i n v e s t i g a t e d . The platinum and P t / I r a l l o y wire 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 behaviour, whereas the Pt/30 I r - T i shows higher coverages at a l l p o t e n t i a l s (which i s in conformity with the observation of the commencement of oxygen e v o l u t i o n at 1.53 v o l t s (RHE) on the coated anodes vs. 1.58-1.65 v o l t s f o r the wire e l e c t r o d e s ) . I r i d i u m shows monolayer coverage at 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 of measurement used i n t h i s work, and has n e a r l y the e q u i v a l e n t of two monolayers when oxygen e v o l u t i o n commences. In a l l 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 l i n e a r above about 1.0 v o l t . 6.3.2.2 Oxygen Layer Development with 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 of the surface oxygen from platinum and P t / I r a l l o y e l e c t r o d e s a f t e r holding at a given c u r r e n t d e n s i t y f o r a predetermined length of time revealed that the surface oxygen coverage increases 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 values approaching two monolayers and beyond. No l i m i t i n g coverage values were apparent from 167 Figure 6.43. Surface oxygen coverage vs. p o t e n t i a l r e l a t i o n s f o r several anodes, over the p o t e n t i a l region p r i o r to commencement of oxygen e v o l u t i o n . E l e c t r o l y t e i s 2M H2SG\, 22°, helium-purged. P o i n t s obtained by g a l v a n o s t a t i c anodic charging to the p o t e n t i a l of i n t e r e s t , followed immediately by cathodic s t r i p p i n g . 168 the present work. Figure 6.44 shows the d i f f e r e n c e s i n the cathodic charge curves f o r a Pt/25 I r a l l o y wire e l e c t r o d e which are produced by various p r i o r anodic treatments. Curve (a) i s t y p i c a l of the surface areas measure-ment technique and represents monolayer coverage. Continued anodic opera-t i o n under c o n d i t i o n s of oxygen e v o l u t i o n causes an increase i n the length of the reduction plateau and a decrease i n the plateau p o t e n t i a l (curves (b) and ( c ) ) . 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 that the p o t e n t i a l reaches about 2.0 v o l t s (RHE) and above, the development of m u l t i l a y e r ("type I I " oxide) f i l m s becomes apparent through the appearance of a l o w e r - p o t e n t i a l plateau which may extend to times e q u i v a l e n t to the removal of several monolayers of surface oxygen (curves (3) and ( 3 ) ) . No data could be obtained by the g a l v a n o s t a t i c technique f o r i r i d i u m , as the shape of the cathodic curve remained unchanged even a f t e r prolonged e l e c t r o l y s i s at high c u r r e n t d e n s i t i e s . The oxygen l a y e r s t r i p p i n g s t u d i e s are summarized i n Table 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 increase of surface oxygen coverage with 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 of 2-3 monolayers. Only data f o r e l e c t r o d e s used i n 2M H2S0n were used, as cathodic s t r i p p i n g of anodes 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 long h i g h - p o t e n t i a l plateau (at 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 r e d u c t i o n of Pb0 2 to 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 " c u p r i c o x i d e ) . M u l t i l a y e r coverage was observed only f o r a Pt/30 I r - T i anode whose p o t e n t i a l was 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 occur f o r platinum. Anodes which had l o s t most of t h e i r platinum loading due to pulsed e l e c t r o l y s i s behaved s i m i l a r l y to i r i d i u m wire and consequently Figure 6.44. Cathodic surface oxygen s t r i p p i n g curves f o r a Pt/25 I r a l l o y wire electrode subjected to various anodic treatments i n helium-purged 2M HzSOh, 22°, (a) p r e - p o l a r i z e d to imminent oxygen e v o l u t i o n ; (b) 3000 sec at 0.06 mA/cm2; (c) 19 hours at 0.06 mA/cm2; (d) 7000 seconds at 6 mA/cm2; (e) 23 hours at 6 mA/cm2. ( A l l current d e n s i t i e s are given with respect to actual e l e c t r o d e area. S t r i p p i n g current density was 0.06 mA/cm2.) 170 Table 6.17 E f f e c t of Anodic P o l a r i z a t i o n a t Various Current D e n s i t i e s and Times on the Surface Oxygen Coverage as Measured by Ga 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 2 S 0 4 , 22° Applied Current Density Time Final Anode P o t e n t i a l ^measured Anode (Geometric Area) mA/cm2 (Actual Area) mA/cm2 Seconds V o l t s vs. SHE ^monolayer Pt wire .18 .07 100 1000 3000 6000 2 . 4 ( 1 0 ) 5 1.74 1.81 1.85 1.87 1 .93 1.25 1.53 1 .59 1.71 2 .33 Pt/20 I r wi re .23 .06 100 1000 3000 2 . 4 ( 1 0 ) 5 1.67 1.76 1.80 1 .27 1.59 1.71 Pt/25 I r wire .24 .06 100 1000 3000 6 . 5 ( 1 0 ) " 1.67 1.75 1.79 1 .27 1.58 1.76 2T4" 0.6 2 . 4 ( 1 0 ) 5 1 .99 ' 2 .80 \ 3 . 3 3 * 24 6 .0 100 1000 7000 8 . 3 ( 1 0 ) " 2.05 2.09 2.08 2.06 1.67 2.46 ' ( 3 . 2 0 ) " ( 0 . 8 0 ) * i 2 .17 \ 7 . 2 4 * Pt/30 I r - T i ( 4 .3 g/m2) 52.1 1.53 100 300 10 " 7 . 6 ( 1 0 ) " 2 . 4 ( 1 0 ) 5 6 . 9 ( 1 0 ) 5 2 . 2 ( 1 0 ) 6 1.66 1.69 1 .74 1.81 1.90 1.99 2.98 1.27 1 .45 1 .59 1.91 1.96 2.83 3.15 CONTINUED Indicates formation of type II oxide. 171 Table 6.17 (Continued) Applied Durrent Density Time Fi n a l Anode P o t e n t i a l Anode (Geometric Area) mA/cm2 (Actual Area) mA/cm2 ^measured Seconds Vo l t s vs. SHE ^monolayer Pt/30 I r - T i (20 g/m2;) 52.1 0.77 100 300 1000 7.2(10)'* 2.4(10) 6 1.61 1.63 1.64 1.72 1.93 1.20 1.28 1.34 1.65 2.47 Pt/30 I r - T i (4.32 g/m2) 15.6 0.39 2.4(10) s 1.96 3.03 Pt/30 I r - T i 4.03 g/m2) 52.1 1.30 2.3(10) 6 1.93 2.02 Pt/30 I r - T i 4.32 g/m2) 104 2.60 1.6(10) 6 2.80 2.83 Pt/30 I r - T i (4.23 g/m2) (2M H 2S0 4, 40°) 15.6 0.39 5.2(10) 6 2.36 -35* 172 i t was not p o s s i b l e to determine the surface oxygen coverage produced on anodic p o l a r i z a t i o n with such anodes by the g a l v a n o s t a t i s s t r i p p i n g technique. 6.3.3 P o l a r i z a t i o n Curves The 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 anodes show two l i n e a r regions separated by an i n f l e c t i o n at about 1.61 v o l t s i n a p o t e n t i a l vs. log (current d e n s i t y ) p l o t s . T a f e l slopes of these l i n e a r regions are .047 and .080 f o r the low- and high-overvoltage regions, respec-t i v e l y . (The l a t t e r slope tended to increase f o r anodes which had been sub-j e c t e d to long-term a n o d i z a t i o n , but was e s s e n t i a l l y constant f o r new e l e c -trodes. 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 of i n d i v i d u a l curves was, at worst, w i t h i n 5 mi 1 1 i v o l t s . The importance of c o n s i d e r a t i o n of the true current d e n s i t y i s shown in Figure 6.45 and 6.46 which show the 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 , with current d e n s i t i e s c a l c u l a t e d both with respect to the geometric area and to the actual area. The coincidence of the curves p l o t t e d with respect to the 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 of the high - o v e r p o t e n t i a l Tafel region to 1.23 v o l t s (SHE) gives an exchange current d e n s i t y , with respect to actu a l surface area, of: i = 9.3(10)" 9 A/cm2 As very many mechanisms have been proposed f o r the oxygen e v o l u t i o n r e a c t i o n , i t i s not useful to consider the Tafel slopes reported here on a mechanistic b a s i s . Indeed, the v a r i a t i o n of the k i n e t i c parameters on nominally i d e n t i c a l e l e c t r o d e s remains one of the major problems i n the e l e c t r o c h e m i s t r y of oxygen. The present values do, however, f a l l w i t h i n the ranges reported f o r the i n d i v i d u a l noble metals. 173 Figure 6.45. 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 anodes of d i f f e r e n t roughness f a c t o r s i n 2M h^SO^, 22°. Current d e n s i t i e s based on geometric area. 174 Figure 6.46. 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 anodes of 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°. Current d e n s i t i e s based on actual surface area. 175 P o l a r i z a t i o n curves measured on platinum and P t / I r a l l o y wire e l e c t r o d e s revealed t h a t i n c r e a s i n g i r i d i u m content 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 could be found f o r platinum, the p o l a r i z a t i o n curves f o r the a l l o y wires showed rather complex behaviour (Figure 6 . 4 7 ) . For the a l l o y s Pt/20 I r and Pt/25 I r the p o l a r i z a t i o n curves assumed an "S-shape" when p l o t t e d i n the conventional manner. Using the measured p o t e n t i a l vs. log i data obtained from both platinum and i r i d i u m over f i v e decades of log i , a computer programme was w r i t t e n to generate p o l a r i z a t i o n curves f o r hypo t h e t i c a l platinum/ i r i d i u m a l l o y s on the assumption that the two metals behaved as separate anodes of i n d i v i d u a l surface areas p r o p o r t i o n a l to t h e i r weight f r a c t i o n s . The r e s u l t s of t h i s c a l c u l a t i o n are given i n Figure 6 .48 and are compared with the measured p o l a r i z a t i o n curve f o r the Pt/30 I r - T i anode. As can be seen, the e f f e c t of even as l i t t l e as one per cent of 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 to cause a decrease i n the oxygen o v e r p o t e n t i a l by over 0.1 v o l t . Higher amounts of i r i d i u m (> 10 per cent) give r e s u l t a n t p o l a r i z a t i o n curves c l o s e to that of i r i d i u m and having a s i m i l a r shape to t h a t of i r i d i u m . 6 .3 .4 Titanium In order to obtain a p o l a r i z a t i o n curve f o r t i t a n i u m which showed an " a c t i v e " region under the c o n d i t i o n s employed f o r measurement, i t was necessary to use a higher a c i d concentration (4M H 2 S 0 i J and temperature ( 6 0 ° ) . The p o l a r i z a t i o n curve of the t i t a n i u m base, cut from a sheet of material which has been etched and otherwise prepared f o r the thermal 176 T r L 0 G ( 'actual> • fJLA/cm2 Figure 6.47. Anodic 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 platinum and platinum/iridum a l l o y s i n 2M HzSOk, 22°. Current d e n s i t i e s r e f e r r e d to actual electrode areas. Figure 6.48. P o l a r i z a t i o n curves f o r platinum and i r i d i u m wire anodes i n 2M H2SG\, 22°, with computer-gener- ^ ated curves f o r various P t / I r a l l o y s . Also included are points 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 . 178 decomposition of a noble metal coating onto i t , shows a c h a r a c t e r i s t i c a c t i v e / p a s s i v e t r a n s i t i o n (Figure 6.49). A s l i g h t t r a n s p a s s i v e region occurred at p o t e n t i a l s above about 2.2 v o l t s (SHE), which i s not charac-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 curves have no r e l a t i o n to those which a c t u a l l y occur during anodic opera-t i o n a t a given p o t e n t i a l . Under p o t e n t i o s t a t i c operation i n the passive region the passive current d e n s i t y was observed to drop continuously with time, without ever reaching a steady value. The cu r r e n t density on a t i t a n i u m "base" anode held at 1.8 v o l t s (SHE) i n 2M H2S0*, 22° decayed to l e s s than 1 uA/cm2 (geometric 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 of t i t a n i u m was inves-t i g a t e d with the Anatek 50-1D dual power supply operated i n s e r i e s , i n the v o l t a g e - c o n t r o l mode used to "buck-out" the p o t e n t i a l - s e n s i n g input of the p o t e n t i o s t a t . Under these c o n d i t i o n s anode p o t e n t i a l s of 100 v o l t s were a t t a i n e d . Current d e n s i t i e s of the order of several mi 11iamps/cm 2 (geometric area) were observed during h i g h - p o t e n t i a l operation as a con-sequence of the rather r a p i d charging rate employed (10 volts/100 seconds). 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 switching to a p o t e n t i a l of 1.8v (SHE) with an anode which had not 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 vigourous 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 to the surface of a t i t a n i u m anode held p o t e n t o s t a t i c a l l y a t high anodic p o t e n t i a l s an extremely large c u r r e n t flows and oxygen i s evolved a t a high r a t e on the noble metal surface. On removal of the 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 th a t part of the t i t a n i u m surface which was touched by 179 Figure 6.49. Anodic p o l a r i z a t i o n curve f o r the t i t a n i u m base of a Pt/30 I r - T i anode, i n 4M H 2 S O 4 , 60°. Curve traced by stepping 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 at 20 mv/50 seconds. 180 the platinum, probably as a r e s u l t of l o c a l f i l m d e s t r u c t i o n . No evidence f o r breakdown was observed i n any of the anodic experiments on t i t a n i u m . The a d d i t i o n of 0.4 gpl t h i o u r e a to a c e l l c o n t a i n i n g a t i t a n i u m anode operating i n the passive region caused no change i n c u r r e n t d e n s i t y whatsoever. 6.4 Anode Deposits 6.4.1 Growth In 2M h^SO^ + 0.5M C U S OL e l e c t r o l y t e s c o n t a i n i n g t r a c e lead impurity the growth of l e a d - c o n t a i n i n g surface deposits was found to increase with time f o r a l l cases studied (Figure 6.50). The amount of surface lead was estimated 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 spectrometer with respect to that observed on a pure,lead sample of 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 that f o r platinum, the highest lead coverages observed correspond to about 5 m i l l i g r a m s of lead on the anode surface (1.92 cm 2 geometric area). A f t e r about 200 hours', o p e r a t i o n , "loadings" of about a m i l l i g r a m are i n d i c a t e d . No obvious r e l a t i o n between the amount of lead deposited and the a p p l i e d c u r r e n t d e n s i t y was found, and there was considerable s c a t t e r of the experimental points above and below the approximate curve drawn on Figure 6.50 f o r higher current d e n s i t y operation. An " i n d u c t i o n " period of greater than 24 hours was necessary i n order to produce det e c t a b l e deposits ( r e l a t i v e i n t e n s i t y of background-correct e d peaks greater than .002). A f t e r 100 hours the deposit became v i s a b l e , showing a blue-black c o l o u r . On removal of f i l m e d anodes from Figure 6.50. Increase i n amount of surface l e a d deposits with 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» ated at 52.1 mA/cm2 (geometric area) i n 2M HzS0h + 0.5M CuSO^, 22°, as i n d i c a t e d by the change in the i n t e n s i t y of the PbLai peak i n X-ray f l u o r e s c e n t spectroscopy. Points denoted by square and t r i a n g l e are discussed i n the t e x t . 182 the c e l l the deposit was found to be e a s i l y removed by wiping with a .. t i s s u e . A f t e r drying the anode deposit r e t a i n e d i t s colour and suscep-t i b i l i t y to removal by gentle wiping. Two s p e c i a l cases are included i n Figure 6.50. The square data point corresponds to 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 lead n i t r a t e was added to the e l e c t r o l y t e to bring the t o t a l lead concentration to 7 g p l . (This c o n c e n t r a t i o n i s not 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 of lead s u l f a t e i s : K s p = 1.06(10- 8) Hence i n 2f*1 H 2 S O 4 , where the a c t i v i t y of s u l f a t e ions may be estimated by aS0, = YS0, ' mS0, = (.002 to .05) • (.70) = .001 to .035 (6) depending on the method used to estimate 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 ions (see Appendix A4). Hence the maximum amount of d i s s o l v e d lead i n s o l u t i o n must correspond to an a c t i v i t y of l e s s than 10~ 5 molal or 5 ( 1 0 ) " 8 g p l . The t r i a n g l e data point i n Figure 6.50 corresponds to the case of pulsed e l e c t r o l y s i s with 4.4 usee, on/1.6 usee, open, and represents the highest lead-loading detected on a l l the anodes used i n s o l u t i o n s con-t a i n i n g trace amounts of lead. Electrodes employed i n the other cases of pulsed e l e c t r o l y s i s , where the frequencies were much g r e a t e r , d i d not 183 have any surface lead deposits e i t h e r by X-ray spectrometry or S.E.M. observation. No e f f e c t was found f o r the presence of surface lead deposits on the p o t e n t i a l vs. time behaviour, as the curves measured under s i m i l a r c o n d i t i o n s i n 2M HzSOk and 2M HzS0^ + 0.5M CuSC\ were e s s e n t i a l l y i d e n t i c a l . The cathodic charge curve of a lead-covered anode showed a long high-p o t e n t i a l plateau (at 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 2S0\ of 1.60 v o l t s , features not observed with "clean" noble metal surfaces. Further, although hydrogen gas bubbling could be used to complete reduce the oxygen coverage w i t h i n minutes on clean s u r f a c e s , the hydrogen p o t e n t i a l could not be a t t a i n e d i n the case of ele c t r o d e s covered with l e a d - c o n t a i n -ing f i l m s even a f t e r several hours. Removal of the f i l m s was r e a d i l y accomplished on immersion i n d i l u t e HC1 s o l u t i o n . 6.4.2 I d e n t i f i c a t i o n X-ray d i f f r a c t o m e t r y with anodes covered with l e a d - c o n t a i n i n g d e p o s i t s , which were removed " l i v e " from the c e l l on termina t i o n of the run, showed intense sharp peaks which could be i d e n t i f i e d e i t h e r with 3-Pb0 2j the anodic c o r r o s i o n product of lead i n s u l f u r i c a c i d s o l u t i o n s , or with the mineral p l a t t n e r i t e (Table 6.18). The f i t was s l i g h t l y b e t t e r f o r the l a t t e r species. In t h e i r recent review on the lead d i o x i d e e l e c -t r o d e , Carr and Hampson [365] i d e n t i f y the d-spacings measured f o r p l a t t n e r i t e with those of 6-Pb0 2. 184 Table 6.18 I d e n t i f i c a t i o n of Lead-Containing Anode Deposits by X-ray D i f f r a c t i o n , f o r 20 up to 80° with CuKaGiRadiation Observed Peak d(A) • I / I i (from chart) Data from A.S. T.M. Index hkl 3-Pb0 2 P l a t t n e r i t e ( P b O , ) d I / I i d I/I i 3.50 100 3.50 100 3.50 100 n o 2.788 98 2.79 80 2.80 100 101 2.475 36 2.46 90 2.48 70 200 (obscured b^ ' Pt and Ti peaks ) - - 2.21 10 210 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 oxide-covered anode was allowed to stand i n the e l e c t r o l y t e at o p e n - c i r c u i t f o l l o w i n g completion of a run, the coating would spon-taneously decompose to lead s u l f a t e with time. This was confirmed by the appearance and i n c r e a s i n g i n t e n s i t y of the x-ray d i f f r a c t i o n peaks cor-responding to lead s u l f a t e , coupled with the simultaneously-decreasing i n t e n s i t i e s f o r the (3-Pb02 peaks. An estimate of the degree of conversion to s u l f a t e was made by holding a given oxide-covered anode f o r various times at o p e n - c i r c u i t i n 2M H 2 S O 4 + 0.5M CuSC\ e l e c t r o l y t e and measuring the i n t e n s i t y at the 20 angle corresponding to the strongest PbSC\ peak o • o (3.001 A) r e l a t i v e to that of the strongest 3-Pb0 2 peak (3.50 A). These experiments are summarized i n Table 6.19. The conversion of the oxide Table 6.19 Conversion of 3-Pb0 2 Deposits to 3-PbSOi* on Holding at Open-Circuit i n 2M H 2S0 4 + 0.5M CuSO^, 22°, f o r Various Lengths of Time, Measured by Comparing the I n t e n s i t i e s of the Strongest PbS0 4 and g-Pb0 2 l i n e s Time at Open-Circuit (minutes) R e l a t i v e Peak I n t e n s i t y : j P b S M 2 1 1 ) 3-Pb0 2(110) 0 0 1 0.04 5 0.18 30 0.35 60 0.40 3 days (3-Pb0 2 barely detected) to the s u l f a t e begins w i t h i n the f i r s t minute of holding at 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 three days. Holding f o r several days 186 in d o u b l e - d i s t i l l e d water, on the other hand, has no e f f e c t on the nature of the anode d e p o s i t , with 3-Pb0 2 being the only species detected. 6.4.3 Morphology The le a d - c o n t a i n i n g species were observed to grow as d i s c r e t e c l u s t e r s on the anode surfaces. I n i t i a l l y , the 6-Pb02 was foundtto appear p r e f e r e n t i a l l y on the "smooth" areas of the Pt/30 I r c o a t i n g , which represent " h i l l s " i n the anode surface s t r u c t u r e (Figure 6.51). Further, the deposits appear to form along ridges i n these " h i l l s . " With increased time of o p e r a t i o n , however, coverage becomes more random (Figure 6.52). Figures 6.53 and 6.54 show B-Pb0 2 coatings t h a t are p a r t i a l l y and almost t o t a l l y converted to PbS0 4, r e s p e c t i v e l y . 6.5 Complete Degradation 6.5.1 Anode P o t e n t i a l Behaviour Complete anode degradation i s preceded by i r r e v e r s i b l e high-p o t e n t i a l operation. The rate of change of 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 exponential increase with time which reaches a maximum of 200 mv/second immediately p r i o r to attainment of the voltage l i m i t of the power supply (50 v o l t s ) . At the voltage l i m i t , the anode cu r r e n t then begins to decay. During the expo-n e n t i a l voltage 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 of i n s t a b i l i t y , beginning with p e r i o d i c " s p i k e s " of p o t e n t i a l which are superimposed on the anode p o t e n t i a l (commencing at about 5.0 v o l t s , and having a height of about 25 mv). The period of these "spikes" v a r i e d from 187 Figure 6.51. 3 - P b 0 2 deposit formed on an anode operated f o r 96 hours at 52.1 mA/cm2 in 2M H 2S0 4 + 0.5M CuS0„, 22°. (lOOOx) Figure 6.52. 3 - P b 0 2 deposit formed on an anode operated f o r 192 hours at 15.6 mA/cm2 i n 2M HaSO,, + 0.5M CuS0 4, 22°. (lOOOx) 188 Figure 6.53. Mixed 3-Pb0 2 and PbSO^ deposit formed on an anode operated f o r 475 hours at 30 mA/cm2 i n 2M H 2S0 4 + 0.5M CuSOi,, 22°, followed by holding at o p e n - c i r c u i t i n the e l e c t r o l y t e f o r several hours. (lOOOx) Figure 6.54. Predominantly PbS0 4 deposit formed on an anode operated f o r 195 hours at 15.6 mA/cm2 i n 2M H 2S0 4 + 0.5M CuSC*. 40°, followed by holding a t o p e n - c i r c u i t i n the e l e c t r o l y t e f o r three days. (lOOOx) 189 several minutes i n i t i a l l y to the order of 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 of the order of several hundred m u l t i v o l t s . H i g h e r - p o t e n t i a l operation i s al s o 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 p o t e n t i a l by n e a r l y two v o l t s , f o l l o w e d by a slow recovery to the r e l a t i v e l y "steady" higher value. 6.5.2 Corrosion Rates The c o r r o s i o n rates of anodes which showed i r r e v e r s i b l e p o t e n t i a l vs. time behaviour, or which s u f f e r e d complete degradation, were not t r e a t e d i n Section 6.2.1. C a l c u l a t i o n of the c o r r o s i o n e f f i c i e n c i e s and rates f o r el e c t r o d e s which, as a r e s u l t of prolonged anodic operation and/or high current 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 vs. SHE) yet were otherwise unimparied i n t h e i r a b i l i t y to s u s t a i n the anodic 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 rates of anodes which operated under lower, r e v e r s i b l e p o t e n t i a l vs. time c o n d i t i o n s (Table 6.20). Corrosion e f f i c i e n c y and r a t e data f o r i n d i v i d u a l anodes which s u f f e r e d complete degradation are given i n Table 6.21. C a l c u l a t i o n s were based on the t o t a l length of the run i n which f a i l u r e occurred, with respect to the changes i n noble metal loadings over t h a t p a r t i c u l a r run. In most cases f a i l u r e occurred a f t e r several r e p e t i t i v e runs at the same (geometric) cu r r e n t d e n s i t y , and the t o t a l time to f a i l u r e i s given i n brackets beside the time to f a i l u r e f o r the run i n which i t occurred. Depending on the length of the run, two groups of 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 Corrosion Data f o r Pt/30 I r - T i Anodes Operating Under I r r e v e r s i b l e Anode P o t e n t i a l Conditions (a) 2M H 2S0 4 + 0.5M CuS0 4, 22° (b) 2M H 2S0 4, 22° E l e c t r o l y t e Current Density (Geometric) mA/cm2 Run Time Hours (Cumulative) Corrosion E f f i c i e n c y yg/A hr Corrosion"Rate (Geometric Area) Weight Fr a c t i o n Pt Fi n a l Anode P o t e n t i a l v o l t s vs. SHE Pt I r Pt V I n i t i a l F i n a l (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 12.5 (a) 260 185 (474) 1.48 0.75 5800 5600 .659 .657 4.86 191 (1) Long-term 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 t h o s e 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 lete 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 f o r the s h o r t e s t runs. The f i r s t observation can be e x p l a i n e d , i n the l i g h t of the c o r r o s i o n rates found i n Table 6.20, by the low c o r r o s i o n rate c o n d i t i o n s t h a t p e r s i s t e d over most of the run. P o t e n t i a l vs. time observations have shown that the f a i l u r e phenomenon i s a r a p i d one, with anode l i f e t i m e being only about ten hours on attainment of 8 v o l t s p o t e n t i a l , and l e s s than an hour on attainment of 10 v o l t s . Thus the c o r r o s i o n rates which occur during the process of f a i l u r e are obscured by the much longer times over which the anode c o r r o s i o n rates were t y p i c a l l y low, as the loadings can only be measured p r i o r to and f o l l o w i n g a given run. Thus the information one may e x t r a c t from anodes belonging to group (1) i s l i m i t e d . Anodes belong-ing to group ( 2 ) , however, c l e a r l y show much higher c o r r o s i o n e f f i c i e n c i e s and rates than are encountered with operation under c o n d i t i o n s where complete degradation d i d not occur. In p a r t i c u l a r , case (8) i n Table 6.21 may be considered to 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 during the f a i l u r e process, as i t was the only example where the c o r r o s i o n was measured w i t h the anode i n i t i a l l y at a high, i r r e v e r s i b l e p o t e n t i a l (11.5 v o l t s vs. SHE) with termination of the run immediately on attainment of the voltage l i m i t of the power supply - that 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. The c o r r o s i o n rates f o r both coating metals were found to be ... extremely high - 1.8 grams/m 2-hr - i n d i c a t i n g t h a t much of the c o a t i n g metal i s l o s t w i t h i n only a very short time. The e q u a l i t y of the c o r r o s i o n rates of i r i d i u m and platinum f o r t h i s and the other 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 D e g r a d a t i o n (a) 2M H2SO,. + 0 . 5 H C u S C , 2 2 ° (b) 2M H2SO,. + 0.5M C u S C , 40" ( c ) 2M H : S 0 4 , 2 2 ° E l e c t r o l y t e C u r r e n t De (Geometr mA/cmz • . i t y c ) Charge P a s s e d t o F a i l u r e A - h r Time t o F a i l u r e Hours ( PIPIT" i " i t - i \/o 1 Run Time Hours * Noble Metal L o a d i n g s g ( P t + I r ) / m 2 * C o r r o s i o n E f f i c i e n c y p g / A - h r C o r r o s i o n r a t e ( G e o m e t r i c A r e a ) p g / h r - m 2 I n i t i a l F i n a l P t I r P t I r 1. (a) 52.1 410 2250 (4098) 2278 2 70 0 33 1 . 4 6 0 . 5 6 1500 590 2. (b) 52.1 263 880 (2628 885 2 96 2 01 1.51 0 . 5 6 1200 800 3. (b) 104 219 524 (1096) 536 3 26 1 39 2 . 2 4 1 . 2 0 3500 3450 4. (b) 260 115 35 (230) 45 3 00 1 12 1 0 . 0 3 6.11 41000 42500 5. ( c ) 104 199 37 (994) 46 2 62 0 99 2 2 . 0 4 1 2 . 2 3 36000 34500 6 . ( c ) 260 311 50 (622) 71 3 59 0 77 1 0 . 3 0 4 . 9 3 40000 38500 7. ( c ) 260 20 39 5 44 2 ** 29 0 16 13.91 6.81 53000 55500 8 . ( c ) 260 228 1610 s e c ( 4 5 5 ) 1610 sec 1 42 0 62 452 228 1 . 8 ( 1 0 ) 6 1 . 8 ( 1 0 ) 6 9 . (d) 521 104 104 163 4 24 0 22 3 . 4 2 1 . 3 2 26000 22000 10. i d ) 521 90 90 95 4 45 0 77 5 . 0 0 2 . 2 4 38000 36000 11. ( a ) +.5 g p l t h i o u r e a 52.1 2.1 21 23 4 53 2 14 138 61 1 . 0 6 ( 1 0 ) 5 99000 12. (a) +.05 g p l t h i o u r e a 52.1 16 160 166 4 46 3 22 1 0 . 1 3 4 . 2 6 770.0 7000 * 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 t h e run i n which d e g r a d a t i o n o c c u r r e d , w i t h r e s p e c t t o t h e t o t a l time o f t h a t p a r t i c u l a r r u n . :* New, l o w - l o a d i n g anode. — 1 CO 193 are f u r t h e r evidence f o r the d i f f e r e n c e of the nature of the c o r r o s i o n processes o c c u r r i n g under c o n d i t i o n s of 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 of the 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 of anodes undergoing complete degradation that c o r r o s i o n rates were c a l c u l a t e d with respect to geometric, r a t h e r than a c t u a l , surface areas.) The charge passed necessary to produce f a i l u r e of the 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/m2) can be determined from the t i m e - t o - f a i l u r e of i n d i v i d u a l anodes i n Table 6.21. This value appears to be more a property of a given anode r a t h e r than i t s operating cur r e n t d e n s i t y or temperature, although anodes employed at the highest current d e n s i t i e s (521 rnA/cm2 geometric area) showed the lowest charge l i f e t i m e s . Charge-to f a i l u r e values were found to vary between 90 and 410 ampere*hours f o r operation i n e i t h e r t e s t e l e c t r o l y t e at current d e n s i t i e s ranging from 52.1 to 521 mA/cm2. A new low-loading anode (case ( 7 ) , Table 6.21) showed a considerably smaller c h a r g e - t o - f a i l u r e value (20 A«hr), but otherwise 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 and r a t e to an ( i n i t i a l l y 4.33 g/m2) anode,whose loading had decreased to a s i m i l a r value p r i o r to commencement of the run i n which f a i l u r e occurred, and which was operated at the same (geometric) current d e n s i t y (compare case (4) i n Table 6.21). Thiourea a d d i t i o n s were found to produce f a i l u r e a f t e r only 2.1 A-hr i n one case (.5 gpl a d d i t i o n ) and 16 A*hr i n another (0.5 gpl addi-t i o n ) . While the coating l o s s rates during operation where f a i l u r e does not occur are higher when thiour e a i s added (Table 6.15) the u l t i m a t e complete anodic degradation i s not ass o c i a t e d with removal of more coating metal than i s the case f o r f a i l u r e i n the cases where no a d d i t i v e s were present. Indeed, the converse i s t r u e , w i t h case (12) i n Table 6.21 showing a r e s i d u a l loading of 3.22 g/m2 a f t e r f a i l u r e . 194 6.5.3 Morphology The predominant features on the surfaces of completely degraded anodes (Figures 6.55-6.58) are bent, 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 o p e ration at the voltage l i m i t (50 v o l t s ) . With extended opera-t i o n at t h i s p o t e n t i a l (60 hours) such features are the only d i s c e r n a b l e aspect of surface morphology with no coating metal evident (as confirmed by X-ray spectroscopy). Higher m a g n i f i c a t i o n (Figure 6.56) shows that the bent, c o n i c a l p r o j e c t s are a c t u a l l y composed of many f i n e l a y e r s arranged perpendicular to the a x i s of the cone. Under c o n d i t i o n s of less-prolonged operation under v o l t a g e - l i m i t c o n d i t i o n s (20 hours), remnants of coating metal are s t i l l v i s i b l e . The c o n i c a l p r o j e c t i o n s , however, appear to grow next to and even beneath the pieces of coating metal (Figure 6.57). The lea d - c o n t a i n i n g surface deposits were found, on f a i l u r e , to have changed i n nature from 3-Pb0 2 to PbSO^ (Figure 6.58). Such a t r a n s -formation, which occurs only under o p e n - c i r c u i t c o n d i t i o n s , suggests that the deposit had become e l e c t r i c a l l y i s o l a t e d from the sub s t r a t e . 6.5.4 Re-coating An anode which had s u f f e r e d complete degradation was given a sputtered platinum coating e q u i v a l e n t to 4 g/m2 and then employed as an anode i n 2M H 2S0 4, 22°. Surface area determination using charging currents of 2mA/cm2 (geometric area) could be performed without d i f f i c u l t y ( g i v i n g a roughness f a c t o r of 32), but anodic operation at 52.1 mA/cm2 (geometric area) caused the;.potential to r i s e to the voltage l i m i t w i t h i n 100 seconds. Recovery of normal anode behaviour with r e c o a t i n g , hence, cannot be Figure 6.55. SEM view of the surface of an anode which has undergone com-p l e t e degradation as a r e s u l t of operation f o r 163 hours at 521 mA/cm2 (geometric area) i n 2M H2SC\, 22°. Total remain-ing loading: 0.22 g/m2. (lOOOx) Figure 6.56. SEM view of 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 continued anodic operation a t the voltage l i m i t (50 v o l t s ) . (lOOOx) Figure 6.57. SEM view o f the surface of an anode which has undergone com-plete degradation as a r e s u l t of operation f o r a t o t a l of 622 hours at 260 mA/cm2 (geometric area) i n 2M H2S0k, 22°. Total remaining loading: 0.77 g/m2. (lOOOx) Figure 6.58. SEM view of the surface of an anode which has undergone com-plete degradation as a r e s u l t of operation f o r a t o t a l of 1096 hours at 104 mA/cm2 (geometric area) i n 2M H 2 S O 4 + 0.5M CuSO^, 40°. Total remaining loading: 1.39 g/m2. (lOOOx) 197 accomplished without p r i o r treatment of the degraded surface to remove the c o r r o s i o n products. 6.5.5 I d e n t i f i c a t i o n of the Surface Degradation Product On degradation, several new peaks appear i n the X-ray d i f f r a c t i o n p a t t e r n , which correspond w e l l to those of T i 0 2 (anatase), the expected c o r r o s i o n product on bare t i t a n i u m at 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 mat e r i a l with respect to the nature of the c r y s t a l l i n e species. Chapter 7 DISCUSSION 7.1 Corrosion 7.1.1 During Rev e r s i b l e P o t e n t i a l vs. Time Operation 7.1.1.1 Anode Ma t e r i a l from the Same Source The c o r r o s i o n behaviour of the Pt/30 I r - T i anode material used f o r most of the present work (average l o a d i n g : 4.33 g/m2) i s con-sidered to be predominantly electrochemical i n nature under the range of co n d i t i o n s i n v e s t i g a t e d (7.8 - 521 mA/cm2 geometric area i n 2M H2S0,+ and 2M H2SO.4 + 0.5M CuSOit, 22-80°). Evidence 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 r e s p e c t t o t h e i r p a r t i a l s u r f a c e a r e a s ( e i t h e r g e o m e t r i c o r a c t u a l ) a r e n o t e q u a l . Under p u r e l y m e c h a n i c a l c o a t i n g detachment, a homogeneous a l l o y must show equal 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 metal becomes e n r i c h e d i n i r i d i u m due t o the 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 p l a t i n u m . O p e r a t i o n under p u l s e d c u r r e n t con-d i t i o n s , i n p a r t i c u l a r , produces h i g h i r i d i u m l e v e l s i n the r e m a i n i n g c o a t i n g m e t a l . Under p u r e l y m e c h a n i c a l detachment c o n d i t i o n s , a homogeneous a l l o y would remain unchanged i n compos i t i o n . 3. t h e r a t i o s 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 o f t h e c o a t i n g m e t a l s ( l r : P t ) , as a consequence o f 198 199 t h e i r w e i g h t f r a c t i o n s i n the a l l o y . The ob s e r v e d r a t i o s t a k e n from the w e i g h t e d mean v a l u e s o f a l l runs in 2M r^SOit and in 2M H 2 S C H + 0.5M CuSC\ were 0 . 3 ^ 8 and 0 . 310 , r e s p e c t i v e l y , compared w i t h the v a l u e o f 0 . 4 5 6 which would o c c u r as a r e s u l t o f p u r e l y mech-a 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 . 0 8 7 ) . h. t h e r e i s a p a r a l l e l i s m between the c o r r o s i o n r a t e s o f both p l a t i n u m and i r i d i u m , and the t o t a l anode r e a c t i o n r a t e (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 ) . T h i s i s m a n i f e s t e d both i n the l i n e a r i t y o f the 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 i n the l a c k o f dependence o f the 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 b e h a v i o u r 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 -c h e m i c a l 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 ruthenium [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 c o n c e r n i n g the 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 metal under s i m i l a r c o n d i t o n s . ) 5. t h e low v a l u e s f o r e f f i c i e n c y . A w e i g h t e d mean v a l u e o f 1.13 u g / A hr was (determined i n 2M H 2 S O 4 + 0.5M CuS0 4) compares f a v o u r a b l y w i t h the v a l u e t h a t can be c a l c u l a t e d from the d a t a o f Chemodanov [ 9 9 , 1 0 1 ] ( 1 . 8 u g / A * h r ) f o r 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 . To be e x a c t , the 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 the amount o f char g e passed on the p a r t i a l a r e a s c o r r e s p o n d i n g t o the i n d i v i d u a l m e t a l s . I f i t i s assumed t h a t the 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 the a p p l i e d c u r r e n t d e n s i t y then the c o r r o s i o n e f f i c i e n c i e s can be c a l c u l a t e d f o r pu r -poses o f d i r e c t c omparison w i t h t h a t o f pure 1".. p l a t i n u m , by d i v i s i o n w i t h the a r e a f r a c t i o n (assumed equal to the 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 the a r e a f r a c t i o n o f p l a t i n u m , the 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 CuS0 4 e l e c t r o l y t e . The s m a l l d i s c r e p a n c y f o r 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 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 due t o the p o s s i b i l i t y t h a t the r e s u l t s o f Chemodanov do not 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 the u n s u i t a b i 1 i t y o f the r a d i o t r a c e r t e c h n i q u e f o r measurement p e r i o d s as long as tho s e employed i n t h i s work. 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 p l a t i n u m , 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 a r e a , a l s o compare w e l l w i t h the 10~6 v a l u e f o r the 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 with the "4.33 g/m2" anode material however, c l e a r evidence was found f o r a predominantly mechanical d i s s o -l u t i o n mechanism. Thus the nature of the c o r r o s i o n behaviour of Pt/30 I r - T i anodes may vary over the same anode sheet. The v a r i a b i l i t y of the nature of the c o r r o s i o n process over the surface of a given 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 experimental anodes cut from t h a t sheet and operated under s i m i l a r c o n d i t i o n s . The anodic c o r r o s i o n behaviour of i r i d i u m was found to be analogous to that of platinum, a l b e i t with smaller c o r r o s i o n rates and e f f i c i e n c i e s . The p a r a l l e l i s m with 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 given by the a p p l i e d current d e n s i t y suggests t h a t , as i n the case of platinum, the p a r t i a l c o r r o s i o n processes and the process of oxygen e v o l u t i o n share common steps.[99,101]. 7.1.1.2 Anode M a t e r i a l From D i f f e r e n t Manufacturing Lots' Anode m a t e r i a l s of nominally i d e n t i c a l a l l o y composition and manufacture, but from d i f f e r e n t manufacturing l o t s , have widely d i f f e r e n t c o r r o s i o n behaviour under s i m i l a r operating c o n d i t i o n s with no obvious dependence on geometric or a c t u a l surface areas, or to i n i t i a l t o t a l noble metal l o a d i n g . Comparison of platinum 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 that most of the other anode m a t e r i a l s i n v e s t i g a t e d showed higher 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 coating metals than the anode material which was used f o r the bulk of the present work (average lo a d i n g : 4.33 g/m2). As the c o r r o s i o n rate 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/m2" anodes, the higher values of c o r r o s i o n e f f i c i e n c y reported f o r other anode m a t e r i a l s of nominally i d e n t i c a l composition may be a t t r i b u t e d to the r e l a t i v e predominance of mechanical c o r r o s i o n i n these cases. Factors which could promote high mechanical d i s s o l u t i o n r a t e s of new anodes i n c l u d e : 1) poor 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 improper 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 -t i o n d u r i n g the c o a t i n g p r o c e d u r e , 2 ) poor a d h e s i o n among s u c c e s s i v e n o b l e metal c o a t -ings due t o improper heat t r e a t m e n t t o promote i n t e r d i f f u s i o n b o n d i n g , 3) h i g h c o a t i n g p o r o s i t y due t o the a p p l i c a t i o n o f h i g h n o b l e metal l o a d i n g s w h i c h would f a c i l i t a t e c o a t i n g metal l o s s by me c h a n i c a l a c t i o n o f bubb l e s g e n e r a t e d w i t h i n the pore. Although the platinum 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 I r - T i anodes from d i f f e r e n t sources ranged from 0.75 to 5.32 ug/A«hr, and those f o r i r i d i u m from 0.26 to 2.07 ug/A-hr, i t i s l i k e l y t hat other anodes may e x h i b i t y e t poorer behaviour due to greater d e v i a t i o n s from optimum manufacturing procedure. From c o n s i d e r a t i o n of the l i t e r a t u r e i t i s c l e a r that the method of manufacture has a profound e f f e c t on the c o r r o s i o n of P t - T i , with anodes having e l e c t r o d e p o s i t e d coatings t y p i c a l l y e x h i b i t -ing higher c o r r o s i o n e f f i c i e n c i e s than those having coatings produced by the thermal decomposition method, due to the predominance of mechanical d i s s o l u t i o n i n the former case. In order to c h a r a c t e r i z e the c o r r o s i o n behaviour of Pt/30 I r - T i anode mat e r i a l from other sources under a p a r t i c u l a r s et of operating c o n d i t i o n s , i t i s thus necessary (or at l e a s t , wise) to perform a d d i t i o n a l 202 experiments of a "standard" type i n order to obtain 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 y values. 7.1.2 Surface Oxygen Coverage and Corrosion The surface oxygen coverage r e s u l t s obtained i n the present work show that the p l a t i n u m / i r i d i u r n a l l o y coatings behave i n a manner s i m i l a r to platinum, showing a l o g a r i t h m i c increase i n coverage from 0 = 1 at the commencement of oxygen e v o l u t i o n to 2 < 0 < 3 over the duration of most of the i n d i v i d u a l runs employed. The c o r r o s i o n r e s u l t s obtained i n the present work thus represent the combined e f f e c t s of c o r r o s i o n phenomena i n two regions of oxygen coverage. 1) 1 - 0 5 2 . The f i r s t s e v e r a l hours 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 c o v e r a g e w i t h t i m e , a r e a l s o c h a r a c t e r i z e d by: (a) a r a p i d . c l i m b i n anode p o t e n t i a l w i t h t i m e [ 9 9 , 1 0 1 ] , (b) a r a p i d drop i n the i n s t a n t a n e o u s c o r r o s i o n r a t e o f p l a t i n u m . 2) 2 ^ 0 5 3 . A f t e r 1-2 days t h e s u r f a c e oxygen c o v e r a g e changes o n l y s l i g h t l y as a consequence o f the l o g a r i t h m i c c o v e r a g e / t i m e r e l a t i o n . F u r t h e r , t h e 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 -a f t e r . The c o r r o s i o n r a t e o f p l a t i n u m , from r a d i o c h e m i c a l work, a l s o assumes a r e l a t i v e l y s t e a d y va1ue. As i n d i v i d u a l run lengths were t y p i c a l l y of 8 days' d u r a t i o n , and over 100 days i n some s p e c i f i c cases, the c o r r o s i o n r e s u l t s may be considered to represent v i r t u a l l y steady s t a t e d i s s o l u t i o n from a surface c o n t a i n i n g the e q u i v a l e n t of 2-3 oxygen monolayers, as the c o n t r i b u t i o n due to the high_non-steady d i s s o l u t i o n rate of platinum i s s m a l l . F u r t h e r , no 203 e f f e c t s of run duration on c o r r o s i o n rate were observed i n the present work. I t i s i n t e r e s t i n g to 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 of platinum only occurs at p o t e n t i a l s above which the e q u i v a l e n t of a second monolayer of oxygen i s deposited on platinum, and 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 of lower oxygen coverage i s s m a l l . This i s a consequence of the "steady s t a t e " nature of his experiments (he only considered the d i s s o l u t i o n r a t e of platinum a f t e r several hours, where i t no longer changed r a p i d l y with time). 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 other work, however, c l e a r l y show th a t the oxygen coverage on platinum increases from 0 = 1 to 0 = 2 a f t e r commence-ment of oxygen e v o l u t i o n , and t h a t t h i s increase i n oxygen coverage takes place simultaneously with the r a p i d drop i n c o r r o s i o n r a t e with time observed by Chenodanov. Thus the attainment of the e q u i v a l e n t of a second monolayer of oxygen, f a r from being a p r e r e q u i s i t e f o r the anodic d i s s o -l u t i o n of platinum 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 the attainment of a "passive" s t a t e with respect to d i s s o l u t i o n . As oxygen coverage does not grow much beyond t h i s l e v e l with continued e l e c t r o l y s i s , the increase i n the d i s s o l u t i o n rate of platinum w i t h c u r r e n t d e n s i t y i s due to the greater surface concentration of intermediates i n the oxygen evolu-t i o n r e a c t i o n , whose involvement i n the platinum 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 of the rates of oxygen e v o l u t i o n and platinum d i s s o l u t i o n . 204 7.1.3 Pulsed E l e c t r o l y s i s Although no d e f i n i t e conclusions can be drawn from the l i m i t e d number of pulsed e l e c t r o l y s i s experiments performed, the r e s u l t s of these runs c e r t a i n l y show the extremely d e l e t e r i o u s nature of p e r i o d i c current r e v e r s a l , p a r t i c u l a r l y with respect to platinum d i s s o l u t i o n from the coatings (as apparent from the high i r i d i u m l e v e l s i n the remaining coat-ing 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 permits operation with r e s i d u a l noble metal loadings well below those encountered on complete degradation of anodes operated under continuous D.C. (as low as .16 g/m2 i n one case) the rates of noble metal l o s s are nevertheless so high (up to 109 ug/A-hr f o r platinum i n one case) that the anode l i f e t i m e i s not s u b s t a n t i a l l y 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 to the high non-steady d i s s o l u t i o n of platinum (and by analogy, i r i d i u m ) which occur immediately on switching on the anodic current during each c y c l e , as the surface oxygen coverage i s reduced during the preceding r e v e r s a l , l e a v i n g a " f r e s h l y reduced" surface f o r each subsequent on-pulse. The p r e f e r -e n t i a l c o r r o s i o n of platinum i s i n f e r r e d from the A.C. c o r r o s i o n behaviour of the two metals discussed i n Section 2.5. Longer r e v e r s a l times, found to produce more intense 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 of hydrogen being absorbed i n t o the s u b s t r a t e , which i s released during the subsequent on-pulse at such a r a p i d rate that the c o a t i n g i s l i f t e d and s p a l l s . Such a mechanism has already 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 e l e c t r o l y s i s i n an e l e c t r o p l a t i n g procedure [73]. 2 0 5 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 e l e c t r o l y s i s w i t h high-frequency on/off p u l s i n g (4.4 usee on/1.6 ysec o f f ) i s a con-sequence of the i n a b i l i t y of the anode to become reduced during the short o f f - c y c l e . During the time the external current 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 tend to discharge through the " f a r a d a i c r e s i s t a n c e " thus maintaining the anodic current. The high-frequency pulsed-current operation produced the t h i c k e s t lead d i o x i d e deposit on the anode that was determined i n the present work. Such a r e s u l t may be r e a d i l y understood i f the l i m i t i n g c urrent d e n s i t y , i ^ , f o r mass-transfer 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: \ = [ 2 F D P b + 2 j [ 5 j ( Any process tending to decrease 6 w i l l r e s u l t i n an increased r a t e of discharge of t r a c e lead ions and hence produce higher amounts of lead d i o x i d e on the e l e c t r o d e surface. With longer 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 drops. The oxygen coverage i s not a f f e c t e d , however, and hence the electrode i s not i n a " f r e s h l y reduced" s t a t e p r i o r to the subsequent "on-pulse." Thus the high l e v e l s of 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 of c u r r e n t - r e v e r s a l operation would not be expected although i t i s apparent that the c o r r o s i o n losses are higher than f o r continuous anodic cu r r e n t operation. These r e s u l t s again suggest t h a t anodes which are not permitted to form oxygen coverages up to and beyond 206 the e q u i v a l e n t of two monolayers show higher c o r r o s i o n losses due to the i n a b i l i t y of the e l e c t r o d e to 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 of these coverages. 7.1.4 Complete Degradation Over the course of i t s operating l i f e t i m e a given anode passes through stages of 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 vs. time behaviour, followed 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 , able to s u s t a i n high a p p l i e d anodic currents at low overvoltages whereas on degradation the 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 to c a r r y the anode current without 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 of the presence of considerable r e s i d u a l coating metal. From the c o r r o s i o n r e s u l t s obtained f o r anodes which sustained f a i l u r e , or imminent f a i l u r e , a change i n the nature of the c o r r o s i o n mechanism from predominantly electr o c h e m i c a l to predominantly mechanical i s i n d i c a t e d with the onset of complete degradation, as manifested by an exponential increase i n anode p o t e n t i a l to the p o t e n t i a l - l i m i t of the power s u p p l i e s . Such behaviour can be explained by the growth of a l a y e r of t i t a n i u m oxide between the coating and substrate 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 nature (due to a high impurity content) to a much lower value [215]. This would e x p l a i n the i r r e v e r s i b l e p o t e n t i a l vs. time behaviour of coated anodes a f t e r prolonged anodic e l e c t r o l y s i s , Complete degradation would be associated with d i e l e c t r i c breakdown of t h i s f i l m when i t s growth causes e l e c t r i c a l i s o l a t i o n of the c o a t i n g . Evidence f o r t h i s appears i n the p e r i o d i c appearance of "spikes" 207 i n the p o t e n t i a l vs. time r e l a t i o n as the p o t e n t i a l begins to climb e x p o n e n t i a l l y . These may be as s o c i a t e d with unsustained e l e c t r o n avalanches across the oxide. Coating losses due to s p a l l i n g would be ass o c i a t e d with such l o c a l d e s t r u c t i o n of the underlying oxide f i l m On attainment of the voltage l i m i t of the 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 over the anode su r f a c e , followed 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 oxide or hydroxide, producing the co n i c a l growths observed on S.E.M. observation of f a i l e d anodes. The i n a c t i v i t y or e l e c t r i c a l i s o l a t i o n of the r e s i d u a l coating metal on a completely degraded anode i s c l e a r l y shown by the presence of the c o n i c a l growths surrounding and even protruding from beneath the remaining coating metal. 7.1.5 Thiourea A d d i t i o n s The increased d i s s o l u t i o n of both coating metals with t h i o u r e a a d d i t i o n , p r i o r to 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 the f o r m a t i o n o f 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 , o r 2) e x t e n s i v e p o i s o n i n g o f the anode s u r f a c e due t o " d e s t r u c t i v e " a d s o r p t i o n o f t h i o u r e a ( w i t h c l e a v a g e o f the C = S bond) [104,1 I 2,116-118], which i n t u r n f o r c e s the 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 the anode s u r f a c e atom e f f e c t i v e l y g r e a t e r c u r r e n t d e n s i t y . Prolonged h i g h - p o t e n t i a l operation as a r e s u l t of thiour e a a d d i t i o n leads to premature anode f a i l u r e ( a f t e r passage of only 2.1 A*hr i n one case, compared with 410 A«hr f o r an e l e c t r o d e operated under s i m i l a r c o n d i t i o n s i n thiourea - f r e e 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 only be r e c o n c i l e d 208 with the large f r a c t i o n of coating metal which remains i f the e l e c t r i c a l i s o l a t i o n of the coating metal i s again considered. The l a r g e increase i n anode p o t e n t i a l which occurs immediately a f t e r t h i o u r e a a d d i t i o n may promote the r a p i d absorption of oxygen through the noble metal (discussed i n S e c t i o n 7.2.4), thus leading to the more-rapid establishment of an i n s u l a t i n g f i l m between the coating and substrate than i n the case of anodic operation 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 P a s s i v a t i o n 7.2.1 Surface Area Studies The gal v a n o s t a t i c charge curves f o r P-f/30 I r r T i e l e c t r o d e s resemble those f o r platinum and p l a t i n u m / i r i d i u m a l l o y wire e l e c t r o d e s inasmuch as they show h y s t e r e s i s between the oxygen l a y e r formation and removal r e g i o n s , y e t e x h i b i t charge balance between the anodic and cathodic processes. I t i s p o s s i b l e , however, t h a t the charge curves may represent a mixed s i t u a t i o n with separate c o n t r i b u t i o n s due to oxygen l a y e r formation and removal on both platinum and i r i d i u m - i n which case the i r r e v e r s i b l e formation and build-up of the e q u i v a l e n t of m u l t i l a y e r i r i d i u m oxides could be postulated by analogy with the known behaviour of i r i d i u m . In such event, the surface areas measured i n the present work would r e f e r only to that p o r t i o n , o f the anode surface which c o n s i s t s of platinum. No evidence 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 found i n t h i s work, however. Observations of a small (up to 10 per cent) increase 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 surface area with prolonged g a l v a n o s t a t i c o p e r a t i o n , 209 followed b y a decrease i n t h i s value, i s c o n s i s t e n t with the competing processes of noble metal surface roughening due to anodic d i s s o l u t i o n and the complete l o s s , or reduction i n s i z e , of i n d i v i d u a l coating f e a t u r e s . Such a mechanism i s a l s o c o n s i s t e n t with the roughness 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 surface areas of e l e c t r o d e s which had l o s t 20-30 per cent of t h e i r loading were lower than those of s i m i l a r e l e c t r o d e s which had higher l o a d i n g s . 7.2.2 Surface Oxygen Coverage For platinum and p l a t i n u m / i r i d i u m wire anodes, the surface oxygen coverage vs. p o t e n t i a l behaviour f o r the p o t e n t i a l region p r i o r to commencement of 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 coatings show s i m i l a r behaviour, but with somewhat higher coverages at 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 intermediate values between the case of platinum (and Pt/5-25 I r wires) and i r i d i u m . No evidence was found f o r g r e a t e r -than monolayer 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 wires or the c o a t i n g s , however. The analogy of the oxygen coverage behaviour of P t / I r a l l o y s with platinum extended i n t o the coverage vs. time conduct under c o n d i t i o n s of simultaneous oxygen e v o l u t i o n , with a maximum oxygen coverage of 2 < 0 < 3 achieved i n the longest runs employed i n the present work. Formation of "type I I " oxide was achieved with both a Pt/25 I r wire anode and a coated e l e c t r o d e (where the f i n a l e l e c t r o d e p o t e n t i a l of 2.3 v o l t s put the anode w i t h i n the r a t h e r small p o t e n t i a l range - 2.1-2.5 v o l t s (SHE) [315] - necessary f o r i t s growth). In g e n e r a l , the formation of m u l t i l a y e r oxide was not observed e i t h e r because the anode p o t e n t i a l 210 remained below the value necessary f o r i t s formation or because i t passed q u i c k l y through the region f o r i t s formation during the " 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 (SHE). 7.2.3 Time-dependence of O v e r p o t e n t i a l : R e v e r s i b l e Behaviour The change i n anode p o t e n t i a l with time observed i n anodic oxygen e v o l u t i o n occurs over the range of oxygen coverages beyond monolayer values. For the "oxide" theory, t h i s e f f e c t i s explained by the increase i n o x i d e - l a y e r thickness with time [25]. Sole [366] derived the time-dependence of the oxygen e v o l u t i o n r a t e at constant 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 of the v a r i a t i o n of the a c t i v a t i o n energy of oxygen evolu-t i o n with f r a c t i o n a l surface oxygen coverage. In the present case, however, monolayer coverage i s already a t t a i n e d p r i o r to commencement of oxygen e v o l u t i o n . This has been pointed out by Go r o d e t s k i i [297], who suggested th a t the increase i n anode p o t e n t i a l with time was due both to the d i f f u -s i o n of oxygen i n t o the metal and to the t i m e - v a r i a t i o n of the platinum-oxygen bond energy (as evidenced by the lowering of the p o t e n t i a l of the reduction plateaus f o r removal of surface 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 s i m i l a r reason f o r the tirne-dependence of o v e r p o t e n t i a l on Pt/30 I r - T i anodes can be postulated from the observed analogous depression of the reduction, pleateau to that of platinum. 7.2.4 Time-dependence of 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 Prolonged 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-mitted to 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 vs. time behaviour, 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 of the anode to s u s t a i n l o w - p o t e n t i a l operation on sub-sequent r e p e t i t i v e runs. Although such an e f f e c t may be postulated to be due to a profound decrease noble metal loading as a r e s u l t of c o r r o s i o n , the onset of i r r e v e r s i b i l i t y has been observed to occur i n one s p e c i f i c instance a f t e r only a 12% l o s s of ooating metal. Further, measurements of the surface area showed t h a t t h i s value had decreased only by 20 per cent p r i o r to the onset of i r r e v e r s i b l e o peration. The i r r e v e r s i b l e p o t e n t i a l vs. time behaviour may be r e c o n c i l e d with the high r e s i d u a l noble metal loadings and surface area i f the e l e c t r i c a l i s o l a t i o n of the coating metal i s again proposed to occur by means of the development of an oxide f i l m having i n s u l a t i n g q u a l i t i e s , between the coating and substrate metals. Bystrov [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 increase i n p o t e n t i a l with time f o r t i t a n i u m substrate anodes. The source of oxygen f o r the growth of an oxide f i l m beneath the coating metal may be oxygen absorbed i n t o the noble metal i t s e l f . Hoare [289-292] has shown that strong anodization of one side of 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 of oxygen to the other s i d e , with D = 1 0 - 1 2 cm 2/sec f o r the d i f f u s i o n c o e f f i c i e n t of oxygen i n platinum. The gradient f o r d i f f u s i o n i s created by the s a t u r a t i o n of the surface l a y e r s of the noble metal with oxygen. In order to c a l c u l a t e the f l u x of oxygen through a noble metal f i l m i t i s necessary to know the s o l u b i l i t y of oxygen i n the metal. This 212 value i s not known, although Hoare [291] suggests t h a t the oxygen s o l u -b i l i t y i n platinum amounts to one oxygen atom per u n i t c e l l of the FCC l a t t i c e . A be t t e r estimate of t h i s value can be made from c o n s i d e r a t i o n of h i s data [290] f o r the d i f f u s i o n of oxygen through a platinum diaphragm. I f the appearance of oxygen on the other side of the f o i l i s assumed to be due to the formation of a monolayer of oxygen on that s u r f a c e , i -31 n n u s p t atoms 1 0 atom 1 mole 0 _ 9 n o / n n N _ 9 moles , 9x 1.31(10] ^2— x 1 p t a t Q m x 6 ( 1 Q ) 2 3 o a t o m s - 2.18(10) - y (2) then the s o l u b i l i t y of oxygen i n platinum can be estimated from the known value of hi 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 required f o r the passage of the 2 .18 (10 ) - 9 moles of 0 atoms through the f o i l . The s o l u b i l i t y i s hence: S = 2.52(10)- 5 M f l ( 3 ) 'i Thus, assuming t h i s value holds f o r the Pt/30 I r a l l o y f i l m s , of average thickness 2 ( 1 0 ) _ 5 cm, the f l u x of oxygen through the c o a t i n g can be estimated by F i c k s ' f i r s t law: i _ Ode J = ~ d ^ m - 1 2 cm 2] f2.52(10)- 5 moles/cm : i o — l l n > 1 0 ) . b " m  sec 1 0 . i 2 m o l e s _ ( 4 ) e n r s e c v ' I f the oxygen which d i f f u s e s through the coating i s u t i l i z e d i n the formation of T i 0 2 (which contains 10" 8 moles 0/cm2 of surface a r e a ) , then a time of 10 4 seconds i s p r e d i c t e d f o r the formation of a s i n g l e mono-l a y e r of t h i s oxide between the coating and sub s t r a t e . As approximately 213 300 hours are required f o r the onset of i r r e v e r s i b l e p o t e n t i a l vs. time behaviour, as i n d i c a t e d i n Figures 6.34 and 6.35, f o r anodes operated at 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 of the surface l a y e r s of the metal with oxygen may be presumed from the work of Hoare [289-292], the t h i c k -ness of the i n s u l a t i n g oxide l a y e r i n such cases may be of the order of 100 monolayers. 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 high-p u r i t y r u t i l e (Table 1.4), namely 1 0 1 7 microohnrcm, and i s presumed to be o r i e n t e d with the c - l a t t i c e parameters (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 correspond to 10 5 v o l t s at the operating current d e n s i t y of 260 mA/cm2. C l e a r l y the r e s i s -t i v i t y of the l a y e r i n the present case i s not so high, or the l a y e r i s of d i f f e r i n g t hickness (which may be expected from the heterogeneous nature of the noble metal coating 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 to show intermediate p o l a r i z a t i o n behaviour between that of platinum and i r i d i u m , as may be expected f o r the case of the combination of two metals which show d i f f e r e n t e l e c t r o c a t a l y t i c a c t i v i t i e s f o r a given e l e c t r o d e r e a c t i o n . The Pt/30 I r - T i e l e c t r o d e s were found to be cl o s e to i r i d i u m i n behaviour as evidenced by the closeness of t h e i r p o l a r i z a t i o n curves. The P t / I r wire a l l o y s , while more a c t i v e than platinum, were i n f e r i o r to the a l l o y c o a t i n g s . S i m i l a r d i f f e r e n c e s i n the a c t i v i t y of other noble metal a l l o y s have been noted i n the l i t e r a -ture [367] and are a t t r i b u t e d to the e f f e c t s of manufacture and pre-treatments which may a l t e r the surface composition of the a l l o y from i t s bulk value. 214 7.2.6 Pulsed Current Operation I n i t i a l l y , pulsed current operation i s advantageous from the standpoint of energy-consumption during the anodic process, as the anode p o t e n t i a l i s not permitted to climb to values c h a r a c t e r i s t i c of the higher degrees of oxygen coverage encountered with prolonged D.C. operation. The c o r r o s i o n losses with p e r i o d i c current r e v e r a l s are so high, however, that the anode surface area i s r a p i d l y depleted to the stage where the anode p o t e n t i a l r i s e s to high values as a r e s u l t of the i n c r e a s e . i n the actual current d e n s i t y . F a i l u r e due to the build-up of an i n s u l a t i n g f i l m between the coating and substrate was not observed even with r e s i d u a l c oating l e v e l s as low as 0.06 g/m2, l i k e l y as a consequence of the lack of establishment of a d i f f u s i o n gradient f o r oxygen through the noble metal. Chapter 8 CONCLUSIONS 8.1 Corrosion The measurement of small changes i n both loading and composi-t i o n of Pt/30 I r - T i anodes produced as a r e s u l t of operation under condi-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 of 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 spectro-scopic technique and fundamental parameter c a l c u l a t i o n described i n Appendix A2. The l o s s of coating metal from such e l e c t r o d e s can, i n ge n e r a l , be explained by the simultaneous occurrence of both electrochemical d i s s o -l u t i o n and mechanical detachment,processes, whose r e l a t i v e predominance depends on the nature of the anode i t s e l f , as a consequence of d i f f e r e n c e s i n manufacture. For the anode material chosen f o r most of the present work - a sheet having 4.337 g/m2 average loading - the d i s s o l u t i o n process was found to be predominantly electr o c h e m i c a l i n nature. For the e l e c t r o l y t e 2M H 2S0 4 + 0.5M CuSO^ at 22° and 40°C, the mean c o r r o s i o n e f f i c i e n c i e s f o r the noble metals i n the coating are: Pt: 1.13 ug/A-hr I r : 0.35 ug/A-hr T o t a l : 1.48 ug/A-hr 215 216 I r i d i u m i s found to have analogous d i s s o l u t i o n behaviour to platinum, inasmuch as i t p a r a l l e l s the r a t e of oxygen e v o l u t i o n , but at a smaller r a t e than platinum 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 coating metal with i r i d i u m . P e r i o d i c current 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 acc e l e r a t e d l o s s of both coating metals, although i r i d i u m i s more r e s i s t a n t i n t h i s regard, leading to i r i d i u m enrichment of the r e s i d u a l c o a t i n g metal to the extent of 89 weight per cent i n the most extreme case. Thiourea does not act as a c o r r o s i o n i n h i b i t o r f o r the coated e l e c t r o d e s . On the c o n t r a r y , t h i s a d d i t i v e acts to promote r a p i d coating d i s s o l u t i o n and premature f a i l u r e of the anodes at concentrations of 0.05 and 0.5 g p l . Ultimate anode f a i l u r e i s preceded by an increase i n the coating metal l o s s rates by about two orders of magnitude over t h e i r e l e c t r o -chemical d i s s o l u t i o n rate values. Coating l o s s under these c o n d i t i o n s i s predominantly mechanical i n nature. Continued operation with a f a i l e d anode which may r e t a i n p o r t i o n s of e l e c t r i c a l l y i s o l a t e d , and hence i n a c t i v e , coating metal r e s u l t s i n intense l o c a l i z e d c o r r o s i o n of the s u b s t r a t e . 8.2 P a s s i v a t i o n Pt/30 I r - T i e l e c t r o d e s showed behaviour t y p i c a l of platinum in g a l v a n o s t a t i c charge s t u d i e s . Surface areas of new anodes from d i f f e r e n t sources di d not show a re g u l a r r e l a t i o n s h i p with noble metal l o a d i n g , although the higher roughness f a c t o r values were observed with anodes having higher loadings. The surface areas of anodes (average i n i t i a l l o a d i n g : 4.337 g/m2) 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 to 10 per cent with time, and then decrease p r o g r e s s i v e l y a f t e r l o s s of 10-20 per cent of the i n i t i a l l o ading. The Pt/30 I r - T i and P t / I r a l l o y wire anodes were found to develop oxygen coverage i n an analogous manner to platinum, a t t a i n i n g monolayer coverage with oxygen immediately p r i o r to the commencement of the e v o l u t i o n of oxygen on anodic charging, and then growing to the equi-v a l e n t of two to three monolayers with prolonged anodic operation with simultaneous oxygen e v o l u t i o n . In an i s o l a t e d case the growth of m u l t i -l a y e r "type I I " oxide was observed on a Pt/30 I r - T i anode. The p o l a r i z a t i o n behaviour of P t / I r a l l o y s i s intermediate between that of platinum and i r i d i u m , with Pt/30 I r - T i showing behaviour very c l o s e to that f o r i r i d i u m metal, the more e f f i c i e n t oxygen e l e c t r o -c a t a l y s t of the two metals considered. The p o t e n t i a l of Pt/30 I r - T i anodes shows both r e v e r s i b l e and i r r e v e r s i b l e behaviour with e l e c t r o l y s i s time. Under r e v e r s i b l e condi-t i o n s , which i s the case with new e l e c t r o d e s subjected to r e p e t i t i v e runs, the change i n anode p o t e n t i a l i s r e l a t e d to the nature of the surface oxygen coverage. Pulsed c u r r e n t operation i s not e f f e c t i v e i n maintaining anode p o t e n t i a l s at low values ( d e s i r a b l e from an economic viewpoint) as the high rates of coating l o s s r e s u l t i n decreased surface area, and con-sequently i n an increase i n the actual c u r r e n t d e n s i t y and hence higher anode p o t e n t i a l s . 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 explained i f the development of an i n s u l a t i n g f i l m i s considered to occur between the coating and substrate metals. The 218 growth of t h i s f i l m i s l i k e l y due to the d i f f u s i o n of absorbed oxygen through the noble metal c o a t i n g . As t h i s f i l m grows, the coa 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 the substrate u n t i l e v e n t u a l l y the el 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 metal, l e a d i n g to an exponential increase i n anode p o t e n t i a l to the voltage l i m i t of the power s u p p l i e s , which i s below the 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 of Pt/30 I r - T i Anodes f o r Copper Electrowinning In Section 1.2, several questions were r a i s e d concerning the a p p l i c a b i l i t y of noble metal coated t i t a n i u m anodes as i n s o l u b l e anodes i n copper e l e c t r o w i n n i n g . The c o r r o s i o n e f f i c i e n c y f o r the p a r t i c u l a r m a t e r i a l used i n most of t h i s work was found to be 1.13 ug/A-hr. The maximum l i f e t i m e of t h i s p a r t i c u l a r anode could thus be estimated a t 800 days f o r operation at 20 mA/cm2 (geometric area). Anodes with t h i c k e r coatings would have p r o p o r t i o n a t e l y longer maximum l i f e t i m e s , provided the coating l o s s rates were s i m i l a r . U nfortunately, the present work has revealed a high degree of v a r i a t i o n i n the coating metal l o s s rates f o r nominally s i m i l a r anodes from d i f f e r e n t manufacturing l o t s . D i f f e r e n t anode sheets, thus, may have considerable 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 current r e v e r s a l were found to enhance coating l o s s r a t e s , these would have to be avoided i n any c e l l s where coated t i t a n i u m anodes were employed. The increase i n anode p o t e n t i a l with time on the coated anodes i s undesirable from an energy consumption standpoint, although anode p o t e n t i a l s of 2.0 v o l t s S.H.E. and above were only a t t a i n e d i n short times 219 with current d e n s i t i e s much higher than those encountered i n copper e l e c -trowinning p r a c t i c e . Although pulsed current operation was found to be d e l e t e r i o u s to c o r r o s i o n behaviour, i n t e r m i t t e n t p u l s i n g ("on-periods" of the order of days) may y e t be found to be d e s i r a b l e from the point of view of maintaining 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 the time i t takes f o r the operating p o t e n t i a l to r i s e above some predetermined maximum (economic) value, such as 2.0 v o l t s S.H.E. 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APPENDIX Al THE COPPER/COPPER SULFATE ELECTRODE Using a reference electrode c o n s i s t i n g of a copper wire .dipped i n t o 2M HzSO^ + 0.5M CuS0 4 s o l u t i o n at 295°K as a "standard" the e f f e c t s of changes i n the e l e c t r o l y t e composition and the e f f e c t s of a d d i t i v e s and gas bubbling were i n v e s t i g a t e d f o r s i m i l a r copper wires dipped i n t o the res p e c t i v e s o l u t i o n s . The data are presented i n Table Al.1 and are pre-sented i n the form of d i f f e r e n c e s i n the measured e l e c t r o d e p o t e n t i a l (plus the 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 that of the "standard" copper/ copper s u l f a t e e l e c t r o d e . As a r e s u l t i t can be seen that the reference e l e c t r o d e p o t e n t i a l i s not g r e a t l y a f f e c t e d (does not change by, say, more than 10 mv) by rat h e r large v a r i a t i o n s i n the e l e c t r o l y t e composition -at l e a s t over the ranges of v a r i a t i o n of composition of experimental 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 lack of s i g n i f i c a n t e f f e c t s of gas bubbling or small p o l a r i z a t i o n s are a conse-quence of the high exchange current of the copper/cupric ion couple (which r e s u l t s i n anodic c o n t r o l of the mixed c o r r o s i o n p o t e n t i a l ) . 238 239 Table A l . l E f f e c t s of V a r i a t i o n s i n the E l e c t r o l y t e Composition on the C e l l : Cu(T)/H 2S0 4(M 1,T)+CuS04M 2,T)/2M H 2 S O - (295° )/H 2 ,Pt(295° ) A l l AE values are r e f e r r e d to the measured p o t e n t i a l with 2M H2SC\ + 0.5M CuSO^ s o l u t i o n a t 298°K, (E + E.) = .298 v o l t s vs. S.H.E. T Mi M2 Other AE ( v o l t s ) 295 8 0.5 -.012 4 0.5 -.010 2 0.5 0 1 0.5 + .018 .1 0.5 + .045 295 2 satd. + .002 2 0.5 0 2 0.2 -.016 2 0.1 -.025 2 0.01 -.034 350 2 0.5 + .023 295 2 0.5 +.03M CoS0 4 -.001 2 0.5 + a few drops EDTA 0 2 0.5 + 1M HN03 -.001 2 0.5 + 0 2 bubbling + .002 2 0.5 + He bubbling 0 295 2 0.5 + 1 uA p o l a r i z a t i o n 0 + 10 uA + .001 + 100 uA + .003 1 uA p o l a r i z a t i o n 0 - 10 uA -.007 - 100 uA -.012 APPENDIX A2 X-RAY FLUORESCENCE SPECTROSCOPY A2.1 Fundamental Parameter C a l c u l a t i o n of 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 The P t L ^ i and I r l _ a 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 the purposes of t h i s c a l c u l a t i o n as they are the most intense peaks observed during X-ray spectroscopy with continuous tungsten r a d i a t i o n , and because they are not cl o s e to any major tungsten c h a r a c t e r i s t i c r a d i a -t i o n peak which may otherwise i n t e r f e r e with the i n t e n s i t y measurement. The wavelengths of the c h a r a c t e r i s t i c r a d i a t i o n are: PtLcxi: 1. 313 angstroms I r L c t i : 1.352 angstroms Only t h a t part of the i n c i d e n t (primary) r a d i a t i o n having wavelengths l e s s than the L J T J adsorption edges i s capable of 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 r a d i a t i o n . Secondary fluorescence i s a l s o p o s s i b l e when on element has c h a r a c t e r i s t i c r a d i a t i o n peaks at wavelengths below the L J J J edge of the other element. B e r t i n [351] has described the t h e o r e t i c a l c a l c u l a t i o n of 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 of primary r a d i a t i o n impinging on a metal specimen, which enable the conversion of i n t e n s i t y data to 240 241 a n a l y t i c a l (composition) data. He a l s o reviews the various techniques f o r measurement of the thicknesses of metal f i l m s . For the present work, the t h e o r e t i c a l treatment has been extended to the p a r t i c u l a r case of P t / I r a l l o y coatings of v a r i a b l e thickness and composition. The f o l l o w i n g n otation i s used throughout t h i s s e c t i o n : I 0 ( 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 of the primary X-ray beam. r. the absorption jump r a t i o f o r element i . For L 1 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 of the absorption c o e f f i c i e n t of the element on the short wavelength s i d e to that on the long-wave-length side of the corresponding ( L T T T ) absorption edge. 1 U r . . - l the f r a c t i o n of 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 r i a given subshel1. the fluorescence y i e l d , or the p r o b a b i l i t y that vacancy created i n a given subshell w i l l r e s u l t i n a r a d i a t i v e t r a n s i t i o n . y(x). the p r o b a b i l i t y of a given e l e c t r o n t r a n s f e r i n a given s u b s h e l l , defined as the r e l a t i v e i n t e n s i t y of the l i n e i n question over the sum of 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 that p a r t i c u l a r subshel1. the mass absorption c o e f f i c i e n t vs. wavelength r e l a -t i o n . For each element t h i s r e l a t i o n has discon-t i n u i t i e s at the absorption edges. For an a l l o y , at a given wavelength, al 1 oy y i p where Wn- are the weight f r a c t i o n s of the a l l o y con-s t i t u e n t s . he mass absorption 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 . This 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 at a l l absorption edges corresponding to a l l of the a l l o y elements. y ( A ) a 1 1 o y P 242 u ( A ) . the l i n e a r absorption c o e f f i c i e n t vs. wavelength r e l a t i o n (obtained by m u l t i p l y i n g the mass absorp-t i o n c o e f f i c i e n t by the d e n s i t y of e l e m e n t . i i . u ( A ) ,, the l i n e a r absorption c o e f f i c i e n t vs. wavelength a y r e l a t i o n f o r the a l l o y (obtained by m u l t i p l y i n g the mass absorption c o e f f i c i e n t f o r the a l l o y by the d e n s i t y of the a l l o y ) . I ,1 the i n t e n s i t y of the primary r a d i a t i o n at some depth x x x x, and an inc r e m e n t a l l y greater depth, w i t h i n the specimen. 01 the angle of incidence of the primary r a d i a t i o n . 0 2 the angle of e x i t o f the f l u o r e s c e n t r a d i a t i o n to the c o l l i m a t o r . dfi 4TT the f r a c t i o n of 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 r a d i a -t i o n which passes through the c o l l i m a t o r . k i n t e n s i t y losses due to absorption i n aitr, i n d i f f r a c t i o n from the analy z i n g c r y s t a l , and i n the detector. k, • the "Lorenz f a c t o r " due to the d i f f e r e n c e i n the ' n Bragg c o n d i t i o n f o r the d i f f r a c t i o n of the P t L ^ i and IrLcti wavelengths. 1 K, L , i s i n 20 n Bragg,i I , . the i n t e n s i t y of c h a r a c t e r i s t i c r a d i a t i o n of element ' i produced by an incremental amount of primary r a d i a t i o n , w i t h i n a l a y e r Ax i n the specimen. iV . the i n t e n s i t y of c h a r a c t e r i s t i c r a d i a t i o n of element c a r ' n i produced by an incremental amount of primary r a d i a t i o n , at the surface of the specimen. i'' 1 . the i n t e n s i t y of c h a r a c t e r i s t i c r a d i a t i o n of element a r ' n i produced by an incremental amount of primary r a d i a t i o n , at the det e c t o r . I . . the t o t a l i n t e n s i t y of primary c h a r a c t e r i s t i c r a d i a -primary,! t i o n Q f e l e m e n t ^ a t t h e d e t e c t o r : (a) f o r an i n f i n i t e l y t h i c k specimen, (1^) primary, i , (b) f o r a specimen of t o t a l thickness x,°°(I ) primary, i A 243 I char/sec, j I sec, i I s e c , i I sec, l I t o t a l , i the i n t e n s i t y of other 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 of primary r a d i a t i o n w i t h i n a l a y e r Ax i n the specimen, which i s i t s e l f capable of . . 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 of element i . the amount of 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 the specimen. the amount of 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 the specimen that reaches the detector. .. the t o t a l i n t e n s i t y of 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 of element i at the detector: (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 of t o t a l thickness x, (I ) X SGC 51 the t o t a l i n t e n s i t y of (primary + 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 of element i at the detector: (a) f o r an i n f i n i t e l y t h i c k specimen, ( I O 0 ) t 0 ^ a ] -j (b) f o r a specimen of t o t a l thickness x, ( I ^ ) t Q t a - j ^ The d e r i v a t i o n of fundamental equations f o r the t o t a l charac-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 i n t e n s i t y f o r element i in v o l v e s the c a l -c u l a t i o n of the v a r i a t i o n of the i n t e n s i t y of that p o r t i o n of the primary r a d i a t i o n whose wavelengths are s u f f i c i e n t to e x c i t e the 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 , with depth i n the specimen metal f i l m , assumed to be p e r f e c t l y smooth and homogeneous. Consequently, the e f f i c i e n c y of conversion of the primary r a d i a t i o n to the de 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 can be c a l c u l a t e d , followed by a s i m i l a r c a l c u l a t i o n of the v a r i a t i o n of the i n t e n s i t y of the escaping 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 the c o l l i m a t o r , as i t reaches the specimen surface from various depths. I n t e n s i t y losses i n the 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 considered to give a value f o r the i n t e n s i t y of t h i s r a d i a t i o n at the d e t e c t o r . Secondary fluorescence e f f e c t s are evaluated by c o n s i d e r i n g the production of other c h a r a c t e r i s t i c r a d i a t i o n of s u f f i c i e n t energy to e x c i t e the des 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 the other element. This c a l c u l a t i o n i n v o l v e s , as above, the determination of the i n t e n s i t i e s of such other 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 capable of secondary f l u o r e s -cent e x c i t a t i o n , the e f f i c i e n c i e s of such e x c i t a t i o n , and the r e s u l t a n t a t t e n u a t i o n of the 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 the specimen at d i f f e r e n t depths towards the c o l l i m a t o r and u l t i -mately to the detector. As several terms i n the above c a l c u l a t i o n cannot be r e a d i l y evaluated, the approach taken i n v o l v e s the determination of r e l a t i v e i n t e n s i t y values, where an i n f i n i t e l y t h i c k pure platinum specimen i s considered f o r t h i s purpose. By t h i s means, the indeterminate terms cancel. Consider an incremental amount of primary r a d i a t i o n , I 0 A A , impinging on the specimen. The wavelength range of t h i s p o r t i o n of the i n c i d e n t beam i s s u f f i c i e n t l y small that i t can be considered monochromatic. Consequently, the equation f o r the absorption of the primary r a d i a t i o n [ 3 5 2 ] , g i v i n g the i n t e n s i t y of t h a t p o r t i o n of the primary r a d i a t i o n at some depth x w i t h i n the specimen, can be w r i t t e n : [ x = I 0 A A exp ' u ( X ) a n o y . x s i n 91 0) S i m i l a r l y , the equation f o r the i n t e n s i t y of the incremental amount of the primary r a d i a t i o n a t an inc r e m e n t a l l y greater depth x + Ax: 245 Jx+Ax = I o A A e x P •y(A) a l l o y (x + Ax) s i n 91 (2) Thus the amount of the incremental p o r t i o n 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 : lx ' W = I o A X e X P ^ y ( A ) a i l o y _ * x s i n 01 1 - exp -u(A) a l loy s i n 01 Ax (3) As Ax approaches zero, equation (3) s i m p l i f i e s t o : I -. W 5 I o A A e x p f -u (A ) a 11 oy s i n 01 'u(A) a l l o y s i n 01 (4) Of the amount of absorbed r a d i a t i o n , the amount absorbed by element i depends on the actual amount of th a t element present. Only the f r a c t i o n P ^ a l l o y (5) can absorb the i n c i d e n t photons and thus be re s p o n s i b l e f o r the production of the des 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 f o r element i . The e f f i c i e n c y of production of t h i s r a d i a t i o n i s given by the product of the 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 des 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 the 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 of the des i r e d l i n e . Thus, the i n t e n s i t y of 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 the increment of primary r a d i a t i o n w i t h i n the l a y e r Ax i s given by: I char, i r . - l r i w i ' g i I - I , » x x+Ax W. l P U ; a l l o y (6) 246 The i n t e n s i t y of 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 c o r r e c t l y to pass through the c o l l i m a t o r i s given by c o n s i d e r a t i o n of the absorp-t i o n of t h i s r a d i a t i o n as i t passes through the specimen. Thus, at the specimen s u r f a c e , the i n t e n s i t y of the primary c h a r a c t e r i s t i c r a d i a t i o n which was produced over the depth x to x + Ax by an incremental amount of primary i n c i d e n t r a d i a t i o n i s : * c h a r , i * c h a r , i d f i diT exp c h a r , i s i n 62 a l l o y (7) Further, the i n t e n s i t y of t h i s r a d i a t i o n at the de t e c t o r i s : I c h a r , i v L , i I c h a r , i (8) The i n t e n s i t y of 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 the detector as a r e s u l t of i r r a d i a t i o n of the specimen by the continuous range of wave-lengths capable of 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 r a d i a t i o n , and a r i s i n g from a l l depths w i t h i n the specimen i s hence: I primary,l I I SWL fOO W . i d x d x (9) where i n t e g r a t i o n s are performed over the short wavelength l i m i t of the primary i n c i d e n t r a d i a t i o n (SWL) to the wavelength corresponding to the L J J J edge, and over a l l depths i n the specimen. For, convenience i f we l e t 247 K i 4TT V 1 oi i • g i • Wn- (10) then the i n t e n s i t y f o r primary 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 primary,l sinOl I o U ) SWL P U ; a l l o y p s i n 61 + _ dX (11) char, i s i n 02 al 1 oy For a specimen of thickness x, on the other hand: ^ I x ) primary,i -in s i n G 1 Io(X) 1 - exp -c h a r , i sin..01 s i n 02 a Hoy P u ; a l 1 o , y + p s i n 01 s i n (12) char, i In order to account f o r the secondary f l u o r e s c e n t enhancement due to those peaks of element j which are capable of 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 consider an analogous d e r i v a t i o n f o r the production of t h i s other c h a r a c t e r i s t i c r a d i a t i o n of element j . By comparison with equation ( 6 ) , and c o n s i d e r i n g a l l n l i n e s capable of causing 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 : 248 y i = y L char/sec,j L n n r . - l _ J r • J J co . • q . J,n s j , n x x+Ax w. J P ( X ) a n o y J j That p o r t i o n of the above r a d i a t i o n which i s converted to secondary f l u o -rescent r a d i a t i o n of element i i s : i . = y sec,n L n (I , , .) v c h a r / s e c , j ' V 1 r. • co. • q. • W. cha r , j ,n char,j,n a l l o y The amount of 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 the l a y e r Ax which reaches the detector i s : I. s e c , i I s e c , i l , i dfi 4T ' e x P [ Xchar , i J s i n 62 a l l o y On c o n s i d e r a t i o n of the t o t a l i n t e n s i t y of 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 various terms f o r each of the n c h a r a c t e r i s t i c l i n e s capable of producing secondary fl u o r e s c e n c e , f -Is* r . - l r . I J J J,n 3 j , n y [ char,j,n char,j,n a l l o y then i t can be shown that the r e s u l t a n t equations f o r the secondary f l u o -rescent i n t e n s i t y from a specimen of thickness x i s : 249 ^ x ^ s e c , i P -W. n J L ^-(x) edge,j,n p v"M, T m . f s i n e ! I o ( A ) 1 -exp v y a 1 l o y _ . s i n 01 SWL p u ' a l l o y p + — C h a r > 1 ^ l l o y ] . ) s i n 62 j x char , i s i n 61 s i n 02 a l l o y (17) In the present case, the i n t e n s i t y of a pure platinum specimen of i n f i n i t e thickness was evaluated according to equation (11) and a computer programme was w r i t t e n i n order to provide values of the i n t e n s i t y r a t i o s according t o : I t o t a l , i I primary,i I t o t a l , P t sec, i I primary, Pt (18) Relevant 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 , e x c i t a -t i o n 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 r a d i a t i o n wave-lengths and absorption edges v/ere obtained from several sources [351-365], For the purposes of the c a l c u l a t i o n the 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 estimated by measuring the r e l a t i v e i n t e n s i t y p r o f i l e of the s c a t t e r e d beam produced by i n s e r t i n g a piece of p l a s t i c i n the specimen chamber of the X-ray spectrograph. The r e l a t i o n was subse-quently expressed as a 5th-order polynomial f o r the c a l c u l a t i o n . The mass absorption c o e f f i c i e n t vs. wavelength r e l a t i o n s f o r platinum and i r i d i u m were f i t t e d to curves of the form: 250 e = a + bA3 (19) between each d i s c o n t i n u i t y (absorption edge) over the r e l e v a n t range of wavelengths (up to the I j j j edge). The i n t e g r a t i o n s shown i n equations (11) and (12) are consequently not p o s s i b l e due to the presence of several d i s c o n t i n u i t i e s i n the mass absorption c o e f f i c i e n t vs wavelength r e l a t i o n f o r the a l l o y . This problem was circumvented by c o n s i d e r i n g a l l i n t e g r a -t i o n s to be the summation of a l l 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 over the d e s i r e d wavelength range. For the production of IrLcn charac-t e r i s t i c r a d i a t i o n , f o r example, equations (11) and (12) are evaluated over the sum: IrL II swL PtL, swL IrL, PtL I PtL IrL II I IrL PtL PtL II IrL II IrL PtL I I I I I I (20) Secondary fluorescence was only considered f o r those other char-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 at l e a s t 10 per cent that of the Lai l i n e . Under these c o n d i t i o n s , only the I r L y i l i n e was considered to be capable of producing secondary PtLcti r a d i a t i o n , and the PtL|3 2 and PtLy6 l i n e s were considered capable of e x c i t i n g secondary I r L a i r a d i a t i o n . A2.2 Preparation of Platinum-Coated Titanium Standards A Hummer D.C. Sput t e r i n g System was employed to produce platinum loadings on f l a t t i t a n i u m sheet substrates cut i n t o d i s c s of the same s i z e as the anodes prepared from the commercial Pt/30 I r - T i sheet described i n Section 4.1.1. 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 strong 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. S p u t t e r i n g was performed i n a vacuum of 150-200 m i l l i t o r r at 1000 v o l t s and 10 mA. A f t e r s p u t t e r i n g , the specimens were re-weighed. Platinum loadings were determined by d i v i d i n g the weight gain by the exposed area of the t i t a n i u m sheets. The specimens were then placed i n a P h i l i p s X-ray Spectrograph such t h a t they were centred w i t h i n the primary beam, provided by a tungsten X-ray tube operated at 35 KV and 15 mA. 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 of 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 at 38.05° 20. Measurements performed on an X-ray spectrometer have an inherent l i m i t of accuracy which depends on the counting procedure and the s t a b i l i t y of the equipment. As the a r r i v a l of photons at the d e t e c t o r i s purely random with time, the arrangement of a large number of counts i s found to obey a Gaussian d i s t r i b u t i o n . The c o e f f i c i e n t of v a r i a t i o n e i s the s t a -t i s t i c a l term most commonly a p p l i e d i n spectroscopy. (The p r o b a b i l i t i e s are 68.3, 95.4 and 99.7% that the true mean value l i e s w i t h i n ± e, 2e, and 3e% of the measured average value, r e s p e c t i v e l y . ) In the absence of systemic e r r o r s the accuracy of a measurement i s r e l a t e d to the number of counts N, taken i n a given i n t e r v a l and the number of r e p l i c a t e counting periods, n: tri'lOO 1% / i o o \ to me = • — (m=l,2,3 ) (2 > ¥ v^ n The p r e c i s i o n of the measured average value under i d e a l c o n d i t i o n s i s hence l i m i t e d only by.the t o t a l number of counts taken. Actual operating 252 c o n d i t i o n s are 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 i s : e = Jz1 counting s t a t i s t i c s + E e z equipment e r r o r s (22) Indeed, when counting i s performed on a peak of high i n t e n s i t y (thus pro- -ducing a large number of t o t a l counts) the predominant e r r o r i s due to equipment i n s t a b i l i t y . This s i t u a t i o n makes i t impossible to 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 counting e r r o r i s , say, an order of magnitude l e s s than the equipment e r r o r . In the present case, measurements of 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 with 25 r e p l i -cate determinations. The counting e r r o r and the measured 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 f o r the various specimens are shown i n Table A2.1. Instrument i n s t a b i l i t y (long term e r r o r ) can be compensated to a great degree by repeated measurements on a s u i t a b l e standard. The equipment i s i n i t i a l l y adjusted to provide a given count t o t a l on the standard and a l l subsequent measurements on the standard are compared with t h i s "standard" value. That i s , subsequent determinations of the average of a number of r e p l i c a t e counting i n t e r v a l s with the standard are d i v i d e d i n t o the accepted standard value to provide 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 to the count t o t a l s measured f o r other specimens e i t h e r immediately p r i o r to or a f t e r measure-ment with the standard. The standard may be a s t a b l e specimen of any convenient kind. The average t o t a l count f i g u r e s i n Table A2.1 must be c o r r e c t e d f o r both the dead-time of the p r o p o r t i o n a l counter, and f o r the background 253 Table A2.1 P r e c i s i o n of Measured Count Values ( P t L a i I n t e n s i t y & Background) on Prepared P t - T i Standards, and Comparison with E r r o r Due to Counting S t a t i s t i c s Specimen Average Total Counts (17 sec) No. of Count Periods ^ e ^ c o u n t i n g = 300 TuTn-^ £ ^measured = 300(f) 0 gPt/m 2 1467 15 2.02% 10.06% 0.872 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 Pt f o i l 152446 25 0.15 3.47 254 r a d i a t i o n . The former i s r e a d i l y evaluated according to the equation: -r . . Measured count rate / 0 0 \ True count rate = 1 _ ( M e a s u r e d C Q U n t r a t e ) ( D e a d t i m e ) (23) For a p r o p o r t i o n a l counter, the dead time i s about 2 microseconds [356]. Since i t i s impossible to d i r e c t l y measure the i n t e n s i t y of the background of platinum-coated specimens at the P t L a i angle, the v a r i a t i o n of the background with loading was measured at 35° 20, where no i n t e r -ference i s encountered from t i t a n i u m , tungsten, or platinum l i n e s . The v a r i a t i o n was then added to the background determined at the P t L a i angle (38.05° 20) f o r a bare t i t a n i u m specimen. The background was found to increase s l i g h t l y with measured P t L a i counts. Table A2.2 summarizes the 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 with the prepared P t - T i standards, with the f i n a l r e s u l t s being expressed as an i n t e n s i t y r e l a t i v e to a t h i c k platinum standard. A2.3 Conversion of Measured I n t e n s i t y Data to A n a l y t i c a l Data The r e l a t i v e i n t e n s i t y vs. noble metal loading data f o r se l e c t e d P t / I r a l l o y s of various composition are given i n Figures A2.1 and A2.2 f o r P t L a i and I r L a x r a d i a t i o n , r e s p e c t i v e l y . As can be seen, the 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 values f o r the prepared P t - T i standards c l o s e l y conform to the t h e o r e t i c a l l y p r e d i c t e d curve. For P t / I r a l l o y coatings of unknown loading and composition, the a n a l y t i c a l data were determined by means of another computer programme which incorporated the r e s u l t s of the fundamental parameter c a l c u l a t i o n 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 Radiation Measured with Prepared P t - T i Standards, with the Resultant True Peak I n t e n s i t i e s and Ratios R e l a t i v e to a Thick Platinum Standard (17 Second Counting I n t e r v a l ) Specimen Measured PtLct! Counts Dead-Time Corrected Counts C a l c u l a t e d Background True PtLai Counts (I ) A Peak: Background Ratio I X I 0 0 0 gPt/m 2 1467 1467 2380 0 0 0.0000 0.872 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 Pt f o i l 152446 155070 2380 152690 64 1.000 256 Noble metal loading , g (Pt -Hr ) /m 2 Figure A2.1 R e l a t i v e P t L a i i n t e n s i t y vs. loading r e l a t i o n f o r various P t / I r a l l o y s , f o r 9 i = 60°, 9 2 = 35°, W-tube operated at 35 kv. Data points r e f e r to the 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 the prepared Pt - T i standards. 257 Noble metal loading, g(Pt+lr)/m 2 Figure A2.2. R e l a t i v e IrLcti i n t e n s i t y vs. loading r e l a t i o n f o r various P t / I r a l l o y s f o r 9 i = 60°, 6 2 = 35°, W-tube operated at 35 kv. 258 described in Section 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 to provide an i n i t i a l estimate of the weight f r a c t i o n s of the a l l o y components. Background c o r r e c t i o n s f o r I r L a i and P t L a i i n t e n s i t i e s were estimated by using l i n e a r sums (weighted to the estimated wiehgt f r a c t i o n s of the components) of the background vs. t o t a l counts r e l a t i o n s estimated or d i r e c t l y measured f o r the I r L a i and P t a x wavelengths on Pt-Ti and I r - T i specimens. On background c o r r e c t i o n , the r a t i o of the I r L a i and P t L a i i n t e n s i t i e s was used to determine a second estimate of the weight f r a c t i o n s of the a l l o y components, according to a r e l a t i o n determined from the fundamental parameter c a l c u l a t i o n : W I r H = f n W P t ^ I r L a i , t o t a l , I n . . , t o t a l [ P t L a i (24) The second estimate was compared with the i n i t i a l estimate, and i f the d i f f e r e n c e was greater than 0.1 per cent, the backgrounds were r e c a l c u l a t e d . This i t e r a t i v e procedure was repeated u n t i l the weight f r a c t i o n estimates converged. The next step then in v o l v e d the loading determination. I t was found that curves of the type shown i n Figure A2.1 could be expressed by equations of the form: Pt loading = p x l n ( l + y) + p 2 In(1 - y) (25) where PtL a i i n t e n s i t y produced from an a l l o y of thickness x  W t • 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 Pt standard (26) 259 and p! and p 2 are fu n c t i o n s of Wpt- The form of equation (25) i s par-t i c u l a r l y useful i n that i t permits the mathematical r e l a t i o n of the e n t i r e f a m i l y of curves of the type shown i n Figure A2.1 with only two parameters, y et pr 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 to the t h e o r e t i c a l equations. Once the platinum loading was determined, the i r i d i u m loading was r e a d i l y provided from the weight f r a c t i o n of th a t metal. APPENDIX A.3 I K - D K O P C A L C U L A T I O N S FOR E L E C T R O L Y T E SOLUT IONS EMPLOYED IN THE PRESENT WuRK ACCORD ING TO T H E EQUAT ION OF BMRNARTT C 3 5 7 ] FOR PLANAR E L E C T R O D E S : i 6 "(D 4) = -3 ; k where IR i D d k "IR k IR drop i n mi 1 1 i v o l t s a p p l i e d (geometric) current d e n s i t y i n mA/cm2 working electrode/Luggin c a p i l l a r y t i p separation i n cm diameter of the 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 - 1 c n r 1 ) E l e c t r o l y t e Temperature °C k (from r e f . ohm-1 cm-[358]) i i mA/cm2 V [R ^ 6=0.1 cm 6=0.2 cm 6=1 cm 2M H 2S(H 22 0.71 7.8 1 2 11 15.6 2 4 22 52.1 7 15 73 104 15 29 146 260 37 73 366 521 73 147 734 2M H 2S0 4 40 0.80 52.1 7 13 65 104 13 26 130 260 33 65 325 521 65 130 561 2M H2SC\ + 22 0.62 52.1 8 17 84 0.5M CuS0 4 104 17 34 168 117 19 38 189 260 42 84 419 521 84 168 840 2M H 2S0 4 + 40 0.68 52.1 8 15 77 0.5M CuS0 4 104 15 31 153 117 17 34 172 260 38 76 382 521 77 153 766 260 APPENDIX A4 ESTIMATION OF INDIVIDUAL ION ACTIVITIES A4.1 A c t i v i t y of the Hydrogen Ion i n S u l f u r i c A c i d S o l u t i o n s The problem of determining i n d i v i d u a l ion a c t i v i t i e s i s two-f o l d , as i t involve s the es t i m a t i o n of both the concentrating of the ion in the system of i n t e r e s t and i t s i n d i v i d u a l i o n i c a c t i v i t y c o e f f i c i e n t . For s u l f u r i c a c i d s o l u t i o n s , the problem i s made more d i f f i c u l t by the v a r i a t i o n of the second concentration 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: „ aH a S 0 , mH mSL\ . YH Y S 0 , 1/ aHSCU HS0 4 YHS0, C Q K Young [359,360] has determined the concentrations of the i n d i -vidual i o n i c species 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 his data these c o n c e n t r a t i o n s , expressed as m o l a l i t i e s , can be determined.for any e l e c t r o l y t e s of i n t e r e s t . Further, the concentration e q u i l i b r i u m con-s t a n t , IO> , and the r a t i o of the 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 -mated. These c a l c u l a t i o n s are summarized i n Table A4.1. In d i v i d u a l 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 estimated 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 of Harned [362], who measured the EMF of the c e l l : 261 262 Table A4.1 I n d i v i d u a l Ion M o l a l i t i e s , Ionic Strengths, E q u i l i b r i u m Constants and Ionic A c t i v i t y C o e f f i c i e n t Ratios C a l c u l a t e d from the Raman Spectra Data of Young [736] f o r Aqueous S u l f u r i c A c i d S o l u t i o n s (K 2 values from Robinson [361]) t °c H ?S0 4 HSOV m so;2 H + I K 2 K2,c y _ Y H ^ S 0 , M m R YHS0\ 25 .1 .1 .07 .03 .13 .16 .0106 .056 .19 1 1.04 .73 .31 1.35 1 .66 II .57 .019 2 2.17 1.5 .70 2.9 3.60 II 1 .35 .0079 4 4.75 3.1 1.7 6.4 8.15 II 3.51 .0030 8 12.2 9.5 2.8 15.0 17.9 n 4.42 .0024 40 .1 .1 .075 .025 .125 .15 .0064 .042 .15 1 1.04 .78 .26 1.3 1.56 n .43 .015 2 2.17 1.6 .54 2.7 3.23 M .91 .0070 4 4.75 3.4 1 .3 6.1 7.35 II 2.73 .0023 8 12.2 10.0 2.3 14.6 16.9 3.36 .0012 60 .1 .1 .08 .02 .12 .14 .0036 .030 .12 1 1.04 .83 .21 1.25 1.46 II .32 .011 2 2.17 1.7 .43 2.6 3.01 M .66 .0055 4 4.75 3.8 .95 5.7 6.65 II 1.43 .0025 8 12.2 10.6 1.7 14.0 15.7 II 2.25 .0016 80 .1 .1 .085 .015 .115 .13 .002 .020 .10 1 1.04 .88 .16 1.2 1.36 n .22 .0091 2 2.17 1 .8 .32 2.5 2.79 n .44 .0045 4 4.75 4.0 .71 5.5 6.17 II .98 .0020 8 12.2 11.1 1.2 13.4 14.7 1.45 .0014 263 Pt, H 2|H 2SC\(m)|PbS0 4|Pb0 2, Pt (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 to the c e l l EMF by E = E° + §- ln(4m 3 Y ± 3 ) - f i In a 2 , ^ (3) where a l l terms have t h e i r conventional s i g n i f i c a n c e . Such an expression assumes complete d i s s o c a t i o n of 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 ions. As t h i s i s not true 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 values of 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 ) = mY±(complete d i s s o c i a t i o n ) ( 4 ) m H 2 m S 0 , using the i n d i v i d u a l ion m o l a l i t i e s c a l c u l a t e d from Young's data. (The mean a c t i v i t y c o e f f i c i e n t s determined by equation (4) do not d i f f e r g r e a t l y from those tabulated by Harned, and c o i n c i d e f o r the case of complete d i s -s o c i a t i o n . ) 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 are given i n Table A4.2. The a c t i v i t y of the hydrogen ion may be estimated by several means. Four methods are described here: 1) Assume = 1. The a^ values are hence d i r e c t l y c a l c u l a b l e from the values given i n Table A4.1, using the mean i o n i c a c t i v i t y coef-f i c i e n t values i n Table A4.2 and the equation: V Y S 0 , 1/3 (5) 264 2) Assume Y U / Y ^ Q ^ = "I- The r e s u l t a n t Y S Q value determined from the r a t i o y R c a l c u l a t e d i n Table A4.1 can then be used, i n conjunction with 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 values i n Table A4.2 and equation (5) to permit estimation of y ^ . 3) Assume that y< -Q can be estimated from the tabulated 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 other binary s u l f a t e s a l t : s o l u t i o n s of s i m i l a r i o n i c s t r e n g t h , on the assumption that y ± - y . . = y c r i f o r c a n o n o U 4 t h a t e l e c t r o l y t e . 4) Assume that y ^ can be estimated from the tabu 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 of s i m i l a r i o n i c s t r e n g t h , on the assumption that y ± = y H = y ^ f o r that e l e c t r o l y t e . The r e s u l t s of these c a l c u l a t i o n s are summarized i n Table A4.3. The d i v e r -gence of the hydrogen a c t i v i t y values f o r s o l u t i o n s above 0.01M, as c a l c u -l a t e d by the above methods, can be r e a d i l y seen i n Figure A4.1 which shows the v a r i a t i o n of c a l c u l a t e d s o l u t i o n pH with c o n c e n t r a t i o n . A4.2 A c t i v i t i e s i n Copper-containing S u l f u r i c A c i d S o l u t i o n s 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 a c i d / c u p r i c s u l f a t e s o l u t i o n s . 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 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 can be estimated, however, by co n s i d e r i n g t h a t they are the same as those i n s o l u t i o n s of the i n d i v i d u a l e l e c t r o l y t e s at the same i o n i c strength as the mixed e l e c t r o l y t e . The i o n i c s t r e n g t h s , 265 Table A4.2 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 A c i d S o l u t i o n s , Y ± , from the Data of Harned [362], Recalculated to Account f o r the Incomplete D i s s o c i a t i o n of S u l f u r i c A c i d t H 2S0 4 Y ± Complete Y ± Incomplete Y ± Incomplete 3 °c M m D i s s o c i a t i o n D i s s o c i a t i o n D i s s o c i a t i o n 25 .1 .1 .265 .332 .0366 1 1.04 .130 .163 .00433 2 2.17 .130 .156 .00380 4 4.75 .212 .245 .0147 8 12.2 .835 1.19 1.69 40 .1 .1 .227 .311 .0301 1 1.04 .111 .152 .00351 2 2.17 .105 .144 .00299 4 4.75 .160 .209 .00913 8 12.2 .535 .829 .570 60 .1 .1 .197 .298 .0265 1 1.04 .092 .139 .00269 2 4.75 .120 .182 .00603 8 12.2 .347 .611 .228 80* .1 .1 .185 .317 .0319 1 1 .04 .080 .136 .00252 2 2.17 .075 .129 .00215 4 4.75 .100 .171 .00500 8 12.2 .275 .560 .176 Estimated y± values f o r complete d i s s o c i a t i o n . Table A4.3 Estimated 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 Acid S o l u t i o n s , Based on the Four Methods Described i n the Text, U t i l i z i n g the In 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 Table A4.1 and the Mean Ionic 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 H 2 S O 4 I Method (1) Method (2) Method (3) Method (4) M m YH Y S 0 , aH YH Y S 0 , aH YH Y S 0 , aH YH Y S 0 , aH 25 .1 .1 .16 1 .037 .13 .44 .19 .057 .39 .24 .051 .779 .060 .10 1 1.04 1.66 1 .0043 1.35 .48 .019 .65 .25 .068 .34 .929 .0050 1.25 2 2.17 3.60 1 .0038 2.9 .69 .0079 2.00 .29 .046 .84 1.567 .0015 4.54 4 4.75 8.15 ] .015 6.4 2.21 .0030 14.1 .63 .037 4.03 6.21 .00038 40. 8 12.2 17.9 1 1.69 15.0 26.5 .0024 398. 6.06 .046 91. 55. 8 ( 1 0 ) - 7 825. 40 .1 .1 .15 1 .030 .125 .45 .15 .056 .35 .25 .043 .782 .049 .10 1 1.04 1.56 1 .0035 1.3 .48 .015 .63 .22 .070 .29 .908 .0039 1.18 2 2.17 3.23 1 .0030 2.17 .65 .0070 1.69 .25 .048 .67 1.409 .0015 3.80 4 4.75 7.35 1 .0091 6.1 1.99 .0023 11.3 .50 .037 3.02 4.91 .00038 30. 8 12.2 16.9 1 .57 1.4.6 21.8 .0012 318. 3.56 .045 52. 45. .00028 657. 60 .1 .1 .14 1 .026 .12 .47 .12 .056 .32 .26 .038 .784 .043 .094 1 1.04 1.46 1 .0027 1.25 .50 .011 .62 .19 .073 .24 .888 .0034 1.11 2 2.17 3.01 1 .0022 2.6 .63 .0055 1.63 .21 .050 .54 1.320 .0012 3.43 4 4.75 6.65 1 .0060 5.7 1.55 .0025 8.9 .40 .038 2.27 3.97 .00038 22.6 8 12.2 15.7 1 ,.23 14.0 11.9 .0016 167. 2.30 .043 32. 39. .00014 559. 80 .1 .1 .13 1 .032 .115 .56 .10 .065 .34 .27 .040 .787 .052 .091 1 1.04 1.36 1 .0025 1.2 .53 .0091 .63 .18 .076 .22 .869 .0033 1.04 2 2.17 2.79 1 .0022 2.5 .69 .0045 1.73 .21 .048 .53 1.089 .0018 2.72 4 4.75 6.17 1 .0050 5.5 1.58 .0020 8.7 .36 .039 1.97 3.42 .00043 18.8 8 12.2 14.7 1 ...18 13.4 11.2 .0014 150. 2.10 .040 28. 32.1 .00017 430. cn cn 267 Figure A4.1. pH of concentrated s u l f u r i c a c i d s o l u t i o n s by various c a l c u l a t i o n s . 268 however, cannot be r e a d i l y determined as the a d d i t i o n of s u l f a t e ions a f f e c t s the values of the concentrations of hydrogen and b i s u l f a t e ions i n order to conform to the second d i s s o c i a t i o n constant. The i o n i c strength can be obtained by an i t e r a t i v e procedure of e s t i m a t i o n , c a l c u l a t i o n of i o n i c e q u i l i b r i a , comparison, and refinement of the estimate, e t c e t e r a . For a p a r t i c u l a r e l e c t r o l y t e , 2M H 2S0 4 + 0.5M CuSC\, the estimated a c t i v i t i e s of the hydrogen ion and c u p r i c ions are given i n Table A4.4. The copper ion a c t i v i t y was c a l c u l a t e d on the assumption that ( Y ± ) r c-n = ~ Yen ' A4.3 Reference Electrode P o t e n t i a l s The reference 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 the Nernst equations f o r the hydrogen electrode and copper/copper s u l f a t e e l e c t r o d e using the estimated i n d i v i d u a l 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 of the standard e l e c t r o d e p o t e n t i a l s were obtained from De Bethune [363]. These r e s u l t s are summarized i n Table A4.5. In a d d i t i o n , the el e c t r o d e p o t e n t i a l values f o r the commercial mercury/mercurous s u l f a t e e l ectrode as given by Caton [364] are included f o r completeness. 269 Table A4.4 Ionic M o l a l i t i e s , I o n i c Strength, Second D i s s o c i a t i o n Constant, Mean Ionic 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 the E l e c t r o l y t e : 2M H 2S0 4 + 0.5M CuS0 4 Temperature (°C) 25 40 60 80 ""rlSO., 1.51 1.64 1.9 2.05 mso 4 1.16 1.03 .77 .61 mR 2.85 2.72 2.47 2.32 I 5.5 5.2 4.7 4.4 K2c 2.2 1.7 1.0 .7 ( Y ± ) H 2 S ( V .192 .175 .154 .149 ( Y ± ) C U S 0 , .037 .038 .039 .040 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 .74 .65 a u (method 4) n 7.9 6.7 5.4 4.6 aCu .019 .019 .020 .020 270 Table A4.5 Ca l c u l a t e d Reference Electrode P o t e n t i a l s with Respect to the Hypothetical Standard Hydrogen Electrode (Neglecting L i q u i d Junctions) f o r the C e l l : E l e c t r o d e ( T ) / E l e c t r o l y t e CO/SHE (298° K) Reference Electrode t°C E° ( v o l t s ) E ( v o l t s ) Pt,H 2/2M H 2S0 4 25 0 1.35 (method 1) .65 (method 2) .34 (method 3) 1.25 (method 4) .027 .018 -.004 .039 40 .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 Cu/2M H 2S0 4 + .5M CuSO,, 25 .337 .019 .286 40 .350 .019 .297 60 .368 .020 .312 80 .385 .020 .325 Hg/HgS0 l tsatd/K 2S0 4 25 .612 .656* 40 .613 .657 60 .614 .658 80 .615 .659 Data from Caton [364]. 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 of oxygen p r e v i o u s l y deposited on an anode p o l a r i z e d to 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 to the amount of charge consumed i n t h i s process, which i n turn can be equated with the amount of charge necessary to remove a monolayer of oxygen atoms (1:1 sto i c h i o m e t r y with surface noble metal atoms) [219]. surfaces i s not known, r e l i a b l e estimates of these values were made by W i l l [328] and hence on the basis of the 2-electron t r a n s f e r r e a c t i o n : Although the exact atom "density" of a re a l cm 2 of noble metal M + H 20 M-0 + 2H + + 2e (1) monolayer charge values can be c a l c u l a t e d . These are: monolayer = 420 uC/real cm2 (2) Q, monolayer] = 440 uC/real cm2 (3) I r 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 values. 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 i s thus given by the product of the t r a n s i t i o n time, T , and the a p p l i e d c u r r e n t , I , d i v i d e d by the monolayer charge f o r a rea l cm 2 of e l e c t r o d e surface: A....... = -rn (4) actual Q monolayer a l l o y Roughness f a c t o r s are i n turn c a l c u l a t e d by d i v i s i o n of the value given b-(4), by the geometric electrode area: 

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