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Corrosion behaviour of nickel and monel in aqueous fluoride media. Ney, Hugh Daniel Wallingford 1964

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CORROSION BEHAVIOUR OF NICKEL AND MONEL' IN AQUEOUS FLUORIDE MEDIA  by  HUGH D . W. NEY  \  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED  SCIENCE  IN THE DEPARTMENT of METALLURGY  We a c c e p t t h i s t h e s i s as conforming t o r e q u i r e d standard  •Members o f the Department  the  of Metallurgy  THE UNIVERSITY OF BRITISH COLUMBIA February  196^  In p r e s e n t i n g the  t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of  requirements f o r an advanced degree at the  University  o f B r i t i s h Columbia, I agree that the  L i b r a r y s h a l l make  it  study.  f r e e l y a v a i l a b l e f o r reference  agree that  and  I further  permission f o r extensive copying of t h i s t h e s i s  f o r s c h o l a r l y purposes may  be granted by the  Department or by h i s r e p r e s e n t a t i v e s .  Head of  my  It i s understood  that  copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l  gain  s h a l l not be allowed without my  Department of  METALLURGY  The U n i v e r s i t y of B r i t i s h Vancouver Canada. Date  February  Columbia,  1964  written  permission.  ABSTRACT  The c o r r o s i o n b e h a v i o u r of n i c k e l and monel i n aqueous f l u o r i d e s o l u t i o n s was s t u d i e d by p o t e n t i o s t a t i c surface  examination of the  corroded  p o l a r i z a t i o n t e c h n i q u e s and  specimens.  N i c k e l does not e x h i b i t t h e u s u a l a c t i v e - p a s s i v e for 0 <  4.0 but corrodes  pH •<  In the range k.O < active  regions.  6.5  pH <  <  pH <  r a p i d l y e s p e c i a l l y at  transition  the g r a i n b o u n d a r i e s .  6.5 the n i c k e l - p o l a r i z a t i o n curve c o n t a i n s  two  N i c k e l i s p a s s i v e i n contaat w i t h a f l u o r i d e s o l u t i o n t w i t h  12.0.  P o l a r i z a t i o n curves  o f n i c k e l i n f l u o r i d e s o l u t i o n s of v a r i e d  p H ' s and f l u o r i d e i o n c o n c e n t r a t i o n s that the current  i n the range 4.0 <  as a f u n c t i o n - o f p o t e n t i a l i n the  pH <  7.0  f i r s t active  revealed  region is'  independent of f l u o r i d e i o n c o n c e n t r a t i o n but dependent on p H . -The i n the f i r s t p a s s i v e and second a c t i v e fluoride ion concentration.  r e g i o n s a r e a f u n c t i o n o f pH and  S u r f a c e examinations  at the g r a i n b o u n d a r i e s i n t h e second a c t i v e proposed which accounts  currents'  showed t h a t n i c k e l  region.  corrodes  A mechanism has been  f o r c o r r o s i o n i n the second a c t i v e  r e g i o n by F  a d s o r p t i o n and p a s s i v a t i o n by e i t h e r H 0 or OH a d s o r p t i o n on the a n o d i c a l l y 2  p o l a r i z e d metal s u r f a c e . • A mathematical a n a l y s i s based on c o m p e t i t i v e a d s o r p t i o n of these s p e c i e s t o be c o n s i s t e n t  as a f u n c t i o n o f e l e c t r o d e  with the experimental  p o t e n t i a l i s shown  data.  Monel corrodes at: ^.ess t h a n h a l f the. r a t e o f n i c k e l at the mixed potential i n fluoride solutions with 0 < overvoltage.  Monel e x h i b i t s  pH <  active-passive  w i t h the p a s s i v e c u r r e n t up t o 6 times as  6.5 due t o i t s  l a r g e r hydrogen  b e h a v i o u r s i m i l a r t o n i c k e l but  large.  ACKNOWLEDGEMENT  . The a u t h o r wishes t o express a p p r e c i a t i o n t o members of  the  Department o f M e t a l l u r g y , p a r t i c u l a r l y t o D r . E . P e t e r s and M r . W . M. Armstrong, who d i r e c t e d t h i s work. Mrs.  S p e c i a l thanks a r e extended  W. M. Armstrong f o r many h e l p f u l  to  discussions.  . F i n a n c i a l support from Aluminium L a b o r a t o r i e s  L i m i t e d i n the  form o f a F e l l o w s h i p and from the N a t i o n a l Research C o u n c i l under Grant No. A-1463, i s g r a t e f u l l y acknowledged.  TABLE OF CONTENTS Page  INTRODUCTION .  .  .  P r e v i o u s Work .  . .  . . . . . . . . . . . . .  1  • Potent i o s t a t i c - . P o l a r i z a t i o n  5  p H - P o t e n t i a l Diagram f o r N i - H 0 2  . . . . .  Purpose and Scope of P r e s e n t I n v e s t i g a t i o n APPARATUS AND EXPERIMENTAL Electrochemical  12 .•  14  . . . . . . . . . .  C e l l and E l e c t r i c a l Apparatus  Materials  16  . . . . . .  16  '  20  P o l a r i z a t i o n Curves S u r f a c e Examination  V  .........  2k  Nickel in Acid Fluoride.Solutions Nickel i n Neutral Fluoride Solutions N i c k e l i n Basic Fluoride Solutions  . . . . . . . . . . . . . . . .  Mechanism of N i c k e l C o r r o s i o n i n F l u o r i d e Media Monel  21  23  RESULTS AND DISCUSSION  "  1  .  24 27 44  . . . . .  46 56  CONCLUSIONS: i fl  60  RECOMMENDATIONS FOR FUTURE INVESTIGATIONS  62  REFERENCES  63  APPENDIX A  65  LIST OF FIGURES Figure 1.  Page T y p i c a l P o l a r i z a t i o n Curve of a M e t a l w i t h an A c t i v e Passive T r a n s i t i o n  2.  3.  4  C a t h o d i c Curves w i t h D i f f e r e n t Redox Exchange Superimposed' on an Anodic Curve ....... P o l a r i z a t i o n Curves Showing E f f e c t Redox Exchange C u r r e n t  .  .  .  Currents ... . . .  9  o f Change i n C a t h o d i c .  .  .  .  .  .  ...  .  .  .  10  4.  Anodic or C a t h o d i c Rate C o n t r o l of the C o r r o s i o n C u r r e n t  H  5.  N i c k e l - W a t e r p H - P o t e n t i a l Diagram .  13  6.  Electrochemical  7.  Schematic Diagram o f E l e c t r i c a l Apparatus  8.  P o l a r i z a t i o n Curve o f N i c k e l i n F l u o r i d e S o l u t i o n  .  .  .'  Corrosion C e l l  17 ,  19 at  pH = 2 . 2 9.  2  Mixed P o t e n t i a l  v e r s u s pH .  26  10.  S u r f a c e s o f N i c k e l C o r r o d e d i n F l u o r i d e s at  11.  P o l a r i z a t i o n Curve of N i c k e l i n F l u o r i d e S o l u t i o n  Low pH  .  .  29  12.  Exchange C u r r e n t v e r s u s pH  .  13.  C r i t i c a l Anodic C u r r e n t v e r s u s pH .  .  31 .  .  .  . . . . . . .  31  . L o g a r i t h m of the Minimum P a s s i v e C u r r e n t v e r s u s pH + pF  15.  C u r r e n t - T i m e Curves at S e l e c t e d  16.  P o l a r i z a t i o n Curve i n 0.42 M NaF s o l u t i o n at pH = 6 . 2 S u r f a c e s o f N i c k e l i n the F i r s t A c t i v e S t a t e C o r r o d e d i n 0.42 M-NaF at pH = 6 . 2 . . . . . . . . . . . . S u r f a c e s o f N i c k e l i n the P a s s i v e S t a t e C o r r o d e d i n  17.  Regions  18.  Potential  the  19.  Log C u r r e n t D e n s i t y at  20.  pH p l u s p F The P o t e n t i a l a t the I n i t i a t i o n Region v e r s u s pH p l u s 2pF  32  of N i c k e l  34 35  S o l u t i o n s o f pH = 6 . 2 at  28  at  pE = 6.2 . . . .  14.  5  36  Second A c t i v e Peak v e r s u s . p H p l u s p F  .  38  the Second A c t i v e Peak v e r s u s 38 o f the Second A c t i v e 39  L i s t of Figures. C o n t i n u e d . . . . Figure  ^  Page  21;  Positive  Slope i n Second A c t i v e Region v e r s u s pH +. pF .  22.  P o l a r i z a t i o n Curve of N i c k e l i n 0.2 M NaCl S o l u t i o n at pH =  23.  6.2  .  .27.  J  .Potential  ^5  F u n c t i o n of Ion i n V i c i n i t y o f a Charged ?  M i c r o - s t r u c t u r e ' of N i c k e l Showing Second Phase Flade P o t e n t i a l ' / E p , ,  and P o t e n t i a l a t  Initiation .  .  .  .  .  .  5  2  of  .  5^  P o l a r i z a t i o n Curve o f Mionel i n Q.k2 M NaF S o l u t i o n of pH =  29.  51  . . . . . . . . .  Second A c t i v e Region v e r s u s pH . 28.  ^3  .  11 J3  Electrode 26.  .  Solutions-at  P o l a r i z a t i o n Curve o f N i c k e l i n 0.^2 M-NaF S o l u t i o n at pH =  . 25.  hi  S u r f a c e s of N i c k e l C o r r o d e d i n F l u o r i d e pH =  2k.  6.1  ^-0  6". 2  57  P o l a r i z a t i o n Curve of Monel i n 0.^2 M NaCl S o l u t i o n at pH =  6.2  59  LIST OF TABLES  Table I .  Experimental  Conditions  Table I I .  P o l a r i z a b i l i t i e s and R e f r a c t i o n s o f the H a l i d e Ions OH"" and H 0 2  Table I I I .  Exchange^Currents of Monel and N i c k e l i n S i m i l a r Solutions . . . . .  INTRODUCTION  .Previous.Work  The c o r r o s i o n r e s i s t a n c e o f n i c k e l and. n i c k e l a l l o y s t o f l u o r i n e , hydrogen f l u o r i d e and aqueous f l u o r i d e s i s w e l l known.  A review"*" o f •materials  r e s i s t a n t t o f l u o r i n e and i t s ..compounds s t a t e s t h a t n i c k e l and' monel --(70$ nickel,  30^  copper) are the most v e r s a t i l e .  N i c k e l and n i c k e l a l l o y s  are 2  used i n uranium f u e l d i f f u s i o n p l a n t s , and  i n alkylation plants,  plants  a l l o f which use f l u o r i n e , hydrogen f l u o r i d e ,  o t h e r f l u o r i d e s , • a n h y d r o u s , or Some q u a l i t a t i v e  uranium f u e l r e p r o c e s s i n g  or  aqueous.  r e s e a r c h e s . have been done on the c o r r o s i o n  of  k n i c k e l and i t s  alloys in fluorides.  Takhtarova and Antonovskaya  monel and " n i c k e l t o - b e r e s i s t a n t t o K F . H F , vapor phases i n the absence of oxygen.  aqueous,  Both S c h u s s l e r  4  4  Of t h e s e ,  5 most c o r r o s i v e .  NH F, NH F.HF,  found  HF i n l i q u i d  h y d r o f l u o r i c a c i d was  s i l v e r solders  and Braun  found monel t o be r e s i s t a n t  Braun a l s o found, t h a t g a l v a n i c  A l t h o u g h none of t h e . p r e v i o u s  in  the a t t a c k on monel.  work on n i c k e l c o r r o s i o n  in fluoride solutions,  to  c o u p l i n g w i t h s i l v e r and .  i n the presence of oxygen i n c r e a s e d  p o l a r i z a t i o n studies  the  6  c o n c e n t r a t e d HF but t h a t exposure t o the atmosphere r e s u l t e d  severe a t t a c k .  and  described  some work has been-done  on the  e f f e c t o f o t h e r halogen i o n s on the p o l a r i z a t i o n o f n i c k e l i n s u l p h a t e s . 7 ft Turner , u s i n g 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 t e c h n i q u e s , found t h a t c h l o r i d e  ft T h i s r e f e r s t o the method: o f a p p l y i n g a c o n s t a n t e x t e r n a l c u r r e n t an e l e c t r o d e and f o l l o w i n g the change i n e l e c t r o d e p o t e n t i a l w i t h t i m e .  to  - 2  i o n s i n c r e a s e d the electrode.  c u r r e n t t h a t was r e q u i r e d t o p a s s i v a t e a n i c k e l  T h i s was a t t r i b u t e d  than NiS04 which was suggested passivating  -  t o the f a c t t h a t N i C l t o be the  first  2  i s more  soluble  f i l m . f o r m e d i n the  mechanism. 8  More r e c e n t l y , p o l a r i z a t i o n techniques  Trueriipler a n d . K e l l e r , u s i n g p o t e n t i o s t a t i c (holding electrode p o t e n t i a l  o f c u r r e n t and time) s t u d i e d the a f f e c t o f C l b e h a v i o u r of n i c k e l i n s u l p h a t e ions increased  the a c t i v i t y  solution.  constant  independent  on the  passivation  and Br  They t o o found t h a t  halogen  o f n i c k e l ( d e f i n e d here as the anodic d i s s o l u t i o n  current of n i c k e l i n a c t i v e p o t e n t i a l r e g i o n s ,  see F i g u r e ' 1 ) .  o f halogen i o n s a l s o caused a second a c t i v e r e g i o n above  -300  The p r e s e n c e mV.  In both  ++ active regions  the c o r r o s i o n p r o c e s s y i e l d e d N i  ions.  . H a l i d e i o n s have a l s o been r e p o r t e d t o produce a secondary  activity  9 i n z i r c o n i u m , magnesium and aluminium  s i m i l a r t o t h a t f o u n d . b y Truempler  and K e l l e r except t h a t the e l e c t r o d e d i d not p a s s i v a t e a g a i n . a c t i v e peak r e s u l t e d  in p i t t i n g corrosion.  T h i s second  The mechanism as e x p l a i n e d by  K o l o t y r k i n ^ w i l l be d i s c u s s e d l a t e r i n r e f e r e n c e t o the p r e s e n t work. he a t t r i b u t e s the  second a c t i v e peak t o t h e a d s o r p t i o n of the h a l i d e  due t o t h e i r p o l a r i z a b i l i t y a t r e p l a c e the p a s s i v a t i n g  higher electrode p o t e n t i a l s  oxygen a t  random s i t e s .  so t h a t  concentration  o f h a l i d e i o n s " i s i n i t i a t e d and m a i n t a i n e d a t  s i t e s as the h a l i d e i o n s c a r r y the c u r r e n t t o  ions  they  The h a l i d e i o n s cause  n i c k e l t o i o n i z e by f o r m i n g complexes w i t h the n i c k e l atoms.  The the  dissolution  electrolytes.  11 Tronstad  s t u d i e d the  t h a t the p a s s i v i t y  f i l m s on n i c k e l by p o l a r i z e d l i g h t .  i s due t o the  the  critical  them.  Other work has been done on n i c k e l i n s u l p h a t e  Briefly,  He proposes  f o r m a t i o n o f an oxide f i l m which grows  -3 t o a constant  thickness  of about kO A .  M a c G i l l a v r y et a l .  p o t e n t i a l s . i n s o l u t i o n s of v a r i o u s . f o r e i g n - i o n s . of n i c k e l i n the presence i o n s w i t h the oxide  of 0  He proposed t h a t d i s s o l u t i o n  takes p l a c e by. the i n t e r a c t i o n o f hydrogen  2  film.  Vett.er and A r n o l d results  studied n i c k e l  13  and Osterwald and U h l i g  f o r the p o t e n t i o s t a t i c  lk  obtained s i m i l a r  p o l a r i z a t i o n of n i c k e l i n sulphuric a c i d .  However,. t h e y p o s t u l a t e d d i f f e r e n t mechanisms f o r the p a s s i v a t i n g  process.  V e t t e r f o l l o w s E v a n ' s s c h o o l i n p r o p o s i n g t h a t the p a s s i v a t i o n r e s u l t s  from  oxide or hydroxide f i l m f o r m a t i o n whereas U h l i g m a i n t a i n s t h a t chemisorbed oxygen from water p a s s i v a t e s  the metal  surface.  The q u e s t i o n of. the mechanism,of m e t a l p a s s i v a t i o n i s p r o b a b l y one  15 of d e f i n i t i o n as much as c o n c e p t .  Evans  , in suggesting t h i s ,  t h a t a. t h r e e - d i m e n s i o n a l f i l m i s n e c e s s a r y  has  stated  f o r a m e t a l t o remain passive, under  changing c o n d i t i o n s . .He r e a d i l y admits t h a t , upon a t t a i n i n g the F l a d e p o t e n t i a l (E-p i n F i g u r e 1) rosion.  l e s s than a monolayer o f oxygen i s s u f f i c i e n t t o stop  The monolayer w i l l t e n d t o grow t o an oxide f i l m ,  depending on the p o t e n t i a l and c h a r a c t e r of the s u p p o r t i n g  the  cor-  thickness  electrolyte.  16 Bune a n d . K o l o t y r k i n  have used, v a r i o u s , o x i d a n t s  i n sulphate  solutions  t o show t h a t these produce', anodic c u r r e n t s which p o l a r i z e the n i c k e l i n the same way t h a t a n i c k e l e l e c t r o d e  can be p o l a r i z e d p o t e n t i o s t a t i c a l l y .  concluded t h a t the d i s s o l u t i o n c u r r e n t  o f n i c k e l is. a f u n c t i o n of the  p o t e n t i a l o n l y . . T h i s i s important as i t  They electrode  i n d i c a t e s t h a t m e t a l s . c a n be c h e m i -  c a l l y p a s s i v a t e d by the a d d i t i o n o f o x i d i z i n g agents t o the c o r r o s i v e medium.  Potential F i g u r e 1.  T y p i c a l P o l a r i z a t i o n Curve of a M e t a l w i t h an Active-Passive Transistion.  i i  Potentiostatic  Polarization  P o l a r i z a t i o n of metals i n s o l u t i o n has l o n g been used as a method t o e l u c i d a t e c o r r o s i o n mechanisms and t o determine the p o s s i b i l i t i e s of a n o d i c p r o t e c t i o n of the m e t a l .  However, i t  i s o n l y w i t h i n the past  ten  y e a r s t h a t p o t e n t i o s t a t i c . t e c h n i q u e s have been e x t e n s i v e l y . u s e d .  Modern  t e c h n i q u e s are based on the use o f an e l e c t r o n i c  potentiostat.  device c a l l e d a  T h i s instrument d e t e c t s any v a r i a t i o n of the e l e c t r o d e respect  to a reference  p o t e n t i a l with  e l e c t r o d e which i s . i n the c i r c u i t .  then brought back' t o t h e p r e s e t  The p o t e n t i a l i s  v a l u e by a u t o m a t i c a l l y a d j u s t i n g - t h e  f l o w i n g between the working e l e c t r o d e  and an a u x i l i a r y e l e c t r o d e  current  (see.Figure  7).  17 - 2 3 Much has been w r i t t e n about 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  The essence of these w i l l be b r i e f l y summarized h e r e .  Potentiostatic  niques are u s u a l l y a p p l i e d t o the study of p a s s i v a t i n g e l e c t r o d e s  tech-  and i t  is  i n t h i s c o n n e c t i o n t h e y w i l l be d i s c u s s e d . F i g u r e 1 shows t y p i c a l p o t e n t i o s t a t i c curves f o r a m e t a l showing ah a c t i v e - p a s s i v e p o t e n t i a l o f any e l e c t r o d e t h a t p o t e n t i a l at on i t s  or i f oxygen i s  transition.  +  +  2e  In many i n s t a n c e s t h e c a t h o d i c r e a c t i o n H  -  2  ,  l/20  2  +  a n d t h e anodic r e a c t i o n —$>-  H 0  +  2  2e"  is M  + +  +  2e~.  20H",  (b)  The mixed or r e s t  which the r a t e o f the c a t h o d i c and a n o d i c r e a c t i o n s  present  M  and g a l v a n o s t a t i c  i n s o l u t i o n (no e x t e r n a l l y a p p l i e d c u r r e n t )  s u r f a c e are e q u a l . 2H  (a)  is  is occuring.. „ ;  - 6  I f the e l e c t r o d e  i s p o l a r i z e d c a t h o d i c a l l y , from t h e mixed p o t e n t i a l  "by the a p p l i c a t i o n of an e x t e r n a l and i t s  rate increases  current  the  cathodic reaction  .The r e l a t i o n between the l o g a r i t h m of  and the p o t e n t i a l of the e l e c t r o d e  I f the anodic r e a c t i o n  a n o d i c c u r r e n t may f a l l  as shown the e l e c t r o d e  the f o r m a t i o n o f a p r o t e c t i v e  becomes p a s s i v e .  This  is  - A s l o n g as  the anodic p o t e n t i a l causes no f u r t h e r  increase  The c o r r o s i o n r a t e i s . u s u a l l y c o n t r o l l e d by the r a t e of m e t a l .  ion d i f f u s i o n through the passive  layer:..,  ••At some h i g h e r p o t e n t i a l i n the anodic d i r e c t i o n , the becomes t r a n s p a s s i v e  (i.e.  the c u r r e n t b e g i n s t o r i s e  steeply)  breakdown or. the i n i t i a t i o n o f another anodic r e a c t i o n . curve can t h e n be s u b d i v i d e d i n t o a . c a t h o d i c p a s s i v e and. t r a n s p a s s i v e  p o l a r i z a t i o n curve which c h a r a c t e r i z e s t h a t p a r t i c u l a r medium.  i  0  due t o  film  The p o l a r i z a t i o n region  parts.  be determined from the  the m e t a l ' s  By e x t r a p o l a t i n g . t h e  c o r r o s i o n behaviour i n  l i n e a r p o r t i o n s o f the  r e g i o n , the c o r r o s i o n or mixed p o t e n t i a l s  or exchange c u r r e n t ,  electrode  r e g i o n and a n . a n o d i c  C e r t a i n e l e c t r o c h e m i c a l parameters.may  i n the a c t i v e  active.  oxide or hydroxide f i l m becomes k i n e t i c -  a l l y f a v o u r a b l e or oxygen i s chemisorbed on the metal s u r f a c e .  with a c t i v e ,  is  p o l a r i z a t i o n i n the anodic d i r e c t i o n the  s h a r p l y as the e l e c t r o d e  the passive state p e r s i s t s  current  i s known as the p o l a r i z a t i o n c u r v e .  increases  However,.on f u r t h e r p o t e n t i o s t a t i c  in current.  predominates  with further p o l a r i z a t i o n . ..Similar considerations  apply to anodic p o l a r i z a t i o n .  because  -  E  m  and the  corrosion  , are determined by the i n t e r s e c t i o n , . F i g u r e  exchange c u r r e n t  i s the r a t e o f the anodic and c a t h o d i c r e a c t i o n s  on the e l e c t r o d e  s u r f a c e w i t h no e x t e r n a l l y a p p l i e d c u r r e n t .  curve  1.  The  occuring  I f the  anodic  -7c u r r e n t i s the o x i d a t i o n of the m e t a l t o m e t a l i o n s , then the,exchange c u r r e n t g i v e s the c o r r o s i o n r a t e  19  .  The s l o p e s o f t h e l i n e a r r e g i o n s of the a n o d i c p r o c e s s ,  ( T a f e l slopes)  are  characteristics  u s u a l l y metal d i s s o l u t i o n +n  -M and the c a t h o d i c  —  M  +  ne  process  2H  +  +  2e~  — ^  H  2  or l/2  0  .+  2  H 0 2  +  2e~  _ ^  20H~  L i n e a r r e g i o n s o f t h e p o l a r i z a t i o n curve i n d i c a t e an a c t i v a t i o n or c o n c e n t r a t i o n c o n t r o l l e d , r e a c t i o n , b o t h o f which f i t the T a f e l - e q u a t i o n  '  -  E  where. E •E % a i  0  - o E  =  * l  a.+  =  b log  i  i s the p o t e n t i a l o f the e l e c t r o d e , i s . t h e r e v e r s i b l e p o t e n t i a l f o r the e l e c t r o d e i s the o v e r v o l t a g e or p o l a r i z a t i o n , and b are T a f e l c o n s t a n t s , and i s the c u r r e n t d e n s i t y .  The T a f e l e q u a t i o n can be d e r i v e d , 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 an a c t i v a t i o n c o n t r o l l e d p r o c e s s  and  b  =  a  =  reaction,  2k  .  For  i t can be shown t h a t  2.$ RT F  c<n  2.3 RT F  log,i  Q  <Xn  where R i s the u n i v e r s a l gas c o n s t a n t , •T i s the a b s o l u t e t e m p e r a t u r e , n i s the number o f e l e c t r o n s b e i n g exchanged at the a c t i v a t i o n barrier F i s the Faraday constant and c< i s a f a c t o r . b e t w e e n o and 1 r e p r e s e n t i n g the symmetry of the a c t i v a t i o n b a r r i e r .  -.8 -  These r e l a t i o n s h i p s show,that measurement p e r m i t s the e s t i m a t i o n of  n.  I f n i s . known ( e . g . . i f  step i s i d e n t i c a l t o t h e o v e r a l l Ni i n which n = 2)  —>•  Ni  + +  the  slope  rate-determining  reaction  ' .+  2e~  then o< i s o b t a i n e d .  number of e l e c t r o n s  of the T a f e l  However, n may not be known because  t a k i n g p a r t p e r molecule i n the r a t e - d e t e r m i n i n g  i s n o t n e c e s s a r i l y i d e n t i c a l with'the number i n . the can be assumed t h a t  overall reaction.  step If  i s a p p r o x i m a t e l y 0 . 5 , the number of e l e c t r o n s  the r a t e - d e t e r m i n i n g s t e p may be a s c e r t a i n e d .  I f b becomes  r a t e - c o n t r o l l i n g s t e p i s not e l e c t r o c h e m i c a l .  Thus an examination of  T a f e l p l o t o f an e l e c t r o c h e m i c a l o f the r a t e - d e t e r m i n i n g  infinite  r e a c t i o n can sometimes r e v e a l the  the  it  in then-the the  nature  step.  Three important parameters from the a n o d i c p o l a r i z a t i o n curve o f a m e t a l showing an a c t i v e - p a s s i v e density,  _"_ crit,  p o t e n t i a l at  1  t r a n s i t i o n are  the c r i t i c a l a n o d i c  the p r i m a r y p a s s i v e p o t e n t i a l Epp (the  the a c t i v e  current  current  d e n s i t y and  maximum) and the F l a d e p o t e n t i a l Ep (the most noble  p o t e n t i a l at which an e l e c t r o d e  w i l l be s e l f - a c t i v a t e d  after  passivation).  These i n d i c a t e the ease of p a s s i v a t i o n and s t a b i l i t y o f t h e p a s s i v e c o n d i t i o n of a metal.  . Low v a l u e s o f : . i c r i t and a c t i v e  ease o f p a s s i v a t i o n .  An a c t i v e  i n u n s t a b l e environments.  v a l u e s o f Bgp and-Ep  v a l u e of Ep a l s o i n d i c a t e s  stable  lead;  to  passivation  A l o n g range o f p o t e n t i a l t h r o u g h which t h e  metal  remains p a s s i v e and a s m a l l p a s s i v e c o r r o s i o n r a t e i p , enhance the p o s s i b i l i t i e s for protecting  the metal, by a n o d i c p o l a r i z a t i o n .  . Th'is can be i l l u s t r a t e d by  c o n s i d e r i n g d i f f e r e n t a c t i v a t i o n - c o n t r o l l e d r e d u c t i o n p o l a r i z a t i o n curves when superimposed on the anodic c u r v e . -exchange c u r r e n t s  are  shown i n F i g u r e  Three such curves w i t h d i f f e r e n t 2.  23  redox  - 9 -  ©^  1  •—N  ©\  .!>» -P •H  C  ©^  CQ  a a  V  -p <D  u u  o o  Active  Potential  Figure 2.  Noble  C a t h o d i c Curves w i t h D i f f e r e n t Redox Exchange C u r r e n t s Superimposed on a n A n o d i c C u r v e . 1  F o r case 1 the  rate of o x i d a t i o n equals the r a t e o f r e d u c t i o n  p o i n t A i n d i c a t i n g t h a t the r a p i d l y ) at  electrode i s  the r e s t p o t e n t i a l .  where the a n o d i c and c a t h o d i c  i n the a c t i v e s t a t e  For case 2 however t h e r e are  r a t e s are  equal.  p o i n t s B and D r e p r e s e n t s t a b l e p o t e n t i a l s . c o n d i t i o n s of the or the p a s s i v e will  corrode  at  .It  c o n d i t i o n i s most e a s i l y a t t a i n e d  points  can be shown t h a t  only  E n v i r o n m e n t a l and h i s t o r i c a l i n the  I n case 3 the e l e c t r o d e i s p a s s i v e  the r a t e i n d i c a t e d , by the p a s s i v e  3 r e p r e s e n t s the most f a v o u r a b l e  corrodes  three  e l e c t r o d e w i l l govern whether the e l e c t r o d e i s  condition o r b o t h .  t o the r e v e r s i b l e  (i.e.  at  current.  and  O b v i o u s l y case  condition for corrosion resistance. i f Epp and Ep are  active  This  f a i r l y a c t i v e with respect  hydrogen p o t e n t i a l and i f i-.^-crif i s  small, i t  s t a b l e i f Ejp i s a c t i v e and. the p o t e n t i a l range of p a s s i v a t i o n  is  is. most extensive  - 10 -  and i t  i s most r e s i s t a n t  i f the p a s s i v e c u r r e n t  s t a t i c p o l a r i z a t i o n of e l e c t r o d e s  \  •P Ci QJ  U U  (dashed l i n e s  indicate  /  •rl CO  a  b , and c .  above  current.)  -p  P  Potentio-  i n c o n d i t i o n s r e p r e s e n t e d , by the  cases y i e l d s curves as.shown i n F i g u r e s J a , cathodic  is negligible.  / \  \  1 1 1 1  /  / \ / \l ll  \  J  o  O  Potential  Potential (a)  Case  (b). Case  1.  .  Potential (c)  F i g u r e 3•  2.  Case 3•  P o l a r i z a t i o n Curves Showing E f f e c t o f Change i n C a t h o d i c Redox Exchange C u r r e n t .  P o l a r i z a t i o n curves are a l s o u s e f u l i n i n d i c a t i n g the c o n t r o l l i n g r e a c t i o n a t the" mixed p o t e n t i a l :  that i s ,  rate-  they i n d i c a t e  whether  the r a t e o f c o r r o s i o n i s c a t h o d i c a l l y , a n o d i c a l l y o r j o i n t l y c o n t r o l l e d and i f  - 11  i n each case the r a t e - c o n t r o l l i n g Four c a s e s , activation  step i s  electrochemical  i n which.the r a t e - c o n t r o l l i n g step i s . e i t h e r or. d i f f u s i o n ,  are  and d .  Potential Xontrol-  •-(-b) ~ O a t h o d i c _ A c t i v a t i o n . C o n t r o l  Potential  Potential  (c). Anodic D i f f u s i o n C o n t r o l  Figure 4.  electrochemical  i l l u s t r a t e d . i n F i g u r e s 4 a , b , c,  Potential 4 a.). AnodLc...Actj  or p h y s i c a l .  (d) C a t h o d i c D i f f u s i o n C o n t r o l  Anodic or C a t h o d i c Rate C o n t r o l o f the C o r r o s i o n Current.  There are c e r t a i n drawbacks t o the use o f p o l a r i z a t i o n curves the above p u r p o s e s . a l l electrochemical  One i s t h a t e x t e r n a l reactions  currents  only represent  o c c u r i n g at the e l e c t r o d e  surface.  for  the sum of However,  -  - 12  the reaction•which, i s a c c o u n t i n g f o r a l l or most o f the c u r r e n t  -  is usually  known. The  current  at a set  potential  i s n e a r l y always  time-dependent  and may take days t o become s t a b l e .  .But w i t h the passage o f c u r r e n t  time the c o n d i t i o n o f the e l e c t r o d e  surface  and i t s  environment  and  change.  T h e r e f o r e t h e shape of the curve may g r e a t l y , depend on the t e c h n i q u e used i n o b t a i n i n g the p o t e n t i a l - c u r r e n t i n surface  The above-mentioned  change  condition i s e s p e c i a l l y s i g n i f i c a n t i f p i t t i n g occurs.  the a n o d i c c u r r e n t at  data^.  the p i t .  i s n e a r l y c o m p l e t e l y accounted  O f t e n the c a t h o d i c  reaction  Then  f o r by the m e t a l  i s o c c u r i n g on the  oxidation  non-corroding  a r e a s even when p o l a r i z e d a n o d i c a l l y so t h a t the t r u e a n o d i c c u r r e n t g r e a t e r than the apparent  p H - P o t e n t i a l Diagram f o r The  current^.  Ni-H 0 2  energy, r e l a t i o n s h i p s  o f a metal w i t h aqueous  environments  may be summarized by means o f a.. Pourbaix or p H - p o t e n t i a l diagram. relationships  is  i n d i c a t e the c o n d i t i o n s o f c o r r o s i o n ,  The  p a s s i v a t i o n or immunity. 27  Figure 5 .  i s the p H - p o t e n t i a l diagram f o r n i c k e l i n water  r e p r e s e n t e q u i l i b r i a between t h e . s o l i d phases of n i c k e l d i s s o l v e d i n water.  and s e l e c t e d  These c o n c e n t r a t i o n s  c o n d i t i o n s of p a s s i v i t y and n o b i l i t y , and i t  .  The l i n e s concentrations  may be u s e d . t o  define  has become a c o n v e n t i o n  use 1 0 " ,M d i s s o l v e d metal i o n s as the a r b i t r a r i l y d e f i n e d l i m i t o f  .corrosion).  6  W i t h f l u o r i d e s . i n the except t h a t N i F . ^ H 0 2  concentrations  2  system the diagram i s changed v e r y  very s i m i l a r t o N i form an adherent  + +  little  i s thermodynamically s t a b l e w i t h r e s p e c t t o N i  g r e a t e r than 1 0 ~  6  M.  i n pure H 0 . 2  The b o u n d a r i e s o f i t s  stability  However i n most circumstances  f i l m on the n i c k e l s u r f a c e .  it  to  + +  are does  The e f f e c t o f f l u o r i d e i s  f o r e d i s r e g a r d e d i n d i s c u s s i n g the p H - p o t e n t i a l diagram.  at  not there-  Figure 5.  N i c k e l - W a t e r p H - P o t e n t i a l Diagram  - 14 -  N i c k e l i n a s o l u t i o n containing 10".. M .• . Ni a t any p o t e n t i a l i n the pH range 9-12, T h i s i s because N i ( 0 H ) s o l u b i l i t i e s below 10~ Ni(0H)  2  2  according to the pH-potential.diagram.  or h i g h e r o x i d e s are M and are  6  s h o u l d he p a s s i v e  stable  i n t h a t t h e y have  considered t o form a p r o t e c t i v e  forms at the r e s t p o t e n t i a l so t h a t a s s o c i a t e d  film.  p o l a r i z a t i o n curves  • i  s h o u l d s h o w , l i t t l e o r no a c t i v i t y . to passivate.  , However,. t h i s  A t pH < 9 n i c k e l would not be  i s not always t h e -case as. water m o l e c u l e s . o r  h y d r o x i d e i o n s may be adsorbed at p o t e n t i a l s i n i t i a t e t h e f o r m a t i o n of a hydroxide f i l m . curve u s u a l l y shows an a c t i v e ions i n s o l u t i o n .  above the r e s t p o t e n t i a l , ,In t h i s  pH r e g i o n due t o the f o r m a t i o n o f hexavalent  case the p o l a r i z a t i o n  s t i l l higher potentials n i c k e l as N i o /  would a l s o be expected  t o corrode  pH y 12 t o f orm . N i 0 H  i n s t e a d of the p a s s i v a t i n g ' N i ( O H . ) .  t h e y are good e l e c t r o n i c  lattice  t.-UA....  12 h i g h e r o x i d e s of n i c k e l are  These may form an adherent  f i l m but  and i o n i c conductors t h e y w i l l not be  They w i l l p r o b a b l y , b e good i o n i c conductors  2  -Nickel  2  s u f f i c i e n t l y noble p o t e n t i a l s .  which the N i 0  .  in this  i n aqueous s o l u t i o n s w i t h a  Throughout the pH r e g i o n 0 . t o at  and  r e g i o n , , the e x t e n t o f which may depend on the  N i c k e l s h o u l d corrode at  2  expected  i s known t o  have  stable if  protective.  i f t h e y have a d e f e c t  structure,  28 i  Purpose and Scope o f the Present  Investigation  The purpose o f t h i s i n v e s t i g a t i o n was t o examine . p o l a r i z a t i o n curves f o r n i c k e l and monel i n s o l u t i o n s o f v a r y i n g pH and f l u o r i d e f o r evidence o f c o r r o s i o n - r e s i s t i n g p r o p e r t i e s mechanisms o f these metals  i n t h i s range  content  and of p o s s i b l e c o r r o s i o n  o f environments. ••• I n d i c a t i o n s were  a l s o sought f o r methods o f i m p r o v i n g c o r r o s i o n r e s i s t a n c e i n those s o l u t i o n c o m p o s i t i o n s where c o r r o s i o n r a t e s may be c o n s i d e r e d  excessive.  - 15  I n i t i a l experiments  -  e n t a i l e d p o l a r i z a t i o n s t u d i e s at widely-  v a r y i n g pH i n b u f f e r e d e l e c t r o l y t e s .  The curve i n a., f l u o r i d e  electrolyte  of pH = 6 . 2 showed a second a c t i v e peak which was r e p o r t e d but not  explained  8 by Truempler a n d - K e l l e r  i n chloride solutions.  .Further  conducted i n an attempt t o e x p l a i n t h i s anomaly.  These  studies.were consisted.of  p o l a r i z a t i o n s t u d i e s at d i f f e r e n t f l u o r i d e and hydrogen i o n current-time  curves a t  c o r r o d e d specimens.  c o n s t a n t p o t e n t i a l and s u r f a c e  concentrations,  examinations  of  P o l a r i z a t i o n curves were a l s o o b t a i n e d f o r n i c k e l  i n c h l o r i d e s o l u t i o n , t o determine any d i f f e r e n c e  i n corrosion behaviour  w i t h , o t h e r h a l i d e i o n s , and i n n i t r a t e s o l u t i o n as a h a l o g e n - f r e e Potentiostatic  curves  reference.  f o r monel i n f l u o r i d e s and c h l o r i d e s were  o b t a i n e d i n an attempt t o r e l a t e the b e h a v i o u r of monel and n i c k e l .  -.16 -  APPARATUS AND EXPERIMENTAL  E l e c t r o c h e m i c a l C e l l and E l e c t r i c a l The h i g h l y c o r r o s i v e materials  Apparatus  nature o f . f l u o r i d e s g r e a t l y  l i m i t e d the  and thus the d e s i g n of the t e s t c e l l . ' T e t r a f l u o r o e t h y l e n e  was used because of i t s  inertness  and h i g h temperature. ( 5 0 0 ° F )  The c e l l designed f o r the p r e s e n t work ( F i g u r e  6)  (Teflon)  strength.  is similar to a c e l l  29 d e s c r i b e d by Weininger and Grams ^ and c o n t a i n s  a l l the. e s s e n t i a l  elements,  which are 1.  working, a u x i l i a r y and. r e f e r e n c e e l e c t r o d e s  2.  circulating  3.  gas  k.  electrolyte  saturation  i n l e t and  outlet  . p r o v i s i o n . f o r h e a t i n g and temperature  control.  The c e l l d e s i g n a l s o l e n d s i t s e l f t o ease of changing the working e l e c t r o d e and  electrolyte. A l l p a r t s are machined from . r o d , b a r and.tube  by the Crane P a c k i n g Company. Teflon c y l i n der (a),  The c e l l c o n s i s t s  stock Teflon supplied  of a 2 i n c h I . D . , 3 inch O.D.  w i t h disks, or e l e c t r o d e h o l d e r s above  (b)  and below  (c).  The c e l l i s s e a l e d by O - r i n g s and clamped t o g e t h e r by b e a r i n g p l a t e s and threaded r o d s ,  (p).  . The working e l e c t r o d e  (d)  i s wrapped w i t h T e f l o n . t a p e and. drawn  i n t o p l a c e by t i g h t e n i n g the nut at the end of the t h a t a l s o a c t s as an e l e c t r i c a l p l a t e by a b a k e l i t e washer. of the  (h)  steel bolt  c o n n e c t i o n and i s i n s u l a t e d . f r o m the  • The a u x i l i a r y e l e c t r o d e  same metal as the working e l e c t r o d e ,  •copper w i r e  stainless  that i s soldered:to  it.  (f)  bearing  is a circular  and i s h e l d i n p l a c e by the  The r e f e r e n c e e l e c t r o d e  (e)  (r)  is  disk stiff  Figure  6.  Electrochemical  Corrosion  Cell  - 18 -  l o c a t e d i n a-..Teflon i n s e r t  (i)  f i l l e d w i t h s a t u r a t e d KC1.  The bottom o f  the i n s e r t i s p e r f o r a t e d and f i l l e d w i t h agar agar g e l o f s a t u r a t e d - K C 1 which i s i n c o n t a c t  w i t h the e l e c t r o l y t e  i n tube ( j ) .  A small hole i n  the top o f . t h e c e l l n e x t t o the working e l e c t r o d e a l l o w s e l e c t r o l y t e  into  t h i s tube.  The e l e c t r o l y t e by the gas l i f t  at  (1).  i s poured i n t o the f u n n e l at  (k) and c i r c u l a t e s  . T h i s p r o v i d e s c i r c u l a t i o n through the c e l l and  p a r a l l e l t o the e l e c t r o d e  faces.  T h i s a l s o s a t u r a t e s the e l e c t r o l y t e  with  t h e p a r t i c u l a r gas u s e d , i n t h i s case n i t r o g e n (< 0.7$ 0 ) from compressed 2  ,gas tanks s u p p l i e d by*Canadian L i q u i d A i r Company. with a small hole i n i t  A T e f l o n sheet  (m)  i s clamped over the f u n n e l and m a i n t a i n s s m a l l  p o s i t i v e p r e s s u r e of the s a t u r a t i n g . g a s  over the  electrolyte.  T h i s c e l l has the advantage o f b e i n g f l e x i b l e i n use and simple i n d e s i g n but does not p e r m i t the c e l l r e a c t i o n t o be o b s e r v e d .  -A schematic diagram of the e l e c t r i c a l system i s shown i n F i g u r e 7.The p o t e n t i o s t a t  i s a D u f f e r s Model 600.  C u r r e n t i s s u p p l i e d from a 12 v o l t  c e n t e r - t a p e d D e l c o c a r b a t t e r y and i s measured b y a Simpson Model 29MC 50 microampere (uA) ammeter t o ' w h i c h i s ' added shunts and a s h o r t i n g type r e v e r s i n g s w i t c h g i v i n g f u l l s c a l e r e a d i n g s o f 50 y&> 200 u A , ^00 u A , 2 mA 5 mA a n d . 2 0 mA i n b o t h d i r e c t i o n s . lead.  The ammeter i s i n the a u x i l i a r y e l e c t r o d e  The p o t e n t i a l between the s a t u r a t e d c a l o m e l r e f e r e n c e e l e c t r o d e and  the working e l e c t r o d e  i s measured by a . M o d e l 7569P Pye P o t e n t i o m e t e r u s i n g  a Beckman Model G . S . p H meter as a h i g h s e n s i t i v i t y , h i g h impedance n u l l detector.  Potentiostat ©-  Potentiometer  -©  Reference  0-  Null.Detector —  ©  © -  Electrode  -Working E l e c t r o d e Auxiliary Electrode  i  h1  F i g u r e 7•  Schematic Diagram, o f E l e c t r i c a l Apparatus  VO  - 20 -  U n f o r t u n a t e l y the p o t e n t i o s t a t r e q u i r e d service.  frequent-maintenance  To overcome d e l a y s caused by-breakdowns, a " c l a s s i c a l "  s t a t was a l s o u s e d . wet c e l l s  It consisted  o f two 2.2 v o l t Hart D . H . S .  i n p a r a l l e l , from which a p o l a r i z i n g c u r r e n t  an Ohmite t h r e e - p o l e  8.5 ohm v a r i a b l e r e s i s t o r .  potentio-  15 g l a s s  i s drawn by a d j u s t i n g  The c u r r e n t  was measured  by. t h e m o d i f i e d Simpson ammeter but i n order t o f o l l o w t h e p o t e n t i a l more c l o s e l y a Beckman " Z e r o m a t i c " potential.  (±  10  drift  pH meter was used, f o r measuring the  T h i s h a s . t h e r e q u i r e d . h i g h impedance but much lower  sensitivity  mV).  . Materials Electrodes A.D.  (5 cm  2  i n a r e a ) were machined from r o d s u p p l i e d : by  MacKay I n c . w i t h s t a t e d p u r i t i e s  o f 99.9$> f o r b o t h monel and. n i c k e l .  When i m p u r i t i e s were s u s p e c t e d . t o be p r e s e n t i n the n i c k e l , i t was a n a l y z e d , with t h e . f o l l o w i n g  results:  0.015 %  :Sulphur Carbon Iron' Copper Cobalt The MacKay, : I n c . of S h e r r i t t  0.030  0.040 0.150  O.35O  monel sheet f o r t h e a u x i l i a r y e l e c t r o d e was a l s o s u p p l i e d by A . D . .The n i c k e l a u x i l i a r y e l e c t r o d e was s u p p l i e d by Mr.. V . - N . Mackiw  Gordon Mines L i m i t e d .  -All  reagents used i n . t h e e l e c t r o l y t e s were Baker and'Adamson  reagent grade chemicals  d i l u t e d i n d i s t i l l e d w a t e r . - The e l e c t r o l y t e s , were  b u f f e r e d w i t h phosphate  or  succinate.  - 21  -  P o l a r i z a t i o n Curves The c o r r o s i o n specimen was mounted-in the t e s t c e l l a f t e r  polishing  w i t h l/2 g r i t emery'paper and c l e a n i n g w i t h C h l o r o t h a n e * -The specimen was aged d u r i n g n i t r o g e n s a t u r a t i o n o f the e l e c t r o l y t e  f o r about l/2 hour and  t h e n c a t h o d i c a l l y reduced f o r l/2 hour t o remove oxide f i l m . settings  Potential  were made i n c r e a s i n g l y a n o d i c b e g i n n i n g from t h i s c a t h o d i c  i n 10 t o 100 mV i n c r e m e n t s . made f o l l o w e d by at  least  A t each s e t t i n g i n i t i a l c u r r e n t  one a d d i t i o n a l r e a d i n g a f t e r  d e t e c t time-dependent c u r r e n t v a r i a t i o n s .  5  o  r  region  r e a d i n g s were 10 m i n u t e s ' t o  The pH of the e l e c t r o l y t e  measured w i t h s h o r t - r a n g e „ H y d r i o n pH paper b e f o r e and a f t e r specimen was r e t a i n e d f o r subsequent e x a m i n a t i o n o f the  was  each r u n .  The  surface.  Curves were o b t a i n e d f o r n i c k e l and monel i n f l u o r i d e . s o l u t i o n s at w i d e l y v a r i e d p H . -Most work was.done on n i c k e l i n f l u o r i d e media i n . t h e pH range o f 4 . 0 was e v i d e n t . reference  t o 7-0 because under these c o n d i t i o n s secondary  Curves were a l s o o b t a i n e d i n sodium n i t r a t e  activation  s o l u t i o n s as a  and i n sodium c h l o r i d e s o l u t i o n s f o r c o m p a r i s o n .  . S e v e r a l experiments were made w i t h n i c k e l i n f l u o r i d e s o l u t i o n s at pH = 6.2 t o determine t h e time-dependence o f c u r r e n t at p o t e n t i a l s responding t o s e l e c t e d p a r t s  of the p o l a r i z a t i o n c u r v e .  cor-  The growth of a  i protective  f i l m can u s u a l l y be i n t e r p r e t e d from d e c a y i n g c u r r e n t  values.  I n t h e s e experiments the c o r r o s i o n specimens were l e f t at  the  p o t e n t i a l s f o r one or more days d u r i n g w h i c h time c u r r e n t  r e a d i n g s were  recorded at  intervals.  i n Table  I.  A  1,1,1  -  selected  E x p e r i m e n t a l c o n d i t i o n s f o r a l l runs are summarized  trichloroethane  - 22 TABLE . I . Experimental Conditions Experiment Number  8 10 11 12  13 lk  15 16 17 18 19 20 21 22  23  24  25B 26  27B 28 29 30 31 32 33  34B  „55B  36B  37B 38B 39B 40B  B  Anode Material Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Monel Monel Monel Monel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel Nickel  Electrolyte NaF NaF NaF HF HF + NaF NaN0 NaF HF NaF NaN0 NaCl NaCl NaF NaFNaF NaF NaF NaF NaF NaF HF-+ NaF NaF NaCl NaF NaF HF NaF NaF NaF NaF NaN0 NaF 3  3  3  i n d i c a t e s current-time experiments.  • Concentration  0.39 0.42  0.45  0.1  O.38 0.08 0.42  0.08  0.41  0.083  0.42 0.2 0.042 0.21  0.29 0.42  0.42 0.42  0.42 0.42  0.30 0.42 0.42 0.42 0.42 0.1  0.42 0.42  0.42 0.42 0.1 0.42  Buffer  pH  • Phosphate 4.0 Phosphate 11.3 NaOH >12 HF 1.0 Phosphate 5-8 Phosphate 6.0 Succinate 5-2 Phosphate 2.2 6.0 Succinate Succinate 4.8 Phosphate h.9 Phosphate 6.1 Phosphate 6.2 Phosphate 6.2 6.2 Phosphate ' Phosphate 6.2 Phosphate 6.2 Phosphate 6.2 Phosphate 6.2 6.0 Phosphate 4 .1 Phosphate 11.2 Phosphate 4.0 Phosphate 7.0 Phosphate 5.^ 1.0 Phosphate 6.2 Phosphate 6.2 Phosphate 6.2 Phosphate 6.2 6.0 Phosphate Phosphate 6.2  - 23 -  Surface•Examinat i o n The s u r f a c e s  i o f a l l c o r r o d e d specimens were examined under a  b i n o c u l a r microscope f o r c h a r a c t e r i s t i c s . p e c u l i a r  t o the c o r r o s i o n system.  Specimens c o r r o d e d at  constant  cularly significant.  Micrographs were made of t h e s e , u s i n g a R e i c h e r t  microscope and P o l a r o i d camera.  p o t e n t i a l over extended times were p a r t i -  - 2k  RESULTS AND DISCUSSION  N i c k e l specimens p o l a r i z e d i n 0.08 M HF at pH = 2.2 and. i n 0.42  M NaF at  curves  pH = 6.2 and 11..3 each b u f f e r e d by phosphate gave p o l a r i z a t i o n  as shown i n F i g u r e 8, 11, and 22 r e s p e c t i v e l y .  t h r e e d i s t i n c t types  These curves  show  o f b e h a v i o u r of n i c k e l i n f l u o r i d e s as a f u n c t i o n  of pH. Nickel i n Acid Fluoride Solutions The curve at pH = 2.2 and another which was done i n u n b u f f e r e d 0.1 M HF at pH = 1.0 show t h a t n i c k e l does not p a s s i v a t e f l u o r i d e i o n s at  low p H .  that n i c k e l passivates acid  In c o n t r a s t , V e t t e r " ^ ,  i n the presence  of  Uhlig"'"^ and Bune"^ found  on a t t a i n i n g a p o t e n t i a l o f about  -90 mV i n s u l p h u r i c  solutions. A The mixed p o t e n t i a l s  are p l o t t e d v e r s u s pH w i t h other, d a t a  n i c k e l i n p h o s p h o r i c and s u l p h u r i c a c i d s o l u t i o n , F i g u r e 9. of 310 mV at pH = 2.2 i s i n t e r m e d i a t e  The v a l u e  between the mixed p o t e n t i a l s , i n 1 N  s u l p h u r i c a c i d and 0.1 M p h o s p h o r i c a c i d . have some e f f e c t  Presumably these f o r e i g n i o n s  on the r e a c t i o n s on the n i c k e l e l e c t r o d e  exchange c u r r e n t was determined t o be about 50 u A / c m , 2  surface.  A  experiments  were done u s i n g t h i s  See Appendix I f o r C a l i b r a t i o n of  Potentials  The  indicating a f a i r l y  h i g h c o r r o s i o n r a t e of 13Q mdd or 0.02 i p y . No t i m e - c u r r e n t  for  system.  -  -  +400  +200  0  -200 Potential  F i g u r e 8.  -^00  -600  (mV? v e r s u s SCE) r  P o l a r i z a t i o n Curve o f N i c k e l i n 0.08 M NaF S o l u t i o n a t pH = 2.2"  rfnh  *  25  -  - 26 -  +100 N i c k e l i n - S u l p h a t e , . Osterwald and U h l i g ^  A +200  O  N i c k e l in-Phosphate,- M a c G i l l a v r y  D  N i c k e l i n Phosphate work  and F l u o r i d e ,  1-300  et  al ^ 1  present  •a  •a w CO  U  -,CU  +hoo  •H -P OJ -P  O P-  +500  T3  a; X •H  s  +600  +700 11 pH F i g u r e 9-  Mixed P o t e n t i a l v e r s u s pH  - 27 -  In making the r e a d i n g s change  i n current  f o r t h i s p o l a r i z a t i o n curve  i t was noted t h a t  the  w i t h time was s m a l l except f o r the p o t e n t i a l r e g i o n from  + 100 t o 0 mV where t h e r e may have been some f i l m growth.  . O p t i c a l examination o f the  specimen p o l a r i z e d i n the  p h o s p h a t e - b u f f e r e d s o l u t i o n showed i n t e r f e r e n c e film, formation. corrosion.  c o l o u r s which may i n d i c a t e  The m i c r o g r a p h , F i g u r e 10a..shows e x t e n s i v e  intergranular  ..The mechanism, o f the c o r r o s i o n which p r e f e r e n t i a l l y  attacks  g r a i n 'boundaries w i l l be d i s c u s s e d l a t e r i n r e l a t i o n t o the r e s u l t s nickel corrosion i n neutral fluoride solutions.  Another specimen was  c o r r o d e d i n 0.1 M HF w i t h no a p p l i e d p o t e n t i a l , f o r 1 d a y . F i g u r e 10b.  of  The m i c r o g r a p h  shows e x t e n s i v e p i t t i n g c o r r o s i o n and no g e n e r a l c o r r o s i o n  f i l m . f o r m a t i o n between the p i t s .  Obviously,  p r o d u c i n g p i t s w h i l e the c a t h o d i c  reaction  From these o b s e r v a t i o n s  it  c o r r o s i o n by f l u o r i d e s at  the a n o d i c areas are  stationary  occurs, on t h e r e s t of the  i s evident that n i c k e l i s  or  surface.  s u b j e c t . t o extensive  low pH independent o f the a p p l i e d p o t e n t i a l .  . Nickel in Neutral.Fluoride Solutions  F i g u r e 11 shows t h a t n i c k e l i s a c t i v e i n c o n t a c t w i t h a s o l u t i o n c o n t a i n i n g f l u o r i d e s a t pH = 6.2. typical active-passive  transition.  On r a i s i n g i t s . p o t e n t i a l .Further  n i c k e l has  i n the anodic d i r e c t i o n t h e r e  i s a second a c t i v e r e g i o n s i m i l a r t o t h a t f o u n d b y Truempler and K e l l e r i n sulphate  a  s o l u t i o n s c o n t a i n i n g c h l o r i d e s and. b r o m i d e s .  8  This behaviour  "P5 12 was not found by V e t t e r ^ or U h l i g working w i t h n i c k e l i n pure  sulphates.  (a)  At noble p o t e n t i a l s  pH =  2.2  X  (b)  At mixed p o t e n t i a l pH =  1.0  X F i g u r e 10.  §00  300  S u r f a c e s o f N i c k e l Corroded i n F l u o r i d e s Low pH  at  - 29 -  E  +500  F  0  E-  c  -500  P o t e n t i a l ( m V v e r s u s SCE) F i g u r e 11.  -1000 V  P o l a r i z a t i o n Curve o f N i c k e l i n F l u o r i d e • a n d N i t r a t e S o l u t i o n at pH = 6.2 Auh  -30 _  The c a t h o d i c  r e g i o n , the exchange c u r r e n t ,  the f i r s t a c t i v a t i o n peak are  f u n c t i o n s o f pH; o n l y .  o b t a i n an a c c u r a t e v a l u e of the exchange c u r r e n t i s n o t l i n e a r and t h e r e f o r e mixed p o t e n t i a l .  f l u o r i d e ion concentrations.  cathodic  which corresponds  t o a c o r r o s i o n r a t e o f 0.17  E ••= 0.2O0 t  as shown i n F i g u r e 9.  f o l l o w s the  approximate  varying  f a i r l y passive  at pH = 7.0  mdd.or 27 X 1 0 "  o f the anodic  Two e x p e r i m e n t s . ( N o s .  probably, because n i c k e l complexes w i t h s u c c i n a t e  p r e d i c t e d by the p H - p o t e n t i a l diagram.  15 and 17)  Even i f the  potentials,  concentration  is  not  of Ni-^" i n  the n i c k e l would not  be expected t o f o r m a:...hydroxide f i l m , u n t i l a pH of 8 . 0  attained.  f o r p a s s i v a t i o n i s not a f f e c t e d by  widely varying f l u o r i d e ion concentrations, numbered Ik, 21,  using  which i s a c h e l a t i n g i o n .  mg o f d i s s o l v e d n i c k e l ,  The c r i t i c a l anodic: c u r r e n t  is  reaction  The p a s s i v a t i o n of n i c k e l i n aqueous s o l u t i o n at pH = 6.5  M/i., or 1.5  ipy.  6  equation  as a b u f f e r gave much more a c t i v e mixed and F l a d e  experiments  is  The abrupt change i n p o t e n t i a l a t about pH = 6.5  w i t h the p a s s i v a t i o n o f the e l e c t r o d e .  s o l u t i o n i s 10"  the  0.050 pli  due t o a.change from a c t i v a t i o n t o ohmic o v e r v o l t a g e  succinate  to  v e r s u s . pH at  The exchange c u r r e n t  m  to  curve t o the mixed p o t e n t i a l .  exchange c u r r e n t  At low p H ' s the mixed p o t e n t i a l E  m  However ah  T h i s i n d i c a t e s . t h a t n i c k e l becomes  i n f l u o r i d e s o l u t i o n s above pH 6.52  was not p o s s i b l e  because the T a f e l p l o t  T h i s i s i l l u s t r a t e d i n F i g u r e 11.  F i g u r e 12 i s a p l o t of the e s t i m a t e d  uA/cm  .It  does not permit a c c u r a t e e x t r a p o l a t i o n  v a l u e was o b t a i n e d b y e x t r a p o l a t i n g . t h e  0.3  the mixed p o t e n t i a l and  22 and 23.  at  c o n s t a n t pH as shown by ex  The c r i t i c a l a n o d i c c u r r e n t  is  Figure l j .  C r i t i c a l Anodic C u r r e n t D e n s i t y v e r s u s pH  60jik/cm  i n each c a s e .  On t h e o t h e r hand Truempler and K e l l e r  t h a t an a d d i t i o n o f 0.05 M o f c h l o r i d e i o n i n c r e a s e d current  from 30 mA/cm  2  t o 100 m A / c m . 2  found  the c r i t i c a l  However, t h e c r i t i c a l  anodic  anodic  current  i s d e f i n i t e l y , a . : f u n c t i o n o f pH as shown i n F i g u r e 13.  The F l a d e p o t e n t i a l  :E  p  = .*  follows the equation:  0.240 + 0.065 pH  w h i c h compares w i t h the r e l a t i o n g i v e n by U h l i g a s :  E  F  =  - 0 . 1 2 0 +O.O59 pH  The p r e s e n c e o f f l u o r i d e i o n s i n s o l u t i o n l e s s e n s t h e r e s i s t a n c e of p a s s i v a t e d  n i c k e l to corrosion.  i n hydrogen i o n c o n c e n t r a t i o n negative  This e f f e c t  as i l l u s t r a t e d  i s increased with increase  i n F i g u r e 14, where pF i s t h e  logarithm of f l u o r i d e i n i o n concentration.  h  6  8  In c o n t r a s t ,  changes  10  pH + pF F i g u r e 14.  Minimum P a s s i v e  Current Density versus  pF + p H .  - 33 i n pH have l i t t l e e f f e c t  . ,  .  sulphuric acid  on the p a s s i v a t i n g c h a r a c t e r i s t i c s  of nickel i n  -13,.14  C u r r e n t - t i m e curves shown i n F i g u r e 15, (a) and (b) f i l m growth i n the p a s s i v e r e g i o n . c u r r e n t d e n s i t y o f about 0.5 nA/cm df 8 |iA./cm  2  after  indicate  Curve (a) a t +200.mV g i v e s a f i n a l after  2  2 hours as compared t o the v a l u e  5-10 minutes i n d i c a t e d on the p o l a r i z a t i o n c u r v e .  emphasizes t h a t the p o l a r i z a t i o n curve c u r r e n t f a r from t h e s t e a d y - s t a t e v a l u e s .  This  readings i n t h i s region are  The g r a d u a l l y s l o p i n g curve i n the a c t i v e -  p a s s i v e t r a n s i t i o n may be more a c c u r a t e as a v e r t i c a l l i n e on the b a s i s o f . f i n a l steady-state currents.  The f i n a l c u r r e n t a t +200 mV i s l e s s than t h e  f i n a l c u r r e n t a t -I50 mV, curve ( b ) . the p o l a r i z a t i o n c u r v e s ,  T h i s agrees w i t h the t r e n d shown i n  f o r t h e c u r r e n t t o i n c r e a s e w i t h the a p p l i e d p o t e n t i a l  i n the p a s s i v e r e g i o n .  F i g u r e 16 (a) and ( b ) , and F i g u r e 17ajr ' show photomicrographs o f ' n i c k e l specimens.corroded at constant p o t e n t i a l s s o l u t i o n a t a pH o f 6.2.  i n 0.42 M sodium f l u o r i d e  The specimen c o r r o d e d a t t h e mixed p o t e n t i a l f o r  1.25 d a y s , F i g u r e s . l 6 a , shows a l a r g e amount o f g e n e r a l c o r r o s i o n which has n e a r l y o b l i t e r a t e d the p o l i s h s t r i a t i o n s .  . T h i s . c o n f i r m s an expected  r a t e o f c o r r o s i o n - s u g g e s t e d by the exchange c o r r o d e d i n the a c t i v e  c u r r e n t measurement.  r e g i o n a t a p o t e n t i a l o f +300 mV f o r i  i s c h a r a c t e r i z e d b y b o t h p i t t i n g and g e n e r a l c o r r o s i o n . surface  The specimen,  d a y , F i g u r e l6b,  Figure. 17a, i s the  o f a specimen c o r r o d e d i n the p a s s i v e r e g i o n f o r 1.75 d a y s .  shows.less general c o r r o s i o n . w h i l e another  high  Nevertheless corrosion i s s t i l l  This  apparent,  specimen, • F i g u r e 17b h e l d i n the p a s s i v e r e g i o n w i t h n i t r a t e  r e p l a c i n g f l u o r i d e as t h e e l e c t r o l y t e  shows no c o r r o s i o n .  This confirms that  - 3h 1000  10  ,100  Log Time (minutes) Figure  15.  C u r r e n t - T i m e Curves a t S e l e c t e d Regions o f N i c k e l P o l a r i z a t i o n Curve i n 0.^2 JVPNaF S o l u t i o n at pH = 6 . 2  (a)  At the mixed  potential X  Figure 1 6 .  300  S u r f a c e s of N i c k e l i n the F i r s t A c t i v e S t a t e Corroded i n F l u o r i d e S o l u t i o n s at pH = 6 . 2  (a)  At - 1 5 0 mV i n 0 . 4 2 M NaF X  300  (b) A t -200 mV i n 0.1 M NaN0 ( C o m e t - l i k e marks are p o l i s h s t r i a t i o n s ) X 300 3  F i g u r e 17.  S u r f a c e o f N i c k e l i n the P a s s i v e S t a t e Corroded i n S o l u t i o n s o f pH = 6 . 2  -37 -  f l u o r i d e i o n s cause n i c k e l t o corrode  even when a p p a r e n t l y p a s s i v e .  mechanism whereby f l u o r i d e produces t h i s c o r r o s i o n i s not c l e a r . f l u o r i d e i o n s are  i n c o r p o r a t e d i n t o the l a t t i c e o f the hydroxide  c a u s i n g i t t o become an i o n i c c o n d u c t o r .  The  Possibly film  A l t e r n a t i v e l y , f l u o r i d e s may  cause the hydroxide f i l m t o d i s s o l v e by complexing w i t h the n i c k e l i o n s i n the l a t t i c e of the  film.  The f a c t t h a t f l u o r i d e causes c o r r o s i o n o f n i c k e l i n the  passive  s t a t e h e l p s e x p l a i n the photomicrograph o f r, n i c k e l corroded i n . t h e  active  30 state,  F i g u r e 16b.  U . F . Franck  has e x p l a i n e d p i t t i n g c o r r o s i o n i n the  a c t i v e s t a t e as b e i n g due t o the e x i s t e n c e o f a c t i v e and p a s s i v e the same p o l a r i z e d e l e c t r o d e . electrolyte  resistance.  The d i f f e r e n c e  s i t e s on  i n p o t e n t i a l i s caused by  Thus the p i t s are presumed t o be a c t i v e s i t e s and  the a r e a s o f g e n e r a l c o r r o s i o n are  "passive"  sites.  . Both p o s i t i o n and e x t e n t of the second a c t i v e r e g i o n are of f l u o r i d e and hydrogen or hydroxide i o n c o n c e n t r a t i o n s . of the a c t i v e peak, F i g u r e 19,  dependence appears  The p o t e n t i a l  F i g u r e 18 and the l o g a r i t h m o f the c u r r e n t  are p l o t t e d as f u n c t i o n s o f pH p l u s p F . t o be l i n e a r .  . The p o t e n t i a l at  at  the  In b o t h cases the  o f the d a t a .  F i g u r e 21  t o 6.5  a change  i n the r a t e ^ c o n t r o l l i n g  s t e p at  i s a p l o t of  sharply.  w h i l e at  the  The  a . p H p l u s pF e q u a l  s i n c e below t h i s v a l u e the T a f e l s l o p e i s c o n s t a n t of 135  an o< n v a l u e o f 0 A 5 ,  the  This  p o s i t i v e T a f e l s l o p e o f the second a c t i v e r e g i o n v e r s u s pH p l u s . p F . l a t t e r indicates  peak,  the  i n i t i a t i o n of  second a c t i v e r e g i o n i s p l o t t e d v e r s u s pH p l u s 2pF,, i n F i g u r e 20. seems t o g i v e the b e s t l i n e a r f i t  functions  h i g h e r v a l u e s the T a f e l s l o p e  mV, g i v i n g  increases  - 38 -  h'y  5  6  7  8  9  pF +..pH F i g u r e 18.  k  5  P o t e n t i a l a t Second A c t i v e ' v e r s u s - pH p l u s p F .  6  7  Peak  8  .pF + pH F i g u r e 19.  L o g ' C u r r e n t D e n s i t y a t the Second A c t i v e Peak v e r s u s pj? p l u s p H .  9  - 39 -  pH +  F i g u r e 20.  P o t e n t i a l at  2pF  the I n i t i a t i o n o f  Second A c t i v e R e g i o n / E pH .+  2pF.  the  versus  - ko -  ^/  600  0  *** /o a  4"00  /  *r~t  %  CJ fut H CO  /  200  1 5.0  1 6.0  1 7.0  1 8.0  pH + pF F i g u r e 21.  P o s i t i v e Slope i n Second A c t i v e Region v e r s u s pH p l u s p F .  The second a c t i v e r e g i o n was absent done i n . n i t r a t e  ,  i n those two  s o l u t i o n s w i t h no f l u o r i d e i o n s p r e s e n t ,  experiments  as shown i n F i g u r e 11.  The minimum p a s s i v e c u r r e n t was'Tower i n these p o l a r i z a t i o n curves but i n a l l o t h e r r e s p e c t s t h e y resembled e x p e r i m e n t a l curves solutions.  for fluoride-containing  O n . t h e o t h e r hand 0.2 M c h l o r i d e s o l u t i o n s , - F i g u r e 22, showed a  8 second a c t i v e r e g i o n i n agreement w i t h o b s e r v a t i o n s T h i s curve  by o t h e r  investigators  .  shows t h a t s m a l l e r c o n c e n t r a t i o n s o f c h l o r i d e i o n s . i n d u c e g r e a t e r  c o r r o s i o n r a t e s than f l u o r i d e i o n s . I n f l u o r i d e s o l u t i o n s the n i c k e l became p a s s i v e a g a i n a t more noble t h a n about  -950 mV.  In c h l o r i d e solutions  p a s s i v e r e g i o n was not observed because the c u r r e n t a c t i v e r e g i o n was so.much l a r g e r t o the p o t e n t i a l o f maximum  t h i s probable  d e n s i t y i n the  potentials final second  t h a t i t was i m p o s s i b l e t o p o l a r i z e the  current.  electrode  - kl -  -p  •Potential F i g u r e 22.  (mV v e r s u s SGE)  P o l a r i z a t i o n o f N i c k e l i n 0 . 2 ' M NaCl S o l u t i o n a t pH = 6.1 ^un  - k2 -  C u r r e n t - t i m e curves a t p o t e n t i a l s a c t i v a t i o n show i n i t i a l decreases i n c u r r e n t F i g u r e 15.  It  i s p o s s i b l e these r e p r e s e n t  i n the r e g i o n o f  secondary  f o l l o w e d by a . s m a l l  f i l m growth  rise,  i n p a s s i v e areas  i n c o m p e t i t i o n w i t h the i n c r e a s i n g a r e a of the d i s s o l u t i o n s i t e s .  A photomicrograph o f a.specimen used i n the  time-current  experiments, - F i g u r e 23a, shows t h a t the d i s s o l u t i o n s i t e s are boundaries. are  grain  I n t e r f e r e n c e c o l o u r s on b i n o c u l a r examination o f the  specimens  i n t e r p r e t e d t o i n d i c a t e f i l m growth between the g r a i n b o u n d a r i e s .  C o r r o s i o n between the g r a i n boundaries i s a l s o apparent but t h i s  is  s i m i l a r t o the c o r r o s i o n o f n i c k e l i n the p a s s i v e r e g i o n , F i g u r e 17a.  C o r r o s i o n i n the second p a s s i v e r e g i o n shows g e n e r a l over the whole s u r f a c e w i t h s l i g h t g r a i n boundary e t c h i n g ,  corrosion  F i g u r e 23b.  T h i s a l s o i s s i m i l a r t o t h e . c o r r o s i o n by f l u o r i d e s . i n the f i r s t  passive  region. Specimens p o l a r i z e d i n sodium c h l o r i d e s o l u t i o n s were  extremely  The above o b s e r v a t i o n s  protected  pitted. i n d i c a t e t h a t n i c k e l cannot be  by anodic p o l a r i z a t i o n i n f l u o r i d e s o l u t i o n s w i t h [ F - ]  )  0.G1 and at pH ^  6.5,  because the second a c t i v e . r e g i o n predominates a t p o t e n t i a l s where n i c k e l wdul&u o r d i n a r i l y be p a s s i v e . agent i s p r e s e n t .  At pH )  6.5 n i c k e l i s p a s s i v e i ' £ no s t r o n g o x i d i z i n g  The c o r r o s i o n c u r r e n t  i n the.second active  r e g i o n at  \[F~] { . 0.01 M i s . s m a l l enough so t h a t n i c k e l c o u l d be p r o t e c t e d at pH ^ 6 . 5 by anodic p o l a r i z a t i o n , e i t h e r a p p l i e d by a c h e m i c a l o x i d a n t or by an current.  external  (a)  At the  second a c t i v e peak,  -700 mV  X 300  (b)  I n the  second p a s s i v e  region,  - I O 5 O mV  X 300  F i g u r e 25.  Surfaces  o f N i c k e l Corroded i n F l u o r i d e S o l u t i o n s at pH =  6.2  - kk  'Nickel i n B a s i c : F l u o r i d e S o l u t i o n s The p o l a r i z a t i o n c u r v e ,  F i g u r e 2k, i n d i c a t e s t h a t n i c k e l i s  i n c o n t a c t w i t h an aqueous s o l u t i o n o f f l u o r i d e s a t  high pH.  passive  T h i s curve  is  an example o f the type drawn i n F i g u r e 3(c) and would be p r e d i c t e d from the p H - p o t e n t i a l diagram, F i g u r e 5-  (The s m a l l a c t i v e peak i n F i g u r e 2k would  p r o b a b l y be e l i m i n a t e d w i t h l o n g e r times a t  constant  I f the f i l m i s assumed t o be N i ( O H ) , 2  o c c u r i n g on the s u r f a c e are  o f the e l e c t r o d e  potential).  then the two  w i t h no e x t e r n a l l y  reactions  applied current  probably 2H  • +  2e~  ~ ^  H  2  and • Ni  + 20H~  Thus the abrupt change e x p l a i n e d by a change  Ni(OH)  2  +  2e~  i n mixed p o t e n t i a l as a f u n c t i o n o f pH ( F i g u r e 9) i n the  to a change i n r a t e - c o n t r o l . the c a t h o d i c  —>-  s t a t e o f the o x i d i z e d n i c k e l which would l e a d I n the a c t i v e s t a t e the  r a t e i s c o n t r o l l e d by  r e a c t i o n as i l l u s t r a t e d i n F i g u r e 4{b), whereas  r e g i o n the r e a c t i o n  is  i n the  passive  i s p r o b a b l y c o n t r o l l e d by d i f f u s i o n o f the n i c k e l i o n s  t h r o u g h the oxide as i n F i g u r e kfc). The-exchange  current  nickel is self-passivated  The f i r s t of N i 0 . 2  i s v e r y low (0.3  i n the pH. range 6.5  transpassive  uA/cm ) c o n f i r m i n g ' t h a t  to  2  12.0.  r e g i o n i s p r o b a b l y due t o the  formation  K o l o t y r k i n and Knyasheva"^ o b t a i n e d the same b e h a v i o u r w i f h  n i c k e l i n potassium sulphate,.solutions w i t h a T a f e l slope of compares v e r y w e l l w i t h the v a l u e o f 8 0 mV i n t h i s work. the phenomena as due t o the f o r m a t i o n o f N i 0  2  "JO mV which  They e x p l a i n e d  on the e l e c t r o d e  surface.  28 Presumably the f i l m would have a n i c k e l d e f e c t  lattice  and thus  higher  nickel ion conductivity. The f i n a l r i s e  i n current  at about  -1200  mV i s due t o the e v o l u t i o n  U  1000  -p  S 100  F i g u r e 2k.  P o l a r i z a t i o n Curve f o r N i c k e l i n 0.k2 S o l u t i o n a t pH = 11.3  M NaF  5  - 46 of oxygen and i s not n e c e s s a r i l y  accompanied by r a p i d c o r r o s i o n .  Mechanism of N i c k e l C o r r o s i o n i n F l u o r i d e Media. The mechanism whereby c h l o r i d e i o n s , i n s o l u t i o n i n i t i a t e a  second  a c t i v e r e g i o n on the p o l a r i z a t i o n curve o f zirconium,., magnesium and aluminum has been proposed by K o l o t y r k i n " ' " ^ .  The r e s u l t s  based are v e r y s i m i l a r t o the r e s u l t s  on w h i c h t h i s mechanism i s  o b t a i n e d i n the p r e s e n t  work f o r n i c k e l 8  i n f l u o r i d e and c h l o r i d e s o l u t i o n s and, the r e s u l t s f o r n i c k e l i n c h l o r i d e and bromide s o l u t i o n s . therefore  be expected t o be the same i n  o f Truempler and K e l l e r  The c o r r o s i o n mechanism might  all.cases.  K o l o t y r k i n ' s mechanism i s . b a s e d on the premise t h a t h a l i d e adsorb on z i r c o n i u m , e t c , at p o t e n t i a l s  p r e f e r e n t i a l l y to h y d r o x i d e i o n s o r water molecules  above E , but not below,, because o f t h e i r g r e a t e r p o l a r i z a b i l i t y . c  C h l o r i d e , bromide and i o d i d e i o n s i n f a c t have l a r g e r than e i t h e r  H 0 or OH 2  and n e v e r t h e l e s s region.  ions  (Table I I )  p o l a r i z a b i l i t i e s 1 i; \  but f l u o r i d e i o n s have a s m a l l e r p o l a r i z a b i l i t y  g i v e r i s e t o a s i m i l a r though l e s s  T h e r e f o r e t h i s , mechanism must be i n v a l i d at  e x t e n s i v e second a c t i v e l e a s t f o r c o r r o s i o n by  fluorides. TABLE  II. Gram I o n i c R e f r a c t i on  Polarizability  Species  o^o X 10 cm 24  0.99 3.02 4.17 6.28 1.80 2.76  F" Cl" Br I" OH"  0  . R  3  2  A  Value extrapolated  A  from.Refractive  3  2.60 9.03 12.60 19.00 5.10  -  1,44  H0  cm  —  Index.  A new mechanism i s proposed t h a t accounts f o r c o r r o s i o n by a l l h a l i d e i o n s i n the  second a c t i v e  region.  -47  The f o l l o w i n g mechanism i s based on the premise t h a t the charge on n i c k e l ,  surface  zirconium e t c . . i s negative at t h e i r r e v e r s i b l e p o t e n t i a l s - ^ .  The n e g a t i v e s u r f a c e charge a r i s e s  from the d i s s o l u t i o n o f p o s i t i v e m e t a l  i o n s from an e l e c t r i c a l l y i n s u l a t e d m e t a l i n s o l u t i o n thus l e a v i n g a n e g a t i v e charge on the s u r f a c e .  T h i s i s c a l l e d the e l e c t r o l y t i c  i s opposed by the e l e c t r o s t a t i c  attractive force,  s o l u t i o n e f f e c t and  the f o r c e o f  between the d i s s o l v e d p o s i t i v e i o n s and. t h e . s u r f a c e ,  attraction  and t h e . o s m o t i c  effect  which i s e f f e c t i v e l y - the sum o f a l l other f o r c e s t e n d i n g towards d e p o s i t i o n o f the c a t i o n s  on the m e t a l s u r f a c e .  The osmotic and e l e c t r o s t a t i c  f o r c e s are c o n s i d e r e d t o be g r e a t e r than the e l e c t r o l y t i c o n l y f o r v e r y n o b l e metals l i k e p l a t i n u m and g o l d , p o s i t i v e charge active  on the s u r f a c e . i n such c a s e s .  enough t o m a i n t a i n a n e g a t i v e charge  attractive  solution effect  g i v i n g r i s e t o a net  N i c k e l i s c o n s i d e r e d t o be  on i t s  surface at  its  reversible  redox p o t e n t i a l p r o d u c i n g a p o t e n t i a l f i e l d a t the m e t a l - s o l u t i o n i n t e r f a c e of unknown v a l u e .  T h e r e f o r e , the e l e c t r o d e r e p u l s e s n e g a t i v e i o n s at  n i c k e l - h y d r o g e n mixed p o t e n t i a l , which, i n i n d i f f e r e n t e l e c t r o l y t e s , to i t s  the  i s . close  r e v e r s i b l e redox p o t e n t i a l i n the pH range c o n s i d e r e d , . F i g u r e 5.  water molecules would, be adsorbed u n d e r - t h e s e c o n d i t i o n s . o f the e l e c t r o d e  Only  As the p o t e n t i a l  i s made more n o b l e , , n i c k e l i o n i z e s more r e a d i l y , h y d r o l y z i n g or  forming h a l i d e complexes away from the n i c k e l s u r f a c e . the f i r s t a c t i v e r e g i o n i n . t h e p o l a r i z a t i o n c u r v e .  . T h i s , gives, r i s e  to  Thus the c o r r o s i o n mechanism  in t h i s region i s : H 0  H 0  2  2  Ni(H 0) 2  Ni  the l a s t  ++  2 a d s  (H 0) 2  2  .....(la)  a d s  — N i  ( H  2  — N i ( O H )  2  +  +  0 )  2  +  s t e p occurringaaway from the s u r f a c e .  the e l e c t r i c  .+ . 2e~ + -2H  (lb) .....(lc)  A t even more n o b l e p o t e n t i a l s ,  f i e l d i n i t i a t e s h y d r o l y s i s of the o x i d i z e d n i c k e l near the  surface;  - 48 -  the more p o s i t i v e the. charge on the the h y d r o l y s i s r e a c t i o n u n t i l i t f i l m . o n . the e l e c t r o d e .  s u r f a c e , , the  occurs close  closer  enough t o  . T h i s mechanism p r e d i c t s  t o the  surface  form an  adherent  the pH dependence o f  F l a d e p o t e n t i a l Ep., the p o t e n t i a l at which the h y d r o l y s i s r e a c t i o n i n an adherent  film.  As noted p r e v i o u s l y the d a t a o f the F l a d e  as a f u n c t i o n o f pH f i t s .E  =  F  the  the  results  potential  relation:  -0.240:  T h i s mechanism a l s o p r e d i c t s  +  O.O65 PH  t h a t the  of the f l u o r i d e i o n c o n c e n t r a t i o n . the p r e s e n t  is  first  .  active region i s  -(2)  independent  T h i s i s c o n f i r m e d by, the r e s u l t s  of  work,, F i g u r e s 12 and 1 3 .  • As the e l e c t r o d e  surface  i s : made more p o s i t i v e ,  a t t r a c t s n e g a t i v e l y charged i o n s such as OH ,• C l water m o l e c u l e s f o r a d s o r p t i o n s i t e s .  and F  the e l e c t r i c  which compete  1  where 0  X  and Q  = e  K  l  t ~]  isotherm,  ....-(3)  F  2  are the p r o p o r t i o n s o f the  2  f l u o r i d e and h y d r o x i d e i o n s , : l i . ^ ,0^. - 0 by water m o l e c u l e s ,  with  T h i s a d s o r p t i o n i n aqueous f l u o r i d e  s o l u t i o n s may be d e s c r i b e d q u a n t i t a t i v e l y b y t h e - L a n g m u i r a d s o r p t i o n Qjr. - Si -  .field  and K n i s  2  covered by  i s the p r o p o r t i o n of s u r f a c e  an e q u i l i b r i u m  K i n e t i c considerations  surface  covered  constant.  o f a d s o r p t i o n and d e s o r p t i o n show t h a t  AH K  1 V  . =  k  e  ~RT  •  where k i s a c o n s t a n t and ^ H i s ^ t h e e n t h a l p y may'be  enthalpy of a d s o r p t i o n .  (4) The  written: AHi.=  AH  Q  I  - £  n- F. E  '(5)  _k 9  where  A. H  is.the  0  E i s the a p p l i e d f i e l d , field,  standard  e n t h a l p y w i t h no a p p l i e d  n i s the charge on the  field,  i o n moving i n the  electric  /& i s the p r o p o r t i o n of the a p p l i e d p o t e n t i a l through-which  i o n moves when a d s o r b e d ,  From t h i s  it  and F i s . the Faraday  the  constant.  follows that  AE =.  •9,,  1 - 0! - 9  [F-]  ,k  - /9FE  Ql  e.  .  ,(6)  2  A s i m i l a r e q u a t i o n can be. obtained, f o r a d s o r p t i o n of h y d r o x i d e i o n s which when added t o the above r e l a t i o n  _0 _+_0 _ = 1 - ©! - ©2 1  / [F~] k  2  gives  e~RT~  x  +  [ 0 H  -]  k 2  e  ~ W ]  e  ^ ••(7)  where Q±, 0 ,  [F~]/  2  However, the other f o r adsorption  [OH ] and E are  considered  variables.  f l u o r i d e and h y d r o x i d e i o n s a l s o compete w i t h each  sites,  OH" •+. F ^  d s  " .JL  (0H")  .....(8)  +:-F"  ads  with K  " 0  which w i t h e q u a t i o n  X  [OH ] -  l -  e  (1 + K  X  i  y  j  (7) y i e l d s  •[OH"]  0  (  "frT )  •  :  =i  .  • [F ] _  e  x  • + [OH"]  ( l - K l f | )  e  2  }  e  '  -£FE RT  •••••< > 10  ^ H where e From t h i s  it  1 > 2  = kx,2  e  RT  follows that: [OH"] '  0x (1 + K " [ F T ) —  -/?FE  e ~ W = .[F*]ex + [OH ] e  2  - ©JF"^  - Oi[OH~]e  : 2  [0H"1  2  - KOxlOH ]e±-- KQ  ±  [g- j - e  (11)  g  - 50 -  The f o u r t h and s i x t h terms i n the denominator are n e g l i g i b l e because K i s c o n s i d e r e d l a r g e and [0H~] i s i n - t h e o r d e r .of I O  Thus, by i g n o r i n g t h e s e  e  =  a n d t a k i n g logarithms  less.  _K  *  i (UK~- .TT 1)  Q  RT  or  terms, 1  -_£FE  - 7  +  .(12)  =  [F~1  mrr  61 + 62  -  (1  KOl)  :  = S ° i + log (0H=  -2,3  l o  F=) " 1 ° S [ jJiPT  +  i  e  +  e  (1 - K Q i ) ]  2  (13) I t may be assumed t h a t a c r i t i c a l c o n c e n t r a t i o n o f adsorbed f l u o r i d e i o n s tira n e c e s s a r y second a c t i v e a constant,  t o i n i t i a t e the c o r r o s i o n which r e s u l t s  i n . the  r e g i o n on the p o l a r i z a t i o n curve and thus 9 ^ may be c o n s i d e r e d  Qi c r i t .  Four l i m i t i n g c a s e s a r e c o n s i d e r e d t o a s c e r t a i n ;  pH and pF dependence o f E , the p o t e n t i a l a t the i n i t i a t i o n o f the  second  c  1 >>>> K In case 1. i t i s assumed t h a t \-.bf[QH J [F J IF"] e - K OjJ'eg.- > > [OH~ J . T h i s r e s u l t s i n the r e l a t i o n :  active (1  region.  _  - 2.3/fF  E  RT  ' =  l o g Qi  which shows no dependence o f E  c  c  r  i  t  - PH - l o g (1  l  f  [OH ]  e  ~2" 1S 5  a  »  E  c  (1 - K 0,' e  2  = l o g Oj.  i  c  r  case  - log e '+  t  r  c  )„  ....,(14)  :  Case 2 a p p l i e s when  d  1  is  >„>,. .  [QH"]  K  >  ,....(15)  pF  on pH and i s a l s o i n c o n s i s t e n t w i t h experiment.  1  K  3, -rp=] >  crit  n  and. y i e l d s  T h i s shows no dependence of E Similarly^!'  0  a  on f l u o r i d e c o n c e n t r a t i o n and t h e r e f o r e  inconsistent with experimental evidence. and  - K  the  TQIT]  pH-dependence and may be d i s r e g a r d e d .  a n d  " )  Case 4^  K  Q  e £  K [p -] 1  ^ ^  [F~] y  ToITT  i e  1  [0H~ J  a n c  ^  l  d s  n  o  - 51. -  — F — e - ,  \>  >  -2.3^  -  (1  [OH ]  K 0)  V  E  i n which E  c  c  = l o g Oi  7  c  r  i  e  2  results  i n the  f  + l o g K + pF - l o g  t  expression  2  i s a f u n c t i o n o f ,pH + 2pF.  e  i  + pF .+. pH  (16)  T h i s . i s . i n accordance w i t h t h e  e x p e r i m e n t a l r e s u l t s , • F i g u r e 2 0 , and may be r a t i o n a l i z e d , by c o n s i d e r i n g K very large.  Ec=G  . The above e x p r e s s i o n ,  "'537t  i°s  =  RLT  -  _ J | (  when w r i t t e n i n t h e form  P  H  +  2PF)  (  1  7  )  RT indicates that a p l o t o f E or 5 9 . I mV.  c  v e r s u s pH p l u s -2pF would have a. s l o p e o f y . ^ —  I n F i g u r e 20 t h e s l o p e i s a p p r o x i m a t e l y 230 mV which agrees  w i t h t h a t p r e d i c t e d f o r t h e above mechanism i f ^ = 0.26. • The energy o f a d s o r p t i o n i s . o n l y a p r o p o r t i o n o f t h e t o t a l energy o f a n i o n moving t h r o u g h the t o t a l a p p l i e d p o t e n t i a l , as i l l u s t r a t e d i n F i g u r e 25.  Electrode Surface  Double Layer  D i s t a n c e from Electrode  F i g u r e 25.  Surface  Potential- Function of Ion i n V i c i n i t y o f a.Charged Electrode  -  f? i s d e f i n e d as the applied potential  change i n t h e  (i.e.  E  =  ds applied  a  small, that i s ,  f l u o r i d e ion concentration.:  ^ relatively active  as p r e d i c t e d by e q u a t i o n  ( 17)  Recent w o r k ^  shown t h a t a d s o r p t i o n of  chloride  i o n s i s f a v o u r e d by low pH and l a r g e c h l o r i d e i o n c o n c e n t r a t i o n s . found t h a t c h l o r i d e a d s o r p t i o n i n h i b i t s coverage o f the  adsorbed  potentials  i f t h e r e i s s m a l l hydroxide i o n and l a r g e  w i t h p l a t i n u m i n c h l o r i d e s o l u t i o n s has  also  -  energy o f a d s o p t i o n w i t h change i n the  A d s o r p t i o n of f l u o r i d e i o n s occurs at i f pH and pF are  52  It  was  s u r f a c e by  oxygen.  A d s o r p t i o n of f l u o r i d e i o n s can be expected t o i n the n i c k e l h y d r o x i d e f i l m . location sites,  In such p l a c e s as  occur at weak spots  g r a i n boundaries and d i s -  the m e t a l i s p r o b a b l y s l o w l y d i s s o l v i n g .  As f l u o r i d e i o n s  carry •  c u r r e n t t o these s i t e s ,  rapidly.  In the p r e s e n t work, n i c k e l s u l p h i d e has been i d e n t i f i e d at  grain boundaries, weak on the  Figure 2 6 .  f l u o r i d e a d s o r p t i o n w i l l occur t h e r e most  Any p a s s i v a t i n g f i l m i s l i k e l y t o be  particularly  s u l p h i d e phase and t h e r e f o r e c o r r o s i o n w i l l be i n i t i a t e d  f e r e n t i a l l y at  the g r a i n b o u n d a r i e s .  Figure 2 6 .  Micro-structure  of N i c k e l Showing Second  the  Phase.  pre-  The adsorbed, f l u o r i d e i o n s w i l l form a s o l u b l e n i c k e l - h a l i d e complex,  probably N i F  , and. thus i n i t i a t e  3  a corrosion reaction.  The  mechanism i s  nads  N i ( H 2 0 )  Ni(F ) 3  +  5  F  "  N 1  <  F 3 )  !ds  MF3 • + 2  a d s  H a l i d e i o n s c a r r y p a r t of the c u r r e n t sufficient  concentration  g r a i n boundaries i n t h i s  .The  increase  1J5  m  large  exchanged.  H  °  2  e".  t o these s i t e s - a n d thus m a i n t a i n a  case,-Figure  these l o c a l i z e d  i n c u r r e n t w i t h p o t e n t i a l w i l l f i t the T a f e l the n i c k e l o x i d a t i o n s t e p .  an o x i d a t i o n p r o c e s s i n which one e l e c t r o n at  At h i g h e r pF and pH h y d r o x i d e a d s o r p t i o n i n t e r f e r e s  equation  This i s  and low pH ( F i g u r e 21).  i s c o n t r o l l e d - b y c o m p e t i t i o n between the two a d s o r b i n g a n i o n s .  a time  is  and the  rate  At h i g h pF ;  ions  site.  The above mechanism which g i v e s r i s e t o , t h e second a c t i v e i n n e u t r a l s o l u t i o n s can be a p p l i e d t o c o r r o s i o n p r o c e s s e s at up. t o  -800 mV i n a c i d , f l u o r i d e s o l u t i o n s ,  p o t e n t i a l and the  (Figure 8).  pH = 3> 5Figure 27. ;  potentials Flade  pH show t h a t  than the F l a d e p o t e n t i a l  at  A d d i t i o n a l support i s found i n F i g u r e 10a, which shows  e x t e n s i v e g r a i n boundary c o r r o s i o n on a,specimen anodic p o t e n t i a l s  anodic  region  The p l o t s . o f the  c r i t i c a l - h a l i d e adsorption potential;;;versus  f l u o r i d e i o n s adsorb at more a c t i v e p o t e n t i a l s  the  the  The slope  and low pH the r a t e may a l s o be c o n t r o l l e d by d i f f u s i o n of t h e ' f l u o r i d e t o the  sites,  22a.  f l u o r i d e ion concentrations  v y indicates  n  f o r the r e a c t i o n t o p r o c e e d at  i f the r a t e - c o n t r o l , - - i s 1 a c t i v a t i o n : . o f case at  +  subjected to corrosion  at  i n acid f l u o r i d e s , apparently i d e n t i c a l with corrosion i n  second a c t i v e r e g i o n i n n e u t r a l s o l u t i o n s - ( F i g u r e  27). - Thus, n i c k e l  corrodes  - 54 -  2  4  6 pH  Figure 2 7 .  F l a d e P o t e n t i a l , • E , and P o t e n t i a l a t A c t i v e Region v e r s u s pH F  I n i t i a t i o n of Second  -55  i n low p H . s o l u t i o n s a t  a l l anodic p o t e n t i a l s  "by complexing w i t h f l u o r i d e  ions. The g r a d u a l l y s l o p i n g p o l a r i z a t i o n curve at noble p o t e n t i a l acid fluoride solutions concentration sites.  indicates  specific,  p e r p e n d i c u l a r t o the p o t e n t i a l be p o t e n t i a l  t h a t the r a t e i s c o n t r o l l e d . b y mixed  p o l a r i z a t i o n and d i f f u s i o n of f l u o r i d e i o n s t o the  - S i n c e these s i t e s are  field.  the  anions must t r a v e l  between  d i s s o l v e f r o m . p i t s i t e s as d i v a l e n t  solutions  -950 and -1100 mV. to  metal  oxidation state.  can be expected t o occur as the e l e c t r o d e p o t e n t i a l  p o s i t i v e and t h e r e f o r e d i s p l a c e s h a l i d e  will  T a f e l slope.  t h a t oxygen p r e f e r e n t i a l l y adsorbs  atoms w i t h bonds c o r r e s p o n d i n g t o t h e i r h i g h e s t reaction  partly  for nickel i n neutral fluoride  shows t h a t n i c k e l i H p a s s i v e , a t p o t e n t i a l s K o l o t y r k i n " ^ has suggested  dissolution  T h i s . p a r t o f t h e i r movement  independent and g i v e s r i s e t o a l a r g e r  .The p o l a r i z a t i o n curve  in  ion adsorption.  This i  i s made more  I r o n i s known t o  i o n s and'-from p a s s i v e  a r e a s as  tri-  29 valent  ions,  i n c h l o r i d e and bromide s o l u t i o n s  - p o t e n t i a l diagram ( F i g u r e  5)  predicts  .  In a d d i t i o n , the ..pH-  t h a t M 2 O 3 would form at  more noble than -750 mV i n . t h e absence o f h a l i d e i o n s . r e g i o n f o r n i c k e l i s t h e r e f o r e concluded t o r e s u l t to n i c k e l t o . f o r m a of h a l i d e  ions.  trivalent  oxide f i l m which i s  potentials  The second  from oxygen  passive  adsorption  s t a b l e even i n the  presence  - 6 5  Monel P o l a r i z a t i o n s t u d i e s . o n monel i n f l u o r i d e s o l u t i o n s b u f f e r e d at pH = 4.0, 6.2  (Figure  28) and 11.3 showed b e h a v i o u r s i m i l a r t o  However,•monel has a h i g h e r o v e r v o l t a g e  . 2H  +  + - 2e~  The r e s u l t i n g exfchange  — ^  currents  are  f o r the c a t h o d i c  H  nickel.  reaction:  2  lower t h a n f o r n i c k e l (see  Table  III)  •TABLE. I I I . .Exchange. C u r r e n t f o r Monel and N i c k e l i n S i m i l a r Solutions  Monel  4,0  6 uA/cm  6.0 11.0  5  16 u A / c m  2  2 uA/cm 0.3 u A / c m  8 uA/cm 0.6 u A / c m  2  These d a t a . i n d i c a t e s  acid solution .  Nickel  pH  2  .The exchange or c o r r o s i o n c u r r e n t r e a c t i o n as  o f monel i n f l u o r i d e s  i l l u s t r a t e d i n F i g u r e ^b). • Any  e n v i r o n m e n t a l change which i n c r e a s e s the r a t e o f . t h e . c a t h o d i c  reaction  shape o f the a n o d i c curve  i n f l u o r i d e s at  i n t h e presence, o f f l u o r i d e s .  i s v e r y s i m i l a r t o the anodic curve f o r n i c k e l  low p H .  In n e u t r a l f l u o r i d e s o l u t i o n , pH = 6 the a n o d i c c u r r e n t s l i g h t l y on r a i s i n g the p o t e n t i a l p a s t the a c t i v e p e a k , • F i g u r e 28. "passive"  will  the c o r r o s i o n of monel. •At pH = 4 monel does, not p a s s i v a t e  The  2 2  t h a t monel i s more r e s i s t a n t than n i c k e l t o f l u o r i d e s i n  i s c o n t r o l l e d by the c a t h o d i c  increase  2  current  was about 12 pA/cm.  2  as compared t o 2 p A / c m  2  for  decreases The nickel.  - 57'  1000  -500  0  Potential Figure 28.  -1000  (mV v e r s u s - S C E )  P o l a r i z a t i o n Curve o f M o n e l ' i n O.H^-M. NaF S o l u t i o n at pH = 6.0 A'«» ?*'*-8  -  At h i g h e r p H , monel behaves, s i m i l a r l y t o n i c k e l but a g a i n "passive" current 0.6 u A / c m  2  d e n s i t y i s much l a r g e r .  and the  f i r s t active  The exchange c u r r e n t  region i s missing.  The f i r s t  a p o t e n t i a l o f -700 mV compared t o  r e g i o n s t a r t s at  O p t i c a l examination of the specimens  is  the  about  transpassive  -600.mV f o r  nickel.  showed more p i t t i n g and l a r g e  cavities. .No. experiments  to ascertain  time dependence of the  current  at  a f i x e d p o t e n t i a l were done. '  Monel was a l s o p o l a r i z e d i n c h l o r i d e s o l u t i o n a t pH- = 6> F i g u r e 29. The ' . r e s u l t i n g . c u r v e was;;.very::.,similar.-.to t h a t o b t a i n e d for. n i c k e l i n . c h l o r i d e s but w i t h a d e c r e a s e d T a f e l s l o p e i n the second a c t i v e compared t o 180 mV. f o r  r e g i o n , 75  V  a  s  nickel.  A l l o y i n g copper t o n i c k e l produces a l a r g e r and t h i s accounts  m  f o r t h e lower exchange c u r r e n t .  hydrogen q v e r v o l t a g e  However, i t a l s o  t h e e x t e n t of p a s s i v a t i o n at more n o b l e p o t e n t i a l s .  reduces  . T h i s may be caused by  23 breakdown of t h e f i l m due t o copper d i s s o l u t i o n .  Arai  has found t h a t . i n  n i c k e l - c o p p e r a l l o y s c o n t a i n i n g more t h a n about 50$ copper, f i l m breakdown occurs  due t o the f o r m a t i o n o f CuO at a f a s t e r r a t e than N i ( 0 H ) . 2  has.indicated that increases  i n copper c o n t e n t  increase  the  of n i c k e l - c o p p e r a l l o y s when p o l a r i z e d i n s u l p h u r i c a c i d . decrease i n t h e r e s i s t a n c e as a r e s u l t by e l e c t r o n s  of the f i l l i n g  "passive" He e x p l a i n s  o f the n i c k e l  from copper w h i c h i s . n o t a t r a n s i t i o n element.  current the  d-orbitals  This decreases  the  s u s c e p t a b i l i t y o f t h e a l l o y t o oxygen a d s o r p t i o n and t h e r e f o r e  its  a b i l i t y to  passivate.  Uhlig  decreases  -  ——I +300  :  .  -LJ +200  I +100  •  I 0  -100  • P o t e n t i a l (mV v e r s u s - S C E ) F i g u r e 2 9 . - P o l a r i z a t i o n Curve o f Monel i n 0.42 S o l u t i o n at pH = /fu/\ ^SJ  NaCl  5 9 -  I -200  - 60 -  CONCLUSIONS  1.  N i c k e l does not p a s s i v a t e  i n a c i d f l u o r i d e s o l u t i o n s and i s  p r e f e r e n t i a l l y c o r r o d e d a t the g r a i n b o u n d a r i e s a t anodic p o t e n t i a l s up to  -800 mV ( v e r s u s S C E ) .  2.  The p o l a r i z a t i o n curve f o r n i c k e l i n n e u t r a l f l u o r i d e s o l u t i o n s  shows an a c t i v e r e g i o n a t anodic p o t e n t i a l s potential.  s l i g h t l y above the mixed  . N i c k e l i s thought t o corrode by f o r m i n g aquo-complexes  in this  r e g i o n i n which f l u o r i d e i o n s have no e f f e c t .  3.  N i c k e l becomes p a s s i v e  i n n e u t r a l s o l u t i o n s at p o t e n t i a l s E , F  according t o the r e l a t i o n E  F  = -0.240 + 0.065; pH  by h y d r o l y s i s o f t h e aquo-complexes hydroxide f i l m . i n the passive  4.  a t the s u r f a c e  t o form an adherent  F l u o r i d e ions i n s o l u t i o n increase  the corrosion  current  region.  Fluoride ions i n neutral s o l u t i o n i n i t i a t e  on the n i c k e l p o l a r i z a t i o n curve a t p o t e n t i a l s  a second a c t i v e  region  which a r e more a c t i v e w i t h  i n c r e a s i n g f l u o r i d e i o n and d e c r e a s i n g h y d r o x i d e i o n c o n c e n t r a t i o n s .  A  mechanism i s proposed whereby the c o r r o s i o n i s i n i t i a t e d by t h e a d s o r p t i o n o f f l u o r i d e at s u f f i c i e n t l y noble p o t e n t i a l s .  These i o n s compete w i t h h y d r o x i d e  ions so t h a t b y c o n s i d e r i n g t h e Langmuir a d s o r p t i o n i s o t h e r m , between the c r i t i c a l h a l i d e a d s o r p t i o n p o t e n t i a l , negative  E , and pH and pF (the c  logarithm of f l u o r i d e ion concentration), E  c  =C  -  -5|  (log  G  l  b  r  i  t  a relation  + P H + 2pF)  - 61 -  is derived.  The mechanism f o r c o r r o s i o n i n the second a c t i v e r e g i o n i s  Ni(H 0) 2  Ni(F ) 3  n a d s  + : F 3  3 "  N i  NiF  ads  3  ( 3)ads' F  + n  H 0 2  + 2e  N i c k e l becomes p a s s i v e a t p o t e n t i a l s more noble t h a n -950 mV  5.  i n n e u t r a l f l u o r i d e s o l u t i o n s , p r o b a b l y due t o the f o r m a t i o n o f film(Ni 0 2  3  6.  6-5 < PH <  7.  or h i g h e r  oxide).  N i c k e l i s passive i n contact with f l u o r i d e s solutions with  12. Monel c o r r o d e s  less  r a p i d l y at  the mixed p o t e n t i a l i n aqueous  f l u o r i d e s o l u t i o n s because o f i t s h i g h e r hydrogen o v e r v o l t a g e . anodic p o t e n t i a l s , • monel does not r e a l l y  8.  a.passive  However,  at  passivate.  . N i c k e l and monel c o r r o d e more r a p i d l y i n c h l o r i d e s o l u t i o n s than  i n f l u o r i d e at a l l anodic  potentials.  -  RECOMMENDATIONS FOR FUTURE INVESTIGATIONS  1.  62  -  V  The c o r r o s i o n o f n i c k e l i n f l u o r i d e media c o u l d be s t u d i e d as  a f u n c t i o n o f temperature t o e s t a b l i s h the temperature range over which the c o r r o s i o n mechanisms may be a p p l i c a b l e , and t o o b t a i n more v thermodynamic 2.  detailed  relationships.  . P o l a r i z a t i o n studies  of n i c k e l i n s o l u t i o n s of c h l o r i d e , bromide  and i o d i d e would r e v e a l the e x t e n t t o which the mechanism f o r c o r r o s i o n by f l u o r i d e s i n the second a c t i v e . r e g i o n  3.  P o l a r i z a t i o n techniques  a p p l i e s to other h a l i d e i o n s .  i n which a square wave a l t e r n a t i n g  i s superimposed on the a p p l i e d p o t e n t i a l are properties  k.  u s e f u l i n d e t e r m i n i n g the  o f the p a s s i v a t i n g f i l m and the double l a y e r .  to n i c k e l corrosion i n h a l i d e s o l u t i o n s ,  Corrosion studies  of metals  These may be a p p l i e d  e x p e c i a l l y i n passive  such as  on.these  in chloride)  metals  regions.  z i r c o n i u m , aluminum, and  magnesium i n f l u o r i d e s o l u t i o n s t o e s t a b l i s h the r e l a t i o n s h i p s mechanisms  of  corrosion  (which were found t o y i e l d a second a c t i v e  t o the mechanisms  current  of n i c k e l c o r r o s i o n , would be v a l u a b l e .  region  - 63 REFERENCES  1. 2. 3. 4.  . G.C.-WMttaker,.-Corrosion,  6 ,-.= 283 (1950).  F . Maness, U . S . - Atomic Energy Comm. Report HW-68426 ( 1 9 6 l ) . .J.  Bergman and G.- W. C . MacDonald, C o r r o s i o n , 17, 9 and 12  E . I.  Antonovskaya and L . V . . Takhtarova,• Z h u r . Vsesoyuz... Khim. . Obshohestva im D . I . Mendeleeva, 6,. 477  5.  M. S c h u s s l e r , , I n d . . E n g . . Chem. 4j_,  6.  W. J .  8.  9. 10.  (196l). ( C A .  5_6:292g)  133 (19,55).  B r a u n , F . W . . F i n k and G.•Lee E r i c k s o n , U . S . • A t o m i c Energy Comm.Report BMI-1237  7.  (I96I).  D . R. T u r n e r , J .  (1957).  Electrochem. S o c , , . 9 8 ,  434 (1951).  . G . Truempler and R. K e l l e r , H e l v . Chim. • A c t a 44,.I69I ( I 9 6 I ) . Y . . K o l o t y r k i n and G . W. G i l m a n , - D o k l . A k a d . Nauk. • SSSR, Y..Kolotyrkin, 1961,  13_7_,  642 ( I 9 6 I ) .  " F i r s t I n t e r n a t i o n a l Congress on M e t a l l i c C o r r o s i o n "  B u t t e r w o r t h s , . London, 1 9 6 2 , - p .  11.  . L . T r o n s t a d , - T r a n s . Faraday S o c . 2<?,  12.  . D. H . M a c G i l l a v r y , J .  10.  502 (1933) •  H . . Rosenbaum and R. W.Stevenson, J .  Electrochem.  S o c . 89, 22 (I952.)... . 13.  . K. J .  14.  . J.  15.  V e t t e r and K . A r n o l d ,  Osterwald and H . H . U h l i g ,  U. R.-Evans,  I96I, 16. 17. 18.  Z . E l e k t r o c h e m . , 6k,  "First  J.  244  Electrochem.. S o c . ,  (i960). 108, 515 ( I 9 6 I ) .  I n t e r n a t i o n a l Congress on M e t a l l i c C o r r o s i o n "  B u t t e r w o r t h s , - London, 1962, p .  N . Y a . Bune and Y . K o l o t y r k i n , . N . D . Greene, C o r r o s i o n ,  3-  Z h u r . F i z . . Khim,  3J>,  1543  (I96I).  15_, 369 (1959).  M. S t e r n and A . L . . G e a r y , J .  Electrochem.  Soc,  104, 56, 559 and 645  (1957).  19.  ,M... S t e r n , C o r r o s i o n , 14,  440t  20.  W . - A . M u e l l e r , C o r r o s i o n , 18,  21.  V . C h i k a l and M. • Prazak,• J .  .22. 23.  (I958). 349.(1962).  I r o n and S t e e l I n s t . . 1 9 3 ,  - T . . P . H o a r , - J . A p p l . Chem., 11,  121  N . D . Greene, C o r r o s i o n , 18, 136t  (i960). (1962).  36O ( I 9 5 9 ) .  - 64  .24.  T . P.. Hoar, " A n o d i c - B e h a v i o u r o f M e t a l s " in Modern Aspects o f Electrochemistry, I I , e d . J . . O'M. B o c k r i s , B u t t e r w o r t h s , London, 1959, p . 272.  .25.  I b i d , p . 325.  26.  w  R. L i t t l e w o o d , . C o r r o s i o n S c i e n c e 3, 99  (I963).  .27.  M. P o u r b a i x , " A t l a s - D . E q . u i l i b r e s E l e c t r o c h i m i q _ u e s " , • P a r i s , p . 333, (1963).  .28.  A r a i Y o s h i , Kogyo Kagaku Z a s s h i ,  29. .30-  31. 32..  33.  -  ;  Gauthier-Villars,  6k, 600 (I96I).  . J . . L . Weininger and W. .R. Grams, J . -Electrochem. - S o c .  109, 9&k  (I962).  U . . F . . F r a n c k , • " F i r s t I n t e r n a t i o n a l Congress on M e t a l l i c Corrosion',' 1961, B u t t e r w o r t h s , London, 1962, p . 113. Y . K o l o t y r k i n and V . M. K n y a s h e v a / Z h u r . F i z . . C h i m . . R. J . H a r t m a n ,  " C o l l o i d Chemistry",  1 9 4 7 , p . .222-229.  3_0, 1990 (1956).  Houghton " M i f f l i n Company,  N . Hackerman and M. C . B a n t a , J . . Electrochem... S o c .  Ill,  Cambridge,  114 (1964).  - 6  5  APPENDIX A . Potential Standardization  The  standard calomel reference  experiments was checked a g a i n s t  •A  99*999$  s o l u t i o n was i n t r o d u c e d i n t o the p o t e n t i a l between the r e f e r e n c e  is  —>•  A O.9O7 molar cadmium s u l p h a t e  c e l l and s a t u r a t e d electrode  w i t h n i t r o g e n . - The  and cadmium e l e c t r o d e  Cd  + +  potential  E  the e l e c t r o d e  =  + 2e~  E  or  .  Using-an  o f 0.05, and the Nernst e q u a t i o n  0  RT - ~p  In  r  c  d  [Cd  +  +  1  iCdT  ]  Cd  p o t e n t i a l f o r the above system was  hydrogen e l e c t r o d e  read  of  0.4-03 v o l t s on t h e hydrogen s c a l e as r e p o r t e d i n L a t i m e r  activity coefficient  was  a f t e r 30 minutes and 60 minutes t o be 680 mV.  standard electrode  Cd  the  was f i t t e d i n the working e l e c t r o d e  holder a f t e r ageing i n sulphuric a c i d ;  The  used i n a l l of  the cadmium-cadmium s u l p h a t e h a l f - c e l l .  cadmium e l e c t r o d e  on the Pye p o t e n t i o m e t e r  electrode  0.684  versus- the s a t u r a t e d  The e x p e r i m e n t a l v a l u e o f  0.680  0.433  v o l t s versus  the  calomel.  v o l t s is' w e l l w i t h i n  experimental  accuracy.  A  Latimer/W. M./'Oxidation Potentials"  Hall,.1952.  Englewood C l i f f s /  Prentice  

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