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Corrosion behaviour of nickel and monel in aqueous fluoride media. 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 accept t h i s t h e s i s as conforming t o the required standard •Members of the Department of Metal lurgy THE UNIVERSITY OF BRITISH COLUMBIA February 196̂ In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It i s understood that copying or publication of this thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of M E T A L L U R G Y The University of B r i t i s h Columbia, Vancouver Canada. Date F e b r u a r y 1964 ABSTRACT The c o r r o s i o n behaviour of n i c k e l and monel i n aqueous f l u o r i d e so lut ions was s tudied by 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 techniques and surface examination of the corroded specimens. N i c k e l does not e x h i b i t the u s u a l a c t i v e - p a s s i v e t r a n s i t i o n f o r 0 < pH •< 4.0 but corrodes r a p i d l y e s p e c i a l l y at the g r a i n boundaries . In the range k.O < pH < 6.5 the n i c k e l - p o l a r i z a t i o n curve contains two a c t i v e r e g i o n s . N i c k e l i s passive 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 6.5 < pH < 12.0. P o l a r i z a t i o n curves 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 s of v a r i e d pH's and f l u o r i d e ion concentrations i n the range 4.0 < pH < 7.0 revealed that the current as a f u n c t i o n - o f p o t e n t i a l i n the f i r s t a c t i v e r e g i o n i s ' independent of f l u o r i d e ion concentrat ion but dependent on pH. -The c u r r e n t s ' i n the f i r s t passive and second a c t i v e regions are a f u n c t i o n of pH and f l u o r i d e i o n concentra t ion . Surface examinations showed that n i c k e l corrodes at the g r a i n boundaries i n the second a c t i v e r e g i o n . A mechanism has been proposed which accounts f o r c o r r o s i o n i n the second a c t i v e region by F adsorpt ion and p a s s i v a t i o n by e i t h e r H 20 or OH adsorpt ion on the a n o d i c a l l y p o l a r i z e d metal sur face . • A mathematical a n a l y s i s based on competitive adsorpt ion of these species as a f u n c t i o n of e lec t rode p o t e n t i a l i s shown t o be consis tent w i t h the experimental d a t a . Monel corrodes at: ^.ess than h a l f the. rate of n i c k e l at the mixed p o t e n t i a l i n f l u o r i d e s o l u t i o n s w i t h 0 < pH < 6.5 due to i t s l a r g e r hydrogen overvoltage. Monel e x h i b i t s a c t i v e - p a s s i v e behaviour s i m i l a r t o n i c k e l but w i t h the passive current up to 6 times as l a r g e . ACKNOWLEDGEMENT . The author wishes to express a p p r e c i a t i o n t o members of the Department of M e t a l l u r g y , p a r t i c u l a r l y to D r . E . Peters and M r . W . M. Armstrong, who d i r e c t e d t h i s work. S p e c i a l thanks are extended to Mrs. W. M. Armstrong f o r many h e l p f u l d i s c u s s i o n s . . F i n a n c i a l support from Aluminium Laborator ies L i m i t e d i n the form of a Fel lowship 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 . . . . 1 Previous 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 20 . . . . . 12 Purpose and Scope of Present I n v e s t i g a t i o n .• 14 APPARATUS AND EXPERIMENTAL . . . . . . . . . . 16 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 Apparatus . . . . . . 16 M a t e r i a l s ' 20 P o l a r i z a t i o n Curves V 21 Surface Examination . . . . . . . . . 23 RESULTS AND DISCUSSION 2k N i c k e l i n A c i d F l u o r i d e . S o l u t i o n s . 24 N i c k e l i n N e u t r a l F l u o r i d e Solut ions . . . . . . . . . . . 27 " N i c k e l i n Basic F l u o r i d e S o l u t i o n s . . . . 44 Mechanism of Nickel C o r r o s i o n i n F l u o r i d e Media . . . . . 46 Monel . 56 CONCLUSIONS: 60 i fl RECOMMENDATIONS FOR FUTURE INVESTIGATIONS 62 REFERENCES 63 APPENDIX A 65 LIST OF FIGURES Figure Page 1. T y p i c a l P o l a r i z a t i o n Curve of a Metal with an A c t i v e - Passive T r a n s i t i o n 4 2. Cathodic Curves w i t h D i f f e r e n t Redox Exchange Currents Superimposed' on an Anodic Curve . . . . . . . . . . . . . 9 3 . P o l a r i z a t i o n Curves Showing E f f e c t of Change i n Cathodic Redox Exchange Current . . . . . . . . . . . . . . . 10 4 . Anodic or Cathodic Rate C o n t r o l of the C o r r o s i o n Current H 5. Nickel-Water p H - P o t e n t i a l Diagram . . . . ' 13 6. E l e c t r o c h e m i c a l C o r r o s i o n C e l l 17 7. Schematic Diagram of E l e c t r i c a l Apparatus , 19 8 . 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 at pH = 2 .2 2 5 9 . Mixed P o t e n t i a l versus pH . 26 10. Surfaces of N i c k e l Corroded i n F l u o r i d e s at Low pH . . 28 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 at pE = 6.2 . . . . 29 12. Exchange Current versus pH . 31 13. C r i t i c a l Anodic Current versus pH . . . . . . . . . . . . 31 14. . Logarithm of the Minimum Passive Current versus pH + pF 32 15. Current-Time Curves at Se lec ted Regions of N i c k e l 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 34 16. Surfaces of N i c k e l i n the F i r s t A c t i v e State Corroded i n 0 .42 M-NaF at pH = 6 .2 . . . . . . . . . . . . 35 17. Surfaces of N i c k e l i n the Passive State Corroded i n Solut ions of pH = 6 .2 36 18. P o t e n t i a l at the Second A c t i v e Peak versus .pH p l u s p F . 38 19. Log Current Densi ty at the Second A c t i v e Peak versus pH p l u s p F 38 2 0 . The P o t e n t i a l at the I n i t i a t i o n of the Second A c t i v e Region versus pH plus 2pF 39 L i s t of Figures . C o n t i n u e d . . . . Figure ^ Page 21; P o s i t i v e Slope i n Second A c t i v e Region versus pH +. pF . -̂0 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 = 6.1 hi 23. Surfaces of 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 - a t pH = 6.2 . . J . ^3 2k. 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-NaF S o l u t i o n at pH = 11 J3 ^5 . 25. . P o t e n t i a l Funct ion of Ion i n V i c i n i t y of a Charged Elec t rode ? . . . . . . . . . 51 26. M i c r o - s t r u c t u r e ' of N i c k e l Showing Second Phase . . . . 5 2 .27. Flade P o t e n t i a l ' / E p , , and P o t e n t i a l at I n i t i a t i o n of Second A c t i v e Region versus pH . . . . 5^ 28. P o l a r i z a t i o n Curve of Mionel i n Q.k2 M NaF S o l u t i o n of pH = 6". 2 57 29. 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 Condi t ions 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 of the H a l i d e Ions OH"" and H 2 0 Table I I I . Exchange^Currents of Monel and N i c k e l i n S i m i l a r Solut ions . . . . . INTRODUCTION .Previous.Work The c o r r o s i o n r e s i s t a n c e of 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"*" of •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 states that n i c k e l and' monel --(70$ n i c k e l , 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 , uranium f u e l reprocess ing p l a n t s and i n a l k y l a t i o n p l a n t s , a l l of which use f l u o r i n e , hydrogen f l u o r i d e , or other f luorides ,•anhydrous, or aqueous. Some q u a l i t a t i v e researches. 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 a l l o y s i n f l u o r i d e s . Takhtarova and Antonovskaya found monel and " n i c k e l to-be r e s i s t a n t to K F . H F , N H 4 F , N H 4 F . H F , HF i n l i q u i d and vapor phases i n the absence of oxygen. Of these , h y d r o f l u o r i c a c i d was the 5 6 most c o r r o s i v e . Both Schuss ler and Braun found monel t o be r e s i s t a n t t o aqueous, concentrated HF but that exposure t o the atmosphere r e s u l t e d i n severe a t t a c k . Braun a l s o found, that ga lvanic c o u p l i n g w i t h s i l v e r and . s i l v e r solders i n the presence of oxygen increased the a t tack on monel. Although none of t h e . p r e v i o u s work on n i c k e l c o r r o s i o n descr ibed p o l a r i z a t i o n s tudies i n f l u o r i d e s o l u t i o n s , some work has been-done on the e f f e c t of other halogen ions on the p o l a r i z a t i o n of n i c k e l i n sulphates . 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 techniques , found that c h l o r i d e ft This r e f e r s to the method: of a p p l y i n g a constant e x t e r n a l current t o an electrode and f o l l o w i n g the change i n e lec t rode p o t e n t i a l w i t h t ime. - 2 - ions increased the current that was required to pass ivate a n i c k e l e l e c t r o d e . This was a t t r i b u t e d to the f a c t that N i C l 2 i s more soluble than NiS04 which was suggested to be the f i r s t f i l m . f o r m e d i n the p a s s i v a t i n g mechanism. 8 More r e c e n t l y , Trueriipler a n d . K e l l e r , us ing 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 techniques (holding electrode p o t e n t i a l constant independent of current and time) s tudied the a f f e c t of C l and Br on the p a s s i v a t i o n behaviour of n i c k e l i n sulphate s o l u t i o n . They too found that halogen ions increased the a c t i v i t y of n i c k e l (defined 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 ) . The presence of halogen ions a l s o caused a second a c t i v e region above -300 mV. In both ++ a c t i v e regions the c o r r o s i o n process y i e l d e d N i i o n s . . H a l i d e ions have a l s o been reported t o produce a secondary a c t i v i t y 9 i n z i rconium, magnesium and aluminium s i m i l a r t o that found.by Truempler and K e l l e r except that the e lec t rode d i d not pass ivate a g a i n . T h i s second a c t i v e peak r e s u l t e d i n p i t t i n g c o r r o s i o n . The mechanism as expla ined by K o l o t y r k i n ^ w i l l be discussed l a t e r i n reference to the present work. B r i e f l y , he a t t r i b u t e s the second a c t i v e peak t o the adsorpt ion of the h a l i d e ions due to t h e i r p o l a r i z a b i l i t y at higher e lec t rode p o t e n t i a l s so that they replace the p a s s i v a t i n g oxygen at random s i t e s . The h a l i d e ions cause the n i c k e l to i o n i z e by forming complexes w i t h the n i c k e l atoms. The c r i t i c a l concentrat ion of h a l i d e i o n s " i s i n i t i a t e d and maintained at the d i s s o l u t i o n s i t e s as the h a l i d e ions carry the current to them. Other work has been done on n i c k e l i n sulphate e l e c t r o l y t e s . 11 Tronstad s tudied the 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 . He proposes that the p a s s i v i t y i s due to the formation of an oxide f i l m which grows - 3 - to a constant thickness of about kO A . M a c G i l l a v r y et a l . s tudied n i c k e 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 . He proposed that d i s s o l u t i o n of n i c k e l i n the presence of 0 2 takes place by. the i n t e r a c t i o n of hydrogen ions wi th the oxide f i l m . 13 lk Vett.er and A r n o l d and Osterwald and U h l i g obtained s i m i l a r r e s u l t s f o r the 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 of n i c k e l i n s u l p h u r i c a c i d . However,. they postula ted 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 fo l lows Evan's school i n proposing that 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 formation whereas U h l i g maintains that chemisorbed oxygen from water pass ivates the metal sur face . The question of. the mechanism,of metal p a s s i v a t i o n i s probably one 15 of d e f i n i t i o n as much as concept. Evans , in suggesting t h i s , has s tated that a. three-dimensional f i l m i s necessary f o r a metal to 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 Flade p o t e n t i a l (E-p i n Figure 1) l e s s than a monolayer of oxygen i s s u f f i c i e n t t o stop c o r - r o s i o n . The monolayer w i l l tend to grow to an oxide f i l m , the thickness depending on the p o t e n t i a l and character of the supporting e l e c t r o l y t e . 16 Bune a n d . K o l o t y r k i n have used, various , oxidants i n sulphate s o l u t i o n s to show that these produce', anodic currents which p o l a r i z e the n i c k e l i n the same way that a n i c k e l e lectrode 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 . They concluded that the d i s s o l u t i o n current of n i c k e l is. a f u n c t i o n of the electrode p o t e n t i a l o n l y . . This i s important as i t i n d i c a t e s that metals . can be chemi- c a l l y pass ivated by the a d d i t i o n of o x i d i z i n g agents t o the c o r r o s i v e medium. P o t e n t i a l 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 Metal with an A c t i v e - P a s s i v e T r a n s i s t i o n . i i 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 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 long 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 anodic p r o t e c t i o n of the metal . However, i t i s only w i t h i n the past ten years that p o t e n t i o s t a t i c . techniques have been e x t e n s i v e l y . u s e d . Modern techniques are based on the use of an e l e c t r o n i c device c a l l e d a p o t e n t i o s t a t . T h i s instrument detects any v a r i a t i o n of the e lectrode p o t e n t i a l with respect to a reference e lec t rode which i s . i n the c i r c u i t . The p o t e n t i a l i s then brought back' to the preset value 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 current f lowing between the working e lec t rode and an a u x i l i a r y e lectrode (see .F igure 7 ) . 17 -23 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 here . P o t e n t i o s t a t i c t e c h - 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 lec t rodes and i t i s i n t h i s connection they w i l l be d i s c u s s e d . F igure 1 shows t y p i c a l p o t e n t i o s t a t i c (a) and g a l v a n o s t a t i c (b) curves f o r a metal showing ah a c t i v e - p a s s i v e t r a n s i t i o n . The mixed or res t p o t e n t i a l of any electrode 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 current) i s that p o t e n t i a l at which the rate of the cathodic and anodic reac t ions occuring. . ; „ on i t s surface are e q u a l . In many instances the cathodic r e a c t i o n i s 2 H + + 2 e - H 2 , or i f oxygen i s present l / 2 0 2 + H 2 0 + 2e" 20H", a n d t h e anodic r e a c t i o n i s M —$>- M + + + 2e~. - 6 - I f the e lec t rode 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 current the cathodic r e a c t i o n predominates and i t s rate increases w i t h f u r t h e r p o l a r i z a t i o n . . .S imilar considerat ions apply to anodic p o l a r i z a t i o n . .The r e l a t i o n between the logar i thm of current and the p o t e n t i a l of the e lec t rode i s known as the p o l a r i z a t i o n curve . I f the anodic r e a c t i o n increases as shown the electrode i s a c t i v e . However, .on f u r t h e r 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 i n the anodic d i r e c t i o n the anodic current may f a l l sharply as the electrode becomes p a s s i v e . T h i s i s because the formation of a p r o t e c t i v e oxide or hydroxide f i l m becomes k i n e t i c - a l l y favourable or oxygen i s chemisorbed on the metal sur face . -As long as the passive state p e r s i s t s the anodic p o t e n t i a l causes no f u r t h e r increase i n c u r r e n t . The c o r r o s i o n rate i s . u s u a l l y c o n t r o l l e d by the rate of metal . ion d i f f u s i o n through the passive layer:. . , ••At some higher p o t e n t i a l i n the anodic d i r e c t i o n , the e lec t rode becomes t ranspassive ( i . e . the current begins t o r i s e s teeply) due t o f i l m breakdown or. the i n i t i a t i o n of another anodic r e a c t i o n . The p o l a r i z a t i o n curve can then be subdivided i n t o a . ca thodic region and an.anodic region with a c t i v e , passive and. t ranspassive p a r t s . C e r t a i n e l e c t r o c h e m i c a l parameters.may be determined from the p o l a r i z a t i o n curve which charac ter izes the meta l ' s c o r r o s i o n behaviour i n that p a r t i c u l a r medium. By e x t r a p o l a t i n g . t h e l i n e a r p o r t i o n s of the curve i n the a c t i v e 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 E m and the c o r r o s i o n or exchange c u r r e n t , i 0 , are determined by the i n t e r s e c t i o n , . F i g u r e 1. The exchange current i s the ra te of the anodic and cathodic r e a c t i o n s occuring on the e lectrode surface 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 . I f the anodic - 7 - current i s the o x i d a t i o n of the metal t o metal i o n s , then the,exchange 19 current gives the c o r r o s i o n rate . The slopes of the l i n e a r regions ( T a f e l slopes) are c h a r a c t e r i s t i c s of the anodic process , u s u a l l y metal d i s s o l u t i o n +n -M — M + ne and the cathodic process 2 H + + 2e~ — ^ H 2 or l / 2 0 2 .+ H 2 0 + 2e~ _ ^ 20H~ Linear regions of the 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 concentrat ion c o n t r o l l e d , r e a c t i o n , both of which f i t the T a f e l - e q u a t i o n ' E - - E o = * l = a . + b l o g i where. E i s the p o t e n t i a l of the e l e c t r o d e , •E 0 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 lec t rode r e a c t i o n , % i s the overvoltage or p o l a r i z a t i o n , a and b are T a f e l constants , and i i s the current d e n s i t y . 2k The T a f e l equation can be derived, from t h e o r e t i c a l considerat ions . For an a c t i v a t i o n c o n t r o l l e d process i t can be shown that b = 2.$ RT c<n F and a = 2.3 RT l o g , i Q < X n F where R i s the u n i v e r s a l gas constant , •T i s the absolute temperature, n i s the number of e lec t rons being exchanged at the a c t i v a t i o n b a r r i e r F i s the Faraday constant and c< i s a factor .between o and 1 represent ing 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 of the T a f e l slope permits the es t imat ion of n . I f n i s . known ( e . g . . i f the ra te -determining step i s i d e n t i c a l t o the o v e r a l l r e a c t i o n Ni —>• N i + + ' . + 2e~ i n which n = 2) then o< i s obtained. However, n may not be known because the number of e lec t rons t a k i n g part per molecule i n the ra te -determining step i s not 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 o v e r a l l r e a c t i o n . I f i t can be assumed that i s approximately 0 . 5 , the number of e lec t rons i n the ra te -determining step may be a s c e r t a i n e d . I f b becomes i n f i n i t e then-the r a t e - c o n t r o l l i n g step i s not e l e c t r o c h e m i c a l . Thus an examination of the T a f e l p l o t of an e l e c t r o c h e m i c a l r e a c t i o n can sometimes r e v e a l the nature of the ra te -determining s tep . Three important parameters from the anodic p o l a r i z a t i o n curve of a metal showing an a c t i v e - p a s s i v e t r a n s i t i o n are the c r i t i c a l anodic current d e n s i t y , _"_ 1crit, the primary passive p o t e n t i a l Epp (the current d e n s i t y and p o t e n t i a l at the a c t i v e maximum) and the Flade 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 lec t rode w i l l be s e l f - a c t i v a t e d a f t e r p a s s i v a t i o n ) . 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 of the passive c o n d i t i o n of a metal . . Low values of : . i c r i t and a c t i v e values o f Bgp and-Ep lead; t o ease of p a s s i v a t i o n . An a c t i v e value of Ep a l s o i n d i c a t e s s table p a s s i v a t i o n i n unstable environments. A long range of p o t e n t i a l through which the metal remains passive and a smal l passive c o r r o s i o n rate i p , enhance the p o s s i b i l i t i e s f o r p r o t e c t i n g the metal, by anodic 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 reduct ion p o l a r i z a t i o n curves when superimposed on the anodic curve . Three such curves w i t h d i f f e r e n t redox 23 -exchange currents are shown i n Figure 2. - 9 - .!>» -P •H CQ a a -p <D u u o o © ^ •—N © \ © ^ C 1 V A c t i v e Figure 2 . P o t e n t i a l Cathodic Curves w i t h D i f f e r e n t Redox Exchange Currents Superimposed on an 1 Anodic Curve . Noble For case 1 the rate of o x i d a t i o n equals the rate of r e d u c t i o n at point A i n d i c a t i n g that the electrode i s i n the a c t i v e state ( i . e . corrodes r a p i d l y ) at the r e s t p o t e n t i a l . For case 2 however there are three points where the anodic and cathodic rates are e q u a l . . I t can be shown that only p o i n t s B and D represent s table p o t e n t i a l s . Environmental and h i s t o r i c a l condi t ions of the e lec t rode w i l l govern whether the e lec t rode i s i n the a c t i v e or the passive c o n d i t i o n o r b o t h . In case 3 the e lec t rode i s pass ive and w i l l corrode at the rate indica ted , by the passive c u r r e n t . Obviously case 3 represents the most favourable c o n d i t i o n f o r c o r r o s i o n r e s i s t a n c e . This 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 i f Epp and Ep are f a i r l y a c t i v e wi th respect to the r e v e r s i b l e hydrogen p o t e n t i a l and i f i-.^-crif i s s m a l l , i t is. most s table 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 i s extensive - 10 - and i t i s most r e s i s t a n t i f the passive current i s n e g l i g i b l e . 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 of e lec t rodes i n condi t ions represented, by the above cases y i e l d s curves as.shown i n Figures J a , b , and c . (dashed l i n e s i n d i c a t e cathodic current . ) -p • r l CO a P •P Ci QJ U U o O \ / / 1 \ / 1 \ / 1 \ / 1 \l ll \ J P o t e n t i a l P o t e n t i a l (a) Case 1. (b). Case 2. . P o t e n t i a l (c) Case 3• Figure 3• P o l a r i z a t i o n Curves Showing E f f e c t of Change i n Cathodic 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 r a t e - c o n t r o l l i n g r e a c t i o n at the" mixed p o t e n t i a l : that i s , they i n d i c a t e whether the rate of 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 or 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 step i s e lec t rochemical or p h y s i c a l . Four cases, 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 e l e c t r o c h e m i c a l a c t i v a t i o n or. d i f f u s i o n , are i l l u s t r a t e d . i n Figures 4 a , b , c, and d . P o t e n t i a l 4 a.). AnodLc...Actj P o t e n t i a l X o n t r o l - •-(-b) ~Oathodic_Act i v a t i o n . C o n t r o l P o t e n t i a l (c). Anodic D i f f u s i o n C o n t r o l P o t e n t i a l (d) Cathodic D i f f u s i o n C o n t r o l Figure 4. Anodic or Cathodic Rate C o n t r o l of the Corros ion C u r r e n t . There are c e r t a i n drawbacks t o the use of p o l a r i z a t i o n curves f o r the above purposes. One i s that e x t e r n a l currents only represent the sum of a l l e l e c t r o c h e m i c a l reac t ions occuring at the e lec t rode s u r f a c e . However, - 12 - the reaction•which, i s accounting f o r a l l or most of the current i s u s u a l l y known. The current at a set p o t e n t i a l 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 wi th the passage of current and time the c o n d i t i o n of the e lec t rode surface and i t s environment change. Therefore the shape of the curve may greatly , depend on the technique used i n obta ining the p o t e n t i a l - c u r r e n t d a t a ^ . The above-mentioned change i n surface c o n d i t i o n 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 . Then the anodic current i s n e a r l y completely accounted f o r by the metal o x i d a t i o n at the p i t . Often the cathodic r e a c t i o n i s occuring on the non-corroding areas even when p o l a r i z e d a n o d i c a l l y so that the t rue anodic current i s greater than the apparent c u r r e n t ^ . p H - P o t e n t i a l Diagram f o r N i - H 2 0 The energy, r e l a t i o n s h i p s of a metal with aqueous environments may be summarized by means of a.. Pourbaix or p H - p o t e n t i a l diagram. The r e l a t i o n s h i p s i n d i c a t e the condi t ions of c o r r o s i o n , 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 . The l i n e s represent e q u i l i b r i a between t h e . s o l i d phases and s e l e c t e d concentrat ions of n i c k e l d i s s o l v e d i n water. These concentrat ions may be u s e d . t o def ine condi t ions of p a s s i v i t y and n o b i l i t y , and i t has become a convention t o use 1 0 " 6 ,M d i s s o l v e d metal ions 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 of .corrosion). With f l u o r i d e s . i n the system the diagram i s changed very l i t t l e except that N i F 2 . ^ H 2 0 i s thermodynamically s table w i t h respect t o N i + + at concentrations greater than 1 0 ~ 6 M. The boundaries of i t s s t a b i l i t y are very s i m i l a r to N i + + i n pure H 2 0 . However i n most circumstances i t does not form an adherent f i l m on the n i c k e l s u r f a c e . The e f f e c t of f l u o r i d e i s t h e r e - fore disregarded 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. Figure 5 . Nickel-Water 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 conta ining 10" . . M . .• N i should he passive at any p o t e n t i a l i n the pH range 9-12, according to the p H - p o t e n t i a l . d i a g r a m . This i s because N i (0H) 2 or higher oxides are s table i n that they have s o l u b i l i t i e s below 10~ 6 M and are considered t o form a p r o t e c t i v e f i l m . N i ( 0 H ) 2 forms at the res t p o t e n t i a l so that a s s o c i a t e d p o l a r i z a t i o n curves • i should s h o w , l i t t l e or no a c t i v i t y . At pH < 9 n i c k e l would not be expected t o p a s s i v a t e . , However,. t h i s i s not always the -case as. water molecules .or hydroxide ions may be adsorbed at p o t e n t i a l s above the res t p o t e n t i a l , and i n i t i a t e the formation of a hydroxide f i l m . , In t h i s case the p o l a r i z a t i o n curve u s u a l l y shows an a c t i v e r e g i o n , , the extent of which may depend on the ions i n s o l u t i o n . N i c k e l should corrode at s t i l l higher p o t e n t i a l s i n t h i s pH region due to the formation of hexavalent n i c k e l as N i o / . - N i c k e l would a l s o be expected to corrode i n aqueous s o l u t i o n s with a t.-UA.... pH y 12 t o f orm .Ni0 2 H instead of the p a s s i v a t i n g ' N i ( O H . ) 2 . Throughout the pH region 0 . to 12 higher oxides of n i c k e l are s table 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 may form an adherent f i l m but i f they are good e l e c t r o n i c and i o n i c conductors they w i l l not be p r o t e c t i v e . They w i l l probably ,be good i o n i c conductors i f t h e y have a defect s t r u c t u r e , 28 which the N i 0 2 l a t t i c e i s known t o have i Purpose and Scope of the Present I n v e s t i g a t i o n The purpose of 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 of v a r y i n g pH and f l u o r i d e content f o r evidence of 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 and of p o s s i b l e c o r r o s i o n mechanisms of these metals i n t h i s range of environments. •• I n d i c a t i o n s were a l s o sought f o r methods of improving 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 compositions where c o r r o s i o n rates may be considered excess ive . - 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 tudies at widely- varying 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 e l e c t r o l y t e of pH = 6 .2 showed a second a c t i v e peak which was reported but not explained 8 by Truempler a n d - K e l l e r i n c h l o r i d e s o l u t i o n s . . F u r t h e r studies .were conducted i n an attempt to e x p l a i n t h i s anomaly. These c o n s i s t e d . o f p o l a r i z a t i o n s tudies at d i f f e r e n t f l u o r i d e and hydrogen i o n concentra t ions , current- t ime curves at constant p o t e n t i a l and surface examinations of corroded specimens. P o l a r i z a t i o n curves were a l s o obtained 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 , to determine any d i f f e r e n c e i n c o r r o s i o n behaviour with, other 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 halogen-free r e f e r e n c e . P o t e n t i o s t a t i c curves 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 obtained i n an attempt t o r e l a t e the behaviour 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 Apparatus The h i g h l y corros ive 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 mater ials and thus the design 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 (Teflon) was used because of i t s iner tness and h i g h temperature. (500°F) s t rength . The c e l l designed f o r the present work (Figure 6) i s s i m i l a r t o a c e l l 29 descr ibed by Weininger and Grams ^ and contains 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. reference e lec t rodes 2. c i r c u l a t i n g e l e c t r o l y t e 3. gas s a t u r a t i o n i n l e t and o u t l e t k. . p r o v i s i o n . f o r heat ing and temperature c o n t r o l . The c e l l design a l s o lends i t s e l f t o ease of changing the working e lec t rode and e l e c t r o l y t e . A l l parts are machined from .rod, bar and.tube stock T e f l o n suppl ied by the Crane Packing Company. The c e l l c o n s i s t s of a 2 i n c h I . D . , 3 inch O.D. T e f l o n c y l i n d e r (a) , with disks, or e lec t rode holders above (b) and below ( c ) . The c e l l i s sealed by O-rings and clamped together by bear ing p l a t e s and threaded rods , (p ) . . The working e lec t rode (d) i s wrapped w i t h T e f l o n . t a p e and. drawn i n t o place by t i g h t e n i n g the nut at the end of the s t a i n l e s s s t e e l b o l t (r) that a l s o acts as an e l e c t r i c a l connection and i s i n s u l a t e d . f r o m the bearing p l a t e by a b a k e l i t e washer. • The a u x i l i a r y e lec t rode (f ) i s a c i r c u l a r d i s k of the same metal as the working e l e c t r o d e , and i s h e l d i n place by the s t i f f •copper wire (h) that i s s o l d e r e d : t o i t . The reference e lec t rode (e) i s F i g u r e 6. E l e c t r o c h e m i c a l C o r r o s i o n C e l l - 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 saturated KC1. The bottom of 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 wi th agar agar g e l of saturated-KC1 which i s i n contact with 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 next to the working e lec t rode allows e l e c t r o l y t e i n t o t h i s tube. The e l e c t r o l y t e 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 by the gas l i f t at ( 1 ) . . T h i s provides 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 to the e lectrode f a c e s . This a l s o saturates the e l e c t r o l y t e with the p a r t i c u l a r gas used, i n t h i s case n i t r o g e n (< 0.7$ 0 2 ) from compressed ,gas tanks suppl ied by*Canadian L i q u i d A i r Company. A T e f l o n sheet (m) w i t h a small hole i n i t i s clamped over the f u n n e l and maintains small p o s i t i v e pressure of the sa tura t ing .gas over the e l e c t r o l y t e . This c e l l has the advantage of being f l e x i b l e i n use and simple i n design but does not permit the c e l l r e a c t i o n t o be observed. -A schematic diagram of the e l e c t r i c a l system i s shown i n Figure 7- .The p o t e n t i o s t a t i s a Duffers Model 600. Current i s suppl ied from a 12 v o l t center- taped Delco car 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 is ' added shunts and a s h o r t i n g type r e v e r s i n g switch g i v i n g f u l l scale readings of 50 y&> 200 uA, ^00 uA, 2 mA 5 mA and.20 mA i n both d i r e c t i o n s . The ammeter i s i n the a u x i l i a r y e lec t rode l e a d . The p o t e n t i a l between the saturated calomel reference e lec t rode and the working electrode i s measured by a .Model 7569P Pye Potentiometer us ing 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 d e t e c t o r . P o t e n t i o s t a t ©- Potentiometer -© 0- Reference E lec t rode -Working Elec t rode A u x i l i a r y Elec t rode N u l l . D e t e c t o r — © © - F i g u r e 7• Schematic Diagram, of E l e c t r i c a l Apparatus i h-1 VO - 20 - Unfor tunate ly the p o t e n t i o s t a t r e q u i r e d frequent-maintenance s e r v i c e . To overcome delays caused by-breakdowns, a " c l a s s i c a l " p o t e n t i o - s ta t was a l s o used. I t cons is ted of two 2.2 v o l t Hart D . H . S . 15 g lass wet c e l l s i n p a r a l l e l , from which a p o l a r i z i n g current i s drawn by a d j u s t i n g an Ohmite three-pole 8.5 ohm v a r i a b l e r e s i s t o r . The current was measured by. the modif ied Simpson ammeter but i n order t o f o l l o w the p o t e n t i a l d r i f t more c l o s e l y a Beckman "Zeromatic" pH meter was used, f o r measuring the p o t e n t i a l . This has . the r e q u i r e d . h i g h impedance but much lower s e n s i t i v i t y (± 10 mV). . M a t e r i a l s E lec t rodes (5 cm 2 i n area) were machined from rod supplied: by A . D . MacKay Inc . with s ta ted p u r i t i e s of 99.9$> f o r both monel and. n i c k e l . When i m p u r i t i e s were suspected. to be present i n the n i c k e l , i t was analyzed, wi th t h e . f o l l o w i n g r e s u l t s : :Sulphur 0.015 % Carbon 0.030 I ron ' 0.040 Copper 0.150 Cobalt O.35O The monel sheet f o r the a u x i l i a r y e lec t rode was a l s o s u p p l i e d by A . D . MacKay, :Inc. .The n i c k e l a u x i l i a r y e lectrode was s u p p l i e d by Mr.. V.- N. Mackiw of S h e r r i t t Gordon Mines L i m i t e d . - A l l 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 water. - 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 s u c c i n a t e . - 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 p o l i s h i n g w i t h l/2 g r i t emery'paper and c l e a n i n g with Chlorothane* -The specimen was aged dur ing n i t r o g e n s a t u r a t i o n of the e l e c t r o l y t e f o r about l/2 hour and then 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 . P o t e n t i a l se t t ings were made i n c r e a s i n g l y anodic beginning from t h i s cathodic region i n 10 t o 100 mV increments. At each s e t t i n g i n i t i a l current readings were made fol lowed by at l e a s t one a d d i t i o n a l reading a f t e r 5 o r 10 minutes ' to detect time-dependent current v a r i a t i o n s . The pH of the e l e c t r o l y t e was measured with s h o r t - r a n g e „ H y d r i o n pH paper before and a f t e r each r u n . The specimen was r e t a i n e d f o r subsequent examination of the s u r f a c e . Curves were obtained 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 widely v a r i e d pH. -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 of 4 .0 to 7-0 because under these condi t ions secondary a c t i v a t i o n was e v i d e n t . Curves were a l s o obtained i n sodium n i t r a t e s o l u t i o n s as a reference and i n sodium c h l o r i d e s o l u t i o n s f o r comparison. . S e v e r a l experiments were made with 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 the time-dependence of current at p o t e n t i a l s c o r - responding t o s e l e c t e d par ts of the p o l a r i z a t i o n curve . The growth of a i p r o t e c t i v e 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 decaying current v a l u e s . In these experiments the c o r r o s i o n specimens were l e f t at the se lec ted p o t e n t i a l s f o r one or more days during which time current readings were recorded at i n t e r v a l s . Experimental condi t ions f o r a l l runs are summarized i n Table I . A 1 ,1 ,1 - t r i c h l o r o e t h a n e - 22 - TABLE . I. Experimental Condi t ions Experiment Number Anode M a t e r i a l E l e c t r o l y t e • Concentrat ion B u f f e r pH 8 N i c k e l NaF 0 .39 • Phosphate 4 . 0 10 N i c k e l NaF 0.42 Phosphate 11.3 11 N i c k e l NaF 0 .45 NaOH >12 12 N i c k e l HF 0.1 HF 1.0 13 N i c k e l HF + NaF O .38 Phosphate 5-8 lk N i c k e l NaN03 0 .08 Phosphate 6 . 0 15 N i c k e l NaF 0.42 Succinate 5-2 16 N i c k e l HF 0 .08 Phosphate 2.2 17 N i c k e l NaF 0.41 Succinate 6 . 0 18 N i c k e l NaN03 0 .083 Succinate 4 . 8 19 N i c k e l NaCl 0.42 Phosphate h.9 20 N i c k e l NaCl 0.2 Phosphate 6 . 1 21 N i c k e l NaF 0.042 Phosphate 6 .2 22 N i c k e l NaF- 0.21 Phosphate 6 .2 23 N i c k e l NaF 0 .29 Phosphate 6.2 24 N i c k e l NaF 0.42 ' Phosphate 6 .2 25B N i c k e l NaF 0 .42 Phosphate 6 .2 26 N i c k e l NaF 0.42 Phosphate 6 .2 27B N i c k e l NaF 0 .42 Phosphate 6 .2 28 Monel NaF 0.42 Phosphate 6 . 0 29 Monel HF-+ NaF 0 .30 - 4 . 1 30 Monel NaF 0 .42 Phosphate 11.2 31 Monel NaCl 0.42 Phosphate 4 . 0 32 N i c k e l NaF 0.42 Phosphate 7 . 0 33 N i c k e l NaF 0.42 Phosphate 5.^ 34B N i c k e l HF 0.1 - 1.0 „55B N i c k e l NaF 0 .42 Phosphate 6 .2 36B N i c k e l NaF 0.42 Phosphate 6 .2 37B N i c k e l NaF 0 .42 Phosphate 6 .2 38B N i c k e l NaF 0.42 Phosphate 6 .2 39B N i c k e l NaN03 0.1 Phosphate 6 . 0 40B N i c k e l NaF 0.42 Phosphate 6 .2 B i n d i c a t e s current - t ime experiments. - 23 - Surface•Examinat ion i The surfaces of a l l corroded 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 to the c o r r o s i o n system. Specimens corroded at constant p o t e n t i a l over extended times were p a r t i - c u l a r l y s i g n i f i c a n t . Micrographs were made of these, u s i n g a Reicher t microscope and P o l a r o i d camera. - 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 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 curves as shown i n Figure 8, 11, and 22 r e s p e c t i v e l y . These curves show three d i s t i n c t types of behaviour 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. N i c k e l i n A c i d F l u o r i d e S o l u t i o n s The curve at pH = 2.2 and another which was done i n unbuffered 0.1 M HF at pH = 1.0 show that n i c k e l does not passivate i n the presence of f l u o r i d e ions at low pH. In c o n t r a s t , V e t t e r " ^ , Uhlig"'"^ and Bune"^ found that n i c k e l pass ivates on a t t a i n i n g a p o t e n t i a l of about -90 mV i n s u l p h u r i c a c i d s o l u t i o n s . A The mixed p o t e n t i a l s are p l o t t e d versus pH w i t h other, data f o r n i c k e l i n phosphoric and s u l p h u r i c a c i d s o l u t i o n , F igure 9. The value of 310 mV at pH = 2.2 i s intermediate between the mixed potent ia ls , i n 1 N s u l p h u r i c a c i d and 0.1 M phosphoric a c i d . Presumably these f o r e i g n ions have some e f f e c t on the reac t ions on the n i c k e l e lec t rode s u r f a c e . The exchange current was determined t o be about 50 u A / c m 2 , i n d i c a t i n g a f a i r l y high c o r r o s i o n rate of 13Q mdd or 0.02 i p y . No t ime-current experiments were done u s i n g t h i s system. A See Appendix I f o r C a l i b r a t i o n of P o t e n t i a l s - 25 - +400 +200 0 -200 -^00 -600 P o t e n t i a l (mVr? versus SCE) Figure 8. P o l a r i z a t i o n Curve of N i c k e l i n 0.08 M NaF S o l u t i o n at pH = 2.2" rfnh * - 26 - +100 A +200 N i c k e l i n - S u l p h a t e , . Osterwald and U h l i g ^ O N i c k e l in-Phosphate,- M a c G i l l a v r y et a l 1 ^ D N i c k e l i n Phosphate and F l u o r i d e , present work •a w CO U -,CU •H -P OJ -P O P- T3 a; X •H s 1-300 +hoo +500 +600 • a +700 11 pH Figure 9- Mixed P o t e n t i a l versus pH - 27 - In making the readings f o r t h i s p o l a r i z a t i o n curve i t was noted that the change i n current w i t h time was small 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 there may have been some f i l m growth. . O p t i c a l examination of the specimen p o l a r i z e d i n the phosphate-buffered s o l u t i o n showed i n t e r f e r e n c e colours which may i n d i c a t e f i l m , format ion . The micrograph, Figure 10a..shows extensive i n t e r g r a n u l a r c o r r o s i o n . ..The mechanism, of 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 at tacks g r a i n 'boundaries w i l l be discussed 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 of n i c k e l c o r r o s i o n 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 . Another specimen was corroded 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 day. The micrograph Figure 10b. shows extensive p i t t i n g c o r r o s i o n and no general c o r r o s i o n or f i l m . f o r m a t i o n between the p i t s . O b v i o u s l y , the anodic areas are s t a t i o n a r y producing p i t s while the cathodic r e a c t i o n occurs, on the res t of the s u r f a c e . From these observations i t i s evident that n i c k e l i s s u b j e c t . t o extensive c o r r o s i o n by f l u o r i d e s at low pH independent of the a p p l i e d p o t e n t i a l . . 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 Figure 11 shows that n i c k e l i s a c t i v e i n contact wi th a s o l u t i o n containing f l u o r i d e s at pH = 6 .2. On r a i s i n g i t s . p o t e n t i a l n i c k e l has a t y p i c a l a c t i v e - p a s s i v e t r a n s i t i o n . . F u r t h e r i n the anodic d i r e c t i o n there 8 i s a second a c t i v e region s i m i l a r to that found b y Truempler and K e l l e r i n sulphate s o l u t i o n s conta ining c h l o r i d e s and. bromides. This behaviour "P5 12 was not found by V e t t e r ^ or U h l i g working with 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 §00 (b) At mixed p o t e n t i a l pH = 1.0 X 300 Figure 10. Surfaces of N i c k e l Corroded i n F l u o r i d e s at Low pH - 29 - +500 E F 0 E- c -500 -1000 Potential (mV versus SCE) V Figure 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 •andNitrate S o l u t i o n at pH = 6.2 Auh - 3 0 _ The cathodic r e g i o n , the exchange c u r r e n t , the mixed p o t e n t i a l and the f i r s t a c t i v a t i o n peak are funct ions of pH; o n l y . . I t was not p o s s i b l e to obtain an accurate value of the exchange current because the T a f e l p l o t i s not l i n e a r and therefore does not permit accurate e x t r a p o l a t i o n to the mixed p o t e n t i a l . This i s i l l u s t r a t e d i n Figure 11. However ah approximate value was obtained by e x t r a p o l a t i n g . t h e cathodic curve t o the mixed p o t e n t i a l . Figure 12 i s a p l o t of the estimated exchange current versus. pH at v a r y i n g f l u o r i d e ion concentra t ions . This i n d i c a t e s . that n i c k e l becomes f a i r l y passive i n f l u o r i d e s o l u t i o n s above pH 6.5- The exchange current at pH = 7 .0 i s 0.3 uA/cm 2 which corresponds t o a c o r r o s i o n rate of 0.17 mdd.or 27 X 10" 6 i p y . At low pH's the mixed p o t e n t i a l E m f o l l o w s the equation Em••= 0.2O0 t 0.050 pli as shown i n Figure 9. The abrupt change i n p o t e n t i a l at about pH = 6.5 i s due to a.change from a c t i v a t i o n t o ohmic overvoltage of the anodic r e a c t i o n w i t h the p a s s i v a t i o n of the e l e c t r o d e . Two experiments . (Nos. 15 and 17) u s i n g succinate as a b u f f e r gave much more a c t i v e mixed and Flade p o t e n t i a l s , probably, because n i c k e l complexes w i t h succinate which i s a c h e l a t i n g i o n . 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 i s not p r e d i c t e d by the p H - p o t e n t i a l diagram. Even i f the concentrat ion o f N i - ^ " i n s o l u t i o n i s 10" M/i., or 1.5 mg of d i s s o l v e d n i c k e l , the n i c k e l would not be expected to form a:...hydroxide f i l m , u n t i l a pH of 8 .0 a t t a i n e d . The c r i t i c a l anodic: current 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 v a r y i n g f l u o r i d e i o n concentrat ions , at constant pH as shown by ex experiments numbered Ik, 21, 22 and 23 . The c r i t i c a l anodic current i s Figure l j . C r i t i c a l Anodic Current Densi ty versus pH 60jik/cm i n each case. On the other hand Truempler and K e l l e r found that an a d d i t i o n of 0.05 M of c h l o r i d e i o n increased the c r i t i c a l anodic current from 30 mA/cm 2 to 100 mA/cm 2 . However, the c r i t i c a l anodic current i s d e f i n i t e l y ,a . : funct ion of pH as shown i n Figure 13. The Flade p o t e n t i a l fo l lows the e q u a t i o n : : E p = . * 0.240 + 0.065 pH which compares w i t h the r e l a t i o n given by U h l i g a s : E F = -0.120 +O.O59 pH The presence of f l u o r i d e ions i n s o l u t i o n lessens the r e s i s t a n c e of pass ivated n i c k e l to c o r r o s i o n . This e f f e c t i s increased w i t h increase i n hydrogen i o n concentrat ion as i l l u s t r a t e d i n Figure 14, where pF i s the negative logar i thm of f l u o r i d e i n i o n c o n c e n t r a t i o n . In c o n t r a s t , changes h 6 8 10 pH + pF Figure 14. Minimum Passive Current Densi ty versus pF + pH. - 33 - i n pH have l i t t l e e f f e c t 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 n i c k e l i n . , . -13,.14 sulphuric a c i d Current - t ime curves shown i n Figure 15, (a) and (b) i n d i c a t e f i l m growth i n the passive r e g i o n . Curve (a) at +200.mV gives a f i n a l current densi ty of about 0.5 nA/cm 2 a f t e r 2 hours as compared to the value df 8 |iA./cm2 a f t e r 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 curve . This emphasizes that the p o l a r i z a t i o n curve current readings i n t h i s region are f a r from the steady-state v a l u e s . The gradual ly s l o p i n g curve i n the a c t i v e - passive t r a n s i t i o n may be more accurate as a v e r t i c a l l i n e on the b a s i s of . f i n a l s teady-state c u r r e n t s . The f i n a l current at +200 mV i s l e s s than the f i n a l current at -I50 mV, curve ( b ) . This agrees w i t h the t rend shown i n the p o l a r i z a t i o n curves , f o r the current to increase w i t h the a p p l i e d p o t e n t i a l i n the passive r e g i o n . Figure 16 (a) and (b) , and Figure 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 i n 0.42 M sodium f l u o r i d e s o l u t i o n at a pH of 6.2. The specimen corroded at the mixed p o t e n t i a l f o r 1.25 days, F i g u r e s .l6a, shows a large amount of general 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 h i g h rate of corrosion-suggested by the exchange current measurement. The specimen, corroded i n the a c t i v e r e g i o n at a p o t e n t i a l of +300 mV f o r i day, Figure l6b, i s c h a r a c t e r i z e d by both p i t t i n g and general c o r r o s i o n . Figure. 17a, i s the surface of a specimen corroded i n the passive region f o r 1.75 days. This shows.less general c o r r o s i o n . Nevertheless c o r r o s i o n i s s t i l l apparent, while another specimen, • Figure 17b h e l d i n the passive region 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 the 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. Current-Time Curves at Selec ted Regions of 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 p o t e n t i a l X 300 Figure 1 6 . Surfaces of N i c k e l i n the F i r s t A c t i v e State Corroded i n F l u o r i d e Solut ions at pH = 6 . 2 (a) At - 1 5 0 mV i n 0 .42 M NaF X 300 (b) At -200 mV i n 0.1 M NaN03 (Comet-l ike marks are p o l i s h s t r i a t i o n s ) X 3 0 0 Figure 17. Surface of N i c k e l i n the Passive State Corroded i n S o l u t i o n s of pH = 6 . 2 - 3 7 - f l u o r i d e ions cause n i c k e l t o corrode even when apparently p a s s i v e . The 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 . P o s s i b l y f l u o r i d e ions are incorporated i n t o the l a t t i c e of the hydroxide f i l m causing i t to become an i o n i c conductor. 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 wi th the n i c k e l ions i n the l a t t i c e of the f i l m . The f a c t that f l u o r i d e causes c o r r o s i o n of n i c k e l i n the passive s tate helps e x p l a i n the photomicrograph of r, n i c k e l corroded i n . t h e a c t i v e 30 s t a t e , F igure 16b. U . F . Franck has explained 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 state as being due to the existence of a c t i v e and passive s i t e s on the same p o l a r i z e d e l e c t r o d e . The d i f f e r e n c e i n p o t e n t i a l i s caused by e l e c t r o l y t e r e s i s t a n c e . Thus the p i t s are presumed to be a c t i v e s i t e s and the areas of general c o r r o s i o n are " p a s s i v e " s i t e s . . Both p o s i t i o n and extent of the second a c t i v e region are f u n c t i o n s of f l u o r i d e and hydrogen or hydroxide i o n concentra t ions . The p o t e n t i a l of the a c t i v e peak, F igure 18 and the logar i thm of the current at the peak, Figure 19, are p l o t t e d as f u n c t i o n s of pH plus p F . In both cases the dependence appears to be l i n e a r . . The p o t e n t i a l at the i n i t i a t i o n of the second a c t i v e region i s p l o t t e d versus pH plus 2pF,, i n Figure 20. This seems t o give the best l i n e a r f i t of the d a t a . Figure 21 i s a p l o t of the p o s i t i v e T a f e l slope of the second a c t i v e region versus pH p l u s . p F . The l a t t e r i n d i c a t e s a change i n the r a t e ^ c o n t r o l l i n g step at a .pH plus pF equal t o 6.5 s ince below t h i s value the T a f e l slope i s constant of 135 mV, g i v i n g an o< n value of 0 A 5 , while at higher values the T a f e l slope increases s h a r p l y . - 38 - h'y 5 6 7 8 9 pF +..pH Figure 18. P o t e n t i a l at Second A c t i v e Peak ' versus - pH plus p F . k 5 6 7 8 9 .pF + pH Figure 19. Log 'Current Densi ty at the Second A c t i v e Peak versus pj? plus pH. - 39 - pH + 2pF Figure 20. P o t e n t i a l at the I n i t i a t i o n of the Second A c t i v e R e g i o n / E versus pH .+ 2pF. - ko - 600 ^0/ *** /o 4"00 / a *r~t / CJ ft % u H CO 200 1 1 1 1 5.0 6.0 7.0 8.0 pH + pF , Figure 21. P o s i t i v e Slope i n Second A c t i v e Region versus pH plus p F . The second a c t i v e region was absent i n those two experiments done i n . n i t r a t e s o l u t i o n s wi th no f l u o r i d e ions present , as shown i n F igure 11. The minimum passive current was'Tower i n these p o l a r i z a t i o n curves but i n a l l other respects they resembled experimental curves f o r f l u o r i d e - c o n t a i n i n g s o l u t i o n s . On.the other 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 region i n agreement wi th observations by other i n v e s t i g a t o r s . This curve shows that smaller concentrations of c h l o r i d e i o n s . i n d u c e greater c o r r o s i o n rates than f l u o r i d e i o n s . In 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 passive again at p o t e n t i a l s more noble than about -950 mV. In c h l o r i d e s o l u t i o n s t h i s probable f i n a l passive region was not observed because the current d e n s i t y i n the second a c t i v e region was so.much l a r g e r that i t was impossible to p o l a r i z e the electrode to the p o t e n t i a l of maximum c u r r e n t . - kl - -p •Potential (mV versus SGE) Figure 22. P o l a r i z a t i o n of N i c k e l i n 0 .2 'M NaCl S o l u t i o n at pH = 6 .1 ^un - k2 - Current- t ime curves at p o t e n t i a l s i n the region of secondary a c t i v a t i o n show i n i t i a l decreases i n current fol lowed by a . s m a l l r i s e , Figure 15. I t i s p o s s i b l e these represent f i l m growth i n passive areas i n competi t ion w i t h the i n c r e a s i n g area of the d i s s o l u t i o n s i t e s . A photomicrograph of a.specimen used i n the t ime-current experiments, - F igure 23a, shows that the d i s s o l u t i o n s i t e s are gra in boundaries . Interference colours on b i n o c u l a r examination of the specimens are 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 boundaries . 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 i s s i m i l a r t o the c o r r o s i o n of n i c k e l i n the passive r e g i o n , Figure 17a. C o r r o s i o n i n the second passive region shows general c o r r o s i o n over the whole surface w i t h s l i g h t g r a i n boundary e t c h i n g , F igure 23b. This a l s o i s s i m i l a r to 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 r e g i o n . 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 p i t t e d . The above observations i n d i c a t e that n i c k e l cannot be protected 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 at 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 . At pH ) 6.5 n i c k e l i s pass ive i '£ no strong o x i d i z i n g agent i s present . The c o r r o s i o n current i n the .second a c t i v e region at \[F~] { . 0.01 M i s . small enough so that n i c k e l could be protected 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 chemical oxidant or by an e x t e r n a l current . (a) At the second a c t i v e peak, -700 mV X 300 (b) In the second passive r e g i o n , - IO5O mV X 300 Figure 25. Surfaces of 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 curve, Figure 2k, i n d i c a t e s that n i c k e l i s passive i n contact with an aqueous s o l u t i o n of f l u o r i d e s at high pH. This curve i s an example of the type drawn i n Figure 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, Figure 5- (The small a c t i v e peak i n Figure 2k would probably be e l i m i n a t e d w i t h longer times at constant p o t e n t i a l ) . I f the f i l m i s assumed t o be N i ( O H ) 2 , then the two reac t ions occuring on the surface of the e lec t rode w i t h no e x t e r n a l l y a p p l i e d current are probably 2H • + 2e~ ~ ^ H 2 and • N i + 20H~ —>- N i ( O H ) 2 + 2e~ Thus the abrupt change i n mixed p o t e n t i a l as a f u n c t i o n of pH (Figure 9) i s explained by a change i n the state of the o x i d i z e d n i c k e l which would l e a d to a change i n r a t e - c o n t r o l . In the a c t i v e state the rate i s c o n t r o l l e d by the cathodic r e a c t i o n as i l l u s t r a t e d i n F igure 4{b), whereas i n the passive region the r e a c t i o n i s probably c o n t r o l l e d by d i f f u s i o n of the n i c k e l ions through the oxide as i n Figure kfc). The-exchange current i s very low (0.3 uA/cm 2) confirming' that n i c k e l i s s e l f - p a s s i v a t e d i n the pH. range 6.5 to 12.0. The f i r s t t ranspassive region i s probably due to the formation of N i 0 2 . K o l o t y r k i n and Knyasheva"^ obtained the same behaviour w i f h n i c k e l i n potassium sulphate , . solut ions w i t h a T a f e l slope of "JO mV which compares very w e l l w i t h the value of 80 mV i n t h i s work. They explained the phenomena as due to the formation of N i 0 2 on the e lec t rode surface . 28 Presumably the f i l m would have a n i c k e l defect l a t t i c e and thus higher n i c k e l i o n c o n d u c t i v i t y . The f i n a l r i s e i n current at about -1200 mV i s due to the e v o l u t i o n 1000 U5 -p S 100 Figure 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 M NaF S o l u t i o n at pH = 11.3 - 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 ac t ive region on the p o l a r i z a t i o n curve of zirconium,., magnesium and aluminum has been proposed by Kolotyrkin" '"^ . The r e s u l t s on which t h i s mechanism i s based are very s i m i l a r to the r e s u l t s obtained i n the present 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 of Truempler and K e l l e r 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 . The c o r r o s i o n mechanism might therefore be expected to be the same i n a l l . c a s e s . K o l o t y r k i n ' s mechanism i s . b a s e d on the premise that h a l i d e ions adsorb on z i rconium, e t c , p r e f e r e n t i a l l y to hydroxide ions or water molecules at p o t e n t i a l s above E c , but not below,, because of t h e i r greater p o l a r i z a b i l i t y . C h l o r i d e , bromide and i o d i d e ions i n f a c t have l a r g e r p o l a r i z a b i l i t i e s 1 i; \ than e i t h e r H 20 or OH (Table II) but f l u o r i d e ions have a smaller p o l a r i z a b i l i t y and nevertheless give r i s e to a s i m i l a r though l e s s extensive second a c t i v e r e g i o n . Therefore t h i s , mechanism must be i n v a l i d at l e a s t f o r c o r r o s i o n by f l u o r i d e s . TABLE I I . Species P o l a r i z a b i l i t y o ^ o X 10 2 4 c m 3 Gram Ioni c Refrac t i on . R cm 3 F " 0.99 2.60 C l " 3.02 9.03 Br 4.17 12.60 I " 6.28 19.00 OH" 1.80 A 5.10 0 2.76 - H 20 1,44 — A Value extrapolated f r o m . R e f r a c t i v e Index. A new mechanism i s proposed that accounts f o r c o r r o s i o n by a l l h a l i d e ions i n the second a c t i v e r e g i o n . - 47- The f o l l o w i n g mechanism i s based on the premise that the surface charge on n i c k e l , z i rconium 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 negative surface charge a r i s e s from the d i s s o l u t i o n of p o s i t i v e metal ions from an e l e c t r i c a l l y i n s u l a t e d metal i n s o l u t i o n thus l e a v i n g a negative 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 s o l u t i o n e f f e c t and i s opposed by the e l e c t r o s t a t i c a t t r a c t i v e f o r c e , the force of a t t r a c t i o n between the d i s s o l v e d p o s i t i v e ions and. t h e . s u r f a c e , and the .osmotic e f f e c t which i s e f f e c t i v e l y - the sum of a l l other forces tending towards d e p o s i t i o n of the cat ions on the metal sur face . The osmotic and e l e c t r o s t a t i c a t t r a c t i v e forces are considered to be greater than the e l e c t r o l y t i c s o l u t i o n e f f e c t only f o r very noble metals l i k e plat inum and g o l d , g i v i n g r i s e to a net p o s i t i v e charge on the s u r f a c e . i n such cases . N i c k e l i s considered t o be a c t i v e enough t o maintain a negative charge on i t s surface at i t s r e v e r s i b l e redox p o t e n t i a l producing a p o t e n t i a l f i e l d at 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 lectrode repulses negative ions at the nickel -hydrogen 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 , i s . c lose to i t s 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. Only water molecules would, be adsorbed under-these c o n d i t i o n s . As the p o t e n t i a l of the electrode i s made more noble , , 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 sur face . . This , gives, r i s e to the f i r s t a c t i v e region i n . t h e p o l a r i z a t i o n curve . Thus the c o r r o s i o n mechanism i n t h i s region i s : H 2 0 H 2 0 a d s . . . . . ( l a ) N i ( H 2 0 ) 2 a d s — N i + + ( H 2 0 ) 2 .+ . 2e~ ( lb) ++ + Ni ( H 2 0 ) 2 — N i ( O H ) 2 + -2H . . . . . ( l c ) the l a s t step occurringaaway from the surface . At even more noble p o t e n t i a l s , the e l e c t r i c 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 s u r f a c e ; - 48 - the more p o s i t i v e the. charge on the surface , , the c l o s e r to the surface i s 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 occurs c lose enough to form an adherent f i l m . o n . the e l e c t r o d e . . T h i s mechanism p r e d i c t s the pH dependence of the Flade 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 r e s u l t s i n an adherent f i l m . As noted p r e v i o u s l y the data of the Flade p o t e n t i a l as a f u n c t i o n of pH f i t s the r e l a t i o n : . E F = - 0 . 2 4 0 : + O.O65 PH . - (2 ) This mechanism a l s o p r e d i c t s that the f i r s t a c t i v e region i s independent of the f l u o r i d e ion c o n c e n t r a t i o n . This i s confirmed by, the r e s u l t s of the present work,, Figures 12 and 1 3 . • As the e lectrode surface i s : made more p o s i t i v e , the e l e c t r i c . f i e l d a t t r a c t s n e g a t i v e l y charged ions such as OH ,• C l and F which compete with water molecules f o r adsorpt ion s i t e s . This adsorpt ion i n aqueous f l u o r i d e s o l u t i o n s may be descr ibed q u a n t i t a t i v e l y b y the-Langmuir adsorpt ion isotherm, Qjr. = K l t F ~] ....-(3) 1 - S i - e 2 where 0 X and Q 2 are the proport ions of the surface covered by f l u o r i d e and hydroxide i o n s , : l i . ^ ,0̂. - 0 2 i s the p r o p o r t i o n of surface covered by water molecules , and K n i s an e q u i l i b r i u m constant . K i n e t i c considerat ions of adsorpt ion and desorpt ion show that A H K 1 V . = k e ~RT • (4) where k i s a constant and ^ H i s ^ t h e enthalpy of a d s o r p t i o n . The enthalpy may'be w r i t t e n : A H i . = A H Q I - £ n- F. E '(5) _ k9 - where A. H 0 i s . t h e standard enthalpy with no a p p l i e d f i e l d , E i s the a p p l i e d f i e l d , n i s the charge on the i o n moving i n the e l e c t r i c f i e l d , /& 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 the ion moves when adsorbed, and F i s . the Faraday constant . From t h i s i t fo l lows that AEQl - /9FE •9,, = . [F-] ,k e . . ,(6) 1 - 0! - 9 2 A s i m i l a r equation can be. obtained, f o r adsorpt ion of hydroxide ions which when added t o the above r e l a t i o n gives ••(7) _01_+_02_ = / [F~] k x e ~ R T ~ + [ 0 H - ] k 2 e ~ W ] e ^ 1 - ©! - ©2 where Q±, 02, [ F ~ ] / [OH ] and E are considered v a r i a b l e s . However, the f l u o r i d e and hydroxide ions a l s o compete with each other f o r adsorpt ion s i t e s , OH" •+. F ^ d s " .JL (0H") a d s +:-F" . . . . . ( 8 ) with K " 0X [OH - ] ( y j which w i t h equation (7) y i e l d s •[OH"] • . • • -£FE 0X (1 + K " f r T ) : = i [ F _ ] e x + [OH"] e 2 } e RT l - e i ( l - K l f | ) ' •••••<10> ^ H where e 1 > 2 = kx,2 e RT From t h i s i t fo l lows that : [OH"] ' -/?FE 0x (1 + K " [ F T ) e ~ W = — : [0H"12 .[F*]ex + [OH ]e 2 - © J F " ^ - O i [ O H ~ ] e 2 - KOxlOH ]e±-- KQ± [g- j - e g (11) - 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 considered large and [0H~] i s i n - t h e order .of I O - 7 or l e s s . Thus, by i g n o r i n g these terms, 1 _K * - _ £ F E Q i (UK~-+.TT=1) e RT = [ F ~ 1 mrr 61 + 62 (1 -KOl) a n d : t a k i n g logarithms .(12) -2,3 = l o S ° i + l o g (0H= + F=) " 1 ° S [ jJiPT e i + e 2 (1 - K Q i ) ] (13) I t may be assumed that a c r i t i c a l concentrat ion of adsorbed f l u o r i d e ions tira necessary to 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 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 and thus 9^ may be considered a constant, Qi c r i t . Four l i m i t i n g c a s e s ; a r e considered to a s c e r t a i n the pH and pF dependence of E c , the p o t e n t i a l at the i n i t i a t i o n of the second 1 >>>> K a c t i v e r e g i o n . In case 1. i t i s assumed that \-.bf[QH_J [F J a n d IF"] e (1 - 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 : - 2.3/fF E ' = l o g Qi c r i t - PH - l o g (1 - K 0crit):„ . . . . , ( 1 4 ) RT which shows no dependence of E c on f l u o r i d e concentrat ion and therefore i s 1 >„>,. K i n c o n s i s t e n t w i t h experimental evidence . Case 2 a p p l i e s when [QH"] . and lf e a » (1 - K 0,' e 2 and. y i e l d s [OH ] ~2"51S E c = l o g Oj. c r i t - l o g e r ' + pF ,....(15) This shows no dependence of E c 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. K 1 [F~] S i m i l a r l y ^ ! ' case 3, -rp=] > > T Q I T ] a n d " K Q) e £ ^ ToITT y i e l d s n o K 1 pH-dependence and may be d i s r e g a r d e d . Case 4̂  [p1-] ^ [0H~ J a n c ^ - 51. - — F — e - , \> > ( 1 - K 0) e 2 r e s u l t s i n the expression [OH ] V 7 2 f - 2 . 3 ^ E c = l o g Oi c r i t + l o g K + pF - l o g e i + pF .+. pH ( 16) i n which E c i s a f u n c t i o n of ,pH + 2pF. T h i s . i s . i n accordance w i t h the experimental r e s u l t s , • F i g u r e 20, 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 l a r g e . . The above express ion , when w r i t t e n i n the form E c = G " ' 537 t i°s =RLT - _ J | ( P H + 2 P F ) ( 1 7 ) RT i n d i c a t e s that a p l o t of E c versus pH p l u s -2pF would have a. slope of y . ^ — or 59 . I mV. In Figure 20 the slope i s approximately 230 mV which agrees w i t h that p r e d i c t e d f o r the above mechanism i f ^ = 0.26. • The energy of adsorpt ion is. only a p r o p o r t i o n of the t o t a l energy of an i o n moving through 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 Figure 25. Elec t rode Double Surface Layer Distance from Elec t rode Surface Figure 25. P o t e n t i a l - Funct ion of Ion i n V i c i n i t y of a.Charged E l e c t r o d e - 52 - f? i s def ined as the change i n the energy of adsoption with change i n the E a p p l i e d p o t e n t i a l ( i . e . = a d s ^ a p p l i e d Adsorpt ion of f l u o r i d e ions occurs at r e l a t i v e l y a c t i v e p o t e n t i a l s i f pH and pF are s m a l l , that i s , i f there i s smal l hydroxide ion and large f l u o r i d e ion concentration. : as p r e d i c t e d by equation ( 17) Recent w o r k ^ with plat inum i n c h l o r i d e s o l u t i o n s has shown that adsorpt ion of c h l o r i d e ions i s favoured by low pH and large c h l o r i d e i o n concentra t ions . I t was a l s o found that c h l o r i d e adsorpt ion i n h i b i t s coverage of the surface by adsorbed oxygen. Adsorpt ion of f l u o r i d e ions can be expected t o occur at weak spots i n the n i c k e l hydroxide f i l m . In such places as g r a i n boundaries and d i s - l o c a t i o n s i t e s , the metal i s probably slowly d i s s o l v i n g . As f l u o r i d e ions carry • current to these s i t e s , f l u o r i d e adsorpt ion w i l l occur there most r a p i d l y . In the present work, n i c k e l sulphide has been i d e n t i f i e d at the g r a i n boundaries , F igure 2 6 . Any p a s s i v a t i n g f i l m i s l i k e l y to be p a r t i c u l a r l y weak on the sulphide phase and therefore c o r r o s i o n w i l l be i n i t i a t e d p r e - f e r e n t i a l l y at the g r a i n boundaries . Figure 2 6 . M i c r o - s t r u c t u r e of N i c k e l Showing Second Phase. The adsorbed, f l u o r i d e ions w i l l form a soluble n i c k e l - h a l i d e complex, probably N i F 3 , and. thus i n i t i a t e a c o r r o s i o n r e a c t i o n . The mechanism i s N i ( H 2 0 ) n a d s + 5 F " N 1 < F 3 ) ! d s + n H 2 ° N i ( F 3 ) a d s MF3 • + 2 e" . Hal ide ions c ar ry par t of the current t o these s i t e s - a n d thus maintain a s u f f i c i e n t concentrat ion f o r the r e a c t i o n t o proceed at these l o c a l i z e d s i t e s , g r a i n boundaries i n t h i s c a s e , - F i g u r e 22a. .The increase i n current 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 equation i f the rate -control,--is 1 a c t i v a t i o n : . o f the n i c k e l o x i d a t i o n s tep . This i s the case at large f l u o r i d e i o n concentrat ions and low pH (Figure 21). The slope 1J5 m v y i n d i c a t e s an o x i d a t i o n process i n which one e l e c t r o n at a time i s exchanged. At higher pF and pH hydroxide adsorpt ion i n t e r f e r e s and the rate i s c o n t r o l l e d - b y competit ion between the two adsorbing anions . At h i g h ;pF and low pH the rate 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 ions t o the s i t e . The above mechanism which gives r i s e t o , t h e second a c t i v e region 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 processes at anodic p o t e n t i a l s up. to -800 mV i n ac id , f l u o r i d e s o l u t i o n s , (Figure 8). The p l o t s . o f the Flade p o t e n t i a l and the c r i t i c a l - h a l i d e adsorpt ion potential ; ; ;versus pH show that f l u o r i d e ions adsorb at more a c t i v e p o t e n t i a l s than the Flade p o t e n t i a l at pH = 3> ;5Figure 27. A d d i t i o n a l support i s found i n Figure 10a, which shows extensive g r a i n boundary c o r r o s i o n on a,specimen subjected t o c o r r o s i o n at anodic p o t e n t i a l s i n a c i d f l u o r i d e s , apparent ly i d e n t i c a l w i t h 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 n n e u t r a l s o l u t i o n s - (F igure 27). - Thus, n i c k e l corrodes - 54 - 2 4 6 pH Figure 2 7 . Flade P o t e n t i a l , • E F , and P o t e n t i a l at I n i t i a t i o n of Second A c t i v e Region versus pH -55 i n low p H . s o l u t i o n s at a l l anodic p o t e n t i a l s "by complexing wi th f l u o r i d e i o n s . 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 i n a c i d f l u o r i d e s o l u t i o n s i n d i c a t e s that the rate i s c o n t r o l l e d . b y mixed concentrat ion 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 ions to the d i s s o l u t i o n s i t e s . - S i n c e these s i t e s are s p e c i f i c , the anions must t r a v e l p a r t l y perpendicular t o the p o t e n t i a l f i e l d . T h i s . p a r t of t h e i r movement w i l l be p o t e n t i a l independent and gives r i s e t o a l a r g e r T a f e l s l o p e . .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 that 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 between -950 and -1100 mV. K o l o t y r k i n " ^ has suggested that oxygen p r e f e r e n t i a l l y adsorbs to metal atoms w i t h bonds corresponding t o t h e i r highest o x i d a t i o n s t a t e . This i r e a c t i o n can be expected t o occur as the e lectrode p o t e n t i a l i s made more p o s i t i v e and therefore d i s p l a c e s h a l i d e i o n a d s o r p t i o n . Iron i s known t o 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 ions and'-from passive areas as t r i - 29 valent i o n s , i n c h l o r i d e and bromide s o l u t i o n s . In a d d i t i o n , the ..pH- - p o t e n t i a l diagram (Figure 5) p r e d i c t s that M2O3 would form at p o t e n t i a l s more noble than -750 mV i n . t h e absence of h a l i d e i o n s . The second passive region f o r n i c k e l i s therefore concluded t o r e s u l t from oxygen adsorpt ion to n i c k e l t o . f o r m a t r i v a l e n t oxide f i l m which i s s table even i n the presence of ha l ide i o n s . - 5 6 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 behaviour s i m i l a r t o n i c k e l . However,•monel has a higher overvoltage f o r the cathodic r e a c t i o n : . 2 H + + - 2e~ — ^ H 2 The r e s u l t i n g exfchange currents are lower than f o r n i c k e l (see Table I I I ) •TABLE. I I I . .Exchange. Current f o r Monel and N i c k e l i n S i m i l a r S o l u t i o n s pH Monel N i c k e l 4,0 6 uA/cm 2 16 uA/cm 2 6.0 2 uA/cm 2 8 uA/cm 2 11.0 0.3 uA/cm 2 0.6 uA/cm 2 These d a t a . i n d i c a t es that 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 5 a c i d s o l u t i o n . .The exchange or c o r r o s i o n current of monel i n f l u o r i d e s i s c o n t r o l l e d by the cathodic r e a c t i o n as i l l u s t r a t e d i n Figure ^b). • Any environmental change which increases the ra te of . t h e . c a t h o d i c r e a c t i o n w i l l increase the c o r r o s i o n of monel. •At pH = 4 monel does, not passivate i n the presence, of f l u o r i d e s . The shape of the anodic curve i s very s i m i l a r to the anodic curve f o r n i c k e l i n f l u o r i d e s at low pH. 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 anodic current decreases 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 past the a c t i v e peak,•Figure 28. The " p a s s i v e " current was about 12 pA/cm. 2 as compared t o 2 pA/cm 2 f o r n i c k e l . - 57' - 1000 0 -500 -1000 P o t e n t i a l (mV versus-SCE) Figure 2 8 . P o l a r i z a t i o n Curve of 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 higher pH, monel behaves, s i m i l a r l y t o n i c k e l but again the " p a s s i v e " current d e n s i t y i s much l a r g e r . The exchange current i s about 0.6 uA/cm 2 and the f i r s t a c t i v e region i s m i s s i n g . The f i r s t t ranspassive region s t a r t s at a p o t e n t i a l of -700 mV compared t o -600.mV f o r n i c k e l . O p t i c a l examination of the specimens showed more p i t t i n g and large c a v i t i e s . .No. experiments to a s c e r t a i n 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 at pH- = 6> F igure 29. The ' . resul t ing.curve was;;.very::.,similar.-.to that obtained for. n i c k e l i n . c h l o r i d e s but with a decreased T a f e l slope i n the second a c t i v e r e g i o n , 75 m V a s compared t o 180 mV. f o r n i c k e l . A l l o y i n g copper to n i c k e l produces a l a r g e r hydrogen qvervoltage and t h i s accounts f o r the lower exchange c u r r e n t . However, i t a l s o reduces the extent of p a s s i v a t i o n at more noble p o t e n t i a l s . . T h i s may be caused by 23 breakdown of the f i l m due to copper d i s s o l u t i o n . A r a i 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 than about 50$ copper, f i l m breakdown occurs due t o the formation of CuO at a f a s t e r rate than N i ( 0 H ) 2 . U h l i g h a s . i n d i c a t e d that increases i n copper content increase the " p a s s i v e " current 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 . He explains the decrease i n the r e s i s t a n c e as a r e s u l t of the f i l l i n g of the n i c k e l d - o r b i t a l s by e lec t rons from copper which i s . n o t a t r a n s i t i o n element. This decreases the s u s c e p t a b i l i t y of the a l l o y t o oxygen a d s o r p t i o n and therefore decreases i t s a b i l i t y t o p a s s i v a t e . - 5 9 - ——I : . -LJ I I • I +300 +200 +100 0 - 1 0 0 - 2 0 0 •Potential (mV versus-SCE) Figure 2 9 . - P o l a r i z a t i o n Curve of Monel i n 0.42 NaCl S o l u t i o n at pH = /fu/\ ^SJ - 60 - CONCLUSIONS 1. N i c k e l does not passivate 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 corroded at the g r a i n boundaries at anodic p o t e n t i a l s up to -800 mV (versus SCE) . 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 region at anodic p o t e n t i a l s s l i g h t l y above the mixed p o t e n t i a l . . N i c k e l i s thought to corrode by forming aquo-complexes i n t h i s region i n which f l u o r i d e ions have no e f f e c t . 3 . N i c k e l becomes passive 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 of the aquo-complexes at the surface to form an adherent hydroxide f i l m . F l u o r i d e ions i n s o l u t i o n increase the c o r r o s i o n current i n the passive r e g i o n . 4. F l u o r i d e ions i n n e u t r a l 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 n i c k e l p o l a r i z a t i o n curve at p o t e n t i a l s which are more a c t i v e with i n c r e a s i n g f l u o r i d e i o n and decreasing hydroxide i o n concentra t ions . 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 the adsorpt ion of 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 ions compete w i t h hydroxide ions so that by c o n s i d e r i n g the Langmuir adsorpt ion isotherm, a r e l a t i o n between the c r i t i c a l h a l i d e adsorpt ion p o t e n t i a l , E c , and pH and pF (the negative logar i thm of f l u o r i d e ion concentra t ion) , E c = C - -5| ( log G l b r i t +PH + 2pF) - 61 - i s d e r i v e d . The mechanism f o r c o r r o s i o n i n the second a c t i v e region i s N i ( H 2 0 ) n a d s + : 3 F N i ( F 3 ) a d s ' + n H 20 N i ( F 3 ) 3 " ads N i F 3 + 2e 5. N i c k e l becomes passive at p o t e n t i a l s more noble than -950 mV 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 , probably due t o the formation of a . p a s s i v e f i l m ( N i 2 0 3 or higher o x i d e ) . 6. N i c k e l i s passive i n contact wi th f l u o r i d e s s o l u t i o n s wi th 7. Monel corrodes l e s s 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 of i t s higher hydrogen overvol tage . However, at anodic p o t e n t i a l s , • monel does not r e a l l y p a s s i v a t e . 8. . N i c k e l and monel corrode 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 p o t e n t i a l s . 6-5 < PH < 12. - 62 - RECOMMENDATIONS FOR FUTURE INVESTIGATIONS V 1. The c o r r o s i o n of n i c k e l i n f l u o r i d e media could be s tudied as a f u n c t i o n of 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 obta in more d e t a i l e d v thermodynamic r e l a t i o n s h i p s . 2 . . P o l a r i z a t i o n s tudies 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 extent 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 a p p l i e s t o other h a l i d e i o n s . 3 . P o l a r i z a t i o n techniques i n which a square wave a l t e r n a t i n g current i s superimposed on the a p p l i e d p o t e n t i a l are u s e f u l i n determining the p r o p e r t i e s of the p a s s i v a t i n g f i l m and the double l a y e r . These may be a p p l i e d to n i c k e l c o r r o s i o n i n h a l i d e s o l u t i o n s , e x p e c i a l l y i n passive r e g i o n s . k. C o r r o s i o n s tudies of metals such as z i r conium, 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 of c o r r o s i o n mechanisms on.these metals (which were found t o y i e l d a second a c t i v e r e g i o n i n c h l o r i d e ) t o the mechanisms 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 . - 63 - REFERENCES 1. . G . C . - W M t t a k e r , . - C o r r o s i o n , 6 ,-.= 283 (1950). 2. F . Maness, U . S . - Atomic Energy Comm. Report HW-68426 ( 1 9 6 l ) . 3. . J . Bergman and G.- W. C . MacDonald, C o r r o s i o n , 17, 9 and 12 ( I 9 6 I ) . 4 . E . I . Antonovskaya and L . V . . Takhtarova,• Zhur . Vsesoyuz... Khim. . Obshohestva im D . I . Mendeleeva, 6,. 477 ( 1 9 6 l ) . ( C A . 5_6:292g) 5. M. S c h u s s l e r , , I n d . . Eng. . Chem. 4j_, 133 (19,55). 6. W. J . Braun, F . W . . F i n k and G.•Lee E r i c k s o n , U.S.•Atomic Energy Comm.Report BMI-1237 (1957). 7. D. R. Turner , J . Electrochem. S o c , , . 98 , 434 (1951). 8. . G . Truempler and R. K e l l e r , H e l v . Chim. • Acta 44,.I69I ( I 9 6 I ) . 9. Y. . K o l o t y r k i n and G. W. G i l m a n , - D o k l . Akad. Nauk. • SSSR, 13_7_, 642 ( I 9 6 I ) . 10. Y . . K o l o t y r k i n , " 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 " 1961, Butterworths, . London, 1 9 6 2 , - p . 10. 11. . L . Tronstad,- T r a n s . Faraday Soc . 2<?, 502 (1933) • 12. . D. H . M a c G i l l a v r y , J . H . . Rosenbaum and R. W.Stevenson, J . Electrochem. Soc . 89, 22 (I952.)... . 13. . K. J . V e t t e r and K. A r n o l d , Z . E lekt rochem. , 6k, 244 ( i 9 6 0 ) . 14. . J . Osterwald and H . H . U h l i g , J . Electrochem. . S o c . , 108, 515 ( I 9 6 I ) . 15. U . R . - E v a n s , " 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 " I96I, Butterworths, - London, 1962, p . 3- 16. N. Ya. Bune and Y. K o l o t y r k i n , Zhur . F i z . . Khim, 3J>, 1543 (I96I). 17. . N . D . Greene, C o r r o s i o n , 15_, 369 (1959). 18. M. Stern and A . L . . Geary, J . Electrochem. S o c , 104, 56, 559 and 645 (1957) . 19. ,M.. . S t e r n , C o r r o s i o n , 14, 440t ( I 9 5 8 ) . 2 0 . W . - A . M u e l l e r , C o r r o s i o n , 18, 349 . (1962). 21 . V . C h i k a l and M. • Prazak,• J . I ron and S t e e l I n s t . .193, 36O ( I 9 5 9 ) . . 2 2 . - T . . P . H o a r , - J . A p p l . Chem., 11, 121 ( i 9 6 0 ) . 23. N . D. Greene, C o r r o s i o n , 18, 136t (1962). - 64 - .24. T . P.. Hoar, "Anodic-Behaviour of Metals" in w Modern Aspects of E l e c t r o c h e m i s t r y , I I , e d . J . . O'M. B o c k r i s , Butterworths, London, 1959, p . 272. .25. I b i d , p . 325. 26. R. L i t t l e w o o d , . C o r r o s i o n Science 3, 99 ( I 9 6 3 ) . .27. M. Pourbaix, " A t l a s - D . ; E q . u i l i b r e s Electrochimiq_ues", G a u t h i e r - V i l l a r 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 , 6k, 600 (I96I). 29. . J . . L . Weininger and W. .R. Grams, J . -Electrochem. - Soc . 109, 9&k ( I 9 6 2 ) . .30- U . . F . . Franck, • " 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, Butterworths, London, 1962, p . 113. 31. Y. K o l o t y r k i n and V . M. Knyasheva/ Zhur . F i z . . C h i m . 3_0, 1990 (1956). 32.. . R. J . H a r t m a n , " C o l l o i d Chemis t ry" , Houghton " M i f f l i n Company, Cambridge, 1 9 4 7 , p . .222-229. 33. N. Hackerman and M. C . Banta, J . . Electrochem... Soc . I l l , 114 (1964). - 6 5 APPENDIX A . P o t e n t i a l S t a n d a r d i z a t i o n The standard calomel reference e lec t rode used i n a l l of the experiments was checked against the cadmium-cadmium sulphate h a l f - c e l l . •A 99*999$ cadmium elec t rode was f i t t e d i n the working electrode holder a f t e r ageing i n s u l p h u r i c a c i d ; A O .9O7 molar cadmium sulphate s o l u t i o n was introduced i n t o the c e l l and saturated wi th n i t r o g e n . - The p o t e n t i a l between the reference e lec t rode and cadmium electrode was read on the Pye potentiometer a f t e r 30 minutes and 60 minutes to be 680 mV. The standard e lec t rode p o t e n t i a l of Cd —>• C d + + + 2e~ i s 0.4-03 v o l t s on the hydrogen scale as reported i n Latimer . Using-an a c t i v i t y c o e f f i c i e n t of 0.05, and the Nernst equation RT r c d + + 1 i C d T E = E 0 - ~p In [Cd ] Cd the e lectrode p o t e n t i a l f o r the above system was 0.433 v o l t s versus the hydrogen electrode or 0.684 versus- the saturated calomel . The experimental value of 0.680 v o l t s is ' w e l l w i t h i n experimental accuracy. A L a t i m e r / W . M . / ' O x i d a t i o n P o t e n t i a l s " Englewood C l i f f s / Prent ice Hall , .1952.

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