STRESS CORROSION CRACKING OF 316 STAINLESS STEEL IN CAUSTIC SOLUTIONS by DAVID CHARLES CROWE B . S c , ( M e c h a n i c a l E n g i n e e r i n g ) , The U n i v e r s i t y o f M a n i t o b a , 1977 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE : " i n THE FACULTY OF GRADUATE STUDIES Depar tmen t o f M e t a l l u r g i c a l E n g i n e e r i n g We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRIT ISH COLUMBIA May 1982 0 D a v i d C h a r l e s C r o w e , 1 9 8 2 . I n p r e s e n t i n g 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 o f t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e a n d s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e h e a d o f my d e p a r t m e n t o r by h i s o r h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l n o t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t o f M&M/fu'ry/cq-l E^/i/ie 6 4 D u r i n g h e a t t r e a t i n g t h e s p e c i m e n s were e n c l o s e d i n s t a i n l e s s s t e e l (Sen Pak) e n v e l o p e s . The m e c h a n i c a l p r o p e r t i e s f o r the 3 m a t e r i a l h i s t o r i e s a r e l i s t e d i n T a b l e I I . The s p e cimen gage s e c t i o n s were p o l i s h e d w i t h 3/0 emery paper and d e g r e a s e d wi t h c h l o r e t h a n e . Each specimen was wrap-ped w i t h T e f l o n t a p e , l e a v i n g o n l y t h e gage s e c t i o n e x p o s e d . T a b l e II M e c h a n i c a l P r o p e r t i e s o f SSRT Specimens - M a t e r i a l Hi s t o r y Y i e l d S t r e n g t h a t 92 °C MPa U l t i m a t e T e n s i Te S t r e n g t h a t 92 °C MPa' Hardness HRB A s - r e c e i ved 283 530 87 A n n e a l e d 170 - 72 A n n e a l e d & S e n s i t i z e d 170 - 70 20 The s p e c i m e n s were s t o r e d i n a d e s i c c a t o r u n t i l n eeded. The c e l l used f o r t h e SSRT i s i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 2. I t s c o n s t r u c t i o n was s i m i l a r t o t h a t o f t h e c e l l used i n t h e p o l a r i z a t i o n s t u d i e s . The s p e c i m e n f i t t e d t i g h t l y i n t o the c e l l bottom t o p r e v e n t l e a k a g e o f t h e s o l u t i o n . The t h r e a d e d ends o f t h e s p e c i m e n screwed i n t o g r i p s w hich p i n n e d i n t o a f l o o r model I n s t r o n t e s t i n g m a chine. H e a t i n g t a p e , wrapped a r o u n d the c e l l , was r e g u l a t e d w i t h a t h e r m i s t o r probe and t e m p e r a t u r e c o n t r o l l e r as d e s c r i b e d i n S e c t i o n 2.1.1. The p o t e n t i a l was measured w i t h r e s p e c t t o a room tempera-t u r e . 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 (SCE) and c o n t r o l l e d t o + 0.005 V d u r i n g t h e t e s t w i t h t h e same p o t e n t i o s t a t used i n t h e p o l a r i z a t i o n s t u d i e s . A r o l l t y p e c h a r t was used t o r e c o r d l o a d v e r s u s e l o n g a t i o n d u r i n g t h e t e s t . S o l u t i o n s were made as d e s c r i b e d i n S e c t i o n 2.1.1. 2.2.2 P r o c e d u r e The SSRT s p e c i m e n was i n s e r t e d i n t h e t e s t c e l l and p l a c e d i n t h e I n s t r o n . The s o l u t i o n f o r t h e t e s t , f r e s h l y mixed and h e a t e d t o t e m p e r a t u r e was p oured i n t o t h e c e l l . N i t r o g e n purge was begun i m m e d i a t e l y t o p r o v i d e s t i r r i n g and t o p r e v e n t o x i d a t i o n o f s u l f i d e i n t h o s e t e s t s u s i n g s u l f i d e . H e a t i n g t a p e was wrapped around t h e c e l l and h e a t a p p l i e d . Slow strain rate test c e l l a) specimen b) Luggin c a p i l l a r y c) c e l l d) l i d e) reflux condenser f) temperature probe g) nitrogen purge h) Teflon c e l l bottom i) counter electrode 22 A c a t h o d i c p o t e n t i a l o f - 1 . 2 5 w a s a p p l i e d t o t h e s p e c i m e n f o r 3 0 m i n u t e s p r i o r t o t h e t e s t t o r e d u c e a n y s u r -f a c e f i l m . T h e p o t e n t i a l w a s t h e n s e t t o t h e t e s t p o t e n t i a l . A f t e r a n o t h e r 30 m i n u t e s , t h e c r o s s h e a d o f t h e I n s t r o n w a s - 6 -1 s e t i n m o t i o n t o g i v e a s t r a i n r a t e o f 3 . 3 x 1 0 " S~.•; a n d t h e l o a d r e c o r d e d . E a c h t e s t l a s t e d ; -\. 4 8 h o u r s , e n d i n g w i t h f r a c t u r e . I f S C C o c c u r r e d , t h e t e s t t i m e w a s s h o r t e r . When t h e s p e c i m e n w a s r e m o v e d f r o m t h e c e l l , t h e d i a m e t e r o f t h e f r a c t u r e s u r -f a c e s w a s m e a s u r e d w i t h a t r a v e l l i n g m i c r o s c o p e a n d t h e p e r - ; c e n t r e d u c t i o n i n a r e a c a l c u l a t e d . T h e f r a c t u r e s u r f a c e s o f t h e s p e c i m e n s w e r e e x a m i n e d a n d p h o t o g r a p h e d i n a s c a n n i n g e l e c t r o n m i c r o s c o p e . I n s o m e o f t h e t e s t s , o n e - h a l f o f t h e f a i l e d g a g e s e c t i o n w a s m o u n t e d i n ' Q u i c k m o u n t ' e p o x y a n d p o l i s h e d t o 1 ym w i t h d i a m o n d p a s t e . T h e m o u n t e d s p e c i m e n w a s t h e n e t c h e d w i t h a m i x t u r e o f 10 ml H N 0 3 , 10 ml C H 3 C 0 0 H , a n d 1 8 ml H C l w i t h 4 d r o p s o f g l y c e r o l . E x a m i n a t i o n a n d p h o t o g r a p h y w e r e c o n d u c t e d w i t h a Z e i s s U l t r a -p h o t o p t i c a l m i c r o s c o p e . A n a l i q u o t o f s o l u t i o n w a s t a k e n f r o m o n e t e s t t o d e t e r -m i n e c h l o r i d e c o n c e n t r a t i o n r e s u l t i n g f r o m l e a k a g e o f KC1 f r o m t h e L u g g i n c a p i l l a r y . A n a u t o m a t i c t i t r a t o r ( R a d i o -m e t e r T T T 8 0 ) a n d a n a u t o m a t i c b u r e t t e ( R a d i o m e t e r A B U - 8 0 ) 23 were employed i n t h e a n a l y s i s . 2. 3 F r a c t u r e M e c h a n i c s T e s t i n g ' 2.3.0 I n t r o d u c t i o n P o l a r i z a t i o n s t u d i e s and s l o w s t r a i n r a t e t e s t s t o g e t h e r p r o v i d e i n f o r m a t i o n on t h e p o t e n t i a l a t w h i c h SCC may o c c u r i n a g i v e n e n v i r o n m e n t , and on t h e e l e c t r o c h e m i c a l r e a c t i o n s p r o m o t i n g s u s c e p t i b i l i t y . F r a c t u r e m e c h a n i c s t e s t i n g can add t o t h i s u n d e r s t a n d i n g by g i v i n g i n f o r m a t i o n on k i n e t i c f a c t o r s and p r o v i d i n g u s e f u l i n f o r m a t i o n f o r e q u ipment d e s i g n e r s and o p e r a t o r s . Specimens used i n f r a c t u r e m e c h a n i c s t e s t i n g a r e p r e -n o t c h e d and f a t i g u e p r e - c r a c k e d t o overcome i n i t i a t i o n p r o b -lems and p r o v i d e d a t a on c r a c k growth k i n e t i c s . The s p e c i m e n s a r e d e s i g n e d so t h a t l o a d , c r a c k l e n g t h , and s t r e s s i n t e n s i t y may be r e l a t e d by a known K j - c a l i b r a t i o n r e l a t i o n . The s t r e s s i n t e n s i t y , K j , can t h e n be c a l c u l a t e d f o r a g i v e n l o a d and c r a c k l e n g t h t o g i v e a measure o f t h e i n t e n s i t y o f s t r e s s a t 4. ft fi R the c r a c k t i p . ' Rate o f c r a c k growth may be r e l a t e d t o s t r e s s i n t e n s i t y t h r o u g h o u t th e t e s t . In c o r r o s i v e e n v i r o n -ments, t h e r e i s o f t e n a s t r e s s i n t e n s i t y below which no 6 5 c r a c k i n g can be d e t e c t e d ; t h i s i s c a l l e d K ^ ^ Q . A S KJ. I S r a i s e d above t h i s v a l u e , c r a c k growth r a t e r i s e s t o o . T h i s r e g i o n o f K j - d e p e n d e n t v e l o c i t y i s c a l l e d R e g i o n I. A r e g i o n o f K T - i n d e p e n d e n t c r a c k growth r a t e e x t e n d s t h r o u g h 24 i n t e r m e d i a t e v a l u e s o f K^; t h i s i s Region I I . F i n a l l y t h e growth r a t e b e g i n s t o i n c r e a s e a g a i n u n t i l f a i l u r e o c c u r s a t KI(.; t h i s i s R e g i o n I I I . T h i s b e h a v i o r has been o b s e r v e d by a number o f i n v e s t i g a t o r s f o r a v a r i e t y o f a l l o y s and s o l u -t i c s . 1 3 ' 4 8 ' 4 9 ^ISCC l s d i f f i c u l t t° d e t e r m i n e b e c a u s e c r a c k i n g r a t e becomes i n f i n i t e l y slow as Kj i s d e c r e a s e d t oward ^ j 5 Q Q * I n i t i a t i o n would have t o t a k e p l a c e j u s t above K J C . Q C b e c a u s e the Kj v a l u e i n c r e a s e s d u r i n g c r a c k i n g f o r t h i s s p e c i m e n d e s i g n R e g i o n I i s o f i n t e r e s t t o d e s i g n e r s b e c a u s e equipment which i s c r a c k i n g spends a l a r g e p o r t i o n o f i t s l i f e t h e r e . R e g i o n II i s a l s o i m p o r t a n t and i t i s o f s p e c i a l i n t e r e s t t o r e s e a r c h e r s s t u d y i n g t h e c a u s e s o f SCC because t h e c r a c k growth k i n e t i c s i n t h a t r e g i o n a r e c o n t r o l l e d s o l e l y by s t r e s s i n d e p e n d e n t p r o c e s s e s . The T - n o t c h d o u b l e c a n t i l e v e r beam (TN-DCB) was o r i g i -1 4 n a t e d by R u s s e l and Tromans. I t i s i l l u s t r a t e d i n F i g u r e 3. The Kj c a l i b r a t i o n f o r the TN-DCB specimen i s g i v e n by: Kj'= (T.172. x 1 0 5 ) P ( a ) 0 ' 5 ' [2 . 43-3 . 62 ( a / 0 . 032 ) + 1 4 . 5 ( a / 0 . 0 3 2 ) 2 - 24. 6(a/0 .03'2) 3 + 2 6 . 5 ( a / 0 . 0 3 2 ) 4 ] . . . (1 ) 2 5 F i g u r e 3 T - n o t c h d o u b l e c a n t i l e v e r b e a m s p e c i m e n . 26 E r r o r s due t o t h e downward growth o f c r a c k s have been c a l c u -48 l a t e d by R u s s e l , but have been c o n s i d e r e d t o be n e g l i g i b l e i n t h i s p r e s e n t s t u d y . The TN-DCB has been used h e r e b e c a u s e : 1. I t i s t h i n , so t h e c r a c k l e n g t h measured i n t h e s u r f a c e w i l l n o t d i f f e r g r e a t l y from th e c r a c k f r o n t a t t h e c e n -t e r . 2. R e s e a r c h has been done on t h i s m a t e r i a l i n M g C ^ u s i n g 14 48 t h i s s p e c i men d e s i g n . T h i s s i m i l a r i t y w i l l a i d c o m p a r i s o n o f b e h a v i o r i n t h e s e d i f f e r e n t e n v i r o n m e n t s . The specimen has some d i s a d v a n t a g e s : 1. The v e r y low l o a d s r e q u i r e d t o p r o d u c e a f a t i g u e p r e -c r a c k a r e d i f f i c u l t t o a p p l y a c c u r a t e l y and t h e s t r e s s i n t e n s i t i e s r e q u i r e d t o p r e - c r a c k p r e c l u d e SCC t e s t i n g below t h a t s t r e s s i n t e n s i t y . 2. The s p e c i m e n i s t h i n , so p l a n e s t r a i n c o n d i t i o n s would n o t be s a t i s f i e d a t h i g h s t r e s s i n t e n s i t i e s , d e p e n d i n g on y i e l d s t r e n g t h . To f u l f i l t h e c r i t e r i o n f o r p l a n e - s t r a i n t e s t i n g t h e 2 66 s p e c i m e n t h i c k n e s s must be >'2.5 ( K / a y ) . For t h e m a t e r i a l u s e d , p l a n e s t r a i n c o n d i t i o n s a r e s a t i s f i e d o n l y t o a s t r e s s 27 i n t e n s i t y v a l u e o f . 17 MPa/nT (a s s u m i n g -a = 475 MPa). In s p i t e o f t h i s , t e s t i n g was c o n d u c t e d up t o 100 •-.MRa/m" a t which p o i n t g r o s s d e f o r m a t i o n began. The c r a c k s were s t r a i g h t and d i d n o t seem t o a v o i d p r o p a g a t i o n t h r o u g h p l a n e -s t r e s s p l a s t i c zones as has been s u g g e s t e d . ^ 2.3.1 M a t e r i a l s and P r e p a r a t i o n Type 316 s t a i n l e s s s t e e l from a s i n g l e p l a t e was u s e d . I t s c o m p o s i t i o n has been l i s t e d i n T a b l e I . The p l a t e was c o l d - r o l l e d t o an i n t e r m e d i a t e t h i c k n e s s , t h e n ?. .a itn.e.a l e d:. .'at. 1050 °C f o r 30 m i n u t e s i n a s t a i n l e s s s t e e l f o i l e n v e l o p e (Sen P a k ) . I t was t h e n quenched i n w a t e r . F u r t h e r c o l d r o l l i n g l e f t t h e m a t e r i a l w i t h a t h i c k n e s s o f 3.2 mm and 25% c o l d work. The c o l d worked p l a t e had a y i e l d s t r e n g t h o f 475 MPa and u l t i m a t e t e n s i l e s t r e n g t h o f 830 MPa. Ha r d n e s s measured HRC 25. The p l a t e m a t e r i a l and worked 14 48 c o n d i t i o n were i d e n t i c a l t o t h o s e employed by R u s s e l . ' One s p e c i m e n was made o f m a t e r i a l w hich was r o l l e d t o i t s f i n a l t h i c k n e s s t h e n a n n e a l e d .at 1050 °C f o r 1 h o u r . T h u s , i t was f r e e b f c o l d work. Ha r d n e s s measured HRB 76. Specimens were machined from t h e r o l l e d m a t e r i a l so t h a t c r a c k s would p r o p a g a t e i n t h e r o l l i n g d i r e c t i o n . The s p e c i -mens were p o l i s h e d t o 600 g r i t on one s i d e and t h e s u r f a c e 2 8 was s c r i b e d w i t h f i d u c i a l l i n e s 1 mm a p a r t t o be used t o measure c r a c k v e l o c i t y . S t a r t e r c r a c k s were sawn i n each s p e c i m e n w i t h a j e w e l -l e r s saw. The s p e c i m e n s were f a t i g u e p r e - c r a c k e d t o r e d u c e i n i t i a t i o n t i m e . The f a t i g u i n g was c a r r i e d out a t a maximum s t r e s s i n t e n s i t y v a l u e l o w e r than t h a t t o be used i n t h e t e s t . A S o n n t a g f a t i g u e machine was u t i l i z e d f o r p r e - c r a c k i n g . F i n a l l y t h e s p e c i m e n s were wrapped w i t h T e f l o n t a p e l e a v i n g a band o f e x p o s e d a r e a i n t h e v i c i n i t y o f t h e c r a c k and i t s r e g i o n o f p r o p a g a t i o n . The c o r r o s i o n c e l l s used i n t h e t e s t s were o f a s i m i l a r c o n s t r u c t i o n t o t h a t used i n t h e p o l a r i z a t i o n s t u d i e s ( S e c t i o n 2 . 1 . 1 ) . The s o l u t i o n s were mixed i n t h e same way, n i t r o g e n p u r g i n g c o n t i n u e d t h r o u g h o u t t h e t e s t s , t e m p e r a t u r e was co n -t r o l l e d t o + 1 °C, and p o t e n t i a l was c o n t r o l l e d t o + 0.005 V w i t h r e s p e c t t o an e x t e r n a l s t a n d a r d c a l o m e l e l e c t r o d e . Each s p e c i m e n was mounted i n i t s t e s t c e l l and l o a d e d as shown i n F i g u r e 4. T e n s o m e t e r s ( H o u n s f i e l d and Monsanto) were used m o s t l y , w i t h a s p r i n g i n s e r i e s w i t h t h e specimen t o m i n i m i z e l o a d drop r e s u l t i n g from c r a c k i n g . Some s p e c i -mens were l o a d e d by a c a l i b r a t e d homemade d e v i c e which used a F r a c t u r e m e c h a n i c s t e s t i n g c e l a) specimen b) g r i p s c ) p i n s d ) c e l l l i d e) b e a k e r . 30 w e i g h t e d l e v e r t o a p p l y the l o a d . The p o t e n t i a l was a p p l i e d i n t h e same manner as f o r the sl o w s t r a i n r a t e t e s t s . A v a r i e t y o f p o t e n t i o s t a t s was u s e d , i n c l u d i n g ECO model 549 and Wenking models OPA 69 and 68TS10. 2.3.2 P r o c e d u r e The s p e c i m e n and c e l l were p l a c e d i n t h e t e n s o m e t e r , the s o l u t i o n was added, and p o t e n t i a l was a p p l i e d . No c a t h o -d i c p o t e n t i a l was a p p l i e d t o r e d u c e s u r f a c e f i l m s p r i o r t o t h e t e s t . Load was a p p l i e d 30 m i n u t e s a f t e r e s t a b l i s h i n g t h e p o t e n t i a l c o n t r o l . The L u g g i n c a p i l l a r y had t o be r e p l a c e d a b out once p e r week b e c a u s e t h e c o t t o n t h r e a d d i s s o l v e d o u t o f i t s t i p and a b u b b l e f o r m e d , b r e a k i n g t h e c i r c u i t . . The t e s t s o l u t i o n was r e p l a c e d p e r i o d i c a l l y w i t h f r e s h s o l u t i o n . The l e v e l o f s o l u t i o n i n t h e c e l l was m a i n t a i n e d d a i l y by a d d i n g a few m i l l i l i t e r s o f b o i l e d d i s t i l l e d w a t e r . The c e l l was opened e v e r y few days t o measure t h e c r a c k l e n g t h . T h i s was done v i s u a l l y w i t h t h e a i d o f t h e . s c a l e s c r i b e d on the specimen s u r f a c e . Time, l o a d , c r a c k l e n g t h and c a l c u l a t e d s t r e s s i n t e n s i t y were r e c o r d e d . In t h o s e c a s e s where c r a c k i n g o c c u r r e d on one s i d e o f t h e specimen o n l y , the o t h e r s i d e was sawed p e r i o d i c a l l y to a m a t c h i n g l e n g t h t o m a i n t a i n p r o p e r l o a d i n g g e ometry. 31 At t h e c o m p l e t i o n o f some o f t h e t e s t s , t h e c o r r o s i o n c e l l was l o w e r e d away from t h e s p e c i m e n and t h e s u r f a c e o f t h e s p e c i m e n was d r i e d q u i c k l y w i t h a b s o r b e n t p a p e r t i s s u e . pH i n d i c a t o r p a p e r ( p H y d r i o n ) w i t h pH i n t e r v a l s o f 0.5, was p r e s s e d a g a i n s t t h e specimen a t t h e c r a c k , so t h a t t h e p a p e r a b s o r b e d s o l u t i o n d r a i n i n g from t h e c r a c k . The i n d i c a t o r p a p e r showed t h a t t h i s c r a c k s o l u t i o n has a pH o f <\* 14 a t room t e m p e r a t u r e . The T e f l o n t a p e was removed and t h e s p e c i m e n was r i n s e d w i t h d i s t i l l e d w a t e r and e t h a n o l b e f o r e b e i n g s e c t i o n e d . Specimens w i t h heavy c o r r o s i o n d e p o s i t s were c l e a n e d u l t r a -s o n i c a l l y w i t h an i n h i b i t e d a c i d s o l u t i o n composed o f 3 ml H C l , 4 ml 2 - B u t y n e - l , 4 d i o l ( 3 5 % aqueous s o l u t i o n ) p l u s 50 ml 6 8 d i s t i l l e d w a t e r which p r o d u c e d no a r t i f a c t s . The f r a c t o g r a p h y was examined i n an ETEC s c a n n i n g e l e c t r o n m i c r o s c o p e a t 20 keV. The c r a c k l e n g t h s measured d u r i n g t h e t e s t were p l o t t e d v e r s u s t i m e . A l i n e was f i t t e d to t h i s d a t a u s i n g t h e l e a s t s q u a r e s method i f t h r e e a p p r o x i m a t e l y c o l i n e a r p o i n t s were a v a i l a b l e . O t h e r w i s e t h e l i n e t h r o u g h two p o i n t s was c a l c u l a -t e d . T h i s was t h e c a s e f o r most R e g i o n I d a t a . D i f f e r e n t i a -t i o n o f t h e e q u a t i o n f o r t h e l i n e y i e l d e d t h e v e l o c i t y . The v e l o c i t y has been p l o t t e d as a c o n s t a n t v a l u e f o r R e g i o n II ( s t r e s s i n t e n s i t y i n d e p e n d e n t c r a c k i n g ) . E r r o r was c a l c u l a -t e d f o r 95% c o n f i d e n c e l i m i t s u s i n g c o n s t a n t s from S t u d e n t ' s 69 t - d i s t r i b u t i o n as d e s c r i b e d by S t a n t o n . V a l u e s f o r a p p a r e n t a c t i v a t i o n e n e r g y were c a l c u l a t e d i n t h e same manner u s i n g 90% c o n f i d e n c e l i m i t s . 2.4 E l e c t r o n D i f f r a c t i o n ; A n a l y s i s o f S u r f a c e F i l m s 2.4.0 I n t r o d u c t i o n A n a l y s i s o f the c o r r o s i o n f i l m s r e m a i n i n g on t h e f r a c t u r e s u r f a c e s ^ may p r o v i d e i m p o r t a n t c l u e s about the mechanism o f c r a c k i n g . D i f f e r e n c e s i n c o m p o s i t i o n between the f i l m and t h e m a t r i x may show which e l e m e n t s have been d i s s o l v e d d u r i n g c r a c k i n g . The f i l m s t r u c t u r e may be formed o f t h e p r o d u c t s o f v a r i o u s d i s s o l u t i o n r e a c t i o n s . Examina-t i o n o f the c r y s t a l s t r u c t u r e may p r o v i d e i n s i g h t i n t o t h e s e r e a c t i o n s . 2.4.1 S a m p l i n g f o r C o r r o s i o n F i l m A n a l y s i s F r a c t u r e m e c h a n i c s (TN-DCB) s p e c i m e n s from t e s t s i n 3.35 mol/kg NaOH a t 92 °C were s e c t i o n e d w i t h a j e w e l l e r s saw t o e x p o s e the f r a c t u r e s u r f a c e s . S e c t i o n s were p l a c e d i n s h a l l o w d i s h e s and submerged i n a s o l u t i o n o f 1 volume p e r c e n t bromine i n methanol as d e s c r i b e d by N i k i f o r u k . ^ A f t e r 2 'days, th e s e c t i o n s were t a p p e d l i g h t l y t o d i s l o d g e 3 3 the f i l m and t h e n removed. S m a l l p i e c e s o f t h e s u r f a c e f i l m were l e f t f l o a t i n g i n the m e t h a n o l - b r o m i n e m i x t u r e . These were p i c k e d up w i t h a f i n e c o p p e r e l e c t r o n m i c r o s c o p e s p e c i m e n g r i d and p l a c e d i n methanol t o remove t r a c e s o f b r o m i n e . Sub-s e q u e n t l y , t h e f i l m s were p i c k e d up w i t h a f i n e c o p p e r g r i d c o v e r e d w i t h a s u p p o r t f i l m o f c a r b o n and p l a c e d on a b s o r b e n t f i l t e r p a p e r t o d r y . A f t e r w a r d s t h e g r i d w i t h f i l m s was s t o r e d i n a d e s i c c a t o r . 2.4.2 P r o c e d u r e The g r a t i n g w i t h f i l m on i t , was i n s e r t e d i n a sample h o l d e r and p l a c e d i n t h e t r a n s m i s s i o n e l e c t r o n m i c r o s c o p e , H i t a c h i model HU-11A, f o r e x a m i n a t i o n a t 100 keV. A f i e l d l i m i t i n g a p e r t u r e was used t o s e l e c t t h e a r e a from which an e l e c t r o n d i f f r a c t i o n r i n g p a t t e r n would be o b t a i n e d . A f t e r t h e p h o t o g r a p h i c p l a t e was e x p o s e d t o t h e e l e c t r o n d i f f r a c t i o n p a t t e r n , t h e d i f f r a c t i o n r i n g p a t t e r n o f an e v a p o r a t e d g o l d f i l m s t a n d a r d was p h o t o g r a p h e d . T h i s s e r v e d as a c a l i b r a t i o n s t a n d a r d . The d i s t a n c e s , d, between c r y s t a l p l a n e s i n t h e l a t t i c e were known f o r t h e g o l d s t a n d a r d . The d i a m e t e r , D, o f the c o r r e s p o n d i n g d i f f r a c t i o n r i n g s were measured on t h e d i f -f r a c t i o n p a t t e r n . U s i n g e q u a t i o n 2, i t was p o s s i b l e t o d e t e r m i n e t h e camera c o n s t a n t , . <: 34 Dd = . . . ( 2 ) The d i a m e t e r s o f t h e r i n g s were measured on t h e d i f f r a c -t i o n p a t t e r n o f t h e u n i d e n t i f i e d s u r f a c e f i l m , and u s i n g the camera c o n s t a n t , t h e , d - s p a c i n g was c a l c u l a t e d . The d - s p a c i n g and t h e r e l a t i v e i n t e n s i t i e s were matched w i t h t h o s e l i s t e d i n t h e powder d i f f r a c t i o n c a t a l o g ^ 1 t o i d e n t i f y t h e compound i n t h e f i l m . These were c o n f i r m e d by c o m p a r i s o n w i t h t h e o r e t i -c a l e s t i m a t e s o f r e l a t i v e i n t e n s i t i e s f o r e l e c t r o n d i f f r a c -72 t i o n i n FegO^ as c a l c u l a t e d by B i r l e y . The p a t t e r n s were f o u n d t o be c h a r a c t e r i s t i c o f s p i n e l s t r u c t u r e s e x h i b i t i n g c u b i c symmetry f o r w h i c h : d h k l = a o / ( h 2 + k 2 + l 2 ) 0 - 5 , . . . ( 3 ) where a i s t h e l a t t i c e p a r a m e t e r and h, k and 1 a r e the M i l l e r i n d i c e s o f t h e p l a n e o f i n t e r e s t . The f a c t o r / h +k +1 ; was p l o t t e d v e r s u s t h e d i a m e t e r o f the r i n g s o f t h e d i f f r a c t i o n p a t t e r n and a l i n e was f i t t e d to the d a t a . From t h i s , t h e l a t t i c e p a r a m e t e r , a Q , c o u l d be c a l c u l a t e d u s i n g e q u a t i o n 3. 3 . RESULTS 3 . 1 A n o d i c P o l a r i z a t i o n C u r v e s 3 . 1 . 0 NaOH The a n o d i c p o l a r i z a t i o n c u r v e s have been drawn w i t h r e s p e c t t o two d i f f e r e n t p o t e n t i a l s c a l e s : t h e s t a n d a r d c a l o m e l e l e c t r o d e , V $ C E ' a n d t h e s t a n d a r c l h y d r o g e n e l e c t r o d e , ^SHE" These a r e r e l a t e d by: VSHE = - ° ' 2 4 1 6 V S C E • • ' ( 4 ) No c o r r e c t i o n was made t o t h e p o t e n t i a l t o a c c o u n t f o r t h e r m a l g r a d i e n t e f f e c t s and l i q u i d j u n c t i o n s . F i g u r e 5 shows t h e c u r v e f o r 3 1 6 s t a i n l e s s s t e e l r o d i n 3 . 3 5 mol/kg NaOH a t 9 2 °C. The a c t i v e - p a s s i v e t r a n s i t i o n " nose" l o c a t e d a t - 1 . 0 0 V S C E was f l a t and d o u b l e peaked. The p r i m a r y p a s s i v e range e x t e n d e d from - 0 . 7 5 t o - 0 . 2 5 V ^ F . A N C ' A p r i m a r y t r a n s p a s s i v e r e g i o n was o b s e r v e d a t - 0 . 1 0 V ^ r ^ . From 0 t o 0 . 2 5 V S C E t h e r e was a s e c o n d a r y p a s s i v e r e g i o n . F i g u r e 6 shows t h e p o l a r i z a t i o n c u r v e f o r t h e p l a t e m a t e r i a l . I t was s i m i l a r t o F i g u r e 5 e x c e p t t h a t t h e a c t i v e -p a s s i v e "nose" e x h i b i t e d a s i n g l e peak. The e f f e c t o f NaOH i s shown i n F i g u r e 7 . I n c r e a s i n g t h e c o n c e n t r a t i o n i n c r e a s e d t h e c u r r e n t d e n s i t y , and d i s p l a c e d 36 0.50 CURRENT DENSITY A/m2 F i g u r e 5 A n o d i c p o l a r i z a t i o n c u r v e , 316 s t a i n l e s s s t e e l r o d , 3.35 mol/kg NaOH, 92 °C. ure 6 A n o d i c p o l a r i z a t i o n c u r v e , 316 s t a i n l e s s s t e e l p l a t e , 3.35 mol/kg NaOH, 92 °C. 38 0.50 CURRENT DENSITY A/m2 • i F i g u r e 7 A n o d i c p o l a r i z a t i o n c u r v e s a t s e l e c t e d NaOH c o n c e n t r a t i o n s , 316 s t a i n l e s s s t e e l r o d , 92 °C. t h e p r i m a r y t r a n s p a s s i v e ( m o r e a c t i v e ) v a l u e s . p e a k a n d c o r r o s i o n p o t e n t i a l 3 9 t o l o w e r R a i s i n g t h e t e m p e r a t u r e i n c r e a s e d t h e c u r r e n t d e n s i t y i n 3 . 3 5 m o l / k g NaOH a n d l o w e r e d t h e p o t e n t i a l o f t h e t r a n s p a s s i v e p e a k a s i l l u s t r a t e d i n F i g u r e 8 . F i g u r e 9 i l l u s t r a t e s t h e p o l a r i z a t i o n c u r v e s f o r c h r o m i u m i n t w o c o n c e n t r a t i o n s o f N a O H . T h e t r a n s p a s s i v e c u r r e n t d e n s i t y w a s s h i f t e d t o l o w e r p o t e n t i a l i n t h e m o r e c o n c e n t r a t e d s o l u -t i o n . T h e c o r r o s i o n p o t e n t i a l , E ^ O R R ' w a s i n t h e p a s s i v e r e g i o n i n 3 . 3 5 m o l / k g NaOH a n d i n t h e a c t i v e r e g i o n i n 8 m o l / k g N a O H , a c c o u n t i n g f o r t h e a c t i v e - p a s s i v e t r a n s i t i o n a t - 1 . 1 0 V ^ r f n t h e l a t t e r . T h e s o l u t i o n w a s y e l l o w a f t e r c o m p l e t i o n o f t h e a n o d i c s c a n . T h e p o l a r i z a t i o n b e h a v i o r o f n i c k e l i s s h o w n i n F i g u r e 1 0 . T h e r e w a s a n a c t i v e - p a s s i v e p e a k a t - 0 . 8 5 V ^ ^ ' a n d a p a s s i v e r e g i o n a t m o r e n o b l e p o t e n t i a l s . I r o n h a d a n a c t i v e - p a s s i v e c u r r e n t p e a k a t - 1 . 0 5 V < - C E a s s h o w n i n F i g u r e 1 1 . 3 . 1 . 1 NaOH + N a 2 S F i g u r e 1 2 s h o w s t h e p o l a r i z a t i o n c u r v e f o r t h e 3 1 6 s t a i n l e s s s t e e l r o d i n 2 . 5 m o l / k g NaOH + 0 . 4 2 3 m o l / k g N a 2 S 4 0 F i g u r e 8 A n o d i c p o l a r i z a t i o n c u r v e s a t s e l e c t e d t e m p e r a t u r e s , 316 s t a i n l e s s s t e e l r o d , 3.35 mol/kg NaOH. 41 F i g u r e 9 A n o d i c p o l a r i z a t i o n c u r v e s , chromium, 3.35 and 8 mol/kg NaOH, 92 °C. F i g u r e 10 A n o d i c p o l a r i z a t i o n c u r v e , n i c k e l , 3.35 mol/kg NaOH, 92 °C. F i g u r e 11 A n o d i c p o l a r i z a t i o n c u r v e , i r o n , 3.35 mol/kg NaOH, 92 °C. 44 F i g u r e 12 A n o d i c p o l a r i z a t i o n c u r v e , : ! ;i 316 s t a i n l e s s s t e e l r o d , 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S, 92 °C. 45 a t 92 °C. The c u r r e n t d e n s i t y between -1.15 and -0.75 V^^. was o f t h e same o r d e r o f m a g n i t u d e as t h a t o f t h e a c t i v e -p a s s i v e c u r r e n t peak i n the s o l u t i o n w i t h o u t s u l f i d e . F i g u r e 1 3 i 11 u s t r a t e s t h e e f f e c t o f i n c r e a s i n g c o n c e n t r a - i <> t i o n s o f NaOH. The' c o r r o s i o n , p o t e n t i a l , E Q O R R ' was i n t h e a c t i v e r e g i o n and an a c t i v e - p a s s i v e t r a n s i t i o n was e v i d e n t . The c u r r e n t peak became l a r g e r as t h e NaOH c o n c e n t r a t i o n was i n c r e a s e d , and was much l a r g e r t h a n t h a t i n t h e s u l f i d e f r e e s o l u t i ons. The e f f e c t o f t e m p e r a t u r e i n 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S i s i l l u s t r a t e d i n F i g u r e 14 f o r p l a t e m a t e r i a l . The c u r r e n t d e n s i t y i n c r e a s e d and E Q Q R R d e c r e a s e d as t h e t e m p e r a t u r e was i n c r e a s e d . Two s m a l l c u r r e n t peaks were o b s e r v e d a t the a c t i v e - p a s s i v e t r a n s i t i o n , s i m i l a r t o t h o s e ii seen i n t h e NaOH s o l u t i o n s . The c u r r e n t peaks were i n s i g n i -f i c a n t compared t o t h o s e shown i n F i g u r e 13. 3.2 Slow S t r a i n Rate T e s t s 3.2 . 0 NaOH The p e r c e n t r e d u c t i o n s i n c r o s s - s e c t i o n a l a r e a o f t h e SSRT s p e c i m e n s a r e p l o t t e d v e r s u s p o t e n t i a l i n F i g u r e 15. The minimum r e d u c t i o n i n a r e a o c c u r r e d at -0.50 t o -1.50 V c r c . T h i s i n d i c a t e d t h a t t h e g r e a t e s t s u s c e p t i b i l i t y t o SCC 46 F i g u r e 13 A n o d i c p o l a r i z a t i o n c u r v e s i n s o l u t i o n s o f s e l e c t e d NaOH c o n -c e n t r a t i o n w i t h Na^S, 316 s t a i n l e s s s t e e l r o d , 92 °C. F i g u r e 14 A n o d i c p o l a r i z a t i o n c u r v e s a t s e l e c t e d t e r m p e r a t u r e s , 316 s t a i n l e s s s t e e l p l a t e . 2.5 mol/kg NaOH + 0.423 mol/kg Na 9S. 48 F i g u r e 15 E f f e c t o f p o t e n t i a l on p e r c e n t r e d u c t i o n i n a r e a d u r i n g s l o w s t r a i n r a t e t e s t s , 3.35 mol/kg NaOH, 92 °C. A n o d i c p o l a r i z a t i o n c u r v e f o r 316 s t a i n l e s s s t e e l r o d . 49 was n e a r t h e p r i m a r y t r a n s p a s s i v e peak. F i g u r e 16 shows t h e a p p e a r a n c e o f t h e f r a c t u r e r e g i o n o f th e s p e c i m e n t e s t e d a t -0.10 V S Q E - The s u r f a c e was s e v e r e l y a t t a c k e d and e x h i b i t e d i n t e r g r a n u l a r c r a c k s . The s u r f a c e s o f s p e c i m e n s t e s t e d a t 0.10, 0, -0.50 and -0.15 V s c £ , w e r e c r a c k e d s i m i l a r l y . The f r a c t u r e s e c t i o n s i n d i c a t e d t h a t c r a c k i n g o c c u r r e d i n t h e s e c o n d a r y p a s s i v e r e g i o n b u t not i n t h e p r i m a r y p a s s i v e r e g i o n . S u r f a c e c r a c k i n g was a b s e n t a t l o w e r p o t e n -t i a l s n e a r t h e a c t i v e - p a s s i v e t r a n s i t i o n , as shown a t -0.95 V"sCE i n F i g u r e 17. An a n n e a l e d s p e c i m e n t e s t e d a t -0.10 V S C E showed s l i g h t l y l e s s s u s c e p t i b i l i t y t o SCC t h a n the a s - r e c e i v e d m a t e r i a l . A s e n s i t i z e d s p e c i m e n t e s t e d a t -1.15 V , , ^ was s l i g h t l y more s u s c e p t i b l e . One t e s t e d a t -0.85 V ^ i r showed no d i f -f e r e n c e from t h e a s - r e c e i v e d m a t e r i a l . The s u r f a c e f i l m o b s e r v e d on t h e s p e c i m e n s v a r i e d w i t h the p o t e n t i a l . I t was dark red-brown above about -0.50 V ^ ^ ^ At 0.25 V < . £ £ , i t a p p e a r e d t o be t h i c k and f a i r l y b r i t t l e , as seen i n F i g u r e 18a. At -1.0 V j - ^ , t h e s u r f a c e f i l m a p p e a r e d t o be v e r y much t h i n n e r w i t h a d i s t o r t e d a p p e a r a n c e as shown i n F i g u r e 18b. 50 F i g u r e 16 SSRT specimen a f t e r t e s t i n g a t -0.10 V - r F O L C i n NaOH s o l u t i o n . F i g u r e 17 SSRT s p e c i m e n a f t e r t e s t i n g a t -0.95 V i n NaOH s o l u t i o n . b F i g u r e 1 8 S u r f a c e f i l m on SSRT speci m e n s t e s t e d i n NaOH s o l u t i o n a) 0.25 V,,ri-, b) - 1.00 V - r F . 52 The s o l u t i o n from one t e s t (-0.25 V c ^ r r ) was a n a l y z e d f o r [ C l " ] . The c o n c e n t r a t i o n was ^ 0.03 mol/kg a f t e r a 2 day t e s t . 3.2.1 NaOH + Na^S; The p e r c e n t r e d u c t i o n s i n c r o s s - s e c t i o n a l a r e a o f t h e SSRT s p e c i m e n s a r e i l l u s t r a t e d i n F i g u r e 19. Some s u s c e p t i -b i l i t y t o SCC i n t h e a c t i v e r e g i o n was i n d i c a t e d by t h e de-c r e a s e i n r e d u c t i o n i n a r e a a t -1.15 Vc-^^. T e s t s were not c o n d u c t e d a t h i g h e r p o t e n t i a l s due t o o x i d a t i o n o f s u l f i d e i o n s as e v i d e n c e d by t h e i n c r e a s e i n c u r r e n t above -0.75 V S C E . The f r a c t u r e d r e g i o n o f t h e specimen t e s t e d a t -1.15 ^SCE ^ s "* 1 1 u s t r a t e d i n F i g u r e 20. T h e r e were no c r a c k s i n the s u r f a c e f i l m between -0.85 and -0.95 V ^ Q ^ . No e f f e c t o f s e n s i t i z a t i o n was o b s e r v e d i n sp e c i m e n s t e s t e d a t -1.00 and -1.15 V$CE" 3.3. F r a c t u r e M e c h a n i c s T e s t i n g 3.3.0 NaOH 3.3.0.0 E f f e c t o f S t r e s s I n t e n s i t y F i g u r e 21 i s a p l o t o f c r a c k growth r a t e v e r s u s Kj i n 3.35 mol/kg NaOH, 92 °C, -0.10 V $ C E . The d a t a show Region I 53 % REDUCTION IN AREA 50 60 70 80 90 -i 1 1 1 1 CURRENT DENSITY A/m 2 F i g u r e 19 E f f e c t o f p o t e n t i a l on p e r c e n t r e d u c t i o n i n a r e a d u r i n g slow s t r a i n r a t e t e s t s , 2.5 mol/kg NaOH + 0 . 4 2 3 mol/kg N a 2S, 92 °C. A n o d i c p o l a r i z a t i o n c u r v e f o r 316 s t a i n l e s s s t e e l r o d . Figure 20 SSRT specimen after testing at -1.15 V $ C E in NaOH + Na2S solution 10 >—• t> r • e—e— T°C e- 92 A - 8 2 B - 7 2 10 20 30 40 50 60 70 80 STRESS INTENSITY MPavTTT C r a c k g r o w t h r a t e v e r s u s s t r e s s i n t e n s i t y a t s e l e c t e d t e m p e r a t u r e s i n 3 . 3 5 m o l / k g N a O H , - 0 . 1 0 V c r c . 56 (Kj. d e p e n d e n t ) and R e g i o n I I (Kj. i n d e p e n d e n t ) b e h a v i o r . Region I d a t a were d i f f i c u l t t o o b t a i n b e c a u s e c r a c k i n g was so slow. K i s c C a P P e a r e c ! t 0 D e l e s s t h an TO MPa/m . The d a t a used t o c o n s t r u c t F i g u r e 21 a r e summarized i n T a b l e I I I . 3.3.0.1 T e m p e r a t u r e E f f e c t C r a c k growth r a t e d a t a i n 3.35 mol/kg NaOH a t 92, 82 and 72 °C a r e l i s t e d i n T a b l e I I I and i l l u s t r a t e d i n F i g u r e 21. R e s u l t s o b t a i n e d i n 12 mol/kg NaOH a t 92 and 82 °C a r e a l s o l i s t e d i n T a b l e I I I and a r e shown i n F i g u r e 22. The p o t e n t i a l was m a i n t a i n e d a t -0.10 V^^^ i n a l l t h e s e t e s t s . C r a c k growth r a t e d e c r e a s e d w i t h d e c r e a s i n g t e m p e r a t u r e . The e f f e c t o f t h e t e m p e r a t u r e on Re g i o n II c r a c k ! growth r a t e was e v a l u a t e d assuming a s i m p l e A r r h e n i u s : r a t e law: V = V Q exp (-Q/RT) .. . ( 5 ) where V = c r a c k growth r a t e V Q= e x p e r i m e n t a l c o n s t a n t Q = a p p a r e n t a c t i v a t i o n e n e r g y R = gas c o n s t a n t T = t e m p e r a t u r e (°K) The l o g a r i t h m o f c r a c k growth r a t e was p l o t t e d v e r s u s 1/T as shown i n F i g u r e 23. A l i n e was f i t t e d t o t h e 3.35 5 7 TABLE I I I Summary of F r a c t u r e Mechanics Test Data NaOH Concentration mol/kg 3.35 3.35 3.35 12 12 3.35 3.35 T 92 82 72 92 92 82 92 92 SCE -0.10 -0.10 -0.10 -0.10 -0.10 -0.10 0.0 -0.175 Kj Range MPa^n 10 - 10.5 18 - 22.6 22.6 - 27 27 33 50 40 40 30 - 30.7 - 52.7 - 39.3 - 98.3 - 110.3 - 45.7 26 35.6 35.5 72.1 20 - 23.5 23.5 - 34 34 - 75.4 15 27.6 27.6 73.1 16 - 52 15 - 28.8 36.3 - 63.1 25.8 - 30.9 30.9 - 94.4 30 - 35.2 35.2 - 64.8 25 46 Crack Growth Rate m/s 4.29 + 2.15)xlC' 4.04 + 0.81)x!0 6.17 + 1.23)xlO" 7.26 + 1.82)xl0" 1.02 + 0.08)xl0" 0.94 + 0.27)xl0 1.02 + 0.34)xl0 1.04 + 0.17)xl0 0.90 + 0.18)xl0 •10 -9 -8 -8 -8 -8 2.5 +0.19)xl0" 5.39 + 0.90)xl0" 1.05 + 0.55)xl0 2.09 + 0.22)xl0 3.09 + 0.25)x1O -9 0.82 1.50 + 0.16)xl0 + 0.55)xl0 -8 1.36 0.99 1.35 + 0.16)xl0 + 0.09)xl0 + 0.11 )xl0 -8 -8 -8 i 2.29 + 0.02)xl0 0.94 + 0.13)xl0 -9 2.23 3.01 + 0.29)xl0" + 0.17)xlC" (0.31 + 0.04)xl0" Number of Observations 6 17 58 10 20 30 AO 50 60 70 STRESS INTENSITY (K,) MPa/m" F i g u r e 22 C r a c k g r o w t h r a t e v e r s u s s t r e s s i n t e n s i t y a t s e l e c t e d t e m p e r a t u r e s i n 12 m o l / k g NaOH, -0.10 V c r r . [ NaOH ] mol/kg 0 - 3 . 3 5 A-12.0 _l I I I L_ 2 7 2.8 2.9 1 / T x 10 3 Arrhen iu s p l o t of the Region II c rack growth ra te s in 3.35. and 12 mol/kg NaOH, -0.10 V,,-,-. 60 mol/kg NaOH d a t a u s i n g t h e l e a s t s q u a r e s method. The ap-p a r e n t a c t i v a t i o n e n e r g y , c a l c u l a t e d from t h e g r a d i e n t o f the l i n e , was 60 ± 8 kJ/mol ( 9 0 % c o n f i d e n c e ) . The a c t i v a -t i o n e n e r g y i n 12 mol/kg NaOH was e s t i m a t e d t o be ^ 37 k J / m o l . 3.3.0.2 E f f e c t o f NaOH C o n c e n t r a t i o n The e f f e c t o f NaOH c o n c e n t r a t i o n i s i l l u s t r a t e d i n F i g u r e 24 and t a b u l a t e d i n T a b l e I I I . The e f f e c t o f c o n c e n t r a -t i o n a p p e a r e d t o be s m a l l and w i t h i n t h e e r r o r s o f measurement a t 8 and 12 mol/kg NaOH. The d a t a o f F i g u r e 23 s u g g e s t s t h a t , a l t h o u g h t h e e f f e c t o f c o n c e n t r a t i o n was s m a l l a t 92 °C, i t had a g r e a t e r e f f e c t a t 82 °C. 3.3.0.3 E f f e c t o f A p p l i e d P o t e n t i a l T e s t r e s u l t s o b t a i n e d a t 0, -0.110 and -0.175 i n 3.35 mol/kg NaOH, 92 °C a r e shown i n T a b l e I I I and F i g u r e 25. The c r a c k growth r a t e was f a s t e s t a t -0.10 V ^ Q ^ , i n agreement w i t h t h e p o t e n t i a l o f maximum s u s c e p t i b i l i t y i n t h e SSRT. In a d d i t i o n , f r a c t u r e m e c h a n i c s t e s t s were c o n d u c t e d a t a c t i v e - p a s s i v e p o t e n t i a l s . A t e s t c o n d u c t e d i n 3.35 mol/kg NaOH a t 92 °C, -1.15 V $ C E and 30 MPa/jrT, showed no c r a c k i n g i n 10 d a y s . A n o t h e r t e s t , a t -0.85 V $ C E and 30 MPa/IT, showed no c r a c k i n g a f t e r 15 d a y s . C r a c k growth r a t e s would have been < 2.89 x 1 0 " 1 0 m/s and < 1.92 x 1 0 - 1 0 m/s r e s p e c t i v e l y t o be i n d e t e c t a b l e i n t h e t e s t p e r i o d . 10 o x 1.0 LU 5 o or ID 0.1 i£ o < or o WW P T t NaOH ] m o l / k g 0 - 3.35 A - 8.0 B - 12.0 61 10 20 30 40 50 60 70 STRESS INTENSITY i-ob-< or o cr „ ID 0.1 < cr o -J 1 1 1 i i i I 10 20 30 40 50 60 70 80 STRESS INTENSITY (K,) MPa^rfT f i g u r e 35 C r a c k growth r a t e v e r s u s s t r e s s i n t e n s i t y i n 12 mol/kg NaOH + 0.423 mol/kg N a 2S, 92 °C, -1.175 V c r c . 79 -1.15 V S C E . 3.3.1.2 F r a c t o g r a p h y In a l 1 . f r a e t o g r a p h s t h e d i r e c t i o n o f c r a c k p r o p a g a -t i o n i s from t h e top t o t h e bottom o f the p h o t o g r a p h . The f r a c t o g r a p h y o f t h e sp e c i m e n c r a c k e d i n t h e NaOH + Na 2S s o l u t i o n was mixed t r a n s g r a n u l a r and i n t e r g r a n u l a r . I t i s i l l u s t r a t e d i n F i g u r e 36. Th e r e was no s i g n i f i c a n t d i f -f e r e n c e i n c r a c k i n g m o r p h o l o g y w i t h i n the range o f s t r e s s i n t e n s i t y t e s t e d . The s u r f a c e s were c o n s i d e r a b l y r o u g h e r t h a n t h o s e o b t a i n e d i n NaOH. C o r r o s i o n d e p o s i t s shown i n F i g u r e 37 were b l a c k i n c o l o r , w i t h a t i n g e o f g r e e n when r i n s e d w i t h d i s t i l l e d w a t e r . D u r i n g removal o f d e p o s i t s i n t h e i n h i b i t e d a c i d s o l u t i o n , a hydrogen s u l f i d e s m e l l was p r o d u c e d i n d i c a t i n g the d e p o s i t s were metal s u l f i d e s . 3.3.1.3 S o l u t i o n A p p e a r a n c e The s o l u t i o n t a k e n from t h e c e l l d u r i n g t h e t e s t was brown i n c o l o r . When d i l u t e d w i t h t h e a d d i t i o n o f d i s t i l -l e d w a t e r ( i . e . r e d u c t i o n o f pH), the s o l u t i o n t u r n e d g r e e n . D i l u t i o n w i t h f r e s h s t r o n g c a u s t i c s o l u t i o n d i d n o t have t h i s e f f e c t . b F i g u r e 36 F r a c t u r e s u r f a c e s a f t e r t e s t i n g i n 12 mol/kg NaOH + 0.423 mol/kg N a 2S, 92 °C, -1.175 V S C E a) 37-39 MPa/m b) 50-54 MPa/m. 81 F i g u r e 37 C o r r o s i o n d e p o s i t s a f t e r t e s t i n g i n 12 mol/kg NaOH + 0.423 mol/kg N a 2 S , 92 °C, -1.175 V Q p p j 50-54 MPa/m. 82 3.4 E l e c t r o n D i f f r a c t i o n A n a l y s i s o f S u r f a c e F i l m s i n NaOH An example o f a t y p i c a l e l e c t r o n d i f f r a c t i o n p a t t e r n i s shown i n F i g u r e 38. T a b l e IV i s an a n a l y s i s , o f t h e d a t a f o r t h i s sample. F i g u r e 39 i s a p l o t o f . D, t h e d i a m e t e r o f t h e d i f f r a c t i o n p a t t e r n r i n g s , v e r s u s v h +k +1 o b t a i n e d from t h e d a t a i n T a b l e IV. The l a t t i c e p a r a m e t e r , a Q , f o r t h e o c u b i c s p i n e l was 8 . 5 0 + 0.01 A a c c o r d i n g t o e q u a t i o n 3. The i n t e n s i t i e s o f t h e d i f f r a c t i o n r i n g s were c o n s i s t e n t w i t h Fe^O i n the x - r a y powder d i f f r a c t i o n f i l e 7 1 and w i t h t h e i n t e n s i -72 t i e s c a l c u l a t e d f o r e l e c t r o n d i f f r a c t i o n i n Fe^O^. O t h e r v a l u e s o b t a i n e d f o r t h e l a t t i c e p a r a m e t e r were 8.5 + 0.1, 8.8 + 0.2, 9.3 + 0.3 and 9.3 + 0.2 A. F i g u r e 38 E l e c t r o n d i f f r a c t i o n p a t t e r n from c o r r o s i o n f i l m . 84 / h 2 + k 2 + l 2 F i g u r e 3 9 D i a m e t e r o f d i f f r a c t i o n r i n g s v e r s u s A W . T a b l e IV E l e c t r o n - Di f f r a c t i o n P a t t e r n ' Data 1 R i n g Number Di ameter Inches d 6 A I n t e n s i t y h, k, 1 / h 2 + k 2 + l 2 1 1.1.2 2.97 weak 2 2 0 2.83 2 1 .22 2.73 s p o t s c - -3 1 .31 2.54 weak 3 1 1 3.32 4 1 .56 2.13 wea k 4 0 0 4. 5 1.72 1 .94 s p o t 3 3 1 4.36 6 1 .95 1 .71 v e r y weak 4 2 2 4.90 7 2.10 1 .60 v.v. weak 3 3=: 3 5.20 5 1 1 8 2.25 1 .48 medi um 4 4 0 5.66 9 2.44 1 .36 s p o t 6 2 0 6.32 10 2.62 1 .27 v.v. weak 5 3 3 6.56 11 2.72 1 .22 medi um 4 4 4 6.93 12 3.00 1.11 v.v. weak 7 3 1 7.68 6 4 2 13 3.16 1 .05 v.v. weak 8 0 0 8.00 86 4. DISCUSSION 4.1 I n t e r p r e t a t i o n o f A n o d i c ' P o l a r i z a t i o n C u r v e s E l e c t r o c h e m i c a l r e a c t i o n s o c c u r r i n g on the s u r f a c e may be i d e n t i f i e d by e x a m i n i n g t h e p o l a r i z a t i o n c u r v e s . A n o d i c c u r r e n t peaks r e s u l t from r e a c t i o n s which t a k e p l a c e t o change a metal s p e c i e s t o a form more t h e r m o d y n a m i c a l l y s t a b l e a t t h a t p o t e n t i a l . The thermodynamic s t a b i l i t y o f a s p e c i e s may be d e t e r m i n e d from an E-pH d i a g r a m , and t h u s t h e p o s s i b l e r e a c t i o n s w hich c a u s e t h e c u r r e n t peaks may be i d e n t i f i e d . The p o l a r i z a t i o n c u r v e f o r the 316 s t a i n l e s s s t e e l r o d m a t e r i a l i n 3.35 mol/kg.NaOH a t 92'°C has been p r e s e n t e d i n F i g u r e 5. I t i s b e l i e v e d t o be a c o m p o s i t e c u r v e which may be i n t e r p r e t e d i n terms o f t h e b e h a v i o r o f t h e major a l l o y c o n -s t i t u e n t s . The p o l a r i z a t i o n b e h a v i o r o f chromium was c o n s i d e r e d w i t h r e f e r e n c e t o t h e E-pH e q u i l i b r i u m d i a g r a m c o n s t r u c t e d by L e e 7 3 f o r t h e C r - H 2 0 s y s t e m a t 100 °C. The pH i s -v 12.5-13 i n 31 3.35 mol/kg NaOH a t 92 °C. The t r a n s p a s s i v e r e g i o n i n F i g u r e 9 c o r r e s p o n d e d t o t h e d i s s o l u t i o n o f chromium o x i d e t o 2 - 7 3 C r 0 4 . The y e l l o w c o l o r o f t h e s o l u t i o n a f t e r p o t e n t i o -2-dynamic s c a n n i n g i n NaOH c o n f i r m e d t h e p r e s e n c e o f CrO^ i o n s . By i n s p e c t i o n , i t was c o n c l u d e d t h a t the p r i m a r y t r a n s -p a s s i v e r e g i o n o b s e r v e d above -0.25 V^r i n F i g u r e 40 r e s u l t e d from f o r m a t i o n o f CrO^ 87 2-The p o l a r i z a t i o n b e h a v i o r o f n i c k e l was a n a l y z e d u s i n g 74 an E-pH d i a g r a m c o n s t r u c t e d by Cowan and S t a e h l e f o r t h e N i - H 2 0 s y s t e m a t 100 °C. The i n c r e a s i n g c u r r e n t o b s e r v e d above t h e c o r r o s i o n p o t e n t i a l i n F i g u r e 10 may have been due 74 to f o r m a t i o n o f HNi0^- v i a e q u a t i o n 7. HNi0 2- + 3H + + 2e" Ni + 2H 20 ...(7) EHNiO-/Ni = -°-837-0.074 pH + 0.037 log [HNi0~], V $ H E ...(7a) The p o t e n t i a l o f t h e c u r r e n t peak, -0.85 V < , ^ E was a t t r i b u t e d t o f o r m a t i o n o f a p a s s i v a t i n g f i l m . I t was n o t e d t h a t t h e a c t i v e - p a s s i v e peak a t -0.85 V ^ ^ E f o r n i c k e l ( F i g u r e 10) c o r -r e s p o n d e d to t h e peak a t -0.9 V -0.75 V<.£ E by o x i d a t i o n o f s u l f i d e . 75 76 B i e r n a t and Robins ' have c o n s t r u c t e d E-pH d i a g r a m s f o r t h e Fe-S-H 20 and S-H 20 systems a t 100 °C. A c c o r d i n g t o t h e i r d a t a , i r o n s u l f i d e s a r e s t a b l e a t low p o t e n t i a l . 91 F i g u r e 41 A n o d i c p o l a r i z a t i o n c u r v e s , 316 s t a i n l e s s s t e e l , NaOH + Na 2S, 92 °C. I d e n t i f i c a t i o n o f r e a c t i o n s . S i m i l a r l y , MacDonald and S y r e t t have c o n s t r u c t e d an E-pH d i a g r a m f o r t h e Ni-S-H^O s y s t e m a t 25 °C, i n d i c a t i n g p o s s i b l e f o r m a t i o n o f n i c k e l s u l f i d e s a t low p o t e n t i a l . No E-pH d i a -gram was a v a i 1 a b l e f o r t h e Cr-S-H^O s y s t e m . For h i g h e r c o n c e n t r a t i o n s o f NaOH, l a r g e r peak a n o d i c c u r r e n t d e n s i t i e s were o b s e r v e d ( F i g u r e 1 3 ) . S i m i l a r peaks 49 61 62 have been o b s e r v e d f o r m i l d s t e e l . ' ' The p o s i t i o n o f th e m a j o r c u r r e n t peak has been a t t r i b u t e d t o d e p o s i t i o n o f fi l FeS when i t s s o l u b i l i t y p r o d u c t i s e x c e e d e d . 4.2 SCC S u s c e p t i b i l i t y C o m p a r i s o n o f t h e slow s t r a i n r a t e t e s t s w i t h p o l a r i z a -t i o n b e h a v i o u r i n d i c a t e d t h a t SCC s u s c e p t i b i l i t y i n NaOH s o l u t i o n was a s s o c i a t e d w i t h i n s t a b i l i t y o f t h e p a s s i v e f i l m . The p o l a r i z a t i o n s t u d y showed t h a t breakdown o f p a s s i v e f i l m and d i s s o l u t i o n t o chromate o c c u r r e d i n t h e p r i m a r y t r a n s p a s ^ s i v e r e g i o n where SCC s u s c e p t i b i l i t y was g r e a t e s t ( F i g u r e 1 5 ) . The s u r f a c e o f a sp e c i m e n t e s t e d i n t h e s u s c e p t i b l e p o t e n t i a l range was d e e p l y c r a c k e d ( F i g u r e 1 6 ) . The c r a c k s may have formed where t h e s u r f a c e f i l m was b r o k e n , t h u s a l l o w i n g f a s t l o c a l i z e d d i s s o l u t i o n . R u p t u r e o f t h e f i l m may have o c c u r r e d by t h e sudden r e l e a s e o f d i s l o c a t i o n s p i l e d up b e h i n d t h e f i l m . A c c o r d i n g t o t h e f i l m r u p t u r e and d i s s o -1 9 l u t i o n m o d e l, t h e r e l e a s e d d i s l o c a t i o n s would form a s l i p 93 s t e p o f b a r e metal which would th e n be d i s s o l v e d . F o r m a t i o n o f chromate i o n s would have c a u s e d r a p i d d i s s o l u t i o n r e s u l t -i n g i n f o r m a t i o n o f deep c r a c k s b e f o r e r e p a s s i v a t i o n w i t h an i r o n - n i c k e l f i l m c o u l d o c c u r . A SCC mechanism i n v o l v i n g h y d r o g e n e m b r i t t l e m e n t would be r u l e d o u t a t p r i m a r y t r a n s p a s s i v e p o t e n t i a l s . In pH 12.5 75 s o l u t i o n s , h y d r o g e n e v o l u t i o n v i a e q u a t i o n 10 would o c c u r o n l y below -1.17 Vc,^. 2 H + +• 2e" H 2 _ ( 1 0 ) E = 0.057-0.074 pH - 0.037 l o g a . ..(10a) H /H 2 H 2 A pH measurement o f s o l u t i o n d r a i n i n g out o f a s t r e s s c o r r o s i o n c r a c k i n a f r a c t u r e m e c h a n i c s specimen showed t h a t pH was not m e a s u r a b l y l o w e r i n t h e c r a c k s o l u t i o n t h a n i n t h e b u l k s o l u -t i o n . Thus l o c a l h y drogen e v o l u t i o n was u n l i k e l y even i n s i d e c r a c k s . 49 In c o n t r a s t t o b e h a v i o r o f m i l d s t e e l i n NaOH s o l u t i o n s , t h e 316 s t a i n l e s s s t e e l showed no s u s c e p t i b i l i t y t o SCC a t t h e a c t i v e - p a s s i v e p o t e n t i a l . The a b s e n c e o f s u s c e p t i b i l i t y a t t h e s e p o t e n t i a l s may have r e s u l t e d from a number o f f a c t o r s . No s i g n i f i c a n t f i l m i n s t a b i l i t y may o c c u r i n t h i s r a n g e . A s t a b l e chromium f i l m may m i n i m i z e d i s s o l u t i o n o f i r o n and 94 n i c k e l and t h u s p r e v e n t i n s t a b i l i t y . An a l t e r n a t e f a c t o r c a u s i n g t h e a bsence o f s u s c e p t i b i l i t y may have been t h a t t h e s t r a i n r a t e was t o o h i g h . In t h a t c a s e , f a i l u r e would have o c c u r r e d b e f o r e c r a c k i n g had had t i m e t o p r o g r e s s . I f c r a c k i n g i n t h e slow s t r a i n r a t e t e s t s o c c u r r e d by c y c l e s o f f i l m r u p t u r e , d i s s o l u t i o n and r e p a s s i v a t i o n t h e n th e r a t e and d u r a t i o n o f d i s s o l u t i o n and r e p a s s i v a t i o n c y c l e s would have a f f e c t e d t h e c r a c k i n g r a t e . The low c u r r e n t d e n s i t y a t t h e a c t i v e - p a s s i v e peak i n d i c a t e d t h a t d i s s o l u t i o n r a t e was r e l a t i v e l y low a t t h i s p o t e n t i a l . V ery l i t t l e d i s -s o l u t i o n m i ght have t a k e n p l a c e b e f o r e r e p a s s i v a t i o n was c o m p l e t e . Time i n t h e d i s s o l u t i o n s t a g e would have been de-c r e a s e d by r a p i d r e p a s s i v a t i o n k i n e t i c s , p e r h a p s r e d u c i n g d i s -s o l u t i o n below t h a t r e q u i r e d t o s u s t a i n c r a c k i n g . A d d i t i o n -a l l y , i f h y drogen e m b r i t t l e m e n t were i m p o r t a n t i n t h e c r a c k i n g mechanism a t t h i s p o t e n t i a l , t h e n r a p i d r e p a s s i v a t i o n m ight r e d u c e t h e p e r i o d d u r i n g w hich hydrogen i s a b s o r b e d i n t o t h e l a t t i c e . o Park e t a l . have e x p l a i n e d SCC o f 304 s t a i n l e s s s t e e l i n 20N NaOH i n terms o f t h e r a t i o o f c u r r e n t d e n s i t y on a b a r e m e t a l s u r f a c e t o t h a t on a f i l m e d m e t a l . T h i s r a t i o was l o w e s t i n t h e p r i m a r y t r a n s p a s s i v e r e g i o n . A l t h o u g h t h e y o b s e r v e d c r a c k i n g n e a r t h e c o r r o s i o n p o t e n t i a l ( / v-0.9 V S H E ) , t h e r a t e o f c r a c k i n g was l o w e r f o r a l l o y s w i t h h i g h e r chromium 95 c o n t e n t . The p r e s e n t i n v e s t i g a t i o n , d i f f e r e d from t h a t o f Park e t a l . i n t h a t c r a c k i n g was not o b s e r v e d i n NaOH s o l u -t i o n s a t t h e c o r r o s i o n p o t e n t i a l . The d i f f e r e n c e may have been due t o t h e i r more c o n c e n t r a t e d s o l u t i o n s , f a s t e r s t r a i n r a t e , and d i f f e r e n t a l l o y c o m p o s i t i o n . S a n t a r i n i 7 8 t e s t e d a 17 C r - 1 3 N i s t e e l i n 50% NaOH a t 130 °C. D u r i n g t h e t e s t , t h e open c i r c u i t p o t e n t i a l r o s e from -1.17 t o -1.13 t h e n -1.00 V$QI£- T h i s was i n t h e p o t e n t i a l r e gime where i r o n i n t h e a l l o y would be d i s s o l v i n g , chromium would form a p a s s i v e f i l m and n i c k e l would be thermodynamic-a l l y s t a b l e . S a n t a r i n i i d e n t i f i e d t h i s r e g i o n as d a n g e r o u s f o r SCC. The same r e s u l t was n o t o b s e r v e d i n t h e p r e s e n t s t u d y , p e r h a p s b e c a u s e o f t h e l o w e r t e m p e r a t u r e and c o n c e n t r a -t i o n h e r e . A l s o , t h e p r e s e n t s t u d y was p o t e n t i o s t a t i c and so would n o t a l l o w a s h i f t i n p o t e n t i a l , as was o b s e r v e d by S a n t a r i n i and i n c l u d e d i n t h e model he p r o p o s e d . In t h e 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S ( s i m u l a t e d w h i t e l i q u o r ) s o l u t i o n , s u s c e p t i b i l i t y t o SCC was d e t e c t e d by SSRT a t -1.15 t o -1.175 V ^ ^ ^ . A c c o r d i n g t o e q u a t i o n 10, H^ 2-may be e v o l v e d a t t h i s p o t e n t i a l . The p r e s e n c e o f S i o n s may r e t a r d t h e hydrogen e v o l u t i o n and so promote a b s o r p t i o n o f a d s o r b e d h y d r o g e n . T h i s d i s s o l v e d h y d r o g e n may t h e n d i f -f u s e i n t o t h e metal t o c a u s e h y d r o g e n e m b r i t t l e m e n t . The e f f e c t o f s u l f i d e (H 9S s o l u t i o n ) i n p o i s o n i n g t h e r e c o m b i n a t i o n 96 79 r e a c t i o n 10 has been shown e x p e r i m e n t a l l y . The s u l f i d e a l s o may have weakened t h e p a s s i v e s u r f a c e f i l m formed a t t h i s p o t e n t i a l , making i t l e s s p r o t e c t i v e and t h u s a l l o w i n g d i s s o l u t i o n t o o c c u r . A n o t h e r p o s s i b i l i t y may be t h a t 2-the s u l f i d e a f f e c t e d t h e r e p a s s i v a t i o n r a t e . S may have a d s o r -bed o n t o t h e s u r f a c e i n s t a d o f 0H~, t h u s r e d u c i n g the r a t e o f f o r m a t i o n o f p a s s i v e f i l m and s l o w i n g r e p a s s i v a t i o n . The s l o w e r r e p a s s i v a t i o n r a t e may l e a v e the s u r f a c e bare f o r a l o n g e r p e r i o d o f t i m e . T h i s bare s u r f a c e may c a t a l y z e e v o l u t i o n o f a d s o r b e d hydrogen t h e r e b y l e a d i n g t o g r e a t e r h y d r o g e n a b s o r p t i o n 79 as shown by B e r k o w i t z and H o r o w i t z . S e n s i t i z a t i o n c a u s e d some s u s c e p t i b i l i t y i n 3.35 mol/kg NaOH a t -1.15 i n t h e slow s t r a i n r a t e t e s t s ( F i g u r e 1 5 ) . The s e n s i t i z a t i o n would have t i e d up some o f t h e chromium i n i n -t e r g r a n u l a r c a r b i d e s and hence t h e f i l m may n o t have formed p r o -p e r l y a d j a c e n t t o g r a i n b o u n d a r i e s . Lower chromium c o n t e n t may a l s o have r e s u l t e d i n s l o w e r r e p a s s i v a t i o n t h u s a l l o w i n g more r d i s s o l u t i o n b e f o r e r e p a s s i v a t i o n was c o m p l e t e . A l t e r n a t e l y , s l o w e r r e p a s s i v a t i o n k i n e t i c s m i g h t a l l o w l o n g e r t i m e f o r a ! hydrogen e m b r i t t l e m e n t mechanism t o be o p e r a t i v e . S e n s i t i z a t i o n d i d n o t a f f e c t s u s c e p t i b i l i t y i n the NaOH + Na 2S s o l u t i o n . Perhaps i t showed no e f f e c t b e c a u s e s u l f i d e had a l r e a d y s l o w e d t h e r e p a s s i v a t i o n k i n e t i c s . An a n n e a l e d SSRT specimen t e s t e d a t t h e t r a n s p a s s i v e p o t e n t i a l i n NaOH s o l u t i o n had a l a r g e r p e r c e n t r e d u c t i o n i n a r e a a t f a i l u r e t h a n d i d an a s - r e c e i v e d s p e c i m e n . The a n n e a l e d m a t e r i a l was s o f t e r t h an t h e wo r k - h a r d e n e d a s - r e c e i v e d m a t e r i a l . C r i t i c a l s t r a i n s would n o t have d e v e l o p e d i n t h e m a t e r i a l u n t i l l a t e r i n t h e t e s t when e l o n g a t i o n was g r e a t e r . A n n e a l e d s p e c i e mens t e s t e d a t n o n - s u s c e p t i b l e p o t e n t i a l s m i g h t have. v . . 97 showed h i g h e r p e r c e n t r e d u c t i o n s i n a r e a , t o o . The r e s u l t w i t h t h e a n n e a l e d s p e c imen i n d i c a t e d t h a t s t r e s s r e l i e f may r e d u c e s u s c e p t i b i l i t y but does n o t n e c e s s a r i l y p r e v e n t SCC. 4.3 C r a c k Growth Rates and t h e Mechanism o f C r a c k i n g SCC s u s c e p t i b i l i t y was g r e a t e s t i n t h e p o t e n t i a l r e g i o n s i n d i c a t e d by t h e slow s t r a i n r a t e t e s t s , and t h e s e r e g i o n s were c o r r e l a t e d t o p o t e n t i a l r a n g e s f o r f i l m i n s t a b i l i t y by means o f t h e p o l a r i z a t i o n c u r v e s . C r a c k i n g was a t t r i b u t e d t o the o p e r a t i o n o f a f i l m r u p t u r e and d i s s o l u t i o n mechanism. As d e s c r i b e d below, a s t u d y o f t h e c r a c k growth r a t e s under a v a r i e t y o f c o n d i t i o n s p r o v i d e d c o n c l u s i v e e v i d e n c e r e g a r d i n g t h e n a t u r e o f t h e d i s s o l u t i o n mechanism. 4.3.1 P o t e n t i a l Dependence o f C r a c k Growth Rate F r a c t u r e m e c h a n i c s t e s t i n g c o n f i r m e d t h e SSRT r e s u l t t h a t SCC s u s c e p t i b i l i t y was p o t e n t i a l d e p e n d e n t . In t h e 3.35 mol/kg NaOH, Re g i o n II c r a c k growth was s l o w e r a t 0.0 and -0.175 V $ C E t h a n a t -0.10 V $ C E ( F i g u r e 2 5 ) . T h i s r e s u l t was e x p e c t e d b ecause 0.0 and -0.175 V S C E were a t t h e e x t r e m i t i e s o f t h e r e g i o n o f SCC s u s c e p t i b i l i t y d e t e r m i n e d by SSRT. F r a c t u r e m e c h a n i c s t e s t s a t -0.85 and -1.15 V c o n f i r m e d t h e ab s e n c e o f c r a c k i n g a t a c t i v e - p a s s i v e p o t e n t i a l s i n t h e 3.35 mol/kg NaOH. 98 The c r a c k i n g i n s u l f i d e c o n t a i n i n g s o l u t i o n s was p o t e n t i a l d e p e n d e n t , a l s o . The c r a c k i n g o b s e r v e d i n 12 mol/kg NaOH + 0.423 mol/kg Na 2S a t -1.175 V $ C E was s t o p p e d when the p o t e n -t i a l was r a i s e d t o -1.15 V S C E > Specimens t e s t e d i n 2.5 mol/kg NaOH + 0.423 mol/kg Na 2S d i d n o t c r a c k a t -1.00 o r -1.10 V $ C E . The SSRT r e s u l t s i n d i c a t e d some SCC s u s c e p t i b i l i t y a t ^ - 1.15 V S C E b u t t n i s w a s n o t o b s e r v e d f o r t h e f r a c t u r e m e c h a n i c s s p e c i m e n s made from p l a t e m a t e r i a l . P e rhaps t h e d i f f e r e n c e was due t o d i f f e r e n t c o m p o s i t i o n s o f t h e r o d and p l a t e , m a i n l y o f chromium and n i c k e l . A l t e r n a t e l y , t h e r e l a t i v e l y h i g h p e r -c e n t r e d u c t i o n i n a r e a a t a c t i v e - p a s s i v e p o t e n t i a l s may have i n d i c a t e d m a r g i n a l SCC s u s c e p t i b i l i t y , w h i c h was i n s u f f i c i e n t t o i n i t i a t e c r a c k i n g i n t h e f r a c t u r e m e c h a n i c s s p e c i m e n s . Thus, t h e i n v e s t i g a t i o n o f p o t e n t i a l dependence o f c r a c k growth r a t e i n d i c a t e d t h a t t h e slow s t r a i n r a t e t e s t s and f r a c t u r e m e c h a n i c s t e s t s were c o n s i s t e n t and t h a t c r a c k growth r a t e was f a s t e s t i n t h o s e p o t e n t i a l r a n g e s o f g r e a t e s t s u s c e p t -i b i l i t y t o SCC as p r e d i c t e d by SSRT. 4.3.2 D i s s o l u t i o n Rate and C r a c k Growth Rate F r a c t u r e m e c h a n i c s t e s t s p r o v i d e d c r a c k growth r a t e s i n R e g i o n s I and II o f t h e v - Kj. p l o t s . Kj i n d e p e n d e n t c r a c k i n g k i n e t i c s i n R e g i o n II were th e r e s u l t o f s t r e s s i n -d e p e n d e n t p r o c e s s e s o n l y , o f which d i s s o l u t i o n i s t h e most p r o b a b l e . 99 An a t t e m p t was made t o r e l a t e t h e c r a c k growth r a t e t o 80 t h e d i s s o l u t i o n r a t e v i a F a r a d a y ' s law. i W ...(11) a where v = c r a c k growth r a t e i = a n o d i c c u r r e n t d e n s i t y a J W = e q u i v a l e n t w e i g h t o f t h e m e tal F = F a r a d a y (9.65 x TO 4 A«S) p = d e n s i t y o f m e tal The a n o d i c c u r r e n t d e n s i t i e s , i , were o b t a i n e d from t h e a p o l a r i z a t i o n c u r v e s . At -0.10 V<- C E t h e c u r r e n t d e n s i t y f o r d i s s o l u t i o n o f chromium was much l a r g e r t h a n t h a t o f i r o n o r n i c k e l ( C r : 14.3 A/m 2, Fe: 2.0 A/m 2, N i : 0.3 A/m 2). The c r a c k v e l o c i t y c a l c u l a t e d from t h e a n o d i c c u r r e n t d e n s i t y f o r d i s s o l u t i o n t o C r ( V I ) was: 1.79 x 1 0 ~ 1 0 m/S. S i m i l a r l y f o r i r o n t o F e ( I I I ) t h e c a l c u l a t e d c r a c k growth r a t e was 0.49 x 1 0 " 1 0 m/s,and f o r n i c k e l t o N i ( I I ) i t was 1.02 x 1 0 " 1 1 m/s. C r a c k growth r a t e i n s u l f i d e s o l u t i o n was not compared w i t h e x p e c t e d c r a c k growth r a t e c a l c u l a t e d from d i s s o l u t i o n r a t e b e c a u s e o f u n c e r t a i n t y a b o u t what s p e c i e s would be f o r m e d ; o r what t h e i r d i s s o l u t i o n r a t e s would be. None of t h e c a l c u l a t e d c r a c k growth r a t e s was l a r g e enough t o a c c o u n t f o r t h e o b s e r v e d r a t e i n NaOH (^1 x l O m/s). 1 00 However, d u r i n g c r a c k i n g , d i s s o l u t i o n would not have been o c c u r -r i n g u n i f o r m l y o v e r t h e s u r f a c e , but would have been c o n c e n t r a -t e d a t d e f e c t s i n t h e o x i d e f i l m . T h e r e f o r e , t h e t r u e c u r r e n t d e n s i t y a t d e f e c t s i t e s would have been much h i g h e r t h a n t h a t i n d i c a t e d by t h e p o l a r i z a t i o n d i a g r a m s . 8 Park e t a l . have shown t h a t c u r r e n t d e n s i t y on a ba r e s t a i n l e s s s t e e l e l e c t r o d e i n NaOH i s about 100 t i m e s t h a t on a 91 f i l m e d e l e c t r o d e . T h i s i s c o r r o b o r a t e d by Hoar and Jon e s 82 f o r m i l d s t e e l s t r a i n i n g i n NaOH, and by D i e g l e and V e r m i l y e a f o r i r o n i n NaOH u s i n g a drop w e i g h t a p p a r a t u s . I f c u r r e n t d e n s i t i e s on s t r a i n i n g e l e c t r o d e s a r e 100 t i m e s h i g h e r t han on s t a t i c e l e c t r o d e s , t h e n c r a c k i n g r a t e s may be a l s o . T h i s would a c c o u n t f o r t h e o b s e r v e d c r a c k i n g r a t e s . I t has been c o n c l u d e d t h a t t h e ma g n i t u d e o f c r a c k growth r a t e c a n n o t be c a l c u l a t e d f r o m t h e magni t u d e o f the c u r r e n t d e n s i t y on t h e p o l a r i z a t i o n d i a g r a m . However, i n c r e a s e s i n ;i. a n o d i c c u r r e n t d e n s i t y s t i l l may c o r r e l a t e w i t h i n c r e a s e s i n c r a c k growth r a t e . In 3.35 mol/kg NaOH a t -0.10 V S C E , i n -c r e a s e s i n c r a c k growth r a t e o v e r a range o f t e m p e r a t u r e ( F i g u r e 21) c o r r e l a t e d w i t h i n c r e a s e s i n c u r r e n t d e n s i t y ( F i g u r e 8 ) . At 92 °C, c r a c k growth r a t e ( F i g u r e 24) c o r -r e l a t e d w i t h i n c r e a s e i n c u r r e n t d e n s i t y o v e r a range o f p o t e n t i a l s ( F i g u r e 7 ) . 101 I t a l r e a d y has been shown t h a t t h e c r a c k growth r a t e s a r e q u a l i t a t i v e l y c o n s i s t e n t w i t h a d i s s o l u t i o n mechanism. Un-f o r t u n a t e l y , c o m p a r i s o n o f c u r r e n t d e n s i t i e s on p o l a r i z a t i o n c u r v e s have been shown t o be i n a d e q u a t e f o r q u a n t i t a t i v e l y p r e d i c t i n g c r a c k growth r a t e s . C u r r e n t d e n s i t i e s were co n -s i s t e n t w i t h a l o c a l i z e d d i s s o l u t i o n mechanism but n e i t h e r c o n f i r m e d i t n o r r e v e a l e d t h e n a t u r e o f t h e d i s s o l u t i o n p r o c e s s . 4.3.3 K i n e t i c s o f C r a c k Growth Rates The a p p a r e n t a c t i v a t i o n e n e r g y f o r c r a c k growth r a t e was f o u n d t o be ^ 60 kJ/mol i n 3.35 mol/kg NaOH and ^ 37 kJ/mol i n 12 mol/kg NaOH ( b o t h a t -0.10 V S C E ) . These a p p a r e n t a c t i v a -t i o n e n e r g i e s were c o n s i s t e n t w i t h a d i s s o l u t i o n p r o c e s s c o n -t r o l l e d by c h a r g e t r a n s f e r , f o r which v a l u e s may range from 21-o o 105 k J / m o l . The a p p a r e n t a c t i v a t i o n e n e r g i e s were t o o l a r g e t o r e s u l t s o l e l y from c o n t r o l by d i f f u s i o n ( t r a n s p o r t ) i n t h e 1 i q u i d p h a s e . An a t t e m p t was made t o c o r r e l a t e t h e a p p a r e n t a c t i v a t i o n e n e r g y f o r c r a c k i n g w i t h t h a t o f t h e d i s s o l u t i o n c u r r e n t measured on t h e p o l a r i z a t i o n d i a g r a m ( F i g u r e 8 ) . The a c t i v a -t i o n e n e r g y v a r i e d from 93.8 + 33.1 kJ/mol a t -0.10 V t h r o u g h 66.8 + 1 9 . 9 kJ/mol a t t h e p r i m a r y t r a n s p a s s i v e c u r r e n t peak t o 33.9 + 3.8 kJ/mol a t t h e s e c o n d a r y p a s s i v e minimum. The f a c t t h a t t h e a c t i v a t i o n e n e r g y o f t h e c u r r e n t peak gave t h e b e s t c o r r e l a t i o n w i t h t h e a p p a r e n t a c t i v a t i o n e n e r g y f o r c r a c k growth r a t e s u p p o r t s t h e f i l m r u p t u r e and d i s s o l u t i o n 102 model i n which t h e m agnitude o f t h e t r a n s i e n t d i s s o l u t i o n c u r -r e n t peak may g r e a t l y a f f e c t c r a c k growth r a t e . The a p p a r e n t a c t i v a t i o n e n e r g y i n t h e 3.35 mol/kg NaOH s o l u t i o n was i n d i c a t i v e o f c h a r g e t r a n s f e r w h i l e t h a t i n t h e 12 mol/kg s o l u t i o n was more r e p r e s e n t a t i v e o f t h e l o w e r v a l u e s 49 o b t a i n e d i n mixed c h a r g e t r a n s f e r - d i f f u s i o n c o n t r o l . In the 12 mol/kg s o l u t i o n , d i f f u s i o n may p l a y a g r e a t e r p a r t i n t h e mixed c h a r g e t r a n s f e r - d i f f u s i o n c o n t r o l o f d i s s o l u t i o n . I n s u f f i c i e n t d a t a p r e c l u d e d c a l c u l a t i o n o f an a c t i v a -t i o n e n e r g y f o r c r a c k i n g i n s u l f i d e s o l u t i o n s . The v a l u e s o f a c t i v a t i o n e n e r g y o b t a i n e d were c o n s i s t e n t w i t h r e s u l t s o b t a i n e d i n o t h e r e n v i r o n m e n t s by o t h e r i n -v e s t i g a t o r s . R u s s e l f o u n d an a p p a r e n t a c t i v a t i o n e n e r g y o f 67 kJ/mol f o r 25% c o l d worked 316 s t a i n l e s s s t e e l from the same p l a t e used i n t h e p r e s e n t s t u d y . 1 4 ' 4 8 S p e i d e l , t e s t i n g 304 s t a i n l e s s s t e e l i n 22% NaCl s o l u t i o n f o u n d an a p p a r e n t 1 3 a c t i v a t i o n e n e r g y o f 72 k J / m o l . S t a e h l e o b t a i n e d 42-75.3 kJ/mol f o r n i c k e l s t r a i n i n g and s t a t i c e l e c t r o d e s i n 1 N H 2 S 0 4 . The l o w e r a c t i v a t i o n e n e r g y o f s t r a i n i n g e l e c t r o d e s was 85 a t t r i b u t e d t o ' l a t t i c e d i s a r r a y ' a f t e r Hoar who p r e d i c t e d t h a t t h e r e m i g h t be a r e d u c t i o n i n t h e a c t i v a t i o n e n e r g y f o r m e t a l d i s s o l u t i o n due t o an i n c r e a s e i n t h e i n t e r n a l e n e r g y o f atoms i n r a p i d l y e m e r g i n g s l i p " p o c k e t s . " 1 0 3 8 6 P e t i t s t u d i e d t h e t r a n s p a s s i v e b e h a v i o r o f n i c k e l i n s o l u t i o n s o f s u l f u r i c a c i d a n d s o d i u m s u l f a t e . He d e t e r m i n e d t h e a c t i v a t i o n e n e r g i e s o f t h e r e a c t i o n r a t e s ( e q u i v a l e n t t o c u r r e n t s a t t h e t r a n s p a s s i v e p e a k o r i n t h e s e c o n d a r y p a s s i v e r e g i o n ) . I m p e d a n c e m e a s u r e m e n t s s h o w e d t h a t t h e r e a c t i o n s o c c u r r e d i n s e v e r a l s t a g e s . T h e a c t i v a t i o n e n e r g i e s q u o t e d w e r e 5 0 . 5 - 7 0 . 0 k J / m o l a t a p o l a r i z a t i o n r a t e o f 3 3 m V / m i n . 1 c S t a e h l e h a s s u g g e s t e d t h a t t h e c u r r e n t t r a n s i e n t a s s o c i -a t e d w i t h c o r r o s i o n o f f i l m - f r e e m e t a l f o l l o w i n g f i l m r u p t u r e i s r e l a t e d t o t h e r a t e o f c r a c k p r o p a g a t i o n . He p r e s e n t e d r e s u l t s t h a t s h o w e d a c t i v a t i o n e n e r g i e s f o r . ' p a r t i a l c u r r e n t s ' , a n d t h a t t h e a c t i v a t i o n e n e r g y i n c r e a s e d w i t h n i c k e l c o n t e n t . T h u s n i c k e l w o u l d r e d u c e c r a c k i n g r a t e b y r e d u c i n g t h e s i z e o f t h e c u r r e n t t r a n s i e n t f o l l o w i n g r u p t u r e , a n d c h r o m i u m w o u l d i n c r e a s e t h e c r a c k i n g r a t e b y i n c r e a s i n g t h e s i z e o f t h e c u r -r e n t t r a n s i e n t . 4 . 4 F r a c t o g r a p h y a n d t h e D i s s o l u t i o n M e c h a n i s m 4 . 4 . 0 C o r r o s i o n D e p o s i t s T h e c o r r o s i o n d e p o s i t s s h o w n i n F i g u r e 2 9 a p p e a r t o h a v e p r e c i p i t a t e d f r o m s o l u t i o n . T h e p r e s e n c e o f t h e s e c r y s t a l l i n e d e p o s i t s i n d i c a t e d t h a t t h e s o l u t i o n w i t h i n t h e c r a c k w a s s a t u r a t e d w i t h m e t a l s p e c i e s . U n f o r t u n a t e l y t h e _ c o m p o s i t i o n o f t h e d e p o s i t s c o u l d n o t be d e t e r m i n e d . T h e c r y s t a l l i n e d e p o s i t s i n F i g u r e 2 9 b r e s e m b l e d h e m a t i t e 104 p l a t e l e t s o b s e r v e d on i r o n i n NaOH s o l u t i o n s . At h i g h e r t e m p e r a t u r e s , where c r a c k i n g was f a s t e r , t h e d e p o s i t s were s l i g h t l y t h i c k e r . The f a s t e r d i s s o l u t i o n a t th e h i g h e r t e m p e r a t u r e s may have c a r r i e d more i o n s i n t o s o l u -t i o n , and t h e s e s u b s e q u e n t l y may have p r e c i p i t a t e d t o form t h e t h i c k e r d e p o s i t . These p r e c i p i t a t e s a p p e a r e d t o have formed d u r i n g c r a c k i n g . They were t h i c k e r n e a r t h e c r a c k mouth where t h e f r a c t u r e s u r f a c e had been e x p o s e d l o n g e r , t h u s a l l o w i n g a l o n g e r t i m e f o r p r e c i p i t a t i o n t o o c c u r . The c o r r o s i o n d e p o s i t s were a f f e c t e d s i g n i f i c a n t l y by p o t e n t i a l . D e p o s i t i o n was g r e a t e r as t h e p o t e n t i a l was i n -c r e a s e d from -0.175 t o 0.0 V S C E ( F i g u r e 3 3 ) . T h i s may have been t h e r e s u l t o f t r a n s i t i o n from p a s s i v e t o s e c o n d a r y p a s s i v e f i l m . I t may a l s o have r e f l e c t e d d i f f e r i n g d e p o s i t i o n r e s u l t i n g from t h e d i f f e r e n t e q u i l i b r i u m c o n c e n t r a t i o n s f o r s o l u b l e s p e c i e s a t t h e s e p o t e n t i a l s . The heavy d e p o s i t s a t 0.0 V m a y have i n t e r f e r e d w i t h d i f f u s i o n p r o c e s s e s o v e r d i s t a n c e s s m a l l e r t h a n a g r a i n . Heavy d e p o s i t i o n was f o u n d i n t h e 12 mol/kg NaOH s o l u t i o n . The c o n t r o l p o t e n t i a l was i n t h e s e c o n d a r y p a s s i v e r e g i o n . C o r r o s i o n d e p o s i t s were t h i c k e r t h a n t h o s e f o u n d a t t h e same p o t e n t i a l i n t h e 3.35 mol/kg NaOH, t h u s i n d i c a t i n g t h a t d i f -f u s i o n may have been a f f e c t e d i n t h e s t r o n g e r NaOH s o l u t i o n . 105 Indeed, t h e a p p a r e n t a c t i v a t i o n e n e r g y c o n f i r m e d t h i s , b e i n g a low v a l u e which was c o n s i s t e n t w i t h mixed a c t i v a t i o n -d i f f u s i o n c o n t r o l . C o r r o s i o n d e p o s i t s were a l s o f o u n d i n t h e s u l f i d e c o n -t a i n i n g s o l u t i o n s . The o b s e r v a t i o n o f metal s u l f i d e s on the f r a c t u r e s u r f a c e s was i n agreement w i t h p r e v i o u s o b s e r v a t i o n s o f i r o n s u l f i d e f o r m a t i o n on i r o n i n NaOH + Na^S s o l u t i o n . 5 1 As p r e v i o u s l y s t a t e d , s o l u t i o n t a k e n from t h e c e l l d u r i n g t h e t e s t t u r n e d g r e e n when n e u t r a l i z e d w i t h t h e a d d i t i o n o f d i s t i l -88 l e d w a t e r . T a y l o r and S h o e s m i t h have s t u d i e d g r e e n a l k a l i n e s u l f i d e s o l u t i o n s o f pH 12-13 and have d e t e r m i n e d t h a t t h e g r e e n c o l o r i s d e r i v e d from c o l l o i d a l NaFeS^ i n s o l u t i o n . T h i s was e v i d e n c e t h a t i r o n d i s s o l v e d d u r i n g t h e f r a c t u r e m e c h a n i c s t e s t s . The f o r m a t i o n o f t h e s e d e p o s i t s would how-e v e r n o t r u l e o ut hy d r o g e n e m b r i t t l e m e n t a t t h i s p o t e n t i a l as th e d e p o s i t s may have formed by d i s s o l u t i o n from the c r a c k w a l 1 s . 4.4.1 F r a c t u r e Mode The c r a c k i n g p r o g r e s s e d m o s t l y i n an i n t e r g r a n u l a r mode. I n t e r g r a n u l a r s e g r e g a t i o n may have been a f a c t o r i n t h e f a s t e r 89 d i s s o l u t i o n a t g r a i n b o u n d a r i e s . A n o t h e r f a c t o r c a u s i n g t h e i n t e r g r a n u l a r c r a c k i n g may have been t h a t s t r a i n was c o n c e n t r a -t e d t h e r e . D i s l o c a t i o n p i l e - u p s a t t h e g r a i n b o u n d a r i e s may have r e s u l t e d i n a g r e a t e r s t r a i n r a t e i n t h e a d j a c e n t a r e a 106 when r e l e a s e d . S l i p bands were o b s e r v e d on t h e g r a i n s u r f a c e s i n F i g u r e s 26-33, o f f e r i n g e v i d e n c e t h a t s u b s t a n t i a l s t r a i n i n g t ook p l a c e t h e r e . L a r g e r and more numerous s l i p s t e p s r e s u l t -i n g f r o m t h i s s t r a i n i n g would have p r o v i d e d s i t e s f o r g r e a t e r d i s s o l u t i o n a t t h e g r a i n b o u n d a r i e s . The g r a i n b o u n d a r i e s t h e m s e l v e s may have i n c r e a s e d t h e d i s s o l u t i o n r a t e due t o t h e i r i n h e r e n t l o c a l a t o m i c d i s a r r a y . The d i s a r r a y . m a y have i n -c r e a s e d t h e r a t e o f a d s o r p t i o n o f damaging s p e c i e s by o f f e r i n g an i n c r e a s e d number o f s u r f a c e i m p e r f e c t i o n s a t which adsorp4r" t t o n c o u l d t a k e p l a c e . T h i s a d s o r p t i o n may have s t i m u l a t e d f a s t e r d i s s o l u t i o n . At t h e p r i m a r y t r a n s p a s s i v e p o t e n t i a l s , a d s o r p t i o n may have been e s s e n t i a l t o t h e d i s s o l u t i o n p r o c e s s . K n o e d l e r and 90 H e u s l e r have s u g g e s t e d t h a t t h e o x i d a t i o n o f chromium t o 2-CrO^ p r o c e e d s v i a s i x c o n s e c u t i v e c h a r g e t r a n s f e r r e a c t i o n s . The i n t e r m e d i a t e s c o v e r t h e s u r f a c e w i t h an a d s o r b e d m o n o l a y e r . Most o f t h e s u r f a c e i s c o v e r e d w i t h C r ( I V ) , i t s o x i d a t i o n b e i n g t h e r a t e d e t e r m i n i n g s t e p . The r e a c t i o n s a r e summarized by e q u a t i o n 12. C r ( 0 H ) x + 1 + e" =F=*Cr(0H) x + OH" ...(12) The f r a c t o g r a p h y t h a t was o b s e r v e d i n t h i s s t u d y was c o n s i s t e n t w i t h 0H~ a d s o r p t i o n , and t h u s s u p p o r t e d t h e d i s s o l u t i o n mechanism s u g g e s t e d by K n o e d l e r and H e u s l e r . 107 F i g u r e 27 i l l u s t r a t e s t h e e f f e c t o f s t r e s s i n t e n s i t y on f r a c t o g r a p h y . T r a n s g r a n u l a r c r a c k i n g was o b s e r v e d o n l y a t h i g h s t r e s s i n t e n s i t y i n 3.35 mol/kg NaOH. The g r e a t e r amount o f d e f o r m a t i o n a t t h i s h i g h s t r e s s i n t e n s i t y may have p r o v i d e d c o m p e t i n g s i t e s w i t h i n t h e g r a i n e i t h e r by s e v e r e d i s r u p t i o n o f o x i d e f i l m o r h i g h s t r a i n e n e r g y s i t e s f o r a d s o r p t i o n . 48 T h i s o b s e r v e d e f f e c t was o p p o s i t e t o t h e r e s u l t s o f R u s s e l f o r 316 s t a i n l e s s s t e e l i n MgCL,,. A d e c r e a s i n g f r a c t i o n o f t r a n s g r a n u l a r SCC and i n c r e a s i n g i n t e r g r a n u l a r SCC were o b s e r v e d as Kj was i n c r e a s e d i n M g C ^ . The d i f f e r e n t b e h a v i o r o b s e r v e d by R u s s e l may have r e s u l t e d from d i f f e r e n t t e m p e r a -t u r e , p o t e n t i a l , pH, or by t h e p r e s e n c e o f C l " . M e t a l l u r g i c a l d i f f e r e n c e s c o u l d be r u l e d o u t as t h e p l a t e and p r e p a r a t i o n o f spe c i m e n s were t h e same as f o r t h e p r e s e n t s t u d i e s . I f g r a i n b oundary s e g r e g a t i o n e f f e c t s were i m p o r t a n t t o t h e i n t e r -g r a n u l a r f a i l u r e i n c a u s t i c , t h e y o b v i o u s l y had a d i f f e r e n t e f f e c t i n MgCl,,. In t h e NaOH s o l u t i o n s , t h e r e was no change i n t h e mode o f c r a c k i n g o v e r t h e range o f p r i m a r y t r a n s p a s s i v e , t r a n s p a s s i v e peak, and s e c o n d a r y p a s s i v e p o t e n t i a l s i n v e s t i g a t e d . Beyond t h i s r a n g e , c r a c k i n g mode may change. Some e v i d e n c e i n t h e 27 l i t e r a t u r e s u p p o r t s t h i s p o s s i b i l i t y . Okada e t a l . have q u o t e d a s t u d y by Subramanyam and S t a e h l e w hich showed t h a t c r a c k i n g mode depended on e l e c t r o d e p o t e n t i a l . At t r a n s p a s s i v e p o t e n t i a l s , i n t e r g r a n u l a r c r a c k i n g was f a v o r e d i n 304 s t a i n l e s s 108 s t e e l i n 70% NaOH but a t t h e a c t i v e - p a s s i v e p o t e n t i a l t r a n s g r a n u l a r c r a c k i n g p r e d o m i n a t e d . The c r a c k i n g mode, i n NaOH s o l u t i o n s d i d not change w i t h t e m p e r a t u r e ( F i g u r e s 26 and 30) i n s p i t e o f t h e s u b s t a n t i a l r ange o f c r a c k growth r a t e . The ab s e n c e o f an e f f e c t i n d i ^ c a t e d t h a t change i n c r a c k i n g mode a t h i g h Kj v a l u e s a t 92 °C was n o t due t o h i g h e r c r a c k growth r a t e a t t h o s e h i g h e r s t r e s s i n t e n s i t i e s . C a u s t i c c o n c e n t r a t i o n a f f e c t e d t h e mode o f c r a c k i n g . F i g u r e 31 i l l u s t r a t e s t h a t some t r a n s g r a n u l a r c r a c k i n g o c c u r -r e d i n 12 mol/kg NaOH pe r h a p s when the o r i e n t a t i o n was f a v o r -a b l e . A c l u e t o u n d e r s t a n d i n g t h e change i n f r a c t o g r a p h y w i t h c o n c e n t r a t i o n s m i g h t be f o u n d i n t h e v a l u e o f t h e a c t i v a t i o n e n e r g y . T h a t v a l u e s u g g e s t e d t h a t i n t h e c o n c e n t r a t e d s o l u -t i o n , d i f f u s i o n was p l a y i n g a l a r g e r r o l e i n mixed c o n t r o l o f d i s s o l u t i o n . T h i s may i n t u r n have a f f e c t e d t h e f r a c t o g r a p h y . S u r f a c e i m p e r f e c t i o n s a t g r a i n b o u n d a r i e s w h i c h may have been i m p o r t a n t i n i n c r e a s i n g d i s s o l u t i o n r a t e f o r a c t i v a t i o n c o n -t r o l l e d d i s s o l u t i o n may have been l e s s i m p o r t a n t i n a c r a c k i n g p r o c e s s a l r e a d y s l o w e d down by d i f f u s i o n i n f l u e n c e s . Thus t r a n s g r a n u l a r c r a c k i n g c o u l d be as f a v o r a b l e as i n t e r g r a n u l a r . In c o n t r a s t t o t h e i n t e r g r a n u l a r c r a c k i n g i n t h e NaOH, th e f r a c t u r e s u r f a c e s i n t h e 12 mol/kg NaOH + 0.423 mol/kg Na^S s o l u t i o n e x h i b i t e d mixed i n t e r g r a n u l a r - t r a n s g r a n u l a r 109 c r a c k i n g . The l a r g e r p r o p o r t i o n o f t r a n s g r a n u l a r c r a c k i n g i n the s u l f i d e c o n t a i n i n g s o l u t i o n t h a n i n t h e s t r a i g h t NaOH s o l u t i o n may have r e s u l t e d from t h e p r e s e n c e o f t h e s u l f i d e i o n s . The s u l f i d e may have a d s o r b e d on t h e l a t t i c e imper-f e c t i o n s a t the g r a i n b o u n d a r i e s t h e r e b y d e c r e a s i n g t h e a d s o r p -t i o n o f 0H~ and l e s s e n i n g t h e d i s s o l u t i o n r a t e a t t h e g r a i n 2-b o u n d a r i e s . I f a d s o r p t i o n o f S r e d u c e s d i s s o l u t i o n r a t e t h e n , s i m i l a r l y , i t may be r e s p o n s i b l e f o r the s l o w e r c r a c k growth r a t e i n t h e NaOH + Na 2S s o l u t i o n . A l t e r n a t e l y , t h e d i f f e r e n c e i n f r a c t o g r a p h y may have r e s u l t e d from t e s t i n g i n a much d i f f e r e n t p o t e n t i a l r a n g e . Hydrogen e m b r i t t l e m e n t was p o s s i b l e a t t h i s p o t e n t i a l and 2_ may have c a u s e d t h e t r a n s g r a n u l a r c r a c k i n g . S may have been i m p o r t a n t i n c a u s i n g h y drogen e m b r i t t l e m e n t by p o i s o n i n g h y d r o -gen r e c o m b i n a t i o n . Lack o f e x p e r i m e n t a l r e s u l t s a t t h e s e c o n -d i t i o n s has p r e c l u d e d e v a l u a t i o n o f t h i s p o s s i b i l i t y . In c o n t r a s t to the p r e s e n t r e s u l t , A s a r o 3 4 found t h a t 304 s t a i n l e s s s t e e l showed m o s t l y i n t e r g r a n u l a r c r a c k i n g i n 50% NaOH w i t h 0.03 m o l / l i t e r Na 2S a t 180 °C. The d i f f e r e n c e 2-may have r e s u l t e d from e i t h e r l o w e r S c o n c e n t r a t i o n o r d i f -f e r e n t p o t e n t i a l (unknown). 4.5 E l e c t r o n D i f f r a c t i o n A n a l y s i s o f S u r f a c e F i l m s i n NaOH The e l e c t r o n d i f f r a c t i o n r i n g p a t t e r n s were c o n s i s t e n t n o 71 72 w i t h a s p i n e l t y p e o f s t r u c t u r e ' but t h e l a t t i c e para-m e t e r s were g e n e r a l l y a l i t t l e h i g h e r t h a n t h a t e x p e c t e d from 91 ° p u b l i s h e d d a t a f o r Fe-b.ased s p i n e l s e.g. 8.50 A v e r s u s o ^ 8.38 A f o r Fe^O^. The d i f f e r e n c e may have been due t o a d i f -f e r e n t v a l e n c y s t a t e o r an i r r e g u l a r l a t t i c e a r r a n g e m e n t h e r e . N i k i f o r u k 7 0 o b t a i n e d r e a s o n a b l e v a l u e s f o r s p i n e l l a t t i c e p a r a m e t e r s formed on s t a i n l e s s s t e e l s i n MgCl^ u s i n g t h e same method. T h e r e were some f u n d a m e n t a l p r o b l e m s w i t h t h i s method o f a n a l y s i s . The f i l m a n a l y z e d was not a r e p r e s e n t a t i v e sample o f t h e f i l m on t h e f r a c t u r e s u r f a c e . Much o f t h e s u r f a c e d e p o s i t was t h i c k and as su c h was opaque t o e l e c t r o n s . The p a t t e r n s may n o t have been f o r m a t e r i a l from t h e f r a c t u r e s u r f a c e but from t h e s i d e s o f t h e s p e c i m e n . The method 72 d e s c r i b e d by B i r l e y f o r o b t a i n i n g e l e c t r o n d i f f r a c t i o n p a t t e r n s from t h e u n s t r i p p e d f r a c t u r e s u r f a c e would a v o i d t h e s e p r o b l e m s . In t h a t method, t h e e l e c t r o n beam p a s s e s t h r o u g h o x i d e s on s u r f a c e a s p e r i t i e s . The s u r f a c e f i l m was t e n t a t i v e l y i d e n t i f i e d as Fe^P^. E-pH d i a g r a m s i n d i c a t e t h a t FeOOH m i g h t be formed a t p r i m a r y t r a n s p a s s i v e p o t e n t i a l s . The FeOOH may have d e h y d r a t e d on removal from t h e s o l u t i o n o r i n s u b s e q u e n t h a n d l i n g t o form H F e 5 0 8 ( Y - F e 2 0 3 ) ^ v i a : 5 FeOOH H F e 5 0 8 + 2H 20 ...(13) I l l 7 2 B i r l e y has n o t e d t h a t i t i s a l m o s t i m p o s s i b l e t o d i s t i n g u i s h between y-Fe^O^ ( H F e 5 0 g ) and F e 3 0 4 by t h e e l e c t r o n d i f f r a c t i o n t e c h n i q u e b e c a u s e o f t h e l i m i t a t i o n s i n t h e a c c u r a c y o f t h e t e c h n i que. 5. SUMMARY P o l a r i z a t i o n s t u d i e s , s l o w s t r a i n r a t e t e s t s , f r a c t u r e m e c h a n i c s t e s t s , e l e c t r o n d i f f r a c t i o n , and f r a c t o g r a p h i c t e c h n i q u e s have been employed t o o b t a i n e n g i n e e r i n g d a t a and to r e v e a l t h e mechanism o f cr.ackihgj o f 316 s t a i n l e s s s t e e l i n c a u s t i c s o l u t i o n s . In NaOH s o l u t i o n , c r a c k i n g o c c u r r e d i n t h e p r i m a r y t r a n s p a s s i v e p o t e n t i a l range where t h e chromium p a s s i v e f i l m 2 -was u n s t a b l e and d i s s o l u t i o n o f chromium t o CrO^ was o c c u r -r i n g . At a c t i v e - p a s s i v e p o t e n t i a l s , o n l y s e n s i t i z e d m a t e r i a l showed some s u s c e p t i b i l i t y , and t h a t may have r e s u l t e d f r o m f i l m i n s t a b i l i t y and i r o n d i s s o l u t i o n . The mechanism o f c r a c k i n g a t p r i m a r y t r a n s p a s s i v e p o t e n t i a l s i n NaOH s o l u t i o n a p p e a r e d t o be d i s s o l u t i o n , which may i n v o l v e an a d s o r p t i o n s t e p . T h e r e was e v i d e n c e t h a t d i f f u s i o n i n t h e l i q u i d phase a l s o a f f e c t e d c r a c k i n g k i n e t i c s a t p o t e n t i a l s i n t h e s e c o n d a r y p a s s i v e r e g i o n . In t h e NaOH + Na 2S s o l u t i o n s , c r a c k i n g o c c u r r e d a t a c t i v e p o t e n t i a l s . F i l m i n s t a b i l i t y a t t h e s e - p o t e n t i a l s may have r e s u l t e d from the p re sence o f s u l f i d e . Hydrogen e m b r i t t l e m e n t c o u l d not be r u l e d out at a c t i v e - p a s s i v e p o t e n t t i a 1 s . BIBLIOGRAPHY 11 3 1. J . E. Truman and R. P e r r y , B r i t . C o r r . J . 1 pp.60-66 ( 1 9 6 6 ) . 2. P. P. Snowden, J . I . S . I . . 197 pp. 1 36-141 (1 961 ). 3. P. P. Snowden, J . 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