Effect of segregated sulfur on the stress corrosion susceptibility of nickel
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i n v e s t i g a t i o n s which have suggested that the p r e s e n c e of s e g r e g a t e d i m p u r i t i e s in g r a i n b o u n d a r i e s cause i n t e r g r a n u l a r c o r r o s i o n and i n t e r g r a n u l a r s t r e s s c o r r o s i o n c r a c k i n g (IGSCC). Most of these s t u d i e s have been on s t a i n l e s s s t e e l s . N o n s e n s i t i z e d a u s t e n i t i c s t a i n l e s s s t e e l has been shown to c o r r o d e i n t e r g r a n u l a r l y in b o i l i n g solutions of HNO3 which c o n t a i n oxidizing ions such as d i c h r o m a t e . ~-~ The p r e s e n c e of s e g r e g a n t s in the g r a i n b o u n d a r i e s was d e t e r m i n e d by i n d i r e c t m e a n s such as m i c r o h a r d n e s s m e a s u r e m e n t s or by c o n t r o l l e d additions to high p u r i t y a l l o y s . J o s h i and Stein s have m e a s u r e d the g r a i n b o u n d a r y c o m p o s i t i o n of solution a n n e a l e d Type 304 s t a i n l e s s s t e e l and have found l a r g e q u a n t i t i e s of s u l f u r which they c o r r e l a t e with s u s c e p t i b i l i t y to i n t e r g r a n u l a r attack. High n i c k e l alloys a r e s u s c e p t i b l e to IGSCC as well. 7,s A s the n i c k e l content is i n c r e a s e d , the r a t e of intergranular corrosion also increases. This interg r a n u l a r c o r r o s i o n s e n s i t i v i t y is not r e l a t e d to the p r e c i p i t a t i o n of c h r o m i u m c a r b i d e . Sulfur s e g r e g a t i o n has b e e n suggested as the p r o b a b l e c a u s e . T h i s study e x a m i n e s the effects of s e g r e g a t e d s p e cies on the c h e m i c a l - m e c h a n i c a l s t a b i l i t y of g r a i n b o u n d a r i e s of n i c k e l . It has long b e e n known that s e g r e g a t e d i m p u r i t i e s can cause i n t e r g r a n u l a r f r a c t u r e in n i c k e l . Sulfur has been shown to be r e s p o n s i b l e for changes in d u c t i l i t y and yield s t r e n g t h . 9-n Other r e s u l t s suggest that the p r e s e n c e of s e g r e g a t e d i m p u r i t i e s in the b o u n d a r y which a r e hydrogen r e c o m b i n a t i o n p o i s o n s a r e r e s p o n s i b l e for i n t e r g r a n u l a r c r a c k i n g in the p r e s e n c e of hydrogen. 1~ However, t h e r e have b e e n few i n v e s t i g a t i o n s of the effects of s e g r e g a t e d i m p u r i t i e s on the s t r e s s c o r r o s i o n s u s c e p t i b i l i t y . T h e r e a r e s e v e r a l ways in which s e g r e g a t e d i m p u r i t i e s in g r a i n b o u n d a r i e s can affect the c h e m i c a l m e c h a n i c a l s t a b i l i t y . T h e s e s p e c i e s can d i s s o l v e , f o r m i n g high l o c a l i z e d c o n c e n t r a t i o n s of c o r r o s i v e ions which a t t a c k the c r a c k tip r e g i o n . The r e p a s s i v a tion r a t e of the r u p t u r e d p a s s i v e f i l m can be r e d u c e d c a u s i n g l o c a l i z e d r a p i
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