The influence of sulfides on the oxidation behavior of nickel-base alloys
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F. S. PETTIT
Th e o x i d a t io n of p r e s u l f i d i z e d c h r o m i u m , N i - C r , and Ni-A1 a l l o y s , and c o m p l e x n i c k e l b a s e a l l o y s w a s studied at 1000~ in 1.0 a t m of o x y g e n . S u l f u r - r i c h s u r f a c e l a y e r s w e r e p r o d u c e d in the p r e t r e a t m e n t by u s in g H2S-Hz m i x t u r e s . P r e s u l f i d i z e d c h r o m i u m o x i d i z e d at a r a t e s i m i l a r to that of s u l f u r - f r e e c h r o m i u m . T h e o x i d a t i o n r a t e of p r e s u l f i d i z e d N i - C r a l l o y s w a s a f f e c t e d by s u l f u r only when liquid n i c k e l s u l f i d e w as p r e s e n t w h i c h a c c e l e r a t e d the o x i d a tion r a t e by c r e a t i n g r a p i d diffusion paths t h r o u g h the Cr2Os s c a l e . Th e oxidation b e h a v i o r of p r e s u l f i d i z e d Ni-A1 a l l o y s , with a l u m i n u m c o n t e n t s sufficient f o r the f o r m a t i o n of a p r o t e c t i v e A12Oa l a y e r in t h e s u l f u r - f r e e c o n d i t i o n , w a s influenced by s u l f u r only when a l u m inum sulfide was f o r m e d in the p r e s u l f i d a t i o n t r e a t m e n t which c a u s e d the AlzOs s c a l e to be p o r o u s . The o x i d a ti o n b e h a v i o r of n i c k e l - b a s e a l l o y s containing both c h r o m i u m and a l u m inum was i n s e n s i t i v e to the p r e s e n c e of s u l f i d e s when the c o n c e n t r a t i o n of a l u m i n u m in t h e a l l o y was such that a p r o t e c t i v e AlzOs s c a l e w a s f o r m e d during o x i d a t i o n of the s u l f u r - f r e e a l l o y and a l u m i n u m s u l f i d e w a s not f o r m e d in the p r e s u l f i d i z i n g t r e a t m e n t .
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a p r e v i o u s p a p e r I it h a s b e e n shown that the Na2SO4induced a c c e l e r a t e d o x i d a ti o n of p u r e n i c k e l is c a u s e d by r e a c t i o n b e t w e e n NiO and o x i d e ions in the Na2SO4 ( i. e. Na20) r a t h e r than by o x i d a t i o n of the n i c k e l s u l fide w h i c h is a l s o p r e s e n t . In t h a t p a p e r the i n f l u e n c e of s u l f u r on the oxidation r a t e of p u r e n i c k e l w a s e x a m i n e d by o x i d i z i n g n i c k e l s p e c i m e n s that had b e e n p r e s u l f i d i z e d in a H2S-H2 g a s m i x t u r e . In the Na2SO4induced a c c e l e r a t e d o x i d a ti o n of n i c k e l - b a s e s u p e r a l l o y s , s u l f i d e p h a s e s a r e c o m m o n l y p r e s e n t in the a l l o y s u b s t r a t e . To e x a m i n e the i n f l u e n c e of s u l f id e p h a s e s on t h e o x i d a t i o n p r o p e r t i e s of n i c k e l - b a s e a l l o y s , the o x i d a t i o n b e h a v i o r of p r e s u l f i d i z e d c h r o m i u m , N i - C r a l l o y s , Ni-A1 a l l o y s , N i - 2 5 C r - f i A 1 , and t h r e e n i c k e l b a s e s u p e r a l l o y s (B- 1900, W a s p a l o y , U d i m e t - 7 0 0 ) h a s been studied. EXPERIMENTAL A l l o y s and m e t a l s u s e d in t
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