The effects of molybdenum additions to nickel-chromium alloys on their sulfidation properties
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W. W. SMELTZER,
AND
J. S. KIRKALDY
The e f f e c t of m o l y b d e n u m a d d i ti o n s to ) , - p h a s e N i - C r on the r e a c t i o n of the a l l o y s with H 2/H2S a t m o s p h e r e s at 700~ has b e e n i n v e s t i g a t e d . The e f f e c t is to r e d u c e the r a t e of i n t e r n a l s u l f i d a t i o n of the a l l o y s , d i m i n i s h g r e a t l y the e x t e n t of liquid s u l f i d e f o r m a t i o n and slow the g r o w t h r a t e of e x t e r n a l s o l i d s u l f i d e s c a l e . The f i r s t two e f f e c t s a r e t h e r m o d y n a m i c in o r i g i n w h i l s t the l a s t e f f e c t is due to f a v o r a b l e doping of CrzS 3 and to the f o r m a t i o n within a Cr3S 4 s c a l e of MoS s p r e c i p i t a t e s which p a r t i a l l y b l o c k o u t w a r d m e t a l diffusion.
"~-~OT c o r r o s i o n " of a s u p e r a l l o y l e a d s to the f o r m a tion of a t h i ck s c a l e on the a l l o y s u r f a c e . B e c a u s e o x ides a r e c o n s i d e r a b l y m o r e s t a b l e than the a n a l o g o u s s u l f i d e s and b e c a u s e the a c t i v i t y of s u l f u r in the gas p h a s e is in m a n y c a s e s m u c h l e s s than that of oxygen, the e x t e r n a l s c a l e is c o m p o s e d of o x i d e s . At the s c a l e a l l o y i n t e r f a c e , h o w e v e r , the o x y g e n a c t i v i t y is c o n t r o l l e d by the m e t a l - o x i d e e q u i l i b r i a r a t h e r than the e n v i r o n m e n t and is t h e r e f o r e v e r y low. If s u l f u r can penetrate the oxide s c a l e its a c t i v i t y can e x c e e d that of o x y g e n at the a l l o y s u r f a c e and in this c a s e i n t e r n a l s u l f i d a t i o n is to be e x p e c t e d . In fact, s u l f u r has b e e n found 1 to diffuse t h r o u g h o x i d e s , and i n t e r n a l s u l f i d a tion and e v e n the f o r m a t i o n of a s u l f i d e s u b s c a l e h a v e f r e q u e n t l y b e e n o b s e r v e d in s u p e r a l l o y hot c o r r o s i o n . F o r this r e a s o n the study of s u p e r a l l o y s u l f i d a t i o n (as d i s t i n c t f r o m hot c o r r o s i o n ) is of s o m e i n t e r e s t . The m a t r i x of n i c k e l - b a s e s u p e r a l l o y s is e s s e n t a i l l y a N i - C r ) ' - p h a s e , s o l u t i o n s t r e n g t h e n e d by the addition of r e f r a c t o r y m e t a l s . It is i m p o r t a n t to know how r e f r a c t o r y m e t a l a d d i t i o n s a f f e c t the s u l f i d a t i o n p r o p e r t i e s of the a l l o y s , h o w e v e r l i t t l e q u a n t i t a t i v e i n f o r m a tion is a v a i l a b l e . Attention is f o c u s s e d in this w o r k on the e f f e c t s of m o l y b d e n u m a d d i t i o n s . The s u l f i d a t i o n p r o p e r t i e s of the b i n a r y s y s t e m s C r Mo and N i - M o have both been i n v e s t i g a t e d . P u r e c h r o m i u m r e a c t s with H2/H~S a t m o s p h e r e (PS2 = 1 x 10 -3 afm) in the t e m p e r a t u r e r a n g e 8
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