Effects of transient carbon deposition on the sodium sulfate-induced corrosion of nickel-base alloys
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D. W. McKEE and G. ROMEO are Physical Chemists at General Electric Research and Development Center, Schenectady, N. Y. 102301. Manuscript submitted March 8, 1974. METALLURGICALTRANSACTIONSA
hot c o r r o s i o n has g e n e r a l l y b e e n r e g a r d e d as an oxidation phenomenon. The m o s t widely held view, 7 is that the fluxing action of the deposited s o d i u m s u l f a t e p r o m o t e s the d i s s o l u t i o n of p r o t e c t i v e oxide l a y e r s on the alloy s u r f a c e . It has b e e n d e m o n s t r a t e d 7 that this s o l u b i l i t y effect is s t r o n g l y dependent on the oxide ion a c t i v i t y in the s a l t phase. The oxide ion a c t i v i t y of the sulfide c o n d e n s a t e can be s h a r p l y d e c r e a s e d by m e t a l s such as m o l y b d e n u m , t u n g s t e n o r v a n a d i u m , the p r e s e n c e of which in the a l loy r e s u l t s in c a t a s t r o p h i c c o r r o s i o n r a t e s when sulfate deposition o c c u r s . In the a b s e n c e of t h e s e e l e m e n t s , a c c e l e r a t e d s a l t - i n d u c e d c o r r o s i o n of n i c k e l alloys c a n also o c c u r as a r e s u l t of the r e a c t i o n b e t w e e n Na2SO4 and n i c k e l which c a u s e s s u l f u r to diffuse into the alloy, l e a d i n g to an i n c r e a s e in the oxide ion a c t i v i t y and d i s s o l u t i o n of o t h e r w i s e p r o t e c t i v e oxide s e a I e s , a c c o r d ing to Pettit and c o - w o r k e r s . 7 C o n c u r r e n t l y n i c k e l and c h r o m i u m sulfides a r e f o r m e d b e n e a t h the alloy s u r face. It is not c l e a r , however, how this explanation would apply to the a c c e l e r a t e d c o r r o s i o n o b s e r v e d with nickel alloys that have b e e n p r e - o x i d i z e d to f o r m c o h e r e n t d e n s e oxide l a y e r s , as the s o l u b i l i t y of NiO and Cr203 in s t o i c h i o m e t r i c Na2SO4 is v e r y low under o v e r all oxidizing c o n d i t i o n s . 2'9 However, in the p r e s e n c e of solid c a r b o n , b e t w e e n 1073 and 1273 K (800 and 1000~ NiO is r e a d i l y r e d u c e d to the m e t a l l i c state, 1~ CrsOs is r e d u c e d to CrO 11 and Na2SO~ is c o n v e r t e d to sodium sulfide, x2 so that p e n e t r a t i o n of s u l f u r into the alloy is facilitated. P r e v i o u s p a p e r s in this s e r i e s 12'~s have d e s c r i b e d the effects of m e t h a n e on the s u l f u r dioxide- and sodium s u l f a t e - i n d u c e d c o r r o s i o n of n i c k e l alloys. It was d e m o n s t r a t e d that the a l l o y s u r f a c e can act as a c a t a l y s t in d e c o m p o s i n g the gaseous h y d r o c a r b o n to solid c a r b o n at a r a t e which is dependent on the c h e m i c a l c o m p o s i tion of the alloy. The p r e s e n t p a p e r e x p l o r e s the s i g n i f i c a n c e of the f o r m and s t r u c t u r e of the c a r b o n d e posit in d e t e r m i n i n g c o r r o s i o n r a t e s u n d e r the o v e r a l l oxidizing c o n d i t i
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