Effect of sequential sulfidation and oxidation on the propagation of sulfur in an 85 Ni-15 Cr alloy
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S P E N G L E R AND R. VISWANATHAN
It has been p r o p o s e d in the l i t e r a t u r e that s u l l i d a t i o n of n i c k e l - b a s e alloys in the p r e s e n c e of Na2SO4-ash d e p o s i t s , is a s e l f - s u s t a i n i n g r e a c t i o n , in that once c h r o m i u m sulfides a r e f o r m e d in the m e t a l , s u b s e q u e n t oxidation of the sulfides can l i b e r a t e the s u l f u r and n u c l e a t e new c h r o m i u m sulfide p a r t i c l e s along a n a d v a n c i n g front. The a p p l i c a b i l i t y of this m e c h a n i s m to sulfidation r e a c t i o n s o c c u r r i n g in the a b s e n c e of Na2SO4-ash d e p o s i t s was i n v e s t i g a t e d by exposing s a m p l e s of a n 85 N i - 1 5 Cr alloy which had b e e n p r e s u l f i d i z e d in a SO2-N2 m i x t u r e , to a n O2-N2 m i x t u r e for 30, 200, and 300 h r at 1600~ A p r o g r e s s i v e i n w a r d m i g r a t i o n of c h r o m i u m s u l tides was r e v e a l e d by m e t a l l o g r a p h i c m e a s u r e r c e n t s . By c h e m i c a l a n a l y s i s the t o t a l a m o u n t of s u l f u r was found to be c o n s e r v e d in the s a m p l e even up to 300 h r of oxidation. T h e s e r e s u l t s a r e c o n s i s t e n t with the following s e l f - s u s t a i n i n g model. 2Cr3S4 + 9 0 ~
3Cr203 + 8S
8S+6Cr~2Cr3S4
HIGH t e m p e r a t u r e
c o r r o s i o n is well r e c o g n i z e d as a p o t e n t i a l p r o b l e m in m a r i n e and l a n d - b a s e d gas t u r b i n e s o p e r a t i n g in e x c e s s of 1400~ w h e r e , for econ o m i c r e a s o n s , the t u r b i n e s a r e run on low g r a d e c o n t a m i n a t e d r e s i d u a l fuels 1-7 C o m b u s t i o n of this low q u a l i t y fuel r e s u l t s in a c o m b i n a t i o n of s u l f a t e a s h ( g e n e r a l l y c o n t a i n i n g Na, Mg, and Ca) and s u l f u r b e a r i n g g a s e s , which c a u s e a s u l f i d a t i o n - a c c e l e r a t e d oxidation of the m e t a l c o m p o n e n t s , c o m m o n l y r e f e r r e d to a s hot c o r r o s i o n . L a b o r a t o r y i n v e s t i g a t i o n s8 - n on i r o n - b a s e and nickel-13ase h i g h - t e m p e r a t u r e a l l o y s u n d e r hot c o r r o s i o n conditions have indicated that the sulfidation r e a c t i o n s lead to enhanced r a t e s of o x i d a tion and that s u l f u r p e n e t r a t i o n into the b a s e m e t a l p e r s i s t s even a f t e r r e m o v a l of the s u l f u r - b e a r i n g g a s e s . T h e s e o b s e r v a t i o n s have led some i n v e s t i g a t o r s z'8'1~ to suggest that the s u l l i d a t i o n r e a c t i o n in t h e s e m a t e r i a l s is s e l f - s u s t a i n i n g . In g e n e r a l m a n y of the e a r l y i n v e s t i g a t i o n s w e r e of hot c o r r o s i o n r e a c t i o n s a s they o c c u r in the p r e s e n c e of NazSO4 dep o s i t s . S i m o n s , B r o w n i n g , and Liebhafsky 1~ p r o p o s e d that r e d u c t i o n of Na2SO4 by a n unspecified agent R co
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