Thermodynamics and phase relationships of transition metal-sulfur systems: I. The cobalt-sulfur system
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p r e s e n c e of s u l f u r - b e a r i n g gases in h i g h - t e m p e r a t u r e s y s t e m s such as gas t u r b i n e s , coal g a s i f i e r s , b o i l e r s , and so forth m a y pose a s e r i o u s c o r r o s i o n p r o b l e m . In o r d e r to u n d e r s t a n d the m e c h a n i s m of c o r r o s i o n due to s u l f u r attack with the u l t i m a t e goal of m i n i m i z i n g c o r r o s i o n , it is e s s e n t i a l to have a thorough knowledge of the t h e r m o d y n a m i c p r o p e r t i e s and phase e q u i l i b r i a of the m e t a l - s u l f u r s y s t e m s i n volved. With this in mind, a r e s e a r c h p r o g r a m was i n i t i a t e d to study the t h e r m o d y n a m i c s and phase r e l a t i o n s h i p s of b i n a r y and t e r n a r y m e t a l s u l f u r s y s t e m s with the m e t a l b e i n g e i t h e r i r o n , nickel, cobalt o r c h r o m i u m . T h e s e four m e t a l s were chosen for the i n i t i a l study b e c a u s e they a r e i m p o r t a n t a l l o y i n g e l e m e n t s in i n d u s t r i a l m a t e r i a l s . The p r e s e n t p a p e r on the c o b a l t - s u l f u r s y s t e m is the f i r s t of a s e r i e s of p a p e r s d e a l i n g with the t h e r m o d y n a m i c s and p h a s e r e l a t i o n s h i p s of b i n a r y and t e r n a r y m e t a l - s u l f u r s y s tems. The b i n a r y Co-S phase d i a g r a m given by H a n s e n and A n d e r k o ~ is e s s e n t i a l l y c o r r e c t and has b e e n updated by Elliott 2 and Shunk. 3 T h e r e a r e five i n t e r m e d i a t e p h a s e s i n this s y s t e m . They a r e C04S3~y, C09S8, C01-yS, C03S4 and COS2. The C04S3• and Cox-yS p h a s e s have r a t h e r l a r g e r a n g e s of h o m o g e n e i t y and a r e stable only at high t e m p e r a t u r e s . The s u b s c r i p t s (3 + y) and (1 - y) a r e used to designate the d e v i a t i o n s f r o m s t o i c h i o m e t r i c c o m p o s i t i o n s for these two p h a s e s . Rau, 4 R o s e n q v i s t , 5 Laffitte, 6 Kolbina, B a r b a n e l , N a z a r ova and A r i y a , 7 and L e e g a a r d and R o s e n q v i s t 8 m e a s u r e d the activity of s u l f u r as a function of t e m p e r a t u r e and c o m p o s i t i o n in the Co-S s y s t e m . However, none of t h e s e i n v e s t i g a t o r s made a n e x t e n s i v e study for the C04S3+y phase at a l l t e m p e r a t u r e s and for the C o , - y S p h a s e at lower t e m p e r a t u r e s . The p u r p o s e s of the p r e s e n t study a r e 1) to d e t e r m i n e the a c t i v i t i e s of s u l f u r in C04Ss~y, C09S8 and Co~_yS, 2) to e v a l u a t e the data obtained in the p r e s e n t study and those a v a i l a b l e in the l i t e r a t u r e for these t h r e e p h a s e s as well as for data in the l i t e r a t u r e f o r other phases so that t h e r m o d y n a m i c equations of s t a t e may be d e r i v e d for a l l the Y. O. CHEN, formerly Research Assistant, MaterialsDepartment, College of Engineeringand
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