Stability of chromium (III) sulfate in atmospheres containing oxygen and sulfur
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T H E c o r r o s i o n of a l l o y s e x p o s e d to e n v i r o n m e n t s c o n t a i n i n g o x y g e n a n d s u l f u r can b e a n a l y z e d with the aid of c h e m i c a l p o t e n t i a l d i a g r a m s . Such d i a g r a m s a r e e i t h e r p l o t s of 1) the l o g a r i t h m of the p a r t i a l p r e s s u r e of s u l f u r a s a f u n c t i o n of nA//(nA + riB), the m o l e f r a c t i o n of the m e t a l l i c c o m p o n e n t A , f o r a q u a r t e r n a r y s y s t e m A-B-S-O a t a c o n s t a n t t e m p e r a t u r e and o x y g e n p o t e n t i a l , o r 2) the l o g a r i t h m of the p a r t i a l p r e s s u r e of o x y g e n a s a f u n c t i o n of hA~ (nA + nB) a t a c o n s t a n t t e m p e r a t u r e and s u l f u r p o t e n t i a l . In the c a s e of the f i v e - c o m p o n e n t s y s t e m , A - B C-S-O, the c o m p o s i t i o n of m e t a l l i c c o m p o n e n t s A , B , and C c a n be r e p r e s e n t e d on a G i b b s ' t r i a n g l e with the p a r t i a l p r e s s u r e of o x y g e n p l o t t e d p e r p e n d i c u l a r to the p l a n e of the t r i a n g l e . To construct such chemical potential plots, reliable t h e r m o d y n a m i c d a t a a r e n e e d e d for all the p h a s e s involved. Unfortunately, thermodynamic data for some of the b i n a r y and t e r n a r y o x i d e and s u l f i d e p h a s e s a r e not a v a i l a b l e in the l i t e r a t u r e . S o m e of the m i s s Lug d a t a can b e e s t i m a t e d with a c c e p t a b l e a c c u r a c y u s ing c u r r e n t m o d e l s and r e c e n t l y e s t a b l i s h e d c o r r e l a tions. Using such techniques, sulfur potential plots w e r e d e v e l o p e d to e x p l a i n the s u l f i d a t i o n b e h a v i o r of SAE 310 s t a i n l e s s s t e e l .1 H o w e v e r , t h e s e c o r r e l a t i o n s a r e not well e s t a b l i s h e d f o r c o m p o u n d s c o n t a i n i n g both o x y g e n and s u l f u r ( s u c h a s o x y s u l f i d e s a n d s u l -
f a t e s ) . C o n s e q u e n t l y , the only w a y of o b t a i n i n g the inf o r m a t i o n f o r o x y s u l f i d e s and s u l f a t e s is by c o n d u c t i n g p r o p e r experiments. A s a p a r t of a p r o g r a m on the c o r r o s i o n of the int e r n a l c o m p o n e n t s of g a s i f i e r s i t b e c a m e n e c e s s a r y to d e v e l o p the c h e m i c a l p o t e n t i a l d i a g r a m s f o r i n d u s t r i a l a l l o y s in m u l t i c o m p o n e n t g a s e o u s e n v i r o n m e n t s . It w a s f o u n d that no t h e r m o d y n a m i c d a t a f o r c h r o m i u m (IH) s u l f a t e w e r e r e p o r t e d in the l i t e r a t u r e and h e n c e t h e s e experimental studies were undertaken. This c o m m u n i c a t i o n r e p o r t s the m e a s u r e d f r e e e n e r g i e s of f o r m a t i o n of c h r o m i u m (IH) s u l f a t e . EXPERIMENTAL MATERIALS
A n h y d r o u s c h r o m i u m s u l f a t e , Cr2(SO4)3, w a s p r e p a r e d by t h r e
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