Phase instability of manganese-iron carbides
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long b e e n known that c a r b o n s a t u r a t e d f e r r o manganese decrepitates during atmospheric storage when its i r o n content is not s u f f i c i e n t l y high. R o b i e t t e 1 s t a t e s that many R u s s i a n m a n g a n e s e o r e s , low in iron, have to be m e l t e d with s c r a p to attain the c o r r e c t i r o n content in the f i n al p r o d u c t if that p r o d u c t is to be s t a b l e d u r i n g s t o r a g e . H o w e v e r , to date no s a t i s f a c t o r y e x p l a n a t i o n has b e e n o f f e r e d on the r o l e of i r o n in s t a bilizing carbon saturated f e r r o m a n g a n e s e during storage. No w o r k has b e e n done to d e t e r m i n e the r o o m t e m p e r a t u r e s t a b i l i t y in d r y and m o i s t a i r and in w a t e r of c a r b o n s a t u r a t e d m a n g a n e s e - i r o n a l l o y s in w h i ch the i r o n content was v a r i e d in a s y s t e m a t i c m a n n e r . T a n a b e et alZ found that c a r b o n s a t u r a t e d f e r r o m a n g a n e s e d e c r e p i t a t e s in wet a i r if the i r o n content is l e s s than 5 pct* but is s t a b l e if the i r o n content is in e x c e s s *Concentrationsare expressedin wt pct throughout. of 5 pct. When they r e a c t e d the c a r b o n s a t u r a t e d f e r r o m a n g a n e s e with w a t e r , no d i s i n t e g r a t i o n was o b s e r v e d when the i r o n content was h i g h e r than 10 pct. T o p c h i a s h v i l i 3 a t t r i b u t e s the e f f e c t of i r o n on the s t a b i l i t y of c a r b o n s a t u r a t e d f e r r o m a n g a n e s e to the f o r m a t i o n of a d e n s e and s t r o n g p r o t e c t i v e l a y e r on the s u r f a c e of the c a r b i d e . H o w e v e r , no i n f o r m a t i o n is given about the n a t u r e of this l a y e r . He o b s e r v e d no d e c r e p i t a t i o n a f t e r t r e a t m e n t in w a t e r o r a KOH solution when the i r o n content of the f e r r o m a n g a n e s e was in e x c e s s of 10 pct. M y e r s and F i s h e l 4 s tu d i e d the h y d r o l y s i s of a m a n g a n e s e c a r b i d e to wh ic h they a s s i g n e d the f o r m u l a MnaC. H o w e v e r , in the light of the c u r r e n t l y a c c e p t e d phase d i a g r a m and b e c a u s e of the way in which they p r e p a r e d t h e i r c a r b i d e s a m p l e s , it is l i k e l y that they s t u d i e d a n o t h e r c a r b i d e and not Mn3C. A c c o r d i n g to M y e r s and F i s h e l the r e a c t i o n of m a n g a n e s e c a r b i d e with w a t e r at t e m p e r a t u r e s b e t w e e n 20 and 90~ y i e l d s Mn(OH)2 and Hz, CH4, and C2H6 as g a s e o u s p r o d u c t s . In p r a c t i c e , d e c r e p i t a t i o n of c a r b o n s a t u r a t e d f e r r o m a n g a n e s e o c c u r s m a i n l y v i a r e a c t i o n with the w a t e r v a p o r in the a t m o s p h e r e . In c a s e of s t o r a g e in the open, r e a c t i o n with r a i n w a t e r a l s o p la y s a r o l e . F o r m o s t of th
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