Kinetics of oxide formation in liquid copper and silver

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LURASCHI

AND JOHN

F. ELLIOTT

The p r o c e s s of f o r m a t i o n of oxide p h a s e s in liquid copper and liquid s i l v e r has b e e n s t u d ied b y p u m p i n g oxygen into the liquid alloy with the use of a s o l i d - e l e c t r o l y t e e l e c t r o c h e m ical cell. The oxygen potential at the e l e c t r o l y t e - m e t a l i n t e r f a c e is m o n i t o r e d s i m u l t a n e o u s l y by the e l e c t r o c h e m i c a l cell. A wide r a n g e of effects were o b s e r v e d r a n g i n g f r o m the v e r y r a p i d f o r m a t i o n of a l a y e r of oxide at the e l e c t r o l y t e - m e t a l i n t e r f a c e to what a p p e a r s to b e homogeneous n u c l e a t i o n of the oxide in the m e t a l . The r e s u l t s m a y help to explain some of the difficulties that s o m e t i m e s have b e e n o b s e r v e d in u s i n g this type of cell to m e a s u r e the oxygen potential of a liquid m e t a l . The r e s u l t s indicate that a s u p e r s a t u r a t i o n r a t i o of about 9 is n e c e s s a r y for homogeneous n u c l e a t i o n of i r o n oxide (most p r o b a b l y Fe304(s)) in liquid copper c o n t a i n i n g 0.01 to 0.07 pet iron. The i n t e r f a c i a l t e n s i o n (XCu_Fe304 is c a l c u l a t e d to be 0.74 J / m 2. In e x p e r i m e n t s with higher c o n c e n t r a t i o n s of deoxidant (0.2 pet Fe in Cu and 0.2 to 0.4 pet Ni i n Ag) e q u i l i b r i u m p r e c i p i t a t i o n of oxides a p p a r e n t l y p r e d o m i n a t e s over homogeneous n u c l e a t i o n for the e x p e r i m e n t a l conditions employed. A m a t h e m a t i c a l model which p a r t l y explains the different effects o b s e r v e d is p r e s e n t e d . k i n e t i c s of n u c l e a t i o n and growth of oxides in liquid m e t a l s a r e i m p o r t a n t i n r e f i n i n g o p e r a t i o n s , e s p e c i a l l y in deoxidation of s t e e l . Because of this, much effort has b e e n spent i n t r y i n g to c h a r a c t e r i z e the n u cleation p r o c e s s in m e t a l l i c m e l t s . This is a difficult e x p e r i m e n t a l task, s i n c e the a i m is to m e a s u r e a p r o c e s s o c c u r r i n g in a homogeneous melt, and the s e n s ing device used in a d i r e c t m e a s u r e m e n t i n t r o d u c e s h e t e r o g e n e i t i e s . The a l t e r n a t i v e is an i n d i r e c t m e a s u r e m e n t , which p r o d u c e s i n d i r e c t r e s u l t s , and its i n t e r p r e t a t i o n is complex and s u b j e c t to s e r i o u s e r r o r . As a p a r t of a r e s e a r c h p r o g r a m intended to a s s e s s the i m p o r t a n c e of homogeneous n u c l e a t i o n in deoxidation of s t e e l , l-s Sigworth and E l l i o t t 4,s developed a d i r e c t technique to m e a s u r e the n u c l e a t i o n p r o c e s s . It used a solid e l e c t r o l y t e e l e c t r o c h e m i c a l cell both to pump oxygen into a melt, and to m e a s u r e the oxygen c o n c e n t r a t i o n in the liquid m e t a l at the m e t a

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