Steady-state electrotransport of carbon in iron
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E l e c t r o t r a n s p o r t of c a r b o n in r - F e was s t u d i e d by a s t e a d y - s t a t e method. P r e v i o u s e x p e r i m e n t a l s t u d i e s of e l e c t r o t r a n s p o r t in solid m e t a l l i c s y s t e m s have b e e n l i m i t e d to the use of u n s t e a d y s t a t e methods. The s t e a d y - s t a t e d i s t r i b u t i o n in a solid s y s t e m can be a t t a i n e d in a r e a s o n a b l e t i m e p r o v i d e d c e r t a i n conditions a r e fulfilled. This method has the advantage that the effective c h a r g e , Z*, can be e v a l u a t e d d i r e c t l y f r o m the r a t i o of the c h e m i c a l c o m p o s i t i o n s at the anode and cathode ends without involving the c h e m i c a l diffusivity. C 14 was i n t r o d u c e d into A r m c o i r o n s p e c i m e n s by c a r b u r i z i n g . After e l e c t r o t r a n s p o r t for a s u i t a b l e t i m e , the s t e a d y - s t a t e c o n c e n t r a t i o n - d i s t a n c e c u r v e was d e t e r m i n e d by the method of r e s i d u a l r a d i o a c t i v i t y . The effective c h a r g e , Z*, of c a r b o n in r - F e u n d e r the e x p e r i m e n t a l conditions e m p l o y e d (0.11 to 0.50 wt pct C, 927 ~ to 1027~ and 1100 to 2500 amp p e r sq c m c u r r e n t density) is +3.99 + 0.08. No d e p e n d e n c e of Z* on c a r b o n content, t e m p e r a t u r e , or c u r r e n t d e n s i t y was detected. Although the v a l u e Z* = + 4 c o i n c i d e s with the c l a s s i c a l v a l e n c e of c a r b o n , the m e c h a n i s m of t r a n s p o r t p r o b a b l y i n v o l v e s the e l e c t r o n i c c a r r i e r s as well as the field f o r c e on the s c r e e n e d c a r b o n ion. P R E V I O U S s t u d i e s of e l e c t r o t r a n s p o r t in solid m a t e r i a l s have b e e n made using an u n s t e a d y - s t a t e method. 1 Although the m a t h e m a t i c a l r e l a t i o n s for p o s s i b l e s t e a d y - s t a t e p r o c e d u r e s a r e well known, ~ only s t u d i e s of r a p i d l y - d i f f u s i n g g a s e s in m e t a l s s have been r e p o r t e d in the l i t e r a t u r e . The p r e s e n t r e s e a r c h d e m o n s t r a t e s the f e a s i b i l i t y of the s t e a d y - s t a t e method and m a k e s full use of the fact that the o r d i n a r y diffusion coefficient, D, does not e n t e r into the q u a n t i t a t i v e d e t e r m i n a t i o n of the effective c h a r g e , Z*, the e s s e n t i a l p a r a m e t e r of e l e c t r o t r a n s p o r t . The following b r i e f r e v i e w of e l e c t r o t r a n s p o r t in the F e - C s y s t e m r e f e r s to c o n v e n t i o n a l u n s t e a d y - s t a t e e x p e r i m e n t s . The f i r s t s t u d i e s w e r e made over t h i r t y y e a r s ago 4 and, although n u m e r o u s i n v e s t i g a t i o n s have b e e n r e p o r t e d s i n c e that t i m e , 5 the e s s e n t i a l n a t u r e of e l e c t r o t r a n s p o r t in this s y s t e m r e m a i n s u n c l
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