The effect of oxygen on tantalum-sodium compatibility
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KLUEH
The effect of oxygen in both p h a s e s on the c o m p a t i b i l i t y of t a n t a l u m with s o d i u m was i n v e s t i g a t e d at 600~ by exposing t a n t a l u m s p e c i m e n s to liquid s o d i u m in static c a p s u l e s . I n c r e a s i n g the i n i t i a l oxygen c o n c e n t r a t i o n of the s o d i u m f r o m 50 to 12,000 ppm led to i n c r e a s e d d i s s o l u t i o n of the t a n t a l u m in the s o d i u m ; oxygen m i g r a t e d f r o m the s o d i u m to the t a n t a l u m . As the i n i t i a l oxygen c o n c e n t r a t i o n of the s o d i u m i n c r e a s e d , the a c c o m p a n y ing t a n t a l u m s p e c i m e n s f i r s t gained and then lost weight. We concluded that the addition of oxygen to sodium r e s u l t s in the f o r m a t i o n of a l o o s e l y a d h e r i n g t e r n a r y oxide s c a l e on the t a n t a l u m s u r f a c e . The i n c r e a s e d d i s s o l u t i o n of t a n t a l u m and the weight change b e h a v i o r of the t a n t a l u m s p e c i m e n s when the i n i t i a l oxygen c o n c e n t r a t i o n of the sodium is i n c r e a s e d i n d i c a t e s that the n o n a d h e r e n t t e r n a r y oxide flakes off in p r o g r e s s i v e l y g r e a t e r a m o u n t s a s its t h i c k n e s s i n c r e a s e s . When the c o n c e n t r a t i o n of oxygen in t a n t a l u m b e f o r e t e s t exceeded 200 ppm, sodium p e n e t r a t e d the t a n t a l u m . P e n e t r a t i o n , a s in other r e f r a c t o r y m e t a l - o x y g e n - a l k a l i m e t a l s y s t e m s p r e v i o u s l y i n v e s t i g a t e d , p r o c e e d s with the f o r m a t i o n of a t e r n a r y oxide by a wedging m e c h a n i s m . At 10w oxygen c o n c e n t r a t i o n s attack o c c u r s along g r a i n b o u n d a r i e s , and a s the oxygen c o n c e n t r a t i o n of the t a n t a l u m is i n c r e a s e d , the n u m b e r of g r a i n b o u n d a r i e s a t t a c k e d and the depth of attack i n c r e a s e . Still higher oxygen c o n c e n t r a t i o n s induce t r a n s g r a n u l a r a t t a c k .
BECAUSEof the
high o p e r a t i n g t e m p e r a t u r e s of the c o m p a c t n u c l e a r r e a c t o r s p r o p o s e d for R a n k i n e - c y c l e space power s y s t e m s , 1 the liquid a l k a l i m e t a l s will be u s e d a s c o o l a n t s and the r e f r a c t o r y m e t a l s and t h e i r a l l o y s m u s t be c o n s i d e r e d a s s t r u c t u r a l and cladding m a t e r i a l s . Also, c e r t a i n r e f r a c t o r y m e t a l s m a y be u s e d in the l i q u i d - m e t a l fast b r e e d e r r e a c t o r , w h e r e sodium is the coolant and heat t r a n s f e r m e d i u m . Hence, s e v e r a l i n v e s t i g a t o r s have e x a m i n e d the c o m p a t i b i l i t y between the r e f r a c t o r y m e t a l s n i o b i u m and t a n t a l u m and the liquid a l k a l i m e t a l s Na, K, and Li. 2-s In the a b s e n c e of i m p u r i t i e s , e s p e c i a l l y oxygen, r e
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