Diffusivity and solubility of oxygen in platinum and Pt-Ni alloys
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VELHO AND R . W . B A R T L E T T
The diffusivity and s o l u b i l i t y of oxygen in m e t a l s p e c i m e n s w e r e d e t e r m i n e d f r o m m e a s u r e m e n t s of p e r m e a t i o n through thin wall tubes containing oxygen and heated by e l e c t r i c a l r e s i s t a n c e . The p e r m e a t i n g oxygen d e s o r b e d in v a c u u m as m o n o a t o m i c oxygen and the flux was m o n i t o r e d m a s s - s p e c t r o m e t r i c a l l y . A known helium leak r a t e and e x p e r i m e n t a l l y - d e t e r m i n e d s e n s i t i v i t i e s of the two g a s e s were u s e d for flow c a l i b r a t i o n . The diffusion coefficient of oxygen in pure p l a t i n u m , c a l c u l a t e d f r o m the t i m e lag to r e a c h a steady state flux, i s : F.-(78,000_e 25,000) t D = (9.3 + 1.8) exp k RT
'
cm2/sec
in the t e m p e r a t u r e r a n g e of 1435 ~ to 1504~ The s o l u b i l i t y of oxygen in pure p l a t i n u m was obtained from the s t e a d y - s t a t e flux u s i n g the p r e v i o u s l y d e t e r m i n e d diffusivity. The s o l u 1/2 bility is p r o p o r t i o n a l to p 02 and at 1 a t m of oxygen the s o l u b i l i t y is: ~-(114,000 • 3 4 , 0 0 0 ) ] C~) = (0.2 • 0.1) x 10 ~2 exp [~
, wt pct.
S m a l l a m o u n t s of n i c k e l , l e s s than r e q u i r e d for i n t e r n a l oxidation, had a n e g l i g i b l e effect on the oxygen s o l u b i l i t y and diffusivity in p l a t i n u m a l l o y s . T H E diffusion coefficient and solubility of oxygen in p l a t i n u m a r e of i n t e r e s t for high t e m p e r a t u r e a p p l i c a t i o n s of p l a t i n u m p a r t i c u l a r l y as claddings for s u p e r a l l o y s and r e f r a c t o r y m e t a l s . To afford effective p r o t e c t i o n of the s u b s t r a t e m e t a l s the r a t e of oxygen t r a n s p o r t through the cladding m u s t be s m a l l . E x p e r i m e n t s ~ with p l a t i n u m - c l a d m e t a l s at high t e m p e r a t u r e s show little oxidation for extended p e r i o d s followed by a c c e l e r a t i n g }-ates of oxidation and e v e n t u a l l y c a t a s t r o p h i c f a i l u r e . T h e s e r e s u l t s suggest that s u b s t r a t e m e t a l s diffuse into p l a t i n u m claddings d u r i n g e x p o s u r e at high t e m p e r a t u r e s and a l t e r the oxygen t r a n s p o r t p r o p e r t i e s of p l a t i n u m . Oxidation s t u d i e s of p l a t i n u m - b a s e a l l o y s show that while an oxide s c a l e f o r m s when a p p r e c i a b l e r e a c t i v e solute m e t a l is p r e s ent 2 i n t e r n a l oxidation o c c u r s r e a d i l y for dilute a l l o y s .3 Although oxidation of p l a t i n u m 4-8 and oxygen exchange r a t i o s on p l a t i n u m s u r f a c e s 7-9 have b e e n ext e n s i v e l y studied t h e r e have b e e n few a t t e m p t s to det e r m i n e the t r a n s p o r t p r o p e r t i e s of oxygen in p l a t i n u m . N o r t o n ~~ t r i e d to m e a s u r e the p e r m e a t i o n of oxygen t
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