The deformation behavior of the matrix near a planar interface in a two-phase medium
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x 10 4 p x / z e x p
8095R T+ 8 8
m o l e s -1 cm-Z t ~
over a r a n g e of h y d r o g e n p r e s s u r e s f r o m 21 t o r r to 766 t o r r and o v e r a t e m p e r a t u r e r a n g e of 342 to 619 K. V a r i a t i o n of p e r m e a t i o n r a t e with the s q u a r e root of hydrogen p r e s s u r e was used as evidence that the p e r m e a t i o n p r o c e s s was diffusion r a t h e r than s u r f a c e reaction controlled. The p e r m e a t i o n data w e r e used in c o n j u n c t i o n with the s o l u b i l i t y e x p r e s s i o n of Gonzalez ~ to d e t e r m i n e the diffusivity of h y d r o g e n through alpha i r o n as: D = 1.01 • I0-a e x p - 159____55 cm2/s RT
T H E m e a s u r e m e n t of h y d r o g e n p e r m e a t i o n r a t e s in alpha i r o n below 450 K has g e n e r a l l y b e e n l i m i t e d to t r a n s i e n t t e c h n i q u e s due to a low flux r a t e and difficulty with s u r f a c e c o n t r o l l i n g p r o c e s s e s . Deviations f r o m high t e m p e r a t u r e data, in the d i r e c t i o n of higher a c t i v a t i o n e n e r g i e s and lower t r a n s p o r t r a t e s , have b e e n o b s e r v e d in r e c e n t work for t e m p e r a t u r e s below 473Kfi In this i n v e s t i g a t i o n a highly s e n s i t i v e steady state gas p e r m e a t i o n technique has b e e n used to m e a s u r e h y d r o g e n p e r m e a t i o n r a t e s in alpha i r o n over a t e m p e r a t u r e r a n g e of 342 to 619 K. A v a c u u m s y s t e m c a pable of o p e r a t i o n b e t w e e n 10 -e t o r r and a t m o s p h e r i c p r e s s u r e was e m p l o y e d to supply hydrogen in the gas phase to the s p e c i m e n . The s p e c i m e n was coated with a thin a c t i v a t e d p a l l a d i u m l a y e r to p r o v i d e s u r f a c e k i n e t i c s c o m p a t i b l e with S i e v e r t ' s Law down to a t e m p e r a t u r e of 342 K. The outlet s y s t e m p r e s s u r e (See Fig. 1) was m e a s u r e d with an i o n i z a t i o n gauge and the flux computed with the p u m p i n g speed of the outlet s y s t e m . The m e a s u r e d p e r m e a b i l i t y coupled with Gonz a l e z ' s o l u b i l i t y e x p r e s s i o n 1 has b e e n used to compute the diffusivity of hydrogen in alpha i r o n .
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r e c r y s t a l l i z e d in an induction f u r n a c e by c y c l i n g through the lower t r a n s f o r m a t i o n t e m p e r a t u r e , u n d e r a v a c u u m of 10 -6 t o r r . After a n n e a l i n g , both s i d e s of the d i s c s were given a vapor deposit of 99.95 pet p u r e p a l l a d i u m , a p p r o x i m a t e l y 1000A thick, to p r o v i d e a s u r f a c e which would allow a c t i v a t i o n by s u i t a b l e t e c h n i q u e s . Due to the v e r y high r a t e of diffusion of h y d r o g e n through p a l l a d i u m , and the e x t r e m e t h i n n e s s of t h e s e l a y e r s r e l a tive to the i r o n s a m p l e , it is felt that t h e s e l a y e r s should not c o n t r i b u t e s i g n i f i c
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