An adsorption study of hydrogen on iron and its relation to hydrogen embrittlement
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The c h e m i c a l r e a c t i o n s of h y d r o g e n gas on i r o n s u r f a c e s have b e e n d e t e r m i n e d by s i m u l t a n e o u s l y m e a s u r i n g the v o l u m e of g a s a d s o r b e d and the c o r r e s p o n d i n g m a g n e t i z a t i o n change. The c o m b i n a t i o n of t h e s e e x p e r i m e n t a l r e s u l t s as well as the k i n e t i c s of the r e a c t i o n s is used to e x p l a i n the t e m p e r a t u r e d e p e n d e n c e o b s e r v e d in c r a c k growth s t u d i e s p e r f o r m e d in g a s e o u s h y d r o g e n . The r e a c t i o n is shown to be a two step p r o c e s s involving the f o r m a t i o n of a n a d s o r b e d m o l e c u l a r p r e c u r s o r p r i o r to the f o r m a t i o n of the e m b r i t t l i n g hydrogen ion. The a d s o r p t i o n i s o t h e r m and m a g n e t i z a t i o n - a d s o r p t i o n i s o t h e r m for H2 on Fe at 77 K were d e t e r m i n e d to be L a n g m u i r i a n . T h i s , plus a f i r s t o r d e r a d s o r p t i o n r a t e a r e given as e v i d e n c e for the e x i s t e n c e of a c h e m i s o r b e d m o l e c u l a r ~ p r e c u r s o r at this t e m p e r a t u r e . The m e c h a n i c a l t e s t data of other i n v e s t i g a t o r s for slow c r a c k growth in g a s e o u s H2, which show a n o n m o n o tonic change of c r a c k growth r a t e with t e m p e r a t u r e , b e c o m e e x p l a i n a b l e b a s e d on a m e a s u r e d d e c r e a s e in the a d s o r p t i o n of H2 at t e m p e r a t u r e above 300 K and the two step a d s o r p t i o n p r o c e s s . At t e m p e r a t u r e s below 300 K the f o r m a t i o n of an H- ion f r o m the a d s o r b e d p r e c u r s o r H~ ion is the r a t e c o n t r o l l i n g p r o c e s s in g a s e o u s h y d r o gen e m b r i t t l e m e n t . At t e m p e r a t u r e s above 300 K, the d e c r e a s i n g net a d s o r p t i o n r a t e of W2 b e c o m e s the l i m i t i n g p r o c e s s .
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the effects of hydrogen on the m e c h a n i c a l p r o p e r t i e s of s t e e l have b e e n o b s e r v e d s i n c e the late n i n e t e e n t h century, it was in 1944 that O r o w a n 1 f i r s t s u g g e s t e d that a d s o r p t i o n of the gas i t s e l f could play an i m p o r t a n t r o l e . He t h e o r i z e d that if the s u r f a c e free e n e r g y could be lowered by a d s o r p t i o n , it could e x p l a i n the o b s e r v e d d e c r e a s e in f r a c t u r e s t r e n g t h . Petch, 2 also, r e c o g n i z e d the i m p o r t a n t role of ads o r p t i o n in e m b r i t t l e m e n t and a r g u e d that it d e c r e a s e d the s u r f a c e e n e r g y t e r m in the G r i f f i t h e q u a t i o n . T o day it is s t i l l not c l e a r what part a d s o r p t i o n plays in a g e n e r a l m e c h a n i s m for gaseous hydrogen e m b r i t t l e ment (GHE), but in e v e r y GHE s i t u a t i o n a d s o r p t i o n n e c e s s a r i l y m u s t o c c u r and could, t h e r e f
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