Selective adsorption and hydrogen embrittlement
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THE
p r e m a t u r e f a i l u r e of s t e e l s in h y d r o g e n cont a i n i n g e n v i r o n m e n t s has b e e n w i d e l y i n v e s t i g a t e d f r o m a m e c h a n i c a l s t r e n g t h point of view. H o w e v e r , the c h e m i s t r y of the e n v i r o n m e n t and in p a r t i c u l a r the s u r f a c e r e a c t i o n s b e t w e e n the e n v i r o n m e n t and the m e t a l s p e c i m e n has not b e e n p u r s u e d with the s a m e v i g o r . It is w e l l known that the pH of a liquid e n v i r o n m e n t and e x t e r n a l l y a p p l i e d e l e c t r i c a l p o t e n t i a l s can c o n t r o l the k i n e t i c s of c r a c k growth. T h e s i m i l a r i t y b e t w e e n s t r e s s c o r r o s i o n and h y d r o g e n gas e m b r i t t l e m e n t has b e e n pointed out by Sandoz and B e a c h e m ? It i s t h e r e f o r e r e a s o n a b l e to conclude that the c o n t r o l of s u r f a c e c h e m i c a l r e a c t i o n s can be u s e d to m o d i f y the e f f e c t s of a h y d r o g e n a t m o s p h e r e . I n d e e d it h a s a l r e a d y been shown that c e r t a i n g a s e o u s a d d i t i v e s to a h y d r o g e n a t m o s p h e r e can s t o p a r u n n i n g c r a c k in 4340 s t e e l , s T h i s p a p e r s h a l l f i r s t p r e s e n t the r e s u l t s of a s e r i e s of a d s o r p t i o n e x p e r i m e n t s and then c o r r e l a t e t h e m with e x p e r i m e n t a l o b s e r v a t i o n s of c r a c k g r o w t h in 4340 s t e e l . T h e p r o c e s s of h y d r o g e n e m b r i t t l e m e n t m a y be b r o k e n down into the following g e n e r a l s t e p s . 1) The a d s o r p t i o n of the h y d r o g e n c o n t a i n i n g m o l e c u l e ; 2) the d i s s o c i a t i o n of the m o l e c u l e to p r o v i d e a t o m i c o r ionic hydrogen; 3) diffusion to c r i t i c a l l y s t r e s s e d r e gions a h e a d of the c r a c k tip; and 4) l o s s of m e c h a n i c a l s t r e n g t h and f a i l u r e . Steps 1 and 2 a r e c h e m i c a l in n a t u r e . If t h e s e s t e p s a r e c o n s e c u t i v e then c o n t r o l o v e r the c h e m i s t r y would l e a d to c o n t r o l o v e r the mechanics. Steps 1 and 2 have been w i d e l y s t u d i e d in the f i e l d of c a t a l y s i S . It i s w e l l known that r e a c t i o n s of t h i s kind can b e inhibited o r p r o m o t e d b y the p r e s e n c e of s m a l l a m o u n t s of c e r t a i n i m p u r i t i e s . In the p r e s e n t work, a d s o r p t i o n e x p e r i m e n t s on i r o n f o i l s w e r e c a r r i e d out to d e t e r m i n e s o m e of the r e l e v a n t p a r a m e t e r s of s e l e c t i v e a d s o r p t i o n that a r e a p p l i c a b l e to the cont r o l of h y d r o g e n e m b r i t t l e m e n t . EXPERIMENTAL
A l l a d s o r p t i o n i s o t h e r m s were d e t e r m i n e d With a Cahn v a c u u m m i c r o b a l a n c e (RG 2000) in the p r e s s u r e r a n g e of 0.13 m P a (10 -6 T o r r ) to 40 k P a (3
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