Effect of electrodeposited metals on the permeation of hydrogen through iron membranes
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T H E p r o b l e m of h y d r o g e n e m b r i t t l e m e n t of i r o n and its alloys has r e c e n t l y a t t r a c t e d c o n s i d e r a b l e attention in view of its t e c h n o l o g i c a l i m p o r t a n c e . 1-4 In a t t e m p t s to delay the i n c i d e n c e of hydrogen e m b r i t t l e m e n t , s o m e w o r k e r s alloyed i r o n with m e t a l s which dec r e a s e d the diffusivity of h y d r o g e n e.g., Ni 5 and Cr, 6 w h e r e a s o t h e r s coated the b a s e m e t a l with a n o t h e r m e t a l , e.g., e l e c t r o d e p o s i t e d Ni, 7-13 Cu,7-11,13,14 Cr, 7-~1 Pb, 9-1~ Co, 9 Sn; 7'1~ v a p o r deposited Cd, A1, Mg, Zn and Z n - P b alloys;11 and c h e m i c a l l y deposited Au and A u - N i composite l a y e r s 3 1 V a r i o u s other w o r k e r s 15,~6 have studied the p e r m e a b i l i t y of composite m e t a l m e m b r a n c e s to hydrogen charged f r o m the gas phase. With all of these coatings, hydrogen e n t r y into the b a s e m e t a l was d e l a y e d and the r a t e of its p e r m e a t i o n dec r e a s e d . In s o m e of these c a s e s , this was a t t r i b u t e d to the b a r r i e r effect of the coating, F o r coatings to be effective b a r r i e r s , they should be continuous, r e l a tively thick and i m p e r v i o u s , much like p r o t e c t i v e s c a l e s on m e t a l s . When the d i i f u s i v i t y D of hydrogen is much lower in the coating m a t e r i a l than in the s u b s t r a t e , the coating m a t e r i a l , in g e n e r a l , may be cons i d e r e d a candidate b a r r i e r m a t e r i a l . In this ease, t h e r e will be a n a p p r e c i a b l e t i m e lapse, even for thin l a y e r s , before the a c t i v i t y (concentration) of hydrogen at the c o a t i n g / s u b s t r a t e i n t e r f a c e r i s e s a p p r e c i a b l y , or in the l i m i t a p p r o a c h e s that e x i s t i n g in the e n v i r o n m e n t . On this b a s i s , the t h r e e coating m a t e r i a l s cons i d e r e d in this paper, Pt, Ni and Cu, a r e good candidate b a r r i e r m a t e r i a l s for i r o n s u b s t r a t e s ; the hydrogen diffusivity at 25~ in each of the t h r e e coating m e t a l s is about 10 -~ cm 2 s -z, as c o m p a r e d to about 10 "~ cm z s -1 for i r o n . F o r e v a l u a t i n g the e f f e c t i v e n e s s of coatings of different t h i c k n e s s , the m a s s t r a n s f e r coefficient, m = D//L where L is the t h i c k n e s s of the coating, is a useful p a r a m e t e r . A n o t h e r i n t e r e s t i n g and potentially effective method S. S. CHATTERJEE, formerly Research Associate, The Pennsylvania State University, is now with the Fertilizer Company of India, Ltd., Sindri, Dhanbad, Bihar, India, B. G. ATEYA is Research Associate and H. W. PICKERINGis Professor and Chairman, Metallurgy Section, Materials Science Department, The PennsylvaniaState University, University Park, PA 16802. Manuscript submitted February 24, 1977. METALLURGICALTRANSACTIONSA
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