A STUDY OF ADSORPTION AND DIFFUSION OF HYDROGEN IN AN AUSTENITIC STAINLESS STEEL BY AP-FIM

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A STUDY OF ADSORPTION AND DIFFUSION OF HYDROGEN IN AN AUSTENITIC STAINLESS

STEEL BY AP-FIM

D. Ren

To cite this version:

D. Ren. A STUDY OF ADSORPTION AND DIFFUSION OF HYDROGEN IN AN AUSTENITIC STAINLESS STEEL BY AP-FIM. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-453-C6-458.

�10.1051/jphyscol:1987674�. �jpa-00226882�

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A STUDY OF ADSORPTION AND DIFFUSION OF HYDROGEN IN AN AUSTENITIC STAINLESS STEEL BY AP-FIM

D. Ren

Institute of Metal Research, Academia Sinica, Shenyang, P.R. China

A b s t r a c t

The p u r p o s e of t h e p r e s e n t work i s t o s t u d y h y d r o g e n a d s o r p t i o n a n d d i f f u s i o n on t h e s u r f a c e of a u s t e n i t i c s t a i n - l e s s s t e e l . Hydrogen i s n o t i c e a b l y a d s o r b e d on t h e s u r f a c e a t low t e m p e r a t u r e ( 8 5 ~ ) and d i f f u s e s i n t o t h e b u l k t o a d e p t h of a b o u t 40 atom l a y e r s . The d i f f u s i o n a c t i v a t i o n e n e r g y i s c a l c u l a t e d t o be 0.22 eV ( o r 21.2 ~ ~ / r n o l ) . It i s a l s o f o u n d t h a t t h e m i c r o s t r u c t u r e of a s p e c i m e n c h a r g e d w i t h h y d r o g e n i s c h a n g e d . The d i s t r i b u t i o n of h y d r o g e n h a s a s u b s u r f a c e f l u c t u a t i o n .

1. I n t r o d u c t i o n

D i f f u s i o n of h y d r o g e n i n m e t a l s h a s b e e n a s u b j e c t of g r e a t i n t e r e s t , i n b o t h m a t e r i a l s s c i e n c e a n d p h y s i c s . Hy- d r o g e n a t o m s occupy i n t e r s t i t i a l s i t e s i n m e t a l l a t t i c e s a n d d i f f u s e much f a s t e r t h a n any o t h e r a t o m s i n s o l i d s . T h e r e a r e many r e f e r e n c e s a b o u t s t u d i e s of a d s o r p t i o n and d i f f u - s i o n of hydrogen:" w i t h many d i f f e r e n t methods b e i n g u s e d t o i n v e s t i g a t e t h e s o l u b i l i t y , d i f f u s i o n a n d t r a p p i n g of hy- d r o g e n i n m e t a l s . The p u r p o s e of t h e p r e s e n t work i s t o s t u d y t h e a d s o r p t i o n a n d d i f f u s i o n of h d r o g e n on t h e s u r - f a c e of a u s t e n i t i c s t a i n l e s s s t e e l (ASST by AP-FIH. The d i f - f u s i o n a c t i v a t i o n e n e r g y i s c a l c u l a t e d t o be 0.22 eV a t low t e m p e r a t u r e (85K) by t h e TLK model and t h e mechanism o f d i f - f u s i o n of h y d r o g e n i n g r a i n b o u n d a r i e s i s d i s c u s s e d .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987674

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JOURNAL DE PHYSIQUE

2. The method and r e s u l t s of e x p e r i m e n t

The e x p e r i m e n t s were c a r r i e d o u t i n a n atom p r o b e f i e l d i o n m i c r o s g o p e . The i n s t r u m e n t h a s a s t r a i g h t f l i g h t t u b e and i s c o n t r o l l e d by computer. The c o l d f i n g e r of t h e m i c r o - s c o p e was c o o l e d c o n t i n u o u s l y w i t h l i q u i d n i t r o g e n . The back- g r o u n d p r e s s u r e i s T X I O - ~ t o r r . The image g a s i s neon a t a p r e s s u r e of 5 x 1 0 ' ~ t o r r . The a d s o r p t i o n of h y d r o g e n i s c a r - r i e d o u t a t room t e m p e r a t u r e and l o w t e m p e r a t u r e (85K). P n i - t i a l l y , a c l e a n s u r f a c e ( w i t h o u t h y d r o g e n ) must be p r e p a r e d i n o r d e r t o s t u d y t h e a d s o r p t i o n of hydrogen. T h e r e f o r e a f t e r e l e c t r o l y t i c p o l i s h i n g a sample i s p u t i n t o t h e FIM vacuum chamber where o b s e r v a t i o n of t h e f i e l d i o n image and AP a n a l y s i s a r e c a r r i e d o u t u n t i l t h e c o n c e n t r a t i o n o f - h y d r o - g e n on t h e s u r f a c e i s l e s s t h a n one p r e c e n t . A f t e r w a r d , t h i s t i p was moved i n t o a p r e p a r a t o r y vacuum chamber whose p r e s s u r e i s 3x1 0'6 t o r r

,

and hydrogen i s i n t r o d u c e d a t a t m o s p h e r i c p r e s - s u r e . During t h i s t i m e , t h e t e m p e r a t u r e of t h e t i p i s a b o u t 300 K , and r e m a i n s 3 0 m i n u t e s i n t h e a t m o s p h e r e of h y d r o g e n a l l o w i n g h y d r o g e n a d s o r p t i o n on t h e t i p s u r f a c e . Then t h i s

t i p was r e t u r e d t o t h e FIM vacuum chamber. The o b s e r v a t i o n of t h e f i e l d i o n image and AP a n a l y s i s were t h e n c a r r i e d

o& again, The e x p e r i m e n t shows tha* bydrag- could nat

a d s o r b on t h e s u r f a c e of ASS a t room t e m p e r a t u r e . For a d - s o r p t i o n of hydrogen a t low t e m p e r a t u r e , a t i p n o t c o n t a i n - i n g hydrogen was p r e p a r e d by t h e method mentioned a b o v e . The t i p i s c o o l e d f i r s t t o a b o u t 85K w i t h l i q u i d n i t r o g e n and neon i s f i l l e d i n t o t h e vacuum s y s t e m t o 3 x 1 0 - ~ t o r r . Then 5x10-' t o r r of h y d r o g e n i s i n t r o d u c e d i n a d d i t i o n t o t h e Ne and r e m a i n s f o r 3 0 m i n u t e s . A f t e r w a r d , t h e o b s e r v a - t i o n of f i e l d i o n image and AP a n a l y s i s a r e c a r r i e d o u t . The main sample c o n s t i t u e n t s a r e Fe, C r , N i a n d Mn, and t h e m a t r i x s t r u c t u r e i s FCC. F i g . l a shows t h e f i e l d i o n image of ASS w i t h o u t c h a r g e d h y d r o g e n . Because t h e c o m p o s i t i o n of t h e m a t e r i a l i s complex, t h e d e g r e e of o r d e r i s l e s s t h a n d e s i r e d w i t h o n l y a s m a l l f a c e of low o r d e r and homogeneous m a t r i x a p p e a r i n g on t h e p i c t u r e . F i g . 2 shows t h e r e s u l t s of AP a n a l y s i s , which a r e n e a r l y c o n s i s t e n t w i t h t h e c h e m i c a l a n a l y s i s r e s u l t s . F i g .3 shows t h e

AP

p r o f i l e : hydrogen h a s d i f f u s e d i n t o ASS t o a d e p t h of 40 atom l a y e r s . A n o t h e r

?ample was c h a r g e d w i t h hydrogen a t 1 0 0 a t m o s p h e r e s a t 300 a C and AP-FIM e x p e r i m e n t s were c a r r i e d o u t by t h e method mentioned above. F i g . l b shows t h e f i e l d i o n image of ASS a f f e c t e d by h y d r o g e n - c h a r g i n g . The image shows t h a t t h e s t r u c t u r e of t h e m a t r i x was c h a n g e d and h y d r i d e s o r p r e c i - p i t a t e s a p p e a r e d . The AP p r o f i l e of hydrogen shows t h a t t h e d i s t r i b u t i o n of h y d r o g e n h a s a f l u c t u a t i o n ( i n F i g . 4 ) a l o n g t h e d e p t h . The c o n c e n t r a t i o n of h y d r o g e n i n t h e u n d e r l y i n g r e g i o n a t a d e p t h of f i v e a t o m i c l a y e r s , i s h i g h e r t h a n a t t h e s u r f a c e . The many hydrogen atoms were t r a p p e d i n t h e g r a i n b o u n d a r i e s .

3 . D i s c u s s i o n

D i f f u s i o n mechanisms of h y d r o g e n i n metals s t r i c t l y depend on t e m p e r a t u r e . I n a g e % e r a l way, a t t h e l o w e s t tem- p e r a t u r e s H atoms a r e e x p e c t e d t o m i g r a t e by c o h e r e n t t u n - n e l i n g . I n t h i s p r o c e s s , H atoms d i s p l a c e i s r a i s e d ; H atoms move by phonon p r o c e s s e s . The t r a n s p o r t of H atoms

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e x e c u t e d t h r o u g h b a r r i e r jumps by t h e r m a l e x c i t a t i o n . I n complex a l l o y s , t h e r e a r e many d i s l o c a t i o n s and g r a i n boun- d a r i e s ; t h e r e f o r e hydrogen i s f i r s t a d s o r b e d on t h e s u r f a c e of ASS a t low t e m p e r a t u r e (85K) and t h e n H atoms d i f f u s e i n t o t h e b u l k t h r o u g h t h e l a t t i c e o r g r a i n b o u n d a r i e s . It c a n be r e g a r d e d a p r o c e s s of t h e r m a l e x c i t a t i o n . D i f f u s i o n of a s u r f a c e l a y e r w i t h lower c o n c e n t r a t i o n was approxima- t e l y r e g a r d e d a s b u l k d i f f u s i o n . Hence t h e d i f f u s i o n of hydrogen i n ASS i s d e s c r i b e d by t h e TLK model. The mean s q u a r e d i s p l a c e m e n t c a n be shown t o be < r 2 > = 4 ~ t d u r i n g a t i m e i n t e r v a l t , and D i s t h e d i f f u s i o n c o e f f i c i e n t . We a p - p l y t h e a r r h e n i u s e x p r e s s i o n g i v e n by

D = D~ e x p ( E ~ / ~ T ) _{( 1}

1

Ea = kT l o g e (4D,/r2) ( 2 ) where Ea i s t h e d i f f u s i o n a c t i v a t i o n energy,. Do i s t h e p r e - e x p o n e n t i a l t e r m D o = ~ v 1 2 e x p ( ~ s / k ) = 3 x 1 0 - 3 . According t o o u r e x p e r i m e n t c o n d i t i o n , r=120A, T=85K, t = 1 8 0 0 s e c o n d , k=1.38 x 1 0 - l 6 e r g K", t h e d i f f u s i o n a c t i v a t i o n e n e r y of h y d r o g e n i n ASS i s c a l c u l a t e d a c c o r d i n g t o e q u a t i o n ( 2 5 as Ea=0.22eY

( o r 21.2 K ~ / m o l ) , D=2.10-l6 c m 2 / s . The 21.2 KJ/mol of d i f - f u s i o n a c t i v a t i o n e n e r g y c a l c u l a t e d u s i n g t h e TLK model i s a p p r o x i m a t e l y e q u a l t o 20.5 KJ/mol of t r a p b i n d i n g e n e r g y which a p p l i e s t o hydrogen t r a p p e d by g r a i n b o u n d a r i e s i n p u r e n i c k e l . I t i s shown t h a t t r a p p i n g by g r a i n b o u n d a r i e s p l a y s a n i m p o r t a n t r o l e f o r d i f f u s i o n of h y d r o g e n i n ASS.

The f i e l d i o n image of F i g . l a shows t h a t t h e sample of un- c h a r g e d hydrogen e x h i b i t s l i t t l e o r d e r i n i t s s t r u c t u r e . I n t h i s o r d e r e d r e g i o n , t h e d i f f u s i o n of hydrogen c a n p r o c e e d t h r o u g h t h e l a t t i c e . I n o t h e r d i s o r d e r e d r e g i o n s , atom of hydrogen c o u l d u n d e r g o r e f l e c t i o n and a d s o r p t i o n a t g r a i n b o u n d a r i e s . Thus t h e d i f f u s i o n of hydrgoen i s c e r t a i n l y t h r o u g h jum i n g . The mean-square d i s p l a c e m e n t of d i f f u s r o n i s g i v e n by

P

where N i s t h e number *of hydrogen atoms w i t h jumping d i s - t a n c e 1, a i s t h e l e n g t h of t h e d i f f u s i o n c h a n n e l . Accor- d i n g t o t h e s h o r t - o r d e r e d s t r u c t u r e on t h e f i e l d i o n image

( F i g . l a ) , one c a n e s t i m a t e a=3OA, ln4A. When N=25 one c a n c a l c u l a t e <r2>=120A from e q u a t i o n ( 3 ) . These r e s u l t s show t h a t t h e d i f f u s i o n of hydrogen i n ASS c a n be e x e c u t e d t h r o u g h t h e l a t t i c e b u t underwent r e f l e c t i o n and a d s o r p t i o n a t g r a i n b o u n d a r i e s , t h e d i f f u s i o n c o e f f i c i e n t D b e i n g small a t 85K.

The f i e l d i o n image i n F i g . l b shows t h a t t h e m i c r o s t r u c - t u r e of a h y d r o g e n - c h a r g e d s p e c i m e n i s changed t o c r e a t e t h e s t r u c t u r e of p r e c i p i t a t e s o r h y d r i d e s . The AP p r o f i l e shows t h a t t h e c o n c e n t r a t i o n of h y d r o g e n on t h e s u r f a c e i s h i g h e r t h a n t h e s p e ~ i m e n uncharged w i t h hydrogen. The d i s t r i b u t i o n of hydrogen h a s a f l u c t u a t i o n t h a t f o l l o w s t h e d e p t h . The c o n c e n t r a t i o n of hydrogen i n t h e s u b s u r f a c e i s h i g h e r t h a n t h a t a t t h e s u r f a c e a t a d e p t h of f i v e l a y e r s . It i s shown t h a t i n tHe specimen a f t e r h y d r o g e n - c h a r g i n g a r e c r e a t e d p r e - c i p i t a t e s of 100A200 A s i z e and s m a l l e r . Hydrogen of h i g h e r c o n c e n t r a t i o n c a n l e a d t o f o r m a t i o n of m e t a l - h y d r i d e s . I t i s a l s o shown t h a t t h e t r a p p i n g s i t e of h y d r o g e n were i n c r e a s e d f o r a h y d r o g e n - c h a r g e d specimen. When hydrogen d i f f u s e s f r o m t h e s u r f a c e i n t o t h e b u l k , atoms of h y d r o g e n a r e t r a p p e d

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p r e f e r e n t i a l l y i n t h e t r a p p i n g s i t e s t o l e a d t o hydrogen of h i g h e r c o n c e n t r a t i o n . Hence t h e c o n c e n t r a t i o n of hydrogen changes w i t h d e p t h and t h e r e i s a h i g h e r c o n c e n t r a t i o n i n t h e g r a i n boundary

.

Acknowledgement

I am g r a t e f u l t o Mr. Zhang Xiumu f o r p r o v i d i n g t h e samp- l e of a u s t e n i t i c s t a i n l e s s s t e e l .

R e f e r e n c e s

1 . N. R. Louthan Acta M e t a l l u r g i c a 23(1975) 745.

2. V. P. Zhdanov S u r f a c e S c i e n c e m ( 1 9 8 5 ) L614.

3. Y. Fukai Advances P h y s i c s & ( I 985) 263.

4. Sung-man Lee Metal. Trans. A 1 7 ~ ( 1 9 8 6 ) 181.

5. G. A y r o l t J . Chem. Phys. 7 281.

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c h a r g e w i t h h y d r o g e n ( a ) c h a r g e w i t h h y d r o g e n ( b ) .

F i g . 2 The mass s p e c t r u m of a u s t e n i t i c s t a i n l e s s s t e e l by AP.

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F i g . 3 The AP p r o f i l e of h y d r o g e n i n t h e a u s t e n i t i c s t a i n l e s s s t e e l u n c h a r g e w i t h h y d r o g e n .

HIM 2.0.

F i g . 4 The AP p r o f i l e of h y d r o g e n i n t h e a u s t e n i t i c s t a i n l e s s s t e e l c h a r g e w i t h hydrogen.