Selective Adsorption and Hydrogen Embrittlement V. SRIKRISHNAN AND P . J . F I C A L O R A T h i s p a p e r p r e s e n t s a d e f i n i t e c o r r e l a t i o n b e t w e e n h y d r o g e n e m b r i t t l e m e n t and a d s o r p tion. The effects of the p r e s e n c e of g a s e o u s a d d i t i v e s on h y d r o g e n e m b r i t t l e m e n t and hyd r o g e n a d s o r p t i o n w e r e s t u d i e d . T h o s e g a s e o u s a d d i t i v e s which h a l t a r u n n i n g c r a c k in 4340 s t e e l loaded in a h y d r o g e n a t m o s p h e r e a l s o halt h y d r o g e n a d s o r p t i o n . T h o s e g a s e ous a d d i t i v e s which a c c e l e r a t e the c r a c k g r o w t h i n c r e a s e the s u p p l y of h y d r o g e n a t o m s at the m e t a l s u r f a c e . It is concluded that the effect of g a s e o u s a d d i t i v e s to inhibit o r p r o mote c r a c k growth i s a c o n s e q u e n c e of t h e i r a b i l i t y to i n c r e a s e o r d e c r e a s e the s u p p l y of h y d r o g e n a t o m s at t h e m e t a l s u r f a c e by s o m e c h e m i c a l p r o c e s s .
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 (300 T o r r ) ,
at r o o m t e m p e r a t u r e . The p r e c i s i o n of t h e b a l a n c e is 0.1 ~g, the m a x i m u m l o a d is 1 g and the t o t a l output is 1 inV. The v a c u u m - g a s handling s y s t e m has b e e n d e s c r i b e d e l s e w h e r e . 4 T h e i r o n f o i l used in the s t u d y had a n o m i n a l t h i c k n e s s of 25 / l m . It had a c a r b o n content of 0.1 p e t and the t o t a l m e t a l l i c i m p u r i t i e s w e r e l e s s than 1000 p p m . In o r d e r to m a x i m i z e the s u r f a c e a r e a , while k e e p i n g the t o t a l weight below a g r a m , the f o i l w a s c h e m i c a l l y thinned in a 80 pet m e t h a n o l - 2 0 p e t n i t r i c a c i d solution. The f o i l was f r e q u e n t l y r i n s e d in d i s t i l l e d w a t e r d u r i n g thinning, and final c l e a n i n g was done with ethyl a l c o h o l in an u l t r a s o n i c bath. T h e foil was s u s p e n d e d f r o m the b a l a n c e , the s y s t e m was e v a c u a t e d to 0.13 m P a and p u r i f i e d h y d r o g e n was i n t r o d u c e d in s m a l l d o s e s . T h e h y d r o g e n was p u r i f i e d by m e a n s of a p a l l a d i u m - s i l v e r m e m b r a n e . The s t e a d y s t a t e change in weight of the i r o n s p e c i m e n a t v a r i o u s p r e s s u r e s was n o t e d to o b t a i n the a d s o r p t i o n i s o t h e r m s . A s q u a r e m e t e r of i r o n s u r f a c e has a p p r o x i m a t e l y 2 • 1019 a t o m i c s i t e s . If one H a t o m p e r s i t e a d s o r b s , the weight of the r e s u l t ing m o n o l a y e r i s 3 tzg. m -2. By thinning the i r o n to obtain a p o r o u s foil, it was p o s s i b l e to o b t a i n s u r f a c e a r e a s of a p p r o x i m a t e l y 0.1 m s . g-l. T h e s e s u r f a c e a r e a s w e r e e s t i m a t e d b y a s s u m i n g that the " k n e e " in the h y d r o g e n a d s o r p t i o n i s o t h e r m r e p r e s e n t s monolayer coverage. The a d s o r p t i o n of SO2, Os, N20, COs and H2S w e r e a l s o s t u d i e d . A l l t h e s e g a s e s w e r e o b t a i n e d in high p u r i t y r e s e a r c h g r a d e . T h e H2S, which was the m o s t i m p u r e of the a b o v e g a s e s , had a m i n i m u m p u r i t y of 99.5 p c t . The SOs, Os, N20 and COs had m i n i m u m p u r i t i e s of 99.98 pct, 99.99 pct, 99.6 p c t and 99.99 p c t r e s p e c t i v e l y . The i m p u r i t i e s in Os and CO2 w e r e l a r g e l y i n e r t s p e c i e s e x c e p t <5 p p m THC, <5 p p m H20, <5 p p m CH4 and <1 p p m Hz. It i s quite l i k e l y that the s u r f a c e of the i r o n f o i l s c o n t a i n e d s o m e s u r f a c e oxide, s i n c e it was e x p o s e d to a i r a f t e r thinning. We b e l i e v e , h o w e v e r , that the s u r f a c e s of the f o i l s c l o s e l y a p p r o x i m a t e s the s u r f a c e s found in c r a c k g r o w t h e x p e r i m e n t s s i n c e the a d s o r p t i o n r e s u l t s a r e c o n s i s t e n t with t h e s e e x p e r i ments. E X P E R I M E N T A L RESULTS a) H y d r o g e n A d s o r p t i o n
V. SRIKRISHNAN and P. J. FICALORA are Postdoctoral Fellow and Professor, respectively, at Syracuse University, Department of Chemical Engineering and Materials Science, Syracuse, NY 13210. Manuscript submitted January 28, 1976. METALLURGICAL TRANSACTIONS A
The r o o m t e m p e r a t u r e a d s o r p t i o n i s o t h e r m of hyd r o g e n on the i r o n foil e x h i b i t s a " k n e e " s u g g e s t i n g that the a d s o r p t i o n w a s m u l t i l a y e r . T h e h y d r o g e n a d VOLUME 7A, NOVEMBER 1976 1669
s o r p t i o n was quite r e v e r s i b l e and a l l the a d s o r b e d h y d r o g e n could be r e m o v e d by p u m p i n g at 0.13 m P a in l e s s than 30 rain. T h e r e m a i n i n g p a r t s of t h i s s e c t i o n show the e f f e c t of SO2, O2, Nee, CO2 and HeS on the a d s o r p t i o n of h y d r o g e n . T h e h y d r o g e n a d s o r p tion i s o t h e r m w i l l b e shown in the s u b s e q u e n t p a r t s of this s e c t i o n .
PRESSURE,k Po 133 20.0 26.7 Hz OPTAKE OKI CLEAN F()IL
67
0
I
53.5 I
40.0 I
(a) I0-..,g....--.---
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0 H~, UPTAKE ON FOIL EXPOSED "I0.67 kPo(STORR)SO
b) E f f e c t of s e e H y d r o g e n uptake on a c l e a n e d foil, a s o u t l i n e d e a r l i e r , was e v a l u a t e d in t h r e e e x p e r i m e n t a l r u n s ( F i g . l(a)). T h i s i r o n f o i l was d e g a s s e d t h e n e x p o s e d to 0.67 klaa (5 T o r r ) of s e e f o r 10 rain and the c h a m b e r w a s e v a c u a t e d to 0.13 m P a . Seven m i c r o g r a m s of SO2 r e m a i n e d i r r e v e r s i b l y a d s o r b e d even a f t e r 1 h of p u m p i n g at 0.13 m P a . T h i s foil did not a d s o r b any h y d r o g e n when s u b j e c t e d to a p r e s s u r e r a n g e of 0.13 m P a to 33 k P a (250 T o r r ) (Fig. l(b)). T h i s s a m e foil w a s kept a t 0.13 m P a f o r 18 h. A s u b s e q u e n t h y d r o g e n a d s o r p t i o n e x p e r i m e n t showed no uptake o v e r t h e s a m e h y d r o g e n p r e s s u r e r a n g e . T h i s i n d i c a t e d that the a d s o r b e d SO2 w a s s t i l l p r e s e n t . The foil w a s h e a t e d at 300~ in 0.13 m P a f o r about 20 rain. When t h i s foil was u s e d to s t u d y h y d r o g e n a d s o r p t i o n , it w a s found that the c a p a b i l i t y of h y d r o g e n uptake had b e e n c o m p l e t e l y r e s t o r e d ( F i g . l(c)). The a d s o r b e d s e e m u s t have been t h e r m a l l y d e s o r b e d . The next e x p e r i m e n t s w e r e p e r f o r m e d in a He + 3 p c t SO2 (by volume) e n v i r o n m e n t . A new i r o n f o i l was f i r s t e x p o s e d to 0.67 k P a SO2 and then u s e d to d e t e r m i n e the gas uptake c h a r a c t e r i s t i c s in the H2 + 3 pct SO2 e n v i r o n m e n t . Continued a d s o r p t i o n of p r e s u m a b l y SO2 was found (Fig. l ( d ) ) . T h e s a m e foil w a s h e a t e d at 0.13 m P a to d e s o r b the s e e . T h i s d e s o r p t i o n w a s conf i r m e d by a s u b s e q u e n t h y d r o g e n a d s o r p t i o n - d e s o r p t i o n e x p e r i m e n t . T h i s s a m e foil was then u s e d to s t u d y the a d s o r p t i o n - d e s o r p t i o n in the a b o v e He-SO2 m i x t u r e . The r e s u l t s a r e shown in F i g . 2(a) and (b). It was found that 12 lzg of gas had b e e n i r r e v e r s i b l y a d s o r b e d ( F i g . 2(a)). A s u b s e q u e n t a d s o r p t i o n e x p e r i m e n t with p u r e h y d r o g e n on this s a m e foil s h o w e d no h y d r o g e n uptake ( F i g . 2(b)). T h i s i n d i c a t e d that the r e t a i n e d gas was e s s e n t i a l l y SO2.
(b)
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( IO-6 ) Fig. I--Adsorption
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GAS ADSORPTION-DESORPTION ON FOIL EXPOSED TO A H z - 3 % SOzGAS MIXTURE (o)
N ~ New f o i l s w e r e u s e d to study the effect of oxygen on h y d r o g e n a d s o r p t i o n . With a foil in p o s i t i o n , t h e s y s t e m was p u m p e d down to 0.13 m P a and 0.33 k P a of oxygen w a s added. In t h i s e x p e r i m e n t , t h e f o i l a d s o r b e d 4.5 ~g of oxygen at 0.33 k P a . When the c h a m b e r was e v a c u a t e d to 0.13 m P a a l l but 0.7 ~ g of oxygen w a s d e s o r b e d within 60 m i n . A s u b s e q u e n t h y d r o g e n a d s o r p t i o n e x p e r i m e n t showed that the p r e s e n c e of t h i s s m a l l amount of a d s o r b e d oxygen d i d not have any influence on h y d r o g e n a d s o r p t i o n . In the s e c o n d s e t of e x p e r i m e n t s , h y d r o g e n a d s o r p tion was s t u d i e d in the p r e s e n c e of 0.33 k P a of oxygen. T h e f o i l s w e r e i n i t i a l l y e x p o s e d to oxygen and then h y d r o g e n was a d d e d to a t o t a l p r e s s u r e of 33 k P a ( F i g . 3). The h y d r o g e n a d s o r p t i o n i s o t h e r m s w e r e found to be v e r y s i m i l a r . H o w e v e r , the a d s o r b e d oxygen did have a r e t a r d i n g effect on the t o t a l u p t a k e of h y d r o g e n , e s p e c i a l l y at the l o w e r h y d r o -
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AOOITIONAL Hz UPTAKE - -
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F i g . 3 - - E f f e c t of t h e p r e s e n c e hydrogen.
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3,4
250
300
of o x y g e n o n t h e a d s o r p t i o n of
METALLURGICAL TRANSACTIONS A
gen p r e s s u r e . An i n c r e a s e of the initial oxygen p r e s s u r e to 0.67 k P a did not change the nature of subsequent hydrogen adsorption. d) Effect of CO2 and N~O The p r o c e d u r e was the s a m e as in the case of oxygen. In the f i r s t set of runs, the f r e s h l y cleaned foil was exposed to 0.33 k P a of CO2 (N20). The total amount a d s o r b e d in the c a s e of CO2 was 2.5 #g and in the c a s e of N20 was 1.5 #g. Both g a s e s could be c o m p l e t e l y d e s o r b e d by pumping at 0.13 mPa. H y d r o gen a d s o r p t i o n on foils exposed to these gases and subsequently pumped out, was found to be identical to that on a clean foil. Next, 0.33 k P a o r COs (N~O) was added to the foil at 0.13 m P a . Without removing the gas, hydrogen was added in s m a l l doses until the total p r e s s u r e was 33 kPa. The r e s u l t i n g i s o t h e r m s (Fig. 4) showed that the i n i t i a l CO2 (N~O) a d s o r b e d was not d e s o r b e d by hydrogen, that hydrogen continued to adsorb. e ) R o l e of H2S A new iron foil was suspended from the balance. A s s o o n a s 0 . 3 3 k P a of h y d r o g e n s u l f i d e w a s i n t r o d u c e d , t h e f o i l r a p i d l y a d s o r b e d 2 7 . 0 # g of g a s . W h e n
0
6.7 I
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ADDITIONAL Hz UPTAKE
the chamber was evacuated to 0.13 m P a , 21 /zg of H2S r e m a i n e d a d s o r b e d . Even after pumping at 0.13 m P a for 12 h, the bulk of the hydrogen sulfide did not des o r b . A hydrogen adsorption e x p e r i m e n t on this foil showed continued hydrogen uptake (Fig. 5). The ads o r b e d H2S had e l i m i n a t e d the initial portion of the hydrogen a d s o r p t i o n i s o t h e r m . This was c l e a r l y in c o n t r a s t to the effect of SO2. In a second s e r i e s of e x p e r i m e n t s , 0.33 kPa of H2S was initially introduced to the c h a m b e r . Without r e moving the H2S, hydrogen was added in s m a l l d o s e s until the total p r e s s u r e was 33 kPa. F i g . 6 shows the net weight gained by the foil. The weight gain c o r r e s ponding to 0.33 k P a was c l e a r l y due to H2S adsorption. The additional gain in weight was much m o r e than that o b s e r v e d if p u r e hydrogen alone continued to be a d s o r b e d . Also, the previous e x p e r i m e n t showed that the a d s o r b e d H2S did not have any p r o m o t i o n a l effect on the subsequent hydrogen uptake. In the l a s t s e r i e s of e x p e r i m e n t s , the hydrogen uptake c h a r a c t e r i s t i c s of a new iron foil was d e t e r m i n e d (Fig. 7(a)). This foil was exposed to 0.67 k P a of SO2 for 25 min. A f t e r the s y s t e m was evacuated, 16 tzg of SO2 r e m a i n e d i r r e v e r s i b l y a d s o r b e d . Subsequent hydrogen a d s o r p t i o n runs showed that no hydrogen was taken up in the p r e s s u r e range of 0.13 m P a to 33 kPa (Fig. 7(b)). This foil with the a d s o r b e d SO2 gas now exposed to 0.33 k P a of H2S for 25 min. When the s y s tem was evacuated to 0.13 mPa, the foil had a net gain in weight of 27 #g. A subsequent hydrogen adsorption e x p e r i m e n t (Fig. 7(c)) showed that the hydrogen a d s o r p t i o n was identical to that obtained when the foil was exposed to H2S alone and pumped out. This indicated that a s u b s t a n t i a l amount of the s u r f a c e was covered with hydrogen sulfide.
CO;, UPTAKE
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DISCUSSION
]-,
It is appropriate to consider the surface chemical reactions before proceeding to correlate the adsorption studies with crack growth observations. Kumnick and Johnson 5 determined the activation energies for permeation and diffusion of hydrogen in
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Fig. 5--Hydrogen uptake on a H2S a d s o r b e d surface. METALLURGICAL TRANSACTIONS A
(10-6 )
50
I00
150
200
250
500
PRESSURE, TORR
Fig. 6--Effect of the p r e s e n c e of H2S on hydrogen uptake on an iron f o i l VOLUME 7A, NOVEMBER 1976-1671
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250
300
Fig. 7--Competitive adsorption of H~S and SO2 on an iron surface.
a n n e a l e d i r o n to be 39.3 • 2.1 kJ 9mol -~ and 17.6 • 2.1 kJ 9mol -~ r e s p e c t i v e l y . The r a t e of a d s o r p t i o n of hyd r o g e n on i r o n v a r i e s with the s q u a r e r o o t of the p r e s s u r e . This is c o n s i s t e n t with the g e n e r a l view that hydrogen in the a d s o r b e d l a y e r i s p r e s e n t a s a t o m s . Since the a c t i v a t i o n e n e r g y for p e r m e a t i o n is equal to the s u m of the a c t i v a t i o n e n e r g i e s of diffusion and d i s s o c i a t i v e a d s o r p t i o n , the a c t i v a t i o n e n e r g y of the d i s s o c i a t i v e a d s o r p t i o n p r o c e s s of h y d r o g e n on i r o n should be 21.8 9 2.1 kJ 9mol -~. P o r t e r and T o m p k i n s 6 concluded that the r a t e d e t e r m i n i n g step in the a d s o r p tion p r o c e s s is the a c t i v a t e d m i g r a t i o n of h y d r o g e n a t o m s such that s u r f a c e s i t e s a r e made a v a i l a b l e for f u r t h e r a d s o r p t i o n . They f u r t h e r d e t e r m i n e d that a c t i v a t i o n e n e r g y f o r m i g r a t i o n was a f u n c t i o n of the f r a c t i o n of the s u r f a c e c o v e r e d (0) and v a r i e d f r o m 12.6 to 25.2 kJ 9m o F 1 while 0 v a r i e d f r o m 0.75 to 0.85. One would expect that the s u r f a c e c o v e r a g e d u r i n g a diffusion or p e r m e a t i o n e x p e r i m e n t to be quite high. The a g r e e m e n t b e t w e e n t h e s e two i n v e s t i g a t i o n s is t h e r e f o r e quite good. If the s u r f a c e c h e m i c a l r e a c t i o n ( d i s s o c i a t i v e adsorption) is one of the e a r l y steps in h y d r o g e n e m b r i t t l e m e n t one would expect that the a m o u n t of s u r face a r e a a v a i l a b l e and the n a t u r e of the h y d r o g e n c o n t a i n i n g s p e c i e s to be i m p o r t a n t f a c t o r s . R e c e n t l y Bowker and H a r d i e 7 e v a l u a t e d the i m p o r t a n c e of a v a i l a b l e s u r f a c e a r e a on the t e n s i l e d u c t i l i t y of a high s t r e n g t h s t e e l (H50) i n a hydrogen e n v i r o n m e n t . They s e l e c t i v e l y coated the gage length of the s p e c i m e n , t h e r e b y exposing d i f f e r e n t a m o u n t s of the s u r f a c e to hydrogen gas. T h e y found that the loss of ductility depended on the f r a c t i o n of s u r f a c e a r e a exposed to hyd r o g e n . F u r t h e r , K e r n s and Staehle 8 found that the c r a c k growth r a t e widely differed for d i f f e r e n t h y d r o gen containing gas (H2, HC1, HBr and H2S). The L a n g m u i r a d s o r p t i o n model s can be used to u n d e r s t a n d a d s o r p t i o n when m o r e than one g a s e o u s s p e c i e s is p r e s e n t in the e n v i r o n m e n t . If e q u a l p r e s 1672-VOLUME 7A, NOVEMBER 1976
s u r e s of two d i f f e r e n t m o l e c u l e s A and B a r e comp e t i n g f o r a d s o r p t i o n on the s a m e s u r f a c e , at a given t e m p e r a t u r e , the r a t i o
w h e r e 0 is the f r a c t i o n of the s u r f a c e covered, P the p r e s s u r e and AHa the heat of a d s o r p t i o n . T h i s exp r e s s i o n i n d i c a t e s that the gas s p e c i e s with the h i g h e r heat of a d s o r p t i o n ( m o r e negative) will p r e f e r e n t i a l l y adsorb. The heat of a d s o r p t i o n of oxygen on i r o n is g r e a t e r than that of hydrogen. 1~ Hancock and J o h n s o n 11 have found that the p r e s e n c e of s m a l l a m o u n t s of oxygen in a h y d r o g e n e n v i r o n m e n t i n h i b i t e d c r a c k growth in H-11 s t e e l . Liu and F i c a l o r a 2 found that c r a c k growth in 4340 s t e e l can be inhibited o r p r o m o t e d by s u i t a b l e a d d i t i o n s of s p e c i f i c g a s e s . A d s o r p t i o n s t u d i e s m e a s u r e the d y n a m i c e q u i l i b r i u m b e t w e e n the g a s e o u s and a d s o r b e d p h a s e s . In h y d r o gen e m b r i t t l e m e n t , t h e r e is continued d i s s o c i a t i o n of h y d r o g e n m o l e c u l e s and diffusion of the H atom to the c r i t i c a l l y s t r e s s e d r e g i o n s . A new d i m e n s i o n is added by the c r e a t i o n of e n e r g e t i c a l l y f a v o r a b l e s i t e s by the p r o p a g a t i n g c r a c k . Studies on m e t a l s u r f a c e s ~2'13 have shown that t h e r e is an i n c r e a s e in c a t a l y t i c a c t i v i t y , when s u r f a c e defects a r e i n c r e a s e d by t h e r m a l o r m e c h a n i c a l m e a n s . T h e n a i v e approach b a s e d on the L a n g m u i r model cannot t h e r e f o r e fully d e s c r i b e the d y n a m i c e m b r i t t l i n g p r o c e s s for the following r e a s o n s : a) In the p r e s s u r e and t e m p e r a t u r e r a n g e of d i s c u s s i o n t h e r e may be m u l t i l a y e r a d s o r p t i o n . The d i s t r i b u t i o n of A and B i n v a r i o u s l a y e r s could be i m p o r t a n t . When a f r e s h s u r f a c e is exposed, the m i g r a t i o n of a t o m s to this new a r e a could d e t e r m i n e the f i n a l o u t c o m e , b) The s t r o n g l y c h e m i s o r b e d l a y e r could a c t as a hydrogen t r a n s f e r stage. F o r e x a m p l e , H2S d i s s o c i a t e s on a d s o r p t i o n and the r e s u l t i n g H a t o m s could diffuse into the l a t t i c e . T h i s site is now f a v o r a b l e for hydrogen d i s s o c i a t i o n , c) The b a s i c a s s u m p t i o n i n the L a n g m u i r m o d e l that a l l s i t e s a r e e n e r g e t i c a l l y equivalent is not v a l i d . In hydrogen e m b r i t t l e m e n t the p r o p a g a t i n g c r a c k c r e a t e s e n e r g e t i cally a c t i v e s i t e s . T h e r e s u l t s of the c r a c k growth e x p e r i m e n t s ~ a r e shown i n F i g s . 8 and 9. A p r e c r a c k e d 4340 WOL s p e c i m e n t e n s i o n loaded in a h y d r o g e n e n v i r o n m e n t f a i l e d in a few m i n u t e s (Fig. 8(a)). T h e load applied in this e x p e r i m e n t was the m a x i m u m used in the c r a c k growth e x p e r i m e n t s . At the s a m e load a s p e c i m e n in a H2 + 3 pct SO2 (by volume) gas m i x t u r e did not fail f o r 3.9 h (Fig. 8(b)). At a r e d u c e d load, in a hydrogen a t m o s p h e r e , a r u n n i n g c r a c k was i n i t i a t e d in a s p e c i m e n . At the t e n t h m i n u t e 50 cc of SO2 was added to the e n v i r o n m e n t and c r a c k i n g c e a s e d (Fig. 8(c)). The ads o r p t i o n e x p e r i m e n t s showed that SO2 was i r r e v e r s i b l y t a k e n up on the i r o n s u r f a c e and excluded hydrogen a d s o r p t i o n . F u r t h e r when a n i r o n foil was exposed to a H2 + 3 pct SO2 m i x t u r e , SO2 was t a k e n up p r e d o m i n a n t l y . It is c l e a r , t h e r e f o r e , that SO2 can a b s o r b on a n i r o n s u r f a c e and exclude the d i s s o c i a t i v e a d s o r p t i o n n e c e s s a r y for h y d r o g e n e m b r i t t l e m e n t . T h e a d s o r p t i o n e x p e r i m e n t s with oxygen indicated that the bulk of a d s o r b e d oxygen was d e s o r b e d by e v a c u a t i n g the s y s t e m . The s m a l l a m o u n t which r e m a i n e d a d s o r b e d did not affect s u b s e q u e n t h y d r o g e n METALLURGICAL
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VOLUME 7 A , N O V E M B E R 1 9 7 6 - 1 6 7 3
a d s o r p t i o n . H o w e v e r it w a s found t h a t h y d r o g e n a d s o r p t i o n was r e t a r d e d when both oxygen and h y d r o g e n w e r e p r e s e n t in the g a s p h a s e . It i s known 11 t h a t the a d d i t i o n of s m a l l a m o u n t s of oxygen to a h y d r o g e n e n v i r o n m e n t a r r e s t s c r a c k p r o p a g a t i o n in s t e e l . Howe v e r when the oxygen i s r e m o v e d f r o m the e n v i r o n m e n t cracking resumes. T h e r o l e of H2S in both a d s o r p t i o n and c r a c k g r o w t h s t u d i e s is v e r y i n t e r e s t i n g . F i g . 8(d) shows t h e r e s u l t of a t e s t in which a s p e c i m e n was l o a d e d in a h y d r o g e n a t m o s p h e r e . T h e l o a d w a s s u f f i c i e n t l y low s u c h t h a t t h e r e was no c r a c k g r o w t h f o r 31 min. When 50 cc of H2S was added the c r a c k i m m e d i a t e l y b e g a n to g r o w . An a d d i t i o n of SO2 did not halt the c r a c k g r o w t h and the s p e c i m e n f a i l e d . The a d s o r p t i o n e x p e r i m e n t s c l e a r l y showed that H2S a d s o r b e d v e r y s t r o n g l y and i r r e v e r s i b l y . An a d s o r p t i o n - m a g n e t i z a t i o n s t u d y ~ of H2S a d s o r p t i o n on n i c k e l has shown that t h e f o l l o w i n g process occurs: HeScg, + Ni ( s u r f a c e ) -
H. . . . . . I -Ni-
/ Ni-
S ..... ~ Ni-
H I Ni-
If we assume that the adsorption-dissociation process for H~S on nickel is similar to that for H2S on iron then the accelerating effect of H~S can be explained. In embrittlement experiments with H2S, the hydrogen atoms produced from the adsorption-dissociation process can diffuse readily into the critically stressed zone. Hydrogenmolecules may then interact with the sites left vacant. The ability of H2S to make hydrogen atoms readily available was shown by Kemball.le He showed that two hydrogen atoms per molecule of H2S in the adsorbed layer were readily exchanged for deuterium atoms. On the basis of energetics alone the accelerating effect of HeS is reasonable. The H-H bond energy is 434 kJ 9mol-~ and H-S is 346 kJ 9mol-1.17 Hydrogen atoms are available from HeS at a lower energy than from H2. The adsorption experiments (Fig. 7) showed that the SO2 adsorbed surface could be reactivated with r e s p e c t to h y d r o g e n u p t a k e by i n t r o d u c i n g HzS. T h e r e s t o r a t i o n of h y d r o g e n uptake i n d i c a t e d that H2S d i s p l a c e d the bulk of SO2. If the amount of SOs in the g a s m i x t u r e is r e d u c e d , the d i s p l a c e m e n t of SO2 on the surface by H2S would be more complete. This is confirmed by the crack growth experiments. The restoration of hydrogen uptake by the addition of H2S in the adsorption study explains the renewed crack g r o w t h in F i g . 8(e) a f t e r the a d d i t i o n of H2S. F i g . 9(a) shows n i t r o g e n h a s l i t t l e o r no effect on the c r a c k growth r a t e . On iron, n i t r o g e n i s a d s o r b e d to a s m a l l extent only (T -- 273 K) and the p r o c e s s i t s e l f i s slow.X8 T h e a b i l i t y of a m m o n i a to poison an i r o n s u r f a c e and inhibit c r a c k g r o w t h i s s u r p r i s i n g ( F i g . 9(b), c o n s i d e r i n g that a m m o n i a can p r o v i d e h y d r o g e n a t o m s . W a h b a and K e m b a l l ~9 have s t u d i e d a m m o n i a c h e m i s o r p t i o n on i r o n at r o o m t e m p e r a t u r e and found that the amount a d s o r b e d i s r o u g h l y equal to that in hyd r o g e n c h e m i s o r p t i o n . T h e y concluded that the a m m o n i a m o l e c u l e m u s t c o v e r two s i t e s . T h e r e w a s a l s o s o m e h y d r o g e n l i b e r a t i o n f r o m the a d s o r b e d a m m o n i a . The o v e r a l l p r o c e s s m a y be w r i t t e n a s
2Fe+NH3r
Fe-H
+Fe-NH2
1674-VOLUME 7A, NOVEMBER 1976
Fe-H~
1
(1-x) Fe-H+(x),zH2~g~
w h e r e x is s m a l l . When the s u r f a c e was e x p o s e d to e x c e s s a m m o n i a , t h e r e was a t e n d e n c y f o r the c h e m i s o r b e d h y d r o g e n to be d i s p l a c e d by the a m i n e r a d i c a l s . It is l i k e l y that the a m i n e r a d i c a l s do not f u r t h e r d i s s o c i a t e to supply the h y d r o g e n . It is i n t e r e s t i n g to note that the t i m e d e l a y e x h i b i t e d in the inhibiting a c t i o n of a m m o n i a i s not p r e s e n t with SO2. The s t r o n g e r c h e m i s o r p t i o n of H2S d i s p l a c e s a b s o r b e d a m i n e r a d i c a l s t h e r e b y r e s t o r i n g the h y d r o g e n supply to the s u r f a c e of the m e t a l and the c r a c k p r o p a g a t e d (Fig. 9(b)). The e f f e c t of CO2 o b s e r v e d in the c r a c k g r o w t h e x p e r i m e n t s i s e v i d e n t f r o m the r e s u l t s of the a d s o r p t i o n e x p e r i m e n t s . C a r b o n d i o x i d e could be c o m p l e t e l y d e s o r b e d by p u m p i n g the s y s t e m down to a v a c u u m of 0.13 m P a . When CO2 was a d m i t t e d into the s y s t e m (0.33 k P a ) p r i o r to a h y d r o g e n a d s o r p t i o n e x p e r i m e n t , it was found that t h e r e was no d e s o r p t i o n of the i n i t i a l l y a d s o r b e d CO2. H y d r o g e n continued to a d s o r b . F i g . 9(c) s h o w s that the a d d i t i o n of CO2 did not s u b s t a n t i a l l y affect the c r a c k g r o w t h in a p u r e h y d r o g e n a t m o s p h e r e . Although no c r a c k g r o w t h e x p e r i m e n t s have b e e n done to d e t e r m i n e the inhibiting effect of n i t r o u s oxide on a c r a c k g r o w i n g in a h y d r o g e n a t m o s p h e r e , the r e s u l t s of the a d s o r p t i o n e x p e r i m e n t s c l e a r l y i n d i c a t e that the effect of n i t r o u s oxide will be s i m i l a r to C02. P o r t e r and T o m p k i n s ~ found t h a t an i n i t i a l e x p o s u r e of i r o n to c a r b o n monoxide i n h i b i t e d s u b s e q u e n t hyd r o g e n u p t a k e . It should be p o i n t e d out that the m a x i mum h y d r o g e n p r e s s u r e u s e d in t h e i r e x p e r i m e n t s was 13.3 P c . T h e heat of c h e m i s o r p t i o n of c a r b o n monoxide i s s l i g h t l y l a r g e r than the heat of c h e m i s o r p t i o n of h y d r o g e n on i r o n . One t h e r e f o r e e x p e c t s that the a d d i tion of CO to a c r a c k g r o w i n g in h y d r o g e n would inhibit the h y d r o g e n e n t r y into i r o n . T h i s was conf i r m e d in F i g . 9(d). The s u b s e q u e n t c y c l i n g of the load r e n e w e d the c r a c k growth. Since the amount of CO was s m a l l (50 cc) it is l i k e l y t h a t it was c o m p l e t e l y r e m o v e d f r o m the e n v i r o n m e n t by a d s o r p t i o n . When the c y c l i n g c r e a t e d new s u r f a c e s i t e s the hyd r o g e n e f f e c t was r e n e w e d . CONCLUSION T h e r e i s a definite c o r r e l a t i o n b e t w e e n h y d r o g e n e m b r i t t l e m e n t and a d s o r p t i o n . In f a c t t h i s c o r r e l a t i o n m a y find p r a c t i c a l value in a l l e v i a t i n g the 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 . The c o r r e l a t i o n f u r t h e r shows the i m p o r t a n c e of the c h e m i c a l r e a c t i o n . U n f o r t u n a t e l y the e l e c t r o n i c i n t e r a c t i o n s of a d s o r b a t e - s u r f a c e and a d s o r b a t e - a d s o r b a t e a r e not y e t w e l l enough u n d e r s t o o d to t r e a t the p r o b l e m in a m o r e q u a n t i t a t i v e m a n n e r . The p r o b l e m s b e c o m e even m o r e c o m p l e x when m o r e than one k i n d of a d s o r b a t e s p e c i e s i s i n v o l v e d . A l though e x p e r i m e n t a l o b s e r v a t i o n s f r o m c a t a l y s i s can be u s e d to p r e d i c t the effect of a p a r t i c u l a r gas on c r a c k g r o w t h in a h y d r o g e n e n v i r o n m e n t , caution m u s t b e e x e r c i s e d in t h i s p r o c e d u r e . F o r e x a m p l e , a l though H2S is a w e l l known c a t a l y t i c p o i s o n , it a c c e l e r a t e s c r a c k growth. F u r t h e r c r e a t i o n of a f r e s h s u r f a c e a s a c r a c k p r o p a g a t e s i n t r o d u c e s a new v a r i a b l e . A c o m b i n a t i o n of a d s o r p t i o n s t u d i e s , s u c h a s METALLURGICAL TRANSACTIONS A
those presented herein, crack growth experiments and permeability measurements c a n be u s e d to d e d u c e m o r e r e l i a b l e c o n c l u s i o n s on t h e p h e n o m e n o n of s e l e c tive adsorption as related to hydrogen embritt le m e nt. ACKNOWLEDGMENT The financial support provided by the Niagara Mohawk Power Corporation is gratefully acknowledged. REFERENCES 1. G. Sandoz: Met. Trans., 1972, vol. 3, p. 169. 2. H. W. Liu, Ya-Lung Hu, and P. J. Ficalora: Eng. Fract. Mec., 1973, vol. 5, p. 281. 3. S. P. Wolsky and E. J. Zdanuk: UltraMicro WeightDetermination in Controlled Environments, chapt. 1l, Wiley, N. Y. 1969. 4. V. Srikrishnan, H. W. Liu, and P~ J. Ficalora: Scr. Met., 1975, vol. 9, p. 663.
METALLURGICAL TRANSACTIONS A
5. A. J. Kumnick and H. H. Johnson: Met. Trans., 1974, vol. 5, p. 1199. 6. A. S. Porter and F. C. Tompkins: Proc. Roy. Soc., 1953, voL A 217, p. 544. 7. P. Bowker and D. Hardie: L 'Hydrogene dans les Metaux-Congre's Internationate, p. 284, Editions Science Et Industrie, Paris, 1972. 8. G. E. Kerns and R. W. Staehle: Scr. Met., 1972, vol. 6, p. 1189. 9. G. C. Bond: Catalysis by Metals, 1st ed., p. 72, Academic Press, New York, 1962. 10. Ibid: p. 79. 11. G. G. Hancock and H. H. Johnson: Trans. TMS-AIME, 1966, vol. 236, p. 513. 12. M. J. Duell and A. J. B. Robertson: Trans. Faraday Soc., 1961, vol. 57, p. 1416. 13. J. Eckell: Z. Etektrochem., 1939, vol. 39, p. 433. 14. Y. Hu: M.S. Thesis, Syracuse University, 1972. 15. I. E. Den Besten and P. W. Selwood: J. Catal., 1962, vol. 1, p. 93. 16. C. Kemball: Act. Cong. lntern. Catalyse, Paris, 1961, vol. 2, p. 1811. 17. JANAF Thermochemical Tables, The Dow Chemical Corp., Midland Mich., Aug. 1965. 18. V. Ponc and Z. Knoc: J. CataL, 1968, vol. 10, p. 73. 19. M. Wahba and C. Kemball: Trans. Faraday Soc., 1953, vol. 49, p. 1351.
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