Discussion of "Evaluation of Hydrogen Embrittlement Mechanisms"* D. P. WILLIAMS

AND

H. G. NELSON

In a recent paper, ~9 Barth and Steigerwald attempted to clarify the controversy which exists with regard to the mechanism responsible for reversible hydrogen embrittlement of hydrogenated steels. They based their discussion on two sets of experiments concerned with i) the recovery during aging of the "incubation time" normally observed prior to the initiation of hydrogen-induced crack growth; and 2) the observation of hydrogen embrittlement at very low temperatures (-321~ Based on their interpretation of these experiments, they concluded that neither the "pressure theory ''2~ nor the "adsorption theory ''22'23 of hydrogen embrittlement can account for these observations and that the results of both sets of experiments support the "lattice-interaction theory" of Troiano. 24 The purpose of this note is to show that this conclusion is in no way justified and that, in fact, the experimental observations neither support nor contradict any of the three major theories. The "incubation-time" is a well-known characteristic of the hydrogen-embrittlement of hydrogenated steels and is related in each of the three major theories 21'z3'z4 to the time required for hydrogen transport to the location(s) of hydrogen-metal interaction. The hydrogen transport mechanism which determines the "incubationtime" and which exhibits an activation energy of about 9000 cal per mole is not simple lattice diffusion as implied by Barth and Steigerwald but is, rather, transient diffusion thought to be the movement of lattice hydrogen in equilibrium with hydrogen in "traps". 2~ In the "incubation time"-recovery experiments of Barth and Steigerwald, the kinetics of transport away from the hydrogen-metal interaction site were also found to exhibit an activation energy of about 9000 cal per mole. On the basis of this similarity, they conclude that the hydrogen-metal interaction at "voids" as required by both the "pressure theory" and the "adsorption theory" must not be involved and that the observation of this activation energy for recovery supports the "lattice-interaction theory". They state that, if the interaction site were at a void, the kinetics of the recovery of the "incubation time" would necessarily be characterized by a thermally-activated process having an activation e n e r g y e q u a l to t h e a c t i v a t i o n e n e r g y f o r d i s s o c i a t i o n and absorption. This statement is totally incorrect as will be shown below. O n l o a d i n g of a h y d r o g e n a t e d , n o t c h e d s p e c i m e n i n i tially having a uniformly distributed hydrogen concent r a t i o n , a s o l u b i l i t y g r a d i e n t i s e s t a b l i s h e d in t h e v i c i n i t y of t h e n o t c h d u e to t h e a p p l i e d s t r e s s g r a d i e n t . B e c a u s e of t h i s n o n e q u i l i b r i n m c o n d i t i o n , t r a n s i e n t d i f f u s i o n of h y d r o g e n w i l l o c c u r to the p o i n t of m a x i m u m *C. F. BARTH and E. A. STEIGERWALD, Met. Trans., 1970, vol. 1, 3451-55. D. P. WILLIAMS and H. G. NELSON are Research Scientists, NASAAmes Research Center, Moffett Field, Calif. Discussion submitted January 29, 1971. METALLURGICAL TRANSACTIONS

tensile stress. 26 Additionally, on loading, preexisting voids can coalesce and new voids can be created--primarily at the point of maximum tensile stress. 27 Presumably, the "incubation-time" is the time required to establish a sufficient concentration of hydrogen at the point of maximum tensile stress, either in the lattice or in the voids. As can be seen, regardless of the form of the interaction sites, the kinetics of the "incubation-time" will be given by the kinetics of hydrogen transport (transient diffusion). Considering the "incubation-time"-recovery experiments of Barth and Steigerwald, when the stress is released, a driving force will be established for hydrogen transport away from the point where maximum tensile stress occurred in order to again establish a uniform concentration of hydrogen within the specimen. When the uniform concentration has again been established, the "incubation-time" will have been fully recovered. For the hydrogen concentrated in the metal lattice beneath the root of the notch, the important step in reestablishing equilibrium will again be transient lattice diffusion. Therefore, the activation energy for the recovery should again be about 9000 cal per mole. For molecular hydrogen located in voids beneath the root of the notch, the hydrogen transport required to reestablish equilibrium when the load is relaxed will include the reaction steps of dissociation, adsorption, and absorption, as well as transient diffusion. The kinetics of the recovery process of embrittlement caused by hydrogen in voids will be related to the slowest of these necessary steps. These conditions are exactly analogous to experimental conditions which exist during gasphase permeation of a membrane. Based on most known hydrogen permeation studies in iron and steel, ~8'29 transient hydrogen diffusion is the slowest of these reaction steps and its kinetics are observed to control hydrogen transport under nonequilibrium conditions. Therefore, regardless of whether the hydrogen-metal interaction occurs in the lattice or at voids beneath the notch root, the kinetics of the recovery of the incubation time should be described by the kinetics of transient hydrogen diffusion and should be characterized by an activation energy of about 9000 cal per mole. Therefore, the statement t h a t the results of the recovery experiments support the "lattice-interaction theory" in perference to the other two theories is incorrect. Barth and Steigerwald also demonstrated that, if a specimen is hydrogenated to a sufficiently high level and subsequently tested at liquid-nitrogen temperature (-321~ the ductility of the specimen would be considerably lower than for an unhydrogenated specimen. They concluded that this observation also supports the "lattice-interaction theory" of embrittlement to the exclusion of the "adsorption-theory" and the "pressure theory". This conslusion is based on their contention that only the "lattice-interaction theory" would predict reversible embrittlement under conditions where rate processes occur negligibly slowly. Again, in our opinion, this conclusion is unjustified. Actually, it is difficult to reasonably explain the occurrence of reversible hydrogen embrittlement at -321~ by any of the three major theories, especially if the fracture is accompanied by appreciable plastic flow. All of the theories were developed to explain the generally observed cause of reversible embrittlement; that is, hydrogen-induced, subcritical crack growth. VOLUME 2, JULY 1971-1987

Since the rate of hydrogen-induced crack growth is dependent on the rate of arrival of hydrogen at, or near, the tip of the growing crack, no reversible embrittlement would be expected at a temperature as low as -321~ where the rate of hydrogen transport is negligible. This expectation is generally consistent with most experiments 3~ where the degree of embrittlemerit is found to decrease with decreasing temperature in the cryogenic temperature regime. However, under changing conditions which produce very high concentrations of hydrogen in the s p e c i m e n , S t e i g e r w a l d et al. ,32 F a y e t , 3~ and now, B a r t h and S t e i g e r w a l d have m e a s u r e d e m b r i t t l e m e n t at v e r y low t e m p e r a t u r e s . As s u g g e s t e d by B a r t h and S t e i g e r w a l d , this e m b r i t t l e m e n t m a y be due to a g e n e r a l r e d u c t i o n in d u c t i l i t y of the la tt ic e c a u s e d by the i n t e r a c t i o n of highly c o n c e n t r a t e d h y d r o gen with the m e t a l l a t t i c e ; h o w e v e r , it could just as w e l l be due to i n t e r a c t i o n s b e tw e e n d i s l o c a t i o n s and the s t r e s s fields a s s o c i a t e d with a high c o n c e n t r a t i o n of h y d r o g e n - f i l l e d voids. P r o v i d i n g that the s t r e s s e s w e r e r e d u c e d when the hydrogen was r e m o v e d , both of t h e s e m e c h a n i s m s would lead to r e v e r s i b l e e m b r i t t l e m e n t . In o u r opinion, it is m o r e l ik e l y , h o w e v e r , that as obs e r v e d by F ay et 33 e m b r i t t l e m e n t is due to the s h a r p e n ing of a propagating c r a c k by the linking of h y d r o g e n p r o d u c e d voids with the c r a c k tip. T h i s s h a r p e n i n g r e d u c e s the ductility of a s p e c i m e n b e c a u s e it a l l o w s the d e v e l o p m e n t of a h i g h e r s t r e s s s t a t e ahead of the notch. This m e c h a n i s m would a l s o c a u s e e m b r i t t l e m e n t at v e r y low t e m p e r a t u r e s ; h o w e v e r , this type of e m b r i t t l e m e n t is not r e v e r s i b l e u n l e s s the voids r e h e a l , o r c l o s e , on r e m o v a l of the hydrogen. B a r t h and S t e i g e r w a l d a t t e m p t e d to p r o v e , on the b a s i s of the data shown in t h e i r F i g . 8, that e m b r i t t l e m e n t at 321~ was r e v e r s i b l e . T h e y have i n t e r p r e t e d the data for the a s - c h a r g e d s p e c i m e n s and the data f o r the baked (hydrogen r e m o v e d ) s p e c i m e n s a s being best r e p r e s e n t e d by two d i f f e r e n t c u r v e s , t h e r e b y indicating s o m e d e g r e e of r e v e r s i b i l i t y of the l o w - t e m p e r a t u r e e m b r i t t l e m e n t . T h e s e data m a y a l s o be i n t e r p r e t e d d i f f e r e n t l y , h o w e v e r . The data can be equally w e l l r e p r e s e n t e d by a s i n g l e c u r v e having a shape s i m i l a r to that u s e d to d e s c r i b e the e a r l i e r l o w - t e m p e r a t u r e data of S t e i g e r w a l d e t a l . (Ref. 32, F i g . 5), if it is a s s u m e d that the s c a t t e r in the p r e s e n t data is of the s a m e o r d e r a s that on c h a r g i n g c u r r e n t vs hydrogen c o n c e n t r a t i o n i ndi c a t ed in the e a r l i e r w o r k . If this w e r e done, the data of F i g . 8 could be i n t e r p r e t e d as e v i d e n c e that the l o w - t e m p e r a t u r e e m b r i t t l e m e n t is i r r e v e r s i b l e r a t h e r than as e v i d e n c e that it is r e v e r s i b l e . In c o n c l u s i o n , n e i t h e r the r e c o v e r y e x p e r i m e n t s n o r the l o w - t e m p e r a t u r e e m b r i t t l e m e n t e x p e r i m e n t s conducted by B a r t h and S t e i g e r w a l d can be c o n s i d e r e d as c o n c l u s i v e e v i d e n c e fo r support of the " l a t t i c e - i n t e r a c t i o n t h e o r y " of h y d r o g e n e m b r i t t l e m e n t in p r e f e r e n c e to the " p r e s s u r e t h e o r y " o r the " a d s o r p t i o n t h e o r y " .

Authors' Reply C. F. BARTH

AND

E.A.

STEIGERWALD

Williams and Nelson in their discussion have failed to accurately appreciate the experimental technique involved and have not correctly interpreted the data presented in the paper "Evaluation of Hydrogen Embrittlement Mechanisms." Many of the criticisms which 1988-VOLUME 2, JULY 1971

they raise relative to the lattice embrittlement theory have been previously presented 34 and discussed, 3s'3~ over the past decade. In the case of the reversibility experiments Williams and Nelson agree that the incubation time is controlled by lattice diffusion and this incubation time is reversible with respect to the applied stress. They then question the interpretation on the basis that although the measured activation energy is consistent with a lattice diffusion model it is merely the slowest step and not necessarily the critical one. Work reviewed by Barrer, 37 data presented by Chang and Bennett, 3~ and permeability measurements by Barth, Steigerwald, and Troiano 39 indicate the importance of surface reactions and show that at small thicknesses and/or low temperatures surface reactions can indeed be rate controlling and lattice diffusion is not necessarily the slowest step in the process. On this basis the fact that lattice diffusion behavior is measured is a meaningful indication that the lattice embrittlement theory is operative. The most important point relative to the active mechanism however is that the incubation t i m e is r e v e r s i b l e w i t h r e s p e c t to the a p p l i e d s t r e s s . N e i t h e r the p r e s s u r e t h e o r y n o r the a d s o r p t i o n t h e o r y can d i r e c t l y account f o r this b e h a v i o r . In fact the e x planation that W i l l i a m s and N e l s o n p r o v i d e in t h e i r d i s c u s s i o n s u p p o r t s this c l a i m . F o r e x a m p l e , they s t a t e that hydrogen is a t t r a c t e d to r e g i o n s of m a x i m u n t e n s i l e s t r e s s in the vicinity of a void. P r e s u m a b l y , they a r e alluding to the p r o c e s s involving hydrogen diffusion into r e g i o n s of high t r i a x i a l s t r e s s s u r r o u n d i n g d e f e c t s o r n o t c h e s . E x a m i n a t i o n of this situation f r o m the standpoint of e i t h e r the p r e s s u r e t h e o r y o r the a d s o r p tion t h e o r y r e v e a l s that if the ap p l i ed s t r e s s is r e l a x e d there is little driving force for the hydrogen to dissociate and return into solution since the solubility in the region surrounding the void is actually decreased on removal of the external load. The fact that crack initiation is paced by the diffusion of hydrogen and that these cracks nucleate not at the surface but in the triaxial zone ahead of a notch or defect, a5 critically supports the lattice embrittlement mechanism. Secondary processes such as void coalescence or formation of additional voids can be complicating factors but are not considered basic to the embrittlement because experiments performed on specimens prestressed prior to c h a r g i n g and t e s t i n g p r o d u c e d no v a r i a t i o n in the delayed failure kinetics. As W i l l i a m s and Nelson i n d i c a t e , the low t e m p e r a ture r e v e r s i b l e embrittlement experiments are contrad i c t o r y to both the a d s o r p t i o n and p r e s s u r e t h e o r i e s . C o n t r a r y to t h e i r s t a t e m e n t s the r e s u l t s a r e in a g r e e ment with the l at t i ce e m b r i t t l e m e n t t h e o r y as i n i t i a l l y p r e s e n t e d by T r o i a n o . 35 The c o m p l e t e r a t i o n a l e behind t h e s e e x p e r i m e n t s have been p r e v i o u s l y r e v i e w e d 3s'4~ and w i l l not be r e p e a t e d h e r e . As s t a t e d in the c u r r e n t p ap er the low t e m p e r a t u r e r e v e r s i b l e e m b r i t t l e m e n t is an e x t r e m e l y c r i t i c a l r e s u l t . T h e s e r e s u l t s a r e in c o m p l e t e a g r e e m e n t with the p r e v i o u s low t e m p e r a t u r e s t u d i e s . E a c h set of data is i n t e r n a l l y c o n s i s t e n t and c l e a r l y i n d i c a t e s that r e v e r s i b l e low t e m p e r a t u r e hyd r o g e n e m b r i t t l e m e n t does ex i st o v e r a r e l a t i v e l y low l e v e l hydrogen content below that r e q u i r e d to p r o d u c e i r r e v e r s i b l e e m b r i t t l e m ent. 19. C. F. Barth and E. A. Steigerwald:Met. Trans., 1970, vol. 1, pp. 3451-55. 20. C. Zapffe: Trans. ASM, 1947,vol.39, p. 191. METALLURGICAL TRANSACTIONS