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Hydrogen dissociation poisons and hydrogen embrittlement
Scripta
METALLURGICA
Vol. I01 pp, 8 7 1 ~ 8 7 3 1976 P r i n t e d in the U n i t e d ' S t a t e s
Pergamon
PreSS,
Inc.
HYDROGEN DISSOCIATION POISONS AND HYDROGEN EMBRITTLEMENT
B. J. Berkowltz, J. J. Burton, C. R. Helms, and R. S. Polizzotti Corporate Research Laboratories Exxon Research and Engineering Company Linden, New Jersey 07036 (Received
May
12,
1976)
Many metallurgical phenomena such as hydrogen embrittlement, corrosion, and sulfide stress cracking involve surface interactions between a metal and its environment. While these phenomena are of considerable importance and have been extensively explored in the metallurgical literature, they have rarely been approached from a catalysis/surface science point of view. In thls report we show that such a perspective can provide novel insight. A nember of elements, particularly from groups VA and VIA in the periodic chart, have been identified as hydrogen recombination poisons in electrochemical and metallurgical studies of hydrogen embrittlement (1-4). Although the mechanistic details are uncertain, it appears that the general effect of these elements in electrolytes is to inhibit the recombination of hydrogen adatoms at the metal cathode, and thereby promote the absorption of hydrogen by the metal. Similarily, when these elements are present at grain boundaries and free surfaces, intergranular fracture is promoted by enhanced hydrogen absorption (5-7). Interestingly many of these same elements have been identified as poisons for catalytic reactions which involve hydrogen dissociation (8). It appears that those elements which do not adsorb hydrogen are, in fact, hydrogen dissociation poisons, as they do readily adsorb atomic hydrogen (9). In Figure i wecompare known hydrogen recombination poisons [from the electrochemical literature (1-4)] with hydrogen dissociation poisons [poor hydrogen adsorbers from the surface science literature (i0)]. There are remarkable similarities between the two lists. It appears that elements which are hydrogen recombination poisons are in fact also hydrogen dissociation poisons. This presents an apparent paradox: A hydrogen dissociation poison inhibits the formation of atomic hydrogen, whereas a hydrogeu recombination paison inhibies the formation of molecular hydrogen. How can the same eleman~ play this dual role? We will show here that the resolution of this apparent paradox has metallurgi~al implications. First let us examine what a hydrogen recombination poison does. In a typical electrochemical experiment, a metal is cathodically polarized in an acid solution. Hydrogen ions are attracted to the metal surface where they are neutralized. The resultant hydrogen atoms can either be absorbed by the metal or recombine to form molecular hydrogen, which bubbles off the surface. This process is represented schematically as
~ k d1/2 H2(ads°rbed) )
kr
H+(solution) + e-
)
H°(adsorbed)
1/2
H2(bubbles) (i)
Ho
(absorbed)
871
872
HYDROGEN
DISSOCIATYON
POISONS
Vol,
i0,
NO,
I0
A hydrogen recombination poison slows the rate of formation of H2(adsorbed ) from H°(adsorbed) by reducing the recombination rate constant, ~ . Since adsorbed hydrogen atoms can either recombine to f o ~ H 2 or be absorbed~ slowing the recombination step may lead to the absorbtlon of more hydrogen atoms. A similar reaction scheme also describes hydrogen dissociation. In this Case, the source of the hydrogen is the molecular gas and the reactions can be represented by
1/2 ILl(gas) . ~ 1/2 H2(adsorbed ) x
~ H°(adsorbed)
(2)
The dissociation and recombination rate constants in Equation 2, kd and ~ , are the same rate constants which appear in Equation i. A hydrogen dissociation poison, reduces the dissociation rate constant, k d. At this point, the reason that hydrogen dissociation poisons and hydrogen recombination poisons are the same elements becomes apparent. A catalyst (or poison) generally affects the rate of a reaction in both directions; it does not change the equilibrium. The equilibrium constant for the reaction H 2 ~ H ° is kr/k d and therefore should not be seriously affected by a catalyst (or poison). Hence, a hydrogen recombination poison, which reduces ~ , should also be a dissociation poison and reduce k d. The crucial experimental difference between a recombination poison and a dissociation poison is a direct consequence of the nature of the external source of the hydrogen. In the catalysls experiment, the source of the hydrogen is molecular and the production of atomic hydrogen is hindered. In the electrochemical experiment, the source of the hydrogen is atomic and the production of molecular hydrogen is hindered. This has some very important metallurgical consequences. Consider an element which is a hydrogen embrittling agent in a cathodic charging experiment. It acts by poisoning hydrogen recombination and thereby assists hydrogen absorbtion into the metal. This same element on the surface of the metal can give the opposite effect when the source of the hydrogen is molecular. It slows the dissociation of H2, thereby reducing the amount of atomic hydrogen on the surface and hence the rate of absorbtion of hydrogen by the metal. In those cases where hydrogen absorbtion is the limiting step in embrlttlement, this effect could actually reduce the susceptibility.
Acknowledgements We thank T. E. Fischer and R. M. Latanision for their discussions of this work.
References 1.
M. Smlalowskl, Hydrogen in Steel, Pergamon Press, Oxford and Addison-Wesley Publishing Co., Reading, Massachusetts, (1962).
2.
T. P. Radhakrishnan and L. L. Shreir, Electrochem/ca Acre, ii, 1007 (1966).
3.
J. F. Newman and L. L. Shrelr, Corro. Scl., 9, 631 (1969).
4.
R. D. McCrlght and R. W. Staehle, J. Electrochem. Soc., 121, 609 (1974).
5.
R. M. Latanislon and H. Opperhauser, Jr., Met. Trans., 5, 483 (1974).
6.
R. M. Latanlsion and H. Opperhauser, Jr., Met. Trans., 6a, 233 (1975).
7.
I. M. Bernsteln and A. W. Thompson, Strengthening Mechanisms and Alloy Design, Chapter ii, Edited by J. K. Tien and G. S. Ansell, to be published, Academic Press~ 1976.
Vol.
I0, No.
i0
HYDROGEN
DISSOCIATION
POISONS
873
8.
D. O. Hayward and B. M. W. Trapnell, Chemisorptlon, (Butterworth & Co., Ltd.), London, 234 (1964).
9.
J. Pritchard and F. C. Tompkins, Trans. Faraday Soc., 56, 540 (1960).
i0.
G. Ehrlic~, Proceeding of Third International Congress on Catalysis, North Holland, Amsterdam, p. 113, (1964).
IA
IIA
IB
liB
IIA
IVA
VA
VIA
C
M~
Cu
Zn
Ag
Cd
Cs
Figure i. Comparison of known hydrogen recombination poisons (shaded) with hydrogen dissociation poisons. Note that all known hydrogen recombination poisons are also dissociation poisons.
METALLURGICA
Vol. I01 pp, 8 7 1 ~ 8 7 3 1976 P r i n t e d in the U n i t e d ' S t a t e s
Pergamon
PreSS,
Inc.
HYDROGEN DISSOCIATION POISONS AND HYDROGEN EMBRITTLEMENT
B. J. Berkowltz, J. J. Burton, C. R. Helms, and R. S. Polizzotti Corporate Research Laboratories Exxon Research and Engineering Company Linden, New Jersey 07036 (Received
May
12,
1976)
Many metallurgical phenomena such as hydrogen embrittlement, corrosion, and sulfide stress cracking involve surface interactions between a metal and its environment. While these phenomena are of considerable importance and have been extensively explored in the metallurgical literature, they have rarely been approached from a catalysis/surface science point of view. In thls report we show that such a perspective can provide novel insight. A nember of elements, particularly from groups VA and VIA in the periodic chart, have been identified as hydrogen recombination poisons in electrochemical and metallurgical studies of hydrogen embrittlement (1-4). Although the mechanistic details are uncertain, it appears that the general effect of these elements in electrolytes is to inhibit the recombination of hydrogen adatoms at the metal cathode, and thereby promote the absorption of hydrogen by the metal. Similarily, when these elements are present at grain boundaries and free surfaces, intergranular fracture is promoted by enhanced hydrogen absorption (5-7). Interestingly many of these same elements have been identified as poisons for catalytic reactions which involve hydrogen dissociation (8). It appears that those elements which do not adsorb hydrogen are, in fact, hydrogen dissociation poisons, as they do readily adsorb atomic hydrogen (9). In Figure i wecompare known hydrogen recombination poisons [from the electrochemical literature (1-4)] with hydrogen dissociation poisons [poor hydrogen adsorbers from the surface science literature (i0)]. There are remarkable similarities between the two lists. It appears that elements which are hydrogen recombination poisons are in fact also hydrogen dissociation poisons. This presents an apparent paradox: A hydrogen dissociation poison inhibits the formation of atomic hydrogen, whereas a hydrogeu recombination paison inhibies the formation of molecular hydrogen. How can the same eleman~ play this dual role? We will show here that the resolution of this apparent paradox has metallurgi~al implications. First let us examine what a hydrogen recombination poison does. In a typical electrochemical experiment, a metal is cathodically polarized in an acid solution. Hydrogen ions are attracted to the metal surface where they are neutralized. The resultant hydrogen atoms can either be absorbed by the metal or recombine to form molecular hydrogen, which bubbles off the surface. This process is represented schematically as
~ k d1/2 H2(ads°rbed) )
kr
H+(solution) + e-
)
H°(adsorbed)
1/2
H2(bubbles) (i)
Ho
(absorbed)
871
872
HYDROGEN
DISSOCIATYON
POISONS
Vol,
i0,
NO,
I0
A hydrogen recombination poison slows the rate of formation of H2(adsorbed ) from H°(adsorbed) by reducing the recombination rate constant, ~ . Since adsorbed hydrogen atoms can either recombine to f o ~ H 2 or be absorbed~ slowing the recombination step may lead to the absorbtlon of more hydrogen atoms. A similar reaction scheme also describes hydrogen dissociation. In this Case, the source of the hydrogen is the molecular gas and the reactions can be represented by
1/2 ILl(gas) . ~ 1/2 H2(adsorbed ) x
~ H°(adsorbed)
(2)
The dissociation and recombination rate constants in Equation 2, kd and ~ , are the same rate constants which appear in Equation i. A hydrogen dissociation poison, reduces the dissociation rate constant, k d. At this point, the reason that hydrogen dissociation poisons and hydrogen recombination poisons are the same elements becomes apparent. A catalyst (or poison) generally affects the rate of a reaction in both directions; it does not change the equilibrium. The equilibrium constant for the reaction H 2 ~ H ° is kr/k d and therefore should not be seriously affected by a catalyst (or poison). Hence, a hydrogen recombination poison, which reduces ~ , should also be a dissociation poison and reduce k d. The crucial experimental difference between a recombination poison and a dissociation poison is a direct consequence of the nature of the external source of the hydrogen. In the catalysls experiment, the source of the hydrogen is molecular and the production of atomic hydrogen is hindered. In the electrochemical experiment, the source of the hydrogen is atomic and the production of molecular hydrogen is hindered. This has some very important metallurgical consequences. Consider an element which is a hydrogen embrittling agent in a cathodic charging experiment. It acts by poisoning hydrogen recombination and thereby assists hydrogen absorbtion into the metal. This same element on the surface of the metal can give the opposite effect when the source of the hydrogen is molecular. It slows the dissociation of H2, thereby reducing the amount of atomic hydrogen on the surface and hence the rate of absorbtion of hydrogen by the metal. In those cases where hydrogen absorbtion is the limiting step in embrlttlement, this effect could actually reduce the susceptibility.
Acknowledgements We thank T. E. Fischer and R. M. Latanision for their discussions of this work.
References 1.
M. Smlalowskl, Hydrogen in Steel, Pergamon Press, Oxford and Addison-Wesley Publishing Co., Reading, Massachusetts, (1962).
2.
T. P. Radhakrishnan and L. L. Shreir, Electrochem/ca Acre, ii, 1007 (1966).
3.
J. F. Newman and L. L. Shrelr, Corro. Scl., 9, 631 (1969).
4.
R. D. McCrlght and R. W. Staehle, J. Electrochem. Soc., 121, 609 (1974).
5.
R. M. Latanislon and H. Opperhauser, Jr., Met. Trans., 5, 483 (1974).
6.
R. M. Latanlsion and H. Opperhauser, Jr., Met. Trans., 6a, 233 (1975).
7.
I. M. Bernsteln and A. W. Thompson, Strengthening Mechanisms and Alloy Design, Chapter ii, Edited by J. K. Tien and G. S. Ansell, to be published, Academic Press~ 1976.
Vol.
I0, No.
i0
HYDROGEN
DISSOCIATION
POISONS
873
8.
D. O. Hayward and B. M. W. Trapnell, Chemisorptlon, (Butterworth & Co., Ltd.), London, 234 (1964).
9.
J. Pritchard and F. C. Tompkins, Trans. Faraday Soc., 56, 540 (1960).
i0.
G. Ehrlic~, Proceeding of Third International Congress on Catalysis, North Holland, Amsterdam, p. 113, (1964).
IA
IIA
IB
liB
IIA
IVA
VA
VIA
C
M~
Cu
Zn
Ag
Cd
Cs
Figure i. Comparison of known hydrogen recombination poisons (shaded) with hydrogen dissociation poisons. Note that all known hydrogen recombination poisons are also dissociation poisons.