Influence of stress on H 2 S adsorption on iron
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INTRODUCTION
A wide variety of experiments has shown that surface and interface adsorption can affect the bulk mechanical properties of materials. Examples range from the influence of C12 adsorption on crack growth in steel z to the effects of oxide films on the yield strength of crystals. 2 It has also been suggested ~that material deformation could alter surface adsorption. Because adsorption influences embrittlement and crack growth, such an effect should be included in models of environmentally enhanced crack growth. 3'4 The primary objective of this work was to measure directly any influence of stress on H2S adsorption on iron. Stress enhanced adsorption could also have implications for catalytic, corrosion, and wear properties of materials. Although no direct measurements of stress or strain enhanced adsorption have been reported, there are good reasons to expect such an effect. The higher dislocation density in strained materials provides a first reason to expect stress to influence reactivity and adsorption. The presence of either line, 5 step, 6 or point defects 7 at a surface are known to enhance surface reactivity. Specifically, stepped Si surfaces were found to be ten times more reactive to O than are smooth unstepped surfaces, 8 H and O have been observed to bind to defects on Si 9 and Ni, ~~respectively, and ion beam damage has been found to increase the sticking coefficient of O adsorption on Pt(111). ~1Second, the applied stress may itself influence adsorption rates. Calculations by Chiftan and Ruck ~a indicate that mechanical stresses in a substrate will alter the chemical potential of an adsorbed atom. This effect would influence both adsorption and diffusion and could play a role in stress corrosion and crack propagation. In spite of the many observations of defect enhanced reactivity, only recently have systematic studies of such effects been undertaken. 6 Because reactivity is strictly well defined only for local equilibrium conditions,~3 most studies have involved careful examination of single crystal surfaces with known orientation and step density. 6 Although such experiments show that defects often can and do influence reactivity, they do not fully duplicate the circumstances likely to be present during crack initiation and growth,
D. R. BAER, Senior Research Scientist, and M. T. THOMAS and R. H. JONES, Staff Scientists, are with the Materials Department, Battelle, Pacific Northwest Laboratory, P.O. Box 999, Richland, WA 99352. Manuscript submitted January 31, 1983.
METALLURGICALTRANSACTIONSA
which are the motivation of the current study. This study of adsorption on stressed surfaces was undertaken to help the understanding of adsorption as it might relate to crack initiation and growth. Initial results, showing that an applied stress enhances adsorption of HzS on iron, are reported in this paper. The adsorption of H2S on iron was chosen because H2S enhances fracture of many engineering alloys and effects of stress on its adsorption would be of particular interest. There have been r
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