Dissociative adsorption of hydrogen at edge dislocation emergence on α-Fe studied by density functional theory

Abstract

Pipelines provide an efficient and economical method for hydrogen transportation with a high capacity over wide ranges and long distances, contributing to accelerated realization of a full-scale hydrogen economy. However, steel pipelines are prone to hydrogen embrittlement in high-pressure gaseous environments once the generated hydrogen (H) atoms enter the steels. Dislocations are one of the most common defects contained in steels. The H atom generation and adsorption at dislocation emergences on pipeline steels have not been fully understood. This work investigated and determined the stable hydrogen adsorption configurations at the emergence of an edge dislocation on iron (Fe) (100) crystalline plane by density functional theory. The partition function was used to study the thermodynamics of dissociative adsorption of hydrogen, as well as the effects of dissociated impurity gases (i.e., oxygen, methane and water vapor) existing in the carried fluid, under pipeline operating conditions. Results demonstrate that the dislocation core is the stable site for hydrogen adsorption, followed by the sites with a tensile strain at the dislocation, although the dissociative adsorption of hydrogen can also occur at the compressive strain sites. An elevated hydrogen gas pressure and low temperature favor the hydrogen adsorption. The three impurity gases can have dissociative adsorption occur at the edge dislocation. Moreover, their adsorptions are more thermodynamically favorable than hydrogen. Oxygen has the most stable adsorption configuration, followed by water vapor and then methane. Pre-adsorbed O-containing gases (i.e., oxygen and water vapor) compete with the hydrogen adsorption and decrease the stability of the hydrogen adsorption configuration. The competing effect of the impurity gases with hydrogen adsorption occurs only when they adsorb at the compressive sites of the dislocation. When the impurity gases pre-adsorb at the tensile strain sites of the dislocation, they have little influence on hydrogen adsorption.