It was reported that small additions of oxygen to hydrogen gas could mitigate the hydrogen embrittlement of steels. Since atomic hydrogen dissolution in steels is responsible for embrittlement, catalysis of molecular hydrogen by the steel surface is an essential step in the embrittlement process. The most probable role of oxygen in mitigating hydrogen embrittlement is to inhibit the reactions between molecular hydrogen and the steel surface. In order to elucidate the mechanism of such surface reaction of hydrogen with the steel in the presence of oxygen, hydrogen and oxygen adsorption, dissociation, and coadsorption on Fe(100) surface were investigated using density functional theory. Potential energy surface scans were performed for gas molecules approaching the surface. It was found that H2 is captured by the surface at 2.3 � distance. The H2 molecule is dissociated and the H atomsa are relaxed on hollow sites over the iron surface. O2 is captured significantly deeper in the gas volume at a distance of 3.6 � from the top iron layer. The dissociation proceeds without formation of stable chemisorption states. The first derivatives of the potential energy change with respect to the distance from the surface, e.g., the forces acting on O2 and H2, are compared. The results show that the attractive force to the surface acting on O2 is an order of magnitude greater compared to the force acting on H2. H2 coadsorption is significantly hindered on iron surfaces with predissociated oxygen. The results are in good agreement and provide theoretical understanding for previously published experimental data regarding the inhibition of hydrogen embrittlement for steels exposed to hydrogen gas containing part-per-million traces of O2 impurities.

Hydrogen and oxygen coadsorption on iron surface: Insights from theory

246th National Meeting (2013)

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