Effect of gas impurities on the dissociative adsorption of hydrogen on Fe(100) surface of pipeline steel

Abstract

As common impurities in natural gas and hydrogen compatibility experiments, oxygen (O₂) and water (H₂O) have a significant impact on hydrogen compatibility of pipeline materials. The competition among gas molecules primarily occurs on the metal surface, where differences exist in the adsorption states of these gases. However, the mechanisms underlying their effects on hydrogen dissociative adsorption remain unclear. In this study, density functional theory (DFT) and first-principles molecular dynamics (FPMD) calculations were employed to model the adsorption of H₂, O₂, and H₂O on Fe(100) surface. The aim was to elucidate the mechanisms by which O₂ and H₂O affect hydrogen dissociative adsorption and to clarify the dynamics of the hydrogen dissociation process. The results reveal that O₂ exhibits high adsorption and dissociation activity. The adsorbed O atoms increase the energy barrier for hydrogen dissociative adsorption and weaken the interaction between hydrogen and iron atoms. In contrast to strong inhibitory effect of O₂, H₂O presents a more complex behavior. On one hand, H₂O can physically adsorb onto the pipeline surface, and reduces the stability of subsequently adsorbed H atoms without inhibiting hydrogen dissociation. On the other hand, charge exchange between H₂O molecule and substrate atoms causes the Fe surface to acquire a negative charge. This charge redistribution decreases the surface potential, making the steel more prone to corrosive cathodic hydrogen precipitation reactions, thereby promoting hydrogen entry. This study provides valuable insights into the mechanisms of competitive adsorption of impurity gases on pipeline steel surfaces, enhancing the understanding of hydrogen-material compatibility in pipeline systems.