Surface-to-bulk hydrogen transport in BCC iron: a computational review of adsorption and diffusion mechanisms

Abstract

Hydrogen embrittlement poses a significant challenge in various engineering applications. In this context, the iron/hydrogen system serves as a crucial prototype for examining the interaction between hydrogen and steel. Despite the critical importance of this topic, there is a notable lack of knowledge regarding the influence of surface characteristics on the penetration of hydrogen into materials. To address this gap, we have conducted an extensive review of the existing literature, particularly focusing on how different surface orientations of body-centered cubic (bcc) iron affect the adsorption, diffusion, and subsequent penetration of hydrogen into an iron crystal. The review primarily focuses on computational methods, incorporating experimental data for comparative analysis wherever feasible. This comprehensive synthesis of scattered information leads to several key conclusions. First, there is a systematic relationship between surface geometry and adsorption energy that has previously been overlooked. Second, bulk diffusion characteristics are recovered just a few atomic layers beneath the surface, emphasizing the importance of the initial surface layers in determining initial penetration. Third, penetrating through the surface layers is generally more challenging than further diffusion through bulk iron, with the specifics heavily dependent on the surface orientation. Studies on high-index surfaces are limited. We identify this as an area needing further research, and this review provides a solid foundation for such studies.