One strategy to approach the challenge of hydrogen embrittlement in steels is to examine the metal surface to come up with strategies to minimize hydrogen penetration into the material.

This study explores how the orientation of monocrystalline grains of bcc-iron (as a model system for steel) at the surface influences the diffusion of atomic hydrogen into the material. Our objective was to identify surface characteristics that minimize hydrogen penetration, thereby informing strategies for improved steel microstructure design.

We conducted an extensive literature review of computational studies and performed static density functional theory (DFT) calculations on nine different low-index and vicinal bcc-iron surfaces. Using DFT, we mapped the potential energy surface of atomic hydrogen adsorption. Nudged elastic band (NEB) method calculations were then employed to determine energy barriers for hydrogen diffusion into the subsurface, using local adsorption sites as starting points. Electronic structure calculations complemented these methods, providing insights into the bonding mechanisms between hydrogen and iron atoms.

Our results reveal a previously unreported relationship between surface geometry, as described by stereographic projection, and hydrogen adsorption energy. This finding suggests that hydrogen adsorption on high-index surfaces can be characterized by local effects reminiscent of low-index (100), (110), and (111) surfaces. Furthermore, we found that the adsorption energy at the initial site significantly influences the energy barrier for subsurface diffusion.

These insights allow us to propose guidelines for the steel industry to design microstructures that limit hydrogen ingress at the surface. By preferentially orienting grains to minimize hydrogen adsorption and maximize diffusion barriers, it may be possible to enhance steel's resistance to hydrogen embrittlement, particularly in pipeline applications.