Crystallographic control of hydrogen ingress in bcc-iron: Insights from ab initio simulations

Hydrogen uptake into body-centered cubic (bcc) iron as a root cause for subsequent hydrogen embrittlement, is initiated at the surface. In this paper, we quantify how readily H diffuses from the surface into the bulk. We consider a set of 10 different Fe surfaces and treat H-permeation as a two-step process. First, density-functional calculations determine the adsorption energy of an isolated H atom at every crystallographically distinct surface site. Second, for each adsorption site we map the minimum-energy pathway between the surface and the lattice. Across all orientations studied, a clear trend emerges: sites that bind hydrogen most weakly are the starting point of the lowest-barrier diffusion channels into the metal interior. Thus, the least-favorable adsorption pockets act as “gateways” for subsurface penetration. This insight provides a practical design rule: minimizing exposure of such high-energy adsorption motifs should make bcc-iron components less susceptible to hydrogen uptake and the associated embrittlement. • Weak surface adsorption sites correspond to easy channels for H uptake. • Linear relationship between H diffusion barrier and local adsorption energy. • Weakening of adsorption energy near (110)-vicinal step explicable by d-band theory. • (100)-like surfaces exhibit two-step diffusion pathway facilitating H uptake.