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

TL;DR

This study uses ab initio simulations to analyze how crystallographic surface orientations of bcc-iron influence hydrogen ingress, revealing that weakly binding sites facilitate easier hydrogen penetration into the metal.

Contribution

It provides a detailed atomic-level understanding of hydrogen diffusion pathways on various Fe surfaces, informing strategies to reduce hydrogen embrittlement.

Findings

Weakly binding surface sites serve as gateways for hydrogen entry.

Surface orientation significantly affects hydrogen adsorption and diffusion.

Designing surfaces to minimize high-energy adsorption sites can reduce hydrogen uptake.

Abstract

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 low-index, vicinal and general 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 that carries the atom beneath the surface and into the lattice. Across all ten orientations studied, a clear trend emerges: sites that bind hydrogen most weakly (highest adsorption energy) are the starting point of the lowest-barrier diffusion channels into the metal interior. Thus, the least-favorable adsorption pockets act as gateways for efficient subsurface penetration. These insights provide a practical design rule: suppressing or minimizing exposure of such high-energy adsorption motifs - through appropriate surface texturing or orientation control - should make bcc-iron components less susceptible to hydrogen uptake and the associated embrittlement.