Hydrogen diffusion analysis in austenite phase of high-strength low-alloy steel

A major safety challenge of pipelines transporting hydrogen is embrittlement and its associated failures. Past incidents suggests that hydrogen alters steel's mechanical properties, potentially leading to catastrophic failure. Hydrogen embrittlement prevention (or delay) strategies require a fundamental understanding of hydrogen dissociation, adsorption modes, and subsequent diffusion into steels. This work presents an atomistic analysis of hydrogen diffusion in austenite microstructure, representative of the segment of high-strength low-alloy steels. It elucidates the diffusion energetics that govern the early stages of hydrogen ingress. This work used density functional theory calculations on two variants of austenitic microstructure: pure iron, and iron-carbon-manganese slab systems. Results indicate that in both cases, molecular hydrogen undergoes catalytic dissociative adsorption leading to exothermic adsorption at hollow sites. Diffusion into subsurface is primarily controlled by the first two layers of the surface, while subsequent diffusion into subsurface layers encounters a relatively low energy barrier. This study confirms that lateral surface diffusion is the energetically favored pathway upon catalytic dissociation of hydrogen molecules. Consequently, these findings provide DFT-informed criteria for surface coating design strategies targeting the surface layers to mitigate hydrogen dissociation and surface diffusion.