Why does hydrogen embrittle metals? Insights from quantum mechanics

This review consolidates density functional theory insights into hydrogen embrittlement accumulated over the past decade. Fundamental hydrogen-metal interactions spanning adsorption, dissolution, and diffusion are analysed across BCC, FCC, and HCP structures, revealing order-of-magnitude differences in transport kinetics governed by interstitial site geometry rather than crystal class alone. The competing HEDE, HELP, and HESIV mechanisms are critically evaluated, with computational evidence increasingly supporting synergistic operation whose balance depends on local microstructure and stress state. The paradoxical role of hydrogen traps is reconciled: efficacy depends on spatial distribution relative to failure sites, not merely binding strength. Electronic structure descriptors, particularly the d-band centre, provide a transferable screening framework for hydrogen-resistant compositions. Material-specific design principles encompassing alloying strategies, precipitate engineering, and multilayer coatings are extracted. Methodological advances including machine learning potentials and thermodynamic integration are assessed, and five future research priorities are identified.