Hydrogen embrittlement of steels: trap the hydrogen before it finds the crack

A tensile specimen cut open to its grains and read as the path of one hydrogen atom: H₂ splits at the surface, the atoms hop between tetrahedral sites of the iron lattice, defects hold some back, fine carbides lock them away and, where hydrogen gets through, a crack opens. Five cards explain each station, from the sources of hydrogen to the two mechanisms (HEDE and HELP) and the designed traps that make steels more resistant.

1. Entry: H₂ dissociates into atoms at the surface, probably the rate-limiting step of entry (H–H bond about 4.48 eV, lowered at a surface); gas, electrochemical reactions and corrosion all supply hydrogen (Yu et al. 2024; Shulin et al. 2026). 2. Diffusion: In iron, H sits in tetrahedral interstitial sites and hops between them over a barrier of 0.088 eV; its diffusivity in α-Fe can be five orders of magnitude higher than in nickel (Yu et al. 2024). 3. Trapping: Vacancies, dislocations, grain boundaries and phase boundaries trap H reversibly; carbides and microvoids trap it irreversibly (binding energy > 60 kJ/mol); deeper traps release H at higher temperature in thermal desorption (Yu et al. 2024). 4. Designed traps: Atom probe shows H inside fine V–Mo–Nb carbides (Chen et al. 2017) and at incoherent NbC interfaces, dislocations and grain boundaries (Chen et al. 2020); 2–4 nm κ-carbides mitigate embrittlement, coarse grain-boundary carbides worsen it (Xiong et al. 2024). 5. Embrittlement: HEDE (weakened lattice and interfaces) and HELP (more mobile dislocations) act together; H cut a pipeline steel’s strength by over 50 % and its ductility by over 90 % (Yu et al. 2024); Mn-rich zones that arrest microcracks doubled HE resistance (Sun et al. 2021).

Starting materials: Hydrogen from gas, electrochemical charging or corrosion. Product: A steel whose designed traps keep hydrogen away from crack-prone sites.

Yu et al., “Hydrogen Embrittlement as a Conspicuous Material Challenge─Comprehensive Review and Future Directions”, Chemical Reviews 2024 (doi:10.1021/acs.chemrev.3c00624); Chen et al., “Direct observation of individual hydrogen atoms at trapping sites in a ferritic steel”, Science 2017 (doi:10.1126/science.aal2418); Chen et al., “Observation of hydrogen trapping at dislocations, grain boundaries, and precipitates”, Science 2020 (doi:10.1126/science.aaz0122); Sun et al., “Chemical heterogeneity enhances hydrogen resistance in high-strength steels”, Nature Materials 2021 (doi:10.1038/s41563-021-01050-y); Xiong et al., “Impact of Size and Distribution of k-Carbides on the Hydrogen Embrittlement and Trapping Behaviors of a Fe-Mn-Al-C Low-Density Steel”, Materials 2024 (doi:10.3390/ma17112698); Shulin et al., “Mechanisms of hydrogen embrittlement, mitigation strategies, and emerging materials for hydrogen energy and advanced manufacturing”, Frontiers in Chemistry 2026 (doi:10.3389/fchem.2026.1957974).

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