Suppressing gaseous hydrogen embrittlement of Cr–Mo steel by introducing water vapor: Insights from experiments and calculations

As hydrogen emerges as a key energy carrier for carbon-neutral technologies, mitigating gaseous hydrogen embrittlement (GHE) in existing structural materials becomes a critical challenge for a seamless transition to a hydrogen economy, alongside the development of new hydrogen embrittlement-resistant materials. This work combined experimental studies and first-principles calculations to investigate the role of water vapor in mitigating GHE in the SCM435 low alloy steel. Fatigue crack growth (FCG) tests revealed that adding 991 vol ppm water vapor to a hydrogen environment markedly suppressed hydrogen-induced acceleration in the SCM435 steel. The crack growth rate in moist hydrogen was reduced by approximately 8 times compared to dry hydrogen for both strength levels, reaching levels comparable to those in air. Scanning electron microscopy analysis indicated that fracture surfaces in the moist hydrogen exhibited ductile transgranular fracture, contrasting with the quasi-cleavage and intergranular fracture features observed in dry hydrogen, confirming the protective effect of water vapor. Computational modeling showed that water molecules tended to adsorb on the clean Fe(110) surface in molecular form with an adsorption energy of −0.32 eV. Increasing water coverage raised the hydrogen dissociation barrier from 0 to 0.39 eV, reducing the dissociation rate constant by over 10⁷. These results suggest that trace amounts of water vapor can act as a practical GHE inhibitor, offering new perspectives for enhancing the reliability of structural materials in hydrogen-rich environments.