Mitigation of hydrogen embrittlement of iron and steels by carbon monoxide in gaseous H2

This paper provides a comprehensive review of the authors’ recent achievements in mitigating hydrogen embrittlement (HE) in iron (Fe) and steels using carbon monoxide (CO) in gaseous H2. CO contained in H2 gas preferentially adsorbs on the Fe surface, and effectively suppresses hydrogen dissolution into the material, resulting in mitigating HE. Fracture toughness test results under various conditions demonstrated the effectiveness of CO in mitigating HE, with its impact influenced by factors such as loading rate, gas pressure, strength, and chemical composition. The CO mitigation effect diminishes with the lower loading rate, higher gas pressure, higher strength, and increased chromium content. The mechanisms underlying the effects of these factors on the susceptibility to HE are theoretically elucidated based on the surface reactions of the H2 and CO molecules with the Fe surface through density functional theory and molecular dynamics simulations.