CO + O2 atmosphere as a superior inhibitor of gaseous hydrogen embrittlement in a martensitic steel: Fatigue crack growth and atomistic insights

The development of effective gaseous inhibitors is a pivotal strategy for mitigating gaseous hydrogen embrittlement in metallic materials. In this work, we demonstrate that a mixed carbon monoxide and oxygen (CO+O 2 ) atmosphere acts as a superior inhibitor, fully suppressing hydrogen-assisted cracking under the present test conditions in the JIS SCM435 (UTS = 954 MPa). Fatigue crack growth tests in 0.2 MPa H 2 revealed that, whereas 500 volume ppm CO or 50 volume ppm O 2 alone provided only partial mitigation, the CO+O 2 environment restored the fatigue resistance to the inert-environment baseline. Fractographic analysis confirmed a transition from GHE-induced facet and quasi-cleavage features to a typical ductile transgranular fracture surface under the CO+O 2 atmosphere. Density functional theory calculations elucidated that CO/O co-adsorption broadened the range of accessible high-coverage surface compositions on Fe(110), forming a passivating adlayer that increased the hydrogen dissociation barrier to ∼2.12 eV and thereby suppressed hydrogen uptake.