Mitigation of hydrogen environment embrittlement of carbon steels A106 and A333 by addition of CO to H₂ gas and its gas pressure dependence

The addition of carbon monoxide (CO) to hydrogen (H₂) gas can mitigate the hydrogen environment embrittlement (HEE) of steels. The effect of gas pressure on the mitigation of the HEE of low carbon steels by the addition of CO was investigated in this study by a concerted effort of fracture toughness tests in H₂ gas containing CO and molecular dynamics (MD) simulations of the surface chemical reactions of H₂ and CO with the iron (Fe) surface. The degree of mitigation of the HEE depended on the gas pressure and CO concentration; the CO concentration to achieve complete HEE prevention was enhanced with an increase in the gas pressure, in other words, the CO mitigation effect was reduced by the increased gas pressure. The MD simulations revealed that the dissociation rate of dihydrogen molecules to atomic hydrogen on the Fe surface was significantly enhanced with an increase in the gas pressure, whereas the adsorption rate of CO on the Fe surface was almost independent of the gas pressure. These simulation results suggest that the hydrogen uptake into a steel is more effectively prohibited in a lower pressure H₂ gas containing CO, which accounts for the gas pressure dependence of the CO mitigation effect observed in the fracture toughness tests.