Comprehending the Inhibitory Effects of Oxygen and Carbon Monoxide on the Hydrogen Embrittlement of Pipeline Steel Via Both Experimental Investigation and Simulation Calculation

For hydrogen-blended natural gas pipelines, trace O2 and CO inevitably exist as impurity gases and affect the interaction between hydrogen and pipeline materials, thereby altering hydrogen embrittlement (HE). In this study, gaseous hydrogen permeation and slow strain rate tensile (SSRT) tests were conducted to quantitatively compare the effects of O2 and CO on the HE of X42 pipeline steel dominated by body-centered cubic (bcc) α-Fe within the 0–100 vppm range. Combined with fracture surface and cross-section characterization of post-fracture specimens, the hydrogen-induced failure mechanism was clarified. Experimental results demonstrate that O2 has a stronger inhibitory effect on HE than CO at equivalent contents. As the blending content increases, the inhibition is continuously enhanced with a gradually weakening enhancement trend. Fracture characteristic analysis results indicate that the increase of tensile-directional grain length and average kernel average misorientation (KAM) value in the gas-side region after O2 or CO blending verifies the mitigated HE. Furthermore, O2 or CO blending mitigates the impacts induced by the synergistic action of the hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanisms on the tensile fracture behavior of the material. The density functional theory (DFT) calculation results show that pre-adsorbed O atoms or CO molecules on the Fe(110) surface increase the energy required for hydrogen dissociation, adsorption and diffusion. In particular, O atoms induce a greater energy increment, leading to stronger suppression of hydrogen permeation.