Content dependence of CO2 effect on the hydrogen embrittlement sensitivity of P110 steel for underground hydrogen storage: Experiments and DFT calculations

• Hydrogen-induced FCG in P110 steel occurs at Δ K = 17 MPa·m 1 / 2 . • CO 2 accelerates the emergence of the turning point Δ K T for HAFCG. • At the critical CO 2 concentration, HE in steel is at its most severe. • The dual impact of CO 2 is explained from a microscopic perspective. The use of underground hydrogen storage enables large-scale and cost-effective storage of hydrogen. However, the presence of carbon dioxide (CO 2 ) in underground hydrogen reservoirs can influence the interaction between hydrogen and the tubing materials. This study investigated the effects of varying CO 2 concentrations on the hydrogen-assisted cracking in P110 steel by a combined study of fatigue crack growth rate (FCGR) experiments and density functional theory (DFT) calculations. Experimental results indicated that the presence of CO 2 accelerates the FCGR compared to pure hydrogen. As the concentration of CO 2 increased, the FCGR initially rose and then declined, peaking in 0.008 MPa CO 2 + 7.992 MPa H 2 (0.1 vol% CO 2 + H 2 ). Additionally, the presence of CO 2 promoted the emergence of the critical stress intensity factor for hydrogen-accelerated fatigue crack growth, bringing the turning point forward from 27 MPa·m 1/2 to approximately 17 MPa·m 1/2 . DFT calculations elucidated that the activation energy for hydrogen dissolution first decreased and then increased as the partial pressure of CO 2 in the hydrogen environment rose. This suggested that the ease of hydrogen permeation into the steel surface initially decreased and then increased, in alignment with the experimental findings.