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

The use of underground hydrogen storage enables large-scale and cost-effective storage of hydrogen. However, the presence of carbon dioxide (CO₂) in underground hydrogen reservoirs can influence the interaction between hydrogen and the tubing materials. This study investigated the effects of varying CO₂ 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 CO₂ accelerates the FCGR compared to pure hydrogen. As the concentration of CO₂ increased, the FCGR initially rose and then declined, peaking in 0.008 MPa CO₂ + 7.992 MPa H₂ (0.1 vol% CO₂ + H₂). Additionally, the presence of CO₂ 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 CO₂ 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.