Hydrogen embrittlement susceptibility of X65 pipeline steel pre-charged with hydrogen in hydrogen-blended natural gas with CO2: Influences of pressure and strain rate

• Investigated HE of X65 steel under varying strain rates and gas compositions. • Lower strain rates intensified HE, with a 25% reduction in elongation. • CO 2 addition enhanced hydrogen adsorption and diffusion, promoting HE. Hydrogen embrittlement (HE) behavior of X65 pipeline steel was investigated under varying strain rates and gas environments relevant to hydrogen transport systems. Slow strain rate tensile (SSRT) tests were performed at 1 × 10⁻⁶ s⁻¹ and 1 × 10⁻⁵ s⁻¹ in pure nitrogen, 30 vol.% H₂, and H₂-CO₂ mixtures at 4 and 12 MPa following 72 h of hydrogen pre-charging. At 4 MPa, elongation loss increased by ∼25% at a lower strain rate, indicating a strain-rate-dependent HE mechanism. Fractographic analysis revealed a transition from ductile dimples to quasi-cleavage features and secondary cracks in the presence of CO₂. Addition of CO₂ enhances hydrogen adsorption and diffusion, leading to increasing hydrogen trapping and accelerating crack propagation. These findings advance the mechanistic understanding of HE under mixed gas conditions and indicate combined influences of environmental composition and strain rate on structural integrity of pipeline steels in hydrogen services.