CO₂ effect on the fatigue crack growth of X80 pipeline steel in hydrogen-enriched natural gas: Experiment vs DFT

Utilizing the existing natural gas grid presents a promising method for transporting hydrogen on a large scale. However, the effects of natural gas and its impurities on hydrogen-assisted cracking in pipeline steel have not been adequately studied. This study aims to investigate the fatigue performance of X80 pipeline steel in hydrogen-enriched natural gas (HENG) environments and in mixtures containing the impurity CO 2 . This is achieved through fatigue crack growth rate (FCGR) tests and density functional theory (DFT) calculations. The experimental results show that the FCGR in H 2 is slightly faster than that in HENG, whereas it is slower than that in the N 2 /CO 2 /H 2 mixtures. The enhanced FCGR by CO 2 further increases with the increasing CO 2 content. DFT computational results indicate that the adsorbed CO 2 on the iron surface significantly accelerates the migration of H atoms from surface to subsurface. This promotes the entry of hydrogen into the steel.