Microscale observation of hydrogen embrittlement induced by hydrogen traps in L245NS pipeline steel after eight years of hydrogen transportation

The mechanical properties and microstructure of decommissioned L245NS pipeline steel are investigated after 8 years of service in a 4 MPa pure hydrogen environment. It utilized China’s first samples of long-serviced hydrogen pipelines, which are valuable for evaluating the long-term performance of hydrogen pipeline steels. A key scientific regularity was revealed: In long-term high-pressure gaseous hydrogen, hydrogen in low-strength pipeline steels preferentially segregate at MnS/AlN precipitates, dislocations, grain boundaries, and micropores. This segregation reduces local cohesive energy (e.g., MnS lattice) and induces stress concentration, synergistically initiating and propagating secondary cracks—the core mechanism of hydrogen embrittlement (HE). Notably, MnS precipitates exhibit intrinsic HE susceptibility (hydrogen causes internal cracking), a previously unreported phenomenon. Slow strain tensile tests (significant cross-sectional shrinkage decrease, HE sensitivity index up to 17.79%) quantitatively validated this mechanism.