Hydrogen-assisted degradation in API 5L X52 pipeline steel: Mechanical behaviour, fractography and microstructural mechanisms

As hydrogen pipeline transportation is expected to play a key role in the future energy transition, understanding hydrogen-assisted cracking is essential for ensuring the safe and reliable operation of hydrogen infrastructure. The present work evaluated the hydrogen-assisted cracking behaviour of API 5L X52 pipeline steel through slow strain rate tensile testing, Charpy V-notch impact testing, Vickers hardness measurements, fractographic analysis, and microstructural characterisation. Electrochemical charging is used to provide hydrogen to specimens using 3.5 wt% NaCl solution containing 2 g/L thiourea. The mechanical properties, fracture behaviour, and microstructural features of hydrogen-charged specimens are compared with uncharged reference specimens. Hydrogen charging reduced ductility by 40.09-74.76%, confirming the susceptibility of API 5L X52 steel to hydrogen embrittlement. Similar brittle fracture morphologies across all charging durations suggest that hydrogen pre-charging beyond 24 h is ineffective. Increasing cathodic potential intensified hydrogen-assisted cracking, while transgranular crack propagation through ferrite grains remained the dominant cracking mode.