Hydrogen-assisted degradation of base metal and heat affected zone in X65 pipeline steel welds with varying surface finishes evaluated by in-situ microcantilever bending

Repurposing the existing natural gas pipeline infrastructure for hydrogen applications poses safety concerns regarding hydrogen embrittlement. Pipeline weldments are particularly considered as vulnerable sites for hydrogen-induced degradation effects. To this end, this study evaluates the micromechanical interaction of hydrogen with the base metal (BM) and heat affected zone (HAZ) of API 5 L X65 pipeline steel. In-situ microcantilever bending complemented with high-resolution post-mortem characterisation was employed to reveal the hydrogen-assisted degradation of the BM and HAZ microstructure. As a second novel variable, the role of the surface condition in hydrogen environment was highlighted by comparing a polished reference surface with one containing a non-protective oxide layer. This unique approach reveals that hydrogen facilitates localised deformation by enhancing dislocation nucleation and reducing dislocation mobility. The polished HAZ exhibits a marginally better resistance to hydrogen embrittlement than the BM, attributed to the microstructural features. However, the oxide layer interacts synergistically with hydrogen, significantly deteriorating the mechanical integrity. Thus, the complete state of the pipeline determines the degree of hydrogen-assisted degradation, including both the microstructure and the surface condition. • Hydrogen embrittlement evaluation via in-situ cantilever bending of pipeline welds • Hydrogen-dislocation interactions drive localised deformation and degradation • The microstructure determines the hydrogen degradation of the BM and HAZ • Synergistic deterioration effects between hydrogen and a corrosion surface layer