Molecular hydrogen is widely proposed as an important energy carrier in the future sustainable energy society as a means to de-carbonize and green the energy economy. This increased use of hydrogen requires an extensive distribution system. For this reason, the transport of molecular hydrogen and natural gas mixtures via existing and refurbished gas pipelines is currently being explored.

However, there exist material durability concerns for existing pipelines that are repurposed to distribute hydrogen. The main materials of natural gas transmission pipelines are carbon steels which may be susceptible to hydrogen embrittlement (HE), especially at high hydrogen concentrations and pressures. Hydrogen embrittlement can cause irreparable damage to natural gas pipelines which are long-lived and hard to replace infrastructures.

This work studied the influence of steel chemical composition and impurity concentration (P and S), process parameters, initial strength and ductility levels, microstructure and stress triaxiality on the extent of HE in two pipeline steels, X42 and X70, in order to predict the material durability.

Process parameters influenced the initial strength and ductility of the steels while P and S levels only affected the initial ductility. It was found that, during slow strain rate tensile tests with in-situ Hydrogen charging, the initial ductility decreased by a maximum of 82% and the time to fracture decreased by a maximum of 64%. Increasing the stress triaxiality led to a further decrease in ductility of up to 71% and in time to fracture of up to 72% due to HE. Additionally, the fracture surfaces changed from ductile to quasi-cleavage after Hydrogen charging. Overall, hydrogen had a significant negative effect on the mechanical properties of all the steels examined. However, under the test conditions applied, there was no major influence of the P and S content on the degree of embrittlement.