Anisotropy of hydrogen embrittlement in ferrite-pearlitic steel considering operational degradation

Green hydrogen currently plays an important role as a decarbonized energy carrier. It can be transported cost-effectively over long distances by pipelines. It is potentially possible to use existing gas transmission networks for this purpose. However, a significant part of gas transit pipelines is near the end of their design life, that is in-service degradation of gas pipeline steels should be taken into account at assessment of possibility of usage of existing gas network for hydrogen transportation safely. The main objective of this study was to assess whether degraded pipeline steels became more susceptible to hydrogen embrittlement than the as-delivered one and whether long-term operation influenced on hydrogen embrittlement anisotropy of pipeline steels. The low-carbon and low-alloyed pipeline steels from the point of view of their sensitivity to hydrogen embrittlement in the as-delivered state and after long-term operation were investigated. It was revealed that the anisotropy of strength properties was insignificant. However, the anisotropy is particularly pronounced in both plasticity characteristics, reduction in area and elongation, and hydrogen embrittlement susceptibility as well. A noticeable resistance anisotropy of pipeline steels to hydrogen embrittlement depending on the plane of fracture propagation relative to the rolling direction of the pipe, and also on degree of their operational degradation was revealed. Fractographic observation confirmed hydrogen embrittlement of pipeline steels caused by permeated hydrogen inside them. The pronounced anisotropy of hydrogen embrittlement susceptibility of operated pipeline steels can influence on the safe exploitation of natural gas pipelines at transportation of hydrogen or its mixture with natural gas.