Study on the hydrogen compatibility and hydrogen diffusion behavior of X42 steel with notch under gaseous hydrogen environment

Pipeline defects can significantly affect the hydrogen embrittlement behavior of materials during the transportation of hydrogen-blended natural gas through in-service pipelines. This study introduces an inner notched specimen that more accurately simulates wall thickness reduction at defect sites. The impact of the notch on the material's mechanical properties was assessed through slow strain rate tensile tests. The hydrogen diffusion process was also analyzed using gaseous hydrogen permeation tests, which quantified changes in hydrogen flux due to the presence of the notch. Furthermore, microstructural characterization near the notch was performed using Electron Backscatter Diffraction (EBSD) to investigate the causes of the observed differences in test results induced by the defect. The findings indicate that susceptibility to hydrogen embrittlement at standard notches was lower than at inner notches with the same relative depth. Consequently, the critical safe hydrogen blending ratio determined from the standard notch may be conservative and should be reconsidered alongside results from the inner notch in practical applications. Notches positioned on the hydrogen-contacting side of the specimen increased the contact area between hydrogen and the metal surface, facilitating the dissociative adsorption of hydrogen molecules. This enhanced hydrogen adsorption and its subsequent penetration into the steel surface. In contrast, outer notches provided a different diffusion path for hydrogen atoms, with a greater decay in hydrogen concentration along the diffusion direction compared to smooth specimens. This led to fewer hydrogen atoms accumulating at the notch on the hydrogen-measuring side.