Modeling of hydrogen atom distribution at corrosion defect on existing pipelines repurposed for hydrogen transport under pressure fluctuations

While repurposing the existing pipelines for hydrogen transport contribute to accelerated development of the full-scale hydrogen economy, the suitability of the aged pipelines should be assessed on their hydrogen embrittlement (HE) susceptibility in high-pressure gaseous hydrogen environments. In this work, a threedimensional mechanics-hydrogen diffusion coupling finite element model was developed to determine the distribution of hydrogen (H) atoms at corrosion defect on pipelines under pressure fluctuations. Parametric effects, including corrosion defect dimensions (i.e., width, length, and depth) and pressure fluctuating parameters (i.e., cyclic load ratio and loading frequency), were determined. A high stress concentration exists in the longitudinal edge of the defect, while the stress level in the circumferential edge is low. The defect center is associated with the greatest stress and stress variation amplitude. H atoms tend to concentrate at the corrosion defect, especially the defect center, representing the most vulnerable site to initiate hydrogen-induced cracks. Most H atoms reside at the lattice sites, rather than the traps, indicating a limited capacity of the traps to host H atoms as compared to the crystalline lattice sites. With the increase in defect depth and length, both the stress level and stress variation amplitude at the corrosion defect are apparently elevated. More H atoms accumulate at the defect center, increasing the susceptibility to HE. As a comparison, the HE susceptibility of the corroded pipelines decreases with increased corrosion width. As the cyclic load ratio decreases, less H atoms accumulate at the corrosion defect. A decreasing cyclic loading frequency results in decreased stress variations but an increased H atom concentration at the corrosion center. The results provide a base to control pipeline HE by properly adjusting the operating pressure.