Coupled stress–diffusion modeling of hydrogen embrittlement in API X52 and X70 pipeline steels

This present study numerically investigated the susceptibility of API 5L X52 and X70 pipeline steels to hydrogen embrittlement using a stress-assisted diffusion model implemented in COMSOL Multiphysics. Both materials were modelled with triangular fracture geometries having angle between 10° and 90°, aspect ratios ranging from 0.1 to 0.5, temperatures between 50 °C and 200 °C, initial hydrogen concentrations from 4 to 10 mol/m 3 , and prescribed displacements between 0.5 mm and 2 mm. The results for the percentage change in the hydrogen concentration and von Mises stress revealed significant material dependent differences in stress behavior and hydrogen accumulation. X52 exhibited pronounced stress relaxation at sharp flaws with aspect ratios of 0.1, where von Mises stress reductions approached 100%, attributed to extensive plastic deformation. In contrast, X70 maintained higher stress levels due to its greater strength and strain-hardening capacity but experienced abrupt stress collapses up to –69.3% at critical fracture lengths, reflecting localized instability. Hydrogen accumulation was also strongly influenced by geometry and temperature. X52 reached a maximum hydrogen concentration change of 496.9% at a 30° fracture angle, aspect ratio ( ) of 0.2, and 50 , while X70 recorded a substantially higher peak of 757.65% at a 30° fracture angle, aspect ratio of 0.1, and 50 ° C . Increasing temperature reduced hydrogen concentration in both steels, from 0.7034 to 0.4754 mol/m 3 in X52 and from 0.7419 to 0.5012 mol/m 3 in X70, confirming that low temperatures promote hydrogen trapping. X52 is more prone to plasticity-driven hydrogen accumulation, whereas X70 is more susceptible to abrupt stress-induced embrittlement. • A stress-assisted diffusion model in COMSOL Multiphysics predicted hydrogen embrittlement in X52 and X70 pipeline steels. • Sensitivity of fracture angle, aspect ratio, temperature, and hydrogen concentration was investigated. • X52 exhibited plasticity-driven hydrogen uptake peaking at 496.9% (30 0 , = 0.2, 50 ° C ). • X70 exhibited extreme hydrogen accumulation of 757.65% at sharp fractures. • Temperature demonstrated a profound influence on hydrogen accumulation due to trap destabilization.