Numerical analysis of hydrogen atom diffusion and trapping at an unconstrained dent on pipelines

Abstract

In this work, a three-dimensional finite element-based mechanics-diffusion coupling model is developed to determine the stress/strain distribution and hydrogen (H) atom concentration at an unconstrained dent on an X52 steel pipe which is to be used for transport of high-pressure hydrogen gas. Both von Mises stress and equivalent plastic strain concentrations and a high hydrostatic stress gradient exist at the dent. The distribution of H atom concentration coincides with that of the hydrostatic stress. The maximum H atom concentration in the steel lattice locates at the dent bottom, while the distribution of the H atom concentration at metallurgical traps is consistent with that of the equivalent plastic strain. A competitive effect exists between the hydrostatic stress and the plastic strain on the H atom distribution. At low initial H atom concentrations (e.g., 0.001 mol/m3), the equivalent plastic strain dominates the distribution of H atoms. At high initial H atom concentrations (e.g., 0.1 and 1 mol/m3), the hydrostatic stress is more important to determine the distribution of H atom concentration. As the dent depth increases, both the von Mises stress and plastic strain at the dent increases, while the total H atom concentration decreases due to decreased hydrostatic stress. When the internal pressure increases from 5 MPa to 15 MPa, there are slightly increases in von Mises stress, equivalent plastic strain and the total H atom concentration.