Finite element modeling of hydrogen atom diffusion and distribution at pore defects in circumferential welds of pipelines

Abstract

Assessing the hydrogen embrittlement (HE) susceptibility of circumferential welds of pipelines with pore defects in gaseous hydrogen environments contributes to the safe operation of hydrogen-blended pipelines. A three-dimensional finite element model coupling defect-mechanics-hydrogen diffusion was developed in this study to determine the variation of hydrogen concentration in both lattice and trap sites in the pore region, influenced by pore defect location, size, and cyclic loading frequency. Results show that the distribution of the hydrogen atom concentration at lattice sites resembles the distribution of hydrostatic stress and distribution at trap sites resembles the distribution of plastic strain. Most hydrogen atoms reside in lattice sites rather than dislocation traps. The hydrostatic stress and plastic strain in the pore region change with the pore location due to the constraint conditions during the welding process. The hydrogen atom concentration in pore region at WL-4 and WL-5 (i.e., the filling layers) is relatively high, making these regions susceptible to hydrogen-induced cracking. The strain level at pore defects increases significantly with pore size, accompanied by a rise in hydrogen concentration at trap sites. When the pore diameter is 1 mm, the hydrogen concentration in the pore region reaches its peak level. Even smaller pore defects (with diameter is 0.7 mm) can result in hydrogen enrichment. Cyclic internal pressure reduces the hydrostatic stress level and total hydrogen concentration in the pore region. Under fluctuating internal pressure, low-frequency loading poses the greatest threat to the structural integrity and hydrogen embrittlement resistance of pipeline joints.