Gaseous hydrogen permeation behavior of the base metal and weld metal of X42 and X52 pipeline steels under different hydrogen pressure

The effect of different hydrogen blending ratios in CH₄+H₂ environment and varying hydrogen pressures in pure H₂ environment on the gaseous hydrogen permeation behavior of base metal (BM) and weld metal (WM) of X42 and X52 pipeline steels were investigated in this study. The appropriate sample thickness for the gaseous hydrogen permeation tests was determined through preliminary experiments. Under the experimental conditions involved in this research, the hydrogen permeation process in all four materials is controlled by bulk diffusion and exhibited thermodynamic non-ideality. The steady-state hydrogen permeation current density is found to be proportional to the 0.5–0.75 power of hydrogen pressure ($P_{H_2}$), while the effective hydrogen diffusion coefficient is proportional to approximately the 0.3 power of $P_{H_2}$. Furthermore, the microstructure of four materials was analyzed, and their influence on hydrogen permeation behavior was evaluated. X42 WM contains a large amount of acicular ferrite and numerous large-angle grain boundaries, which impose the greatest resistance to hydrogen diffusion. X52 BM has banded F/P structure, where adjacent ferrite bands provide shorter diffusion paths for hydrogen, resulting in the highest effective hydrogen diffusion coefficient. Besides, the hydrogen permeation parameters of X42 BM and X52 WM are comparable.