Hydrogen permeation behavior at different positions in the normal direction of X42 and X52 pipeline steels

Microstructure is an important factor affecting hydrogen diffusion in pipeline steels. Due to the different stress and heat conditions, the surface and internal microstructure of the rolled pipeline steels is different. The ferrite grain size in surface layer of X42 pipeline steel is smaller than other locations, while the surface layer of X52 pipeline steel has continuous banded ferrite/pearlite structure and grains with uneven size. Hydrogen permeation behavior is investigated at different positions in the normal direction of X42 and X52 pipeline steels using the electrochemical hydrogen permeation technique. The results show that the values of the effective diffusion coefficient of surface layer in X42 and X52 pipeline steels are the smallest, while the values of the subsurface hydrogen concentration at steady state are the largest. Hydrogen microprint experiment results indicate hydrogen atoms mainly escape at pearlite, ferrite grain boundaries and near inclusions in X42 and X52 pipeline steels.