The electrochemical hydrogen charging of pipeline steel was studied in a model solution of groundwater under a galvanostatic regime which was close to conditions of free corrosion. This corresponds to the real conditions of steel hydrogenation during the operation of the pipeline. The specimens were hydrogen-charged at different times to achieve the different total concentrations of hydrogen in the steel. Using a special experimental procedure, the total hydrogen concentration as well as the concentration of diffusible hydrogen, and the concentration of residual hydrogen in the steel were determined. It was found that within hydrogen concentration range CH ≤ 0.227 ppm all hydrogen in the steel is diffusible. Under CH ≥ 0.227 ppm some amount of the hydrogen remains diffusible, but the residual hydrogen appears in the steel. Based on these results, it was supposed that two different effects of the electrochemical hydrogen on the steel microstructure can exist: for hydrogen concentration range CH ≤ 0.227 ppm and for CH ≥ 0.227 ppm. The study of the case where only a low concentration of diffusible hydrogen is present in the steel showed that after the action of the diffusible hydrogen the micro and macro porous defects (voids) appear in the steel leading to an increase in the overall defectiveness of the steel and the number of defects and their total area increase with the increase in the concentration of diffusible hydrogen. This defectiveness affects the strength and other mechanical properties of the pipeline steel.
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