Hydrogen induced cracking behaviors of X70 pipeline steel and its welds under electrochemical charging

The hydrogen induced cracking (HIC) of X70 pipeline steel was studied in H₂SO₄ solutions with different concentrations by means of electrochemical charging. Larger current densities, longer charging time and lower pH values of solution would promote the hydrogen atoms to enter into X70 steel. Non-metallic inclusions such as nitrides and oxides play different roles in HIC of X70 steel. Microscopic observation showed that there is no dependent relation between cracks induced by hydrogen and the inclusions of nitrides. Otherwise, the oxides of magnesium, aluminum and calcium etc are more harmful as the sources of hydrogen induced cracks. According to the results of hydrogen permeation test, the effective diffusivity (D_eff) of hydrogen at room temperature is 3.34 × l0^-9 cm²/s. Tensile specimens of X70 steel and welds showed severe plasticity loss after hydrogen charging, while the welds have lower plasticity than that of X70 base metal.