Hydrogen promoted initiation and propagation of pitting and crevice corrosion of X70 micro-alloyed steel were characterized by potential dynamic measurements, the scanning reference electrode technique (SRET) and electrochemical impedance spectra (EIS). At open circuit potential, in situ SRET results show that hydrogen accelerates the nucleation and propagation of pitting of X70 steel. The pitting potential E-p of X70 steel gradually decreases with an increase of chloride ion concentration in NaHCO3 solution. Pre-charged hydrogen does not have a significant effect on the pitting potential E-p and open circuit potential E-corr of the steel in 0.5 M NaHCO3 solution. However, a synergistic effect of hydrogen and Cl- on the anodic dissolution and pitting potential of X70 steel is observed in 0.5 M NaHCO3 solution containing chloride ions. When crevices are present in X70 steel, hydrogen accelerates the initiation and progress of crevice corrosion. The mechanisms by which hydrogen promotes the initiation and propagation of pitting and crevice corrosion are proposed and discussed. (C) 2003 Elsevier Ltd. All rights reserved.