Mechanistic investigation of hydrogen-enhanced anodic dissolution of X-70 pipe steel and its implication on near-neutral pH SCC of pipelines

The effects of hydrogen-charging on anodic dissolution of pipe steel under near-neutral pH condition were studied by electrochemical techniques. Hydrogen-charging enhances the anodic dissolution rate of the steel. The hydrogen-enhanced dissolution increases with increasing charging current density. The hydrogen effect is attributed to the alteration of chemical potential and exchange current density of steel. Hydrogen-charging affects the corrosion process of the steel. In particular, at a high charging current density, a layer of corrosion product forms on the electrode surface to change corrosion potential and interfacial double-charge layer capacitance as well as charge-transfer resistance. The hydrogen effect factor for enhanced anodic dissolution of steel at an anodic potential of -0.4 V (SCE) is 1.53 only. Hydrogen-enhanced anodic dissolution of steel by itself may not be the major factor contributing to the high rate of crack growth in pipe steel in near-neutral pH electrolyte. A further investigation of the synergistic effect of hydrogen and stress on dissolution at the crack-tip is essential to determine the mechanism of near-neutral pH stress corrosion cracking of pipelines. (C) 2007 Elsevier Ltd. All rights reserved.