Photo-electrochemical studies of the local dissolution of a hydrogen-charged X80 steel at crack-tip in a near-neutral pH solution

In this work, the photocurrent densities of a pre-cracked X80 pipeline steel under various applied forces in the absence and presence of hydrogen-charging were measured in a near-neutral pH solution through a photo-electrochemical system. Electrochemical impedance spectroscopy (EIS) was also measured on the steel electrode to obtain the local impedance under various test conditions. It was determined that, of the total photocurrent density measured at the crack-tip of the charged steel, the photo-induced current density, hydrogen-enhanced dissolution current density and photo-oxidative current density of hydrogen atoms contribute approximately 65.8%, 12.8% and 21.4%, respectively. An applied force increases the photocurrent density measured at crack-tip due to the enhancing stress concentration and the local electrochemical activity. In the presence of hydrogen-charging, the measured photocurrent density at individual applied force was higher than that measured on the uncharged electrode, which was associated with the hydrogen-induced dissolution current density and the photo-oxidation of hydrogen atoms. Under an identical applied force there was a higher photocurrent density at crack-tip than the region ahead of the crack for both charged and uncharged electrodes due to the high electrochemical activity at the crack-tip, resulting in more hydrogen atoms accumulating locally. The local electrochemical impedance measurements were consistent with the photo-electrochemical results to demonstrate the dependence of local dissolution rate of the steel at crack-tip on the applied force and hydrogen-charging.