Micro-electrochemical characterization of corrosion of welded X70 pipeline steel in near-neutral pH solution

The local corrosion behavior of welded X70 pipeline steel in near-neutral pH solution was studied by micro-electrochemical measurements, including scanning vibrating electrode and local electrochemical impedance spectroscopy. The microstructure of the welded steel was observed by optical microscopy and scanning electron microscopy. It is demonstrated that the microstructure of weld metal consists of acicular ferrite and grain boundary ferrite, while that of heat-affected zone is a mixture of acicular ferrite, bainitic ferrite and a few martensite/austenite microconstituents. The microstructure of base steel is typically ferrite and pearlite. Electrochemical corrosion mechanism of welded X70 steel does not experience change upon hydrogen-charging, or stressing, or both. Hydrogen-charging is capable of enhancing the local anodic dissolution of the steel. The resistance of corrosion product layer decreases with hydrogen-charging, and heat-affected zone has the largest dissolution current upon hydrogen-charging. The increase of applied stress enhanced the anodic dissolution of welded X70 steel, especially the heat-affected zone, in near-neutral pH solution. Maximum current is observed in heat-affected zone, and increases with the increase of applied stresses. The total synergistic effect of hydrogen-charging (10 mA/cm2) and applied stress (550 MPa) on anodic dissolution of welded X70 steel in near-neutral pH solution is determined to be within the range of 5.7 and 6.5, with a maximum value encountering in heat-affected zone.