Micro-electrochemical characterization and Mott-Schottky analysis of corrosion of welded X70 pipeline steel in carbonate/bicarbonate solution

The electrochemical corrosion behavior of welded X70 pipeline steel in a bicarbonate/carbonate solution was studied by scanning vibrating electrode technique and localized electrochemical impedance spectroscopy as well as Mott-Schottky analysis. The effect of applied stress on corrosion of various zones in welded steel specimen was investigated. Results demonstrated that passivity can be established on various zones, i.e., weld metal, heat-affected zone (HAZ) and base steel, of welded X70 steel. There is a higher passive current density for the passivated HAZ than that for weld metal and base steel. Applied stress enhances the anodic dissolution of steel, as indicated by the decreasing charge-transfer resistance and increasing dissolution current density of steel with the increase of stress. A maximum current density is observed in HAZ at all stressing levels. With the positive shift of anodic potential, the stability of passive film is enhanced, and the dissolution current density decreases. When the applied potential approaches the water oxidation potential, the current density increased significantly again due to the oxidation of water. The passive film formed is an n-type semiconductor. There are higher donor density and thinner space-charge layer in passive film formed at HAZ than those formed on weld metal and base steel. Moreover, the donor densities in all zones increased significantly with the applied stresses. The metallurgical transformations and formation of low temperature hardening phase transformation products during welding result in the enhanced electrochemical activity in HAZ. (C) 2009 Elsevier Ltd. All rights reserved.