Electrochemical polarization behavior of X70 steel in thin carbonate/bicarbonate solution layers trapped under a disbonded coating and its implication on pipeline SCC

Potentiodynamic polarization measurements were performed on X70 pipeline steel in thin carbonate/bicarbonate solution layers trapped under a disbonded coating. A conceptual model was developed to illustrate the effects of the thickness and concentration of the trapped solution layer, cathodic protection (CP) potential and stress on stress corrosion crack initiation and propagation in pipelines. It was found that the passivity of the steel depended on the solution layer thickness, and the passive current density decreased with the thinning of the solution layer. With an increasing solution concentration, the role of the solution layer thickness in the steel passivity became unapparent, which was attributed to a strong passivating ability of bicarbonate and carbonate ions. Furthermore, with the decrease of the solution layer thickness, the pitting potential was shifted negatively. However, an increase of the solution concentration enhanced the resistance of the steel to pitting. A pre-application of CP would degrade the passivity of the steel due to the hydrogen-enhanced activity of the steel. Moreover, an applied stress shifted the pitting potential negatively, and decreased the passive potential range.