Localized EIS characterization of corrosion of steel at coating defect under cathodic protection

Localized electrochemical impedance spectroscopy (LEIS) technique was used to investigate localized corrosion of steel at defect of coating and, furthermore, to determine the effects of cathodic protection (CP) on local electrochemical environment and the resultant corrosion reaction at the base of coating defect. The results demonstrated that corrosion of steel is dependent on CP potential and the defect geometry. For coated steel containing defect under appropriate CP potentials, cathodic reaction is dominated by reduction of oxygen. Mass-transfer of oxygen through solution layer and the defect with a narrow, deep geometry (the depth/width ratio is about 5.5) is the rate-limiting step for the corrosion process of steel. Even at a very negative potential, e.g., -1200 mV (SCE), the measured impedance spectroscopy is still associated with the diffusion-controlled charge-transfer reaction of steel at the base of defect. It is attributed to the fact that the applied CP is partially shielded by the defect with a narrow, deep geometry. With the negative increase of cathodic potential, charge-transfer resistance and the local impedance of electrode increase. It is attributed to the elevation of local alkalinity at the base of defect when the coated steel is under CP. This conclusion is subject to the condition that a significant cathodic disbonding of coating has not occurred. Furthermore, with the increase of test time, the charge-transfer resistance increases, which is attributed to the enhancement of the alkaline environment at the base of defect under CP with time.