Accelerated tests for the evaluation of the corrosion performance of coil-coated steel sheet: EIS under cathodic polarisation

Cathodic polarisation has been used as an accelerated test method to assess the degradation of coil-coated architectural cladding. Data were recorded via electrochemical impedance spectroscopy (EIS) measurements. Coil-coated steel (CCS) sheet with three different intermediate zinc-based metallic coatings were investigated, namely galvanised, galfan and aluzinc. Corrosion tests were performed by polarizing the painted panels to two cathodic potentials corresponding to either aluminium or magnesium sacrificial anodes while immersed in 3% NaCl aqueous solution in a horizontal flat-cell. The cathodic potential values considered were −1.082 and −1.522 V versus saturated calomel electrode (SCE). Cathodic polarisation conditions were found for which differences in the corrosion resistance of the three systems were detected from the analysis of the EIS data at significantly shorter exposure times than in the case of EIS data measured at their corresponding open circuit potential, thus providing an accelerated test method for determining the quality of the CCS system.