AC impedance study on corrosion of 555%Al-Zn alloy-coated steel under thin electrolyte layers

The corrosion behavior of 55%Al-Zn alloy-coated steel under a chloride-containing electrolyte layer ranging from 15 to 888 mu m in thickness has been investigated using electrochemical impedance spectroscopy (EIS). The interface of the coating-electrolyte layer can be represented by an equivalent circuit consisting of a solution resistance, a charge transfer resistance, a double layer capacitance and a Warburg element. The corrosion current density estimated from the polarization resistance significantly decreases as the thickness of electrolyte layer increases between 15 and 100 mu m and is almost independent of the thickness up to 888 mu m. Furthermore, corrosion monitoring of 55%Al-Zn alloy-coated steel has been performed under alternate conditions of 1 h-immersion in solution of 0.05 M NaCl or 0.05 M Na2SO4 and 7 h-drying at 298 K and 60% RH. During the wet-dry cycles, the instantaneous corrosion rate was monitored by AC impedance method together with the corrosion potential. The corrosion rate increases at the initial stage of the wet-dry cycles because the native oxide on the coating surface is dissolved into the solution and converted to hydrated zinc and aluminum oxides, and then decreases slowly owing to the accumulation of the corrosion products over the coating surface. The corrosion mechanism of 55%Al-Zn coating is discussed on the basis of the monitoring results. (C) 2000 Elsevier Science Ltd. All rights reserved.