EIS behavior of anodized zinc in chloride environments

Electrochemical impedance spectroscopy (EIS) technique was applied to the corrosion study of the anodic coatings (ANC) on zinc. The ANC consists of a porous out-layer over a barrier layer and a modified one (CoANC) by adding cobalt ions to the electrolyte in the anodic treatment would refine the pores of the outer layer. The limited layer diffusion effect was found in the impedance plot of CoANC-coated zinc (CoANC-ZN) due to its finer pores. By comparing the EIS, the corrosion of both ANC-coated zinc (ANC-ZN) and CoANC-ZN was found changing from charge transfer control process to diffusion control process during the prolonged immersion in chloride environments. Two modes of corrosion process based on the immersion time and their equivalent circuits were proposed in considering the chemical products and physical structures resulting from the corrosion reactions. All parameters in equivalent circuits were adjusted simultaneously to obtain an optimum fit to the measured data. The simulated diagrams for the proposed equivalent circuits are in good agreement with the measured data.