Quantification of protective properties of the coating/corrosion product/steel interface by integration of transmission line model with EIS results

An experimental set up was designed to characterize and quantify the protective properties of coated steel samples over time. A porous layer of corrosion products (Fe _x O _y ) was formed on the metal surface prior to coating application to simulate the coating disbondment condition. The detailed sample system configuration is as follows: coal tar epoxy coating/corrosion product/steel substrate. A continuous damage evolution mechanism was proposed which contains the following three stages: (I) mass transport within the coating and the corrosion porous layer, (II) mass transport and charge transfer mechanism mixture due to the formation of active sites beneath the coating layer, and (III) charge transfer dominant mechanism due to adsorption and electrochemical reaction. This damage evolution concept was supported by the experimental results in this work. Based on the electrochemical impedance spectroscopy (EIS) technique, we observed that the sample system underwent stage I when initially immersed in an electrolyte solution. By evaluating the induced disbondment conditions, transitions from stage I to stages II and III were observed during the experimental period. After 55 days of exposure, stage III was identified in the EIS spectra. A transmission line model and EIS analysis was applied to each experimental condition. The simulated impedance magnitude has a good agreement with the experimental results.