Abstract
The results of electrochemical impedance studies on an epoxy polyamide coated steel and an epoxy/chromate conversion coated magnesium alloy are discussed here. Both coating systems were immersed in room temperature aqueous sodium chloride solution. Correlation of impedance parameters obtained early in exposure with long term visual appearance demonstrates that certain impedance parameters (i.e. low frequency impedance, coating resistance, and breakpoint frequency) provide information on the long-term performance of the coated metal system. However, it will be shown that more conservative threshold impedance criteria must be utilized in the case of the coated magnesium alloy system in comparison to the coated steel system when attempting to predict long term performance. The differences in impedance parameter thresholds, it is hypothesized, is caused by the rapid corrosion rate that occurs at even the smallest defect sites in the case of the coated magnesium alloy system. The measurement, analyses, and equivalent circuit modeling procedures used to obtain EIS parameters and their correlation with changes in coating characteristics are reviewed.
