Performance Assessment and Prediction of Corrosion Prevention Compounds with Electrochemical Impedance Spectroscopy

Abstract

The performance of corrosion prevention compounds (CPC) on aluminum alloy (AA)7075-T6 (UNS A97075) was assessed using electrochemical impedance spectroscopy (EIS). The good correlation between the protection performance of CPC and both the interfacial impedance and double-layer capacitance allowed two assessment criteria to be defined. Excellent protection was exhibited by CPC-coated surfaces with interfacial impedances above 0.1 MΩ-cm2 or double-layer capacitances below 7.6 × 10-8 F/cm2. A correlation between the doublelayer capacitance (Cdl) and corroded area also was obtained. Thus, one also can evaluate CPC performance by calculating the corroded area from the measured capacitance. In this way, CPC performance could be ranked not only by the time period offull protection, but also by the rate at which the degradation spreads. In addition, a prediction method was demonstrated based on impedance parameters that showed the feasibility of using data from less than 50 days to predict the performance of CPC after 180 days of exposure. It was found that the CPC failure can be greatly accelerated without changing the relative ranking among the CPC used by introducing intentional scratches on CPC-coated specimens. In particular, the minimum time needed to rank various CPC was reduced to five days for scratched specimens from several months for unscratched ones. Other impedance parameters such as coating capacitance and resistance did not correlate with performance for any of the CPC studied.