Evaluating organic coating performance by EIS: Correlation between long-term EIS measurements and corrosion of the metal substrate

In this study, the possibility of obtaining a corrosion indicator from a time series of electrochemical impedance spectra acquired during immersion of intact organic coatings was inspected. To generate a wide distribution in performance, coatings with different pigment volume concentrations were applied on AA2024-T3 and cold-rolled mild steel and tested in four different electrolytes and for different exposure times. The relative ranking of anticorrosion performance was based on the visual appearance of the metal substrates after stripping the coating and it correlated well with the estimation of the corrosion charge estimated from the time series of electrochemical impedance spectroscopy spectra. Corrosion testing of painted metals was performed in a range of conditions inducing different levels of degradation. Electrochemical impedance spectroscopy was measured periodically and, for each condition, the series of impedance data was used to calculate a numeric corrosion indicator that correlates with the corrosion on the metal substrate.image