ZINC-RICH PAINTS ON STEELS IN ARTIFICIAL SEAWATER BY ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

Electrochemical impedance spectroscopy (EIS) was applied over a wide frequency range to characterize the corrosion protection behavior by two types of zinc-rich paints (ZRP) (zinc-rich epoxy polyamide and zinc-rich ethyl silicate) on steels. Information concerning the properties of ZRP coatings has been evaluated from the changes of open-circuit potential and impedance spectrum with exposure time in ASTM artificial seawater. The influence of pigment volume concentration (PVC) and coating thickness was studied in order to determine the appropriate values that enable beneficial effects for improving corrosion protection. Characteristic coating parameters obtained from nonlinear fit routines of experimental impedance spectra at different exposure times, according to a circuit model consisting of a series combination of the uncompensated R(OMEGA) and a parallel R(t)-CPE element, correlate well with the gradual coating deterioration as the barrier-like properties of ZRP are diminished progressively. Results can be interpreted in terms of corrosion processes going on extensively through pores and cracks in the ZRP coatings.