The determination of coating performance with impedance measurements—IV. Protective mechanisms of anticorrosion pigments

Red lead and chromate pigments have been applied successfully to improve the performance of protective coatings for many years, but are now subjected to legislative restrictions. Zinc phosphate and aluminium flakes are popular but the protective mechanisms are not fully investigated. In this paper, which is Part IV of a series, results are presented on the anti-corrosion properties of two epoxy primers on steels containing small amounts of aluminium and chromium specially developed for application in seawater. One coating contained aluminium flakes and the other zinc phosphate. The anti-corrosion mechanisms of these non-barrier coatings was investigated with impedance measurements and SEM/EPMA. Results are compared to long term immersion performance in seawater. The interpretation of the impedance measurements is based on analysis techniques that have been presented in earlier papers in this series. The primers show in principle a similar impedance behaviour as model epoxy barrier type clearcoats. This facilitates the determination of water uptake, swelling of the coating polymer and loss of adhesion from the results of the impedance measurements based on the use of constant phase elements in the equivalent circuit. However the analysis of the results of impedance measurements on these primers requires the use of somewhat more complex circuits. It shows that the contribution of the coating film, the pores in the coating, the corrosion reaction and the diffusion of reactive species can be separated.