The electrochemical behavior and microstructure of chromium-free passivation film on Galvanized steel

In this paper, the chromium-free passivation solution was prepared by organic / inorganic process. The surface morphology and corrosion resistance of the composite films were investigated by scanning electron microscopy (SEM), atomic force microscope (AFM), and electrochemical impedance spectroscopy (EIS). The chromium-free passivation film was immersed in 3.5% NaCl solution, and the EIS changed with the immersed time. The results showed that the total impedance of chromium-free passivation film increased more than one order of magnitude compared with the galvanized steel, which is the same as that of chromate passivation film.With immersed time increasing, the impedance decreased, and the impedance stabilized after 8h. AFM and SEM results indicated that the film surface that was very flat was uniformly suspended by a lot of solid particles, in which the little hollow portion was 8nm. The framework structure of the film was cross-linked by resin emulsion and organic silane. After 96h NSS test, the galvanized steel surface appeared only a little pitting corrosion, which proved that corrosion resistance of galvanized steel increased obviously by the chromium-free passivation.