Effects of Silica Addition on Corrosion Resistance of Dry-in-place Chromate

The effects of fine silica particles on dry-in-place chromate coating system were examined in terms of the amount of silica addition, the drying temperature in treatment, and the corrosive environment on corrosion resistance. The mechanism behind corrosion resistance was analyzed in the terms of the corrosion prevention of silica particles in the chromate coatings by AC impedance and XPS. When silica is added to a solution of dry-in-place chromate, the anticorrosion effect is clearly improved due to the chemical and physical effect of silica particles, i. e., the chemical adsorption-desorption action of chromium ion and the physical resistance against the transmission of corrosive factors through the coating. We found that the optimum amount of silica addition and the optimum drying temperature for obtaining high corrosion resistance differed with the environment. These results indicate that the coating properties of silica contained in dry-in-place chromate are extrinsically determined by the corrosive environment.