Mechanism of Corrosion Control by Silica in Organic Composite Coated Steel Sheet

Synopsis : Organic composite coated steel sheets have been widely adopted by automobile and appliance manufacturers. Silica is usually used for organic composite coatings due to its advantages with regard to corrosion resistance. Although a number of reports have appeared on the mechanism of corrosion control by silica recently, its function in organic composite coating is not yet well understood. In this study, the influence of corrosion test conditions on the corrosion protection effect of silica was studied by exposing organic composite with silica coated electrogalvanized and Zn-Ni plated steel sheets in a cyclic corrosion test, salt spray test and humidity cabinet test. The influence of silica on the formation of zinc hydroxide was also studied by the potentiometric titration of zinc chloride solution with silica. The results indicate that silica accelerates the formation of zinc hydroxide chloride during the corrosion process, and corrosion is controlled as a result of the controlled reduction of oxygen by the zinc hydroxide chloride. Moreover, silica which dissolves during the corrosion process produces stable zinc silicate, and the zinc silicate acts as a barrier to control corrosion.