Characterization of the silica conversion film formed on zinc-electroplated steel

Abstract

The chemical conversion of zinc-electroplated steel sheet using colloidal silica was investigated as an alternative to chromate conversion. The corrosion resistance of zinc-electroplated steel sheet with and without this treatment was estimated from electrochemical impedance spectroscopy, open circuit potential in 3 mass% NaCl solution and the salt spray test. The solution used for the colloidal silica coating consisted of colloidal silica, titanium sulfate, cobalt sulfate, and succinic acid. This solution produced an adhesive film and maintained good corrosion resistance. Adding cobalt ions to the colloidal silica solution enhanced the adhesion between the silica film and the zinc layer electroplated on the steel sheet. In the electrochemical impedance analysis, the corrosion protection conferred by the colloidal silica film was about five times greater than that of a normal zinc-electroplated steel sheet, but not as good as that of a chromate film. In the salt spray test, rust appeared more quickly on chromate-coated specimens than on colloidal silica film surfaces.