Corrosion protection property of colloidal silicate film on galvanized steel

Chemical conversion treatment of galvanized steel using colloidal silica was investigated as an alternative treatment to chromate conversion. The solution used for the colloidal silicate coating consisted of colloidal silicate, titanium sulfate, and nitrate ions to maintain good corrosion resistance. Adding CoSO4 to the colloidal silicate solution enhanced the adhesion between the silicate film and the zinc coat on the steel. The corrosion of galvanized steel with and without this treatment was investigated in 3 mass% NaCl solution using electrochemical techniques, such as electrochemical impedance spectroscopy, open circuit potential, and the salt spray test (SST). With immersion in NaCl solution, rust appeared on the surface of galvanized steel after 15–20 days, and after 40–50 days on silicate-conversion-coated specimens. In the SST, rust appeared on chromate-coated specimens more quickly than on those with a colloidal silicate film surface.