Electrochemical characteristics and surface morphology in non-chromate chemical conversion coating for Zn-electroplated steel sheets

The corrosion protection property and morphology of colloidal silica conversion films as an alternative to chemical conversion coating(CCC) films were examined. The corrosion behavior was investigated in 3%NaCl solution using electrochemical techniques. Corrosion was implied by the appearance of red rust on the specimen surface. The results show that in 3%NaCl solution, red rust appears at 15-20, 55-70, and 75-85 d on Zn-electroplated steel, colloidal silica conversion-coated specimens, and chemical conversion-coated, specimens, respectively. In the salt spray test, the colloidal silica film provides better corrosion protection than CCC film, i.e., red rust appears at 96 h on the Zn-electroplated steel sheet, at 432 h on the CCC films, and at 888 h on silica conversion coating.