Silica-sol solution was prepared using tetraethoxysilane as a starting material. A silica-gel film was formed on both silica glass substrates and hot-dip galvanized steel plates by single-step dip coating and drying at room temperature. X-ray diffraction (XRD) analysis was performed on the films formed on the silica glasses and the steel plates; it revealed that the film formed on the former consisted of H2Si2O5 and both Zn2SiO4 and ZnNO3(OH) · H2O were detected in the film on the latter. From these results, it was cleared that the as-prepared silica-sol itself was H2Si2O5, however the silica-sol was changed to Zn2SiO4 when coated on the hot-dip galvanized steel plates resulting from a reaction with Zn on the surface of plates. The thickness of tripe-layered film formed by three-times dipping was estimated to be about 1.3μm by direct SEM observation. Using a salt spray test and a stack test, corrosion resistance of the hot-dip galvanized steel plates coated with the silica-sol was examined, in comparison with a chromate-coated and a naked hot-dip galvanized steel plates. The silica-sol coated plates exhibited superior corrosion resistance to both former and latter samples in each test. This might be explained in terms of the formation of Zn2SiO4 and ZnNO3(OH) · H2O. The coating materials, insoluble in water, can protect the steel plates from corrosive oxygen and electrolyte ions etc.
