Patent ID: 7588801

Claim:
A chromium-free rust inhibitive treatment method for a metal product having a zinc surface comprising: coating the zinc surface of the metal product with an aqueous chromium-free conversion coating liquid to form a conversion-coated film on the zinc surface and applying a solution of a chromium-free surface treatment agent to the surface coated with the conversion coated film to form a siliceous film having an average thickness of 0.5 to 3 μm on the surface coated with the conversion coated film, the solution of the chromium-free surface treatment agent being alkoxysilane oligomer in an alcohol solvent and containing a component that changes into silica of 10 to 25% by weight as an amount converted to silica, wherein the alkoxysilane oligomer has been prepared by hydrolysis and condensation polymerization of tetraalkoxysilane together with acid catalyst, and the weight averaged molecular weight of the alkoxysilane oligomer is 1000 to 10000.