Phosphate conversion coating treatment composition for metal surfaces, process for forming a protective coating on a metal, and manufactured product

摘要

A phosphate surface-treatment bath which contains a cationic organic polymeric compound (or salt thereof) that contains at least one cationic nitrogen atom and has a molecular weight of 1,000 to 1,000,000 forms a highly corrosion resistant film on metal surfaces (e.g., iron, zinc, aluminum, etc.) that is also an excellent paint undercoat. The film preferably is one in which the resin composed of the cationic organic compound (or salt thereof) has penetrated into the grain boundaries between phosphate crystals with an accompanying formation of a phosphate crystal/resin composite. A film formed in accordance with the present invention affords a remarkable improvement in corrosion resistance and paint adherence, compared with the phosphate conversion achieved with the same treatment, except for the omission of the organic polymer containing cationic nitrogen. In addition, even when the primer coating step is omitted, the present invention exhibits a performance approximately equivalent to the application of a primer coating on a conventional phosphate film.