| 摘要 |
PROBLEM TO BE SOLVED: To improve the electrodeposition coating suitability of a steel sheet in the case of being used for an automobile in particular by forming a silica-added chromate coating film of the surface of a galvanized steel sheet and applying the upper layer of the coating film with an organic resin layer composed of resin and silica. SOLUTION: The surface of a galvanized steel sheet is subjected to silica-added chromate treatment for imparting adhesion with an organic resin layer and corrosion resistance thereto. The coating weight of Cr is regulated to 10 to 200mg/m<2> . The content of Cr<6+> in the chromate layer is regulated to 25 to 70wt.% to the total Cr content. The weight ratio of silica in the chromate layer is regulated to 0.5 to 3 to the total Cr content, and hydrophilic vapor phase silica having 0.005 to 0.1μm primary grain size is used. The weight ratio of phosphoric acid in the chromate layer is regulated to 0.25 to 2.0 to the total Cr content. For improving its adhesion with the plating, a fluorine compound is added thereto by a weight ratio of 0 to 0.4 to the total Cr content. Furthermore, for improving its electrodeposition coating suitability, an Mo compound is added thereto by a weight ratio of 0.01 to 1.0 to the total Cr content.
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