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The surface of Zn-Ni alloy electroplated steel sheet is covered by a thin ZnO layer. The ZnO layer has many effects upon degreasing, formation of phosphate film. Furthermore, wet adhesion of a paint film on Zn-Ni alloy electroplated steel sheet is significantly. affected by the ZnO layer and also by the metal ion balance in the phosphate solution. Also, a change in the structure and the phase of phosphate film and, the stress changes in phosphate and paint films occurring during paint film baking and water immersion for wet adhesion testing, were analyzed to study their correlation with wet adhesion. As a result, the mechanism of wet adhesion of a paint film was clarified.

The newly developed sheet steel lightly coated with an organic composite is as follows. Zn-Ni alloy plated sheet steel with a coating weight of 30 g/m2 and average Ni concentration of 11.5 ∼12.0 % is chromated through electrolysis. The coating weight of chromate film is 50 ∼90 mg/m2 in Cr. Furthermore, emulsified olefin-acrylic acid copolymer resin mixed with colloidal silica of particle size 7 ∼8 nm applied to a thickness of 1.0 ∼1.8 μm. Olefin-acrylic acid copolymer resin and colloidal silica are mixed at the rate of 100 and 30 (parts by weight). It maintains excellent corrosion resistance even after forming, C-ED paint corrosion resistance and paint adhesion. Furthermore, it has excellent perforation resistance. The product has excellent weldability and is well suited to continuous forming, too.