Synopsis Wet adhesion of paint film on Zn-Ni alloy electroplated steel sheet is significantly affected by the metal ion balance in phosphate solution. Variation of the Ni++ ratio in phosphate solution changes the Ni content o f phosphate film formed. As the Ni content of phosphate film increases, the wet adhesion of paint film improves. When the Ni content is sufficiently high, Zn and Zn-Ni alloy electroplated steel sheets show nearly the same wet adhesion of paint film as cold rolled steel

more »

... t. Some of the nickel contained in Zn-Ni alloy electroplated steel sheet dissolves into the phosphate film during phosphate treatment, causing the Ni content of the film to increase. On the other hand, it was found that there is close correlation between the wet adhesion of paint film and the return to Zn3 (PO4)2.4H2O in the water immersion process after baking. Namely, the less the degree of return to Zn3(PO4)2.4H5O, the better becomes the wet adhesion of paint film. When the wet adhesion is low, the precipitation of ZnO can be observed in the bottom of paint film. The occurrence of ZnO has a very significant e ffect on the wet adhesion of paint film. When the Ni content of phosphate film is increased by raising the Ni++ ratio in phosphate solution, both the return of Zn3(PO4)2.4H2O and the formation of ZnO are restrained, and thereby the wet adhesion of paint film is improved appreciably.