Improvement of paintability with Fe–P coating on Zn–Fe and Zn–Ni alloy electroplated steels

Although the Zn–Fe and Zn–Ni alloy electroplated steel sheets show good corrosion resistance, their use in the exposed side of the body is not expected because of inferior phosphatability and paintability of the alloy surfaces to those of the cold rolled steel. To modify these properties, Fe–P(P≤0.5wt%) alloy (2g/m²) was electroplated on the Zn base alloy electroplated steels and the double layered alloy electroplated steels have been developed. The addition of P into the upper Fe layer contributes to the formation of finely packed phosphate crystals in both spray and dip type phosphate treatments. The upper Fe–P layer also helps to suppress the formation of crater form paint defects during the cathodic electrodeposition. Moreover, the Fe–P alloy upper layer reduces blisters at cross cut portions of three–coat painted specimens during corrosion tests. The upper layer does not affect the red rust formation due to its low coating weight. Thus, these pre–coated steel sheets are suitable to be used as exposed side of body panels.