Abstract
Although Zn base alloy coated steel sheets show good corrosion resistance, their use in the exposed side of auto-bodies is not expected because of their poor phosphatability and paintability compared with those of uncoated cold rolled steel sheets.
To modify these properties, an Fe-P alloy layer (26 g/m2) is electroplated on Zn base alloy coated steel and double layer alloy coated steel has 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 painted specimens during corrosion tests. When the upper layer coating weight is below 6 g/m2, the corrosion behavior, such as red rust formation and thickness reduction, of the double layer alloy coated steels is similar to that of the single layer Zn alloy coated steels.
