Effect of Ni Content in Zn-Ni Alloy Electroplated Steel Sheet on Zinc Phosphatability

Abstract

It is important to study the effect of the Ni content on phosphatability of Zn-Ni alloy electroplated steel sheets which are used in the exposed side of automotive bodies. Phosphate coating weights formed on pure Zn and Zn-Ni alloy containing Ni above 11.5% are independent of zinc phoshate solution flow rates and uniform coatings can be obtained. The alloys containing 3 to 9% Ni, however, show a strong flow rate dependency. A number of zinc phosphate crystal nuclei decreases and the final coating weight increases with a decrease in the flow rate. These alloys may cause trouble in the coating uniformity when they are used in the antomotive phosphating line where the flow rate is not uniform.