1999 Volume 85 Issue 1 Pages 39-44
The effect of Fe-15%Zn upper coating on stone chipping resistance of galvannealed steel sheet and Zn-Ni electroplated steel sheet was investigated by using gravelometer under the temperature of -20°C after three-coat painting (phosphate, electropaint, primer surfacer and top coat) for automobile outer panel. ( 1 ) The plating/base-steel interface damage diameter by the stone chipping decreases largely by plating Fe-Zn upper coating on zinc alloy coated steel sheets. On the other hand, the electropaint/phosphate interface damage diameter increases by Fe-Zn upper coating. ( 2 ) P-ratio [Phosphophyllite/(Phosphophyllite+ Hopeite)] of phosphate film of zinc alloy plated steel sheets increases by Fe-Zn upper coating. The peeling resistance between electropaint and phosphate film against impact becomes low when P-ratio is high.
From these results, the reason why the peeling at plating/steel interface decreases when Fe-Zn upper coating is applied seems as follows: Under the condition where Fe-Zn upper coating is applied to zinc alloy plated steel sheets, the peeling resistance at electropaint/phosphate interface is weaker than that at plating/steel interface, then the peeling occured at interface between electropaint and phosphate, which consumes the kinetic energy of gravel, so peeling at plating/steel interface becomes small.
