Effects of Zinc Phosphating on Anti-cratering Properties of Steel Sheets in Electrodeposition Coatings

Effects of zinc phosphating on the cratering phenomena during cathodic electrodeposition coating were studied. Cold rolled(CR), electrogalvanized(EG), double-layer zinc-iron electroplated (Zn-Fe) and galvannealed sheet(GA) were used as test materials. The voltage applied between the anode and the cathode in a plating solution was increased at a constant rate from zero to a certain value (200 to 400 V), and then kept constant. Two peaks were observed in the change of the current with time in the coating process. The peak current ratio, iB (second peak)/ iA(first peak), correlated to the crater density. Phosphate treatment increased the crater density and brought about a high peak current ratio. This tendency was the most remarkable on GA. For all kinds of steel sheets both the value of iA and iA/V(voltage) were decreased by phosphating. The electric resistance calculated from the current-time relation did not depend on the materials.