The Effect of the Electrodeposition Process on Pretreatments

The introduction of zinc coated steels, multication pretreatments and cationic electrodeposition primers has led to improved automobile corrosion resistance. However, zinc phosphates over zinc coated steel are more sensitive to the electrodeposition process than are those over cold rolled steel. This paper is concerned with the dissolution of zinc phosphate pretreatments during electrodeposition. Dissolution was greater than expected, but did not appear to hurt the overall performance of the painted metal. A small amount of dissolution may even allow better anchoring of the E.D. coating. Dissolution was greater at higher voltages and under rupture conditions. There was a definite connection between rupture behavior (electrical discharge) and pretreatment dissolution. Due to their lower alkali solubility, Zn-Mn-Ni phosphate pretreatments were more resistant to attack during electrodeposition than conventional phosphates.