The Effect of Pretreatment Dissolution on Electrocoat Corrosion Resistance

Previous studies have related the cationic electrocoat process to dissolution of phosphate pretreatments from a variety of substrates. Little has been reported however, on the effect the amount of dissolution has on the end-use film properties of the coating such as corrosion resistance.

Results of this work indicate that the corrosion resistance of a coating system can be impaired by phosphate dissolution but in some circumstances, especially with low level of dissolution, an improvement in the corrosion resistance can be obtained. Comparisons are also given of the effects on corrosion resistance of changes in the coat-out conditions from electrodeposition baths. Coated panels were subjected to corrosion conditions then monitored by ordinary visual inspection and by Electrochemical Impedance Spectroscopy (EIS). EIS results provide an insight to the chemical nature of the failure mechanism.