Cratering Mechanism of Cationic Electrodeposition Primer over Precoated Steel Sheets

Cationic electrodeposition primers have been widely adopted by automobile companies due to their advantages with regared to application and corrosion resistance. However, under certain conditions, cationic electrodeposition primers can give craters over precoated steel sheets. Therefore, it is important to establish the exact mechanism of cratering from the viewpoint of cratering control.In this study, resistance-time profile which were calculated by means of measured current and voltage at the initial stage of electrodeposition were investigated using digitized measurements to record data over millisecond time scales. Uncured as-deposited paint film in the course of electrodeposition was observed by optical microscope, and defects remained in the deposited film at the end of electrodeposition was also observed during curing process by optical microscope. The results indicate that electrodeposited paint film is ruptured by violent hydrogen gas generated locally during electrocoating, and the defects remaind in the deposited film at the end of electrodeposition cause cratering after curing paint film.