Cratering in electrodeposition coating of cationic paint on galvannealed steel sheet.

Abstract

Cratering phenomenon in cathodic electrodeposition coating was investigated and the cratering mechanism was discussed. It occurs when part of the paint film undergoes a thermal change due to the spark discharge on the cathode. The thermal change generates nuclei that hinder the leveling of the deposited paint.Graters tend to form on galvannealed steel sheet. Its Zn-Fe alloy coating layer has nonuniform electrical resistance and conductivity compared with other types of steel sheet. Therefore it is easily subjected to local current concentration which causes spark discharge when the electrodeposition voltage is high. Methods to suppress the spark discharge are also studied.