Paint delamination from electrocoated automotive steels during atmospheric corrosion. II: Zinc alloy-coated steels

In order to understand the mechanism of corrosion-induced paint delamination, the corrosion products formed in a series of electrocoated automotive precoated steels have been investigated. The materials were phosphated and electrocoated but not topcoated. The scribed panels were exposed to atmospheric conditions for 15 months. The steels were Zn-coated (electro-galvanized, hot-dip galvanized) and Zn alloy-coated (ZnNi, ZnFe, and ZnAl). In most systems two distinct regions were observed, namely a delamination zone without corrosion products which preceeded a zone where the coating forms corrosion products. Most systems form ZnO/Zn(OH) sub 2 ; ZnNi forms a 4Zn(OH) sub 2 ZnCl sub 2 . The mechanism of delamination is postulated to be mechanical in nature, i.e. by osmotic effects. Paint from ZnFe-coated steels does not delaminate. Suggestions for the further improvement of the corrosion resistance of painted automotive materials are given. 32 ref.--AA