Mechanisms of Cosmetic Corrosion in Painted Zinc and Zinc-Alloy-Coated Sheet Steels

A major consideration in autobody application of coated sheets is paint performance on electrogalvanized coatings. This paper describes a study in which a number of electrogalvanized coatings, including Zn, 9%Ni-Zn, 13%Ni-Zn, and 18%Fe-Zn, were characterized for composition, structure, and phosphatability and compared for paint performance in accelerated corrosion tests. Metallographic examination and microanalysis of scribe cross sections were conducted to understand the mechanisms of cosmetic corrosion. Results indicate that paint creep-back is determined by two interrelated factors, namely, (1) anodic dissolution of the coating and (2) metal coating/e-coat interfacial failure. The relative importance of these factors in relation to test conditions and the coating characteristics is examined, and approaches to improve cosmetic corrosion performance of electrogalvanized coatings are discussed.