Abstract
In an effort to provide improved protection against inside-out perforation corrosion for automotive body panels, organic/metallic composite coatings, such as Durasteel (thin organic coated ZnNi) and Zincroplex®, have been developed for sheet steels. In the present study, we evaluated two thin organic coatings, one solvent-borne and one water-borne, in combination with three metallic coated sheet steels of electroplated pure zinc, hot-dipped galvanneal and electroplated ZnNi. Perforation corrosion resistance was evaluated by measuring the weight loss and thickness reduction of flat panels in two long-duration cyclic corrosion tests. The results showed that (1) the organic coatings improved corrosion resistance through their barrier protection, supplementing the sacrificial protection of the zinc-base metallic coatings; (2) while all composite coatings outperformed their respective metallic-only coated steels, ZnNi performed the best with either organic coating; (3) a higher perforation rate was obtained in the test with higher salt concentration in the salt-spray test cycle and with the longer time of wetness; and (4) in both tests, all materials exhibited the same stepwise pattern of corrosion behavior.
