Under-Vehicle Corrosion Testing of Primed Zinc and Zinc Alloy-Coated Steels

The mechanism of underfilm corrosion propagation and the type of corrosion products formed in electrocoated zinc and zinc alloy coated steels exposed in an under-vehicle corrosion test (UCT) have been studied using scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX), and x-ray diffraction (XRD). The precoated steels were electrogalvanized (EG), hot-dip galvanized (HDG), galvannealed (GA), and electrodeposited ZnFe (18% Fe), exposed for 1 to 3 years. All cathodically electrocoated (EC-coated) materials were found to form the same type of product, namely, crystalline zinc hydroxychloride, but the crystallographic orientation of this product is different for the four materials. EG forms, in addition, some ZnOHCl. The layer of corrosion products develops between the phosphate coating and the metal surface. Initially, the propagation is anodic, but reverses in a later stage to cathodic. The Fe-containing coatings form smaller amounts of corrosion products than EG and HDG. The phosphate crystals were found to rehydrate during exposure for all substrates studied. A model for the corrosion mechanism is described.