Corrosion of primed and fully painted precoated automotive steels in atmospheric and scab tests

Abstract

The mechanism of cosmetic corrosion propagation and the types of corrosion products formed in painted precoated automotive steels during exposure to atmospheric and scab corrosion tests have been studied using SEM, EDX, XRD and static SIMS methods. The precoated steels were hot-dip galvanized (HDG), galvanneal (GA) and zinc-nickel electroplated steel (ZnNi). Exposure to atmospheric, marine conditions was for 15 months and in the Volvo scab test for 30 months. Corrosion was compared for electrocoated (EC) and for fully painted panels. In all systems studied and under both test conditions, underfilm corrosion propagated considerably further in the topcoated systems than in the panels that were EC-coated alone. In all EC-coated systems the corrosion product was identified as zinc hydroxychloride, whereas in all topcoated systems a mixture of zinc hydroxy-chloride and zinc hydroxychlorosulfate was found. Sulfur-containing pigments in the topcoat system were identified as the cause of the formation of the sulfur-containing corrosion product. A model is presented which accounts for the different corrosion rates of the EC-coated and topcoated systems.