Corrosion Propagation Under Paint Films on Galvanized Steel: A Comparison of Phosphating and Thin Film Corrosion Pretreatment Technologies

Abstract

The pretreatment of car bodies prior to applying the paint is crucial to promote paint adhesion and corrosion inhibition. The current phosphating pretreatments’ inability to pretreat magnesium and a body with aluminum content of over 30%, as well as future environmental legislation, will force a change in the pretreatment technology. The sustainable alternative pretreatment systems do not contain heavy metals, reducing the need for post-treatment of the waste-water and sludge from the process. The change will enable more lightweight materials in car bodies resulting in CO emissions. The process baths for the environmentally friendly pretreatments do not require much energy input as they can run at ambient temperature compared to phosphating baths that are heated to 60° C. The understanding of corrosion and the role of new pretreatments increase the confidence to build safe and reliable automobiles. The environmentally friendly pretreatments in this study are commercially available zirconium oxide-based pretreatments with silane or ceramic additives. The corrosion mechanisms were studied on hot dipped galvanized steel and painted. The study was performed by exposing the samples in a cyclic corrosion test for 3 and 13 weeks. The corrosion products were analyzed using XRD, SEM/EDS, and FT-IR techniques. The corrosion products found on the samples for all pretreatments are (Zn Cl (OH) •H O) and (Zn (CO ) (OH) ), as well as zinc chlorides and Zn(OH) . The corrosion front of the samples pretreated with the zirconium-based pretreatment exhibited corrosion propagation in the zinc grain boundaries. The grain boundary corrosion, as opposed to the diffuse corrosion front previously reported in the literature, is believed to stem from a decoupling of the cathodic site situated in the scribe when the corrosion blister is drying up. A more local anode/cathode pair is believed to form near the corrosion front giving rise to this behavior.