The Effect of Surface Treatment on the Performance of a Zirconium-Based Conversion Coating on AA7075 Automotive Alloys for Protection Against Filiform Corrosion

The resistance to filiform corrosion (FFC) of AA7075 automotive aluminum alloys pretreated with a Zr-based conversion coating was investigated as a function of Surface Treatment. Two different Surface Treatments were studied (1) alkaline cleaning and (2) alkaline cleaning with a subsequent acid deoxidation step. Specimens were compared to a control and studied with and without the application of a Zr-based pretreatment conversion coating after which a polyvinyl butyral (PVB) primer coating was applied. Data from FFC experiments are compared with surface characterization data to study the FFC kinetics and to elucidate the mechanism by which the Surface Treatment processes affect both the formation of the Zr-based conversion coating and the FFC filament propagation rate on the coated system. The effect of Surface Treatment on intermetallic particle (IMP) density and surface enrichment of alloying elements is discussed. It was found that specimens prepared by a Surface Treatment with alkaline clean-only resulted in a substantially thicker (60 nm) Zr-rich oxide layer that was 3x the thickness of the control specimen. These specimens provided superior resistance to FFC where the thick Zr-rich oxide is thought to provide a dense blocking layer to prevent electron transfer at the interface.