Effects of the Addition of Polyvinyl Pyrrolidone (PVP) as a Corrosion Inhibitor to Zr/V-based Conversion Coating on Aluminum Alloy

Abstract

Abstract Polyvinyl pyrrolidone (PVP) was selected as an additive for zirconium-based conversion treatment on aluminum alloy substrate. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were used to investigate the effect of PVP on the morphology and composition of the conversion coating. Neutral salt spray (NSS), electrochemical impedance spectroscopy (EIS), and Tafel polarization tests were used to characterize the corrosion resistance of the conversion coating. The results show that the conversion coating was mainly composed of metal oxide (ZrO2, VO2, V2O5, etc.), metal fluoride (ZrF4, Na3AlF6) and PVP. The density of Na3AlF6 crystal is increased by adding PVP into the conversion bath. This is due to the adsorption of PVP molecules on the surface of the substrate, which trap metal ions and thus facilitate the coating formation reaction. The corrosion resistance of the substrate treated by conversion coating containing PVP was significantly improved. When the amount of PVP introduced into the conversion bath is 1.5 g/L, the conversion coating can provide excellent corrosion resistance to the substrate. In addition, the non-polar groups in the PVP molecule can effectively improve the adhesion between the conversion coating and the electrophoretic coating.