Enhanced corrosion resistance of a chromium-free conversion/organic composite coating on an AZ91D magnesium alloy

Abstract

A composite coating was prepared on an AZ91D magnesium alloy; first, a chromium-free potassium permanganate conversion substrate was deposited and then the substrate was further coated with epoxy resins. The surface morphology, chemical composition and deposited products of the conversion coating were investigated with scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The results show that the conversion coating mainly consists of MnO2 and Al2O3 oxides, whose Pilling-Bedworth ratios (PBR) are larger than 1, indicating that the conversion coating is dense. Electrochemical-impedance-spectroscopy (EIS) plots reveal that the conversion coating shows a strong resistance to the flow of ions and electrons, demonstrating that the corrosion resistance of the AZ91D magnesium alloy is considerably enhanced. Neutral salt spray tests show that the corrosion resistance of the AZ91D magnesium alloy is substantially improved due to a composite coating consisting of a conversion deposit and an organic coating.

Corrosion and Protection of Magnesium Alloys: Recent Advances and Future Perspectives

“…The organic coating is a protective film formed by coating organic films on the surface of the sample via chemical methods [134]. Geng et al [135] synthesized a chromium-free potassium permanganate/epoxy composite coating on AZ91D magnesium alloy. The composition of the coating primarily consisted of MnO 2 and Al 2 O 3 .…”