Effect of Cation Incorporation in the Plasma Electrolytic Oxide Layer Formed on AZ31 Magnesium Alloy

Abstract

The role of anion in the formation of oxide coating by plasma electrolytic oxidation (PEO) has been extensively studied. However, there was no clear evidence of the role of cation. This work is aimed to study the mechanism of sodium (Na) incorporation and its effect on the PEO coating formed on AZ31 magnesium (Mg) alloy in Na3PO4 electrolyte by post-mortem investigation. The study was performed on a porous and a dense coating developed at 1 and 5 min, respectively. Investigation using scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS) revealed incorporation of Na as high as ∼10 at% in the coating as whiskers accumulated in the outer layer and as filler inside tunnels. Whiskers were composed of Mg3PO4 phase enclosed by Na-O compounds which was formed by chemical reactions of Na+ ions, oxygen (O2), and water (H2O) in the vicinity of plasma discharge. The polarization test demonstrated an order magnitude improvement of corrosion resistance provided by the porous coating while two orders of magnitude for the dense coating. The incorporation of Na compounds shifted the corrosion potential of the coated specimen ∼135 mV below the substrate. The EIS test indicated that Na compounds contributed to temporary passivation with exposure time.