A model describing the growth of a PEO coating on AM50 Mg alloy under constant voltage mode

Abstract

Plasma electrolytic oxidation (PEO) process is employed to coat AM50 magnesium alloy in an alkaline phosphate solution applying constant voltage. Effects of treatment time on the morphology, composition and thickness of the resulting coatings are investigated by XRD and SEM respectively. Based on the analysis of the experimental results, a mathematical model is developed to simulate PEO coating growth including porosity and coating thickness. It is assumed that the current density of electrical discharges generated at the bottom of the pores remains constant and the coating growth is determined by Faraday’s law. This model describes a 2D transient finite element approach including necessary physical system parameters in mathematically coupled conditions. It demonstrates how the coating thickness is affected by current density, phase composition and porosity. The model reproduces the PEO process reasonably well and simulation results are in good agreement with experimental results, especially for predicted coating thicknesses. Although the model has a drawback in describing the oxygen release and hydroxyl ion concentration due to an empirical kinetic description, it may still be a useful tool in explaining and predicting PEO coating growth on magnesium alloy under constant DC voltage.