Growth Process and Dielectric Breakdown of Micro Arc Oxidation Coating on AZ31 Mg Alloy Pretreated by Alkali Treatment

A dense magnesium hydroxide coating was established on the surface of Mg alloy by alkali treatment before the micro arc oxidation (MAO) process, which can serve as the barrier layer in the MAO process. The generation mechanism of microdischarges during MAO was investigated by applying a constant voltage in an alkaline aqueous solution. Real-time imaging was employed to follow the gas evolution and microdischarge characteristics. It has been detected that the alkali treatment coating was breakdown during the MAO process, which is a typical solid film breakdown model. Through combining current density-time response, real-time imaging with the surface morphologies at varying applied voltages, a breakdown discharge model for the growth of MAO ceramic coating was proposed. The initial discharge ignited in the gas films at the bottom of the micropore and then induced the breakdown of the dielectric barrier layers. Which is typically “discharge-in-pore” breakdown model. The growth direction of the MAO coating on Mg alloy is outwards to the ceramic coating surface and inwards to the Mg alloy simultaneously.