Influence of non-valve metal phase on growth behavior and performance of PEO coating on Mg–Zn–Mn alloys with high Mn content

Abstract

The influence of pure metal phases (precipitation or reinforcement phases) on the PEO discharge and coating growth in Mg alloys remains underexplored. This study investigated the effects of the α-Mn phase on voltage evolution, coating morphology, phase composition, and corrosion resistance by subjecting Mg–4Zn-xMn (x = 0, 0.6, 1.2, 2.4 wt%) alloys to PEO treatment in alkaline silicate-based electrolyte. Results indicate that the presence of α-Mn does not prevent spark discharge. However, an increase in α-Mn content leads to a downward shift in the voltage-time curve. The α-Mn phase significantly slows down the voltage ascent rate and prolongs the duration of Stages I to III of PEO process. Electrochemical oxidation of the surrounding Mg matrix by the α-Mn phase induces the formation of pits and porous local morphologies, exacerbating coating unevenness. With the increasing in Mn content, the thickness and porosity of PEO coatings gradually decrease for the same treatment time. The coated Mg–4Zn-1.2Mn alloy exhibits the highest corrosion resistance. The differences in oxidizability between the α-Mn phase and the Mg matrix, as well as the stability and dielectric properties of the oxide products may be the underlying reasons for these effects. Despite Mn oxides' instability under alkaline conditions at high potentials, reaction products with the electrolyte offset this drawback. This study enhances understanding of pure metal phases' influence on Mg alloy PEO and contributes to research on PEO processes for Mg-based materials containing pure metal phases.