Effect of copper additive on microstructure and anti-corrosion performance of black MAO films grown on AZ91 alloy and coloration mechanism

Abstract

Black micro-arc oxidation (MAO) films on AZ91 alloy were successfully fabricated using MAO technique in a designed bath of silicate electrolyte with addition of colorant copper additive aiming to extend the application scope of magnesium and its alloys. It was found that the color of MAO films on AZ91 alloy changed gradually from white to black with the increasing copper ion concentration. The black color MAO films formed on AZ91 alloy may mainly be owing to the existence of CuO particulates in the porous outer layer. In addition, incorporation of CuO in the uniform and compact MAO films could significantly improve the anti-corrosion property of AZ91 substrate, thus effectively prevent the inward migration of corrosive medium. Copper additive concentration of electrolyte also affected the surface roughness of MAO films. The increase of copper additive concentration may result in an increase in the thickness and compactness of the black MAO films, and finally an enhancement in their corrosion resistance. Therefore, the MAO technique method coupled with silicate electrolyte containing copper additive was a promising surface modification technique for magnesium alloys.