Coating of Al-Mg and Al-Mg-Zn alloys in two different electrolytes by microarc oxidation method

In this study, the effect of zinc as an alloying element on the ceramic-based coatings synthesized by MAO on the surface of Al-Mg alloys was investigated. First, Al, Al-Mg (atomic 2% Mg) and Al-2Mg-Zn (atomic 3, 6, 9, 12, 15 % Zn) alloys were prepared by induction melting in a controlled atmosphere and casting into copper mold. After their characterization, these substrates were coated by MAO method in silicate and aluminate electrolytes at constant voltage and current for 20 minutes. The coated samples were characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersion spectrometer (EDS), eddy current tester, microhardness tester and profilometer. The phase contents of the coatings synthetized in both electrolytes are similar. The coating formed on the surface of pure aluminum and Al-2Mg consists of Al and -alumina and -alumina. Al-Mg-Zn alloys containing up to 6 % Zn are composed of Al and -alumina phases while Al-Mg-Zn alloys with 9 % and more Zn contains also ZnO phase. ZnAlO phase was also evident on the surface of Al-2Mg-15Zn sample coated in aluminate solution. The coating thicknesses obtained were 90-110 µm and 18-38 in sodium silicate solution and aluminate solution respectively. There was no significant change in coating thicknesses with the change of alloy element content in the substrates. Sponge-like, porous coating morphology formed in silicate electrolyte while crater like with homogeneously distributed circular pores coating morphology formed in aluminate electrolyte. Surface roughness values of all coated samples are higher than those of substrates before MAO. The increase in surface roughness was much more pronounced for the samples coated in silicate electrolyte. The highest coating hardness value (2098 HV) was obtained on the surface of pure aluminum coated in silicate solution. The hardness values of the coatings formed in the aluminate solution are higher than those of formed in the silicate solution.