Microstructure and properties of a self-sealing MAO coating on 6063 aluminum alloy fabricated with stepwise current density

Different MAO coatings were prepared on 6063 aluminum alloy via micro arc oxidation (MAO) technology with a normal constant current density and novel stepwise current densities. The microstructure, wear resistance and corrosion behavior of the alloy with various coatings were comprehensively studied by multiple characterization methods. The results show that a compact coating (MAO-G3) with narrowed or sealed micro-pores was formed under a stepwise decreased current density as the ejection of molten metals were stranded in discharge channels by a gradient reduced input voltage. The CoF and wear rate of the sample with MAO-G3 coating reduced by 17 % and 65 %, respectively, when compared with those of the MAO-G1 coating formed by a stepwise increased current density, due to the compact and less porous structure of the MAO-G3 coating. Meanwhile, severe localized corrosion during potentiodynamic polarization occurred in the substrate but was eliminated in the samples with MAO coatings. The corrosion current density (I corr ) of the alloy with MAO coatings were one order of magnitude lower and the corrosion potentials (E corr ) were more positive than that of the substrate, revealing improved corrosion resistance. The sample with MAO-G3 coating showed the best corrosion resistance due to the self-sealing effect which was manifested as the highest E corr , the lowest I corr and the excellent electrochemical impedance performance, although its thickness was the lowest among the coatings.