Study On Process, Microstructure And Performances Of Microarc Oxidation Coatings Fabricated On AZ91D Magnesium Alloy

Ceramic coatings fabricated on AZ91D magnesium alloy were prepared by microarcoxidation （MAO） and a new dual electrolyte system with relevant electrical parameters ofmicroarc oxidation was developed. Characteristics of MAO coatings such as morphologies,coating thickness, surface roughness, structure and chemical composition werecharacterized by scanning electron microscopy （SEM）, layer thickness meter, digitalmicroscope, X-ray diffraction （XRD） and energy disperse spectroscopy （EDS）, respectively.The corrosion resistance of the coatings in a3.5%NaCl neutral solution was evaluated byimmersion test, electrochemical impedance spectroscopy （EIS） and potentiodynamicpolarization test. The results are showed as follows:The corrosion resistance of MAO coatings is affected by the concertration ofelectrolyte in a composite electrolyte composed of Na2SiO3, NaAlO2and some additives.Corrosion resistance of MAO coatings increases when the concentration of eachcomposition of the eclectrolyte increases, however it decreases when the concentrationincreases further. The variation trend of coating thickness is the same as that of corrosionresistance. After the test optimized through orthogonal experiment, a composite electrolyteconsisted of15g/L Na2SiO3,9g/L NaAlO2,2g/L Na2B4O7,7g/L C6H5Na3O7,5mL/LC3H8O3and3g/L NaOH is obtained.The electrical parameters have a significant effect on the characteristics of MAOcoatings, and compact coatings with good corrosion resistance can be prepared only in anelectrolyte with proper electrical parameters. Single factor experiment shows that corrosionresistance of MAO coatings increases first and then decreases gradually with the increase ofcurrent density, duty cycle, oxidation time, respectively. Moreover, with the increase offrequency, the corrosion of MAO coatings decreases gradually. A proper electricalparameter is obtained, containing current density （15A/dm²）, frequency （520Hz）, dutycycle （38%） and oxidation time （15min）.Microarc oxidation process can be divided into three stages at a constant current, whichare anodic oxidation stage, microarc oxidation stage and local microarc oxidation stage. Thecoating obtained in the optimized microarc oxidation process exhibites a typical porousmicrostructure, but with a smooth and uniform surface, indicating a small surface roughness.Moreover, the coating is characterized with a typical outer loose layer and inner dense layer, also with a thin boundary layer between MAO coating and substrate. EDS results show thatMAO coating primarily contains Mg, Al, O and Si elements which are distributed on thesurface of MAO coating uniformly. XRD analyses indicate that the coating is mainlycomposed of MgO, Mg2SiO4and MgAl₂O₄. MgAl₂O₄is mainly distributed on the outerloose layer and boundary layer, and MgO is mainly distributed on the inner dense layer.Moreover, Mg2SiO4is distributed on the whole MAO coating uniformly. The optimizedMAO coating reveals a lower current density which decreases five orders of magnitude ascompared to the magnesium substrate. Meanwhile, the corrosive potential of the coatedsamples increases nearly by104mV. EIS results show that the corrosion resistance of thecoating is mainly determined by the inner dense layer.