Research On The Composite Coating Combining Micro-arc Oxidation With Sol-gel On The Surface Of Magnesium Alloy

Micro-arc oxidation (MAO) coatings were successfully prepared on ZE10 magnesium alloy by micro-arc oxidation in silicates electrolyte system. The effects of voltage, oxidation time, frequency and duty ratio on the thickness, morphology and corrosion resistance of the micro-arc oxidation coatings were investigated by using eddy current thickness measuring gauge, X-ray diffraction, scanning electron microscopy, energy disperse spectroscopy (EDS) and electrochemical workstation, etc. In order to realize the better corrosion resistance, the rare earth was added in the silicates electrolyte and the coating, and sealing treatment by using SiO2 sol was performed respectively, and the thickness, morphology and corrosion resistance of the micro-arc oxidation coatings were discussed.The results indicated that the thickness and hole of MAO coatings was increased with anode voltage increasing from 200V to 350V. The thickness is increased obviously with increasing in the oxidation time in the beginning and basically stable after 20 minutes. Frequency and duty ratio has little effect on the film thickness, smaller pore size obtained at a high-frequency. The corrosion resistance of MAO coatings increase firstly, then decrease with the applied voltage and time increasing; and duty ratio and frequecy are little influential factors of the corrosion resistance of MAO coatings.The coating formed contained three layers, outer loose layer, middle compact layer, inner transitional layer. XRD and EDS show that the MAO coatings are mostly composed of MgO, MgF2, Mg2SiO4.The experiment of different methods addition of rare earth element for MAO show that the Ce element addition in electrolyte and the pre-treatment of rare earth Ce conversion coating can reduce the thickness and cracks, raise density and further improve the corrosion resistance of coating. The least cracks and highest density and best corrosion resistance appeared in the micro-arc oxidation coating as the pre-treatment of rare earth Ce conversion coating was performed and Ce element was added in the electrolyte.Silica sol was prepared using TEOS as the precursor and HNO3 as the catalyst by alkoxide solution. SiO2 sol-gel film was prepared on the surface of MAO coating by dip-coating method and spin-coating method. The results show that dip-coating and spin-coating can be spread uniform on MAO coating surface, sol can enter into the hole of MAO coating and seal the hole effectively. The hole-sealed treatment can further improve the corrosion resistance of MAO coating of magnesium alloy, and dip-coating method is better than spin-coating method.