| Microarc oxidation (MAO) of magnesium alloys is a plasma discharge that occurs at the metal/electrolyte interface when the applied voltage exceeds a certain critical breakdown value. The process produces thick, hard and well-adhered ceramic-like coatings to be obtained with high corrosion resistance, thermal stability and dielectric properties. Therefore, MAO is one of the most prospective methods of surface treatment for magnesium and its alloys. In this investigation, the MAO processing on magnesium alloy AZ91D in (NaPO3)6 aqueous solution was carried out by using of the power supply manufactured with our own design. The effect of pulse parameters (voltage or current, frequency and duty cycle) on microstructure and corrosion resistance was investigated. The experimental results show that, the voltage and current change regularly. The growing rate of coatings increases with the elevating of voltage, current and duty cycle. Accordingly, coating surface becomes rougher and the percent of pore increases, as well. But, when frequency increases, the growing rate of coatings reduces, the surface becomes smoother and the percent of pore decreases. The ceramic-like coatings formed during MAO processing can enhance the corrosion resistance of AZ91D alloy significantly. The coating microstructure and morphology were characterized by optical and scanning electron microscopy, observing the structure both parallel to the substrate and also in polished cross-sections. Phase composition was studied by XRD analysis. The coatings formed in (NaPO3)6 aqueous solution contained two layers: an outer loose layer, which has a lot of small pores, and an inner compact layer, which grew into the metallic substrate. The coatings was mainly composed of MgO,MgAl2O4,Mg3(PO4)2. |
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