Effects of Voltage on the Microstructure and Corrosion Performance of Microarc Oxidation Coated AZ31 Magnesium Alloys

To study the effect of process parameters on the microstructure and corrosion resistance of microarc oxidation(MAO) coatings,the MAO coatings were prepared on AZ31 Mg alloys in the electrolyte of sodium phosphate(Na3PO4) at various applied voltages.The microstructures of the samples before and after corrosion were observed by scanning electron microscope(SEM) and optical micrograph.The composition of the MAO coatings and corrosion products were determined by X-ray diffraction(XRD).The corrosion behaviors were analyzed by the potentiodynamic polarization and electrochemical impedance spectroscopy(EIS).The results show that the MAO coatings are primarily composed of Mg3(PO4)2,MgO,Mg and MgAl2O4;and the corrosion products are composed of Mg(OH)2,quintinite and Ca10(PO4)6(OH)2.The coatings produced at 325 V and 3 000 Hz for 5 min show the best corrosion resistance with the densest morphology and the least corrosion current density.