| A micro-arc oxidation process for AZ91, AZ31 and AM60 magnesium alloys based on an environmentally friendly alkaline electrolyte of 100 g·L-1 NaOH + 20 g·L-1 Na2B4O7?10H2O + 50 g·L-1 C6H5Na3O7?2H2O + 60 g·L-1 Na2SiO3?9H2O has been studied in this paper. The micro-arc oxidation process can form anodized film with good protective properties on magnesium alloys. At the same time, the electrochemical corrosion behaviors of different magnesium alloys and their anodized films were also studied. Finally, the micro-arc oxidation process in this study has been compared with a commonly-used micro-arc oxidation process in dilute solution.The micro-arc oxidation process was carried under constant current mode, different films were formed on AZ91, AZ31 and AM60 magnesium alloys by changing the processing parameters such as current density and processing time. The properties of the films were determined by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization method. The investigation shows that when the applied current density is 20mA?cm-2 and the anodization time is 40min, the anti-corrosion properties of AZ91 and AZ31 magnesium alloys are best, while for AM60 magnesium alloy, when the applied current density is 20mA?cm-2 and the anodization time is 50min, its corrosion resistance has the optimum value. Microstructure and composition of films were examined by SEM, XRD and EDS. The micro-arc oxidation films are porous and their components consist of MgO and Mg2SiO4. The thicknesses of micro-arc oxidation films were measured by Coating Thickness Gauge and they increase linearly with time and current density respectively. Besides, the relationship between the corrosion resistance and the thickness was also studied and the result shows that there is an optimal thickness corresponding to the best corrosion resistance.The corrosion resistance of both the matrix of magnesium alloys and micro-arc oxidation samples were determined by neutral salt immersion test and EIS to study their corrosion mechanisms. The results show that the order of the corrosion resistance of the anodized films has the same order of the corrosion resistance of magnesium alloy matrix. The corrosion morphology of the matrix alloy is complete corrosion, while that of the anodized film is pitting, indicating the superior in corrosion resistance of the micro-arc oxidation coatings. Finally, the anodized films obtained in this study was compared to that based on a commonly-used dilute electrolyte of 5 g·L-1 NaOH + 15 g·L-1 Na2SiO3·9H2O. The results show that the thickness and corrosion resistance of the micro-oxidation film in this study is higher than the films acquired in dilute solution, this indicate that the current efficiency in this process is higher than that in the dilute solution and thus more practical. |
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