A Study On A Micro-arc Oxidation Process For AZ91 And ZK60 Magnesium Alloys And The Properties Of The Micro-are Oxidized Films

A micro-arc oxidation process for AZ91 and ZK60 magnesium alloys based on an environmentally friendly alkaline electrolyte of 0.1 mol/L NaOH + 0.1 mol/L Na2SiO3.9H2O has been studied in this paper. The micro-arc oxidation process can form anodized film with good protective properties on magnesium alloys. Simultaneously, the electrochemical corrosion behaviors of micro-arc anodized films and the effects of variety of electrolytes concentration on micro-arc process and corrosion resistance of anodized films were also investigated. Finally, the differences of micro-arc process and properties of anodized films between two different electrolyte systems were present.The micro-arc oxidation process was carried out using constant current mode, different anodized films were formed on AZ91 and ZK60 magnesium alloys by changing the processing parameters such as processing time and electrolyte temperature. The properties of micro-arc anodized films were evaluated by electrochemical impedance spectroscopy (EIS), amperometric i-t curves and potentiodynamic polarization curve method. The results show that with the applied current density of 20 mA.cm-2, the anodization time is 30 min and the electrolyte temperature is 10℃,the anti-corrosion property of AZ91 magnesium alloys is best, while for ZK60 magnesium alloys, when the anodization time is 30 min and the electrolyte temperature is 15℃, its corrosion resistance has the optimum value.Microstructure and composition of anodized films in micro-arc electrolyte were examined by SEM, XRD and EDS. It is found that the micro-arc oxidation films are porous and components in AZ91 magnesium alloys consist of Mg0.976O0.024,Mg17Al12 and Mg2SiO4 and those in ZK60 magnesium alloys consist of MgO and Mg2SiO4. Besides, variety of film thickness with increase of oxidation time was tested by Coating Thickness Gauge, and the result show the there is an optimum thickness value with a good corrosion resistance.Effect of variety of electrolytes concentration on the properties of micro-arc oxidation films was studied by using electrochemical methods, SEM, XRD and EDS. The results show that anti-corrosion properties of micro-arc oxidation films increase with the reduce of NaOH concentration in electrolyte and the optimum value is 0.1 mol/L, while the corrosion resistance of oxidation films increase with the increase of Na2SiO3 in electrolyte and its optimum value is 1.0 mol/L. Finally, the micro-arc oxidation process and the anti-corrosion properties of the anodized films obtained in silicate electrolyte of 4.0 g/L (0.1 mol/L) NaOH + 28.422 g/L (0.1 mol/L) Na2SiO3.9H2O were compared to those based on an aluminate electrolyte of 20 g/L NaOH +10 g/L Na2AlO3+ 5 g/L Na3PO4.12H2O + Na2SiO3.9H2O 1 g/L. The results show that the cell voltage in silicate electrolyte is about 100V higher than that in aluminate electrolyte. According to the electrochemical tests results, the anti-corrosion property of the micro-arc oxidation films formed in silicate electrolyte is better than that of the films obtained in new electrolyt.