The Micro-arc Oxidation Process And Characteristics Of The Colored Film On ZK60Magnesium Alloy

The color obtained by Micro-arc oxidation （MAO） process was white or grey. For thefurther development and utilization of MAO, this topic studies oxidation coloring coating onmagnesium alloy, and then gets different colors of coloring coating on ZK60magnesiumalloy. A dual electrolytes system containing aluminate and phosphate by adding CuSO₄anddifferent power mode were employed to investigate effect of different concentration andelectric parameters on MAO taupe coatings. Yellow coating was obtained by addingKMnO₄in developed aluminate and phosphate system, getting the optimal processparameters of coloring salt and corresponding electrical parameters. It also had a initialdisscuss about growing mechanism and properties of the coatings.Taupe coating was obtained by adding CuSO₄in aluminate and phosphate system, andevaluated by coating color and roughness and corrosion resistance. After the test optimizedthrough univariate experiment, a composite electrolyte consisted of17.5g/L NaAlO2,3.5g/LNa₃PO₄,1.5g/L Na₂B₄O₇,3g/L NaOH,2g/L CuSO₄and3.6g/L EDTA is obtained. NaAlO2,Na₃PO₄, NaOH and CuSO₄are effect on properties of the coloring coating. After the testoptimized through orthogonal experiment on the main influence factors, a compositeelectrolyte consisted of17.5g/L NaAlO2,3.5g/L Na₃PO₄,3.5g/L NaOH and1.2g/L CuSO₄isobtained. The various properties of coloring coating was increase, the corrosion resistance is0.103g/m³h, hardness is510HV, roughness is0.735μm. Compared with the ZK60substrate,electrochemical impedance spectroscopy of MAO coating increases by4order whichexhibit excellent corrosion resistance.Compared with constant current mode, MAO coloring coating had uniform color andlow roughness under constant voltage mode. Constant voltage power mode were employedto investigate effect of different electric parameters on MAO taupe coatings. Corrosionresistance of MAO coatings increases when the voltage increases, however the coatingobtained uneven color when voltage was lower. MAO taupe coating had the best color whenthe voltage was360V. Thickness of MAO coatings decreases when the duty cycle andfrequency increases. Corrosion resistance of MAO coatings was0.153g/m²h when the dutycycle was30%. Constant voltage power mode was divided into three stages, coating wasmainly generated in the second phase of the reaction, the third stage was the finishing stageof coating. Thickness of MAO coatings increases when the time increases, however it decreases when the time increases further.Phase analysis shows that the main phase of coating were MgAl₂O₄, CuAl₂O₄and MgO,the coloring coating was cause by taupe CuAl₂O₄. White coating was obtained at thebeginning of reaction, as the reaction progresses the copper ions reacted in loose layer whenthe voltage was360V. The coloring coating was obtained on the loose layer.Yellow coating was obtained by adding KMnO₄in developed aluminate and phosphatesystem. It was little impact on the rest of the components in the solution for low content ofKMnO₄,only to optimize the KMnO₄content in the electrolyte. The coating was evaluatedby color and roughness and corrosion resistance. After the test optimized through univariateexperiment, the optimal process parameters of coloring salt and corresponding electricalparameters0.2g/L KMnO₄,16Adm-2current,35%duty cycle,13min time is obtained. Afterthe test optimized through orthogonal experiment, the optimal process parameters ofcoloring salt and corresponding electrical parameters0.1g/L KMnO₄,16Adm-2current,35%duty cycle,9min time is obtained. The various properties of coloring coating was increase,the corrosion resistance is0.053g/m³h, hardness is630HV, roughness is0.516μm.Phase analysis shows that the main phase of coating were MgAl₂O₄and MgO, there isno phase containing Mn. Element distribution of MAO coating by line scan shows that theyellow coating contains Mn. The coloring coating was cause by Mn.