Study On The Micro-arc Oxidation Electrolyte Formula And Film Coloring Of Magnesium Alloy

Micro-arc oxidation (MAO) is an advanced surface protective treatment technology of forming ceramic coatings on the metals. It is a development direction of magnesium alloy surface treatment technology. In this paper through a lot of tests, a micro-oxidation electrolyte formula and some process parameters of AZ91D magnesium alloy are obtained, then the micro-oxidation and coloring technology are studied.On the parts of electrolyte optimization, according to contrast test and orthogonal test, adopting visual analysis and variance analysis, a good MAO composition of the electrolyte system for magnesium AZ91D alloy is found out: NaAoO₂5g/L, NaOH4g/L, H₂O₂3ml/L; and the best technical parameters is: current density 1.0A/dm², frequency 600Hz, duty cycle 20%, oxidation time 20min. Through the best electrolyte formula and process parameters, we make the film of MAO which has good corrosion resistance, thickness, density and hardness. By using SEM the surface and cross-section morphology of the ceramic coatings have been analyzed, the results show that coating main including many network structures, and the cross-section is composed of the compact layer and loose layer. The XRD observation analysis shows the film is mainly made up of the MgO, Al₂O₃ and MgAl₂O₄.On the parts of coloring study, tests show that the depth of ceramic coating color can be controlled by colored salt content and micro-oxidation time. The more coloring salt content, the more deeply film color, and the longer micro-oxidation time, the more deeply film color. At last by using the new best electrolyte and the best technical parameters: NaAlO₂5g/L, NaOH4g/L, H₂O₂3ml/L, K₂Cr₂O₇0.5g/L, current density 1.0A/dm², frequency 600Hz, duty cycle 20%, oxidation time 20min, we can get the green ceramic coating with smooth and evenly coloring surface. The XRD analysis shows: the main composition of the green ceramic coating is MgO, Al₂O₃ and MgCr₂O₄, and the MgCr₂O₄ colored the film. The performance testing shows: the ceramic coating has relatively good surface appearance, excellent corrosion resistance and color sustainability, better thickness, hardness than basal magnesium alloy and tight combine with basal magnesium alloy.