Study On Preparation And Properties Of Fuscous Micro-arc Oxidation Film On Mg Alloy

Magnesium is the lightest structural metal, with high strength/weight ratio, high stiffness ratio, excellent electromagnetic shield, good machinability and damping characteristics and so on. Therefore magnesium alloys are widely applied to the fields of aerospace, electronic, automobile, etc. However, magnesium is intrinsically highly reactive and its alloys usually have relatively poor corrosion resistance, which is actually one of the main obstacles to the application of magnesium alloys in practical environments. Micro-arc oxidation (MAO) is an advanced surface protective treatment technology of forming ceramic coatings on the metals. Currently the ceramic films which formed on magnesium alloys by MAO is silvery white. In this paper, the fuscous ceramic films were obtained through micro-arc oxidation on AM50 magnesium alloys, at the same time the influence of ultrasonic wave was investigated.By the method of adding some coloring salt into electrolytes, the fuscous ceramic films were directly obtained on the surface of magnesium alloy by MAO. The effects of different electrolyte systems and different coloring salt on the color of films were investigated. The basis composition of micro-arc oxidation electrolyte was established. The parameters of MAO were optimized according to orthogonal design method, the orthogonal experiment of four factors with three levels was conducted. The process parameters for preparing the dark green ceramic film is NaAlO₂ 13g/L, NaVO₃ 2g/L, C₃H₅(OH)₃ 3ml/L, U = 410V, t = 30min; the process parameters for preparing the black ceramic film is NaAlO₂ 10g/L, C₄H₄KNaO₆·4H₂O 1g/L, CuSO4 1g/L, pH = 11, t = 20min; the process parameters for preparing the blackish green ceramic film is Na₂SiO₃ 10g/L, KF 10g/L, KOH 5g/L, CuSO₄ 1.5g/L, NH₃·H₂O 20ml/L, t = 7¹0min, U = 245²70V. The SEM results showed the porosity and microarc channel diamter of the fuscous micro-arc oxidation film decrease. The XRD results showed the dark green ceramic film was mainly composed of VO₂, V₃O-5 and Mg₃(VO₄)₂; the black ceramic film was mainly composed of MgO, MgAl2O4 and Cu₃MgO-4; the blackish green ceramic film was mainly composed of MgO, SiO₂, Mg₂SiO₄, CuN₆ and CuSiO₃·H₂O. The main reason of color-display is that V oxide or Cu oxide exists in the film. Corrosion experiment and electrochemical treatment showed that the anticorrosion property of the fuscous ceramic films was improved.In this paper, a new method, chemical conversion and micro-arc oxidation, was presented to prepare the fuscous ceramic films. The gray black oxidation film and dark red oxidation film were obtained through the method. The effects of process of chemical conversion and micro-arc oxidation on the color of the films were investigated. The technology conditions of the fuscous ceramic films were optimized through orthogonal test. The chemical conversion technology conditions for preparing the gray black oxidation film is NaMoO₄ 230g/L, pH = 3, T = 60℃, t = 8min; The chemical conversion technology conditions for preparing dark red oxidation film is KMnO₄ 50g/L, NH₄H₂PO₄ 100g/L, T = 50℃, t = 11min. The micro-arc oxidation technology conditions both are Na₂SiO₃ 10g/L, NaOH 5g/L, t = 10²0min. The SEM results showed the uniform and compact films were obtained. The XRD results showed gray black oxidation film was mainly composed of SiO₂, Mo₉O₂₆, MgSiO₃ and Mg₂SiO₄; dark red oxidation film was mainly composed of SiO₂,MnO,MnP,MgSiO₃ and Mg₂SiO₄. Electrochemical treatment showed that the corrosion resisting property of the brunet ceramic films was very good. This method has not seen in the literature.In process of micro-arc oxidation, the influence of ultrasonic wave on MAO films was investigated. The results showed the properties of the films were improved.