Study On The Formation Mechanism Of The Self-sealing Pores Micro-arc Oxidation Film On Magnesium Alloy

Mg alloys are the lightest metal structural materials with the density of2/3that of aluminumand1/4that of steel. They are widely used in aerospace, military, civilian and other fields due totheir excellent physical and mechanical properties. However, the application scope of Mg alloyshas been limited for their poor corrosion resistance. Recently, Micro-arc oxidation (MAO) withlow cost, less pollution and simple process is an ideal surface treatment technology formagnesium alloys. However, there are many pores in the traditional MAO film to reduce theirprotection ability. In our previous research, a fluorotitanate solution system has already beendeveloped. The MAO film prepared by this fluorotitanate solution contains more chemicallystable titanium oxides. Also, the pores in film are self-sealed during film formation process, whichgreatly improves the film’s compactness. The corrosion resistance of the self-sealing pore MAOfilm is enhanced5~6times over the film obtained with traditional MAO solution. In view of thisnovel MAO technology, the aim of this paper is to optimize the electric parameters, clarify thefilm formation mechanism and deposition process of the pore sealing materials.A single factor method was used to investigate the effect of current density, applied voltage,oxidation time, frequency, and duty cycle on the film quality. The optimum electrical parametersare obtained as follows: current density of3A dm-2, applied voltage of420V without constantvoltage processing, frequency of800Hz and the positive duty cycle of30%.As for the film formation process, a dense passive film which consists of MgO, MgF2andAlPO4is formed on the surface of Mg substrate in the initial oxidation. The addition of F ions tothe electrolyte solution has a positive effect on the compactness of the initial passive film. Afterthe passive film is broken down, MgO becomes the main film constituents. With the rise ofoxidization voltages, MgF2and MgO continue to be formed accompanying the increase of poredimensions and decrease of pore numbers. When the applied voltage reaches a certain value, partsof titanium oxides are deposited as Ti3O5. With the gradual increase of Ti3O5content, the milk-white film is gradually converted into a blue appearance. As the oxidization voltage is300V,some of pores are self-sealed. Almost all the pores are self-sealed until the applied voltage of420V.As for the deposition process of the pore sealing constituents, the deposition of Ti3O5(PillingBed-worth Ratio (PBR) is greater than1) on the film surface or the pore wall is beneficial forimproving the film compactness, decreasing the pore numbers and enhancing the power release from the pores. The air pressure difference between the inside and outside of pores caused by ahuge power release results in the easy deposition of film constitutes in the pore interior not thefilm surface. In the subsequent condensation process, the MgF2with the lowest melting point isdeposited at last, and just seals the pores to achieve the self-sealing pore MAO film. Theexperimental data shows that the Ti content of over5at.%is helpful in depositing MgF2insidethe pores to realize the pore self-sealing.