Study On The Micro-arc Oxidiation Processing Of AZ91D Die-Casting Magnesium Alloy And Corrosion Behavior Of The Films

Micro-arc oxidation (MAO) is a kind of surface processing technique by which a ceramic film can be prepared in-situ on the light metals and its alloys surface. After treated by the technique, the corrosion resistance, wear resistance, and integrated property of the magnesium alloy can be improved significantly. Especially, due to its simple processing, easy and efficient operation, MAO has been widely applied in the field of airspace, automobile, electron, mechanical, and so on. However, some deficiencies of this technique, including poor corrosion resistance of magnesium alloy substrate, complicated corrosion mechanism of the films, few MAO solutions for magnesium alloy, short solutions lifetime and instability of the solutions, limit its further applications.In this paper, both the MAO processing for AZ91D die-casting magnesium alloy and the growth mechanism of the films were investigated. On the other hand, both the corrosion behavior and the anti-corrosion mechanism of the magnesium alloy were analyzed. Through detail investigation, some results can be obtained as follows:A steady and long lifetime zirconium-salt (MZ) solution was developed successfully. The corrosion resistance of the film MZ400 prepared in MZ solution is superior to that prepared in silicate (MS) and aluminate (MA) solutions. The lifetime of the specimens under salt spray condition reached maximum 384h. The corrosion resistance of the films prepared in MZ solution enhanced with the increasing of films thickness.The electrochemical impedance spectrometry (EIS) and corrosion models fitted by equivalent circuit indicate that the corrosion reaction resistance Rt for the specimens produced in MZ solution is the highest. During testing, the uniform corrosion reactions occur at the interfaces between the medium and the films because the corrosion medium penetrates into the films uniformly. The pit corrosion occurs in the films produced in MS and MA solutions.The surface of the films produced in MS and MZ solutions exhibits a net-pore shaped morphology, which is comprised of nets linked each other and pores acting as the channels of MAO reaction. The surface of the films produced in MA solution exhibits a grain-piled shape. The MZ400 film, with 30μm thickness, exhibits a dense structure. The pores formed previously are filled with increasing of the film thickness. Therefore, no obvious pores retained in the film are observed.XRD results show that ZrO2 is the main phase in the films produced in MZ solution. This indicates that the zirconium added in the solution has diffused into the films through complex MAO reactions. The existence of ZrO2 phase results in the high hardness of the films.