| Magnesium alloys have characteristics of strong chemical, electrochemical activity and poor corrosion resistance, particularly in salt-spray conditions. This has affected its application in the automotive and aerospace industries. AZ91D and AZ31 are the most widely used cast magnesium alloy and wrought magnesium alloy. It is very important to prolong their service life and expand the application areas by adopting surface protection method.This paper presented a new process for protecting magnesium alloy, that was molybdate chemistry conversion. AZ31 and AZ91D magnesium alloy were selected to test. The factors of affecting molybdate conversion process were investigated, such as the concentration of , pH value, temperature, conversion time, theα(Mg) phase,β(Mg17Al12) phase and the magnesium alloy element. Conversion processes of molybdate chemistry conversion on AZ31 and AZ91D were determined, the corrosion resistance, composition and forming mechanism about the conversion film were researched.During the molybdate conversion, the smallest corrosion rate is obtained in AZ31 chemical conversion system when the pH value is 3.5 ~ 4, the temperature 70 ~ 75℃, the concentration of Na2MoO4 20 g/L, the concentration of NaF 4g/L. Uniform and continuousβ(Mg17Al12) phase in AZ91D provides favorable conditions for the conversion film completely covering the metal matrix. AZ31 alloy is partly covered by conversion film because it contains moreα(Mg) phase and fewβ(Mg17Al12) phase. The testing results show that conversion film mainly have composion of Mg, F, Al, O, Mo and their oxides, fluoride, such as MoO2, MoO3, Al2O3, MgF2. AZ91D has high Mo content and Mo is distributed uniformly, while AZ31 has low Mo content and Mo is distributed locally. The ability of forming conversion film on the AZ91D surface is better than AZ31, so the corrosion resistance of AZ91D is better than AZ31. The conversion film surface is rough and uneven, it provides a good base for organic coating without any crude process. |
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