| Magnesium and magnesium alloys are widely used in aerospace,military products,electronic products and medical biomimetic materials because of their low density and light weight.However,magnesium and magnesium alloys have some disadvantages,such as poor wear resistance,low hardness and easy corrosion,which seriously restrict and affect the use value and wide application of magnesium and magnesium alloys.In order to improve the corrosion resistance of magnesium alloys,the molybdate conversion film was prepared on the surface of AZ91D magnesium alloy with sodium molybdate instead of traditional potassium dichromate.The process of molybdate conversion film was optimized.It was confirmed that the optimum process and system of molybdate conversion film was:the NazMoO4 was 30g/L,the KMnO4 was 8 g/L,the pH is 5.5,the temperature is 50 ?,and the film forming time is 40 min.The optimal molybdate conversion film and AZ91D magnesium alloy substrate bond is very good,can reach zero.Molybdate conversion film can obviously improve the corrosion resistance of AZ92D magnesium alloy,but as an anticorrosive coating on magnesium alloy surface,the corrosion resistance of molybdate conversion film is general and can be used as lower layer decoration.Epoxy resin is widely used as anticorrosive coating because of its good corrosion resistance,high bond strength,low curing shrinkage and high mechanical properties.In this paper,the optimal molybdate conversion film was prepared on the surface of AZ91D.The epoxy resin coating was prepared by spraying method,and the molybdate conversion film/epoxy resin composite coating was obtained.The corrosion resistance of the coating was studied by means of the adhesion test before and after soaking and the polarization curves.The results show that the molybdate conversion film/epoxy composite coating has good protective effect and corrosion resistance on AZ91D magnesium alloy.Because of the large activity of Cl-ion and the small radius of ion,it is easy to enter into the crack of molybdate conversion film,so that the adsorption of Cl-will replace the O2-in theSurface oxide of the conversion film,and the conversion film will be destroyed,and the crack of the conversion membrane will be coarsened after a long period of immersion.The probability of Cl-and H2O entering into the conversion film and AZ91D magnesium alloy substrate was increased.The molybdate conversion film finally cracked and exposed the AZ91D magnesium alloy substrate.Cl-and O2 dissolved in water attack the unstable C position of epoxy resin polymer chain,and then replace it,and hydrolyze it,resulting in corrosion pit.After the composite coating was soaking for a long time,the hydrophilic swelling of the epoxy resin layer resulted in serious corrosion of the epoxy resin layer under the combined action of H2O and O2,and finally revealed the conversion film. |
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