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MgO@Nb2O5 composite coating was prepared on the surface of Mg–7Y alloy by micro-arc oxidation technique. The microstructure of the composite coatings was tested by scanning electron microscopy, energy spectrum analysis, X-ray diffraction and scanning kelvin probe force microscope. The hydrothermal performance and corrosion resistance of MgO@Nb2O5 composite coatings were analyzed by electrochemistry tests and laser confocal. The results showed that microspore diameter, porosity and roughness of the composite coatings were all decreased with the increasing of Nb2O5 concentration. The micro-arc oxidation composite coatings were mainly composed of MgO and Nb2O5. Compared with the micro-arc oxidation coating without Nb2O5, the corrosion voltage of the micro-arc oxidation coating with Nb2O5 was increased and the corrosion current was reduced. The corrosion resistance of Mg–7Y alloy was effectively improved by the MgO@Nb2O5 composite coatings, and the best corrosion resistance of the micro-arc oxidation coating was achieved when the Nb2O5 concentration was 2 g/L. The hydrothermal test demonstrated that the hydrothermal resistance of the coatings was significantly improved due to the addition of Nb2O5.

Due to its excellent performance, aluminum alloys are frequently used in wheels, but the corrosion of aluminum alloy wheels is still a major problem to be further studied. To understand the corrosion of wheels, it is important to consider the role of the organic coating in the corrosion process. Meanwhile, it is also meaningful to find an effective method to remove the organic coating properly to investigate the corrosion products under the organic coating. In this paper, the corrosion status of the A356 aluminum alloy substrate under an organic coating was observed firstly; lipid solutions such as NMP solution and butyl acrylate solution were used to remove the coating, and comparative experiments were carried out by controlling factors such as the soaking temperature, time, composition of solution etc. To find out the optimized soaking method to remove the organic coating; finally, the morphology of the localized corrosion on the aluminum alloy surface and the composition and structure of the corrosion products on the aluminum alloy substrate were analyzed. Results show that the corrosion products mainly contain ZrO2, Zr3O2F8, AlO(OH), and Al2O3. The corrosion process prefers to initiate at the defects where the coating and the substrate are not tightly bonded. This work shows the corrosion behavior of the aluminum alloy under an organic coating and provides a useful method to remove the organic coating from the aluminum alloy substrate effectively, and the results can be valuable for the understanding and mitigation of metal corrosion under an organic coating.

The powder thermal diffusion alloying method is an important method to fabricate intermetallic coatings on magnesium alloy substrates. The diffusion source, the diffusion time and the diffusion temperature are key parameters for a diffusion alloying process. While the effective time of an active diffusion source at the diffusion temperature is still unclear. Clarification of the effective time of a diffusion source is important, as the diffusion source could be utilized effectively and economically only if the effective time of the active diffusion source is known. Here, the effective time of a diffusion source at 400 °C was investigated. Results show that, the effective time of the diffusion source which contains 10 g ZnO and 1 g NH4Cl is around 150 h at 400 °C. For a more compact diffusion source, the effective time is relatively shorter. Then, chemical reactions in the diffusion source at elevated temperature were analyzed. The failure of the diffusion source is related to the reduction in the number of active metal ions, while an ineffective diffusion source can be reactivated by addition of new halide powder. These results could deepen the understanding of the role of the diffusion source in a powder thermal diffusion alloying process and indicate a limit under which the diffusion source could be utilized effectively.

Metal oxides can be doped by various metal ions to tune the properties. While the migration of metal ions especially extrinsic metal ions in metal oxides still needs to be clarified in order to adjust the composition and the structure of metal oxides properly. Here, 14 kinds of metal oxide powders were utilized to investigate the key factors of Zn ion migration in metal oxide powders experimentally. Results show that chemical reactions and/or phase transitions in metal oxides can hinder the migration of Zn ions in metal oxide powders. Compared with unstable metal oxides, it is easier for Zn ions to pass through stable metal oxides which maintain the same component phases at elevated temperatures. Transition from unstable metal oxide medium to stable metal oxide medium will make the transport of Zn ions in the medium from impossible to possible. These findings are helpful for selecting proper metal oxides in related industries such as battery designing, gas sensor development and so on.