| The surface corrosion problem of 2A12 aluminum alloy in the process of use is one of the reasons that seriously restrict its further use.Therefore,it is necessary to adopt a reasonable surface treatment technology to improve the corrosion resistance of 2A12 aluminum alloy.In this paper,ceramic coatings containing rare earth oxides(La2O3 and Nd2O3)were prepared on the surface of 2A12 aluminum alloy by micro-arc oxidation.Because the coatings had good corrosion resistance,it can effectively protect the substrate.On the one hand,in the silicate-phosphate electrolytic system,scanning electron microscope(SEM),energy spectrometer(EDS),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and electrochemical workstation were used to study the effects of different concentrations of rare earth oxides:La2O3(0 g/L,0.4 g/L,0.8 g/L,1.2 g/L and 1.6 g/L)and Nd2O3(0 g/L,0.1 g/L,0.2 g/L,0.3 g/L and 0.4 g/L)particles on the micro-arc oxidation coatings growth process,surface and cross section morphology,phase composition,properties(hardness,thickness and corrosion resistance),and the doping mechanism of rare earth oxides micro-arc oxidation was discussed.The results showed that with the increase of rare earth oxides concentration(La2O3:0～1.2 g/L and Nd2O3:0～0.3 g/L),the coating formation rate gradually increased.This was because the rare earth oxides migrated to the surface of 2A12 aluminum alloy(anode)under the combined action of electric field force and mechanical agitation,resulting in the increase of the coatings resistance,which increased the micro-arc oxidation voltage.As the rare earth oxides were adsorbed on the surface of the micro-arc oxidation coatings and formed discharge center,the discharge on the surface of the coatings was more uniform.In addition,the rare earth oxides were mixed with the molten material generated in the micro-arc oxidation process into the discharge channel,so that the discharge channel was filled and the density of the coatings increased.The increase of the coating thickness was due to the increase of micro-arc oxidation voltage which increased the coating formation rate.The increase of coating hardness was due to the coating thickness increased and the formation of hard phases Al2O3 and SiO2 in the coatings.The corrosion resistance of the coatings was enhanced because the thickness,hardness and density of the micro-arc oxidation coatings increased,and the probability of the corrosive medium Cl-entering the coatings was greatly reduced.With the further increase of the concentration of rare earth oxides(La2O3:1.6 g/L and Nd2O3:0.4 g/L),the rare earth oxides adsorbed on the surface of the coatings increased,and the violent partial discharge generated on the coatings surface,resulting in the ablation of the coatings.Therefore,the resistance of the coatings decreased,resulting in the decrease of the micro-arc oxidation voltage,which reduced the coating formation rate,the density,thickness and hardness of the coatings,so that the corrosion resistance of the micro-arc oxidation coatings was also lower than that of La2O3 and Nd2O3 particle concentrations of 1.2 g/L and 0.3 g/L.XRD test results showed that the micro-arc oxidation coatings with La2O3 was mainly composed of γ-Al2O3,SiO2 and La2O3 phases,while the micro-arc oxidation coatings with Nd2O3 was mainly composed of α-Al2O3,γ-Al2O3 and SiO2 phases.XPS test results verified the conclusion of the coating phase compositions in the XRD analysis,and it was found that the micro-arc oxidation coatings prepared in the two systems contained the corresponding rare earth oxides,which futher verified the feasibility of preparing the coatings containing rare earth oxides on the surface of 2A12 aluminum alloy by micro-arc oxidation.On the other hand,the surface macroscopic morphology and main phases composition of the coatings prepared by micro-arc oxidation with different concentrations of rare-earth oxides(La2O3 and Nd2O3)in the salt fog environment were analyzed.The results showed that:After 24 hours of salt spray corrosion,obvious corrosion phenomenon appeared on the surface of 2A12 aluminum alloy.When the rare earth oxide particles(La2O3:0～0.4 g/L and Nd2O3:0 g/L)were added,only a small amount of pitting corrosion occured on the surface of the coatings.With the increase of the concentrations of rare earth oxide particles(La2O3:0.8～1.6 g/L and Nd2O3:0.1～0.4 g/L),the micro-arc oxidation did not show obvious corrosion phenomenon,this was because the micro-arc oxidation coatings containing this concentrations of rare earth oxides had a large thickness and density,and Al(OH)3,Al2O3·3H2O and AlCl3 generated in the corrosion process were covered on the coatings surface and filled in the discharge channel,thus further reducing the probability of corrosion medium Cl-infiltrating into the coatings,and reducing the corrosion of CI-on the coatings. |
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