Preparation And Densification Of La/Ce Double Rareearth Conversion Film On AZ91D Magnesium Alloy

Magnesium alloy has excellent performance and broad application prospects, but its poor corrosion resistance is one of the main factors restrict ing its development, so the surface protection technology of magnesium al loy has been the focus of research in the world. In this paper, the prepar ation and densification of La/Ce double rare earth conversion film on AZ91D magnesium alloy were investigated. The double rare earth conversion film was prepared on the surface of AZ91D magnesium alloy by chemic al immersion method, and the mixed solution of Ce（NO₃）₃ and La（NO₃）₃was used as the conversion solution, and the（NH₄）₃PO₄ solution was used to carry out the densification treatment. The single factor experiments and orthogonal experiments were used to determine the optimum film form ing process and densification process of La/Ce double rare earth conversion film. The surface morphology of the film was investigated by scanning electron microscope（SEM）; the composition of La/Ce double rare earth conversion film was analyzed by X-ray diffraction（XRD）, and the corrosion resistance of the film was investigated by spot test, hydrogen evolution experiment, tafel polarization curves,and electrochemical impedance spect roscopy. The formation mechanism and densification mechanism of La/Ce double rare earth conversion film were discussed by the mixed potential time curve（E-t）.The optimum process parameters of film forming obtained by the ort hogonal experiment are as follows:immersion temperature is 40℃,molar ratio of lanthanum nitrate and cerium nitrate is ₃:1,the amount of H2_O2 is 5ml, immersion times is 60 min. The optimum densification process para meters are as follows: densification treatment temperature is 50℃,densific ation treatment time is 4 min, and the concentration of（NH₄）₃PO₄ is 5%.The SEM results show that La/Ce double rare earth conversion film is of amulti-layer structure and its surface is covered with cracks, and the surface of the conversion film after densification treatment has been completely covered by granular aggregate, without cracks and bare matrix.The XRD results show that the La/Ce dual rare earth conversion film is mainly composed of MgO,CeO₂,La（OH）₃,Ce（OH）₄, and the conversion film after densification treatment is mainly composed of Mg O,CeO₂,La（OH）₃,Ce（OH）₃,Mg₃（PO₄）₂,Ce PO₄ 和 La PO₄.The electrochemical test results show that the free corrosion potential and the free current of the La/Ce dual rare earth conversion film after densification treatment are higher than that of the conversion film without densification treatment, so the results show that the corrosion resistance of the conversion film after densification is enhanced.