| Magnesium alloys, as the 21st green engineering material, present a wide range of applications due not only to their better electrical conductivity, high thermal conductivity, high dimensional stability and good electromagnetic shielding characteristics, but also their good cast ability and good welding ability. Unfortunately, it is prone to oxidation in common environments, especially in the acidic environments and in salt-water conditions containing Cl-, Br-SO42- and so on, which leads to a severe erosion. Poor corrosion resistance is the obstruction to broaden applications of magnesium alloys. So it is very important to study the surface anti-corrosion technology of magnesium alloys.At present, surface coating treatment on the magnesium alloys is one of the most effective and simple ways. The coating employed between the substrate and the environment could offer an effective barrier to improve its corrosion resistance. In this thesis, anti-corrosion technologies of magnesium alloys were investigated. Based on corrosion mechanism and anticorrosion treatment of magnesium alloys, through some appropriate pre-treatment, and then the oxide ceramic coatings were prepared on the AZ91D magnesium alloy surface through sol-gel method. The main contents and conclusions in this paper are as follows:The ZrO2 ceramic coatings were first successfully prepared on the stannate-coated AZ91D magnesium alloy surface by sol-gel method taken zirconium acetate as precursor, anhydrous ethanol as solvent and proper surfactant. The morphology and structure of the coatings were investigated by SEM and XRD. The potentiodynamic polarization curves and EIS were employed to characterize the anti-corrosion properties of the coatings. The results indicated that: (1) Transparent steady sol was achieved by keeping the concentration of zirconium at a range of 0.08 mol/L-1 and the volume ratio of solvent and surfactant at 8:2; (2) Anti-corrosion ZrO2 ceramic coatings were obtained on AZ91D magnesium alloy surface through the optimal technique parameters. The chemical conversion coating was mainly consisted of MgSnO3·H2O. The ZrO2 sol sintered at 400℃for 2 h turned into nanocrystalline ZrO2 ceramic coating. The ZrO2 ceramic film was mainly composed of tetragonal phase ZrO2 and the average grain size of crystalline ZrO2 was 10 nm.The nonpoisonous, compact and friendly CeO2 ceramic coatings had been successfully prepared on AZ91D magnesium alloy surface to improve its corrosion resistance. The effect factors on the preparation of cerium-based conversion coating such as the different composition of the reacted solution, surface morphology and so on were investigated. The optimal technique parameters of the reacted solution were filtrated. The preparation and anticorrosion properties of the conversion coatings and CeO2 ceramic coatings on AZ91D magnesium alloy surface were analyzed by OM, SEM, XRD, EIS test and potentiodynamic polarisation curve test. The results showed that the corrosion resistance of AZ91D magnesium alloy was found to be greatly improved by means of this novel environmental-functionally CeO2 ceramic coatings.To improve the corrosion resistance of AZ91D magnesium alloy, on the basis of quondam experiments, we successfully combined the advantage of phytic-acid conversion coating and sol-gel technology, a novel anti-corrosion sol-gel based A12O3 ceramic coating was developed on the AZ91D magnesium alloy. The performance of the coatings were investigated by TG-DTA, FT-IR, SEM, XRD, EDS, Contact angles test, EIS test and potentiodynamic polarisation curve test. The results indicated that: (1) The phytic-acid chemical conversion coating was mainly composed of C, O, Mg, Al and P. The weight percents of C, O, Mg, Al and P were roughly in line on different places of the coating, indicating that the phytic acid conversion coating could be homogeneously formed on the magnesium alloy. This provided good conditions for the preparation of the follow-up homogeneous Al2O3 ceramic coating. (2) The Al2O3 gel begun to transformate from amorphous to crystalline phase when the sintering temperatures was 280℃. The Al2O3 sol-gel coated specimen sintered at 380℃had the best corrosion resistance properties as compared to the specimens sintered at 120℃and 280℃.A functional gradient coating was prepared on the AZ91D magnesium alloy surface to improve corrosion resistance of AZ91D magnesium alloy. The phytic acid conversion coating was employed as chemical pre-treatment and a barrier to environment before applying the sol-gel-based CeO2 ceramic coating. The performance of the coatings were investigated by SEM, XRD, EDS, Contact angles test, EIS test and potentiodynamic polarisation curve test. The results indicated that: (1) The optimal technique parameters of the functional gradient coating were 0.5 M CeO2 sol, two layers obtained for sol-gel spin-coating process with the speed of 1000 rpm on phytic-acid chemical-pretreated specimens and the sintered temperature was 280℃. (2) The weight percents of C,O, Ce were nearly equivalent no matter where cracks were seriously or the coatingss were relatively compact, P and Al disappeared in the functional gradient coating, which indicated that the magnesium alloy substrates were fully and homogeneously covered by the functional gradient coating. The uniformly distributed micro-cracks in the CeO2 ceramic coating were lobed, didn't reach the phytic acid conversion coating. (3) The anticorrosion properties of the AZ91D magnesium alloy were improved greatly by the functional gradient coating.
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