| Magnesium-lithium alloys are widely used in aerospace as the lightest metallic materials.The potential difference at the interface of the two phasesα-Mg andβ-Li of Mg-Li alloy leads to a higher frequency of corrosion in the working environment,limiting its application.Therefore,in this thesis,a polyaniline composite coating was applied to the surface of Mg-Li alloy to improve its corrosion resistance.Before coating the polyaniline composite layer,the molybdate conversion film needs to be prepared on the surface of magnesium-lithium alloy to study the effect of process parameters on the conversion film,and then the polyaniline composite coating is prepared to study the corrosion resistance mechanism.The chemical transformation method was used to prepare molybdate conversion film on the surface of magnesium-lithium alloy,and the influence of process parameters on the corrosion performance of molybdate conversion film was studied by single-factor experimental method to control the Na2MoO4 concentration,solution pH and temperature.Scanning electron microscopy（SEM）,energy spectroscopy（EDS）,electrochemical tests,electrical conductivity tests and X-ray diffractometer（XRD）were used to test the morphological characteristics,corrosive properties and composition of the conversion films to determine the optimal process conditions.The experimental results showed that when the Na2MoO4 concentration was 20 g/L,the solution pH=5 and the temperature was 45℃,the conversion film was treated in the conversion solution for 10 min,and its corrosion current density was 7.052×10-5 A/cm2 in 3.5 wt.%Na Cl solution,and the film surface resistance was 165 mΩ/cm,indicating that the molybdate conversion film improved the corrosion resistance of Mg-Li alloy.Since cracks exist in the molybdate conversion film,the conversion film was modified to study the effect of nano-TiO2 on the morphology of the molybdate conversion film and the electrochemical corrosion behavior in 3.5 wt.%NaCl solution.The experimental results showed that（1）when the concentration of nano-TiO2 was 2 g/L,the corrosion resistance of the molybdate conversion film was optimal,and its corrosion current density was 2.936×10-7A/cm2.（2）When the concentration of nonionic surfactant polyethylene glycol-4000 was 0.8g/L,the nano-TiO2 was dispersed,and the corrosion current density of the conversion film obtained was 1.694×10-7 A/cm2.（3）When the nano-TiO2 was modified by sodium dodecylbenzene sulfonate cationic surfactant with a concentration of 0.3 g/L,the corrosion current density of the converted film was reduced to 1.146×10-7 A/cm2.（4）When the two surfactants were combined,the corrosion current density of the converted film was 1.143×10-7 A/cm2,and the corrosion resistance of the converted film was the best.The best performance,indicating that the corrosion resistance of the Mg-Li alloy substrate is significantly improved.The polyaniline epoxy resin composite coating and the modified polyaniline composite coating were prepared on the magnesium-lithium alloy molybdate conversion film.When the content of polyaniline was 4%,the corrosion current density of polyaniline epoxy resin coating in 3.5 wt.%NaCl solution was 8.730×10-8 A/cm2,and the corrosion current density of pure epoxy coating was 1.765×10-7 A/cm2;when the content of nano-TiO2 was 3%,the corrosion current density of nano-TiO2 modified polyaniline epoxy resin coating indicates that the modified polyaniline epoxy coating shows excellent corrosion resistance. |
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