Application Study On The Composite Films Technology On AZ91D Magnesium Alloy Via Sol-gel Method

Magnesium alloys have the advantageous of low density and high strength/weight ratio make them attractive materials for many industrial applications,especially automotive,electronics,aircraft etc.However,the high corrosion susceptibility limits their use.The sol-gel process has been demonstrated to be one of the most effective routes to the surface modification technology of magnesium alloy.In this work,the sol-gel method and composite technology are used to improve the corrosion resistance ofAZ91D magnesium alloy.The major research efforts of the present study are as follows:1.Anti-corrosive nano TiO₂ thin films were successfully prepared on AZ91D magnesium alloy substrate.Since AZ91D magnesium alloy can not endure the acidic TiO₂ sol,CeO₂ thin films were used as the inner layer,and then TiO₂ thin films with excellent performance were applied to AZ91D magnesium alloy.The preparation condition of composite CeO₂/TiO₂ thin films was investigated. Composition and surface morphology of the thin films were analyzed using X-ray diffraction(XRD), contact angles measurements and scanning electron microscope(SEM).Potentiodynamic polarization curves and electrochemical impedance spectroscopy(EIS) tests were used to evaluate the corrosion behavior of the coated samples in 3.5 wt.%NaC1 solution.The results demonstrated that TiO₂ thin films mainly dominated the corrosion resistance of samples,the two layer CeO₂ and two layer TiO₂ composite films showed the best corrosion resistance.In addition,the TiO₂ thin films with excellent corrosion resistance could significantly improve the application of AZ91D magnesium alloy.2.Ce,Mg single thin film and composite films technology were developed to improve the anticorrosion performance of AZ91D magnesium alloy via novel sol-gel process.In the novel sol-gel process,the appropriate additive celloidin was used to stabilize and disperse uniformly the inorganic salts.The sols with appropriate pH could be applied directly on magnesium alloy.Compared with the traditional sol-gel process using alkoxide precursors,the novel sol-gel technology offers important advantages such as lower cost,more environment-friendly and simpler application procedures easily adaptable within industry.The structure of xerogels and films,morphology of films were analyzed using thermo-gravimetric(TG) and differential thermal analysis(DTA),Fourier transformed infrared spectroscopy(FT-IR),XRD and SEM.The anticorrosion performance of the sol-gel coated and uncoated samples in 3.5 wt.%NaC1 neutral solution had been evaluated by potentiodynamic polarization tests and EIS.The results demonstrated that the single Ce or Mg film did not show a significant corrosion inhibition,the composite films "Ce inner film/Mg outer film" exhibited better corrosion resistance.The optimized heat treatment temperature is 250℃.3.Phosphate conversion coating with interesting morphology helps to anchor further deposits and the sols have the flow characteristics.On the basis of previous works,combination of both of the advantages,Ce and Si thin films has been deposited onto AZ91D magnesium alloy by sol-gel process after a pre-functionalization of the substrate in a conversion bath,respectively.Duplex systems "conversion layer/Ce film" and "conversion layer/Si film" have been prepared on AZ91D magnesium alloy.TG-DTA and EIS were used to study the preparation technology and corrosion resistance of composition films.The effect factors on the preparation such as the depositing methods of sols,the number of layers and thermal treatment temperature were investigated.The results indicate that the composite films on AZ91D magnesium alloy exhibit an excellent corrosion resistance.