Doubie Rare-earth Conversion Films Formed On AZ31 Magnesium Alloy And Study Its Protection

The double rare-earth treatment for AZ31 Mg alloy substrates were studied by soaking in cerium nitrate and lanthanum nitrate mixture solutions to discuss the effects of immersion temperature,immersion times and solutions with KMnO₄ and so on and to characterize the influence of conversion coatings on corrosion resistance properties.The corrosion protection capability of double rare-earth conversion films was evaluated by titration test,weight lossing test and Tafel polarization curves.With the increase of immersion time,changing colors time in titration test adds from 6s to 79 s.The corrosion speed of conversion films in 3.5 %NaCl solutions is 1/5 times than that of Mg alloy substrates.Polarization curve shows that the improvement of corrosion potential is 0.313 V.And corrosion current density reduces from 4.03×10^-2 mA/cm² to 2.31×10^-4mA/cm².SEM test investigates that the morphology of conversion coating is cracked and dry-muddy. The corrosion resistance property presents the trend of increasing firstly and decreasing later with the extending of immersion time.Design uniformity test,the capability of conversion films was evaluated by SEM,titration test,weight lossing test and Tafel polarization curves and make certain optimal treatment technics:immersion temperature 48.679℃,immersion times3min,immersion concentration of solution 0.006 mol·L^-1,solutions with KMnO₄ 7.2 mL·L^-1,molar ration of cerium and lanthanum in solution 0.5.New film was formed on the surface of double rare-earth conversion film, treated by TiO₂ sol.The SEM micrograph shows that the new substance arises in the crack of the surface of the double rare-earth conversion films.The inside film are unexposed and the cracks are decreased,then the films become more uniform. The double rare-earth conversion films treated by TiO₂ sol are tested by potentiodynamic polarization curve and electrochemical impedance spectroscopy (EIS).The results indicate that the corrosion potential of specimen is improved from -1.167 V to -0.777 V,the corrosion current density is reduced two magnitudes,the radius of capacitance loops of EIS is enlarged,and the polarization resistance is improved from 1.62 kΩto 1.81 kΩ.The corrosion resistance of specimen is increased obviously.The E-t test analysis come into being property of conversion films.