Research On Preparation And Corrosion Resistance Of Anodic Films For AZ31 Magnesium Alloy

As the lightest metal structural material,magnesium alloys are applied to automotive,electronic and aeronautical industries increasingly because of a number of desirable features,including high specific strength,high specific rigidity,damping performance and recycled easily.However,their applications are limited severely for their low corrosion resistance.Suitable surface treatments which were used to improve the corrosion resistance of magnesium alloys were very significant.Anodizing process is a common and effective surface treatment method for magnesium alloys.Anodizing process is simple and the cost is lower in contrast to other surface treatment methods.The anodic films with porous structure are a good base for organic coatings.In this paper,an anodizing process based on an environmentally friendly electrolyte bath was obtained through experiments for the anodizing of AZ31 magnesium alloy.The electrolyte bath contained none of chromate,fluoride or any other poisonous ingredient.With adding additives and sealing treatment,the corrosion resistance of anodic films had been improved.Orthogonal design which could choose the influence factors and levels was employed in tests to produce optimum anodizing composition of AZ31 magnesium alloy.Eventually,the anodizing parameters were obtained through range analysis,and the result of which indicated that the composition of the electrolyte was 100g/L Na₂B₄O₇,50g/L NaAlO₂ and 30g/L NaOH,the constant anodizing voltage was 50V and the anodizing time was 10min.XRD results showed that the main components of the anodic film were periclase MgO and spinel MgAl₂O₄.Potentiodynamic polarization curves investigated that the corrosion resistance of AZ31 magnesium alloy was greatly enhanced through the optimum technic. On the basis of the study above,the effects of additives and sealing treatment on the properties of anodic films were investigated.The test results showed that addition of sodium citrate and TiO₂ sol in the basic anodizing electrolyte not only improved the surface morphology,but also enhanced the corrosion resistance of the anodic films formed on AZ31 Mg alloy.When the addition of sodium citrate was 10g/L,the self-corrosion potential was -0.614V. When the addition of titania sol was 4%,the corrosion potential was -0.769V.The XPS results indicated that Ti element existed in the forms of Ti(â…¡),Ti(â…¢)and Ti(â…£) in the anodic film.Ti(â…¡),Ti(â…¢) and Ti(â…£) was from TiO species,Ti₂O₃ species and TiO₂ species,respectively.The ratio of peak area(r_i(%)=A_i/A) was Ti(â…£)62.99%,Ti(â…¢)27.16%,Ti(â…¡)9.85%.Hot water sealing,SiO₂ sol sealing and TiO₂ sol sealing were used for anodic films.The anodic film through TiO₂ sol sealing had better corrosion resistance than sealed in other sealing processes.After being sealed in TiO₂ sol,the self-corrosion potential of the anodic film was -0.487V,the self-corrosion current density was 0.120μA/cm⁽²⁾,the polarization resistance and the corrosion rate was 296494.7ohm/cm² and 0.0005 g/(m².h),respectively.