A Study On The Plasma Electrolytic Oxidation Behavior On Pure Aluminum In Sodium Tungstate Electrolytes System

Plasma electrolytic oxidation（PEO）can form ceramic oxide films on the surface of Al,Mg,Ti and Zr,and contributes to an excellent wear resistance,corrosion resistance and other surface properties of the metal surface.Meanwhile,the tungsten oxides（WOx≦₃）have excellent electro-optical and gas-sensitive properties.If tungsten oxides（WOx≦₃）thin film or powder can be prepared by plasma electrolytic oxidation technology,the application of PEO technology will be largely widened,and it has significant value both in scientific research and pratical application.In this paper,PEO behavior of pure aluminum in the sodium tungstate electrolyte with different concentrations was studied.We are the first to have fabricated WO_x≦₃ powder by PEO technology,and pure WO₃ film layer was also obtained under certain conditions.The main results are as follows:In the PEO process of pure aluminum in the sodium tungstate electrolyte with a concentration of 10 g·L^-1、16 g·L^-1,the solution was obviously green and turbid..The test found that this process produced a large amount of WO ₃ powder.The size of the powders obtained in 10 g·L^-1 electrolyte is micro and nano magnitude,with an average particle size of about 544 nm.Tungsten oxide particles synthesized by this method have certain photocatalytic properties,and 10 mg of WO₃ powder can degrade34%of methyl orange solution with a concentration of 5 mg·L^-1 and a total amount of50 ml in 8 hours.Thermodynamic analysis of the formation of micro and nano tungsten oxide particles in PEO process was carried out and found to be related to the physical and chemical properties of WO₃.The melting point and boiling point of WO₃ are much lower than that of Al₂O₃,and WO₃ is easy to sublimate.High temperature in the plasma discharge channel of the PEO process makes WO₃ sublimate and vaporized and then WO₃ is solidified and dispersed in the electrolyte after it encounters a cold electrolyte.pure aluminum can also generate a few microns thick film in the 5 g·L^-1 and 10g·L^-11 Na₂WO₄·2H₂O electrolytes apart from producing powder in 10 g·L^-1 and 16 g·L^-11 Na₂WO₄·2H₂O electrolytes,the main component of the film is Al₂O₃ and including a small amount of W and WO₃.While in the electrolyte of 16 g·L^-1 Na₂WO₄·2H₂O,a thick rough green film was formed on the surface of the sample.The main component was WO₃,and the film had a certain photocatalytic property.It is considered that the content of W in the initial film formed on the surface of pure aluminum is higher because of the high concentration of the electrolyte of 16 g·L^-1.In addition,the surface roughness of the specimen increases because of the erosion of the initial oxide film by the alkaline electrolyte.These factors lead to the formation of WO₃ on the pure aluminum surface.Finally,the film formation behavior of 5 g·L^-1 Na₂SiO₃ in sodium tungstate electrolyte was studied.It was found that the PEO film formed at the concentration of5 g·L^-1 Na₂SiO₃+10 g·L^-11 Na₂WO₄·2H₂O has better corrosion resistance and the corrosion current was reduced to 6.9×10^-8 A·cm².