Preparation And Properties Of Alumina Ceramic Coating On Depleted Uranium By Cathodic Micro Arc Electrodeposition

Uranium has been widely used in military, nuclear energy and other fields because of its unique nuclear performance. But it shows strong chemical activities because of its special electronic structure and low standard electrode potential. It can react with the surrounding atmosphere gases (like water vapor, oxygen and so on) very easily, which not only results in the deterioration of the uranium performance, but also pollution the environment and causes a radiological hazard to operators. Preparation surface coating on uranium metal is one of effective methods to slow or even prevent its corrosion. Currently, many methods or technologies on anti-corrosion for uranium material have been reported, which have their several advantages and disadvantages. The coating materials include metal, inorganic and organic materials. Oxide ceramic coatings are widely used as anti-corrosion coating materials due to their excellent wear and corrosion resistance and simple preparation process, but to my knowledge, there has a few literature reports on the application of oxide ceramic materials as surface corrosion coating for uranium and its alloys. Cathodic micro arc deposition is a recently developed method to produce ceramic oxide on metal as surface coating. Compared with the micro-arc oxidation technique for preparation ceramic oxide coatings, cathodic micro-arc deposition method has little limit to the substrate materials if only they are conductive, and the composition of the ceramic oxides can be adjusted easily. Therefore, it may be suitable for preparing ceramic anti-corrosion coating on the surface of metallic uranium. In this paper, the cathodic micro-arc deposition method was used to prepare alumina ceramic coating on uranium surface. The influence of electrolyte parameters (such as electrolyte concentration), power supply parameters (such as average current density, frequency, duty cycle) and deposition time on the magnitude of arc voltage, the phase composition and the morphology of the coating was studied. Then, the optimized parameters were obtained for producing alumina ceramic coating on depleted uranium samples, and the stability and the corrosion resistance of the coating prepared under such optimized parameters and the bonding force between the substrate and the coating were also analyzed. Some results as followed were obtained.When preparing alumina ceramic coating on the depleted uranium surface by cathodic micro-arc deposition method, the surface roughness of depleted uranium sample, the electrolyte concentration and stirring speed are the key factors in determining whether there occurs arc or not during the experimental process. Only with a proper surface roughness of depleted uranium and stirring speed to the electrolytic solution, such as Ra.0.10 μm and 250-300 r/min, respectively, under this experimental condition, can arcs occur at voltage of 200-220 V and stabilize at approximately 290 V with a constant concentration of electrolyte of 1.0mol/L.SEM morphologies of alumina ceramic coatings indicated that the coating surface was rough and porous and its bulk consisted of rock-like sinters and a large amount of holes located on the sinters, but no penetrating hole or crack was observed from the cross-section morphologies of the coatings. The phase of the coating was composed of mainly α-Al2O3 and γ-Al2O3, and also a small amount of UO2.Procedure parameters will affect the microstructure and the phase composition of the alumina ceramic coating on uranium prepared by cathodic micro arc deposition method in some degree. Proper electrolyte concentration and average current density will help to improve the surface roughness of the coating, higher the frequency and lower the duty cycle of the power supply help to increase the coating density, larger the electrolyte concentration and the average density in favor of the generation of α-Al2O3.The thickness of the coating can be controlled by adjusting parameters such as average current density and power frequency because of their intensive effects on the thickness of the coating, rather than the deposition time because the prolonged deposition will deteriorate the microstructure of the coating.An optimized combination of parameters for preparing alumina ceramic coating on depleted uranium was proposed as follow:the sample surface roughness was of approximately Ra 0.10 μm; the aluminum nitrate concentration and stirring speed of electrolyte was of 1.0 mol/L and 200-300 r/min, respectively; the current density, operating frequency and duty cycle of power supply was of 6 A/dm2,900 Hz, and 10%, respectively. Alumina ceramic coatings with proper microstructures and excellent bonding strength with uranium substrate were prepared under these combined parameters. These coatings showed good stabilities in atmosphere at ambient temperature and under temperature of 90℃ and relative humidity of 90%, and an excellent corrosion resistance in 3.5% NaCl solution, because the corrosion current density was of approximately two orders lower than that of uncoated depleted uranium sample.