U-2.5wt% Nb Alloy Oxidation Kinetics And Atmosphere Corrosion On The Mechanical Properties

U-Nb alloys have been widely used in the field of nuclear engineering because of a combination of good corrosion resistance and mechanical properties. However, owing to the metastable structure in use, in recent years, the aging behavior of U-Nb alloys has been concentrated. As we know, two main reasons will lead to loss of ductility of U-Nb alloy. One is the phase transformation, and the other is surface corrosion. It is well known that uranium is a reactive element, and easily adsorb oxygen and water vapor when exposed to air, subsequently react with those gases. With the extent of time, the thickness of corroded layer will be increased, which can lead to the decrease of the cross-section area and formed much more cracks in the scale, which could affect its mechanical properties. In addition, the interaction of uranium with water vapor produces hydrogen, the ability to adsorb hydrogen of U-Nb alloy is stronger than uranium, and the hydrogen will also affect the mechanical properties of U-Nb alloy. As a structural material, the mechanical properties of U-Nb alloys are very important. If the mechanical properties decrease to a critical point arisen by interior and exterior factors during storage, the catastrophic results will probably happen. Thus, study of the effect of surface corrosion on the mechanical properties has scientific and engineering valueIn the present work, the oxidation behavior of U-2.5wt%Nb has been studied systemically, and more surface sensitive-techniques have been used to investigate the scale morphology and microstructure. The results show that, at a temperature range below 80℃, the scale formed on the surface is dense and compact, and the oxidation behavior follows a parabolic law. At temperature range of 100-150℃, the oxidation is made of para-linear law. However, above 200℃, the oxidation mainly follows linear oxidation law, but a very short parabolic law was also observed. The scale was characterized by XRD, AES and XPS. The results show that, below 200℃, the scale is mainly consisted of UO₂, Nb₂O₅, NbO, and a very trace of NbO₂ observed. The analysis of XPS revealed that alloy niobium into uranium doesn't change the chemical environment of each element. The oxidation process of niobium and uranium is similar to pure metal of each element. Temperature significantly affected on the scale structure. Under UHV condition, increasing temperature will result in diffusion of oxygen into the bulk and result in the reduction of oxide. Utilizing the second Fick'law and the model of interface movement, the oxidation process of uranium has been simulated, and the relationship between oxidation thickness and time has been obtained. The simulation predictions are in good agreement with experimental results.The effect of oxidation on the mechanical properties of U-2.5wt%Nb alloy was investigated by accelerated test. Meanwhile, the tensile properties of U-2.5wt%Nb stored in humid air, sealed humid air, sealed nitrogen-saturated water, respectively, have also been carried out. The results indicate that a brittle layer and a lot of defects, such as cracks, corrosion pits have been formed on the surface of tensile test bar, which have greatly influence on the tensile properties, can lead to the decrease of ductility and strength. the morphologies of fracture show that the fracture is a mix of dimple and brittle fracture .Under the experimental condition, the structure and composition . in the humid air and sealed vessel, at 60℃. The strength did change slightly, but the ductility decrease slightly. At 45deg, during the experimental conditions, the strength and the ductility did not show evidently change. In nitrogen-saturated water vapor, the results are same as that in 45℃. The main reason is that at low temperature, the reaction is very slow, and the product of hydrogen is very small. So we did not observe the change of mechanical properties. From the results of dynamic test result, it can be concluded that corrosion has effect on the dynamic mechanical properties, but the mechanism need to be further studied.