Effect Of Fe, Al And Si On The Reaction Of U-0.7wt.% Ti Alloy With Water Vapor In Closed System

U-0.7wt.%Ti alloy is an important nuclear material which is readily to react with gaseous molecules (H2O, H2, O2, etc). The corrosion of U-0.7wt.%Ti alloy in atmosphere is a complicated process, which is sensitive to many parameters, including heat treatment, surface condition, and impurities in material, etc. The effect of the impurities, such as iron, aluminum and silicon, on the oxidizing corrosion and hydriding corrosion of uranium and uranium alloys has not been widely concerned, especially for U-0.7wt.%Ti alloy. The effect of impurities (Fe, Al, Si), such as impurity content and existence form, on the oxidizing corrosion and hydriding corrosion of U-0.7wt.%Ti alloy was studied in this paper. At the same time, the change of micro structure on the effect of corrosion of U-0.7wt.%Ti alloy was investigated.1) The effect of impurities (Fe, Al, Si) on the microstructure of U-0.7wt.%Ti alloy was studied by SEM-EDX, LSCM and DSC, etc. Three sets of U-0.7wt.%Ti samples with different impurity forms were used, including cast sample (as received sample), quench cooled sample and slow cooled sample. In argon protective atmosphere, the cast coupon was heated from room temperature up to 800℃ with the heating rate at 10 K/min, and kept in 800℃ for 2 hours. Subsequently, the coupon was cooled to room temperature with the cooling rate at 10 K/min or quenched by water to obtain the slow cooled coupon or the quench cooled coupon. The impurities exist mainly in dissolved form (Fe, Al, Si) for the quench cooled sample and intermediate compound (U3Si, U6Fe, UAl2) for the slow cooled sample, respectively. A small part of impurities (Fe, Al, Si) react with titanium to form TixSiy phase and TixAly phase. And the grains of U-0.7wt.%Ti alloy can be refined by the impurities (Fe, Al, Si).2) The influence of impurities (Fe, Al, Si) on the reaction of U-0.7wt.%Ti alloy and hydrogen was systematically studied by in-situ microscope and PVT method. Before experiment, each set of U-0.7wt.%Ti samples was treated to obtain uniform surface conditions by a vacuum thermal pre-treatment (VTPT) cycle. Induction time measurements demonstrate that in the early stage of hydriding reaction,the reaction kinetics display a strong dependence on the impurity contents and existence form. Silicon impurity can accelerate the hydriding corrosion of U-0.7wt.%Ti alloy for both the slow cooled sample (silicon mainly exists in U3Si phase) and quench cooled sample (silicon mainly exists in dissolved form). Because silicon impurity can lower the activation barrier of uranium hydriding, the dissolved silicon or U3Si phase may act as nucleation sites. Iron impurity also can accelerate the hydriding corrosion of U-0.7wt.%Ti alloy for both the slow cooled sample (iron mainly exists in U6Fe phase) and quench cooled sample (iron mainly exists in dissolved form). Because iron impurity can refine the grains of U-0.7wt.%Ti alloy to generate much more grain boundaries, which can enhance the hydriding rate of U-0.7wt.%Ti alloy. The results also suggest that aluminum impurity can accelerate the hydriding corrosion of U-0.7wt.%Ti alloy for the quench cooled sample(aluminum mainly exists in dissolved form), but reduce the hydriding rate for the slow cooled sample (aluminum mainly exists in UAl2 phase). On the one hand aluminum impurity can enhance the activation barrier of uranium hydriding hence reduce the hydriding rate of U-0.7wt.%Ti alloy, on the other hand aluminum impurity can refine the grains of U-0.7wt.%Ti alloy hence increase the hydriding rate of U-0.7wt.%Ti alloy. This may cause the difference of aluminum impurity on the hydriding rate for the slow cooled U-0.7wt.%Ti alloy and quench cooled U-0.7wt.%Ti alloy.3) Raman spectroscopy combined with weight gain method was used to evaluate the effect of impurities(Fe, Al, Si) on the corrosion behavior of U-0.7wt.%Ti alloy with moisture at 80℃. The results show that both dissolved iron and U6Fe phase can enhance the oxidative corrosion resistance of U-0.7wt.%Ti alloy. This may be relevant with iron impurity can suppress the surface corrosion product of UO2 to form a higher valence uranium oxide.The experimential results also indicate that the dissolved silicon and U3Si phase have little influence on the oxidative corrosion of U-0.7wt.%Ti alloy. The dissolved aluminum and UA12 phase have little influence on the oxidative corrosion of U-0.7wt.%Ti alloy.4) The reaction of U-0.7wt.%Ti alloy with water-hydrogen mixture was studied. The effect of water content on the reaction of U-0.7wt.%Ti alloy and hydrogen was investigated by measuring pressure drop and in-situ micrographic observation at 200℃. The results indicate that moisture can inhibit the reaction of U-0.7wt.%Ti alloy with hydrogen. At the same time, we find that hydriding corrosion can accelerate the oxidative corrosion of U-0.7wt.%Ti alloy with moisture. We also demonstrated that uranium hydride can induce the ignition of U-0.7wt.%Ti alloy, which may severely affect the safety and reliability of U-0.7wt.%Ti alloy.5) Raman spectroscopy was employed to characterize uranium hydride and deuteride in this paper. For the Raman shift of UH3 and UD3 is inversely proportional to the mass square of hydrogen isotope approximatively. The typical Raman peaks of UH3 and UD3 can be identified by the mass difference of hydrogen isotope for one another. The results show that the characteristic peaks of UH3 powder are at 725 and 938 cm-1,the characteristic peaks of UD3 powder are at 518 and 669 cm-1, respectively. The present work demonstrates that Raman spectroscopy can be successfully used to characterize uranium hydride and deuteride.The results of our paper reveal the intrinsic relationship between the impurities (Fe, Al, Si) and the oxidizing and hydriding corrosion of U-0.7wt.%Ti alloy, and give some benefit to improve the corrosion resistence of uranium and uranium alloys.