A gas chromatographic study of uranium oxidation mechanism in moist gases

1 8 , Details of the plot, hydrogen formation vs. time, indicating repetitive appearance of parabola-like curve segments 88 19# Effect of replacing old corrosion environment with fresh moist gas of the original composition . .• 89 20, Water-vapor pressure dependencies of corrosion rate in moist argon before and after refreshing the e n v i r o n m e n t ............... • • • 90 21, Comparison of the surface temperatures of the uranium specimen and the reference inert (stain less steel) specimen at various stages of oxidation , 91 22, Features of surface morphology of corrosion product formed on the surface of uranium oxidized by moisture , , , 92 2 3 , Electron micrograph of the surface of inner compact oxide.Microcracks are formed on the flat surface of the compact oxide ........ 9^ 24.Electron micrograph of the metal-corrosion product interface.Network pattern is formed by the influence of the microcracks formed on the surface of the compact oxide layer , , ........... 95 2 5 .Fine structure of each layer in shattered oxide ., 96 2 6 .An idealized model for the pseudo-linear rate law in uranium oxidation 97 2 7 .Schematic representations of the interfacial morphology of the uranium corrosion products formed in moist environments 98 vii \ TABLES Table Page 1* Comparison of formation voltage and thickness of air films 99 2* Material balance in uranium-water reaction at 60°C, 0 ~ mmHg ......................... 100 viii particularly stressed the importance of the formation of a uranium hydride phase at the oxide-metal Interface and its continual fracture which keeps the surface from being protected by the oxide film.The present study is intended to provide some critical aspects of the proposed mechanism, through the detailed analysis of the oxidation behavior, as well as to add a more quantitative basis to the proposed model.A better understanding of this type of oxidation would provide helpful information on related areas, such as aqueous or gaseous oxidation of thorium, plutonium, zirconium, titanium, and other metals that are chemically similar to uranium.SUMMARY AND CONCLUSIONS Kinetics of oxidation of uranium in moist gases and its correlation to the morphology of the surface product layer were studied in detail by quantitative gas chromatographic and metallographic techniques.The results obtained substantiated earlier work which lead to the propose! of a "hydride-fracture" mechemism.The principal conclusions are as follows; 1) The process of uranium oxidation in moist inert gas con sisted of three different time-dependent stages: direct logarithmic, time-1«5 powered, and linear time dependencies.2) Oxygen reduced the reaction rate by stretching the intervals of hydride fractures rather than by reducing the rate of each elementary diffusion-controlled reaction, 3) Oxygen combined with active corrosion product hydrogen to form water.The kinetics of oxygen consumption involved linear, parabolic, and asymptotic time dependencies.The transition from one to the other was explained in terms of the morphology change.The process repeated, suggesting the occurrence of hydride fracture.4) The Arrhenius plot for the linear stage in moist inert gas consisted of two components, one being diffusion of oxygen through oxide layer and the other being believed to be hydride formation.The former contributes to the rate of the elementary parabolic process, while the latter to the hydride fracture frequency.5