3140. Analysis of solid surface monolayers by mass and energy spectrometry methods

Abstract

The corrosion resistance of uranium is greatly enhanced by alloying with niobium. In this study the initial stages of corrosion of a specific uranium-base alloy (U–14.1 at.% Nb) by O2 or D2O have been examined using the surface specific techniques of X-ray photoelectron spectroscopy (XPS), thermal programmed desorption (TPD), static secondary-ion mass spectroscopy (SSIMS), and sputtered neutrals mass spectroscopy (SNMS). XPS studies of the U–14.1 at.% Nb surface following oxidation using O2 at 300 K indicate production of a thin oxide overlayer of stoichiometric UO2.0 intermixed with Nb2O5. The same stoichiometry is exhibited for uranium when the oxide is prepared at 500 K with O2; although, niobium is much less oxidized exhibiting a mixture of NbO and Nb. Contrary to previous XPS literature, SNMS depth profiling studies reveal that oxidation by O2 is much greater (as judged by oxide layer thickness) than that exhibited by D2O. An oxide layer thickness of less than 20 Å was created using D2O as an oxidant at 300 K with exposures >3500 L (oxide layers created from O2 are significantly greater at much smaller exposures). Formation of a critical density of Nb2O5 is suggested to be responsible for the enhanced corrosion resistance by preventing diffusion of O− (O2−) or OD−/OH− into the oxide/metal interface region. The domains of stability of hydroxyl formation have also been followed using TPD, SSIMS and XPS. Maximal surface hydroxyl concentrations (Θrel=0.30) are obtained at a surface temperature of 175 K for these experimental conditions.