An examination of the initial oxidation of a uranium-base alloy (U–14.1 at.% Nb) by O 2 and D 2O using surface-sensitive techniques

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 O 2 or D 2O 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 O 2 at 300 K indicate production of a thin oxide overlayer of stoichiometric UO 2.0 intermixed with Nb 2O 5. The same stoichiometry is exhibited for uranium when the oxide is prepared at 500 K with O 2; 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 O 2 is much greater (as judged by oxide layer thickness) than that exhibited by D 2O. An oxide layer thickness of less than 20 Å was created using D 2O as an oxidant at 300 K with exposures >3500 L (oxide layers created from O 2 are significantly greater at much smaller exposures). Formation of a critical density of Nb 2O 5 is suggested to be responsible for the enhanced corrosion resistance by preventing diffusion of O − (O 2−) 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 ( Θ rel=0.30) are obtained at a surface temperature of 175 K for these experimental conditions.