Anodic oxidation of UO 2: Part I. Electrochemical and X-ray photoelectron spectroscopic studies in neutral solutions

Abstract

The anodic oxidation of polycrystalline UO 2 has been studied in neutral Na 2SO 4 solution (6<pH<11) using potentiostatic and cyclic voltammetric techniques, the nature of the surface films formed has been determined using X-ray photoelectron spectroscopy. The oxidation proceeded via a sequence of surface oxide films. At low potentials ( E<−0.5 V vs. SCE), or at short times (<1 min) at more anodic potentials ( E≅+0.3 V), a thin film (t molecular layers) of UO 2+ x ( x≤0.25) was formed. At slightly more positive potentials and longer times (1< t<10 min), a film of U 3O 7 grew on top of this UO 2+ x base layer. For potentials >+0.1 V, dissolution as U(VI) occurred, and for E=+0.3 V the surface film was transformed gradually to U 2O 5 (≅1 h) and eventually to U 3O 8 ( tt16 h). For E≥+0.4 V a film of UO 3· zH 2O precipitated on the electrode surface in the absence of stirring. Also some dissolution of the U 2O 5 film occurred due to a decrease in the local pH at the electrode surface. Both the precipitation and the film dissolution were prevented by stirring.