The oxidation behavior of uranium treated by ultrasonic surface rolling process

Abstract

The oxidation behavior of uranium treated by ultrasonic surface rolling process (USRP) was investigated. Optical microscopy (OM), transmission electron microscopy (TEM), x-ray diffraction (XRD), x-ray photo-electron spectroscopy (XPS), auger electron spectroscopy (AES), scanning electron microscopy (SEM), focused ion beam (FIB) and ultraviolet-visible reflectance spectroscopy were used to characterize the microstructure and oxidation behavior of uranium. The oxidation kinetics of USRP treated uranium with pure O2 at 343–388 K was subdivided into two different regimes: a linear-like stage and a parabolic-like stage. The calculated activation energies of USRP treated uranium were approximately 52.3kJ/mol and 75.1kJ/mol for linear and parabolic stages, respectively. The oxide thickness of untreated uranium generally followed a parabolic growth model and the activation energy was 61.4kJ/mol. After the USRP treatment, the nano-grained layer and the close-packed U(002) texture were formed, which promoted the formation of a passive and stable oxidation film on the surface. Meanwhile, the improved surface integrity and the residual compressive stress were obtained, which slowed down the rupture of the oxide layer. As a result, it was concluded that the enhancement of oxidation resistance after USRP treatment was attributed to the nano-grained microstructure, the U(002) texture, the residual compressive stress and the smooth surface.