The Experimental Research on Oxidation Kinetics of Uranium Surface Treated by Laser Nitriding*

The oxidation kinetics of uranium surface treated by pulsed laser nitriding，which has been proved to be protective during storing at room temperature in the atmosphere, were studied using reflectance spectroscopy and X-ray diffraction (XRD) technique.
The growth of the oxide thickness firstly follows a parabolic growth model for the nitriding uranium with O₂, which is coincided with the diffusion barrier model. And the initial regime of slow oxide-film growth is followed by a much faster growth stage of a linear curve, resulting from the cracking of the initial oxide layer. It has been shown that the critical thickness reached at the onset of the transition is about 300 nm in this work.
The activation energy equaling to 98 kJ/mol was obtained by monitoring the oxide growth in the first stage, and 85.9 kJ/mol in the second stage . In comparison, the growth of oxide thickness of untreated uranium was also monitored, indicating that corrosion resistance could be obviously improved after nitriding treatment. It may result from the formation of U-N-O compound after oxidation, which would resist the diffusion of oxygen ions in the oxide layer.