Nitriding technologies are promising surface modification techniques of uranium based on pulsed laser irradiating and glow plasma treatment. Nitrided layers with different nitrogen contents (UN0.35, UN0.75, UN1.08 and UN1.5) were prepared on the surface of uranium. The present study aims to investigate the microstructure and corrosion properties of the reaction of the UNx layers with ultra-low water vapor at room temperature. The electronic structures were analyzed in situ by X-ray photoelectron spectroscopy in high vacuum.
The results showed that the UN0.35, UN0.75, and UN1.08 samples were mainly composed of uranium nitride (UN) and metallic uranium, while the surface microstructure of the UN1.5 sample was U2N3. The dense and uniform nitride layer with a grain size of 20 to 50 nm was obtained on the uranium surface, which acted as a barrier and prevented the further diffusion of anions into the matrix. The corrosion products of the UN0.35, UN0.75, and UN1.08 samples were mainly UO2-xNy and UO2 after reaction with the water vapor. The contents of UO2-xNy increased with increasing nitrogen contents, and the corrosion rate decreased significantly. The intermediate compounds UO2-xNy reacted slowly with the water vapor, and eventually converted to UO2. Meanwhile, the corrosion products of the UN1.5 sample were mainly U2N3+xOy and UO2-xNy after reaction with the water vapor. The percentage of U2N3+xOy and UO2-xNy remained almost stable over a long period of time, which indicated that the high contents of U2N3+xOy and UO2-xNy prolonged the time for complete conversion to UO2. It can be concluded that the U-N-O ternary compounds retarded the corrosion process and the UNx layers with high nitrogen contents showed excellent corrosion resistance.