In Situ Electronic Structure Analysis of Water-Corroded UN_x in High Vacuum

Nitriding technologies are promising surface modification techniques of uranium based on pulsed laser irradiating and glow plasma treatment. Nitrided layers with different nitrogen contents (UN_0.35, UN_0.75, UN_1.08 and UN_1.5) were prepared on the surface of uranium. The present study aims to investigate the microstructure and corrosion properties of the reaction of the UN_x 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 UN_0.35, UN_0.75, and UN_1.08 samples were mainly composed of uranium nitride (UN) and metallic uranium, while the surface microstructure of the UN_1.5 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 UN_0.35, UN_0.75, and UN_1.08 samples were mainly UO_2-xN_y and UO₂ after reaction with the water vapor. The contents of UO_2-xN_y increased with increasing nitrogen contents, and the corrosion rate decreased significantly. The intermediate compounds UO_2-xN_y reacted slowly with the water vapor, and eventually converted to UO₂. Meanwhile, the corrosion products of the UN_1.5 sample were mainly U₂N_3+xO_y and UO_2-xN_y after reaction with the water vapor. The percentage of U₂N_3+xO_y and UO_2-xN_y remained almost stable over a long period of time, which indicated that the high contents of U₂N_3+xO_y and UO_2-xN_y 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 UN_x layers with high nitrogen contents showed excellent corrosion resistance.