The Uranium Surface Plasma Nitriding Preliminary Study

Uranium possesses a wide range of usage in nuclear engineering. As metallic uranium is reactive in ambience or application environment, it is essential to improve the corrosion resistance via surface modification. Forming a passive nitride layer on uranium surface has been proved to be an effective measurement for the protection against corrosion. In this study we prepared surface nitrided samples with plasma nitriding. Analyses for composition and chemical valence are carried out using the data of Auger Electron Spectroscopy (AES), X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). An optimized setup of nitriding process is established, while the mechanism of plasma nitriding on uranium surface is proposed and discussed. Various corrosion tests are performed to examine the anti-corrosion properties of surface nitride layer.The examination of nitride layers prepared with different parameters reveals that oxide layer on sample surface and the oxygen-contained contaminants in base vacuum has crucial influence over nitriding rate. Oxygen is more reactive with uranium than nitrogen, thus uranium would be oxidized preferentially and the surface oxide layer acts as a barrier against nitriding. Higher base vacuum, as well as lower leak rate would be beneficial for the nitriding process, and it is recommended before nitriding, the vacuum chamber should be roasted and the oxide layer on sample surface should be sputtered away. Hydrogen assists to remove oxygen in working atmosphere and deoxidize sample surface, induce nitrogen's dissociation in gas phase and its diffusion in the nitride layer. Thus hydrogen would accelerate nitriding and lower the base vacuum requirement.Analyses of XRD data show the nitride layer prepared by plasma nitriding mainly consisted ofαuranium sesquinitride, but varies slightly with experiment details. Nitriding with pure nitrogen and a high base vacuum (5×10-1Pa), the thin nitride layer obtained, which contains a few metallic uranium, a minor phase of uranium dioxide and uranium carbide; nitriding with nitrogen-hydrogen mixture and a lower base vacuum (5×10-4 Pa), the nitride layer is much thicker. No peaks of metallic uranium or carbide are detected, while the signal of dioxide is intensified. AES depth profiling reveals the nitride layer (about 200 nm thick) prepared by hydrogen-free process exhibits a structure composed by oxidized surface, U-N compound layer, nitride-substrate interface and N diffused metal bulk. The atomic concentration near the interface varies gradually, indicates a good adhesion to the substrate. The hydrogen-assisted process is more likely to produce thicker layers and a deeper oxygen distribution. Reaction-diffusion mechanism is supported by the analysis of interface evolution. During nitriding the oxide layer and nitride compound layer act as diffusion barriers successively, which could be avoid by introducing hydrogen into work atmosphere or sample heating.The corrosion resistance of nitride surface was tested by various experiments. The nitrided sample retains its luster after a year's storage in ambience. Nitride layer decreases hydride nucleation rate and density, and the compressive stress impedes the expansion of corrosion sites. In the test of long term storage in hot and wet environment, the sample surface covered by thin nitride layer remains integrity and thus protective.Nitride layer on uranium, of a certain thickness, can be prepared by plasma nitrding with finely tuned working parameters like vacuum and working atmosphere. The obtained nitride layer possesses a gradual interface thus a tight nitride-substrate adhesion, would provide a long term protection against corrosive agents.