The corrosion of depleted uranium in terrestrial and marine environments

Chronospeciation of uranium released in soil during a long-term DU shell weathering experiment

Corrosion process was investigated of depleted uranium (DU) ammunition fragments buried for three years in aerobic soils continuously irrigated with water. The continuing corrosion process was triggered through formation of soluble uranyl oxyhydrate phases such as metaschoepite and becquerelite, which were identified by micro-Raman and X-ray diffraction spectroscopy. The soil was not amended by phosphates and, therefore, no uranyl phosphates were found as corrosion products on the DU surfaces by X-ray photoelectron spectroscopy. A speciation modelling at high temporal sequence (chronospeciation approach) indicated that the abundant Fe oxyhydroxides in the soil immobilized the U(IV) released through DU corrosion. During the first two years, therefore, only <10 mg of U(VI) was thus found in the leachates from the soil columns, even though >3 g of DU had been corroded. However, the degree of this immobilization was found to be controlled by the amount of dissolved inorganic and organic carbon (DIC and DOC) in the soil pore water providing for U(VI) complexation competing with surface complexation by the Fe hydroxides. The chronospeciation approach applied is useful to improve our understanding and ability to predict the long-term fate of U(VI) and the mechanisms controlling U(VI) mobility in soil contaminated with DU shells.

Laboratory simulation of uranium metal corrosion in different soil moisture regimes

A novel laboratory simulation system has been developed for the study of the corrosion of uranium metal in soils. Corrosion and transportation of depleted uranium (DU) as the metal undergoes weathering as a buried material within the soil environment. The corrosion of uranium metal in soil was not well understood due to the gas-liquid-solid phase of the soil. This study presents a novel method to investigate the change of uranium species during the process of process of oxidation of metallic uranium in these environments. Compared with other techniques used for the study of environmental corrosion of metals in soils, this method has the advantage of low secondary uranium pollution, no energy consumption, and ease of operation. The simulation system has been used for the following studies:

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Simultaneously simulate the corrosion of uranium metal in different soil moisture regimes

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Study the influence of biogeochemical factors on the corrosion of uranium metal

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Investigate the change of uranium species during oxidation

Corrosion Behavior of the Ti–6Al–4V Alloy in Sulfate-Reducing Bacteria Solution

The corrosion behavior of the Ti–6Al–4V alloy was investigated in a sulfate-reducing bacteria (SRB) solution. The results showed that sulfate-reducing bacteria has good affinity with the surface of the Ti–6Al–4V alloy after 5 days. A potentiodynamic polarization test demonstrated that the corrosion resistance of the Ti–6Al–4V alloy was initially improved but deteriorated quickly in the subsequent period. The corrosion mechanism of the Ti–6Al–4V alloy was revealed by analyzing its microstructure with the aid of scanning electron microscopy, X-ray photoelectron spectroscopy and X-ray fluorescence. The pitting corrosion was deemed to be a typical cause of the corrosion behavior of the alloy in the SRB solution. The underlying mechanism of the pitting corrosion was proposed for the alloy.

Influences of pH Values’ Changes on the Oxide Film of U-0.79 wt.% Ti Alloy in Aqueous Solution—A Combined Study of Traditional Electrochemical Tests and Scanning Reference Electrode Technique

By combining traditional electrochemical tests including Tafel extension method and Mott-Schottky fitting, spectroscopic ellipsometry (SE) and a micro-region analysis technique, which is an integrated system of a scanning reference electrode technique and scanning tunneling microscope (SRET/STM), the changes in properties of the oxide film that formed on the surface of the U-0.79 wt.% Ti alloy (U-Ti alloy in short) in 0.1 M NaNO3 were carefully investigated as the pH value changed. The results show that the properties of the oxide film are strongly pH-dependent. The corrosion potential and corrosion current density decrease with the increasing pH value. The oxide film appears to be a p-type semiconductor at pH = 2.43. However, the transition from n-type to p-type for the oxide film as a semiconductor is observed with the increasing applied potential when the solution pH value varies from 2.43 to 7.0. The oxide film presents as an n-type semiconductor when the pH value varies from 7.0 to 11.44. In addition, during the transition of the pH, the roughness and the number of active points of the alloy surface decreases while the oxide film is thicker. It can be concluded that the corrosion resistance of the oxide film formed on the U-Ti alloy surface is enhanced in neutral or alkaline solutions.

TiN Films Deposited on Uranium by High Power Pulsed Magnetron Sputtering under Low Temperature

Depleted uranium (DU) is oxidized readily due to its chemical activities, which limits its applications in nuclear industry. TiN film has been applied widely due to its good mechanical properties and its excellent corrosion resistance. In this work, TiN protection films were deposited on DU by direct current magnetron sputtering (DCMS) and high power pulsed magnetron sputtering (HPPMS), respectively. The surface morphology and microstructures were investigated by atomic force microscope (AFM), scanning electron microscopy (SEM), and grazing incidence X-ray diffraction (GIXRD). The hardness and Young’s modulus were determined by nano-Indenter. The wear behavior and adhesion was analyzed by pin-on-disc tests and scratch adhesion tests and the corrosion resistance was evaluated by electrochemical measurements. The results show that the TiN films that were deposited by HPPMS outperformed TiN film deposited by DCMS, with improvements on surface roughness, mechanical properties, wear behavior, adhesion strength, and corrosion resistance, thanks to its much denser columnar grain growth structure and preferred orientation of (111) plane with the lowest strain energy. Besides, the process of Ti interlayer deposition by HPPMS can enhance the film properties to an extent as compared to DCMS, which is attributed to the enhanced ion bombardment during the HPPMS.

Influence of macroscopic defects on the corrosion behavior of U-0.79 wt%Ti alloy in sodium chloride solution

Uranium alloys containing a low concentration of titanium have received wide attention due to their greatly enhanced corrosion resistance and outstanding mechanical performances. Herein, we investigated the effect of macroscopic defects on the corrosion behavior of U-0.79 wt%Ti (denoted as U-Ti) alloy in 0.01 M NaCl solution using traditional electrochemical testing technologies and a novel scanning electrochemical composite probe (SECP). The results demonstrate that pitting corrosion occurs rapidly on the alloy surface due to macroscopic defects. Moreover, macroscopic defects led to a decrease in corrosion potential and polarization resistance, and an increase in corrosion current density. Furthermore, the potential and pH value distributions were detected in the same region using the composite probe. The results show that the region around the macroscopic defects become corrosion-active positions and the potential difference (vs. the average potential of the alloy surface) in this area is significantly higher than that at positions without macroscopic defects, while the opposite was observed for the pH value distribution. In addition, the distribution of the vertical direction (Z) potential at the active point was clearly different from that at the inactive point. A possible reason for this could lie in the difference in the electric field distribution and electrode reaction type between the active point and inactive point on the alloy surface.

Removing uranium (VI) from aqueous solution with insoluble humic acid derived from leonardite

The occurrence of uranium (U) and depleted uranium (DU)-contaminated wastes from anthropogenic activities is an important environmental problem. Insoluble humic acid derived from leonardite (L-HA) was investigated as a potential adsorbent for immobilizing U in the environment. The effect of initial pH, contact time, U concentration, and temperature on U(VI) adsorption onto L-HA was assessed. The U(VI) adsorption was pH-dependent and achieved equilibrium in 2 h. It could be well described with pseudo-second-order model, indicating that U(VI) adsorption onto L-HA involved chemisorption. The U(VI) adsorption mass increased with increasing temperature with maximum adsorption capacities of 91, 112 and 120 mg g^-1 at 298, 308 and 318 K, respectively. The adsorption reaction was spontaneous and endothermic. We explored the processes of U(VI) desorption from the L-HA-U complex through batch desorption experiments in 1 mM NaNO₃ and in artificial seawater. The desorption process could be well described by pseudo-first-order model and reached equilibrium in 3 h. L-HA possessed a high propensity to adsorb U(VI). Once adsorbed, the release of U(VI) from L-HA-U complex was minimal in both 1 mM NaNO₃and artificial seawater (0.06% and 0.40%, respectively). Being abundant, inexpensive, and safe, L-HA has good potential for use as a U adsorbent from aqueous solution or immobilizing U in soils.