Uranium Hydride Formation Study as Observed by Scanning Surface Potential Imaging
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0986-OO05-06
Uranium Hydride Formation Study as Observed by Scanning Surface Potential Imaging Marilyn E. Hawley, Mary Ann Hill, Yongqiang Wang, and Roland K. Schulze Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM, 87545 Abstract Uranium is an extremely important material for commercial and military applications (i.e. nuclear power, nuclear weapons, conventional weapons, and armor systems) and, like a number of other materials, is vulnerable to corrosion by environmental gases that affect their properties, leading to component degradation, shortened lifetimes and materials failure. For uranium this is particularly true in the case of corrosion by hydrogen. A fundamental understanding of the corrosion process at the nucleation stage is of critical importance. The goal of this work is to study the role of common chemical impurities in uranium with initiation sites for the formation of destructive hydride blisters. After samples were implanted with various ions, they were first annealed under vacuum at 200ºC, and then exposed to one atm of ultra-pure hydrogen at 200ºC to accelerate the aging process. Scanning force microscopy surface potential imaging was used to characterize the structure and corresponding electrical properties of polycrystalline uranium surfaces that resulted from the implantation of different suspect atoms and exposure to hydrogen gas. Surface potential images revealed features related to different oxide structures, hydride spots/blisters, and other features not obvious in the corresponding topograph. In surface potential images, blisters appear as bright (higher potential) features in sharp contrast to the uranium oxide background. In a previous study possible inclusions were observed in the center of some blisters. Blister formation did not appear to correlate with implantation of any specific specie, however, distinct differences were seen between implanted and non-implanted sides of the same sample. Introduction Uranium, like other metals, is susceptible to the corrosive effects of H2 gas, which because of its small size readily diffuses through the UO2 surface layer into the bulk material. In the case of U, hydrogen attack results in the formation of first spots then blisters that break through and eventually cover the surface. The surface oxide thickness is already known to influence blister nucleation. These areas often appear as local circular features identifiable by their color difference from the surrounding UO2 surface. The uranium hydride features formed as a result of this process have significantly different electrical properties than the base metal or UO2 coating. Identification of specific sites responsible for nucleation of hydride blisters is complicated by the presence of many potential candidates. Polycrystalline depleted uranium (DU) samples contain grain boundaries, different grain orientations, local and extended defects, inclusions (strain), and chemical impurities that can affect the formation of these blisters. In all likelihood
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