Electrochemical Evaluation and Surface Characterization of Josephinite as a Natural Analog for Container Materials
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Electrochemical Evaluation and Surface Characterization of Josephinite as a Natural Analog for Container Materials Yi-Ming Pan and Gustavo A. Cragnolino Center for Nuclear Waste Regulatory Analyses, Southwest Research Institute 6220 Culebra Road, San Antonio, TX 78238-5166, USA ([email protected]) ABSTRACT A sample of josephinite, a rock containing predominantly a Ni-Fe metallic phase, was evaluated as a natural metal analog to increase confidence in the assessment of waste package performance for the potential high-level radioactive waste repository at Yucca Mountain, Nevada. The josephinite sample was characterized electrochemically in simulated groundwater environments using cyclic potentiodynamic polarization and potentiostatic tests. The passive surface layers formed potentiostatically were examined by X-ray photoelectron spectroscopy. These results were compared to those obtained with a synthetic Ni3Fe alloy with a chemical composition similar to that of josephinite. Electrochemical studies showed that josephinite exhibited passivity at a slightly higher pH than did the cast Ni3Fe alloy and was found to be slightly more susceptible to pitting corrosion. The passive films formed on the josephinite and the cast Ni3Fe alloy have a duplex structure consisting of an Fe-rich hydroxide outer layer and a Ni-rich oxide inner layer. Results obtained from this study provide an appropriate characterization of the environmental conditions leading to the passivity and localized corrosion of josephinite. Extended persistence of a stable passive film, however, is essential for the long-term stability of the josephinite sample. The relationship between the passive behavior and the formation of alteration layers needs to be established for assessing the survivability of josephinite. INTRODUCTION Alloy 22, the alloy proposed by the U.S. Department of Energy as the outer container material for the waste package at the potential repository at Yucca Mountain, Nevada, has existed only for slightly more than 20 years. The long-term corrosion resistance of the alloy depends on the stability of a protective oxide film on the alloy surface through which slow uniform corrosion occurs. Natural analogs that have survived for an extraordinarily long time have been considered appropriate instruments for increasing confidence on container life estimations. Several investigations have been conducted to assess the validity of josephinite, a rock containing a naturally occurring Ni-Fe alloy, as an appropriate natural analog for the assessment of container life [1–3]. Josephinite, in the form of mechanically weathered pebbles and nuggets, was originally collected from placer gravel deposits in Josephine Creek in southwestern Oregon [4-6]. This investigation was conducted using a sample of “new” josephinite provided by Professor J. Bird (Cornell University). The sample, collected in the proximity of Cave Junction in Oregon and less altered than the Josephine Creek samples, exhibits a predominant Ni-Fe metallic phase containing small di
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