Electrochemical Corrosion of Electrodes in a Simulated Nuclear Waste Glass Melt
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ABSTRACT Corrosion of potential candidate electrode materials, molybdenum and tantalum, in a simulated nuclear waste glass melt was investigated using electrochemical (dc-powered) and non-powered tests. Electrochemical corrosion data showed that tantalum was more corrosion-resistant than molybdenum in this melt. Tantalum also showed passivation. The non-powered test data showed that tantalum corroded more than molybdenum. This was attributed to penetration of protective passivation layer at the tantalum-glass interface by the glass melt. Microstructural features and chemistry across selected electrode-glass interfacial regions supported these results. INTRODUCTION An integrated waste approach was evaluated under the Department of Energy's Landfill Focus Area (formerly the Minimum Additive Waste Stabilization or MAWS program). The approach consisted of blending waste streams of different compositions to reduce the need for additives to produce a stable, high quality waste form. The program considered vitrification technology for stabilization of these wastes. Vitrification is a well-established technology for processing temperatures below I I50'C due to service temperature limitation of the commonly used electrode, Inconel 690. As some of the blended wastes processed at temperatures higher than 1200'C, an investigation was undertaken in the FY 95 to extend the waste vitrification technology to high temperatures. Additionally, greater electrode life at processing temperature in the range of I 100°C was also considered. Corrosion of materials of construction of melter for nuclear waste vitrification had been investigated since the 7 0 's1'4. Electrochemical corrosion of electrodes for high temperature vitrification was first reported in 1995'". The authors demonstrated the prospect of electrochemical protection of electrode materials in contact with highly corrosive high-level waste simulated glass melts. In the present paper, electrochemical corrosion of candidate electrode materials in a simulated nuclear waste is reported and the electrochemical corrosion tests data are compared to non-powered tests data. EXPERIMENTAL Candidate Electrode Materials Potential candidate electrode materials were molybdenum and tantalum. Molybdenum was chosen, as it was the most used melt-contact material for electrode/booster applications in commercial glass melters. Tantalum was chosen based on thermodynamic stability of its oxide and the prospect of electrochemical protection"5.
309 Mat. Res. Soc. Symp. Proc. Vol. 500 ©1998 Materials Research Society
Glass Composition and Preparation The selected glass composition represented a waste glass composition from Idaho site. The glass was prepared using oxides, carbonates, and hydroxides. The glass was melted in an electrically heated furnace and quenched by pouring on a stainless steel plate. Then, the glass was crushed into - 40 mesh and used for the corrosion tests. The targeted glass composition and corresponding chemical analytical results are presented in Table I. Table I Glass Com
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