Current density effects on the corrosion of ceramic and metallic electrode materials in waste glasses

Vitrification by joule-heating requires suitable electrode materials. Molybdenum electrodes are used often in high temperature (∼ 1,500 C) joule-heated melters producing commercial glass while Inconel 690 has been the material of choice for lower temperature (∼ 1,150 C) high-level nuclear waste vitrification. Vitrification of diverse waste streams at higher-temperatures places increasingly severe demands upon the electrode materials. Some commercially available ceramics possess sufficient conductivity at high temperatures to be used as electrodes. Many mixed and low-level waste vitrification applications involve significant amounts of reducible species that accelerate the corrosion process for metallic electrodes. In addition, the corrosion rate of all electrode materials depends on the electrode current density. A special test rig was designed that measures the dependence of corrosion rate on both current density and temperature over extended periods of time, Molybdenum, Inconel 690, and three types of commercial ceramic coupons were tested over a range of conditions in simulated waste glasses containing key reducible species (e.g., PbO, CuO, ZnO, SO3). The corrosion rates show strong dependencies on the current density. Coupons were sectioned and the glass-coupon interface examined using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDS). Cu, Pb, Ni and Sn were found to be reduced to their metallic states at the molybdenum coupon surfaces and were active in the corrosion process