Corrosion coupons were exposed in a 1/10 scale demonstration melter system in order to evaluate the performance of alternate materials for use as top head and off gas components in the Defense Waste Processing Facility (DWPF) melter at the DOE’s Savannah River Site. The demonstration system is a prototypic version of the DWPF melter, in which high activity radioactive waste will be vitrified and encapsulated for long term storage. Fifteen different nickel and cobalt base alloys were exposed for five months to corrosive vapors and molten glass, characteristic of the DWPF melter except for radioactivity. Process conditions included temperatures approaching 900°C and a synthetic acid waste containing formates, halides, sulfates, phosphates, nitrates, and mercury. Severe oxidation of N06625 occurred, and this was attributed to the formation of Mo03. Cobalt containing alloys experienced significant degradation in this environment. Results of the metallurgical examination showed that alloys containing 50 Wt% Ni, 25 Wt5 Cr, and additions of 2 to 3 Wt% of either aluminum or silicon performed comparably or better than the N06690 in both the splash zone and in the vapor space near the lid. Corrosion rates for these alloys did not exceed 0.2 mm per year (8 mpy). Based on the observations, N06690 material is expected to perform acceptably for the 2-year design life in the DWPF.
Keywords: high temperature corrosion, hot corrosion, environmental degradation, nickel base alloys, cobalt base alloys, glass melter, off-gas, nuclear waste processing