Corrosion of Metals by Liquid Bismuth-Tellurium Solutions

Corrosion of candidate materials of construction for stills to extract radioactive polonium-210 from bismuth by distillation at temperatures of 450 to 950 C (842 to 1742 F) was evaluated. Tellurium, being chemically similar to polonium, was used as a nonradioactive stand-in for polonium. Tantalum appears to be the most satisfactory material from the standpoint of long term corrosion and fabricability. Tantalum was corroded at rates up to 2 × 10⁻⁵ in/hr during the initial 100 to 200 hours exposure; the rate decreased to below 2 × 10⁻⁶ in/hr after 400 hours for concentrations of tellurium in bismuth of less than 30%. Graphite was not attacked chemically, but was penetrated through pores. TZM alloy exhibited average corrosion rates of less than 6 × 10⁻⁶ in/hr during 500 hours exposure in less than 20% tellurium in bismuth, but corroded rapidly (7.2 × 10⁻⁵ in/hr) in 50% tellurium. The stainless steels, particularly Type 430, might be used to approximately 550 K (276.85 C); but corrosion rates increased above 3 × 10⁻⁵ in/hr for a 15% solution of tellurium in bismuth at higher temperatures.