Oxidation and Volatilization of TZM Alloy in Air

The excellent high temperature strength and thermal conductivity of molybdenum-base alloys provide attractive features for components in advanced magnetic and inertial fusion devices. Refractory metal base alloys react readily with oxygen and other gases, and molybdenum alloys are susceptible to losses from highly volatile molybdenum trioxide (MoOsub3) species. Transport of radioactivity by the volatilization, migration, and re-deposition of MoO3 during a potential accident involving a loss of vacuum or inert environment represents a safety issue. We have experimentally measured the oxidation, volatilization and re-deposition of molybdenum from TZM in flowing air between 400 and 800°C. Calculations using chemical thermodynamic data for vapor pressures over pure MoOsub3 and a vaporization mass transfer model correlate well with experimental data between 600 and 800°C. Partial saturation of (MoOsub3) gas species account for influences of flow rate at 700°C. Some anomalies in oxidation rate below 650°C, suggesting that other phases, e.g., MoOsub2 or other non-stoichiometric oxides may influence oxidation and volatilization processes under some limited conditions.