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 (MoO3) species. Transport of radioactivity by the volatilization, migration and re-depssition 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 degreesC and 800 degreesC. Calculations using chemical thermodynamic data for vapor pressures over pure MoO3 and a vaporization mass transfer model correlate well with the experimental data between 600 degreesC and 800 degreesC. Partial saturation of(MoO3) gas species accounts for influences of flow rate at 700 degreesC. Some anomalies in oxidation rate below 650 degreesC suggest that other phases, e.g., MoO2 or other non-stoichiometric oxides may influence oxidation and volatilization processes under some limited conditions. (C) 2000 Elsevier Science B.V. All rights reserved.