Kinetic mechanisms of high temperature tungsten oxidation: Experimental evaluation with modeling

The high temperature heterogeneous oxidation of bulk tungsten (W) was studied using thermogravimetric analysis over a range of flow reactor temperatures and steam (H₂O) pressures, supporting previous work involving O₂, CO₂, and CO reactions with W. Isothermal reaction rates were determined at temperatures ranging from approximately 1100 to 1700°C. Constant system pressures of 1 atm were employed, however H ₂O partial pressures ranged from 7.6 to 25.8 torr. Using Arrhenius reaction rate kinetics, the activation energy of the W-H₂O reaction was found to be 51 kcal/mol, along with a pressure exponent of 0.83. Oxidation rates of the H₂O oxidizer were determined to lie between that of O₂ and CO₂. Using kinetic correlations developed for tungsten oxidation by H₂O, CO₂, and O₂ it was determined that H₂O is the dominant oxidizing species involved in tungsten rocket nozzle erosion when an AP/HTPB propellant is used. It was also determined that under common rocket motor conditions the nozzle oxidation rates are limited either by molecular diffusion, or by the number of active sites at the nozzles surface. These results emphasize the need for a detailed mechanism describing tungsten oxidation in order to accurately model W nozzle erosion. A numerical model of the flow reactor used in this study is presented which will allow such a mechanism to be developed.