The high-temperature heterogeneous reaction rates of bulk tungsten (W) were studied using thermogravimetric analysis in steam (H2O) and hydrogen (H2) atmospheres. Reaction rates were determined at isothermal conditions for temperatures ranging from approximately 1100 to 1700 C. Constant system pressures of 1 atm were employed; however,H2O partial pressures ranged from 3.04 to 51.3 torr. Using Arrhenius reaction rate kinetics, the activation energy of the tungsten–H2O oxidation reactionwas calculated to be 51 kcal=mol, while theH2O pressure exponent, or reaction order, was determined to be 0.82. Increasing concentrations ofH2 were found to exponentially reduce the desorption rates of the tungsten oxides involved in the oxidation reaction. Addition of H2 slightly reduced the apparent activation energy of the reaction as well, indicating a stronger inhibiting effect at higher temperatures. Thederived oxidation rates indicate that, undermost rocketmotor conditions, corrosion of tungsten nozzles aremost likely limited by gas-phase diffusion of oxidizers to the nozzle surface and/or lack of available reaction sites (i.e., surface area limited or pressure independent).
Read full abstract