High-temperature oxidation kinetics of tungsten-water reaction with hydrogen inhibition

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,H2Opartial pressures ranged from 3.04 to 51.3 torr. Using Arrhenius reaction rate kinetics, the activation energy of the tungsten-H₂O oxidation reaction was calculated to be 51 kcal=mol, while theH2Opressure exponent, or reaction order, was determined to be 0.82. Increasing concentrations of H₂ 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. The derived oxidation rates indicate that, under most rocket motor conditions, corrosion of tungsten nozzles are most 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).