Surface reaction controlled oxygen absorption in a Ta-8 W-2 Hf alloy: kinetics and concentration gradients

It is known that the rate of oxygen absorption in several bcc metals and alloys under conditions of low oxygen pressure and high temperature is controlled by the absorption rate of oxygen at the gas-metal interface, but the relationships between absorption rate, oxygen concentration gradient, and average oxygen concentration have not been studied in detail. In this paper these relationships are examined using several different solutions of the diffusion equation to represent the absorption process, and the relations derived are compared to oxygen absorption measurements for a Ta-8 W-2 Hf alloy under the appropriate experimental conditions. It was found that the absorption rate and oxygen gradient were accurately described by a solution of the diffusion equation using a constant oxygen flux into the specimen as the external boundary condition. The rate limiting step in the process is the oxygen absorption rate at the gas-metal interface. This phenomenological model should also properly describe the oxygen absorption behavior of pure metals under the same conditions when the process is controlled by the absorption rate at the gas-metal interface, provided the sticking coefficient does not change during absorption.