Rate-Controlling Processes in the High-Temperature Oxidation of Tantalum

Abstract

The oxidation of tantalum in the temperature range 500~176 is approximately linear. At atmospheric pressure, the rate constant increases with temperature in the temperature range 5000-650 ~ and 800~176 but decreases slightly as the temperature increases from 650 ~ to 800~ The reaction rate depends on the square root of the oxygen pressure at low pressures and high temperatures, but at lower temperatures the pressure dependence decreases as the pressure increases. On the basis of experimental evidence in the literature, it is concluded that in the temperature range 500~176 the rate-controlling process is a reaction at the interface between the atmosphere and a layer of tantalum pentoxide growing adherently on the metal surface. This interface reaction is preceded by an equilibrium adsorption of oxygen on the interface, the adsorption taking place with dissociation. In the temperature range 800~176 the over-all linear rate is a consequence of the diffusioncontrolled growth of adherent pentoxide to a critical thickness, at which the scale fails from the metal. The corollary to this conclusion is that the rate of the diffusion process must be strongly dependent on the oxygen pressure.