The Oxidation Behavior of Tantalum at 700°–1000°C

Abstract

The oxidation behavior of tantalum has been studied at 700°–1000°C at oxygen pressures ranging from 1 atm to 0.01 Torr O2. The oxidation involves oxide formation and oxygen dissolution in the metal. The rate of oxygen dissolution is governed by diffusion of oxygen in the tantalum metal. Except for the very initial stage of the oxidation, the weight gain due to oxygen dissolution is of minor importance compared to the total weight gain during oxidation. However, the relative importance of oxygen dissolution increases with decreasing oxygen pressure. The oxide scale consists of . At and below 800°C the metallic oxide phase is formed at the metal/oxide interface, while above this temperature gradually becomes the intermediate reaction product at the metal/oxide interface. Below 800°C and at the higher oxygen pressures the formation follows a linear rate from the start of the reaction. At about 800deg;C one begins to observe a change in the oxidation behavior, and at higher temperatures the initial formation is parabolic. The parabolic oxidation is concluded to be governed by a rate‐determining diffusion of oxygen through the oxide scale. The parabolic stage is followed by a linear oxidation. The paralinear transition is due to cracking of the protective scale down to the metal/oxide phase boundary, and the subsequent linear oxidation above 800°C is concluded to be determined by the rate of nucleation and growth of on at the metal/oxide interface. The linear oxidation is dependent on oxygen pressure and the pressure dependence is interpreted in terms of an oxygen chemisorption equilibrium prior to the rate‐determining part‐process. The linear rate of oxidation at 1 atm O2 exhibits a reversal in the temperature dependence at 700°–750°C. At 100 Torr O2 the oxidation rate is independent of temperature and at lower pressures the reversal is absent. Above 800°C the linear rate constant increases rapidly with temperature at all oxygen pressures. The reversal in temperature dependence is a result of the change in oxygen pressure dependence associated with the change in the reaction scheme at about 800°C. An additional factor may be a decreased thermal stability of the intermediate reaction product with increasing temperature.