Kinetics of Oxidation of Columbium And Other Refractory Metals^★

The oxidation of tungsten and molybdenum occurs by two consecutive reactions, forming first a suboxide then the trioxide. Tantalum and columbium oxidize by four simultaneous reactions: solution of oxygen in the metal, nucleation and growth of a suboxide phase at the metal surface and two phase boundary processes giving rise to two different modifications of the pentoxide. By assuming that all reactions are first order complex chain reactions, rate equations are formulated giving the rate of oxidation as a function of pressure, temperature and time. Regression rate expressions for the metals tungsten, tantalum and columbium above 700 C are given as $dxWdt=3.05×102×10−5,290/TPO21/2$⁠; $dxTadt=33.2×10−4,000/TPO21/2$⁠; and $dxCbdt=1.19×10−2,060/TPO21/2$⁠, respectively. The rate is expressed in cm/hr, T is in degrees K and $PO2$ in atmospheres pressure of oxygen. 3.8.4, 2.1.1, 6.3.5, 6.3.16, 6.3.13