Institute of Metals Division - Kinetics of the Reactions of Columbium and Tantalum with O2, N2 and H2

THIS paper. will present the results of our studies on the kinetics of the gas phase reactions of co-lumbium and tantalum with O2, N2 and H2. Studies on zirconium and titanium have been previously reported.9,11 Due to their high melting points and their many excellent physical and chemical properties, colurn-bium and tantalum have been finding their places as useful metals in science and industry. Chemically the metals are very inert to gas and liquid phase corrosion at room temperature. However, at moderate temperatures of 250°C and higher the metals react readily with oxygen and hydrogen. It is of interest to study the gas phase reactions of these metals for the following reasons: (1) to compare the rates of the several reactions with those of other metals and to correlate the rate data with theory and structure predictions, (2) to add basic information on the practical difficulties of the reduction, refining and working of columbium and tantalum, (3) to anticipate new uses for the metals and (4) to understand the role that the metals play as components in high temperature alloys. Literature Survey A number of papers exist in the literature on the gas phase reactions of columbium and tantalum with O2, N2 and H2; however, only fragmentary data have been reported on the kinetics of these reactions. Although the free energy of formation of the oxide Ta2O520at 25°C has been determined and some thermodynamic work has been reported on columbium and its oxides, the data are not sufficient to make equilibria calculations at elevated temperatures. Oxidation: 1. Kinetics: The rate of oxidation of columbium and tantalum has been studied by McAdam and Geil23 using the interference color method. Tantalum is reported to oxidize more slowly than columbium, zirconium and iron. The amounts of oxidation of columbium after 20 hr in air for several temperatures are reported by the Fansteel Metallurgical Corporation.36 Columbium starts to oxidize in air at about 200°C. The oxide is said to be adherent and prevents further oxidation until the temperature is raised. Columbium is reported not to become brittle on heating in air for short periods like tantalum because the oxide film prevents further reaction. The columbium oxides dissolve into the metal when heated in a vacuum at temperatures between red heat and 1200°C. Above 1200°C the oxides are reported to evaporate. Columbium, in the oxide free state, is an active getter in vacuum tubes. 2. Preparation and Structure: G. Grube, O. Kuba-schewski and K. Zwiauer15 have studied the decomposition of Cb2O5 in argon. They find that CbO2 is formed slowly at 1150° and rapidly at 1300°C. Cb2O5 can be reduced to CbO2 by moist H2. The reduction of this dioxide to the lower oxides is rapid. By varying conditions CbO2, Cb2O3, CbO and Cb2O can be formed. CbO2, CbO and Cb2O have independent lattices while Cb2O3 is a stoichiometric mixture of CbO and CbO2. In a later paper O. Kuba-schewski" has shown that CbO is cubic and that CbO2 has a rutile type of lattice. The most exhaustive work on the oxides of columbium is that of G. Brauer.8 The number and constitution of the solid phases in the binary system Cb-O were investigated by the preparative, analytical and X ray methods. He indicates that only the oxides Cb2O5, CbO2 and CbO exist with limited regions of homogeneity. The solid oxide Cb,O, exists