Oxidation of Molybdenum 550° to 1700°C

Weight change and oxygen consumption measurements were used to study the oxidation of molybdenum from 550° to 1704°C for pressures of 5 to 76 Torr. For temperatures of 550°–700°C two processes occurred simultaneously, oxide scale formation and molybdenum trioxide volatility. Above 800°C at pressures up to 76 Torr molybdenum trioxide volatilized as fast as it formed. At 900°C and 76 Torr using 1.2 cm2 samples the primary chemical reaction gave a rate of about 1018 at. molybdenum/cm2/sec. Above this temperature for 1.2 cm2 specimens the reaction was limited by gaseous diffusion of oxygen. Little change was found in the rate of oxidation to 1615°C. Pressure had only a small effect on the rate of reaction for these reaction conditions. However, in the chemically controlled region pressure had an important effect on the rate of oxidation. To extend the temperature region where the primary chemical reaction was rate controlling, samples of small area were used. A sample having a total area of 0.12 cm2 gave a reaction rate of . For these very fast reactions, appreciable temperature rises occurred, and the actual sample temperature had to be estimated. A log K vs. 1/T plot of the primary chemical reaction data gave an energy of activation of 19.7 kcal/mole. Reaction conditions where gaseous diffusion processes are rate controlling were determined. All of the earlier studies were made for these reaction conditions. The activated state theory of surface reactions was applied to the primary chemical reaction in the oxidation of molybdenum. A mechanism of mobile adsorption was found to be the primary chemical reaction. This adsorption process probably occurred on a surface already covered with a layer of adsorbed oxygen atoms since was volatilized.