Kinetics of Oxidation of Pure Tungsten, 1150°–l615°C

Abstract

Weight change and oxygen consumption measurements were used to study the kinetics of oxidation of tungsten from 1150° to 1615°C at oxygen pressures of 2–100 Torr. The data were collected in a static type reaction system. Volatilization of tungsten trioxide above 1100°C changed the nature of the oxidation reaction for temperatures between 1100° and 1250°C. At 1150°C and 76 Torr oxygen pressure, oxide scale formed and some tungsten trioxide volatilized, while at 1250 °C and 76 Torr oxygen pressure, all of the oxygen reacted to form volatile tungsten trioxide.Initial oxidation rates were used to evaluate the effect of temperature and pressure on the kinetics of oxidation. Between 1100° and 1250°C, the initial rates of oxidation were nearly independent of temperature. A rapid increase in the rates of oxidation occurred between 1250° and 1350°C for oxygen pressures above 5 Torr. Above 1350 °C the rate data also increased with temperature but to a smaller extent. Due to oxide volatility, oxygen pressure had a major influence on the formation of oxide scale for temperatures between 1000° and 1250 °C. Low oxygen pressures favored volatilization of the oxide for a given temperature. Above 1300 °C, where oxide scales were absent, the effect of oxygen pressure on the rate of reaction followed the equation Here is the rate of oxidation, is the pressure, and is a constant. Above 1300°C, the rate‐controlling process was probably the diffusion of oxygen gas through a barrier layer of tungsten trioxide vapor. Comparison of the rates of oxidation with those for carbon and molybdenum obtained under similar reaction conditions showed tungsten to have the fastest rate of oxidation. This may be related to the nature of the barrier layer of reaction product surrounding the specimen undergoing reaction.