Kinetics of the Oxidation of Tungsten by CO2 at High Temperatures

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Abstract
The kinetics of oxidation of tungsten by CO2 was studied in a stagnation‐flow reactor at CO2 partial pressures from 20 to 337 torr. The study covered the temperature range 2200°—3200°K and included measurements of the effect of CO on the reaction kinetics at CO/CO2 ratios up to 5:1. The rates observed below about 2650°K were essentially independent of the velocity of the impinging gas stream, indicating that the surface kinetics primarily determined the rates up to 2650°K at the lower gas‐flow rates used and to higher temperatures at higher flow rates. Where surface kinetics were rate determining, the data could be explained simply by incorporating a temperature‐dependent ``sticking coefficient'' for CO2 into the two‐layer adatom model previously proposed for the W–O2 reaction. The effect of CO was small and was readily incorporated into the model. Velocity‐dependent deviations from the rates predicted by the model were observed at temperatures above 2650°K. These were largely explained by simple mass‐transfer theory. At low flow rates and high temperatures it appears that the reactions of desorbed oxygen became important.

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