Quasiequilibrium treatment of gas-solid reactions. III. Rate of volatilization of tungsten by high-temperature oxidation

Abstract

Theoretical predictions computed on the basis of the quasiequilibrium treatment of gas-solid reactions are compared with existing experimental data on the rate of volatilization (erosion) of solid tungsten by reaction with gaseous O₂ at high temperature (∼ 1300° ≲ T ≲ 3600° K) and low pressure (4.5 × 10⁻⁷ ≤$p_{O_2 }$ ≤11.5 Torr). The only unknown parameter in the analysis is the equilibrium probability,$\zeta '_{O_2 }$, defined as the fraction of the impinging O₂ molecules that attain thermochemical equilibrium at the tungsten surface rather than undergoing nonreactive scattering (e.g., reflection). An approximate expression for$\zeta '_{O_2 }$ is estimated by a straightforward empirical procedure that is consistent with the quasiequilibrium treatment. The theoretical results based on this expression for$\zeta '_{O_2 }$ T because appears to be an exponential function ofT; (b) In the intermediate region, the formation of volatile oxides decreases sharply with increasingT because atomic oxygen becomes the thermodynamically favored reaction product, thereby causing Σ_W to decrease with increasingT; (c) In the highest region, Σ_W again increases withT as a result of the formation of WO and the sublimation of W.