Rhenium oxidation by steam at high temperatures

The kinetics of rhenium oxidation were studied under flowing steam + argon mixtures, at atmospheric total pressure and steam partial pressure in the range 24 - 74 kPa, and at temperatures from 1600degreesC up to 2200degreesC. No regime change was observed in the temperature dependence of the oxidation rate, which shows that the same chemical regime extends up to the highest temperature investigated. Moreover, at high temperature, the results showed a dependence of the oxidation rate on the gas velocity and the steam partial pressure, in good agreement with a gas-phase mass-transfer correlation for laminar flow, revealing that, for the conditions used, mass-transport processes in the gas phase also act as a limiting factor. It was assumed that the mass-transport limiting factor is the steam diffusion through a volatile oxides gaseous layer above the rhenium surface. A comparison of these results with rhenium oxidation rates reported for air leads us to propose the dissociative adsorption of the H2O molecule as the limiting chemical stage of the oxidation process.