KINETIC AND AERODYNAMIC ASPECTS OF THE OXIDATION OF METALS BY PARTIALLY DISSOCIATED OXYGEN

Using a resistance monitoring technique, the oxidation rate of electrically heated molybdenum and tungsten filaments has been studied over a wide range of oxygen atom concentrations and surface temperatures at flow rates such that the observed oxidation kinetics are not falsified by diffusional limitations. Partially dissociated steams are produced in the pressure range 0. 710 Torr using a microwave discharge and O-atom concentrations are determined using the NO2 light titration technique. Under the conditions investigated the oxides of molybdenum and tungsten latilize as rapidly as they are formed and the unprotected metal is subjected to direct attack by oxygen atoms. On molybdenum and tungsten surfaces oxidation probabilities for atomic oxygen are found to be higher than corresponding values for molecular diatomic oxygen by up to one to two orders of magnitude, whereas the corresponding activation energies are considerably lower. Moreover, while present studies of the pressure dependence of the rates reveal complex departures from simple power-law kinetics for O2 attack, the O-atom data reported follow simple first order kinetics.