High-Temperature Oxidation

The oxidation of ZrC was studied at temperatures of 1130°–2160°K and oxygen partial pressures around 3.9 and 20 Torr. The rate of oxidation was monitored with a thermal conductivity cell. Independent measurements were made of net weight gain and quantities of CO ( g ( and CO 2 ( g ( evolved. Oxidation was shown to be nonpreferential, i.e. , zirconium was oxidized at the same rate as carbon. Gas phase diffusion control imposed by the experimental system was frequently encountered. Where it was possible to observe a true chemically controlled reaction rate, the kinetics appeared to be linear. Microscopic examination of the oxidized specimens revealed preferential oxidation along grain boundaries. Between 1130° and 1560°K this preferential oxidation resulted in intercrystalline fracture. At higher temperatures stresses were apparently sufficiently relieved so that the samples remained intact. The oxidation of HFC between 1790° and 2000°K, at oxygen pressures near 10 Torr, was also found to be linear and preferential along grain boundaries.