Kinetics of the Oxidation of Chromium

Abstract

The oxidation of high purity chromium was studied over the temperature range 700°–1100°C using the vacuum microbalance method. Below 900°C conventional oxidation curves were obtained which can be fitted to the parabolic rate law. Above 900°C and for a film thickness of approximately 4800Aå, the rate of oxidation increased in an unusual manner. This increase in the rate of oxidation disappeared on further oxidation. At temperatures of 1050°C and higher a large increase occurred in the rate of oxidation, suggesting that the oxide film was no longer protective for film thicknesses greater than 42,000Aå.A logarithmic plot of the parabolic rate law constant vs. shows two straight lines separated by a transformation region. This gives 37,500 cal/mole and −15.3 entropy units for heat of activation and entropy of activation between 700° and 900°C and 59,400 cal/mole and +6.2 entropy units for 1000°–1100°C.The rate of evaporation and the rate of oxidation of Cr are equal at about 950°C. This corresponds to the transformation region between the two mechanisms of oxidation. It is concluded that the failure of Cr in oxidation is closely related to the high vapor pressure of Cr above 900°C.