Kinetics of the Oxidation of Chromium

The oxidation of high purity chromium was studied over the temperature range 700~176 using the vacuum microbalance method. Below 900~ conventional oxidation curves were obtained which can be fitted to the para-bolic rate law. Above 900 ~ and for a film thickness of approximately 4800A, the rate of oxidation increased in an unusual manner. This increase in the rate of oxidation disappeared on further oxidation. At temperatures of 1050 ~ 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 42,000A. A logarithmic plot of the parabolic rate law constant vs. 1/T 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 be-tween 700 ~ and 900 ~ and 59,400 cal/mole and +6.2 entropy units for 1000 ~ 1100~ The rate of evaporation and the rate of oxidation of Cr are equal at abou