Oxidation of iron within the scanning electron microscope: Oxide crystallite growth kinetics during the oxidation of iron in CO2

The growth of magnetite (Fe₃O₄) on a polycrystalline iron substrate has been continuously followed in situ in the S.E.M. at 500°C in 13.3 N m^?2 CO₂. Medium resolution (250 Å) micrographs of the same area have been taken at time intervals over periods of oxidation in excess of 200 hr. Successive layers of oxide crystallites grow according to the same logarithmic rate law, but the overall thickening rate of the oxide film conforms to the parabolic rate law. A theoretical analysis of this situation suggests that the dominant factor determining the overall growth kinetics is the frequency of oxide nucleation steps. It can be shown that the basic logarithmic growth of individual oxide crystallites could lead to a variety of overall growth processes, depending upon the conditions required for nucleation.