THE USE OF OXYGEN ISOTOPIC LABELING TO UNDERSTAND OXIDATION MECHANISMS

Isotopic labeling is a powerful tool to evaluate transport and reactio n mechanisms of oxidation, The evaluation can answer the essential que stion in high-temperature oxidation, which is the dominating migrating species? Isotopic labeling can be used in two different ways. analysi s of the gas phase and analysis of the oxide formed. By using as-repor ted, depth-profiling data from the literature, the oxygen exchange was evaluated and compared with oxide growth. Gas-phase analysis was used to evaluate the oxygen-exchange reactions, O-2 <----> H2O, O-2 <----> O-2, and O-2 <----> MexOy, in relation to the oxidation of Si and Fe- Cr-Al alloys in similar to 10 mbar isotopically-labeled H2O/O-2-gas mi xtures at 900 to 950 degrees C. The time dependence of the rare of the oxygen exchange was used to explain the deviation from parabolic oxid ation kinetics. The results of this study suggest that atomic oxygen i s the migrating species during the oxidation of Si and alumina-forming alloys.