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Using grazing-emission X-ray fluorescence (GEXRF), isothermal oxidation of the alloys 55Fe-25Cr-20Ni and 55Fe-25Cr-20Ni(+0.3Y) (wt.%) were studied as a function of oxidation time at 750 8C in O 2 . In addition, the effect of thermal cycling was studied. Using GEXRF, oxide thickness, the Cr-depletion zone in the substrate, and Fe and Ni concentrations in the oxide were monitored as a function of oxidation time. Scanning-electron microscopy was used to independently measure the Cr-depletion zone. Raman spectroscopy was used to measure the concentration of Fe 2 O 3 appearing in the oxides in early oxidation (less than 2 h). Both GEXRF and Raman measurements show that the thermally-grown chromium oxide purifies with extended oxidation; initially abundant Fe 2 O 3 became undetectable after 2 h of oxidation. However, the total Fe concentration was still *3% after 2 h but systematically decreased with further oxidation. Thermal cycling had no effect on these results.

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