DOI: 10.1007/s11085-005-5715-0 Oxidation of Metals, Vol. 64, Nos. 1/2, August 2005 (© 2005) Abnormal High Growth Rates of Metastable Aluminas on FeCrAl Alloys H. El Kadiri, ∗ †‡ R. Molins, ∗ Y. Bienvenu, ∗ and M. F. Horstemeyer† Received July 21, 2004; revised March 22, 2005 Experimental evidence in high temperature oxidation of alumina-forming alloys has accumulated that the overall growth kinetics of the oxide scale are slower for 1000 ◦ C, where the stable α-Al 2 O 3 phase predominates, than for 900 ◦ C where metastable γ -Al 2 O 3 and/or θ -Al 2 O 3 polymorphs predomi- nate. This intriguing behaviour has been unanimously related to the substan- tial presence of twin boundaries and the cation vacancy network intrinsic to the metastable aluminas allowing faster diffusion than in the nearly close packed corundum structure. This paper shows that this abnormal growth rate accom- panying the presence of stable alumina polymorphs in platelets or needle-like morphology is rather due to the formation of a corundum-alumina-rich com- pact layer from an outer metastable layer by the concomitant sintering at the intersection vertices of the platelets and secondary recrystallization in these platelets. These phenomena are illustrated from oxidation tests performed on thin FeCrAl foils in both a conventional muffle furnace (designed by AET) and thermogravimetric analysis furnace (TGA) over the temperature range of 800– 1300 ◦ C using field emission scanning electron microscope (FEG-SEM), trans- mission electron microscope (TEM), electron probe microanalysis (EPMA), atomic force microscope (AFM), grazing incidence X-ray diffraction (GIXRD) and image analysis (IA) techniques. KEY WORDS: kinetics; platelet nucleation; metastable alumina; transformation; sintering; grain growth; platelet thickening. ∗ Ecole des Mines de Paris, Centre des Mat´ eriaux, CNRS UMR 7633, BP 87, 91003 Evry, France. † Center for Advanced Vehicular Systems, Mississippi State University, MS 39762-9627 USA. ‡ To whom correspondence should be sent. e-mail: elkadiri@cavs.msstate.edu 63 0030-770X/05/0800–0063/0 © 2005 Springer Science+Business Media, Inc.