High-temperature-oxidation behavior of iron-aluminide diffusion coatings

Aluminide diffusion coatings were oxidized in air under atmospheric pressure under isothermal and cyclic conditions. The high-temperature efficiency of the pack-aluminized alloys was tested by comparing their oxidation behavior in the temperature range 800-1080 degrees C. The k(p) values deduced from the parabolic plots of weight-gain curves showed that alpha-Al2O3 composed the major phase of the oxide scale on samples oxidized at T > 1000 degrees C. For lower temperatures, transient-alumina phases were observed. The aluminide materials also exhibited excellent resistance to cyclic oxidation at 1000 degrees C. The second aim of this study was to dope the aluminide compounds obtained by a pack-cementation process with yttria, which was introduced by metal-organic chemical-vapor deposition (MOCVD). The beneficial effect of the reactive-element-oxide coating is strongly dependent on its mode of introduction, since the oxidation resistance is drastically increased when the Y2O3 coating was applied prior to the aluminization process. When applied after the aluminization, the reactive element gave negative effects on the high-temperature oxidation behavior of the iron aluminides. The oxide morphologies, X-ray diffraction patterns and two-stage experiments helped to understand the oxide-scale-growth mechanisms.