Influence of reactive element oxide coatings on the high temperature oxidation behavior of alumina-forming alloys

Reactive-element-oxide coatings were processed by a metal-organic chemical-vapor-deposition technique on the surface of a model FeCrAl alloy. The high-temperature performances of Nd2O3-, Y2O3-coated and uncoated alloys were tested in air under atmospheric pressure at 1050, 1100 and 1200degreesC. The coated samples did not exhibit the expected reactive-element effects since the oxidation rates were not decreased, and the oxide-scale adherence was only slightly improved. The study of the oxide-scale morphology revealed very convoluted oxide scales, except for alumina scales formed on uncoated materials at 1100 and 1200degreesC. Two-stage oxidation experiments showed that the reaction proceeded by a mixed anionic - cationic diffusion process; consequently, the growth of alumina within the existing alumina layer results in convoluted scales. It is proposed that the weak incorporation of the reactive elements within the thermally growing alumina scales was responsible for the limited reactive element effects, when reactive-elements were applied as oxide coatings on alumina-forming steels.