EFFECT OF LANTHANUM OR CERIUM ADDITION ON HIGH-TEMPERATURE OXIDATION RESISTANCE OF FE-CR-AL ALLOY FOILS

The oxidation behavior of Fe-20 approximately 25Cr-5Al alloys of 50 and 500-mu-m thicknesses and containing up to 0.09%La or up to 0.06%Ce was examined in an isothermal oxidation test at 1423 and 1473 K in air. Mass gain measurements were made, and an investigation of the oxide layer by scanning electron microscopy, X-ray diffraction, electron probe microanalysis, and chemical analysis was carried out after the test. The oxidation process proceeds in three stages in the 50-mu-m foil. In the first stage, an Al2O3 layer grows governed by the parabolic rate law, which is the same as the rate law for thicker specimens. In the second stage, a Cr2O3 layer grows between the Al2O3 layer and the substrate bulk after the exhaustion of the Al in the bulk. The kinetic behavior of the oxidation of the Cr2O3 layer follows the linear oxidation law, and the linear rate constant is proportional to the parabolic rate constant in the first stage. In the third stage, Fe in the bulk begins to oxidize after the Cr content of the bulk falls below a critical value as a result of Cr oxidation. The oxidation of Fe then leads to "Breakaway" oxidation. The addition of La decreases the growth rate of the Cr2O3 layer by reducing the growth rate of the Al2O3 layer and improves the oxidation resistance of the Cr2O3 layer. The duration of the second stage is thereby extended, and the La-containing foil exhibits good oxidation resistance. The addition of Ce, in contrast, increases the growth rate of the Cr2O3 layer by accelerating the growth of the Al2O3 layer and thus leads to quick "Breakaway" oxidation.