Effect of Al Content on the Internal Oxidation of Fe-Low Al Alloys at 1073 K

Oxidation properties of Fe-3, 5 and 7 mass%Al alloys were investigated at 1073 K in an oxygen gas atmosphere for up to 14.4 ks using thermo-gravimetry, scanning electron microscope, electron probe microanalyzer, and X-ray diffraction analysis. Time dependence of oxidation amounts can be divided into three regions, i.e., an initial stage with very high oxidation rate, a transient stage with a rapid decrease in the rate, and then a steady state oxidation. The critical times among these stages are 0.1 and 3.6 ks. The 7Al alloy oxidized very slowly, due to the formation of a duplex Al2O3 and Fe2O3 scale, in contrast to rapid oxidation of both 5Al and 3Al alloys. The 5Al alloy oxidized faster than the 3Al alloy during the initial stage, while for longer oxidation the 3Al alloy became faster. The 3Al alloy oxidized to form a thick scale with a triplex structure of Fe2O3, Fe3O4 and FeAl2O4, accompanied with an internal oxidation of duplex FeAl2O4 and Al2O3 zones. These internal oxides grew perpendicular to the alloy/scale interface. Oxidation of the 5Al alloy, close to the internal-external transition composition, is very complex depending on portions and oxidation times. It has a thin scale like the 7Al alloy and a thick scale with internal oxides like the 3Al alloy. Further, a precipitate free zone (PFZ) formed in the internal oxidation, sandwiched in between the FeAl2O4 and Al2O3 zones, and the formation mechanism of PFZ was proposed