Internal oxidation and nitridation behavior of an Fe-5 mass%Al alloy at 1073 K

The internal oxidation and nitridation behavior of an Fe-5 mass%Al alloy was investigated at 1073 K in oxygen, nitrogen-20 vol%oxygen (mix), and air atmospheres for up to 57.6 ks using thermo-gravimetry, scanning electron microscope, electron probe micro analyzer, and X-ray diffraction analysis. The time dependence of the amount of oxidation showed very high oxidation rate at the initial stage, a transient stage with a rapid rate decrease, and then steady state oxidation. The critical times between the transient and steady states were 900 s in air, and 2 ks in both the mix and in O-2. In the "steady state" condition, the oxidation rate in O-2 was very slow, while it was relatively faster in the mix and in air. In O-2, both a thin duplex (Fe2O3 and Al2O3) and a thick triplex (Fe2O3, Fe3O4, and FeAl2O4) scales formed and the film-like internal oxides grew beneath the thick triplex scale. In the mix and in air a thick scale formed with a triplex structure of Fe2O3, Fe3O4, and FeAl2O4 accompanied by an internal oxidation of duplex FeAl2O4 and Al2O3 zones, which grew perpendicularly to an alloy/scale interface, and spike-like AlN precipitates formed beneath the internal oxidation zone. Further, film-like Al2O3 formed locally at the top of internal AlN precipitates. A precipitate free zone (PFZ) formed between the internal oxidation and nitridation zone, and a formation mechanism for the PFZ was proposed.