The oxidation of alumina-forming alloys

The growth and development of protective Al2O3, scales on NiAl and Fe-Cr-Al alloys, during oxidation at temperatures from 900 degrees to 1300 degrees C, is complicated by transformations of the metastable y-Al2O3 and theta-Al2O3 phases to the most stable alpha-Al2O3 phase; the rates of these transformations increase with increasing temperature, so alpha-Al2O3 predominates very quickly at 1300 degrees C but much more slowly at 1000 degrees C. Growth of alpha-Al2O3 scales involves both inward diffusion of oxygen and outward diffusion of Al3+ ions, via short-circuit paths, probably scale grain boundaries; however; the addition of small amounts of reactive element to the alloy inhibits outward transport of Al3+ ions and causes significant changes to the configuration of the scale. Although alpha-Al2O3 scales are effective barriers to transport of reactants at high temperatures, they are susceptible to spallation from the alloy, particularly on thermal cycling. Important factors in determining the resistance to spallation include the fracture toughness, particularly at, or close to, the scale/alloy interface, the configuration of the scale and the number and size of physical defects at the interface. Sulphur impurities decrease the resistance of the scale to spallation, while reactive elements increase this resistance; these effects are correlated with the latest hypotheses to account for the mechanical integrity of the scales.