Oxidation of NiAl and FeAl

Recent studies on the oxidation of the beta-phases NiAl and FeAl are reviewed. Generally, these aluminides form an alumina layer which should be protective, however, certain special features render the pure phases rather susceptible to accelerated oxidation and corrosion. At 1000 degrees C fast growing metastable modifications gamma-, delta- and theta-Al2O3 are formed, which are undergrown by alpha-Al2O3 and transform to that stable modification only slowly. The latter process is accelerated by alloying elements such as Cr, Y, Ce..., their oxides promote alpha-Al2O3 nucleation. A further inherent drawback is the formation of voids and cavities beneath the oxide scale, due to consumption of Al and inward diffusion of Ni and Fe, respectively. The cavities lead to decreased oxide adherence and spalling of the scale, especially upon temperature changes. When formed at grain boundaries, the cavities and channels may allow intergranular oxidation and even severe internal oxidation is observed if no protective scale is formed due to impurities from the atmosphere. (C) 1999 Elsevier Science Ltd. All rights reserved.