Oxidation of NiAl and FeAl

Recent studies on the oxidation of the beta-phases NiAl and FeAl are review ed. Generally, these aluminides form an alumina layer which should be prote ctive, however, certain special features render the pure phases rather susc eptible to accelerated oxidation and corrosion. At 1000 degrees C fast grow ing metastable modifications gamma-, delta- and theta-Al2O3 are formed, whi ch 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 inher ent 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 c avities and channels may allow intergranular oxidation and even severe inte rnal oxidation is observed if no protective scale is formed due to impuriti es from the atmosphere. (C) 1999 Elsevier Science Ltd. All rights reserved.