Heat-resistant alloys, Haynes 230 and SS310, were exposed to air and humidified H-2 at 750 degrees C for up to 1000h, respectively, simulating the environments in reduced temperature solid oxide fuel cells (SOFCs). The oxidized samples were characterized by using SEM, EDS and X-ray diffraction to obtain the morphology, thickness, composition and crystal structure of the oxide scales. A mechanism for the formation of metallic Ni-rich nodules on top of the oxide scale in Haynes 230 sample oxidized in humidified H2 was established. Thermodynamic analysis confirmed that MnCr2O4 is the favored spinel phase, together with Cr2O3, in the oxide scales. (c) 2005 Elsevier B.V All fights reserved.