Effects of Cr and Al additions on the oxidation of FeCoNi-based medium-and high-entropy alloys in CO2/CO mixed gases

Abstract

The effects of Cr and Al additions on the oxidation of FeCoNi-based medium- and high-entropy alloys containing FeCoNi, FeCoNiCr, and FeCoNiCrAl0.3 were studied at 600-950 degrees C in a 9:1 CO2/CO gas mixture. The oxidation kinetics of all three alloys obeyed the parabolic-rate law, and their oxidation rates steadily increased with temperature. The scales formed on FeCoNi consisted of an intermixed layer of CoO and Fe3O4 and internal FeO precipitates (observed at 950 degrees C). Those formed on FeCoNiCr consisted of an outer layer of CoNiO2 intermixed with FeCoCrO4 and an inner layer of Cr2O3. The scale formed on FeCoNiCrAl0.3 was strongly temperaturedependent, consisting of discontinuous Cr2O3 granules at 600 degrees C, internal Al2O3 precipitates formed beneath a Cr2O3 layer at 760 degrees C, and an exclusive alpha-Al2O3 layer at 950 degrees C.