Oxidation mechanism of Ni-20Cr foils and its relation to the oxide-scale microstructure

The oxidation behavior of Ni-20 Cr foils of 100- and 200-mu m thickness was studied in air between 500 and 900 degrees C. Simultaneously, the morphology, micro-structure, and composition of the oxide layers were determined by scanning and transmission electron microscopies. Depending on the temperature, the oxide layer differed significantly. The scale formed at all temperatures was complex, with an outer NiO layer having columnar grains, and an inner layer of equiaxed NiCr2O4 + NiO +Cr2O3 grains. At low temperatures (500 and 600 degrees C), the chromium content was insufficient to form a continuous Cr2O3 layer, while such a continuous layer formed at the inner interface at oxidation temperatures of 700 to 900 degrees C, internal oxidation of chromium occured in the substrate. The oxidation mechanism is described taking into account these morphologies and the oxidation kinetics. The observation of no significant differences between the oxidation behavior of thin strips and thick materials is related to the limited exposure times of the study.