Oxidation behaviour in air of thin alloy 601 fibres

The oxidation behaviour in air of 12 mu m diameter continuous alloy 601 fibres has been studied using thermo-gravimetric analysis (TGA) for kinetics identification and transmission electron microscopy (TEM) for determination of the nature of the oxide layers. The TGA allows two stages in the formation of the oxide layer to be distinguished: the first stage corresponds to the growth of a continuous layer of NiO above a discontinuous sub-layer of Cr2O3 whereas the second stage is attributed to the parabolic growth of the Cr2O3 sub-layer, from the time it becomes continuous. A third stage can be observed for high oxidation temperatures. The TEM observations of oxide layers formed after 30 min at 650, 750 and 900 degrees C confirm these results. One common characteristic of these 3 oxidation conditions is the appearance of large cavities under the oxide layer. These cavities seem to be the consequence of the oxidation mechanism of Cr and of the particular morphology of the material (i.e. small diameter cylinders).