High temperature oxidation of Co-Mn alloys

Abstract

The oxidation of cobalt-manganese alloys in the range 0–45 wt%Mn corresponding to the stability of the α-phase of cobalt has been studied in the range 750–950°C as an example of binary alloys producing solid solution scales. The alloys oxidize according to a parabolic rate law with a rate constant intermediate between those of the pure metals. The scales were formed mainly by a CoO-MnO solid solution with a spinel phase present either as dispersed particles or a continuous layer in the cubic oxide, particularly for the Mn-rich alloys: some degree of internal oxidation has also been observed. The scale properties show a time dependence at constant temperature which is unusual in these systems and is probably related to the presence of the spinel phase. The internal oxidation and the structure and composition of the external scale are discussed with reference to the phase diagram for this system and to the recent theories for the growth of solid solution scales.