OXIDATION OF TWO Co-Nb ALLOYS UNDER 1 ATM O2 AT 600℃～800℃

The oxidation behavior of two Co-Nb alloys containing 15 and30 Wt.%Nb respectively was studied at 600-800℃ under 1 atm of pure oxygen. The oxidation kinetics followed parabolic law only approximately, whilst, especially at 600℃, the transient rate constant decreased with time. The oxidation rates of the two alloys were faster than that of pure cobalt, but the increment was rather small. The two alloys underwent both external and internal oxidation under all the conditions tested. The external scales were composed of a continuous layer of pure cobalt oxide close to the scale/gas interface, followed by an inner layer containing a complex mixture of two double Co-Nb oxides (CoNb2O6 and Co4Nb2O9) with the basemetal oxide. The region of internal oxidation contained a mixture of cobalt metal, niobium oxides (Na2O5or/and NbO2) and in the outermost region there was a double oxide formed from the Nb-rich metal phase.No niobium depletion region was observed at the alloy/scale interface. The oxidation behavior of these alloys was discussed in terms of the low solubility of niobium in cobalt and the microstructure of the alloys and the oxide scales.