THE CORROSION OF 2 CO-NB ALLOYS UNDER 1 ATM O-2 AT 600-800-DEGREES-C

The oxidation behavior of Co-Nb alloys containing 15 and 30 wt% Nb has been studied at 600-800 degrees C under 1 atm of pure oxygen. The cor rosion kinetics followed the parabolic law only approximately, because , especially at 600 degrees C, the instantaneous rate constant was dec reasing with time. The corrosion rates of the two alloys were faster t han that of pure cobalt, but the rate increase was rather small. The t wo Co-Nb alloys underwent both external and internal oxidation under a ll the conditions examined. The external scales were composed of a con tinuous layer of pure cobalt oxide close to the scale-gas interface, f ollowed by an inner layer containing a complex mixture of two double C o-Nb oxides (CoNb2O6 and Co4Nb2O9) with the base-metal oxide. The regi on of internal oxidation contained a mixture of cobalt metal, niobium oxides (Nb2O5 or/and NbO2) and in the outermost region the double oxid e, which formed from the Nb-rich metal phase. Under no conditions was a region of niobium depletion observed at the alloy-scale interface. T he corrosion behavior of these alloys is discussed with special refere nce to the effects of the low solubility of niobium in cobalt and to t he relation between the microstructure of the alloys and of the scales .