THE OXIDATION OF CO-NB ALLOYS UNDER LOW-OXYGEN PRESSURES AT 600-800-DEGREES-C

The oxidation of two Co-Nb alloys containing 15 and 30 wt.% Nb has been studied at 600-800 degrees C in H-2-CO2 mixtures providing an oxygen pressure of 10(-24) atm at 600 degrees C and 10(-20) atm at 700 and 800 degrees C, below the dissociation pressure of cobalt oxide. At 600 and 700 degrees C both alloys showed only a region of internal oxidation composed of a mixture of alpha cobalt and of niobium oxides (NbO2 and Nb2O5) and at 700 degrees C also the double oxide CoNb2O6, which formed from the Nb-rich Co3Nb phase. No Nb-depleted layer formed in the alloy at the interface with the region of internal oxidation at these temperatures. Upon oxidation at 800 degrees C a transition between internal and external oxidation of niobium was observed, especially for Co-30 Nb. This corrosion mode is associated with the development of a single-phase, Nb-depleted region at the surface of the alloy. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in cobalt and to the relation between the microstructures of the alloys and of the scales.