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 bee n 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 8 00 degrees C, below the dissociation pressure of cobalt oxide. At 600 and 700 degrees C both alloys showed only a region of internal oxidati on composed of a mixture of alpha cobalt and of niobium oxides (NbO2 a nd Nb2O5) and at 700 degrees C also the double oxide CoNb2O6, which fo rmed 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 int ernal and external oxidation of niobium was observed, especially for C o-30 Nb. This corrosion mode is associated with the development of a s ingle-phase, Nb-depleted region at the surface of the alloy. The corro sion mechanism of these alloys is examined with special reference to t he effect of the low solubility of niobium in cobalt and to the relati on between the microstructures of the alloys and of the scales.