The oxidation of Co?Nb alloys under low oxygen pressures at 600–800°C

The oxidation of two Co–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600°C and 10^–20 atm at 700 and 800°C, below the dissociation pressure of cobalt oxide. At 600 and 700°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°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°C a transition between internal and external oxidation of niobium was observed, especially for Co–30Nb. 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.