THE CORROSION OF TWO Co-Nb ALLOYS UNDER 1 ATM O_2 AT 600-800℃

摘要

The oxidation behavior of Co-Nb alloys containing 15 and 30 wt% Nb has been studied at 600-800℃ under 1 atm of pure oxygen. The corrosion kinetics followed the parabolic law only approximately, because, especially at 600℃. the instantaneous rate constant was decreasing with time. The corrosion rates of the two alloys were faster than that of pure cobalt, but the rate increase was rather small. The two Co-Nb alloys underwent both external and internal oxidation under all the conditions examined. 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 (CoNb_2O_6 and Co_4Nb_2O_9) with the base-metal oxide. The region of internal oxidation contained a mixture of cobalt metal, niobium oxides (Nb_2O_5 or/and NbO_2) and in the outermost region the double oxide, which formed from the Nb-rich metal phase. Under no conditions was a region of niobium depletion observed at the alloy-scale interface. The corrosion behavior of these alloys is discussed with special reference to the effects of the low solubility of niobium in cobalt and to the relation between the microstructure of the alloys and of the scales.