AN ANALYSIS OF THE INTERNAL OXIDATION OF BINARY M-NB ALLOYS UNDER LOW-OXYGEN PRESSURES AT 600-800-DEGREES-C

The corrosion of M-Nb alloys based on iron, cobalt, and nickel and con taining 15 and 30 wt% Nb has been studied at 600-800 degrees C under l ow oxygen pressures (10(-24) atm at 600 degrees C and 10(-20) atm at 7 00-800 degrees C). Except for the Co-Nb and Ni-Nb alloys corroded at 8 00 degrees C, which formed external scales of niobium oxides, corrosio n under low O-2 pressures produced an internal oxidation of niobium. T his attack was much faster than expected on the basis of the classical theory. Furthermore, the distribution of the infernal oxide in the al loys containing two metal phases was very close to that of the Nb-rich phase in the original alloys. These kinetic, microstructural, and the rmodynamic aspects are examined by taking into account the effects of the limited solubility of niobium in the various base metals and of th e two-phase nature of the alloys.