THE OXIDATION OF FE-NB ALLOYS IN 1-ATM OF PURE OXYGEN AT 600-800-DEGREES-C

The corrosion of two Fe-Nb alloys containing 15 and 30 wt% Nb has been studied at 600-800 degrees C under 1 atm of pure oxygen. The reaction followed the parabolic rate law to a reasonable approximation with ra te constants much lower than for the oxidation of pure iron under the same conditions. Internal oxidation and depletion of niobium in the al loy have never been observed. The external scales were composed of a m atrix of Fe2O3 containing a dispersion of double Fe-Nb oxide, while Fe O and Fe3O4 were not observed. The oxidation behavior of these alloys is interpreted with reference to an approximate phase diagram of the t ernary system Fe-Nb-O, taking into account the effects of the low solu bility of niobium in the base metals and of the presence of two metal phases in the alloy.