THE CORROSION OF 2 NI-NB ALLOYS UNDER 1 ATM O-2 AT 600-800-DEGREES-C

The oxidation of two Ni-Nb alloys containing 15 and 30 wt% Nb has been studied at 600-800 degrees C in pure oxygen under 1 atm O-2 at 600-80 0 degrees C. The scales formed on both alloys under all conditions sho w an external scale, generally duplex, containing an outermost layer o f nearly pure NiO and an innermost region of NiO mixed with the double Ni-Nb oxide NiNb2O6. Moreover, the samples corroded at all temperatur es also show a region of internal oxidation composed of a mixture of a lpha nickel and niobium oxides (Nb2O5 or/and NbO2), which formed from both alloy phases Ni8Nb and Ni3Nb. No important depletion of niobium w as observed in the alloy close to the interface with the zone of inter nal oxidation, while the depth of this region is generally much higher than measured for the corrosion of the same alloys under low oxygen p ressures at the same temperatures. The corrosion mechanism of these al loys is examined with special reference to the effects of the low solu bility of niobium in nickel.