The Oxidation of Ni-Nb Alloys at Low-Oxygen Pressures at 600-800℃

摘要

The oxidation of two Ni~Nb alloys containing 15 and 30 wt.% Nb has been studied at 600-800℃ in H_2-CO_2 mixtures providing an oxygen pressure of 10~(-24) atm at 600℃ and 10~(-20) atm O_2 at 700 and 800℃, these pressures being less than the dissociation pressure of nickel oxide. The scales formed on both alloys at 600 and 700℃ show only a region of internal oxidation composed of a mixture of alpha nickel and niobium oxides (Nb_2O_5 or/and NbO_2), which formed from both the metal phases present, i.e., Ni_8Nb and Ni_3Nb. Only small, or even no, Nb depletion was observed in the alloys close to the interface with the zone of internal oxidation at these temperatures. On the contrary, samples of both alloys corroded at 800℃ produced a continuous external scale of niobium oxides without internal oxidation. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in nickel.