Abstract
The oxidation of niobium and dilute niobium/copper alloys has been studied at 76 mm of oxygen in the temperature range 548-749°C, and at 0.76-760 mm of oxygen at 690°C. It has been shown that additions of copper decreased the oxidation resistance of niobium and that the oxidation reaction obeyed the modified parabolic law before breakaway oxidations. The two controlling reactions have been isolated and have been shown to be the parabolic growth of a dense NbO[2] layer, containing oxygen interstitial defects, and the dissociation of oxygen molecules at the NbO[2]/Nb[2]O[5] interface. From conductivity experiments the diffusion of oxygen interstitials across the NbO[2] scale has been considered. A mechanism of short-circuit diffusion has been proposed which has incorporated an hypothesized effect of copper additions on the NbO[2] structure. To account for the layered nature of the oxide scale that had been observed on the surface of all samples, a cracking mechanism has been proposed which may be reconciled with the oxidation data.