Kinetics and mechanisms of niobium corrosion in molten glasses

The kinetics and mechanisms of corrosion of pure niobium in three molten borosilicate glasses (Zn or Fe-enriched) were investigated in the 1050 - 1400°C temperature range. The corrosion layers formed at the metal/glass interface, the elemental concentration profiles in the glass, and the corrosion rates were determined. Corrosion rates are reported on an Arrhenius plot. They lead to the determination of the activation energies of corrosion processes that depend on the glass composition. The rate-limiting process is probably the diffusion of oxygen through one of the corrosion layers. The corrosion mechanisms are described in the form of successive redox reactions between the glass and the substrate. They are characteristic of a metal-glass couple. From these results, the Nb^II/Nb⁰ standard potential is estimated to be below -1.4V/YSZ, while the Nb^IV/Nb^II and Nb^V/Nb^IV standard potentials are estimated to be in the -0.96V to -0.76V range. The formation of an intermetallic compound containing boron (NbB) as a corrosion layer is seen for the first time.