Corrosion behaviour of niobium in sodium hydroxide solutions

The electrochemical behaviour of niobium was investigated in sodium hydroxide solutions at different temperatures, using open-circuit potential (OCP) measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). OCP and polarization measurements show that Nb is spontaneously active in 10, 15 and 30 wt % NaOH at 25, 50 and 75°C. The anodic polarization curves in all cases show a dissolution/passivation peak followed by a current plateau, corresponding to Nb2O5 formation. The spontaneous active corrosion of Nb leads to the formation of soluble niobates that precipitate to sodium niobates. The evaluation of the corrosion current densities obtained from Tafel extrapolation of polarization curves and the polarization resistance values determined from EIS measurements indicates that the corrosion rates of niobium increase with increasing NaOH concentration and temperature.