Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95 °C

New Pourbaix diagrams were calculated at 25, 75 and 95 °C for the Nb–H2O system. The species Nb(OH)4+ and NbO3- were considered. Potentiodynamic polarization and mass loss experiments (14 days) were conducted in concentrated H2SO4 (20, 40 and 80 wt%) and HCl (20 and 38 wt%) solutions at 75 and 95 °C. Nb forms a metastable pentoxide (Nb2O5) in H2SO4 and HCl solutions which dissolves as Nb(OH)4+. Corrosion rates decrease between the 40% and the 80% H2SO4 solutions. SEM micrographs show generalized pitting in the 20% and 40% H2SO4 solutions. Mass loss corrosion rates did not exceed 306 μm/yr. Corrosion rates estimated by Tafel extrapolation were within two orders of magnitude of those measured by mass loss and it is shown that this finding is consistent with the thickening of the oxide.