The Mechanism of Corrosion of Columbium in Sodium Hydroxide

Abstract A mechanism for the corrosion of columbium in sodium hydroxide is proposed. Corrosion proceeded by the nucleation and growth of Na8Cb6O19 · 13 H2O, having an orthorhombic structure. Weight Change vs Time behavior was found to be linear, logarithmic, or stifling and the process either concentration or activation controlled depending upon experimental conditions. When activation controlled, the rate controlling step was the conductance of columbium ions through the oxide lattice and the activation energy for the process was 34.8 K cal/mol. When concentration controlled, formation of the corrosion product was controlled by diffusion of hydroxide ions to the sodium columbate crystals. For both mechanisms, growth occurred at the oxide/solution interface.