Influence of electropolishing and anodic oxidation on morphology, chemical composition and corrosion resistance of niobium

• Pure niobium was electropolished and subsequently anodised in a H3PO4 solution.
• Phosphorus was successfully introduced into the oxide layers after the treatment.
• Corrosion resistance of niobium in Ringer's solution was improved after anodising.

The work presents results of the studies performed on electropolishing of pure niobium in a bath that contained: sulphuric acid, hydrofluoric acid, ethylene glycol and acetanilide. After the electropolishing, the specimens were subjected to anodic passivation in a 1 mol dm− 3 phosphoric acid solution at various voltages. The surface morphology, thickness, roughness and chemical composition of the resulting oxide layers were analysed. Thusly prepared niobium samples were additionally investigated in terms of their corrosion resistance in Ringer's solution. The electropolished niobium surface was determined to be smooth and lustrous. The anodisation led to the growth of barrier-like oxide layers, which were enriched in phosphorus species.