Electropolishing of Nitinol Wires and Its Influence on Corrosion Mechanisms

Nitinol is a superelastic and shape-memory alloy often used for the production of vascular stents and surgery implants. Electrochemical polishing is normally used at the preparation stage of stent production. The Electropolishing treatment was performed in 1M Sulfuric acid in 75% Methanol and 25% Glycerine for several treatment times. The corrosion behaviours of Nitinol wires in Hanks' Balanced Salts Solution and in 3% NaCl at room temperature before and after the treatment were compared. It was found that Nitinol is less active in Hank's solution that in Sodium Chloride. Tafel slopes built to polarization curves indicate that the mechanisms of corrosion of Nitinol in Hanks' solution and in NaCl are different. On the other hand, total corrosion current densities are similar in both kinds of media. Plausible equivalent circuits hint that the surface layers formed in Hanks' solution are not the same as in NaCl. Specimens after the Electrochemical Polishing do not show any differences in the equivalent circuits either in Tafel slopes.