Electrochemical Etching of NiTi Alloy in a Neutral Fluoride Solution

摘要

The NiTi alloy is a material of great technological interest for its unique thermo-mechanical properties (1), including excellent shape memory effect - ability to recover apparently permanent deformations of about 8-10% by heating - and superelasticity -with a pseudoelastic field up to 10-12% in strain. For these reasons the Ni-Ti alloy is successfully applied in manufacturing special devices for several industrial applications (2), and appears especially attractive in the biomedical field (3).rnDue to the stability of Ni in alkaline media and of the native oxide TiO_2 in ordinary acids, only a few aggressive mixtures are suitable for chemical or electrochemical NiTi dissolution. The use of methanolic sulfuric acid has been e.g. recently proposed to perform electropolishing of freshly polished samples (4,5). However, to efficiently etch samples aged and/or heated, with a tough surface oxide layer, HF solutions are the media of choice (6). HF is a very dangerous substance for the operator: it is a clear, colorless, volatile substance (b.p. 19.5 °C) and its small undissociated molecule penetrates tissues by non-ionic diffusion, causing deep damage such as necrosis of soft tissues, bone erosion and systemic poisoning (7, 8) [Search the Web for hydrofluoric acid safety information]. Even a trivial skin contact without immediate pain may cause severe delayed damage. Operation with HF requires strict safety measures, rigorous personnel training and uncomfortable cohabitation with a severe risk. Neutral fluoride solutions are