Electrochemical Etching of NiTi Alloy in a Neutral Fluoride Solution

Abstract

Electrochemical etching of NiTi alloy in a pH 6 fluoride solution is proposed as a convenient, less dangerous procedure than traditional chemical etching in oxidizing HF media. Anodic polarization produces protons at the electrode surface, and local acidification increases the concentration of HF that efficiently dissolves the surface oxides, promoting a sustained dissolution current that decreases with increasing mass-transfer rate. The voltammetric pattern includes a "pseudoplateau" at low potentials, in which only a metal dissolution takes place, and a steady current increase at higher potentials due to the onset of oxygen evolution. The treatment produces a semilustrous rough surface, showing a structure of shallow microscopic cavities in scanning electron microscopy images. Impedance spectra recorded in the pseudoplateau region are similar to those observed during anodic polarization of Ti in HF, and their analysis indicates a formation of a thin barrier film whose thickness increases linearly with potential.The NiTi alloy is a material of great technological interest because of its unique thermomechanical properties, 1 including an excellent shape memory effect, the ability to recover apparently permanent deformations of about 8-10% by heating, and superelasticity, with a pseudoelastic field of 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 Due 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 recently proposed to perform electropolishing of freshly polished samples. 4,5 However, to efficiently etch aged and heated samples 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 ͑with a boiling point of 19.5°C͒, and its small undissociated molecule penetrates tissues by nonionic 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 much less dangerous but are also ineffective etchants, so there seems to be an inherent contradiction between effectiveness and safety.Electrochemical treatments can change the local conditions at the electrode surface by current flow, e.g., the local pH by generation of protons or hydroxyl ions, and in some cases, this property may be used to drive in a desired way the chemical reactivity at the electrode/solution interface. This strategy has been recently applied to prepare h...

Electrochemical Surface Structuring for Strong SMA Wire–Polymer Interface Adhesion

“…The latter results from the short treatment time and the experimental setup, in which the SMA wire is connected to the positive pole, and positively charged hydrogen ions are thus pushed away from the treated surface. So far, electrochemical treatments were mostly applied to increase the corrosion resistance and biocompatibility of NiTi SMA surfaces for biomedical applications − or the fabrication of microactuators from SMA sheets. , Some sporadic attempts have been made to use electrochemical treatments to improve the interface adhesion, but these were mainly based on anodizing, ,, which typically leads to the formation of a thick rough oxide layer on the surface. In our approach, the dissolution and oxide formation rates are balanced, allowing us to control the surface topography and chemical composition of the surface, as a result of which the metal surface is covered only with a thin unavoidable oxide layer.…”

Adsorptive Purification of Aqueous Solution from Fluoride Ions by Carbonaceous Materials

“…However, special forms, in which fluorine occurs (Fig. 1) are dependent on the solutions pH [16]. In solutions with a very low pH the dominant forms are undissociated HF molecules, whereas with increasing pH of the solution their contribution lowers towards the dissociated form of fluoride ions (F -).…”

“…In solutions with a very low pH the dominant forms are undissociated HF molecules, whereas with increasing pH of the solution their contribution lowers towards the dissociated form of fluoride ions (F -). as a pH function [16] Fluorine compounds entering the human body play a significant role, both their surplus, as well as the deficiency of this element, has a negative impact on living organisms. In trace amounts it is necessary for the correct functioning of plants and animals.…”