Electrochemical Etching of NiTi Alloy in a Neutral Fluoride Solution

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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.