High Voltage Pulse Anodization of a NiTi Shape Memory Alloy

bMax-Planck-Institut fur Eisenforschung, D-40237, Dusseldorf, Germany In an attempt to improve the corrosion resistance of a NiTi shape memory alloy, thick oxide films were fabricated on the surface by an anodic oxidation process. High-voltage square shape pulse anodization was carried out in various electrolytes over the whole range of pH values from sulfuric acid, acetate buffer, ammonium pentaborate, sodium diphospho heptoxide, sodium aluminate, and potassium hydroxide. Microscopic observation showed that the films are porous or tile structured in nature and the structural analyses by X-ray diffraction and Raman spectroscopy demonstrated their amorphous nature. From the chemical analyses, the oxide films were determined to consist mainly of titanium as the metal component, whereas the nickel content of the oxide films depends on the pH value of the electrolyte as well as the amount of anions incorporated from the electrolyte. The application of an asymmetrical rectangle waveform was effective in increasing the density of the films. An electrochemical technique was used for screening the corrosion resistance of the anodized films and demonstrating the improvement in the resistance against localized corrosion of NiTi.