Microanalytical Characterization and Surface Modification of TiNi Orthodontic Archwires

Orthodontic archwires (equiatomic TiNi alloy) of both used (4 weeks) and unused conditions were microanalyzed by optical and scanning electron microscopes, energy dispersive X-ray spectroscopy, and electron diffraction to characterize the surface layers. They were also subjected to immersion and polarization corrosion tests in a 0.9% NaCI aqueous solution. Based on results obtained from these analytical and experimental studies, surfaces of TiNi archwires were further electrochemically treated to etch away nickel selectively and reform the surface morphology to uniform and porous surface layers. Main conclusions were: (a) surface layers of used archwires were covered contaminants causing the discoloration, and the contaminants were identified as mainly KCl crystals, (b) surfaces of both used and unused wires were observed to be inegular features characterized by lengthy island-like structures, where nickel was selectively dissolved, (c) corrosion tests in a 0.9% NaCl aqueous solution in immersion and polarization methods indicated that by increasing temperature from 3°to 60°C and acidity from pH 11 to pH 3, calculated corrosion rates increased, and (d) surface layers of TiNi archwires can be electrochemically modified to selectively etch nickel away, leaving a Ti-enriched surface layer and forming a uniformly distributed porous surface that may reduce the coefficient of friction against the orthodontic brackets.