Effect of surface treatment and sterilization processes on the corrosion behavior of NiTi shape memory alloy

Nickel-titanium (NiTi) alloy derives its biocompatibility and good corrosio n resistance from a homogeneous oxide layer mainly composed of TiO2, with a very low concentration of nickel. In this article, we described the corros ion behavior of NiTi alloys after mechanical polishing, electropolishing, a nd sterilization processes using cyclic polarization and atomic absorption. As a preparative surface treatment, electropolishing decreased the amount of nickel on the surface and remarkably improved the corrosion behavior of the alloy by increasing the mean breakdown potential value and the reproduc ibility of the results (0.99 +/- 0.05 V/SCE vs. 0.53 +/- 0.42). Ethylene ox ide and Sterrad(R) sterilization techniques did not modify the corrosion re sistance of electropolished NiTi, whereas a steam autoclave and, to a lesse r extent, peracetic acid sterilization produced scattered breakdown potenti al. In comparing the corrosion resistance of common biomaterials, NiTi rank ed between 316L stainless steel and Ti6A14V even after sterilization. Elect ropolished NiTi and 316L stainless-steel alloys released similar amounts of nickel after a few days of immersion in Hank's solution. Measurements by a tomic absorption have shown that the amount of released nickel from passive dissolution was below the expected toxic level in the human body. Auger el ectron spectroscopy analyses indicated surface contamination by Ca and P on NiTi during immersion, but no significant modification in oxide thickness was observed. (C) 2000 John Wiley & Sons, Inc.