The corrosion resistance of Nitinol alloy in simulated physiological solutions Part 2: The effect of surface treatment

The effect of surface treatment – boiling in water and thermal oxidation at temperatures up to 600 °C – on the corrosion behavior of Nitinol was investigated in simulated Hanks physiological solution using electrochemical polarization methods. Morphological and compositional properties of the modified surfaces were analyzed by scanning electron microscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy depth profiling. Surface preparation – grinding or polishing – is shown to have a decisive role in the degree of improvement of corrosion properties by surface treatments. Low temperature treatments like boiling in water and thermal oxidation at 100 °C resulted in the formation of oxide layers only a few nanometers thick, and composed mainly of TiO2 and a small amount of NiO. These layers are well able to protect the underlying Nitinol substrate. Up to 500 °C, surface preparation directly determines the thickness of the oxide scale, as a 20-fold difference in thickness is observed between ground and polished samples. At higher temperatures, the oxide thickness was similar for the two samples. A multilayer structure is observed at all temperatures investigated. The outermost layer at the oxide/air interface is composed of TiO2 and NiO, while the interior of the oxide scale is composed exclusively of TiO2. Oxide layers formed by thermal oxidation at elevated temperatures also improve the corrosion characteristics of Nitinol, especially for polished substrates.