The corrosion stability of Nitinol in simulated physiological solution is strongly dependent on the surface preparation – grinding, polishing or chemical etching. Whereas a ground surface is not resistant to localized corrosion, polished and chemically etched surfaces are resistant to this type of corrosion attack. The reasons for this behaviour were investigated through metallurgical, topographical and chemical properties of the surface as a function of surface preparation. For that purpose, scanning electron microscopy combined with chemical analysis, confocal microscopy and X-ray photoelectron spectroscopy were used. The surface roughness decreased in the following order: chemically etched ) ground ) polished surface. Besides differences in topography, distinct differences in chemical composition of the outermost surface are observed. Ground, rough surfaces comprised mainly titanium oxides and small amounts of nickel metal. Chemically etched and, especially, polished surfaces are composed of a mixture of titanium, nickel and titanium oxides, as studied by angle resolved X-ray photoelectron spectroscopy. These results emphasize the importance of detailed investigation of the metal surface since small differences in surface preparation may induce large differences in corrosion stability of material when exposed to corrosive environments.
The effect of surface treatment − boiling in water and thermal oxidation at temperatures up to 600 ºC – on the corrosion behaviour 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 ºC resulted in the formation of oxide layers only a few nanometres 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 ºC, 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.
Prolongation of the average life expectancy and an active life-style in old age are related to the constant increase in the number of joint diseases which eventually require a surgical procedure. The diseased joint is replaced with a joint prosthesis, the functionality of the joint is recovered, and pain is reduced. In the last decade the number of joint replacement operations has increased several times over and is expected to increase further. In order to enable patients a painless and active life-style, it is necessary to develop materials which are long lasting in vivo. Metallic biomaterials must exhibit high corrosion and wear resistance. In vitro research on materials under simulated physiological conditions is presented. These experiments are complemented by examples from clinical practice performed in collaboration with orthopedic surgeons. Morphological and chemical changes in the material during the course of in vivo performance are related to processes of wear and corrosion. The local and systemic consequences of these processes in the human body are presented.