Surface structure and corrosion resistance of short-time heat-treated NiTi shape memory alloy
Introduction
Nearly equi-atomic NiTi alloys (nitinol) have attracted much attention due to their shape memory effect, superelastic behavior, high tensile strength, good corrosion resistance and biocompatibility. These characteristics make NiTi alloys attractive for biomedical applications, such as stents, which are generally utilized to restore a damaged blood vessel or an oesophagus, among others.
NiTi alloys are generally regarded as highly corrosion resistant, similar to stainless steel or titanium. The reason is the spontaneous formation of a native passive TiO2 layer on the surface. Even a weak oxidizing environment, such as water, air or humidity, in contact with nitinol at low temperature is sufficient to produce a few nanometers of a passive protective layer. However, in the processing of NiTi alloys they experience various forming, heat- or surface-treating steps, which strongly influence the surface structure, chemistry and, therefore, corrosion performance.
In a human body, various fluids may come into contact with NiTi implants. In particular, serious problems may arise when a strongly acidic fluid, like gastric juice, attacks the surface of the alloy. In general, the corrosion of nitinol has two aspects. First, corrosion is accompanied with nickel release from an implant into the surrounding body fluid and tissue, which may enhance an allergic reaction in a sensitive organism. Second, corrosion may cause pitting and a reduction of mechanical performance of an implant. In an extreme case, an implant broken due to corrosion may produce sharp fragments that are dangerous for surrounding tissue [1].
For these reasons, the corrosion of nitinol has become an important issue in biomedical engineering during the last decade. To slow down this corrosion process, a highly protective adherent layer with a low concentration of nickel and internal defects must cover the surface of nitinol. A number of oxidation treatment procedures have been reported so far to support the preferential Ti oxidation and increase the protective effect against corrosion. These include thermal oxidation either in air or under a low oxygen pressure, etching, passivation in nitric acid, anodization, electrochemical polishing, oxygen ion implantation, laser oxidation, hydrothermal oxidation in water, sol–gel techniques, cathodic deposition and others [2], [3], [4], [5], [6], [7], [8], [9].
Most often, the last processing step in the manufacture of nitinol stents is the shape setting. The shape setting treatment means a short-time heating of nitinol at around 500 °C and serves to achieve a desired shape and superelastic behavior of an implant. Heating can be performed in air, argon, vacuum or a salt bath. In oxidizing environments, titanium in nitinol generally oxidizes preferentially over nickel due to the higher thermodynamic stability of TiO2 as compared to NiO [3]. At sufficiently high temperatures, titanium reacts rapidly even with traces of oxygen present in a surrounding environment. As a result, a TiO2-enriched and Ni-depleted surface oxide layer forms [10], [11], [12], [13]. The corrosion performance of a nitinol implant thus depends on properties of such an oxide layer, particularly, on its thickness, chemistry, adherence and internal structure.
In the literature, there are few studies on the influence of the shape setting treatment on corrosion behavior. For this reason, our study is concerned with short-time heat treatments of nitinol at around 500 °C. Treatments were carried out in various environments in order to simulate different oxidation potentials and the purpose was to prepare and compare various surface states of the Ni–Ti alloy. Corrosion tests were performed in a relatively aggressive solution with pH 2. The solution simulates gastric fluid, so that information obtained by these tests is important for implants, such as oesophageal stents, for example. The fracture of a stent in a patient's body due to corrosion may cause serious problems [14], [15].
Section snippets
Experimental
NiTi alloy (50.9% Ni; hereafter, all concentrations are in at.% unless otherwise stated) was used for heat treatments in our experiments. The 0.45-mm-thick wire with a tensile strength of 1580 MPa and Af of 25 °C was provided by an industrial supplier. According to the supplier specifications, the wire was cold drawn (45% deformation) and annealed. Detailed specifications of the annealing conditions were not given.
The NiTi wire was first subjected to chemical etching in a 1HF + 4HNO3 + 5H2O (by
Structure, chemical and phase composition
Fig. 1a presents a SEM view of the CHE surface. One can see than the surface is not flat. Instead, chemical etching causes the formation of small dimples of a few micrometers in size on the surface. There are also scratch marks resulting from the cold drawing process in which spherical or elongated non-metallic inclusions are observed. These inclusions consisting mainly of titanium carbides originate from the melting process of nitinol and from its contamination by the melting crucible.
Conclusions
In this work, we studied several heat treatments of a NiTi alloy at about 500 °C for 5–10 min in various environments, i.e., under conditions close to the shape setting procedure generally used in NiTi implant manufacture. The chemically etched surface was investigated for comparison. The treated materials were studied with respect to surface structure and chemistry and their impact on corrosion resistance. The corrosion rate was measured as the rate of Ni-release in this study, which is an
Acknowledgements
The research on NiTi alloys and on surface nanostructures is financially supported by the Czech Academy of Sciences (projects no. IAA200100902 and KAN300100801) and by the Ministry of Education, Youth and Sports of the Czech Republic (project no. MSM6046137302).
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2020, MaterialiaCitation Excerpt :Therefore, long implantation durations of Nitinol parts in the human body have the potential to release harmful nickel ions and possibly lead to implant failure secondary to fracture [19–22]. The in vitro corrosive behavior of Nitinol in simulated body fluid (SBF) medium and artificial physiological environments such as artificial saliva and manufactured gastric juice showed good corrosion resistance for medical use [23–29]. However, in vitro studies performed in real physiological fluids showed a change in the release of nickel ions, thus affecting the biocompatibility of the alloy [24,30–32].