Nearly equiatomic NiTi alloys (nitinol) have been considered as excellent biomaterials desirable for biomedical implant devices since they possess unique properties, such as shape memory effect, superelasticity and good biocompatibility. They have been widely used as successful orthodontic wires, self-expanding cardiovascular, bone fracture fixation plates and staples, and so on. The NiTi orthodontic wires have been developed to replace the traditional metallic materials, such as stainless steel, pure titanium and Ti6Al4V. Due to their unique superelasticity, Nitinol can offer the constant and desirable driving force to rectify misaligned teeth.
It has been entirely clear that Ni ion release can lead to allergenicity, toxicity and carcinogenicity. Therefore, the corrosion resistance of Nitinol is considerably concerned. In addition, the combination of aggressive medium and complex stress state in the oral environment has caused the frequent premature fracture failure of NiTi orthodontic wires in service. Thus, it increases the remedy cost and even brings much suffers to the patients.
As documented in literatures, chloride and fluoride, which are introduced into the oral cavity through the daily diet, oral health care and dental remedies, are the most aggressive medium for NiTi orthodontic wires. The available researches showed that fluoride negatively influence the corrosion resistance of Nitinol in the simulated physiological solutions. However, there is little understanding of the detailed aggravation effect of fluoride and there lacks specific work to clarify whether different mechanisms exist for NiTi orthodontic wires regarding their corrosion behavior in fluoride and chloride medium. Therefore, in this work, traditional electrochemical measurements and scanning electronic microscopy were used to investigate the influence of equal molar concentration of chloride and fluoride on the electrochemical corrosion behavior of Nitinol.
Up to date, most of the relative work is concerning the biocompatibility caused by Ni ion release under the static corrosion process of Nitinol. The mechanochemical interaction of Nitinol, however, was not given enough concerns. Therefore, the other part of work includes the influence of static corrosion on the mechanical behavior and the slow strain rate test (SSRT) of Nitinol in chloride and fluoride medium.
Based on the above experiments, the following conclusions are drawn:(1) NiTi shape memory alloy is primarily susceptible to localized corrosion when exposed to solutions containing chloride, while it is susceptible to general corrosion when subjected to solutions containing fluoride; (2) NiTi shape memory alloy exhibits much higher Ni ion release rate in fluoride than in chloride; (3) the synergistic interaction of fluoride and chloride on the corrosion behavior of NiTi shape memory alloys will aggravate their corrosion resistance; (4) the critical stress to induce the martensitic transformation of NiTi shape memory alloys will decline under the static corrosion. This will lead to the decrease of the desired driving force to rectify the misaligned teeth and hence negative their biofuntionality; (5) slow strain rate tests indicated that fluoride will promote the fracture failure of NiTi alloys compared with chloride and Nitinol will experience fatigue-like failure in the phase of stress induced martensitic transformation in higher concentrations of fluoride. While NiTi orthodontic wires will crack from the localized pitting under applied anodic potential in Chloride.
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