Effect of fluoride ions on corrosion behavior of nickel Essay

1. Introduction: Titanium alloys are very reactive materials and reacts instaneusely with oxygen, forming an oxide film, which give them excellent anti corrosive properties []. Thus, titanium alloys have proven to be the most tissue compatible inorganic materials, especially for dentistry []. Nickel-titanium wires, which have a near equiatomic composition, were introduced to orthodontic clinical use by Andersen and colleagues in the early 1970’s []. They have been highly popular in dentistry because have a much lower elastic modulus and wider elastic range than other orthodontic wires such as those composed of beta-titanium, stainless steel, and cobalt-chromium-nickel []. NiTi alloys have excellent biocompatibility with living tissue and corrosion resistance in various simulated body fluids []. These properties originating from the spontaneously formed thin oxide passivation layer mainly consist of TiO2 []. For patients with orthodontic fixed appliance, mouth rinsing with fluoride-containing products is an effective method for the prevention of caries because such appliances have complicated morphologies []. However, reduced corrosion resistance of pure titanium and titanium alloys in fluoride containing environments that attack the protective surface oxide layer and cause allergic reactions to NiTi wires after insertion had been reported []. The corrosion resistance of pure titanium and titanium alloys largely depends on the fluoride concentrations []. Some studies concerning the corrosion behavior of titanium alloys in fluoride containing environments have been reported []. It is claimed that the fluoride containing solution with fluoride ions more than 20 ppm may destroy the protective oxide la... ...tive layer increases. 5. Conclusions The corrosion resistance of NiTi was studied in ringer’s solution for different chloride ion concentrations. Our results show that the increasing of the chloride ion concentration can accelerate the corrosion of NiTi alloy. The OCP values in KCl and NaCl contained solutions are less negative at lower concentrations than in higher ones and the resistance to corrosion decreases as the chloride concentration increases. According to the EIS analysis, NiTi presents a good corrosion resistance even at higher concentration of chloride ions but the polarization resistance and capacitance value of protective oxide layer decreases as the concentration of ions increases. Acknowledgements The author acknowledges the Shiraz University, school of materials science and engineering for the financial supports. (Grant No. 89-GR-Eng-58)