Introduction: NiTi based alloys are widely used in biomedical applications due to their special mechanical properties, corrosion behaviour and biocompatibility[1]. As orthodontic wires, they deliver light and relative constant force, resulting in a more physiological dental movement.
Addition of copper (Cu) as a third element can improve some mechanical properties desirable to their application as orthodontic wires, such as narrowing of stress hysteresis and a more stable stress-strain plateau[2]. However, the better mechanical properties are not accompanied by an increase in the corrosion behaviour. A recent report revealed a lower corrosion resistance of CuNiTi in physiological medium[3].
Using different metals in oral environment, for example as orthodontic arch wires and brackets, may result in galvanic corrosion. The aim of this work is to assess the galvanic corrosion of CuNiTi with other alloys commonly used in dentistry, such as stainless steel (SS) and beta titanium.
Materials: The materials used for this work were the following: CuNiTi, SS and beta titanium arch wires. CuNiTi was an 0.18-inch heat activated (35 oC) arch wire from Ormco Corp. (Orange, USA). SS and beta titanium wires were 0.16-inch arch wires ordered from GAC International Inc. (New York, USA).
Methods: Electrochemical tests were performed in 0.9% NaCl electrolyte in order to simulate physiological solution. A conventional three electrode cell with a saturated calomel electrode as a reference electrode and platinum as a counter electrode was used. This cell was connected to a potentiostat (Metrohm Autolab). Potentiodynamic polarization was conducted at a scan rate of 20 mV/min starting from -600 mV to 800 mV.
Cathodic curves of the materials with higher corrosion potentials (SS and beta titanium) were overlaid with the anodic curve of the CuNiTi. At this intersection point, the value corresponding to x axis was determined as the estimated potential of the galvanic couple (Ecouple) and y axis as the estimated current from the galvanic couple (icouple).
Results: Initial results indicate an electrode potential gap between CuNiTi and SS and titanium alloys in 0.9 NaCl solution. This suggests that galvanic corrosion might occur in this environment. Mean Ecouple measured for the CuNiTi/SS couple was -0.1 mV. For CuNiTi/titanium was -0.8 mV.
A typical polarization curve for CuNiTi and titanium is shown in Figure 1. Direct galvanic coupling and potentiostatic experiments will be performed for CuNiTi/SS and CuNiTi/titanium couples.
Discussion: In dentistry, there are usually two or more different metals together in a potential corrosive environment. Therefore, galvanic corrosion must be investigated. Potentiodynamic curves revealed that for both couples, CuNiTi/SS and CuNiTi/titanium, CuNiTi would act as an anode, and therefore, would corrode faster than it would alone. As a result of corrosion, deterioration of the arch wire surface may occur and the mechanic properties of CuNiTi wires could be altered, delaying orthodontic treatment. Moreover, CuNiTi corrosion might affect the material biocompatibility due to release of potential allergic Ni ion.
Conclusion: CuNiTi might be susceptible to galvanic corrosion in oral environment when coupled to more noble materials, such as SS and titanium.
This work was supported by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); and the Fundação Coordenação de Projetos, Pesquisas e Estudos Tecnológicos (Fundação COPPETEC).
References:
[1] Thierry, B., & Tabrizian, M. (2003). Biocompatibility and biostability of metallic endovascular implants: state of the art and perspectives. Journal of Endovascular Therapy, 10(4), 807-824.
[2] Gil, F. J., Solano, E., Pena, J., Engel, E., Mendoza, A., & Planell, J. A. (2004). Microstructural, mechanical and citotoxicity evaluation of different NiTi and NiTiCu shape memory alloys. Journal of materials science: Materials in medicine, 15(11), 1181-1185.
[3] Kassab, E., Neelakantan, L., Frotscher, M., Swaminathan, S., Maaß, B., Rohwerder, M., Jockenhoevel, S., Gomes, J., Eggeler, G. (2014). Effect of ternary element addition on the corrosion behaviour of NiTi shape memory alloys. Materials and Corrosion, 65(1), 18-22.