Surface Modification of Superelastic NiTi Alloys by Electrochemical Dealloying Method

Superelastic NiTi alloys are extensively used in biomedical field especially in orthodontics and orthopedics applications due to their desirable properties [1]. These alloys are susceptible to surface corrosion and serious problem might be raised due to release of toxic nickel ions from surface to tissues and body fluids. Therefore, optimizing the surface and electrochemical and biocompatibility investigations of the NiTi SMA alloys are required [2]. In this survey, it is tried to improve surface characteristics of NiTi specimens by electrochemical dealloying methd. Superelastic Ti-50.8 at.% Ni alloy has been produced by powder metallurgy was used in this study. After polishing and cleaning, the specimens surface were treated by electrochemical method in a two component electrolyte of nitric acid and methanol with a volume ratio of 25:75 at 5.5 V for 30, 60, 120 and 180s at room temperature. After electrochemical dealloying, surface structure of the specimens was investigated by SEM coupled with EDX analysis. Also, corrosion behavior of the specimens is examined by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in Ringer solution at 37°C. Figure 1 SEM micrograph of NiTi specimen’s surface after electrochemical dealloying in different magnification. 165 Figure 2 potentiodynamic polarization and EIS tests results of the base and dealloyed NiTi specimens. Results show the porosity of the NiTi surface has been increased with electrochemical dealloying and 3D nano/micro structure on surface has been created as shown in figure 1. EDX analysis proves that the 3D structures are titanium/oxygen rich phases. Thus, Ni removal was effectively occurred from the NiTi specimen’s surface during electrochemical dealloying. From electrochemical corrosion tests (figure 2), the corrosion potential is shifted to more positive values after dealloying. In other hand, the corrosion current density increases in contrast of base NiTi (not dealloyed) because of higher porosity of dealloyed NiTi specimens. However, increasing surface porosity leads to improve biocompatibility due to better adhesion of cells and enhance tissue ingrowth [3]. The semicircle diameter of the base NiTi is higher than the electrochemical dealloyed specimens which indicate higher corrosion resistance of the base NiTi that is in agreement with polarization results. Among dealloyed specimens, the specimen with 60s dealloying time shows higher corrosion resistance. Consequently, the electrochemical dealloying process effectively removes Ni from the surface of NiTi specimens and 3D nano/micro titanium oxides structures are formed on the surface. This porous structure could be improved biocompatibility of the NiTi alloy.