Effect of rhodium plating on the ion dissolution from nickel-titanium and pure nickel wires

Abstract

Purpose: Rhodium-plated orthodontic wires are utilized for esthetic purposes. However, the decline in the corrosion resistance of rhodium (Rh)-plated nickel-titanium (Ni-Ti) wires has been a concern. In this study, Rh-plated nickel (Ni) wires and Ni-Ti orthodontic wires were prepared via electroplating, and the microstructures of the plated layers were investigated. Methods: The surface microstructure and thickness of the plated layers were analyzed using scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX). The corrosion resistance was tested by immersion into an acidic solution in addition to applying plated and non-plated wires to the oral cavity or subcutaneous tissues of animals. Ni dissolution and distribution into the contacted oral mucosa and implanted subcutaneous tissues were visualized using synchrotron radiated X-ray fluorescence (SR-XRF). Results: In Ni wires, the 1-2 μm plated rhodium layer was formed in contact with the substrate. This layer suppressed the corrosion of the Ni wire in acidic solutions. However, in Ni-Ti wires, defects in the plated layer were observed and subsequently, Ni and Ti dissolution in the acidic solution was enhanced because of galvanic corrosion. In Ni-Ti wires, Ni and Ti could not be detected in the tissues because of their low contents. Conclusion: The plated Rh layer was homogeneously formed in contact with the substrate in Ni wire compared to the plated layer on Ni-Ti wires. For the clinical use in orthodontic treatment, it might be suggested that Rh-plated Ni-Ti wires should be more stable in oral cavity without ion dissolution, however, the effect of dissolved Ni and Ti from Rh-plated Ni-Ti wires would not be a concern due to their extremely lower contents.