Influence of surface layer on mechanical and corrosion properties of nickel-titanium orthodontic wires

To analyze the effect of various coating formulations on the mechanical and corrosion properties of nickel-titanium (NiTi) orthodontic wires. Uncoated, rhodium-coated, and nitrified NiTi wires were observed with a three-point-bend test, surface roughness (Ra) measurement, scanning electron microscopy, energy dispersive spectroscopy, and electrochemical testing (open circuit potential, electrochemical impedance spectroscopy, and cyclic polarization scan). Differences in the properties of tested wire types were analyzed with analysis of variance and Tukey post hoc test. Uncoated and nitrified NiTi wires showed similar mechanical and anticorrosive properties, while rhodium-coated NiTi wires showed the highest Ra and significantly higher modulus of elasticity, yield strength, and delivery of forces during loading but not in unloading. Rhodium-coated NiTi wires also had the highest corrosion current density and corrosion potential, lowest impedance modulus, and two time constants on Bode plot, one related to the Rh/Au coating and the other to underlying NiTi. Working properties of NiTi wires were unaffected by various coatings in unloading. Nitrification improved corrosion resistance. Rhodium coating reduced corrosion resistance and pronounced susceptibility to pitting corrosion in artificial saliva because of galvanic coupling between the noble coating and the base alloy.