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

Abstract

To analyze the effect of various coating formulations on the mechanical and corrosion properties of nickel-titanium (NiTi) orthodontic wires. Materials and Methods: 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. Results: 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. Conclusion: 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.

Effect of pH, fluoride and hydrofluoric acid concentration on ion release from NiTi wires with various coatings

“…Nitrogen ion implantation leads to surface composition changes from NiTi to TiN (titanium nitride), replacing known allergen nickel with inert nitrogen. Improvement of the corrosion resistance of NiTi wire with nitrified surface compared to the uncoated NiTi wire was confirmed in previous electrochemical researches performed in artificial saliva and fluoride mouth rinse solution 1,3,4) . Rhodium is a noble metal with white reflective appearance and excellent anti-corrosion properties 5) , but in the commercially available wires it was noted that this aesthetic feature diminishes fast within oral cavity 6,7) .…”

“…These prophylactic agents are usually applied once a week on teeth, as recommended by manufacturers. Testing media were heated in a water bath at 37°C, and their pH value was recorded with the pH meter MP 220 (Mettler Toledo, Greifensee, Artificial saliva solution was prepared from 1.5 g/L KCl, 1.5 g/L NaHCO 3, 0.5 g/L NaH2PO4-H2O, 0.5 g/L KSCN, 0.9 g/L lactic acid, pH value 4.8, as used in previous research 3,4,26,27) . The artificial saliva with the pH of 4.8 was used as imitation of more aggressive environment, which can be found in persons susceptible to caries; in such cases the prescription of adjuvant prophylactic agents would be recommended 15,28,29) .…”

“…Rhodium is a noble metal with white reflective appearance and excellent anti-corrosion properties 5) , but in the commercially available wires it was noted that this aesthetic feature diminishes fast within oral cavity 6,7) . Also, it was found that this surface coating was uneven and non-homogeneous, which caused susceptibility to corrosion in artificial saliva, due to formation of galvanic coupling of underlying NiTi to noble metals in the surface coating [2][3][4] . There are no data available regarding the interaction between the rhodium coating and fluoride solutions.…”

Effect of oral antiseptics on the corrosion stability of nickel‐titanium orthodontic alloys

“…By rhodium‐coating, a thin layer of rhodium and gold is applied to the surface of the wire, which results in better aesthetics of the wire, but the corrosion behavior is obviously not improved. The cause of increased corrosion rate of rhodium‐coated wire is the occurrence of a galvanic cell between the noble coating and NiTi due to the porosity of the coating, whereby the coating becomes a cathode and the basic material becomes an anode . The corrosion process is thereby carried out locally, in places where the coating is porous .…”

“…All the samples were made in triplicate. Released nickel (Ni 2+ ) and titanium (Ti 4+ ) ions were measured by inductively coupled optical emission spectroscopy (ICP‐OES) on the Thermo Elemental IRIS Intrepid II xsp Duo device (Thermo Electron Corporation, Austin, USA) according to the parameters described in previous works . Ion stabilization before the measurement was achieved by adding one drop of ultra‐pure nitric acid.…”