The corrosion of orthodontic wire

Abstract

The corrosion of stainless steel orthodontic wire was investigated using the electrochemical technique of polarization resistance. A potentiostat was used to apply small perturbations about the corrosion potential and the resultant current was measured. The polarization resistance is determined from the slope of the plot, and the corrosion current and rate calculated. Problems associated with the technique are non-linear polarization plots and a prior knowledge of Tafel coefficients. In the present work a non-linear optimization procedure was developed to solve the corrosion equations directly for corrosion current and Tafel coefficients. Six sample groups were investigated, the corrosion rate expressed in terms of corrosion current density (Am−2: as received' (2.89); heat treated (2.49); finger springs (2.64); finger springs sleeved (5.78); Adams clasps (4.32) and Adams clasps sleeved (3.65). The results of the 'as received' were extremely scattered reflecting the random and uncontrollable stress imposed on the wire during the drawing process. The effect of heat treatment was to reduce the scatter and to significantly reduce the corrosion rate. The finger springs did not corrode at a significantly faster rate, but when insulated by the use of heat shrink PVC tubing leaving only the highly stressed coil to corrode, there was a two-fold increase in rate. The highly stressed Adams clasps also had a high corrosion rate. Scanning electron micrographs of the wires before and after corrosion indicated minimal surface damage.