An investigation into the effects of polishing on surface hardness and corrosion of orthodontic archwires

The purpose of this study was to investigate the effect of surface roughnes s on the relative corrosion rates of wires of four alloys-stainless steel, nickel titanium, cobalt chromium, and beta titanium. Batches of wire were d ivided into two groups. Wires in one group were industrially polished to pr ovide a uniform surface finish; wires in the other group were left for comp arison "as received." Wire diameter, hardness, and relative corrosion rates were compared within groups before and after polishing. Comparisons were a lso made across the four groups of alloys. The samples of as-received wires showed variations in surface finish, with beta titanium having the roughes t appearance and cobalt chromium the smoothest. Nickel titanium and stainle ss steel surfaces were similar. Polishing provided a more uniform finish, b ut significantly reduced the diameter of the wires. Microhardness testing o f wire surfaces of each alloy indicated that no significant work-hardening occurred as a result of polishing. The relative corrosion rates (:expressed in terms of corrosion current density) in a 0.9% sodium chloride solution were estimated using the electrochemical technique of polarization resistan ce. Nickel titanium wires exhibited the greatest corrosion current density in the as-received state. Polishing significantly reduced the corrosion rat e of nickel titanium, such that comparison between the four alloys in the p olished state revealed no significant difference in their relative corrosio n rate/corrosion current density.