Evaluation of Corrosion Mechanisms in Stainless Steel Orthodontic Retainer Wires

Objectives: Orthodontic retainers fabricated from stainless steel wire and acrylic are prone to several types of corrosion in the oral environment, including stress, crevice, and galvanic corrosion. The aim of this study was to determine the relative effect of stress corrosion, galvanic corrosion, and crevice corrosion on stainless steel retainer wires.
Methods: Three different brands of 0.032-inch stainless steel wires were tested. Six new segments (2-inch length) of each brand of wire were tested in each of four tests: 1) in an artificial saliva solution to determine a general corrosion ranking; 2) in an artificial saliva solution after a 90-degree bend was placed in the wire to examine stress corrosion; 3) in 6% iron chloride solution to simulate crevice conditions and evaluate crevice corrosion; and 4) in an artificial saliva with the wire coupled to a solder material to measure galvanic corrosion. Corrosion properties were evaluated with a potentiostat. Open circuit potential (mV) and current density (nA/mm²) were compared in tests 1-3, as well as charge (mC) in the galvanic test. Statistical comparisons were completed with analysis of variance (ANOVA) tests and T-tests.
Results: One brand of retainer wire was not superior to the others in all tests. There was no significant difference (p>0.05) in open circuit potential (OCP) or current density between the bent and straight wires when tested in artificial saliva solution. All wires showed significantly (p<0.05) lower OCP and greater corrosion rates when placed in the iron chloride solution compared to artificial saliva. All wires were susceptible to pitting in the crevice conditions whereas pitting was observed less in the artificial saliva. The solder was anodic to the wires and produced the galvanic current.
Conclusions: Stainless steel retainer wires have the potential to corrode via several mechanisms, but crevice corrosion appears to produce the greatest effect.