Types of corrosion in removable appliances: annotated cases and preventative measures

Three different types of electrochemical corrosion cells are discussed - composition, concentration, and stress cells. Annotated cases are presented for each type of corrosion cell in removable appliances using conventional photography, scanning electron microscopy, and X-ray elemental analyses. The complexity that corrosion can attain is illustrated via the case of a palatal expansion device. Specific examples of galvanic cells, which are possible in orthodontic practice, are summarized in terms of the anodic and cathodic half-cells. These include material differences, oxygen gradients, presence of debris, changes in pH, induced stress levels, changes in stress states, and variations in as-received history. A galvanic series is constructed from the orthodontist's perspective that includes titanium, nickel-titanium, cobalt-chromium, and stainless steel alloys, as well as glass-fiber-reinforced polymer-matrix composites. Preventative measures are presented from the perspectives of the raw material manufacturers, the dental laboratories, the chair-side practitioner, and the patient. A list of materials is provided, which are known to promote corrosion in the event that stainless steels are sensitized as a consequence of improper handling. In the final analysis, however, retainers and removable appliances are reliable if corrosion cells are conscientiously averted and preventative measures are routinely employed.