Introduction
Corrosion is the spontaneous destruction of a metal or alloy by physical and chemical reaction with its environment. In the mouth, saliva is an electrolyte, so the most common type is electrochemical corrosion.
How it starts
Polar water molecules pull metal ions into solution. Base metals send out more ions and take on a negative potential, while noble metals, copper and mercury take on a positive one.
When two metals with different potentials are in the mouth, ions move toward the more positive metal and a galvanic current flows. The more negative metal is the anode, and it dissolves. The more positive one is the cathode.
Effects in the mouth
- Weaker restorations: corrosion fatigue can make metal bend or break under normal loads.
- Toxicity and allergy: metal ions and corrosion products are released for years. Nickel, cobalt, palladium, gold and copper are potential allergens.
- Local reactions: gingivitis, stomatitis and blue-gray staining around metal crown margins.
- Pathogalvanism: potentials of 50 to 100 mV are considered normal. Above 100 mV, patients may have burning of the tongue and cheeks, a sour or bitter taste, a dry mouth, ulcers or lichen planus. In one Bulgarian study, removing such metal helped 86.8% of patients.