A tarnish film may intime accumulate elements and compounds that chemically attack metal.
For e.g; egg and certain foods containing sulfur .
sulfides such as hydrogen, or ammonium corrode Ag,Cu,Hg and similar metals present in dental alloys and amalgam.
Specific ions play role in corrosion of certain alloys .
For e.g; Oxygen and chloride in corrosion of amalgam
- When teeth are not in contact – still electric circuit is present due to potential difference between metals. Saliva forms electrolyte and hard and soft tissues forms external circuit.
Electric current generated between gold and amalgam restorations, when they are not in contact –
0.5 – 1 μA with potential difference -500mV.Coating with varnish tends to eliminate galvanic shock.