Tarnish

Tarnish is a thin layer of corrosion that forms over copper, brass, silver, aluminum, magnesium and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in the air. For example, silver needs hydrogen sulfide to tarnish, although it may tarnish with oxygen over time. It often appears as a dull, gray or black film or coating over metal. Tarnish is a surface phenomenon that is self-limiting, unlike rust. Only the top few layers of the metal react, and the layer of tarnish seals and protects the underlying layers from reacting.

Tarnish actually preserves the underlying metal in outdoor use, and in this form is called patina. The formation of patina is necessary in applications such as copper roofing, and outdoor copper, bronze, and brass statues and fittings. Patina is the name given to tarnish on copper based metals.

Chemistry

Tarnish is a product of a chemical reaction between a metal and a nonmetal compound, especially oxygen and sulfur dioxide. It is usually a metal oxide, the product of oxidation. Sometimes it is a metal sulfide. The metal oxide sometimes reacts with water to make the hydroxide; and carbon dioxide to make the carbonate.