Tarnish

Tarnish is a thin layer of corrosion that forms over copper, brass, aluminum, magnesium, neodymium and other similar metals as their outermost layer undergoes a chemical reaction.^[1] 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. The layer of tarnish seals and protects the underlying layers from reacting.

Tarnish preserves the underlying metal in outdoor use, and in this form is called chemical patina,^[2] an example of which is the green or blue-green form of copper(II) carbonate known as verdigris. Unlike patina advantageous in applications such as copper roofing and copper, bronze, and brass statues and fittings exposed to the elements, a chemical patina may be considered undesirable, as on silverware,^[2] or a matter of taste or convention, as in toning on coins.