tarnish

Tarnish is a film of chemically altered material that forms on the surface of a metal, such as silver. It is a form of corrosion, which is the wearing away of a material, often a refined metal, due to chemical reactions, primarily oxidation. This process occurs when a metal's surface reacts with atmospheric agents like moisture and oxygen, and is accelerated by warmth and the presence of acids and salts.12

In metallurgy, surface treatments are employed to enhance a metal's appearance and protect it from corrosion. While some metals form stable, protective oxide layers, others, like silver, develop a superficial sulfide tarnish that generally does not harm the underlying metal. Tarnish is a visible indicator of the chemical reactions occurring on a metal's surface.32