Tarnish

thin layer of corrosion that forms over copper, brass, aluminum, magnesium, neodymium and other similar metals as their outermost layer undergoes a chemical reaction

Tarnish is a layer of corrosion similar to rust. It is caused by oxidisation - when semi - reactive metals such as copper, brass, silver, aluminum meet chemicals in the air. It will cause the metal to darken or go dull by making a thin film on the metal. It is caused by a chemical reaction, between a metal and a non metal (or compound), usually oxygen or sulphur dioxide.