Tarnish
Tarnish is a layer of corrosion similar to rust.
It is caused by oxidisation - when semi - reactive metals such as copper, brass, silver, or 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.
References
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