Tarnish
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Tarnish is a layer of corrosion that develops over copper, brass, silver, or aluminum as they undergo oxidation.
How to treat
When Tarnish occurs, it is treated chemically, usually with strong acids. Many commercial products are dedicated specifically to this.
How to prevent
Most tarnishing is prevented by "Tinning", a process by which the reactive substance is coated in a non-reactive substance (such as tin or wax), and thus protected from oxygen.