Tarnish

Tarnish is a thin layer that forms over copper, brass, silver, aluminum, and other semi-reactive metals as their outermost layer undergoes a chemical reaction. Tarnish is mainly caused by chemicals in the air, such as sulfur dioxide. It often appears as a usually dull, gray or black film or coat over metal. Tarnish is a surface phenomenon, that is self limiting unlike rust. Only the top few layers of the metal react, and the layer of tarnish seals and protects the underlying layers from reacting.

Tarnish actually preserves the underlying metal in outdoor use and is called patina. The formation of patina is necessary in applications such as copper roofing, and outdoor copper, bronze, and brass statues and fittings.

It is the thin layer of aluminum oxide tarnish that protects aluminum in outdoor applications such as screen door frames.

Chemistry

Tarnish is a product of a chemical reaction between a metal and a nonmetal compound, especially oxygen and sulfur dioxide. It is usually a metal oxide, the product of oxidation.

Prevention and removal

Using a thin coat of polish can prevent tarnish from forming over semi-reactive metals. Tarnish can be removed by using steel wool, sandpaper, emery paper, baking soda or a file to rub or polish the metal's dull surface. Fine objects (such as silverware) may have the tarnish electrochemically reversed, or it may be removed with a special polishing compound and a soft cloth.

See also

• Patina
• Rust
• Honing

References

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