Tarnish

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Tarnish is a thin layer of corrosion that forms over metals, often appearing as a dull or discolored film. This process is a result of a chemical reaction between the metal and a non-metal compound, most commonly oxygen and sulfur dioxide. Tarnish is typically seen on metals such as silver, copper, and brass.

Causes

Tarnish occurs when the surface of a metal reacts with substances in the environment. For example, silver tarnishes when it comes into contact with hydrogen sulfide in the air, forming silver sulfide. Similarly, copper tarnishes when it reacts with oxygen, forming copper oxide.

Prevention

To prevent tarnish, metals can be coated with a protective layer. Common methods include:

• Electroplating with a non-reactive metal such as gold or rhodium.
• Applying a thin layer of lacquer or varnish.
• Storing metals in airtight containers with desiccants to reduce exposure to moisture and reactive gases.

Removal

Tarnish can be removed through various methods, including:

• Polishing with a soft cloth and a mild abrasive.
• Using chemical tarnish removers that dissolve the tarnish without damaging the underlying metal.
• Electrochemical reduction, where the tarnished metal is placed in an electrolytic solution and an electric current is applied to reverse the tarnishing reaction.

Commonly Tarnished Metals

• Silver: Forms a black layer of silver sulfide.
• Copper: Develops a green patina of copper carbonate.
• Brass: A mixture of copper and zinc, tarnishes similarly to copper.

Applications

Understanding and managing tarnish is important in various fields, including:

• Jewelry: Maintaining the appearance and value of silver and gold items.
• Antiques: Preserving the integrity and aesthetic of historical artifacts.
• Electronics: Ensuring the functionality of copper and silver components.

See Also

• Corrosion
• Oxidation
• Electroplating
• Patina

References

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