With the single exception of the years of World War II, the American penny, from 1864 to1962, was made of bronze composed of 95 percent copper and 5 percent zinc and tin. From 1962 to 1982 it was composed of 95 percent copper and the rest zinc. After 1982, the penny was changed to a thin shell of copper over zinc. All of these pennies tarnish and can corrode.

Tarnish

Tarnish is different from corrosion. Some metals tarnish and never corrode. Tarnish usually consists of the oxide and sulfide of the base metal. It is thin and not destructive. It merely discolors the base metal.

Corrosion

Corrosion is more substantial, affecting both appearance and dimensions. Chemicals stronger than oxygen or hydrogen sulfide produce corrosion. They are found in the atmosphere.

Gases

Besides pennies, many statues are made of copper or bronze. They well illustrate the process of copper corrosion. At first statues merely darken with tarnish. But especially in modern times, there are gases in the atmosphere that cause corrosion. They include carbon dioxide, sulfur oxides and nitrogen oxides.

Moderate Corrosion

Water in the form of humidity or rain combines with carbon dioxide to produce carbonic acid. This reacts with tarnish to produce the insoluble copper carbonates. On a penny this would be considered corrosion; on a statue this is not undesirable and is called a "patina."

More Serious Corrosion

The other oxides mentioned become sulfurous and sulfuric acids and nitrous and nitric acids. Unfortunately, these react with both copper oxides and copper carbonates to produce soluble copper salts that wash away.

Source:

University of Illinois at Urbana Champaign; Technology Studies in Education; "Penny Composition"

More Information:

Corrosion Doctors; "Corrosion Mechanism"