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Coin Alloy Durability
Copper is too soft to make a good coin, but when combined with a harder material, it works well. A classic example is a pre-1982 American penny. The softer copper is combined with a small amount of zinc, completely changing its characteristics so that it is a much more durable material.
Nickel has become very popular for uses where metal hardness is important. The U.S. "nickel" is actually 75 percent copper and 25 percent nickel, as are the outer layers of dimes and quarters minted from 1965 to the present. Nickel is highly magnetic, which is not a good thing for money especially in today's electronic world, but the copper-nickel alloy used for these coins is not magnetic.
Coin Corrosion Resistance
Copper has excellent corrosion-resistant properties, making it useful in plumbing applications. It will not rust and does not react with very many substances. This is why you can find copper coins over 100 years old that may be "dirty" but are seldom corroded. Nickel remains shiny for a long time making for an attractive coin surface, and is also very corrosion resistant.
Copper and Nickel Are Affordable
Copper and nickel are rather low-cost metals, so minting them from a materials perspective is not expensive. Security for manufacture and transport is less elaborate compared with gold and silver minting, since the incentive to steal small denomination currency is less.
Difficult to Counterfeit
Nickel has a melting point of 1,453 degrees Celsius, making counterfeiting very difficult without extensive resources. The mixture of copper and nickel in today's nickel, dime or quarter must be exact or the coin's color and density will be off, so this again presents a metallurgical barrier to all but the most sophisticated forger, who would no doubt focus on gold or silver coins.