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Steel is an alloy based primarily on iron, with a small proportion of carbon and sometimes other elements added to improve specific properties. It is a strong, hard, and relatively inexpensive, but it is prone to corrosion and rust if unprotected. For coinage, steel is typically used with a protective coating, most commonly copper or nickel plating. Steel was first widely adopted for coinage during the World War II era, when wartime shortages of copper and nickel led many countries to seek cheaper alternatives. It has since remained in use for low-denomination circulation coins in many countries, valued primarily for its low cost. Steel coins are easily identified with a magnet, a property sometimes used to distinguish them from other coinage materials.