Corrosion is a process by which a chemical reaction eats away at a metal. An example of a chemical reaction that causes corrosion is the oxidation of iron by water in an electrolytic process. The product of this reaction is rust.
A broader definition of corrosion is the destructive attack on a metal through interaction with its environment. It is the process by which metallic atoms leave the compounds in the presence of water or gas. Metal atoms leave the metal until it fails or erodes from oxidation.
The tendency for oxidation is dependent on the metal. Metals used in construction, such as steel and copper alloys, are both subject to corrosion. Metals are chemical combinations of multiple elements. These metals are highly vulnerable because of the high energy content of the elements used in metallic form. The energy content is higher than that of their ores, allowing corrosion to become a natural process. Through being present in their natural environments, the ores in the metal revert back to their original form. For example, iron in moist air becomes iron oxide.
The resulting rust is a generic term for when iron corrodes. The reaction between iron and water causes rust to form on a metal object.