Many elements and materials go through chemical reactions with other elements. When steelcomes into contact with water and oxygen there is a chemical reaction and the steel begins to revert to its original form - iron oxide.
In most modern steel applications this problem is easily overcome by coating. Many different coating materials can be applied to steel. Paint is used to coat cars and enamel is used on refrigerators and other domestic appliances. In other cases, elements such as nickel and chromium are added to make stainless steel, which can help prevent rust.