It depends on what the project calls for and the type of environment that it will be exposed to. For the highest amount of strength, carbon steel is a favorite among builders, though it is prone to corrosion and electrochemical reactions if it is not painted, powder-coated or otherwise sealed. It is most often used for the interior structure of commercial buildings, bridges, guardrails and other structures where superior strength is required. In areas that are exposed to moisture or salt water, galvanized steel is often used. It is coated with a thin layer of zinc that helps prevent corrosion and chemical reactions. For many applications that require hygienic, easily-cleaned surfaces, such as hospital or kitchen areas, food storage or medical equipment, stainless steel is used.