Few metallic elements exist in nature in the metallic state- gold is often found in the metallic state, silver and copper are sometimes found as metals and iron and nickel are present in meteorites. Most metals are so readily oxidized by atmospheric oxygen that they are only found naturally as oxides, sulfides or salts. After ores are reduced to produce metals, means must be found to prevent them to be reoxidized by the atmosphere. One method for protecting metal surfaces is anodization.

When a metal surface is anodized, the metal is connected to a source of electrical potential such that it serves as the anode (see Anode). Oxygen is provided by the electrolysis of water. Under carefully controlled conditions, the surface is oxidized to produce a tough oxide coating that is relatively impenetrable to atmospheric oxygen. Natural oxide layers are often too thin to provide sufficient...