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 protection: normal abrasion can remove the oxide allowing further oxidation of the metal or the oxide layer is permeable to additional oxygen. In addition to providing protection, the thickness of the anodized oxide layer, as well as added dyes or pigments, can determine the color of the surface, providing vivid, attractive and durable surfaces.
One of the most common metals that is anodized is aluminum. A wide variety of electrolytes are used to produce the desired surface properties: chromic acid, sulfuric acid, and oxalic acid are the most common but borates, citrates and oxoacids are also sometimes used. The surface that is produced by the electrochemical process is often quite irregular (amorphous) so that there may be regions that atmospheric oxygen can still penetrate. To form a more even, less penetrable surface the electrochemically treated metal may be subjected to a second treatment, such as submersion in boiling water to form a hydrated layer or immersion in hot dichromate salt solutions or sodium silicate to form oxoanion layers. The properties of the surface of the anodized metal are sensitive to the type of electrolyte that is used, its concentration, the temperature at which the process is carried out and the nature of the secondary treatment. When anodized surfaces are colored by the addition of dyes or pigments, a secondary treatment with acetates salts such as cobalt or nickel acetates is used to help seal the coloring agents to the surface. The surface may also be treated with non-polar, insulating substances such as oils or waxes to deter corrosion by aqueous solutions.
One of the more common electrolytes for the anodization of aluminum is sulfuric acid. At the concentrations typically used for this process, of about 1 to 2 molar, the sulfuric acid is able to dissolve any of the naturally produced oxide film on the surface, allowing good control for the formation of a new oxide surface. Because of the high conductivity of sulfuric acid solutions, the process can be carried out quite rapidly, typically less than an hour at about 20 V of DC current with a current density of 1 Ampere for a metal surface about 1 inch by a foot (130 Amperes per square meter). Under these conditions, the anodized surface is about 0.005-0.010 mm thick and is transparent.
Oxalic acid was first used in Japan, then Germany and now worldwide instead of sulfuric acid to produce surfaces that are not only hard and corrosion and abrasion resistant but attractively colored with added dyes or pigments. The colors depend on the conditions used for the electrolysis and, in particular, the thickness of the film. The drawback to the oxalic acid process is its expense relative to the sulfuric acid process.
Impurities such as iron and silicon in aluminum can lead to vulnerable spots for corrosion in the anodized surface and they also decrease the transparency of the oxide layer because compounds other than aluminum oxide are produced by the impurities during the anodization process. For this reason, very pure aluminum must be used to produce the highly reflective surfaces desired for reflectors and for appliance and automobile trim.
Among the metals that are often anodized, magnesium is notable because the untreated metal is so very readily corroded. Anodization of magnesium provides a hard, corrosion resistant surface that is also highly adhesive to paints. Magnesium metal has a low density and magnesium alloys are critically important in the aircraft industry, where integrity of the metal and prevention of corrosion are a must.
Anodization is also used to produce surfaces on metals that will adhere well to paints and other coatings. For these applications, secondary treatments are also used to bind species, typically oxoanions, that form strong linkages to pigments or organic species that bind to the oxide surface, allowing organic coatings to adhere. The feature of anodization that lends itself to this application is that the metal surface can be covered evenly in a predictable manner and the oxide surface is very strongly (covalently) bound to the bulk metal.
This is the complete article, containing 811 words
(approx. 3 pages at 300 words per page).
