Anodizing is a process by which metal is induced to form a surface layer of oxide by immersing it in a liquid electrolyte and applying electric current. The process is called anodizing because the piece being treated forms the anode of an electric circuit. Depending on the alloying metal and the thickness of the coating, this process may change the metal’s hardness, corrosion resistance, thermal or electrical properties. Many parts made by aluminum extrusion, such as those offered at UniqueExtrusions.com, are often anodized to make them harder or for cosmetic reasons.
Anodizing begins with the cleaning of the metal piece when it leaves the aluminum extruder to remove any surface contaminants, after which the piece is subjected an electric current through a component called the cathode. This causes hydrogen to form on the cathode while a layer of oxygen forms on the anode. This creates a layer of oxide on the anode, making a barrier to further oxidation.
The oxide layer is smooth and shiny to the naked eye, giving the piece an attractive appearance. This smoothness is deceptive, since a microscopic examination will reveal a network of tiny fissures. These fissures allow dye to penetrate beneath the metal’s surface, so colors applied to the piece are bright, glossy and permanent. A negative side of the fissures is that they can allow reactive substances to penetrate to the metal substrate, encouraging corrosion. Though anodizing gives the metal some desirable properties, this pitting must sometimes be remedied by the application of protective coatings.
Aluminum alloys are good candidates for anodizing, since the alloying process often deprives them of aluminum’s ability to passivate naturally. Since passivation is the cause of aluminum’s corrosion resistance, the inability to passivate results in alloys that are prone to corrode. Anodizing after the piece leaves the aluminum extruder induces passivation, producing aluminum alloys with the same corrosion resistance as pure aluminum.
Anodized metals, especially aluminum, have many uses in industry and art. Metal panels on the exteriors of buildings are often made by anodized aluminum extrusion, as are railings and other outdoor features. The casings of appliances like refrigerators, microwaves and televisions are often anodized aluminum. Vent covers, awnings, light fixtures and so forth are also made from it, utilizing the metal’s weather resistance. Its lack of corrosion makes it ideal for kitchen items such as pots and pans.