Process

• In the anodizing process, both the metal being anodized, most often aluminum, and an inert material, usually carbon, lead, nickel or stainless steel, are put into an anodizing bath and connected to a direct-current power supply, with the aluminum to the positive terminal of the power supply and the inert material to the negative terminal. When power is applied to the circuit, electrons are drawn from the surface of the aluminum to the inert material, where they react to produce hydrogen gas. The ionization on the surface of the aluminum allows it to react with the water in the bath, creating an oxide layer on the metal's surface. The chemical composition of the bath determines whether the oxide layer will be a barrier layer or a porous layer.
  
  

Barrier Oxides

• When a barrier oxide is grown on the surface of a metal, it forms a dense uniform film that is resistant to the flow of electrical current. Some metals -- aluminum, tantalum and niobium -- are capable of growing a barrier film thick enough to be of commercial use; aluminum coated with a barrier oxide can function as an electrolytic capacitor.
  
  

Porous Anodic Oxides

• Aluminum anodized in certain acidic electrolyte baths acquires a thick oxide layer densely covered with microscopic pores. This oxide layer can prevent corrosion, provide electrical insulation and otherwise protect the surface of the metal. The color of the oxide layer may be altered by the presence of organic acids in the anodizing bath and by a two-step anodizing process that deposits other metals within the pores of the oxide layer. Color can also be added by dyeing the coating.
  
  

Applications

• Anodized aluminum is used for a wide variety of purposes. A clear anodized coating is used on aluminum architectural elements, photolithographic plates and auto trim to prevent corrosion. Colored anodized coatings are used in architectural and decorative applications. A process called hard anodizing produces a wear-resistant oxide layer that is used in engineering applications where durability is required. Porous coatings can be used in nanotechology applications in which the pores are used as patterns for constructing nanowires and nanotubes.