On exposure to air, Aluminium will immediately form a stable oxide layer. This natural oxide film is, however, very thin, approximately 1/100th of a micron, is easily damaged and provides little corrosion resistance under normal atmospheric conditions.
Anodising is the controlled formation of an oxide coating on the surface of components made, primarily, from Aluminium and its alloys. The anodic coating is produced by passing an electric current through a dilute Sulphuric acid solution, Hydrogen is released at the cathode (the negative side of the cell) and Oxygen is formed at the anode (i.e. the work piece) forming Aluminium oxide. The thickness, density, hardness and colour of the coating can be varied depending on the solution and the processing conditions.
By modifying the processing conditions and electrolytes, a wide range of properties and coatings can be achieved, making anodising an ideal surface engineering technique for a wide range of applications.