The process of anodising aluminium involves submerging the metal in an acid electrolyte bath and passing an electrical current through the solution. The bath has a cathode attached to it and the aluminium acts as the anode. When the electrical current is passed through, oxygen forms on the surface of the aluminium creating an aluminium oxide or anodic layer. The quality of this anodic layer depends on many things including the temperature and concentration of the solution as well as the current passed through.
The thicker the anodic layer created, the greater the corrosion resistance and typically this layer is between 5 to 25 microns thick. The thickness of the layer is determined by the length of time the current is passed through the solution and the longer the metal spends in this state, the thicker the anodising layer.
Anodising is most commonly used for improved corrosion resistance on certain types of aluminium alloys. Aluminium alloys that are subject to marine environments typically benefit from anodising. Ship hulls, dock components, and oil rig structures are common examples of these.
BS 1615 / BS EN 12373-1 2001
DEF Stan 03-25
MIL-A-8625 Type III
Ordinarilly, 5-30 microns (ยต)