To oxidize aluminium at the surface by electrolysis. The alminium oxide crystalizes and forms a hard protective layer. This is done to provide chemical or mechanical protection, and also for decorative purposes. Anodized parts are often dyed black, red, or blue. The process works best at low temperatures. Performed by connecting the part to the positive terminal (anode) in an electrolysis circuit, using an acid solution as the electrolyte.