Anodising

Anodising is an electrolytic passivation process used to increase the thickness of the oxide layer on the surface of metal parts. The process is called "anodising" because the treated part forms the anode electrode of an electrical circuit.

Anodic films are most commonly applied to protect aluminum alloys, although similar processes also exist for other metals. 

Anodising increases corrosion resistance, wear resistance and a hardening of the surface. It is one of the more environmentally friendly metal-finishing processes. Through organic (a.k.a. integral colour) anodising, the by-products do not contain heavy metals, halogens, or volatile organic compounds. 

Anodisation changes the microscopic texture of the surface and changes the crystal structure of the metal near the surface. The anodised aluminium layer is grown by passing a direct current through an electrolytic solution, with the aluminium object serving as the anode (the positive electrode). The current releases hydrogen at the cathode (the negative electrode) and oxygen at the surface of the aluminium anode, creating a build-up of aluminium oxide. 

The most common anodising processes, for example sulfuric acid on aluminium, produce a porous surface which soaks up dyes easily. The number of dye colours is almost endless; however, the colours produced tend to vary according to the base alloy. Dyed anodising is usually sealed to reduce or eliminate dye bleed out.

Die so entstandene Eloxalschicht ist sehr hart und gegen viele chemische Substanzen resistent. Ausserdem erhalten Aluminiumteile dadurch ein veredeltes, teures Aussehen. Im Gegensatz zu üblichen Lacken können Eloxalschichten nicht abblättern, da sie aus oberflächlichem Aluminium erzeugt werden und deswegen innigst mit diesem verbunden sind. Bei der Wiederverwertung von eloxiertem Aluminium müssen keine problematischen Lackschichten entsorgt werden, da die Eloxalschicht sehr dünn ist und sich als Schlacke im Schmelzofen ablagert. Somit ist Eloxieren nicht zuletzt eine sehr umweltfreundliche Form der Oberflächenveredelung.