aluminiumoxidefilm

Aluminium oxide film, commonly called the native oxide, is a thin layer of aluminum oxide (Al2O3) that rapidly forms on aluminum surfaces when they are exposed to air or water. The film is typically amorphous, uniform, and adherent, and it provides a first line of defense against further oxidation.

The native oxide is usually only a few nanometers thick, often about 2–5 nm in ambient conditions. As a barrier, it reduces the rate of corrosion for many aluminum alloys, particularly in neutral environments. The film is amphoteric, dissolving in strongly acidic or basic solutions, and its protective quality depends on its continuity and absence of defects.

For many applications, the oxide layer is grown deliberately by anodic oxidation (anodizing). Barrier-type oxide layers can reach tens to hundreds of nanometers, while porous anodic alumina can be formed with pore channels and thicker layers up to several micrometres, expanding the surface area and serving as a template for nanostructures or as a dielectric coating in devices.

Key properties of aluminium oxide films include high hardness, chemical stability, high electrical resistance, and good dielectric strength. The film is self-healing to an extent: if damaged, the exposed aluminum rapidly re-forms oxide in the presence of oxygen. However, chloride-containing environments can promote pitting if the integrity of the film is compromised.

Applications span corrosion protection for aluminum products, decorative finishes, and, when grown by anodizing, production of protective, insulating, and templating oxide layers. The nature of the film—native, anodic barrier, or porous—determines its suitability for a given application.