Anodising is an electrolitic passivation process which form san oxide layer on the aluminium surface under controlled conditions. The anodic film grows with 2/3 of its thickness below the original metal surface and 1/3 above it. It is considerably thicker than natural oxide layer, due to which it effectively protects the aluminium surface from further oxidation, that is corrosion.
Aluminium oxide is a hard, durable, weather resistant substance that protects the base metal. The coating is integral to the metal and cannot peel off or flake. Anodising gives aluminium a deeper, richer metallic appearance than possible with organic coatings.
Which aluminium alloys and what to avoid: