Part of the appeal of Aluminium is its rust proof qualities. This comes from the oxide that forms on its surface. Unfortunately, this same oxide makes aluminium difficult to paint, which is one of the metals few disadvantages. However, if processed electronically this same surface oxide can be thickened and made easier to colour, giving aluminium the best of both worlds – colour and rust resistance.
The electronic process is called anodization. It basically submerges the aluminium in an electrolytic solution, with the aluminium serving as an anode. This is similar to the process used in a battery, or the science experiment where two different metals are immersed in a piece of lemon. The chemical process that generates the electrical voltage across the metals also alters their surface; in this case the surface of the aluminium grows a thicker oxide.
The thicker oxide coating that forms on the aluminium is very porous; it is full of tiny holes about 100 Nano-meters in diameter. This series of holes allows the electrical anodization process to continue contacting the surface of the metal. It allows the metal to be coloured as dye can be used to fill this spaces in the oxide. Failure to fill these spaces allows air and moisture to contact the metal surface underneath the oxide, leaving the object prone to corrosion. As such, anodised aluminium must be either coloured or have its porous surface filled with some other protective substance.
The anodizing process is not new. It has been used on cookware and cups for several generations. Metal iPod cases are made of anodized aluminium.
Aluminium Cladding that is coloured by anodization is quite pleasant looking, and often put to great use in architecture, art, furnishings, or in sculptures.