The lustrous patina that protects the backside of Apple’s new iPad Mini isn’t just some cheap shellac or sealant coat—it’s actually “grown” on the metal itself in an electrochemical process known as anodisation. Here’s how.

Aluminium won’t rust like iron and steel, but the metal is quite susceptible to corrosion. Its corrosion, or oxidation, produces a layer of aluminium oxide across the surface. Luckily, that aluminium oxide layer is actually quite durable, and it can act as a shell to protect the metal from degrading further.

The anodising industrial process starts a controlled oxidation to create an engineered surface layer. It was pioneered in 1923, originally called as the Bengough-Stuart process as a means to seal Duralumin seaplane parts so they could resist corrosion. Today, it’s used on products from roofing to cookware, along with plenty of consumer electronics.

The anodising process is simple. An electric current, as high as 300V DC (but normally only about 15-20V), passes through the metal. Then, the aluminium dips into an electrolytic acid bath, often composed of sulfuric acid. On the interior of the acid tank, a plate of lead (or 6063t6 alloy aluminium) acts as the cathode, or negative terminal. The charged aluminium acts as the anode, completing the circuit.

In the reaction, oxygen ions migrate from the electrolyte onto the surface of the anodising aluminum. The ions build into a protective layer of oxide that is harder, more durable, and roughly 30-per cent thicker than the pure aluminium below it.

The anodised layer is highly ordered, and it contains nanopores 10-150 nanometers in diameter. These nanopores are essential in the anodising process, as they allow oxygen to penetrate the aluminium surface and propagate the reaction. However, these pores also let water and air to penetrate the metal, causing corrosion—the exact thing anodisation is meant to avoid.

So the outermost layer of oxide is typically filled in with dyes, or other corrosion inhibitors, before being sealed. The thickness of the oxide layer makes a difference too. Thicker layers can absorb dyes better, while thinner layers create a brilliant sheen.

Along with corrosion resistance, anodising can provide cosmetic benefits, and help aluminium’s ability to take a coat or primer or glue. But anodising isn’t a magic bullet of metal protection. The performance trade-offs include a harder aluminium that’s more brittle. The anodised layer is less likely to crack through physical wear, but it’s more likely to crack through thermal heat stress. [Anodizing - Wikipedia - Milinc]