What Is Anodising?

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.

Anodisation takes a nonferrous metal...

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.

...and coats it with an oxide layer...

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.

...during an electrolytic process...

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 sulphuric 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 aluminium. 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.

...designed to protect the metal inside.

The anodised layer is highly ordered, and it contains nanopores 10-150 nanometres in diameter. These nanopores are essential in the anodizing 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]


Comments

    except when its done properly it wont scratch and mark..
    unlike the iphone 5, and assuming ipad mini.

      I just looked over my iPhone 5.

      There isn't a mark anywhere on the aluminum. The screen is in worse condition. And the screen is in great condition.

      That is nonsense.. In a previous job, several parts i designed and had produced were anodised aluminium. I saw first hand how it is done.

      Any aluminium product can and will scratch or mark, it is a softer metal vs say Steel or Iron. Its used mainly because it is lighter and will bend vs snapping (steel will more likely crack then an aluminium). In fact having an anodized surface can actually lead to a greater chance for wear marks (scratches and scuff marks) vs a non anodised aluminium. The wear you see is just the outer most layer (die) being removed. Occasionally a deep scratch will reveal the "raw" aluminium.

      I dont own an iPhone 5, but seeing several in the wild i am assuming they are actually quite resilient to normal every day usage. This is not a statement of truth, just an observation i have made from the several i have seen. The anodizing seems to be of pretty good quality, as scuff marks can be buffed out (basic way of saying it) and except for the mirror like finish edge seems resilient to scratches (as resilient as most anodising anyway)

        Aluminium and steel both come in a multitude of different grades. Each grade uses different alloys for different purposes. Believe it or not but some grades of aluminum cannot be bent.

    Yeah cheap anodising is horrible scratches easily but like everything it comes down to good and bad quality.

    "An electric current, as high as 300V DC" that statement still has me scratching my head.

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