How Do You Make Aluminium Tough and Coloured at Once? Grow the Skin

Aluminium is a reactive metal that combines readily with oxygen, and freshly cut aluminium begins reacting with the air immediately, forming a layer of aluminium oxide within milliseconds. That layer is transparent, extremely hard and chemically inert, and it stops the reaction going any further by sealing the metal from the air, which is the entire reason a reactive metal behaves as though it were unreactive. The natural layer is only a few millionths of a millimetre thick, so it is easily scratched through, although it reforms at once. The process under discussion simply takes that natural behaviour and drives it much further deliberately.

The arrangement is an electrical cell with the workpiece as the positive electrode:

• •The part is suspended in a bath of dilute sulphuric acid
• •It is connected as the positive side of a direct current supply
• •Oxygen is liberated at its surface and combines with the metal
• •The oxide layer grows, thousands of times thicker than the natural one
• •It grows downwards into the metal as well as outwards
• •The acid simultaneously etches it, which leaves it full of fine pores

A surface riddled with microscopic vertical channels sounds like a defect and is actually what makes the process commercially valuable. Dipping the freshly treated part in a dye bath lets the colour soak into those pores, where it sits below the surface rather than on top of it, so the colour cannot be rubbed or chipped off in the way paint can, which is why anodised camera bodies, carabiners, cooking pans and phone cases keep their colour under hard use. The part is then sealed, usually in boiling water or steam, which makes the oxide take up water and swell so that the pores close over the dye. Sealing also greatly improves corrosion resistance, and an unsealed part is considerably worse than an untreated one.

Process conditions change the result enormously, and the trade names conceal a fairly simple set of variables. Running the bath colder, at around zero degrees, with a higher current and for longer produces a much thicker and denser layer, tens of times the ordinary thickness, which is used where wear resistance matters more than appearance and which comes out dark grey and cannot be dyed brightly. Chromic acid baths produce a very thin layer used in aerospace, where the loss of metal matters and where the acid left in any crevice is less aggressive. Some baths produce colour without dye at all, by growing a layer that interferes with light or that contains scattered metal particles, which is how architectural bronze finishes are made.

The process is frequently confused with electroplating and is close to its opposite in several respects, which is worth being clear about. Plating deposits a layer of a different metal onto the workpiece, so the part grows and the coating can eventually peel or wear through to the base metal. This process adds nothing, since the layer is made from the workpiece itself by converting its own surface, which means it cannot peel because there is no boundary to peel at. The part also ends up slightly larger and slightly smaller at once, since the layer grows in both directions from the original surface. The electrical polarity is reversed between the two, with the workpiece positive here and negative in plating.

Aluminium grows a thin transparent oxide layer in air by itself, and current in an acid bath drives that same reaction thousands of times further. The acid etches the growing layer at the same time, leaving fine pores that hold dye below the surface, which is then sealed shut with boiling water. Nothing is deposited, since the layer is converted from the metal itself, so it cannot peel.