What is Anodising?

From satellites to skyscrapers (and a few tinier things in between), there are many uses for anodised aluminium. But what exactly is anodising? How does it work? And what are the benefits?

In short

Anodising is an electrochemical process that adds a thick oxide coating to metals, increasing their resistance to corrosion. It gets its name from the method, which uses the metal in question as an anode (more on that below), hence anodising. As well as protecting the surface of metals, the process is often used to add colour for decorative purposes, like the carabiners in the picture below. Anodising was first invented around 100 years ago, and the benefits it brings mean it is still a popular and highly effective choice in modern architecture, engineering, technology and design. Unlike paint or other coatings, the oxide protective layer produced by adonising can’t be chipped off or peeled away because it is an integral part of the metal.

What metals can be anodised?

The metal best suited to adonising is aluminium (and its alloys). You might also see titanium anodised; the technique is often used to create colourful jewellery. Ferrous metals such as iron and steel cannot be anodised because the process would simply immediately corrode (rust) the metal. Magnesium is sometimes anodised, but generally the metal that most commonly undergoes the process is aluminium.

What happens during anodising?

A piece of aluminium, or an aluminium alloy, is submerged in a tank filled with an electrolytic solution of diluted acid. Electrolytic solutions are highly electrically conductive because they’re busy with many negative and positive ions. A positive electrical charge is applied to the aluminium itself so that it acts as an anode, Meanwhile, a cathode plate is attached to the inside of the tank and has a negative charge applied to it. As the electrical current passes through the circuit, it causes positive ions to be drawn to the negative cathode plate. At the same time, the negative ions are attracted to the positive anode, in this case the piece of aluminium. This leaves behind a coating of aluminium oxide on the metal that’s much thicker than the one that would develop through natural oxidisation.

Why anodise?

Anodising protects metal from corrosion, chemical attack and abrasion, and can also be used to add colour. Aluminium is a popular metal because it’s relatively lightweight, but the downside to this is that it is quite soft and vulnerable to corrosion and abrasion. Anodising is the solution to this dilemma, because the aluminium oxide layer created by the process is extremely hard. In fact, aluminium oxide is graded 9 out of 10 on the Mohs hardness scale, meaning it comes in second place after diamond. The oxide is so hard that it’s used as an abrasive in sanding belts. It’s important to remember that anodising won’t strengthen the structure of the metal itself, but it will protect it from the elements and surface scratches.

Aluminium oxide coating doesn’t conduct electricity, meaning another use for anodised aluminium is where electrical insulation is required. And because the oxidised surface is porous and well-ordered, it can hold dyes, which is why the method is useful in producing coloured aluminium products. Anodised and dyed aluminium offers strikingly vibrant colours with an eye-catching metallic sheen that won’t fade, even in the harshest sunlight.

As if all these pluses weren’t impressive enough, anodising is relatively safe for the environment. What’s more, because it doesn’t add anything new to the make-up of the aluminium, it doesn’t affect one of the key benefits of this metal: its unique ability to be infinitely recycled.

So, the main benefits of anodising aluminium are: corrosion resistance, abrasion resistance / lubrication, electrical insulation, environmental safety and aesthetic decoration (colours).

Larger-scale uses for anodised aluminium

Aluminium that has been anodised is often used in shipping and on oil rigs, settings where the metal would otherwise easily corrode. The Willis Tower in Chicago, one of the tallest buildings in the world, located in a famously windy city, is clad in black anodised aluminium. Anodised aluminium is even used on satellite components to keep them in tact in the harsh environment of space!

The three types of anodising

There are three main types of anodising:

  1. Type I – This method uses chromic acid for the electrolyte bath. It produces a relatively thin film on the aluminium, which is harder to colour but that is useful because of its ductility.
  2. Type II: This process uses sulfuric acid in place of the chromic acid. The result is a thicker layer of aluminium oxide that allows for vibrant dyeing results.
  3. Type III (A.K.A. hard anodising): Again, this method uses sulfuric acid, but at a lower temperature and with a higher electrical voltage running through the circuit. The result is an even thicker external layer to the metal. This is usually the best option when making parts that are likely to encounter a lot of friction when in use.

Health & safety

Anodising is a complicated practice that should always be undertaken in a professional environment with suitable health and safety precautions in place.

Anodising is even used in space satellite components.