Anodised aluminium is one of the most widely used materials in the construction sector. It is easy to find anodized parts in doors, bathroom accessories and, of course, in roofs, windows and interior and exterior finishes in buildings.

But what is anodized aluminium? In a few words, we can say that anodizing is a process used to increase the oxide layer on the surface of aluminum. Then, a protective layer of great hardness and high resistance to abrasion is generated.

But let’s explain the process in more detail:

What is anodizing

Anodizing is an electrolytic process, i.e. it uses an electrical current to generate an electrolysis reaction, which includes anodic oxidation and cathodic reduction. It is at this point that the aluminium anode is covered, almost entirely, by an alumina film.

It is during anodic oxidation that the thickness of the surface oxide layer increases. Its structure and characteristics can be modified according to the alteration of:

• The nature of the electrolyte
• Its composition
• Its temperature
• The treatment time
• The voltage used

Types of anodizing

The anodizing process can be done in several different ways. According to the official standardization they are the following:

Chromic acid anodizing

The coating is formed from a chromic acid bath (chromic acid is obtained through chromite, it is a highly oxidizing and corrosive solid). The final coating is notable for its resistance to corrosion and good paint adhesion.

Sulfuric acid anodizing

This is the most common and most widely used procedure for colouring anodised parts. The transparent layer formed after anodizing gives the material, once painted, a high quality metallic appearance.

Silicon anodizing

When the alloy contains silicon, the resulting layer is more resistant to wear and corrosion despite its greyish, opaque colour. This type of anodizing is not used for decoration, but rather for parts that will not be visible.

Hard anodizing

If the part is intended for an environment where it will be most affected by abrasion and corrosion, hard anodizing is usually carried out at low temperature and with the percentages of dissolution reduced to a minimum.

However, this type of parts are not recommended for applications with thermal shocks due to the loss of flexibility suffered by the material during anodizing.

Common thicknesses and uses

Depending on the thickness of the anodised layer, the part will be more or less resistant.

For those cases where the parts need to be resistant, the thickness, instead of the usual 5-10 µm for decorative pieces, can be increased to 50 µm, which is found in parts to be used in a marine environment.

As for its use, it is distinguished according to the thickness of the oxide layer, as follows:

• Classes 5 – 10: 5-9 micron and 10-14 micron – Indoor use only
• Class 15: 15-19 microns – Standard class. Outdoor use
• Class 20: 20-24 microns – Used in aggressive, industrial and marine conditions
• Class 25: 25-29 microns – Used in industrial or marine conditions

In addition to construction, we can find products with anodized aluminum in:

• Household appliances
• Consumer electronics and technology
• Sporting equipment
• Electrical engines

The reason lies in the properties of the metal itself, such as its surface hardness, which makes it resistant to scratching, its thermal conductivity, the possibilities of colouring and, as we saw in the post about the aluminium composite panel, the possibility of aluminium being recycled in an unlimited way.

Colouring

Depending on the part, three different types of colouring will be used:

Electrolytic colouring

This type of colouring is done through metal oxides that are fixed in the alumina layer by means of alternating current. In this type of colouring, the metal ions are deposited at the bottom of the pores of the layer.

The final tone and brightness will depend on the distribution of the metal, which is achieved according to the metallic salts used. For example, with Cu (copper) salts, reddish tones are obtained.

The final result is resistant and suitable for the weather

Anodic colouring

Coloration is achieved through the use of direct current. The tones achieved will be solid and will go from bronze and light grey to bronze and dark grey.

The final result is an oxide layer that is harder and more resistant to abrasion than in the case of electrolytic colouring.

Combined colouring

With this type of process, the colour range that can be achieved is substantially extended. In this case, the electrolytically coloured oxide layers can be re-coloured with organic or inorganic dyes, maintaining the metallic effect.

STACBOND® composite panel with anodised finish

Behaviour of the aluminium type for anodising

Aluminium is classified in series according to its hardness and alloy. The larger the series, the harder the material.

In the case of the aluminium composite panel, due to its properties and the structural needs of the piece itself, is used the 5000 series, which gives an excellent performance in terms of protection and, in general, a good performance for matt and gloss decorative finishes.

In the following table we have compiled the behaviour of the aluminium type according to its series:

Aluminium alloy and behaviour by series

As you have seen, the anodising process of aluminium is one of the most common processes in the construction industry and in many other industries such as technology.

Don’t you think is quite surprising that we can find so many different products carrying anodised aluminium parts in our daily life?

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