Anodising is an electrochemical process which converts the metal surface into a decorative, durable, corrosion-resistant, wear-resistant oxide layer. This aluminium oxide is not applied to the surface like coating or plating, but is fully integrated with the underlying aluminium substrate. This means it cannot chip or peel. It has a highly ordered, porous structure which allows for secondary processes such as colouring. Our anodising layers meet all the quality standards for outdoor use, such as QUALANOD and BS.

Principle of anodising

Following the discovery of how well a very thin oxidation film can protect the aluminium, it was an obvious decision to search for other methods to make the oxide layer thicker so that it offers even more protection. Initially, chemical methods were used. At a later stage, the electrochemical process of ‘anodic oxidation’ was developed. This is usually referred to as ‘anodising’, although this process is also referred to as eloxation, which is derived from the German word ‘Eloxalverfahren’. Sulphuric acid anodising is also referred to as the GS process, from the German ‘Gleichstrom Schwefelsäure’.

A rectifier has been set up next to the anodising bath, with the negative pole connected to the cathode plates and the positive pole connected to the workpiece. The flow of current causes oxygen formation at the anode and hydrogen gas formation at the cathode.

The oxygen (and hydrogen) are initially present in the form of free atoms, which are highly reactive and impact on the aluminium by formation of an oxide layer. Only the atomic oxygen, which causes further oxidation of the aluminium, is used here. In effect, the anodising bath provides a way of bringing pure oxygen – in the form of free atoms – in contact with the metal surface.

In theory, the type of liquid used in the bath is not important, as long as it is an electrolyte, meaning that it conducts the electrical current. The oxide layer that is formed has very different properties to the aluminium itself. This layer has mechanical properties that lie somewhere between glass and porcelain. The layer is hard and brittle and also acts as an electrical insulator.

Anodising in colour

There are many methods to manufacture coloured anodized aluminum. Alumet only uses methods that are colourfast an UV-resistant

Dying with inorganic dyes

Dying with inorganic dyes can be achieved, for example, using a single phase dye, as is the case with use of ferric ammonium oxalate. This give the product a golden colour, but when used at a low concentration it results in the colour ‘new silver’. Alumet offers this technique and colour under the name AluGold.

Electrolytic colouring

In the case of electrolytic colouring, Alumet uses metals obtained from metal compounds such as tin from a solution of tin sulphate. This produces colours ranging from light copper to black. The use of copper sulphate yields colours ranging from light pink to cardinal red. The advantage of this electrolytic dyeing process is that it produces very lightfast colours. Alumet produces these colours under the names AluBronze and AluRed. By electrolytically dyeing 2x, we can produce colours under the name AluOldCopper

Combination colouring: electrolytic and inorganic

The use of new, very lightfast, inorganic dyes in combination with electrolytic colouring is becoming increasingly common, for example in the colours ‘copper’ and ‘brass’. In this process, we first perform electrolytic colouring, followed by ferric ammonium oxalate dyeing. Alumet produces these colours under the names AluCopper and AluBrass.


Interference colouring

Interference colouring is a relatively new colouring process, which can also be considered a colour mixing system. This process not only works with colour, but also with refraction (interference). Following the anodisation process in direct current sulphuric acid by a treatment in acid and alternating current makes it possible to enlarge the lower part of the pore. This is followed by electrolytic metal deposition. This results in the colours grey, blue and green. Alumet also performs this interference technique under the names AluGrey, AluBlue and AluGreen.

Our pre-treatments


Alumet uses special machines and tools to remove the top layer from profiles or sheets.


The effect of brushing is a shiny surface with visible brush marks.


Polishing can be combined with colour anodising to obtain different glossy products.

Cleaning products

Only products that do not damage the anodised surface should be used for cleaning. These are ‘pH neutral’, synthetic cleaning products. These synthetic cleaning products are strongly diluted in water when used for regular maintenance. Warm water can be used if necessary. A number of cleaning products can be ordered from Alumet. Contact for more information.

We must emphasise the warning that alkaline cleaning products, such as soda-containing products and strong detergents, must not be used. Acids such as hydrochloric acid (often used for the cleaning of stone-work façades) and phosphoric acid-containing products should not be used either, as this causes the anodising layer to disappear. Aluminium, including aluminium oxide, is ‘dissolved’ at low pH (acid) or high pH (salt/alkali). Abrasive or scouring materials – such as steel wool, steel wire brushes, sandpaper and hard polishing agents – will also damage the anodising layer.

Cleaning frequency

The cleaning of anodised aluminium in buildings and other structures must be performed according to a set schedule. This schedule is dependent on the atmosphere to which the aluminium is exposed or the extent of natural cleaning (rainfall). The following can be used as a guideline: Under normal circumstances and with natural cleaning through rainfall, it is enough to clean the aluminium once a year with a cloth or brush and plenty of water. A neutral synthetic detergent can be used if necessary and then left to dry. However, cleaning should be performed twice a year in the case of an industrial environment or in the absence of natural cleaning through rainfall.

SG / VEC anodising

Alumet has special printing equipment installed to print dates and production numbers on aluminium profiles in order to fulfil the requirements for “Structural Glazing” or “VEC anodise”. The ink form of this inkjet printer is a permanent mark, which is difficult to remove. This is necessary for facades with structural glazing. The product coding makes it easy to obtain al relevant information concerning anodizing, if something is wrong with the façade.

SG ofwel VEC anodisatie