With its low density, aluminium is one of the lightest kinds of metal and yet it is comparatively very strong. When exposed to the air, this metal gets covered in a thin oxide film, which makes it naturally resistant to corrosion. Aluminium is a good material for cold forming and hot forming, it can be welded and forged and it is a good conductor of electricity and heat. Its high level of conductivity makes it an attractive option for electrical engineering applications, especially within the context of enclosure construction and front panel technology.

Given current quality requirements, many applications can only be realised by constantly seeking advances in terms of surface treatment. The reason for this is that criteria such as design, resistance to an ever longer list of substances and shielding problems, and last but not least environmental factors, are constantly assuming more and more importance.

EL OX AL  stands for ELectrolytic OXidation of ALuminium

During anodic oxidation (anodisation), an electrochemical process transforms the surface of the aluminium into aluminium oxide. The resulting oxide layer is firmly attached to the aluminium. The thickness of the coating layer can be measured in terms of micrometers (10 - 20 µm) and offers increased surface hardness (wear-proof/high surface hardness HV>200),

  • ensures excellent corrosion protection (weather-proof and corrosion-resistant),
  • provides protection against environmental influences and facilitates cleaning (highly durable and easy to care for), meets extremely stringent design requirements (lightfastness of the coloured anodised coating), can be applied in various colours, provides electrical insulation

By contrast, chromate layers serve as an adhesive surface for coats of paint and provide a conductive form of corrosion protection. The thickness of the layer is approx. 2- 5 µm, 50-70% of which penetrates the base material. Chromating is the term used to describe what happens when metal surfaces are treated with solutions containing chrome(VI). The scenario where this process is most commonly used is that of aluminium alloys. A basic distinction is made between yellow and green chromating. Green chromating involves the use of aqueous solutions consisting of chromic acid, fluorides and phosphates. It is the addition of chrome(III) phosphate that gives the layer its green colour. With yellow chromating, the aqueous treatment solution is dominated by chromic acid. The layers that are formed on the surface of the metal are largely made up of chrome/aluminium oxides and hydrated chrome/aluminium oxides. The yellowish colour is achieved through the addition of chromate. With chromating, the coating weight per unit area is between 0.1 and 0.5 g/m². The temperature resistance of these layers ranges up to approx. 150°C.

As far as aluminium surfaces (plus galvanised steel surfaces) are concerned, the various chromating processes covered by DIN 50939 defined the state of the art for a long time.

However, in recent years a fierce debate has been raging concerning the use of chromium compounds.

In the light of the RoHS Directive1 and following a series of constant cost increases and an increasing awareness of quality, alternative rust-proofing methods (i.e. those that do not involve the use of chrome) can now be used. This approach combines the advantages of chrome-free, inorganic passivation systems with those of organic thin film coatings, creating inorganic conversion coatings with an overlying polymer film. The layer is colourless or slightly yellowish. The approvals for the process are issued by both the legislature and the quality associations (GSB International, Qualicoat).

The results of the investigation into the level of corrosion resistance and paint adhesion are as follows:

Test method

Test result

Acetic acid salt spray test in acc. with DIN 50021 ESS

> 1000 h, < 1 mm corrosion creep, no blistering

Testing in a saturated atmosphere with presence of SO2 in acc. with DIN 50018 SFW

> 30 cycles, <1 mm corrosion creep, no blistering

Steam test in acc. with DIN 55632-1

No blistering after 2 h

Cross cutting in acc. with ISO 2409


1 Directive 2002/95/EC of the European Parliament and of the Council of 27 January 2003 on the restriction of the use of certain hazardous substances in electrical and electronic equipment

The Directive is available at:


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