The process

Chromating of aluminium

Chromating describes the chemical surface treatment with solutions containing chromate or passivation with solutions that are free of chromium VI. Metalux offers yellow chromating, colourless chromating and chromium-(VI)-free chromating.

Chromating of aluminium

Adhesive primer and corrosion protection

During chromating, chromic acid acts on metallic surfaces. This causes complex chromic acid salts (chromates) to be formed. The base material is dissolved in the process.

The dissolved metal ions of the base material are incorporated into the chromate layer. These belong to the passivation layers, i.e. they are inorganic non-metallic protective layers.

Anti-corrosion properties

Adhesive primer

The most common function of chromate coatings is corrosion protection. However, they are also used to obtain an adhesive base primer for painting or gluing. Chromating processes work on aluminium, magnesium and zinc. Due to legal regulations (EU End-of-Life Vehicle Directive, RoHS Directive), chromates containing carcinogenic Cr (VI) compounds may no longer be used in the automotive industry and many other areas since 01 July 2007. We therefore use the process exclusively for the aerospace and defence sector.

Chromating is a corrosion protection in its own right. No further surface protection is needed. Corrosion resistance depends on the alloy, with copper-free alloys achieving the highest corrosion protection.

The electrical contact resistance of chromate layers is very low. It is usually better than the electrical resistance of untreated parts. Chromate coatings are therefore very suitable for parts of aluminium enclosures, which must be conductively connected to each other to ensure the shielding effect of the enclosure.

The conversion coatings can be subjected to temperatures up to 80°C without a reduction in corrosion protection.

Zinc, zinc die casting, aluminium and their alloys

The main cause for the poorer corrosion protection of chromium-free conversion coatings is either a coating that is too thin or one that is still too reactive. Good corrosion protection is only achieved if the conversion layer is thick, i.e. forms a good barrier effect against corrosive media – and if the conversion layer consists of very inert substances, i.e. substances that are inert and as insoluble as possible in water, acids and alkalis.

We therefore work with chromium (III) as a substitute. The chromium (III) based passivation process SurTec 650 ChromitAL is suitable for high-alloyed aluminium, castings and forgings. It forms a weakly iridescent, slightly yellowish, reddish to blue layer (depending on the alloy used). SurTec 650 chromitAL is a process to refine an aluminium surface with a corrosion protecting conversion coating. It is compliant with the EU End-of-Life Vehicles Directive and the EU Directive on Waste Electrical and Electronic Equipment. Regarding corrosion protection, electrical conductivity of the surface and adhesion of subsequent coatings, SurTec 650 chromitAL is a fully adequate substitute for yellow chromating.

Versatile application

All process technologies at a glance!

Would you like to learn more about how we refine surfaces? We offer these process technologies:

Powder coating

Hard Eloxal anodising

Eloxal anodising

Chromating of aluminium

Chromic acid anodising

Chemical nickel plating