Why aluminium finishing?

Being the third most common element in the Earth's crust, aluminium is present in various compounds in almost all rocks and soil. Premium aluminium is produced in a two-stage process from bauxite containing approximately 50 % Al²O³.

This material has gained considerably in importance in industry over recent decades due to its specific characteristics, such as high dimensional stability and low specific weight.

A considerable role in this development can be attributed to the finishing of aluminium surfaces. Aluminium finishes are found in decorative elements for indoor and outdoor use and are applied as functional surfaces to system components in a broad range of branches of industry.

A natural passivation layer forms on the surface of aluminium when it is exposed to the air. This layer, however, only prevents further corrosion of the base metal within the neutral range of pH 5-8. The very thin oxide layer, however, is neither decorative nor hardwearing. Aluminium finishing, on the other hand, helps achieve the desired functionality for your components as well as provide them with a highly appealing look.

• Pickling
• Glass bead blasting
• Chromate conversion coating
  passivation
• Electropolishing

• Anodised aluminium, transparent
• Anodised aluminium, black
• Anodised aluminium in numerous colours

• Hard anodised aluminium in various shades of colour

Preliminary treatments

Degreasing and pickling
All workpieces are firstly always degreased before surface finishing is commenced, so as to remove oils, fats and other contamination. In the E0 pickling solution, oxides are removed without any further surface abrasion. If a silk-matt finish is required or fine scratches need to be smoothed out, the workpieces are put through the E6 pickling solution. Afterwards, an acidification/neutralisation step is performed to remove alkali residues and foreign metals from the surface.

Glass bead blasting
In certain situations, an especially even satin surface is desired on the aluminium components. This effect can be achieved by blasting the workpieces with fine glass beads before further treatment. On larger-sized components, this step is performed manually in a special blasting booth. In the case of smaller components, this can be performed automatically and relatively inexpensively in a rotating perforated basket.

Chromate conversion coating, passivation
After the preliminary treatment described above, many aluminium components are given a transparent or yellow chromate conversion coating. The coating produced serves in some cases as an excellent adhesion promoter for subsequent paint finishes. It is visually quite appealing, also on turned parts, and at the same time offers rather good corrosion protection. Large numbers of small parts can be treated in bulk in baskets and barrels. CrVI-free passivation, which in accordance with the EU End-of-Life Vehicles Directive will soon be required, is currently undergoing testing in our laboratory and will be available for series production after trials have been carried out.

Electropolishing
To provide particular components with a highly polished surface, we have an electropolishing system that involves very special pre-treatment and after-treatment steps. In the polishing bath itself, the workpieces are connected up to form the anode in the circuit. This causes both aluminium metal and aluminium oxide to be dissolved. The electric current has a propensity for acting on the surface’s peaks, elevations and irregularities. When this process is used on appropriate materials, it produces mirror-like reflectance.

Anodised aluminium

The electrolytic oxidation of aluminium, known as anodising, represents the final stage in the production of many aluminium workpieces. It gives the components their final look. During anodising, the workpiece is – as the name implies – connected up as the anode in an acid electrolyte, and the oxygen produced as a result of this converts the aluminium on the surface to aluminium oxide. The oxide layer grows by up to approximately 2/3 into the materialand – due to the greater volume of the formed oxide – up to 1/3 out of the material . It is therefore very important that this be taken into account when considering the dimensions of the article. The layers produced in this way offer excellent corrosion protection on pure aluminium and on most aluminium alloys. It is, however, recommended that when purchasing the aluminium, one orders material of a quality suitable for anodising.

The layers, commonly referred to as being "naturally coloured" in accordance withC-0 (formerly EV1), usually have a thickness of 10-15 µm for engine parts and components used in interiors and approximately 20-25 µm in accordance with DIN 17611 for outdoor applications. "Naturally coloured" in this context means that the oxide layer in the case of pure aluminium is transparent, colourless and glasslike, while when it comes to various aluminium alloys – depending on their material composition – the oxide layers produced appear somewhat dull or matt, occasionally also with a yellow tinge or light brown colour.Aluminium oxide is a good Isolatorwith regard to surface conductivity as well as volume conductivity. The dielectric strength for dry layers is, depending on thickness, 100-600 volts

The layers are harder than glass, cutlery steel and hard chromium, though in this case we are dealing with the intrinsic hardness of the layer, which one can only measure – with adequate layer thickness – in cross-section. Immediately after anodising, the layers are very porous and are therefore excellently suited (in a similar way to textiles) to accepting dyes that are dispersed sufficiently finely. The colour used most is black in accordance with C-35 (formerly EV6)
However, we also have many other colours in use: from blue, green and red to purple, "gold" and champagne. For good dying results to be achieved, the oxide layers must have a minimum thickness of 20-25 µm, and one must have suitable recipes, sufficient temperatures and immersion times, and good sealing. During sealing, the layer is transformed from the amorphous state to a crystalline state by moisture expansion of the oxide (boiling in deionised water).

After sealing, the oxide layer primarily comprises aluminium oxide hydrate (Al²O³ · 1H²O).

Note: Components intended for subsequent painting as well as signs and front panels that are later to be printed must not be sealed before these operations are carried out, or else the colours will not adhere.

Hard anodising

Hard anodising is a special variant of the process of anodic oxidation. Especially hard, thick and abrasion-proof oxide coatings are produced on aluminium for technical applications that meet industry's requirements with regard to wear resistance, lubricity, dielectric strength, and corrosion resistance. Depending on the material, the layers are between 25 and 150 µm thick. For standard technical demands, 30 to 80 µm are sufficient. No cosmetic demands are made on the appearance of these generally grey to brown coloured oxide layers. Before hard anodising is performed, the predominantly machined and functional components are degreased and pickled as required to remove the aluminium's natural oxide layer. The pickling strength depends on the amount of surface roughness permissible for the component. With regard to the desired layer properties – such as tensile strength, breaking elongation, and hardness – the selection of the correct material is of great significance.

The alloying elements have a fundamental influence on the layer formation. They can be grouped as follows:

• Alloying elements that are not corroded during anodising, such as silicon and lead
• Alloying elements that are soluble but which remain as oxide or another insoluble compound in the layer, such as magnesium and zinc
• Alloying elements that are highly soluble and which do not form stable compounds in the layer, such as copper and nickel

The alloying elements can have the effect of an imperfection in the oxide layer or can form macropores if they are largely dissolved out of the oxide layer, as is the case with copper and nickel.

The layer growth on sharp edges does not lead to the formation of a closed layer, and thus corrosion resistance in these areas is no longer given. Already during the design process, one should therefore take care to choose radiuses that are as large as possible.

 Aluminiumveredelung(DE) 528.64 Kb