Anodising

Anodising is used for the functional and decorative finishing of aluminium materials.

Anodising is carried out in an acidic electrolyte at slightly below room temperature. Here the bath temperature is therefore higher than in hard anodising. The component to be coated is connected as an anode. During treatment, the workpiece surface is converted into an aluminium oxide layer. The result is a porous, dense, hard and wear resistant surface which is tightly bonded to the base material. In contrast to hard anodising, up to one third of the oxide layer builds up on the original metal surface and up to two-thirds penetrates into the base material. This should be considered during the design phase of a component with respect to their fit.

The properties of an anodised layer, for example, chemical resistance, wear resistance and hardness, depend on several factors. These include the alloy of the base material, particularly the alloying additions, as well as the surface pretreatment, the parameters of the anodisation and the layer thickness. The latter depends on the intended use, the demands that are placed on the layer, and the subsequent loading. As a rule, layer thicknesses are produced from 5 to 25 µm.

With regard to the resistance to chemicals and corrosion, the anodised layer is resistant to chemically neutral substances in the pH range of 5 to 8. The process-related pores in the layer can be closed by special sealing processes: the corrosion resistance is greatly improved. Even brief acidic or weakly alkaline attacks are resisted by the layer when it has been sealed accordingly.

The hardness of an anodised layer is subject mainly to the influence of the alloying constituents. Hardnesses of 250 to 350 HV can be achieved.

The anodised layer increases the wear resistance of the aluminium. Highly wear-resistant surfaces are, however, only produced by hard anodising (HART-COAT® layer). (see HART-COAT®-process).

We at the Hangzhou plant expressly point out to our customers that, due to different operating conditions, only a practical test on the performance of the layer system by the user himself can provide information.

Almost any aluminium alloy can be anodised.

Aluminium components with an anodised surface have proven themselves in virtually all industries. Anodising improves wear protection, increases the corrosion protection and prolongs the life of parts. Anodising can also be used as a corrosion protective sub-layer before painting of aluminium components, for example for use near the sea. Not least, this coating process is suitable for decorative purposes.

Process variants

The oxide layers obtained from anodising can also be black. The combination of anodising with PTFE coatings results in very good anti-friction properties of the layer. Anodised layers can also satisfy different decorative requirements. For example, pretreatments such as pickling, grinding, blasting and polishing are carried out.