Anodizing

The anodizing is a process for surface treatment of aluminum. By means of anodic oxidation, a protective layer can be produced on the metal, which prevents the natural, uncontrolled oxidation of the metal by reaction with oxygen from the ambient air.
The special feature of the method is that, unlike for example inElectroplating, no layer is applied, but a conversion of the uppermost aluminum layer takes place. This layer serves not only for corrosion protection, it can also be colored by means of an additional process step. Resistance and optics of the anodized layer can be influenced by the suitable choice of the process parameters.

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Technical basics for anodizing

Aluminum as well as iron or lead reacts in the raw state with the oxygen from the air as unedle metal, whereby a thin layer of aluminum oxide forms on the surface. This oxide layer is a few nanometers thin and transparent. The formation of the layer does not alter the hardness and wear resistance of the workpiece surface, but protects the aluminum from corrosion.

Eloxing - Color and Surface Variants of Schüco International KG

The acronym of ectrolytic idation of uminium can be reinforced in a targeted manner by the anodizing, also called anodizing process (acronym of ectrolytic idation of uminium). In the classification of Production methods according to DIN 8580, the anodization counts as the 5th main group (coating). The anodization is assigned to the electroplating (5.9.1). With the other methods of surface technology, anodizing has in common that a targeted influence on the properties of the material surface is sought. In addition to the decorative effect, the surface properties often have a functional meaning:

Corrosion resistance
Wearing behaviour
Electrical contact resistance
Catalytic effect

Layer thicknesses of 5 to 25 micrometers can be achieved with anodizing. The oxide layer or anodoxal layer protects the workpiece from corrosion as long as its surface is intact.

Process sequence of anodizing

The anodizing can be divided into 4 steps, the color bath being considered optional. This process is only necessary if a specific coloring of the workpiece is intended.

Eloxal plant of Schüco International KG

Pretreatment

The pretreatment primarily serves to create a dust-free and fat-free surface as a starting condition for a good result. With a high optical claim to the workpiece, a chemical pretreatment can also be part of the process. Unevennesses in the surface caused by previous processing steps are thereby equalized by means of pickling.

Anodically Oxidizing

In the actual step of anodizing, the aluminum parts are immersed in an electrolyte (oxalic or sulfuric acid). The workpiece is connected as an anode, i.e. connected to the positive pole of a direct current source. Plates of lead which are not attacked by the electrolyte frequently serve as the cathode. If the current flows, water is electrolytically decomposed at the cathode. The oxygen at the anode reacts with the aluminum and the oxide layer forms.

Coloring

The capillary-like pores produced during anodically oxidizing in the workpiece surface can be partly filled with dyes by introducing them into a color bath. The color effect thus does not arise on the surface, but on the pore bottom.

Sealing

In the last step, the open pores are sealed in the workpiece surface. By boiling in demineralized water, a transparent aluminum oxide hydrate forms which closes the pores of the anodized layer.

Hardening between 200 and 400 HV can be realized by anodizing. A hardness of up to 600 HV can be achieved by means of a hard anodizing process.

Harteloxal from TK - Oberfläche GmbH

Anodized aluminium part of the company S + W Eloxal Oberflächentechnik GmbH

Applications of anodizing

The anodizing is used primarily as corrosion protection, but is also used in thinner layers for mainly decorative applications. The method is suitable for finishing pipes, profiles, rods, sheets and all other semi-finished products of aluminum. Its manifold advantages are the reason for the wide application of anodizing:

Optimal suitability for outdoor applications
No electrical conductivity
Very hard and scratch-resistant surface
High resistance to corrosion
Optically attractive surface
Very low layer thickness
No splintering or peeling

Eloxal with individually coloured surface - Holzapfel Group

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