What does anodised layer thickness mean for aluminium sheets?

What is an anodised layer?

Anodising (also known as anodisation) is an electrochemical process that controllably oxidises the surface of aluminium. This creates a protective oxide layer that is firmly bonded to the base material. This so-called anodised layer improves resistance to corrosion, wear and mechanical influences.

Why is the layer thickness important for aluminium sheets?

The thickness of the anodised layer has a direct influence on the properties and the service life of an aluminium sheet. Depending on the requirements placed on the component, too thin a layer thickness can offer insufficient protection, while too thick a layer can make mechanical post-processing more difficult. Particularly in industrial use, the right balance is what matters: while in the decorative sector just a few micrometres are often sufficient, components for outdoor use or with high mechanical loading require significantly thicker layers. The anodised layer therefore influences not only the resistance, but also the functionality and appearance of the aluminium sheet.

How is the anodised layer formed?

During anodising, the aluminium sheet is immersed in an acidic electrolyte bath, typically sulphuric acid, and connected as the anode in a direct current circuit. The electrical voltage initiates targeted oxidation on the surface of the aluminium. This forms a porous yet dense oxide layer that grows from the inside out. The depth of the pores and thus the layer thickness can be influenced by duration, current intensity and electrolyte composition. In a final step, the anodised layer is closed by so-called sealing, which seals the pores and further significantly improves corrosion resistance.

Typical anodised layer thicknesses at a glance

Application	Usual layer thickness	Notes
Decorative applications	5 – 10 µm	for indoor areas, low stress
Components with medium load	10 – 20 µm	standard for many construction applications
High stress/industry	20 – 25 µm	high abrasion & corrosion resistance
Special applications	> 25 µm	e.g. seawater environment, engineering

Influence of the anodised layer thickness on the properties

A thicker anodised layer brings several advantages:

• Increased corrosion protection, e.g. against moisture or chemical exposure
• Better abrasion resistance, especially in moving parts or friction
• Increased hardness of the surface
• Improved electrical insulation, depending on the design

However, too thick a layer can lead to stress cracks during mechanical machining (e.g. drilling, milling).

How is the anodised layer thickness measured?

The layer thickness is usually stated in micrometres (µm). Both non-destructive and destructive methods are used for testing. The eddy current method is a common method for non-destructive measurement and is particularly suitable for serial inspections. Here, an electromagnetic field is generated via a probe, and its interaction with the metallic layer is measured. Alternatively, a micrograph of a cross-section and microscopic evaluation can provide a very exact statement about the layer thickness – but only once and with material loss. The choice of measurement method depends on the field of application and on the required quality documentation.

Standards and specifications for layer thickness

Binding guidelines and standards exist for anodised aluminium sheets, defining the permissible anodised layer thicknesses and tolerances. Particularly relevant here is DIN EN ISO 7599, which specifies requirements for decorative and protective anodising. Depending on the stress class, it regulates which minimum layer thicknesses must be maintained. Further standards such as DIN 17611 or Qualanod certifications additionally regulate aspects such as adhesion, colouring and sealing quality. These standards ensure that anodised aluminium sheets offer consistent quality and reliability even in series production and are measurable and usable in international comparison.

Selection criteria for the right anodised layer

When choosing the right aluminium sheet with an anodised surface, the following should be considered:

• Place of use: indoor or outdoor?
• Stress: mechanical, chemical or climatic?
• Design requirements: visible parts often require flawless, uniform surfaces
• Machining: will the sheet be milled or drilled afterwards?

Conclusion: the layer thickness determines the suitability for use

The anodised layer thickness is a decisive criterion for the function, service life and quality of aluminium sheets. Those who select in a targeted way save costs in the long term and increase the operational safety of their constructions.

For technical applications, detailed coordination between manufacturer, designer and user is advisable in order to achieve the optimum combination of material, layer thickness and post-processing.