Anodising aluminium

Anodising aluminium (anodising) is one of the most important processes for protecting aluminium permanently and enhancing its appearance. Anodic oxidation creates a hard, dense oxide layer – the so-called anodic coating. This anodic layer is firmly bonded to the base material, increases the strength of the surface, protects against corrosion and enables a wide range of colours and surface effects.

If you use aluminium parts – such as CNC-milled parts, turned parts, profiles or sheet metal parts – anodising is often the first choice: for long-lasting components in mechanical engineering, high-quality housings, design components or technically stressed functional surfaces.

In this article you will receive a comprehensive overview of the anodising of aluminium: from the basics and layer thicknesses through to colours and strength and on to typical applications in industry.

Basics: What happens during the anodising of aluminium?

Anodising is an electrochemical process in which the natural oxide layer of aluminium is deliberately reinforced. Aluminium already forms a thin protective layer in air, but during anodising this process is technically controlled so that a substantially thicker, uniform and extremely resistant oxide layer is created. To this end, the components are first thoroughly cleaned, degreased and, depending on the desired subsequent appearance, pickled or given a bright pre-treatment. They are then hung as the anode in an electrolytic bath – usually based on sulphuric acid – and treated under direct current. In the process, a porous oxide layer grows that is firmly bonded to the base material and cannot flake off.

After oxidation, the layer remains open-pored so that it can be coloured, for example in black, bronze, gold or other decorative shades. In a final step, the layer is sealed, usually by hot water or steam. This closes the pores, which makes the resulting surface significantly more resistant to abrasion, moisture, environmental influences and UV radiation. The result is a homogeneous, robust and visually high-quality aluminium surface that convinces both functionally and decoratively and is particularly well suited to precise turned parts, milled parts and technical components.

Anodic layers at a glance: layer thicknesses, hardness and resistance

The properties of anodised aluminium depend decisively on the layer thickness and the process.

Typical layer thicknesses in anodising

• Decorative anodising: approx. 5–15 µm Suitable for visible parts in interior areas, design components, furniture profiles, housings with low mechanical load.
• Technical anodising: approx. 15–25 µm Standard for many industrial applications, e.g. machine components, housings, guide elements, profiles in normally loaded environments.
• Hard anodising (hard anodisation): approx. 30–60 µm (in some cases more) For highly loaded functional surfaces, sliding and guideways, heavily stressed components in automation, pneumatics, hydraulics or in toolmaking.

Hardness and wear resistance

The hardness of an anodic layer is, depending on the process, significantly above that of the base material. Classic hard anodic layers achieve microhardnesses in the range of approx. 300–500 HV and are therefore significantly more abrasion-resistant than untreated aluminium.

Advantages at a glance:

• High abrasion resistance, especially with hard anodising
• Lower wear on sliding and guide surfaces
• Higher surface strength, e.g. in clamping and gripping areas

Corrosion protection

Anodised aluminium offers significantly improved corrosion protection:

• The dense anodic layer protects against moisture and environmental influences.
• Sealing closes the pores – the surface becomes more resistant to media such as water, humidity or certain cleaning agents.
• In combination with suitable alloys, stable results can also be achieved in demanding environments (e.g. industry, outdoor areas).

Colours in anodising: scope for design in function and design

A key advantage of anodising is the possibility of finishing aluminium components in a wide variety of shades without losing the metallic appearance. In electrolytic colouring, metal particles such as tin or nickel are deposited in the pores of the anodic layer, which leads to very durable, UV-stable shades that are particularly suitable for outdoor areas. Typical colours range from black through bronze and champagne to various shades of grey. In organic colouring, dyes are introduced into the open-pored anodic layer, creating a great variety of shades – including strong colours such as red, blue or green.

This colouring appears particularly intense, but is less UV-stable and therefore suitable above all for interior applications and decorative components. A special process is interference colouring, in which brilliant, iridescent shades are produced through defined layer thicknesses and light refraction. This process is preferably used in architecture and for high-quality visible parts when a special visual effect is to be achieved. Overall, anodising thus opens up diverse design possibilities that optimally combine both functional requirements and aesthetic demands.

Influence of the alloy: Why not every aluminium anodises the same

Not every aluminium alloy reacts the same way to the anodising process. Alloys with a high copper, silicon or zinc content can:

• produce darker, inhomogeneous shades
• show streaks, spots or clouding
• form different layer thicknesses and hardnesses

Proven for anodising aluminium include:

• EN AW-6060 / 6063: often for profiles and visible parts, good decorative results
• EN AW-6082: widely used in mechanical engineering, good combination of strength and anodisability
• EN AW-5005 (for sheets): specially for anodisable sheets with uniform colour effect

If you are planning to anodise aluminium, it is worth coordinating the alloy, surface requirement and colour with the processor or anodising plant at an early stage.

Dimensional accuracy and tolerances: Planning the anodic layer correctly

Dimensional accuracy plays a central role in anodising, since the anodic layer does not simply rest on top as an additional coating, but partly grows into the base material and partly outwards. Typically about one third of the layer grows outwards, while two thirds are formed within the material, which means that the overall dimensions of a component can change slightly. With a layer thickness of 20 µm, deviations of around 5–10 µm outwards per side often result – a value that should definitely be taken into account for precise fits, bores or guides. The internal structure of the layer can also influence the final fit accuracy, which is why sensitive areas such as precise bores, guides or functional surfaces must be planned accordingly in advance or, if necessary, reworked.

Threads present a particular challenge, since the anodic layer in them can impair functionality. Threads are therefore often cut after anodising or specifically masked during the process in order to ensure flawless screwability. Sharp edges should also be slightly broken, since the anodic layer can turn out too thin on very sharp edges, which reduces resistance in the long term. Overall: the tighter the tolerances and the more complex the fits, the more important careful coordination between design, machining and the anodising plant.

Strength & surface properties: What does anodic coating deliver in everyday use?

Anodised aluminium surfaces offer a combination of high mechanical load capacity, improved wear resistance and durable corrosion protection. The anodic oxide layer significantly increases the hardness of the surface, especially with hard anodising, as a result of which components remain dimensionally accurate for longer and functional surfaces wear only slowly even under intensive use. CNC-milled parts and turned parts benefit from this, since sliding surfaces, edges or clamping areas remain stable for longer and retain a high-quality appearance. At the same time, anodised aluminium is less susceptible to scratches and frictional influences than untreated or painted aluminium.

The strength of the anodic coating is also evident with regard to corrosion resistance: the sealed oxide layer effectively protects against moisture, environmental influences and many media, as a result of which components can be used reliably in the long term. In demanding environments such as industrial plants, outdoor areas or coastal regions, the combination of a suitable alloy and optimum layer thickness is decisive. Added to this is the electrical insulating character of the anodic layer, which, depending on the application, can be either a functional advantage or a design requirement. While insulating surfaces make housings and components safer, for EMC-relevant applications or defined conductivity certain contact points must be specifically masked or machined afterwards.

Anodising in practice: possible applications in industry and design

Anodised aluminium is standard in many sectors. Typical fields of application:

• Mechanical engineering Guide and sliding surfaces, machine frames, mounts, clamping devices, covers and protective hoods.
• Automation and plant engineering Profiles, sensor holders, gripper parts, assembly components.
• Electronics and housing technology Housings for controllers, power supplies, electronic modules, heat sinks – functionally and visually high-quality.
• Medical technology Components with high hygiene requirements, easy-to-clean surfaces, colour-coded components.
• Vehicle construction and mobility Lightweight components, holders, profiles, add-on parts in interior and exterior areas.
• Architecture, furniture, design Visible profiles, handles, panels, façade elements, furniture parts – often in coloured anodised finish.

If you have aluminium anodised, you can combine technical requirements (strength, corrosion protection) with a clearly defined appearance – including corporate colours, contrast surfaces or functional coding.

Anodising vs. powder coating vs. painting

Process	Advantages	Disadvantages	Typical layer thickness	Suitable for
Anodising	Very abrasion-resistant, metallic appearance is retained, firmly bonded to the material, good dimensional accuracy	Limited colour variety with UV-stable colours, dependent on alloy	5–60 µm (depending on process)	Precise CNC parts, technical components, housings, profiles
Powder coating	Large selection of colours and structures, also suitable for alloys that do not anodise well	Thick layer, less suitable for fits, can flake off	60–120 µm	Visible parts, large-area components, robust outdoor applications
Painting	Maximum colour variety, effects possible, also on complex geometries	Lower abrasion resistance, sensitive to scratches, risk of flaking	20–80 µm	Design parts, one-offs, components without high mechanical load

Collaboration with the anodising plant: What you should clarify in advance

So that your anodised aluminium parts correspond exactly to your requirements, you should define a few points at an early stage:

• Desired layer thickness (e.g. 15–20 µm or hard anodising 40–50 µm)
• Colour and area of use (indoor/outdoor, decorative/technical)
• Alloy and pre-treatment (ground, blasted, milled, turned)
• Requirements for dimensional accuracy and fits
• Marking or colour coding of functional surfaces

The more precisely the requirements are described, the more reproducible the results – especially in series production of milled parts, turned parts or aluminium profiles.

Typical sources of error and how to avoid them

Similar problems occur time and again when anodising aluminium. These include:

• Inconsistent colours: due to different alloys, batches or pre-treatments.
• Visible machining marks: scores, milling marks or grinding marks often become more visible through anodising – clean surface preparation is decisive.
• Edge flaking with other processes: not with anodising itself, but with incorrectly chosen alternatives such as paint, flaking can occur.
• Unplanned dimensional changes: failure to take the layer thickness into account with tight tolerances.

If you align your design to anodising from the outset, many of these errors can be ruled out from the start.

Practical recommendations for anodised turned parts and milled parts

For anodised turned parts and milled parts made of aluminium, well thought-out preparation is decisive in order to reliably meet both technical and visual requirements. The right alloy forms the basis: while EN AW-6082 is particularly suitable for technical and mechanically loaded components, EN AW-6060 and 6063 deliver very good results for profiles and decoratively demanding visible parts. Breaking the edges is equally important, since sharp edges weaken the anodic layer and can favour uneven results. A cleanly defined surface quality – whether fine-turned, ground or blasted – ensures that the subsequent appearance looks homogeneous and can be reproduced without difficulty.

During planning, consideration should also be given to how and where the component is hung during anodising, since contact points can inevitably leave small marks. Fits and functional surfaces also require particular attention: they should either be designed generously in advance, masked during the process or reworked after anodising in order to maintain the required precision. Overall, the best results are achieved when the requirements are already taken into account in the design and coordinated with the anodising plant at an early stage.

Anodising aluminium – technical function and high-quality appearance in one step

The anodising of aluminium combines corrosion protection, wear resistance and design freedom in one process. Through the targeted choice of layer thickness, colour, alloy and pre-treatment, precisely coordinated solutions can be realised for the most diverse applications.

Whether decorative coloured anodising for visible parts or hard anodising for highly loaded machine elements: if you have aluminium anodised, you increase the service life of your components, improve the strength of the surface and create a professional, long-lasting appearance – ideal for modern turned parts, milled parts, profiles and assemblies in industry.

FAQ – Frequently asked questions about the anodising of aluminium

What advantages does anodic coating have over paint or powder coating? Anodic coating is harder, more abrasion-resistant and firmly bonded to the base material. The layer cannot flake off, the metallic appearance is retained, and dimensional accuracy can be better controlled.

Which aluminium alloys are best suited to anodising? Proven in practice include EN AW-6060, 6063, 6082 and special anodisable sheets such as EN AW-5005. They deliver uniform colours and high-quality anodic layers.

How do the dimensions change during the anodising of aluminium? The layer grows partly inwards, partly outwards. With typical layer thicknesses of 15–25 µm, small but relevant dimensional changes result, which should be planned for with tight tolerances.

Is anodised aluminium electrically conductive? The anodic layer itself is electrically insulating. If a component is to remain conductive, contact points must be machined accordingly, masked or exposed afterwards.

Which colours are possible when anodising aluminium? From natural tone (silver) through black, bronze, champagne and gold to many organic shades, much is possible. For outdoor applications, electrolytically coloured, UV-stable colours are to be preferred.

When is hard anodising worthwhile? Hard anodising is worthwhile whenever aluminium parts are exposed to high mechanical loads, friction or heavy stress from media – for example with sliding surfaces, guides, tool holders or heavily stressed machine parts.