Aluminium anodising is one of the most commonly used methods for protecting and improving the surface properties of aluminium components. In industrial practice, we most commonly encounter two types of anodising: standard (Type II) and hard (Type III). Although both processes are based on the same electrochemical principles, their results, properties and applications are diametrically opposed. So when should one choose Type II anodising, and when Type III?
In this article, you will learn:
Anodising is an electrochemical process involving the controlled oxidation of an aluminium surface, the aim of which is to produce a durable, uniform layer of aluminium oxide. Unlike coatings applied to the surface, the anodised layer is formed by transforming the base material itself, which ensures excellent adhesion and resistance to mechanical damage. This process is extremely important in modern industry, as aluminium – despite its many undeniable advantages – is, in its raw state, susceptible to scratching, abrasion and environmental factors.
The anodised layer increases the component’s durability, improves its tribological properties and allows the surface to be given a specific colour. Depending on the process parameters, it is possible to obtain coatings that are either decorative or highly functional, designed for use in demanding industrial conditions.
The anodising process involves immersing an aluminium component in an electrolyte and passing a direct current through it. The aluminium acts as the anode, which means that a controlled oxidation reaction takes place on its surface. This reaction results in the formation of a porous aluminium oxide structure, which grows both into the material and on its surface.
A characteristic feature of the anodised layer is its microporous structure. This enables colouring and additional sealing of the coating, which increases corrosion resistance. It is the control of the process parameters that determines whether the resulting layer will be thin and purely decorative, or thick and highly wear-resistant.
The selection of the electrolyte and process parameters determines the properties of the final coating. The differences between Type II and Type III anodising stem mainly from the bath temperature, current density and duration of the process.
The most important parameters of the anodising process are:
The thickness of the anodised layer is one of the key parameters determining the properties of the component. Thin coatings primarily provide corrosion protection and aesthetic benefits, whilst thick coatings increase resistance to wear and mechanical stress.
In industrial practice, the choice of coating thickness is always based on an analysis of the component’s operating conditions. Different requirements apply to decorative components than to those forming part of mechanical systems.
Type II anodising focuses primarily on aesthetics and basic corrosion protection. The anodised layer is porous in nature, allowing it to be coloured in various shades. This solution is widely used in the consumer electronics sector, architecture and for the visual elements of devices.
In manufacturing, this process enables a consistent visual finish whilst maintaining good weather resistance. Soft anodising in 12 colours is particularly popular, as it allows the appearance of components to be tailored to the product’s visual identity.
Type II anodised coatings are relatively thin, which is due to their intended use. The 5–25 µm range provides effective protection against corrosion but does not significantly increase abrasion resistance. This thickness allows for the maintenance of high surface quality and dimensional precision of the component.
Thanks to the thin coating, Type II anodising can be used on components requiring high assembly accuracy. This process does not cause significant dimensional changes, which is crucial in mass production.
One of the greatest advantages of decorative anodising is the ability to colour the surface. The porous structure of the layer allows dyes to be introduced, which are then fixed during the sealing process.
This makes it possible to achieve a wide variety of colours – from natural silver, through black, to vibrant corporate colours. The colour is an integral part of the layer, so it does not flake or chip.
The anodised layer effectively protects aluminium against moisture and weathering. The sealing process closes the pores in the layer, significantly increasing its corrosion resistance.
In practice, this ensures the long-term durability of components used outdoors and in damp environments.
Type III anodising is a process designed to deliver maximum durability and wear resistance. It is used for components operating under demanding mechanical and environmental conditions. This solution is specifically designed for the industrial sector – particularly where aluminium is used as a substitute for steel.
It is precisely hard anodising for industrial parts that allows for a significant increase in the durability of components whilst maintaining their low weight.
A Type III anodised coating is many times thicker than that used in decorative anodising. Such a high thickness significantly increases wear resistance, but at the same time requires design tolerances to be taken into account.
A thick layer also means greater resistance to chemical and environmental factors.
A Type III coating achieves a hardness similar to that of ceramics. This enables aluminium to operate under conditions of intense friction. This significantly extends the service life of mechanical components and reduces the need for their replacement.
The anodised layer has very good insulating properties. This is crucial in electronics and the power industry. The coating protects components against short circuits and current flow.
The choice between Type II and Type III anodising depends on the component’s operating conditions. In many projects, it is essential to strike a balance between aesthetics, durability and cost:
It is clear that decorative Type II anodising is used wherever not only corrosion protection but also aesthetic appearance is important. For this reason, it is used:
Hard anodising (Type III) is used in demanding industrial applications. The use of hard anodising enables the production of:
Decorative anodising is cheaper and quicker; however, hard anodising significantly extends the service life of parts, which in many applications translates into operational savings.
The final cost is influenced by factors such as coating thickness, batch size, surface preparation, quality requirements and the need for additional processing steps. We must emphasise here that the anodising process is closely linked to prior CNC machining – the quality of the surface prior to anodising directly affects the quality of the final coating. This is why, in industrial projects, solutions such as comprehensive aluminium machining with anodising carried out in a single location are increasingly being chosen. This helps to reduce logistical costs and shorten lead times.
Michał Adamczyk, Anodising Plant Manager at RADMOT, explains that:
“In production practice, the most important thing is to view anodising in the context of the entire project. Customers often compare only the price of the process, whilst overlooking the savings resulting from the greater durability of the parts. At RADMOT, we always help to select a solution so that the total cost of ownership of the component is as low as possible.”
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Which type of anodising should be chosen for components in service?
Type III anodising is recommended for components in service. It provides greater resistance to wear.
Is Type II anodising sufficient for corrosion protection?
Yes, it is sufficient for most applications, particularly for decorative purposes.
What colours are available for Type II anodising?
A wide range of colours is available, although the most popular are black, silver and graphite.
Can Type III anodising be coloured?
Colouring options are limited. Natural or black are the most commonly used colours.
How does the cost of anodising compare to other protective coatings?
The cost depends on the thickness of the layer and the size of the batch. It is often more cost-effective than paint coatings.