When one wants to improve the qualities of an aluminium part, the most commonly used process is undoubtedly Anodising, or Anodic Oxidation. Conceptually, we can liken it to other metal surface treatments such as Nickel Plating, Burnishing, Phosphating and Chrome Plating, although each differs in the substances used and specific procedures. What they all have in common is the creation of a thin surface layer that refines the properties of the workpiece being processed.
Below we go into detail on Anodising, both Hard and Natural, with its characteristics and differences.
What Is Anodising Good For?
Anodising is an electro-oxidation process used to create a layer of aluminium oxide on the surface of certain materials, primarily aluminium alloys. The treatment involves immersing the workpiece in an electrolytic solution and then applying an electric current.
These operations result in the formation of the porous anodic oxide film, which has high hardness, wear resistance and corrosion resistance. The coating also acts as an electrical insulator, perfect for applications where a reliable dielectric barrier is required.
It should be noted that processing can be influenced by a number of factors, including the chemical composition of the aluminium, the temperature of the electrolyte, the duration of the process and the intensity of the electric current used.
However, when done correctly, anodising provides excellent protection for aluminium alloys while improving their mechanical and chemical properties. For this reason, it is advisable to only turn to professionals with a proven track record in precision mechanics.
How Anodising is Done
To achieve effective anodising, it is essential that the surface of the workpiece to be treated is completely clean. It is therefore necessary to carry out preliminary treatments in order to degrease and clean: a seemingly trivial step, without which it would not be possible to proceed optimally. These operations may include degreasing, alkaline pickling or acid pickling.
This is done by immersing the workpiece in a 20% sulphuric acid solution, then releasing electric current: anodic current is applied to the part, cathodic current to the cathode. In doing so, oxygen ions are released, which in turn create bonds with aluminium atoms: what results is the aluminium oxide that coats the surface of the part.
The steps in the process do not end there and further work may be required. One of the characteristics of the oxide layer, in fact, is its porosity, which can be exploited for colouring. Thus the process takes on the features of an aesthetic as well as functional action, acting on the appearance of the part.
After being applied, the colour is then fixed by hot or cold fixing: the former uses water at 95 degrees; the latter a nickel fluoride solution at room temperature. The choice of one or the other depends on the intended use of the artefact.
Depending on the intensity of the treatment, a distinction is made between Hard Anodising, which produces a thicker and stronger oxide layer, and Natural Anodising, which is mainly used for aesthetic purposes.
Hard Anodising is mainly used in the industrial sector or in contexts requiring an intervention aimed at improving the physical-mechanical properties of a part. The structure of aluminium oxide is identical in both types of processing. It consists of hexagonal columns arranged in a honeycomb pattern, and the hollow space within them is commonly referred to as the 'pore'.
In the case of Hard Anodising, the pores are small, generally between 20-40 nm, which gives the layer a dense and compact configuration. This, together with the increased thickness of the film, contributes to its high strength. However, these factors also lead to a dark colour, tending to black, which makes it difficult to dye the part.
Natural Anodising, or Decorative Anodising, is instead indicated for ornamental applications. The main objective of the treatment is not to improve performance, but instead aims to standardise and enhance the appearance of the part.
The protective power of Natural Anodising is reduced compared to its counterpart, due to the larger pore size, which usually ranges between 50-100 nm. Consequently, the thickness also decreases: these characteristics affect the aesthetics of the surface, which becomes light grey and easy to dye.
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