Even before surface treatment, the appearance and surface quality of extruded aluminium profiles is perfectly satisfactory for many applications. Thanks to good corrosion resistance, surface treatment is rarely necessary simply to provide corrosion protection. However, there are many other reasons for treating the surfaces of profiles. Examples of attributes that can be changed by surface treatment include:
– surface structure
– colour
– corrosion resistance
– hardness
– wear resistance
– reflectivity
– electrical insulation
Surfaces do not always need treatment after extrusion. Load-bearing structures and machine parts are examples of products where the surface quality is satisfactory without any treatment.
Anodising, one of the most common surface treatments, is used to (amongst other things):
– maintain a product’s “as-new” appearance.
– enhance corrosion resistance.
– create a dirt repellent surface that satisfies stringent hygiene requirements.
– create a decorative surface with durable colour and gloss.
– create a “touch-friendly” surface.
– create function-specific surfaces, for example, slip surfaces, abrasion-resistant surfaces for use in machine parts, etc.
– give surfaces an electrically insulating coating.
– provide a base for the application of adhesives or printing inks.
Recommended layer thickness when anodising
The anodising process
There are normally four stages in the process: pre-treatment, anodising, colouring (where required) and sealing. The most frequent type of anodising is natural anodizing or silvery anodized. The electrolytic process takes place once the metal surface has received the appropriate mechanical or chemical pre-treatment and has been thoroughly cleaned.
The profile is connected to a direct current source and becomes the anode (hence anodising). An electrolytic cell is formed. Dilute sulphuric acid at room temperature is normally used as the electrolyte. During electrolysis, the surface of the metal is oxidised. The process continues until the desired layer thickness (usually 5 – 25μm) is reached.
Maintenance – cleaning
The anodic oxide layer has good corrosion resistance in most environments. With the proviso that the surface is cleaned, anodized profiles are virtually maintenance-free. The surface cleans easily in both water with a little neutral detergent and in white spirits. Although solvents do not affect aluminium, strong alkaline solutions should be avoided.
Resistance to corrosion, discoloration and abrasion increases with layer thickness. Recommendations for suitable thicknesses are given in the table above.
As the anodic oxide layer has poor cold formability, forming should take place before anodising. Cutting and drilling can be carried out after anodising but the exposed surfaces will, of course, be untreated. Welding is to be carried out before anodising.
Properties of anodized aluminum
Corrosion resistance is very good, especially where pH is between 4 and 9. In contact with strongly alkaline substances, surfaces can stain and be damaged. Thus, it has to be borne in mind that aluminum should be protected against lime, cement and gypsum (e.g. on building sites). Visible surfaces can be protected using tape.
The hardness of the oxide layer depends on the anodising process used. Generally, the layer is harder than glass and as hard as corundum. The oxide layer is transparent. Whether natural or coloured, its appearance depends on the viewing angle.
At temperatures above 100°C, fine cracks form in the oxide layer. From an aesthetic point of view, this may be an undesirable effect.
The reflectivity of bright etched aluminium is high. The gloss value is 90 units (ISO 7599, 60° viewing angle). This decreases slightly with anodising. The oxide layer is an electrical insulator. A sealed, 15μm oxide layer has a breakdown voltage of 500 – 600 V. An anodised profile can be recycled with no pre-treatment. Before remelting, painted profiles must first have the paint removed.
Painting offers a limitless choice of colours and very good colour matching (repeatability). Powder coating is now easily the most widespread method of painting aluminium profiles.
To ensure the right adhesion for the paint, it is important that pre-treatment, paint application and subsequent curing are all carried out correctly. As maximum adhesion and durability are prime goals, pre-treatment is of crucial importance. Pre-treatment normally comprises degreasing and pickling of the surface, followed by a chemical treatment.
The chemical treatment (chrome-free or chrome-based) gives good adhesion and effective corrosion resistance. The chrome-free titanium based process is GSB approved and is now our standard method. It has undergone extensive testing.
Rinse water from the chromating process is treated in efficient cleaning plants. The sludge is drawn off and sent away for appropriate disposal. Pre-treatment is the same for both powder coating and wet painting.
Broadly speaking, there are absolutely no limits to the choice of colour. Powder coatings are applied and cured without solvents. This gives a good work environment and has no negative impact on the external environment.
In a wet coating plant, half the paint is lost through evaporation and the waste involved in over-spraying. In a modern powder coating plant, up to 98% of the powder is used. Powder that does not adhere to the product is recirculated via a reclamation system.
Powder coating qualities
The prime qualities of powder coating and powder coats are:
– No risk of running or blistering.
– High repeatability.
– Powder coatings withstand knocks and abrasion far better than
wet paint coatings.
– Good formability (e.g. can be formed after coating).
– Suitable for outdoor use – good resistance to UV and corrosion.
Coating thickness is normally 60 – 140μm. In some designs, the thickness of the coating has to be taken into consideration when determining profile dimensions and tolerances.
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