Finishing Aluminium – Mechanical and Chemical Finishes and Anodising Aluminium

As with other materials, finishes for aluminium are designed to either preserve an existing surface or promote a new one which is visually or functionally more desirable.

They are normally identified in one of four categories:

• Mechanical finishes
• Chemical finishes
• Anodising
• Applied coatings

The first three categories are covered in this article.

Mechanical Finishes for Aluminium

Being a metal, aluminium is amenable to mechanical finishing processes commonly used with other metals, and can usually be used with the same equipment without disadvantage, although to exploit the more unique characteristics of aluminium alloys, special equipment is frequently desirable.

Generally, mechanical finishes rely either on processes which remove metal by abrasion (grinding, polishing, buffing, blasting) or processes which apply a texture by mechanical force (embossing, shot peening etc). Compared with steel, abrasive finishing processes need to take into account the relative softness of aluminium, so, lubrication and lower application pressures are necessary.

Chemical Finishes

These rely on specific chemical reactions, and are peculiar to aluminium. There are five principal classifications of these:

• Chemical cleaning
• Etching
• Chemical brightening
• Conversion coatings
• Immersion plating

Chemical Cleaning

Treatments designed to remove soils and oxides inhibitive to final finishes or other work operations.

Etching

Treatments designed to produce various degrees of surface mattness.

Chemical Brightening

Treatments designed to produce a high surface lustre.

Conversion Coatings

Treatments which promote surface films desirable for functional or visual purposes.

Immersion Plating

Treatments which deposit metallic surface coatings.

Anodising

Once aluminium is exposed to air it immediately combines with the oxygen in the air to form aluminium oxide, an inert protective layer. In nature this layer of oxide is only millionths of an inch thick.

Anodising is a method of thickening the layer of aluminium oxide to provide greater protection. It is a controlled oxidation of the aluminium surface achieved by immersion in an electrolyte (usually dilute sulphuric acid).

Production of Anodised Aluminium Coatings

In the process, aluminium is used as an anode and a low voltage, high amperage, direct current is passed through the metal. A hard, inert oxide film initially forms on the aluminium surface and is followed by a less dense oxide layer in which there are capillary pores. The capillary pores allow oxidation to proceed further.

Unlike plating and painting which are coatings added on to the base metal, anodic coatings grow downward into the parent metal and are an integral part of the metal itself. Anodic coatings cannot lift, flake or peel.

Properties of Anodised Aluminium Coatings

After anodising, the oxide file is porous and it is through these pores that colour can later be added to the anodic film and in practice these pores must be sealed off to prevent the later entry of atmospheric contaminations or the leaching away of any colourants.

As the layer of aluminium oxide formed is so inert it might be asked why more than one thickness of film is required. Normally aluminium is alloyed with various constituents to give different characteristics.

The usual architectural alloy of 6060 contains both magnesium and silicon at about 0.5% each as a stiffening agent, plus a number of small impurities. These alloying elements form magnesium silicide which becomes trapped in the anodic film.

These particles can be as long as seven microns and tend to scatter light. Therefore, as a film becomes thicker it becomes duller, but in the thinner films the particles can extend the full depth of the film leaving it subject to corrosive attack.

For this reason the Australian Standards define the following areas:

Characteristics Anodised Coatings for Decorative Applications

• High lustre applications in a protected environment or subject to frequent washing (motor car trims, appliances, decoration, shower screens etc). - 5 MICRON

Characteristics Anodised Coatings for Architectural Applications

• Internal partitions, handrails, internal fittings subjected to robust handling, or frequent washing. Not for industrial/marine environment. - 10-15 MICRON
• External fittings such as window/door assemblies - commercial. Infrequent cleaning. Subject to wind abrasion. - 20-25 MICRON

Characteristics Anodised Coatings for Engineering Applications

• "Hard coatings". - 50-100 MICRON

Protecting Anodised Coatings

Dust particles which settle on the surface of the film, if not washed off, will hold moisture. This moisture, in turn, will absorb air born contaminants such as sulphur dioxide.

Due to a small amount of oxygen also absorbed and its relationship to the large amount of oxygen in the atmosphere, a field of electrical potential will be created which will attack the film.

Frequent washing will remove these particles and ordinarily this attack is self limiting. However should the film be too thin, or sufficiently abraded so that the alloying elements are closed to or at the surface, and close to the dust particle, crevice corrosion can occur which will reach the base aluminium.

Aluminium Extrusion Alloys for Decorative and Architectural Applications Suited to Anodising

The most appropriate extrusion alloys for decorative and architectural anodising are those in the 6000 series.

Other alloys may be anodised but the finish cannot be guaranteed to meet requirements.

Anodising Aluminium and Weld/Braze Joints

In anodised components, the heat affected zones of welded or brazed joints will show a variation in colour from the remainder of the section. This can vary from a slightly darker tone to a very dark grey, or even black when a silicon filler wire is used in brazing.

Colour Variations between Batches

Since there may be slight variations in colour between batches of anodising, top and bottom colour limits should be agreed upon between purchaser and supplier.

Colour Variances between Cast and Wrought Aluminium Alloys

Where cast and wrought components are anodised and used in combination, an exact colour match is rarely possible because of the marked difference in the chemical composition of the two materials.

This information has been sourced, reviewed and adapted from materials provided by Capral Aluminium - Aluminium Extrusions.

For more information on this source, please visit Capral Aluminium - Aluminium Extrusions.