About Anodising

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Anodising is an electrochemical process used to produce durable and decorative finishes on components made of aluminium or aluminium-based alloys. Aluminium, in an untreated form produces a protective oxide coating on exposure to the atmosphere. This oxide is inherently protective, but being very thin provides little resistance to long term corrosion. The coating can, however, be thickened by anodising. Sulphuric acid anodising which is extensively used for general industrial and architectural applications, produces a coating (typically silver or pale grey) which can be left natural or dyed to produce a wide range of colours.

(various other colours can also be matched providing sample colours are provided)

The Anodising Process.

The principle of anodising developed out of aluminium's ability to form this oxide layer which can be electrochemically produced under controlled conditions to provide thick protective coatings. In essence the process consists of passing an electric current through an acid solution in which the immersed aluminium component acts as the anode, and aluminium or lead typically provides the cathode.

During the process electrolysis breaks down the water into oxygen and hydrogen. The oxygen combines with the aluminium to produce aluminium oxide and the hydrogen is liberated.

The aluminium component is loaded onto an appropriate jig or racking system.
The jig allows the electric current to pass from the rectifier power source to the work pieces. The jig also enables the components to be physically handled throughout the process.

Effective jigging is an essential pre- requisite of successful anodising
The component is pretreated
The appearance of the aluminium surface will depend markedly on the alloy type being used.

Once loaded on to a jig and pretreated, the aluminium component is immersed in the dilute acid solution and subjected to the passage of an electrical current. The type of acid used, the electrical conditions, the alloy type, and the processing time will all affect the properties and thickness of the anodic oxide coating, and coating thicknesses can be produced that range from a few micrometers to around 75 micrometers (0.0003").

The table below indicates the range of coating thicknesses typically called for :

The thickness of the oxide coating will have a direct effect on dimensions and tolerances on a coating thickness of AA10 (10 microns. i.e. 0.010 mm, 0.0004"), we can represent the formation of the coating thus:

The anodic coating can be chemically stripped from the aluminium but please note, the specification and quality of alloy used affects the cosmetic appearance of the stripped component.

Under high magnification the anodic oxide coating resembles a sponge with a very fine columnar structure. This columnar structure enables the anodic oxide coating to absorb dyes and pigments into its pores. The aluminium component can therefore be dyed during the process, giving an extremely attractive cosmetic finish.

Benefits of Anodising!

The anodic coating is much harder than the underlying aluminium providing protection against handling and abrasion. The resistance to wear depends upon the hardness and thickness of the coating.
Anodising protects aluminium against many forms of corrosion. In industrial and marine atmospheres it is essential that the coating should be of adequate thickness and correctly sealed. As a general rule a film thickness of 25 microns is recommended for normal exterior applications.
Anodic coatings have a dielectric constant of 8 to 10 and a voltage breakdown of about 50 volts per micrometre of film thickness in a dry atmosphere.
Very thin anodic coatings of the order of 1 - 2 micrometers have excellent reflectivity for infrared radiation. Thicker anodic films of 5 micrometers and over are good heat emitters or absorbers having emissivity exceeding 70%. The thermal conductivity of the anodic coating is about 1/10th that of aluminium metal but it has been found that anodic coatings of 5 microns and thicker produce an excellent heat dissipating surface where the maximum rate of radiant heat transfer is required. A black anodised finish is therefore excellent for heat sinks.
A wide range of anodic coatings having different light reflecting properties can be produced by choosing metal in the correct purity and applying the appropriate pretreatment.