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Anodising Process

The anodising process involves sequentially treating the aluminium extrusion in a series of long tanks (19 at Fletcher Aluminium's Auckland plant). Some tanks contain chemical solutions – either heated or operated at near ambient temperature. Others contain chemicals and have electrical current applied. The treatment results in a thickened oxide layer of between five and twenty five microns thick.

The anodising process has four major stages, which are explained in detail in this section:

1. Racking
2. Chemical cleaning and etching
3. Anodic oxidation (anodising)
4. Colouring and sealing

Each stage is followed by rinsing in town supply water.

1. Racking
Good electrical contact is essential for effective anodising. So the objective of racking is to obtain a secure contact between the aluminium to be anodised and the material carrying the electrical current from the power supply point.
The electrical contact will leave a contact point at each end of the anodised piece (within 25mm of the respective end). Sometimes with very flimsy sections, a central support wire is required. This will also leave a contact point.

2. Chemical cleaning and etching
The processes in this stage are degreasing (cleaning), etching (caustic etching) and desmut (deoxidise). These processes result in metal that is chemically clean and smooth with a satin finish.

3. Anodic oxidation (anodising)
Anodising itself is an electro-chemical process that allows anodic film to grow on metal. The anodic film is porous. So the oxidisation mechanism continues reaching the parent metal and the film grows in a controlled manner.
The anodic film is therefore integral with the parent metal from which it grows. There is no joint, as for example, in electroplating.
With the preferred aluminium alloy for anodising purposes (6060 T5), the anodic film is virtually transparent. So you can see the surface of the metal beneath the film. With stronger alloys, the anodic film is usually more opaque. This is due to some of the alloying constituents being trapped within the film.

4. Colouring and sealing
As anodic film is porous, anodised aluminium can be coloured in various ways. It must also be sealed before going into service, to prevent water from reaching the parent metal.

The preferred option for colouring at Custom Solutions is electrolytic colouring.

Electrolytic colouring involves electrodepositing stable metal compounds from a solution of metal salts deep into the pores of the anodic film, away from the effects of sunlight and corrosive elements. Electrolytic colours are limited to a range of bronzes and black.

After the colouring process and before an anodised product is used, the anodic film must be sealed to close the pores. This prevents water that may contain corrosive substances from reaching the parent metal.

Sealing can be done using boiling water or special chemicals. The Fletcher Aluminium plant uses special chemicals.