A unique combination of properties puts aluminium and its alloys among the most versatile of engineering and construction materials. All alloys are light in weight, yet some have strengths that are greater than structural steel. Most alloys can be easily worked into a variety of forms and can accept a wide range of surface finishes. The majority of alloys are highly durable under most service conditions. Aluminium and most of its alloys also have the benefit of good electrical and thermal conductivities and high reflectivity to both heat and light.
A key benefit of aluminium is only a small fraction of energy is required to recycle into new material, a major advantage in the present economic climate
THE ANODISING PROCESS
The sulphuric anodising processes first made their appearance around 1929. The thick, hard coatings produced could be sealed and were corrosion resistant. They have since been widely accepted for the treatment of aluminium in domestic, industrial and marine uses.
Aluminium has the natural ability to produce a protective oxide film on exposure to atmosphere. This natural oxide is hard and resistant to both water and normal atmospheric conditions, but the protection it offers is inadequate if other agents are present to start a corrosive attack on the aluminium. For this reason processes were sought whereby the natural oxide film could be reinforced by anodising to form a coating which is hard, could be coloured and was even more resistant to wear and corrosion; especially in aggressive coastal and industrial environments.
Anodising is an electro-chemical process whereby the aluminium surface is converted to a hard transparent film of oxide which is an integral part of the aluminium. The anodising process takes place in a diluted solution of sulphuric acid in which the aluminium forms the positive pole (the anode), and the electrode – the negative pole (the cathode) of the cell. A current is passed though the cell and oxide is formed in the pores of the aluminium surface.
The anodised aluminium can now be coloured or left in its natural silver colour, ready for sealing.
ANODISING AND COLOURING PROCESS
The process starts with a clean and rinse to remove cutting fluids etc from the substrate. An etch follows which is used to remove the naturally formed oxide layer and to give the metal an even matt appearance over the whole surface area. The metal is then rinsed. Continuing the process, the metal is placed in cold water to neutralize any residue from the etch. The metal is then rinsed again. The anodising process then takes place which is followed by a final rinse. The colouring process is next. This is achieved by using an electrochemical process to deposit nickel or tin into the open pores created by the anodising process. The pores are then sealed to ensure no airborne contaminants can enter which could cause premature attack of the newly formed oxide layer. The colour is retained and the metal is then rinsed
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