What is anodising?
Anodising is the process by which the natural film on aluminium is greatly increased in thickness.
Aluminium metal is on the anodic side of the galvanic series. Its position is similar to zinc and magnesium, ie it is readily oxidised. The oxide on aluminium is naturally corrosion resistant, very hard, abrasion resistant, an insulator and very tenacious. In its natural form the oxide film on aluminium is less than 0.50 microns thick.
Because the naturally occurring film is very thin and attached to a soft ductile metal, it is easily damaged. Building up this coating provides very useful properties for the aluminium surface.
The basis of every good coating is preparation. In anodising, good preparation is essential. Oils and greases are removed in weak alkali solutions and the surface is etched to remove heavy oxides. After rinsing, the aluminium is dipped into a desmut solution to remove the insoluble components of the aluminium which remain on the surface after etching, rinsed and presented for anodising.
How is it done?
Anodising is electroplating in reverse. During anodising the part is made the anode (positive electrode) in an electrolytic cell.
The aluminium is immersed in an electrolyte consisting of an acid/water solution. A range of acids may be used, eg sulphuric acid for relatively soft, easily dyed coatings and organic acids for hard integral coatings. The temperature of the solution is controlled to give the desired properties, eg at 20oC a sulphuric acid anodising solution will give a soft, transparent clear, easily dyed coating whereas at 5oC a hard, dense, dull grey coating is produced (hard anodising).
A DC (direct current) electric current is passed between the aluminium that is made the anode (positive terminal), the electrolyte and a cathode (often lead).
When the current is applied, the water in the electrolyte breaks down and oxygen is deposited at the anode. This oxygen combines with the aluminium to form oxide and thus builds on the oxide film always present on the surface.
The acid in the electrolyte tries to dissolve this oxide and produces a porous oxide film on the aluminium surface. Coating thickness up to 25 micron is recommended for external use. The oxide grains are hexagonal in shape and each grain contains a hexagonal hole within it.
Once the required thickness of anodic film is obtained, the aluminium is removed from the electrolyte and rinsed thoroughly to remove the acids from the pores in the film. The anodic film produced from sulphuric-based electrolytes is now ready for colouring, if required.
The anodic film thus produced is quite porous and will accept or trap any material into its pores, either advantageous or disadvantageous to its properties. To prevent this occurring these pores are closed or the coating is "sealed". This is done by adding water to the oxide (hydrolysing). The oxide swells and in so doing the pores close-up. The resultant film is now smooth, hard, homogenous and transparent. The sealing process may be carried out in boiling water, or in chemically enriched water at room temperature.
How is colour introduced?
There are three basic methods for introducing colour into anodised films.
1 Surface dyes or pigments
The clear anodised aluminium with its unsealed porous film is immersed in a bath containing organic dyes or inorganic pigments. The colorant is absorbed into the pores of the film and subsequently sealed in.
Almost all organic colours are affected by environmental factors such as UV radiation. Colour change is inevitable with exterior use.
A few inorganic pigments are resistant to UV induced colour changes. Unfortunately, many of these are no longer acceptable environmentally and are not used.
This method of colouring films lends itself to silk screening patterns into the surface.
2 Integral colour
In this process the colours are derived by electro-chemical means. The colour results directly from the alloying elements in the metal, the electrolyte composition, temperature and current density (the amount of current applied per unit of area).
The aluminium is immersed into a special electrolyte under carefully controlled electrical conditions and temperature for various time periods to produce a variety of colours. In some instances special aluminium alloys are required. The colours are due to the colouring of the intermetallic particles that are spread throughout the depth of the anodic film.
3 Electrolytic Deposition
Naturally stable metals and metallic oxide particles are electrolytically deposited at the base of the porous anodic film. The film is then "sealed". The colour is generated through the film and is locked within the clear anodic film.
Anodising produces a high specification metallurgically bonded finish that resists corrosion, abrasion and exposure to industrial, marine and other severe environments. Some bending is possible of anodised aluminium, but this is not recommended, as the film tends to crack.
The choice of finish includes either clear or coloured anodic films. Thickness can be controlled and film thickness range from 10 to 25 microns. For exterior structural and architectural applications 20 to 25 micron films are recommended. Coatings less than 20 micron thick are not recommended for marine environments. Anodising can be used in a diverse range of applications. The aluminium fabricator, builder, or architect may utilise the natural lustrous clear anodised finish or incorporate any of the vast range of coloured finishes.
The permanent and attractive appearance of anodised aluminium makes it an ideal material for use in the home or in industry.
What is colour harmony?
As with timber, brick and other traditional building materials, no two pieces of anodised metal look exactly alike. Slight colour variations are due to differences in alloy composition; the grain size and grain orientation developed during the heat treatment or cold working processes; and variations in the parameters of the colour anodising processes.
The designer can reduce the apparent effect of colour variation by breaking up expanses of colour with other colours, changes of plane or other architectural treatments.
Installations and maintenance
During installations the anodised coating should be protected from damage due to abrasion and materials of construction such as mortar and brick cleaning chemicals.
Once installed, maintaining the initial appearance of anodised aluminium is a simple matter. The soot and grime which builds up on surfaces from time to time contains moisture and salts which will adversely affect the anodised coating and must be removed. Anodised coatings should be washed down regularly (at least once each 6 months in less severe applications and more often in marine and industrial environments). The aluminium should be washed down with soapy water - use a neutral detergent - and rinsed off with clean water. Australian Standard AS 1231:2000-Aluminium and aluminium alloys-Anodic oxidation coatings.
When anodised aluminium is installed without damage to the anodised film and it is maintained regularly, it should be relatively permanent. The film, which is metallurgically bonded to the aluminium, will not crack or peel as with conventional paint films.