What is anodising?
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.
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?
1 Surface dyes or pigments
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
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
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?
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
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.