Mario S Pennisi
and Derek Jones
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
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?
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).
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
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
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