What is Anodising?

Aluminium metal naturally protects itself by immediately forming a protective layer whenever the surface is damaged. Anodising is a process that can provide much thicker protective layers than those that occur naturally. Anodising is about as hard as ruby or sapphire, making it a strongly abrasion and corrosion resistant surface.

Anodising is suitable for both commercial and domestic joinery.

The Anodising Process

The anodising process involves passing a controlled electrical current through extruded aluminium profiles immersed in an acidic (sulphuric) solution which then forms a protective film of aluminium oxide on the surface of the aluminium.

Anodising essentially is an induced thickening of the natural protective oxide film on the metal’s surface. It is a conversion of the parent metal and thus is not a “coating” in the usual sense.

It is proven as a very durable finish, of which the lifetime is dependent on the density, thickness and seal quality of the anodic film.


Following the Anodising process the oxide film can be retained in its clear (natural) condition, or coloured by using various colouring methods. 

The structure of this film consists of many small pores that if required can be used to colour the aluminium during the anodising process.  These small pores are then sealed to provide excellent durability once the colour is added.

The Anodised Finish:

The oxide film renders a hard, durable, weather resistant surface finish to protect the underlying base metal, and unless severely deformed or stressed by excessive thermal movement, the anodic film will not chip, peel, or crack. 

The electrochemical process causes the film to actually grow from the base aluminium metal, and therefore becomes integral, i.e. part of, the base metal.

Conventional sulphuric acid films, produced for external applications, are microscopically porous. Inorganic compounds may be incorporated and sealed within the film to produce a range of colours. 

These durable anodic films, whether coloured or not, are necessary in exposed environments and exhibit longevity, particularly suitable for New Zealand’s high ultraviolet and coastal environments.

Colour Anodising

There are several different processes which can be undertaken to achieve the desired finish and colour of anodised aluminium.

When specifying a finish it is essential that you consider the application, whether it is internal or external. The thinner film thicknesses used for indoor use are unlikely to be satisfactory for outdoor use.

The range of colours suitable for architectural aluminium is limited to a natural finish (i.e. no colouring) through to black. This range of colours will typically start off with a very light bronze (champagne) to light bronze, medium bronze, dark bronze and on to black.

It should be noted that due to the different colouring processes that anodisers are using not all the colours will match up from one supplier to another.

Texture Choice:

Anodising can provide a variety of textures from mechanically polished to heavily etched giving that extra dimension to a project.
As with any textured surface the same colour will vary depending on texture so it is wise to consult a listed finisher for samples prior to selection.