What is Anodising?
Anodising is a methodology used to change the surface chemistry of metals and other substances. With aluminium, the result is decorative and protective film of aluminium oxide, which is built up on the surface.
How do you Anodise Aluminium?
Thoroughly cleaned aluminium is placed into a bath of electrolytic solution (an electrical solution with lots of positive and negative ions.) Most commonly in New Zealand, this solution is sulphuric acid. Aluminium sheet cathodes are then added to the solution, followed by the article for anodising, which has a positive charge applied, thus making it the anode. This causes a reaction whereby the sulphuric acid begins to decompose. Hydrogen ions move to the cathode and change form into hydrogen gas. Meanwhile, aluminium ions move towards the cathode but are met at the anode surface by oxide/hydroxide ions, forming aluminium oxide, which acts as a barrier layer.
Applying a higher voltage for a prolonged period of time will continue the growth of this layer beyond the initial barrier layer. Competing anodising and acid solution processes cause an open, pore-like structure with a depth measured in microns. It should be noted, Aluminium oxide is rated 9 out of 10 on the Moh Hardness Scale (10 being a diamond).
10 Microns: Suitable for internal (or sheltered) applications, or those where frequent cleaning is expected.
15 Microns: Suitable for the majority of architectural requirements.
25 Microns: Recommended for heavy duty external applications where there is little tolerance for deterioration.
What are the benefits of Anodised Aluminium?
Benefits range from the aesthetics to durability. There is a certain appeal to the characteristic anodised aluminium sheen, along with being resistant to scratches and cracking. The barrier layer created during the anodising process also creates resistance to corrosion.
Where a coloured finish is desired, that colour will not peel or otherwise diminish. This is because the colour is deeply embedded in the pores before sealing is completed (the process of taking the anodised article and placing it in hot water for a period of given time). Whilst pigment dyes can be used, more common is electrolyte colouring which tends to produce a uniformed finish which is easily replicated for consistency.
How to care and maintain Anodised Aluminium.
Regular cleaning of the surface is required to maintain and prolong anodised aluminium. If grim deposition is allowed to build up, it will absorb contaminated moisture and store it against the anodised surface. Over time the barrier layer will become compromised and show signs of damage. This is not usually reversible without removal.
The frequency of surface cleaning is dependent upon the environment. Marine and Industrial elements pose an elevated risk and necessitate a cleaning schedule which never has more than 3 months between treatments. In other cases, 6 monthly cleaning may suffice. Given the issues of non-cleaning, it is recommended to err on the side of caution.
Cleaning anodised aluminium can be undertaken using the same methods and materials as windows. Where damage is observed through neglect, it may be possible to restore the finish slightly with the use of solvent such as kerosene and a soft cloth. It should be noted you can damage the film beyond repair all too easily with the use of harsh abrasives.
