Anodising of aluminium is an electrolytic surface treatment process where the work is made anodic in a acid solution. This is normally sulphuric acid, but this can be replaced with other acids for specialised coatings. The process is unlike other finishing methods. Where most processes coat the surface with an organic or inorganic deposit, anodising is a conversion of the base material itself to form a protective oxide coating.
This aluminium oxide film is formed on the surface of the part under a narrow range of temperatures, current densities, agitations and acid strengths. The processing time ranges from 20 minutes to about one hour, depending on end use requirements.
The work is processed on specially constructed racks that are required to conduct the electric current used in the processing. These racks are made from either aluminium or titanium, as other rack materials can interfere with the anodising process. Although we have many standard racks in stock, it may be necessary to construct special racks for some jobs, which may incur extra charges.
Before the anodising process commences, the part must be chemically cleaned. This most often involves a mixture of alkaline cleaners to remove oils and surface contamination. This is generally followed by an acid or caustic etching process.
Although racking marks are kept to a minimum, there always will be some evidence on the part of contact marks where it has been held for processing. With this said, it is usually possible to work with the customer to ensure that the marks occur in cosmetically appropriate places on all components.
Once deposited, anodic coatings can be coloured with a wide range of subtle or vivid dyes, or even electrolytically coloured with metallic compounds when an application requires light-fastness against UV fading.
The final operation in the anodising process is known as sealing. This is where the anodised part is treated in special chemical baths to seal the microscopic pores that the process creates.
It should be noted that components to be anodised must be free from any inserts, screws, bolts and the like. Assuming that they aren’t made from aluminium, they will most likely be attacked by the acids used in the anodising process.