Anodising is the most durable surface treatment method available for aluminium profiles. It is an electrolytic process in which an oxide layer is formed. The layer strengthens the surface which becomes resistant to mechanical wear, UV light and the influence of aggressive environments. Anodising results in durable colour and gloss with the oxide layer thickness ranging from 5 to 20 microns.
The most frequent type of anodising is clear anodising.
The process usually consists of four stages:
- actual anodising
- dyeing (if applicable)
After the metal surface has been subjected to the required mechanical or chemical pre-treatment and has been cleaned thoroughly, the electrolytic process takes place. Direct current is connected to the profile, which becomes the anode (hence the name of anodising). An electrolytic cell is formed. The electrolyte usually consists of diluted sulphuric acid at room temperature. The newly formed oxide layer contains a large number of open pores with a diameter between 120 and 330 angstroms. The pores must be closed in order to obtain a tight surface layer. This is done by sealing – treatment in deionised water at 95-100 degrees C. The aluminium oxide is then transformed into boehmite, increasing its volume and resulting in closing the pores. The oxide layer that is formed through clear anodising is transparent. The clear anodised profiles are delivered with matt finish as standard.
Clear anodised but not yet sealed aluminium is dyed with organic or inorganic pigments. After the dyeing process, the profiles are sealed.
Hardoxal electrolytic dyeing
Like in the simple dyeing process, this dyeing process is a separate step which follows the actual anodising. Under the influence of alternating current, the pigment, which consists of a tin salt, is deposited on the bottom of the pores. The colour scale ranges from champagne to black. The colours are designated as Hx-10 to Hx-50 and have a very good durability.
Aluminium profiles subjected to clear anodising or Hardoxal process meet the requirements of ISO 7599. The standard defines the requirements for sealing, resistance to wear, corrosion and light, as well as gloss durability. Apart from this, the standard defines the requirements for control measures and the testing methods to be used.