Anodizing is the most durable surface treatment 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. Anodizing results in durable color and gloss with the oxide layer thickness ranging from 5 to 20 microns.
The most frequent type of anodizing is clear anodizing.
The process usually consists of four stages:
- Coloring (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 anodizing). An electrolytic cell is formed. The electrolyte usually consists of diluted sulffuric acid at room temperature. The newly formed oxide layer contains many open pores with a diameter between 120 and 330 angstroms. The pores must be closed in order to get a tight surface layer. This is done by sealing – treatment in deionized water at 95–100 °C. The aluminium oxide is then transformed into boehmite, the volume of the coating increases and the pores are closed. The oxide layer that is formed through clear anodizing is transparent. The clear anodized profiles are delivered with a matt surface finish as standard.
Clear anodized but not yet sealed aluminium is dyed with organic or inorganic pigments. After the coloring process, the profiles are sealed.
Hardoxal electrolytic coloring
Just like in the simple coloring process, in this method coloring is a separate stage after anodizing. Under the influence of alternating current, the pigment, which consists of tin salt, is deposited at the bottom of the pores. The scale of colors ranges from champagne to black. The colors, with symbols from Hx-10 to Hx-50, are very durable.
Aluminium profiles subjected to clear anodizing 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. The standard also defines the requirements for control procedures and the testing methods to be used.