Anodizing is a process by which the surface of aluminium profiles is coated with an artificial oxide layer. The anodizing layer is used to increase corrosion resistance, increase wear resistance or prepare the surface for further electrolytic colouring into bronze and black colours.
Anodizing is an electrolytic process where the aluminium workload acts as the anode immersed in sulphuric acid as the electrolyte (the most common electrolyte used for architectural purposes is sulphuric acid). When an electric current is passed through the acid oxygen is given off at the anode side. This oxygen reacts with the aluminium (extrusions) to form an artificial (forced) oxide layer.
Prior to anodizing, the aluminium profiles go through a series of pre-treatment processes with the following as key stages:
Degreasing is carried out by immersing the extrusions in a tank of detergent to remove oil, grease, and other surface contamination from the profile surface. Another critical step is etching whereby the naturally occurring aluminium oxide layer on the extrusions is removed by dipping in a tank of caustic soda to make way for the artificial oxide layer to be applied more uniformly. Any smut that may result from insolubles such as magnesium oxide, intermetallics, silicon, etc. in the etching tank is removed by a process called de-smutting achieved by dipping in a solution of spent-sulphuric acid.
After anodizing, the extrusions may be coloured by dipping into a tank of cobalt sulphate electrolyte to give a colour that ranges from very light to dark bronze or black depending on how long the extrusions stay in the electrolyte and also the electric voltage used. The patented process is called anolok colouring. The anodizing layer is porous at a microscopic level and a process called sealing carried out in water heated to a minimum of 94 degrees Celsius is necessary to close the pores and protect the surface.
Oxide film thickness tests are carried out on the extrusions as well as inspection for visual surface defects before packing the extrusions.
Aluminium is coated with a “self protective” oxide layer which appears soon after extrusion, although attractive in this plain finish, the coating will eventually fade or blotch due to weathering and atmospheric pollution.
The answer is to anodise! This induces growth of the oxide layer under controlled conditions to a specified depth to ensure a long life, maintenance free surface which is always consistent in appearance. Where aluminium is selected for its decorative advantages, or where regular cleaning and maintenance are difficult, anodising is strongly recommended.
A further step is the Alminok colour anodising process, where shades of bronze to black are induced. Colour fastness is absolute; corrosion and abrasion resistance is enhanced.
The range of surface finshes which can be specified are:
- mill finish
- natural anodised
- colour anodised
- chemically brightened
Please seek our advice on the correct surface treatment and finish for your application