Through anodising we strengthen the natural properties of aluminium by increasing the thickness of the protective oxide layer. The surface keeps its fresh appearance and can be coloured in a number of attractive shades
At the same time, the resistance to corrosion and abrasion is increased and the surface becomes dirt-repellent and pleasant to touch. The oxide layer has also electric isolation properties and functions as a good substrate for both printing inks and adhesives.
We also deliver profiles with coloured oxide layers. Some of the shades have very good colour durability and are suitable for outdoor use.
The anodising process
The process usually consists of four stages: pre-treatment, anodising, colouring (if required) and sealing. The most frequent type of anodising is clear anodising. After the metal surface has 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. An electrolytic cell is formed. The electrolyte usually consists of dilute sulphuric acid at room temperature. During the electrolysis, the surface metal is converted into oxide. The process continues until the required layer thickness has been obtained, usually 5-25 µm.
The newly formed oxide layer contains a large number of open pores which must be closed. This is done by sealing – treatment in deionised water at 95-98 degrees C. The surface layer that is formed through clear anodising is transparent. It is also possible to colour the coating. The clear anodised profiles are delivered with matt or semi-gloss finish.
The following thicknesses of anodising coating are recommended:
25 µm – The surface is subjected to strong influence of corrosion or abrasion.
20 µm – Strong or normal influence outside buildings, e.g. in transport or building industry. Strong chemical influence inside buildings, e.g. in food industry.
15 µm – Strong influence of abrasion inside and outside buildings in dry and clean atmosphere.
10 µm – Normal influence inside buildings.
3-5 µm – Protective anodising before processing, short pickling time.
Maintenance – cleaning
The anodising layer has a very good resistance to corrosion in most environments. Anodised profiles are nearly maintenance free, provided that the surface is cleaned. Cleaning with water and neutral detergents or white spirit is sufficient. The resistance of the coating to corrosion, discolouration and abrasion increases in proportion to the layer thickness.
Forming should take place before anodising because the cold forming properties of the anodising layer are poor. Welding is done before anodising. When cutting and drilling the profiles, one should remember that the cutting surface becomes untreated.
Properties of anodised aluminium
The corrosion resistance is very good, especially in the pH value range from 4 to 9. The surface can be stained and damaged in contact with strongly alkaline substances. On should remember to protect aluminium from, for example, lime, cement and gypsum (this applies to, among other things, construction sites). Visible surfaces can be covered with protecting strip.
The hardness of the oxide layer depends on the selected anodising method. Normally, the coating is harder than glass and as hard as corundum. The oxide layer is transparent
Find out more information about anodising in our Design Manual