Anodising is an electrolytic oxidation process in which a controlled oxide layer is formed on the surface of aluminium, increasing its corrosion resistance, hardness and suitability for colouring.
Unlike paint or plating, the anodic oxide layer is not applied on top of the metal — it grows from the aluminium itself, making it extremely durable and resistant to peeling or flaking.
How does anodising work?
The aluminium component is immersed in an acid electrolyte bath, typically sulphuric acid. A direct electrical current is passed through the bath with the aluminium acting as the anode (positive electrode). Oxygen ions are released at the surface and combine with the aluminium atoms to form aluminium oxide (Al2O3).
The result is a porous oxide layer that is many times thicker than the natural oxide film that forms on untreated aluminium. After anodising, the pores can be filled with dyes to produce a wide range of colours, and the surface is then sealed in hot water or a nickel-acetate solution to close the pores and lock in the colour.
Layer thickness and classes
In construction, anodised aluminium is classified by the thickness of the oxide layer:
- Class 5 (5 microns) — interior decorative use.
- Class 10 (10 microns) — interior applications with light wear.
- Class 15 (15 microns) — exterior use in mild environments.
- Class 20 (20 microns) — exterior use in urban or industrial atmospheres.
- Class 25 (25 microns) — exterior use in marine or heavily polluted environments.
The European standard EN 12373 (now replaced by EN 17611) defines quality requirements for architectural anodising.
Applications in construction
Anodised aluminium is used for window and door frames, curtain walls, cladding panels, balustrade components and louvre systems. Its low maintenance, long lifespan and consistent metallic appearance make it a popular alternative to powder coating.
Advantages and limitations
Anodising offers excellent UV stability, scratch resistance and a finish that does not peel. However, colour matching between different production batches can be challenging, and the process is limited to aluminium and its alloys.
Related terms
- Powder coating
- Aluminium alloy
- Surface treatment
- Curtain wall