Anodising - Making A Comeback

We live in a world where architectural aluminium is predominantly powder coated and offered in a multitude of colours, writes Norman Swift of Aluprof.

Applicators offer warranties for colour and gloss retention over an extensive lifetime. Anodising, on the other hand, has all but disappeared apart from it appearing on a few prestigious projects.

So what is anodising and why is it proving popular again?


 “Anodising is simply a highly controlled process of a naturally occurring phenomenon."

– Norman Swift 


Aluminium oxide

In simple terms, anodising is an evenly controlled ‘corrosion’ of the surface of the aluminium, turning aluminium into ‘aluminium oxide’ similar to iron oxide (rust) on steel.

The difference with aluminium oxide is that, once it has been created on the surface, unlike rust on steel it completely seals the base aluminium from further corrosion. So anodising is not an applied finish but a conversion of the surface of the aluminium, thus it will not peel or flake. Done to the right quality, anodising can last many decades without fading.


How it's done

The anodising process is accomplished by immersing aluminium into an acid electrolyte bath and then passing an electric current through the medium. A cathode is mounted to the inside of the anodising tank and the aluminium itself acts as an anode. Oxygen ions are released from the electrolyte to combine with the aluminium atoms on the surface of the profile.


For windows

Anodising aluminium profiles for fenestration requires a specialist plant as the immersive process requires long tanks, often up to 7 metres long, to accommodate full lengths of aluminium profile.



Another key to creating a good anodised finish is the ability to extrude and anodise within a short space of time, reducing the opportunity for natural oxidisation beginning to occur.

Aluprof, one of the largest producers of aluminium fenestration systems in Europe, produces dies that are dedicated to each finish. The more expensive, often hand finished, dies are kept for anodising specifications.

Aluprof profiles are extruded through higher quality dies which offer surfaces with a significant reduction in ‘die lines’ and ‘weld lines’. Once these profiles are etched and anodised they offer a surface which is both very hard wearing and long lasting.

Anodising plants are very costly to design and install. They use high voltage and specialist transformers. Because of its use as a finish in its own right and as a ‘conversion coating’ we have seen two new plants come on-line in the UK in the last couple of years with further plants planned.



Anodising standards generally referred to include BS EN ISO 7599:2018 - ‘anodising of aluminium and its alloys – method for specifying decorative and protective anodic oxidation coatings on aluminium’ and QUALINOD, an independent association located in Zurich. The QUALINOD specification includes a specific anodising specification to meet an architectural standard BS EN ISO 7599.


Powder coating

Anodising is also now becoming a choice for pre-treatment of aluminium. A very thin anodising of just five microns seals the aluminium prior to powder coating. This sealing or ‘conversion coating’ has traditionally been completed in chrome or chrome free coatings. The claim for the use of ‘pre-anodising’ or ‘flash anodising’ prior to powder coating is that it avoids the appearance of a type of corrosion known as filiform corrosion.



Anodising can be specified in both bronze, metallic  finish and shades of colour. Colour is created by the introduction of trace metals such as cobalt or tin during the anodising process. As the anodising is ‘clear’, the introduction of these metals forms an optical colour, which cannot fade over time.

As aluminium hardness will differ from profile to profile and grades differ, for example when using sheet aluminium in cill pressings, it is inevitable that the anodic film will vary in composition which will lead to a natural variation in perceived colour. As aluminium profile is extruded the material takes on a ‘grain’, so the same anodised profile turned through 90º will also vary in shade. This is a natural effect and most specifiers see this offering a richness to the very hard wearing finish that anodising can offer. To limit colour variation it is normal for the supplier of the anodising to offer upper and lower colour limits at the time of specification.

Crucial for specifiers is the knowledge that higher quality dies should be used for producing profiles for anodising and this is something that should find its way into specification for the finish. It then becomes crucial to ensure that the systems company that can offer extrusions at a higher quality, dedicated for anodising and also offer the finished profile.

Picture: Anodising is coming back in to fashion.


Article written by Cathryn Ellis
10th March 2020


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