Architectural Anodising Explained
We live in an architectural aluminium world which is predominantly powder coated in a multitude of colours which offers 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?
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 of the aluminium it completely seals the base aluminium from further corrosion, unlike steel. So anodising is not an applied finish but a conversion of the surface of the aluminium, so it will not peel or flake. Done to the right quality and the materials, life expectancy can last many decades without fading.
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.
Anodising is simply a highly controlled process of a naturally occurring phenomenon.
Anodising aluminium profiles for fenestration takes a specialist plant as the immersive process requires long tanks, often up to 7 metres long to accommodate full lengths of aluminium profile, which is a heavy investment. Once installed, every plant acts differently and the learning curve is as equally complex for the operator.
So what about the ‘die lines’? Simply put, dies which have visible faces require more attention prior to use for an anodised finish. These dies need to be polished and cleaned more regularly as the extrusion process can be quite abrasive on the surface of the die. More complex shaped dies have what is know as ‘weld lines’, this is where the die design incorporates an internal mandrel, often to create a hollow profile. Where the aluminium flows around the mandrel supports and joins again within the die, this often leads to higher temperatures at this point and can affect the anodising by changing its shade. Die design can be changed to put these weld lines on a corner, or recess to make it less visible, but with an increase in die cost.
Aluprof produce dies that are dedicated to each finish. The more expensive, often hand finished dies, are kept for anodising specifications. Another key to creating a good anodised finish is the ability to extrude and anodise within a short space of time, thereby reducing the opportunity for natural oxidisation beginning to occur. Aluprof extrude profile through these higher quality dies which offer higher quality surfaces with a significant reduction in ‘die lines’ and ‘weld lines’. Once these profiles are etched and anodised they offer a high quality surface which is both very hard wearing and long lasting.
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, a independent association located in Zurich. Whilst the QUALINOD specification includes a specific anodising specification to meet an architectural standard; BS EN ISO 7599 mainly deals with general anodising for engineering use and is a method of specifying, not a specification in its own right. Unless the specifier is experienced in agreeing the criterion for the finished product under BS EN ISO 7599:2018, then the QUALINOD specification should be used as it lays down criterion and it’s licenced members are also inspected by an independent test house to ensure compliance with the QUALINOD standard. The QUALINOD standard does encompass almost all of BS EN ISO 7599, but goes much further to ensure anodising is fit for architectural use. Aluprof are a licenced member of QUALINOD.
Anodising is also now becoming a choice for pretreatment of aluminium prior to powder coating. 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 plants are also very costly to design and install, utilising high voltage and specialist transformers to rapidly and evenly create an anodised finish. Because of its combined use of a finish in its own right and as a ‘conversion coating’ prior to powder coating, in the UK alone there has been two new anodising plants come on-line in the last couple of years with further plants planned.
As specifiers tend to specify shades of grey for the majority of fenestration projects, anodising can be specified in both bronze finish and shades of grey. This is created by the introduction of trace metals such as cobalt or tin during the anodising process. Interestingly, 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, also offer the finished profile.