’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.
THE ANODISING PROCESS IS
ACCOMPLISHED BY IMMERSING
ALUMINIUM INTO AN ACID ELECTROLYTE
BATH AND THEN PASSING AN ELECTRIC
CURRENT THROUGH THE MEDIUM.