Clearview National April 2020 - Issue 221 | Page 38

ALUMINIUM
‘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.
38 » A PR 2020 » CL EARVI E W- UK . C O M
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, one of the largest producers of
aluminium fenestration systems in Europe,
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