40 | APRIL 2020
Products
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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, ano-
dising is an evenly con-
trolled ‘corrosion’ of the
surface of the aluminium,
turning aluminium into ‘al-
uminium oxide’ similar to
iron oxide (rust) on steel.
The difference with alumin-
ium 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 alumin-
ium, 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 immers-
ing 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 ac-
commodate 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 ano-
dised 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 temper-
atures 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 larg-
est producers of aluminium
fenestration systems in Eu-
rope, produce dies that are
dedicated to each finish.
The more expensive, often
hand finished dies, are
kept for anodising speci-
fications. Another key to
creating a good anodised
finish is the ability to ex-
trude and anodise within a
short space of time, there-
by reducing the opportu-
nity 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 gen-
erally referred to include
BS EN ISO 7599:2018 -
‘Anodising of aluminium
and its alloys — Method
for specifying decorative
and protective anodic
oxidation coatings on alu-
minium’ and QUALINOD,
a independent association
located in Zurich. Whilst
the QUALINOD specifi-
cation 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 specifica-
tion 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 speci-
fication 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 complet-
ed 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. In-
terestingly, as the anodising
is ‘clear’, the introduction
of these metals forms an
optical colour, which can-
not 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 vari-
ation 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 var-
iation 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 specifica-
tion 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.
With these quality issues
in mind, the specifier will
be aware of the need to
choose a quality company
for the anodising process
and ensure that the alu-
minium is of the highest
quality with its surface free
from natural corrosion. It
is normally best to specify
an aluminium systems sup-
plier who can offer quality
anodising, otherwise issues
can arise where the sys-
tems company and anodis-
er are unwilling to rectify
issues with surface finish
problems. This can lead to
expensive replacements in
finished projects. Unlike
powder coating, anodising
can’t be repaired on-site.
Since setting up the
Aluprof Project Office at
the Business Design Centre
in London the systems
company has rapidly
grown their specification
influence in the UK with
their high performance
architectural aluminium
systems. With overseas
growth across Europe
spreading into the Middle
East and firm roots already
in the East of the USA,
the company is becoming
a global player in facade
supply.
aluprof.co.uk