DOORS & WINDOWS
With fastener corrosion all too o
en the cause of premature window or door failure, Andy Holland, technical
and marketing manager at Rapierstar, looks at the causes and how to best manage this inevitable problem
through correct specifi cation.
In the pursuit of windows and doors
which offer longevity, enhanced security,
optimum safety and robust warranties over
a long, low maintenance service life, the
key considerations are inevitably the design of
the frame, the glass, locking system, handles
and hinges. But what about the quality of the
components that hold windows and doors
together – the fasteners?
It is often assumed that fastener choice
and specifi cation only really matter where
there is a high risk of corrosion, and that it is
an issue confi ned to coastal areas where the
air is particularly aggressive to metals. This
is certainly not the case – corrosion can be a
problem in any location.
When most of us think of corrosion, we
probably think of the typical, highly visible iron
oxide hydrate, commonly referred to as red rust,
but corrosion can also be invisible. Corrosion
does not need a wet or salt-laden atmosphere
to occur. A relative humidity of just 60% is all
it needs, and the chances of corrosion occurring
increase in the presence of common airborne
pollutants and cleaning agents.
Equally, corrosion does not always show on
the surface of a corroded item. Some types of
corrosion, such as stress corrosion cracking,
develop unseen within the structure of a
fastener or other component. In these cases, the
fi rst time the issue becomes apparent is when
something breaks. This could be catastrophic for
an installed door or window, with signifi cant
safety and security risks for building occupants.
Typical causes of corrosion
The principal causes of fastener corrosion in
windows and doors are uniform corrosion and
crevice corrosion, both of which are caused
by the accumulation of moisture triggering
an anodic corrosion reaction. Protection from
Corrosion can be
a problem in any
uniform corrosion is best achieved by selecting
inherently corrosion-resistant fasteners, or
ones with a plating or coating suited to the
environment and application.
Crevice and uniform corrosion share the
same anodic corrosion mechanism, and
therefore share the same protective measures.
The risk of crevice corrosion can be further
reduced by substituting washers for fl anged
fasteners and making all fastener-to-joint
interfaces as smooth as possible.
Another type of corrosion is galvanic
corrosion. This is caused by reactions
between fasteners and the components they
are jointing, where the two metals have
differing electromechanical potentials and
moisture is present. Joining metals of differing
electromechanical potentials causes an electrical
current to fl ow from the less noble anodic
metal, to the more noble cathodic metal,
dissolving the less noble metal in the process.
Fastener corrosion can’t be stopped, it’s an
unrelenting enemy. But it can be managed to
deliver on a service life promise. The key is to
understand the differences between the types of
fastener that are typically used in fenestration. For
example, galvanic corrosion can be avoided by
using fastener materials like austenitic stainless
steel, or protective fi nishes that are as noble or
more noble than the materials being jointed.
The baseline fastener metal is coated carbon
steel – electroplated zinc, plus a clear or yellow
passivation layer. Fasteners of this type will
need to show evidence of being salt spray tested,
showing no corrosion to 240 hours in accordance
with BS EN 9227: 2012. This provides a service
condition of Grade 4 to BS EN 1670: 2007.
Coated carbon steel fasteners are easily
capable of providing a window or door service
life of at least 10 years. But the conditions in
which coated carbon steel fasteners perform
well are not easy to predict and there are two
BUILDING PRODUCTS | MAY 8