October 2020

Galvanic Corrosion
MIXING METALS: AVOID THE ISSUES
Simon Grisewood, Fabrications Manager at Proteus Facades, explains how to avoid bimetallic
corrosion on metal building envelopes.
Bi-metallic or galvanic corrosion occurs
when two dissimilar metals come into
close proximity in the presence of
moisture and air. This causes the more anodic of
the two metals to corrode, whilst the other metal
remains unaffected. An example of bimetallic
corrosion occurred on the Statue of Liberty and
an investigation found that extensive areas of
rust had formed on the iron support frames
where they touched the copper. In this case the
iron supports had to be replaced with 316 grade
stainless steel.
Galvanic corrosion has a bearing on the longevity
of all building elements and in this case
corrective action was taken before it caused
further damage. So, whilst other factors such as
UV exposure, mechanical damage from windblown
debris and freeze thaw action all take their
toll on a building, bi-metallic corrosion is a
serious issue and needs to be taken into
consideration at design stage.
Fortunately, using two or more different metals on
a building’s external envelope does not
necessarily mean corrosion will occur if a few
basic rules are followed. It’s also worth bearing
in mind that the two metals don’t need to be in
contact for galvanic corrosion to take place. For
example, when a more ‘noble’ metal such as a
copper roof corrodes slightly and dissolves in
rainwater and subsequently flows over a less
noble metal such as zinc cladding, it will result in
accelerated corrosion of the latter. This can also
occur where the window surrounds or reveals are
a different metal to the façade cladding as well
as the steel support framework.
Generally, the level of bimetallic corrosion in a
coastal location will be greater because salt in
the air speeds up the galvanic reaction. We
Proteus devised some clever detailing between the zinc roof and TECU Brass cladding panels at Wells Malting in Wells-next-the-Sea.
devised some clever detailing between the zinc
roof and TECU Brass cladding panels at Wells
Malting in Wells-next-the-Sea to create a
seamless transition between the two elements,
removing bimetallic corrosion challenges and
providing a modern twist to a historic building.
However, galvanic corrosion is a challenge on all
buildings and should be a priority when specifying
metallic cladding and roofing systems.
Following the Galvanic Series
It is possible to rank
metals, including those
used for building elements
such as cladding and
roofing on the Galvanic
Series. This lists those that
are least or most
susceptible to corrosive
attack. The further apart
two metals are on the
Galvanic Series, the
greater the potential
difference, and the faster
the corrosion will occur on the less noble
material.
When specifying a metal cladding system, it is
always worth checking where other materials
selected for the roof, rainwater or other façade
elements are positioned on the Galvanic Series.
We routinely check this on all the projects we
work on to ensure that there aren’t issues further
down the line. In most cases, consideration has
been given to this aspect, although sometimes it
can be overlooked.
Early design iterations of
Square Chapel in Halifax
was another example of
where problems with
galvanic corrosion could
have occurred. The original
design concept had a
copper wall above a zinc
roof, running off to a
copper wall. Rainwater
run-off from the copper
(noble) onto the zinc (anodic
50 TC OCTOBER 2020