| Precautions were mentioned previously concerning the use of copper, lead and graphite
with Galvalume and aluminum-coated sheet. The field of corrosion study has defined an
"activity" scale, shown schematically at left, which shows zinc and aluminum
more active than either copper, lead or graphite. The farther they are apart on the scale,
the more dissimilar they are and the stronger the potential for reaction between them.
When a galvanic "couple" is formed by electrical contact, the more active metal
will sacrifice itself (or dissolve) to protect the less active component of the couple. A
couple consisting of aluminum and graphite, for instance, will corrode the aluminum away
much faster than the copper would dissolve in a copper/graphite couple. On a metal
building, when copper, lead or graphite either come in contact with, or in the case of
copper used in close proximity to Galvalume or aluminum-coated sheet coatings, the
coatings will corrode much faster than normal. The protective oxide film which naturally
forms on aluminum surfaces is broken down by copper, lead or graphite in localized areas.
When this occurs, pitting corrosion ensues which is a highly accelerated form of attack.
Zinc coatings are not generally subject to pitting when in contact with the same
materials. Therefore, the galvanic corrosion reaction is slower and spread over a wider
area.
Rainwater Run-off: The compatibility table also contains guidelines for selection of
materials when one component will drain rainwater onto another. Pure rainwater contains
very little dissolved metal. When it contacts galvanized steel, for example, it will
dissolve some zinc from the coating. As the water drains onto another area, such as a
lower roof elevation, it will be less corrosive relative to the pure water. Where there is
a building element with a less active coating over one with an inferior coating, there
will be accelerated corrosion of the lower element. Such a situation occurs where
rainwater drains from a prepainted Galvalume roof into an unpainted galvanized gutter. In
industrial, acid rain or marine environments, these effects will be accelerated due to the
increased corrosivity of the rainwater.
Standing Water:
New applications for metal roofing have required that roof slopes be minimized to as
low as 1/4:12. At times, areas of roofs can be flat depending on the particular building.
These conditions, either by accident or intent, can create areas where water can collect
and remain for extended periods of time with subsequent possibility of accelerated
corrosion. Where an unfavorable galvanic couple exists, as discussed above, the presence
of standing water for prolonged periods will allow the corrosion reaction to continue for
a longer time than it normally would. Even in cases where an adverse couple does not
exist, enough water can complete the necessary electrical contact and corrosion will
proceed as long as the water maintains the circuit.
The appearance of roofing panels can suffer even when all materials within a
water-ponding area are compatible. Aluminum-coated steel panels are not as resistant to
standing water as Galvalume sheet. When the aluminum-coated panel begins to rust, the
standing water can disperse and deposit rust particles on an adjacent Galvalume sheet
panel, resulting in an unsightly stain.
Galvalume� is a registered trademark of BIEC International, Inc.
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