Journal of Environmental Protection, 2011, 2, 956-959
doi:10.4236/jep.2011.27109 Published Online September 2011 (http://www.SciRP.org/journal/jep)
Protective Green Patinas on Copper in Outdoor
Constructions
Yolanda Hedberg, Inger Odnevall Wallinder
Division of Surface and Corrosion Science, Royal Institute of Technology, KTH, Stockholm, Sweden.
Received June 2nd, 2011; revised July 11th, 2011; accepted August 24th, 2011.
ABSTRACT
The last 15 years of research related to atmospheric corrosion and the release of copper to the environment are shortly
summarized. Brown and green patinas with high barrier properties for corrosion are gradually evolved on copper at
atmospheric conditions. The corrosion process and repeated dry and wet cycles results in a partial dissolution of cor-
rosion products within the patina. Dissolved copper can be released and dispersed into the environment via the action
of rainwater, however the major part is rearranged within the patina during drying cycles. The majority of corrosion
products formed h ave a p oor solubility , very different from water soluble copper salts. Th e release process is very slow
and takes place independent of patina color. Its extent has only a marginal effect on the adherent patina. Released cop-
per rapidly interacts with organic matter and in contact with different surfaces already in the close vicinity of the
building, such as drainage systems, storm water pipes, pavements, stone materials and soil systems. These surfaces all
have high capacities to retain copper in the runoff water and to reduce its concentration and chemical form to
non-available and non-toxic levels for aquatic organisms.
Keywords: Copper, Runoff, Patina
Copper surfaces on external buildings such as roofs of
ancient churches, and claddings of historic structures, for
instance the Statue of Liberty in New York, US, have
with time developed a green to turquoise co lor, Figure 1
(left). Green patinas are rapidly evolved with time in
highly polluted environments and their components,
which determine their color, depend on prevailing envi-
ronmental conditions and on concentrations of air pol-
lutants, aerosols and particulates, such as sulfur dioxide
and sodium chloride. In low-polluted environments, the
metallic lustre of copper is of ten maintained , or gradu ally
changed with time to a brown-black shade. The greenish
appearance will hardly ever occur at these conditions, or
evolve extremely slowly. As gaseous concentrations of
sulfur dioxide have decreased substantially during the
last decades in Europe and the US, black-brown patinas
are nowadays predominantly developed in favor of green
patinas. This is unfortunately not the case for rapidly
developing countries in Asia and Africa, due to the high
degree of pollutant s.
Why is a patina developed on copper metal and why is
its color typically brown and/or green? All metals, with
the exception of very noble metals such as gold and
platinum, are oxidized and corroded to different extent
when exposed to ambient outdoor conditions. Well-known
examples where these processes are clearly visible and
evident are red-rust formation on iron or steel construc-
tions, and white rust staining on galvanized steel surfaces.
However, the oxidation of metals or alloys such as tita-
nium and stainless steel is not possible to observe by the
naked eye at identical exposure conditions. These sur-
faces hence often maintain their metallic appearance also
after several decades of exposure. For copper however,
outdoor exposures result in the gradual evolution of cor-
rosion products that exhibit different colors. The initially
formed copper oxide (cuprite) is responsible for a gradu-
ally progressively more brown-black surface appearance,
whereas different basic copper sulfates and chlorides
make the surface greenish, Figure 1 (left).
The evolution of corrosion products within the patina
depends on prevailing environmental conditions, in par-
ticular concentrations of sulfur dioxide and sodium chlo-
ride. In marine environments, deposition of chlorides in
sea-salt aerosols results in the formation of basic copper
chlorides turning the copper surface green-turquoise.
Even though the outer patina layer is greenish in color,

Protective Green Patinas on Copper in Outdoor Constructions 957
Figure 1. Green patina on a copper roof of a church in Stockholm, and released copper re tained by the stone plinth beneath
the roof (left). A cross section of a 350 year old naturally aged corroded copper roof with an outer green patina layer of basic
copper sulfates and an inner brown layer of copper oxide (right).
the inner layer is predominantly black-brownish and
composed of cuprite, Figure 1 (right). This is also true
for green patina evolved in polluted (e.g. sulfur dioxide)
environments where typically different basic copper sul-
fates (and other phases) form the green outer patina layer.
In non-marine and low polluted environments, or when
the copper metal is freshly exposed, the patina turns
brownish in color. This is related to the fact that no, or
low amounts of predominantly basic copper compounds
are formed and integrated with the brown-black patina.
The gradually developed patina, independent of color
and composition, is strongly adherent to the surface and
acts as an efficient barrier that significantly reduces the
corrosion rate of the underlying copper metal. Also the
diffuse dispersion of copper from outdoor constructions
is relatively independen t of patina compos ition and colo r.
This partial dissolution of copper from corrosion prod-
ucts within the patina that may be dispersed into the en-
vironment via the action of rainwater is however a very
slow process and of marginal importance for the adherent
patina. Patinas aged during centuries possess hence still
high barrier properties due to its patina constituents and
most of the underlying copper metal is still non-oxidized ,
a scenario that would not occur in the presence of of eas-
ily soluble corrosion products such as copper salts.
You may believe that a green patina, which is partially
composed of basic copper sulfates and/or basic copper
chlorides, dissolves as rapidly as a water soluble salt of
copper sulfate or copper chloride. This assumption is
highly erroneous for several reasons. Firstly, basic cop-
per compounds evo lved in copper patinas are chemically
very different compared to soluble copper salts, secondly,
the basic copper compounds are integrated within the
patina, predominantly composed of cuprite, and thirdly,
the thin water film conditions combined with repeated
dry and wet periods that govern atmospheric exposure
conditions enable partially dissolved copper released
from patina constituents to re-precipitate during drying
cycles. These conditions are very different from bulk
immersion conditions where no dry period occurs and
dissolved copper has limited possibility to re-precipitate.
It is only via the action of rain water flushing the surfaces
that any dissolved copper can be released from the cop-
per surface into the environment. The action of rainwater
essentially depends on rain characteristics and prevailing
wind directions and to factors such as building g eometry,
orientations and inclinations of roofs and facades, rain
sheltered surfaces etc. As an example, only a very small
surface area of a facade orientated opposite the prevail-
ing wind direction or being partly rain sheltered by
nearby buildings will be exposed to rainfall, whereas the
entire surface of a copper roof will be exposed to rain
able to transport dissolved copper from the patina.
How will released copper interact with the environ-
ment and does this dispersion pose any potential risks or
hazards? Extensive research efforts have been conducted
to understand the metal release mechanisms and the en-
vironmental interaction and fate of released copper from
outdoor constructions, Figure 2. Most ecotoxicological
studies are conducted on easily water soluble salts to
assess adverse effects on aquatic organisms induced by
metals in their ionic form. Such experiments can how-
ever not directly be used to assess the behavior of re-
leased copper from brown or green copper patina due to
the following reasons. Any release of copper requires, as
previously discussed, the action of rainwater to transport
dissolved copper from the surface. Its extent depends on
prevailing meteorological and environmental conditions
including factors such as rain characteristics (intensity,
amount, duration, acidity), duration of wet and dry peri-
ods preceding rain events, pollu tant levels as well as sur-
face orientation and inclination. Released copper will

Protective Green Patinas on Copper in Outdoor Constructions 958
Figure 2. Green and brown patinas formed on copper roofs and facades efficiently reduces the corrosion rate of copper and
acts as protective barriers for the underlying copper metal. Via the action of impinging rainwater, a small amount of copper,
partly dissolved from corrosion products in the patina during repeated dry and wet cycles, can be released to the environ-
ment. Its chemical form and environmental fate depend on interactions with organic matter and different solid surfaces from
source to recipient. Most copper has been shown to be retained in the close vicinity to its source and doesn’t induce any ad-
verse effects on aquatic organisms.
rapidly interact with for instance organic matter forming
non-available complexes and/or be retained by surfaces
and materials already in the drainage system dewatering
for instance a copper roof. Surfaces in drain pipes and
urban storm water systems, such as concrete and cast
iron, have proven to be efficient sinks for released copper.
They also efficiently change the chemical form of copper
to non-available complexes, a condition very different
from ecotoxicological testing with copper salts where all
copper is in a bioavailable chemical form. Actually, more
than 98% of the total amount of released copper in runoff
water is retained by concrete surfaces forming different
stable copper compounds already after 20 m of interac-
tion. These interactions are visible by the naked eye as
greenish areas on for instance pavements were runoff
water from a copper roof is directed and flushed before
entering the storm water system, and on concrete or
limestone plinths beneath ancient bronze statues or cop-
per roofs, Figure 1 (left). These surfaces as well as soil
systems and sediments act as very efficient sinks for
copper released from brown and green patinas on copper
in outdoor constructions. They also have a capacity to
change the chemical speciation of the low fraction of
non-retained copper to copper complexes that are
non-available to sensitive aquatic organisms. Copper
released from green and brown patinas due to the com-
bined action of corrosion and rainwater has clearly been
proven to induce negligible adverse environmental ef-
fects due to its rapid and strong interaction with different
solid surfaces and organic matter upon transport from
source to recipient. Also, to compare the potential impact
of copper levels from roof run-off, the dilution of the
roof run-off into the receiving surface water needs to be
considered. However, if copper-containing runoff water
from a large roof is directly directed into a small volume
of receiving water (e.g. a small pond) without any possi-
bility for natural pre-interactions with organic matter
and/or different surfaces, or dilution, precautionary ac-
tions should be undertaken.
In all, brown and gr een patinas with high barrier prop-
erties for corrosion are gradually evolved on copper at
atmospheric conditions. The corrosion process and re-
peated dry and wet cycles results in a partial dissolution
of corrosion products within the patina. Dissolved copper
can be released and dispersed into the environment via
the action of rainwater, however the major part is rear-
ranged within the patina during drying cycles. The ma-
jority of corrosion products formed hav e a poor solubility,
very different from water soluble copper salts. The re-
lease process is very slow and takes place independent of
patina color. Its extent has only a marginal effect on the
adherent patina. Released copper rapidly interacts with
organic matter and in contact with different surfaces al-
ready in the close vicinity of the building, such as drain-

Protective Green Patinas on Copper in Outdoor Constructions959
age systems, storm water pipes, pavements, stone mate-
rials and soil systems. These surfaces all have high ca-
pacities to retain copper in the runoff water and to reduce
its concentration and chemical form to non-available and
non-toxic levels for aquatic organisms. This chemical
form is very different from copper dissolved from water
soluble copper salts used in ecotoxicological investiga-
tions. Countermeasures may be undertaken if rainwater
from a copper roof is directed into a lake without any
possibility for interactions with organic matter or solid
surfaces adjacent the building.
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