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Fundamentals of Corrosion
The low corrosion rate and interesting patina formation of copper have been
responsible for its use in building structures such as roofs, facades, and gutters. The blue-green patina forms after several years when conditions are
favorable in outdoor atmospheres. Initially it may take on a mottled appearance but after 6 to 12 months, the surface usually turns a uniform dark brown
color and remains as such for several years. It can take 5 to 10 yr before a
green patina begins to appear on sloping surfaces in urban or marine atmospheres. Because vertical surfaces have a much shorter time of wetness, they
remain black much longer. In a marine atmosphere, the surface facing the sea
acquires a green patina sooner than the other surface as a result of a greater
supply of chlorides from the sea winds.
In areas of very low pollution, the patina could conceivably take hundreds
of years to form because of the small supply of anions to form the copper salt.
When the temperature is very low or the exposure conditions are very dry,
the patina may not form at all.
Under acidic conditions, the green patina will not form. Such conditions
can occur near chimneys where acid smoke strikes the roof. Under these
conditions, the corrosion products are soluble.
Traces of dissolved copper will usually be found in rainwater running off
of copper-bearing surfaces. This takes place predominately in urban and
industrial areas that are highly polluted with SO 2 . Such rainwater can cause
blue staining on masonry, stonework, etc.
4.7.6 Copper alloys
To increase the strength and general usefulness of copper, it is alloyed with
other elements such as zinc, aluminum, nickel, tin, silicon, etc. The various
alloy families are as follows:
Principal Alloying Element
Alloy Family
Zinc
Brasses
Tin, aluminum, or silicon
Bronzes
Nickel
Copper nickels
There are other alloy families but these are of little importance in the field of
corrosion resistance and will not be discussed.
4.7.6.1 Brasses
Brasses contain zinc as their principal alloying ingredient. Other alloying
ingredients include lead, tin, and aluminum. Lead is added to improve machineability and not to improve corrosion resistance. The addition of approximately 1% tin increases the de-alloying resistance of the alloys. Aluminum is
Fundamentals of Corrosion
The low corrosion rate and interesting patina formation of copper have been
responsible for its use in building structures such as roofs, facades, and gutters. The blue-green patina forms after several years when conditions are
favorable in outdoor atmospheres. Initially it may take on a mottled appearance but after 6 to 12 months, the surface usually turns a uniform dark brown
color and remains as such for several years. It can take 5 to 10 yr before a
green patina begins to appear on sloping surfaces in urban or marine atmospheres. Because vertical surfaces have a much shorter time of wetness, they
remain black much longer. In a marine atmosphere, the surface facing the sea
acquires a green patina sooner than the other surface as a result of a greater
supply of chlorides from the sea winds.
In areas of very low pollution, the patina could conceivably take hundreds
of years to form because of the small supply of anions to form the copper salt.
When the temperature is very low or the exposure conditions are very dry,
the patina may not form at all.
Under acidic conditions, the green patina will not form. Such conditions
can occur near chimneys where acid smoke strikes the roof. Under these
conditions, the corrosion products are soluble.
Traces of dissolved copper will usually be found in rainwater running off
of copper-bearing surfaces. This takes place predominately in urban and
industrial areas that are highly polluted with SO 2 . Such rainwater can cause
blue staining on masonry, stonework, etc.
4.7.6 Copper alloys
To increase the strength and general usefulness of copper, it is alloyed with
other elements such as zinc, aluminum, nickel, tin, silicon, etc. The various
alloy families are as follows:
Principal Alloying Element
Alloy Family
Zinc
Brasses
Tin, aluminum, or silicon
Bronzes
Nickel
Copper nickels
There are other alloy families but these are of little importance in the field of
corrosion resistance and will not be discussed.
4.7.6.1 Brasses
Brasses contain zinc as their principal alloying ingredient. Other alloying
ingredients include lead, tin, and aluminum. Lead is added to improve machineability and not to improve corrosion resistance. The addition of approximately 1% tin increases the de-alloying resistance of the alloys. Aluminum is