Corrosion of Metallic Coatings 265
Mild atmosphere: Malachite CuCO
3
:Cu(OH)
2
SO
x
atmosphere: Brochonite CuSO
4
:3Cu(OH)
2
Chloride atmosphere: Atacamite CuCl
2
:3Cu(OH)
2
In most coastal areas, the amount of sulfates in the atmosphere exceeds
the amount of chlorides. As a layer of copper salt grows on the surface of
the corrosion product layer, the protective ability of the corrosion layer
increases. As the exposure time increases, the average corrosion rate of
copper gradually decreases. After 20 years, the corrosion rate of copper is
reduced to half the value of the rst year as a result of the secondary barrier
of corrosion products.
The initial corrosion rate of a copper coating depends on atmospheric con-
ditions such as time of wetness, and type and amount of pollutants. Time of
wetness is the most important factor affecting the corrosion rate of copper.
The corrosion rate of copper usually obeys parabolic law:
M
2
= kt
where M is the mass increase, k is a constant, and t is the exposure time.
Accordingly, the average corrosion rate decreases with increased exposure
time, which means that the surface of the copper is covered with basic salts
by degrees and thereafter the corrosion rate approaches a constant value.
Twenty-year exposure tests found the average corrosion rate of copper to
be as follows:
0.0034 mil/yr in dry rural atmospheres
0.143 mil/yr in rural atmospheres
0.0476–0.515 mil/yr in industrial atmospheres
0.0198–0.0562 mil/yr in marine atmospheres
Until the base metal is exposed, the corrosion process of a copper-coated
layer is similar to that of copper plate. Galvanic corrosion of copper-coated
steel is induced when the steel substrate is exposed. However, in the case of
copper-coated stainless steel, the occurrence of galvanic action depends on
the composition of the stainless steel.
In chloride atmospheres, galvanic pitting takes place at the pores in cop-
per layers and galvanic tunneling at cut edges on types 409 and 430 stainless
steels; whereas in SO
x
atmospheres, uniform corrosion takes place on the
copper coating.
Copper coatings are used for both decorative purposes and for corrosion
protection from the atmosphere. Copper-coated steels are used as roofs,
ashings, leaders, gutters, and architectural trim. Copper undercoat also
improves the corrosion resistance of multilayered coatings, specically in
the plating of nickel and chromium.