for the formation of the original barrier action. The electrochemical reactions
in the corrosioncells on copper area sfollows:
Anodic reaction:
Cu/ Cu
C
C e
Cu/ Cu
2 C
C 2 e
Cathodic reaction: O 2 C H 2 O / 4 e C 4OH
K
Chloride ionsinanatural environment stabilize cuprous ions. Cupric ions
are more stable. Since the EMF of corrosion on copper is less than that on
iron, the reactivity of as teel surfacei sd ecreased by coating it with copper.
Over aperiod of time, corrosion products graduallybuildupasecondary
layer against corrosion. Initially,acuprous oxide layer is formed, followed
by the copper surfacec overedw ith basic copper salts. Pollutantsi nt he
atmosphere determine the formation of basic copper salts as follows:
Mild atmosphere
Malachite CuCO 3 :Cu(OH) 2
SO x atmosphere
Brochanite CuSO 4 :3Cu(OH) 2
Chloride atmosphere
Atacamite CuCl 2 :3Cu(OH) 2
In most coastal areas, the amount of sulfatesi nt he atmosphere exceeds
the amount of chlorides. As al ayer of copper salt grows on the surface
of the corrosion product layer,the protective ability of the corrosion layer is
increased. As the exposure time increases,t he average corrosionr ate of
coppergradually decreases. After twenty years, the corrosion rate of copper
is reducedt oh alf the value of the first year as ar esulto ft he secondary
barrierofc orrosion products.
The initialcorrosion rate of acopper coating is dependent on atmospheric
conditions suchastime of wetness and typeand amount of pollutants. Time
of wetness is the mostimportant factor affecting the corrosion rate of copper.
The corrosion rate of copperusuallyo beys parabolic law:
M
2 Z kt;
where
M Z mass increase
k Z aconstant
t Z exposure time
Accordingly,the average corrosion rate decreases with increased exposure
time,t hat means the surface of the copper is covered with basic salts by
degrees, and thereafter,t he corrosionrate approaches aconstant value.
Copson
6
conducted twenty year exposure tests and found the average
corrosion rate of coppertob ea sf ollows:
0.0034 mil/year in dry rural atmospheres
0.143 mil/year in rural atmospheres
Corrosion of Linings and Coatings
434
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485
in the corrosioncells on copper area sfollows:
Anodic reaction:
Cu/ Cu
C
C e
Cu/ Cu
2 C
C 2 e
Cathodic reaction: O 2 C H 2 O / 4 e C 4OH
K
Chloride ionsinanatural environment stabilize cuprous ions. Cupric ions
are more stable. Since the EMF of corrosion on copper is less than that on
iron, the reactivity of as teel surfacei sd ecreased by coating it with copper.
Over aperiod of time, corrosion products graduallybuildupasecondary
layer against corrosion. Initially,acuprous oxide layer is formed, followed
by the copper surfacec overedw ith basic copper salts. Pollutantsi nt he
atmosphere determine the formation of basic copper salts as follows:
Mild atmosphere
Malachite CuCO 3 :Cu(OH) 2
SO x atmosphere
Brochanite CuSO 4 :3Cu(OH) 2
Chloride atmosphere
Atacamite CuCl 2 :3Cu(OH) 2
In most coastal areas, the amount of sulfatesi nt he atmosphere exceeds
the amount of chlorides. As al ayer of copper salt grows on the surface
of the corrosion product layer,the protective ability of the corrosion layer is
increased. As the exposure time increases,t he average corrosionr ate of
coppergradually decreases. After twenty years, the corrosion rate of copper
is reducedt oh alf the value of the first year as ar esulto ft he secondary
barrierofc orrosion products.
The initialcorrosion rate of acopper coating is dependent on atmospheric
conditions suchastime of wetness and typeand amount of pollutants. Time
of wetness is the mostimportant factor affecting the corrosion rate of copper.
The corrosion rate of copperusuallyo beys parabolic law:
M
2 Z kt;
where
M Z mass increase
k Z aconstant
t Z exposure time
Accordingly,the average corrosion rate decreases with increased exposure
time,t hat means the surface of the copper is covered with basic salts by
degrees, and thereafter,t he corrosionrate approaches aconstant value.
Copson
6
conducted twenty year exposure tests and found the average
corrosion rate of coppertob ea sf ollows:
0.0034 mil/year in dry rural atmospheres
0.143 mil/year in rural atmospheres
Corrosion of Linings and Coatings
434
CAT8247—CHAPTER 7—17/10/2006—15:02—TRGANESH—XMLM ODEL B–pp. 403–485