Atmospheric Corrosion
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surface moisture film created by rain or dew. This reduces the time of wetness, which in turn reduces the corrosion rate. In addition, as the temperature increases, the solubility of oxygen and other corrosive gases in the
electrolyte film is reduced.
When the air temperature falls below 32°F (0°C), the electrolyte film might
freeze. As freezing occurs, there is a pronounced decrease in the corrosion
rate, which is illustrated by the low corrosion rates in the sub-arctic and arctic regions.
In general, temperature is a factor that influences corrosion rates but it is of
little importance except under extreme conditions.
4.2.4 initial Exposure
Atmospheric corrosion rates proceed through three stages or periods: the
induction period, the transition period, and the stationary period. During
the induction period, the metal is covered with a spontaneously formed
oxide and the aqueous layer. This oxide provides some degree of protection,
depending on the metal and the aggressiveness of the atmosphere. During
the transition period, the oxide layer transforms into a fully developed layer
of corrosion products. The final or stationary period is characterized by the
surface being fully covered by corrosion product and eventually reaching
constant properties with respect to chemical composition and stationary
corrosion rates. The more aggressive the exposure conditions, the shorter
the two initial periods (i.e., induction and transition). For example, in a
benign (indoor) atmosphere, these initial periods for steel will last for years;
but in highly polluted industrial areas, these initial periods might last only
a few months.
Initial exposure conditions have a marked influence on the subsequent
corrosion rate. During the first days of exposure, wet conditions (caused by
high relative humidity or rainfall) cause higher corrosion rates than dry conditions. These effects will vary from one material to another. For example,
zinc is more sensitive than steel. Differences are explained by the fact that
different materials form different corrosion products with different protective properties. A wide variety of structurally related corrosion products can
be found on zinc, the nature of which depends on initial exposure conditions. The seasonal dependence on the concentrations of peroxide and ozone
in the atmosphere might also be a contributing factor.
During the third period of exposure, a constant corrosion rate is established — and thus the terminology “stationary period.”
4.2.5 Sheltering
The design of the installation should eliminate all possible areas where
water, dirt, and corrosion products can accumulate. The degree of shelter
from particle fallout and rainfall can affect the initial and ultimate corrosion
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surface moisture film created by rain or dew. This reduces the time of wetness, which in turn reduces the corrosion rate. In addition, as the temperature increases, the solubility of oxygen and other corrosive gases in the
electrolyte film is reduced.
When the air temperature falls below 32°F (0°C), the electrolyte film might
freeze. As freezing occurs, there is a pronounced decrease in the corrosion
rate, which is illustrated by the low corrosion rates in the sub-arctic and arctic regions.
In general, temperature is a factor that influences corrosion rates but it is of
little importance except under extreme conditions.
4.2.4 initial Exposure
Atmospheric corrosion rates proceed through three stages or periods: the
induction period, the transition period, and the stationary period. During
the induction period, the metal is covered with a spontaneously formed
oxide and the aqueous layer. This oxide provides some degree of protection,
depending on the metal and the aggressiveness of the atmosphere. During
the transition period, the oxide layer transforms into a fully developed layer
of corrosion products. The final or stationary period is characterized by the
surface being fully covered by corrosion product and eventually reaching
constant properties with respect to chemical composition and stationary
corrosion rates. The more aggressive the exposure conditions, the shorter
the two initial periods (i.e., induction and transition). For example, in a
benign (indoor) atmosphere, these initial periods for steel will last for years;
but in highly polluted industrial areas, these initial periods might last only
a few months.
Initial exposure conditions have a marked influence on the subsequent
corrosion rate. During the first days of exposure, wet conditions (caused by
high relative humidity or rainfall) cause higher corrosion rates than dry conditions. These effects will vary from one material to another. For example,
zinc is more sensitive than steel. Differences are explained by the fact that
different materials form different corrosion products with different protective properties. A wide variety of structurally related corrosion products can
be found on zinc, the nature of which depends on initial exposure conditions. The seasonal dependence on the concentrations of peroxide and ozone
in the atmosphere might also be a contributing factor.
During the third period of exposure, a constant corrosion rate is established — and thus the terminology “stationary period.”
4.2.5 Sheltering
The design of the installation should eliminate all possible areas where
water, dirt, and corrosion products can accumulate. The degree of shelter
from particle fallout and rainfall can affect the initial and ultimate corrosion