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Fundamentals of Corrosion
appreciably, whereas below this value, atmospheric corrosion is negligible.
The critical relative humidity varies for different metals and pollutants.
4.3.2 Wet atmospheric Corrosion (Phase Layers)
Wet atmospheric corrosion results from repeated wet and dry cycles, the
presence of pollutants, and the formation of an aqueous layer in which the
atmospheric pollutants dissolve. The wet cycles result from dew, fog, rain,
or snow. In many cases, the dew, fog, rain, or snow may already contain the
dissolved corrodent, which then deposits on the surface.
4.3.2.1 Dew
Dew is an important source of atmospheric corrosion — more so than rain —
and particularly under sheltered conditions. Dew forms when the temperature of the metal surface falls below the dew point of the atmosphere. This
can occur outdoors during the night when the surface temperature of the
metal is lowered as a result of radiant heat transfer between the metal and
the sky. It is also common for dew to form during the early morning hours
when the air temperature rises more quickly than the metal temperature.
Dew may also form when metal products are brought into warm storage
after cold shipment.
Under sheltered conditions, dew is an important cause of corrosion. The
high corrosivity of dew is a result of several factors:
1. Relatively speaking, the concentration of contaminants in dew
is higher than in rainwater, which lowers the pH values. Heavy
industrialized areas have reported pH values of dew in the range
of 3 or lower.
2. The washing effect that occurs with rain is usually slight or negligible with dew.
With little or no run-off, the pollutants remain in the electrolyte and continue
their corrosive action. As the dew dries, these contaminants remain on the
surface to repeat their corrosive activity with subsequent dew formation.
4.3.2.2 Rain
Depending on the conditions, rain can either increase or decrease the effects
of atmospheric corrosion. Corrosive action is caused by rain when a phase
layer of moisture is formed on the metal surface. Rain causes thicker layers of electrolyte on the surface than dew. The corrosive activity is increased
when the rain washes corrosive promoters such as Η + and SO 4
− from the air
(acid rain). Rain has the ability to decrease corrosive action on the surface of
Fundamentals of Corrosion
appreciably, whereas below this value, atmospheric corrosion is negligible.
The critical relative humidity varies for different metals and pollutants.
4.3.2 Wet atmospheric Corrosion (Phase Layers)
Wet atmospheric corrosion results from repeated wet and dry cycles, the
presence of pollutants, and the formation of an aqueous layer in which the
atmospheric pollutants dissolve. The wet cycles result from dew, fog, rain,
or snow. In many cases, the dew, fog, rain, or snow may already contain the
dissolved corrodent, which then deposits on the surface.
4.3.2.1 Dew
Dew is an important source of atmospheric corrosion — more so than rain —
and particularly under sheltered conditions. Dew forms when the temperature of the metal surface falls below the dew point of the atmosphere. This
can occur outdoors during the night when the surface temperature of the
metal is lowered as a result of radiant heat transfer between the metal and
the sky. It is also common for dew to form during the early morning hours
when the air temperature rises more quickly than the metal temperature.
Dew may also form when metal products are brought into warm storage
after cold shipment.
Under sheltered conditions, dew is an important cause of corrosion. The
high corrosivity of dew is a result of several factors:
1. Relatively speaking, the concentration of contaminants in dew
is higher than in rainwater, which lowers the pH values. Heavy
industrialized areas have reported pH values of dew in the range
of 3 or lower.
2. The washing effect that occurs with rain is usually slight or negligible with dew.
With little or no run-off, the pollutants remain in the electrolyte and continue
their corrosive action. As the dew dries, these contaminants remain on the
surface to repeat their corrosive activity with subsequent dew formation.
4.3.2.2 Rain
Depending on the conditions, rain can either increase or decrease the effects
of atmospheric corrosion. Corrosive action is caused by rain when a phase
layer of moisture is formed on the metal surface. Rain causes thicker layers of electrolyte on the surface than dew. The corrosive activity is increased
when the rain washes corrosive promoters such as Η + and SO 4
− from the air
(acid rain). Rain has the ability to decrease corrosive action on the surface of