Atmospheric Corrosion
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depends on the specific metal and the surface contaminants. When the relative humidity exceeds the value at which the salt starts to absorb water and
dissolve, the corrosion increases sharply. The critical relative humidity corresponds to the vapor pressure above a saturated solution of the salt present.
The corrosion rate is directly affected by the amount of water present on the
corroding surface.
4.2.1.2 Phase Layers
Phase layers may arise from precipitation of rain, fog, or wet or melting snow,
or from dew formed by condensation on cold metallic surfaces.
4.2.1.2.1 Dew
Dew formation occurs when the temperature of the metal surface is below
the dew point of the atmosphere. This may occur outdoors during the night,
when the surface temperature may decrease by radiant heat transfer between
the structure and the sky. Another reason for dew formation may be the conditions in the early morning, when the temperature of the air increases faster
than the temperature of the metal, especially if the mass, and thus the heat
capacity, of the metal is great. Dew may also form when metal products are
brought into warm storage after cold transport.
Dew is a very important cause of corrosion, particularly under sheltered
conditions. Compared to the amount of water on surfaces covered by adsorption layers, the amount of water on the surface resulting from dew is considerably greater. Because the washing effect is usually slight or negligible,
periods of dew are considered very corrosive.
One factor responsible for the high corrosivity of dew is the large content
of atmospheric contaminants present in dew. In heavily polluted industrial
atmospheres, pH values lower than 3 have been reported. This applies to inert
substrates, such as painted surfaces on which dry deposition between periods of rain can lead to the accumulation of fairly acidic corrosive pollutants.
4.2.1.2.2 Rain
Rain creates even thicker layers of electrolyte on the surface than dew. The
thickness of the water layer retained on the surface has been estimated to be
approximately 100 g/m 2 . Precipitation in the form of rain affects corrosion
by giving rise to a phase layer of moisture on the material surface and by
adding corrosion stimulators in the form of, for example, Η + and SO 4
2− . On
the other hand, rain washes away pollutants deposited on the surface during the dry period. Whereas the first two processes promote corrosion, the
third — at least in the case of steel — decreases corrosion. The significance of
the two latter processes is dependent on the ratio between the dry and wet
deposition periods.
In a strongly polluted atmosphere, corrosion on the skyward side of
steel plates is substantially lower than on the downward side. In a strongly
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depends on the specific metal and the surface contaminants. When the relative humidity exceeds the value at which the salt starts to absorb water and
dissolve, the corrosion increases sharply. The critical relative humidity corresponds to the vapor pressure above a saturated solution of the salt present.
The corrosion rate is directly affected by the amount of water present on the
corroding surface.
4.2.1.2 Phase Layers
Phase layers may arise from precipitation of rain, fog, or wet or melting snow,
or from dew formed by condensation on cold metallic surfaces.
4.2.1.2.1 Dew
Dew formation occurs when the temperature of the metal surface is below
the dew point of the atmosphere. This may occur outdoors during the night,
when the surface temperature may decrease by radiant heat transfer between
the structure and the sky. Another reason for dew formation may be the conditions in the early morning, when the temperature of the air increases faster
than the temperature of the metal, especially if the mass, and thus the heat
capacity, of the metal is great. Dew may also form when metal products are
brought into warm storage after cold transport.
Dew is a very important cause of corrosion, particularly under sheltered
conditions. Compared to the amount of water on surfaces covered by adsorption layers, the amount of water on the surface resulting from dew is considerably greater. Because the washing effect is usually slight or negligible,
periods of dew are considered very corrosive.
One factor responsible for the high corrosivity of dew is the large content
of atmospheric contaminants present in dew. In heavily polluted industrial
atmospheres, pH values lower than 3 have been reported. This applies to inert
substrates, such as painted surfaces on which dry deposition between periods of rain can lead to the accumulation of fairly acidic corrosive pollutants.
4.2.1.2.2 Rain
Rain creates even thicker layers of electrolyte on the surface than dew. The
thickness of the water layer retained on the surface has been estimated to be
approximately 100 g/m 2 . Precipitation in the form of rain affects corrosion
by giving rise to a phase layer of moisture on the material surface and by
adding corrosion stimulators in the form of, for example, Η + and SO 4
2− . On
the other hand, rain washes away pollutants deposited on the surface during the dry period. Whereas the first two processes promote corrosion, the
third — at least in the case of steel — decreases corrosion. The significance of
the two latter processes is dependent on the ratio between the dry and wet
deposition periods.
In a strongly polluted atmosphere, corrosion on the skyward side of
steel plates is substantially lower than on the downward side. In a strongly