Atmospheric Corrosion
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indoor conditions, however, other criteria seem to be valid, but have not yet
been fully clarified.
The time of wetness can also be measured directly with electrochemical
cells. The cell consists of thin metal electrodes (0.5 to 1 mm) separated from
each other by thin insulation (0.1 to 0.2 mm). When the surface becomes wetted, a current starts to flow and the time of wetness is defined as the time
when the cell current or the electromotive force exceeds a certain value. Not
even the direct electrochemical measurement of the time of wetness will
give an unambiguous value, because the result depends — among other factors — on the type of cell, its dimensions, the presence of corrosion products,
and the definition of the threshold value of current or voltage that is considered the lower limit for the time of wetness. Most electrochemical techniques
indicate mainly the time of wetness caused by phase layers of electrolyte,
and they usually give lower values than calculations from meteorological
data. Values between 1000 and 2700 hours per year (h/yr) are often reported
from sites in the temperate climatic zone.
4.2.2 Composition of Surface Electrolyte
The electrolyte film on the surface will contain various species deposited
from the atmosphere or originating from the corroding metal. The composition of the electrolyte is the determining factor of the corrosion process.
The following provides a brief survey of the origin, transformation reactions,
deposition mechanisms, and pollutants.
4.2.2.1 Oxygen
Oxygen is readily absorbed from the air, so that at least the outer region of the
thin water film on the metal surface may be considered saturated with oxygen.
4.2.2.2 SO x
The main part of anthropogenic SO x pollution is caused by combustion of
fossil fuels (i.e., oil and coal in industrial regions), which can cover less than
5% of the Earth’s surface.
Most of the sulfur derived from the burning of fossil fuels is emitted in
gaseous form as SO 2 . Both the chemical composition and the physical state
of the pollutants change during their transport in the atmosphere. SO 2 is oxidized on moist particles or in droplets of water to sulfuric acid (H 2 SO 4 ).
4.2.2.3 NO x
Emissions of NO x originate primarily from different combustion processes,
road traffic, and energy production. Since 1940, the atmospheric emission of
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indoor conditions, however, other criteria seem to be valid, but have not yet
been fully clarified.
The time of wetness can also be measured directly with electrochemical
cells. The cell consists of thin metal electrodes (0.5 to 1 mm) separated from
each other by thin insulation (0.1 to 0.2 mm). When the surface becomes wetted, a current starts to flow and the time of wetness is defined as the time
when the cell current or the electromotive force exceeds a certain value. Not
even the direct electrochemical measurement of the time of wetness will
give an unambiguous value, because the result depends — among other factors — on the type of cell, its dimensions, the presence of corrosion products,
and the definition of the threshold value of current or voltage that is considered the lower limit for the time of wetness. Most electrochemical techniques
indicate mainly the time of wetness caused by phase layers of electrolyte,
and they usually give lower values than calculations from meteorological
data. Values between 1000 and 2700 hours per year (h/yr) are often reported
from sites in the temperate climatic zone.
4.2.2 Composition of Surface Electrolyte
The electrolyte film on the surface will contain various species deposited
from the atmosphere or originating from the corroding metal. The composition of the electrolyte is the determining factor of the corrosion process.
The following provides a brief survey of the origin, transformation reactions,
deposition mechanisms, and pollutants.
4.2.2.1 Oxygen
Oxygen is readily absorbed from the air, so that at least the outer region of the
thin water film on the metal surface may be considered saturated with oxygen.
4.2.2.2 SO x
The main part of anthropogenic SO x pollution is caused by combustion of
fossil fuels (i.e., oil and coal in industrial regions), which can cover less than
5% of the Earth’s surface.
Most of the sulfur derived from the burning of fossil fuels is emitted in
gaseous form as SO 2 . Both the chemical composition and the physical state
of the pollutants change during their transport in the atmosphere. SO 2 is oxidized on moist particles or in droplets of water to sulfuric acid (H 2 SO 4 ).
4.2.2.3 NO x
Emissions of NO x originate primarily from different combustion processes,
road traffic, and energy production. Since 1940, the atmospheric emission of