Atmospheric Corrosion
79
corrosion problems. The corrosivity of an industrial atmosphere diminishes
with increasing distance from the city.
Marine environments are subject to chloride attack resulting from the deposition of crystals formed by the evaporation of spray that has been carried
by the wind from the sea. The quantity of chloride deposition from marine
environments is directly proportional to the distance from the shore. The
closer to the shore, the greater the deposition and corrosive effect. The atmospheric test station at Kure Beach, North Carolina, shows that steels exposed
80 ft from the ocean corrode 10 to 15 times faster than steels exposed 800 ft
from the ocean. In addition to these general air contaminants, there may also
be specific pollutants found in a localized area. These may be emitted from
a manufacturing operation on a continuous basis and can result in a much
more serious corrosion problem than that caused by the presence of general
atmospheric pollutants.
Because of these varying conditions, a material that is resistant to atmospheric corrosion in one area may not be suitable in another area.
To compound the problem, there is no clear line of demarcation between
these atmospheric types. In many cases, there is no “pure” rural or urban area.
Contamination from industrial or marine areas can find its way into these
areas based on the prevailing winds and other atmospheric conditions.
Indoor atmospheres might be free of corrosion in “clean rooms” or subject
to severe corrosion, as around a pickling bath in a steel mill.
Atmospheric conditions should be defined in terms of temperature,
humidity, and contaminants, as well as their corrosivity to specific materials of construction being considered. In addition to the general atmospheric
condition, special conditions such as cooling tower drift or spray, spills, or
releases of water or chemicals should not be overlooked and must be taken
into account.
4.2 Factors Affecting Atmospheric Corrosion
Atmospheric corrosion is an electrochemical process and as such depends on
the presence of an electrolyte. The usual electrolyte associated with atmospheric corrosion is water resulting from rain, fog, dew, melting snow, or
high humidity. Because an electrolyte is not always present, atmospheric
corrosion is considered a discontinuous process. Corrosion takes place only
during the time of wetness. It can be described by the equation:
K
L
h
= ∑ t V (n)
n k
79
corrosion problems. The corrosivity of an industrial atmosphere diminishes
with increasing distance from the city.
Marine environments are subject to chloride attack resulting from the deposition of crystals formed by the evaporation of spray that has been carried
by the wind from the sea. The quantity of chloride deposition from marine
environments is directly proportional to the distance from the shore. The
closer to the shore, the greater the deposition and corrosive effect. The atmospheric test station at Kure Beach, North Carolina, shows that steels exposed
80 ft from the ocean corrode 10 to 15 times faster than steels exposed 800 ft
from the ocean. In addition to these general air contaminants, there may also
be specific pollutants found in a localized area. These may be emitted from
a manufacturing operation on a continuous basis and can result in a much
more serious corrosion problem than that caused by the presence of general
atmospheric pollutants.
Because of these varying conditions, a material that is resistant to atmospheric corrosion in one area may not be suitable in another area.
To compound the problem, there is no clear line of demarcation between
these atmospheric types. In many cases, there is no “pure” rural or urban area.
Contamination from industrial or marine areas can find its way into these
areas based on the prevailing winds and other atmospheric conditions.
Indoor atmospheres might be free of corrosion in “clean rooms” or subject
to severe corrosion, as around a pickling bath in a steel mill.
Atmospheric conditions should be defined in terms of temperature,
humidity, and contaminants, as well as their corrosivity to specific materials of construction being considered. In addition to the general atmospheric
condition, special conditions such as cooling tower drift or spray, spills, or
releases of water or chemicals should not be overlooked and must be taken
into account.
4.2 Factors Affecting Atmospheric Corrosion
Atmospheric corrosion is an electrochemical process and as such depends on
the presence of an electrolyte. The usual electrolyte associated with atmospheric corrosion is water resulting from rain, fog, dew, melting snow, or
high humidity. Because an electrolyte is not always present, atmospheric
corrosion is considered a discontinuous process. Corrosion takes place only
during the time of wetness. It can be described by the equation:
K
L
h
= ∑ t V (n)
n k