82
Fundamentals of Corrosion
polluted atmosphere, where dry deposition is considerably greater than the
deposition of sulfur pollutants, the washing effect of rain dominates. In a
less polluted area, the situation is reversed, which indicates that the corrosive action of rain, in this case, is more important.
The pH value of precipitation seems to be significant for metals whose corrosion resistance may be ascribed to a protective layer of basic carbonates or
sulfates, as on zinc or copper. If the pH of rainwater falls to values close to 4 or
even lower, this may lead to accelerated dissolution of the protective coatings.
4.2.1.2.3 Fog
Especially high acidity and high concentrations of sulfates and nitrate can
be found in fog droplets in areas of high air pollution. In California, the pH
of fog water has been found to be in the range of 2.2 to 4.0. The processes
controlling fog water chemistry appear to be condensation of water vapor
on and its evaporation from preexisting aerosol and scavenging of gas-phase
nitric acid.
4.2.1.2.4 Dust
On a weight basis in many locations, dust is the primary air contaminant.
When in contact with metallic surfaces and combined with moisture, dust
can promote corrosion by forming galvanic or differential cells that, because
of their hygroscopic nature, form an electrolyte on the surface. Suspended
particles of carbon and carbon compounds, metal oxides, sulfuric acid,
ammonium sulfate, sodium chloride, and other salts will be found in industrial atmospheres. It is these materials, when combined with moisture, that
initiate corrosion.
The settled dust may promote corrosion by absorbing sulfur dioxide and
water vapor from the air. Hygroscopic salts such as chlorides or sulfates form
a corrosive electrolyte on the surface. Carbonaceous particles can start the
corrosion process by forming cathodes in microcells with a steel surface.
Dust-free air is less likely to cause corrosion.
4.2.1.2.5 Measurement of Time of Wetness
For practical purposes, the time of wetness is usually determined on the
basis of meteorological measurements of temperature and relative humidity.
The period when the relative humidity is ≥80% at temperatures greater than
32°F (0°C) is often used for estimating the actual time of wetness. The time of
wetness determined by this method may not necessarily be the same as the
“actual” time of wetness because wetness is influenced by the type of metal,
the pollution in the atmosphere, the presence of corrosion products, and the
degree of coverage against rain. The expression for time of wetness mentioned above, although not based on a detailed theoretical model, usually
shows good correlation with corrosion data from field tests under outdoor
conditions. This implies that this parameter corresponds to the kinetically
decisive time periods during which corrosion proceeds. Under sheltered and
Fundamentals of Corrosion
polluted atmosphere, where dry deposition is considerably greater than the
deposition of sulfur pollutants, the washing effect of rain dominates. In a
less polluted area, the situation is reversed, which indicates that the corrosive action of rain, in this case, is more important.
The pH value of precipitation seems to be significant for metals whose corrosion resistance may be ascribed to a protective layer of basic carbonates or
sulfates, as on zinc or copper. If the pH of rainwater falls to values close to 4 or
even lower, this may lead to accelerated dissolution of the protective coatings.
4.2.1.2.3 Fog
Especially high acidity and high concentrations of sulfates and nitrate can
be found in fog droplets in areas of high air pollution. In California, the pH
of fog water has been found to be in the range of 2.2 to 4.0. The processes
controlling fog water chemistry appear to be condensation of water vapor
on and its evaporation from preexisting aerosol and scavenging of gas-phase
nitric acid.
4.2.1.2.4 Dust
On a weight basis in many locations, dust is the primary air contaminant.
When in contact with metallic surfaces and combined with moisture, dust
can promote corrosion by forming galvanic or differential cells that, because
of their hygroscopic nature, form an electrolyte on the surface. Suspended
particles of carbon and carbon compounds, metal oxides, sulfuric acid,
ammonium sulfate, sodium chloride, and other salts will be found in industrial atmospheres. It is these materials, when combined with moisture, that
initiate corrosion.
The settled dust may promote corrosion by absorbing sulfur dioxide and
water vapor from the air. Hygroscopic salts such as chlorides or sulfates form
a corrosive electrolyte on the surface. Carbonaceous particles can start the
corrosion process by forming cathodes in microcells with a steel surface.
Dust-free air is less likely to cause corrosion.
4.2.1.2.5 Measurement of Time of Wetness
For practical purposes, the time of wetness is usually determined on the
basis of meteorological measurements of temperature and relative humidity.
The period when the relative humidity is ≥80% at temperatures greater than
32°F (0°C) is often used for estimating the actual time of wetness. The time of
wetness determined by this method may not necessarily be the same as the
“actual” time of wetness because wetness is influenced by the type of metal,
the pollution in the atmosphere, the presence of corrosion products, and the
degree of coverage against rain. The expression for time of wetness mentioned above, although not based on a detailed theoretical model, usually
shows good correlation with corrosion data from field tests under outdoor
conditions. This implies that this parameter corresponds to the kinetically
decisive time periods during which corrosion proceeds. Under sheltered and