The Equipment Design Process
341
Under exposure to most atmospheres, the corrosion rate becomes stabilized
within 3 to 5 years. A dark brown to violet patina or protective film forms
during this period. This patina is a rust formation that tightly adheres to the
surface and cannot be wiped off. In rural areas with little or no pollution, a
longer period may be required to form this protective film. In areas that are
highly polluted with SO 2 , the weathering steels exhibit a much higher corrosion rate and the formation of loose particles of rust. Under these conditions,
the film provides little or no protection.
These steels do not form this protective film in marine environments
where chlorides are present. Corrosion rates will be as high as for conventional carbon steels. Table 11.1 compares the corrosion rate of carbon steel
and weathering steels in various atmospheres.
The formation of a patina depends on a series of wet and dry periods.
Periods of flushing followed by periods of drying are necessary to develop
the patina. In areas where the steel is sheltered from the rain, the dark patina
does not form. Instead, a layer of rust in lighter color forms, which has the
same effect.
Because the patina formed has a pleasant aesthetic appearance, the weathering steels can be used without the application of any protective coating
of antirust paint, zinc, or aluminum. This is particularly true in urban and
rural areas.
When employing these weathering steels, design consideration should be
given to the elimination of possible areas where water, dirt, and corrosion
products can accumulate. If pockets are present, the time of wetness will be
increased, which will lead to the development of corrosive conditions. The
TabLE 11.1
Corrosion of Carbon Steels in Various Types of Atmospheres
Atmosphere
Exposure
Time
(yrs)
Average Reduction in Thickness (mil)
Carbon
Steel
A242
(K11510) a
Cu-P steel
A588 (K11430) a
Cr-V-Cu Steel
Urban industrial
3.5
3.3
1.3
1.8
7.5
4.1
1.5
2.1
Rural
3.5
2.0
1.1
1.4
7.5
3.0
1.3
1.5
Severe marine 2 5M/80 ft from ocean
0.5
7.2
2.2
3.8
2.0
36.0
3.3
12.2
3.5
57.0
—
28.7
5.0
Destroyed
14.4
38.8
a Weathering steels.
341
Under exposure to most atmospheres, the corrosion rate becomes stabilized
within 3 to 5 years. A dark brown to violet patina or protective film forms
during this period. This patina is a rust formation that tightly adheres to the
surface and cannot be wiped off. In rural areas with little or no pollution, a
longer period may be required to form this protective film. In areas that are
highly polluted with SO 2 , the weathering steels exhibit a much higher corrosion rate and the formation of loose particles of rust. Under these conditions,
the film provides little or no protection.
These steels do not form this protective film in marine environments
where chlorides are present. Corrosion rates will be as high as for conventional carbon steels. Table 11.1 compares the corrosion rate of carbon steel
and weathering steels in various atmospheres.
The formation of a patina depends on a series of wet and dry periods.
Periods of flushing followed by periods of drying are necessary to develop
the patina. In areas where the steel is sheltered from the rain, the dark patina
does not form. Instead, a layer of rust in lighter color forms, which has the
same effect.
Because the patina formed has a pleasant aesthetic appearance, the weathering steels can be used without the application of any protective coating
of antirust paint, zinc, or aluminum. This is particularly true in urban and
rural areas.
When employing these weathering steels, design consideration should be
given to the elimination of possible areas where water, dirt, and corrosion
products can accumulate. If pockets are present, the time of wetness will be
increased, which will lead to the development of corrosive conditions. The
TabLE 11.1
Corrosion of Carbon Steels in Various Types of Atmospheres
Atmosphere
Exposure
Time
(yrs)
Average Reduction in Thickness (mil)
Carbon
Steel
A242
(K11510) a
Cu-P steel
A588 (K11430) a
Cr-V-Cu Steel
Urban industrial
3.5
3.3
1.3
1.8
7.5
4.1
1.5
2.1
Rural
3.5
2.0
1.1
1.4
7.5
3.0
1.3
1.5
Severe marine 2 5M/80 ft from ocean
0.5
7.2
2.2
3.8
2.0
36.0
3.3
12.2
3.5
57.0
—
28.7
5.0
Destroyed
14.4
38.8
a Weathering steels.