104
Fundamentals of Corrosion
4.7.2 Weathering Steels
Weathering steels are steels to which small amounts of copper, chromium,
nickel, phosphorus, or various combinations thereof have been added. This
results in a low-alloy carbon steel that has improved corrosion resistance in
rural areas exhibiting low pollution levels. Factors that affect the corrosion
resistance of these steels are:
Climatic conditions
•
Pollution levels
•
Degree of sheltering from the atmosphere
•
Specific composition of the steel
•
Exposure to most atmospheres results in a corrosion rate that becomes stabilized in 3 to 5 years. Over this period of time, a protective film or patina,
dark-brown to violet in color, forms. This patina is a tightly adhering rust
formation on the surface of the steel that cannot be wiped off. Because the
formation of this film depends on the pollution in the air, in rural areas
where there may be little or no pollution, a longer time may be required to
form this film. In areas that have a high level of SO 2 , loose rust particles are
formed with a much higher corrosion rate. This film of loose particles offers
little or no protection against continued corrosion.
When chlorides are present, such as in a marine environment, the protective film will not form. Under these conditions, the corrosion rates of the
weathering steels are equivalent to that of unalloyed carbon steel.
To form the patina, a series of wet and dry periods is required. If the steel
is installed in such a manner as to be sheltered from the rain, the dark patina
does not form. Instead, a rust lighter in color forms that provides the same
resistance. The corrosion rates of the weathering steels will be the same as
the corrosion rate of the unalloyed steel when it is continuously exposed to
wetness such as in water or soil.
Because the dark brown patina formed has a “pheasant” 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.
To receive the maximum benefit from the weathering steels, consideration
must be given to the design. The design should eliminate all possible areas
where dirt, water, and corrosion products can accumulate. When pockets are
present, the time of wetness increases, which leads to the development of
corrosive conditions. The design should make maximum use of exposure to
the weather. Sheltering from rain should be avoided.
While the protective film is forming, rusting will proceed at a relatively high
rate, during which time rusty water will be produced. This rusty water may
stain masonry, pavements, and the like. Consequently, steps should be taken
Fundamentals of Corrosion
4.7.2 Weathering Steels
Weathering steels are steels to which small amounts of copper, chromium,
nickel, phosphorus, or various combinations thereof have been added. This
results in a low-alloy carbon steel that has improved corrosion resistance in
rural areas exhibiting low pollution levels. Factors that affect the corrosion
resistance of these steels are:
Climatic conditions
•
Pollution levels
•
Degree of sheltering from the atmosphere
•
Specific composition of the steel
•
Exposure to most atmospheres results in a corrosion rate that becomes stabilized in 3 to 5 years. Over this period of time, a protective film or patina,
dark-brown to violet in color, forms. This patina is a tightly adhering rust
formation on the surface of the steel that cannot be wiped off. Because the
formation of this film depends on the pollution in the air, in rural areas
where there may be little or no pollution, a longer time may be required to
form this film. In areas that have a high level of SO 2 , loose rust particles are
formed with a much higher corrosion rate. This film of loose particles offers
little or no protection against continued corrosion.
When chlorides are present, such as in a marine environment, the protective film will not form. Under these conditions, the corrosion rates of the
weathering steels are equivalent to that of unalloyed carbon steel.
To form the patina, a series of wet and dry periods is required. If the steel
is installed in such a manner as to be sheltered from the rain, the dark patina
does not form. Instead, a rust lighter in color forms that provides the same
resistance. The corrosion rates of the weathering steels will be the same as
the corrosion rate of the unalloyed steel when it is continuously exposed to
wetness such as in water or soil.
Because the dark brown patina formed has a “pheasant” 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.
To receive the maximum benefit from the weathering steels, consideration
must be given to the design. The design should eliminate all possible areas
where dirt, water, and corrosion products can accumulate. When pockets are
present, the time of wetness increases, which leads to the development of
corrosive conditions. The design should make maximum use of exposure to
the weather. Sheltering from rain should be avoided.
While the protective film is forming, rusting will proceed at a relatively high
rate, during which time rusty water will be produced. This rusty water may
stain masonry, pavements, and the like. Consequently, steps should be taken