Atmospheric Corrosion
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protective film on the base metal. Due to the lack of protective film formation, white rust corrosion will continue until the protective zinc coating is
entirely removed from the underlying steel, which is then subjected to accelerated corrosion and premature failure. White rust will occur whenever zinc
or galvanized steel is exposed to water that has a pH value above 8.2. The
corrosion rate is governed by the alkalinity of the water and the presence of
any accelerating agents. Why the basic zinc carbonate cannot form has not
been established. It may be due to long-lasting moistening or to a poor supply of CO 2 . White rust is more prevalent during rainy seasons. It may also
form on moist zinc surfaces in contact with mineral wool, which prevents
access of air and drying. This condition may happen in heat-insulated wall
structures containing galvanized steel members.
In years gone by, zinc coating of steel (galvanizing) was considered a satisfactory corrosion-resistant material for structures in outdoor atmospheres.
The corrosion rate was only 0.5 to 1.0 µm/yr. However, the corrosion rate has
been increasing in many urban or industrial areas as a result of increasing
pollution by SO 2 . Corrosion rates have reached 5 µm/yr or more in many
areas. Because of this, it is often necessary to apply to galvanized steel a coat
of anticorrosion paint for added protection.
4.7.4 Zinc and Zinc alloy Coatings on Steel
Galvanized steel is steel that has been coated with zinc. The galvanizing process is widely used to protect steel from atmospheric corrosion. Structures,
sheet steel, wire, and piping are all forms that are protected by galvanizing.
The protection afforded in rural areas is greater than that in urban or
industrial atmospheres. In the latter areas there is a greater concentration
of industrial pollutants. The air in these areas is contaminated with various
sulfur compounds, which together with the moisture in the air convert the
normally impervious corrosion-resistant zinc carbonate and zinc oxide layer
into zinc sulfate and zinc sulfite. These water-soluble compounds have poor
adhesion to the zinc surface and therefore are washed away relatively easily
by rain. This exposes the underlying surface to attack by oxygen in the air.
A galvanized steel coating normally corrodes in the same manner as solid
zinc. In areas where the zinc coating is defective, the exposed steel, under
most conditions, is cathodically protected. Because of the iron-zinc alloy
present in most galvanized coatings, galvanized surfaces have a higher corrosion resistance than zinc in neutral and acid conditions. Zinc coatings on
steel exposed to air corrode in the same manner as solid zinc, and there is an
approximate linear relationship between weight loss and time. In industrial
atmospheres, the purity of the coating has little effect on the corrosion rates.
Protection of galvanized surfaces from atmospheric corrosion is due to the
formation of basic salts, primarily carbonate. The most widely accepted formula is Zn 5 (OH) 6 (CO 3 ) 2 . The galvanized surface of zinc is rapidly attacked
when environmental conditions lead to the formation of soluble film. One of
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protective film on the base metal. Due to the lack of protective film formation, white rust corrosion will continue until the protective zinc coating is
entirely removed from the underlying steel, which is then subjected to accelerated corrosion and premature failure. White rust will occur whenever zinc
or galvanized steel is exposed to water that has a pH value above 8.2. The
corrosion rate is governed by the alkalinity of the water and the presence of
any accelerating agents. Why the basic zinc carbonate cannot form has not
been established. It may be due to long-lasting moistening or to a poor supply of CO 2 . White rust is more prevalent during rainy seasons. It may also
form on moist zinc surfaces in contact with mineral wool, which prevents
access of air and drying. This condition may happen in heat-insulated wall
structures containing galvanized steel members.
In years gone by, zinc coating of steel (galvanizing) was considered a satisfactory corrosion-resistant material for structures in outdoor atmospheres.
The corrosion rate was only 0.5 to 1.0 µm/yr. However, the corrosion rate has
been increasing in many urban or industrial areas as a result of increasing
pollution by SO 2 . Corrosion rates have reached 5 µm/yr or more in many
areas. Because of this, it is often necessary to apply to galvanized steel a coat
of anticorrosion paint for added protection.
4.7.4 Zinc and Zinc alloy Coatings on Steel
Galvanized steel is steel that has been coated with zinc. The galvanizing process is widely used to protect steel from atmospheric corrosion. Structures,
sheet steel, wire, and piping are all forms that are protected by galvanizing.
The protection afforded in rural areas is greater than that in urban or
industrial atmospheres. In the latter areas there is a greater concentration
of industrial pollutants. The air in these areas is contaminated with various
sulfur compounds, which together with the moisture in the air convert the
normally impervious corrosion-resistant zinc carbonate and zinc oxide layer
into zinc sulfate and zinc sulfite. These water-soluble compounds have poor
adhesion to the zinc surface and therefore are washed away relatively easily
by rain. This exposes the underlying surface to attack by oxygen in the air.
A galvanized steel coating normally corrodes in the same manner as solid
zinc. In areas where the zinc coating is defective, the exposed steel, under
most conditions, is cathodically protected. Because of the iron-zinc alloy
present in most galvanized coatings, galvanized surfaces have a higher corrosion resistance than zinc in neutral and acid conditions. Zinc coatings on
steel exposed to air corrode in the same manner as solid zinc, and there is an
approximate linear relationship between weight loss and time. In industrial
atmospheres, the purity of the coating has little effect on the corrosion rates.
Protection of galvanized surfaces from atmospheric corrosion is due to the
formation of basic salts, primarily carbonate. The most widely accepted formula is Zn 5 (OH) 6 (CO 3 ) 2 . The galvanized surface of zinc is rapidly attacked
when environmental conditions lead to the formation of soluble film. One of