Atmospheric Corrosion
87
patina, or protective film, develops over 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
film. In areas that are highly polluted with SO 2 , the weathering steels exhibit
a much higher corrosion rate and loose rust particles form. Under these conditions, the film formed offers little or no protection.
Additional information regarding weathering steels and other metals or
alloys as to their resistance to atmospheric corrosion can be found in the section dealing with the specific material.
4.2.8 Pollutants Present
One of the most important factors affecting atmospheric corrosion is the
presence of specific pollutants. In areas having low atmospheric pollution,
corrosion rates are correspondingly low. The presence of atmospheric pollutants such as the various oxides of nitrogen, sulfur-containing compounds,
chlorine-containing compounds, and other, less common pollutants will
stimulate corrosion.
It has been proven that the following gaseous constituents are of significant importance in contributing to atmospheric corrosion: O 2 , H 2 O 2 , SO 2 , H 2 S,
COS, NO 2 , HNO 3 , NH 3 , HCl, HCHO, and HCOOH. Typical ranges of these
materials found under outdoor and indoor conditions are given in Tables 4.1
and 4.2. They may be present as the result of either natural or anthropogenic
processes, and may undergo a variety of chemical changes during transport
in the atmosphere.
All of the species are reactive and as such have a certain average lifetime
that is limited by their ability to react with atmospheric oxidizers, primarily the hydroxyl radicals OH − and O 3 . OH – is generated by photoinduced
dissociation of O 3 (ozone) and the subsequent reaction of the electronically
excited, energy-rich oxygen atom O ('D) and water vapor:
O + hv O('D) + O , ( <310nm)
O(C'D) + H O 2O
3
2
2
→
→
λ
H H
−
It is possible for the OH − molecules to oxidize several of the species, such
as SO 2 , H 2 S, and NO. However, a large portion of the OH − molecules is consumed through reactions with hydrocarbon molecules producing an end
product of HO 2 (the hydroperoxyl radical). This radical converts to hydrogen
peroxide (H 2 O 2 ) and O 2 according to:
HO +HO
H O +O
2
0
2
0
2 2
2
→
87
patina, or protective film, develops over 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
film. In areas that are highly polluted with SO 2 , the weathering steels exhibit
a much higher corrosion rate and loose rust particles form. Under these conditions, the film formed offers little or no protection.
Additional information regarding weathering steels and other metals or
alloys as to their resistance to atmospheric corrosion can be found in the section dealing with the specific material.
4.2.8 Pollutants Present
One of the most important factors affecting atmospheric corrosion is the
presence of specific pollutants. In areas having low atmospheric pollution,
corrosion rates are correspondingly low. The presence of atmospheric pollutants such as the various oxides of nitrogen, sulfur-containing compounds,
chlorine-containing compounds, and other, less common pollutants will
stimulate corrosion.
It has been proven that the following gaseous constituents are of significant importance in contributing to atmospheric corrosion: O 2 , H 2 O 2 , SO 2 , H 2 S,
COS, NO 2 , HNO 3 , NH 3 , HCl, HCHO, and HCOOH. Typical ranges of these
materials found under outdoor and indoor conditions are given in Tables 4.1
and 4.2. They may be present as the result of either natural or anthropogenic
processes, and may undergo a variety of chemical changes during transport
in the atmosphere.
All of the species are reactive and as such have a certain average lifetime
that is limited by their ability to react with atmospheric oxidizers, primarily the hydroxyl radicals OH − and O 3 . OH – is generated by photoinduced
dissociation of O 3 (ozone) and the subsequent reaction of the electronically
excited, energy-rich oxygen atom O ('D) and water vapor:
O + hv O('D) + O , ( <310nm)
O(C'D) + H O 2O
3
2
2
→
→
λ
H H
−
It is possible for the OH − molecules to oxidize several of the species, such
as SO 2 , H 2 S, and NO. However, a large portion of the OH − molecules is consumed through reactions with hydrocarbon molecules producing an end
product of HO 2 (the hydroperoxyl radical). This radical converts to hydrogen
peroxide (H 2 O 2 ) and O 2 according to:
HO +HO
H O +O
2
0
2
0
2 2
2
→