96
Fundamentals of Corrosion
acids such as Cu + and Ag + coordinate with reduced sulfur compounds.
Intermediate acids such as Fe 2+ , Cu 2+ , and Zn 2+ would be expected to coordinate with a broader range of bases.
The corrosion products formed may be amorphous or crystalline, depending on the rate of crystallization and formation. It is known that slow growth
or aging of amorphous phases may result in a change to the crystalline state.
This process can occur through slow transformation in the solid state or
through dissolution–reprecipitation processes. Such is the case in the transformation from amorphous to crystalline state of nickel sulfates, with the
former being less corrosion resistant than the latter.
4.5 Specific Atmospheric Corrodents
The atmospheric region closest to Earth is known as the troposphere and
contains nitrogen (N 2 ), oxygen (O 2 ), and the rare gases neon (Ne), krypton
(Kr), helium (He), and xenon (Xe). Of all the molecules involved, these make
up 99.9% by weight and of these, only oxygen plays a part in atmospheric
corrosion. The remaining constituents, nitrogen and the rare gases, due to
their inability to react with metal surfaces, are not of significant importance
to atmospheric corrosion.
Oxygen, because of its ability to accept electrons and its involvement in
chemical transformations of the atmosphere, is particularly important to
atmospheric corrosion. Other materials present in the troposphere that play
a part in atmospheric corrosion are water and carbon dioxide. Water acts as
an electrolyte; and carbon dioxide, which has a concentration of approximately 330 ppm and is highly soluble in water, contributes to the acidity of
the aqueous layer.
Other trace gases present with a total concentration of less than 10 ppm,
and also of importance in atmospheric corrosion, are O 3 , H 2 O 2 , SO 2 , H 2 S, COS,
NO 2 , HNO 3 , NH 3 , HCl, Cl 2 , HCHO, and HCOOH. Either natural or anthropogenic processes are responsible for their presence, and they may undergo
a variety of chemical changes during their presence in the troposphere. All
species are reactive and exhibit a specific average lifetime, which is limited
by their ability to react with important atmospheric oxidizers, specifically
the hydroxyl radical OH − and O 3 . The hydroxyl radical is formed by the photoinduced dissociation of ozone and the subsequent reaction of the electronically excited, energy-rich oxygen ion O('D) and water vapor:
O +hv O('D)+O
n m
O('D)+H O 2OH
2
2
2
→
<
→
−
(
)
λ 310
Fundamentals of Corrosion
acids such as Cu + and Ag + coordinate with reduced sulfur compounds.
Intermediate acids such as Fe 2+ , Cu 2+ , and Zn 2+ would be expected to coordinate with a broader range of bases.
The corrosion products formed may be amorphous or crystalline, depending on the rate of crystallization and formation. It is known that slow growth
or aging of amorphous phases may result in a change to the crystalline state.
This process can occur through slow transformation in the solid state or
through dissolution–reprecipitation processes. Such is the case in the transformation from amorphous to crystalline state of nickel sulfates, with the
former being less corrosion resistant than the latter.
4.5 Specific Atmospheric Corrodents
The atmospheric region closest to Earth is known as the troposphere and
contains nitrogen (N 2 ), oxygen (O 2 ), and the rare gases neon (Ne), krypton
(Kr), helium (He), and xenon (Xe). Of all the molecules involved, these make
up 99.9% by weight and of these, only oxygen plays a part in atmospheric
corrosion. The remaining constituents, nitrogen and the rare gases, due to
their inability to react with metal surfaces, are not of significant importance
to atmospheric corrosion.
Oxygen, because of its ability to accept electrons and its involvement in
chemical transformations of the atmosphere, is particularly important to
atmospheric corrosion. Other materials present in the troposphere that play
a part in atmospheric corrosion are water and carbon dioxide. Water acts as
an electrolyte; and carbon dioxide, which has a concentration of approximately 330 ppm and is highly soluble in water, contributes to the acidity of
the aqueous layer.
Other trace gases present with a total concentration of less than 10 ppm,
and also of importance in atmospheric corrosion, are O 3 , H 2 O 2 , SO 2 , H 2 S, COS,
NO 2 , HNO 3 , NH 3 , HCl, Cl 2 , HCHO, and HCOOH. Either natural or anthropogenic processes are responsible for their presence, and they may undergo
a variety of chemical changes during their presence in the troposphere. All
species are reactive and exhibit a specific average lifetime, which is limited
by their ability to react with important atmospheric oxidizers, specifically
the hydroxyl radical OH − and O 3 . The hydroxyl radical is formed by the photoinduced dissociation of ozone and the subsequent reaction of the electronically excited, energy-rich oxygen ion O('D) and water vapor:
O +hv O('D)+O
n m
O('D)+H O 2OH
2
2
2
→
<
→
−
(
)
λ 310