Marcus P. Corrosion mechanisms in theory and practice
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522 Rohwerder et al.
Figure 37 Progress of the delamination front as a function of time after the defect
activation for the different surface pretreatments presented in Figure 31 [105].
Figure 38 XP survey spectra of the interface of the delaminated region in comparison
with the primer composition [105].
Copyright © 2002 Marcel Dekker, Inc.
primer/plasma polymer interface. Figure 38 shows the survey spectra of the
underside of the polymer and the metal surface in the delaminated area. The
underside of the polymer clearly shows the presence of HMDS plasma polymer,
indicated by the Si2p and Si2s peaks. The metal surface, on the other hand, exhibits
only negligible amounts of silicon but a significant amount of sodium. Thus, the XP
spectra in Figure 38 confirm that the interface failure takes place between the plasma
polymer and the oxide.
Corrosion Protection of Aluminum and Copper Surfaces
For passive metals such as aluminum and copper, plasma polymers have been used
as thin corrosion-protecting layers without an additional top coat [123–125]. Oehr
and Schindler [124] deposited plasma polymer barrier layers on aluminum and gold.
Vinyltrimethylsilane showed good adhesion to gold and a sufficient degree of
stability against sulfuric acid. Iriyama et al. [123] studied plasma polymerization of
tetraethoxysilane as a corrosion-protecting layer for aluminum granules. Corrosion
resistance was tested in an alkaline medium. The dissolved aluminum was compared
for unmodified and plasma polymer–covered granules. Effective corrosion protection
could be achieved at pH 11.
Lin and Yasuda [125] considered the effect of plasma polymer deposition
methods on the corrosion protection of copper. The properties of trimethylsilane
plasma polymers were investigated in DC cathodic polymerization, with and without
anode magnetron enhancement, af magnetron glow discharge polymerization, and rf
glow discharge polymerization. The corrosion test consisted of wet/dry cycles using
0.1 M NaCl. DC magnetron cathodic polymerization offered the best corrosion
protection based on enhanced deposition uniformity and adhesion. The application of
plasma polymers with permeation resistance and surface dynamic stability greatly
reduced the pitting densities.
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Corrosion Prevention by Adsorbed Monolayers 527
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529
15
Atmospheric Corrosion
Christofer Leygraf
Royal Institute of Technology, Stockholm, Sweden
INTRODUCTION
Atmospheric corrosion is the oldest type of corrosion recognized, and the atmosphere
is the environment to which metals are most frequently exposed [1]. The impact
on high-technology societies has been exemplified by the assumption that the cost of
protection against atmospheric corrosion is about 50% of the total costs of all
corrosion protection measures [2].
Atomospheric corrosion is the result of interaction between a material—mostly
a metal—and its atmospheric environment. When exposed to atmospheres at room
temperature with virtually no humidity present, most metals spontaneously form a
solid oxide film. If the oxide is stable, the growth rate ceases and the oxide reaches a
maximum thickness of 1 to 5 nm (1 nm = 10
–9
m). Atmospheric corrosion frequently
occurs in the presence of a thin aqueous layer that forms on the oxidized metal under
ambient exposure conditions; the layer may vary from monomolecular thickness to
clearly visible water films. Hence, atmospheric corrosion can be said to fall into two
categories: “damp” atmospheric corrosion, which requires the presence of water vapor
and traces of pollutants, and ‘‘wet” atmospheric corrosion, which requires rain or other
forms of bulk water together with pollutants [3].
It was the pioneering work of Vernon [4] that developed the field of
atmospheric
corrosion to a scientific level. He discovered that corrosion rates
increase rapidly above a “critical humidity” level, and he was also the first scientist
to demonstrate the combined accelerating effects of SO
2
and relative humidity on
atmospheric corrosion rates. Later, Evans [3], Rozenfeld [5], Barton [2], and others
demonstrated that electrochemical processes play important parts, and it became
evident that atmospheric corrosion of metals involves a broad range of electro-
chemical, chemical, and physical processes. The progress in the understanding of
atmospheric corrosion has been reviewed in several books and review chapters
[2, 5–7] and updated in various conference proceedings [8–10].
There are obvious reasons why scientists’ interest in atmospheric corrosion has
become even more widespread during the past decades and why atmospheric
corrosion has become a truly interdisciplinary field of science. Environmental effects
on outdoor constructions—not least those that are part of our cultural and historical
Copyright © 2002 Marcel Dekker, Inc.
heritage—and on vehicles have triggered national and international exposure
programs with comprehensive characterization of acidifying pollutant levels. Today
the environmental effects not only concern what the environment does to metals but
also what the metal does to the environment after running off any outdoor
construction and reaching the biosphere. Frequently observed failures of highly
miniaturized electronic components and equipment—even at low levels of
pollutants—have focused technicians’and scientists
’
interest on indoor conditions
too. Furthermore recent progress in the development of new analytical techniques
has resulted in a more detailed understanding of surface and interface phenomena as
well as a better characterization of the corrosion products formed.
The purpose of this chapter is to outline briefly some of the new insights that
have emerged as a result of basic studies of the atmospheric corrosion of metals. The
chapter is entirely devoted to metallic materials, which means that the reader will
have to turn to other sources for reviews of atmospheric corrosion of, for example,
calcareous stones, glasses, plastics, painted materials, and textiles.
The complex interaction between the metal and the environment ranges from
the atmospheric region over the thin aqueous layer to the solid surface region.
Hence, the next three sections are devoted to various reactions and other phenomena
in each of the regions involved. They are followed by two sections that deal with
selected aspects of atmospheric corrosion outdoors and indoors, respectively.
THE ATMOSPHERIC REGION
The main constituents of the troposphere—the atmospheric region closest to the
earth—are N
2
, O
2
, and rare gases (Ne, Kr, He, and Xe). They comprise more than
99.9% by weight of all molecules involved [11, p. 9]. Among these, N
2
and the rare
gases are of no significant importance to atmospheric corrosion because of their
inability to react with metal surfaces. O
2
, on the other hand, is of paramount impor-
tance due to its participation as an electron acceptor in cathodic reactions and its
involvement in chemical transformations of the atmosphere. Other tropospheric
constituents conducive to atmospheric corrosion are H
2
O and CO
2
. H
2
O is of
critical importance in several respects (its role is discussed in more detail in the
following sections). Its concentration in the troposphere varies over several orders
of magnitude—from around 100 volume parts per million (ppm) to 10,000 ppm. The
role of CO
2
has been strikingly overlooked in studies of atmospheric corrosion. This
naturally occurring molecule, which has a concentration of about 330 ppm and high
solubility in water, contributes to the acidification of the aqueous layer.
The remaining constituents of importance in atmospheric corrosion are presentonly
as trace gases with a total concentration of less than 10 ppm. Numerous investi-
gations based on both laboratory and field exposures have proved at least the
following gaseous constituents to be of significant importance [12]: O
3
, H
2
O
2
, SO
2
,
H
2
S, COS, NO
2
, HNO
3
, NH
3
, HC1, Cl
2
, HCHO, and HCOOH. Typical ranges for
most of these constituents are given in Table 1 for outdoor and indoor conditions. Their
presence may be the result of either natural or anthropogenic processes, and they
may undergo a variety of chemical transformations during transport in the tropo-
sphere. Because all species are reactive, they have a certain average lifetime, which
is often limited by the ability to react with important atmospheric oxidizers,
530 Leygraf
Copyright © 2002 Marcel Dekker, Inc.
Atmospheric Corrosion 531
a
Corresponding to 5wt % HCl according to Ref. 17.
primarily the hydroxyl radical, OH·, and O
3·
OH· is generated by photoinduced
dissociation of O
3
and the subsequent reaction of the electronically excited, energy-
rich, oxygen atom O(
1
D) and water vapor:
Although OH. molecules can oxidize several of the corrosion-stimulating species,
such as SO
2
, H
2
S, and NO
2
, a large fraction of OH· molecules are consumed through
reactions with hydrocarbon molecules, whereby one of the end products is the
hydroperoxyl radical, HO
2
·. This is another important component in gas-phase
chemistry because it disproportionates into hydrogen peroxide, H
2
O
2
, and O
2
according to:
H
2
O
2
, being highly soluble in aqueous environments, is another powerful oxidizer.
The importance of oxidizing species in atmospheric chemistry and the multitude of
reaction paths can be exemplified by considering different sulfur- , nitrogen- , and
chlorine-containing compounds of significance in atmospheric corrosion.
Sulfur-Containing Compounds
SO
2
has long been recognized as the most important gaseous stimulant in atmospheric
corrosion. It has many anthropogenic sources, such as combustion of sulfur-
containing coal and oil, and emission from metal, petrochemical, and pulp and paper
industries. Depending on the environment, there are several possible ways for SO
2
to
oxidize, some of which are summarized below. In the gas phase, SO
2
can oxidize
according to [13]:
Copyright © 2002 Marcel Dekker, Inc.
where M designates another gaseous molecule (e.g., N
2
or O
2
) that collides with a
generated HSO
3
· molecule and removes some of the excess energy released. In the
aqueous phase [14]:
532 Leygraf
The oxidation of S(IV) to S(VI) in the aqueous phase may also slowly proceed
through:
or be promoted by O
3
, Fe(III)- or Mn(II)-containing catalysts, or an increased
pH [14].
H
2
S, together with COS, is another principal atmospheric corrosion stimulant.
It is emitted both from natural biological sources (e.g., volcanoes, moss, or swamp
areas) and from anthropogenic sources (e.g., pulp and paper industries, catalyst
converters in motor vehicles, sewage plants, garbage dumps, animal shelters, and
geothermal power plants). H
2
S can react with OH· to form SO
2
according to [15]:
Nitrogen-Containing Compounds
NO and NO
2
are mainly formed through high-temperature combustion processes in
power plants, vehicles, etc. The fraction of NO in the combustion gas is much higher
than that of NO
2
but it is rapidly converted to NO
2
according to:
Farther away from the emission source NO may also form NO
2
through:
NO
2
plays a most important part in atmospheric chemistry by absorption of solar
light and the subsequent formation of O
3
through:
NO
2
can also be oxidized to nitric acid according to:
Copyright © 2002 Marcel Dekker, Inc.