Corrosion of constructional steels in marine and industrial environment

Chapter 1
Theoretical Evidences
1.1 Rusting of Iron
Iron, in its various forms, when exposed to the different facets of environment it
tends to be highly reactive owing to its natural tendency to form iron oxide. This
degradation of iron is known as corrosion, more particularly rusting, when oxi-
dation occurs in presence of moisture. However, if a thin film of iron oxide
develops on its surface which is impervious and tenacious, it protects iron from
further oxidation loss and it is called protective oxide film. This spontaneous
formation of protective oxides which forms only on certain type of alloy steels is
known as passivation. This hard nonreactive surface film (1–4 nm) inhibits further
corrosion.
Corrosion is an electrochemical phenomenon leads to the generation of very
low electric currents. A mathematical relationship is available between the oxi-
dation rate and the electrical properties. Good resistance to oxidation may gen-
erally be expected when the electrical resistance of the oxide formed is high. J.C.
Hudson worked on ferrous metals and established its relative resistance in different
atmospheric conditions [1].
1.2 Corrosion of Steel
Steel corrodes when exposed to myriad conditions including outdoor atmosphere.
It is noteworthy that all types of steel including the low alloy type are prone to rust
in moist atmosphere. Rusting is an electrochemical process characterised by
exchange of electrons. In some cases, the additions of 0.3 % copper to carbon steel
can reduce the rate of rusting to a greater extent. The elements Cu, P, Cr and Ni
have all been shown to improve resistance to atmospheric corrosion. Formation of
a dense, tightly adhering rust scale is responsible in lowering the corrosion rate
J. K. Saha, Corrosion of Constructional Steels in Marine and Industrial Environment,
Engineering Materials, DOI: 10.1007/978-81-322-0720-7_1, Ó Springer India 2013
1

leading to use without protection and can also extend paint life by decreasing the
amount of corrosion underneath the paint. The rate of rusting is usually higher in
the first year of exposure to atmosphere than in subsequent years, and increase
significantly with the degree of pollution and moisture in the air. Alloying ele-
ments contribute to a more compact and less porous corrosion product as surface
film. Adherent, protective films on these steels seal the surface against further
penetration of water, which does not easily wet the oxide surface. Compact surface
oxide films develop more rapidly in industrial atmospheres containing SO2, which
is probably involved in film formation in presence of moisture by forming
sulphurous and sulphuric acids which are very corrosive. This theory was based
largely on the observation that the corrosion products formed on steel when
exposed to industrial atmosphere were usually rich in sulphates. However, the
corrosion rates of weathering steels are not reduced in industrial atmospheres to
levels lower than those in non-corrosive rural or semi rural atmosphere. Periodic
drying is required for the surface film to develop its protective properties [2].
In acidic solution, it involves an oxidation reaction (anodic reaction) where the
metal gets into an ionic state by dissolution and releases electrons. Simultaneously,
a reduction reaction (cathodic reaction) consumes the free electrons released by
the anodic reaction and either a metal gets deposited or more usually the cathodic
reaction. In order to continue the corrosion process in steel, formation of distinct
anodic and cathodic areas is a prerequisite, which are electrically connected. These
anodic and cathodic reactions occur in presence of an electrolyte, which can be a
common aqueous solution, acidic medium or a thin film of moisture present on the
surface, pores and crevices. At higher humidity, corrosion increases due to con-
densation of moisture film on the metallic surface leading to formation of innu-
merable galvanic cells. These cells are formed due to generation of electromotive
force between surface film and trapped film in pores and crevices, which act as
cathode and anode. Intrinsically, two important factors influence the corrosion
phenomenon at a fundamental level. These are electromotive forces generated
between the two electrodes and pH of the aqueous media. The electromotive
force–pH relation was first proposed by M. Pourbaix [3] as shown in Fig 1.1 and
these are useful in predicting zones of corrosion, passivity and immunity in metal–
aqueous system. Pure iron exhibits formation of protective scale whereas carbon
steel shows formation of incoherent layers of scale which easily flakes off to
expose fresh areas for further attack.
There are different forms of corrosion of which most important one is uniform
corrosion which occurs over the majority of the surface of a metal at a steady and
often predictable rate. Rusting can be slowed or stopped by using paint, controlling
conductivity of solution, by applying current to metals and/or by stopping oxygen
to reach the surface. Other forms of corrosion commonly encountered during
service exposure of iron and steel is localised corrosion which is more severe than
the uniform one as failure occurs without warning in a short period of use or
exposure. Galvanic corrosion can occur when two different metals are placed in
contact with each other and caused by the greater tendency of one of the metal to
give up electrons than the other. Pitting corrosion occurs in materials that have a
2 1 Theoretical Evidences

protective film such as a corrosion product or when a coating breaks down.
Metallurgical factors that can affect corrosion in steel are crystal imperfections,
grain size and shape, grain heterogeneity, impurity inclusions and residual stress.
1.2.1 Mild Steel
In mild steels, passivation in the stricter sense is not possible. The passive region
of iron is characterised by a thin film of cubic oxide of c Fe2O3/Fe3O4 in neutral
solution. This type of film is formed by the reaction of clean iron with oxygen or
dry air. The composition of the passive film depends on the type of electro-
chemical reactions and the nature of solution to which it is subjected. In such a
situation, Fe2+
in solution may anodically form on the surface to give an outer c
FeOOH layer. Another passive film on iron is Fe(OH)2, which is a polymeric
layered structure [1]. However, it is reported to change character on removal from
the passivating medium and long-term drying, to a form more closely resembling
to c Fe2O3. It is reported that with the exception of those formed at very low
passivating potentials, passive films do not seem to undergo significant local
structural changes upon drying in the air. It is also reported that the passive film on
iron composed of small particle size of c Fe2O3/Fe3O4 [4]. Cahan and Chen [5]
suggested that the passive film is not a semiconductor but a highly doped film with
Fe2+
and Fe3+
as defects. The oxide film near the iron electrode contains Fe2+
and
Fe3O4 on outer surface. Raman spectroscopy study of the passive film indicates
that the film consists of a layered structure with at least two components. The inner
layer is most likely Fe3O4 and the outer layer primarily Fe3+
species. X-ray dif-
fraction data shows a spinel oxide (c Fe2O3, Fe3O4), which is inconsistent with
other crystalline bulk oxides, hydroxides or oxyhydroxides [6].
Fig. 1.1 E-pH (Paurbaix
Diagram) of iron in sulphate
containing aqueous media
1.2 Corrosion of Steel 3

1.2.2 Weathering Steel
Weathering steels comprise a group of high strength, low alloy steels containing
alloying elements to give an enhanced resistance to rusting compared with carbon
steels. These steels have 1–2.5 % of alloying elements (Cr, Cu, Si, and P) and have
a tendency to form rust at a rate depending on the access of oxygen in the presence
of moisture and air. As the process progresses, the rust layer acts as a barrier to the
ingress of oxygen and the rate of rust growth slows down. In mild steels, the rust
layer becomes non-adherent and detaches after specific time to exposure condi-
tions. In weathering steels, the rusting process is initiated in the same way, but the
alloying elements help to produce less porous and more adherent rust film. This
rust system develops with time, becomes protective by impeding further access of
oxygen and moisture to the metal surface and hence reduces considerably the rate
of rust growth. The rust colour and its characteristic of weathering steel depend
upon the nature of the environment and exposure time. In an industrial atmosphere,
the weathering process will generally be more rapid and the final colour becomes
darker. In the rural atmosphere, the oxide formation is usually slower and the
colour becomes lighter. The tightly adherent oxide usually forms over a period of
18 months to 3 years in industrial atmosphere. Weathering steel promotes for-
mation of an adherent rust layer after about 8 years of service and retards the
corrosion by 75 % compared to mild steel [7] and in presence of relatively high
airborne sea salt (coastal environment) the protective layer cannot be formed.
Weathering steel is not advised to be used in bare conditions involving severe
marine and severe industrial environments [8].
Pourbaix [9] showed that the typical behaviour of weathering steel is due to
passivation during drying and lack of activation during wetting. Rust reduction of
weathering steel is slower than mild steel while not much difference has been
found in chemical analysis of the rust films on MS and WS, the morphology is
quite different [10, 11]. Rust formed on weathering steel is rather compact in
comparison to the loose rust found on mild steel. However, favourable atmo-
spheric conditions are required to get stable rust on weathering steel, like air borne
chloride (0.5 mg/100 cm2
/day), average wetness time 60 %, industrial pollu-
tants (SO2 2.1 mg/100 cm2
/day) [12, 13].
1.3 Atmospheric Corrosion
Atmospheric corrosion is an electrochemical process with the electrolyte being a
thin layer of moisture on the metal surface. The composition of the electrolyte
depends on the deposition rates of the air pollutants and varies with the wetting
conditions. The factors influencing the corrosivity of atmospheres are gases in the
atmosphere, critical humidity and dust content. Two rural environments can differ
widely in average yearly rainfall and temperature and can have different corrosive

tendencies and corrosion products [14, 15]. During atmospheric exposure, steel
gets a reddish brown corrosion product consisting of different constituents. The
electrochemical reactions at wet surface of steel as proposed by Evans [16] in
neutral alkaline condition are:
Anodic half-cell reaction Fe ! Fe2þ
þ2eÀ
ð1:1Þ
Cathodic half-cell reaction H2O + 1/2 O2þ2eÀ
! 2OHÀ
ð1:2Þ
The anodic and cathodic reactions are only the ﬁrst step in the process of
creating rust. Several more stages must occur for rust to form:
Fe2þ
+ 2OHÀ
! Fe OHð Þ2 ð1:3Þ
Ferrous hydroxide [Fe(OH)2] and hydrated ferrous oxide (FeO.nH2O) is ﬁrst
diffusion barrier layer formed on the surface. As the pH of saturated Fe(OH)2 is
about 9.5, the surface of steel corroding in aerated pure water is always alkaline.
Due to incipient oxidation green coloured Fe(OH)2 is formed. Ferrous oxide is
converted to hydrous ferric oxide or ferric hydroxide at the outer rust layer as
dissolved oxygen is available by the following reaction.
2 Fe OHð Þ21=2 O2 ! Fe2O3:2H2O ð1:4Þ
In weathering, steel rust formed on atmospheric corrosion in different envi-
ronments is composed of crystalline compounds like haematite, magnetite and
oxyhydroxides of iron like goethite, akaganeite, lepidocrocite and feroxyhite apart
from amorphous ferric oxyhydroxide rust. These rust constituents transform to one
another during wet–dry cycles of atmospheric exposure [17].Various phases of
corrosion products formed in progressive exposure to atmosphere are given in
Table 1.1 [6, 18].
The alloying elements play a major role in modifying the oxyhydroxide rust
layer which inhibits the ingress of oxygen and iron cations. Orange to reddish
brown in colour hydrated ferric oxide formed is called rust and available as non
magnetic a Fe2O3 and magnetic c Fe2O3. Rust layers are not protective because
they are permeable to air and water and steel continues to corrode even after rust
has formed [1].The rusting of steel in the atmosphere is given by:
2 Fe + H2O + 3/2 O2 ! 2FeOOH ð1:5Þ
FeOOH is the main component of the rust formed in presence of water on steel
at room temperature. Misawa [10] has summarised these processes and according
to him metal dissolution is the anodic reaction, while the dominant cathodic
reaction is oxygen reduction. The presence of a thick electrolyte layer on the
surface can limit oxygen reduction rate. In such situations, the following reduction
reaction supports oxidation of steel.
8 FeOOH + Fe2þ
+ 2e- ! 3Fe3O4 + 4H2O ð1:6Þ
1.3 Atmospheric Corrosion 5

The oxidation of Fe2+
ions to green rust transformed to c FeOOH in well-
aerated systems and in turn is transformed to Fe3O4 in oxygen depleted systems.
The phases of change in rust with time in mild steel are c FeOOH transforms to the
more stable a FeOOH. With increasing time, a FeOOH converts to either c Fe2O3
or a Fe2O3, while conversion to a Fe2O3 usually requires higher temperatures.
The rust on weathering steel after 16 years of exposure in a rural environment
was found to be composed of two layers, with the inner dull layer comprising
nano-sized particles of a FeOOH and the outer bright layer, c FeOOH [19]. The
rust on weathering steel after 25 years of exposure in an industrial environment
exhibited similar characteristics [20].
1.3.1 Corrosion Products
Weathering steels develop a compact adherent protective oxide film that protects
the surface against further corrosion with prolonged exposure to the atmosphere.
The first oxyhydroxide form is c FeOOH and part of it begins to transform to a
FeOOH. The remaining part at later stage is composed of both oxyhydroxides.
These hydroxides are less protective against corrosion and they readily crack
allowing for ingress of oxygen and moisture to reach the metal surface and cause
Table 1.1 Phases of corrosion products in atmospheric exposure
Phases Lattice Crystal system
/Habits
Density
(gm/
cm3
)
Free
energy
(DG kJ/
mol)
Features
a b c
a FeOOH
Goethite
4.60 9.96 3.02 Orthorhombic/
Acicular
4.28 -490.4 Yellowish brown to
dark brown,
Scaly/fibrous
c FeOOH
Lepidocrocite
3.06 12.51 3.87 Orthorhombic/
Lath
4.09 -471.4 Polymorph of
goethite, platy,
orange colour,
Red Rust
d FeOOH
Feroxyhite
2.94 4.49 3.8 Hexagonal/
Plates
– – Thin rolled films
a Fe2O3
Haematite
5.035 13.72 5.26 Hexagonal/
Plates
5.24 -742.4 Reddish brown to
black flaky rust,
characterised by
red streak
c Fe2O3
Maghemite
8.33 – 24.99 Cubic/Lath 4.69 -540.2 Black and similar to
magnetite
Fe3O4
Magnetite
8.396 – Inverse spinel/
Octahedra
5.18 -1014.2 Black colour, mill
scale
b FeOOH
Akaganeite
10.48 10.48 3.02 Tetragonal/
Somatoids,
3.55 – Brown/white colour
Contains Cl-
ions
a,b,c: Relative lengths of crystallographic axes DG : Free energy of adsorption

further corrosion. With time, a part of the FeOOH transforms to magnetic oxides
of iron, which are much more protective than these oxyhydroxides. In addition to a
and c FeOOH, there is another oxyhydroxide of amorphous nature called d
FeOOH. In mild steel, this does not form in a continuous manner and amorphous d
FeOOH forms are not protective in nature for this reason. The formation of
amorphous d FeOOH as a continuous layer next to the metal surface is catalysed
by the presence of P, Cu and Cr in the steel. The presence of this amorphous layer
was thought to be the reason for the excellent corrosion resistance of the weath-
ering steels [21]. However, some findings [22, 23] show that the stable rust layer
was not necessarily composed of amorphous rust but densely packed nano-size Cr
substituted goethite. Cr substituted a FeOOH is very fine through which oxygen,
water and corrosive substances are difficult to penetrate. Furthermore, chloride
ions are also difficult to pass through this. However, for the formation of this
protective rust layer, it is necessary that favourable atmospheric conditions exist
for application of steel in bare condition.
After extensive studies it has been found that Cr compounds are effective for
obtaining the protective rust layer in a short period of time [24]. Cr substituted a
FeOOH forms rapidly in presence of Cr2(SO4)3 solution. This accelerates the
dissolution of steel and promotes the formation of goethite. On the other hand,
Cr3+
forms fine particles of Cr substituted goethite and improves the protection
ability of the rust layer.
1.3.2 Atmospheric Corrosion Mechanism
Misawa et al. [10, 25] first investigated the mechanism of formation of constituents
of atmospheric rust in aqueous solution and identified amorphous oxyhydroxide,
FeOx(OH)3–2x, besides a FeOOH and c FeOOH in atmospheric rust. Again Mis-
awa et al. [11] and Yamashita et al. [26] have reported that the c FeOOH forms at
early stages of rusting and transforms into amorphous rust before converting to a
FeOOH. Both Misawa [10] and Suzuki et al. [27] also concluded that the presence
of Cu favours the formation of amorphous, crack-free uniform rust layer.
It is reported that the formation of a FeOOH and c FeOOH results from water
loss and crystallization of Fe(OH)3, the main corrosion product, with amorphous d
FeOOH as an intermediate phase during drying cycle. In presence of high
humidity, the reduction of c FeOOH results in the formation of Fe(OH)2 and
finally Fe3O4. Under dry and oxidising conditions, when oxygen is easily able to
penetrate into the rust layer, the ferrous layer/Fe3O4 is oxidised to unordered
Fe(OH)3 and/or amorphous FeOOH which again transforms into crystalline a or c
FeOOH by water loss and crystallization [28].
According to Larrabee et al. [22] Cu inhibits the formation of crystalline a
FeOOH and c FeOOH and thus prevents microcracking in rust. This is attributable
to crystallization of a FeOOH and c FeOOH during drying cycle of rust. Stratmann
et al. [29] found significant differences between the rusting of iron and iron with

0.5 % Cu during drying cycle and attributed this to the formation of a dense
corrosion product on Cu containing iron. Kishikawa et al. [30] reported that
weathering steel alloyed with Cr, Cu, P and Ni forms a non-amorphous densely
packed nano-sized Cr substituted a FeOOH, which prevents the permeation of
water, oxygen and corrosive substances. On the other hand, the c FeOOH mem-
brane was found to possess anion selective property. The rust layer formed on the
weathering steel has double-layered structure and protects steel from corrosion
because of the formation of bipolar membrane which suppresses the cathodic
reaction. Thus, the formation of a FeOOH in inner layer and c FeOOH in outer
layer is important for protection ability against rust. The ratio of a FeOOH to c
FeOOH in rust increases gradually as time passes. Corrosion rate decreases to
almost zero when this ratio exceeds 1.4. A value of 2 is considered as rust stability
index for maximum protection [31].
Yamashita et al. [32] observed that atmospheric rusts on weathering steels are
composed of Cr substituted a FeOOH, c FeOOH and a small amount of c Fe2O3
and/or Fe3O4. The dark Cr substituted a FeOOH area was located in the inner layer
while the bright c FeOOH area was in the outer layer. Thus, the innermost Cr
substituted a FeOOH layer may be the ﬁnal form of the protective rust layer which
suppresses and prevents the transport of corrosive species through the rust layer to
retard further corrosion.
Study conducted in Taiwan by Wei [33] on carbon steels and weathering steel
with high Phosphorous and exposed to rural, urban, coastal and coastal industrial
environments concluded that the characteristics of the protective rust layer and the
corrosion resistance of weathering steels depend on the environment and the test
period. It was observed that c FeOOH formed in the inner rust layer along with
some a FeOOH, a Fe2O3 and Fe3O4 in the initial exposure period and the amount
of a FeOOH gradually increased in weathering steels. The enrichment of crack
free and dense rust layer with Cr, Cu and P is attributed to the corrosion protection
of the substrate steel.
The rust layers on plain carbon and weathering steels exposed to coastal
industrial environment in Japan for 17 years had been characterised by Asami
et al. [34]. They found that the rust was composed of a FeOOH, b FeOOH,
c FeOOH, Fe3O4 and amorphous rust. a FeOOH was predominant on all steels and
appeared uniformly distributed throughout the rust layer. Concentration of a
FeOOH was higher and c FeOOH was lower on weathering steels than on plain
carbon steel. Amorphous rust was located at the bottom of the rust layer irre-
spective of the steel types. Previous studies by authors on distribution of phases
and alloying elements in the rust layers in weathering and plain carbon steels
reported that the rust oxides consist of three layers: inner, outer and outermost
[35]. The outermost layer was found to be about 3 lm thick and enriched with
atmospheric deposits. The concentration of b FeOOH was reportedly higher on the
skyward surface of both steels. In weathering steel, the alloying elements Cu and
Cr enriched the inner layer of rust, while Si, P and Ni were not found to exhibit any
characteristic distribution.

According to Yamamoto et al. [36], the amorphous and the crystalline con-
stituents were intermingled in the inner rust layer of weathering steels exposed to
rural environment for 35 years. Asami et al. [37] analysed the rust layers formed
after 17 years on weathering steel bridge exposed to coastal industrial environment
reported that c FeOOH and b FeOOH existed in outer layer while the amorphous
rust and a FeOOH with enriched Cr, Ni and Cu were found widespread in the inner
rust. Kihira et al. [38] referring to the work of Sakashita and Sato [39] tried to
explain the difference in protectiveness of weathering steel rust by the phenom-
enon of ion selectivity of the rust layer. Keiser et al. [40] found d FeOOH in the
weathering steel rust along with about 10 % c FeOOH and a small amount of a
FeOOH. Yamashita et al. [41] investigated the rust layers formed on weathering
steels exposed for 17 years in Japan and reported that the protective properties of
the rust layer on weathering steels containing alloying elements (Cr, Cu etc.)
composed of a FeOOH. The amount of b FeOOH phases increased with the
progressive increasing level of airborne salt. They also suggested that the pro-
tective properties of the rust layer were related to the suppression of ion transport
due to its densely packed structure.
Most of the authors have indicated that long-term exposure results are impor-
tant in terms of protective rust formation on weathering steels and also supported
the view that the early stages of exposure determine the subsequent corrosion rate.
Thus, corrosion rates during the early months of exposure are far more important
than the ultimate rate in the context of a study of the mechanisms of protection
[42].
It is proposed that during rust formation of steel in alkaline solutions the oxi-
dation of Fe2+
proceeds via Fe(OH)2 and yields magnetite as the end product [43].
In another study of transformation of Fe(OH)2 at pH 11 and at temperature 65 °C,
it was noticed that initially both Fe3O4 and a FeOOH form but a FeOOH formation
takes place at an early stage of reaction. It was suggested that excess Fe+2
ions
interacted with Fe+3
oxides, resulting in Fe3O4 formation. It is further reported that
Cl-
ions retard magnetite formation by binding the neighbouring OH-
ions groups
to form Fe–O–Fe linkage in alkaline pH 11. Sulphate has a tendency for a FeOOH
formation of 0.1 M concentration of Fe(OH)2 [2]. Moreover, sulphate also plays
role during transformation of green rust to oxides/oxy hydroxides.
Subramanium [44] have shown that the presence of small amount of cations
(Cu, Mn) in rusts may accelerate the oxidation of Fe+2
in solution due to their
compound forming tendency with Fe+3
. Thus, magnetite is a major constituent
in the weathering steel grades, which contains 0.3–0.4 % (wt.) of Cu. This
corroborates the previous studies of Inouye et.al [45]. They indicated the strong
magnetite promoting tendency of Cu during the formation of magnetite from
Fe(OH)2. However, they have speciﬁed the maximum limit of Cu (3 % by wt.)
above which it suppresses the magnetite formation. In another study [46], it was
noticed that the presence of traces of Cr decreases the amount of Fe3O4 in oxide
and increase unstable a FeOOH content in the corrosion product. It can be noted
here is that the tendency of Cr ions to form ﬁne particles of a FeOOH which may
increase with passage of time and help in stabilizing the rust layer and protect the

surface from corrosion attack. High Si content in weathering steel also gives the
similar protective effect [47].
1.3.3 Effect of Acidity of Solution
Acid solutions (low pH) are more corrosive than neutral or alkaline solutions. In
ordinary iron or steel, the dividing line between rapid corrosion in acid solution
and moderate or low corrosion is nearly neutral or alkaline solution at pH 7.5. In
case of corrosion of iron or steel in aerated water, anodic reaction takes place at all
pH values as per Eq. (1.1), but the corrosion rate varies due to changes in the
cathodic reduction reaction as per Eq. (1.2). In the intermediate pH 4–10 ranges,
loose, porous, ferrous oxide deposit shelters the surface and maintains the pH at
about 9.5 beneath the deposit. The corrosion rate is nearly constant and is deter-
mined by uniform diffusion of dissolved oxygen through deposit in this range of
pH. In more acidic solutions (pH 4), the oxide is soluble and corrosion increases,
due to availability of H+
ions for reduction by the equation
2Hþ
+ 2eÀ
! H2 ð1:7Þ
The absence of the surface deposit also enhances access of dissolved oxygen,
which, if present, further increases corrosion rate. Dissolved oxygen is cathodi-
cally reduced in acid according to
O2 + 4Hþ
+ 4eÀ
! 2H2O ð1:8Þ
Reactions (1.7) and (1.8) occur simultaneously in acid solutions with dissolved
oxygen. Diffusion of dissolved oxygen controls the corrosion rate at a constant
level in the pH range 4–10. Thus, metallurgical variables affecting the anodic
reaction [1] have no effect on the corrosion rate. Such is not the case for acid,
where the cathodic reaction is under activation control. The carbide phase shows
low overvoltage (higher rate) for reduction of H+
ions. Thus, high carbon steels
have a higher corrosion rate in acid solution than that of the low carbon steels
[48, 49].
1.3.4 Effect of Alloying Elements
Metallurgical factors affect metal loss and tend to corrode at a lower rate with
higher alloy content. Atmospheric corrosion resistance of steel was improved by
alloying with Cu, P or Cr to form passive oxide layer [50]. Studies have shown that
these steels show superior corrosion resistance in particular during atmospheric
exposure but not so much for immersed exposure as in seawater and close to the
coastline in the presence of high chloride concentrations. Alloying elements like

Cu, Cr, Ni, Si, P, etc. are added in carbon steel to achieve compact, adherent and
pore free rust layer which in turn provide good corrosion protection to the steel
surface depending on the environment [27–30, 51, 52]. Weathering process of steel
with Cu and P promote the formation of a tightly adherent, protective and stable
rust layer to act as a barrier to electrochemical attack under wet–dry cycle. Cor-
rosion of weathering steels containing Cu, P and Cr virtually ceases after 3 years
of exposure. It is noteworthy that the formation of protective rust layer does not
depend only on the alloying elements but also on the environment. It is observed
that the formation of protective rust on the steel surfaces containing Cu is easier in
industrial and rural atmospheres and difficult in marine atmospheres containing
chloride ions. In their research work, Larrabee and Coburn [53] have revealed the
effect of alloying elements on the corrosion resistance of steel where Cu and P
additions are most beneficial in improving the resistance of steels. They have
showed that the corrosion rate of plain carbon steels increases progressively with
exposure time whereas same decreases in Cu containing steels in industrial
atmosphere.
Atmospheric attacks on steels have been studied on field exposed steel in
industrial, rural and marine environments and found that P, Cu, Ni, Cr and Si
improve the resistance to corrosion while Mn does not seem to affect it and S
increases nucleation rate. The relative importance reported for marine atmosphere
is P, Si, Cu (up to 0.3 %) and Cr, Ni, Cu (above 0.3 %) [52–54].
Horton et al. [55] observed that when steels containing Cu and Ni are exposed
in industrial and marine atmospheres, the Cu and Ni appear in the rust layers both
in the loose outer and adherent inner rust on skyward and ground ward surfaces.
Also it was shown by chemical analysis that Ni, Cu, Cr and Mn from weathering
steel appear in the rust layer and provides protection. Presence of chlorides in the
atmosphere accelerates corrosion of steels leading to the formation of basic Fe2+
,
Fe3+
chlorides and b FeOOH. Townsend et al. [56] conducted 8-year atmospheric
corrosion tests on weathering steel in rural, industrial and marine environments
with different heated conditions and indicated that heat treatments have no effect
on the corrosion resistance/performance of weathering steels.
1.3.5 Environmental Factors
The environmental factors that tend to accelerate metal loss include high humidity,
high temperature and proximity to the ocean, extended periods of wetness and the
presence of pollutants in the atmosphere. The small amount of carbon dioxide
normally present in the air neither initiates nor accelerates corrosion. Vernon [57]
was the first to study the corrosion rate of steel coupons in the presence of well-
defined atmospheres. Atmospheric gases such as CO2, SO2, NO2, HCl, etc. after
getting dissolved in the moisture layer on the metal surface, these gases result in a
number of ions and ionic species like H+
, Cl-
, CO3
2-
, NO3
-
, SO4
2-
, etc. They
measured corrosion rate was as a function of time, relative humidity and

atmospheric pollutants like SO2 and showed that corrosion starts in non-contam-
inated atmospheres only at a relative humidity near to 100 %. Thus, they suggested
that weather resistant steel is not to be used in uncoated condition in severe marine
and severe industrial environments. Weathering steel is 5–8 times more corrosion
resistant than plain carbon steel in industrial atmosphere and in marine atmo-
sphere, superiority of the weather resistant steel depends on the salt content of the
atmosphere [58, 59].
Effect of Temperature. Temperature plays an important role in atmospheric
corrosion. There is normal increase in corrosion activity which can theoretically
double for each 10° increase in temperature. As the ambient temperature drops
during the evening, metallic surfaces tend to remain warmer than the humid air
surrounding them and do not allow condensation until some time after the dew
point has been reached. As the temperature begins to rise in the surrounding air,
the lagging temperature of the metal structures will tend to make them act as
condensers, maintaining a film of moisture on their surfaces [60–63].
Effect of Relative Humidity. Relative humidity is defined as the ratio of the
quantity of water vapour present in the atmosphere to the saturation quantity at a
given temperature. Atmospheric corrosion takes place in presence of a thin film
electrolyte that can form on metallic surfaces when exposed to a critical level of
humidity. While this film is almost invisible, the corrosive contaminants are
known to reach relatively high concentrations, under conditions of alternate
wetting and drying. The critical humidity level is a variable and depends on the
nature of the corroding material, the tendency of corrosion products and surface
deposits to absorb moisture, and the presence of atmospheric pollutants. It has
been shown that, this critical humidity level is 60 % for iron if the environment is
free of pollutants [60]. In the presence of thin film electrolytes, atmospheric
corrosion proceeds by balanced anodic and cathodic reactions. The anodic oxi-
dation reaction involves the corrosion attack of the metal, while the cathodic
reaction is naturally the oxygen reduction reaction. The most important factor in
atmospheric corrosion is moisture, either in the form of rain, dew, condensation or
high relative humidity. In the absence of moisture, most contaminants would have
little or no corrosive effect. Rain also may have a beneficial effect in washing away
atmospheric pollutants that have settled on surfaces. This effect has been partic-
ularly noticeable in marine atmospheres. On the other hand, if the rain collects in
pockets or crevices, it may accelerate corrosion by supplying continued wetness in
such areas. Continuous wetting and drying of the surface is required during
atmospheric corrosion which makes distinctly different from the usual corrosion
mechanism under immersed conditions [61].The atmospheric corrosion cannot be
described as a simple oxidation reaction but associated with electrochemical
reaction kinetics [62, 63]. Atmospheric corrosion experiences metal dissolution
and the oxygen reduction during wet–dry cycles. In industrial atmosphere having
sulphur bearing compounds, the corrosion product on steel is a basic iron sulphate.
The Cu bearing steels form Cu compound which plugs the pores in the corrosion
product, resulting in better corrosion resistance [64–66].

Effect of Sulphates. Sulphur dioxide comes from combustion of fuels and is
identified as one of the most important air pollutants which contribute to the
corrosion of metals. In the presence of SO2, corrosion starts at 60 % relative
humidity and the rates are considerably higher than in the absence of any con-
taminate. There is a close relation between the uptake of SO2 and the corrosion
rate measured during field exposure over a period and it depends considerably on
the acid hydrolysis of SO2 in water and the fast oxidation of SO2 on iron surfaces.
The cycle consists of two reactions, the oxidative hydrolysis of iron sulphate and
the subsequent acid corrosion of iron [67].
4FeSO4 + O2 + 6H2O ! 4FeOOH + 4H2SO4 ð1:9Þ
4H2SO4 + 4Fe + 2O2 ! 4FeSO4 + 4H2O ð1:10Þ
The acid rain triggered atmospheric corrosion of steel and presence of SO2
accelerate corrosion rate due to formation of sulphuric acid resulting the deposi-
tion of iron oxides in zones of decreased pH. Misawa [10] recognised the
importance of amorphous and less crystalline iron oxide phases for the chemistry
involved between iron dissolution and formation of stable FeOOH. Iron ions
initially form green complexes, which are transformed into green rust, magnetite,
FeOOH and finally after long exposure times transformed to a FeOOH. Hence, a
large number of thermodynamically metastable phases exist which are transformed
to stable oxides with the environmental conditions [68]. Stratmann [69] developed
a number of experimental techniques to investigate the electrochemical reaction
mechanism during the atmospheric corrosion even under wet/dry conditions.
Studies indicate the adsorption of gaseous species into the corroding surface and
subsequent chemical reactions occurring in the thin electrolyte layer [66].
Thus, in an industrial environment SO2 in presence of moisture, leads to the
formation of a very thin oxide film composed of an inner layer of Fe3O4 covered
by an outer layer FeOOH. The weak outer layer of FeOOH is penetrated by
fissures and moisture enters into the pores in Fe3O4 layer in form of condensed
water and dissolved O2, SO2 forms H2SO4 in the oxide pores.
2SO2 + 2H2O + O2 ! 2H2SO4 ð1:11Þ
The oxide pores is to permit easier access for the electrolyte solution to the
underlying surface. This H2SO4 partially dissolves the oxide, producing FeSO4 and
thereby opens the oxide pores to permit easier access for the electrolyte to
underlying surface.
Fe3O4 + 4H2SO4 ! FeSO4 + Fe2 SO4ð Þ3 + 4H2O ð1:12Þ
Fe2 SO4ð Þ3 + H2O ! FeSO4 + O2 + H2SO4 ð1:13Þ
Also, FeSO4 is also hygroscopic in nature which enhances the atmospheric
corrosion rate by attracting moisture from the atmosphere.

Effect of Chlorides. Marine environments have high percentage of relative
humidity and airborne salt. Studies have found that the thickness of the adsorbed
layer of water on zinc surface increases with % relative humidity and that cor-
rosion rates increase with the thickness of the adsorbed layer. There also seems to
be a finite thickness to the water layer that, when exceeded, can limit the corrosion
reaction due to limited oxygen diffusion. However, when metallic surfaces become
contaminated with hygroscopic salts their surface can be wetted at lower % RH.
The presence of magnesium chloride on a metallic surface can make a surface
apparently wet at 34 % RH while sodium chloride on the same surface requires
77 % RH to create the same effect. Corrosion rate in chloride environment can also
be reduced in special grade of weathering steel as shown by the study. Maghemite
as oxide phase formed on the carbon steel exposed in the marine environment is
responsible for high corrosion rate [70, 71] although this rust phase is stable and
protective in industrial environment.
1.4 Corrosion Protection by Coating
The major protective coatings applied to structural steelwork are paints, metal
coatings and combinations of both. The choice is partly governed by the actual
environmental conditions and partly by economic considerations [72, 73]. The
protection methods can be divided into three categories like applying organic or
inorganic coatings, controlling electrode potential to make metal immune or
passive by adding alloying elements in steels to promote formation of passive layer
and addition of corrosion inhibitors to the environment [74].
1.4.1 Passive Rust Coatings
Passivity is due to the presence of a thin film which isolates the metal surface from
a corrosive aqueous environment. A major impetus for work in neutral solutions
came from the research of Nagayama and Cohen [75]. They considered that in the
passive region, iron is covered by a thin film of cubic oxide of c Fe2O3/Fe3O4.
Other compositions and structures were proposed for the passive film, some
involving the inclusion of hydrogen or the presence of water [76, 77]. In fact, the
composition of the passive film on iron depends on the type of electrochemical
parameters during the formation of the film and the nature of solution in which it is
formed. In passivity study, it is reported that the passive film on iron is composed
of Fe(OH)2, c Fe2O3/Fe3O4. [78, 79].

1.4.2 Organic Coatings
Organic coatings used on steel provide an effective barrier protection by isolating
steel from the attacking species. Scientific investigations have determined that the
limiting factor in the protective mechanisms of barrier coatings is their resistance
to the flow of ionic current [80]. Coatings which contain large quantities of
metallic zinc provide corrosion protection by galvanic action. The barrier prop-
erties of the coating are improved by increased thickness, by the presence of
pigments and fillers that increase the diffusion path for water and oxygen, and by
the ability to resist degradation. Degradation allows access of reactants to the
coating–substrate interface without the necessity for diffusion through the coating
and it is an electrochemical process which follows the same principles as corrosion
of uncoated steel. The total corrosion process comprises of components like
transport of water, oxygen and ions through coating, development of aqueous
phase at the coating–metal interface, activation of the metal surface for the anodic
and cathodic reactions and deterioration of the coating–metal interfacial bond [81].
As coatings age in a corrosive environment, the interconnecting network of
pores within the coating eventually become saturated with water, salts, etc.,
exposing the metal substrate to a corrosive environment. The saturation of the
pores also creates paths of lower electrical resistance through the coating. Aged
organic coating systems also possess dielectric properties, which cause them to act
as capacitors to electrical current. Corrosion occurring at a metal surface has a
polarisation resistance related to the corrosion rate, and an electric double layer
that also behaves as a capacitor.
An important property of a coating is its resistance to water penetration and two
related properties are coating dielectric strength and coating resistance to ionic
movement. Water penetration in coating decreases the dielectric strength, resis-
tivity and makes the coating less insulative. Once corrosion has begun, the corro-
sion products formed can cause undercutting and loss of adhesion of the coating.
Water penetration may swell the coating and produce stresses that eventually lift
the coating from the substrate. The presence of water increases coating deteriora-
tion and substrate corrosion, since they can accumulate underneath the coating,
cause delamination by blistering or accelerate corrosion of the substrate. Bacon,
Smith and Rugg [82] determined a direct correlation between resistances and the
ability of the coating to protect the underlying steel from corrosion. All coatings
were found to exhibit an initial decrease in resistance, which varied in terms of rate
and duration. For a good coating, this initial decrease was followed by an abrupt
recovery to around the original value. They found that the resistance of a poor
coating continued to decrease resulting in failure within 60 days. A coating that
maintained a resistance of 108
ohm-cm2
provided good corrosion protection while
those between 106
and 108
ohm-cm2
were fair and resistance less than 106
ohm-cm2
were poor performers [83, 84]. The dissolution of environmental water into the
coating was more important than the uptake of salt from the solution by the coating.
1.4 Corrosion Protection by Coating 15

The level of dissolved salts of the exposure environment had an effect on the
properties of coatings [85].
Mayne [86] showed that, upon immersion in an aqueous solution, most organic
coatings acquire a negative charge and the acquisition of this charge has the effect
of creating a selectively permeable membrane, which is preferentially permeable
to cations; that is, a film that has gained a negative charge due to immersion.
1.5 Degradation of Organic Coatings
There are several types of corrosion found beneath organic coatings and these are
blistering, filliform corrosion, rusting, anodic undermining and cathodic delami-
nation. Blistering is one of the first signs of breakdown in the protective nature of
the coating. The blisters are local regions where the coating has lost adherence from
the substrate and where water may accumulate and corrosion may begin. The blister
formation occurs by volume expansion due to swelling, gas inclusion and gas
formation [87, 88]. In all the cases, the blister provides a locale for collection of
water at the coating–substrate interface. Oxygen penetrates through the coating and
leaching of ionic materials from the interface. All the constituents are available for
electrochemical corrosion and oxygen is necessary for the cathodic reaction:
Again filliform corrosion is encountered on steel underneath organic coatings in
a humid air environment. Corrosion initiates in the presence of soluble ionic
species at defects in the coatings and propagates at the metal–coating interface as
worm like filaments due to differential aeration oxygen concentration. Oxygen
diffuses through the tail and leads to the separation of anodic and cathodic reaction
zones. The primary cathodic region is near the back of the head (at the head–tail
boundary) where oxygen is supplied and the primary anodic region is at the front
edge of the head of the filament.
Organic coatings slow the mass transport process of water, oxygen and ionic
species which is necessary for corrosion. Non-sacrificial coatings show good barrier
properties but corrosion phenomena can occur at corrosion defects where the steel
substrate is not protected. Once corrosion starts on steel protected by an organic
coating system, growing of blisters appear and a rapid deterioration occurs. This
leads to the second type of protection that a coating can provide, sacrificial pro-
tection. In addition, corrosion products from the sacrificial layers promote pore
blockage in organic coatings preventing environmental intrusion [89].
1.5.1 Delamination of Coatings
Blistering and delamination are the most common forms of failure found in organic
coatings. Factors affecting the performance of a system include surface prepara-
tion, coating application, cure regime and film integrity. Soluble salts at the

interface can form a concentrated salt solution and that acts to draw water through
the coating, which behaves as a semipermeable membrane from the exposure
environment. Anodic blistering mode of failure was addressed by Koehler [90]
who considered liquid filled blisters to be anodic in nature. Cathodic blistering is
the result of an alkaline environment under the coating caused by the cathodic
reaction, associated with corrosion that occurs at a damaged site of the film
[91]. The fault may take the form of mechanical damage to the coating or may be
inherent coating faults like pores/holidays.
Anodic half-cell reaction:
Fe ! Fe2þ
+ 2eÀ
ð1:14Þ
Fe2þ
+ O2 + H2O ! Fe2O3 + H2O ð1:15Þ
Cathodic half-cell reaction:
O2 + 4Hþ
+ 4eÀ
! 2H2O reduction in acidic solutionð Þ ð1:16Þ
H2O + 1/ 2 O2 + 2eÀ
! 2OHÀ
reduction in neutral/basic solutionð Þ ð1:17Þ
Some pathways must exist through the film to allow the sodium ions to the
interface in order to produce the alkaline environment. These pathways could be
due to pores. However, an alternative theory is proposed by Leidheiser [92]
suggests that beyond a given concentration, alkali cations may have a deleterious
effect on the coating, which leads to morphological changes and introducing
conductive pathways to the interface.
2Naþ
+ 2OHÀ
! 2NaOH in presence of alkaline solutionð Þ ð1:18Þ
Similar to cathodic blistering, cathodic delamination is also the result of
alkalinity at the interface. Again, this alkalinity is the result of cathodic activity
under the coating. It is associated with faults, either inherent or induced, in the
coating. Cathodic polarisation may be a consequence of either corrosion at the
point of damage or the application of cathodic protection. Resulting from exper-
iments carried out by Smith and Dickie [93] on primer failure, it has been shown
that under impressed cathodic conditions, corrosion inhibitive pigments play no
part in the reduction of disbonding.
A number of explanations have been put forward for delamination mechanism
whereby the alkaline environment under the film affects the integrity of the metal–
polymer interface, or perhaps more properly the interface between the oxide and
the polymer. Koehler [94] showed that this form of failure only occurs when there
are alkali metal cations available in the environment to act as counter ions to the
cathodically generated OH-
ions.
Considering a coated steel substrate, immersed in an electrolyte of neutral or
near neutral pH, the half-cell reaction responsible for the delamination process is
to be oxygen reduction. This reaction generates OH-
ions at the cathodic site and
is responsible for the alkaline environment at the delamination front. The elements
1.5 Degradation of Organic Coatings 17

required for the process to proceed are water, oxygen and free electrons. The
electrons may be generated by either an anodic reaction or through the application
of cathodic protection [95].
Cathodic delamination is a result of a damaged coating; there are two possible
routes that the reactants for the cathodic reaction may take. The two alternatives
are either through the coating or along the polymer–metal interface. An extensive
review of the delamination process was carried out by Leidheiser et al. [96] and the
results indicated that water was transported to the reaction zone through the
coating. It was suggested that a certain fraction of this could be in the form of a
cation as the cathodic nature and may favour the transmission of water associated
with an ion possessing a positive charge. The supply of oxygen to the cathodic site
was found to be largely through the coating, with a small contribution from
interfacial transport, in the case of the epoxy coating studied.
Adhesion plays an important role in the protective mechanism of coatings.
When one considers the process of cathodic delamination it is clear that, once the
paint has become detached from the substrate, the underlying metal is exposed to
the environment and is no longer accorded any protection from the coating sys-
tem. Whilst the loss of adhesion, resulting from the delamination process, effec-
tively reduces the protection afforded by the coating, it is important to consider
whether the original adhesion is the deciding factor in the delamination process.
Gowers and Scantlebury [97] suggested that the beneficial role of the adhesion of a
paint/coating is due to the impairment of the formation of a layer of electrolyte at
the coating–substrate interface, preventing ionic current flow and the spread of
corrosion over the surface. Gosselin [98] showed that good surface preparation is
the key to good adhesion but the type, as well as the condition, of the substrate has
been found to have a strong influence upon the initial dry, and the subsequent wet,
adhesion of a metal/coating .
1.6 Corrosion Measurement and Analysis
Laboratory corrosion testing and evaluation of uncoated and coated materials is an
integral part of corrosion studies. This involves immersion, salt spray and elec-
trochemical testing techniques. These simulative tests may prove to be very useful
in generating data for estimation of corrosion performance and subsequent deg-
radation. The testing must consider procedures which either reproduce a service
environment or use an environment with higher severity. Emphasis is placed on
coatings applied to steel surfaces and not many accelerated test methods are
available for predicting reliably the service performance of paints.
The permeability of organic coatings increases with time or the resistance to
penetration decreases with time. The degradation is associated with corrosive ions
and water penetration into the coating, transport of ions through the coating, and
subsequent corrosion reactions at the coating–metal interface [99]. Standard
coating immersion tests can take hundreds to thousands of hours, whereas

electrochemical impedance spectroscopy (EIS) can provide reliable data on per-
formance in a short time. Capacitance and electrical properties of the coating are
measured as a function of time. Since corrosion is an electrochemical process, it
appears logical that the electrical resistance of a coating would be related to its
protective ability. The DC resistance of the coating was essentially considered to
be the internal resistance of the cell metal/coating/aqueous environment. For good
coatings, the resistance changed slowly but for poor coatings the resistance
dropped more rapidly. Rusting generally was not noted on the test panels until the
DC resistance dropped 106
ohm-cm2
. Since it is difficult to accelerate evenly all
the various factors involved, an accelerated method of detecting the deterioration,
or the lack of continued protection, of a coating could be more useful and accurate
than method of actually speeding up the deterioration or the corrosion process.
This is one reason why electrical methods for detecting paint breakdown appear to
show a comparatively high degree of correlation with actual breakdown in the
same environment. The extension of electrical methods for measuring the degree
of deterioration to coatings and uncoated steels exposed in atmospheric environ-
ments may thus be promising [100, 101].
1.6.1 Corrosion Rate Measurement
Corrosion occurs at a rate determined by equilibrium between opposing electro-
chemical reactions. The rate of any given electrochemical process depends on the
rates of two conjugate reactions proceeding at the surface of the metal. Transfer of
metal atoms from the lattice to the solution (anodic reaction) with the liberation of
electrons and consumption of these electrons by some depolarisers (cathodic
reaction). When these two reactions are in equilibrium, the flow of electrons from
each reaction of balanced and no net electron flow (current) occurs. Various
methods are available for the determination of dissolution rate of metals in cor-
rosive environments but electrochemical methods employing polarisation tech-
niques are by far most widely used. The corrosion rate (CR) is evaluated by mass
loss method considering uniform corrosion. The Corrosion rate is determined by
the following formula as per standard [102].
CR lm=yearð Þ¼
87600W
A Â T Â D
ð1:19Þ
Where, W is weight loss (mg), A is area of the specimen (cm2
), D is density of
the specimen (gm/cm3
), T is exposure time (hours) and unit lm/year is micro-
metre/year. Indirect methods of corrosion rate measurement involve anodic/
cathodic reaction, consideration of current potential relationship or polarisation
resistance values. Tafel extrapolation method is the most popular laboratory
methods for measuring corrosion rate of a metal from electrochemical data in a
corrosive medium.
1.6 Corrosion Measurement and Analysis 19

1.6.2 Electrochemical Methods
Since kinetics and mechanism of corrosion is controlled by electrochemical
principles, the technique based on electrochemical methods is used to determine
the corrosion rate and understand the mechanism of corrosion process. The testing
methods are based on principle of accelerating the corrosion process without
changing the environment and the corrosion rates can be measured without
removing the test specimens.
These processes require anodes and cathodes in electrical contact and an ionic
conduction path through an electrolyte. The electrochemical process includes
electron flow between the anodic and cathodic areas; the rate of this flow corre-
sponds to the rates of the redox reactions that occur at the surfaces. Monitoring this
electron flow provides the capability of assessing the kinetics of the corrosion
process. This also records the thermodynamic tendencies (potential) with the
accumulated metal loss registered. This is used to manipulate potential of test
specimen beyond its equilibrium value (OCP), a phenomenon called polarisation,
to effect measurements and magnitude of polarisation is called the overvoltage or
overpotential. It can have a plus or minus sign depending on whether it is above or
below the equilibrium potential value. The test electrode polarisation can be
accomplished by either DC/AC based polarisation measurements using a power
supplying equipment called Potentiostat. Three electrode corrosion testing cell is
employed with test electrolyte, specimen, counter and reference electrodes. The
counter electrodes are usually conducting, noble materials such as graphite, plat-
inum, etc. Reference electrode is used to measure and record the potential of the
test electrode during the testing process. Normally, the more negative the potential,
the higher the metal tendency to corrode [103].
Open Circuit Potential. Metal immersed in an aqueous solution develops an
electric potential at its surface called open circuit potential (OCP) which is a
characteristic of the metal solution system. The magnitude of OCP is measured
with respect to reference electrode with the help of high impedance voltmeter and
potentiostat is used to polarise or displace equilibrium potential of specimen in the
negative (cathodic) or positive (anodic) direction with reference to OCP. This is
manipulating the rates (ionic currents) of respective cathodic and anodic half-cell
electrochemical reactions. The electrochemical potential of a metal in a certain
solution is dependant on the type of the metal, the composition of the solution and
its pH, oxygen content and temperature [104, 105].
Polarisation Test Method. This method is used to determine the corrosion rate.
Polarisation resistance (Rp) is the resistance of specimen to oxidation during the
application of an external potential in DC corrosion measurement methods. The
CR and Icorr are related to Rp and can be calculated from equation given below and
polarisation resistance is related to Icorr according to Stern Geary relation [106].
Rp ¼
babc
2:303Icorr ba þ bcð Þ
ð1:20Þ

Where, ba and bc are anodic and cathodic Tafel slopes (mV/decade), Icorr is
corrosion current density (A/cm2
) and Rp is polarisation resistance (ohm-cm2
).
This involves a potential scan ± 250 mV of Ecorr at a scan rate of 0.1–1.0 mV/s.
The technique is used to determine the equilibrium corrosion current, potential,
Tafel constants and corrosion rates. The corrosion rate (CR) is determined from
the Faraday’s law:
CR ¼
0:13IcorrðEWÞ
q
ð1:21Þ
Where, CR is corrosion rate in mpy (1 mpy = 0.054 lm/yr), EW is equivalent
weight, q is density of material in gm/cm3
and Icorr is corrosion current in A/cm2
.
Tafel extrapolation is used to determine the equilibrium corrosion current, where
linear extrapolations of anodic and cathodic branches of the plot beyond ±50 mV
of OCP are made to intersect at OCP to measure the Icorr. This is a destructive
technique as it can cause some degree of surface roughening on the test specimen
[107]. General corrosion occurs in the active region, little or no corrosion occurs in
the passive region and pitting corrosion can occur in the transpassive region [108].
Cyclic Polarisation. Cyclic polarisation curves are considered as an extension
of potentiodynamic polarisation curves and used to measure the pitting tendencies.
The potential scan begins at Ecorr (OCP) and continues in the positive (anodic)
direction up to the transpassive region, where a large increase in current (corro-
sion) occurs. At a threshold current density, the scan is reversed and continued in
the negative (cathodic) direction back. The applied potential versus the log values
of the measured current density are plotted. The cyclic polarisation plots can show
positive hysteresis, negative hysteresis, repassivation or protection potential.
Negative hysteresis occurs when reverse scan current density is less than that for
the forward scan and positive hysteresis occurs when reverse scan current density
is greater than that for the forward scan. A passive film is damaged when potential
is raised into the transpassive region and pits can initiate when film damage is at
discrete (localised) locations on the metal surface. Pits will continue to grow when
protection potential (Epp) is greater than Ecorr and pits will not grow when Epp is
less than Ecorr. In cyclic polarisation curve, hysteresis can provide information on
pitting corrosion rates and how readily a passive film repairs itself. Positive hys-
teresis occurs when passive film damage is not repaired and/or pits initiate; neg-
ative hysteresis occurs when a damaged passive film repairs itself and pits do not
initiate. Area of hysteresis is very important as more the area, more aggravated the
corrosion is. Generally, the reverse scan is at a higher current level than the
forward scan. The size of the pitting loop is a rough indication of pitting tendency;
the larger the loop, the greater the tendency to pit [109, 110].
Electrochemical Impedance Spectroscopy. Electrochemical Impedance Spec-
troscopy (EIS), a non-destructive investigative technique enables an insight into
the corrosion process not obtained by DC techniques. EIS provides information on
reaction parameters, corrosion rates, oxide characteristics and coating integrity,
data on electrode interfacial capacitance and charge transfer resistance. It provides

kinetic and mechanistic information on electrochemical systems such as corrosion
processes. The slow electrode kinetics, slow preceding chemical reactions and
diffusion impede electron ﬂow in electrochemical cells much in the same way as
resistors, capacitors and inductors do in AC circuits. The working electrode
interface undergoing an electrochemical reaction is analogous to an electronic
circuit with a speciﬁc combination of resistors and capacitors and AC circuit
theory can be used to characterise an electrochemical system in terms of equiv-
alent circuit. The technique broadly involves subjecting an electrochemical system
to a range of small magnitude AC polarising voltage frequencies and corroding the
system response in the form of complex impedance plots. The complex impedance
diagrams are correlated with an equivalent AC circuit model with unique values
for circuit elements. These values can then be used to infer kinetic and mechanistic
information about an electrochemical system [111–113]. The response of a cor-
roding metal to small amplitude AC signal (10–20 mV) of widely varying fre-
quency (0.001–100 kHz) can be analysed by EIS following the absorbance of
electrical energy at a certain frequency at the metal solution interface. On appli-
cation of a sinusoidal alternating potential signal of the form:
V tð Þ ¼ V0Sin xt ð1:22Þ
Time dependence current response of the form:
I tð Þ ¼ I0Sin ðxt þ hÞ ð1:23Þ
Where V(t) is applied potential,V0 is amplitude of applied potential, I0 is
amplitude of generating current, electrode surface expressed as an angular fre-
quency (x) and h is phase between V and I. Due to the applied potential frequency
(x) dependent impedance Z(x) may be expressed as:
Z xð Þ ¼ Ru þ
Rp
1 þ x2R2
pCdl2
!
þ j
xR2
P
1 þ x2R2
PCdl2

¼ Zreal þ j Zimg ð1:24Þ
Where Ru is the solution resistance, Rp is the polarisation resistance and Cdl is
the double-layer capacitance. Various electrochemical phenomena at the metal
solution interface causes a time lag and a measurable phase angle h. These pro-
cesses will be simulated by resistive and/or capacitive electrical networks. The
impedance behaviour of an electrode may be expressed in Nyquis plot of Zimg
(imaginary part of impedance) as a function of Zreal (real part of impedance) or in
Bode plots of mod Impedance and h versus frequency, where x = 2 pf.
To evaluate a coating, along with Ru, Rp, Cdl and Wd (Warburg impedance) two
additional circuit elements, namely coating capacitance (Cc) and resistance of
coating pores (Rpo) come into account. The presences of Cdl or Cc can be idealised
by a constant slope in Zimg versus frequencies plot and peaks in h versus fre-
quencies plots.
For uncoated sample, Zimg versus frequencies plot shows early low impedance
at all frequencies. In coated specimen, Rpo measures the early deterioration at low

frequency impedance. The corrosion product accumulation at the coating–metal
interface can induce coating defects and thereby reduces Rpo. The coating in these
cases is applied to the surface, which is not completely derusted. Rp for corrosion
beneath the coating is apparently quite high, as stated earlier and would require
still lower frequency measurements, which are difficult and time consuming. In
absence coating, the Zimg versus frequency plot measures the low value of Rp at
low frequency resulting from the comparatively high corrosion rate.
Electrified interfaces called electric double layers (Cdl) are set up at metal–
electrolyte boundaries during electrochemical process. These interfaces are char-
acterised by impedances to electron flow and ionic movement. The impedance of
an electrified boundary manifests as interfacial capacitance and associated charge
transfer resistance. The electrified interfaces are typified by time constants, which
are given by product of magnitudes of associated capacitances and resistances. The
time constants are noticeable in EIS spectra as semicircles in Nyquist plots,
negative slopes in Bode magnitude plots and negative inflections in Bode phase
plots. Mathematical regression of time constants in EIS spectra with equivalent
electrical AC circuit models leads to quantification of associated resistances and
capacitances. A higher charge transfer (ohmic) resistance implies greater polari-
sation or corrosion resistance of the metal in a given aqueous environment. The
capacitances themselves can be used to identify the corrosion, coating and diffu-
sion processes with different time constants.
Thus, the metal–electrolyte interface behaves and responds like an AC circuit
with a specific combination of resistors and capacitors under the influence of the
AC polarising voltage frequencies. Figs. 1.2, 1.3, 1.4, 1.5 show metal solution
interface (single time constant system) where Ru is solution resistance, Rp is po-
larisation resistance.
The EIS spectra for coated metal–electrolyte systems are characterised by two
time constants, two semicircles in Nyquist plots, two negative slopes in Bode
magnitude plots and two negative inflections in Bode phase plots [114, 115].
Figs. 1.6, 1.7, 1.8, 1.9 show coated metal solution interface (two time constant
system) and Cdl is double-layer capacitance.
The coating time constants are smaller and manifest at higher frequency regions
of the impedance spectra, whereas time constants corresponding to metal corrosion
appear at lower frequency regions. An impedance plot obtained can be correlated
with one or more equivalent pore resistance (Rpo), coating capacitance (Cc) and
polarisation resistance (Rp).
CPE is used in a model in place of a capacitor to compensate for non-homo-
geneity in the system. A rough or porous surface can cause double-layer capaci-
tance to appear as CPE and Warburg element [116, 117].
Kihira et al. [118] applied EIS to investigate the condition of the rust film
formed on the weathering steel, and proposed new corrosion monitoring method
based on rust film resistance. Nishimura et al. [119] measured the electrochemical
impedance of a carbon steel covered with rust film formed in a wet/dry environ-
ment containing chloride ions. They reported that the charge transfer resistance
(Rp) increased with the wet–dry cycles of exposure.

Itagaki et al. [120] used EIS to investigate the electrochemical properties of the
rust film membrane formed on low alloy steels. The electrochemical impedance of
the actual rust film membrane formed by wet–dry cycles showed the capacitive
semicircle on Nyquist plot corresponding to a single time constant. The time
constant of the capacitive semicircle was found composed of the rust film resis-
tance and the film capacitance. The value of rust film resistance was shown to
depend on the alloying elements in weathering steel and it was shown that the
Fig. 1.2 Metal electrolyte
interface of uncoated
corroding steel
Fig. 1.3 EIS spectra for
single time Constant of bode
magnitude plot
single time constant of bode
phase plot

addition of alloying elements increases the diameter of the capacitive semicircle.
The result meant the low permeation rate of chloride ions in the rust ﬁlms of these
alloys. The Nyquist plots were found to diverge from a true semicircle due to the
current distribution in the ﬁlm [121].
Feliu et al. [122] have applied EIS to study the corrosion and electrochemical
activity at the metal–rust interface in connection with the application of protective
single time constant nyquist
plot
Fig. 1.6 Electrolyte
interface of corroding
coated steel
Fig. 1.7 EIS spectra for two
time constant of Bode
magnitude plot

treatments with rust converters to rusted steel. Mild steel with mill scale were
prerusted for 2 years in a rural atmosphere before applying conversion treatments.
They found that the shape of the low frequency areas of the Nyquist plots are
markedly inﬂuenced by diffusion processes in the rust layer and/or by the porous
nature of the rusted steel electrode itself.
With alloying and increasing period of exposure to saline atmospheres, the
magnitudes of rust pore resistances are expected to increase and rust capacitive
reactance will decrease since capacitance is inversely proportional to AC imped-
ance. It is also likely that the charge transfer resistance which is indicative of metal
corrosion, itself will undergo an increase with alloying thereby signifying higher
corrosion resistance for alloyed steels. Corrosion, coating and diffusion processes
are not always associated with same frequency ranges. Corrosion resistances are
observed at low frequencies, but coating pore resistances can also be observed at
low frequencies, particularly, when a coating is saturated with electrolyte and
metallic corrosion does not occur. Capacitance values can be used to guide
interpretation as to what type of process is associated with each time constant.
Corrosion time constants have capacitance values (1–20 lF/cm2
), coating time
constants have capacitance values (nF/cm2
) and oxides have capacitance values
(1000 lF/cm2
). Capacitance values of the order of C100 lF/cm2
are found when
surface adsorption occurs in conjunction with corrosion [103].
time constant of Bode phase
plot
time constant nyquist plot

EIS data are analysed by fitting them to an equivalent electrical circuit model
consisting of resistors, capacitors, and inductors. The working electrode interface
undergoing an electrochemical reaction is analogous to an electronic circuit and
can be characterised as an electrochemical system in terms of equivalent circuit.
Typical circuits are shown in Figs. 1.10, 1.11, 1.12 and 1.13 where Yo is admit-
tance (ohm-cm2
),Cf is double-layer capacitance and a is the exponents [114]. (R.E:
Reference Electrode and W.E: Working Electrode)
1.7 Rust Characterisation
Several techniques are used in different stages of rust characterisation for steels
and microscopy related techniques are useful in understanding the topological state
of the corroded layers and in analysing their cross sections.
1.7.1 SEM and EDX
The degree of corrosion, surface morphology, particle size and texture can be
effectively studied by scanning electron microscope (SEM) and energy dispersive
X-ray analysis (EDX). The optical microscope can be used for imaging the surface
but it has limitations of resolution and depth of field at higher magnifications. SEM
can be used for high-resolution imaging of the surface, with a large depth of focus.
Atmospheric corrosion of weathering steel in the presence of NaCl and SO2 was
investigated by A.Q. Qu found that NaCl can accelerate the corrosion [123]. The
relationship between mass loss and amount of NaCl deposition follows the qua-
dratic function both in SO2 free air and in air containing SO2. The combined effect
of NaCl and SO2 on the corrosion of steel is greater than that caused by each single
component [124]. SEM and EDX are used to characterise the corrosion products of
steel. In the absence of SO2, a FeOOH, b FeOOH, c FeOOH, Fe3O4 and c Fe2O3
are the dominant corrosion products, while b FeOOH, c FeOOH, Fe3O4 and
FeSO4.H2O dominate in the presence of SO2 [125].
1.7.2 X-ray Diffraction
X-ray diffraction (XRD) is used for identifying the oxides in rust and sometime
provides incorrect identification of the composition of the rust formed on weath-
ering and carbon steels [126, 127]. Separate identification of Fe3O4 and c Fe2O3 is
not possible as both oxides have cubic structure and nearly identical lattice
parameters at room temperature. Analysis of rust coatings by XRD significantly
underestimates the goethite fraction in the corrosion products, especially for

weathering steel [128]. This is due to the presence of the nanophase oxides whose
diffraction lines are very broad and are frequently overlooked owing to their
overlapping with sharper peaks for larger particles of the same oxide phases in the
rust and are believed to be incorrectly referred to as amorphous. XRD measure-
ments have lead to general conclusion that weathering steel forms a protective
coating with ratio a FeOOH/c FeOOH [ 2 [129, 130].
In another study, corrosion rates of Mn–steel and Cu–Mn weathering steel in a
simulated coastal environment were measured by wet–dry cyclic test. The rust
layer was observed and analysed by SEM and XRD. The experimental results
Fig. 1.10 Representative
randle equivalent circuit
CPE equivalent circuit
CPE with diffusion
equivalent circuit
REAP equivalent circuit

showed that the corrosion rate of Cu–Mn weathering steel was lower than that of
Mn–steel, due to the formation of a denser rust layer. The rusts on the two steels
consisted of Fe3O4, a FeOOH, b FeOOH, c FeOOH and amorphous phases. The
amount of a FeOOH and b FeOOH in the rust of Mn–steel was larger than that of
Cu–Mn weathering steel. The addition of Cu increased the amount of Fe3O4, while
the addition of Mn decreased the amount of c FeOOH in the rusts [131].
1.7.3 Raman Spectroscopy
Raman spectroscopy is used to study the internal structure of molecules and
provides unique information about molecular patterns, spacing, and bonding. This
is based on Raman Effect, which is the inelastic scattering of photons by molecules
as every compound possesses a typical Raman spectrum. In order to be Raman
active a molecular rotation or vibration must cause some change in any component
of molecular polarisability. This is defined as the induced dipole moment set up in
the molecule by applied electric field. Practically, stokes lines (high k, low t low
t‹ ) are intense in the spectrum than anti stokes lines (low k, high t high t‹ ) and the
shift are measured with respect to the reference Rayleigh lines (unshifted with
same k, t t‹ ). Fine structure effects are not considered in practical situation as the
corresponding effects are of less important. It is essentially an emission spec-
troscopy. The source is a monochromatic (laser) and the instrumentation is simply
a typical visible range (He–Ne) spectrometer. Raman Effect can take place for any
frequency of the incident light which is simply a light scattering phenomenon.
Spherical top molecules are completely Raman inactive whereas asymmetrical top
one is Raman active [132–134]. This case is corresponding to rotation of Raman
mode (some phases identified by XRD not by Raman due to inactive Raman). For
vibrational Raman mode, symmetrical vibration always produce intense Raman
lines whereas unsymmetrical ones are normally weak and sometimes unobservable
[135, 136]. Table 1.2 provides the important bands of some common corrosion
products of iron and [R] indicates the published references.
Thibeau et al. [143]. have used Raman and infrared spectroscopy to investigate
the structure of the inner rust layer formed on weathering steels exposed to an
industrial environment for 4.5 and 8 years. The inner rust layer on weathering steel
was composed primarily of d FeOOH with 10–20 %, c FeOOH and some
a FeOOH irrespective of the exposure period.
Dunnwald and Otto [137] found phase transformation of iron corrosion product
to Fe(OH)3 in the atmosphere containing SO2 with humidity by Raman spec-
troscopy. Subsequently, Fe(OH)3 gets transformed to crystalline FeOOH with
amorphous FeOOH. It has been shown that the amorphous rust is the primary
product of atmospheric corrosion, which later transforms to crystalline forms in
the absence of copper. Yamashita et al. [144] studied the long-term growth of the
protective rust layer formed on weathering steel under atmospheric corrosion in an
industrial region involving an exposure for 26 years. The outer layer of rust was
1.7 Rust Characterisation 29

Table1.2ImportantBandsofCorrosionProducts
OxidesDescriptionWavenos(cm-1
)PublishedWavenos(cm-1
)[R]
aFeOOH
Goethite
Givesrelativelystrongpeaks205,247,300,386,418,481,549245,300,390,420,480,550,685
248,303,397,485,554,680,1002,1120
245,300,390,485,550,675
298,397,414,474,550
[138]
[137]
[139]
[140]
bFeOOH
Akaganeite
Characterised
by4peaks
314,380,549,722310,386,497,538,723
310,385,415,480,535,615,675,725
[141]
[139]
cFeOOH
Lepidocrocite
Charactersed
by7peaks
219,252,311,349,379,528,648255,380,528,654,1054,1307
252,380,660
[140]
[142]
dFeOOHGivesrelativelyweakpeaks297,392,666400,655
220,295,385,495,670
[141]
[143]
aFe2O3
Haematite
Givesstrongestpeaks226,245,292,411,497,61227,245,293,298,414,501,612
225,245,295,415,500,615,1320
[143]
[138]
cFe2O3
Maghemite
Characterisedby4peaks381,486,670,718265,300,345,395,515,645,670,715,1440
350,505,660,710,1425
[138]
[141]
Fe3O4
Magnetite
Characterisedby2peaks532,667616,663
298,319,418,550,676,1322
[143]
[137]
BoldStrongestPeakinSpectrum,UnderlinedNextStrongestPeakinSpectrum

composed of c FeOOH while the inner layer was comprised mainly of densely
packed nanoparticles of a FeOOH. Further, a FeOOH was found enriched with Cr
and reported to be the stable and protective uniform rust layer. It was proposed that
the c FeOOH, as an initial rust layer of weathering steel, formed after a few year of
exposure, is transformed eventually into the final stable rust layer consisting of
nano-size a FeOOH after decades with amorphous ferric oxyhydroxide as an
intermediate transition product which is formed after several years of exposure in
atmosphere. The mean diameter of the rust particles was found to be approxi-
mately 0.5 lm in the outer loose layer aggregate whereas the inner layer was
composed of densely packed fine particles within the larger secondary particles. In
contrast, the corrosion product formed on mild steel contained number of voids
and microcracks.
Microscopic observation of weathering steel exposed outdoors during stable
protective rust coating development, reveal two phases in layers parallel to the
steel surface. The layer adjacent to the steel is grey and compact and the external is
reddish and porous. The thickness of the inner phase increases up to outdoor
exposure periods longer than 5 years, when it becomes the only component of the
patina. It is responsible for the electrochemical potential increase and low corro-
sion rate of steel, restricting oxygen and water access as a barrier to elements
controlling further corrosion. In carbon steels corrosion products form also as two
optically different phases, but they are mixed up. They experience lower increase
of electrochemical potential during natural or simulated outdoor exposure. Their
corrosion rate remains up to an order of magnitude above those determined for
weathering steels in the respective atmospheres [145, 146].
1.8 Rust Simulation
Pourbaix diagram [3] maps out possible stable equilibrium phases of an aqueous
electrochemical system and indicates that pure iron is passive at pH values from 9
to 12.5 to form iron hydroxide. Considering the interplay of atmospheric factors
this diagram was used as guide to the steel dissolution process to form passivity on
WS in laboratory. The passive films formed on pure iron are not so stable and
consequently the passivation state of iron is not maintained for prolonged time
periods [147, 148]. The rust layers of steels play a role as a barrier against cor-
rosion, and their growth rate is decreased to a rate similar to that of the passive
films, when suitable elements are added to the steel [149, 150].
Rust on weathering steel changes over time and the final protective rust has a fine
a FeOOH and is dispersed as amorphous rust. It is reported that the addition of seed
rust, which is a stage in rust formation, results in the preferential formation of
homogeneous rust. This phenomenon suggested the possibility that protective rust
will also form preferentially in atmospheric environments when protective rust is
present [151, 152].
1.7 Rust Characterisation 31

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