Materials Chemistry and Physics 112 (2008) 844–852
Contents lists available at ScienceDirect
Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
The effect of ␤-FeOOH on the corrosion behavior of low carbon steel
exposed in tropic marine environment
Yuantai Ma, Ying Li ∗ , Fuhui Wang
State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Wencui Road 62, Shenyang 110016, China
a r t i c l e
i n f o
Article history:
Received 14 January 2008
Received in revised form 19 May 2008
Accepted 15 June 2008
Keywords:
Corrosion
Monolayer
Oxide
Infrared spectroscopy
a b s t r a c t
The atmospheric corrosion performance of carbon steel exposed in Wanning area, which located in the
south part of China with tropic marine environment characters, was studied at different exposure periods (up to 2 years). To investigate the effect of ␤-FeOOH on the corrosion behavior of carbon steel in
high chloride ion environment, rust layer was analyzed by using infrared spectroscopy, scanning electron
microscope, X-ray diffraction, and the rusted steel was measured by electrochemical impedance spectroscopy method. The weight loss test indicated that the corrosion rate of carbon steel sharply increased
during 6 months’ exposure and gradually reduced after longer exposure. The results of rust analysis
revealed that the underlying corrosion performance of the carbon steel was dependent on the inherent properties of the rust layers formed under different conditions such as composition and structure.
Among all the iron oxide, ␤-FeOOH exerted significant influence. The presence of a monolayer of the rust
as well as ␤-FeOOH accelerated the corrosion process during the initial exposure stage. EIS data implied
that ␤-FeOOH in the inner layer was gradually consumed and transformed to ␥-Fe2 O3 in the wet-dry
cycle, which was beneficial to protect the substrate and reduced the corrosion rate.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Structural materials are often used under different rust layers,
and their corrosion performances depend upon a large number
of factors including the rust formation mechanism, rust evolution
process and the inherent properties of the rust layers such as composition and structure. The characteristic of the rust layers formed
on the surface of low carbon steel exposed in different environments may give rise to distinct consequences which act either to
reduce the corrosion rate, which means the corrosion product is
protective; or to accelerate the corrosion process, which means
the corrosion product is non-protective. Stratmann et al. [1] has
pointed out that the protective effect of the rust layers depended on
the chemical composition, adherence, compactness, hygroscopic
capacity and morphology. Yamashita et al. [2] also proved that the
rust formed on the low-alloy steels consisted of two layers: the
outer layer, which was looser and easily flaked off; the inner layer
mainly composed of ␣-FeOOH was compact and denser, and was
the protective layer. Different compositions have been reported for
the protective inner layer. Okada et al. [3] reported the formation
of an amorphous layer of fine magnetite next to the steel surface.
∗ Corresponding author. Tel.: +86 24 2392 5323; fax: +86 24 2389 3624.
E-mail address: [email protected] (Y. Li).
0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2008.06.066
Misawa et al. [4] suggested the existence of an amorphous ferric
oxyhydroxide. It has also been shown that the presence of ␦-FeOOH
in the rust layer has a key effect [5]. Phase transformations in rust
have been attributed to atmospheric influences, for instance [4] the
hygroscopic SO2 in industrial atmospheres often lowered the pH of
water, wetted the rust layer and dissolved the initial corrosion products of ␥-FeOOH, and also promoted the phase transformation of
␥-FeOOH to amorphous ferric oxyhydroxide and ␣-FeOOH.
Chloride ion is the primary aggressive species in marine environment. Allam [6] observed that the chlorides (FeCl2 , FeCl3 ) were
only formed during the initial stages; as the rust layer grew in
thickness, the supply of fresh chloride ions may gradually diminish, slowing down the rate of chloride formation. Some researcher
has found that ␤-FeOOH formed on the surface of steel in high Cl−
containing environment. For example, Nishimura et al. [7] investigated the effect of chloride ion on the transformation of rust formed
on the carbon steel and observed that the content of ␤-FeOOH
increased with the concentration of Cl ions in the environment
and ␤-FeOOH was transformed from green rust I (GRI) automatically in the dry process of the test. The transports of chloride ions
through the rust layer permit direct attack on the underlying bulk
steel, which accelerate corrosion process due to forming ␤-FeOOH
in high Cl− containing environment. Yamashita et al. [8] interpreted
the effect of environmental corrosiveness on the formation process
of the rust layer and concluded that ␣-FeOOH was preferentially
Y. Ma et al. / Materials Chemistry and Physics 112 (2008) 844–852
Table 1
Chemical compositions of the carbon steel studied (wt. %)
Steel
C
S
P
Mn
Si
Cu
Q235
0.176
0.023
0.019
0.57
0.233
0.033
formed in the case of the Na2 SO4 solution film, whereas ␤-FeOOH
appeared only under the NaCl solution film. Kamimura et al. [9] also
reported that ␤-FeOOH was electrochemical active and the existence of ␤-FeOOH influenced the corrosion behavior of weathering
steel exposed at marine site. It approved that ␤-FeOOH actually
formed on the surface of steel exposed at high Cl− containing
environment and sharply influenced the corrosion performance.
However, the exact mechanism of this process is still unclear. And
that, the corrosion mechanism of steel obtained by wet-dry corrosion test in the laboratory cannot explain the corrosion behavior of
steel exposed in the real environment suitably.
In the present paper, carbon steel was exposed at 25 m from the
sea line in Wanning area, an exposure station with high concentration Cl− containing environment. The corrosion rate of carbon steel
exposed for various periods in a marine site has been determined
by weight loss techniques and the effect of structure and composition of the corrosion products on the corrosion rate is assessed.
The resistances and capacitances of the rust layers were obtained
by fitting the EIS results and the mechanism of protective effect
of the rust layer was discussed. The emphasis of our research was
attracted in the effect of ␤-FeOOH to protective property of the rust
layer.
2. Experimental
2.1. Materials preparation
Carbon steel (Q235) was used as the test material and its composition
was listed in Table 1. Before experiment the specimens were sectioned into
100 mm × 45 mm × 5 mm coupons and polished down to 800# grade emery paper,
cleaned ultrasonically in acetone and then rinsed with distilled water. All the specimens were weighed and prepared for exposure test.
2.2. Atmospheric exposure tests
Wanning district is located at Hainan Province in the south part of China,
between 110◦ 05 eastern longitude and 18◦ 58 northern latitude, and is a part of
the tropical marine zone with high temperature, high humidity, high Cl− concentration and low pollution. The climatic characterization and the main environmental
parameters measured are listed in Tables 2 and 3. For investigating the corrosion
behavior of low carbon steel in high Cl− containing environment, the exposure location was about 25 m away from the sea line. According to ISO-4542 specification, the
carbon steel specimens (Q235) were exposed at 30◦ to the horizontal, with skyward
surface facing sea, Test specimens were retrieved from the marine site for analyses
after 3, 6, 9, 12, 18 and 24 months’ exposure. During every periods of sampling, five
samples were picked up, two samples for electrochemical measurement and the
other three samples for weight loss test. For weight loss measurements, corrosion
products on the specimen surfaces were removed chemically by immersion in a specific solution (500 ml HCl + 500 ml distilled water + 3.5 g hexamethylenetetramine)
that was vigorously stirred for ∼10 min at 25 ◦ C. After corrosion products had been
completely removed, the specimens were rinsed with distilled water, dried with
blower, and then weighed to determine their mass loss.
2.3. Characterization of rust layers
The specimens retrieved from exposure site were stored in the desiccators for
analysis. For IR analysis, the outer rust layers were scraped off using a coarse wire
brush and inner layers by a surgical blade. About 3 mg of rust was mixed with about
100 mg of pure anhydrous KBr and ground to fine size in a mortar with a pestle. The
mixture was then pressed in a simple die using suitable pressure to result in circular
disc of about 1 mm thickness. A Magna-IR 560 infrared spectrophotometer was used
to measure spectra of the rust layers in the ranges 400 cm−1 and 4000 cm−1 with
the accuracy of 4 cm−1 . For SEM observation, the exposure specimen was cut into
bit samples with size 15 mm × 15 mm × 5 mm by hand-operated saw, and then small
samples were encapsulated into the PVC pipe filled with epoxy resin. The ring sample
was polished down to 1000# grade emery paper and polished with terylene by using
coal oil for chloride enriched in the inner layer-substrate interface. The morphologies
of surface and cross-section of the rust layers were characterized by using the SEM
(XL30FEG) with EDAX for analyzing the change of the element chlorine in the inner
layers. The rust phase was ground to fine powders sample in a mortar with a pestle
for XRD and the XRD conditions were 40 kW intensity, 2.0◦ /min−1 scanning speed,
and 2 = 5–40◦ of range using a Cu target.
2.4. EIS measurements
The EIS measurements were taken using PARSTAT 2273 potentiostat/galvanostat, equipped with Powersuite software. Impedance measurements of
the rusted steel specimens were performed using a three-electrode electrochemical
cell, with a saturated calomel electrode (SCE) as reference electrode and a platinum
foil as counter electrode. The original samples were stored in the desiccators to
avoid the influence of different environment. The whole sample was manually
sectioned into 45 mm × 10 mm × 5 mm coupons using hand-operated saw, and
the coupon was connected with copper lead by using soldering tin as a working
electrode. The working electrode was covered with the mixture of olefin and rosin
to leave an exposed area of 3–5 cm2 of the front surface.
The measurements were carried out in 0.1 mol L−1 Na2 SO4 aqueous solution at
25 ± 2 ◦ C in view of the annual average temperature of Wanning area was 24.7 ◦ C and
frequency range of 100 kHz to 10 mHz, and signal amplitude perturbation 10 mV.
3. Results and discussion
3.1. Corrosion rates of carbon steel
Fig. 1 shows the variation of the thickness loss of the carbon steel
with natural exposure at Wanning exposure site. The thickness loss
(␮m) was obtained from the formula:
Table 2
Meteorological data of atmosphere at exposure site
Site
Average temperature (◦ C)
Average humidity (%)
Wetness time (h a−1 )
Precipitation (mm a−1 )
Days of precipitation (day a−1 )
Sun shine (h a−1 )
Distance to sea (m)
845
Wanning
24.7
86
6736
1782.4
122
2123.4
25
Wt × 104
A
(1)
Where Wt is the weight loss (g), is the density (7.86 g cm−3 ) of
the carbon steel and A is the exposed area (cm2 ) of the specimen.
Fig. 2 shows the dependence of corrosion rate on exposure time.
Table 3
Corrosion factors of atmosphere at exposure site
Site
Wanning
Cl− (deposition
(mg 100 cm−2 day−1 ))
3.767
SO2 (deposition
(SO3 mg 100 cm−2 day−1 ))
0.060
NO2 content
(mg m−3 )
0.005
H2 S content
(mg m−3 )
0.000
pH
5.0
Rain
Cl− (mg m−3 )
SO4 2− (mg m−3 )
11229
3.16E−4 mol L−1
3552
3.70E−5 mol L−1
846
Y. Ma et al. / Materials Chemistry and Physics 112 (2008) 844–852
more, the corrosion rate can be classified by PAI [11]. Yamashita et
al. [10] also argued that the formation of ␣-FeOOH was promoted
and/or crystal growth of ␥-FeOOH was suppressed by the surface
treatment, that this process lowers the corrosion rate of weathering
steel.
Fig. 1. Thickness loss of carbon steel vs. exposure time in marine site.
The corrosion rate (␮m year−1 ) is obtained from the relationship:
CRn (corrosion rate) =
dn − dn−1
tn − tn−1
(2)
where d is the thickness loss (␮m), t is the exposure time (month), n
is the period of sampling (when n = 1, means the sample is exposed
for 3 months, in turn n = 2, 3, 4, 5, 6, means the sample is exposed
for 6, 9, 12, 18, 24 months). It is observed that the corrosion rate
increases with the prolonged exposure time during the first 6
months and slightly decreases until 12 months. However, when the
carbon steel was exposed for up to 18 months, the corrosion rate
reached the vertex; after exposure for 24 months, the corrosion rate
again showed some decrease.
3.2. Analysis of rust phase
3.2.1. Rust composition
The fluctuation of the corrosion rate for various periods of exposure is related with the change of composition and structure of the
rust layer, the deposition of chloride and the change of weather
during the period of exposure. Some authors [10,11] suggested the
protective ability index (PAI) as a paramount parameter to evaluate the protectiveness of rust layer. Kamimura et al. [9] figured out
that the definition of PAI hinged on different environment. Further-
Fig. 2. Corrosion rate of carbon steel vs. exposure time in marine site.
Fig. 3. Infra-red absorption spectras of rust layer: (a) outer layer; (b) inner layer.
Y. Ma et al. / Materials Chemistry and Physics 112 (2008) 844–852
847
which is transformed from the GRI in the dry process, accelerates the corrosion of the carbon steel in a wet-dry environment
containing chloride ions. In the present study, that the ␤-FeOOH
and intermediate phase, FeOCl, are detected by X-ray diffraction,
as shown in Fig. 4. The corrosion rate increases markedly from 3
months’ exposure to 6 months’. It is expected that the protective
effect of the rust layer is weakened because of the formation of ␤FeOOH in the dry process. The decrease in corrosion rate observed
after 6 months’ exposure means that the protective effect of the
rust layer is enhanced with the formation of ␥-Fe2 O3 . With further
increase in exposure time, the reduction of ␤-FeOOH continued and
the amount of ␤-FeOOH decreased. Furthermore, ␣-FeOOH exists
in the inner layer again, which is protective and stable, and lowers
the corrosion rate.
Fig. 4. XRD pattern for inner layer formed on carbon steel exposed for 6 months.
The compositions of the rust layers were analyzed by infrared
spectroscopy as shown in Fig. 3. The rust phase ␤-FeOOH was not
accurately detected by using IR, so the inner rust layer formed at
six months’ exposure was analyzed by X-ray diffraction as shown
in Fig. 4. Table 4 lists the change of the rust composition formed on
the carbon steel exposed for various exposure periods.
A monolayer of rust was formed on the carbon steel exposed for
3 months, and this layer was composed of ␥-FeOOH, ␣-FeOOH and
amorphous oxyhydroxide, as well as ferrihydrite. After another 3
months’ exposure (6 months), a second layer of rust was formed
on the surface of carbon steel. From the beginning of sixth month
exposure, the composition of the outer layer comprising ␥-FeOOH,
␣-FeOOH and amorphous oxyhydroxide, as well as Fe3 O4 , did not
vary with the exposure time (up to 24 months’ exposure). However, the composition of the inner layer showed some variation.
The inner layer formed during 6 months’ exposure was composed
of only ␤-FeOOH and amorphous oxyhydroxide. After exposure
for 9, 12 and 18 months, the inner layer composition was fairly
constant with exposure time, consisting of ␤-FeOOH, ␥-Fe2 O3
and amorphous oxyhydroxide. But at the end of natural exposure test (24 months’ exposure), the inner layer was composed
of ␥-FeOOH, ␣-FeOOH and amorphous oxyhydroxide, as well as
Fe2 O3 .
It is obvious that the phase transformation happens in the
rust layer formed during various exposure times. Misawa et al.
[12,13] suggested that dissolved chromium and phosphorus ions
enhance the formation of uniform amorphous ferric oxyhydroxide,
which protects the steel substrate, and that this amorphous ferric
oxyhydroxide will further transform into the more stable and protective structure of ␣-FeOOH. Some workers reported that ␥-FeOOH
formed first and was then transformed to ␣-FeOOH and Fe3 O4
[14–16]. Nishimura et al. [7] suggested the existence of ␤-FeOOH,
Table 4
The rust composition formed on the carbon steel exposed for various periods
Exposure time (month)
3
6
9
12
18
24
Outer layer
L, G, AM
Ferrihydrite
L, G, AM
Fe3 O4
L, G, AM
Fe3 O4
L, G, AM
Fe3 O4
L, G, AM
Fe3 O4
L, G, AM
Fe3 O4
Inner layer
L, G, AM
Ferrihydrite
A, AM
A, AM
␥-Fe2 O3
A, AM
␥-Fe2 O3
A, AM
␥-Fe2 O3
L, G, AM
␥-Fe2 O3
L: lepidocrocite (␥-FeOOH); G: goethite (␣-FeOOH); A: akaganetite (␤-FeOOH); AM:
amorphous rust mix.
3.2.2. Rust structure
The surface appearance of carbon steels gradually changed to a
dark reddish color during the initial stage of exposure and gradually became rougher. After 6 months’ exposure, double rust layers
appeared and the outer layer looked very loose, easily flaked off.
Examination of the surface rust showed that it consisted of very
porous discrete globules during the initial exposure stage; with
the exposure time extended, the surface gradually became compact
and the morphology of the rust changed, as shown in Fig. 5. Kassim et al. [17] reported that the initial oxyhydroxide product in the
presence of the Cl− ion was ␥-FeOOH which was lath shaped after
its formation by rapid oxidation of Fe(OH)+ and aging. According
to Dias et al. [18], ␥-FeOOH which formed on steel after exposure
in a humid tropical climate was platelike. In addition, Smith and
McEnaney [19] observed that acicular ␥-FeOOH developed on the
outside surface of grey cast iron. From these observations, the platlike porous corrosion product should be the ␥-FeOOH. According
to Misawa et al. [4], ␥-FeOOH was first transformed to amorphous
ferric oxyhydroxide by dissolution and precipitation, and then the
amorphous ferric oxyhydroxide transformed to ␣-FeOOH by deprotonation using hydroxyl ions provided by rain. It is reasonable to
assume that the nebulous phase is the amorphous ferric oxyhydroxide and the clubbed phase is ␣-FeOOH, as shown in Fig. 5. The
appearance of the two phases prevents the penetration of water,
oxygen and other corrosives to the rust-steel interface and lowers the corrosion rate of the carbon steel during the later stage of
exposure. Nevertheless, ␤-FeOOH exists in the inner layer during
exposure of 6 months, and is very porous and loose, as can be seen in
Fig. 6. This structure can provide the tunnels for the penetration of
corrosive species such as chloride ions. So the existence of ␤-FeOOH
accelerates the corrosion of steel and deteriorates the protective
effect of rust layer.
The cross-sectional morphologies of the outer layers are shown
in Fig. 7. The outer layer gradually grows, containing more microcracks and is easily scaled off. Fig. 8 shows that the crack-free
inner layer is dense, adherent and is thinner than the outer layer.
Corrosive species especially chloride ion can thus easily penetrate the outer layer and attack the inner layer and the corrosion
process mostly occur at the rust/steel interface. Examination of
the cross-section of the inner layer by EDAX revealed significant
presence of chloride in the inner layer during the initial stage
of exposure. With the exposure time prolonged, the amount of
chloride decreases gradually as can be seen in Fig. 9. After 24
months’ exposure, however, chloride was not detected to any significant extent in the inner layer. It is reasonable to consider that
the chloride ions play a primary role on the corrosion of steel at
the beginning of exposure. Chloride ions can easily penetrate the
thin monolayer and deposit on the rust-steel interface to form the
intermediate containing chloride, which cause the rust to become
loose. This rust structure cannot prevent the penetration of the
848
Y. Ma et al. / Materials Chemistry and Physics 112 (2008) 844–852
Fig. 5. The morphologies of rust surfaces on the carbon steel samples during various exposure times: (a) 3 months; (b) 6 months; (c) 18 months; (d) 24 months.
chloride ions and accelerate the corrosion of steel. With the process of the corrosion, some intermediate phase close to the inner
layer gradually transformed to the other stable and dense oxides
phase, which resulted in the outer layer growing thicker. The penetration of chloride ion through the outer layer becomes more
difficult. On the other hand, the chloride ions deposited during
the initial stage exposure is used up. These two effects promote
the formation of a dense and adherent inner layer, which enhances
the protective effect of rust layer and lowers the corrosion rate of
steel.
3.3. Impedance measurements
EIS is a powerful method to study the surface condition of metals and also a tool to evaluate the protective effect of the rust
layer formed on the steel. Kihira et al. [20] used this technique
to investigate the condition of the rust layer formed on weathering steel, Bousselmi et al. [21] also employed the technique to
resolve the layer of corrosion products on carbon steel submerged
in seawater, while Santana et al. [22] characterized of the layers
of corrosion products on various substrates in different atmospheric environments. It is obvious that the variation of capacity
(CR ) and resistance (RR ) of the rust layer are related with the characteristics of corrosion products. It is useful for the capacity and
resistance of rust layer to clarify the protective mechanism of rust
layer.
The influence of the rust structure on the atmospheric corrosion
of the tested steels was further investigated by EIS measurements.
In order to evaluate the protective effect of the rust layers, EIS measurements of various rusted steels were carried out in 0.1 mol L−1
Fig. 6. The morphologies of inner layers on the carbon steel samples during various exposure time: (a) 6 months; (b) 24 months.
Y. Ma et al. / Materials Chemistry and Physics 112 (2008) 844–852
849
Fig. 7. The cross-sectional morphologies of outer layers during various exposure time: (a) 6 months; (b) 9 months; (c) 18 months; (d) 24 months.
Na2 SO4 aqueous solution. Fig. 10 presents the Nyquist and Bode
diagrams of carbon steel after exposure for various periods. Generally, these Nyquist diagrams illustrate two compressed semicircles
and a diffusion tail in the low-frequency region. The compressed
semicircle is attributed to a dispersion of the time constant, which
may be caused by surface roughness and/or heterogeneities in the
metal/electrolyte interface [23–28]. Accordingly, a general equivalent circuit is proposed and depicted in Fig. 11. The optimum-fit
result according to this proposed equivalent circuit for carbon steel
exposed to Wanning marine atmosphere is depicted in Fig. 12.
Obviously, this simulation is in agreement with the experiment
data.
The rust layer resistance RR , deduced from extrapolation to the
real axis of the semicircle of the high frequencies, represents the
barrier against migration of ions in association with the corrosion
reaction and is the most useful for estimating the protective properties of the rust layers [29,30]. Pan et al. [31] investigated the
protective effect of rust films using the parameter:
C = εε0
A
d
(3)
where C is the capacitance, ε is the relative permittivity of the oxide,
ε0 is the vacuum permittivity, A is the effective area and d is the
thickness of the rust film.
Fig. 8. The cross-sectional morphologies of inner layers: (a) exposure for 6 months; (b) exposure for 24 months.
850
Y. Ma et al. / Materials Chemistry and Physics 112 (2008) 844–852
Fig. 13 shows the variations of the RR and CR as a function of
exposure time. The RR values for both layers remarkably increased
with exposure time. According to Wang et al. [32], the more protective the rust layer is, the higher the value of RR . Nevertheless,
the observed variation of RR conflicts with the proposed protective properties of the rust layers. For the double-layer system, it is
not appropriate to evaluate the protective effect of the rust layer
by using the property of a certain rust layer; we must take into
account the effects of both outer and inner layers. Therefore, the
influence of the variation of CR to the protective property of the
rust layer must be discussed. What the value of CR of the outer
layer gradually decreases means that the outer layer grows with
the exposure time according to Eq. (3). The inner layer exhibits
the same trend until after 12 months’ exposure when the value
of CR increases sharply; it implies that the thickness of the inner
layer diminishes according to Eq. (3). It is reasonable assume that
some phases dissolve and new chemical reactions occur. According to the change of the rust composition and above discussion, the
mechanism of the protective effect of the rust layer is suggested as
follows:
Fig. 10. Impedances diagrams for carbon steel after various exposure periods in
marine site. (a) Nyquist diagrams for tested steel; (b) bode diagrams for tested steel.
In wet process:
Fe → Fe2+ + 2e,
Anodic reaction :
Cathodic reaction :
Total reaction :
Fe2+ → Fe3+ + e
O2 + 2H2 O + 2e → 4OH−
(Fe2+ , Fe3+ ) + Cl− + OH− → FeOCl + HCl
In dry process:
FeOCl → ␤-FeOOH
Anodic reaction :
Cathodic reaction :
Fig. 9. Elemental analysis for inner rust layer: (a) elements distribution of point A
in Fig. 8(a); (b) elements distribution of point B in Fig. 8(b).
Fe3 O4 → ␥-Fe2 O3 + e
␤-FeOOH + e → Fe3 O4
Fig. 11. Equivalent circuit for rusted steel in an electrolyte solution. Routerlayer and
Couterlayer: the resistance and capacitance of outer layer. Rinnerlayer and Cinnerlayer: the resistance and capacitance of inner layer. Q is CPE (constant phase element)
parameter. Rct : charge transfer resistance. ZW : Warburg diffusion impedance.
Y. Ma et al. / Materials Chemistry and Physics 112 (2008) 844–852
851
The wet-dry cycle accelerates these rust transformations and ␤FeOOH in the inner layer is gradually used up. The rust layer
becomes dense and inhibits the permeation of chloride ion, leading
to better protective effect.
4. Conclusions
Fig. 12. Experimental and simulated impedance spectra for carbon steel after 9
months exposure.
The protective effect of rust layers formed on carbon steel in
a marine environment was assessed after 3–24 months of exposure. The protective effect varied with the exposure time, which
is related with the change of the composition and structure of the
rust layer. The rust presented a monolayer structure during the initial stages of exposure (0–3 months) and then an outer layer was
formed after 6 months. The thickness of the outer layer gradually
increased with exposure time and was much larger than that of the
inner layer.
The composition of the outer layer, consisting of ␥-FeOOH,
␣-FeOOH, Fe3 O4 and an amorphous substance remained almost
unchanged for all exposure times, while that of the inner layer
changed with the various periods of exposure. ␤-FeOOH appeared
after 6 months and was the key factor to influence the protective
effect of the rust layer. The existence of ␤-FeOOH accelerated the
corrosion rate of carbon steel and weakened the protective property
of the rust layer.
A decrease of in the value of the CR of the outer layer is related
with an increase in the thickness of the outer layer. The variation of
capacitance of the inner layer implied the appearance of the new
chemical reaction; in the dry process, ␤-FeOOH was transformed
to ␥-Fe2 O3 in the inner layer and enhanced the protective effect of
the inner layer.
Acknowledgements
The investigation is supported by the National Natural Science
Fund of China under the contract Nos. 50499331-6 and 50671113.
The authors are also grateful to Dr. E.E. Oguzie for the modification
of English.
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Fig. 13. The variation of resistances and capacities of both rust layers: (a) outer layer;
(b) inner layer.
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