Download Effect of soil compositions on the electrochemical corrosion behavior

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Mat.-wiss. u. Werkstofftech. 2010, 41, No. 4
DOI 10.1002/mawe.201000578
Effect of soil compositions on the electrochemical corrosion
behavior of carbon steel in simulated soil solution
Einfluss der Erdbodenzusammensetzung auf das elektrochemische
Verhalten von Kohlenstoffsthlen in simulierten Erdbodenlsungen
T. M. Liu1, Y. H. Wu1, 2, S. X. Luo2, C. Sun3
In this study, effect of cations, Ca2+, Mg2+, K+, and anions, SO42 – , HCO3 – , NO3 – on electrochemical
corrosion behavior of carbon steel in simulated soil solution was investigated through potentiodynamic polarization curves and electrochemical impedance spectroscopy. The results indicate
that the Ca2+and Mg2+ can decrease the corrosion current density of carbon steel in simulated
soil solution, and K+, SO42 – , HCO3– , and NO3– can increase the corrosion density. All the above
ions in the simulated soil solution can decrease its resistivity, but they have different effect on
the charge transfer resistivity. This finding can be useful in evaluating the corrosivity of certain
soil through chemical analysis, and provide data for construction engineers.
Keywords: Soil corrosion / simulated soil solution / carbon steel / corrosion behavior /
Schlsselwrter: Korrosion im Erdboden / simulierte Erdbodenlsungen / Kohlenstoffsthle / Korrosionsverhalten /
1 Introduction
With the development of Chinese economy and society, more
and more pipelines for natural gas and oil transport and land
buried structures are constructed. They are often expected to
have a longer working life. The fundamental cause of the deterioration of land buried structures is soil corrosion. Soil corrosion is
an electrochemical interaction between underground structures
and the ambient soil environment. Actually, the concern with the
environment is of great importance and a better understanding
of the soil as a corrosive agent becomes necessary for the use of
adequate protection for buried structures, avoiding the occurrence of leakiness and, as a consequence, the contamination of
the soil [1]. An increasing awareness and understanding of the
soil corrosion concept has been noticed since the National Association of Corrosion Engineers (NACE) was founded in 1943.
Nowadays, in China, with the rapid development of petroleum
industries and constant increase in the numbers of steel pipelines buried underground, more and more attention needs to be
paid to this area of study [2].
1
College of Materials Science and Engineering, Chongqing University,
Congqing 400044, P.R. China
2
Department of Chemistry, Zunyi Normal College, Zunyi 563002, P.R.
China
3
State Key Laboratory for Corrosion and Protection, Institute of Metal
Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China
Correspondence author: Y. H. Wu, College of Materials Science and
Engineering, Chongqing University, Congqing 400044, P.R. China
E-mail: [email protected]
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Due to the complexity of soil and its porous, heterogeneous
and discontinuous environment constituted by mineral or
organic solid phase, water liquid phase, air and other gas phases
[3 – 4], corrosion behavior mechanism for underground structure
is still unclear. The factors that influence corrosion in soil are
numerous, such as, soil type, moisture content, and position of
water table, soil resistivity, soluble ion content, soil pH, oxidation-reduction potential and, the role of microbes in soil environment [3]. So, the studies of soil corrosion are generally studied in
simulated solutions. Chemical composition plays a key role in
understanding how a soil influences the corrosion of buried
steel. The chemical compositions of soil usually include NaCl,
CaCl2, MgCl2, KCl, Na2SO4, NaHCO3, and NaNO3. Carlos et al. [1]
studied the corrosivity of the soil in the Southeastern region of
Brazil. Nie et al. [5] studied the effect of temperature on the electrochemical corrosion characteristics of carbon steel in a salty
soil. However, there has been little work investigating the effect
of soil compositional cations (Na+, Ca2+, Mg2+, K+) and anions
(Cl – , SO42 – , HCO3– , NO3– ) on the electrochemical corrosion
behavior of carbon steel in simulated soil solution directly.
On account of its physical, mechanical and economic advantages, steel is almost always used in underground structures. In
this study, 0.01 M NaCl solution, which was selected as the simulated soil solution based on the composition of soil in Yingtan,
China, was used as matrix soil solution, and the effect of compositional cations and anions on the corrosion behavior of Q235
carbon steel in simulated soil solution was discussed. The soil
corrosivity can be considered as the capacity of this environment
to produce and to develop the phenomenon of corrosion. The soil
is defined as an electrolyte and this can be understood by means
of the electrochemical theory [1]. So, the potentiodynamic polarization curves technique and electrochemical impedance spec-
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Mat.-wiss. u. Werkstofftech. 2010, 41, No. 4
Effect of soil compositions on the electrochemical corrosion
Table 1. Chemical compositions of the carbon steel studied (wt.%)
Steel
C
S
P
Mn
Si
Cu
Q235
0.176
0.023
0.019
0.057
0.233
0.033
Table 2. Corrosion electrolyte type and compositions used in this
test
Electrolyte type Compositions
troscopy (EIS) were used in this investigation. It is expected that
a better understanding will be obtained about the compositional
ions on the corrosion of buried carbon steel.
1
2
3
4
5
6
7
0.01 M NaCl
0.01 M NaCl + 0.01 M CaCl2
0.01 M NaCl + 0.01 M MgCl2
0.01 M NaCl + 0.01 M KCl
0.01 M NaCl + 0.01 M Na2SO4
0.01 M NaCl + 0.01 M NaHCO3
0.01 M NaCl + 0.01 M NaNO3
2 Experimental
Specimens for electrochemical tests were made from Q235 carbon steel whose chemical composition was shown in Table 1.
Samples were cut into small squares of 10 mm 6 10 mm, and
then covered with epoxy resin except test surface, with a working
area of 1 cm2. Each sample was successively polished using silicon carbide emery papers from grit 150, 240, 400 to 600, then
rinsed with deionized water, and degreased with acetone.
The corrosion of buried structures is related to soil conditions
in which they are buried. The simulated soil solution was based
on the soil composition from Yingtan, China. 0.01 M NaCl solution was used as the matrix corrosion electrolyte, and other cations or anions were added into it in order to study the effect of
compositional ions on the corrosion behavior of carbon steel in
soil solution. The chemical compositions of the used corrosion
electrolytes were listed in Table 2.
All the electrochemical measurements were carried out by
means of Parstat 2273 equipment at room temperature. Potentiodynamic polarization measurements were carried out in a conventional three electrodes glass cell with a platinum counter electrode and a saturated calomel electrode (SCE) as reference electrode with luggin capillary bridge. All tests have been performed
in aerated solutions. The potentiodynamic polarization curves
were recorded by a constant sweep rate of 10 mV/min. Before
recording the polarization curves, the open-circuit potential was
stable for 30 min. The cathodic branch was always determined
first, the open-circuit potential (OCP) was then re-established
and the anodic branch determined. The samples were polarized
from – 0.3V to 0.6 V versus OCP.
Electrochemical impedance spectroscopy (EIS) has proved its
usefulness to follow the degradation of metals [6 – 7]. EIS measurements were performed in a frequency range of 105 – 10 – 2 Hz
Figure 1. Potentiodynamic polarization curves and EIS diagram about effect of Ca2+, Mg2+, K+ on Q235 carbon steel in NaCl solution.
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T. M. Liu et al.
Figure 2. Potentiodynamic polarization curves and EIS diagram about effect of SO42 – on Q235 carbon steel in NaCl solution.
with 51 points settled. Square sheet steel of the same size as
described above was used as the working electrode. The corrosion electrolyte and test equipment were the same as that used of
potentiodynamic polarization curves measurement. The above
electrochemical measurements in all the tested solutions have
been conducted for three times, but almost no differences were
obtained. So, only was one curve for each test given in this study.
3 Results
Effect of Ca2+, Mg2+, K+ on electrochemical corrosion behavior of
Q235 carbon steel in simulated soil solution is shown in Figure 1.
Figure 1(a) stands for the potentiodynamic polarization curves.
Figure 1(b) and Figure 1(c) stand for the Nyquist diagram and
Bode plot for EIS, respectively. Effect of SO42 – on electrochemical
corrosion behavior of Q235 carbon steel in simulated soil solution is shown in Figure 2. Effect of HCO3– on electrochemical corrosion behavior of Q235 carbon steel in simulated soil solution is
shown in Figure 3. Effect of NO3– on electrochemical corrosion
behavior of Q235 carbon steel in simulated soil solution is shown
in Figure 4.
In order to further understand the corrosion mechanisms, the
corresponding equivalent circuit for EIS tests is shown in Figure
5. The capacitance loop can be described with Rs, Rt and Q. Rt
was charge transfer resistance representing the resistance of
electron transfer during electrochemical reaction process. Q was
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the electric double layer capacitance. Rs referred to the solution
resistance between working electrode and reference electrode.
The plots were fitted using ZSimpWin software and the results
were listed in Table 3. Effect of Ca2+, Mg2+, K+ on the Rt of Q235
carbon steel in NaCl solution is shown in Figure 6. Effect of SO42 – ,
HCO3– , NO3– on the Rt of Q235 carbon steel in NaCl solution is
shown in Figure 7. It was well known that the low frequency
capacitance loop was mainly related to the characteristics of electric double layer formed in the interface of metal surface and corrosion electrolyte, which can be described by Rt.
4 Discussion
4.1 Effect of Ca2+, Mg2+, K+ on electrochemical corrosion
behavior of carbon steel
In order to study the effect of Ca2+, Mg2+, and K+ on electrochemical corrosion behavior of carbon steel in simulated soil solution.
0.01 M CaCl2, 0.01 M MgCl2 and 0.01 M KCl were added into the
0.01 M NaCl solution, respectively. Clearly, for all the potentiodynamic polarization curves, the cathodic process are all controlled
by the reduction of dissolved oxygen, and the anodic process are
all controlled by the dissolution of carbon steel electrode. For the
addition of CaCl2, it reduces the cathodic current density a little,
and increases the anodic current density obviously. For the addition of MgCl2, it reduces the cathodic current slightly, but greatly
increases the anodic current density. For the addition of KCl, it
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Effect of soil compositions on the electrochemical corrosion
Figure 3. Potentiodynamic polarization curves and EIS diagram about effect of HCO3– on Q235 carbon steel in NaCl solution.
Table 3. EIS fitting results of Q235 carbon steel in all the simulated soil solutions
Solutions
0.01 M NaCl
0.01 M NaCl + 0.01 M CaCl2
0.01 M NaCl + 0.01 MgCl2
0.01 M NaCl + 0.01 KCl
0.01 M NaCl + 0.01 Na2SO4
0.01 M NaCl + 0.01NaHCO3
0.01 M NaCl + 0.01NaNO3
Equivalent
circuit
R(QR)
R(QR)
R(QR)
R(QR)
R(QR)
R(QR)
R(QR)
Rs
(ohm/cm2)
118.1
40.08
36.61
57.82
44.50
61.12
44.66
increases the cathodic current density obviously, and firstly
decreases and then increases the anodic current density. From
the potentiodynamic polarization curves, it can be concluded
that the aggressiveness of the three added compositions are in
the order of KCl A MgCl2 A CaCl2. As the anions are the same, we
can say the aggressiveness of the cations is in order of K+ A Mg2+
A Ca2+.
For the Nyquist diagram and Bode plot, it is seen that there is a
similar feature. Usually, the magnitude of impedance at high frequency stands for the solution resistance, and low frequency
stands for the charge transfer resistance. As can be seen from
Figure 1(b), the size of the high frequency semicircle decreased
in the order of effect of CaCl2 A MgCl2 A NaCl A KCl. It means
that, for the NaCl matrix solution, the aggressiveness of the three
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Capacitance Q
Rt
(ohm/cm2)
Q-Yo (ohm/cm2/s)
Q-n
0.001084
0.0008365
0.000976
0.001126
0.001532
0.0004054
0.001.070
0.7925
0.8278
0.8579
0.8011
0.8179
0.7229
0.8058
1522
1684
1586
1496
1296
1374
1459
added compositions are in the order of KCl A MgCl2 A CaCl2. For
the Figure 1(c), the addition of the KCl, MgCl2, and CaCl2 all
decreased the solution resistance. From the results in Table 3 and
Figure 6, it can be seen that the fitting result is in accordance
with the results in Figure 1(b) and Figure 1(c). So, the EIS study
results are in accordance with the potentiodynamic polarization
results.
4.2 Effect of SO42 – , HCO3– , NO3– on electrochemical
corrosion behavior of carbon steel
In order to study the effect of SO42 – , HCO3– , NO3– on the electrochemical corrosion behavior of carbon steel in simulated soil solution. 0.01 M Na2SO4, 0.01 M NaHCO3 and 0.01 M NaNO3 were
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Mat.-wiss. u. Werkstofftech. 2010, 41, No. 4
Figure 4. Potentiodynamic polarization curves and EIS diagram about effect of NO3– on Q235 carbon steel in NaCl solution.
Figure 5. Equivalent circuit for EIS plots of Q235 carbon steel in all
the test solutions. Rs is the resistance of corrosive electrolyte; Q is
constant phase element parameter; Rt is the charge transfer resistance.
added into 0.01 M NaCl solution, respectively. For all the cases,
the cathodic processes are characterized by reduction process of
dissolved oxygen, and the anodic processes are dissolution of carbon steel electrode. For Figure 2, the addition of Na2SO4 can
increase the cathodic and anodic current density of carbon steel
in NaCl solution (Figure 2(a)), decrease the size of capacitive
semicircle (Figure 2(b)), and can decrease the charger transfer
resistance in low frequency and solution resistance in high frequency (Figure 2(c)).
For the results of addition of NaHCO3 in Figure 3, it can
increase both the cathodic and anodic current density of carbon
steel in NaCl solution as seen in Figure 3(a). The diffusive impedance in low frequency range in Figure 3(b) indicates that the diffusion process is the step control process. Figure 3(c) also indicates that the addition of NaHCO3 can decrease the solution
resistance. Higher corrosion current density and lower charge
transfer resistance indicate the corrosivity of NaHCO3 on the cor-
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Figure 6. Effect of Ca2+, Mg2+, K+ on the Rt of Q235 carbon steel in
NaCl solution.
rosion behavior of carbon steel in NaCl solution. The presence of
low-frequency diffusive impedance in Nyquist diagram in Figure
3(b) suggests that mass-transfer of dissolved oxygen plays an
essential role in carbon steel corrosion, and the whole corrosion
process is mixed-controlled by activation and diffusion steps, and
this may due to the emission of carbon dioxide.
For the results of addition of NaNO3 in Figure 4, including Figure 4(a), Figure 4(b), and Figure 4(c), the NaNO3 can slightly
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Mat.-wiss. u. Werkstofftech. 2010, 41, No. 4
Effect of soil compositions on the electrochemical corrosion
higher corrosion current density. The lower aggressiveness of
cations than anion lies in the ability of their precipitation on the
carbon steel electrode surface, as a sequence, inhibited its dissolution rate.
5 Conclusions
Figure 7. Effect of SO42 – , HCO3– , NO3– on the Rt of Q235 carbon steel
in NaCl solution.
increase the corrosion current density, decrease the charger
transfer resistance, and decrease the solution resistance.
From the results in Table 3 and Figure 7, it can be seen that the
fitting results are in accordance with the results in Figure 2, Figure 3, and Figure 4. Based on the above discussion, we can gain
the conclusion that the corrosivity of the addition chemicals are
in the order of Na2SO4 A NaHCO3 A NaNO3. As the cations are
the same, we can say the aggressiveness of the anions is in order
of SO42 – A HCO3– A NO3– .
4.3 Discussion on the carbon steel corrosion mechanism
in simulated soil solution
Generally, the anodic and cathodic reactions of carbon steel corrosion in aerated solution can be expressed as follows:
Fe fi Fe2+ + 2e
(1)
O2 + 2H2O + 4e fi 4OH –
(2)
Effect of cations, Ca2+, Mg2+, K+, and anions, SO42 – , HCO3– , NO3– ,
on electrochemical corrosion behavior of carbon steel in simulated soil solution was investigated via potentiodynamic polarization curves and electrochemical impedance spectroscopy. The
obtained conclusions are as follows:
1. In simulated soil solution, the aggressiveness of the compositional cations is order of K+ A Mg2+ A Ca2+, and that of anions is
in order of SO42 – A HCO3– A NO3– .
2. Addition of Ca2+ and Mg2+ can increase the charge transfer
resistance and K+ can decrease it. All the anions, SO42 – ,
HCO3– , and NO3– can decrease the charge transfer resistance.
But, both the cations and anions can reduce the solution
resistivity in 0.01 M NaCl solution.
3. For soil solution, the corrosivity of cations is more aggressive
than that of anions. This may due to their difference in
radius.
As soil is a very complex mixture, it is quite difficult to understand the real corrosion mechanism and the effect of different
ions, and we studied this in simulated soil solutions. This may
provide information for the forecast of resistance of certain soil,
and can guide the engineers to select more suitable construction
materials.
Acknowledgement
This work was supported by Science and Technology Foundation
of Guizhou Province of China (No. 20082008) and Science and
Technology Foundation of Zunyi City of China (No. 200724).
6 References
The dissolution of carbon steel from the steel matrix to corrosive electrolyte stands for the anodic reaction, and oxygen dissolving and diffusion through the soil solution towards the steel electrode surface stands for the cathodic reaction. The corrosion rate
is often expressed by corrosion current density. The corrosion
current density has relation to the resistance of electrolyte
besides the anodic dissolution rate and cathodic dissolved oxygen
reduction rate. Both the cations and anions can decrease the solution resistance in simulated soil solution as can be seen in Figure 1(c), Figure 2(c), Figure 3(c), Figure 4(c), and Table 3. The difference of the ions on the aggressiveness of corrosion electrolyte
may lies in their radii and adsorption energy. Ion with shorter
radius and high adsorption energy can preferentially adsorb at
the special sites of carbon steel electrode surface, leading to
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2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
[1] C. Alberto, M. Ferreira, A. C. Ponciano, Science of the Total
Environment 2007, 388, 250.
[2] Y. T. Li, Corrosion Engineering Science and Technology 2008,
44, 91.
[3] A. Benmoussa, M. Hadjel, M. Traisnel, Materials and Corrosion 2006, 57, 77.
[4] Y. H. Wu, T. M. Liu, S. X. Luo, C. Sun, Materialwissenschaft
und Werkstofftechnik 2010, 41, 142.
[5] X. H. Nie, X. G. Li, C. W. Du, Y. F. Cheng, Journal of Applied
Electrochemistry 2007, 39, 277.
[6] E. O. Olorunniwo, I. B. Imasogie, A. A. Adeniyi, Anti-Corrosion Methods and Materials 2004, 54, 346.
[7] K. Belmokre, N. Azzouz, F. Kermiche, Materials and Corrosion 1998, 49, 108.
Received in final form: March 18th 2010
T 578
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