investigation of inhibition effect of some amino acids
TRANSCRIPT
TU DELFT
Investigation of inhibition effect of some amino acids against steel
corrosion in chloride-containing alkaline solution
Jun Liu
4236572
2014/8/13
MSc Materials Science and Engineering
Department of Materials Science and Engineering
Faculty of Mechanical, Maritime and Materials Engineering
Delft University of Technology
II
Abstract
The corrosion inhibitory effect against reinforced steel corrosion of four amino acids was
evaluated comparing with sodium nitrite which is a widely used commercial inhibitor. The
experiments were carried out in simulated pore solutions using electrochemical methods as
well as optical microscopy. Electrochemical impedance spectroscopy (EIS) was mainly used
to screen the resistance of different inhibitor systems to compare the efficiency of their
inhibition against chloride-induced corrosion. Moreover, corrosion current density and
corrosion rates were also calculated for each inhibitor at different chloride concentrations.
Cyclic voltammetry (CVA) was conducted to analyze the corrosion process on the steel
surface. Pitting potentials as well as repassivation potentials were recorded and the difference
between them was compared. Optical microscopy was used for observing corrosion spots on
steel surface at different time during the measurements.
From the images and spectra, information can be interpreted as how chloride concentration
influenced corrosion rate and to what extent can the four amino acids inhibit corrosion
process. In general, pAB had the best inhibition effectiveness among the amino acids,
followed by 11AUA. 6ACA and Gly showed similar performance during the experiments,
both of whose corrosion inhibiting ability were not satisfactory. The difference of corrosion
inhibition in amino acids may be owing to their carbon chain length and the existence of
functional group. In total, the corrosion inhibitive effect of amino acids was not as good as
that of sodium nitrite.
III
Contents
Abstract ................................................................................................................................. II
Scientific paper ...................................................................................................................... 4
1. Introduction ................................................................................................................. 21
2. Background .................................................................................................................. 22
2.1 Corrosion of reinforced steel .............................................................................. 23
2.2 Inhibitors against corrosion of rebar in concrete ................................................. 25
2.2.1 Classification of inhibitors ....................................................................... 25
2.2.2 Nitrites ................................................................................................... 26
2.2.3 Organic inhibitors ................................................................................... 27
2.3 Inhibition mechanism ........................................................................................ 28
2.4 Aspects influencing corrosion inhibition ............................................................. 32
2.5 Electrochemical analysis methods ...................................................................... 33
2.5.1 Electrochemical impedance spectroscopy ................................................ 33
2.5.2 Cyclic voltammetry ................................................................................. 35
3. Experimental ................................................................................................................ 36
3.1 Materials and specimens ................................................................................... 36
3.2 Solution preparation .......................................................................................... 36
3.3 Test methods ..................................................................................................... 37
4. Results and discussion .................................................................................................. 39
4.1 OCP evolution ................................................................................................... 39
4.2 Investigation by EIS ............................................................................................ 40
4.2.1 Effect of five inhibitors on impedance when added into NaOH solution .... 42
4.2.2 Inhibition effect of five inhibitors with admixture of NaCl ........................ 45
4.2.3 Surface images ....................................................................................... 49
4.2.4 Inhibition effect of three selected inhibitors with admixture of NaCl as a
function of chloride concentration ......................................................................... 51
4.2.5 Discussion of equivalent circuit parameters ............................................. 53
4.2.6 Discussion of corrosion rate and inhibition efficiency ............................... 57
4.3 Investigation by CVA .......................................................................................... 59
4.3.1 Cyclic voltammograms of five inhibitors .................................................. 60
4.3.2 Optical microscopy photos ...................................................................... 64
4.3.3 Discussion of potentials .......................................................................... 65
5. Conclusion and Recommendation ................................................................................. 69
5.1 Conclusion......................................................................................................... 69
5.2 Recommendation .............................................................................................. 69
Acknowledgement ............................................................................................................... 71
Reference ............................................................................................................................ 72
4
Scientific paper
The key findings of this master thesis have been summarized in the form of a scientific paper.
This paper is presented on the next fifteen pages.
5
Investigation of inhibition effect of some amino acids against
steel corrosion in chloride-containing alkaline solution
Abstract
Four amino acids were investigated in simulating pore solution with addition of different chloride
concentration comparing with sodium nitrite to evaluate their inhibiting ability in this article.
Electrochemical impedance spectroscopy was conducted for five successive days to record the
impedance and cyclic voltammetry was performed to study the corrosion process. The impedance
spectra were analyzed to investigate the chloride corrosion threshold of the candidates using an
equivalent circuit and the elements in the circuit were further discussed. Small differences were found
in the case of pitting potential while repassivation potential changed as a function of chloride
concentration. Optical microscopy was used to confirm the pitting spots. The results suggest that these
amino acids have different levels of inhibition effect against chloride induced corrosion, though not as
effective as NaNO2 did. Among the amino acids, pAB performed best and Gly as well as 6ACA had the
poorest inhibitory ability. This result may be related to the benzene function group in pAB and the
difference in carbon chain length.
1. Introduction Reinforcement corrosion, as one of the main causes of
degradation in concrete structures, occurs when concrete is
suffering from carbonation or exposed to chloride salts. In this
situation, the embedded rebar may become depassivated and
start to corrode at a significant rate. The chloride ions cause
localized breakdown of the passive layer which is initially
formed on the steel surface because of the high pH of the
concrete pore solution. Once corrosion has initiated, corrosion
products accumulate on the steel surface [1]. Since the volume
of these corrosion products is several times larger than that of
the original steel, an increase of the internal tensile stress in
the concrete induces cracking and can lead to spalling
eventually. This situation facilitates further intrusion of
aggressive agents and consequently, accelerates the corrosion
process [2].
Such problems are widespread and the worldwide cost of
treating them sums up to billions of Euros annually [3].
Therefore, the development of economic and eco-friendly
techniques for dealing with reinforcement corrosion has been
a high priority within the construction repair industry for
many years. Nowadays, there are several commercial methods
available for remedial treatment of corrosion-damaged
concrete, such as replacement of cracked concrete with fresh
concrete and cathodic protection of the contaminated concrete
part [3].
Among available methods, the use of corrosion inhibitors
seems to be attractive because of their low cost and easy
handling, comparing to other preventive methods [4]. During
the past decades, a number of repair systems have been
devised which are intended to restore the protective ability of
carbonated or chloride-contaminated concrete cover by
introducing corrosion inhibitors into the affected material [5].
A lot of commercial corrosion inhibitors are now in the
market among which sodium nitrite and calcium nitrite are
most widely used. The nitrites are the first admixtures
commercialized on a large scale and are believed to have good
inhibition effects despite their toxicity to some extent [6, 7]. In
recent days many alternative eco-friendly corrosion inhibitors
such as rare earth elements and organic compounds have been
developed. The organic compounds based on alkanolamines
and amines or organic acids have attracted increased attention
because of their effectiveness in terms of corrosion inhibition
and relatively low cost [8]. Among them, amino acids were
reported as a type of good, safe corrosion inhibitors for many
metals in various aggressive media [9, 10]. They are nontoxic,
relatively cheap and easy to produce with purities greater than
99%. At the present time, there are more than 200 different
amino acids known to occur in nature. Most of the natural
6
amino acids are the alpha amino acids which contain carboxyl
and amino groups bonded to the same carbon atom [11]. It is
known that only in the presence of passive oxide film,
formation of hydrogen bond accounts for most of the
inhibitory action [10]. However, the inhibition mechanisms
are still not well-known yet though lots of research has been
conducted.
In this project, the corrosion inhibition of four amino acids
was investigated based on electrochemical impedance
spectroscopy (EIS) and cyclic voltammetry (CVA). Sodium
nitrite was used in this study for the comparison purpose
because its well-known inhibition property in cement-based
material system.
2. Experimental
2.1. Materials and solution preparation
Five inhibitors under evaluation were NaNO2 and sodium salts
of p-aminobenzoic acid (pAB), 11-aminoundecanoic acid
(11AUA), 6-aminocaproic acid (6ACA) and glycine (Gly).
The four amino acids were chosen considering different length
of carbon chain and how functional groups (i.e. –NH2,
–COOH) are arranged in the molecular structure. The four
sodium salts of amino acids were prepared by neutralization
of the corresponding individual amino acids with equivalent
molarity of sodium hydroxide. For convenience, the
abbreviations (i.e. pAB, 11AUA, 6ACA, Gly) are used to
represent the sodium salts of the four amino acids in 0.1M
NaOH solution in this paper. And NaNO2 represents sodium
nitrites in 0.1M sodium hydroxide solution.
Low-carbon steel (St 37) coupons with an exposed surface
area of 3.14 cm2 were used as working electrode. The steel
was ground with emery paper starting from 320 down to 2400,
and then degreased with acetone and further cleaned with
double distilled water before drying with a hair dryer. Prior to
subject to any assigned test, the steel coupons were immersed
in the testing solution for 48h to achieve a stable passivation.
0.1M sodium hydroxide solution was used to simulate the
high alkalinity environment of concrete pore liquid. The
following four solutions were prepared for testing:
1) 0.1M NaOH as a reference solution
2) 0.1M NaOH + NaCl—different concentrations
3) 0.1M NaOH + 0.1M inhibitors
4) 0.1M NaOH + 0.1M inhibitors + NaCl—different
concentrations
Afterwards, tests were conducted by adding NaCl solution
from 0.05M up to 0.4M to each test solution until corrosion
was detected.
2.2. Test methods
Electrochemical impedance spectroscopy (EIS) was
performed using Solartron 1286 potentiostat connected with a
frequency response analyzer. A common three-electrode
system with carbon steel as working electrode, Pt as counter
electrode and a standard calomel electrode (SCE) as reference
electrode were used. The EIS measurements were conducted
by polarizing the working electrode at ±10 mV around its
OCP using sinusoidal perturbations range between 60 kHz
and 10 mHz. EIS tests were conducted starting from 1h after
NaCl was added and until 4 days, so for each solution, data of
five days were acquired.
Open circuit potential values were recorded before each EIS
tests, all the values are referred to SCE.
Cyclic voltammetry (CVA) was performed with the same
potentiostat and three-electrode system as EIS measurements
did. The test was conducted as the following steps which was
also used by M. Cabrini et al [12]: specimen conditioning at
−1 V vs. SCE for 60 s in order to clean the surface avoiding
relevant damage of the passive film, followed by 15 s
equilibration at open circuit potential and two consecutive
voltammetry cycles from −1.7 to +0.7 V vs. SCE at 50 mV/s
scan rate.
Optical microscope photos were taken using Olympus
BX60M. Pictures were acquired before immersion in solution,
after 48h passivation and after 4d EIS test when corrosion
occurred.
3. Results and discussion
3.1. OCP evolution
The OCP values were recorded before every EIS tests and Fig.
2 presents the OCP evolution of the steel electrode in the six
test solutions. Generally, the potential gradually increased
during 5 days tests for both pure NaOH solution and the five
inhibitor solutions, indicating that the addition of these
inhibitors had no adverse effects on the passivation of steel
electrode. Furthermore, the OCPs of the five inhibitor
solutions exhibited higher values than the pure NaOH solution
which may suggest that a better passivation was achieved in
7
the inhibitor solutions. This finding is in agreement with the
work done by H.E. Jamil and his co-workers [2], which
suggested that the adsorption of inhibitors on the steel surface
that enhance the passivation process, therefore passivated
state was achieved more quickly and better.
Fig. 2. Open circuit potentials for 0.1M NaOH solution and
NaOH admixed with five inhibitors during 5 testing days
Fig. 3 gives the OCP evolution of steel electrodes in six
solutions at their own critical chloride concentrations when
corrosion initiated. Generally speaking, steel is regarded as in
passive state when its OCP value is more anodic than -270
mV [13]. In the graph, all specimens exhibited passive state in
the first testing day (2d in Fig. 3). However after 3d, only the
ones in pAB solution and in NaNO2 solution were still
passivated with the OCP higher than -270 mV. While from 4d
on, all the specimens were corroded. It can be seen that the
steel electrode in pure NaOH solution corroded at the lowest
chloride concentration (0.05M NaCl) compared to the ones in
inhibitor solutions, suggesting an inhibitory effect can be
expected against chloride-induced corrosion when inhibitors
were added and they could raise chloride corrosion threshold
in varying degrees.
Fig. 3. OCP evolution of steel electrode in test solutions with
addition of NaCl: NaOH at 0.05M; Gly and 6ACA at 0.1M;
11AUA at 0.2M; pAB at 0.3M; NaNO2 at 0.4M.
It can also be found from Fig. 3 that except for those in pAB
solution and in NaNO2 solution, all other OCP values
decreased sharply at 3d which was just 24h after chlorides
were added in. For potentials in pAB solution, this rapid
reduction occurred at 4d and remained stable after then. This
phenomenon was also found in other inhibitor solutions
except for NaNO2, whose potentials decreased gradually
during the five-day measurements. Considering the chloride
corrosion threshold as the highest one, it is believed that
NaNO2 has the best inhibiting effect among five inhibitors.
And for amino acids candidates, pAB performed noticeably.
These results derived from OCP tests will be further discussed
and confirmed in the following text with electrochemical
impedance spectroscopy and cyclic voltammetry.
3.2. Investigation by EIS
To interpret the EIS data, an equivalent circuit was used as
shown in Fig. 4. The circuit has been previously reported for
steel response in alkaline environment with addition of
inhibitors [14-17]. In the circuit, five elements are included:
Rs is the electrolyte resistance, CPEf is the constant phase
element for passive film, Rf is the film resistance, Rct is the
charge transfer resistance and CPEdl is capacitance for
metal/solution double layer. The CPE is a simple distributed
element and its behavior can be owing to the fractal nature of
the electrode interface or heterogeneity of the steel surface.
Associated with the CPE, there is a variable value n in the
equation: ZCPE = (jω)-n
/Y0, which ranges between 0 and 1
describing the distribution of the dielectric relaxation times in
the frequency domain. When n equals to 1, the CPE represents
a capacitor and n = 0 represents a pure resistor; when 0 < n <
1, CPE shows a non-ideal capacitive response [13, 17, 18].
Table 1 gives some fitting results from EIS test with fitting
errors smaller than 7%.
Fig. 4. Equivalent circuit for analysis of impedance spectra.
R1 = Rs; CPE1 = CPEf; R2 = Rf; CPE2 = CPEdl; R3 = Rct.
8
Table 1
Best fit parameters from EIS measurements for steel electrodes in solutions without NaCl during five-day test duration. Rp=Rct+Rf
Time Rs (Ω) CPEf, Y0 (Ω
-1 S
n) n Rf (kΩ cm
2) CPEdl, Y0 (Ω
-1 S
n) n Rct (kΩ cm
2) Rp (kΩ cm
2)
NaOH 1d 37.6 1.14E-04 0.939 41.2 5.91E-05 0.750 136.9 178.1
5d 46.7 1.09E-04 0.925 121.4 1.71E-05 0.920 365.5 486.9
NaOH+0.05M
NaCl 1d 33.3 8.18E-05 0.969 107.4 2.73E-05 0.932 338.7 446.1
2d 38.3 8.99E-05 0.953 4.9 1.27E-06 0.934 586.2 591.1
3d 38.1 1.01E-04 0.951 29.5 2.46E-04 0.916 14.8 44.3
4d 40.2 1.16E-04 0.946 4.3 1.88E-04 0.656 13.9 18.2
5d 38.7 1.29E-04 0.931 2.2 4.21E-04 0.629 13.2 12.9
Gly 1d 30.6 7.97E-05 0.977 7.8 1.17E-05 0.835 536.5 544.3
5d 32.9 8.20E-05 0.951 13.3 3.03E-06 0.908 1100.1 1113.4
Gly+0.1M
NaCl 1d 30.0 8.10E-05 0.979 6.0 7.86E-06 0.857 334.2 340.2
2d 27.8 1.27E-04 0.935 15.1 4.03E-04 0.607 13.9 29
3d 31.8 1.55E-04 0.929 1.9 4.88E-04 0.447 8.9 10.8
4d 33.0 1.92E-04 0.900 1.2 1.07E-03 0.516 9.8 11
5d 32.8 2.48E-04 0.868 0.7 1.86E-03 0.515 6.4 7.1
6ACA 1d 19.3 8.39E-05 0.956 9.5 6.64E-06 0.949 576.9 586.4
5d 20.3 5.74E-05 0.933 10.2 1.06E-06 0.951 1535.1 1545.3
6ACA+0.1M
NaCl 1d 35.6 6.72E-05 0.962 4.6 1.16E-05 0.774 835.4 840
2d 36.2 2.36E-04 0.877 2.0 1.36E-04 0.458 5.9 7.9
3d 33.8 7.01E-04 0.759 2.1 1.34E-03 0.420 2.4 4.5
4d 34.5 7.28E-04 0.764 1.7 9.60E-04 0.462 2.7 4.4
5d 31.7 8.50E-04 0.746 1.0 8.48E-04 0.457 3.3 4.3
11AUA 1d 13.9 3.88E-05 0.925 2.6 1.05E-06 0.913 891.0 893.6
5d 18.9 4.52E-05 0.913 4.4 1.06E-06 0.967 1462.0 1466.4
11AUA+0.2M
NaCl 1d 21.7 8.97E-05 0.975 19.2 3.57E-05 0.652 125.7 144.9
2d 20.6 1.77E-04 0.913 2.0 1.02E-03 0.449 2.1 4.1
3d 22.5 1.73E-04 0.920 1.2 4.87E-04 0.438 4.1 5.3
4d 22.1 2.00E-04 0.902 0.8 5.33E-04 0.436 3.6 4.4
5d 22.3 2.54E-04 0.865 0.7 6.97E-04 0.486 4.3 5.0
pAB 1d 19.3 6.04E-05 0.948 5.0 1.30E-06 0.962 998.8 1003.8
5d 20.4 5.76E-05 0.932 8.1 1.04E-06 0.950 1467.8 1475.9
pAB+0.3M
NaCl 1d 17.7 1.19E-04 0.944 18.2 1.27E-05 0.832 354.1 372.3
2d 17.4 1.25E-04 0.941 80.2 2.36E-05 0.813 374.2 454.4
3d 18.3 1.72E-04 0.922 7.9 1.52E-04 0.411 10.8 18.7
4d 19.9 1.81E-04 0.928 5.2 2.78E-04 0.441 13.7 18.9
5d 18.7 1.88E-04 0.924 4.5 1.89E-04 0.588 18.9 23.4
NaNO2 1d 14.5 2.72E-05 0.960 6.9 3.65E-06 0.690 992.0 998.9
5d 14.5 2.57E-05 0.953 7.9 2.32E-06 0.720 1044.1 1052.0
9
NaNO2+0.4M
NaCl 1d 14.8 7.48E-05 0.978 2.6 3.79E-05 0.734 378.1 380.7
2d 14.8 7.62E-05 0.977 1.7 3,84E-05 0.695 330.4 332.1
3d 14.7 9.57E-05 0.958 1.1 5.25E-05 0.450 90.1 91.2
4d 14.1 1.17E-04 0.932 7.2 9.76E-05 0.458 53.8 61
5d 13.9 1.50E-04 0.914 1.9 3.48E-04 0.448 6.6 8.5
A convenient way to evaluate the corrosion resistance of
specimens is to compare the diameters of the curves in
Nyquist plot. With larger diameter, the better corrosion
resistance of the sample can be expected [19]. Fig. 5 and 6
gives the Nyquist and Bode plots of the EIS results obtained
on samples immersed in solutions with or without inhibitors.
We can see that in Nyquist plot, the diameter of NaOH curve
is much smaller than those of inhibitors at 1d (Fig. 5)
indicating that the corrosion resistance is much smaller. As
time goes, at 5d the impedance gap became smaller, however,
the impedance of solutions with inhibitors was still at least
double of that in NaOH solution. It is suggested that a passive
film formed on the iron surface as a consequence of the
alkaline solution with high pH. With the influence of
inhibitors, the film was reinforced so that impedance became
larger. Moreover, the phase angle in the low frequency region
was also higher in solutions with inhibitors, indicating a good
corrosion inhibition offered by the inhibitors. It is possible
that the passive layer becomes thicker, or more homogeneous.
It could strongly inhibit the corrosion activity on the surface,
creating an insulating layer, which behavior approaches that
of a capacitor (some curves in phase angle plot approaching
90 degrees in Fig. 6). Furthermore, it was reported that this
layer seems to have the capacity to bind the chlorides through
the amino groups present in the inhibitor molecules [2].
Fig. 5. Impedance spectra for steel electrode in pure NaOH solution and in solutions admixed with five inhibitors at 1d: a) Nyquist
plot; b) magnitude Bode plot; c) phase angle Bode plot.
a)
b) c)
10
Fig. 6. Impedance spectra for steel electrode in pure NaOH solution and in solutions admixed with five inhibitors at 5d: a) Nyquist
plot; b) magnitude Bode plot; c) phase angle Bode plot.
Fig. 7 and Fig. 8 give the EIS results of steel electrodes in Gly
solution and in 6ACA solution, respectively. The shape of
phase angle plot suggests the presence of two time constants
partially overlapped which can be described by the equivalent
circuit in Fig. 4. At 1d when sodium chloride was added, the
steels in both solution remained passive state, especially the
system in 6ACA which behaved like a capacitor (Fig. 8),
suggesting that a homogeneous protective layer was present
on the steel surface. However, significant decrease in
impedance value and phase angle at low frequencies indicated
that corrosion initiated after 2 days which was in accordance
with the OCP results (Fig. 3). The impedance values at 5d in
both situations were almost 2 orders of magnitude lower than
those at 1d. It is worth noting that in Bode plot, phase angles
recovered at low frequencies region at 4d and 5d of the
specimen in solution containing Gly, which may suggest the
corrosion process was controlled by diffusion [14]. After three
days, impedance of the two inhibitors stopped to decrease and
stabilized. Looking at the results in 5d, it is hard to tell which
of the two inhibitors has a better inhibition effect.
Fig. 7. EIS spectra of steel specimen in 0.1M Gly solution at 0.1M NaCl from 1d to 5d: a) magnitude Bode plot; b) phase angle
Bode plot.
a)
b) c)
a) b)
11
Fig. 8. EIS spectra of steel specimen in 0.1M 6ACA solution at 0.1M NaCl from 1d to 5d: a) magnitude Bode plot; b) phase angle
Bode plot.
The EIS response of specimen in 11AUA solution at 0.2M
NaCl was very similar to that in 6ACA solution at 0.1M NaCl
(Fig. 8). As shown in Fig. 9, the corrosion of the steel started
rapidly soon after chloride was added and after one day, phase
angle decreased and maintained a smaller value in the
following several days, which was accordant with impedance
response. The behavior of steel in pAB solution at critical
chloride concentration (0.3M) differed slightly with that of
11AUA. In the first two days, the steel sample remained a
passive state having some capacitive behavior, as shown in
Fig. 10. However, starting from 3d, corrosion was initiated
leading to a decrease of total impedance values and phase
angle.
Fig. 9. EIS spectra of steel specimen in 0.1M 11AUA solution at 0.2M NaCl from 1d to 5d: a) magnitude Bode plot; b) phase
angle Bode plot.
Fig. 10. EIS spectra of steel specimen in 0.1M pAB solution at 0.3M NaCl from 1d to 5d: a) magnitude Bode plot; b) phase angle
Bode plot.
Fig. 11. EIS spectra of specimen in 0.1M NaNO2 solution at 0.4M NaCl from 1d to 5d: a) magnitude Bode plot; b) phase angle
Bode plot.
The impedance spectra were in agreement with the OCP
results as well in NaNO2 solution. The trend of impedance
spectra in Fig. 11 shows a gradual reduction. Initially, the
inhibitor layer seemed to cover the entire surface because of
a) b)
a) b)
a) b)
a) b)
12
good adsorption, while the total impedance of the system
decreased continuously with time. Technically, corrosion
initiated at 3d and the pits kept growing and propagating,
some obvious spots could be found after the testing period, as
shown in the optical images (Fig. 12).
Fig. 12. Optical microscope pictures with 5× magnification: a)
48h passivation; b) after 5d EIS tests
Fig. 13-15 show the impedance spectra for three inhibitors
with increasing amount of NaCl in 5d are shown. From these
figures, we can see that under critical chloride concentration,
impedance changed to a small extent with increasing chloride
concentrations, especially for specimens in pAB solution and
in NaNO2 solution. This phenomenon indicated that strong
inhibition of the dissolution processes occurred on the steel
surface, even if chloride concentration became higher [2]. As
far as pAB is concerned, the effect could be neglected while
for NaNO2, only at the chloride concentration of 0.05M that
the impedance was slightly larger than higher Cl-
concentration. As for specimen in 11AUA solution, the curves
at 0.05M NaCl and 0.1M NaCl do not overlap to a great
extent; nevertheless, we can still find that under these
conditions, the steel electrodes were passivated. As for Gly
and 6ACA, similar spectra were obtained which can correlate
to Fig. 7 and Fig. 8, respectively.
Fig. 13. Impedance spectra in 0.1M 11AUA solution with different NaCl concentrations at 5d: a) magnitude Bode plot; b) phase
angle Bode plot.
Fig. 14. Impedance spectra in 0.1M pAB solution with different NaCl concentrations at 5d: a) magnitude Bode plot; b) phase
angle Bode plot.
a)
b)
a) b)
b) c)
a) b)
13
Fig. 15. Impedance spectra in 0.1M NaNO2 solution with different NaCl concentrations at 5d: a) Nyquist plot; b) magnitude Bode
plot; c) phase angle Bode plot.
In addition to the impedance spectra, analysis of the
parameters in equivalent circuit gives us a more specific way
to evaluate the inhibiting effect of these amino acids. The
capacitance element values for 5 inhibitor solutions at 0.1M
NaCl are illustrated in Fig. 16. The evolution of film
capacitance is an indicator of thickness of the protective film
and/or its homogeneity [16]. For steel electrodes in Gly and
6ACA solutions which corroded at this chloride concentration,
the Y0f values increased. Especially for 6ACA specimen,
value in 5d was almost one order of magnitude higher than
that in 1d. As for specimens in the 3 rest inhibitor solutions,
there is a slightly decreasing trend of Y0f values. Taking film
resistance into consideration (not shown in figure, but can be
refer to Table 1), it can be suggested that a reinforcement of
the protective behavior of the inhibitor film formed on the
surface leading to a better passivity [14]. However, whether
the film thickness or its homogeneity controlled the corrosion
behavior is not clear because the difference during five-day
tests of this parameter was not distinct.
Fig. 16. Capacitance values of film for the five inhibitor
solutions with addition of 0.1M NaCl
Fig. 17. Capacitance values of double layer for the five
inhibitor solutions with addition of 0.1M NaCl
a)
b) c)
14
Fig. 17 gives the double layer capacitance for five inhibitor
solutions at 0.1M NaCl. The Y0dl values of corroding
specimens (Gly and 6ACA) increased sharply, with the
increase of 2 orders of magnitude. However, for other
specimens (11AUA, pAB, NaNO2) whose chloride corrosion
thresholds are higher, values fluctuated except for pAB which
showed a marginally rising trend. It was reported that the
inhibitor may induce some corrosion activity during tested
period to create more favorable anchorage sites for the
formation of the protective inhibitor film, probably by
exchanging some hydrated substances of the iron
oxides/hydroxides surface layer [2].
The evolution of charge transfer resistance is shown in Fig. 18,
with inhibitor solutions at 0.1M NaCl. Generally, Rct can be
seen as an indicator of corrosion rate [16]. As can be seen that
for corroding specimens (Gly, 6ACA), Rct dropped sharply in
the second day owing to breakdown of passive film attacked
by chloride ions. In the next 4 days, it continued to decrease
marginally suggesting increasing of pitting spots and/or
corrosion area in pits [14]. While for non-corroded specimens
(11AUA, pAB, NaNO2), the resistance increased gradually
revealing the steel specimens were in a sound passive state.
Fig. 18. Charge transfer resistance for the five inhibitor
solutions with addition of 0.1M NaCl
Fig. 19 and 20 show the evolution of CPE parameters as a
function of chloride concentrations. It can be seen that with
increasing chloride concentration, Y0 of film capacitance
increased gradually (Fig. 19) likely due to the passive film
becoming less homogeneous and thinner. It was also possible
that the passive film was destroyed at some spots. On the
other hand, Y0 of double layer (Fig. 20) rose sharply at critical
Cl- concentration. This increase could be related to the
corrosion products over pits inducing an increase in electrode
surface area [20]. Meanwhile, charge transfer resistance
corresponding to Y0dl has an opposite trend that the values
decreased sharply at critical chloride concentration, indicating
a remarkable increase of corrosion current density which
means pitting corrosion initiated.
Fig. 19. Capacitance values of film for the five inhibitor
solutions at different NaCl concentrations at 5d
Fig. 20. Capacitance values of double layer for the five
inhibitor solutions at different NaCl concentrations at 5d
Apart from the parameters discussed above, the corrosion
behavior of reinforced steel in concrete is generally
determined by corrosion current density icorr quantitatively.
This parameter is usually related to polarization resistance Rp
and is calculated using the equation icorr = B/Rp, where B is the
Stern-Geary constant dependent on the nature of corrosion
reactions [14]. The value of B is 52 mV for passive steel while
equals to 26 mV in corroded situation. Table 2 shows
polarization resistance and corrosion current density of the
inhibitors at critical chloride concentration as well as the one
just below threshold. Reference NaOH solutions are also
included. The polarization resistance was calculated on basis
of Rf and Rct [21] and the values we got were a little larger
than the impedance values at very low frequency (usually 10-4
Hz). It is proposed that when corrosion current density is
lower than 0.1 μA/cm2, the corrosion rate is negligible [14]. In
the table, the corrosion current density for specimens in
solution with addition of inhibitors is lower than that in pure
NaOH solution, thus corrosion rate is lower even at high
chloride concentration. At their critical NaCl concentration,
the performance of pAB was quite remarkable due to its
lowest corrosion current density.
15
Table 2
Polarization resistance, corrosion current density and
corrosion rate of NaOH and five inhibitors at different
chloride concentration
Rp (kΩ cm2) icorr (μA cm
-2)
NaOH 487 0.107
NaOH+0.05 12.9 2.010
Gly+0.05 1113 0.047
Gly+0.1 13.8 1.886
6ACA+0.05 1149 0.045
6ACA+0.1 4.2 6.119
11AUA+0.1 988 0.053
11AUA+0.2 5.03 5.166
pAB+0.2 822 0.063
pAB+0.3 20.3 1.279
NaNO2+0.3 1789 0.029
NaNO2+0.4 8.6 3.030
From the EIS results, we can conclude that inhibition effect
varies among the selected five inhibitors. NaNO2 remains the
most effective inhibitor, while among the other four amino
acids pAB has the best inhibition effect, followed by 11AUA.
6ACA and Gly have the poorest inhibitory ability within the
candidates. In the following text, some cyclic voltammetry
results are shown to verify the consequence we got from
above results.
3.3. Investigation by CVA
The manner in which these inhibitors can influence the
corrosion process is pointed by their electrochemical behavior
in test solutions studied by cyclic voltammetry. Measurements
were carried out in solutions at the same chloride
concentrations as studied in EIS test.
Fig. 21 shows typical cyclic voltammograms for steel
electrode in alkaline solution under the attack of chlorides. On
the anodic scan, current decreased continuously starting from
a negative value and changed the sign at corrosion potential
Ecorr [22]. As shown in Fig. 21, the peaks 1 and 2 may be
attributed to the first oxidation of iron to iron (II) hydroxide
according to the reaction Fe+2OH−→Fe(OH)2+2e
−. The peaks
3 and 3‘ are assigned to ferrous–ferric transformations in an
outer oxide layer (γ-Fe2O3 and/or its hydrated form γ-FeOOH),
and in a relatively compact inner oxide layer (Fe3O4),
respectively. The processes could be ascribed to reactions
3Fe(OH)2+2OH−→Fe3O4+4H2O+2e
− and
Fe3O4+H2O+OH−→3FeOOH+e
−. The hydroxide γ-FeOOH
may dehydrate subsequently to give γ- Fe2O3 [17]. When the
anodic current fell to a relatively low value, the onset of
passivation may occur. The sudden increase of the current
density at some critical potential indicating initiation of pitting
attack is pitting potential Ep [22]. When the potential scan
reversed at oxygen evolution potential to a cathodic direction,
the current gradually decreased in the positive branch and
reached zero value at the repassivation potential, or protection
potential Erp [23]. Peaks 4 and 3 are conjugated, indicating a
partial reversibility of the reaction of peak 3. Peaks 4 and 5,
represent the reduction of ferric to ferrous oxide (α-Fe2O3
and/or γ-Fe2O3→Fe3O4→Fe(OH)2) and of ferrous oxide to
iron, respectively [18, 24].
The pitting potential in the first cycle relates with the
capability of inhibitors to inhibit the onset of localized attack.
With high inhibiting substances such as pAB or 11AUA at
0.05M NaCl (can be found in Fig. 25 and Fig. 24,
respectively), this potential is expected to reach the value of
oxygen evolution potential before localized corrosion initiates.
In the second cycle, it may be somehow associated with the
ability to improve the repassivation of the steel during the
cathodic scan, thus to reduce the risk of corrosion propagation
[24]. The repassivation or protection potential depends on the
chloride concentration in the solution and generally becomes
more negative with higher chloride concentration.
16
Fig. 21. Typical CVA curves: 0.1M NaOH with addition of 0.05M NaCl
Fig. 22 shows the cyclic voltammetric curves of specimens in
6ACA solution at different chloride concentrations. As can be
clearly seen, at chloride concentration of 0.1M, the pitting
potential shifted to a smaller value than the other two curves.
Almost identical voltammograms were found with respect to
Gly (Fig. 23) that the curves at concentrations under critical
chloride concentration have similar passive behavior for the
two cycles due to the restoration of passivity during the
potential scan in the cathodic direction of the first cycle [24].
It was suggested that the current remains stable in the passive
domain, and the reverse curve corresponds to lower current
values than those obtained during the forward scan, this
behavior is typical of a product layer thickening during the
forward scan [15]. However, at 0.1M NaCl, a completely
active behaviour was observed in the second cycle, denoting
the absence of repassivation after the first cycle.
Fig. 22. CVA curves of 0.1M 6ACA at different chloride
concentrations
Fig. 23. CVA curves of 0.1M Gly at different chloride
concentrations
In Fig. 24, CVA curves are related to steel specimen in
11AUA solution. It is well-marked that the current value at
0.2M NaCl in passive region (around -0.5V to 0.55V)
increased significantly compared to curves at lower chloride
concentration. The region corresponded to pitting potential
accordingly moved towards a negative direction. Moreover,
The current peak associated with the oxidation of iron(II)
hydroxide to the iron(III) oxide increased as well. However,
for lower chloride concentrations, the curves overlapped with
each other to some extent, suggesting that below chloride
corrosion threshold, the influence of increase of chloride
concentration on passive film was negligible.
1 2 3’
3
4
5
17
Fig. 24. CVA curves of 11AUA at different chloride
concentrations
Fig. 25. CVA curves of 0.1M pAB at different chloride
concentrations
In terms of the CVA curves of pAB (Fig. 25), a hysteresis
could be obviously noticed when chloride concentration
increased after inversion of potential scan, and this could be
ascribed to the initiation of pitting corrosion. This was further
confirmed by the second cycle in which pitting potential was
lower and hysteresis became more evident. This hysteresis
drove repassivation potential to a more cathodic value and
made the positive current density stay in a wider region.
El-Haleem et al [23] suggested that these positive current
during cathodic scan may be related to continuous
propagation of formed pits. If comparing the whole Cl-
concentration range, it could be found that pitting potential
shifted gradually to a lower value when chloride concentration
increased. And the passive potential region moved upwards
slightly.
Fig. 26. CVA curves of NaNO2 at different chloride
concentrations
As for NaNO2 solution, the difference among the curves is
very small as we can see from the curves of solution without
chloride and solution with 0.3M NaCl. At 0.4M NaCl, the
decrease of pitting potential became obvious. Moreover, the
peaks related to Fe2+
→Fe3+
(peak 3 in Fig. 21) were more
distinguishable than those in amino acids solution which
could be ascribed to the quick oxidation ability of ferrous ions
to ferric oxide offered by nitrites [20]. This ability could also
enhance the surface oxide film passivity since the product
layers based on magnetite are more conductive than those
based on hydroxides. The magnetite-based layers were also
reported as the most stable and corrosion-resistant ones [15].
At the end of CVA test, optical microscopy pictures were
taken. Fig. 27 shows the photo with 10× magnification. It can
be seen that the surface color changed to brownish instead of
original metallic silver. Several pits can be observed which
indicated that pitting process dominated during the tests. This
is in agreement of the cyclic voltammograms at critical
chloride concentrations.
Fig. 27. Optical microscopy photo at the end of CVA
measurement with 10× magnification
Normally, the difference of pitting potential and repassivation
potential was taken as a relative measurement of inhibition
ability against localized corrosion [23]. With smaller
difference between Ep and Erp, greater tendency for pitting
repassivation would be and better inhibiting ability can be
expected. These potentials were recorded from the cyclic
voltammograms in order to compare the repassivation ability
of the selected inhibitors. The differences between pitting
potential and repassivation potential in different test solutions
are listed in Table 3. There is globally a trend that with
increasing chloride concentration, the value becomes larger,
suggesting that steel specimen has higher risk to be corroded
18
and accordingly the inhibitory effect is weaker. The low
values in Gly and 6ACA at 0.1M NaCl are owing to their
considerably low pitting potentials. This could be related to
their critical chloride thresholds which are actually lower than
0.1M NaCl. Therefore, tests with some more precise
concentrations between 0.05M and 0.1M or even higher
chloride concentrations may be needed to acquire a more
accurate trend. However, detailed investigation was not made
in this paper because these two inhibitors were not as effective
as other candidates.
Table 3
Values of (Ep-Erp) for different solution as a function of
chloride concentration
Cl- (mol
L-1)
Ep – Erp (mV / SCE)
0 0.05 0.1 0.2 0.3 0.4
NaOH 544 837 934 - - -
Gly 627 751 462 - - -
6ACA 513 601 475 - - -
11AUA 612 611 703 944 - -
pAB 498 563 718 896 928 -
NaNO2 644 422 474 794 773 997
4. Conclusions The four amino acids we studied in this paper showed
inhibitory effect on steel in alkaline solution against chloride
attack. Among them, pAB exhibited the best inhibiting ability
at chloride concentrations below 0.3M, probably owing to the
spatial effect of its benzene ring. The threshold of 11AUA was
0.2M NaCl, more effective than the other two amino acids
which may be owing to its long carbon chain. 6ACA and Gly
can similarly inhibit chloride-induced corrosion to some
extent, in whose solutions the steel electrodes corroded at
0.1M NaCl. In general, the effect on retardation of chloride
corrosion threshold of these four amino acids is not as good as
sodium nitrite, which showed excellent inhibiting effect at
concentrations below 0.4M NaCl.
Cyclic voltammograms indicated that pitting potentials for
these inhibitors at different chloride concentrations had slight
difference, while repassivation potentials varied to a large
extent. The difference between Ep and Erp was larger with
increasing chloride concentration, except for that in 6ACA
and Gly which exhibited extraordinary pitting potentials at
0.1M NaCl.
Optical microscopy photos confirmed the existence of pitting
spots and were in concordance with the results obtained from
electrochemical methods. When the CVA measurements were
done, the surface color changed from the original color to a
brownish color and evident pitting spots could be observed.
References
[1] Tuutti, K. (1982). Corrosion of steel in concr (No.
Monograph).
[2] Jamil, H. E., Shriri, A., Boulif, R., Bastos, C., Montemor,
M. F., & Ferreira, M. G. S. (2004). Electrochemical
behaviour of amino alcohol-based inhibitors used to
control corrosion of reinforcing steel. Electrochimica
acta, 49(17), 2753-2760.
[3] Page, C. L., Ngala, V. T., & Page, M. M. (2000).
Corrosion inhibitors in concrete repair
systems. Magazine of Concr Res, 52(1), 25-38.
[4] Ormellese, M., Berra, M., Bolzoni, F., & Pastore, T.
(2006). Corrosion inhibitors for chlorides induced
corrosion in reinforced concrete structures. Cem and
concr res, 36(3), 536-547.
[5] Ngala, V. T., Page, C. L., & Page, M. M. (2002).
Corrosion inhibitor systems for remedial treatment of
reinforced concrete. Part 1: calcium nitrite. Corrosion
Sci, 44(9), 2073-2087.
[6] Jamil, H., Montemor, M. F., Boulif, R., Shriri, A., &
Ferreira, M. G. S. (2003). An electrochemical and
analytical approach to the inhibition mechanism of an
amino-alcohol-based corrosion inhibitor for reinforced
concrete. Electrochimica acta, 48(23), 3509-3518.
[7] Gaidis, J. M. (2004). Chemistry of corrosion
inhibitors. Cem and Concr Composites, 26(3), 181-189.
[8] Ormellese, M., Lazzari, L., Goidanich, S., Fumagalli, G.,
& Brenna, A. (2009). A study of organic substances as
inhibitors for chloride-induced corrosion in
concrete. Corrosion Sci, 51(12), 2959-2968.
[9] Li, L., & Sagues, A. A. (2002). Chloride corrosion
threshold of reinforcing steel in alkaline solutions-cyclic
polarization behavior. Corrosion, 58(4), 305-316.
[10] Amin, M. A., Khaled, K. F., Mohsen, Q., & Arida, H. A.
(2010). A study of the inhibition of iron corrosion in HCl
solutions by some amino acids. Corrosion Sci, 52(5),
1684-1695.
[11] Ashassi-Sorkhabi, H., Majidi, M. R., & Seyyedi, K.
(2004). Investigation of inhibition effect of some amino
acids against steel corrosion in HCl solution. Appl surf
sci, 225(1), 176-185.
19
[12] Cabrini, M., Lorenzi, S., Pastore, T. (2014). Cyclic
voltammetry evaluation of inhibitors for localised
corrosion inalkaline solutions. Electrochimica Acta, 124,
156–164.
[13] Hu, J., Koleva, D. A., De Wit, J. H. W., Kolev, H., & Van
Breugel, K. (2011). Corrosion performance of carbon
steel in simulated pore solution in the presence of
micelles. J of the Electrochemical Soc, 158(3), C76-C87.
[14] Królikowski, A., & Kuziak, J. (2011). Impedance study
on calcium nitrite as a penetrating corrosion inhibitor for
steel in concrete. Electrochimica Acta, 56(23),
7845-7853.
[15] Koleva, D. A., Boshkov, N., Van Breugel, K., & De Wit,
J. H. W. (2011). Steel corrosion resistance in model
solutions, containing waste materials. Electrochimica
Acta, 58, 628-646.
[16] Jamil, H. E., Shriri, A., Boulif, R., Montemor, M. F., &
Ferreira, M. G. S. (2005). Corrosion behaviour of
reinforcing steel exposed to an amino alcohol based
corrosion inhibitor. Cem and Concr Composites, 27(6),
671-678.
[17] Andrade, C., Keddam, M., Nóvoa, X. R., Pérez, M. C.,
Rangel, C. M., & Takenouti, H. (2001). Electrochemical
behaviour of steel rebars in concrete: influence of
environmental factors and cement
chemistry. Electrochimica Acta,46(24), 3905-3912.
[18] Valek, L., Martinez, S., Mikulić, D., & Brnardić, I.
(2008). The inhibition activity of ascorbic acid towards
corrosion of steel in alkaline media containing chloride
ions. Corrosion Sci, 50(9), 2705-2709.
[19] Lin, C. H., & Duh, J. G. (2009). Electrochemical
impedance spectroscopy (EIS) study on corrosion
performance of CrAlSiN coated steels in 3.5 wt.% NaCl
solution. Surf and Coat Technol, 204(6), 784-787.
[20] Monticelli, C., Frignani, A., & Trabanelli, G. (2002).
Corrosion inhibition of steel in chloride-containing
alkaline solutions. J of appl electrochem,32(5), 527-535.
[21] Scully, J. R. (2000). Polarization resistance method for
determination of instantaneous corrosion
rates. Corrosion, 56(2), 199-218.
[22] Deyab, M. A., & Keera, S. T. (2012). Cyclic
voltammetric studies of carbon steel corrosion in
chloride-formation water solution and effect of some
inorganic salts. Egypt J of Petroleum, 21(1), 31-36.
[23] Abd El-Haleem, S. M., & Abd El-Wanees, S. (2011).
Chloride induced pitting corrosion of nickel in alkaline
solutions and its inhibition by organic amines. Mater
Chem and Phys, 128(3), 418-426.
[24] Pastore, T., Cabrini, M., Coppola, L., Lorenzi, S.,
Marcassoli, P., & Buoso, A. (2011). Evaluation of the
corrosion inhibition of salts of organic acids in alkaline
solutions and chloride contaminated concrete. Mater and
Corrosion, 62(2), 187-195.
20
21
1. Introduction
Corrosion of reinforced steel in concrete is one of the major reasons of degradation in
concrete system which is of great importance to both safety issues and economic effects [1, 2].
In order to minimize the loss and maintain a longer service time of the concrete, there are
numerous ways to prevent or to mitigate the reinforcement corrosion. Among them, the use of
inhibitors appears to be an effective method.
Calcium nitrite is the most widely used commercial inhibitor at present, but due to its toxicity,
researchers are searching for green inhibitors as alternatives. Therefore, organic inhibitors are
under investigation and among them, amino acids are noteworthy because of their
non-toxicity and environmentally friendly characteristics.
In this project, the corrosion inhibition of four amino acids was investigated based on
electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CVA). Optical
microscopy images were used for surface observation such as the confirmation of corrosion
spots. Sodium nitrite was used in this study for the comparison purpose because its
well-known inhibition property in cement-based material system.
In chapter 2, some background information of reinforcement corrosion and the use of
inhibitors are given. Chapter 3 briefly introduces the experimental aspects such as the
specimens and procedure, etc. The results are shown in chapter 4 and some discussions are
also included. In chapter 5, conclusions are drawn and some recommendations are given for
future research.
22
2. Background
Reinforcement corrosion, as one of the main causes of degradation in concrete structures,
occurs when concrete is suffering from carbonation or exposed to chloride salts. In this
situation, the embedded rebar may become depassivated and start to corrode at a significant
rate. In the corrosion process, the chloride ions can cause localized breakdown of the passive
layer which is initially formed on the steel surface because of the high pH in surrounding
environment. Once corrosion has initiated, corrosion products accumulate on the steel surface
[2]. Since the volume of these corrosion products is several times larger than that of the
original steel, the internal tensile stress in the concrete will increase. This increase may induce
cracking and lead to spalling eventually. This situation facilitates further intrusion of
aggressive agents and accelerates the corrosion process, which may consequently result in
collapse of the whole construction [3].
Such problems are widespread and the worldwide cost of treating them sums up to billions of
Euros annually [4]. Therefore, the development of economic and eco-friendly techniques for
dealing with reinforcement corrosion has been a high priority within the construction repair
industry for many years. Nowadays, there are several commercial methods available for
remedial treatment of corrosion-damaged concrete, such as replacement of cracked concrete
with fresh concrete and cathodic protection of the contaminated concrete part [4].
The most widely practiced method of repairing such deterioration has been removal of the
cracked or significantly contaminated concrete part and mending with fresh concrete. This
can be both costly and inconvenient, particularly in cases where large amounts of physically
sound but contaminated materials have to be removed. Therefore, there has been widespread
interest in developing alternative approaches for restoration of the protective nature of
concrete cover in cases where carbonation and/or chloride contamination has taken place but
where corrosion-damaged concrete has developed to a minor degree [5]. Among available
methods, the use of corrosion inhibitors seems to be attractive due to their low cost and easy
handling, comparing to other preventive methods [6].
Plenty of commercial corrosion inhibitors are now in the market among which sodium nitrite
and calcium nitrite are most widely used. The nitrites are the first admixtures commercialized
on a large scale and are believed to have good inhibition effects despite their toxicity to some
extent [1, 7]. Besides the nitrites, some organic inhibitors based on alkanolamines and amines
or organic acids have attracted increased attention because of their effectiveness in terms of
corrosion inhibition and relatively low cost [8]. Among the organic compounds, amino acids
are competitive since they are known as green, low-cost and easily produced with high purity
[9]. However, the inhibition mechanisms are still not well-known yet though lots of research
has been conducted.
In this chapter, a brief introduction of reinforced steel corrosion will be given followed by the
inhibitors that are used at present and may be potentially applied to the concrete in future. The
inhibition mechanisms and the aspects that have influence on inhibiting ability will be
23
discussed as well. Then the two electrochemical testing methods that we used in the
experiment will be introduced in the last section.
2.1 Corrosion of reinforced steel
Under normal circumstances, reinforced steels embedded in concrete structures are in passive
state that they are protected by a thin film due to high alkalinity (pH = 12.6–13.5) of the
concrete pore solution. Corrosion may initiate only when passivity of steel is destroyed,
leading to loss in performance. This breakdown of passivity occurs in principle in two ways:
One of them is carbonation of concrete. The reaction of carbon dioxide with cement paste can
result in a pH drop (to about 8–9) leading to general corrosion, which can be explained by Eq.
(2.1);
Ca(OH)2 + CO2 → CaCO3 + H2O (2.1)
The other is chloride-induced corrosion of rebar. This kind of corrosion occurs when the
chloride concentration at the reinforced steel surface reaches a critical threshold level, though
this level was reported to show much variability [10]. It was also suggested by some authors
that the chloride content in the range of 0.4–1% by cement weight may cause localized
corrosion [11].
In terms of the corrosion on steel surface, the most important reaction in the corrosion process
representing iron dissolution can be over-simplified by Eq. (2.2):
Fe → Fe2+
+ 2e- (2.2)
However, the reactions of the whole corrosion process in concrete are far more complicated.
For instance, although ferrous ions are slightly soluble, they can travel some distance possibly
enough to reach the concrete surface, especially under corrosion conditions (in the presence
of chloride ions and relatively low pH). If these ferrous ions, usually with a pale green color,
reached the surface of the concrete, they would be easily oxidized by the oxygen in air to
ferric ions, and would precipitate there as one of several reddish, brown or black oxides
(which are hydrated to some extent). The reactions can be explained by the following
equations:
O2 + 2H2O + 4e- → 4OH
- (2.3)
Fe2+ → Fe
3+ + e
- (2.4)
xFe2+
+ yO2 + H2O → Fe3O4, Fe2O3, Fe(OH)3, Fe(OH)3·3H2O (2.5)
If, on the other hand, the precipitates formed rapidly enough and close enough to the
corrosion spot, possibly oxidized by nearby air bubbles, they could become a protective layer
and stifle corrosion [7]. A typical pitting corrosion process is schematically shown in Figure
2.1.
24
Figure 2.1 Schematic diagram of pitting corrosion
When chloride-induced corrosion initiates, the local pH decreases resulting in fast corrosion
propagation. This will cause accelerated damage of the reinforcing steel and possibly failure
of the concrete. However, the mechanism how chlorides accelerate corrosion of reinforced
steel is very complex and not thoroughly explained, but some descriptions may be applicable
such as [7]: complex formation between chloride ions and some forms of iron, adsorption on
the surface against protective species and field effect to attract ferrous ions out of the metal.
The chloride-induced corrosion is a localized corrosion and the corrosion current density at
the pits is basically higher than that in normal condition. The difference could be up to several
orders of magnitude and it is then quite detrimental. However, since only a small area of the
steel surface is initially under attack, the measured corrosion rate over the entire exposed area
of steel shows only a 4–10 fold increase initially. As corrosion continues in the pitting spot,
lower pH and more active corrosion process will result in larger surface areas being corroded,
thereby the measured corrosion rate increases. In other words, the entire corrosion process is
accelerated at the propagation stage [12]. This is a major threat to the durability and
performance of concrete structures as mentioned before because once chloride initiates
corrosion, pitting corrosion products of the reinforcement will lead to cracking and spalling of
the concrete cover due to the volume expansion, and decrease of ductility and reduction of
cross section of the reinforcing steel can be expected. The series of consequences result in
large amounts of economic costs and waste of materials and energy [13].
The introduction of chlorides may be from the mix water or the aggregates, even if nowadays
it is restricted by standards. Moreover, chlorides can also penetrate from outside, such as in
highway structures where de-icing salts are used, or in offshore applications exposed to
seawater [6]. Nevertheless, it is suggested that without the presence of chloride or carbonation
happened, even in the passive state there is always a small corrosion current stream in the
reinforced steel which is related to the process of maintaining the protective passive oxide
[12]. This current is certainly at very low levels in the passive state promoted by the high
alkaline environment existing in concrete. Such phenomenon prevents significant
accumulation of rust or outspread corrosion processes, therefore the steel will remain sound
and safe for centuries.
25
2.2 Inhibitors against corrosion of rebar in concrete
In order to prevent the reinforced steel corrosion or to repair the contaminated concrete, a lot
of methods have been applied and among them, corrosion inhibitors have significant effect. A
corrosion inhibitor is defined as a chemical substance, either liquid or powder, which can
result in the reduction of corrosion rate effectively when added into the concrete system,
usually in very small concentrations as an admixture, without significantly changing the
concentration of other corrosion agent (generally water, oxygen, chloride ions) [4, 14]. It is
thought that cement itself have the ability to bind some chloride ions as well [7], though it
cannot be regarded as an inhibitor. According to the features of corrosion inhibitors, they
should be viewed as an additional protective measure rather than as an alternative to the
design specifications for durable concrete [14].
The use of corrosion inhibitors can be dated back to decades ago primarily because of their
relatively low cost comparing to the protective lifetime they provide [7]. The advantages of
using inhibitors for corrosion protection are that they are uniformly distributed throughout the
cement paste so that they can protect the entire steel surface; and that the concrete‘s low
permeability prevents the inhibitor from leaching out, though on the other hand hinders the
osmosis of migrating concrete inhibitors to some extent [14]. Generally, inhibitors are thought
to counteract chloride ions. The effects are postponing of the corrosion onset by raising the
threshold level of chloride ions before corrosion initiates, or reduction of the corrosion rate
once it starts. Some inhibitors may have secondary effect taking a water-based organic
inhibitor as an example which consists primarily of amines and fatty-acid esters. This
water-based organic inhibitor helps to reduce deterioration induced by the ingress of other
aggressive species such as sulfate, because of its permeability reducing property [15].
2.2.1 Classification of inhibitors
There are several classification systems of corrosion inhibitors based on the way they are
added in, the mechanism they work, etc. In the concrete level, inhibitors can be divided into
two groups: admixed inhibitors, which are added to fresh concrete for new structures, and
migrating inhibitors, which can penetrate into the hardened concrete and are usually oriented
to repair system [6]. Admixed inhibitors are commercially available since 1970s, while
migrating inhibitors for concrete structures were proposed in the last 20 years. Nowadays,
there is a number of inhibitors available on the market based on this classification: inorganic
admixtures such as nitrites (used as additives in particular), and sodium monofluorophosphate
(Na2PO3F, whose structure is shown in Figure 2.2) as migrating inhibitors used as surface
applied liquid on hardened concrete; organic compounds based on mixtures of alkanolamines,
amines or amino-acids, or based on an emulsion of unsaturated fatty acid ester (with one or
more double bonds between carbon atoms) of an aliphatic carboxylic acid and a saturated
fatty acid (having no double bonds between carbon atoms), proposed both as admixed and
migrating inhibitors [6, 8].
26
Figure 2.2 Structure of sodium monofluorophosphate
Meanwhile in view of the corrosion process on reinforced steel surface, inhibitors can be
subdivided into three types: anodic, cathodic, and mixed, depending on whether they interfere
with the corrosion reaction preferentially at the anodic or cathodic sites or both are involved
[14]. An ‗anodic inhibitor‘ is one which primarily retards the anodic process of metal
dissolution, causing the corrosion potential of the inhibited metal to shift in the more noble
(positive) direction, whereas a ‗cathodic inhibitor‘ retards mainly the cathodic process
(usually oxygen reduction or hydrogen discharge) and induces a negative shift in the
corrosion potential [4]. Calcium nitrite is a typical anodic inhibitor while most organic
corrosion inhibitors are regarded as mixed inhibitors because through adsorption on the metal
surface, they form an organic layer, which may inhibit both the anodic and cathodic processes
[6].
2.2.2 Nitrites
The ideal situation for inhibitors is to prevent corrosion onset without detrimental effects on
the concrete quality [1]. However, at the first investigating stage, none of the proposed
corrosion-inhibitive compounds had satisfactory performance without any detrimental effect
on the mechanical behavior of the concrete to which they were added at meantime. Among
the various candidates, sodium nitrite appeared to be the most effective inhibitor when used at
adequate dosages. It had been used for corrosion inhibition in non-concrete applications
previously and was commercially available in Europe. However, the addition of such an alkali
salt was not advantageous in concrete. It may induce substantial strength losses in the
concrete and enhance the risk of alkali silica reaction (ASR) problems. Moreover, it was also
suggested to aggravate the corrosion process if its concentration was below the level required
for complete passivation of steel against chloride-induced corrosion. The disadvantage of
such an alkali-metal salt admixture is even more pronounced now, as alkali levels in cements
drift higher and as alkali-aggregate reactions become more prominent [7].
During the 1970s, however, an important progress was achieved through the introduction of
calcium nitrite as a commercially available admixture since it has similar efficiency to sodium
nitrite as a corrosion inhibitor for steel, but without known detrimental effects on the
mechanical properties of concrete or its susceptibility to ASR [4]. It was the first corrosion
inhibitor admixture commercialized on a large scale for reinforced concrete. Calcium nitrite
was first used in Japan, where at relatively small doses to counteract the salt present in sea
27
sand which was used in construction of reinforced concrete. Since then, calcium nitrite has
been used in concrete in Europe and the United States for long time. Despite the toxicity it
has, extensive testing reveals its effectiveness as a good corrosion inhibitor since it can not
only provide protection against chloride-induced corrosion even in the presence of cracks, but
also improve the compressive strength, thereby the total performance of the concrete structure
[14].
2.2.3 Organic inhibitors
Although the choice of an inhibitor is generally based not only on the electron cloud of the
heteroatom (N, O, P, S), the toxicity degree of tested compounds is rarely considered
seriously, as far as calcium nitrite is concerned [16]. On the other hand, scientists are always
seeking for non-toxic and green corrosion inhibitors. As a consequence, organic inhibitors
came into our sight and are under large amounts of investigations recently. Their merits on
protection of reinforced steel in concrete include non-toxicity and eco-friendly. Moreover,
owing to the ability to diffuse through concrete, they can provide admixed corrosion
inhibition against varying amounts of chlorides [14].
In recent days, many alternative eco-friendly corrosion inhibitors such as rare earth elements
and organic compounds have been developed. Polymer-based organic corrosion inhibitors are
widely applied because they can provide an easy handling and cost-effective corrosion
prevention to delay corrosion initiation. The organic inhibitors are typically based on mixtures
of alkanolamines and amines or amino acids or alternately on organic acids because of their
high water solubility and negligible influence on properties of both fresh and hardened
concrete. They can work either on initiation period of time (increasing chloride threshold
value or reducing chloride penetration rate) or on propagation period, reducing corrosion rate.
The application of organic inhibitors was widely investigated both in concrete and in
simulated pore solution [11].
The most efficient organic inhibitors are compounds with electronegative functional groups
and p electrons are in their triple or conjugated double bonds [17]. Amino acids, as
components of living organisms and precursors for protein formation, were reported as a type
of good, safe corrosion inhibitors for many metals in various aggressive media [18, 19].
Several researchers have investigated the inhibitory potential of some amino acids and the
results obtained from such studies have given some prospects of amino acids as green
corrosion inhibitors [17]. They are nontoxic, relatively cheap and easy to produce with
purities greater than 99%. At the present time, there are more than 200 different amino acids
known to occur in nature. Most of the natural amino acids are the alpha amino acids which
contain carboxyl and amino groups bonded to the same carbon atom [9].
Besides amino acids, there are some other organic inhibitors that are noteworthy. Among
them, alkanolamines such as ethanolamine (H2NC2H4OH, Figure 2.3a), methyldiethanolamine
(CH3N (C2H4OH)2, Figure 2.3b) and triethanolamine (N(C2H4OH)3, Figure 2.3c) were tested
as corrosion inhibitors. Their effect on the concrete mechanical properties was evaluated and
28
the results showed good inhibiting effect [20]. Another popular alkanolamine-based inhibitor
is dimethylethanolamine ((CH3)2NC2H4OH, Figure 2.3d), also known as DMEA and
alkanolamine based salts were also found to delay the occurrence of chloride-induced
corrosion by reducing the ingress of chlorides [6]. Aminoalcohol based inhibitors were
reported to prevent steel corrosion as well as improving the durability of concrete when
applied on the surface [21].
(a) (b)
(c) (d)
Figure 2.3 Structures of alkanolamines: (a) ethanolamine; (b) methyldiethanolamine; (c)
triethanolamine; (d) dimethylethanolamine.
In addition, sodium monofluorophosphate has also been widely studied and applied in the
field to prevent the onset of corrosion or to reduce the corrosion rates, both in the presence of
chlorides and in the presence of carbonation through a hydrolysis reaction explained by the
following equation:
Na2PO3F + H2O → F- + H2PO4
- + HPO4
2- (2.6)
It is used by penetration from the concrete surface because, as an admixture, it induces a
strong retardation of the concrete setting and can be transformed into insoluble compounds
[22]. Other organic substances, which are claimed to have an inhibitive effect, are based on
ternary mixtures of aldonic acid, benzoic acid, and a triazole [23], carboxylic or bicarboxylic
acids [24, 25], tannins, etc.
There is one thing worthy to be notice that even if some substances showed good results in
solution tests, concerns may be with the fact whether they have negative influence on
concrete properties, mainly on the setting time, workability and compressive strength [6].
2.3 Inhibition mechanism
To understand the mechanisms of corrosion inhibition helps us to improve the efficiency of
inhibitors. There are various mechanisms for different inhibitors, but in general, the inhibitive
substance acts on the reinforced steel against corrosion by one or more of the following
mechanisms [14]: (1) to form a barrier layer on the steel surface; (2) to promote the oxidation
29
of iron on the surface to obtain a passive film; (3) to influence the surrounding environment in
contact with the steel. The former two mechanisms could prevent the ingress of chlorides to
the surface, while the latter one may be achieved by buffering the local pH in the pits and
competitive migration of inhibitor molecules and chloride ions into pits.
Before reacting on the surface, the first step in any corrosion inhibition process is the
adsorption of the inhibitor molecules on the steel surface which is competing against chloride
ions. The hetero atoms (such as N, O, P and S) in the functional groups help to facilitate the
adsorption of the inhibitor on the surface, and the aromatic ring has similar function. Apart
from the chemical structure of inhibitors, it is suggested that in general, the nature and surface
charge of the steel which is due to the electrical field forming at the interface between the
steel surface and the electrolyte can influence adsorption phenomenon as well [19].
When adsorption on the surface is completed, the inhibition mechanism is in accordance with
the classification of inhibitors. Anodic inhibitors, such as calcium nitrite, strengthen the
process of forming a passive layer on the steel by reacting with ferrous ions to ferric oxide,
and thereby raise the chloride concentration at which an active or pitting process starts. With
higher concentration of nitrite, the passive layer can be strengthened to resist a higher level of
chloride which has a critical threshold for the [NO2-]/[Cl
-] ratio [12].
Another effective inorganic inhibitor, sodium monofluorophosphate (MFP), acts on the
inhibition of corrosion in some different way. As previously mentioned, MFP hydrolyses in
alkaline media, while in the presence of portlandite (Ca(OH)2), there is another reaction as
indicated by Eq. (2.7):
6Ca(OH)2 + 3PO43-
+ 3F- → Ca5(PO4)3F + CaF2 + 12OH
- (2.7)
The consequence of this reaction is the transformation from portlandite to more insoluble
calcium compounds; meanwhile the concentration of OH- in the pore solution is largely
increased owing to the generated hydroxide ions. This increase may enhance the corrosion
inhibition due to the corresponding increase of [OH-]/[Cl
-] ratio [4].
In the case of organic inhibitors which can be cathodic or mixed inhibitors, mechanisms can
be much more complicated. The corrosion inhibition of reinforced steel in terms of organic
inhibitors can be viewed as a process that involves the formation of chelate on the steel
surface, which includes electron transfer from the functional groups in organic compounds to
the steel surface and the formation of a coordinate covalent bond. In this case, the metal acts
as an electrophile while the nucleophilic centre is in the inhibitor [17]. It is also possible to
state that with the formation of complex compounds, the commercial organic inhibitors
reduce the ingress of chlorides by filling concrete pores and blocking the porosity of concrete
[6]. As far as concrete porosity is concerned, a similar or stronger reduction may be obtained
using low W/C ratio, or by adding pozzolanic or fly ash cement.
In terms of typical organic inhibitors such as amines and alkanolamines which are largely
used commercially, the mechanisms can be explained more specific. The functional group
responsible for the adsorption of these two kinds of inhibitors on steel surface is the lone pair
30
of the nitrogen atom (primarily in amino group); and iron ions on the surface act as a Lewis
acid, because they can accept electrons from the nitrogen atom. The functional groups R,
bound to the nitrogen atom (Figure 2.4a) in amines may influence the adsorption of amines
owing to their electronic properties (might be an electron donor or electron acceptor). Similar
mechanisms apply to alkanolamines as well, but with a hydroxyl it is more likely to form a
chelate on the surface as mentioned above. Moreover, organic carboxylate substances were
investigated to have the delocalization effect of the electrical charge of the carboxylate anion
(–COO-), which is responsible for the adsorption on carbon steel surface (Figure 2.4b).
Carboxylates adsorption is also influenced by the presence of functional groups R, bound to
the carboxylate anion [8].
Figure 2.4 Functional groups of amines (a) and carboxylates (b) [8]
Because of the well-known inhibition effect of the functional groups they have, amino acids
and derivatives were believed to be effective inhibitors. Various researchers confirmed their
inhibitory ability by adsorption on the metal surface, forming a protective film and blocking
the active sites. The adsorption mechanism of amino acids which is controversial plays a very
important role because the protection degree of the metal depends on the adsorption process
[26]. It could be chemical adsorption (Figure 2.5a) as the mechanism of amines discussed
above. Some authors also reported that physical adsorption is a possibility in the case that the
metal surface is oxidized in presence of dissolved oxygen [19]. The ability to inhibit corrosion
of amino acids then is related to its tendency to form hydrogen bonds with the oxide or
hydroxide species on the metal surface since the presence of the oxide film on the metal
surface may promote adsorption via hydrogen bonding. It is known that only in the presence
of passive oxide film, formation of hydrogen bond accounts for most of the inhibitory action
[19]. In that case, physical adsorption is expected assisted by hydrogen bond formation
between amino acids and oxidized surface species, as shown in Figure 2.5b.
31
(a) (b)
Figure 2.5 a. chemical bond; b. hydrogen bond [19]
Apart from the species discussed above, some other organic inhibitors may consist of several
different functional groups; therefore the mechanism related to their corrosion inhibition
could be complex. The work done by Charles [15] reported a water-based organic inhibitor
which functions via a twofold mechanism: first, by reducing chloride ion ingress into concrete
through a hydrophobic property derived from the fatty-acid esters; second, through the
formation of a synergistic protective coating on the surface of the embedded steel by the
film-forming amine components (Figure 2.6) as well as the fatty-acid esters.
Polar group Hydrocarbon tail
Figure 2.6 Adsorption of amine component on steel surface [15]
Although a large number of studies have dealt with inhibitors used to prevent and to control
corrosion of reinforcing steel, there are still conflicting opinions about the effectiveness of
organic inhibitors and the mechanism on reinforcement corrosion protection is not well
understood. Some authors suggest that the organic inhibitors are able to form an adsorbed
layer on the steel surface, hindering steel dissolution [27]. There are also investigations
suggested the organic inhibitors act by blocking both anodic and cathodic reactions which are
mixed mechanisms [28]. Some authors also reported that organic inhibitors decrease the
chloride content and chloride ion diffusion in concrete [29]. On the other hand, there are
references where it was concluded that the migrating corrosion inhibitors are not effective
against chloride induce corrosion for concrete under immersion conditions [30, 31].
Contrarily, other authors suggested that these inhibitors are very effective [32, 33]. It is also
indicated by Jamil et al [3] that the inhibitor molecules are able to induce some corrosion
activity during the early stage when they reach the steel surface, probably by displacing some
hydrated parts of the iron oxides/hydroxides surface film in order to create some sites for the
formation of inhibitor layer. The adsorption of admixed inhibitor seems to involve initial
increase of the activity on the steel surface. Later, the anodic activity is completely hindered.
32
2.4 Aspects influencing corrosion inhibition
It is well-known that reinforced steel in concrete is prone to pitting in contact with aggressive
media containing chloride ions, and the corrosion initiates when the chloride content in
concrete in close proximity to the rebar reaches a critical value. To determine this threshold
value, many studies investigated the aspects that may have influence on it since the threshold
is a key parameter in corrosion control and concrete durability prediction. However, the
corrosion process of reinforced steel in concrete structures is a much more complex
phenomenon than the experiments in the lab. Even reported values of the chloride corrosion
threshold in actual concrete systems vary over a wide range, while the aspects responsible for
that variation are difficult to identify. Fundamental issues examined in the past and of
continuing interest include the pH of the surrounding pore solution and the surface condition
of the rebar [18]. Moreover, the postponing of corrosion initiation was also studied suggesting
that two different effects were related [6]: reduction of chlorides penetration rate in concrete
especially in conditions such as in contact with seawater, and increase of critical chlorides
content for the initiation of corrosion as stated above.
To investigate the effect of steel surface condition, Li and Sagüés [18] examined the behavior
of rebar in simulated solutions in its normal as-produced condition (with a mill scale), an
as-produced plus surface rusting condition, and a sandblasted condition, respectively. The
results showed that with increasing roughness of steel surface, pitting potential decreased
while repassivation potential is nearly independent of it, suggesting a worse effect of
corrosion inhibition.
Nitrites can be taken as an example in terms of pH influence due to their oxidizing properties
as good inhibitors because their inhibitory effect is related to the [OH-]/[Cl
-] ratio, which
should be higher than 0.8–1 to prevent corrosion as generally reported. Besides, in severe
conditions (cracking of concrete, seawater), controversial results were obtained and some
commercial organic products also showed low inhibitive effectiveness [6].
In addition, the behavior of passive film and its improvement against damage caused by
pitting process is of great importance. This is especially important where corrosion inhibitors
are used to mitigate localized corrosion [34]. It is reported by Ngala et al [5] that with a
calcium nitrite-based treatment for corroding reinforced concrete, it appears that nitrite ions
can be transported through realistic thickness of concrete cover of high water/cement ratio in
which case, solubility may be of importance. They can cause some reduction in the corrosion
rate, though are only effective on moderately pre-corroding steel in non-carbonated concrete
with modest levels of chloride contamination and in carbonated concrete without chlorides.
Except for the external factors that may affect the corrosion inhibition efficiency, internal
aspects such as molecular structures can also make a difference. In the work done by Zhao
and his co-workers [35], the inhibition performance of nineteen amino acids was studied by
quantum chemical calculation molecular dynamics simulation and the quantitative
33
structure–activity relationship (QSAR) analysis. Local reactivity results according to the
distribution of HOMO and LUMO as well as the Fukui function suggested that the oxygen
atoms in the carboxyl group and the nitrogen atom in the amino group were the active sites.
But the active sites of molecules, to which phenyl group were introduced, were located on
phenyl group.
Although present researches reported many aspects related to the corrosion inhibition
efficiency, the lack of a standard procedure for the evaluation the effectiveness of corrosion
inhibitors also makes a very difficult comparison among the different experimentation [6].
In summary, to be an effective corrosion inhibitor, the selected chemical or mixture of
chemicals should meet at least the following requirements [14]:
1) The molecules should possess strong electron acceptor or donor properties or both.
2) The solubility should be good enough to be saturated rapidly in the corroding area
without being readily leached out.
3) Induce polarization of the respective electrodes at relatively low current values.
4) Be compatible with the intended system so that adverse side effects are not produced.
5) Be effective at the pH and temperature of the environment in which it is to be used.
2.5 Electrochemical analysis methods
Two popular electrochemical methods which are electrochemical impedance spectroscopy
(EIS) and cyclic voltammetry (CVA) were used in this thesis. A brief introduction of these
two techniques is given in this section presenting their working principles and applicative
areas, etc.
2.5.1 Electrochemical impedance spectroscopy
Electrochemical impedance spectroscopy (EIS), as a powerful tool for the detection of pitting
corrosion initiation and for the monitoring of corrosion rate, has been increasingly popular in
recent years. At the early stage, analysis of double layer capacitance and AC polarography
were the initial areas where EIS was used, while recently it has also been applied to the
characterization of electrode reaction processes and complex interfaces [36].
Unlike most researches which rely on DC techniques to determine parameters such as pitting
potential and repassivation potential, EIS studies the system response to the application of a
periodic small amplitude AC signal. The name impedance spectroscopy is therefore derived
from the way that measurements are carried out [37]. Analysis of the system response
contains information about the interface, its structure and the corresponding reactions [36]. In
terms of the application in concrete industry, EIS allows the characterization of both the
diffusion of aggressive species within the cement-based materials and the kinetics of
electrochemical reactions that occur on the steel electrode surface, in a non-destructive way
[38]. As for the DC techniques (also known as stationary techniques) such as anodic
polarization method, even if the experimental problems in determining reproducible values of
34
these potentials are disregarded, there remains the problem that the pitting potential and the
repassivation potential only indicate a potential region in which localized corrosion might
occur [37].
Impedance is commonly depicted as a complex function, having both real and imaginary
components. The impedance spectra are usually described in both Nyquist plot as imaginary
part vs. real part, and Bode plot as magnitude or phase angle vs. frequency, providing a
convenient tool for determining various electrochemical behaviors. Figure 2.7 presents a
typical Nyquist plot for an electrochemical cell. The salient features of the spectrum are
labeled as follows [38]:
1) The electrolyte resistance (or solution resistance) Re derived from the high-frequency
limit of the diagram.
2) The charge transfer resistance RCT given by the diameter of the high-frequency loop.
3) The polarization resistance RP indicated by the low-frequency limit.
Figure 2.7 Typical Nyquist plot in an electrochemical cell [38]
Despite the numerous advantages EIS has, there are some shortages for this technique as well.
Sensitivity comes in the first place thus it must be used with great care. Even with a different
electrode, the results can be of great difference sometimes. In addition, it is not always well
understood and the interpretation could be quite complicated. This may be related to the fact
that existing reviews on EIS are very often difficult to understand by non-specialists and,
frequently, they do not show the complete mathematical developments of equations
connecting the impedance with the physicochemical parameters. It should be noticed that EIS
is a complementary technique and cannot give all the answers, therefore generally other
methods should also be used to elucidate the interfacial processes [36].
With the help of EIS, we can obtain a series of information about the steel specimen.
Polarization resistance Rp is acquired and corrosion current density as well as corrosion rate
can be calculated. In addition, how the passive layer performs when NaCl is added into the
solution can be analyzed on basis of an equivalent circuit. Therefore, corrosion resistance of
different solutions can be evaluated and inhibition effects of five inhibitors will be
investigated.
35
2.5.2 Cyclic voltammetry
Cyclic voltammetry is increasingly being used as a technique to study all types of
potential-dependent interfacial processes. These include, inter alia, adsorption process,
electro-crystallization phenomena and charge-transfer reactions at semiconductor electrodes
or between non-miscible electrolytes [39].
Cyclic voltammetry curve (or cyclic voltammogram) provides information about electron
transfer kinetics and thermodynamics as well as the consequences of electron transfer [40].
With the aid of a few simple diagnostic criteria, it is possible to obtain, even without
complicated mathematics, a fair amount of information on the electrochemical properties of
the specimens being studied [39].
Besides potentiostats with the electrochemical cell arrangement, the standard device for
voltammetric experiments comprises a voltage scan generator, which supplies the desired
potential program, as well as an XY recorder (or a suitable fast transient recorder) which
registers the current-voltage curve (Figure 2.8). Equipment that consists of modular systems
principally is commercially available [39].
Figure 2.8 Schematic experimental setup for cyclic voltammetry; FG: voltage scan generator,
PT: potentiostat, XY: recorder, WE: working electrode, RE: reference electrode, AE:
auxiliary electrode [39].
With the help of CVA, it is possible to deal with both the transition of valence and the
secondary changes in the film due to oxidation or reduction. The red-ox reaction may lead to
deterioration or improvement of the protective properties of the film. This is reflected in an
increasing or respectively decreasing current density through the film [41].
In an understandable way, the object of using CVA is to study the behavior of protective film
at different chloride concentrations in simulated pore solutions. Pitting potential and
repassivation potential can be read from the voltammograms and their difference Ep-Erp would
be evaluated.
36
3. Experimental
3.1 Materials and specimens
The steel used in the experiment was low-carbon steel St 37, whose chemical composition in
weight percentage is C<0.13, Si=0.1/0.4, Mn=0.2/0.5, P<0.05, S<0.035, Cr=0.5/0.8, N<0.009,
Cu=0.3/0.5. Steel coupons with an exposed surface area of 3.14 cm2 were used as working
electrode. The steel was ground with emery paper starting from 320 down to 2400, and then
degreased with acetone and further cleaned with double distilled water before drying with a
hair dryer. Prior to subject to any assigned test, the steel coupons were immersed in the testing
solution for 48h to achieve a stable passivation.
Five inhibitors under evaluation were NaNO2 and sodium salts of p-aminobenzoic acid (pAB),
11-aminoundecanoic acid (11AUA), 6-aminocaproic acid (6ACA) and glycine (Gly), whose
structures are shown in Figure 3.1. The four amino acids were chosen considering different
length of carbon chain and how functional groups (i.e. –NH2, –COOH) are arranged in the
molecular structure. The four sodium salts of amino acids were prepared by neutralization of
the corresponding individual amino acids with equivalent molarity of sodium hydroxide. For
convenience, the abbreviations (i.e. pAB, 11AUA, 6ACA, Gly) are used to represent the
sodium salts of the four amino acids in 0.1M NaOH solution in this paper. And NaNO2
represents sodium nitrites in 0.1M sodium hydroxide solution.
Figure 3.1 The molecular structures of four amino acids
3.2 Solution preparation
0.1M sodium hydroxide solution was used to simulate the high alkalinity environment of
37
concrete pore liquid. The following four solutions were prepared for testing:
(1) 0.1M NaOH as reference solution
(2) 0.1M NaOH + NaCl—different concentration
(3) 0.1M NaOH + 0.1M inhibitors
(4) 0.1M NaOH + 0.1M inhibitors + NaCl—different concentration
After the passivation period of 48h, NaCl was added to each test solution from 0.05M up to
0.4M before the measurements. The tests were conducted starting from 0.05M NaCl to higher
concentrations until corrosion was detected. The concentration intervals were referred to
Zhengxian‘s unpublished work. The specific testing plan is listed in Table 3.1. Duplicate tests
were conducted for each solution except at some thresholds that a third test was needed.
Table 3.1 Testing plan
Chloride
concentration 0M 0.05M 0.1M 0.2M 0.3M 0.4M
NaOH √ √
Gly √ √ √
6ACA √ √ √
11AUA √ √ √ √
pAB √ √ √ √ √
NaNO2 √ √ √ √ √ √
3.3 Test methods
Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CVA) were the two
electrochemical testing methods. Open circuit potential values were recorded before each EIS
tests. All the potential values in this thesis are referred to SCE. In addition, optical
microscope was used to observe the surface morphology.
Electrochemical impedance spectroscopy (EIS) was performed using Solartron 1286
potentiostat connected with a frequency response analyzer. A common three-electrode system
with carbon steel as working electrode, Pt as counter electrode and a standard calomel
electrode (SCE) as reference electrode were used. The EIS measurements were conducted by
polarizing the working electrode at ±10 mV around its OCP using sinusoidal perturbations
range between 60 kHz and 10 mHz. EIS tests were conducted starting from 1h until 4d after
NaCl was added, so for each solution, data of five days (1h, 1d, 2d, 3d and 4d) were acquired.
For convenience, the data were marked as 1d to 5d representing the first testing day to the
fifth testing day in this thesis.
38
Cyclic voltammetry (CVA) was performed with the same potentiostat and three-electrode
system as EIS measurements did. The test was conducted as the following steps which was
also used by M. Cabrini et al [42]: specimen conditioning at −1V vs. SCE for 60s in order to
clean the surface avoiding relevant damage of the passive film, followed by 15s equilibration
at open circuit potential and two consecutive voltammetry cycles from −1.7V to +0.7V vs.
SCE at 50 mV/s scan rate. CVA tests were performed at the fifth testing day after EIS tests
were finished. Some reproducible tests were conducted in the same procedure (48h
passivation followed by 4d with sodium chlorides added) without EIS tests.
Olympus BX60M optical microscope was used for the surface observation. Pictures were
taken under different resolutions at several time nodes to compare the surface morphology
change and to verify the corrosion spots. Primarily, photos were taken when stable passivation
was reached after 48h immersion in alkaline solution and when electrochemical tests were
finished for specimens with different inhibitors. In addition, some noteworthy moments such
as initiation of corrosion were also recorded.
39
4. Results and discussion
In this chapter, the open circuit potentials will be first given to have a preliminary screen of
the five inhibitors. Then the results obtained from electrochemical impedance spectroscopy
and cyclic voltammetry are presented and discussed. Optical microscopy photos are shown to
observe surface morphology.
4.1 OCP evolution
The OCP values were recorded before every EIS tests and Figure 4.1 presents the OCP
evolution of the steel electrode in the six test solutions. As can be seen from the figure,
generally the potential gradually increased during 5 days tests for both pure NaOH solution
and the five inhibitor solutions, indicating that the addition of these inhibitors had no adverse
effects on the passivation of steel electrode. Furthermore, the OCPs of the five inhibitor
solutions exhibited higher values than the pure NaOH solution which may suggest that a
better passivation was achieved in the inhibitor solutions. This finding is in agreement with
the work done by H.E. Jamil and his co-workers [3], which suggested that the adsorption of
inhibitors on the steel surface that enhance the passivation process, therefore passivated state
was achieved more quickly and better.
Figure 4.1 Open circuit potentials for pure NaOH solution and NaOH with five inhibitors
without addition of NaCl during 5 tested days
Figure 4.2 gives the OCP evolution of steel electrodes in six solutions at their own critical
chloride concentrations when corrosion initiated. Generally speaking, steel is regarded as in
passive state when its OCP value is more anodic than -270 mV [11]. In the graph, all
specimens exhibited passive state in the first testing day (2d in Figure 4.2). However after 3d,
only the ones in pAB solution and in NaNO2 solution were still passivated with the OCP
higher than -270 mV. While from 4d on, all the specimens were corroded. It can be seen that
the steel electrode in pure NaOH solution corroded at the lowest chloride concentration
(0.05M NaCl) compared to the ones in inhibitor solutions, suggesting an inhibitory effect can
40
be expected against chloride-induced corrosion when inhibitors were added and they could
raise chloride corrosion threshold in varying degrees.
Figure 4.2 Critical chloride concentrations for inhibitors: NaOH at 0.05M; Gly and 6ACA at
0.1M; 11AUA at 0.2M; pAB at 0.3M; NaNO2 at 0.4M.
It can also be found from Figure 4.2 that except for those in pAB solution and in NaNO2
solution, all other OCP values decreased sharply at 3d which was just 24h after chlorides were
added in. For potentials in pAB solution, this rapid reduction occurred at 4d and remained
stable after then. This phenomenon was also found in other inhibitor solutions except for
NaNO2, whose potentials decreased gradually during the five-day measurements.
Considering the chloride corrosion threshold as the highest one (0.4M), it is believed that
NaNO2 has the best inhibiting effect among five inhibitors. And for amino acids candidates,
pAB performed noticeably. These results derived from OCP tests will be further discussed
and confirmed in the following text with electrochemical impedance spectroscopy and cyclic
voltammetry.
4.2 Investigation by EIS
To interpret the EIS data, an equivalent circuit was used as shown in Figure 4.3 for analysis
(the circuit has been previously reported for steel response in alkaline environment with
addition of inhibitors [43-46]). In this circuit, five elements are included: Rs is the electrolyte
resistance, CPEf is the constant phase element for passive film, Rf is the film resistance, Rct is
the charge transfer resistance and CPEdl is capacitance for metal/solution double layer. The
CPE element is a simple distributed element and its behavior can be owing to the fractal
nature of the electrode interface or heterogeneity of the steel surface. Associated with the CPE
there is a variable value n in the equation: ZCPE = (jω)-n
/Y0, which ranges between 0 and 1
describing the distribution of the dielectric relaxation times in the frequency domain. When n
equals to 1, the CPE represents a capacitor and n = 0 represents a pure resistor; when 0 < n <
1, CPE shows a non-ideal capacitive response [11, 46, 47]. Table 4.1 gives some fitting
41
results from EIS test with fitting errors smaller than 7%.
Figure 4.3 Equivalent circuit for analysis of impedance spectra.
Table 4.1 Best fit parameters from EIS measurements for steel electrodes in solutions without
NaCl during five-day test duration
Time Rs (Ω) CPEf, Y0 (Ω
-1 S
n) n Rf (kΩ cm
2) CPEdl, Y0 (Ω
-1 S
n) n Rct (kΩ cm
2)
NaOH 1d 37.6 1.14E-04 0.939 41.2 5.91E-05 0.750 136.9
5d 46.7 1.09E-04 0.925 121.4 1.71E-05 0.920 365.5
NaOH+0.05M
NaCl 1d 33.3 8.18E-05 0.969 107.4 2.73E-05 0.932 338.7
2d 38.3 8.99E-05 0.953 4.9 1.27E-06 0.934 586.2
3d 38.1 1.01E-04 0.951 29.5 2.46E-04 0.916 14.8
4d 40.2 1.16E-04 0.946 4.3 1.88E-04 0.656 13.9
5d 38.7 1.29E-04 0.931 2.2 4.21E-04 0.629 13.2
Gly 1d 30.6 7.97E-05 0.977 7.8 1.17E-05 0.835 536.5
5d 32.9 8.20E-05 0.951 13.3 3.03E-06 0.908 1100.1
Gly+0.1M
NaCl 1d 30.0 8.10E-05 0.979 6.0 7.86E-06 0.857 334.2
2d 27.8 1.27E-04 0.935 15.1 4.03E-04 0.607 13.9
3d 31.8 1.55E-04 0.929 1.9 4.88E-04 0.447 8.9
4d 33.0 1.92E-04 0.900 1.2 1.07E-03 0.516 9.8
5d 32.8 2.48E-04 0.868 0.7 1.86E-03 0.515 6.4
6ACA 1d 19.3 8.39E-05 0.956 9.5 6.64E-06 0.949 576.9
5d 20.3 5.74E-05 0.933 10.2 1.06E-06 0.951 1535.1
6ACA+0.1M
NaCl 1d 35.6 6.72E-05 0.962 4.6 1.16E-05 0.774 835.4
2d 36.2 2.36E-04 0.877 2.0 1.36E-04 0.458 5.9
3d 33.8 7.01E-04 0.759 2.1 1.34E-03 0.420 2.4
4d 34.5 7.28E-04 0.764 1.7 9.60E-04 0.462 2.7
5d 31.7 8.50E-04 0.746 1.0 8.48E-04 0.457 3.3
11AUA 1d 13.9 3.88E-05 0.925 2.6 1.05E-06 0.913 891.0
5d 18.9 4.52E-05 0.913 4.4 1.06E-06 0.967 1462.0
11AUA+0.2M
NaCl 1d 21.7 8.97E-05 0.975 19.2 3.57E-05 0.652 125.7
2d 20.6 1.77E-04 0.913 2.0 1.02E-03 0.449 2.1
3d 22.5 1.73E-04 0.920 1.2 4.87E-04 0.438 4.1
4d 22.1 2.00E-04 0.902 0.8 5.33E-04 0.436 3.6
5d 22.3 2.54E-04 0.865 0.7 6.97E-04 0.486 4.3
42
pAB 1d 19.3 6.04E-05 0.948 5.0 1.30E-06 0.962 998.8
5d 20.4 5.76E-05 0.932 8.1 1.04E-06 0.950 1467.8
pAB+0.3M
NaCl 1d 17.7 1.19E-04 0.944 18.2 1.27E-05 0.832 354.1
2d 17.4 1.25E-04 0.941 80.2 2.36E-05 0.813 374.2
3d 18.3 1.72E-04 0.922 7.9 1.52E-04 0.411 10.8
4d 19.9 1.81E-04 0.928 5.2 2.78E-04 0.441 13.7
5d 18.7 1.88E-04 0.924 4.5 1.89E-04 0.588 18.9
NaNO2 1d 14.5 2.72E-05 0.960 6.9 3.65E-06 0.690 992.0
5d 14.5 2.57E-05 0.953 7.9 2.32E-06 0.720 1044.1
NaNO2+0.4M
NaCl 1d 14.8 7.48E-05 0.978 2.6 3.79E-05 0.734 378.1
2d 14.8 7.62E-05 0.977 1.7 3,84E-05 0.695 330.4
3d 14.7 9.57E-05 0.958 1.1 5.25E-05 0.450 90.1
4d 14.1 1.17E-04 0.932 7.2 9.76E-05 0.458 53.8
5d 13.9 1.50E-04 0.914 1.9 3.48E-04 0.448 6.6
4.2.1 Effect of five inhibitors on impedance when added into NaOH solution
A convenient way to evaluate the corrosion resistance of specimens in EIS spectra is to
compare the diameters of the curves in Nyquist plot. With larger diameter, the better
corrosion resistance of the sample can be expected [48]. Figure 4.4 and Figure 4.5 give the
Nyquist and Bode plots of EIS results obtained from samples immersed in solutions with or
without inhibitors at 1d and at 5d, respectively. As can be seen from the Nyquist plot, the
diameter of NaOH curve is much smaller than those of inhibitors in first day (Figure 4.4) indicating that the corrosion resistance is much smaller. As time goes, in the fifth day the
impedance gap became smaller, however, the impedance of solutions with inhibitors was still
at least double of that in NaOH solution. It is suggested that a passive film formed on the iron
surface as a consequence of the alkaline solution with high pH. With the influence of
inhibitors, the film was reinforced so that impedance became larger. Moreover, the phase
angle in the low frequency region was also higher in solutions with inhibitors, indicating a
good corrosion inhibition offered by the inhibitors. It is possible that the passive layer
becomes thicker, or more homogeneous. It could strongly inhibit the corrosion activity on the
surface, creating an insulating layer, which behavior approaches that of a capacitor (some
curves in phase angle plot approaching 90 degrees in Figure 4.5). Furthermore it was reported
that this layer seems to have the capacity to bind the chlorides through the amino groups
present in the inhibitor molecules [3].
43
Figure 4.4 Impedance spectra for steel electrode in 0.1M NaOH reference solution and in
solutions admixed with five inhibitors at 1d: a) Nyquist plot; b) magnitude Bode plot; c)
phase angle Bode plot.
a)
b)
c)
44
Figure 4.5 Impedance spectra for steel electrode in 0.1M NaOH reference solution and in
solutions admixed with five inhibitors at 5d: a) Nyquist plot; b) magnitude Bode plot; c)
phase angle Bode plot.
These figures represent the behavior of specimens in solutions without chlorides. While in the
presence of sodium chloride, corrosion phenomena were observed from the impedance
spectra which are shown in the following section.
a)
b)
c)
45
4.2.2 Inhibition effect of five inhibitors with admixture of NaCl
Figure 4.6 and Figure 4.7 give the EIS results of steel electrodes in 0.1M Gly solution and in
0.1M 6ACA solution respectively, with 0.1M NaCl. The shape of phase angle plot suggests
the presence of two time constants partially overlapped which can be described by the
equivalent circuit in Figure 4.3. At first day when sodium chloride was just added, the steel
electrodes in both solutions remained passive state, especially for the system in 6ACA which
behaved like a capacitor. This can be explained by the plateau in low frequencies in the phase
angle Bode plot (Figure 4.7c), suggesting that a homogeneous protective layer was still
present on the steel surface. However, a significant decrease in impedance value and phase
angle at low frequencies after 2d indicated corrosion initiation which is in accordance with
the OCP results (Figure 4.2). Moreover, the impedance values at 5d in both situations were
almost 2 orders of magnitude lower than those at 1d. It is worth noting that in phase angle plot
of Gly, the values recovered at low frequencies region in the last two testing days, which may
suggest that the corrosion process was controlled by diffusion [43] and another time constant
could be expected at lower frequencies (10-4
to 10-3
Hz). After three days, impedance of
specimens in these two inhibitor solutions stopped to decrease and stabilized at around 2000
Ω·cm2. Considering the results at 5d, it is hard to tell which of these two inhibitors has a
better inhibition effect.
Figure 4.6 EIS spectra of steel specimen in 0.1M Gly solution at 0.1M NaCl from 1d to 5d: a)
Nyquist plot; b) magnitude Bode plot; c) phase angle Bode plot.
a)
b) c)
46
Figure 4.7 EIS spectra of steel specimen in 0.1M 6ACA solution at 0.1M NaCl from 1d to 5d:
a) Nyquist plot; b) magnitude Bode plot; c) phase angle Bode plot.
In Figure 4.8, the impedance spectra of specimen in 0.1M 11AUA solution in presence of
sodium chloride at concentration of 0.2M are given. The figure demonstrates that the
corrosion of the steel started rapidly soon after chlorides were added in because the phase
angle at 1d dropped to around -45 degrees at low frequency. In the following several days,
phase angle decreased and maintained a smaller value, which was accordant with impedance
response. The behavior of steel in 0.1M pAB solution at 0.3M NaCl (Figure 4.9) differed
slightly with that in 11AUA solution. In the first two testing days, the steel sample remained a
passive state exhibiting some capacitive behavior because the phase angle kept in a high value
even in low frequencies. However, starting from 3d, corrosion initiated leading to a decrease
of total impedance values and phase angle. Nevertheless, both impedance values and phase
angle of pAB (around 10 kΩ·cm2 and -20 degrees, respectively) were larger than those of
11AUA (around 3 kΩ·cm2 and -12 degrees, respectively) at 5d, suggesting that pAB offered
better inhibiting effect than 11AUA did even at their own critical chloride concentrations
(pAB at 0.3M NaCl and 11AUA at 0.2M NaCl).
a)
b) c)
47
Figure 4.8 EIS spectra of steel specimen in 0.1M 11AUA solution at 0.2M NaCl from 1d to
5d: a) Nyquist plot; b) magnitude Bode plot; c) phase angle Bode plot.
c) b)
a)
a)
48
Figure 4.9 EIS spectra of steel specimen in 0.1M pAB solution at 0.3M NaCl from 1d to 5d: a)
Nyquist plot; b) magnitude Bode plot; c) phase angle Bode plot.
Figure 4.10 EIS spectra of specimen in 0.1M NaNO2 solution at 0.4M NaCl from 1d to 5d: a)
Nyquist plot; b) magnitude Bode plot; c) phase angle Bode plot.
The impedance spectra were in agreement with the OCP results as well in 0.1M NaNO2
solution. The trend of impedance spectra in Figure 4.10 shows a gradual reduction in all the
three graphs. This trend is particularly obvious in Nyquist plot that the curves become lower
and their lengths become shorter from 1d to 5d accompanying by smaller diameters. And as
can be seen from the two Bode plots, the inhibitor layer seemed to cover the entire surface
initially owing to the good adsorption of nitrites, while the total impedance of the system
decreased continuously as a function of time. The phase angle at low frequency reduced
gradually reaching -15 degrees at 5d, probably suggesting significant corrosion. Technically,
corrosion initiated at 3d and the pits kept growing and propagating, forming some obvious
spots after the testing period, which can be seen in Figure 4.11 in the following section.
b) c)
a)
b) c)
49
4.2.3 Surface images
The pitting corrosion spots were observed using optical microscopy. Figure 4.11 shows
pictures taken at different testing days during the EIS measurements. Basically, the optical
microscope pictures are in accordance with the impedance spectra. When stable passivation
was achieved after 48h immersion in alkaline solution, photo (Figure 4.11a) was taken as a
reference picture. We can see that when there was no corrosion after five-day tests (Figure
4.11b), basically no obvious change can be observed at the steel surface no matter with or
without sodium chloride added into the solution. And some little intensive black spots can be
seen from the picture which might be related to the passive film.
However technically, it is nearly impossible to observe the protective film becoming thicker
or more homogeneous at the surface of the specimen under optical microscopy. When the
decrease of impedance was recorded in EIS test which indicated corrosion initiation, there
were some small pitting spots that can be observed on the steel surface (Figure 4.11c). After
five-day EIS tests, the pits became larger and the number was expected to increase, as shown
in Figure 4.11d. In some situations, it is possible that several small pits may be observed for
specimens at chloride concentration lower than critical value. These pits suggested that with
chloride added in, some spots of the passive layer may be destroyed and corrosion initiated,
while afterwards, the inhibitor molecules succeeded to repassivate the steel surface so that
corrosion did not propagate.
Figure 4.11 Optical microscope pictures with 5×magnification: a) 48h after immersion; b) 5d
after EIS tests without corrosion; c) just after corrosion initiation at 0.4M NaCl in 0.1M
NaNO2 solution; d) 5d after EIS tests at 0.4M NaCl in 0.1M NaNO2 solution.
a) b)
d) c)
50
If we take a microscopic look at the specimen surface, the evolution of interface between
inhibitor layer and steel surface can be expected as a function of time. Surface change of steel
specimen can be illustrated by Figure 4.12 taking the specimen in sodium nitrite solution as
an example at 0.4M NaCl. This specimen was chosen as an example because the gradual
decrease in impedance spectra (Figure 4.10) provides a better understanding on breakdown of
passive film. Conceivably, the following graphs can be applied to other four amino acids with
a different period of time.
Figure 4.12 Illustration of surface change of steel specimen during five-day EIS
measurements in 0.1M NaNO2 solution at 0.4M NaCl.
We can see from the figure that at 1d when steel was passivated owing to the immersion in
alkaline solution, a protective layer of inhibitor molecules was formed on the steel surface
with the help of adsorption, though the film may not be very homogeneous. At 2d when
sodium chlorides had been added into the solution for one day, the passive film was attacked
by the chloride ions and it became thinner and even less homogeneous. However, a passivated
surface was still existed. The corrosive effects of chlorides became more evident with time so
that local breakdown of the protected film can be expected and some small pits appeared on
the steel surface, leading to a dramatic decrease in impedance value. In the last two days, the
damage of passive film was severer and localized corrosion became more serious with
increasing number and area of pitting spots. Therefore, the total impedance and phase angle in
impedance spectra dropped to fairly low values at the end of EIS tests.
a) 1d b) 2d
c) 3d d) 4d
e) 5d
Inhibitor molecules Steel specimen
51
4.2.4 Inhibition effect of three selected inhibitors with admixture of NaCl as a function
of chloride concentration
As can be seen from Figure 4.13-4.15, impedance spectra for 11AUA, pAB and NaNO2 as a
function of NaCl concentration at 5d are shown, respectively. These inhibitors were chosen
because of their better inhibition effect comparing to other two candidates. As for specimen in
0.1M 11AUA solution, the curves at 0.05M NaCl and at 0.1M NaCl do not overlap to a great
extent (at 0.1M NaCl, the behavior was more like a capacitor). Nevertheless, we can still find
that under these conditions, the steel electrodes were still passivated. From these figures, we
can see that under critical chloride concentration, impedance changed on a small scale with
increasing chloride concentrations, especially for specimens in pAB solution and in NaNO2
solution. This phenomenon indicated that strong inhibition of the dissolution processes
occurred on the steel surface, even if chloride concentration became higher [3]. As far as pAB
is concerned, the effect of increasing chloride concentration could be neglected. However for
NaNO2, only at the chloride concentration of 0.05M was the impedance slightly larger than
that at higher Cl- concentrations. At the rest chloride concentrations (except for the corroded
one at 0.4M NaCl), curves in impedance spectra almost overlapped with each other.
Conceivably, as for Gly and 6ACA, similar spectra were obtained which can be correlated to
Figure 4.6 and Figure 4.7, respectively.
a)
52
Figure 4.13 Impedance spectra in 0.1M 11AUA solution with different NaCl concentrations at
5d: a) Nyquist plot; b) magnitude Bode plot; c) phase angle Bode plot.
Figure 4.14 Impedance spectra in 0.1M pAB solution with different NaCl concentrations at
5d: a) Nyquist plot; b) magnitude Bode plot; c) phase angle Bode plot.
b) c)
a)
b) c)
53
Figure 4.15 Impedance spectra in 0.1M NaNO2 solution with different NaCl concentrations at
5d: a) Nyquist plot; b) magnitude Bode plot; c) phase angle Bode plot.
The impedance spectra above give us a perception on how well the inhibitors can militate
under different concentrations of sodium chloride. It is obvious that the chloride corrosion
thresholds for different inhibitors vary to a large extent. However, for all the inhibitors, there
is one similarity that their own inhibitive behaviors at concentrations below chloride
corrosion threshold had little difference, indicating that the increase of chloride concentration
seemed to have bit influence on the inhibiting effect. This is particularly distinct in presence
of pAB and NaNO2. To have a better understanding of how chloride concentration could
influence the inhibition effect, parameters in equivalent circuit are discussed in the following
section to study the change of different elements.
4.2.5 Discussion of equivalent circuit parameters
Besides impedance spectra, parameters in equivalent circuit are important indices to
understand the inhibitory ability. There are five elements in the equivalent circuit that are used
for interpretation. Except for the solution resistance, the other four elements are mutually
a)
b) c)
54
coupled to describe the behavior of passive film and corrosion process, respectively. The
evolutions of three of them are shown below, while as for the film resistance, there is
basically no tendency as a function of chloride concentration. Therefore, it is not
demonstrated in the results.
The film capacitance values in 5 inhibitor solutions at 0.1M NaCl are illustrated in Figure
4.16. It was reported that the evolution of film capacitance is an indicator of thickness of the
protective film and/or its homogeneity [45]. For steel electrodes in Gly and 6ACA solutions
which corroded at 0.1M NaCl, the Y0f values increased as a function of time. Especially for
6ACA specimen, the value at 5d was almost one order of magnitude higher than that at 1d.
Yet for Gly specimen, the capacitance values increased gradually on a smaller scale. As for
specimens in the 3 rest inhibitor solutions, there is a slightly decreasing trend of Y0f values.
Only in the case of sodium nitrite, some fluctuation is observed in the graph. Taking film
resistance into consideration (not shown in figure, but can be refer to Table 4.1), it can be
suggested that a reinforcement of the protective behavior of the inhibitor film formed on the
surface resulting in a better passivity [43]. However, whether the film thickness or its
homogeneity controlled the corrosion behavior is not clear because the difference during
five-day tests of this parameter is not distinct.
Figure 4.16 Capacitance values of film for the five inhibitor solutions with addition of 0.1M
NaCl
Figure 4.17 gives the double layer capacitance in five inhibitor solutions at 0.1M NaCl. The
Y0dl values of corroding specimens (Gly and 6ACA) increased similarly to Y0f, but more
sharply, with the increase of 2 orders of magnitude. However, for other specimens (11AUA,
pAB and NaNO2) whose chloride corrosion thresholds are higher, double layer capacitance
values fluctuated more or less. Within these samples, the amino acids showed a marginally
rising trend. The fluctuation could be possibly related to a phenomenon that the inhibitor may
induce some corrosion activity during test period to create some more favorable sites for the
formation of the protective inhibitor film, probably by exchanging some hydrated substances
of the iron oxides/hydroxides surface layer [3].
55
Figure 4.17 Capacitance values of double layer for the five inhibitor solutions with addition
of 0.1M NaCl
The evolution of charge transfer resistance as a function of time is shown in Figure 4.18, with
inhibitor solutions at 0.1M NaCl. Generally, Rct can be seen as an indicator of corrosion rate
[45]. It can be noticed that for corroding specimens (Gly and 6ACA), Rct dropped sharply in
the second day owing to breakdown of passive film attacked by chloride ions. In the next 4
days, it continued to decrease marginally suggesting increasing of pitting spots and/or
corrosion area in pits [43]. In terms of 11AUA, the values remained stable in a high value
range (around 106 Ω·cm
2) in principle during the tests. Nevertheless, for other two inhibitors
(pAB and NaNO2), the charge transfer resistance increased gradually revealing that the steel
specimens were in a sound passive state.
Figure 4.18 Charge transfer resistance for the five inhibitor solutions with addition of 0.1M
NaCl
56
Figure 4.19-4.21 show the evolution of CPE parameters as a function of chloride
concentrations. It can be seen that with increasing chloride concentration, Y0 of film
capacitance increased gradually for all the inhibitors (Figure 4.19) likely due to the passive
film becoming less homogeneous and thinner. It was also possible that the passive film was
destroyed at some spots. At its chloride corrosion threshold, this increase became steeper
which is particularly evident for that in 6ACA solution. On the other hand, Y0 of double layer
(Figure 4.20) fluctuated at different chloride concentrations under thresholds, but rose sharply
at critical chloride concentration with an increase of at least 2 orders of magnitude (from 10-6
to 10-4
or even 10-3
F·Sn-1
·cm-2
). This increase could be related to the corrosion products over
pits that they formed a structure analogous to capacitor and induced an increase in electrode
surface area at the meantime [49].
Figure 4.19 Capacitance values of film for the five inhibitor solutions at different NaCl
concentrations at 5d
Figure 4.20 Capacitance values of double layer for the five inhibitor solutions at different
NaCl concentrations at 5d
57
The trend of charge transfer resistance (Figure 4.21) is in accordance with that of Y0dl that the
values decreased sharply at critical chloride corrosion concentration. The values at critical
chloride concentration were practically 2 orders of magnitude smaller than those at lower
chloride concentrations, which was similar to the change of double layer capacitance. This
trend indicated a remarkable increase of corrosion current density which meant pitting
corrosion initiated.
Figure 4.21 Charge transfer resistance for the five inhibitor solutions at different NaCl
concentrations at 5d
4.2.6 Discussion of corrosion rate and inhibition efficiency
Apart from the parameters discussed above, the corrosion behavior of reinforced steel in
concrete is generally determined by corrosion current density icorr quantitatively. This
parameter is usually related to polarization resistance Rp and is calculated using the equation
icorr = B/Rp, where B is the Stern-Geary constant dependent on the nature of corrosion
reactions [43]. The value of B is 52 mV for passive steel while equals to 26 mV in corroded
situation. Table 4.2 shows polarization resistance, corrosion current density (also shown in
Figure 4.22) and corresponding corrosion rate of the specimens at their own critical chloride
concentrations as well as the concentrations just below thresholds. The polarization resistance
was calculated on basis of Rf and Rct [50] and the values we got were a little larger than the
impedance values at very low frequency (usually 10-4
Hz). It is proposed that when corrosion
current density is lower than 0.1 μA/cm2, the corrosion rate is negligible [43]. As can be seen
in the table, the corrosion current density for specimens in inhibited solutions with addition of
sodium chlorides (the ones below chloride corrosion thresholds) is lower than that in NaOH
reference solution, accordingly corrosion rate is lower even at high chloride concentration
(e.g. corrosion rate of NaNO2 with 0.3M NaCl is lower than that of pure NaOH solution). It is
clearly shown in Figure 4.22 that at the critical NaCl concentrations, the performance of pAB
was quite remarkable due to its lowest corrosion current density. Contrarily, 6ACA seems to
be the poorest inhibitor because the corrosion current density (also the corrosion rate) is the
58
highest, more than triple of that in Gly solution.
Table 4.2 Polarization resistance, corrosion current density and corrosion rate of NaOH and
five inhibitors at different chloride concentration
Rp (kΩ cm2) icorr (μA cm
-2) corrosion rate (μm y
-1)
NaOH 487 0.11 1.24
NaOH+0.05 12.9 2.01 23.4
Gly+0.05 1113 0.05 0.54
Gly+0.1 13.8 1.89 21.9
6ACA+0.05 1149 0.05 0.53
6ACA+0.1 4.2 6.12 71.1
11AUA+0.1 988 0.05 0.61
11AUA+0.2 5.03 5.17 60.1
pAB+0.2 822 0.06 0.74
pAB+0.3 20.3 1.28 14.9
NaNO2+0.3 1789 0.03 0.34
NaNO2+0.4 8.6 3.03 35.2
Figure 4.22 Corrosion current density for six specimens at different chloride concentrations
Moreover, inhibition efficiency (IE) is also an important indicator for the evaluation of the
inhibiting ability. It can be approximately calculated according to the following equation [43]:
IE = Z inh − Z un
Z inh× 100% (4.1)
Where Z inh is the polarization resistance for specimens in inhibited solutions and Z un is
that in uninhibited solutions. Therefore, at chloride concentration of 0.05M, the inhibition
efficiency of selected inhibitors was calculated based on the data obtained at 5d and the
results are listed in Table 4.3. It can be seen that at 0.05M NaCl, which none of the inhibited
59
specimens corroded, the inhibition efficiencies of all the five inhibitors are larger than 97%
(most of them approaching almost 99%). These high values reveal that both the amino acids
and sodium nitrite have decent inhibitory effectiveness.
Table 4.3 Inhibition efficiency of 5 inhibitors at 0.05M NaCl
IE (%)
Gly 98.8
6ACA 98.9
11AUA 97.1
pAB 98.7
NaNO2 99.0
From the EIS results, it can be concluded that inhibition effect varied among the selected five
inhibitors. Sodium nitrite remained the most effective inhibitor, while among the other four
amino acids pAB had the best inhibition effect, followed by 11AUA. 6ACA and Gly had the
poorest inhibitory ability within the candidates. In the following section, some cyclic
voltammetry results are shown to verify the consequence we obtained from above results.
4.3 Investigation by CVA
The manner in which these inhibitors can influence the corrosion process was pointed by their
electrochemical behavior in test solutions studied by cyclic voltammetry. Measurements were
carried out in solutions at the same chloride concentrations as studied in EIS test. Generally
during the corrosion process, cathodic reaction (oxygen reduction: O2+2H2O+4e-→4OH
-)
proceeds on the passive surface, surrounding pits or crevices, whereas anodic reaction (metal
dissolution: Fe→Fe2+
+2e-) concentrates at the active bottom of these pits [45]. Therefore, the
pitting spots become larger and deeper. With the help of CVA, detailed information of these
reactions as well as some more complex processes will be given and discussed in the
following text.
Figure 4.23 shows typical cyclic voltammograms for steel electrode in alkaline solution under
the attack of chlorides. During the anodic scan, current decreased continuously starting from a
negative value and changed the sign at corrosion potential Ecorr [51]. The peaks 1 and 2 may
be attributed to the first oxidation of iron to iron (II) hydroxide according to the reaction
explained by Eq. (4.2):
Fe + 2OH−
→ Fe(OH)2 + 2e− (4.2)
The peaks 3 and 3‘ are assigned to ferrous–ferric transformations in a relatively compact
inner oxide layer (Fe3O4), and in an outer oxide layer (γ-Fe2O3 and/or its hydrated form
γ-FeOOH), respectively. The processes are ascribed to reactions in the following equations:
3Fe(OH)2 + 2OH−
→ Fe3O4 + 4H2O + 2e− (4.3)
Fe3O4 + H2O + OH−
→ 3FeOOH + e− (4.4)
The hydroxide γ-FeOOH may dehydrate subsequently to give γ- Fe2O3 [48]. When the anodic
current continued to fall to a relatively low value, the onset of passivation may occur. A
60
sudden increase of the current density indicates the initiation of pitting attack at pitting
potential Ep [51]. When the potential scan reversed at oxygen discharge potential to a cathodic
direction, the current gradually decreased in the positive branch and reached zero value at the
repassivation potential, or protection potential Erp [52]. Peaks 4 and 3 are conjugated,
indicating a partial reversibility of the reaction of peak 3. Peaks 4 and 5, represent the
reduction of ferric to ferrous oxide (γ-Fe2O3 → Fe3O4 → Fe(OH)2) and of ferrous oxide to iron,
respectively [47, 53].
The pitting potential in the first cycle relates with the capability of inhibitors to inhibit the
onset of localized attack. With highly inhibiting substances such as pAB or 11AUA at 0.05M
NaCl (can be found in Figure 4.27 and Figure 4.26, respectively), this potential is expected to
reach the value of oxygen evolution potential before localized corrosion initiates. In the
second cycle, this potential may be relevant to the ability of repassivation improvement on the
steel surface during the cathodic scan, thus to reduce the risk of corrosion propagation [53].
Figure 4.23 Typical CVA curves: 0.1M NaOH with addition of 0.05M NaCl
In the next sections, cyclic voltammograms of five inhibitors at different chloride
concentrations are discussed and the specific potentials mentioned in the above graph are
given to describe the possibility of corrosion and inhibiting effect of proposed inhibitors.
4.3.1 Cyclic voltammograms of five inhibitors
Figure 4.24 shows the cyclic voltammetric curves of specimens in 0.1M 6ACA solution at
different chloride concentrations. As can be clearly seen, at chloride concentration of 0.1M,
the pitting potential shifted to a smaller value (Ep‘ in Figure 4.24) than the other two curves.
Moreover, the peak related to ferrous-ferric transformation shifted towards a positive
direction whose potential rose from around -0.66V (0M NaCl) to -0.62V (0.05M NaCl) and
-0.43V (0.1M NaCl). The potentials of peaks correlated to opposite reactions that indicating
Fe3+
→Fe2+
and Fe2+
→Fe increased anodically (towards a positive direction) at 0.1M NaCl as
1 2 3’
3
4
5
Ep
Erp
Ecorr
61
well, especially for the ferric-ferrous transformation that the potential increased from around
-1.1V to -0.6V.
Figure 4.24 CVA curves of steel electrode in 0.1M 6ACA solution at different chloride
concentrations
These phenomena were also found in Figure 4.25 with respect to the specimen in 0.1M Gly
solution. The specimens at concentrations under critical chloride concentration (0M and 0.5M
NaCl) had similar passive behavior in the two successive cycles likely due to the restoration
of passivity during the potential scan in the cathodic direction of the first cycle [53]. The
current remained stable in the passive region, and the reverse curve corresponded to lower
current values than those obtained during the anodic sweep. This behavior was reported as
typical of a product layer thickening during the forward scan and can be observed from the
curves below critical chloride threshold [44]. However, at 0.1M NaCl, a completely active
behavior was observed in the second cycle denoting the absence of repassivation after the first
cycle. In addition, a potential increase was observed from -0.55V (1st cycle) to -0.4V (2
nd
cycle) for the peak describing ferrous-ferric transformation. As with the conjugated
ferric-ferrous transformation peak during cathodic scan, a slight increase of potential can be
noticed from -1.1V to around -0.85V.
Ep Ep’
62
Figure 4.25 CVA curves of steel electrode in 0.1M Gly solution at different chloride
concentrations
Figure 4.26 CVA curves of steel electrode in 0.1M 11AUA solution at different chloride
concentrations
In Figure 4.26, the CVA curves are related to steel specimen in 0.1M 11AUA solution. It is
well-marked that the current density values at 0.2M NaCl in passive domain (around -0.5V to
0.55V) increased significantly compared to curves at lower chloride concentrations. The
region corresponded to pitting potential accordingly moved towards a negative direction.
Moreover, the current peak associated with the oxidation of iron (II) hydroxide to the iron (III)
oxide increased as well. For lower chloride concentrations, the curves overlapped with each
other to some extent, suggesting that below chloride corrosion threshold, the influence of
increased chloride concentration on passive film was negligible.
1st cycle
2nd cycle
Ep
63
Figure 4.27 CVA curves of steel electrode in 0.1M pAB solution at different chloride
concentrations
In terms of the CVA curves of pAB in Figure 4.27, the potential change of the peaks in anodic
sweep showed a very small difference though the increase of current density could still be
observed. In the cathodic sweep, peak 4 became evident at 0.3M NaCl particularly and the
related potential shifted to a smaller value slightly while was generally about -1.1V. Moreover,
a hysteresis could be obviously noticed at 0.3M NaCl after inversion of potential scan and this
could be ascribed to the initiation of pitting corrosion. This was further confirmed by the
second cycle in which pitting potential was lower and hysteresis became more evident. This
greater hysteresis drove repassivation potential to a more cathodic value and made the
positive current density stay in a wider region. El-Haleem et al [52] suggested that these
positive current during cathodic scan may be related to continuous propagation of formed pits.
If comparing the curves of different chloride concentrations, it could be found that pitting
potentials shifted gradually to a lower value when chloride concentration increased. An
increasing trend of the current density can also be found towards positive direction. As for the
passive potential region, it moved upwards slightly.
As for the specimen in 0.1M NaNO2 solution (Figure 4.28), the difference among the curves
at different chloride concentrations is relatively small, which can be seen from the curves of
solution without chloride and solution with 0.3M NaCl. However at 0.4M NaCl, the decrease
of pitting potential became obvious. Moreover, the peaks related to Fe2+
→Fe3+
(peak 3 in
Figure 4.23) were more distinguishable than those in amino acid solutions which could be
ascribed to the quick oxidation ability of ferrous ions to ferric oxide offered by nitrites [49].
This ability could also enhance the surface oxide film passivity since the product layers based
on magnetite were reported to be more stable and corrosion-resistant than those based on
hydroxides. [44].
4
2nd cycle
Ep
Erp
64
Figure 4.28 CVA curves of steel electrode in 0.1M NaNO2 solution at different chloride
concentrations
4.3.2 Optical microscopy photos
The optical microscope pictures not only gave a visualized perception of the surface change
of the specimens, but also helped us to have a better understanding of the results obtained
from electrochemical methods.
In cyclic voltammetry experiments, steel specimens were immersed in test solution for 48h to
reach stable passivation before tests. When the peaks appeared during the anodic sweep, a
brownish color can be observed at the steel surface instead of original metal-like silver. It
darkened after two continuous cycles when test was finished, which can be roughly compared
in Figure 4.29. In this figure, a picture taken in the passive region (usually -0.5 to 0.5 V) in
cyclic voltammograms is shown (Figure 4.29b). Except for the color change at surface, a lot
of black spots can also be observed. These spots may relate to the formation of iron oxides
with increasing potential which formed a protective film on the surface hindering chloride
ingress before pitting potential was reached. At the end of the first cycle of cyclic
voltammetry test, several pitting spots can be observed on the steel surface (Figure 4.29c),
though repassivation occurred during the reverse scan to some extent. Moreover, when the
CVA test was finished after two cycles, the area of pits on steel surface became larger and an
increasing number of pits could also be observed (Figure 4.29d). This observation suggests
that the repassivation was not successful and pitting corrosion continued to propagate.
Ep
65
Figure 4.29 Optical microscope pictures: a) after 48h passivation with 5×magnification; b)
passive region in CVA curves before reaching pitting potential with 5×magnification; c) at the
end of 1st cycle of CVA test with 10×magnification; d) at the end of 2
nd cycle of CVA test with
10×magnification.
4.3.3 Discussion of potentials
Normally, the difference of pitting potential and repassivation potential is taken as a relative
measurement of inhibition ability against localized corrosion [52]. With smaller difference
between Ep and Erp, greater tendency for pitting repassivation would be and better inhibiting
ability can be expected. Table 4.4 shows different potential values for the tested specimens.
From this table, it can be figured out that pitting potentials in test solutions have small
difference though a decreasing trend is exhibited, except for the ones at 0.1M NaCl in Gly
and in 6ACA solutions. In terms of repassivation potentials with the addition of NaCl, in all
the solutions they decreased and reached approximately -400 mV at critical chloride
concentration. The corrosion potentials show similar trend as pitting potentials, with the
values at around -1100 to -1000 mV, while the differences between corroded and protected
specimens become larger.
b) a)
d) c)
66
Table 4.4 Potentials for specimens in test solutions with different chloride concentrations
Pitting
potential Ep
(mV)
Repassivation
potential Erp (mV)
Corrosion
potential Ecorr
(mV)
NaOH: without NaCl 555 11.1 -1160
0.05 M NaCl 553 -284 -1143
0.1 M NaCl 540 -393 -1103
Gly: without NaCl 547 -78.9 -1130
0.05M NaCl 561 -190 -1124
0.1M NaCl 60.9 -401 -1026
6ACA: without NaCl 558 45.4 -1163
0.05M NaCl 566 -34.4 -1121
0.1 M NaCl 86.5 -388 -978
11AUA: without NaCl 560 -51.9 -1140
0.05 M NaCl 567 -43.8 -1152
0.1 M NaCl 564 -140 -1145
0.2 M NaCl 549 -395 -1088
pAB: without NaCl 558 61.3 -1141
0.05 M NaCl 523 -38.6 -1159
0.1 M NaCl 558 -160 -1147
0.2 M NaCl 546 -350 -1138
0.3 M NaCl 534 -393 -1126
NaNO2: without NaCl 618 -26.1 -992
0.05 M NaCl 550 128 -984
0.1 M NaCl 528 -53.6 -990
0.2 M NaCl 579 -215 -996
0.3 M NaCl 556 -217 -995
0.4 M NaCl 578 -419 -1000
To have a better understanding of the potential change, in Figure 4.30 and Figure 4.31 pitting
potential (Ep) and repassivation potential (Erp) of specimens in test solutions were plotted
versus chloride concentration, respectively. As can be seen from the figure, pitting potentials
fluctuate throughout the whole chloride concentration range showing a slightly decreasing
trend. Only the values in Gly and in 6ACA solutions at critical chloride concentration (0.1M)
are exceptional. This phenomenon could be attributed to the poor inhibitory ability of these
two inhibitors that 0.1M sodium chloride is beyond their real chloride corrosion thresholds.
As far as repassivation potential is concerned, below which the initiated pits start to
repassivate, the experimental data approach some linear trend lines. It is suggested that
repassivation potential depends on the extent to which previous pits are grown [52]. It can be
67
seen that although the slopes of trend lines for different inhibitors are not the same,
repassivation potentials tended to decrease with increased chloride concentration, indicating
that pits are more difficult to repassivate in high concentrations of chloride. In addition, there
is a tendency that the slope of the trend line is smaller (the lines are less steep) when the
inhibition effect is better.
Figure 4.30 Evolution of pitting potentials as a function of chloride concentrations
Figure 4.31 Evolution of repassivation potentials as a function of chloride concentrations
The differences between pitting potentials and repassivation potentials in different
experimental solutions are listed in Table 4.5. There is a clear trend that with higher chloride
concentration, the value becomes larger, suggesting steel specimen has higher risk to be
corroded and accordingly the inhibitory effect is weaker. The low values in Gly and in 6ACA
solutions at 0.1M NaCl are owing to their considerably low pitting potentials. This could be
related to their critical chloride thresholds which are actually lower than 0.1M NaCl.
68
Therefore, tests with some more precise concentrations between 0.05M and 0.1M or even
higher chloride concentrations than 0.1M may be needed to acquire a more accurate trend.
However, detailed investigation was not made in this paper because these two inhibitors were
not as effective as other candidates.
Table 4.5 Values of (Ep-Erp) for different solution as a function of chloride concentration
Cl- (mol L
-1)
Ep– Erp (mV)
0 0.05 0.1 0.2 0.3 0.4
NaOH 544 837 934 - - -
Gly 627 751 462 - - -
6ACA 513 601 475 - - -
11AUA 612 611 704 944 - -
pAB 498 563 718 896 928 -
NaNO2 644 422 474 794 773 997
The results indicate that the inhibiting efficiency of proposed amino acids is owing to their
adsorption on steel surface blocking aggressive ions which are primarily chloride ions in this
experiment. The inhibitory efficiency is dependent on the number of adsorption sites [54]
which is only one (the N atom in amine group) in all the amino acids that were tested. The
chemical adsorption of inhibitor molecule reinforces the adhesive strength with steel surface
[19], therefore the inhibited specimens have better performance than the uninhibited NaOH
specimens. In terms of functional groups, amino acids as inhibitors were suggested to have
both film-forming and pore-blocking effects [55]. The film-forming can be explained by the
adsorption of amine group on metals and oxides owing to an unshared electron pair of the
nitrogen atom. Meanwhile the ionized carboxyl group in alkaline solution is responsible for
pore-blocking effects. In addition, carboxyl groups (RCOO-) were also reported to have
strong chemical bond formation properties (competing successfully with Cl- as a complexing
agent for iron) [56]. Moreover, the molecular size influences the effect of inhibition as
previously reported by El-Shafei et al [54] that with higher molecular size, a better inhibitor
could be expected. This statement is in agreement with the consequence that Gly had the
lowest inhibiting effect while 11AUA performed much better. However, the reason why
6ACA exhibited nearly the same inhibition ability as Gly is still unknown, though its
molecular size is much larger than that of Gly and it has longer carbon chain length. Aspects
such as charge density and mode of interaction with the metal surface may also have impact
on the inhibition effect [19, 54]. There is another saying that amino acids as well as other
organic corrosion inhibitors are chelating agents, which can form chelate rings as a result of
the bonding between two or more functional groups from the inhibitor (–NH2 and –COOH)
and the iron cation [55]. Therefore the whole steel surface, both anodic sites and cathodic
sites, can be covered leading to mixed inhibition.
69
5. Conclusion and Recommendation
5.1 Conclusion
The four amino acids we studied in this paper showed inhibitory effect on steel in alkaline
solution against chloride attack. Among them, pAB exhibited the best inhibiting ability at
chloride concentrations lower than 0.3M, probably owing to the spatial effect of its benzene
ring. The chloride corrosion threshold of 11AUA was 0.2M NaCl, more effective than the rest
two amino acids which may be related to the spatial effect of its long carbon chain and larger
molecular size. Despite the longer carbon chain length of 6ACA than that of Gly, they can
similarly inhibit chloride-induced corrosion to some extent. However, in both solutions the
steel electrodes corroded at 0.1M NaCl, which was not satisfactory. In general, the effect on
retardation of chloride corrosion threshold of these four amino acids was not as good as that
of sodium nitrite, which showed excellent inhibiting effect at concentrations below 0.4M
NaCl.
At concentrations lower than chloride corrosion threshold, the total inhibition effectiveness of
four amino acids and sodium nitrite was good, while only at critical chloride concentrations of
each inhibitor that the performance was not satisfactory. The corrosion rate and inhibition
efficiency were more than good for all the inhibitors in inhibited solutions. The equivalent
circuit parameters changed as a function of chloride concentration, but the total impedance
values differed slightly at chloride concentrations lower than threshold.
Cyclic voltammograms indicated that pitting potentials for specimens in the five inhibitor
solutions had slight difference at different chloride concentrations. However, repassivation
potentials were dependent on the chloride concentrations and generally became more negative
with higher chloride concentration. The difference between Ep and Erp was therefore larger
with increasing chloride concentration, reaching around 950 mV at the thresholds. There was
one exception in the case of 6ACA and Gly solutions that the steel electrodes exhibited
extraordinary low pitting potentials at 0.1M NaCl. This exception resulted in even smaller
difference between Ep and Erp at 0.1M NaCl than that without sodium chloride.
Optical microscopy photos confirmed the existence of pitting spots and were in concordance
with the results obtained from electrochemical methods. When the CVA measurements were
done, the surface color changed from the original color to a brownish color and evident
pitting spots could be observed.
5.2 Recommendation
Although critical chloride corrosion threshold was discussed in this thesis, some more
detailed chloride concentrations below threshold were not studied during the experiments. As
70
a consequence, the results in section 4.3.3 showing the pitting potentials in terms of 6ACA
and Gly were fairly abnormal. Moreover, the influence of different concentrations of the
proposed inhibitors can also be studied in future research.
In addition, for optical microscopy it is very hard to observe and verify the existence of the
passive film and protective oxide layer. Therefore, some morphology-detecting techniques
with higher magnification such as SEM or TEM could be introduced for further research.
Also, XRD may be applied to analyze the composition of the protective layer so that the
results obtained from CVA would be better confirmed.
71
Acknowledgement
I would like to thank Arjan Mol as my supervisor who gave me such a good opportunity to do
my graduation project in this fantastic group. I also have much gratitude to my first line
supervisor—Zhengxian Yang, who helped me with my experiments and thesis all the time.
Without his scrupulous instruction, I would not finish all the work so well. Moreover, I would
like to express my gratitude to Rob Polder for his concern and the knowledge he taught at the
first stage of my project. Besides, Min and I supported each other and encouraged mutually
when we ran into difficulties. I am also grateful to Agnieszka, Sander, Sina and Yaiza for their
sincere help during my experiment in the lab. Last but not least, I would express my gratitude
to all my friends and colleagues for the joy and pain that they brought to me during the last
two years.
72
Reference
[1] Jamil, H., Montemor, M. F., Boulif, R., Shriri, A., & Ferreira, M. G. S. (2003). An
electrochemical and analytical approach to the inhibition mechanism of an
amino-alcohol-based corrosion inhibitor for reinforced concrete. Electrochimica
acta, 48(23), 3509-3518.
[2] Tuutti, K. (1982). Corrosion of Steel in Concrete, Swedish Cement and Concrete Research
Institute, Stockholm, Sweden.
[3] Jamil, H. E., Shriri, A., Boulif, R., Bastos, C., Montemor, M. F., & Ferreira, M. G. S.
(2004). Electrochemical behaviour of amino alcohol-based inhibitors used to control
corrosion of reinforcing steel. Electrochimica acta, 49(17), 2753-2760.
[4] Page, C. L., Ngala, V. T., & Page, M. M. (2000). Corrosion inhibitors in concrete repair
systems. Magazine of Concrete Research, 52(1), 25-37.
[5] Ngala, V. T., Page, C. L., & Page, M. M. (2002). Corrosion inhibitor systems for remedial
treatment of reinforced concrete. Part 1: calcium nitrite. Corrosion Science, 44(9),
2073-2087.
[6] Ormellese, M., Berra, M., Bolzoni, F., &Pastore, T. (2006). Corrosion inhibitors for
chlorides induced corrosion in reinforced concrete structures. Cement and concrete
research, 36(3), 536-547.
[7] Gaidis, J. M. (2004). Chemistry of corrosion inhibitors. Cement and Concrete
Composites, 26(3), 181-189.
[8] Ormellese, M., Lazzari, L., Goidanich, S., Fumagalli, G., & Brenna, A. (2009). A study of
organic substances as inhibitors for chloride-induced corrosion in concrete. Corrosion
Science, 51(12), 2959-2968.
[9] Ashassi-Sorkhabi, H., Majidi, M. R., &Seyyedi, K. (2004). Investigation of inhibition
effect of some amino acids against steel corrosion in HCl solution. Applied surface
science, 225(1), 176-185.
[10] Li, L., &Sagüés, A. A. (2001). Chloride corrosion threshold of reinforcing steel in
alkaline solutions-Open-circuit immersion tests. Corrosion, 57(1), 19-28.
[11] Hu, J., Koleva, D. A., De Wit, J. H. W., Kolev, H., & Van Breugel, K. (2011). Corrosion
performance of carbon steel in simulated pore solution in the presence of
micelles. Journal of the Electrochemical Society, 158(3), C76-C87.
[12] Berke, N. S., & Hicks, M. C. (2004). Predicting long-term durability of steel reinforced
concrete with calcium nitrite corrosion inhibitor. Cement and Concrete
Composites, 26(3), 191-198.
[13] Yang, Z., Fischer, H., Cerezo, J., Mol, J. M. C., & Polder, R. (2013). Amino benzoate
modified MgAl hydrotalcites as a novel smart additive of reinforced concrete for
anticorrosion applications. Construction and Building Materials, 47, 1436-1443.
[14] Al-Amoudi, O. S. B., Maslehuddin, M., Lashari, A. N., &Almusallam, A. A. (2003).
Effectiveness of corrosion inhibitors in contaminated concrete. Cement and Concrete
Composites, 25(4), 439-449.
[15] Nmai, C. K. (2004). Multi-functional organic corrosion inhibitor. cement and Concrete
Composites, 26(3), 199-207.
[16] Barouni, K., Bazzi, L., Salghi, R., Mihit, M., Hammouti, B., Albourine, A., & El Issami,
73
S. (2008). Some amino acids as corrosion inhibitors for copper in nitric acid
solution. Materials Letters, 62(19), 3325-3327.
[17] Eddy, N. O. (2011). Experimental and theoretical studies on some amino acids and their
potential activity as inhibitors for the corrosion of mild steel, part 2.Journal of Advanced
Research, 2(1), 35-47.
[18] Li, L., &Sagues, A. A. (2002). Chloride corrosion threshold of reinforcing steel in
alkaline solutions-cyclic polarization behavior. Corrosion, 58(4), 305-316.
[19] Amin, M. A., Khaled, K. F., Mohsen, Q., &Arida, H. A. (2010). A study of the inhibition
of iron corrosion in HCl solutions by some amino acids. Corrosion Science, 52(5),
1684-1695.
[20] European Patent Appl. No. 8630438.2, Publ. No. 0 209 978, published 28.01.87 Bull.
87/5.
[21] Zheng, H., Li, W., Ma, F., & Kong, Q. (2012). The effect of a surface-applied corrosion
inhibitor on the durability of concrete. Construction and Building Materials, 37, 36-40.
[22] Monticelli, C., Frignani, A., &Trabanelli, G. (2000). A study on corrosion inhibitors for
concrete application. Cement and concrete research, 30(4), 635-642.
[23] B.A. Miksic, C. Chandler, M. Kharshan, A. Furman, B. Rudman, L. Gelner, Corrosion
inhibitor for use in reinforced concrete structures, U.S. US 5,597,514 28 Jan 1997, Appl.
377,761, 24 Jan 1995.
[24] K.K. Sagoe-Crentsil, V.T. Yilmaz, F.P. Glasser, Corrosion inhibition of steel in concrete
by carboxylic acids, CemConcr Res 23 (1993) 1380± 1388.
[25] A. Norihico, O. Yoshinori, Study of corrosion inhibitors for ecocement, Chichibu Onoda
KenkyuHokoku 48 (1) (1997) 42.
[26] Bobina, M., Kellenberger, A., Millet, J. P., Muntean, C., &Vaszilcsin, N. (2013).
Corrosion resistance of carbon steel in weak acid solutions in the presence of L-histidine
as corrosion inhibitor. Corrosion Science, 69, 389-395.
[27] Kern, P., &Landolt, D. (2001). Adsorption of organic corrosion inhibitors on iron in the
active and passive state. A replacement reaction between inhibitor and water studied with
the rotating quartz crystal microbalance. Electrochimica Acta, 47(4), 589-598.
[28] Broomfield, J. P. (2006). Corrosion of steel in concrete: understanding, investigation and
repair. CRC Press.
[29] C. K. Nmaiand and D. McDonald, in Proceedings International Symposium, Monterrey,
Mexico, March 1999, RILEM, p. 104 _1999_.
[30] Morris, W., & Vazquez, M. (2002). A migrating corrosion inhibitor evaluated in concrete
containing various contents of admixed chlorides. Cement and Concrete Research, 32(2),
259-267.
[31] Elsener, B., Buchler, M., Stalder, F., &Bohni, H. (2000). Migrating corrosion inhibitor
blend for reinforced concrete: Part 2-Inhibitor as repair strategy. Corrosion, 56(7),
727-732.
[32] U. Maeder, in: R.N. Swamy (Ed.), Corrosion and Corrosion Protection of Steel in
Concrete, Sheffield, UK, 1994, p. 851.
[33] Rosenberg, A. (2000). Discussion: Migrating Corrosion Inhibitor Blend for Reinforcing
Concrete: Part 1-Prevention of Corrosion. Corrosion, 56(10), 986-987.
[34] Saremi, M., &Mahallati, E. (2002). A study on chloride-induced depassivation of mild
74
steel in simulated concrete pore solution. Cement and Concrete Research, 32(12),
1915-1921.
[35] Zhao, H., Zhang, X., Ji, L., Hu, H., & Li, Q. (2014). Quantitative structure–activity
relationship model for amino acids as corrosion inhibitors based on the support vector
machine and molecular design. Corrosion Science, 83, 261-271.
[36] Lasia, Andrzej. "Electrochemical impedance spectroscopy and its applications." Modern
aspects of electrochemistry. Springer US, 2002. 143-248.
[37] Mansfeld, F. (1993). Analysis and interpretation of EIS data for metals and alloys.
Schlumberger Instruments.
[38] Poupard, O., Aı̈t-Mokhtar, A., &Dumargue, P. (2004). Corrosion by chlorides in
reinforced concrete: Determination of chloride concentration threshold by impedance
spectroscopy. Cement and Concrete Research, 34(6), 991-1000.
[39] Heinze, J. (1984). Cyclic Voltammetry—―Electrochemical Spectroscopy‖. New
Analytical Methods (25). AngewandteChemie International Edition in English,23(11),
831-847.
[40] Gosser, D. K. (1993). Cyclic voltammetry: simulation and analysis of reaction
mechanisms (p. 43). New York: VCH.
[41] Gerretsen, J. H., De Wit, J. H. W., &Riviere, J. C. (1990). Passivity and breakdown of
passivity of iron-chromium alloys studied with cyclic voltammetry, ellipsometry and
XPS. Corrosion science, 31, 545-550.
[42] Cabrini, M., Lorenzi, S., &Pastore, T. (2014). Cyclic voltammetry evaluation of
inhibitors for localised corrosion in alkaline solutions. Electrochimica Acta, 124,
156-164.
[43] Królikowski, A., &Kuziak, J. (2011). Impedance study on calcium nitrite as a penetrating
corrosion inhibitor for steel in concrete. Electrochimica acta, 56(23), 7845-7853.
[44] Koleva, D. A., Boshkov, N., Van Breugel, K., & De Wit, J. H. W. (2011). Steel corrosion
resistance in model solutions, containing waste materials. Electrochimica Acta, 58,
628-646.
[45] Jamil, H. E., Shriri, A., Boulif, R., Montemor, M. F., & Ferreira, M. G. S. (2005).
Corrosion behaviour of reinforcing steel exposed to an amino alcohol based corrosion
inhibitor. Cement and Concrete Composites, 27(6), 671-678.
[46] Andrade, C., Keddam, M., Nóvoa, X. R., Pérez, M. C., Rangel, C. M., &Takenouti, H.
(2001). Electrochemical behaviour of steel rebars in concrete: influence of
environmental factors and cement chemistry. Electrochimica Acta, 46(24), 3905-3912.
[47] Valek, L., Martinez, S., Mikulić, D., &Brnardić, I. (2008). The inhibition activity of
ascorbic acid towards corrosion of steel in alkaline media containing chloride
ions. Corrosion Science, 50(9), 2705-2709.
[48] Lin, C. H., & Duh, J. G. (2009). Electrochemical impedance spectroscopy (EIS) study on
corrosion performance of CrAlSiN coated steels in 3.5 wt.% NaCl solution. Surface and
Coatings Technology, 204(6), 784-787.
[49] Monticelli, C., Frignani, A., &Trabanelli, G. (2002). Corrosion inhibition of steel in
chloride-containing alkaline solutions. Journal of applied electrochemistry, 32(5),
527-535.
[50] Scully, J. R. (2000). Polarization resistance method for determination of instantaneous
75
corrosion rates. Corrosion, 56(2), 199-218.
[51] Deyab, M. A., & Keera, S. T. (2012). Cyclic voltammetric studies of carbon steel
corrosion in chloride-formation water solution and effect of some inorganic
salts. Egyptian Journal of Petroleum, 21(1), 31-36.
[52] Abd El-Haleem, S. M., &Abd El-Wanees, S. (2011). Chloride induced pitting corrosion
of nickel in alkaline solutions and its inhibition by organic amines. Materials Chemistry
and Physics, 128(3), 418-426.
[53] Pastore, T., Cabrini, M., Coppola, L., Lorenzi, S., Marcassoli, P., &Buoso, A. (2011).
Evaluation of the corrosion inhibition of salts of organic acids in alkaline solutions and
chloride contaminated concrete. Materials and Corrosion, 62(2), 187-195.
[54] El-Shafei, A. A., Moussa, M. N. H., & El-Far, A. A. (1997). Inhibitory effect of amino
acids on Al pitting corrosion in 0.1 M NaCl. Journal of applied electrochemistry, 27(9),
1075-1078.
[55] Söylev, T. A., & Richardson, M. G. (2008). Corrosion inhibitors for steel in concrete:
State-of-the-art report. Construction and Building Materials, 22(4), 609-622.
[56] Trabanelli, G., Monticelli, C., Grassi, V., &Frignani, A. (2005). Electrochemical study on
inhibitors of rebar corrosion in carbonated concrete. Cement and Concrete
Research, 35(9), 1804-1813.