The enhancing power of iodide on corrosion prevention of mild steel in the presence of a synthetic-soluble Schiff-base: Electrochemical and surface analyses

Article abstract of DOI:10.1016/j.electacta.2010.05.066

The inhibitory action of N,N′-1,3-propylen-bis(3- methoxysalicylidenimine) {PMSI} on mild steel corrosion in sulfuric acid medium was investigated through electrochemical methods and evaluations based on infrared spectroscopy and scanning electron microgr

Full text of DOI:10.1016/j.electacta.2010.05.066

Electrochimica Acta 55 (2010) 6058–6063
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Electrochimica Acta
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The enhancing power of iodide on corrosion prevention of mild steel in the
presence of a synthetic-soluble Schiff-base: Electrochemical and surface analyses
Mohsen Lashgaria,∗, Mohammad-Reza Arshadib, Somaieh Miandaria
a Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Gava-Zang Blvd., Zanjan 45137-66731, Iran
b Department of Chemistry, Sharif University of Technology, PO Box 11365-9516, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
The inhibitory action of N,Nꢀ-1,3-propylen-bis(3-methoxysalicylidenimine) {PMSI} on mild steel corro-
sion in sulfuric acid medium was investigated through electrochemical methods and evaluations based
on infrared spectroscopy and scanning electron micrographs. The studies revealed that the molecule
is a good mixed-type inhibitor (mostly anodic), acts as a multi-dentate ligand and repels the corro-
sive agents by hydrophobic forces. Its adsorption on metal surface has a physicochemical nature and
obeys the Langmuir isotherm. At a critical (optimum) concentration, an anomalous inhibitory behavior
was observed and interpreted at microscopic level by means of desorption/adsorption process and hor-
izontal ↔ vertical hypothesis. The addition of iodide into acid moreover causes a synergistic influence,
a substantial enhancement on inhibitory performance. Finally, using isolated inhibitor calculations at
B3LYP/6-31G + (d,p) level of theory, the equilibrium geometry of PMSI was determined and the energy
required for hindrance avoidance was predicted.
Article history:
Received 14 February 2010
Received in revised form 19 May 2010
Accepted 20 May 2010
Available online 27 May 2010
Keywords:
Mild steel corrosion
Schiff-base ligand
Desorption–adsorption process
Inhibitors in vacuo
Inhibition mechanism
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
high because of various adsorbing centers available in the molecule.
The efficiency of the molecule can further be enhanced through
Due to its high mechanical properties and low cost [1], mild
steel has a wide application in various industries as construction
material for chemical reactors, heat exchange and boiler systems,
storage tanks, and oil and gas transport pipelines.
a synergistic effect, resulting from specific adsorption of a non-
corrosive anion like as iodide which can be bridged between the
inhibitor cation and the positively charged surface in acid solutions
[13,14].
Protection of the equipments and vessels against corrosion is
one of chief concerns of the maintenance and design engineers. The
use of corrosion inhibitors, whenever possible, is one of the most
effective methods for this concern [2]. The materials used as cor-
rosion inhibitors are chemicals, organic or inorganic, which when
added in small quantities to the corrosive environment, reduce the
deterioration of metals. The mechanism of this protection depends
on the metal and inhibitor under consideration. In most cases, how-
ever, it is due to the adsorption of these compounds on the surface
of metals which can reduce the extent of corrosion through ther-
modynamics or kinetics phenomena [3].
Schiff-bases are among azomethine compounds [4,5] that have
recently attracted more attentions in the field of corrosion inhi-
bition science [6–11]. The reason is mainly due to the ease of
synthesis from relatively inexpensive starting-materials and their
eco-friendly properties, harmless for environment [12]. In addition,
the inhibitory performance of these compounds is predicted to be
Besides the factors mentioned above, an industrially suitable
inhibitor should be soluble in aqueous environments, e.g. in acid
washing systems. The molecule synthesized and examined here
(PMSI; Fig. 1), fortunately, has this property. The objective of this
work is to provide a mechanistic understanding of the phenomenon
of mild steel corrosion in sulfuric acid medium in the presence of
PMSI inhibitor. We moreover demonstrate the power of iodide (in
small quantity; 10−3 M), on corrosion prevention of mild steel in
the presence of the synthetic Schiff-base.
2. Experimental
2.1. Materials and solutions
A
2:1 molar ratio mixture of 2-hydroxyl-3-methoxy-
benzaldehyde and 1,3-propandiamine was dissolved in ethanol,
heated and refluxed for about an hour. The condensate was
then cooled and filtered. The filtrate was characterized by NMR
(nuclear magnetic resonance), FT-IR (Fourier transform infrared)
and UV–vis (ultraviolet–visible) spectra. All chemicals used were
analytical grade reagents obtained from Merck Chemical Com-
∗
Corresponding author. Tel.: +98 241 4153205; fax: +98 241 4153232.
E-mail address: Lashgari@iasbs.ac.ir (M. Lashgari).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.05.066

M. Lashgari et al. / Electrochimica Acta 55 (2010) 6058–6063
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Fig. 1. Structural formula of N,Nꢀ-1,3-propylen-bis(3-methoxysalicylidenimine)
{PMSI} Schiff-base.
pany. The steel specimen with the following composition (wt.%;
determined by quantometric analysis): 0.021% Cr, 0.0033% Ni,
0.035% Al, 0.130% C, 0.546% Mn and the remainder iron, was used.
The aggressive solution was made by AnalaR grade sulfuric acid
and double-distilled water. The concentration of acid solution was
0.5 M and those of inhibitor were Cinhib ≤ 0.01 M. For the synergistic
experiments, KI concentration was 0.001 M.
Fig. 2. Polarization curves for mild steel corrosion in 0.5 M H2SO4 solution in the
presence of PMSI at various inhibitor concentrations.
2.2. Electrochemical studies
studies. They were immersed in the respective corrosive solution
for 24 h [17]. Then the specimens were washed with distilled water
and ethanol, dried and stored in a moisture-free desiccator. The
surface morphology was studied ex situ by a SEM VEGA© TESCAN
microscope. Infrared spectra of the inhibitor film on metal sur-
face were also obtained by a Thermo Nicolet NEXUSTM 670 FT-IR
spectrometer.
The mild steel embedded in epoxy resin with exposed area of
0.5 cm2 was used as working electrode. Before each experiment, the
working surface was ground with successive SiC abrasive papers up
to 1200 grit, degreased by acetone and washed with distilled water
and ethanol then dried in air [15].
The auxiliary electrode was a platinum foil and the reference
was Ag/AgCl (3 M KCl) electrode coupled to a Luggin capillary with
the tip placed approximately 1 mm from the working surface. The
reaction vessel was a three-electrode chamber connected to an
AUTOLAB/PGSTAT30 instrument. The experimental data of Tafel
polarization curves and electrochemical impedance spectra (EIS)
were processed by GPES and FRA (4.9) softwares, respectively. In
polarization measurement, the working electrode was immersed in
the test solution at open circuit potential (OCP) for the time needed
to achieve a steady state potential. Then the potential of working
electrode was scanned at a rate of 1 mV/s, from cathodic to anodic
direction.
2.4. Isolated inhibitor calculations
Without going to the details of computational strategies and
quantum electrochemical models developed recently in the field
of corrosion inhibition science [18–20], here, we simply carried out
some quantum chemical calculations on the inhibitor molecule in
vacuo using density functional theory (DFT; B3LYP version [21])
and applying a relatively large standard 6-31G + (d,p) basis set. The
calculations were performed on a PC (3 GHz) via Gaussian 03 suit of
computer codes [22].
The percent inhibition efficiency, IE (%), of PMSI was calculated
from the following equation [16]:
3. Results and discussion
ꢀ
ꢁ
icꢀ orr
icorr
IE (%) = 1 −
× 100
(1)
3.1. Electrochemical measurements
where icorr and icꢀ orr are corrosion current densities measured in the
absence and presence of the inhibitor, respectively.
The polarization curves of mild steel in sulfuric acid medium,
in the presence and absence of the inhibitor compound, are pre-
sented in Fig. 2 and quantified in Table 1. As it can be seen here, the
presence of molecule in acid solution affects on both polarization
branches, which causes the corrosion potential (Ecorr) being more
positive at all inhibitor concentrations as compared with that of
Impedance measurements were carried out at OCP. After
immersion of the specimen in the test solution, prior to each mea-
surement, a stabilization period of 15 min was observed which
proved sufficient to attain a stable value. A sinusoidal potential with
amplitude of 10 mV (rms), superimposed on OCP, was applied. The
frequency of this ac signal was scanned from 100 kHz to 10 mHz. All
EIS measurements were performed at room temperature. The inhi-
bition efficiency for this method was calculated from this equation
[16]:
blank solution. This means that the molecule under consideration
inhib
acts as a mixed-type inhibitor (mostly anodic; E
> Ecbolarnr k).
corr
Data presented in Table 1 also indicate a decrease occurring
in corrosion current density (icorr) as the inhibitor concentration
increases. This trend continues up to an optimum concentration
of 0.005 M. Beyond that the favorable effect will turn suddenly to
an unfavorable one. The distinguished behavior witnessed for this
critical concentration is evidently reconfirmed by impedance mea-
surements as depicted in Fig. 3; at this concentration, the diameter
of semi-circle becomes surprisingly large. To justify this anoma-
lous behavior, it should be noticed that by increasing the inhibitor
concentration, the molecules are generally reoriented from hori-
zontal to vertical mode of adsorption due to area releasing effect
and ensemble energy minimization [23]; the horizontal is predom-
inant at low surface coverages but it changes to the vertical as
the concentration increases. At the critical-optimum concentration,
ꢂ
ꢃ
Rct − Rcot
IE (%) =
× 100
(2)
Rct
where Rcot and Rct are charge transfer resistance in the absence
and presence of the inhibitor, respectively. All experiments were
performed twice and the mean results were reported.
2.3. Surface analysis
The mild steel specimens used here (10 mm × 10 mm) were pre-
pared in the same manner explained above for electrochemical

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Table 1
Potentio-dynamic results of the mild steel corrosion in 0.5 M H2SO4 medium (purged by nitrogen) containing PMSI inhibitor.
obs a
calc b
corr
CPMSI (M)
icorr (␮A/cm2)
−ˇc (mV/dec)
ˇa (mV/dec)
Rp (ꢀ cm2)
−E
(mV Ag/AgCl,3M
)
−E
(mVAg/AgCl,3M
)
IE (%)
corr
Blank
298.80
99.40
80.90
78.13
70.31
54.35
60.81
68
28
32
35
25
27
24
174
182
152
153
169
164
172
28.65
39.34
46.00
53.87
46.09
64.05
53.12
483
453
463
460
445
445
433
488
441
445
441
432
433
426
—
5 × 10−5
66.73
72.93
73.85
76.47
81.81
79.65
10−4
5 × 10−4
10−3
5 × 10−3
10−2
a
Breaking point of the polarization curves where the current direction is reversed.
Calculated from extrapolation of Tafel (anodic and cathodic) lines.
b
Fig. 3. Impedance plots of mild steel in 0.5 M H2SO4 in the presence of PMSI inhibitor
at various concentrations.
where both forms (horizontal and vertical) are counterbalance,
an infinitesimal decreasing in surface coverage causes the verti-
cal mode of adsorption to be completely reverted. The decrease
is taken place instantaneously through accumulative desorption
of the inhibitor molecules adsorbed vertically. The possibility of
accumulative desorption resulting from lateral interactions [24],
moreover provides a reason why the inhibitor efficiency does not
increase above the optimum.
At the balance (optimum), the instantaneous desorption pro-
cess mentioned above is dynamically reversible and synchronized
with the inhibitor adsorption on unoccupied (previously released)
sites. This simultaneous desorption/adsorption process seems to
be responsible for anomalous behavior of the inhibitor molecules
at the critical (optimum) concentration. By accepting this hypothe-
sis, i.e. the synchronized desorption ↔ adsorption process, we have
therefore to observe an inductive loop in impedance spectra at low
frequencies [3], as seen in Fig. 3. This indirectly justifies why the
interesting-inductive loop does not appear below the optimum;
explanatorily, there is no inductive loop since there is no alterna-
tion on the mode of adsorption (the parallel exists only and no
desorption is taken place!).
Fig. 4. Langmuir isotherm for the adsorption of PMSI on mild steel surface in 0.5 M
H2SO4, determined from polarization data.
Kads was found to be 5000 and the corresponding standard free
energy, ꢂGaods was calculated to be −36.8 kJ/mol. The latter is close
to −40 kJ/mol and indicates a relatively strong inhibitor adsorption
[27,28].
The effect of iodide ions on the corrosion rate and inhibition
efficiencies was also studied through impedance measurements.
The results are summarized in Table 2 and the impedance plots are
included in Fig. 5. Concerning this part of study, there are several
points worthy of considerations:
1. Addition of 10−3 M potassium iodide alone to the acid solution
increased the charge transfer resistance, Rct, considerably and
resulted in a percent inhibition efficiency of 64%. This indicates
By applying the following relation [16], the surface coverage (ꢁ)
was calculated.
(icorr − icꢀ orr
)
ꢁ =
(3)
icorr
To determine the adsorption behavior of the molecule under con-
sideration, we applied various linearized isotherms [25,26]. The
best correlation was obtained through Langmuir model with the
regression coefficient, R2 equal to 0.9998 (see Fig. 4). Using the
Langmuir relation, Eq. (4), the value of adsorption equilibrium con-
stant (Kads) was calculated.
Cinhib
ꢁ
1
Kads
Fig. 5. Impedance plots of mild steel in 0.5 M H2SO4 in the presence of iodide
= Cinhib
+
(4)
(10−3M) and PMSI at various inhibitor concentrations.

M. Lashgari et al. / Electrochimica Acta 55 (2010) 6058–6063
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Table 2
Electrochemical impedance parameters for mild steel corrosion in 0.5 M H2SO4 medium (purged by nitrogen) in the presence of PMSI inhibitor.
CPMSI (M)
−Eocp (mVAg/AgCl,
)
Rs (ꢀ cm2)
Rct (ꢀ cm2)
CPE (␮F cm−2
)
n
IE (%)
S
3M
466a
1.078
0.923
36.07
100.89
69.20
73.20
0.889
0.855
—
64.24
—
Blank
472b
470a
446b
0.267
2.082
64.00
396.00
59.75
26.97
0.905
0.857
43.64
90.89
10−4
10−3
2.21
3.14
10.23
457a
448b
1.076
1.925
112.20
988.00
81.05
30.50
0.866
0.830
67.84
96.34
452a
404b
1.447
0.269
115.70
3300.00
68.72
15.52
0.855
0.921
68.82
98.91
10−2
a
Determined in the absence of iodide ions (10−3 M).
Determined in the presence of iodide ions (10−3 M).
b
that the iodide ions can be considered as an inhibitor for mild
steel/H2SO4 system.
parameters. The values of CPE as well as the surface roughness,
n, are given in Table 2.
2. The addition of iodide into the inhibitor-acid solution, improved
the inhibition efficiencies considerably, over 98% for 10−2 M
PMSI.
Included in Table 2 are also the values of S, synergism parameter
calculated by this relation [13,28]:
3. A non-ideal capacitive behavior is clearly observed in Fig. 5, as
occurrence of a depressed semi-circle. This behavior is normally
observed in most EIS studies of the corrosion inhibitor systems
and it is attributed to the surface roughness of the metal. For this
reason, the double layer capacitance, Cdl, has been replaced with
the constant phase element, CPE. The latter quantity is calculated
by the following equation as explained elsewhere [16]:
1 − ÁI − ÁPMSI + ÁIÁPMSI
S =
(6)
1 − ÁI.PMSI
where ÁI and ÁPMSI are the inhibition efficiencies of iodide and PMSI,
respectively, and ÁI.PMSI is that of mixture. From the values of S (all
over one) we can conclude that the synergism occurred in all the
trials and no antagonism was observed [13,28].
Yωn−1
CPE
=
(5)
3.2. Surface analysis studies
sin(nꢃ/2)
where ω is the angular frequency for which the imaginary part of
impedance is maximum. Y and n are two frequency independent
Fig. 6 represents the SEM images of mild steel samples under
various conditions studied in this work. Fig. 6(A) is the image of
Fig. 6. SEM micrographs of mild steel before (A) and after immersion in 0.5 M H2SO4 for 24 h, in the absence (B) and presence (C) of 0.01 M PMSI, containing (D) 0.001 M
potassium iodide.

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M. Lashgari et al. / Electrochimica Acta 55 (2010) 6058–6063
Fig. 8. The equilibrium structure of the Schiff-base, optimized in vacuo at B3LYP/6-
31 + G(d,p) level of theory.
Fig. 7. FT-IR spectra of PMSI inhibitor: (A) in KBr matrix and (B) on metal surface,
measured after 24 h immersion of the specimen in the acid solution containing
0.01 M PMSI and 0.001 M KI.
Fig. 9. Energy variation with methyl rotation around C-O bond, calculated via
B3LYP/6-31G + (d,p) method.
newly polished sample before placing in acid solution. Fig. 6(B)
shows the same sample after immersing in the blank solution for
24 h. The appearance of corrosion products is clearly observed.
Fig. 6(C) was taken from a sample in acid solution containing
PMSI alone and finally Fig. 6(D) represents the metal sample after
immersing in solution containing both PMSI (10−2 M) and potas-
sium iodide (10−3 M). The resemblance of the Fig. 6(D) to that of
(A) is appreciable and proves the excellent corrosion prevention of
PMSI and iodide ions combined. For this combination, moreover, a
study of reflective FT-IR was performed on the adsorbed inhibitor
film. The results are shown in Fig. 7.
Fig. 7(A) was taken from PMSI in KBr matrix and Fig. 7(B) from
the film on metal surface after immersion of the sample in the acid
solution containing 0.01 M PMSI and 0.001 M KI. Table 3 compares
the IR frequencies, between the transmissive and reflective spectra.
From Fig. 7 and Table 3, we observe a broadening effect and shift in
the frequencies of the adsorbing functional groups. Data tabulated
here also indicate that there is no interaction between the aliphatic
moieties and metal surface.
thermodynamically stable and deforms to a handle shape as shown
in Fig. 8. The shrinkage of the aliphatic chain permits both nitrogen
atoms coming close and the distance between them decreases to
the value of 2.88 Å. Furthermore, the figure represents a non-zero
dihedral angle between methoxy and aromatic ring; the torsion
angle is about −64◦. On the basis of geometrical data depicted here,
one may conclude that the methyl group has a hindrance effect on
inhibitor adsorption through aromatic rings and heteroatom cen-
ters, while the empirical data do not support this conclusion. This is
possibly because of hindrance avoidance occurring through inter-
nal rotation of the methyl around C–O bond. Fig. 9 represents the
energy changes during this rotation. The figure also illustrates two
more stable minima. The first is a global minimum, standing for
the structure optimized without any restriction. While, the sec-
ond is slightly unstable (+1.387 kcal/mol) and corresponds with a
hindering-free geometry with dihedral angle of +66◦ at which the
methyl group is being upwards. The destabilization energy seems
to be provided by molecules thermal motions with heat releasing
through strong metal/inhibitor interactions. For this transformed
structure, i.e. the geometry favored for adsorption through aro-
matic rings and heteroatom centers, the lack of interaction between
aliphatic moieties and metal surface is also approved by experi-
mental data; no shift observed in IR frequencies of the aliphatic
moieties (see Table 3). It alternatively indicates that during the
3.3. Theoretical calculations and inhibition mechanism
Fig. 8 illustrates the equilibrium geometry of PMSI optimized
in vacuo. This figure demonstrates that the zigzag shape of the
propylene chain sketching within structural formula (Fig. 1) is not
Table 3
FT-IR results of PMSI inhibitor; see Fig. 7.
Reflective FT-IR frequencies (cm−1
)
FT-IR frequencies (cm−1
)
Functional groups
3363 (broad)
2930, 2857
1620
1250–1520 (broad)
1250–1520 (broad)
3435
2930, 2854
1630
1469
1262
OH of phenol
C–H (aliphatic and aromatic)
C
C
N
C
C–O

M. Lashgari et al. / Electrochimica Acta 55 (2010) 6058–6063
6063
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probably pushed away. This orientation of the aliphatic groups
deems to have a positive effect on the inhibitory action by repelling
the aqueous corrosive agents by hydrophobic forces [29].
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1. The aqueous soluble Schiff-base investigated here (PMSI) is
among easily synthesized eco-friendly compounds, behaves as
a mixed-type (mostly anodic) inhibitor for mild steel corrosion
in sulfuric acid medium. Its inhibitory action enhances with
increasing the inhibitor concentration up to an optimum value of
0.005 M. At this concentration, a dynamic desorption/adsorption
process is responsible for distinct behavior of the molecule at
metal/solution interface.
2. An excellent inhibition power is witnessed for the mixture of
PMSI and iodide ions, while the iodide by itself shows a relatively
good inhibitive performance.
3. PMSI adsorbs on metal surface via aromatic rings, imine groups
(C N) and oxygen centers whereas the aliphatic moieties are
pushed away, towards the solution and repel the aqueous
aggressive species by hydrophobic forces.
4. The adsorption process of PMSI follows the Langmuir isotherm
model.
Acknowledgement
The authors would like to acknowledge the financial support of
the research council of this institute (G2009IASBS124).
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