Journal of Dispersion Science and Technology

ISSN: 0193-2691 (Print) 1532-2351 (Online) Journal homepage: https://www.tandfonline.com/loi/ldis20

Heterocyclic Schiff bases as corrosion inhibitors

for carbon steel in 1 M HCl solution: hydrodynamic
and synergetic effect

Imene Benmahammed, Tahar Douadi, Saifi Issaadi, Mousa Al-Noaimi &

Salah Chafaa

To cite this article: Imene Benmahammed, Tahar Douadi, Saifi Issaadi, Mousa Al-Noaimi &
Salah Chafaa (2019): Heterocyclic Schiff bases as corrosion inhibitors for carbon steel in 1 M HCl
solution: hydrodynamic and synergetic effect, Journal of Dispersion Science and Technology, DOI:
10.1080/01932691.2019.1614038

To link to this article: https://doi.org/10.1080/01932691.2019.1614038

Published online: 28 May 2019.

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JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY
https://doi.org/10.1080/01932691.2019.1614038

Heterocyclic Schiff bases as corrosion inhibitors for carbon steel in 1 M HCl

solution: hydrodynamic and synergetic effect
Imene Benmahammeda, Tahar Douadia, Saifi Issaadia,b, Mousa Al-Noaimic, and Salah Chafaaa
a
Laboratoire d’Electrochimie des Mat
eriaux Mol
eculaires et Complexes (LEMMC), D
epartement de G
enie des Proc
ed
es, Facult
e de
Technologie, Universit
e Ferhat ABBAS, S
etif-1, S
etif, Algeria; bFacult
e des Sciences, Universit
e Ferhat ABBAS de S
etif-1, S
etif, Algeria;
c
Department of Chemistry, Hashemite University, Zarqa, Jordan

ABSTRACT ARTICLE HISTORY

f(1Z)-2-oxo-N0 -phenyl-N-quinolin-8-ylpropanehydrazonamide (H2L-H) and (1Z)-N0 -(4-bromophenyl)- Received 1 January 2019
2-oxo-N-quinolin-8-ylpropanehydrazonamide (H2L-Br)g have been studied as corrosion inhibitors Accepted 28 April 2019
of carbon steel in 1 M HCl medium by the weight loss method, potentiodynamic polarization, elec-
KEYWORDS
trochemical impedance spectroscopy EIS, scanning electron microscopy SEM coupled with Energy
Corrosion; Schiff base;
Dispersion X-Ray Spectroscopy EDS, Atomic Force Microscopy AFM, IR and UV-visible spectroscopy. hydrodynamic; synergy;
The inhibition efficacy has been shown to increase with increasing inhibitor concentration. The MD simulation
corrosion of the carbon steel is mainly controlled by a charge transfer process. The temperature
effect was evidenced and the thermodynamic parameters were calculated. The adsorption of the
inhibitors on the metal surface obeyed the Langmuir adsorption isotherm. The hydrodynamic
effect and the synergetic effect have been implemented. Inhibitory efficiency decreases with
increasing rotation speed and the used additives adsorb cooperatively to form a protective layer
on the metal surface. The results obtained by the experimental measurements and those of the
theoretical calculations (DFT and MD simulation) are in good correlation.

GRAPHICAL ABSTRACT

Equilibrium adsorption configurations of H2L-Br on Fe (1 1 0) surface obtained by molecular

dynamic simulations

Introduction the industry: cleaning, stripping, acidification of oil, desca-

ling and petrochemical processes. In the oil industry, 15%
Metals in general are widely used in industry, especially car-
HCl is used for the acidification treatment, to solubilize the
bon steel which is considered a material of choice, in add- manipulations.[3] The acidic environment causes the corro-
ition to its relatively low cost and good mechanical strength; sion of the metals and consequently their degradation. For
it is often used as a construction material for chemical reac- that, the protection of industrial equipment against corro-
tors, heat exchange systems and boiler, storage tanks, and sion became one of the main concerns of maintenance engi-
oil and gas transportation pipelines.[1,2] Metals require a neers. An important method for protecting metal from
pretreatment most of the time based on acidic solutions corrosion is the addition of inhibitor. These compounds act
such as hydrochloric acid solution which is widely used in

CONTACT Saifi Issaadi issaadi-s@univ-setif.dz Laboratoire d’Electrochimie des Mat
eriaux Mol
eculaires et Complexes (LEMMC), D
epartement de G
enie des
Proc
ed
es, Facult
e de Technologie, Universit
e Ferhat ABBAS, S
etif-1, S
etif, 19000, Algeria.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ldis.
ß 2019 Taylor & Francis Group, LLC
2 I. BENMAHAMMED ET AL.

by physical or chemical adsorption on the metal surface and

lower the corrosion rate of the metals. The adsorption of
the inhibitor on the steel/solution interface can be influ-
enced by the aggressive medium type, the molecular struc-
ture of the inhibitor and the charge distribution on the
molecular skeleton, the nature and the charged surface of
the metal. Heterocyclic compounds are evaluated to be the
effective corrosion inhibitors for metals in acid media as
Figure 1. Molecular structure of synthesized Schiff bases H2L-H and H2L-Br.
well as those with triple or conjugated double bonds or aro-
matic rings in their molecular structures.[4] In the present
C ¼ 1.44; Si ¼ 0.382; Mn ¼ 0.611; S ¼ 0.0666; P ¼ 0.0055
work we are interested in two nontoxic Schiff bases namely
and Fe ¼ 97.5%. Prior to each measurement, the carbon
f(1Z)-2-oxo-N’-phenyl-N-quinolin-8-ylpropanehydrazona-
steel surface had been abraded using different grades of
mide (H2L-H) and (1Z)-N’-(4-bromophenyl)-2-oxo-N-qui-
emery paper (500-800-1200-2400 and 4000 grad). Then, it
nolin-8-ylpropanehydrazonamide (H2L-Br)g to study their
was rinsed with distilled water and degreased with acetone
corrosion inhibition on carbon steel in 1 M HCl solution
before immersion in experimental solution.
using gravimetric measurements, polarization measurements,
electrochemical impedance spectroscopy, scanning electron
microscope SEM and atomic force microscopy AFM. The Working solutions
effect of temperature, the rotation speed and the synergy
were put in evidence. In addition, quantum chemical The aggressive electrolyte solution HCl 1 M was prepared by
approach was very useful to calculate some electronic prop- dilution of analytical grade 37% HCl with distilled water.
erties of the molecules in order to confirm correlation The concentration range of the used inhibitors was 1  104
between the inhibitive effect and molecular structure of the – 2.5  103 M and was cautiously prepared using dilution
techniques. The synergistic effect was achieved in a concen-
studied inhibitors.[5]
tration range (1  104 – 2.5  103 M) of potassium iodide
KI combined with the lowest concentration of inhibitor.
Experimental procedure
Synthesis Gravimetric measurement
The inhibitors were synthesized according to the procedure Gravimetric experiments were performed using cylindrical
reported by M. El-Noaimi et al.,[6] by contacting a solution samples of dimension (h ¼ 1.8 cm and d ¼ 1.8 cm) with the
(20 mmol) of appropriate hydrazonyl chloride in 5.0 mL same carbon steel composition of the working electrode,
absolute ethanol, (2.86 g, 20 mmol) of 8-aminoquinoline and were abraded successively with different grades of emery
triethylamine (2.4 g, 24 mmol) with refluxing for 2 hours. papers (500, 800, 1200, 2400 and 4000), washed with dis-
Condensing the solution followed by cooling produced a tilled water, cleaned with acetone. After being weighed using
yellow solid. The molecular structures of the inhibitors are a ± 0.00001 g precision balance, the specimens were
shown in Figure 1. immersed in 1 M HCl with and without various concentra-
(H2L-H) Anal. Calc. for C18H16N4O: C, 71.04; H, 5.30; N, tions of the studied compounds at room temperature in aer-
18.41. Found: C, 71.21; H, 5.20; N, 18.31%. Yield (3.2 g, ated condition. After a 24 hours immersion, the samples
52%). m.p is 140–142  C. were taken out, thoroughly rinsed with distilled water, dried
1H NMR (CDCl3, d ppm): 8.90 (d, 1 H, H10), 8.49 (s, and further weighed accurately.
1 H, NH), 8.15 (d, 1 H, H5), 7.70 (s, 1 H, NH), 7.50 (t, 1 H,
H9), 7.40 (d, 2 H, H2, H3), 7.35 (m, 3 H, Y ¼ H, H7, H6),
7.05 (d, 2 H, H1, H4), 6.35 (d, 1 H, H8), 2.62 (s, Electrochemical measurements
3 H, COCH3) All electrochemical measurements were done at aerated con-
(H2L-Br) Anal. Calc. for C18H15BrN4O: C, 56.41; H, 3.95; ditions, without agitation and were executed using a Gamry
N, 14.62. Found: C, 56.53; H, 3.78; N, 14.54%. Yield (4.97 g, potentiostat (Reference 3000) integrated with Gamry Echem
65%). m.p. is 165–167  C. Anayst software. All electrochemical measurements were
1H NMR (CDCl3, d ppm): 8.92 (d, 1 H, H10), 8.50 (s,1H, performed in a three-electrode cell, The counter-electrode
NH), 8.18 (d, 1 H, H5), 7.65 (s, 1 H, NH), 7.50 (m, 1 H, H9), CE was a large plate of platinum (5.4 cm2), the saturated
7.45 (d, 2 H, H2, H3), 7.35 (m, 2 H, H6, H7), 7.05 (d, 2 H, calomel electrode SCE was also used as a reference electrode
H1, H4), 6.35(d, 1 H, H8), 2.62 (s, 3 H, COCH3). RE and a carbon steel electrode was utilized as a working
electrode WE, This electrode was prepared from cylindrical
steel and after having undergone the same pretreatment for
Metal samples
gravimetric samples, it was covered with Teflon and only its
The carbon steel metal was analyzed by X-ray fluorescence cross section (0.19 cm2) has been in contact with the aggres-
XRF using a ZSX PRIMUSIV-RIGAKIU instrument. The sive solution. Before all electrochemical measurements, a
results leads to the following composition in (wt %): 30 min period was allowed as a period of stabilization to
 JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY 3

Table 1. Corrosion parameters obtained at 25  C from weight loss measure-

ments of carbon steel after 24 hours immersions in 1 M HCl solution with and
without addition of various concentrations of H2L-H and H2L-Br.
Inhibitor C (M) DW (g) CRW (mg cm2 h1) EW ð%Þ
Blank 0.07342 0.200 /
H2L-H 1  104 0.02957 0.080 59.72
5  104 0.01722 0.047 76.54
1  103 0.01407 0.038 80.83
2.5  103 0.01120 0.030 84.74
H2L-Br 1  104 0.03620 0.098 50.69
5  104 0.02849 0.077 61.19
1  103 0.01543 0.042 78.98
2.5  103 0.00614 0.016 91.63

Figure 2. Variation in corrosion rate and inhibitory efficiency calculated from

the weight loss method as a function of the concentration of H2L-H and H2L-Br
inhibitors at 25  C.

attain a stable value of EOCP : Potentiodynamic polarization

curves were measured at the potential range of ±250 (from
800 to 200 mV/SCE) and scan rate of 0.5 mVs1.
Electrochemical Impedance curves were measured at the fre-
quency range of 100 kHz to 30 mHz with amplitude of
10 mV peak-to-peak, using AC signal at Ecorr.
Hydrodynamic conditions were simulated using a rotating
disc electrode RDE. Rotation rate was controlled between
0 rpm (stagnant solution) and 2000 rpm, using a rotator
(Radiometer CTV101T) instrument. The temperature was
controlled using a thermostatic bath.
Figure 3. Open circuit potential for carbon steel without and with various con-
centrations of H2L-H and H2L-Br inhibitor at 25  C.

Surface analysis
Atomic force microscopy AFM, scanning electron micros-
copy SEM and energy dispersive X-ray spectroscopy EDS
In order to study the inhibitory effect of our Schiff bases an Oxford Instruments company model microscope. The
H2L-H and H2L-Br, carbon steel samples suffered a polish- scanning electron micrographs SEM and Energy Dispersion
ing with Sic emery paper (grade 500, 800, 1200, 2400 and X-Ray Spectroscopy EDS, were been recorded using a
TM3000 Tabletop Microscope.
4000), a cleaning with distilled water and acetone and a dry-
ing with Josef paper, and then they were immersed in 1 M
HCl in the absence and in the presence of 2.5  103 M of Spectroscopic analysis
H2L-H and H2L-Br for 24 hours in aerated state and at FTIR spectra were collected in the 500–4000 cm1 range
room temperature. The surface Morphologies of the carbon using a FT-IR 4200 JASCO spectrometer. UV-Visible spectra
steel samples in the absence and in the presence of inhibi- were recorded in the 200–1000 nm domain using DMF as
tors were analyzed by atomic force microscopy AFM using the solvent and the UV-680 JASCO spectrophotometer.
4 I. BENMAHAMMED ET AL.

Quantum chemical study 7.0 from BIOVIA-Accelrys, USA. The MD simulation was
carried out in a simulation box (24.82 Å  24.82 Å  20.13 Å)
All the theoretical calculations and optimized geometries of
with periodic boundary (Ensemble NVT, T ¼ 298 K, Time of
the inhibitors H2L-H and H2L-Br were performed using the
step ¼ 1 fs, Dynamics time ¼ 5 ps). The box consists of a
Gaussian 09 program at the DFT-B3LYP level, and the 6-
Fe slab and a vacuum layer of 10 Å heights.
31 G base set (d, p).[7]

Results and discussion

Molecular dynamic simulation
Gravimetric measurement
Surface (1 1 0) has been selected as the most stable form for
the molecular dynamic simulation study to visualize the The corrosion rate CRW (mg cm2 h1) was calculated as
adsorption process of inhibitory molecules on the metal sur- follows[8]:
face, using Forcite module in the Material Studio Software W
CRW ¼ (1)
St
Where, W is the weight loss of carbon steel sample, S is the
total area of carbon steel sample and t is immersion time
(24 hours). The inhibition efficiency EW ð%Þ was calculated
using the following equation[9]:

!
CRW CRW
EW ð%Þ ¼   100 (2)
CRW

Where CRW and CRW are uninhibited and inhibited corro-
sion rates, respectively.
Figure 2 shows the variation of the inhibitory efficiency
and the corrosion rate as a function of the inhibitor concen-
tration. Weight loss W (mg), corrosion rate CRW (mg cm2

Figure 4. Nyquist diagrams of carbon steel in 1 M HCl without and with add- Figure 6. Adjusted plots of Nyquist impedance diagrams of carbon steel in 1 M
ition of different concentrations of H2L-H and H2L-Br inhibitors at 25  C. HCl without and with addition of 2.5  103 M of H2L-H and H2L-Br inhibitors.

Figure 5. Electrochemical equivalent circuits used to fit the impedance spectra.

 JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY 5

Table 2. Electrochemical parameters of impedance and corresponding inhibition efficiency for the carbon steel obtained at 25  C in 1 M HCl solution
with and without different concentrations of H2L-H and H2L-Br.
Inhibitor C (M) OCP (mV/SCE) Rs (X cm2) Rct (X cm2) Q (mX1sacm2) 103 a fmax (Hz) Cdl (mF cm2) EIES (%)
Blank 500 0.47 36.08 575.26 838.8 2.96 274.12
H2L-H 1  104 490 0.44 142.74 288.00 744.1 1.88 96.10 74.72
5  104 493 0.57 151.98 419.89 726.3 1.50 183.79 76.25
1  103 484 0.60 213.94 380.36 714.6 1.20 139.66 83.13
2.5  103 597 0.94 216.98 185.42 784.9 1.88 76.86 83.37
H2L-Br 1  104 492 0.37 132.14 87.21 809.1 4.73 30.42 72.69
5  104 489 0.44 233.51 115.78 724.8 3.77 29.39 84.54
1  103 488 0.63 280.63 156.63 703.7 2.35 42.02 87.14
2.5  103 482 0.85 342.76 128.84 676.3 2.96 28.20 89.47

h1) and inhibition efficiency EW ð%Þ values, after 24 hours

of immersion of the carbon steel samples in1 M HCl with-
out and with different concentrations of inhibitors H2L-H
and H2L-Br at 25  C, are reported in Table 1. It is clear that
the corrosion rate decreases while inhibition efficiency
increases by increasing the concentration of inhibitors. At
the optimum concentration 2.5  103 M, the efficiency
reaches 84.74% and 91.63% for H2L-H and H2L-Br, respect-
ively. It is remarkable that H2L-Br is strongly adsorbed on
the surface of mild steel with respect to H2L-H. Bromo sub-
stitute inhibit iron corrosion due to increase the delocaliza-
tion of electron density in the molecule, which makes the
molecule more stable, i.e. better inhibition.

Electrochemical measurement
Concentration effect
Electrochemical impedance spectroscopy (EIS). The evolu-
tion of potential over time is a methodology for monitor the
interface between the metal and the electrolyte. For this, the
EOCP curves of the working electrode in the 1 M HCl solu-
tion without and with the addition of different concentra-
tions of the inhibitors H2L-H and H2L-Br and in the aerated
state for 30 minutes are illustrated in Figure 3. It is observed
that the stable state was reached after 1000 seconds of
immersion for both inhibitors. The experimental Nyquist
plots, the equivalent circuit model and the adjusted Nyquist
plots for carbon steel in 1 M HCl solution in the absence
Figure 7. Polarization curves of carbon steel in 1 M HCl without and with add-
and in the presence of different concentrations of inhibitors ition of different concentrations of H2L-H and H2L-Br inhibitors at 25  C.
are shown in Figures 4, 5, 6, respectively. At high frequen-
cies, Nyquist plots consist of a large capacitive loop. which
means that the corrosion of the carbon steel in the 1 M HCl the constant phase element, Rct is the charge transfer resist-
solution with and without inhibitor is mainly controlled by ance, Rs is the solution resistance, RL and L are attributed to
a charge transfer process,[10] followed by a small inductive the inductive arrangement.
loop at low frequencies, which can be attributed to the The impedance function of the constant phase element
relaxation process obtained by adsorption species like Hþ ads
CPE is represented by the expression[14]:
or inhibitory species on the electrode surface. It could also ZCPE ¼ Q1 ðjxÞa (3)
be related to the re-dissolution of the passivated surface at
low frequencies.[11,12] The imperfection of the semicircles is Where Q is the magnitude of the CPE, the factor j is the
often attributed to frequency dispersion due to surface het- imaginary number ðj2 ¼ 1Þ; the exponent a is the deviation
erogeneity.[13] We can see the increase in the diameter of parameter. When a ¼ 0; the CPE represents a resistance, for
Nyquist plots after adding the inhibitor to the HCl solution, a ¼ 1 an inductor and for a ¼ þ1; a capacitor. x is the
which results from an improvement in the corrosion resist- angular frequency x ¼ 2pfmax and fmax is the maximum fre-
ance of the metal. As shown in Figure 6, a good fit of the quency of the imaginary part of the impedance spectrum.[15]
Nyquist experimental curves has been illustrated by the EIS parameters such as, solution resistance Rs ; charge-
equivalent circuit models shown in Figure 5. Where CPE is transfer resistance Rct ; the amplitude of CPE ðQÞ; the
6 I. BENMAHAMMED ET AL.

Table 3. Electrochemical parameters, corresponding inhibition efficiency and synergic parameter obtained from polarization curves of carbon steel at
25  C in 1 M HCl solution with and without different concentrations of H2L-H, H2L-Br, KI and mixture of (104 inhibitor þ KI).
Inhibitor C (M) Ecorr (mV /SCE) icorr (mA cm2) ba (mV/dec) bc (mV/dec) h Ep ð%Þ S
Blank 488 0.63 116.7 128.6 / / /
H2L-H 1  104 467 0.22 104.3 184.4 0.6458 64.58 /
5  104 468 0.20 101.0 147.0 0.6825 68.25 /
1  103 472 0.15 104.3 152.0 0.7516 75.16 /
2.5  103 585 0.12 84.4 182.3 0.8075 80.75 /
H2L-Br 1  104 483 0.26 103.0 202.2 0.5858 58.58 /
5  104 477 0.25 126.7 340.6 0.6025 60.25 /
1  103 473 0.20 120.5 302.1 0.6700 67.00 /
2.5  103 471 0.14 103.8 277.0 0.7716 77.16 /
KI 1  104 481 0.56 109.6 123.6 0.1083 10.83 /
5  104 479 0.54 115.6 126.4 0.1333 13.33 /
1  103 488 0.49 123.8 127.2 0.2107 21.07 /
2.5  103 549 0.41 121.3 123.3 0.3499 34.99 /
H2L-H 1 KI 104(H2L-H) þ 1  104(KI) 470 0.18 98.1 154.7 0.7141 71.41 1.10
104(H2L-H) þ 5  104 (KI) 477 0.14 83.9 161.2 0.7692 76.92 1.33
104(H2L-H) þ 1  103(KI) 470 0.09 80.0 132.0 0.8541 85.41 1.91
104(H2L-H) þ 2.5  103(KI) 472 0.08 78.7 149.4 0.8617 86.17 1.66
H2L-Br 1 KI 104(H2L-Br) þ 1  104(KI) 478 0.19 89.7 144.1 0.6991 69.91 1.22
104(H2L-Br) þ 5  104 (KI) 478 0.17 95.4 160.7 0.7184 71.84 1.27
104(H2L-Br) þ 1  103(KI) 474 0.16 90.5 160.2 0.7391 73.91 1.25
104(H2L-Br) þ 2.5  103(KI) 472 0.11 85.8 147.0 0.8234 82.34 1.52

heterogeneity a and the double layer capacity Cdl for corro- present the polarization curves of carbon steel in 1 M HCl
sion of carbon steel in 1 M HCl in the absence and presence solution with and without various concentrations of the
of H2L-H and H2L-Br at 25  C are recapitulate in Table 2. inhibitors H2L-H and H2L-Br. It can be observed that the
The inhibition efficiency EEIS was calculated according to addition of the studied Schiff bases reduces the anodic dis-
the following equation[16]: solution and also delays the hydrogen release reaction. The
   addition of inhibitors causes the appearance of cathodic
Rct Rct
EEIS ð%Þ ¼  100 (4) Tafel branches almost parallel. This indicates that the mech-
Rct

anism of hydrogen evolution mainly due to charge transfer
Where Rct and Rct are the uninhibited an inhibited charge- is not modified by the addition of inhibitors.[19]
transfer resistances, respectively. The cathodic hydrogen evolution mechanism is[20]:
The increase in Rct values as a function of concentration
is attributed to the formation of a protective layer at the Fe þ H þ $ ðFeH þ Þads (7)
metal/electrolytic interface, thus protecting the metal against
corrosion. This implies the increase in inhibitory efficiency ðFeH þ Þads þ e ! ðFeH Þads (8)
to reach 83.37% and 89.47%, for H2L-H and H2L-Br,
respectively. The double layer capacity Cdl was calculated ðFeH Þads þ H þ þ e ! Fe þ H2 (9)
using the equation[17]:
 1
Cdl ¼ Q:Rctn1 n (5) The addition of inhibitors H2L-H and H2L-Br causes a
change in anodic tafel slopes. These inhibitors are adsorbed
The decrease in Cdl values may be due to a decrease in
on the metal surface before blocking the active sites without
local dielectric constants and/or an increase in the thickness
modifying the anodic dissolution mechanism which takes
of the electrical double layer. This can be explained by the
place as follows[21–23]:
Helmholtz model.[18]
e e Fe þ Cl $ ðFeCl Þads (10)
Cdl ¼ S (6)
d
ðFeCl Þads $ ðFeClÞads þ e (11)
Where d; S; e ; e are respectively, the deposit thickness, the
surface of the electrode, the permittivity of the medium and
ðFeClÞads ! ðFeClþ Þads þ e (12)
the dielectric constant. It can also be noted that the results
obtained by electrochemical impedance spectroscopy are in
good agreement with those obtained from gravimetric ðFeClþ Þads ! Fe2þ þ Cl (13)
measurements.
There is also an increase in current density for potentials
Potentiodynamic polarization curves. In acid solutions higher than 350 mV in most anode curves. This may be due
containing dissolved oxygen, the anodic reaction of metal to desorption of molecules inhibiting the metal surface.
dissolution is counterbalanced by a hydrogen evolution reac- The relevant electrochemical parameters including the
tion or dissolved oxygen reduction as protons. Figure 7 corrosion potential Ecorr ; cathodic and anodic Tafel slopes
 JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY 7

bc , ba and corrosion current density icorr obtained by

extrapolation of the Tafel lines are listed in Table 3. The
inhibition efficiency Ep ð%Þ was calculated from the follow-
ing equation[24]:

!
icorr icorr
Ep ¼  100 (14)
icorr

Where icorr and icorr are uninhibited and inhibited current
densities, respectively. It is clear that the inhibition efficiency
Ep ð%Þ of H2L-H is increased usually when the inhibitor
concentration increased to exhibit a maximum value of
80.75% at 2.5  103 M. At the same concentration, H2L-Br
reaches a maximum inhibition efficiency value 77.16%. This
suggests that the inhibitory actions are due to their adsorp-
tion on the carbon steel surface. Similar results have been
found by many authors.[25–27] It can also be seen that no
definite displacement of Ecorr values was observed in the
presence of various concentrations of H2L-H inhibitor (the
displacement is less than 85 mV/SCE), proposing that this
compound act as mixed-type inhibitor.[28] Whereas, the Ecorr
values of H2L-Br inhibitor move slightly towards the positive
side. This means that H2L-Br is a mixed inhibitor with
anodic predominance.

Temperature effect
Figure 8 present the effect of temperature (308–338 K) on
the polarization curves of the carbon steel in 1 M HCl solu-
tion with and without optimum concentration of the inhibi-
tors H2L-H and H2L-Br. The electrochemical parameters as
well as the inhibitory power in the absence and in the pres-
ence of different concentrations of inhibitors H2L-H and
H2L-Br in the 1 M HCl solution as a function of the tem-
perature are illustrated in Table 4. In general, icorr values
increase with increasing temperature, whether or not in the
presence of inhibitors in the corrosive solution. In the corro-
sive solution alone (HCl 1 M) icorr increases in a way steady
and fast, which confirms a growing metallic dissolution. On
the other hand, the increase in corrosion current density in
the presence of H2L-H and H2L-Br with temperature is
much lower than that observed for HCl alone. It can be
seen that by increasing the temperature, the inhibitory effi-
ciency is stable for the whole range of concentrations. This
Figure 8. Polarization curves of carbon steel in 1 M HCl without and with add-
may be due to possible specific interactions between the ition of 2.5  103 M of H2L-H and H2L-Br inhibitors at different temperatures.
iron surface and the inhibitor.[29] For the H2L-H inhibitor,
it can be noted that beyond 328 K, the inhibitory efficiency
begins to undergo a decrease which can be explained by a activation energies. Activation parameters values for mild
more sensitive adsorption of the inhibitory molecules on the steel in 1 M HCl in the absence and presence of different
metal surface. concentrations of the inhibitors H2L-H and H2L-Br are
given in Table 5. It has been reported in the literature that
Activation parameters. Figure 9 shows the Arrhenius plots the inhibitors for which the activation energy of the inhib-
 
of lnicorr ¼ f T1 issued from the Arrhenius equation[30]: ited solution is greater than that of the blank solution Einha

  > Ea are adsorbed on the substrate by electrostatic bonds
Ea 
icorr ¼ Aexp  (15) (physisorption) and the inhibitors that possess Einh a < Ea
RT adsorb to the metal surface through strong bonds (chemi-
Where A is a constant (pre-exponential factor), Ea is the sorption).[31,32] The Ea values in the presence of different
activation energy, R is the universal gas constant and T is concentrations of H2L-H inhibitor are higher than that of
the absolute temperature .slopes were used to calculate the the solution without inhibitor. This increase reflects that the
8 I. BENMAHAMMED ET AL.

Table 4. Electrochemical parameters obtained from polarization curves of carbon steel in 1 M HCl without and with addition of H2L-H and H2L-Br inhibitors at
different temperatures.
H2L-H H2L-Br
2 2

T C C (M) Ecorr (mV /SCE) icorr (mA cm ) ba (mV/dec) bc (mV/dec) h Ep ð%Þ Ecorr (mV /SCE) icorr (mA cm ) ba (mV/dec) bc (mV/dec) h Ep ð%Þ
25 Blank 488 0.63 116.7 128.6 / / 488 0.63 116.7 128.6 / /
1  104 467 0.22 104.3 184.4 0.6458 64.58 483 0.26 103.0 202.2 0.5858 58.58
5  104 468 0.20 101.0 147.0 0.6825 68.25 477 0.25 126.7 340.6 0.6025 60.25
1  103 472 0.15 104.3 152.0 0.7516 75.16 473 0.20 120.5 302.1 0.6700 67.00
2.5  103 585 0.12 84.4 182.3 0.8075 80.75 471 0.14 103.8 277.0 0.7716 77.16
35 Blank 595 1.00 111.9 130.5 / / 595 1.00 111.9 130.5 / /
1  104 591 0.29 91.8 129.4 0.7089 70.89 616 0.30 90.6 182.4 0.6937 69.37
5  104 633 0.21 109.7 227.6 0.7879 78.79 611 0.24 111.6 215.7 0.7549 75.49
1  103 637 0.19 107.1 152.2 0.8047 80.47 603 0.19 102.9 204.5 0.8073 80.73
2.5  103 570 0.17 87.1 158.2 0.8256 82.56 607 0.16 111.0 210.3 0.8356 83.56
45 Blank 475 1.55 115.4 129.6 / / 475 1.55 115.4 129.6 / /
1  104 629 0.53 98.1 132.4 0.6542 65.42 626 0.56 97.0 153.2 0.6338 63.38
5  104 646 0.42 144.6 204.6 0.7277 72.77 632 0.40 118.8 174.3 0.7376 73.76
1  103 613 0.31 111.5 182.3 0.7949 79.49 636 0.30 119.0 179.3 0.8047 80.47
2.5  103 623 0.29 133.2 175.6 0.8098 80.98 654 0.29 155.5 209.2 0.8115 81.15
55 Blank 606 2.03 122.3 135.3 / / 606 2.03 122.3 135.3 / /
1  104 507 1.09 132.3 129.3 0.4625 46.25 630 0.94 131.5 128.4 0.5374 53.74
5  104 537 0.75 159.6 154.8 0.6279 62.79 653 0.61 148.6 178.2 0.6976 69.76
1  103 542 0.60 159.2 156.2 0.7028 70.28 669 0.48 148.6 212.8 0.7627 76.27
2.5  103 536 0.56 141.3 156.3 0.7209 72.09 663 0.38 160.4 201.4 0.8090 80.90

inhibitory molecules of H2L-H are physisorbed. On the

other hand, most activation energy values for different con-
centrations of the H2L-Br inhibitor are lower than that of
the blank solution. We can say that this Schiff base (H2L-

Br) is chemisorbed. The activation enthalpy DHa and activa-

tion entropy values DSa were obtained from the equation[33]:
   

RT DSa DHa
icorr ¼ exp exp  (16)
Nh R RT

Where h is the Planck’s

  constant and N is the Avogadro’s
1
number. A plot of ln T ¼ f T showed a straight line. D
icorr

 
Ha and DSa were calculated from the slope and intercept,

respectively. The positive values of DHa confirm the endo-
thermic dissolution process. The negative values of the acti-

vation entropy DSa in the absence and in the presence of
the studied compounds mean that the activated molecules
were in higher order state than that at the initial stage.[34]
The obtained results in Table 5 show that for the Schiff base
 
H2L-H, the values of DHa and DSa in the presence of inhibi-
tor increase compared to the blank, which means that the
energy barrier of the corrosion reaction in the presence of

inhibitor increases. The increase in the value of DSa in the
inhibited solution compared to the uninhibited one explains
that the decrease of the disorder is due to the molecules ori-
ented on the surface and ordered by adsorption through the

active sites.[35] It seems clear that Ea and DHa vary in the
same direction for the H2L-Br inhibitor. They decrease with
increasing concentration of H2L-Br. This phenomenon can
explain to us that the reduction of the steel corrosion rate is
mainly related to the kinetic parameters of the activation.[36] Figure 9. Arrhenius diagram of the corrosion current density of carbon steel in
1 M HCl medium in the absence and presence of the different concentrations of
H2L-H and H2L-Br inhibitors.
Adsorption parameters. The action of Schiff bases as effect-
ive corrosion inhibitors is mainly based on their adsorption
capacity on the metal surface. It is therefore necessary to insights into the interaction of Schiff bases with the surface
know the mode of adsorption helps us to gain valuable of carbon steel. Figure 10 shows the variation of Ch as a
 JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY 9

Table 5. Activation parameters of carbon steel corrosion process in 1 M HCl in

the absence and in the presence of different concentrations of inhibitors H2L-
H and H2L-Br.
 
Inhibitor C (M) Ea ( kJ mol1) DHa (kJmol1) DSa (Jmol1)
Blank 31.91 29.33 150.21
H2L-H 1  104 43.14 40.56 122.57
5  104 37.44 34.86 142.93
1  103 36.41 33.83 147.94
2.5  103 41.43 38.85 132.97
H2L-Br 1  104 35.79 33.21 145.81
5  104 25.55 22.97 180.61
1  103 23.71 21.13 188.50
2.5  103 28.50 25.92 174.81

Table 6. Thermodynamic parameters of adsorption of inhibitors H2L-H and

H2L-Br at different temperatures.
  
Inhibitor T( C) Kads (M1) DGads (kJmol1) DHads (kJmol1) DSads (Jmol1K1)
H2L-H 25 11474.46 33.11 6.540 93.36
35 32605.15 36.89
45 19833.39 36.77
55 10731.91 36.26
H2L-Br 25 6769.10 31.80 10.847 147.01
35 19884.66 35.62
45 21454.62 36.98
55 10690.61 36.251

1
or more, indicates charge transfer between the inhibitory
molecules and the metal surface by forming covalent bonds
(chemisorption).[39] In the present study, the obtained values

of DGads show that both inhibitors undergo a physicochemi-
cal adsorption on the carbon steel surface. The thermo-
dynamic parameters of the inhibitor adsorption process can
be valuable notions to know the mechanisms of corrosion
 
inhibition. The enthalpy DHads and the entropy DSads of
adsorption process were calculated using the following equa-
Figure 10. Langmuir adsorption isotherm plots for carbon steel in 1 M HCl in tion[40]:
the presence of H2L-H and H2L-Br inhibitors at 25  C.
  
DGads ¼ DHads TDSads (19)
function of C: The plot gives a straight line for both Schiff 
It has been reported in the literature that if DHads >0, which
bases, which suggests that the adsorption of these inhibitors
results in an endothermic adsorption process and a chemi-
obeys the Langmuir isotherm, which can be given by equa- 
sorption, if DHads <0, this can be explained by a physical
tion[37]:
adsorption, chemisorption or a mixture of both.[41] In our

C
¼
1
þC (17) case, DHads takes the positive and negative values, for H2L-
h Kads Br and H2L-H, respectively. This is consistent with the
Where h is the surface coverage, C is the inhibitor con- chemical adsorption of H2L-Br and the physicochemical
centration, Kads is the equilibrium adsorption process con- adsorption of H2L-H. The positive value of the adsorption


stant. The standard Gibbs energy of adsorption DGads and entropy DSads shows a decreasing appearance at the solid/
the equilibrium constant of adsorption Kads are linked by liquid interface during the adsorption of these Schiff bases
on the metal surface.[42,43]
the equation[38]:

DGads ¼ RTlnð55:5  Kads Þ (18)
Hydrodynamic effect
Where R is the gas constant, T is the absolute temperature Figure 11 shows the Nyquist plots of carbon steel in 1 M HCl
and 55.5 is the concentration of water in solution (mol/L). solution at different rotational speeds without and with the
Thermodynamic parameters for the adsorption of H2L-H addition of 2.5  103 M of H2L-H and H2L-Br inhibitors.
and H2L-Br on carbon steel in 1 M HCl at different temper- The EIS data were analyzed using the equivalent circuit mod-
atures are listed in Table 6. Research has reported that the els, shown in Figure 6. It is clear that the increase of the rota-

negative values of the standard Gibbs energy DGads refer to tional speed in the uninhibited solution causes a change in
the spontaneity of the Adsorption process and absolute val- the size and shape of the Nyquist plots. The semicircles diam-

ues jDGads j close to 20 kJmol1 or less are generally related eters become wider. The Nyquist graphs show a depressed
to electrostatic interactions between charged molecules and capacitive loop, with an asymmetric shape in the high fre-
metal charges (physisorption), while those around 40 kJmol- quency region. That can be attributed to a diffusion
10 I. BENMAHAMMED ET AL.

r2  x
Re ¼ (20)
k
Where r; x and k are the radius of the rotating disc
electrode RDE active area in mm, angular velocity in rad/s
and kinematic viscosity in stokes (mm2/s), respectively. Re <
105 reveals that the flow regime on the surface of the elec-
trode is laminar.[46] It can be clearly seen from the Reynolds
numbers that the flux is laminar in all studied rota-
tional speeds.
According to Table 7, in the blank solution, the charge
transfer resistance increases by increasing the rotational
speed; this may be due to the formation of oxide and the
reduction of the total active area.[47] In the inhibited solu-
tion, the increase in rotation speed is followed by a remark-
able decrease in the charge transfer resistance for both
inhibitors. This can be explained by desorption of the
inhibitory molecules on the metal surface, caused by the
shear stress.[48] Inhibitory efficacy decreased significantly. It
can be said that increasing rotation speed has a destructive
effect on the inhibitors performance. It is more influential in
the case of H2L-H compared to H2L-Br due to the presence
effect of Br halogen.

Synergistic inhibition effect

Synergy is a combined action between two compounds
whose total effect is greater than their individual effects. The
synergistic effect of inhibitors has become one of the most
important factors in the corrosion inhibition process. In our
study, it was conducted on the combination of the lowest
concentration (1  104 M) of each of the H2L-H and H2L-
Br inhibitors with different concentrations of KI. The polar-
ization curves of the carbon steel in 1 M HCl medium in the
absence and in the presence of the lowest concentration
(1  104 M) of H2L-H and H2L-Br inhibitors in combin-
ation with different concentrations of KI at 25  C are pre-
sented in the Figure 12. It is clear that the shape of the
polarization curves in the presence of the (KI þ inhibitor)
mixture is not substantially different compared to that of
the inhibitors alone, only the current density values decrease
without affecting the aspects of the polarization behavior.
This implies that the addition of KI does not modify the
electrochemical reactions responsible for the corrosion of
carbon steel in the acid medium. The corrosion parameters
such as corrosion potential Ecorr ; anodic and cathodic tafel
slopes ba ; bc ; corrosion current density icorr ; synergism
parameters S and Ep % obtained from the polarization curves
Figure 11. The Nyquist plots of carbon steel in 1 M HCl solution at different are given in Table 3. The combination of (KI þ inhibitor)
rotational speeds without and with the addition of 2.5  103 M of H2L-H and affects a decrease in current density values icorr : This proves
H2L-Br inhibitors.
that KI has a synergistic inhibitory effect on carbon steel in
the 1 M HCl corrosive medium. Therefore the inhibitory
impedance process in a finite thickness layer.[44] While, the efficacy increases by increasing the concentration of KI and
Nyquist plots keep their forms for both inhibitors. The achieves good efficiencies for the mixture (1  104 M of
change in hydrodynamic conditions is followed by a consid- inhibitor þ 2.5  103 M of KI), for both studied inhibitors,
erable decrease in the semicircles diameters. The electrochem- compared with those of the inhibitors alone. No definite
ical parameters extracted from the the Nyquist plots, the change was observed for the Ecorr values in the presence of
inhibitory efficacy as well as the Reynolds numbers Re; calcu- the (KI þ inhibitor) mixture for both H2L-H and H2L-Br
lated using the following equation[45] are presented in inhibitors. The synergism parameters S were calculated using
Table 3. the following equation[49]:
 JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY 11

Table 7. Electrochemical parameters of impedance and corresponding inhibition efficiency of carbon steel in 1 M HCl solution obtained at different rotational
speeds without and with the addition of 2.5  103 M of H2L-H and H2L-Br inhibitors at 25  C.
C (M) Rotation rate Re OCP (mV/SCE) Rs (X cm2) ctI (X cm2) Q (mX1sacm2) a fmax (Hz) Cdl (mF cm2) EEIS (%)
Blank 0 rpm 0 500 0.47 36.08 575.26 838.8 2.96 274.12 /
500 rpm 312.5 473 0.97 50.52 834.21 836.1 1.50 448.42 /
1000 rpm 625 471 0.45 59.16 955.26 777.5 1.50 419.79 /
1500 rpm 937.5 471 0.44 66.23 710.00 461.8 1.88 20.16 /
2000 rpm 1250 493 1.27 67.33 771.57 816.1 2.35 396.24 /
H2L-H 0 rpm 0 597 0.94 216.98 185.42 784.9 1.88 76.86 83.37
500 rpm 312.5 452 1.16 167.97 371.78 747.0 1.50 145.33 69.92
1000 rpm 625 445 1.26 164.50 449.10 735.5 1.50 175.97 64.03
1500 rpm 937.5 453 1.20 145.76 376.57 727.9 1.88 127.25 54.56
2000 rpm 1250 453 1.02 133.36 390.63 711.9 2.35 118.15 49.51
H2L-Br 0 rpm 0 482 0.85 342.76 128.84 676.3 2.96 28.20 89.47
500 rpm 312.5 460 0.64 319.39 464.21 568.5 0.95 109.02 84.18
1000 rpm 625 455 1.72 251.18 401.57 634.0 1.50 106.81 76.44
1500 rpm 937.5 454 8.41 192.47 230.26 724.0 2.35 70.19 65.58
2000 rpm 1250 458 1.20 169.04 374.73 661.1 2.35 91.08 60.17

1h1 h2 þ ðh1 h2 Þ Br, respectively. The metal surface has remarkably improved
S¼ (21)
1  h1þ2 due to the formation of an adsorbed protective layer thus
preventing the aggressive attack of the electrolyte. The pres-
Where h1 and h2 are the surfaces coverage of iodide ions
ence of the inhibitory molecules on the metal surface was
and the inhibitor, respectively. h1þ2 is the surface coverage
confirmed by Energy Dispersion X-Ray Spectroscopy EDS
of iodide þ inhibitor mixture. The halide ions improve the
shown in Figure 15. The mass percentages of the various
stability of the inhibitor on the metal surface either by com-
elements obtained after analysis of the steel surface in 1 M
petitive adsorption or by cooperative adsorption between the
HCl without and with 2.5  103 M of the H2L-H and H2L-
compounds. For competitive adsorption, both compounds
Br inhibitors at 25  C by EDS are collated in the Table 9.
are adsorbed at different sites on the metal surface. While in
The comparison of the EDS spectra of the H2L-H and H2L-
the cooperative adsorption, the anions are chemisorbed on
Br inhibitors (Figure 15b, c) with that of the corroded steel
the metal surface and the cations come to physisorber on a
layer of anions.[50] In general, S < 1 values suggest competi- in 1 M HCl alone (Figure 15a) and the analysis of the results
tive adsorption, whereas S > 1 indicates cooperative adsorp- obtained in Table 9, shows a reduction in the peak of chlor-
tion.[51] According to Table 3, we can notice that all the S ine accompanied by the appearance of nitrogen, oxygen for
values are greater than unity. Implying that increase in both inhibitor and bromine atom for H2L-Br. This confirms
inhibitory efficiency caused by the addition of Iodide ions to the adsorption of the tested inhibitory molecules on the
H2L-H and H2L-Br inhibitors results from a synergistic metal surface.
effect (cooperative adsorption).
Fourier transforms infrared spectroscopic analysis FTIR
Surface analysis FTIR was used to characterize the synthesized Schiff bases.
In addition, the steel samples used for the mass loss study
Atomic force microscopy AFM, scanning electron micros- were immersed in a 2.5  103 M solution for 24 hours,
copy SEM and energy dispersive X-ray spectroscopy EDS then, a thin surface layer was scraped and analyzed by FTIR
Figure 13 and Figure 14 show three-dimensional views to assess the presence of adsorbed inhibitors on the steel
obtained by atomic force microscopy AFM and the scanning surface. Figure 16 represents the FTIR spectra of the pure
electron micrographs SEM of the carbon steel surfaces in H2L-H and H2L-Br inhibitors and those of the recovered
the absence and in the presence of the optimum concentra- layers of the metal surface. The adsorption bands in (cm1)
tion 2.5  103 M of both studied inhibitors before and after corresponding to the groups present in the molecular struc-
24 hours of immersion in 1 M HCl medium in the aerated tures of the inhibitors H2L-H and H2L-Br and the recovered
state and at room temperature. The average roughness of layers are grouped in the Table 10. According to the
the carbon steel surface in the 1 M HCl solution without obtained results, the appearance of characteristic bands such
and with the addition of 2.5  103 M inhibitors H2L-H and that C ¼ N; C ¼ O; C  N; C ¼ CAr in the IR spectrum of
H2L-Br are tabulated in Table 8. It can be seen that the car- the recovered layer suggests that both inhibitors H2L-H and
bon steel surface after 24 hours immersion in 1 M HCl alone H2L-Br were adsorbed on the steel surface to protect it
(Figure 13b and Figure 14b) is heavily damaged and severely against corrosion.
corroded with a roughness of 326.969 nm compared to that
of the control which appears more uniform with some lines
resulting from polishing (Figure 13a and Figure 14a) with a Ultraviolet-visible spectroscopic analysis UV-vis
roughness 6.684 nm. Whereas in the presence of the inhibi- UV-Visible spectroscopy provides information on the for-
tors (Figure 13c, d and Figure 14c, d), the damages are mation of a protective layer on the metal. In order to ensure
reduced, the external morphology appears softer with a the existence of the inhibitors on the carbon steel surface,
roughness equal to 15.819, 12.718 nm for H2L-H and H2L- the samples were immersed during 24 hours in the corrosive
12 I. BENMAHAMMED ET AL.

Figure 12. Polarization curves of carbon steel for different concentrations of KI Figure 13. AFM tridimensional images: (a) carbon steel, (b) carbon steel in 1 M
alone and the mixture (1  104 M of inhibitor þ KI) in 1 M HCl at 25  C. HCl, (c) carbon steel in 1 M HCl in the presence of 2.5  103 M of H2L-H, (d)
carbon steel in 1 M HCl in the presence of 2.5  103 M of H2L-Br.

solution, after the immersion time has elapsed, the steel

to the carbonyl group and imine function, respectively.[52,53]
samples have been removed, dried and their outer layer has
These observations indicate the formation of an inhibitor
been recovered (scraped). This layer has undergone a UV-
protective barrier layer on the metal surface.
Vis analysis to compare the results with those obtained by
the inhibitors alone as shown in Figure 17. The correspond-
ing spectra show the adsorption bands kmax ¼ 269 and
Quantum chemical study
268 nm for H2L-H and H2L-Br, respectively, significant of a
transition pp of C ¼ CAr : While two other bands kmax ¼ The quantum chemical study was carried out to illustrate
300 and 353 nm are barely clear for the H2L-Br inhibitor. the compatibility between the molecular structure and the
These bands can be assigned to transitions np ; attributed inhibitory effect of the studied molecules.
 JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY 13

Figure 14. SEM micrographs of carbon steel samples: (a) polished surface, (b) after 24 hours immersion in 1 M HCl, (c) after 24 hours immersion in 1 M HCl þ
2.5  103 M of H2L-H, (d) after 24 hours immersion in 1 M HCl þ 2.5  103 M of H2L-Br.

Table 8. The average surface roughness of carbon steel in 1 M HCl medium IP þ EA

without and with the addition of 2.5  103 M of H2L-H and H2L-Br inhibitors.
v¼ (27)
2
Carbon steel CS þ HCl CS þ H2L-H CS þ H2L-Br
ELUMO shows the tendency of a molecule to accept electrons,
RMS (nm) 6.68 326.96 15.81 12.71
while EHOMO is related to the electron donor capacity of the
inhibitory molecule to acceptor species with empty molecu-
lar orbitals.[55] The fraction of transferred electron DN was
Quantum chemical parameters such as EHOMO ; ELUMO ; D obtained as follows[56]:
E energy gap, the ionization potential IP and electronic vFe –vinh
affinity EA which are related to EHOMO and ELUMO ; the DN ¼ (28)
2ðgFe þ ginh Þ
dipole moment l; global hardness g; softness rand electro-
negativity v were obtained for the studied molecules to vFe ; vinh and gFe ; ginh indicate the absolute electronegativity
evaluate their activity on the iron surface as follows[54]: and the absolute hardness of iron and the inhibitor, respect-
IP ¼ EHOMO (22) ively. The used values of vFe and gFe are 7 and 0 eV mol1,
respectively. Figure 18 and Figure 19 show the quantum
EA ¼ ELUMO (23) chemical results of H2L-H and H2L-Br molecules calculated
by the DFT/B3LYP method with 6-31 G (d,p) basis set such
DEGAP ¼ ELUMO EHOMO (24) as the optimized structure, highest occupied molecular
orbital HOMO and lowest unoccupied molecular orbital
IPEA LUMO. It is clear that the LUMO energy orbital is located
g¼ (25) on the carbon atoms, while the HOMO energy orbital is dis-
2
tributed throughout the molecule and mainly located on the
1 nitrogens, the carbonyl group, because of the presence of
r¼ (26) free electron pairs in the O and N atoms. They are also
g
localized in the imine groups, as well as the aromatic rings
14 I. BENMAHAMMED ET AL.

Figure 15. EDS spectra of carbn steel surfaces: (a) carbon steel after 24 hours immersion in 1 M HCl, (b) carbon steel after 24 hours immersion in 1 M HCl þ
2.5  103 M of H2L-H, (c) carbon steel after 24 hours immersion in 1 M HCl þ2.5  103 M of H2L-Br.

Table 9. Percentage atomic contents of elements obtained from EDS spectra of DEGAP is weaker, the easier it is for the inhibitory mole-
for H2L-H and H2L-Br.
cules to adsorb on the surface of the metal.[57] The DE value
Inhibitor Fe Cl C N O Br
of H2L-Br is lower than that of H2L-H, so H2L-Br manifests
Blank 63.37 24.90 11.72 / / /
H2L-H 74.17 10.85 9.76 3.42 1.80 / higher inhibition efficiency. This is very well correlated with
H2L-Br 71.28 4.79 9.12 2.71 11.94 0.15 the experimental results. The obtained values of l follow the
order of H2L-Br > H2L-H, which proves the strong adsorp-
present in the H2L-H and H2L-Br inhibitory molecules. tion of H2L-Br.[58] This is similar to experimental inhibitory
These adsorption centers can provide good inhibitory cover- efficiencies. It is reported that inhibitory molecules have a
age. The quantum chemical parameters are listed in Table strong tendency to give electrons to the metal, if
11. The inspection of the results reveals that H2L-Br takes 0 < DN < 3:6:[59] According to the Table 11, DN takes posi-
the lowest value of ELUMO compared to H2L-H, suggesting tive values and lower than 3.6, which suggest the ease of
that H2L-Br has more capacity to accept electrons. The electron transfer from the inhibitor to the vacant d-orbital
adsorption behavior of the inhibitory molecules is also based of the metal surface. It is obvious that the chemical hardness
on the energy gap ðDEGAP ¼ ELUMO EHOMO Þ: More value is the resistance to deformation or polarization of the
 JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY 15

Figure 17. UV-Visible spectra of the inhibitors H2L-H and H2L-Br before and
Figure 16. FTIR of synthesized Schiff bases and (synthesized Schiff bases þ me- after 24 hours immersion of carbon steel in 1 M HCl medium.
tal surface).

Table 10. Spectroscopic adsorption (IR) (cm1) of Schiff bases H2L-H and medium. It has been reported in the literature that mole-
H2L-Br. cules that have atoms with high negative charges are often
Inhibitor rC¼O rC¼N rC¼C Ar rCN Ar rCN more likely to give electrons to the metal surface.[61] In this
H2L-H 1662 1600 1450–1550 1232 1172 case, the inhibitor is able to interfere with the metal surface
H2L-H 1 Metal 1650 1575 1380–1480 1350 1020
H2L-Br 1737 1658 1470–1550 1232 1063
through such atoms. Figure 19 shows Mullikan’s charges for
H2L-Br 1 Metal 1653 1558 1390–1480 1350–1370 1020 H2L-H and H2L-Br. The results obtained show that the
negative charges are specifically centered in the nitrogen
electron cloud of atoms, ions or molecules when the chem- atoms, in the oxygen atom and in the aromatic rings for
ical reaction undergoes a small perturbation. So, the adsorp- both inhibitors. These sites are likely to form adsorption
tion capacity of the inhibitory molecules can also refer to bonds, thus promoting the inhibition of the metal surface
the softness. A high value of softness elucidates a large against corrosion.
adsorption of inhibitory molecules on the metal surface
because the soft molecule has a small difference in energy Molecular dynamic simulation
unlike the hard molecule.[60] The softness values obtained
explain the adsorption of Schiff bases H2L-H and H2L-Br on Figure 20 shows the most appropriate configurations for the
the iron surface by inhibiting its corrosion. adsorption of Schiff bases H2L-H and H2L-Br on the Fe (1 1
Organic molecules can be protonated in an aqueous 0) substrate. It can be seen that the tested products
medium; and also adsorbed on the metal surface. The quan- approach the Fe (1 1 0) surface with an almost flat orienta-
tum parameters obtained by the protonated form shown in tion. It is obvious that the presence of free electron pairs in
Table 11 clearly show that after protonation, the DEGAP val- the molecular structures of the studied inhibitors, especially
ues and those of the dipolar moments l increase for both in terms of aromatic rings, heteroatoms and oxygen, makes
inhibitors. Suggesting that the protonated form can compete it possible to form stable coordination bonds with vacant-d
with the neutral form to ensure good adsorption of inhibi- orbital of iron. This prevents the metal surface from being
tory molecules on the carbon steel surface in the corrosive in contact with the acidic solution. Table 12 summarizes the
16 I. BENMAHAMMED ET AL.

Figure 18. Quantum chemical results of H2L-H and H2L-Br molecules calculated by the DFT/B3LYP method with 6-31 G (d,p) basis set: (a) optimized molecular struc-
ture with Milken charges values, (b) total electron density surface mapped with electrostatic potential.

Figure 19. Frontier molecule orbital density distributions of H2L-H and H2L-Br molecules: (a): HOMO, (b): LUMO.

outputs calculated by the molecular dynamic simulation for configuration is described as the sum of the energies of the
adsorption of Schiff bases H2L-H and H2L-Br on the surface adsorbate components. In our study, the substrate energy
Fe (110). The total energy of the substrate–adsorbate ET (carbon steel surface) is taken as zero.
 JOURNAL OF DISPERSION SCIENCE AND TECHNOLOGY 17

Table 11. The calculated quantum chemical parameters for the inhibitors obtained using DFT at the B3LYP/6-31G (d,p) basis set.
Inhibitor EHOMO E LUMO DE l IP EA g r v DN
Neutral form H2L-H 4.74 1.82 2.92 2.91 4.74 1.82 1.46 0.68 3.28 1.27
H2L-Br 4.88 1.98 2.90 5.13 4.88 1.98 1.45 0.68 3.43 1.23
Protonated form H2L-H 5.29 1.33 3.96 3.05 5.29 1.33 1.98 0.50 3.31 0.93
H2L-Br 5.37 1.52 3.85 5.70 5.37 1.52 1.92 0.51 3.44 0.92

Figure 20. Equilibrium adsorption configurations of H2L-H and H2L-Br on Fe (1 1 0) surface obtained by molecular dynamic simulations. (a) Top view, (b) Side view.

Table 12. Outputs and descriptors calculated by the molecular dynamic simu-
lations for adsorption of H2L-H and H2L-Br on Fe (1 1 0) surface.
Inhibitor H2L-H H2L-Br
Total energy ET (kJ mol1) 131.24 128.07
Adsorption energy Eads (kJ mol1) 138.88 142.95
Rigid adsorption energy (kJ mol1) 140.33 143.80
Deformation energy DE (kJ mol1) 1.44 0.85
dEads =dN i (kJ mol1) 138.88 142.95

The adsorption energy Eads is defined as the energy deliv-

ered (or required) when the relaxed adsorbate components
are adsorbed on the substrate and can be expressed by:
Figure 21. The inhibition mechanism.
Eads ¼ R:A:E þ DE (29)
Where R:A:E defines the rigid adsorption energy and reports
the energy, released (or required) when the unrelaxed adsorption of these compounds on the surface Fe (1 1 0).[63]
adsorbate components (i.e., before the geometry optimiza- It can also be observed that the adsorption energy Eads for
tion step) are adsorbed on the substrate, DE defines the H 2L-Br is  more negative than that of H2L-H
deformation energy and reports the energy released when ðH2 LBr > H2 LHÞ:This reflects the strong adsorption
the adsorbed adsorbate components are relaxed on the sub- behavior of H2L-Br on the Fe surface than that of H2L-H.
strate surface. This correlates well with the experimental inhibitory
(dEads =dN i ) described the energy of substrate–adsorbate efficiencies.
configurations where one of the adsorbate components has
been removed. Small values of ET reflect the stability of our
Proposed mechanism of inhibition
Schiff bases.[62] It is clear from Table 12 that the Eads values
take negative signs for both studied inhibitors during the The anticorrosive power of organic inhibitors generally
simulation process, which suggests the spontaneity of the results from the formation of a protective layer on the metal
18 I. BENMAHAMMED ET AL.

surface due to physical or chemical adsorption behavior or  The action of the studied Schiff bases as effective corro-
both. Based on the results obtained from the electrochemical sion inhibitors relies mainly on their adsorption capacity
experiments and quantum chemistry calculations, the mech- on the metal surface by obeying the Langmuir isotherm.
anisms presented in Figure 21 can be proposed for the  Inhibitory efficiency decreases with increasing rotational
studied inhibitory molecules. After the initial dissolution of speed. This suggests that the hydrodynamic effect is a
the carbon steel, the inhibitory molecules try to bind with destructive effect on inhibitor performance.
the Fe2þ to form a metal/inhibitor complex which is  The study of the synergetic effect revealed that the used
adsorbed on the metal surface by electrostatic binding thus additives adsorb cooperatively to form a protective layer
preventing its subsequent dissolution in the acidic solution on the metal surface.
(physisorption).  SEM and AFM techniques have confirmed the formation
of a protective layer on the carbon steel surface.
Fe $ Fe2þ þ 2 e (30)  Quantum chemical calculations have shown a good cor-
relation with experimental results.
2þ  The inhibitory efficacy follows the order H2L-Br > H2L-
Fe2þ þ inh $ ½Feinh (31)
H. Bromo substitute inhibit iron corrosion due to
Inhibitory molecules can easily be protonated in acidic increase the delocalization of electron density in the mol-
solution. These protonated species can be adsorbed on the ecule, which makes the molecule more stable, i.e. bet-
metal surface by electrostatic interactions. Cl ions are ter inhibition.
adsorbed on the metal surface which consequently becomes
negatively charged. So facilitates the protonated inhibitors Acknowledgements
adsorption.
The authors would like to thank Electrochemistry laboratory of
inh þ xH þ $ ½inhH x 
xþ molecular and complex materials LEMMC, Ferhat ABBAS University-
(32)
Algeria, and Department of Chemistry, Hashemite University-Jordan.
þ
½inhH x xþ þ Fe2þ $ ½inhH x Feð2þxÞ (33)
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