Schiff's bases derived from l-lysine and aromatic aldehydes as green corrosion inhibitors for mild steel: Experimental and theoretical studies

Article abstract of DOI:10.1016/j.molliq.2015.12.027

Three Schiff's bases (SBs) namely, 2-amino-6 (2-hydroxybenzelideneamino) hexanoic acid (SB-1), 2-amino-6-(4-methoxybenzelideneamino) hexanoic acid (SB-2) and 2-amino-6-((4-dimethylamino)benzylideneamino) hexanoic acid (SB-3) derived from lysine (amino aci

Full text of DOI:10.1016/j.molliq.2015.12.027

Journal of Molecular Liquids 215 (2016) 47–57
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Schiff's bases derived from L-lysine and aromatic aldehydes as green
corrosion inhibitors for mild steel: Experimental and theoretical studies
⁎
Neeraj Kumar Gupta, Chandrabhan Verma, M.A. Quraishi , A.K. Mukherjee
Department of Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2015
Received in revised form 21 November 2015
Accepted 8 December 2015
Available online xxxx
Three Schiff's bases (SBs) namely, 2-amino-6 (2-hydroxybenzelideneamino) hexanoic acid (SB-1), 2-amino-6-
(4-methoxybenzelideneamino) hexanoic acid (SB-2) and 2-amino-6-((4-dimethylamino)benzylideneamino)
hexanoic acid (SB-3) derived from lysine (amino acid) and three different aldehydes were synthesized and eval-
uated as corrosion inhibitors for mild steel in 1 M HCl solution using weight loss, electrochemical, scanning elec-
tron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and atomic force microscopy methods. The
results showed that inhibition efficiency increases with the increasing concentration. Among the studied SBs the
SB-3 showed maximum inhibition efficiency of 95.6% at 400 mg L−1 concentration. Potentiodynamic polarization
study revealed that the investigated SBs act as cathodic type inhibitors. Adsorption of the SBs on mild steel sur-
face obeys the Langmuir adsorption isotherms. The weight loss and electrochemical results were well supported
by SEM, EDX and AFM analyses.
Keywords:
GREEN SCHIFF'S BASES
Mild steel
Acid corrosion
SEM/EDX/AFM
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
replaced by environmental friendly non-toxic inhibitors. Amino acid
based Schiff's bases are safe and environmentally benign corrosion in-
Mild steel is widely used in several industries as construction mate-
rial due to its high mechanical properties and low cost [1,2]. However, it
is a highly reactive alloy and is very prone to corrosion during many in-
dustrial processes including acid cleaning, etching, acid pickling, and
acid descaling [3,4]. Among the various available methods of corrosion
protection, the consumption of the synthetic corrosion inhibitors is
one of the most practical and cost effective method [5,6]. Generally, or-
ganic inhibitors inhibit metallic corrosion by adsorbing on the surface
and thereby forming a protective barrier between metal and electrolyte
(1 M HCl) [7]. The adsorption of these inhibitors on metallic surface are
influenced by several factors such as molecular size of inhibitor, nature
of substituents, nature of metal and electrolyte [8–10]. Organic com-
pounds containing heteroatoms including nitrogen, sulfur, and/or oxy-
gen with polar functional groups and conjugated double bonds have
been reported as effective corrosion inhibitor [11]. The Schiff's bases
are important class of compounds characterized by the presence of –
CH_N– group. Schiff's bases constitute potential class of corrosion in-
hibitors and besides this they have many interesting properties and ex-
tensive applications in medicinal, agricultural, pharmaceutical fields
and material science. Due to the great flexibility and diverse structural
aspects, a wide range of Schiff bases have been synthesized and utilized
as corrosion inhibitors for metal in aggressive media [12–21]. Most of
the available Schiff's base inhibitors are toxic in nature and should be
hibitors [22–25]. Previously, some Schiff base compounds have been
studied which have shown good inhibition efficiency (70–98%) in the
concentration range of 350–2700 ppm [26–28]. In view of above obser-
vation it is thought worthwhile to synthesize three Schiff's bases from
lysine (amino acid) with three different aldehydes namely, 2-amino-
6(2-hydroxybenzelideneamino) hexanoic acid (SB-1), 2-amino-6-(4-
methoxybenzelideneamino) hexanoic acid (SB-2), 2-amino-6-(4-
methylamino) benzylideneamino) hexanoic acid (SB-3) to investigate
their corrosion inhibition properties on mild steel in 1 M HCl solution.
In our case, the best inhibition efficiency (95.6%) was shown by SB-3
at 400 ppm. The choice of these compounds is based on the facts that:
(a) they can be synthesized from commercially available cheap and
green starting materials, (b) they are likely to show good inhibition
efficiency, (c) they have aromatic ring, −CH_N– and hetero-atoms
(N, O) through which they can adsorb on the metal surface and inhibit
corrosion, and (d) the presence of the –CH_N− group in Schiff bases
increases their adsorption ability.
2. Experimental section
2.1. Synthesis of corrosion inhibitor
The SBs used in present study were synthesized according to a meth-
od described earlier [29]. The synthetic scheme for SBs is shown in Fig. 1.
The purity of the synthesized SBs was determined by thin-layer chro-
matography using ethyl acetate/n-hexane (4:6) on the silica plate TLC
⁎
Corresponding author.
E-mail addresses: maquraishi.apc@itbhu.ac.in, maquraishi@rediffmail.com
(M.A. Quraishi).
http://dx.doi.org/10.1016/j.molliq.2015.12.027
0167-7322/© 2015 Elsevier B.V. All rights reserved.

48
N.K. Gupta et al. / Journal of Molecular Liquids 215 (2016) 47–57
Fig. 1. Synthetic rout for investigated SBs.
Table 1
IUPAC name, molecular structure, molecular formula, melting point and analytical data of studied SBs.
S.
Name of inhibitor
Chemical structure
Analytical data
No.
1
2-Amino-6 (2-hydroxybenzelideneamino)
hexanoic acid (SB-1)
C13H18N2O3 (mol. wt. 250.29);
IR (KBr cm−1): 3663, 3534, 2848, 2288, 1776, 1733, 1680, 1678, 926, 787, 645; 1H NMR
(300 MHz, DMSO) δ (ppm): 1.58, 1.67, 1.74, 3.42, 3.71, 6.87, 7.27, 7.42, 8.88, 9.42
2
3
2-Amino-6-(4-methoxybenzelideneamino)
hexanoic acid (SB-2)
C14H20N2O3 (mol. wt. 264.14); IR (KBr cm−1): 3665, 3491, 2854, 2358, 1783, 1635, 1250,
1163, 960, 823, 621; 1H NMR (300 MHz, DMSO) δ (ppm): 1.25, 1.57, 1.85, 3.58, 3.71, 5.15,
7.41, 7.52, 7.73, 8.57
2-Amino-6-((4-dimethylamino)
benzylideneamino) hexanoic acid (SB-3)
C15H23N3O2 (mol. wt. 277.36);
IR (KBr cm−1): 3649, 3565, 2818, 2334, 1794, 1741, 1707, 1690, 945, 790, 626; 1H NMR
(300 MHz, DMSO) δ (ppm): 1.27, 1.78, 1.93, 3.09, 3.79, 4.54, 5.14, 6.32, 7.54, 8.98
Table 3
Variation of corrosion rate with temperature in absence and presence of optimum concen-
tration of SBs.
Temperature (K)
Corrosion rate (CR) (mg cm−2 h−1
)
Blank
SB-1
SB-2
SB-3
308
318
328
338
7.60
11.0
14.3
18.6
0.46
2.03
3.40
5.73
0.40
1.90
3.13
5.43
0.33
1.73
2.96
5.23
Fig. 2. Variation of inhibition efficiency with SBs concentration.
Table 2
The weight loss parameters derived for mild steel in 1 M HCl at different concentrations of
SBs.
Inhibitor
Conc (mg L−1
)
CR (mg cm−2 h−1
)
Surface coverage (θ)
η%
Fig. 3. Arrhenius plots of log CR versus 1000/T for mild steel corrosion in 1 M HCl solution
in absence and presence of SBs.
Blank
SB-1
0.0
7.66
1.76
1.20
0.83
0.46
1.70
1.06
0.73
0.40
1.56
0.86
0.60
0.33
–
–
100
200
300
400
100
200
300
400
100
200
300
400
0.770
0.844
0.891
0.939
0.778
0.861
0.904
0.947
0.796
0.887
0.921
0.956
77.0
84.4
89.1
93.9
77.8
86.1
90.4
94.7
79.6
88.7
92.1
95.6
Table 4
Activation parameters for mild steel dissolution in 1 M
HCl in the absence and presence of optimum concentra-
tion of SBs.
SB-2
SB-3
Inhibitor
Ea (kJ mol−1
)
Blank
SB-1
SB-2
SB-3
28.48
70.67
74.71
80.26

N.K. Gupta et al. / Journal of Molecular Liquids 215 (2016) 47–57
49
Fig. 4. Langmuir adsorption plot for mild steel in 1 M HCl solution in presence SBs.
Table 5
The values of Kads and Δ G°ads for mild steel in absence and presence of optimum concen-
tration of SBs in 1 M HCl at different temperature.
Inhibitor
Kads (103 M−1
)
−ΔG°ads (kJ mol−1
)
308
318
328
338
308
318
328
338
SB-1
SB-2
SB-3
18.4
21.4
26.0
5.20
5.73
6.39
3.57
3.97
4.25
2.33
2.52
2.67
35.45
35.82
36.33
33.29
33.81
33.89
33.27
33.56
33.75
33.09
33.31
33.47
Fig. 5. Potentiodynamic polarization plots for mild steel in 1 M HCl solution in the absence
and presence of optimum concentration of SBs.
Fig. 6. (a–c): (a) Nyquist plots for mild steel in 1 M HCl solution in the absence and pres-
ence of an optimum concentration of SBs. (b) Equivalent circuit used to fit the EIS data for
mild steel in 1 M HCl solution. (c) Bode plots for mild steel in 1 M HCl solution in the ab-
sence and presence of SBs at optimum concentration.
plates aluminum (Al) silica. The chemical structure, IUPAC name and
analytical data of synthesized SBs are given in Table 1.
balance Fewere used for chemical, electrochemical and surface experi-
2.2. Materials and chemicals
ments. The specimens' size was 2.5
× 2 × 0.025 cm and
8 × 1 × 0.025 cm for weight loss and electrochemical experiments, re-
spectively. The aggressive test solution of 1 M HCl solution was
The mild steel specimens with chemical composition (wt.%): C =
0.076, Mn = 0.192, P = 0.012, Si = 0.026, Cr = 0.050, Al = 0.023 and
Table 6
Tafel Polarization parameters for mild steel in 1 M HCl solution in absence and presence of
optimum concentration of SBs.
Table 7
Electrochemical impedance parameters obtained from EIS measurements for mild steel in
1 M HCl in absence and presence of optimum concentration of SBs.
Inhibitor
Ecorr
icorr (μA
βa
βc
θ
η(%)
(mV/SCE)
cm−2
)
(mV/dec)
(mV/dec)
Inhibitor Rs (Ω cm2) Rct (Ω cm2) Cdl (μF cm−2
)
n
ϴ
η%
Blank
SB-1
SB-2
SB-3
−445
−519
−531
−519
1150
70.5
83.3
90.7
70.20
114.6
143.9
151.5
121.9
–
–
Blank
SB-1
SB-2
SB-3
1.12
0.96
0.99
0.88
9.58
129.04
164.90
197.91
106.21
37.92
27.75
26.63
0.827
–
–
98.5
91.8
64.2
0.9143
0.9201
0.9441
91.43
92.01
94.41
0.829 0.9257 92.57
0.840 0.9419 94.19
0.852 0.9515 95.15

50
N.K. Gupta et al. / Journal of Molecular Liquids 215 (2016) 47–57
prepared by dilution of analytical grade HCl (37%) with double distilled
water.
2.4. Electrochemical experiment
Electrochemical measurements were performed by the method
as described previously [30]. The electrochemical impedance mea-
surements (EIS) were performed on mild steel specimens in the fre-
quency range of 100 kHz to 0.01 Hz under potentiostatic conditions
using an AC at open circuit potential with amplitude of 10 mV peak
to peak. The charge transfer resistance was calculated from Nyquist
plot from which corrosion inhibition efficiency was calculated
using following equation:
2.3. Gravimetric experiment
The weight loss experiments were performed using the standard
method described earlier [27]. The corrosion rate was calculated using
equation:
W
At
CR
¼
ð1Þ
Rict−R0ct
η% ¼
ꢀ 100
ð4Þ
Rict
where W is the mean value of weight loss of three parallel mild steel
coupons, A is the total area of a mild steel coupon, and t is the immersion
time (3 h). The percentage of inhibition efficiency (η %) and surface cov-
erage (θ) was calculated from the evaluated corrosion rates using fol-
lowing relation:
where Rict and Rc0t are charge transfer resistances in presence and ab-
sence of SBs, respectively.
The potentiodynamic polarization studied were performed on mild
steel specimens by automatically changing the electrode potential
from −250 to +250 mV/SCE versus open circuit potential at a scan
rate of 1 mV s−1. The corrosion current density (icorr) was calculated
by extrapolating the linear segments of the cathodic and anodic Tafel
slopes from which corrosion inhibition efficiency was calculated using
following equation:
CR−C
η% ¼
RðiÞ ꢀ 100
ð2Þ
ð3Þ
CR
CR−CRðiÞ
i0corr−iicorr
θ ¼
CR
η% ¼
ꢀ 100
ð5Þ
i0corr
where CR and CR(i) are the corrosion rate (mg cm−2 h−1) values of mild
where, i0corr and ici orr are the corrosion current densities in absence and
steel coupons in the absence and presence of inhibitors, respectively.
presence of SBs.
Fig. 7. (a–d): SEM image of mild steel surface after 3 h immersion (a) without SBs, (b) with SB1, (c) with SB2, (d) with SB3.

N.K. Gupta et al. / Journal of Molecular Liquids 215 (2016) 47–57
51
Fig. 8. (a–d): EDX image of mild steel surface after 3 h immersion (a) without SBs, (b) with SB1, (c) with SB2, (d) with SB3.
2.5. SEM/EDX study
2.6. AFM analysis
The mild steel was immersed in 1 M HCl solution in absence and
presence of optimum concentration of the SBs for 3 h immersion
time. Thereafter, the mild steel specimens were taken out, washed
with double distilled water, dried and finally analyzed by SEM and
EDX method. The SEM study was carried out using a Zeiss Evo 50
XVP instrument at an accelerating voltage of 5 kV and 500×
magnification.
The AFM analysis was performed using NT-MDT multimode AFM,
Russia, controlled by Solver scanning probe microscope controller. The
single beam cantilever having resonance frequency in the range of
240–255 kHz in semi-contact mode with corresponding spring constant
of 11.5 Nm−1 containing NOVA program was used for image interpreta-
tion. The scanning area in the images was 10 μm × 10 μm.
3. Result and discussion
3.1. Weight loss experiment
Table 8
Percentage atomic contents of elements obtained from EDX spectra for SBs.
3.1.1. Effect of SBs concentration
Inhibitor
Fe
C
N
O
The variation of η% with SBs concentration is shown in Fig. 2 and sev-
eral weight loss parameters are given in Table 2. From the results it is
clear that inhibition efficiency increases on increasing SBs concentration
and maximum efficiency was obtained at 400 mg L−1 concentration.
The order of inhibition efficiency of studied SBs follows the order: SB-
Blank
SB-1
SB-2
SB-3
63.09
62.43
61.73
59.54
36.10
29.31
23.84
22.65
–
–
–
6.48
9.54
8.37
7.89
8.36

52
N.K. Gupta et al. / Journal of Molecular Liquids 215 (2016) 47–57
3 N SB-2 N SB-1. The increase in η% on increasing SBs concentration is
due to increase in the surface coverage.
reaction occurring on the surface [33,34]. Moreover, the decrease in in-
hibition efficiency with increasing temperature signify the physical ad-
sorption that occurs during first stage of adsorption process [34]. The
increased value of Ea also suggest that presence of SBs rate of mild
steel dissolution decreased due to formation of metal-inhibitor complex
[34].
3.1.2. Effect of temperature
To evaluate the effect of temperature on the inhibition efficiency, the
weight loss experiments were also performed in the temperature range
of 308–338 K. The variation of corrosion rate (CR) with temperature is
represented in Table 3. From the results it can be observed that η% de-
creases with increasing temperature. The increase in temperature re-
sulted into desorption of the adsorbed SBs molecules from the mild
steel surface resulting into decrease in η% [31]. The effect of temperature
on corrosion rate can be best represented by Arrhenius equation [32]:
3.1.3. Adsorption isotherms and thermodynamic consideration
The adsorption isotherm is very important in understanding the
mechanism of interaction between metal surface and the inhibitor
[35]. Several adsorption isotherms were tested among which Langmuir
isotherm gave the best fit with values of regression coefficient (R2) very
close to unity. The values of regression coefficient were 0.9998, 0.9998
and 0.9999 for SB-1, SB-2 and SB-3, respectively. The Langmuir isotherm
can be represented as follows [36]:
ꢀ
ꢁ
−Ea
RT
CR ¼ A exp
ð6Þ
where, Ea is the apparent activation energy, T is the absolute tempera-
ture, A is the Arrhenius pre-exponential constant and R is the gas con-
stant. The Arrhenius plots of log CR vs 1/T of mild steel 1 M HCl
solution is shown in Fig. 3 from which the values of Ea were calculated
from the slope and listed in Table 4. The tabulated data revealed that
value of Ea for inhibited solution is greater than that of uninhibited solu-
tion. This increase in Ea in presence of SBs indicates the formation of
higher energy barrier for corrosion process suggesting that adsorbed
film of SBs film on mild steel surface prevents the charge/mass transfer
C
1
K
ðadsÞ
ðinhÞ
θ
¼
þ C
ð7Þ
ðinhÞ
where, Kads is the constant for adsorption–desorption processes occur-
ring on metal surface, θ is the surface coverage and C(inh) is the SBs con-
centration in mg L−1. The Langmuir isotherm plot (Fig. 4) gives a
straight line between log (θ/1–θ) and C(inh) and the value of Kads was cal-
culated and given in Table 4. The Kads values related with the free energy
Fig. 9. (a–d): 3D AFM image of mild steel surface after 3 h immersion (a) without SBs, (b) with SB1, (c) with SB2, (d) with SB3.

N.K. Gupta et al. / Journal of Molecular Liquids 215 (2016) 47–57
53
Fig. 9 (continued).
of adsorption (ΔG0ads) according to the relation [36]:
that investigated SBs are mixed type inhibitors and predominantly act
as cathodic inhibitors [39].
ΔGοads ¼ −RT lnð55:5Kads
Þ
ð8Þ
3.2.2. Electrochemical impedance spectroscopic study
Fig. 6(a) represents the Nyquist plots for mild steel in 1 M HCl solu-
tion in absence and presence of optimum concentration of the SBs. From
the Fig. 6(a) it could be observed that Nyquist plots give similar appear-
ance with and without SBs suggesting that SBs inhibit mild steel corro-
sion without affecting the mechanism [40]. The Nyquist plot consists of
a depressed semicircle at high frequency region which is a characteristic
response of solid metal electrodes in the corrosion process [41]. Imped-
ance parameters such as Rct, n, Cdl, θ and η% were derived from Nyquist
plot by implying equivalent circuit shown in Fig. 6(b) and given in
Table 7. The result showed that addition of SBs causes significant in-
crease in the Rct value suggesting that SBs retard the charge transfer re-
action and corrosion occurring on the mild steel surface by forming
protective film on the surface [42]. From the results it is also clear that
values of Cdl decreased in presence of SBs. The decrease in Cdl value is
due to the decrease in local dielectric constant and/or an increase in
the thickness of the electrical double layer. This increase in thickness
of electric double layer is due to the adsorption of the SBs on the surface.
This finding further suggests that the inhibition of mild steel corrosion is
due to adsorption mechanism [43]. The value of n is related to surface
roughness. Generally surface roughness increases with the decrease in
the value of n [44]. It can be observed that the values on n in presence
of SBs are larger (0.829 for SB-1, 0.840 for SB-2 and 0.852 for SB-3)
where, R is the universal gas constant, T is the absolute temperature and
value 55.5 represent the molar concentration of water in acid solution.
The calculated values of ΔG0
are given in Table 5. The value of
ads
ΔG0
for SBs lies in the range −33.09 to −36.33 kJ mol−1. These
ads
values are in between the threshold values for physical and chemical
adsorption suggesting that the adsorption of SBs on mild steel surface
involves both chemical and physical adsorption [31,37].
3.2. Electrochemical measurements
3.2.1. Polarization study
The Tafel polarization curves obtained for mild steel in absence and
presence of SBs at optimum concentration are shown in Fig. 5. Table 6
represents the polarization parameters i.e., corrosion potential (Ecorr),
cathodic (βc) and anodic (βa) Tafel slopes, corrosion current density
(icorr), surface coverage (θ) and the inhibition efficiency (η %) for mild
steel corrosion with and without SBs. The results showed that in pres-
ence of SBs corrosion current density is decreased due to formation of
protective film [38]. It is also observed that the addition of SBs retards
both cathodic and anodic reactions; however, the cathodic reactions
are comparatively more affected than the anodic reactions suggesting

54
N.K. Gupta et al. / Journal of Molecular Liquids 215 (2016) 47–57
Fig. 10. a–c: Optimized structure of SBs (a) SB-1, (b) SB-2, (c) SB-3.
than that in their absence suggesting that surface smoothness increases
in presence of SBs due to the formation of protective film [45].
Fig. 6(c) illustrates the bode magnitude and phase angle plots re-
corded for mild steel in 1 M HCl solution in absence and presence of
SBs at optimum concentration. It can be seen from the Bode plots that
interfacial impedance increases in presence of SBs. Moreover, bode
plots that give a single narrow peak in the phase angle plot suggest a
single time constant for the corrosion process at the metal–solution in-
terface in absence and presence of SBs. It can be observed that height of
the phase angle peaks increased in presence of SBs suggesting the more
capacitive response of the metal/electrolyte interface due to the pres-
ence of SBs molecules at the interface [46].
additional signals for N and O which suggests that in acid solution SBs
cover the mild steel that acts as barrier between metal and acid solution.
3.3.3. Atomic force microscopy
The 3D AFM micrographs of mild steel surface in absence and pres-
ence of optimum concentration of SBs are shown in Fig. 8. The AFM mi-
crograph of mild steel surface in absence of SBs is highly damaged and
severely corroded (Fig. 9(a)). However, in presence of SBs (Fig. 9b–d)
the surface morphologies significantly improved due to formation of
protective film by SBs. The calculated average surface roughness was
392 nm in absence of SBs, whereas in presence of SB-1, SB-2 and SB-3
the surface roughness were 167 nm, 143 nm and 84 nm, respectively.
This finding again suggests that SBs form protective barrier on the
mild steel surface that protects metal from the corrosion.
3.3. Surface study
3.4. Quantum chemical calculation
3.3.1. SEM analysis
The SEM micrographs for mild steel in absence and presence of SBs
are shown in Fig. 7a–d. Fig. 7(a), represents the SEM micrograph in ab-
sence of the SBs which is severely corroded due to attack of acid on mild
steel. However, in presence of SBs (Fig. 7b–d) the surface morphologies
remarkably improved due to the adsorption of SBs on mild steel surface
that protects the surface from acid corrosion.
The quantum chemical calculation was also performed to
established relation between molecular structure and inhibition effi-
ciency of investigated SBs. In last few decades' quantum chemical calcu-
lation based on the DFT theory have been proposed as a way for
calculating a number of molecular parameters which are directly related
to inhibition efficiency of any chemical inhibitor. Fig. 10 represents the
optimized molecular structure and Fig. 11 represents the frontier mo-
lecular orbital of the investigated SBs. Some common quantum chemical
indices such as, the energy of highest occupied molecular orbital
(EHOMO), the energy of lowest unoccupied molecular orbital (ELUMO),
the energy band gap (ΔE), and the dipole moment (μ) were calculated
from fully optimized SBs molecules and given in Table 9. According to
the Koopmans theorem the values of EHOMO and ELUMO related to the
3.3.2. EDX analysis
The EDX spectra of mild steel in absence and presence of SBs is
shown in Fig. 8 and the results are given in Table 8. The EDX spectrum
of mild steel without SBs is characterized by signals only for C and Fe.
Fig. 8b–d showed the EDX spectra of mild steel in presence of different
studied SBs. The EDX spectra in presence of SBs gives characteristic

N.K. Gupta et al. / Journal of Molecular Liquids 215 (2016) 47–57
55
Fig. 11. a–c: The frontier molecular orbital of studied NTDs (a) SB-1 (left, HOMO; right, LUMO), (b) SB-2 (left, HOMO; right, LUMO) and (c) SB-3 (left, HOMO; right, LUMO).
A ¼ −ELUMO
:
ð10Þ
ionization potential (I) and electron affinity (A) respectively as per fol-
lowing relations [47]:
The value of EHOMO related to the electron donating capability of the
molecules. Generally, high value of EHOMO point out the nature of the
molecule to provide electrons to an appropriate acceptor with vacant
molecular orbitals [48,49]. In our present study the values of EHOMO fol-
low the order: SB-3 N SB-2 N SB-1 which is consisted with the order of
inhibition efficiency. On the other hand a low value of ELUMO associated
with electron accepting tendency of the molecule from the metal sur-
face [49,50]. Literature survey reveals that a lower value of ΔE consisted
with high inhibition efficiency. In our present study the values of ΔE fol-
low the order: SB-3 b SB-2 b SB-1 which just converse with the order of
inhibition efficiency [50]. The calculated values of dipole moment (μ)
are 1.1593, 3.4071 and 3.8529 Debye for SB-1, SB-2 and SB-3,
I ¼ −EHOMO
:
ð9Þ
Table 9
Quantum chemical parameters of the investigated SBs.
Inhibitor
μ
EHOMO
(Hartree)
ELUMO
(Hartree)
ΔE
(Hartree)
σ
ρ
(Debye)
SB-1
SB-2
SB-3
1.1593
3.4071
3.8529
−0.17531
−0.20467
−0.20232
−0.00404
−0.03477
−0.04034
0.17127
0.1699
0.16198
0.0856
0.08495
0.08099
11.682
11.771
12.347

56
N.K. Gupta et al. / Journal of Molecular Liquids 215 (2016) 47–57
Fig. 12. Pictorial presentation of force acting between SBs and mild steel surface.
respectively. The higher dipole moment values of SBs as compared to
water (1.88 Debye) suggested that SBs strongly interact with metal
and form protective surface film by replacing the water present on the
surface [51,52]. The values of global harness (ρ) and global softness
(σ) were calculated using the values of EHOMO and ELUMO as given follow
[53]:
4. Conclusion
The SBs were found to act as effective and green corrosion inhibitors
for mild steel in 1 M HCl solution and their inhibition efficiency related
with concentration and chemical structure. Among the studied SBs, the
SB-3 shows the best inhibition efficiency of 95.6% at 400 mg L−1
concentration.
The adsorption of SBs on mild steel surface obeys the Langmuir ad-
sorption isotherm. The negative sign of ΔG0ads suggests that the SBs
adsorbed spontaneously on the surface. The potentiodynamic study re-
veals that in presence of SBs the cathodic reaction appears to be much
affected than the anodic reaction suggesting that SBs act as predomi-
nantly cathodic inhibitors. SEM/EDX and AFM analyses validate the
weight loss and electrochemical results. The weight loss, electrochemi-
cal and surface measurements were in good agreement.
1
ELUMO−EHOMO ΔE
ρ ¼
¼
¼
:
ð11Þ
ð12Þ
2ðI−AÞ
2
2
σ ¼ 1=ρ:
The calculated values of global hardness and softness are given in
Table 9. It is reported that magnitude of electron transfer from inhibitor
to metal and therefore inhibition efficiency increases with decreasing
the hardness of the inhibitor. Whereas, the high value of global softness
consisted with high inhibition efficiency [54]. In our present study the
values of global softness follow the order: SB-3 N SB-2 N SB-1 which is
in accordance with the inhibition efficiency.
Acknowledgment
Neeraj Kumar Gupta and Chandrabhan Verma, gratefully acknowl-
edged Ministry of Human Resource Development (MHRD), New Delhi
(India) for support.
3.5. Mechanism of inhibition
References
It has been reported that organic compounds inhibit metal corrosion
in acid solution via adsorption mechanism. The adsorption of inhibitor
on the mild steel surface is influenced by several factors including
chemical structure of inhibitor and charge at metal/solution interface
[55,56]. The SBs can exist in protonated form in 1 M HCl solution. More-
over, due to the presence of hydrated chloride ions the surface of mild
steel bears excessive negative charge, attracting the positively charged
SBs [57]. The adsorption of SBs on mild steel surface takes place by
two ways: first, by electrostatic attraction between positively charged
SBs and negatively charged mild steel surface. The second way is the
donor–acceptor interaction between neutral SBs and d-orbital of the
surface Fe-atoms [58]. During first stage of adsorption, the protonated
form of SBs competes with aqueous H+. However, after release of H2
gas, the SBs return in their neutral form [59]. As the hetero-atoms return
to their neutral form, the transfer of unshared electrons pair into d-
orbital takes place [60]. However, this electron transfer causes accumu-
lation of extra negative charge on the metal surface which renders it to
transfer its electrons to the anti-bonding molecular orbitals of the SBs
(retro-donation). Both donation and retro-donation strengthen adsorp-
tion of the SBs molecules on mild steel surface through synergism [61].
The pictorial presentation of the interaction responsible for adsorption
of SBs on mild steel surface is shown in Fig. 12. From the weight loss,
electrochemical and surface study the observed efficiency order was:
SB-3 N SB-2 N SB-1. The highest inhibition efficiency of SB-3 among
three studied SBs is due to presence of strong electron releasing – N
(CH3)2 group.
[1] Y. Sasikumar, A.S. Adekunle, L.O. Olasunkanmi, I. Bahadur, R. Baskar, M.M. Kabanda,
I.B. Obot, E.E. Ebenso, J. Mol. Liq. 211 (2015) 105–118.
[2] C. Verma, M.A. Quraishi, L.O. Olasunkanmi, E.E. Ebenso, RSC Adv. 5 (2016)
85417–85430.
[3] M. Yadav, S. Kumar, N. Tiwari, I. Bahadur, E.E. Ebenso, J. Mol. Liq. 212 (2015)
151–167.
[4] M. Yadav, S. Kumar, R.R. Sinha, I. Bahadur, E.E. Ebenso, J. Mol. Liq. 211 (2015)
135–145.
[5] C. Verma, A. Singh, M.A. Quraishi, J. Mol. Liq. 212 (2015) 804–812.
[6] M. Yadav, S. Kumar, I. Bahadur, D. Ramjugernath, Int. J. Electrochem. Sci. 9 (2014)
6529–6550.
[7] C. Verma, E.E. Ebenso, I. Bahadur, I.B. Obot, M.A. Quraishi, J. Mol. Liq. 212 (2015)
209–218.
[8] H. Vashisht, I. Bahadur, S. Kumar, K. Bhrara, D. Ramjugernath, G. Singh, Int. J.
Electrochem. Sci. 9 (2014) 2896–2911.
[9] C. Verma, P. Singh, M.A. Quraishi, J. Assoc. Arab. Univ. Basic Appl. Sci. xx
(2015)http://dx.doi.org/10.1016/j.jaubas.2015.04.003.
[10] H.L. Wang, H.B. Fan, J.S. Zheng, Mater. Chem. Phys. 77 (2002) 655–661.
[11] M. Yadav, D. Behera, S. Kumar, R.R. Sinha, Ind. Eng. Chem. Res. 52 (2013)
6318–6328.
[12] E. Naderi, M. Ehteshamzadeh, A.H. Jafari, M.G. Hosseini, Mater. Chem. Phys. 120
(2010) 134–141.
[13] K.S. Jacob, G. Parameswaran, Corros. Sci. 52 (2010) 224–228.
[14] M. Behpour, S.M. Ghoreishi, N. Mohammadi, N. Soltani, M.S. Niasari, Corros. Sci. 52
(2010) 4046–4057.
[15] M.A. Hegazy, A.M. Hasan, M.M. Emara, M.F. Bakr, A.H. Youssef, Corros. Sci. 65 (2012)
67–76.
[16] A. Asan, S. Soylu, T. Kıyak, F. Yıldırım, S.G. Oztas, N. Ancın, M. Kabasakaloglu, Corros.
Sci. 48 (2006) 3933–3944.
[17] S. Safak, B. Duran, A. Yurt, G. Türkoglu, Corros. Sci. 54 (2012) 251–259.
[18] A.A. Farag, M.A. Hegazy, Corros. Sci. 74 (2013) 168–177.
[19] L. khandelwal, C.B. Verma, M.A. Quraishi, J. Mater, Environ. Sci. 6 (2015) 810–817.
[20] K.R. Ansari, M.A. Quraishi, A. Singh, Corros. Sci. 79 (2014) 5–15.

N.K. Gupta et al. / Journal of Molecular Liquids 215 (2016) 47–57
[42] B. Zhang, C. He, C. Wang, P. Sun, F. Li, Y. Lin, Corros. Sci. 94 (2015) 6–20.
57
[21] A. Yurt, Ö. Aykın, Corros. Sci. 53 (2011) 3725–3732.
[22] G. Gece, S. Bilgiç, Corros. Sci. 52 (2010) 3435–3443.
[23] K.F. Khaled, Corros. Sci. 52 (2010) 3225–3234.
[24] N.O. Eddy, J. Adv. Res. 2 (2011) 35–47.
[43] K.F. Khaled, N. Hackerman, Mater. Chem. Phys. 82 (2003) 949–960.
[44] A.K. Singh, M.A. Quraishi, Corros. Sci. 53 (2011) 1288–1297.
[45] K.M. Emran, Int. J. Electrochem. Sci. 9 (2014) 4217–4229.
[25] K. Barouni, L. Bazzi, R. Salghi, M. Mihit, B. Hammouti, A. Albourine, S.E. Issami, Mater.
[46] D.K. Yadav, M.A. Quraishi, Ind. Eng. Chem. Res. 51 (2012) 8194–8210.
Lett. 62 (2008) 3325–3327.
[26] S. Issaadi, T. Douadi, A. Zouaoui, S. Chafaa, M.A. Khan, G. Bouet, Corros. Sci. 53 (2011)
1484–1488.
[27] K.C. Emregül, O. Atakol, Mater. Chem. Phys. 82 (2003) 188–193.
[28] M.A. Hegazy, Corros. Sci. 51 (2009) 2610–2618.
[29] A.S.P. Azzouz, R.T. Ali, Natl. J. Chem. 37 (2010) 158–168.
[47] C. Verma, A. Singh, G. Pallikonda, M. Chakravarty, M.A. Quraishi, I. Bahadur, E.E.
Ebenso, J. Mol. Liq. 209 (2015) 306–319.
[48] P.M.A. lvarez, J.F. Garcıˊa-Araya, F.J. Beltran, F.J. Masa, F. Medina, J. Colloid Interface
Sci. 283 (2005) 503–512.
[49] C. Verma, P. Singha, I. Bahadur, E.E. Ebenso, M.A. Quraishi, J. Mol. Liq. 209 (2015)
767–778.
[30] C.B. Verma, M.A. Quraishi, E.E. Ebenso, Int. J. Electrochem. Sci.
8
(2013)
[50] Y. Yana, W. Li, L. Caia, B. Hou, Electrochim. Acta 53 (2008) 5953–5960.
[51] M.S. Masoud, M.K. Awad, M.A. Shaker, M.M.T. El-Tahawy, Corros. Sci. 52 (2010)
2387–2396.
[52] C. Verma, M.A. Quraishi, A. Singh, J. Taiwan Inst. Chem. Eng. 000 (2015) 1–4, http://
dx.doi.org/10.1016/j.jtice.2015.06.020.
12894–12906.
[31] D. Daoud, T. Douadi, H. Hamani, S. Chafaa, M.A. Noaimi, Corros. Sci. 94 (2015) 21–37.
[32] M. Behpour, S.M. Ghoreishi, N. Soltani, M.S. Niasari, Corros. Sci. 51 (2009)
1073–1082.
[33] N.I. Kairi, J. Kassim, Int. J. Electrochem. Sci. 8 (2013) 7138–7155.
[34] M. Faustin, A. Maciuk, P. Salvin, C. Roos, M. Lebrini, Corros. Sci. 92 (2015) 287–300.
[35] R.S.A. Hameed, E.A. Ismail, A.H. Abu-Nawwas, H.I.A. Shafey, Int. J. Electrochem. Sci.
10 (2015) 2098–2109.
[53] Z. El Adnani, M. Mcharfi, M. Sfaira, M. Benzakour, A.T. Benjelloun, M. Ebn Touhami,
Corros. Sci. 68 (2013) 223–230.
[54] S. Deng, X. Li, H. Fu, Corros. Sci. 53 (2011) 760–768.
[55] X. Li, S. Deng, X. Xie, J. Taiwan Inst. Chem. Eng. 45 (2014) 1865–1875.
[56] V. María, F. Bimbi, P.E. Alvarez, H. Vaca, C.A. Gervasi, Corros. Sci. 92 (2015) 192–199.
[57] C. Verma, M.A. Quraishi, A. Singh, J. Taibah Univ. Sci. xx (2015)http://dx.doi.org/10.
1016/j.jtusci.2015.10.005.
[58] B. Xu, Y. Liu, X.S. Yin, W.Z. Yang, Y.Z. Chen, Corros. Sci. 74 (2013) 206–213.
[59] D.K. Yadav, B. Maiti, M.A. Quraishi, Corros. Sci. 52 (2010) 3586–3598.
[60] A.K. Singh, Ind. Eng. Chem. Res. 51 (2012) 3215–3223.
[61] P. Singh, M.A. Quraishi, E.E. Ebenso, C.B. Verma, Int. J. Electrochem. Sci. 9 (2014)
7446–7459.
[36] Y.A. Albrimi, A.A. Addi, J. Douch, R.M. Souto, M. Hamdani, Corros. Sci. 90 (2015)
522–528.
[37] M. Hany, A.E. Lateef, Corros. Sci. 92 (2015) 104–117.
[38] C.M. Goulart, A.E. Souza, C.A.M. Huitle, C.J.F. Rodrigues, M.A.M. Maciel, A. Echevarria,
Corros. Sci. 67 (2013) 281–291.
[39] C. Verma, M.A. Quraishi, A. Singh, J. Taiwan Inst. Chem. Eng. 49 (2015) 229–239.
[40] Q.B. Zhang, Y.X. Hua, Electrochim. Acta 54 (2009) 1881–1887.
[41] A.H.A. Hamzi, H. Zarrok, A. Zarrouk, R. Salghi, B. Hammouti, S.S.A. Deyab, Guenoun,
Int. J. Electrochem. Sci. 8 (2013) 2586–2605.