Evaluating corrosion inhibition property of some Schiff bases for mild steel in 1 M HCl: Competitive effect of the heteroatom and stereochemical conformation of the molecule

Article abstract of DOI:10.1039/c6ra03521c

To evaluate the effect produced by the heteroatom present in organic Schiff bases along with overall stereochemical conformation of such bases towards corrosion inhibition performance, three Schiff bases, namely 2-pyridyl-N-(2′-methylamino phenyl) methyle

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Evaluating corrosion inhibition property of some Schiff bases for
mild steel in 1M HCl: competitive effect of heteroatom and
stereochemical conformation of the molecule
Received 00th January 20xx,
Accepted 00th January 20xx
Alokdut Dutta,a Sourav Kr. Saha,b,c Priyabrata Banerjee,b,c Apurba K. Patraa and Dipankar
Sukula*
DOI: 10.1039/x0xx00000x
www.rsc.org/
To evaluate the effect produced by the heteroatom present in organic Schiff bases along with overall stereochemical
conformation of such bases towards corrosion inhibition performance, three Schiff bases, namely 2-pyridyl-N-(2'-
methylamino phenyl) methyleneimine (PMAM), 2-pyridyl-N-(2'-methylthiophenyl) methyleneimine (PMTM) and 2-pyridyl-
N-(2'-methoxyphenyl) methyleneimine (PMPM), are assessed for mild steel in 1M HCl. Chemical similarity exists among
these three molecules, varying only in the nature of the heteroatom present away from the imine group. Results from
weight loss, potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) techniques demonstrate
that the Schiff bases form a resistive layer on the metal surface, and thereby block the cathodic as well as the anodic
reaction sites effectively. Experimentally obtained corrosion inhibition efficiency follows the sequence:
PMAM>PMTM>PMPM, and this is also confirmed following surface electron microscopy (SEM) images. Langmuir isotherm
is found to provide sufficiently good description towards adsorption behavior of the compounds on the metal surface.
Density functional theory (DFT) calculations and molecular dynamics (MD) simulations are done to correlate the efficiency
of these compounds with their intrinsic molecular parameters, as well as energy optimized molecular orientation.
inhibitors to get adsorbed on the metal surface and thereby help in
reducing the aggressiveness of acid attack on it.7-12 Several reports
suggest the inhibitory effect of these heteroatoms follow the order:
Introduction
Mild steel is the most commonly used ferrous metal. Its widespread
usage is due to some unique properties associated with it, which
includes fairly high tensile strength, ductility, malleability, and most
importantly it is the cheapest form of steel available. One of the
main disadvantages encountered with mild steel is its susceptibility
towards corrosion. Hydrochloric acid (HCl), on the other hand, is a
highly corrosive, strong mineral acid with a wide array of industrial
applications. With growing industrial activities worldwide, the study
of corrosion process of mild steel in several acidic media, including
HCl, and its mitigation has now gained tremendous impetus. Among
many available methods to restrict the acid induced metal
degradation, application of organic corrosion inhibitors has now
become a vibrant field of research and great numbers of scientific
studies are devoted to this subject.1-6 Presence of heteroatoms,
such as O, N, S or P, as well as unsaturated bonds allows organic
P >S >N >O.13-14
Among several classes of organic inhibitors, Schiff bases are very
popular which may be attributable to the ease of synthesis from
relatively inexpensive starting-materials and their eco-friendly or
low toxic properties.15-16 The high inhibitory performance of these
compounds results from the presence of different heteroatoms
(e.g. N, O, Cl, Br) and π electrons in their structure, and most
importantly the imine functional group.17-21 The present work
intends to study inhibitive performance of three Schiff bases,
namely, 2-pyridyl-N-(2'-methylamino phenyl) methyleneimine
(PMAM),
2-pyridyl-N-(2'-methylthiophenyl)
methyleneimine
(PMTM) and 2-pyridyl-N-(2'-methoxyphenyl) methyleneimine
(PMPM) (Table 1) in towards corrosion of mild steel in 1M HCl
solution employing electrochemical as well as weight loss methods.
These inhibitors are having structural similarity, only differing in the
heteroatom at the 2'-substituent of phenyl ring. How these
heteroatoms, together with the stereochemical conformation, are
changing the intrinsic molecular parameters of the molecules,
which in-turn influence their adsorption and subsequent corrosion
inhibition properties, are being explored here. For this purpose,
various molecular parameters like energy of the frontier molecular
orbitals (HOMO and LUMO) in energy optimized geometry,
electronegativity, global hardness and softness, fraction of electron
a.Department of Chemistry, National Institute of Technology, Durgapur 713 209,
India. E-mail address: dipankar.sukul@gmail.com (D. Sukul).
Tel.:+919932244348; fax: +913432547375.
b.Surface Engineering & Tribology Group, CSIR-Central Mechanical Engineering
Research Institute, Durgapur 713209, India.
c. Academy of Scientific and Innovative Research, CSIR-CMERI Campus, Durgapur
713209, India
Electronic Supplementary Information (ESI) available: [Mass spectra, Rest potential
variation, Polarization diagrams, Nyquist plots, Equivalent circuit, Adsorption
isotherms, Fukui functions]. See DOI: 10.1039/x0xx00000x
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Table 1 Structure, abbreviation and characteristics of Schiff-bases.
Structural
Melting
point
ESI-MS (m/z): [L-H+]
(amu)
Molecules
formula
Characteristic IR peaks(cm-1)23
2-pyridyl-N-(2'-methylaminophenyl)
methyleneimine
(PMAM)
1570C
1850C
1680C
1647 (C=N)
1280 (C-N str. for aromatic
secondary amine)
212.1
229.2
213.1
H
N
N
N
N
HN
CH
3
2-pyridyl-N-(2'-methylthiophenyl) methyleneimine
(PMTM)
1622(C=N)
690 (C-S str. for aryl thioethers)
H
N
S
CH
3
2-pyridyl-N-(2'-methoxyphenyl) methyleneimine
(PMPM)
1624 (C=N)
1220 (aryl-O str.)
H
N
O
CH3
transferred have been determined by density functional theory removed completely. The oily residue is kept at 4°C for overnight to
(DFT) calculations. Local reactive sites of the inhibitor molecules are get solid residue, which is then recrystallized from diethyl ether.
analyzed through Fukui indices. Moreover, MD simulation is These compounds are characterized by melting point
employed to ascertain the adsorption behaviour of inhibitors on Fe measurement, molecular ion peak as obtained in the corresponding
(1 1 0) surface. Surface morphology of the corroded metal surface mass spectrum (Advion make compact mass spectrometer, Serial
in absence and presence of inhibitors are assessed by surface No. 3013-0140) and FT-IR (Thermo-Nicolet iS10 IR spectrometer in
electron microscopy (SEM) images.
the range 4000-400 cm-1) (Table 1, Fig. 1 and S1, ESI). Existence of
the molecular ion peak in the corresponding Mass spectrum
confirms the formation of the inhibitor molecules (supported vide
huge abundance ratio in ESI-MS).
Experimental details
Electrochemical measurements
Synthesis of inhibitors
Potentiodynamic polarization and electrochemical impedance
measurement are done by conventional three-electrode system
(model: Gill AC, ACM Instruments, UK) consisting of mild steel
working electrode (WE) with an exposed area of 0.25 cm2, platinum
as counter electrode and saturated calomel electrode (SCE) as
reference. Volume of HCl solution taken for each experiment is 200
mL. Before electrochemical tests, the WE is kept in the test solution
for 45 mins and the rest potential was checked for next 5 mins (Fig.
S2, ESI). No substantial variation of rest potential confirms that the
steady state is reached. Polarization curves are recorded within a
potential ranged from -250 to +250 mV with respect to rest
potential at constant sweep rate of potential 0.5 mV/sec, which is
proved sufficient to meet the demand of Tafel extrapolation tests of
mild steel in 1M HCl. Inhibition efficiency, ꢀꢁ(%) is defined as:
H
NH
X
2
N
MeOH
+
N
O
Refluxed for 6 hr.
N
H
X = -NH, -S, -O
CH
3
X
CH
3
Scheme 1 Mechanistic pathway for synthesis of Schiff-bases.
Inhibitors PMAM, PMPM and PMTM are synthesized as per
reported procedure (Schematically it is represented in scheme 1).22
For preparing PMAM, PMTM and PMPM, methanolic solution of
pyridine-2-carboxaldehyde is added to either of N-
methylphenylenediamine or 2-methyl thioaniline or o-Anisidine,
respectively (1:1 mole ratio) and refluxed under nitrogen for 6h.
After cooling to room temperature, the solvent is removed
completely and the thick oily residue is dissolved in CHCl3 and
washed successively with distilled water, brine solution and finally
again with distilled water. The organic layer is then separated out
and dried over anhydrous Na2SO4 for 1h, filtered and solvent is
icorr −i
(1)
η
P (%) =
corr(inh) ×100
icorr
where, icorr and icorr(inh) are the values of corrosion current density of
uninhibited and inhibited specimens, respectively.
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bristled brush, washed, dried in a vacuum desiccator and final
PMAM
1647
Ads. PMAM
weights are recorded. Percentage inhibition efficiency, ꢀꢋ(%) is
calculated following the relation:
ꢀꢋꢃ%ꢄ = ꢌ ꢇꢌ × ꢊꢉꢉ
(3)
ꢉ
ꢌꢉ
where, w0 and w are the weight loss of the metal coupons in acid
medium without and with inhibitor.
1605
PMPM
3015
All the electrochemical and gravimetric measurements are
carried out thrice and the values of inhibition efficiencies obtained
for three different sets of experiment are found to be very close
(within a range of ±1-2%).
1624
Ads. PMPM
2925
Surface morphology
PMTM
For morphological studies, the mild steel surface is prepared by
keeping the samples for 6h in 1M HCl in the absence and presence
of organic inhibitors at 10 mM concentration. The specimens are
then washed gently with water, carefully dried and analyzed by a
scanning electron microscopy using a Hitachi S-3000N microscope.
1622
Ads. PMTM
Computational details for quantum chemical calculation
DFT calculations are performed in this present study with the ORCA
programme (version 2.7.0).24 This is an open source code developed
by Prof. Dr. Franc Neese (Director, MPI für Chemische Energiekon
version, Muelheim, Germany). Geometry optimizations of all
inhibitor molecules are performed with B3LYP functional level25-26
of DFT employing triple-ζ quality basis sets TZV(P) with one set of
polarization functions on the atoms like N, O and S.27 For carbon
and hydrogens like atoms, slightly smaller polarized split-valence
SV(P) basis sets are used which are double-ζ quality in the valence
region and have polarizing set of d functions on atoms other than
hydrogens. SCF calculations are converged tightly (1 × 10−8 Eh in
energy, 1×10−7 Eh in the density and 1 × 10−7 in maximum element
of the DIIS error vector). Theoretical parameters are calculated in
the liquid phase because electrochemical corrosion always happens
in liquid phase. Therefore, it is necessary to include the effect of a
solvent in the computational outcomes. This method models the
solvent as a continuum of uniform dielectric constant (ε) where
solute is placed as a uniform series of inlocking atomic spheres.
Following Koopmans’ theorem, different intrinsic molecular
quantities like electronegativity (χ, a measure of the power of a
group of atoms to attract electrons towards itself), global hardness
(η, a parameter related to the resistance of an atom to a charge
transfer) and global softness (σ, which shows the reactivity of the
inhibitor molecules in terms of charge transfer), etc., which are
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber ( )
cm-1
Fig. 1 FTIR spectra of the synthesized Schiff base derivatives and
their surface adsorbed forms.
Electrochemical impedance spectroscopy (EIS) measurements
are performed at the rest potential over the frequency range 100
kHz to 10 mHz using 10 mV (rms) voltage excitation. Inhibition
efficiency, ꢀꢂ(%) is defined as:
ꢅꢆꢇꢅꢉꢈ
ꢃ
ꢄ
ꢀꢂ % =
× ꢊꢉꢉ
(2)
ꢅꢆ
0
where, Rp and Rp are the values of polarization resistance in
presence and absence of inhibitor. Rp represents the cumulative
effect arising due to different resistances prevailing at the metal-
electrolyte interface, namely, charge transfer resistance, resistance
due to adsorbed corrosion products as well as inhibitors. All tests
are carried out under unstirred conditions at room temperature of
around 30°C.
Weight loss measurements
The weight loss tests are performed on mild steel coupons (wt%
composition: 0.19 C, 0.21 Si, 0.21 Mn, 0.01 P, 0.01 S and the
remainder iron) with dimensions of 2.5 × 2.5 × 0.1 cm3. Metallic supposed to influence the overall reaction between two interacting
systems, have been calculated.28-30 These quantities are related to
specimens are polished with metallurgical grade emery papers of
electron affinity (A) and ionization potential (I) as follows:
400-1600 grid, washed thoroughly with acetone and distilled water.
Clean and dried coupons are weighed and then immersed in 75 mL
of 1M HCl, taken in a graduated 100 mL beaker covered with a
watch glass, without and with the presence of inhibitor molecules
having 10 mM concentration for 1 to 96h of duration at room
temperature of around 30°C. Very small change in volume of the
electrolytic solution during immersion period is compensated by
adding requisite amount of distilled water. Metallic coupons are
then removed, scales formed on the surface are cleaned with
χ = (I+A)/2
η = (I-A)/2
(4)
(5)
(6)
(7)
(8)
σ = 1/ η = 2/(I-A)
I = -EHOMO
A= -ELUMO
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From these values, the fraction of electrons transferred from the conditions to model a representative part of the interface devoid of
inhibitor molecule to the metallic atom (Δ ) is calculated. As per its any arbitrary boundary effects. The box consisted of a Fe slab, a
N
underlying concept, during the reaction of two systems having water slab (150 H2O molecules) containing the studied compounds
different electronegativity (here, the metallic surface and an and a vacuum layer. During simulations, all the bulk atoms in the Fe
inhibitor molecule) the following mechanism will take place: the (1 1 0) systems were kept ‘‘frozen’’, and the inhibitor molecules and
electron flow will happen from the molecule with lower water molecules are allowed to interact with the metal surface
electronegativity towards that of a higher value, until the chemical freely. The MD simulation is performed under 298 K, NVT ensemble,
potentials become the same. In order to calculate the fraction of with a time step 1.0 and simulation time of 100 ps using the
electrons transferred, a theoretical value for the electronegativity COMPASS force field. The interaction energy, Einteraction between the
of bulk iron was used ꢍ
ꢎꢏ= 7 eV31, and a global hardness of ꢀꢎꢏ = 0, Fe (1 1 0) surface and the inhibitor molecule is calculated according
by assuming that for a metallic bulk I = A.32 The fraction of electrons to the following equation: 37-38
transfers to the metallic surface is calculated as follows: 33
Einteraction
=
Etotal – (Esurface + water
+
Einh(solv)
)
(13)
ꢃꢍꢎꢏꢇꢍꢒꢓꢔ
ꢄ
ꢐꢑ = ꢕꢃꢀ
(9)
ꢎꢏꢖꢀꢒꢓꢔ
ꢄ
where, Etotal is the total energy of the simulation system, Esurface +
The local reactivity of the molecules is analyzed through an
evaluation of the Fukui indices using Dmol3 module, Material
studioTM version 6.1 by Accelrys Inc, San Diego, CA. All the
calculation are performed applying BLYP exchange correlation
functional and the double numeric with polarization (DNP) basis set,
because this set is the best set available in Dmol3. Fukui indices are
used to measure the chemical reactivity, as well as an indicative of
the reactive regions and the nucleophilic and electrophilic
behaviour of the molecule. The regions of a molecule where the
Fukui function is large are chemically softer than the regions where
the Fukui function is small, and by invoking the HSAB principle in a
local sense, one may establish the behaviour of the different sites
with respect to hard or soft reagents. The Fukui function fk is
is the energy of iron surface together with H2O molecules, and
water
Einh(solv) is the energy of the solvent stabilized inhibitor molecule.
These individual energy components are determined by selecting
the respective constituents from the simulation box under final
equilibrated state (presented pictorially in Fig. S3, ESI).
Results
Polarization measurements
The current–potential relationships for mild steel electrode in
1M HCl without and with PMAM, PMTM and PMPM at 10mM
concentration at 303K are shown in Fig. 2. Such polarization
curves for each inhibitor molecule at different concentrations
are included in supplementary section (Fig. S4(A-C), ESI).
Different parameters obtained from polarizations study, like,
corrosion potential (Ecorr), corrosion current density (icorr),
anodic and cathodic Tafel slopes (ba and bc) are shown in Table
2. It is observed that, both the cathodic and anodic
ꢃ ꢄ
defined as the first derivative of the electronic density ꢗ ꢘꢙꢚ with
respect to the number of electrons in a constant external
N
34
ꢃ ꢄ
potential ꢛ ꢘꢙꢚ (equation 10).
ꢜꢝ = ꢞꢟꢗ ꢙꢘꢚꢄꢠ
(10)
ꢃ
ꢟꢑ
ꢛꢃꢘꢙꢚꢄ
The Fukui functions can be written by taking the finite difference
approximations as: 35
ꢜꢖꢝ = ꢡꢝ ꢑ ꢢ ꢊ ꢣ ꢡꢝꢃꢑꢄ (for nucleophilic attack)
(11)
(12)
ꢃ
ꢃ
ꢄ
ꢜꢇꢝ = ꢡꢝ ꢑ ꢣ ꢡꢝꢃꢑ ꢣ ꢊꢄ (for electrophilic attack)
ꢄ
where qk is the gross charge of kth atom in the molecule. The
qk +1), qk N) and qk -1) are defined as the charge of the anionic,
(
N
(
(N
neutral and cationic species, respectively, in their energy optimized
electronic state. Here Fukui functions are obtained through the
finite difference approximation using Hirschfeld population analysis
(HPA).36
Molecular dynamics simulation
Adsorption behaviour of inhibitor molecule is very much important
to understand the corrosion behaviour on metal surface. Molecular
dynamics (MD) simulation is the technique, which has become
popular towards finding the preferential adsorption sites on metal
surfaces. The molecular dynamics (MD) simulations are performed
using the software, studioTM 6.1. Fe (1 1 0) surface is chosen for the
simulation study. The MD simulation of the interaction between
inhibitor molecules and Fe (1 1 0) surface is carried out in a
simulation box (36.83 Å × 36.83 Å × 77.36 Å) with periodic boundary
Fig. 2 Potentiodynamic polarization curves for mild steel in 1 M
HCl in the absence and presence of Schiff-bases having 10 mM
concentration.
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Table 2 Data from potentiodynamic polarization studies for indicating that it is the most effective corrosion inhibitor
mild steel in 1M HCl in various inhibitor systems
among the studied Schiff base molecules.
System
Conc
/mM
-Ecorr
(mV/
SCE)
492
icorr
(μA
ba
(mV
-bc
ꢤꢥ
(%)
Electrochemical impedance spectroscopy (EIS)
(mV
dec-1)
78
cm-2) dec-1)
1200 62
For mild steel, Nyquist plots and Bode diagrams obtained from
EIS in the absence and presence of three inhibitors (e.g.;
PMAM, PMTM and PMPM) in 10 mM concentration are shown
in Fig. 3A and 3B, respectively, while Nyquist plots for all the
three
BLANK
PMAM
0.10
0.50
1.00
5.00
10.00
505
500
503
502
500
378
255
170
104
65
90
79
82
71
72
105
113
107
106
104
69
79
86
91
95
300
10
PMAM
PMTM
PMPM
BLANK
142 Hz
[A]
8
6
4
2
0
250
200
150
100
50
PMTM
PMPM
0.10
0.50
1.00
5.00
10.00
505
504
502
503
515
657
340
232
132
100
84
92
91
82
74
122
120
103
125
127
45
72
80
89
92
3.0 4.5 6.0 7.5 9.0 10.5
62.6 Hz
108 Hz
21 Hz
27.6 Hz
185 Hz
0.10
0.50
1.00
5.00
10.00
497
499
494
515
514
709
399
325
183
124
82
89
83
99
96
101
108
130
104
110
41
66
73
85
90
5.36 Hz
0.25 Hz
82.3 Hz
50 100
0.62 Hz
200
0
0
150
250
300
Z' / ꢀ . cm2
currents are gradually shifting towards lower value for the
whole potential range with increasing concentration of all the
three inhibitors. In presence of the inhibitors, no systematic
variation of anodic and cathodic Tafel slopes with inhibitor
concentration is found. Overall, both are increased from those
observed in the blank solution at lower overpotential range (as
evident from Fig. S5, ESI). This indicates relatively smaller
change of anodic and cathodic currents with polarization
compared to that when no inhibitor is present. Following
Bockris et al., increase in Tafel slope may be attributed to
either adsorption of organic impurities or the presence of
conducting films on the electrode surface.39 In the present
case, overall increase in anodic and cathodic Tafel slopes may
arise from the adsorption of inhibitor molecules on both the
cathodic and anodic reaction sites, and thereby, retarding rate
of the corresponding reactions. Such simultaneous adsorptive
nature of inhibitors may be responsible for irregular variation
of Tafel slopes, which is also been reported by many others for
similar systems.1-16 Mixed-type corrosion inhibition property of
the three inhibitors is also evident from the observed irregular
variation of Ecorr values as a function of inhibitor concentration.
Further, with respect to the uninhibited sample, in presence of
PMTM and PMPM, Ecorr values show a definite tendency of
shifting towards more negative direction. All these
observations suggest that though the Schiff bases used in this
present study act, in general, as mixed type inhibitors, for
PMTM and PMPM, effect of diminishing the rate of cathodic
reduction reaction (hydrogen evolution) is somewhat more
prominent than that of the anodic metal oxidation reaction.
Maximum decrease in icorr is observed in case of PMAM
300
250
200
150
100
50
10
BLAN K
[B]
8
6
4
10-2 10-1 100 101 102 103 104 105
PM A M
PM T M
PM PM
0
10-2
10-1
100
101
102
103
104
105
f / H z
Fig. 3 Electrochemical impedance curves for mild steel in 1M HCl in
the absence and presence of 10 mM concentration of PMAM,
PMTM, PMPM exemplified as: (A) Nyquist and (B) Bode plots.
inhibitors with different concentrations are given in supplementary
section [Fig. S6(A-C), ESI]. For all the three inhibitors with varied
concentration, the shape of the impedance diagrams are
maintained comparing with that of blank solution, suggesting
almost no change in the corrosion mechanism due to inhibitor
addition.40 The capacitive loops in absence and presence of
inhibitors exhibit only one semicircle in the Nyquist plot,
corresponding to one time constant. Bode diagrams also exhibit
one negative fluctuation only. These indicate that corrosion
inhibitive effect by the studied Schiff bases is mostly polarization
resistance controlled. Polarization resistance comprises various
resistive effects acting at the metal-solution interface, which
include charge transfer resistance, resistance due to the adsorbed
organic inhibitors, as well as corrosion products. It is further noticed
that in the Nyquist plots, the obtained semicircle is depressed
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Table 3. Impedance parameters for the corrosion of mild steel in 1M HCl in various inhibitor systems
System
Conc.
(mM)
Rs
Rp
(Ω cm2)
6.3
Q
Cdl
(μF cm-2)
195
n
ꢤ
ꢦ(%)
θ
(Ω cm2)
3.28
(μ
Ω-1sn cm-2)
593
BLANK
PMAM
0.834
0.815
----
80
----
0.80
0.10
2.71
32
256
86.0
0.50
1.00
5.00
10.00
0.10
0.50
1.00
5.00
10.00
0.10
0.50
1.00
5.00
10.00
2.21
2.35
1.75
2.15
3.50
3.23
2.00
2.18
3.08
3.08
3.60
3.56
3.02
2.61
50
88
124
252
17
39
54
89
161
13
209
146
90.4
63.8
348
252
206
143
85.7
378
259
213
178
135
76.3
65.3
44.3
37.7
90.5
77.6
72.2
52.8
41.9
97.6
87.9
76.7
67.9
51.7
0.819
0.844
0.863
0.887
0.792
0.797
0.811
0.812
0.857
0.797
0.819
0.822
0.816
0.811
87
93
95
98
63
84
88
93
96
52
78
85
92
95
0.87
0.93
0.95
0.98
0.63
0.84
0.88
0.93
0.96
0.52
0.78
0.85
0.92
0.95
PMTM
PMPM
29
42
78
121
inhibitors in aqueous HCl solution brings about an irregular
variation in the values of solution resistance Rs). Same
under the real axis, which may be attributable to the roughness and
inhomogeneity of the electrode surface.41-42 Considering all these
facts, the impedance plots are fitted with the equivalent circuit
having one time constant, as depicted in Fig. S7 (ESI). In Nyquist
plots, the polarization resistance (Rp) stands by difference in real
impedance in lower and higher frequencies. Thus, larger the
diameter of the capacitive loop in Nyquist plot, higher is the Rp. It is
seen that with increase in inhibitor concentration, diameter of the
(
phenomenon is reported by many others.45 Generally, increase in
the concentration of Schiff bases, which are non-electrolytic in
nature, should result in increasing the Rs values. Thus to explain the
observed irregular variation, some competing factor should be
taken into consideration. Since in aqueous solution, H+ and and Cl-
ions remain in heavily solvated ion-pair form, so the movement of
one ion gets affected or hindered by the other. Schiff bases readily
get protonated in aqueous acidic solution. Due to large size of the
protonated forms of these Schiff bases, their charge densities
become lower, resulting relatively weaker interaction with the
counter negative Cl- ions. Thus the ions can migrate comparatively
more freely in aqueous solution, resulting in a decrease in Rs values.
Mutual effect of these two opposing factors is most likely being
responsible for the observed irregular variation of Rs with varying
concentration of the Schiff bases studied.
capacitive loop, and thus Rp as well as
ꢤꢦ(%) increases. These
observations suggest that extent of adsorption of the inhibitors
increases with concentration and thereby provides better barrier
towards charge transfer reactions at the metal-solution interface.
The constant phase element (CPE), used in the equivalent circuit, is
related to the double layer capacity (Cdl) and its impedance is given
by:
ZCPE
=
Q-1
(
i
ω)- n
(14)
Further, the inhibitive performance is observed to vary in the series:
where,
Q
is a proportionality coefficient, ω is the angular frequency, PMAM >PMTM>PMPM; i.e, the same trend as observed in the
n is a measure of surface irregularity.41-42 For whole numbers of n = polarization measurement study. This may also be confirmed from
1, 0, -1, CPE is reduced to the classical lumped elements capacitor the corresponding Bode plots (Fig. 3B), where impedance moduli
(C), resistance (R), and inductance (L), respectively. To obtain a for different inhibitors are seen to follow the above mentioned
direct correlation between polarization resistance (Rp) and double order for the whole frequency range.
layer capacitance (Cdl), the later has been recalculated using the
following equation: 43-44
Weight loss measurement
The relative performance among the experimental Schiff bases
1-n 1/n
Cdl=
(Q.Rp
)
(15)
towards inhibition of corrosion of mild steel in 1M HCl is assessed
following weight loss measurement (Fig. 4). Data from weight loss
measurement also confirms the performance trend as obtained
from electrochemical tests. Additionally, it shows that inhibitory
action for all the three inhibitors lasts for considerable time of
exposure in acid solution. This suggests towards strong bond
formation between the Schiff base and metal surface, that results
into a durable inhibitory film formation on the later.
All the fitted parameters, together with
ꢤ
ꢦ(%) values are tabulated
in Table 3. Capacitors oppose or impede the current flow. Thus
decrease in Cdl values with concomitant increase in Rp for gradual
increase in inhibitor concentration clearly indicates towards
corrosion inhibitive performance of the inhibitors by virtue of
enhanced degree of surface coverage, as well as increasing
thickness of the adsorbed layer. It is also seen that the addition of
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Table 4. Adsorption parameters for Schiff bases on mild steel in 1 M
HCl
0.98
0.96
0.94
0.92
0.90
Sample
r2
Kads ( M-1
)
- ΔG°
ads
(kJ mol-1)
PMAM
PMTM
PMPM
0.999
0.999
0.999
15.17 × 103
10.69 × 103
7.84 × 103
34.39
η
η
η
η
0.88
0.86
0.84
0.82
33.50
32.72
PMAM
PMTM
PMPM
0
20
40
60
80
100
time (h)
Observed range of free energies in the present study suggests
adsorption the Schiff bases on Fe-metal surface is of mixed type,
with chemisorption being more dominating over physisorption.
Fig. 4 Variation of inhibition efficiency of PMAM, PMTM, PMPM
having 10 mM concentration at different immersion times towards
corrosion of mild steel in 1M HCl by weight loss measurement.
Surface morphology
SEM images of the corroded metal surface in absence and presence
of inhibitors are shown in Fig. 5. All the three inhibitors are shown
to impart considerable effect in inhibiting corrosion of mild steel in
compared to that in uninhibited sample. But, among the three
corrosion inhibitors, PMAM stands undoubtedly as the best one.
Adsorption isotherm
To understand the mechanism of corrosion inhibition of mild steel
in 1M HCl by the studied Schiff bases, the adsorption behaviour of
these derivatives on the mild steel surface must be known. If simple
adsorptive behaviour is assumed for these derivatives, a direct
relationship between inhibition efficiency and surface coverage, θ
should exist. EIS data are used to evaluate the surface coverage
values, which is based on the relation:46-49
ꢧ = ꢀ ꢃ%ꢄ
(16)
ꢂ
ꢊꢉꢉ
Among several isotherms, Langmuir adsorption isotherm has been
proved to be best fitted to interpret the observed data in this
present study. Here, concentration of adsorbent (
C) is related to the
degree of surface coverage (θ) following the relation:
ꢨ
ꢊ
=
ꢢ ꢨ
(17)
ꢧ
ꢩꢪꢫꢬ
where, Kads is the constant of adsorption. Excellent linear fittings of
the plots of ( /θ) against are given in supplementary section [Fig.
S8 (A-C), ESI], with good correlation coefficient (r2=0.999) and
nearly unit slope, justifies the applicability of the Langmuir model.
The adsorption equilibrium constant (Kads) is related to the standard
Fig. 5 SEM images of mild steel surface after immersion for 6 h in
1M HCl having (a) no inhibitor, and having 10 mM of (b) PMAM, (c)
PMTM and (d) PMPM.
C
C
FTIR spectroscopic studies of the adsorbed layer
In order to ascertain if the inhibitor molecules get adsorbed on the
mild steel surface, FTIR spectroscopic studies are done with the
surface adsorbed layer and these are compared with those of the
prepared Schiff bases (Fig. 1). Prior to measurement, cleaned mild
steel coupons remain exposed in Schiff base solutions (10mM
concentration) prepared in 1M HCl for 24h. Coupons are then
washed repeatedly with distilled water and dried under vacuum
free energy of adsorption, Δ
G
°
by following equation:
ads
Δ
G°
=-RT ln(55.55 Kads
)
(18)
ads
where, 55.55 is the molar concentration of water,
gas constant and
R
is the universal
T
be the temperature (here, 303K). Corresponding
adsorption parameters for PMAM, PMTM and PMPM are given in desiccator. Surface adsorbed layer is collected by scratching it using
a knife. FTIR spectra of adsorbed layers contain almost all the
characteristic bands in the fingerprint region (1500-500 cm-1) along
with vibrational bands corresponding to imine group (1650-1620
cm-1) as well as stretching vibration of aromatic C-H (3100-2950 cm-
1) and methyl C-H groups (2950-2850 cm-1) (Fig. 1). Comparing all
the FTIR spectra, it is revealed that the imine band of PMAM at
1647 cm-1 undergoes a prominent red shift at 1605 cm-1, whereas
for other two Schiff bases these bands get broadened upon
adsorption. Adsorption of Schiff bases are also seen to shift the
aromatic C-H and methyl C-H vibrational frequencies towards lower
wavenumber region. All the information indicates clearly towards
Table 4. The large negative values of Δ
Schiff bases are adsorbed by an appreciable extent on the mild steel
surface at equilibrium. Many references suggest that value of Δ
around -20 kJ mol-1 may be ascribed to the electrostatic interaction
between charged organic molecules and charged metal surface (i.e.,
physisorption).44,46-49 Whereas, that of the order of -40 kJ mol-1 or
higher involves charge sharing or a transfer between the inhibitor
molecule and the metal surface to form a co-ordinate type of bond
(i.e., chemisorption).44,46-49
G°ads indicate that the studied
G°
ads
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adsorption of inhibitor molecules on the mild steel surface, which
ultimately leads towards inhibition of corrosion of the latter in acid
medium. Similar inferences towards adsorption of inhibitors on
metal, following observation of FTIR spectra, have been made by
many other researchers.50-52
Molecular orbital energy level calculations
Electron distribution in HOMO and LUMO energy levels for the
energy optimized neutral inhibitor molecules in aqueous solution
are shown in Fig. 6 and the corresponding energies are tabulated in
Table 5. It is well documented that formation constant for
protonated Schiff bases is much less than the formation constant of
metal-Schiff base complexes.53 Thus it is logical to assume that
though Schiff bases exist preferentially in protonated form in acidic
medium, but it is the neutral form which is mostly responsible
towards interacting with the metal surface. Generally, EHOMO and
ELUMO are related to the susceptibility of the molecule to be
attacked by electrophiles and nucleophiles, respectively. Higher the
value of EHOMO is, stronger is the electron donating ability of the
molecule. Whereas, the lower value of ELUMO signifies the molecule
to be susceptible to accept electrons.54-56 And the molecule with
(PMAM)
(PMTM)
lower absolute values of the energy band gap (ΔE = ELUMO – EHOMO)
exhibits higher inhibition efficiencies because of the possibility of
both ways of electron transfer, i.e., inhibitor to metal and vice
versa. From Table 5, it is seen that the EHOMO values are decreasing
in the order PMAM> PMTM > PMPM, whereas, the same order
represents an increase in the values of both the ELUMO and ΔE. This
manifests that the extent of bonding (inhibitor to metal and back)
between the inhibitor and metal is maximum for PMAM and
decreases following the above order. This result derived from
(PMPM)
HOMO
Optimized
Structure
LUMO
quantum chemical calculation conforms to the experimentally Fig. 6 Optimized molecular structures of Schiff bases with electronic
distribution in HOMO and LUMO.
obtained one. Electron transfer will occur from inhibitor molecule
to metal surface if Δ ˃ 0 and vice versa for Δ
˂ 0. 57-58 According
to Elnga et al., inhibition efficiency increases with increasing
electron-donating ability of the molecule at the metal surface if Δ
< 3.6.59 Inspection of Table 5, shows that all the values of Δ
are
N
N
metal surface and higher inhibition efficiency is expected. From the
values of different molecular parameters (Table 5), it is seen that
electronegativity of three inhibitor molecules follows the trend
PMAM ˃ PMTM ˃ PMPM. Therefore, from this result it is confirmed
that PMAM has the highest ability to accept electrons among the
three inhibitor molecules.
N
N
positive and less than 3.6, indicating that the overall inhibitor-metal
interaction is dominated by donation of electron from inhibitor
molecule to the Fe surface facilitating the formation of coordinate
Global softness (σ) is another important issue concerning the
bond and hence, the corrosion inhibition. Values of ΔN for the three
adsorption of inhibitor molecule on metal surface. Adsorption is
preferable where comparatively higher softness is acquired. In this
study we may consider inhibitor as a soft base and metal surface as
Schiff bases further support the experimentally obtained trend of
inhibition efficiency.
On the other hand, electronegativity (χ), implies electron
attracting capability of the molecule. Higher the electronegativity,
stronger will be the attracting power to accept electron from the
metal surface. Therefore, those inhibitor molecules which possess
higher electronegativity would have strong interaction with the
a soft acid.54 The reported
σ values (vide Table 5) follow the trend
PMAM ˃ PMTM ˃ PMPM, which confirms the inhibiꢀon efficiency is
in the expected order. So, the theoretical outcomes again match
well with the experimental findings.
Table 5. Calculated Quantum chemical parameters for used Schiff-bases obtained from DFT method.
Inhibitors
EHOMO
(eV)
ELUMO
(eV)
Δ
(eV)
E
I
A
(eV)
χ
(eV)
Η
(eV)
σ
ΔN
(eV)
(eV-1)
PMAM
PMTM
-6.1096
-6.1303
-2.1202
-2.0164
3.9894
6.1096
6.1303
2.1202
2.0164
4.1134
4.0734
1.9947
2.0569
0.5013
0.4862
0.7236
0.7114
4.1139
4.2108
PMPM
-6.1519
-1.9471
6.1519
1.9471
4.0495
2.1054
0.4749
0.7006
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obtained by molecular dynamics simulations. Top: top view,
Bottom: side view.
Local reactivity of the corrosion inhibitors
To establish the active site of an inhibitor molecule, three Table 6. Output obtained from MD simulation for adsorption of
influencing and controlling factors; namely, neutral atomic charge, inhibitors on Fe (1 1 0) surface.
distribution of frontier molecular orbital and Fukui indices have to
be considered. Local reactivity is analyzed by means of the
condensed Fukui function.60-62 Condensed Fukui functions allow us
to distinguish each part of the molecule on the basis of its distinct
chemical behaviour due to the different substituent functional
groups. The nucleophilic and electrophilic attack is controlled by the
Systems
Einteraction (kJ mol-1)
Fe (1 1 0) + PMAM + H2O
Fe (1 1 0) + PMTM + H2O
Fe (1 1 0) + PMPM + H2O
‒529.476
+
maximum values of fk and fk–, respectively. The calculated Fukui
‒522.577
indices for nucleophilic and electrophilic attack for the three
selected inhibitors are tabulated in Table S1 (ESI). Regarding to
adsorption phenomenon, high value of fk–on any atom present in
the inhibitor molecule indicates that the atom prefers to form a
chemical bond by donation of electron to the metal, i.e., the atom is
more susceptible towards electrophilic attack. Similarly, an atom
with a high value of fk+ is said to be responsible for bond formation
by accepting an electron from the metal (i.e., back bond or retro
‒494.762
observation regarding the relative anti-corrosive propensity the
three inhibitors studied. MD simulation results under equilibrium
condition in terms of fluctuation in temperature and energy
predicts reasonably the most favourable configuration of the
adsorbed inhibitor on the metal surface. Under this condition,
Einteraction between the inhibitor and Fe (1 1 0) surface can be
provided according to the Eq. (13) and the corresponding values are
summarized in Table 6. The best adsorption configuration of the
inhibitor over Fe (1 1 0) surface as well as the close contacts
between those are depicted in Fig. 7. The strong adsorption
between the inhibitors and the iron surface are reflected from their
high negative interaction energy values. Among the three inhibitors,
PMAM shows the highest value of interaction energy, while PMPM
showing the least (Table 6). Further, estimation of bond distances
among the atoms of the inhibitor molecules and the metal surface
reveals that almost all the atoms lie within a range of 2.5 to 3.5Å
from the metal surface. This reflects strong adsorption property for
all the inhibitors on the Fe (1 1 0) surface in aqueous medium.
Following literature value, it can be argued that the obtained range
–
donation). For all the three inhibitors, fk values are seen to be
higher at imine C and N atoms, C atoms of the phenyl ring,
heteroatom and C atom of the substituent group of the phenyl ring.
fk+ values, on the other hand, are observed to be mostly on imine C
and N atoms, C and N atoms of pyridine group. These are largely in
accordance with the observed electronic distribution of HOMO and
LUMO energy levels of the inhibitors. Most striking observation in
terms of local reactivity analysis reveals that though the S16 atom
present in PMTM possesses the largest fk– value, which should make
PMTM the best inhibitor among all the three inhibitors studied,
–
PMAM containing N16 atom with relatively much lower fk value
–
has come out as the best. Table S1 (ESI) shows that the lower fk
value of N16 atom in PMAM, compared to that of S16 atom in
–
PMTM, is compensated by the relatively high fk values of the C
for bond distance represents the formation of covalent bond which
occurs mostly due to chemisorption.
good agreement with the results obtained from experimental work
as well as from quantum chemical calculations.
atoms of its phenyl ring. fk– values of the C atoms of the phenyl ring
for PMPM are seen to be relatively higher than those for both the
PMAM and PMTM, but O16 atom in PMPM has very low fk– value
than S16 atom of PMTM or N16 atom of PMAM. As far as fk+ values
are concerned, it is observed that all the corresponding atoms
present in the three different inhibitors have very comparable
values.
62-63
All these results are in
Discussion
From the EHOMO and ELUMO trend, it is clear that the binding pattern
is same between the three inhibitor molecules and the metallic
surface, i.e., electron transfer from inhibitor to metal as well as
metal to inhibitor, both are operating during adsorption process.
But the extent of such interaction varies from molecule to
molecule. In general, lone pair of electrons of nitrogen, oxygen or
Molecular dynamics (MD) simulation
MD simulation studies further corroborated the experimental
sulfur atom together with
π-electrons of imine group (C=N),
pyridine and benzene ring can interact with vacant low lying 3d
orbitals of Fe. On the other hand, 4s electrons present in Fe can
form coordinate bond with the LUMO of the inhibitors. From the
torsion angle value (Table 7), it is clear that PMAM has almost
planar structure. This provides PMAM an ideal geometry for parallel
adsorption on the metal surface, and hence, results in higher
surface coverage, and in-turn, higher inhibition efficiency. On the
contrary, PMPM and PMTM are not planar, as evident from torsion
angle values (Fig. 8). This will result in lower degree of surface
Fig. 7 Equilibrium adsorption configurations of inhibitors PMAM (a
and a’), PMTM (b and b’) and PMPM (c and c’) on Fe (1 1 0) surface
Table 7 Torsion angle (°) in the optimized form of the inhibitors.
Torsion angle
PMAM
PMTM
PMPM
C7-N8-C13-C14
C7-N8-C13-C10
-176.78
3.55
-136.79
47.39
-142.48
41.96
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–
formation through electron donation, as evident from its atomic fk
value. Yet, maximum inhibition efficiency is experimentally
observed for PMAM, which is due to favourable energetic
disposition of the frontier molecular orbitals, which, in-turn, is
influenced by its spatial molecular orientation. Further, from the
study of local reactivity of the atoms, it may be concluded by saying
that the effect of S atom present on the substituent group attached
to the phenyl ring in PMTM is less dominating than the overall
effect of the π-electron cloud of the phenyl ring in PMAM in regard
Fig. 8 Schematic representation of probable mode of adsorption of to adsorption of these molecules on the metal surface.
inhibitors, PMAM (a and a’), PMTM (b and b’) and PMPM (c and c’)
on metal surface (arrow represents the direction of most probable
Conclusion
bonding). Top: adsorption involving HOMO, Bottom: adsorption
Present study explicitly manifests that the effect exerted by
involving LUMO.
heteroatom present is not the ultimate criteria in dictating the
coverage by PMPM and PMTM on the metal surface. For all the
molecules, electrons in HOMO are mostly distributed over the
substituted-phenyl ring, the attached heteroatom to the phenyl
ring, and the imine group. This may be responsible for observed
closeness in EHOMO values for all the inhibitors. Electron distribution
in LUMO, on the contrary, is seen to distribute mostly on the
pyridine and the imine groups for PMPM and PMTM, and over the
whole molecular surface for planar PMAM. Thus, it may be
concluded that PMAM utilizes its whole planar molecular surface
during bonding from metal to the LUMO. But as PMPM and PMTM
are not planar, either of the substituted-phenyl moiety or the
pyridine moiety can be adsorbed in a true parallel orientation on
the metal surface by inhibitor to metal electron transfer (Fig. 7).
Though both the aromatic moieties are not disposed perfectly
perpendicular to each other, some probability of simultaneous
interaction between metal surface and PMPM or PMTM through
both the parallel as well as torsionally twisted non-planar
orientation cannot be ruled out. In that case, each molecule will be
involved in simultaneous two-ways bond formation, but obviously
the overall bond strength will be lower than that observed in the
case PMAM. This is manifested in the calculated binding energy
between the Fe (1 1 0) surface and the three Schiff bases, as
observed in MD simulation studies. It may be argued that inhibitors
preferentially occupy the anodic reaction sites on the metal surface
through electron donation from HOMO of the inhibitors, whereas
the electrons present in cathodic reactions sites interact mostly
with LUMO of the inhibitors. Thus, excess negative charge builds up
at the anodic sites, and anodic metal dissolution reaction (i.e.,
oxidation) rate diminishes. On the other hand, deficiency in the
number of available electron for reduction of hydrogen ions leads
to decrease in the rate of hydrogen evolution reaction (i.e.,
reduction reaction) at the cathodic reaction sites. Overall, rate of
extent of adsorption, and hence inhibition of corrosion for
structurally comparable inhibitor molecules, rather their
stereochemical disposition also plays their role in the overall
process. Among structurally resembled three Schiff bases studied,
the one having N atom is seen to provide greater absorptivity and
corrosion inhibition propensity over one having S atom due to
conducive spatial orientation which results into favourable
energetic criteria of lower energy difference between the frontier
molecular orbitals. All the three inhibitors are seen to act broadly as
mixed type inhibitor where chemisorption is somewhat
predominating over physical adsorption as evidenced from
adsorption isotherm and MD simulation studies. The occurrence of
adsorption of the studied Schiff bases on metal surface is
established unambiguously following FTIR spectra of free and
surface adsorbed inhibitor molecules. From electronic distribution
of HOMO and LUMO and from atomic reactivity study for all the
inhibitors, it is concluded that the phenyl ring and the heteroatom
attached to the phenyl ring is mostly involved in bond formation
between the inhibitor and metal surface through electron transfer
from HOMO to metal. For PMAM showing maximum inhibition
effect, simultaneous electron transfer from metal to phenyl ring,
the imine as well as pyridine group is probable, which may be due
to nearly flat orientation of the molecule. On the contrary, for the
other two inhibitors, such simultaneous retro-donation from metal
to phenyl ring is not so pronounced. This disparity along with other
intrinsic molecular parameters has resulted in the observed
sequence of inhibition efficiency, i.e., PMAM >PMTM>PMPM.
Acknowledgements
DS and PB thank Department of Science and Technology (DST),
Govt. of India for supporting research projects (ref no.
SR/FT/CS-110/2010, SB/FT/CS-003/2012 and GAP-183112). SKS
thanks DST, New Delhi, India for his DST Inspire Fellowship.
the corrosion process is mitigated. As in all cases Δ
interaction involving HOMO of the inhibitors is predominant over
that involving LUMO. But as Δ is seen to decrease following the
N is positive,
N
order PMAM>PMTM>PMPM, the effect of retro-donation (electron
transfer from metal to LUMO of the inhibitor) is becoming more
and more important in the above order for the inhibitors. This may
have significance with the experimentally observed fact that though
all the inhibitors are mixed type, PMTM and PMPM are relatively
more cathodic than PMAM. It may be noted further that the S atom
present in PMTM possesses the maximum tendency towards bond
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