Synthesis and comparative study on the anti-corrosion potentials of some Schiff base compounds bearing similar backbone

Article abstract of DOI:10.1016/j.molliq.2018.11.161

The synthesis of (E)-4-((p-tolylimino)methyl) phenol (TMPOL), (E)-4-((benzylimino)methyl) phenol (BMPOL) and (E)-4-((p-phenylimino)methyl) phenol (PMPOL) were carried out by the condensation reaction of 4-methylaniline, phenylmethanamine and aniline, respectively, with 4-hydroxybenzaldehyde. Molecular identities of the Schiff base compounds were probed using NMR (¹H and ¹³C), Fourier transform infrared (FTIR) and mass spectroscopic techniques. The anticorrosion potency of the compounds was investigated for mild steel (MS) in 1 M HCl solution using electrochemical methods. Potentiodynamic polarization (PDP) data revealed that all three compounds were mixed-type inhibitors, with TMPOL showing remarkable cathodic effect at high inhibitor concentrations. Electrochemical impedance spectroscopy (EIS) data revealed improved adsorption of inhibitor species on mild steel surface at increased inhibitor concentrations with TMPOL, BMPOL and PMPOL reaching a maximum efficiency of 75, 88 and 74%, respectively. Scanning electron microscopy (SEM) reaffirmed the conditioning layer on the mild steel surface. Quantum chemical calculations provided molecular based explanations of the roles of heteroatoms and π electron centers on the corrosion inhibition activities of the studied compounds. The better anticorrosion activities of BMPOL compared to its isomeric compound (TMPOL) can be ascribed to the presence of a methylene linkage which enhanced the donor-acceptor interactions. The three inhibitors were inclined towards the Freundlich adsorption isotherm via a spontaneous chemical and physical adsorption of inhibitor molecules on the MS surface.

Full text of DOI:10.1016/j.molliq.2018.11.161

Accepted Manuscript
Synthesis and comparative study on the anti-corrosion potentials
of some Schiff base compounds bearing similar backbone
Elias E. Elemike, Henry U. Nwankwo, Damian C. Onwudiwe
PII:
S0167-7322(18)34112-6
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2018.11.161
MOLLIQ 10071
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Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
8 August 2018
16 November 2018
29 November 2018
Please cite this article as: Elias E. Elemike, Henry U. Nwankwo, Damian C. Onwudiwe
, Synthesis and comparative study on the anti-corrosion potentials of some Schiff base
compounds bearing similar backbone. Molliq (2018), https://doi.org/10.1016/
j.molliq.2018.11.161
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ACCEPTED MANUSCRIPT
Synthesis and comparative study on the anti-corrosion potentials of some Schiff base
compounds bearing similar backbone
1,2,3Elias E. Elemike, 1,2Henry U. Nwankwo, 1,2*Damian C. Onwudiwe
1. Material Science Innovation and Modelling (MaSIM) Research Focus Area, Faculty of
Agriculture, Science and Technology, North-West University, Mafikeng Campus, Private Bag
X2046, Mmabatho 2735, South Africa.
2. Department of Chemistry, School of Mathematics and Physical Sciences, Faculty of
Agriculture, Science and Technology, North-West University, Mafikeng Campus, Private Bag
X2046, Mmabatho 2735, South Africa.
3. Department of Chemistry, College of Science, Federal University of Petroleum Resources,
P.M.B 1221, Effurun, Delta State, Nigeria.
* Corresponding author
Email: Damian.Onwudiwe@nwu.ac.za
Tel: +27 18 389 2545; Fax: +27 18 389-2420
Abstract
The synthesis of (E)-4-((p-tolylimino)methyl) phenol (TMPOL), (E)-4-((benzylimino)methyl) phenol
(BMPOL) and (E)-4-((p-phenylimino)methyl) phenol (PMPOL) were carried out by the condensation
reaction of 4-methylaniline, phenylmethanamine and aniline, respectively, with 4-
hydroxybenzaldehyde. Molecular identities of the Schiff base compounds were probed using NMR
(1H and 13C), Fourier transform infrared (FTIR) and mass spectroscopic techniques. The anticorrosion
potency of the compounds was investigated for mild steel (MS) in 1 M HCl solution using
electrochemical methods. Potentiodynamic polarization (PDP) data revealed that all three compounds
were mixed-type inhibitors, with TMPOL showing remarkable cathodic effect at high inhibitor
concentrations. Electrochemical impedance spectroscopy (EIS) data revealed improved adsorption of
inhibitor species on mild steel surface at increased inhibitor concentrations with TMPOL, BMPOL
and PMPOL reaching a maximum efficiency of 75, 88 and 74%, respectively. Scanning electron
microscopy (SEM) reaffirmed the conditioning layer on the mild steel surface. Quantum chemical
calculations provided molecular based explanations of the roles of heteroatoms and π electron centres
on the corrosion inhibition activities of the studied compounds. The better anticorrosion activities of
BMPOL compared to its isomeric compound (TMPOL) can be ascribed to the presence of a
methylene linkage which enhanced the donor-acceptor interactions. The three inhibitors were inclined
towards the Freundlich adsorption isotherm via a spontaneous chemical and physical adsorption of
inhibitor molecules on the MS surface.
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Keywords: Schiff base; DFT; anticorrosion properties; electrochemistry
1. Introduction
The chemistry of corrosion process and the quest to protect metal from corrosion is of great concern
to chemists and materials scientists. There are continuous research and efforts to inhibit the corrosion
processes of metals, especially steel, due to their wide applications in different industrial process and
fluid transport. Organic compounds have in recent times, emerged as promising candidates for metal
protection. Special interest is on compounds that can be easily synthesized using low cost substrates,
and the compounds which posses high concentration of electron cloud on the aromatic ring or rich
electronegative atoms [1-2].
Schiff bases have recently emerged as compounds of interest in corrosion inhibition due to the
characteristic C=N bond which are formed through simple and straight forward procedure. Moreover,
the C=N bond, in addition to the aromatic rings present in some molecules, create rich electron
platform that facilitates the adsorption of the compounds on the metal surface; thereby block the
corrosion sites and prevent metal dissolution [3, 4]. There are many studies, including some of our
reports, on the use of Schiff bases for metal corrosion prevention. Most of the reports are recent,
which reflects the recent trend in the application of diverse Schiff base compounds as corrosion
inhibitors [5-15]. The merit of using such compounds is the relative ease with which their structures
can be modified by introducing special groups of atoms, thereby varying their properties and
producing compounds with enhanced properties [16].
Negm et al., [17] have reported the corrosion efficiency of cationic isoxazol-2-ium Schiff base
inhibitors. The studies indicated that increase in the hydrophobic chain length of the surfactant Schiff
base is proportional to corrosion efficiency. In another studies by Shaban et al.,[18] they reported that
the enhancd surface characteristics of their synthesized cationic Schiff base can prevent both carbon
steel deterioration and microbial growth.
The chemistry of corrosion inhibition using organic compounds, with high density of electrons on the
electronegative atoms and on the pie-bonds present in the aromatic rings, is all about chelate
formation on the metal surface. This is due to the electron donation from the electron-rich substrates
or ligands resulting in dative bond through chemisorption process [19-24]. The ligand substrate has
the electron rich centers such as the heteroatoms (N, S, O, P) which are nucleophilic, while the metal
surface is electrophilic and ready to accept or share electrons. The inhibition capacity or adsorption
efficiency is a function of the molecular structure of the compound, planarity of atoms or bulkiness of
the group, types of heteroatoms present, type of electrolyte, the nature and surface morphology of
metal, the distribution of charge over the ligand inhibitor molecule and the type of aggressive media
etc [25-28].
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In order to explicitly explain the behaviour of the molecules and electron interaction within the
individual groups, computational chemistry using the relatively common, easy and less expensive
density functional theory (DFT) has been incorporated in this study [29]. The DFT, among other
applications, aids the explaination of the role of electrons on the corrosion inhibition offered by the
Schiff base compounds. The quantum chemical parameters derived from the DFT, such as energy of
the highest occupied molecular orbitals EHOMO, energy of the lowest unoccupied molecular orbital
ELUMO, energy difference ΔE, and ionization energy (IE) are good descriptors that relate the nature of
the molecules to their performance either as corrosion inhibitors or in other applications [30].
This work reports some Schiff base compounds with similar moiety and slight difference in their
substituents groups, in order to evaluate their different potentials towards corrosion inhibition and the
resultant effect of the substituents on their efficiency. It is intended that the work presented will assist
in explaining the effect of isomerism on the electrochemical activities of the studied Schiff bases.
2. Experimental
2.1. Materials
The starting materials, 4-methylaniline, 4-hydroxybenzaldehyde, benzylaniline, aniline were used as
received from the supplier (Merck) without further purification.
2.2. Spectral, electrochemical measurements and theoretical modelling
Spectral characterisations of the compounds were carried out using Bruker alpha-P FTIR
spectrophotometer operated at wavenumber range of 400-4000 cm-1; the different proton and carbon
environments were analysed using 600 MHz Bruker Avance III NMR spectrophotometers (at room
temperature and DMSO as the solvent). The molecular weight of the compounds was obtained using
micrOTOF-Q 1110390 Bruker compass mass spectrometer.
The study of corrosion inhibition of the Schiff bases on mild steel in aggressive acid medium was
carried out using potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy
(EIS). Three-electrode system comprising mild steel, platinum rod and Ag/AgCl with 3 M KCl were
used as working, counter and reference electrodes respectively on an Autolab PGSTAT 302N
obtained from Metrohm. The inhibitor concentrations in the range (20-100 ppm) were prepared using
acetone and distilled water as co-solvent at a ratio of 1:5, respectively. The PDP measurements of MS
substrate in inhibited and uninhibited medium were scanned from cathodic to the anodic directions in
the potential range of ±250 mV, with a scan rate of 0.5 mVs-1. PDP tests were performed after 1800 s
of MS immersion in the aggressive solutions. Corrosion current density values were, thereafter, used
to calculate the inhibition efficiency (%IEPDP) using the following equation [7-9]:
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

i

corr 
%IEPDP 100 1
(1)



icorr


where iocorr and icorr are corrosion current densities without and with inhibitors respectively.
The EIS tests were achieved at the OCP by resolving the frequency response of the corrosion set-up in
the range of 100 kHz to 10 mHz at 10 mV amplitude. The EIS data were fitted into appropriate
equivalent circuit to obtain relevant electrochemical indices such as the charge transfer resistance
(Rct). The Rct values were used to evaluate the %IEEIS using the formula [7-9]:



Rct  R

ct 
%IEEIS 100
(2)


Rct


where Rct and R0 are the charge transfer resistances with and without inhibitor respectively.
ct
Gaussian 09W software package on B3LYP functional and basis set of a 6-31G+ were used for
quantum chemistry calculations [31]. Extensive geometry optimization calculations were performed
on the Schiff bases in both gas and aqueous phases. The water effect on the electronic indices of the
studied Schiff bases was probed using the polarizable continuum model [30]. The quantum indices
recorded are the energy of the lowest unoccupied molecular orbital (ELUMO), the energy of the
highest occupied molecular orbital (EHOMO), energy band gap (ΔE = ELUMO - EHOMO),
chemical hardness (η), chemical softness (σ) and dipole moment. Most of these were derived
from the EHOMO and ELUMO using the following equations 3-5 [32]:
E  ELUMO  EHOMO
(3)
1
(4)
  

EHOMO  ELUMO

2


1
2
(5)




   

EHOMO  ELUMO


2.3. Synthesis of compounds: (E)-4-((p-tolylimino)methyl) phenol (TMPOL), (E)-4-((
benzylimino)methyl) phenol (BMPOL), and (E)-4-(( p-phenylimino)methyl) phenol (PMPOL).
The Schiff base compounds were prepared by refluxing the respective primary amines with 4-
hydroxybenzaldehyde. About 10 mmol of the following amines: 4-methylaniline,
phenylmethanamine, and aniline were utilized in a 1:1 molar ratio with for 4-hydroxybenzaldeyde in
ethanol and 2-3 drops of glacial acetic acid to obtain BMPOL, and PMPOL respectively acid after 2
h at a temperature of about 90 oC . After cooling, the products were filtered out and dried.
TMPOL: Yield, 59.25%. Selected FTIR (cm-1): 3024 υ(H-C=), 2913, 2858 υ(H-C-), 1604 υ(C=N),
1573,1513s υ(C=C), 1385 (Ar-OH), 1189 (Ar-C-H), 837 (p-benzene ring).
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1H NMR: (DMSO, δ ppm): 2.49 (s, CH3), 6.85, 6.93, 7.17, 7.75 (m, C6H4), 8.42 (s, N=CH), 9.76 (s,
OH). 13C NMR (DMSO, δ ppm): 20.60 (CH3), 114.17, 124.30, 128.10, 130.64, 134.11, 146.10,
159.24 (C6H4), 163.43 (HC=N), 191.16 (C-OH). MS (ESI) m/z = C14H13NO. Calcd, 211.26; Found
[M+H]+ 212.10 (100%). Anal. cal for C14H13NO: C, 79.59; H, 6.20; N, 6.63; Found: C, 78.95; H,
6.59; N, 6.84%.
BMPOL: Yield, 59.41%. Selected FTIR (cm-1): 3027 υ(H-C=), 1628,1601 υ(C=N), 1580, 1494
1
υ(C=C), 1307 (Ar-OH), 1169 (Ar-C-H), 987 (p-benzene ring). H NMR: (DMSO, δ ppm): 4.95 (s,
CH2), 6.54, 7.17, 7.28, 7.39, 7.74 (m, C6H4), 8.45 (s, N=CH), 9.78 (s, OH). 13C NMR (DMSO, δ
ppm): 76.77 (CH2), 115.75, 121.99, 124.28, 133.45, 136.36, 138.31, 145.28, (C6H4), 163.41 (HC=N),
191.14 (C-OH). MS (ESI) m/z = C14H13NO. Calcd, 211.26; Found [M+H]+ 212.10 (100%). Anal. cal
for C14H13NO: C, 79.59; H, 6.20; N, 6.63; Found: C, 79.05; H, 6.34; N, 6.99%.
PMPOL: Yield, 17.04%. Selected FTIR (cm-1): 3030 (H-C=), 1600 υ(C=N), 1513 υ(C=C), 1282
1
(Ar-OH), 1188 (Ar-C-H), 838,(p-benzene ring). H NMR: (DMSO, δ ppm): 6.88, 6.97, 7.18, 7.38,
13
7.77 (m, C6H4), 8.45 (s, N=CH), 9.77 (s, OH). C NMR (DMSO, δ ppm): 115.91, 120.93, 127.95,
129.30, 130.72, 148.65, 152.04 (C6H4), 160.13 (HC=N), 191.06 (C-OH). MS (ESI) m/z = C13H11NO.
Calcd. 197.23; Found [M+H]+ 198.09 (100%). Anal. cal for C13H11NO: C, 79.17; H, 5.62; N, 7.10%;
Found: C, 79.34; H, 5.05; N, 7.31%.
2.4. Mild steel preparation for corrosion studies and material surface analysis
The compositions of MS coupons used for electrochemical corrosion tests were as follows: P, S, Si, C
and Fe with percentage weights of 0.019, 0.017, 0.46, 0.17 and 99.334%, respectively. MS coupons of
1 cm x 1 cm dimension were used. The MS coupons were partially suspended in a molten Teflon
holder and allowed to solidify leaving an exposed surface area of 1 cm2. Before measurement was
taken, the MS surface was scraped on Struers MD PianoTM 220. The surface was further abraded
using increasing smoother SiC papers of grit size up to 600 to obtain a shiny surface, followed by
sonicating in acetone and alcohol bath. The cleaned MS specimens were air-dried and used as the
working electrode in the electrochemical tests. For SEM analyses, the cleaned MS coupons were
immersed in 1 M HCl solution with and without 100 ppm of the reported Schiff bases. After 4 h
immersion period, the specimens were retrieved, cleaned with distilled water, dried and used for SEM
analyses.
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3. Results and Discussion
3.1. Synthesis
The three Schiff base compounds (BMPOL, TMPOL, PMPOL) were synthesized in ethanol solution,
and they all gave yellowish colour which turned brownish yellow after drying. All the reactions were
carried out in a 1:1 molar ratio as presented in Scheme 1(a-c). They are soluble in dichloromethane
(DCM), ethanol, chloroform but insoluble in water. BMPOL and TMPOL are isomers, and gave
almost the same moderate yield compared to the low yield of PMPOL.
Scheme 1: Synthetic route for the synthesis of (a) BMPOL, (b) TMPOL, and (c) PMPOL.
3.2. FTIR studies
The absorption obtained around 3020-3030 cm-1 in all the compounds are major indications of
unsaturation due to the C-H vibration of hydrogen attached to an sp2 hybridized carbon. In the
spectrum of TMPOL, the weak absorption at 3025 cm-1 represents the vibrational frequency of the
C-H bonds of the phenyl rings, the symmetrical and assymetrical vibration of the –C-H attached to a
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saturated carbon appeared at 2913 and 2858 cm-1 respectively. This could be ascribed to C-H of the
methyl group attached in the para position of the aromatic ring. The C=N stretching vibrations was
recorded at 1604 cm-1. Other bands which appeared around 1573, 1513 cm-1 could be due to aromatic
ring stretch while the vibrations at 1385 cm-1 may be due to phenolic OH bend. The band around 1189
cm-1 could be attributed to aromatic C-H in plane bend, while the intense bands noticed around 976,
819, 795 cm-1 are due to trans C-H out of plane bend and characteristic bands of the aromatic
unsaturation [33].
In the spectrum of BMPOL, the aromatic =C-H vibrations were recorded at 3027 cm-1, while the
C=N peaks appeared at 1628 cm-1. The bands at 1528 and 1494 cm-1 are due to the aromatic stretching
vibrations and the peak at 1307 cm-1 could be attributed to the O-H of the phenolic group. The other
bands at 1080, 1025, 987 cm-1 could be ascribed to secondary evidence of aromatic C-H in plane and
out of plane bending vibrations coupled with p-substituted aromatic rings [33-35]. The spectrum of
PMPOL gave weak absorption at 3030 cm-1 attributed to =C-H vibrations of the aromatic rings. The
band due to the C=N group showed absorption around 1600 cm-1. The phenolic O-H bending
vibrations appeared at 1282 cm-1 while other bands, 1188, 838, 792 cm-1 were attributed to in plane
and out of plane C-H bending vibrations.
3.3. NMR spectral studies
1
The H and 13C NMR spectra of the Schiff base compounds were recorded in DMSO. Different
signals were obtained which described the structures of the synthesized Schiff base compounds.
Signals at 2.49 ppm in TMPOL can be assigned to the methyl protons in the para position to the imine
group. The aromatic protons appeared as multiplets between 6.85-7.75 ppm, while the imino proton
was observed at 8.42 ppm [34]. A deshielding effect was observed as a result of the position of
attachment of the OH proton, coupled with intramolecular hydrogen bonding that exist in the
compound. This causes a chemical shift in the downfield region of 9.76 ppm. The 13C NMR spectrum
showed the signals due the methyl carbon at 20.60 ppm, and the peaks which appeared in the
range114.17-159.24 ppm reflects the aromatic protons. The signal due to the imino carbon appeared at
163.43 ppm, and the carbon bearing the OH reonated at 191.16 ppm. In the 1H NMR spectrum of
BMPOL, the absence of signals around 2 ppm signifies the absence of any proton due to aliphatic
carbons. The spectrum showed chemical shift at 4.95 ppm, which is an indication of methylene
protons attached to an electron withdrawing group (imino) that exerts some deshielding effect. The
aromatic protons occurred as multiplet in the region of 6.54-7.74 ppm, and the signal at 8.45 ppm
could be attributed to the imino proton. The OH proton in BMPOL exhibited singlet signal at 9.78
ppm. In the 13C NMR, the methylene carbon appeared at 76.77 ppm and the aromatic carbons signals
showed between 115.75- 145.28 ppm. The imino carbon appeared at 163.41 ppm and the carbon
bearing the hydoxyl group resonated in the downfield region around 191.14 ppm [34].
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The NMR signals obtained for PMPOL were similar to the spectra observed for BMPOL and
TMPOL, this affirm the similarity in their backbone. There were no signals obtained between 2.00-
5.00 ppm showing the absence of methyl and methylene protons. The aromatic protons were observed
between 6.88-7.77 ppm as multiplets. The azomethine proton resonated at 8.45 ppm as singlet peak,
whereas the encountered intramolecular hydrogen bonding caused the OH proton to have a downfield
shift at 9.77 ppm similar to the other two compounds. The 13C NMR showed aromatic protons
between 115.91-152.04 ppm, the imino carbon at 160.13 ppm and the OH bearing carbon displayed
signal at 191.06 ppm.
3.4. Mass spectroscopic results
The mass spectra shown in Figures 1-3 reflect the molecular masses of the different synthesized
Schiff base compounds. The TMPOL and BMPOL have the same mass spectra results with molecular
ion peak of 212.10 because they are isomeric, whereas the PMPOL has molecular ion peak of 198.09.
The molecular formula of the compounds were obtained from the mass spectroscopic measurements,
and there were no other fragmentation patterns observed in all the spectra..
Figure 1: Mass spectrum of (E)-4-((p-tolylimino)methyl) phenol (TMPOL).
Figure 2: Mass spectrum of (E)-4-((benzylimino)methyl) phenol (BMPOL).
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Figure 3: Mass spectrum of (E)-4-(( p-phenylimino)methyl) phenol (PMPOL).
3.5.Potentiodynamic polarization measurements
Anodic and cathodic potentiodynamic polarisation was measured to establish the influence of
inhibitors and change in concentration on the overall behaviour of MS in 1 M HCl solution [7, 32, 36,
37]. The results is presented in Figure 4 and the individual kinetic descriptors procured from the Tafel
curves are recorded in Table 1.
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
Blank
Blank
(TMPOL)
(BMPOL
20 ppm
40 ppm
80 ppm
100 ppm
20 ppm
40 ppm
80 ppm
100 ppm
1E-7
1E-6
1E-5
1E-4
1E-3
1E-6
1E-5
1E-4
i (A cm-2)
1E-3
i (A cm-2)
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
(PMPOL)
Blank
20 ppm
40 ppm
80 ppm
100 ppm
1E-6
1E-5
1E-4
0.001
i (A cm-2)
Figure 4: Tafel curves for MS in 1 M HCl without and with different concentrations of PMPOL,
BMPOL and TMPOL.
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The inhibitors display both cathodic and anodic effects on MS corrosion in 1 M HCl solution [8, 38].
A further investigation reveals that cathodic influence is more pronounced in TMPOL at maximum
concentration (100 ppm), which resulted in the most significant reduction in current density recorded
in the present study. In effect, PMPOL, BMPOL and TMPOL were all mixed type corrosion
inhibitors but cathodic effect is more pronounced in TMPOL at elevated concentration [7, 8, 38]. The
values of anodic (βa) and (βc) Tafel slopes recorded in Table 1 corroborate the observed effect on both
cathodic and anodic arms of Figure 4. The minor shift in values of Ecorr connotes that the present set
of inhibitors retard MS dissolution by developing a defensive film that did not alter the reaction
mechanism [7]. TMPOL recorded the least corrosion current density (icorr) values at 20 ppm which
suggests that TMPOL is a better corrosion inhibitor at the lowest studied concentration. However,
there was a pronounced reduction in icorr values for BMPOL at concentration range 80-100 ppm.
Based on this observation, the obvious trend in inhibition efficiency obtained from averaged %IEPDP
values is such that BMPOL ˃ TMPOL ˃ PMPOL.
Table 1: Kinetic parameters from Tafel plots for MS in 1 M HCl in the absence and presence of
PMPOL, BMPOL and TMPOL at various concentrations.
Conc (ppm)
βa (mV/dec)
βc (mV/dec)
-Ecorr (mV) icorr (μA cm-2) %IEPDP
Blank
98.93
72.08
452.72
236
-
PMPOL
20
117.71
135.26
104.13
114.36
90.72
105.59
77.24
85.38
482.20
474.57
488.57
485.17
202
116
96
14
51
59
74
40
80
100
62
BMPOL
20
40
80
100
56.59
86.12
67.47
49.30
92.13
62.40
110.65
15.21
486.83
482.71
472.79
484.08
175
127
48
26
46
80
84
38
TMPOL
20
153.10
136.53
55.05
97.88
77.55
89.33
81.51
477.44
481.31
482.86
491.35
144
141
93
39
40
61
80
40
80
100
130.94
48
3.6.Impedance study
As shown in the Nyquist plots of Figure 5, the kinetics at interfaces can successfully be elucidated
using EIS technique [7, 38]. The impedance spectrum registered at each concentration yielded
imperfect semicircles in high frequency zone which implies charge transfer and development of
defensive film on the MS surface [38, 39].
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60
50
40
30
20
10
0
120
Blank
20 ppm
40 ppm
80 ppm
100 ppm
Blank
(BMPOL)
20 ppm
40 ppm
80 ppm
100 ppm
(PMPOL)
100
80
60
40
20
0
0
10
20
30
40
50
60
0
20
40
60
80
100
120
2
2
z' ( cm )
Z' ( cm )
70
60
50
40
30
20
10
Blank
(TMPOL)
20 ppm
40 ppm
80 ppm
100 ppm
10
20
30
40
50
60
70
2
Z' ( cm )
Figure 5: Impedance spectra for MS in 1 M HCl without and with different concentrations of PMPOL,
BMPOL and TMPOL.
The imperfect nature of the semicircles arose from mass transfer processes, frequency dispersion and
the obvious effect of MS rough surface [39, 40]. The overall impedance Z' for the inhibitor containing
system has been shown to be higher than that obtained for the blank system. This suggests that
electrochemical reactions may have taken place between the inhibitor and MS and/or between the
inhibitor and the 1 M HCl solution (electrolyte) [41].
The impedance parameters which are recorded in Table 2 were used to describe the impedance
characteristics of MS corrosion in blank and various concentrations of the studied Schiff bases [42-
45]. The parameters were obtained after fitting the impedance data in a standard electrical equivalent
circuit given in Figure 7 [32]. Herein, the constant phase element (CPE) was employed instead of the
double layer capacitance (Cdl) in order to achieve a more accurate fit. The capacitive nature of the
11

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imperfect semicircle in the present study was confirmed by the phase shift (n) values that tend to unity
[32]. Slight increament in Rct values was recorded for the inhibitors at low concentrations which
suggested that the inhibitory activities of the reported compounds became significant only at elevated
concentrations (80-100 ppm). This was further reaffirmed by the closeness of phase angle (α) and
slope (S) values (Table 2) obtained for the blank and those of the inhibitor-containing systems
especially at low concentrations. At improved concentrations however, there was obvious
improvement in α and S values that tend to -90 and -1 respectively. The deviation was due to the
rough MS electrode surface and intricate electrochemical activities, whereas the attempt to reach -90
and -1 mirrors an ideal capacitive electrode/electrolyte interface [32, 40]. The Bode phase angle plots
in Figure 6 bore one time constant with widening of their maxima in the presence of inhibitor that
suggests the formation of a protective layer on MS surface [32, 40]. The phenomenon supports the
idea that at increased inhibitor concentrations, more inhibitor species adhere to the exposed surface of
MS and make it more homogeneous. The magnitude of CPE constant (Y0) and solution resistance (Rs)
recorded minor changes with no clear trend. At concentrations range of 80-100 ppm, the values of Rct
followed the order of BMPOL > TMPOL > PMPOL, resulting to the inhibition efficiency order of:
BMPOL > TMPOL > PMPOL, and the maximum %IEEIS reaches up to 88, 75 and 74% for BMPOL,
TMPOL and PMPOL, respectively. These results reassure that the three Schiff bases possess good
corrosion inhibition properties for MS in 1 M HCl solution. In addition, the %IEEIS values display a
similar trend to %IEPDP values.
Table 2: Electrochemical parameters obtained from EIS plots for MS in 1 M HCl in the absence and
presence of PMPOL, BMPOL and TMPOL at various concentrations.
Conc
(ppm)
-
Rs
Rct
(Ω cm-2)
17
Y0
(μSsncm-2)
677
n
%IEEIS
-α
-S
Inhibitor
Blank
(Ω cm-2)
2.50
0.8
-
40
0.30
PMPOL
20
40
80
1.92
2.15
1.57
1.54
19
33
38
65
675
677
582
434
0.8
0.8
0.8
0.8
11
48
55
74
49
59
39
61
45
58
0.45
0.53
0.53
0.78
0.44
0.48
100
BMPOL
TMPOL
20
40
80
3.53
3.58
3.18
2.85
22
32
70
521
429
374
289
0.8
0.8
0.8
0.8
23
47
76
88
100
137
20
40
80
2.08
1.87
4.08
1.67
26
27
38
69
796
621
489
347
0.8
0.8
0.8
0.8
35
37
55
75
100
12

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2.5
70
60
50
40
30
20
10
0
2.0
1.6
1.2
0.8
0.4
0.0
Blank
(PMPOL)
(BMPOL)
60
50
40
30
20
10
0
20 ppm
40 ppm
80 ppm
100 ppm
2.0
1.5
1.0
0.5
Blank
20 ppm
40 ppm
80 ppm
100 ppm
-1
0
1
2
3
4
-1
0
1
2
3
4
log f (Hz)
log f (Hz)
2.5
2.0
1.5
1.0
0.5
60
50
40
30
20
10
0
(TMPOL)
Blank
20 ppm
40 ppm
80 ppm
100 ppm
0.0
-1
0
1
2
3
4
log f (Hz)
Figure 6: Bode impedance modulus and phase angle plots for MS in 1 M HCl in absence and presence
of different concentration of PMPOL, BMPOL and TMPOL.
Figure 7: Electrochemical equivalent circuit.
3.7.Adsorption isotherm
Studying the adsorption isotherm of organic inhibitors on MS in acid solution was justified by the
quasi-substitution action of the inhibitor specie that results in the formation of water-blocking matrix
13

ACCEPTED MANUSCRIPT
[40]. To explore the adsorptive behaviour of PMPOL, BMPOL and TMPOL on MS surface, different
adsorption isotherms such as Freundlich, Frumkin and Langmuir have been examined in order to
establish the potential link between inhibitor concentration (C) and the surface coverage (θ=C/%IEEIS)
is presented in Table 2. We found that the adsorption of PMPOL, BMPOL and TMPOL molecules
  KadsCinh
preferred the Freundlich isotherm of the form [38]:
(6)
where, θ is the degree of surface coverage; Kads is the equilibrium constant of the
adsorption/desorption activity and Cinh is the concentration of the inhibitor.
The fitting lines in Figure 8 show the Freundlich isotherm with values of slopes, intercepts and linear
correlation coefficients (R2) of the studied inhibitors. The choice of the Freundlich isotherm over the
rest was based on the R2 values that are almost unity and the intercept (I) at 0 [32].
Consequently, the values of Kads were calculated from the slopes of the lines. The respective Kads
values are as follows: PMPOL: 28.2
  
105, BMPOL: 19.2 105 and TMPOL: 10.1 105. A higher Kads
value connotes a stronger interaction between adsorbate and adsorbent [37, 38]. As could be observed,
the recorded Kads values showed that TMPOL was the least. This contradicted the weakest inhibition
activity recorded for PMPOL on the acid corrosion of MS. However, such anomaly could be due to
complex mode of physical and chemical interactions. The values of Gibb’s free energy of adsorption,
ΔG°ads were obtained using equation [38]:

(7)
 RT ln(55.5Kads)
Gads
where, R is the universal gas constant, T is the absolute temperature and the value of 55.5 is the
approximate concentration (in mol L-1) of water in the solution. The recorded ΔG°ads values are as
follows: -47.5, -46.6 and -44.9 kJ.mol-1 that represent PMPOL, BMPOL and TMPOL, respectively.
The obtained ΔG°ads values above the threshold of -40 kJ.mol-1 suggest a spontaneous process marked
by both chemical and physical adsorption of inhibitor species on MS surface [46].
PMPOL: I= 0.07: S= 0.01: R2= 0.92
0.9
BMPOL: I= 0.11: S= 0.01: R2= 0.99
TMPOL: I= 0.21: S= 0.01: R2= 0.96
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
20
40
60
80
100
Cinh (ppm)
14

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Figure 8: Adsorption isotherm plot of Freundlich for MS in the presence of PMPOL, BMPOL and
TMPOL.
3.8.SEM studies
The SEM images of MS specimens immersed in 1 M HCl for 3 h in the absence and presence of
PMPOL or BMPOL or TMPOL are shown in Figure 9. As could be observed from Figure 9a, the MS
surface recovered in blank solution showed severely corroded image with densely distributed
corrosion products. This was in sharp contrast to Figures 9b, 9c and 9d where the damage on the MS
surface was reduced, though with appearance of pits. This observation highlights the extent of
protective potential of PMPOL, BMPOL and TMPOL as corrosion inhibitors for MS in acidic media.
The MS coupons retrieved from inhibitor-containing media showed remarkably improved surfaces
that bore traces of roughness that was acquired during MS preparation step.
Figure 9: SEM images of MS samples after immersion in 1 M HCl for 3 h at 30 °C (a) without
inhibitor, (b) with 100 ppm PMPOL, (c) with 100 ppm BMPOL and (d) with 100 ppm TMPOL.
3.9.Computational studies
The efficiency of organic molecules to act as corrosion inhibitors depends to a large extent on their
structural and electronic properties [38, 39]. In exploiting these properties, the frontier molecular
orbital (FMO) density distributions of non-protonated PMPOL, BMPOL and TMPOL were
investigated in both gas and aqueous phases, as shown in Figure 10. The applicable quantum chemical
descriptors most relevant to their corrosion inhibitory activities are listed in Table 3. In accordance
with FMO postulate, molecular reactivity of molecules can be unravelled by analysis of the density
distributions of HOMO and LUMO. EHOMO elucidates the electron donating ability, whereas ELUMO
indicates the electron accepting ability of a molecule [7, 8, 32, 36-38]. Inspection of Figure 10 reveals
that both the HOMO and LUMO are of π-type and spread over the entire structures of the Schiff bases
except for BMPOL and TMPOL, where there is little or no contribution from the phenyl π*-orbital
and methyl alkyl chain to LUMO, respectively. Moreso, the contribution of O and N heteroatoms to
LUMO and HOMO from the Schiff bases is evident. Therefore, the heteroatoms and Schiff base
segment are the principal adsorption centers for donor-acceptor interactions with the unoccupied d-
orbital of the iron atom to form coordinate bonds. Ideally, an efficient corrosion inhibitor is the
15

ACCEPTED MANUSCRIPT
molecule with the least energy gap (ΔE) [7, 32, 37], high dipole moment [32, 47], high chemical
softness () [32, 37] and low chemical hardness () values [32, 48]. These quantum indices and their
illustrations enhanced the reactivity of the studied inhibitors that hugely influenced donor-acceptor
interactions with the mild steel, thereby shielding its surface from direct degradation in acid medium
[49, 50]. Although, there was no general trend of these quantum descriptors with the experimental
corrosion inhibition efficiency of the studied inhibitors, most of them, however, reaffirm that PMPOL
was a better corrosion inhibitor for MS corrosion in 1 M HCl solution.
Optimised structure
PMPOL
HOMO
BMPOL
TMPOL
PMPOL
LUMO
BMPOL
TMPOL
PMPOL
BMPOL
TMPOL
Figure 10: Optimised structures and FMO density distribution of the non-protonated inhibitors in gas
phase.
16

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Table 3: Selected quantum chemical parameters for the studied Schiff bases.
Inhibitor
EHOMO
(eV)
ELUMO
(eV)
ΔE
(eV)
Dipole
moment


Gas Phase
PMPOL
BMPOL
TMPOL
-5.99
-6.30
-5.93
-1.75
-1.41
-1.80
4.24
4.89
4.13
3.04
3.07
1.93
2.12
2.45
2.07
0.47
0.41
0.48
Aqueous
Phase
PMPOL
BMPOL
TMPOL
-6.12
-6.34
-6.06
-1.88
-1.49
-1.93
4.24
4.85
4.13
4.47
4.05
2.96
2.12
2.43
2.07
0.47
0.41
0.48
Conclusion
The synthesis and characterization of (E)-4-((p-tolylimino)methyl) phenol (TMPOL), (E)-4-((
benzylimino)methyl) phenol (BMPOL) and (E)-4-(( p-phenylimino)methyl) phenol (PMPOL) have
been reported in the present study. The Schiff bases were of a common structural backbone with
different substituent groups attached to different centres. The cheaply synthesized compounds were
investigated for corrosion inhibition activities against mild steel corrosion in 1 M HCl solution and the
following conclusions can be drawn:
[1] TMPOL, BMPOL and PMPOL retard mild steel corrosion in 1 M HCl medium with their
inhibition efficiency enhanced at increased inhibitor concentrations.
[2] Both polarization and impedance electrochemical techniques proved that the studied Schiff bases
are mixed type corrosion inhibitors even though cathodic effect is more obvious in TMPOL at
elevated concentration.
[3] The adsorption of the inhibitors on mild steel was spontaneous, marked by both chemical
and physical adsorption of inhibitor species, and prefers the Freundlich adsorption isotherm.
[4] In the presence of the studied inhibitors, SEM images revealed improved surfaces that were
devoid of an extensive layer of corroded porous film that marked the steel specimen immersed in 1 M
HCl solution.
[5] Quantum chemical study indicated that heteroatoms and Schiff base segment were the principal
adsorption centers for donor-acceptor interactions with the unoccupied d-orbital of mild steel. The
better anticorrosion performance of BMPOL and TMPOL could be attributed to among other factors
to the presence of a methylene group and methyl alkyl chain, respectively.
17

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RESEARCH HIGHLIGHTS
 Schiff base compounds, TMPOL, BMPOL and PMPOL were synthesized
 The structure of the Schiff bases was verified using a DFT based calculations.
 The Schiff bases are good inhibitors for mild steel corrosion in 1 M HCl solution
 The adsorption mechanisms preferred the Langmuir adsorption isotherm
 Correlation exist between the inhibition efficiency and quantum chemical indices
22