Synthesis, crystal structures, quantum chemical studies and corrosion inhibition potentials of 4-(((4-ethylphenyl)imino)methyl)phenol and (E)-4-((naphthalen-2-ylimino) methyl) phenol Schiff bases

Article abstract of DOI:10.1016/j.molstruc.2017.06.104

Two Schiff base ligands, 4-(((4-ethylphenyl)imino)methyl)phenol (4EMP) and (E)-4-((naphthalen-2-ylimino) methyl) phenol (4NMP) were synthesized by the reaction of 4-hydroxybenzaldehyde with 4-ethylaniline, 4EMP, or naphthalene-2-amine, 4NMP. The compounds

Full text of DOI:10.1016/j.molstruc.2017.06.104

Accepted Manuscript
Synthesis, crystal structures, quantum chemical studies and corrosion inhibition
potentials of 4-(((4-ethylphenyl)imino)methyl)phenol and (E)-4-((naphthalen-2-
ylimino) methyl) phenol Schiff bases
Elias E. Elemike, Henry U. Nwankwo, Damian C. Onwudiwe, Eric C. Hosten
PII:
S0022-2860(17)30885-2
10.1016/j.molstruc.2017.06.104
MOLSTR 23990
DOI:
Reference:
To appear in:
Journal of Molecular Structure
Received Date:
Revised Date:
Accepted Date:
13 May 2017
20 June 2017
21 June 2017
Please cite this article as: Elias E. Elemike, Henry U. Nwankwo, Damian C. Onwudiwe, Eric C.
Hosten, Synthesis, crystal structures, quantum chemical studies and corrosion inhibition potentials
of 4-(((4-ethylphenyl)imino)methyl)phenol and (E)-4-((naphthalen-2-ylimino) methyl) phenol Schiff
bases, Journal of Molecular Structure (2017), doi: 10.1016/j.molstruc.2017.06.104
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GRAPHICAL ABSTRACT
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Synthesis, crystal structures, quantum chemical studies and corrosion inhibition
potentials of 4-(((4-ethylphenyl)imino)methyl)phenol and (E)-4-((naphthalen-2-ylimino)
methyl) phenol Schiff bases
4
1,2,3 Elias E. Elemike, 1,2Henry U. Nwankwo, 1,2Damian C. Onwudiwe*, Eric C. Hosten
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.
4. Department of Chemistry, Nelson Mandela Metropolitan University, P.O. Box 77000,
Port Elizabeth 6031, South Africa
* Corresponding author
Email: Damian.Onwudiwe@nwu.ac.za
Tel: +27 18 - 389 -2545
Fax: +2718 – 389 -2420
Abstract
Two Schiff base ligands, 4-(((4-ethylphenyl)imino)methyl)phenol (4EMP) and (E)-4-
((naphthalen-2-ylimino) methyl) phenol (4NMP) were synthesized by the reaction of 4-
hydroxybenzaldehyde with 4-ethylaniline, 4EMP, or naphthalene-2-amine, 4NMP. The
13
compounds were characterized using NMR (1H and C), Fourier transform infra-red (FTIR)
and mass spectroscopic techniques. The proton NMR identified the OH peaks at 9.73 and
13
9.77 ppm for 4EMP and 4NMP respectively, while the C-NMR showed the imine carbons
at 172.57 ppm for 4EMP and at 160.89 ppm for 4NMP. The FTIR spectra showed
characteristic peaks at 1605 cm-1 (4EMP) and 1600 cm-1 (4NMP) typical of the azomethine
group, and the mass spectra results gave molecular ion peaks of 226.12 and 248.10
respectively. The structures of the compounds were further established by single crystal X-
ray analysis. The corrosion inhibition potential of the compounds were studied on mild steel
surface in a 1 M hydrochloric acid (HCl) solution, and was analysed using potentiodynamic
polarization (PDP) and electrochemical impedance spectroscopy (EIS). The results of the

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electrochemical methods showed that the studied molecules imparted high resistance in
allowing flow of electrons across the metal–electrolyte platform and behaved as mixed type
inhibitors with 4EMP showing better inhibition properties than 4NMP. Scanning electron
microscopy (SEM) showed the formation of film on the mild steel surface. Quantum
chemical calculations achieved by density functional theory (DFT) was further applied to
explain the adsorption as well as inhibition abilities of the molecules on the mild steel
surface. Thermodynamics studies showed that the two compounds obeyed the Langmuir
isotherm with 4EMP conforming to chemisorption mechanism while 4NMP involved
competitive physisorption and chemisorption mechanism.
Keywords: Schiff base; crystal structure; DFT; corrosion inhibition
1. Introduction
Schiff bases are important category of compounds which could serve as functional models for
the elucidation of structures of biomolecules and biological processes [1]. They are often
used as important ligands especially the ones bearing N, O, S donor atoms which exhibit
structural similarities with neutral biological systems [2-4]. One of their structural
significance has been the inherent interpretation of the mechanism of transformation of
racemization reaction in biological systems attributed to the imine group (C=N). They also
serve as one of the most relevant synthetic ligand systems, with importance in asymmetric
catalysis, conducting materials and polymers, crystal engineering and therapeutic agents [5,
6]. Schiff bases have proven to be cheap starting precursors with relatively easy synthetic
route. The high purity, low toxicity, and eco-friendliness have made the synthesis and
structural studies of new Schiff base compounds very interesting.
In the last few years, extensive efforts have been made, both in academics and industry to
develop Schiff base compounds with interesting anti-corrosion effect, because of their high
thermal stability, complex forming ability and semiconducting properties [7]. Thus, it is the
first step in the search for compounds with potentials to protect the surfaces of metal from
environmental and chemical deterioration. Saha et al. [8] have evaluated the contributions of
the species present in the molecules of (E)-4-((2-(2,4-dinitrophenyl)hydrazono)-
methyl)pyridine, (E)-4-(2-(pyridin-4-ylmethylene)hydrazinyl)benzonitrile and (E)-4-((2-(2,4-
dinitro-phenyl)hydrazono)methyl)phenol towards the corrosion inhibition performances on

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mild steel in 1 M HCl medium. Hamani et al. [9] investigated the corrosion inhibition effect
of some new azomethine compounds on mild steel in 1 M HCl using potentiodynamic
polarization, electrochemical impedance spectroscopy (EIS) and quantum chemical analysis.
It was inferred that the inhibition efficiency increased with increasing inhibitor concentration
and the Schiff bases function as mixed-type inhibitors with the adsorption patterns typical of
Langmuir isotherm. Pordel et al. [10] reported that the intramolecular charge transfer (ICT)
present in (5-hydrox-yimino-1-alkyl-4,5-dihydro-1H-4-indazolyliden)-2-arylacetonitriles due
to the colour intensities could be potential sites for corrosion inhibition properties.
Mild steel is widely used in industries especially in petroleum sector for the transport of the
crude and refined products, and they undergo corrosion with time due to attack by acid or
basic solutions used in pickling. In order to reduce damage and achieve higher life time of the
mild steel, corrosion inhibitors are needed for lower iron dissolution. The incessant
environmental degradation associated with the use of inorganic corrosion inhibitors such as
chromate and nitrite have shifted focus to the more environmental friendly organic
counterparts [11-14]. Stringent environmental legislations and huge penalty for defaulters put
in place by various national agencies, suggest the need to combat corrosion-related problems
in industries holistically. Over the past two decades, extensive research and development
have focused on the use of greener and smarter means to combat corrosion. Macrocyclic
ligands containing N, S, O and P heteroatoms have also been found to possess broad
spectrum anticorrosion properties [15-20]. The nature of the inhibiting molecules, steric
effect, aromaticity, electron concentration on donor sites, type of orbitals donating electrons,
presence of functional groups (such as N=N, C=N, C=S, C=C, R-OH, R=R), surface area of
the molecule, molecular weight of the compound, temperature of reaction and potential effect
at the metal/solution interface contribute immensely to the enhanced anti-corrosive properties
of an inhibitor [22-26]. Recently, Nwankwo et al. [21], explored the corrosion inhibition
properties of 1,8‐dimethyl‐1,3,6,8,10,13‐hexaazacyclotetradecane ligand for mild steel in 1 M
HCl.
In the quest to explore the behaviour of atoms and molecules, quantum chemical calculations
have become a useful method to study reaction mechanism, determine the molecular
structure, and equally elucidate the electronic structure and reactivity of compounds. More
so, the study of corrosion inhibition mechanism in a compound has been made easy by this
technique [27-29]. The possibility of quantum calculations are due to Density Functional

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Theory (DFT) which is reliable and gives correct values for necessary parameters even for
huge complex molecules at low cost, and could explain microcosmic inhibition performance
and their reaction mechanism [30]. Thus, it has become a main source for connecting some
traditional empirical concepts with quantum mechanics, and can be used to understand the
interaction occurring at the material interface and analyse experimental data [31].
In the light of the interesting chemistry of Schiff base and their wide applications, we herein
report the synthesis of two Schiff base compounds which were characterized using
spectroscopic methods and single X-ray analysis. The corrosion inhibition potentials of the
Schiff-base molecules in an aggressive acidic medium were studied using potentiodynamic
polarization and electrochemical impedance spectroscopy (EIS) techniques. In addition,
quantum chemical calculations were performed in order to gain required insights into their
potential mechanism of inhibition. In this perspective, several quantum chemical properties
such as EHOMO, ELUMO, energy gap (ΔE), electronegativity (χ), softness (σ), fraction of
electron transferred from inhibitor to metal surface (DN) were studied.
2. Experimental procedures
2.1 Materials
The chemical reagents used for the synthesis and corrosion studies are of analytical grade. 4-
ethylaniline, naphthalene-2-amine and 4-hydroxybenzaldehyde were purchased from Sigma-
Aldrich and used without further purification. Ethanol, hydrochloric acid (32%) and acetone
were purchased from Merck, South Africa.
2.2 Instrumentation
NMR spectra were recorded on a Bruker Avance III spectrophotometer operating at 600
MHz. DMSO-d6 was used as solvent, and all chemical shifts (δ) are given in parts per million
(ppm). Infrared spectra were measured on a Bruker alpha-P FTIR spectrophotometer in the
frequency range 4000 – 400 cm-1. Melting points were measured by using a Stuart Scientific
melting Apparatus. The mass spectra of the ligands were recorded in ethanol solution in the
positive ionization mode on a micrOTOF-Q 11 10390 Bruker compass Mass spectrometer.
Electrochemical studies using potentiodynamic polarization and electrochemical impedance
spectroscopic measurements were carried out using Autolab PGSTAT 302N obtained from
Metrohm. The instrumental set up includes a three-electrode system; mild steel, platinum rod

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and Ag/AgCl with 3 M KCl as working, counter and reference electrodes respectively. The
surface analysis of the mild steel was carried out using Quanta FEG 250 Environmental
scanning electron microscope (ESEM) under an acceleration voltage of 30 kV with
simultaneous recording of the energy dispersive spectrum (EDS).
2.3 Synthesis of 4-(((4-ethylphenyl)imino)methyl)phenol and (E)-4-((naphthalen-2-ylimino)
methyl) phenol Schiff bases
4-ethylaniline (10 mM) was added slowly to 1.22 g 4-hydroxybenzaldehyde (10 mM)
dissolved in 40 mL ethanol, and drops of glacial acetic acid were added. In a similar
procedure, 1.43 g of 1-naphthylamine (10 mmol) and 1.22 g of 4-hydroxybenzaldehyde (10
mmol) in 40 mL absolute ethanol was refluxed for 2 h, also in the presence of a few drops of
glacial acetic acid. The solutions were filtered and slow evaporation resulted into shiny
crystals suitable for single crystal X-ray analysis.
4EMP, Yield: 75.2%. m.p 156-160 ᵒC. selected FTIR (cm-1): 2962, 2929, 2871 ν (H-C-C),
1605 ν(C=N), 1574, 1511 ν(C=C), 1411 σ(-C=C-N), 1309 ν(C-O), 1284, 1191(Ar-OH),
1
828(p-benzene ring). H-NMR: (DMSO, δ ppm): 1.13, 1.17 (m, CH3), 2.38, 2.41(d, CH2),
6.45, 6.87, 6.91, 7.11, 7.19, 7.61, 7.74 (m, C6H4), 8.43 (s, N=CH), 9.73 (s, OH).13C-NMR
(DMSO, δ ppm): 15.51(CH3), 28.81(CH2), 114.15, 120.95, 126.56, 141.44, 146.55, 150.63,
159.33, 163.88(C6H4), 172.57(HC=N). MS (ESI) m/z =C15H15NO.Calcd 225.29. Found
[M+H]+ = 226.12(100%). Anal. cal for C15H15NO: C,79.97% ; H, 6.71%; N, 6.22%; Found:
C, 80.24%; H, 5.95%; N,6.30%.
4MNP Yield: 65.4%. m.p 194-198 ̊
C. selected FTIR (cm-1): 3046 ν(H-C=C), 2978, 2871 ν (H-C-
C), 1600 ν(C=N), 1580, 1567, 1515 ν(C=C), 1442 σ(-C=C-N), 1383 ν(C-O), 1284, 1152 (O-H), 1039
1
(aromatic C-H ), 831, 772 (naphthalene rings). H NMR (DMSO, δ ppm): 6.91, 6.93 (d, C6H4), 7.17,
7.54, 7.75, 7.91, 8.25 (m, C10H7), 8.52 (s, N=CH), 9.77 (s, OH). 13C NMR (DMSO, δ ppm): 115.83,
123.59, 125.75, 125.10, 126.46, 133.61, 131.01, 148.93 (C10H7), 112.97, 127.77, 127.70, 128.55
(C6H4), 160.37 (HC=N), 160.89 (C-OH).
MS (ESI) m/z = C17H13NO Calcd. 247.29. Found [M+H]+ = 248.10 (100%). Anal. Calc. for
C17H13NO: C, 82.57; H, 5.30; N, 5.66; Found: C, 81.19; H, 5.10; N, 6.33%.

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2.4 Specimen preparation
Mild steel coupons with percentage weight composition: 0.17 % C, 0.46 % Si, 0.017 % S,
0.019 % P, and Fe of the remaining percentage was used for corrosion studies. Coupons of
the mild steel material were made by cutting it into 1 cm x 1 cm and inserted in a Teflon
holder using epoxy resin, with surface area of 1 cm2 been exposed. Before the
electrochemical measurement, there was proper cleaning of the mild steel surface by abrasion
on Struers MD PianoTM 220 (size: 200) mounted on Struers LaboPol-1 machine. The surface
was thoroughly cleaned to achieve a mirror-like surface. The mirror like coupons were
cleaned in a sonicating acetone bath for 30 min, followed by sonication in ethanol, and
subsequent cleaning with paper towel and dried. After drying, the mild steel was used for the
corrosion studies. The aggressive medium of analysis (1 M HCl) was made from (32%) acid
solution. The inhibitor concentrations: 40, 60, 100, 500 and 1000 ppm were prepared in
mixture of acetone and ethanol, and remainder, distilled water and sonicated.
2.5 Mild steel surface analysis
The surface abraded mild steel strips of size 1 × 1 cm were immersed in 1 M HCl both in a
medium without the inhibitor and with 100 ppm inhibitor for 3 h. Adequate cleaning of the
surface with water, acetone and ethanol was done after the time of immersion. The sample
was dried and analysed with SEM. Chemical composition of the mild steel (MS) surface was
recorded by an EDS detector.
2.6 X-ray crystallography
Typical X-ray diffraction studies were carried out at 200 K using a Bruker Kappa Apex II
diffractometer with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The
procedural data collection, cell refinement and data reduction were done by APEXII and
SAINT programs [32]. SHELXT-2014 was used to solve the structures and refined by least-
squares procedures with SHELXLE [33] as a graphical interface. The H-atoms attached to the
carbons were placed in calculated positions and were included in the refinement in the riding
model approximation, with Uiso(H) set to 1.2Ueq(C) while the none-H atoms were
anisotropically corrected. The methyl hydrogens were made to rotate with a fixed angle
around the C—C bond to appropiately fit the experimental electron density (HFIX 137 in the

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SHELX program suite [34]), with Uiso(H) set to 1.5Ueq(C). In the case of the hydroxyl H-
atom, rotation with a fixed angle around the C—O bond was done to best fit the experimental
electron density (HFIX 147 in the SHELX program suite [34]), with Uiso(H) set to 1.5Ueq(O).
The obtained data were corrected for absorption effects by the numerical method using
SADABS [32].
2.7 Quantum chemical calculations
Quantum chemical calculations were performed using density functional theory (DFT)
method on a B3LYP (Becke’s Three parameter hybrid functional using the LYP correlation
functional) theoretical model with electron basis set 6–31G +(d, p) for all atoms [35].
Calculations were performed on the neutral molecules with Gaussian 09 [36]. The molecular
geometries were taken directly from the X-ray diffraction experimental results without any
constraints. Geometry optimization and various quantum chemical parameters were obtained.
The quantum chemical parameters EHOMO, ELUMO, energy gap ΔE (ELUMO−EHOMO), chemical
potential (μ), absolute electronegativity (χ), absolute hardness (η), global softness (σ), and the
fraction of electrons transferred (ΔN) were obtained using the following equations [37-39]:
Ip(Ionization potential) = -EHOMO and EA (electron affinity) = -ELUMO
(1)
(2)
Chemical potential, µ =- 1 (lp+EA)
2
Absolute electronegativity, χ = 1 (EHOMO+ ELUMO
)
(3)
‒
2
Absolute hardness, η = - 1 (EHOMO – ELUMO
)
(4)
(5)
2
Energy gap, ΔE = ELUMO- EHOMO
1
Global softness (σ) =
(6)
(7)
휂
휒퐹푒 ‒ 휒푖푛ℎ
Number of e- transferred from the inhibitor to metal surface (ΔN) = 2(휂
퐹푒 + 휂푖푛ℎ
)
(In equation 7, χFe =7 while ηFe =0).
In a bid to ascertain the prospective centres of the inhibitor molecules that are prone to
electrophilic attacks by the metallic atoms, the electron density surfaces of the electrophilic

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Fukui function (f−) of the studied Schiff bases were visualized using the Multiwfn software
[40].
3. Results and Discussion
3.1 Synthesis
The results of the reaction of 4-ethylaniline with 4-hydroxybenzaldehyde in a 1:1 molar ratio in
ethanol gave 4EMP as shown in scheme 1; while the reaction of naphthalene-2-amine with 4-
hydroxybenzaldehyde readily gave 4MNP, both were obtained in moderately high yield (Scheme 2).
The reaction was completed after 2 h and slow evaporation of the product solution afforded
brownish yellow and brown crystals for 4EMP and 4MNP respectively. Both compounds
were soluble in DMSO, hot ethanol and methanol, partially soluble in chloroform and ethyl
acetate but insoluble in water and toluene.
CHO
NH2
6
14
reflux, 2 h
ethanol
,
glacial acetic acid
15
10
11
1
2
5
8
HO
7
+
H
CH3
H
N
9
C
C
17
2
1
13
4
6
3
12
OH
CH2CH3
4-(((4-ethylphenyl)imino)methyl)phenol
Scheme 1: The synthetic pathway for 4-(((4-ethylphenyl)imino)methyl)phenol
CHO
17
18
11
13
6
10
NH2
reflux, 2 h
N
CH
1
2
19
OH
+
16
12
5
4
9
8
14
15
ethanol
glacial acetic acid
3
7
OH
Naphthalen-2-
amine
4-((naphthalen-2-ylimino)methyl)phenol
4-hydroxybenzaldehyde
Scheme 2: Reaction pathway showing the formation of (E)-4-((naphthalen-2-ylimino) methyl) phenol
Schiff base

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3.2 FTIR Spectral studies
The Infra-red spectroscopic studies of 4EMP show three peaks around 2962, 2929 and
2871cm-1 which are characteristic of alkane homologues due to the CH2CH3 functional
groups. These bands are weak and correspond to C-H stretching vibrations. The sharp peak
observed at 1605 cm-1 could be attributed to –C=N group and the peaks at 1574, 1511 and
1411 cm-1 are attributed to –C=C─ and C-H in plane bending vibrations of the aromatic ring
which forms an important part of the molecule [41, 42, 43]. The band around 1309 cm-1 could
be due to aryl OH deformations while 1284 and 1191 cm-1, which were among the strongest
peaks in the spectra, may be due to C-O stretching vibrations. The sharp and intense band at
828 cm-1 could be ascribed to out of plane vibrations of the p-disubstituted benzene ring [41,
44, 45].
In the spectra of 4MNP, bands due to the azomethine and the phenolic groups modes were
observed. In addition, a few bands due to C-C stretching and symmetric/ asymmetric C-H
modes also appeared. The bands at 3046 cm-1 may be assigned to the OH group, while the
weak bands at 2978 and 2871 cm-1 could be due to the C-H stretching vibrations which are
characteristics of the heteroaromatic structure. The absence of strong bands within 1600-1750
cm-1 range indicates the absence of carbonyl group, a consequence of the formation of the
Schiff base. The appearance of strong bands in the region of 1515–1600 cm-1 could be
assigned to the imine (C=N) group [46, 47]. Intense band that appeared at 1442 cm-1 could be
due to the C-C stretching of the benzene and naphthalene rings and also the C-H in-plane
bending modes of the aromatic rings [41, 45]. The bands which occurred around 1383-1152
cm-1 represent the C-O stretch, the O-H deformation which may have resulted from the
phenolic group in the compound and CH2 wagging and CH2 twisting vibrations [41]. Within
this region, also, aromatic C–H in-plane bending vibrations were observed with weak-to-
medium intensities around 1039 cm-1. The characteristic substituent patterns and also out of
plane vibrations of the naphthalene rings could also be attributed to the band at 831cm-1 [45].
3.3 1HNMR and 13CNMR spectroscopy
The nuclear magnetic resonance analysis of the two compounds gave characteristic signals
1
that conform to the structures. The HNMR spectrum of 4EMP reveals the presence of
protons of C=NH group as one-proton singlet at 8.43 ppm. The observed singlet at 9.73 ppm
could be due to the proton of the OH group attached to the aromatic molecule (Ar-OH), the
appearance of this peak in the downfield region could be due to the inherent hydrogen

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bonding present in the compound [48]. The resonant peaks due to the aromatic protons
13
appeared as multiplets in the region 6.45 to 7.74 ppm [49, 50]. The C NMR spectrum
showed signals at 15.51 and 28.81ppm corresponding to the sp3 (CH3) and sp2 (CH2) carbons
of the ethyl substituent respectively. The signals due to the aromatic carbons were observed
between 114.15- 163.88 ppm. Among the aromatic carbons, C-1 and C-10 resonated
downfield (159.33 and 163.88 ppm respectively) compared to others due to their proximity to
the electronegative atoms [51]. These two carbons also displayed shorter peaks which is
characteristic of quaternary carbons. The carbon atoms: C2 and C6, C3 and C5, C8 and C12,
C9 and C11 are equivalent carbons and presented the same resonant frequencies, thus same
signals were observed. The resonance signal for the C=N was observed at 172.57 ppm.
The 1H-NMR spectrum of 4NMP showed a singlet at 8.52 ppm which can be ascribed to the
imine proton HC=N. A singlet at 8.25 ppm may be due to the 10-H atom which might have
experienced deshielding effect of the electronegative nitrogen atom located at β-position; and
thereby resonated at higher δ-values than the other aromatic protons. The multiplets of peaks
in the region of 7.54-7.91 ppm could be ascribed to some of the aromatic protons. The signal
at 9.77 ppm could be attributed to the hydrogen atom of the OH group, whose high resonance
frequency value could be due to hydrogen bonding thereby raising the signal to higher values
[45].
13
The C-NMR spectra of the compound exhibited bands almost at the same position (160.89
and 160.37 ppm) attributed to imine (HC=N) carbon and the C-16 attached to the OH. The
band located at 148.93 ppm could be attributed to the carbon which is at α-position to the N-
atom (C-9). The proximity to the imine group subjects it to the deshielding effect, hence
resulting into higher resonance frequency (δ-values) compared to other C-atoms in the
naphthalene ring. This peak appeared short compared to others because the carbon atom is a
quaternary carbon. The signals observed in the up-field region in the range 112.97–133.61
ppm, are due to the aromatic carbons.
3.4 X-ray crystallography
Figure 1a presents the thermal ellipsoid representations of the compound 4EMP, while the
schematic drawing of the crystal packing are presented in Figure 1b. The crystal structure of
the compound is orthorhombic with space group Pca21. The molecule contains the imino-
phenolic ring and the ethylbenzene ring. In the asymmetric unit there are four molecules of

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the compound. The structure is non-planar with the least square plane through the phenol ring
making a dihedral angle of 52.03(10)˚ with the least square plane through the ethylbenzene
ring. In turn the ethyl group makes an angle of 73.8(3)˚ with the ethylbenzene ring. The bond
length of C(1)–N(1) (1.282(3) Å) is shorter than the length of normal C–N (1.47 Å) and very
close to that of C=N (1.273(2) Å) and C=N (1.2437(5) Å) [52].
There is one intermolecular O1—H1A…N1i (i: 1/2+x,1-y,z) interaction of length 1.89 Å
which links adjacent molecules and can be described with a C1 (8) graph-set descriptor on the
1
unary level [53, 54]. There is no short π…π ring interactions, but there are two long C—H…π
ring interactions with hydrogen to centroid length of 2.73 and 2.98 Å with the rings of the
phenol and ethylbenzene rings respectively. The bond lengths and angles are within normal
ranges and they are comparable with those of related structures [55-57].
The ortep view and the crystal packing of 4MNP, drawn for the clearest view, are displayed
in Figures 2a and b respectively. The compound crystallized in monoclinic system with space
groups P21/c. In the asymmetric unit there are also four molecules of the compound. The
structure of 4MNP is similar to another reported structure recorded at room temperature [58].
The structure is non-planar with the least square plane through the phenol ring making a
dihedral angle of 54.96(4)˚ with the least square plane through the naphthalene group. The
bond distance obtained for N1 to C1 (C=N) is (1.2805 Å) which is within the interval
reported for other Schiff bases derived from naphthylamine such as the enol-imino (1.28Å)
and keto-imino tautomer (1.30Å) [59]. Analysis of the torsion angles for the C20–N1–C1–
C11 fragment and C20–C21–C22–C23 show values 178.89 and 179.30 (11) respectively
which were the largest recorded values for the synthesized Schiff base. The slightly higher
value for C20–C21–C22–C23 may be due to the OH group in the para position leading to a
little steric effect. There is one long intramolecular C22—H22…N1 hydrogen interaction of
2.55 Å. There is also an intermolecular O1—H1A…N1i (i: -x,1/2+y,3/2-z) interaction of
length 1.93 Å which links adjacent molecules and can be described with a C1 (8) graph-set
1
descriptor on the unary level [60, 61]. This intermolecular hydrogen bond contributes to the
crystal packing, promotes proton transfer and at the same time, contributes to the rigidity of
the molecule. The shortest π…π ring interaction is between naphthalene groups with an inter-
centroid distance of 3.7755(7) Å and a slippage of 0.789 Å. The phenol group has on each
side a C—H…π ring interaction with hydrogen to centroid distances of 2.70 and 2.77 Å. The
crystal packing is governed by hydrogen bond and C–H.....π interactions.
Details of crystal data and structure refinements for the two compounds are provided in Table

ACCEPTED MANUSCRIPT
1, while selected bond lengths and bond angles are shown in Tables 2 and 3 respectively.
(a)
(b)
Figure 1: (a) The molecular structure, and (b) crystal packing of 4-(((4-
ethylphenyl)imino)methyl)phenol with displacement ellipsoids drawn at 50% probability
level

ACCEPTED MANUSCRIPT
(a)
(b)
Figure 2: (a) The molecular structure, and (b) crystal packing of (E)4-(naphthalene-2-
ylimino)methyl)phenol with displacement ellipsoids drawn at 50% probability level

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Table 1: Crystal structure data of 4EMP and 4MNP
Compound
4EMP
4NMP
CCDC
1491359
C15H15NO
1490099
C17H13NO
Empirical formula
Colour/shape
Formular weight
Temperature (K)
Crystal system
Space group
a/Â
Yellowish-brown/needle-like Brown
225.28 g/mol
200
247.28 g/mol
200
orthorhombic
Pca21
monoclinic
P 21/c
10.9790(6)
11.7326(6)
9.2365(5)
1189.77(11)
4
9.9811(5)
10.4585(6)
11.9435(7)
1245.34(12)
4
b/Â
c/Â
V/Å3
Z
Densitycalc/g/cm3
Absorption coefficient (µ)/mm-1
F(000)
1.258
1.319
0.079
0.082
480
520
Crystal size/mm3
Radiation (Å)
Θmin, θmax (º )
Dataset
0.23 x 0.41 x 0.58
0.71073
0.26 x 0.42 x 0.51
0.71073
3.4, 28.3
3.2, 28.3
-12:14, -15:15, -12:12
24923,2907,0.018
-13:12, -13:13, -15:15
21781,3069,0.016
Total uniq data, R(int)
R1 a (I > 2σ(I))
2747
2675
R, wR2 ,S
0.0367, 0.0986, 1.07
-0.20,0.24
0.0373, 0.1038, 1.06
-0.21,0.25
Min and Max resd.dens.(e/ Å‒3)

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Table 2: Selected bond lengths and angles for 4EMP
Bond lengths Bond angles
O(1)-C(14)
1.344(2)
1.282(3)
1.424(2)
0.8400
C(1)-N(1)-C(21)
118.84(16)
109.00
N(1)-C(1)
N(1)-C(21)
O(1)-H(1A)
C(1)-C(11)
C(2)-C(3)
C(14)-O(1)-H(1A)
N(1)-C(1)-C(11)
C(3)-C(2)-C(24)
C(1)-C(11)-C(12)
C(1)-C(11)-C(16)
N(1)-C(21)-C(22)
123.88(17)
113.8(2)
1.462(3)
1.507(4)
1.519(4)
119.07(17)
122.62(18)
122.44(17)
C(2)-C(24)
Table 3: Selected bond lengths and angles for 4NMP
Bond lengths Bond angles
O(1)-C(14)
1.3506(13)
1.2805(13)
1.4264(13)
0.8400
C(1)-N(1)-C(20)
C(14)-O(1)-H(1A)
N(1)-C(1)-C(11)
O(1)-C(14)-C(15)
C(1)-C(11)-C(12)
C(23)-C(22)-H(22)
C(21)-C(26)-C(25)
117.65(9)
109.00
N(1)-C(1)
N(1)-C(20)
O(1)-H(1A)
C(24)-C(25)
C(12)-H(12)
C(22)-H(23)
125.56(9)
117.67(9)
124.76(9)
120.00
1.3651(17)
0.9500
0.9500
118.45(10)
3.5 Quantum chemical methods
In the light of classical chemical considerations, interactions involving a compound and other
species are either by electrostatic or orbital system. The distribution of electrons and their
movement bring about electrostatic interactions within molecules and are true reflections of
their chemical reactions and physico-chemical properties [62, 63]. Figure 3 shows the
optimized geometry, the HOMO density distribution, and the LUMO density distribution of
4EMP and 4NMP molecules as obtained using DFT at the B3LYP/6-31G+ level of theory.

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The interest in DFT quantum chemical technique has grown due to the understanding of
atoms and molecular dynamics, their structural behaviour, properties and reactivities
including clusters and different agents [64]. Further information on Figures 3 shows an even
distribution of electrons on the HOMO and LUMO part of the molecules, which could be due
to the presence of nitrogen and oxygen atoms and several π-electrons on the entire molecules
creating the platform for adsorption by active centres. It is critical to divulge that the ethyl
alkyl chain of 4EMP is not involved in the LUMO distribution. The molecules present
interesting active sites that have strong ability to form coordinate bond with the empty d-
orbital of the iron. The N and O atoms have lone pairs of electrons which bond with the
unfilled 3d orbitals of the iron metal thereby creating protective molecular surface film. The
back donation from the metal atom to the LUMO of the inhibitor molecules creates an
interfacial barrier that would resist the further attack of the metal surface by corrosion agents
[65].
The obtained quantum chemical parameters (such as EHOMO, ELUMO) and electron density
distribution over the frontier molecular orbitals (FMOs) reflect an effective action of the
synthesized compounds as good inhibitors compared to previous reports [39, 65]. Ju et al.
[65] reported the corrosion inhibition of some oxadiazoles and from their quantum chemical
calculations, the obtained energy gap was higher than the obtained value in this work with
lower dipole moments. Similarly, the reports of Yadav et al. [39] on corrosion inhibition of
benzimidazoles derivatives also showed large energy gaps from their quantum calculations
ranging from 7.947 to 8.059 eV with higher dipole moments (3.20-5.02) and lower fraction of
electrons transferred (0.27-0.29). Our results are comparable to the report of Saha et al. [66],
in which the fraction of electrons transferred using their Schiff base molecules as inhibitors
ranged from 0.7377-0.7529 and the energy gaps recorded were 3.3846-3.9517 eV.
Other parameters calculated in this work as shown in Table 4 such as absolute hardness and
softness are important properties that measure stability and reactivity of the molecules. Hard
molecules are characterised by large energy gap while soft molecule possess small energy
gap [67]. In terms of reactivity, soft molecules are more reactive than hard ones since they
have small energy gap that can enable the easy transfer of electrons. This trend is however
consistent with ones observed in the literature and only provides for a qualitative agreement.
It has been reported that effective corrosion inhibitors are molecules that can both donate
electrons to unoccupied orbital of the metal and equally accept free electrons from the metal

ACCEPTED MANUSCRIPT
[68]. The two molecules could be categorized as good corrosion inhibitors from theoretical
studies perspective.
From the calculated absolute softness (σ), 4NMP can be taken as a soft molecule compared to
4EMP and it has lower energy gap thereby placing it as a more reactive compound than
4EMP. On the contrary, experimental corrosion results from impedance spectra shows that at
100 ppm, the % inhibition efficiency of 4EMP is 93% while that of 4MNP is 55%. This
shows that there are some limitations to theoretical. Further explanation to such contrasting
behaviour could be that, the adsorption on the metal surface based on the quantum
calculations, electrochemical and crystallographic studies occurred in different modes
according to the molecular structures. One of such is chemisorption which involves sharing
of lone pair electrons between electronegative atoms and the mild steel. Other modes are
through intermolecular force reflecting physiosorption, which is relevant to the dipole of the
inhibitor and the plane conjugating system of the aromatic ring, which donate π-electrons to
the metal surface, accept electrons from the metal surface and intercalation of
hyperconjugating system [69].
Fukui function f− was employed to analyse the relative susceptibility of the active atomic
sites of an inhibitor molecule to electrophilic attacks. According to Nwankwo et al. [70], the
favourable site for electrophilic attack is the atom in the molecule where the value of f− is the
highest. From the isosurface density of the Fukui function (f−) shown in Figure 3, the
electron density distributions of the f− revealed that the O-atoms of the OH groups and the
C=C π-electrons centres of the benzene rings in both 4NMP and 4EMP are the most
susceptible sites for electrophilic attacks in the molecules. In addition, the N-atoms and the
ethyl alkyl chain attached to the C=C π-electrons centres are also susceptible to electrophilic
attacks.

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Optimized structures
4NMP
4EMP
HOMO
4NMP
4EMP
LUMO
4NMP
4EMP
Electrophilic Fukui function (f−)
4NMP
4EMP
Figure 3: Optimized molecular structures for 4NMP and 4EMP with atom numbering,
HOMO, LUMO electron density isosurfaces and the electrophilic Fukui function (f−,
isosurface value = 0.15) of compounds obtained at B3LYP/6-31G+(d,p) level of theory.

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Table 4: Calculated quantum chemical parameters
Parameters
4NMP values(eV)
4EMP values(eV)
EHOMO
ELUMO
∆E
-5.630
-1.862
3.767
-5.901
-1.722
4.178
Ionization potential(lp)
Electron affinity(EA)
5.630
1.862
-3.746
3.746
0.531
1.884
0.864
5.900
1.722
-3.811
3.811
0.479
2.089
0.763
Chemical potential (µ)
Absolute electronegativity(ɳ)
Absolute softness(σ)
Absolute hardness (χ)
Fraction of electron transferred(ΔN
3.6
Thermodynamic considerations and adsorption isotherms
Adsorption isotherms provide reliable information about the nature of interactions that exist
at the metal/solution interface where both the water and inhibitor molecules are present [71].
It is a well-known fact that the most probable mechanism by which organic compounds
inhibit metal dissolution is by adsorbing onto the metal surface [21]. In a bid to unravel the
adsorptive characteristics of the studied Schiff bases, the data generated from EIS were tested
using Freundlich and Langmuir isotherms.
The general equations for the adsorption models are:
  KadsCinh
Freundlich isotherm:
Langmuir isotherm:
(13)
(14)
Cinh
1

Cinh

Kads
where, θ is the degree of surface coverage; Kads is the equilibrium constant of the
adsorption/desorption process and Cinh is the concentration of the inhibitor. However, both
isotherms do not conform to the expected conditions where the Freundlich adsorption model
in Equation 13 expects intercept at θ = 0 and the slope of the Langmuir adsorption isotherm
in Equation 14 should be unity. The deviation of the slope of a Langmuir adsorption plot
from unity has been attributed to possible interactions between the adsorbed inhibitor

ACCEPTED MANUSCRIPT
molecules [72]. As a result, a modified form of the Langmuir isotherm which is not possible
with the Freundlich isotherm was employed to calculate Kads and ΔGads values. The plots of
the models are shown in Figures 4a-d for both 4EMP and 4NMP. The modified form of
Langmuir isotherm is given in equation 15:
Cinh/θ = 1/Kads + mCinh
(15)
In this modified model, m is the slope of the plot and can be assumed to be an inherent factor
in the surface coverage values. The intercept of the plots was then equated to (1/m)Kads, from
which Kads was calculated. The Kads and ΔGads values are 1611.43 and 28.72 kJ/mol for
4NMP, and 767.28x106 and 61.7 kJ/mol for 4EMP respectively. The reasonably high value of
Kads obtained for 4EMP in the present study suggests strong adsorption of its molecule on the
metal surface [72].
The change in Gibb’s free energy of adsorption (ΔGads) was calculated by using the equation:
Gads  RT ln

55.5Kads

(16)
where ΔGads is the standard free energy of adsorption; R is the gas constant and T is the
absolute temperature. The value of 55.5 is the concentration of water in solution in molL−1.
The ΔGads values obtained for 4NMP lies between threshold of -20 kJmol-1 (for
physisorption) and -40 kJmol-1 (for chemisorption) which corroborates that the studied Schiff
base molecule adsorb on mild steel surface in 1 M HCl via competitive physical and chemical
adsorption mechanisms [70] while for 4EMP, the value of ΔGᴼads obtained was reasonably
above -40 kJmol-1. This showed electronic transfer from the compound to the mild steel
thereby creating coordinate covalent interaction. Such value of ΔGᴼads for the adsorption of
4EMP ligand on MS surface in 1 M HCl therefore, suggests strong chemisorption mechanism
[21, 73]. In addition, the negative sign alludes to the spontaneity of the adsorption process
[70].

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1200
1000
800
1.0
(a)
(b)
0.9
0.8
0.7
0.6
4NMP

4NMP
R2
= 0.85157
600
0.5
Intercept = 0.29922
R2
= 0.9781
0.4
Slope
= 7.55239E-4
400
Intercept = 177.8306
Slope
0.3
0.2
0.1
= 0.8629
200
0
200
400
600
800
1000
0
200
400
Cinh (ppm)
600
800
1000
Cinh (ppm)
12
10
8
(d)
0.94
0.92
0.90
0.88
0.86
(c)
4EMP
R2
=0.7029
Intercept=0.8955
6

Slope
=5.44x10-5
4EMP
R2
Intercept=0.0277
4
=1.000
2
Slope
=0.0106
0
0
200 400 600 800 1000
Concentration of inhibitor (ppm)
0
200
400
600
800
1000
Concentration of inhibitor (ppm)
Figure 4: Representative adsorption isotherms for MS in 1 M HCl containing various
concentrations of the Schiff base: (a,c) Langmuir (b,d) Freundlich.
3.7.
Electrochemical measurements
3.7.1. Potentiodynamic polarization (PDP) study.
The effect of the inhibitor concentration on the anodic and cathodic polarization behaviour of
mild steel in 1 M HCl solution has been investigated using polarization measurements and the
recorded Tafel plots are shown in Figure 5. The respective kinetic parameters derived from
the above plots are given in Table 5. The data from the Tables shows that both anodic metal
dissolution of iron and cathodic hydrogen evolution reaction were significantly inhibited after
the addition of the inhibitor compounds to 1 M HCl solution, and such behaviour is attributed

ACCEPTED MANUSCRIPT
to an excellent mixed-type inhibitor. Meanwhile, the effects on both anodic and cathodic
reactions were significantly enhanced at elevated concentrations of the two Schiff bases. The
inhibition of these reactions became more pronounced on increasing the concentration
between the thresholds of 40 to 1000 ppm. The values of corrosion current density (icorr
)
recorded for 4EMP show that at a very low concentration (40 ppm), inhibitor recorded a
significant drop, which suggests that the studied inhibitor performed greatly in protecting
mild steel surface in 1 M HCl better than 4NMP which recorded similar corrosion current
density at almost 500 ppm. The insignificant shift in Ecorr values as opposed to icorr indicates
that the investigated compounds prevent the corrosion process by forming a protective film
without necessarily altering the reaction mechanism [74].
0.001
0.001
1E-4
1E-4
(a)
Blank
(b)
40 ppm
60 ppm
100 ppm
500 ppm
1000 ppm
Blank
1E-5
1E-6
1E-7
1E-5
1E-6
1E-7
40 ppm
60 ppm
100 ppm
500 ppm
1000 ppm
-0.7
-0.5
-0.4
-0.3
-0.2
E -(0V.6 vs Ag/AgCl, 3 M KCl)
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
E (V vs Ag/AgCl, 3 M KCl)
Figure 5: Polarization curves for MS in absence and presence of different concentrations of inhibitors:
(a) 4NMP and (b) 4EMP.

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Table 5: Tafel polarization parameters for MS in 1 M HCl solution in absence and presence
of different concentrations of the studied 4NMP and 4EMP
Conc
ba
bc
-Ecorr (mV) icorr (μA cm-2)
η%
θ
(ppm)
(mV/dec)
98.93
(mV/dec)
72.08
Blank
452.72
4NMP
236.22
-
-
40
60
111.47
118.32
85.10
61.93
61.52
58.64
71.11
67.14
461.92
469.96
479.68
486.70
452.94
193.56
182.27
100.82
24.70
18.1
0.18
0.23
0.57
0.90
0.91
22.8
57.3
90
100
500
1000
47.57
127.48
20.20
91
4EMP
40
60
69.48
109.87
68.85
74.47
78.63
65.38
76.28
103.89
62.12
73.99
485.12
473.65
473.65
475.55
451.35
34.59
29.30
25.05
24.75
17.43
85
88
89
90
93
0.85
0.88
0.89
0.90
0.93
100
500
1000
3.7.2. Electrochemical impedance spectroscopic (EIS) studies.
The characteristics and kinetics of electrochemical reactions which takes place at metal-
solution interface can interestingly be verified using the electrochemical impedance
spectroscopy [74]. Nyquist plots for the mild steel immersed in 1 M HCl solution in the
absence and presence of the compounds are shown in Figure 6, while impedance parameters,
such as, solution resistance (Rs), charge transfer resistance (Rct), constant phase element (Y0),
the phase shift (n) and the calculated inhibition efficiency (%) values are listed in Tables 6.
The Table shows that Rct values increased and significantly so at high concentrations of the
inhibitor compounds. In addition, at 1000 ppm inhibitor concentration for 4EMP and 4NMP,
larger diameter semicircles were obtained than the other four lower concentrations (Figure 6).
The impedance spectrum recorded at each condition could be seen to give diminished
capacitive imperfect semicircles in high frequency region, which reflects charge transfer and

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protective film formation on the mild steel [21]. The imperfect semicircles are due to mass
transfer processes, frequency dispersion and contributing effect of roughness of metal surface
[21]. In this report, the impedance characteristics of mild steel corrosion was probed in a
simple electrical equivalent circuit matrix with a resistor of solution resistance (Rs), a resistor
of charge transfer (Rct) and a constant phase element (CPE). CPE has successfully been
employed for the description of depressed semicircles with their origin below the real axis
[75]. The impedance nature of CPE is defined as follows [74]:


1
1
(17)



ZCPE

jn
 

Y0


where, Y is the CPE constant; ω is the angular frequency
at which the imaginary

  2fmax

0
part of the impedance (-Zʺ) is maximal; j is the imaginary number and n is the phase shift
(exponent). The phase shift serves as a determinant of the heterogeneity or roughness of the
mild steel surface [75]. In general, CPE can be expressed by the values of n, for instance, if
resistive (n = 0, Y0 = R), capacitive (n = 1, Y0 = C), inductive (n = -1, Y0 = L) and Warburg
impedance (n = 0.5, Y0 = W) [73]. From this study, n ranges from 0.76 to 0.83 for 4EMP and
0.80-0.84 for 4NMP.
Figure 7 shows the Bode plots for mild steel in 1 M HCl without and with different
concentrations of the inhibitors. A close observation of the Bode plot of 4NMP system
reaffirms poor inhibitory action of the inhibitor at lower concentrations due to diminished
adsorptive strength of the molecule on steel surface. This was evident on the absence of phase
angle maxima observed for the 4NMP system at lower concentrations. The figures however
show that at higher frequency, the values of log |Z| and phase angle approaches zero. Such
phenomenon is a characteristic response of resistive behaviour that can be attributed to
solution resistance enclosed between the reference electrode (Ag/AgCl, 3 M KCl) and
working electrode (mild steel) [76, 77]. At intermediate frequencies for 4EMP, however, a
linear relationship between log |Z| vs. log f with a slope near -1 and the phase angle
approaching -70 can be observed. This could be seen as a shift from the ideal capacitive
behaviour at intermediate frequencies since slope of -1 and a phase angle of -90 defines ideal
capacitive behaviour [74, 75]. For 4NMP, reasonable pseudo-capacitive behavior could only
be achieved at maximum inhibitor concentration as revealed in Table 6. That is, at lower
concentrations, 4NMP recorded slope values lower than the blank system.

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The slopes of the Bode plots at intermediate frequencies (S) and the maximum phase angles
(α) for MS in 1 M HCl solution at minimum and maximum concentrations of 4EMP and
4NMP are listed in Table 6. The deviation from an ideal capacitive behaviour is often
attributed to rough working electrode [74]. In addition, the Bode plots shown in Figure 7 are
characterized by single maximum (one time constant) at intermediate frequencies which is
slightly broadened in the presence of inhibitors, thus suggesting the formation of a protective
film on the mild steel surface [74].
The change observed in the values of Rct and Y0 can be related to the gradual removal of water
molecules by the inhibitor compound on the electrode surface, which consequently resulted
to decrease in the number of active sites available for the corrosion process [75].
Figure 8 presents the equivalent circuit employed for the fitting of impedance data for the
studied corrosion inhibitors.
200
250
(a)
160
120
80
40
0
200
150
100
50
Blank
Blank
(b)
40 ppm
60 ppm
100 ppm
500 ppm
1000 ppm
40 ppm
60 ppm
100 ppm
500 ppm
1000 ppm
0
50
100
150
200
250
0
40
80
120
160
200
Z' ( cm2)
Z' ( cm2)
Figure 6: Nyquist plots (a) 4NMP and (b) 4EMP for MS in 1 M HCl in the absence and presence of
different concentrations of inhibitors.

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Blank
40 ppm
60 ppm
100 ppm
500 ppm
1000 ppm
2.8
2.4
2.0
1.6
1.2
0.8
0.4
60
40
20
0
(a)
0
1
2
3
4
log f (Hz)
3.0
2.5
2.0
1.5
1.0
0.5
70
60
50
40
30
20
10
Blank
40 ppm
60 ppm
100 ppm
500 ppm
1000 ppm
(b)
0
1
2
3
4
log f (Hz)
Figure 7: Bode plots for MS in 1 M HCl in absence and presence of different concentrations
of (a) 4NMP and (b) 4EMP.

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Figure 8: Equivalent circuit used to fit the EIS spectra.
Table 6: Electrochemical impedance parameters obtained for MS in 1 M HCl in absence and
presence of different concentrations of 4NMP and 4EMP.
Inhibitor Conc
(ppm)
Rs
Rct
Y0
n
η %
-S
-α
Blank
2.50
16.6
677
0.84
4NMP
0.84
0.84
0.81
0.83
0.80
4EMP
0.82
0.76
0.83
0.83
0.80
-
0.53
0.29
40.47
42.94
40
60
2.52
1.80
4.07
3.73
3.45
20.3
20.8
37.2
197
250
890
793
457
316
303
18.2
20.2
55.4
92.0
93.0
100
500
1000
0.98
0.64
60.68
54.51
40
60
4.49
4.68
2.33
3.08
2.95
129
149
249
261
293
296
308
231
213
215
87.0
89.0
93.0
94.0
94.0
100
500
1000
0.80
61.91
3.8 Mild steel surface analysis
The surface examination by SEM and related EDX spectra for MS in the absence and
presence of the Schiff bases are shown in Figures 9a-c. Figures 9(a) shows MS surface in 1 M
HCl and Figures 9(b) and (c) show MS surface in 1 M HCl containing 100 ppm of 4EMP and
4NMP inhibitors. The MS immersed in 1 M HCl solution showed a highly damaged, rough
and eroded surface full of corrosion products. The observed bumps like features, pits and
cavities were due to acid attack. However, the specimen immersed in the inhibitor solutions
showed improved surface with passive film as well as conditioning layers and little roughness

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due to physical abrasion during metal preparation and cleaning. The observed passive films
on the mild steel surfaces in the presence of inhibitor were composed of carbonaceous
materials emanating from treatment with inhibitor molecules.
For 4EMP, the EDX data reveals the following elemental percentages: C(10.44%),
O(2.37%), Al(0.15%), S(0.1%),Fe(86.94%) for the inhibited sample; C(6.86%), O(23.99%),
Cl(34.6%), Fe(34.55%) for the uninhibited sample while for 4MNP, the EDX data for the
treated surface is as follows: C(7.4%),O(2.34%),Al(0.08%),S(0.21%), Fe(89.96%). The high
content of Fe in the treated surface as compared to the untreated surface revealed the extent
of inhibition and protection offered by the Schiff bases. However, after immersed in 1.0 M
HCl solution for 3 h, a very high chlorine content of 34.6% was observed due to the
aggressive attack of acid solution but such was not observed with the inhibitor showing no
formation of corrosion product (FeCl2·nH2O) on the MS surface [78, 79]. These values
confirm the formation of protective layer by inhibitors on the MS surface in 1M HCl solution.
Figure 9: SEM images of MS surfaces (a) in I M HCl, (b) in the presence of I M HCl and 100
ppm of the 4EMP while (c) is for 4NMP.