Experimental and theoretical study of [N-substituted] p-aminoazobenzene derivatives as corrosion inhibitors for mild steel in sulfuric acid solution

Article abstract of DOI:10.1016/j.molstruc.2014.08.038

[N-substituted] p-aminoazobenzene derivatives (1), (2), (3), (4) and (5) were prepared and investigated as corrosion inhibitors for mild steel in 1 M H₂SO₄solution by weight loss measurements. It has been observed that the corrosion

Full text of DOI:10.1016/j.molstruc.2014.08.038

Journal of Molecular Structure 1076 (2014) 658–663
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Experimental and theoretical study of [N-substituted]
p-aminoazobenzene derivatives as corrosion inhibitors for mild
steel in sulfuric acid solution
⇑
Mehdi Salih Shihab , Hanan Hussien Al-Doori
Al-Nahrain University, College of Science, Department of Chemistry, Al-Jadrya, Baghdad, Iraq
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
In this work, [N-substituted] p-aminoazobenzene derivatives were prepared and investigated as corro-
sion inhibitor for mild steel in 1 M H2SO4 solution by weight loss measurements. Semiempirical molec-
ular orbital calculations were carried out for prepared compounds as molecular models and gave useful
information to explain the interaction between the surface of metal and the organic molecules as corro-
sion inhibitors.
ꢀ
[N-substituted] p-aminoazobenzene
derivatives were prepared as
corrosion inhibitors.
Corrosion inhibitors were tested for
mild steel in 1 M H2SO4 solution.
Weight loss measurements achieved
in absence and presence of inhibitors.
Theoretical approach was studied
adsorption process of corrosion
inhibitors.
ꢀ
ꢀ
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
[N-substituted] p-aminoazobenzene derivatives (1), (2), (3), (4) and (5) were prepared and investigated
as corrosion inhibitors for mild steel in 1 M H2SO4 solution by weight loss measurements. It has been
observed that the corrosion rate decreases, inhibition efficiencies increase and surface coverage degree
increases with increasing inhibitor concentration. Inhibition efficiencies for prepared compounds were
ordered: (1) > (2) > (5) > (4) > (3) with the highest inhibiting efficiency of 63% for 10ꢁ3 M. The values of
Received 24 February 2014
Received in revised form 7 August 2014
Accepted 7 August 2014
Available online 27 August 2014
D
Goads are showing physisorption effect for all prepared compounds. Semiempirical molecular orbital cal-
Keywords:
culations for (1), (2), (3), (4) and (5) could be used as a useful tool to obtain information for explaining the
nature of interaction between the metal surface and the organic molecule as a corrosion inhibitor.
Ó 2014 Elsevier B.V. All rights reserved.
Corrosion inhibitor
Inhibition efficiencies
Physisorption
Introduction
Corrosion is the destructive attacks of metals from its environ-
ment. Corrosion of metals is a major industrial problem that has
attracted many investigators [1]. Use of corrosion inhibitors is
⇑
Corresponding author.
E-mail address: mehdi_shihab@yahoo.com (M.S. Shihab).
one of the most practical methods for protection against metallic
http://dx.doi.org/10.1016/j.molstruc.2014.08.038
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

M.S. Shihab, H.H. Al-Doori / Journal of Molecular Structure 1076 (2014) 658–663
659
corrosion, especially in acidic media [2,3]. Most of the well-known
acid inhibitors are organic compounds containing nitrogen, sulfur
and oxygen atoms [4–6]. Organic molecules of this type may
adsorb on the metal surface and form coordination between their
First, the preparation of azo aniline was carried out by coupling
reaction between the diazonium salt and aniline [18]. Then, a mix-
ture of p-aminoazobenzene (1 g, 0.01 mol), abs. ethanol (20 ml)
and appropriate aromatic aldehyde or ketone (0.01 mol) with a
few drops of glacial acetic acid was refluxed for 8 h. After cooling
to room temperature, the precipitate was filtered and dried. The
product was crystallized from ethanol [19]. All prepared com-
pounds were identified from melting point, FTIR (KBr disc) and
1H NMR (DMSO-d6) techniques. The molecular formula of sug-
gested inhibitors is shown in Scheme 1.
N-electron pair and/or
p electron cloud and the metal surface,
thereby reducing the corrosion in acidic solutions [7–9]. In recent
years, attention has been focused on the investigation of organic
dyes as potential inhibitors for the metals in corrosive environ-
ment [10–17].
In the present work, some Azo dyes of corrosion inhibitors,
namely [N-substituted] p-aminoazobenzene were prepared. The
aim of this work was to investigate the efficiency of these organic
compounds as a corrosion inhibitor for mild steel in a solution of
1 M sulfuric acid.
Inhibitor concentrations of 1 ꢂ 10ꢁ3 to 5 ꢂ 10ꢁ5 M were pre-
pared in 1 M H2SO4 solution at 30 °C. Solutions of 1 M H2SO4 were
prepared by dilution of analytical grade 98% H2SO4 with distilled
water.
Weight loss method
Experimental testing
Mild steel specimens were initially weighed using an electronic
balance. After that the specimens were suspended and completely
immersed in 250 ml beaker containing 1 M sulfuric acid in the
presence and absence of inhibitors. The specimens were removed
after 8 h exposure period at 30 °C, washed with water to remove
any corrosion products and finally washed with acetone. Then,
they were dried and reweighed. Mass loss measurements were
performed per ASTM standard test method described previously
[20,21]. The tests were performed in duplicate to guarantee the
reliability of the results and the mean value of the weight loss is
reported in 1–2% data errors. Weight loss allowed calculation of
the mean corrosion rate in (mg cmꢁ2 hꢁ1). The corrosion rate of
mild steel was determined using the relationship [22]:
Materials
The sheet of mild steel used had the composition percentages
(0.002% P, 0.288% Mn, 0.03% C, 0.0154% S, 0.0199% Cr, 0.002% Mo,
0.065% Cu, 0.0005% V, and the remainder iron). The mild steel sheet
was mechanically press-cut into disc shape with a diameter
(2.5 cm) and thickness (0.05 cm). These disc shapes were polished
with emery papers ranging from 110 to 410 grades to get a highly
smooth surface. However, surface treatments of the mild steel
involved degreasing in absolute ethanol and drying in acetone.
The treated specimens were then stored in a moisture-free desicca-
tor before their use in corrosion studies.
Azo dyes inhibitors, namely: N, N-dimethyl-4-((E)-(4-((E)-phen-
yldiazenyl) phenylimino) methyl) aniline (1), (E)-N-(4-bromobenzyli-
dene)-4-((E)-phenyldiazenyl)aniline (2), (E)-N-(4-nitrobenzylidene)-
4-((E)-phenyldiazenyl)aniline (3), (E)-N-(furan-2-ylmethylene)-4-
((E)-phenyldiazenyl)aniline (4), and N-((1E,4E)-1,5-diphenylpenta-1,
4-dien-3-ylidene)-4-((E)-phenyldiazenyl)aniline (5) were synthesized
as follows:
D
St
M
W ¼
ð1Þ
where
Dm is the mass loss, S the area and t is the immersion
period.
Table 1
Corrosion rate, inhibition efficiency, surface coverage (h) and standard free energy of
adsorption for mild steel in 1 M H2SO4 by using weight loss measurements.
Inhibitor
concentration
(M)
1 M H2SO4
D
M
Corrosion rate IE (%)
h
D
Goads (kJ/mol)
(g)
(mg cmꢁ2 hꢁ1
2.88
)
Uninhibited
(1)
0.113
–
–
ꢁ36.08
(R2 = 0.997)
0.00005
0.0001
0.0005
0.001
0.0614 1.56
0.0575 1.47
0.0491 1.25
0.0415 1.06
45.8
49.00 0.490
56.6
63.2
0.458
0.566
0.632
(2)
ꢁ34.90
0.00005
0.0001
0.0005
0.001
0.0681 1.74
0.0563 1.43
0.0519 1.32
0.0419 1.07
39.6
50.3
54.2
62.9
0.396 (R2 = 0.995)
0.503
0.542
0.629
(3)
ꢁ35.08
0.299 (R2 = 0.998)
0.461
0.521
0.580
0.00005
0.0001
0.0005
0.001
0.0792 2.02
0.0607 1.55
0.0542 1.38
0.0475 1.21
29.9
46.2
52.1
58.0
(4)
ꢁ35.58
0.403 (R2 = 0.991)
0.465
0.514
0.608
0.00005
0.0001
0.0005
0.001
0.0676 1.72
0.0603 1.54
0.0551 1.40
0.0443 1.13
40.3
46.5
51.4
60.8
(5)
ꢁ38.64
0.500 (R2 = 0.995)
0.562
0.580
0.622
0.00005
0.0001
0.0005
0.001
0.0564 1.44
0.0496 1.26
0.0475 1.21
0.0427 1.09
50.0
56.2
58.0
62.2
Scheme 1. The molecular structure of suggested inhibitors.

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M.S. Shihab, H.H. Al-Doori / Journal of Molecular Structure 1076 (2014) 658–663
Table 2
Calculated quantum chemical parameters of suggested inhibitors by using PM3 method.
Inhibitor
HOMO (eV)
LUMO (eV)
D
E (EHOMO ꢁ ELUMO) (eV)
l
(Debye)
Planarity
(1)
(2)
(3)
(4)
(5)
ꢁ9.0140
ꢁ8.3462
ꢁ9.3099
ꢁ8.8004
ꢁ8.5258
ꢁ1.2485
ꢁ1.0692
ꢁ1.7047
ꢁ1.1838
ꢁ1.3765
ꢁ7.7655
ꢁ7.2769
ꢁ7.6051
ꢁ7.6166
ꢁ7.1493
2.09
1.87
6.76
1.58
1.92
Planar
Planar
Planar
Planar
Semi-planar
Theoretical calculations
The correlation between theoretically calculated properties and
experimentally determined inhibition efficiencies has been studied
successfully for uniform corrosion [24–27]. The purpose of this
work was to provide information about the electronic structure
of several organic inhibitors by quantum chemical calculations
and to investigate the relationship between molecular structure
and inhibition efficiency.
All the calculations were performed using the semi-empirical
calculations with PM3 method [28]. For this purpose the Hyper-
chem Program [29] with complete geometry optimization was
used. This computational method has been proven to yield satis-
factory results [26,27]. The easiest way to compare the inhibition
efficiencies of prepared compounds (1), (2), (3), (4) and (5) was
to analyze the energies of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO).
The optimized molecular structures calculated energies EHOMO
ELUMO, energy gap (DE = ELUMO–EHOMO) and other indices are given
in Table 2.
,
Fig. 1. Effect of inhibitor concentration on the efficiencies of mild steel obtained at
30 °C in 1 M H2SO4 containing different concentrations of suggested inhibitors.
The percentage inhibition efficiency (IE (%)) was calculated
using the relationship [23]:
Results and discussion
The results of corrosion rate and inhibition efficiency that were
obtained from weight loss measurements at different concentra-
tions of suggested inhibitors (1), (2), (3), (4) and (5) after 8 h.
immersion at 30 °C are depicted in Fig. 1 and summarized in
Table 1. These values indicate that the mild steel corrosion was
Wcorr ꢁ W
IEð%Þ ¼
corrðinhÞ ꢂ 100
ð2Þ
Wcorr
where Wcorr and Wcorr (inh) are the corrosion rates of mild steel in the
absence and presence of inhibitor, respectively.
Fig. 2. More energetically stable conformations of suggested inhibitors (1), (2), (3), (4) and (5) with PM3 method.

M.S. Shihab, H.H. Al-Doori / Journal of Molecular Structure 1076 (2014) 658–663
661
Fig. 3. The frontier molecular orbital density distributions (HOMO and LUMO) for suggested inhibitors (1), (2), (3), (4) and (5) by using PM3 method.
reduced by the presence of suggested inhibitors in 1 M H2SO4 at all
concentrations that were used in the current study. However, there
is remarkable decrease in the weight of mild steel specimen after
8 h without using an inhibitor. This could be explained by adsorp-
tion of organic compounds on the mild steel surface which makes
an impediment to the corrosive environment.
The increase in efficiency of inhibition in concentration indi-
cates that more inhibitor molecules are adsorbed on the metal sur-
face at higher concentration, leading to greater surface coverage.
An inspection of the values of IE (%) in Table 1 indicates that
the protection efficiency increases with increasing the concentra-
tion of suggested inhibitors, with higher inhibition efficiencies
achieved at 10ꢁ3 M. Thus, the comparative study revealed
that the order of higher inhibition efficiency was as follows:
(1) > (2) > (5) > (4) > (3). This order could be explained by the effect
of molecular structure of organic inhibitors on inhibition efficiency,
as well as adsorption process.
Basic information can be provided from the adsorption iso-
therms to explain the interaction between the organic compounds
and metal surfaces. Specifically, the degree of surface coverage val-
ues (h) at different inhibitor concentrations in 1 M H2SO4 was
achieved from weight loss measurements (h = IE (%)/100) (see
Table 1) at 30 °C and tested with Langmuir isotherm relationship
[30]:
C=h ¼ 1=Kads þ C
ð3Þ
where Kads is the equilibrium constant of the adsorption process.

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M.S. Shihab, H.H. Al-Doori / Journal of Molecular Structure 1076 (2014) 658–663
According to the Langmuir isotherm, Kads values can be calcu-
lated from the intercepts of the straight line of plotting C/h versus
C. Kads is related to the standard free energy of adsorption,
molecules containing N atom. The presence of more than one func-
tional group has been reported to often lead to changes in the elec-
tron density of a molecule, which could influence its adsorption
behavior [36]. The suggested inhibitor could adsorb to the corrod-
ing steel surface via the compact metal–inhibitor complex of ano-
dic sites and thus reduces the Fe electro-dissolution. To study the
relationship between molecular structure and inhibiting effect of
the suggested inhibitors, molecular orbitals of semi-empirical cal-
culations with PM3 method were used. All the theoretical quantum
calculations were performed for suggested inhibitors (1), (2), (3),
(4) and (5) using more energetically stable conformations in the
gas phase at 25 °C (see Fig. 2).
D ,
Goads
with the following equation:
ꢀ
ꢁ
1
55:5
D
Goads
RT
Kads
¼
exp
ꢁ
ð4Þ
The value 55.5 is the molar concentration of water in the solution in
M unit.
In Table 1, the values of standard free energy of adsorption are
negative to indicate that the processes of adsorption of all sug-
gested inhibitors (1), (2), (3), (4) and (5) were spontaneous pro-
cesses on the mild steel surface after 8 h immersion at 30 °C and
that’s given sense for remarkable interaction between suggested
inhibitors and metal surface. Here, adsorbed molecule moves clo-
ser to the surface of metal making electrons and start to overlap
with that of the surface atoms which causes physisorption for sug-
gested inhibitors [31–34].
It is generally accepted that the adsorption of an organic inhib-
itor on a metal surface in acidic media usually involves the forma-
tion of a metal–inhibitor complex by combining an inhibitor with
freshly generated Fe2+ ions on the steel surface [35]:
The calculated quantum chemical parameters of suggested
inhibitors are reported in Table 2.
The PM3 HOMO and LUMO isosurfaces for suggested inhibitors
(1), (2), (3), (4) and (5) are shown in Fig. 3. Depending on the pres-
ence of N atoms in suggesting inhibitors, the repartition density of
the HOMO and LUMO is preferentially localized on N of (C@N) and
(N@N) groups of all molecules. Table 2 also shows different dipole
moments for suggested inhibitors (1), (2), (3), (4) and (5). The val-
ues of dipole moment can be attributed to the non-uniform distri-
butions of positive and negative charges on the various atoms,
which could be related to improving the dipole–dipole interaction
of organic molecules and mild steel surface. Electrostatic potential
maps for suggested inhibitors (1), (2), (3), (4) and (5) are depicted
in Fig. 4. The figure shows the non uniform distribution of electron
density and concentration of negative charges on N (C@N), (N@N)
for all molecules, which reflect the different values of the
calculated dipole moment (see Table 2). From what was described
earlier, different experimental and theoretical results due to differ-
ent molecular structures were obtained, which influenced the
inhibiting effect of the suggested inhibitors. There are steric and
Fe þ 2Hþ ! Feþ2 þ H2
ð5Þ
ð6Þ
Feþ2 þ InhðadsÞ ! ½Fe ꢁ Inhꢃðþa2dsÞꢃðMetal—inhibitor complexÞ
Therefore, formation of a metal–inhibitor complex could work
as a protective layer for the anodic cell to reduce formation of
Fe+2 sites. As such, it could be suggested that at low concentrations
of suggested inhibitors, the probability of forming a compact
metal–inhibitor complex is low. The adsorption mechanism for
giving inhibitors depends on the adsorption behavior of organic
Fig. 4. Electrostatic potential maps for suggested inhibitors (1), (2), (3), (4) and (5) by using PM3 method.

M.S. Shihab, H.H. Al-Doori / Journal of Molecular Structure 1076 (2014) 658–663
663
electronic effects to understand the role of molecular structure on
Appendix A. Supplementary material
inhibiting effect. Inhibitive effect of the suggested inhibitors
depends on N (C@N), (N@N) for all molecules. Semi-planarity of
(5) (see Table 2) improved the E% (see Table 1) to make N atom clo-
ser to the surface of metal and that in turn improved the physi-
sorption at high concentrations. But the planar molecules are still
preferred for improving the inhibiting effect [37] (1), (2), (3) and
(4) (see Table 2).
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molstruc.2014.08.
038.
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For the orders above, the more interesting suggested inhibitor is
(5), which showed physisorption with
D
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is confirmed from the closed values of IE (%) (ꢄ50–62 for different
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Conclusions
The prepared [N-substituted] p-aminoazobenzene derivatives
(1), (2), (3), (4) and (5) were used successfully as corrosion inhibi-
tors on the mild steel surface in 1 M H2SO4 solution at 30 °C. The
results of inhibitive efficiency showed interesting inhibiting effects
of suggested inhibitors. The values free energy of adsorption
revealed physisorption effect for (1), (2), (3), (4) and (5). Molecular
models for prepared compounds (1), (2), (3), (4) and (5) were
achieved by using semiempirical molecular orbital calculations
and gave useful information to explain the interaction between
the surface of metal and the organic molecules.
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Acknowledgments
The authors would like to thank the Department of Chemistry
at Al-Nahrain University for their help and cooperation throughout
this research.