Experimental and theoretical study for corrosion inhibition of mild steel 1 M HCl solution by some new diaminopropanenitrile compounds

Article abstract of DOI:10.1007/s11164-012-0493-1

The inhibition of the corrosion of mild steel 1 M HCl solution by some diamine compounds has been investigated in relation to the concentration of the inhibitor as well as the temperature using weight loss and electrochemical measurements. The effect of the temperature on the corrosion behavior with the addition of different concentrations of new diamine compounds (3-[2-(2-cyano-ethylamino)- methylamino]-propionitrile (P1); 3-[2-(2-cyano-ethylamino)-ethylamino]-propionitrile (P2), and 3-[6-(2-cyano-ethylamino)-hexylamino]-propionitrile (P3), respectively, was studied in the temperature range 40-80 °C. Polarization curves reveal that (P1, P2, and P3) are mixed type inhibitors. The inhibition efficiency of organic compounds is temperature independent, but increases with the inhibitor concentration. Adsorption of inhibitor on the carbon steel surface is found to obey the Langmuir adsorption isotherm. Some thermodynamic functions of dissolution and adsorption processes were also determined. On the other hand, and in order to determine the relationship between the molecular structure of these compounds and inhibition efficiency, quantum chemical parameters were calculated. The theoretically obtained results were found to be consistent with the experimental data. Springer Science+Business Media B.V. 2012.

Full text of DOI:10.1007/s11164-012-0493-1

Res Chem Intermed (2012) 38:1669–1690
DOI 10.1007/s11164-012-0493-1
Experimental and theoretical study for corrosion
inhibition of mild steel 1 M HCl solution by some new
diaminopropanenitrile compounds
L. Herrag
B. Hammouti
•
M. Bouklah
•
•
N. S. Patel
•
•
B. M. Mistry
•
S. Elkadiri
M. Bouachrine
Received: 23 September 2011 / Accepted: 11 January 2012 / Published online: 4 February 2012
Springer Science+Business Media B.V. 2012
ꢀ
Abstract The inhibition of the corrosion of mild steel 1 M HCl solution by some
diamine compounds has been investigated in relation to the concentration of the
inhibitor as well as the temperature using weight loss and electrochemical mea-
surements. The effect of the temperature on the corrosion behavior with the addition
of different concentrations of new diamine compounds (3-[2-(2-cyano-ethylamino)-
methylamino]-propionitrile (P1); 3-[2-(2-cyano-ethylamino)-ethylamino]-propioni-
trile (P2), and 3-[6-(2-cyano-ethylamino)-hexylamino]-propionitrile (P3), respec-
tively, was studied in the temperature range 40–80 ꢁC. Polarization curves reveal
that (P1, P2, and P3) are mixed type inhibitors. The inhibition efficiency of organic
compounds is temperature independent, but increases with the inhibitor concen-
tration. Adsorption of inhibitor on the carbon steel surface is found to obey the
Langmuir adsorption isotherm. Some thermodynamic functions of dissolution and
adsorption processes were also determined. On the other hand, and in order to
determine the relationship between the molecular structure of these compounds and
inhibition efficiency, quantum chemical parameters were calculated. The theoreti-
cally obtained results were found to be consistent with the experimental data.
Keywords Diamine ꢀ Steel ꢀ Hydrochloric acid ꢀ Corrosion inhibitors
L. Herrag ꢀ M. Bouklah ꢀ B. Hammouti ꢀ S. Elkadiri
LCAE-URAC18, Facult e´ des Sciences, Universit e´ Mohammed Premier, 60000 Oujda, Morocco
N. S. Patel (&) ꢀ B. M. Mistry
Applied Chemistry Department, S V National Institute of Technology, Surat 395007, Gujarat, India
e-mail: niketan.ptl@gmail.com
M. Bouachrine
UMIM, Facult e´ Polydisciplinaire de Taza, Universit e´ Sidi Mohamed Ben Abdellah, Taza, Morocco
123

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L. Herrag et al.
Introduction
Carbon steel is widely used in industry. However, in industrial environments, the
metal is subject to acid corrosion. The study of the corrosion of iron is a matter of
tremendous theoretical and practical concern, and as such has received a
considerable amount of interest [1]. Hydrochloric acid (HCl) solutions are widely
used for pickling, cleaning, descaling, etc. of iron and steel. Corrosion inhibitors
are needed to reduce the corrosion rates of metallic materials in this acid medium.
One solution to this problem is the application of a corrosion inhibitor, compounds
containing nitrogen, oxygen, and sulfur atoms generally give rise to satisfying
inhibitor efficiency in the case of iron corrosion in HCl medium [2–5]. However, a
great number of organic compounds have been used in this regard. The influence
of nitrogenated organic compounds, such as amine and heterocyclic compounds, on
the corrosion of steel has been well documented [6–16].
Diamine compounds have recently emerged as a new and potential class of
corrosion inhibitors. A survey of the literature reveals that, despite the high ability
of diamine compounds to interact strongly with the metal surface, little attention has
been paid on the use of these compounds as corrosion inhibitors. In continuation of
our work on the development of diamines as corrosion inhibitors in acidic media,
we report here the inhibitory behavior of new diamine compounds 3-({2-[(2-
cyanoethyl)amino]ethyl}amino)-propanenitrile (P1), 3-({3-[(2-cyanoethyl)amino]
propyl}amino)-propanenitrile (P2), and 3-({6-[(2-cyanoethyl)amino]hexyl}amino)-
propanenitrile (P3), respectively, on corrosion inhibition of mild steel (MS) in 1 M
HCl solutions.
On the other hand, several quantum chemical methods and quantum-chemistry
calculations have been performed to study the molecular structure and the reaction
mechanisms in order to interpret the experimental results as well as to solve
chemical ambiguities and to correlate the inhibition efficiency to the molecular
properties [17, 18]. The structure and electronic parameters can be obtained by
means of theoretical calculations using the computational methodologies of
quantum chemistry. The geometry of the inhibitor in its ground state, as well as
the nature of their molecular orbitals, highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO), are involved in the properties of
activity of the inhibitors. The relationships between the structural parameters and
corrosion inhibition of those compounds have not yet been studied.
The more relevant molecular properties on its action as inhibition of the corrosion
of these three compounds were calculated. These properties are the HOMO energy
(
EHOMO), the LUMO (ELUMO), energy gap (DE), dipole moment (l), electronega-
tivity (v), electron affinity (A), global hardness (g), softness (r), ionization potential
(
I), the fraction of electrons transferred (DN), and the total energy (TE).
The aim of this work is devoted to study the inhibition characteristics of these
compounds for acid corrosion of MS using weight loss and electrochemical
measurements. Thermodynamic parameters such as adsorption heat, adsorption
entropy, and adsorption-free energy can be obtained from experimental data of the
studies of the inhibition process at different temperatures. The kinetic data, such
as apparent activation energy and pre-exponential factor at different inhibitor
1
23

Experimental and theoretical study for corrosion inhibition
1671
concentrations, are calculated, and the effects of the activation energy and pre-
exponential factor on the corrosion rate of steel discussed.
Experimental details
Synthesis of diaminopropanenitrile compounds
The diamine derivatives, namely (3-[2-(2-cyano-ethylamino)-methylamino]-propio-
nitrile (P1), 3-[2-(2-cyano-ethylamino)-ethylamino]-propionitrile (P2), and 3-[6-(2-
cyano-ethylamino)-hexylamino]-propionitrile (P3), respectively, were synthesized as
follows (Fig. 1).
Synthesis of 3-[2-(2-cyano-ethylamino)-methylamino]-propionitrile (P1)
Diaminomethylene solution (83.3 mmol, 5 g) cold at 0 ꢁC and (166.6 mmol,
8.83 g) of acrylonitrile cold were stirred for 24 h in the dark to obtained the yellow
oil with yield 96%.
NC
NH
NH
CN
3
-({2-[(2-cyanoethyl)amino]ethyl}amino)propanenitrile
(
P1)
NC
CN
NH
NH
3
-({3-[(2-cyanoethyl)amino]propyl}amino)propanenitrile
(
P2)
NC
NH
NH
CN
3-({6-[(2-cyanoethyl)amino]hexyl}amino)propanenitrile
(P3)
Fig. 1 Molecular formulae of the diamine compounds studied
123

1672
L. Herrag et al.
1
H NMR (300 MHz, CDCl ): d 2.83–2.87 (t, J = 7.66 Hz, 4H, CH –CH –CN),
3
2
2
2
–
.67 (s, 4H, CH –NH), 2.43–2.47 (t, J = 7.33, 4H, CH –CH –CN), 1.39 (s, 2H,
2 2 2
1
3
NH). C NMR (75 MHz, CDCl ): d 119.32 (C-9, C-11, –CH –CN), 48.68 (C-2,
3
2
C-7, –CH –NH), 45.277 (C-4, C-5, –NH–CH –CH –NH–), 19.148 (C-1, C-8, –CH –CN),
EI-MS (70 eV, m/z): 91 (0.5), 79 (0.1), 67 (0.5), 54 (59.8), 52 (3.2), 40 (2.7).
2
2
2
2
Synthesis of 3-[2-(2-cyano-ethylamino)-ethylamino]-propionitrile (P2)
Diaminoethylene solution (67.5 mmol, 5 g) cold at 0 ꢁC and (135 mmol, 7.16 g) of
acrylonitrile cold were stirred for 24 h in the dark to obtained the yellow oil with
yield 95%.
1
H NMR (300 MHz, CDCl ): d 2.83–2.87 (t, J = 4 Hz, 4H, CH –CH –CN),
3
2
2
2
CH –CN), 1.65 (m, J = 7.33 Hz, 2H, CH –CH –NH), 1.15 (s, 2H, –NH). C NMR
.65–2.78 (t, J = 7.33 Hz, 4H, CH –CH –NH), 2.44–2.50 (t, J = 4 Hz, 4H, CH –
2 2 2
1
3
2
2
2
(
75 MHz, CDCl ): d 119.351 (C-10, C-12, –CH –CN), 48.167 (C-2, C-8, –CH –NH),
3
2
2
4
5.432 (C-4, C-6, –NH–CH –CH –NH–), 30.043 (C-5, –NH–CH –CH –CH –NH–),
2 2 2 2 2
1
9.018 (C-1, C-9, –CH –CN). EI-MS (70 eV, m/z): 91 (2.3), 52 (3.5), 40 (5.6).
2
Synthesis of 3-[6-(2-cyano-ethylamino)-hexylamino]-propionitrile (P3)
Diaminohexamethylene solution (8.6 mmol, 1 g) cold at 0 ꢁC and
(
obtained the yellow oil with yield 84%.
17.2 mmol, 0.91 g) of acrylonitrile cold were stirred for 24 h in the dark to
1
H NMR (300 MHz, CDCl ): d 2.9–3 (t, J = 7.8 Hz, 4H, CH –CH –CN),
3
2
2
2
.55–2.65 (t, J = 7.7 Hz, 4H, CH –CH –NH), 1.2–1.6 (m, 12H, (CH ) –NH), 1.18
2 2 2 6
1
3
(s, 2H, –NH). C NMR (75 MHz, CDCl ): d 119 (C-13, C-15, –CH –CN), 49 (C-4,
3
C-9, –CH –NH), 45 (C-2, C-11, –NH–CH –CH –NH–), 30 (C-5, C-8, –NH–
2
2
2
2
(
CH ) ), 27 (C-2, C-11, –CH –CH –), 19 (C-1, C-12, –CH –CN).
2 6 2 2 2
Material preparation
Corrosion tests were carried out on electrodes cut from sheets of MS. Steel containing
.09% P, 0.38% Si, 0.01% Al, 0.05% Mn, 0.21% C, 0.05% S, and the remainder iron
0
were used for the measurement of weight loss and electrochemical studies. The surface
preparation of the specimens was carried out using emery paper with different grit sizes
ranging from 260 to 1,200; they were degreased with AR grade ethanol and acetone,
and dried at room temperature before use. The solutions (1 M HCl) were prepared by
dilution of an analytical reagent grade 37% HCl with double-distilled water. The
-3
solubility of the tested diamine compounds are about 10 M in 1 M HCl solution.
Methods
Gravimetric measurements
For weight loss measurements, each run was carried out in a double-walled glass
cell equipped with a thermostat-cooling condenser containing 100 ml test solution.
1
23

Experimental and theoretical study for corrosion inhibition
1673
The steel specimens used had a rectangular form (1.5 9 1.5 9 0.05 cm) and was
completely immersed at inclined position in the vessel. The immersion time for the
weight loss was 6 h at 35 ꢁC. After 6 h of immersion, the electrode was withdrawn,
rinsed with double-distilled water, washed with ethanol, dried, and weighed.
Duplicate experiments were performed in each case and the mean value of the
weight loss was reported. The weight loss was used to calculate the corrosion rate
-
2
-1
h ).
(
W) in milligrams per square centimetre per hour (mg cm
Polarisation measurements
Electrochemical measurements were carried out in conventional three-electrode
cylindrical Pyrex glass cells. The working electrode was a disc cut from MS with
the composition described above. The area exposed to the corrosive solution was
2
1
cm . A platinum electrode and saturated calomel electrode (SCE) were used,
respectively, as auxiliary and reference electrodes. All potentials are given in the
SCE scale. The cell was thermostated at 35 ꢁC.
The polarization curves were recorded with a potentiostat type AMEL 550 using
a linear sweep generator type AMEL 567 at a scan rate of 30 mV/min. Before
recording the cathodic potentiokinetic curves up the corrosion potential, the MS
electrode was polarized at -800 mV/SCE for 10 min.
However, for anodic polarization curves, the potential of the working electrode
was swept from its open circuit potential value after 30 min at rest. Solutions were
de-aerated with nitrogen. The nitrogen bubbling was maintained in the solutions
during the electrode chemical measurements.
Computational details
Density functional theory (DFT) methods were used in this study. These methods
have become very popular in recent years because they can reach exactitude similar
to other methods in less time and are less expensive from the computational point of
view. In agreement with the DFT results, energy of the fundamental state of a
polyelectronic system can be expressed through the total electronic density, and, in
fact, the use of electronic density instead of wave function for calculating the energy
constitutes the fundamental base of DFT [19]. All calculations were done by
GAUSSIAN 03 W software [20], using the B3LYP functional [21] and a 6-31G*
basis set [22]. The B3LYP, a version of the DFT method, uses Becke’s three-
parameter functional (B3) and includes a mixture of HF with DFT exchange terms
associated with the gradient-corrected correlation functional of Lee, Yang, and Parr
(
LYP). The geometry of all species under investigation was determined by
optimizing all geometrical variables without any symmetry constraints. Frontier
molecular orbitals (HOMO and LUMO) may be used to predict the adsorption
centers of the inhibitor molecule. For the simplest transfer of electrons, adsorption
should occur at the part of the molecule where the softness, r, which is a local
property, has the highest value. According to Koopman’s theorem [23], the energies
of the HOMO and the LUMO orbitals of the inhibitor molecule are related to the
ionization potential, I, and the electron affinity, A, respectively, by the following
123

1674
L. Herrag et al.
relationships: I ¼ ꢁEHOMO and A ¼ ꢁELUMO. Absolute electronegativity, v, and
IþA
absolute hardness, g, of the inhibitor molecule are given by [24]. v ¼
and
2
IꢁA
1
g ¼
. The softness is the inverse of the hardness: r ¼ .
2
g
Results and discussion
Weight loss measurements
The corrosion of MS in 1 M HCl medium containing various concentrations of
some diamine compounds was studied by weight loss measurements. In this case,
the inhibition efficiency of inhibitor, E (%), is calculated by applying the following
equation:
Wcorr ꢁ W
corrðinhÞ
E% ¼
ꢂ 100
ð1Þ
Wcorr
where Wcorr and W
are the corrosion rates of MS in the absence and presence
corrðinhÞ
of inhibitor molecule, respectively. Table 1 summarizes the corrosion rates (W) of
MS and the E (%) for some diamine compounds studied at different concentrations.
It is obvious from these data that all of these compounds inhibit the corrosion of MS
in 1 M HCl solution at all concentrations used in this study, and that the corrosion
rate was seen to decrease with increasing additive concentrations at 35 ꢁC. We note
-
6
that at a lower concentration (10 M), P3 exhibits the best inhibitory effect near
-
3
those of P1 and P2, because P3 is the longest molecule. At 10 M, E (%) attains
8% for P2. Thus, we assume that this inhibitor is the better inhibitor for the MS,
9
and E (%) was found to be in the following order: P2 [ P1 = P3. The difference in
their inhibitive action can be explained based on the effect of chain length, as
pointed by Touhami et al. [25] in their study on the effect on nature and length of
carbonic chain joining two pyrazolic cycles. They showed that the inhibition effi-
ciency of bipyrazolic compounds decreases with the carbonic chain length joining
the pyrazolic cycles. As a result of this classification, and in order to better
understand the inhibition mechanism of organic compounds studied, a detailed
study using electrochemical polarization was carried out.
Potentiodynamic polarisation
Figure 2 shows the cathodic and anodic polarisation curves of MS in 1 M HCl blank
-
3
-6
solution and in the presence of different concentrations (10 –10 M) of some
diamine compounds studied. With the increase of organic compounds concentra-
tions, both anodic and cathodic currents were inhibited. This result shows that the
addition of inhibitor reduces anodic dissolution and also retards the hydrogen
evolution reaction.
Table 2 gives the values of kinetic corrosion parameters as the corrosion
potential Ecorr, corrosion current density Icorr, Tafel slope bc, and inhibition
efficiency for the corrosion of MS in 1 M HCl solution with different concentrations
of inhibitor.
1
23

Experimental and theoretical study for corrosion inhibition
1675
Table 1 Corrosion parameters
-2
-1
h
Inhibitor
Concentration (M)
W (mg cm
)
E (%)
obtained from weight loss of MS
in 1 M HCl containing various
concentrations of some diamine
compounds at 35 ꢁC
Blank
1
1.44
0.69
0.29
0.16
0.13
0.12
0.09
0.41
0.19
0.16
0.12
0.09
0.03
0.22
0.2
–
-
6
5
P1
10
52.08
79.86
88.88
90.97
91.66
93.75
71.52
86.80
88.88
91.66
93.75
97.91
84.72
86.11
88.19
88.88
90.97
93.75
-
0
1
5
1
5
1
-
-
5
4
9 10
-4
0
9 10
-3
-6
-5
0
P2
10
1
5
1
5
1
0
-
-
5
4
9 10
-4
0
9 10
-3
-6
-5
0
P3
10
1
5
1
5
1
0
-
-
5
4
9 10
0.17
0.16
0.13
0.09
-4
0
9 10
-3
0
The corrosion current densities were estimated by Tafel extrapolation of the
cathodic curves to the open circuit corrosion potential.
From Table 2, it can be concluded that:
•
•
The Icorr values decrease with increasing inhibitor concentration.
Adsorption of organic inhibitor molecules onto a metal surface retards the metal
dissolution, and as a consequence hydrogen evolution by blocking the active
sites on the MS, or can even screen the covered part of the electrode and
therefore protect it from the action of the corrosion medium. The addition of
some compounds produces slight changes in the values of Ecorr and b . The
c
variation of bc values with alteration in type and concentration of inhibitors
indicated that there was an influence of the added compounds on the kinetics of
hydrogen evolution. This indicates [26] that the adsorbed molecules of the
inhibitor do not affect the mechanism of hydrogen evolution.
•
•
The values of inhibition efficiency (EI%) increase with inhibitor concentration
-
reaching a maximum value (88.2%) at 10 M.
3
The diamine compounds have acted as mixed type inhibitors.
Effect of temperature
Temperature can affect the steel corrosion in the acidic media in the presence
and absence of inhibitors. Generally, the corrosion rate increases with the rise of
123

1676
L. Herrag et al.
P1
1
0
1
-6
1
1
1
0 M
-5
0 M
-4
0
,1
0 M
-
3
1
1
0 M
M HCl
0
,01
-
800
-700
-600
-500
-400
-300
-200
E(mV/SCE)
P2
1
0
1
-
3
1
1
1
1
0 M
-4
0 M
0
,1
-5
0 M
-6
0 M
0
,01
1
M HCl
-800
-700
-600
-500
-400
-300
-200
E (mV/SCE)
P3
1
0
1
-3
0 M
1
-
4
1
1
1
0 M
0
,1
-5
0 M
-6
0 M
1
M HCl
0
,01
-800
-700
-600
-500
-400
-300
-200
E (mV/SCE)
Fig. 2 Polarisation curves for carbon steel in 1 M HCl containing different concentrations of diamine
compounds studied at 35 ꢁC
1
23

Experimental and theoretical study for corrosion inhibition
1677
Table 2 Potentiodynamic polarization parameters for corrosion of carbon steel in 1 M HCl with various
concentrations of inhibitor at 35 ꢁC
2
Inhibitor
C (M)
Ecorr (mV)
bc (mV/dec)
Icorr (lA/cm )
E (%)
P1
Blank
-474
-473
-485
-465
-449
-474
-479
-473
-462
-456
-474
-505
-487
-474
-471
177
176
178
180
172
177
151
183
194
163
177
170
169
169
181
1,239.6
485.3
456.2
416.8
239.9
1,239.6
650
–
-
-
-
-
6
5
4
3
10
10
10
10
60.8
63.2
66.3
80.6
–
P2
P3
Blank
-
-
-
-
6
5
4
3
10
10
10
10
47.5
50.1
58.0
88.2
–
618
520
146.3
1,239.6
890.7
531
Blank
-
-
-
-
6
5
4
3
10
10
10
10
28.1
57.2
71.9
77.0
347
284
temperature. To determine the action energy of the corrosion process, gravimetric
measurements were taken at various temperatures (313–353 K) in the absence (W0)
-
3
and presence of inhibitor P1 (W1), inhibitor P2 (W2), and inhibitor P3 at 10
after 1 h of immersion. The corresponding results are given in Table 3.
M
It is clear that the increase of the corrosion rate is more pronounced with the rise
of temperature for the blank solution. In the presence of the tested molecules, Wcorr
is reduced even at high temperature. E (%) passed from 97.3 to 60.3% when the
-
3
temperature rises from 313 to 353 K at 10 M of P2. We note that the efficiency of
the inhibitors tested depends on the temperature and decreases with the rise of
temperature from 313 to 353 K.
The logarithm of the corrosion rate of steel Wcorr can be represented as a straight-
line function of 1,000/T (Arrhenius equation; Fig. 3), where T is the temperature in
Kelvin.
The activation energy could be determined from Arrhenius plots for the steel
corrosion rate, presented in Fig. 3, by the following relationship:
ꢀ
ꢁ
ꢀ
ꢁ
0
E
a
Ea
0
corr
0
w
¼ K exp ꢁ
and wcorr ¼ K exp ꢁ
ð2Þ
RT
RT
0
a
where Ea and E are the activation energies for the corrosion in the presence and
absence of inhibitors, respectively.
From these results, we note that the addition of inhibitors in acid solution
apparently affects the value of the activation energy of the corrosion reaction. This
phenomenon is often interpreted with chemical adsorption.
123

1678
L. Herrag et al.
Table 3 Effect of temperature on the corrosion rate of steel in 1 M HCl solution (W0) without and with
-
P1, P2, and P3 at 10 M and the corresponding corrosion inhibition efficiency
3
-2
-1
Inhibitor
Temperature (K)
313
Concentration (M)
W (mg cm
h
)
E (%)
h
P1
Blank
2.6
0.3
–
–
-
5
4
5
1
5
1
9 10
88.46
88.84
90.76
92.30
–
0.884
0.888
0.907
0.923
–
-4
0
0.29
0.24
0.2
-
9 10
-3
0
3
3
3
3
23
33
43
53
Blank
4.8
-
5
4
5
1
5
1
9 10
0.76
0.7
84.16
85.41
87.50
91.66
–
0.841
0.854
0.875
0.916
–
-4
0
-
9 10
0.6
-3
0
0.4
Blank
8.8
-
5
4
5
1
5
1
9 10
2.2
75.00
80.22
82.95
87.84
–
0.750
0.802
0.829
0.878
–
-4
0
1.74
1.5
-
9 10
-3
0
1.07
13.2
4.2
Blank
-
5
4
5
1
5
1
9 10
68.18
72.65
81.66
87.12
–
0.681
0.726
0.816
0.871
–
-4
0
3.61
2.42
1.7
-
9 10
-3
0
Blank
26.5
9.8
-
5
4
5
1
5
1
9 10
63.01
64.52
76.18
77.73
–
0.630
0.645
0.761
0.777
–
-4
0
9.4
-
9 10
6.31
5.9
-3
0
P2
313
Blank
2.6
-
5
4
5
1
5
1
9 10
0.36
0.32
0.17
0.07
4.8
86.15
87.69
93.46
97.30
–
0.861
0.876
0.934
0.973
–
-4
0
-
9 10
-3
0
3
3
23
33
Blank
-
5
4
5
1
5
1
9 10
0.7
85.41
86.87
92.33
94.58
–
0.854
0.868
0.923
0.945
–
-4
0
0.63
0.32
0.26
8.8
-
9 10
-3
0
Blank
-
5
4
5
1
5
1
9 10
2.3
73.86
82.72
90.34
91.59
0.738
0.827
0.903
0.915
-4
0
1.52
0.85
0.74
-
9 10
-3
0
1
23

Experimental and theoretical study for corrosion inhibition
1679
Table 3 continued
-2
-1
Inhibitor
Temperature (K)
Concentration (M)
Blank
W (mg cm
h
)
E (%)
h
3
43
53
13.2
4.7
–
–
-
5
5
1
5
1
9 10
64.39
76.13
83.63
85.15
–
0.643
0.761
0.836
0.851
–
-4
0
3.15
2.16
1.96
26.5
10.5
8.24
6.4
-4
9 10
-3
0
3
Blank
-
5
4
5
1
5
1
9 10
60.37
68.90
75.84
79.24
–
0.603
0.689
0.758
0.792
–
-4
0
-
9 10
-3
0
5.5
P3
313
Blank
2.6
-
4
1
5
1
0
0.3
88.46
90.76
92.69
–
0.884
0.907
0.926
–
-4
-4
-4
-4
-4
9 10
0.24
0.19
4.8
-3
0
3
3
3
3
23
33
43
53
Blank
-4
1
5
1
0
0.56
0.5
88.33
89.58
92.50
–
0.883
0.895
0.925
–
9 10
-3
0
0.36
8.8
Blank
-4
1
5
1
0
1.3
85.22
89.09
90.9
–
0.852
0.890
0.909
–
9 10
0.96
0.8
-3
0
Blank
13.2
3.1
-4
1
5
1
0
76.51
82.42
88.63
–
0.765
0.824
0.886
–
9 10
2.32
1.5
-3
0
Blank
26.5
7.9
-4
1
5
1
0
70.18
77.35
87.92
0.701
0.773
0.879
9 10
6
-3
0
3.2
Adsorption
Basic information on the interaction between the inhibitor and the carbon steel can
be provided by the adsorption isotherm. Two main types of interaction can describe
the adsorption of the organic compounds: physical adsorption and chemisorption.
These are influenced by the chemical structure of the inhibitor, the type of the
electrolyte and the charge and nature of the metal. The surface coverage h of the
metal surface by the adsorbed inhibitor was calculated [27] assuming no change in
the mechanism of the cathodic reaction using the equation:
123

1680
L. Herrag et al.
4
2
0
2
P1
-
3
1
0 M
-
4
5
.10
M
-
4
1
0 M
-
5
5
.10
HCl(1M)
M
-
2
,8
2,9
3,0
3,1
3,2
3,3
-1
1000/T(K )
4
P2
2
0
2
4
1
M HCl
-5
5
.10
-4
M
-
-
10 M
-4
5
.10
-3
M
10 M
2
,8
2,9
3,0
3,1
3,2
3,3
-1
1
000/T(K )
P3
2
0
2
-
3
1
0
M
-
4
5
.10
M
-4
1
0
M
-
1
M HCl
2,9
2
,8
3,0
3,1
3,2
3,3
-
1
1
000/T(K )
Fig. 3 Arrhenius plots for steel in 1 M HCl in the absence and presence of P1, P2, and P3
W0 ꢁ W
h ¼
W0
ð3Þ
where W and W are the corrosion rates in the absence and presence of the inhib-
0
itors, respectively.
1
23

Experimental and theoretical study for corrosion inhibition
1681
The h values for different inhibitor concentrations at 35–80 ꢁC were tested by
fitting to various isotherms. By far the best fit was obtained with the Langmuir
adsorption isotherm. According to this isotherm, h is related to concentration
inhibitor C via;
C
1
¼
þ C
ð4Þ
ð5Þ
h
K
ꢀ
ꢁ
0
1
DG
ads
K ¼ 5
exp ꢁ
5:5
RT
0
ads
where K is the adsorptive equilibrium constant and DG is the free energy of
adsorption.
It was found that Fig. 4 (plot of h/C vs. C) gives straight lines with slope equal or
nearly equal to 1 for 40, 50, 60, 70, and 80 ꢁC. This result indicates that the
adsorption of compound under consideration on the steel/acidic solution interface at
all temperatures follows the Langmuir adsorption isotherm.
Thermodynamic parameters are important to study the inhibitive mechanism.
0
ads
The values of DG at different temperatures were estimated from the values of K
0
ads
and Eq. 5. The negative values of DG show that the adsorption of the organic
compounds studied is a spontaneous process [28] under the experimental conditions
-1
0
ads
described. It is well known that values of DG of the order of 20 kJ mol or lower
-
1
indicate a physisorption, while those of the order of 40 kJ mol or higher involve
charge sharing or a transfer from the inhibitor molecules to the metal surface to
form a co-ordinate type of bond [29, 30]. On the other hand, Metiko sˇ -Hukovi c´ et al.
0
-1
[31] describe the interaction between thiourea and iron (DGads = -39 kJ mol ) as
chemisorption. The same conclusion was given by Wang et al. [32] concerning the
0
ads
-1
interaction between mercapto-triazole and MS (DG = -32 kJ mol ). Moreover,
Bayoumi and Ghanem [33] consider that the adsorption of naphthalene disulphonic
-1
0
ads
acid on the MS was principally by chemisorption (DG = -28.47 kJ mol ). Thus,
0
ads
the DG
value obtained here shows that, in the presence of 1 M HCl,
chemisorption of inhibitors on the MS may occur.
Considering the values of enthalpy and entropy of the inhibition process have no
distinct changes in the temperature range studied, the thermodynamic parameters
0
ads
0
ads
DH and DS for the adsorption of P1, P2, and P3 on carbon steel can be
calculated from the following equation:
0
ads
0
ads
0
ads
DG ¼ DH ꢁ TDS
ð6Þ
0
ads
0
ads
where DH and DS are the variation of enthalpy and entropy of the adsorption
0
process, respectively. The values of DGads were plotted against T. This plot is shown
0
ads
in Fig. 5. It should be noted that the entropy change of adsorption ꢁDS is the
0
ads
slope of the straight line DG ꢁ T according to Eq. 6. The intercept of the straight
0
ads
line is used to calculate the heat of adsorption DH . The calculated values are
0
ads
0
ads
depicted in Table 4. The negative sign of DH indicates that the adsorption of
inhibitor molecules is an exothermic process. DS in the presence of inhibitor is
123

1682
L. Herrag et al.
P1
-3
-4
-4
-4
3
3
3
08K
13K
23K
1,2x10
9,0x10
6,0x10
3,0x10
333K
43K
353K
3
0,0
-
4
-4
-4
0,0
3,0x10
6,0x10
9,0x10
C(M)
P2
3
3
3
53K
43K
33K
-
-
-
-
3
4
4
4
1,2x10
9,0x10
6,0x10
3,0x10
323K
3
3
13K
08K
0
,0
-
4
-4
-4
0
,0
3,0x10
6,0x10
C(M)
9,0x10
-
-
-
-
-
-
3
3
4
4
4
4
1,2x10
1,0x10
8,0x10
6,0x10
4,0x10
2,0x10
3
3
3
08K
13K
23K
P3
333K
43K
3
353K
0
,0
0
-
4
-4
-4
-4
-3
,0
2
,0x10
4,0x10
6,0x10
8,0x10
1,0x10
C(M)
Fig. 4 Langmuir adsorption plots for carbon steel in 1 M HCl containing different concentrations of
inhibitor at different temperatures
1
23

Experimental and theoretical study for corrosion inhibition
1683
-
-
-
-
0,120
0,125
0,130
0,135
P1
-
3
-3
-3
2,8x10
3,0x10
3,2x10
-
1
1
/T(K )
-
-
-
-
-
-
-
-
-
-
0.121
0.122
0.123
0.124
0.125
0.126
0.127
0.128
0.129
0.130
P2
0
.0028 0.0029 0.0030 0.0031 0.0032 0.0033
-1
1
/T(K )
-
-
-
-
115
120
125
130
P3
-
3
-3
-3
2,8x10
3,0x10
3,2x10
-
1
1
/T(K )
0
Fig. 5 The relationship between DGads and T
123

1684
L. Herrag et al.
Table 4 The thermodynamic parameters for some diamine compounds studied at the carbon steel sur-
face in 1 M HCl solution
a
b
B
0
ads
0
ads
0
Inhibitor
Temperature
K)
K
DG
DH
DS
ads
-1
)
-1
-1 -1
K )
(
(kJ mol
(kJ mol
)
(J mol
P1
308
215,616.68
232,885.80
102,139.20
61,616.95
38,578.46
44,736.32
88,958.47
76,091.34
103,711.63
73,297.66
55,388.46
41,141.42
107,220.43
105,260.83
93,820.95
90,883.47
32,119.94
18,974.64
1.07
1.08
1.09
1.13
1.13
1.26
1.01
1.02
1.05
1.08
1.16
1.24
1.06
1.07
1.07
1.09
1.1
-41.76
-42.64
-41.78
-41.68
-41.59
-43.24
-39.49
-39.72
-41.83
-42.16
-42.63
-43.00
-39.97
-40.57
-41.56
-42.76
-41.07
-40.72
-38.56
-38.56
-38.56
-38.56
-38.56
-38.56
-14.29
-14.29
-14.29
-14.29
-14.29
-14.29
-34.19
-34.19
-34.19
-34.19
-34.19
-34.19
10.4
3
3
3
3
3
13
23
33
43
53
13
9.9
9.3
8.8
13.2
-
2
P2
P3
a
308
8.2 9 10
8.12 9 10
8.52 9 10
8.37 9 10
8.26 9 10
8.13 9 10
18.77
-
-
-
-
-
2
2
2
2
2
3
3
3
3
3
13
23
33
43
53
308
3
3
3
3
3
13
23
33
43
53
20.72
23.54
26.53
20.07
1.1
18.51
Equilibrium constant of adsorption
b
Slope value of Langmuir adsorption isotherm
large and positive, meaning that an increase in disordering takes place in going from
reactants to the metal adsorbed species reaction complex [34].
Moreover, since adsorption enthalpy is small and negative and adsorption
entropy is large and positive, it can be assumed that the driving force for the
adsorption of adsorbate is the increase in entropy during the process of adsorption
rather than the decrease in enthalpy.
0
ads
On the other hand, DH can also be calculated according to the Van’t Hoff
equation:
0
ads
ꢁ
DH
RT
ln K ¼
þ constant
ð7Þ
Figure 6 shows the plot of ln K versus 1/T which gives straight lines with slopes
0
ads
0
ads
0
ads
of DH =R and intercept of DH . The calculated DH using the Van’t Hoff
equation confirms the exothermic behavior of the adsorption of some diamine
0
ads
compounds on the steel surface. Values of DH obtained by both methods are in
0
ads
good agreement. Moreover, the assumed DS value is very close to that given in
Table 4.
In the present study, chemisorption is evident from the apparent activation energy
of the corrosion, that is lower in the presence of some diamine compounds than in
1
23

Experimental and theoretical study for corrosion inhibition
1685
1
1
1
1
1
2,5
2,0
1,5
1,0
0,5
P1
-
3
-3
-3
2,8x10
3,0x10
3,2x10
-
1
1
/T(K )
1
1
1
1
1
1
1.6
1.4
1.2
1.0
0.8
0.6
P2
0.0028
0.0029
0.0030
0.0031
0.0032
0.0033
-
1
1
/T(K )
P3
1
1
1
1
1,5
1,0
0,5
0,0
-
3
-3
-3
2
,8x10
3,0x10
3,2x10
-
1
1
/T(K )
Fig. 6 The relationship between ln (K) and 1/T
123

1686
L. Herrag et al.
Fig. 7 Calculated HOMO and LUMO molecular orbitals of the studied molecules P1, P2, and P3 in their
neutral form
Table 5 The calculated quantum chemical parameters of the studied molecules
P1
P2
P3
-5.8476
E
HOMO (eV)
LUMO (eV)
-6.1703
-6.4333
E
0.7343
6.9046
6.1967
0.8038
7.2371
4.4668
-0.7401
5.1075
6.0001
93.1
DE (eV)
l (debye)
EW (%)
93.3
98
I ¼ ꢁEHOMO (eV)
6.1703
-0.7343
2.718
6.4333
-0.8038
2.814
5.8476
0.7401
3.2938
A ¼ ꢁELUMO (eV)
IþA
v ¼
g ¼
r ¼
(eV)
2
IꢁA
(eV)
3.452
0.289
3.618
0.276
2.553
0.391
2
1
g
TE (u.a.)
-532.22998
-571.54711
-685.67387
their absence, the inhibition efficiency, which is temperature independent, and the
0
large negative values of DGads. Therefore, organic compounds may adsorb on a
metal surface in the form of a neutral molecule via the chemisorption mechanism
[
35] involving the sharing of electrons between the nitrogen atoms and iron.
1
23

Experimental and theoretical study for corrosion inhibition
1687
Table 6 The calculated quantum chemical parameters of the studied molecules in their protonated form
P1 P2 P3
-10.283 -9.707 -8.386
E
HOMO (eV)
LUMO (eV)
E
-4.504
5.779
8.8937
93.3
-4.396
5.311
8.7181
98
-4.317
4.069
20.7674
93.1
DE (eV)
l (debye)
EW ð%Þ
I ¼ ꢁEHOMO (eV)
10.283
4.504
7.166
9.707
4.396
7.0515
8.386
4.317
6.351
A ¼ ꢁELUMO (eV)
IþA
v ¼
g ¼
r ¼
(eV)
2
IꢁA
(eV)
2.889
0.346
2.655
0.376
2.034
0.491
2
1
g
TE (u.a.)
-532.58827
-571.9066
-689.8502
The covalent bond with the metal is most probably formed between the unpaired
electrons of the N-atom.
Theoretical results
Quantum chemical calculations were performed to investigate the effect of
structural electronic parameters on the inhibition efficiency of inhibitors. Previous
studies confirmed the fact that, in aqueous acidic solution, these types of compounds
get protonated and exist either as neutral molecules or in the form of cations.
The studied compounds may absorb on the metal surface in the form of neutral
molecules or in the form of protonated molecules. Geometric and electronic
structures of the inhibitors were calculated by the complete geometrical optimiza-
tion in their neutral and protonated form. The obtained molecular structures and
HOMO and LUMO orbitals of the neutral inhibitor molecules by DFT/B3LYP/6-
3
1G*(d) obtained from the DFT calculations are given in Fig. 7.
Quantum chemical parameters are obtained from the calculations which are
responsible for the inhibition efficiency of inhibitors such as the energies of HOMO
EHOMO), energy of LUMO (ELUMO), energy gap (DE), dipole moment (l),
(
electronegativity (v), electron affinity (A), global hardness (g), softness (r),
ionization potential (I), and the TE are shown in Table 5.
The energy of HOMO is often associated with the electron-donating ability of a
molecule. Therefore, the energy of LUMO indicates the ability of the molecule to
accept electrons [36, 37]. So, when we compared the studied compounds P1, P2,
and P3, the calculations show that the compound P2, which has the highest
inhibition efficiency, has the highest LUMO level and lowest HOMO. On the other
hand, when we examine the obtained values of gap energies, the results obtained
show that the compound P2 has a high value of DE gap. This parameter provides a
measure for the stability of the formed complex on the metal surface. Even in the
literature the correlation between DE gap and inhibition efficiency is often cited.
123

1688
L. Herrag et al.
Our theoretical results indicate that no significant relationship was detected between
DE gap and inhibition efficiency. The most widely used quantity to describe the
polarity is the dipole moment of the molecule [38]. Dipole moment is the measure
of polarity of a polar covalent bond. It is defined as the product of the charge on the
atoms and the distance between the two bonded atoms. The total dipole moment,
however, reflects only the global polarity of a molecule. For a complete molecule,
the total molecular dipole moment may be approximated as the vector sum of
individual bond dipole moments. The calculations show that the compound P2 has
the lowest value of dipole moment. Indeed, the inhibition efficiency decreases with
increasing dipole moment. The theoretical study has shown that quantum chemical
parameters such as hardness (g) and softness (r) were well correlated with the
inhibition efficiency when we compared the studied compounds, the calculations
showing that the compound P2 has the highest value of hardness (g) and a low value
of softness (r). Indeed, the inhibition efficiency increases with increasing hardness.
Other quantum chemical parameters such as electronegativity (v), electron affinity
(
A), ionization potential (I), and the TE were also calculated. Our theoretical results
indicate that no significant relationship was detected between these parameters and
inhibition efficiency.
On the other hand, in an acidic medium, these molecules could be easily
protonated; the calculated quantum chemical parameters of the protonated
compounds are shown in Table 6. The results show that protonation produces a
remarkable change of all the calculated quantum chemical parameters. So, there is a
considerable effect on the reactivity which should be considered. Comparing
quantum chemical results for neutral and protonated inhibitors, EHOMO as well as
ELUMO values decreased in the case of the protonated inhibitors which reflects the
decreasing of reactivity due to the protonation, but there is no correlation with the
experimental result. Also, it could be noticed that the protonated molecules exhibit
high dipole moment in the following order P3 [ P1 [ P2. The protonated
compound P2 has the lowest value of dipole moment. Indeed, the inhibition
efficiency decreases with increasing dipole moment. So, we expect that the dipole
moment will play an important role in the adsorption of protonated inhibitors on the
metal surface.
Conclusion
The following conclusions are drawn:
•
The diamine derivatives inhibit the corrosion of steel in 1 M HCl solution. The
inhibition efficiency increases with the inhibitor concentration.
Inhibitors act essentially as mixed type inhibitors.
•
•
The adsorption of inhibitors on the steel surface from 1 M HCl solution follows
the Langmuir adsorption isotherm. The adsorption process is a spontaneous and
exothermic process.
•
The inhibition efficiency of organic compounds decreases in the temperature
range 313–353 K.
1
23

Experimental and theoretical study for corrosion inhibition
1689
•
•
The results obtained from weight loss and potentiodynamic polarisation are all
in good agreement.
From the obtained results and by using the DFT calculations, the inhibition
efficiency of the studied compounds P1, P2, and P3 was investigated leading to
the following conclusions:
–
–
Using the B3LYP/6-31 G(d) method, the inhibition efficiency of the studied
compound may be correlated to their molecular structure.
The calculated dipole moments, the softness, and hardness show reasonably
good correlation with the efficiency of corrosion inhibition and no significant
relationship was found with the other parameters.
–
–
All studied inhibitors have a considerable tendency for protonation and this
process produces a remarkable change of all the calculated quantum
chemical parameters. Also, it could be concluded from the calculations that
the protonated molecules exhibit high dipole moment in the following order
P3 [ P1 [ P2.
Comparing quantum chemical results for neutral and protonated inhibitors,
EHOMO as well as ELUMO values decreased in the case of the protonated
inhibitors which reflects the decreasing of reactivity due to the protonation,
but there is no correlation with the experimental result.
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