Synergistic effect of 1-(2,5-dioxoimidazolidin-4-yl)urea and Tween-80 towards the corrosion mitigation of mild steel in HCl

Article abstract of DOI:10.1039/c9nj03336j

Herein, the synergistic effect of 1-(2,5-dioxoimidazolidin-4-yl)urea (DMU) and Tween-80 on the corrosion of mild steel in a 1 mol L^-1 HCl solution was investigated by weight-loss tests, electrochemical methods and surface analysis (SEM/SECM). The experiments revealed that the combination of DMU and Tween-80 strongly inhibited the corrosion of mild steel as compared to individual inhibitors, and the adsorption mode belonged to the Langmuir isothermal type. Based on the electrochemical results, DMU and DMU-Tween-80 behaved as mixed-type inhibitors, and their best inhibition efficiencies were 77.31% and 96.35%, respectively. The calculated synergistic parameter value was larger than unity, indicating that the enhancement of inhibition efficiency was due to the synergistic effect of DMU and Tween-80. Surface analysis techniques confirmed the presence of inhibitors on the mild steel surface. Quantum chemical parameters based on density functional theory provided a further insight into the mechanism of inhibition. Moreover, molecular dynamics simulations were carried out to explore the configurational adsorption behaviour of DMU on an Fe(110) surface.

Full text of DOI:10.1039/c9nj03336j

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Synergistic effect of 1-(2,5-dioxoimidazolidin-4-yl)urea and
Tween-80 towards corrosion mitigation of mild steel in HCl
Received 00th January 20xx,
Accepted 00th January 20xx
Weiwei Zhang,a Hui-Jing Li,*a,c Chen Wang,a Li-Juan Wang,*b Gen Li,a Hengyu Ma,a Qianwen Pan,a
and Yan-Chao Wu*a,c
DOI: 10.1039/x0xx00000x
Abstract: The synergistic effect of 1-(2,5-dioxoimidazolidin-4-yl)urea (DMU) and Tween-80 on mild steel corrosion in 1 mol
L-1 HCl solution was investigated by weight-loss tests, electrochemical methods and surface analysis (SEM/SECM). The
experiments revealed that the combination of DMU and Tween-80 strongly inhibit the corrosion of mild steel compared to
individual inhibitors, and the adsorption mode belongs to the Langmuir isothermal type. Based on electrochemical results,
DMU and DMU–Tween-80 behaved as mixed-type inhibitors, and their best inhibition efficiencies were 77.31% and
96.35%, respectively. The calculated synergistic parameter value was larger than unity, indicating the enhancement of
inhibition efficiency is due to the synergistic effect of DMU and Tween-80. Surface analysis techniques confirmed the
presence of inhibitors on the mild steel surface. Quantum chemical parameters based on density functional theory provided
further insight into the mechanism of inhibition. Molecular dynamics simulations were carried out to explore the
configurationally adsorption behaviour of DMU on Fe (110) surface.
www.rsc.org/
in a hydrochloric acid solution.13-15 Solomon et al. and Oukhrib
et al. have also reported the effect of chitosan on St37 steel
and copper in a 15% H2SO4 environment and 3.5% NaCl
Introduction
Mild steel is the earliest and most used basic metal material in
modern industry. Due to its good mechanical properties and
low cost, mild steel is widely used in machinery, chemical,
metallurgy, electric power, and other industries. However,
mild steel is impressionable to corrode during acid cleaning,
and corrosion not only causes great economic damage but also
media.16,17 Another option is the use of 1-(2,5-
dioxoimidazolidin-4-yl)urea (DMU) as corrosion inhibitor,
which is a rich natural resource and widely used in the fields of
cosmetics, medicine, agriculture and bioengineering, so it has
the potential to become an ecofriendly corrosion inhibitor.18-23
DMU contains a plurality of nitrogen heteroatoms and amino
acids, and thus might be easily dissolved in the hydrochloric
acid and adsorbed on the steel surface. The corrosion
inhibition efficiency could be increased by the additional use of
a surfactant. Yousefi et al.24 studied the synergistic effect of
imidazolium-based ionic liquids with sodium dodecyl sulfate on
mild steel in HCl solution. The results showed that the
mixtures had good inhibition effect compared to the individual
surfactants and inhibitors. Mobin et al.25 also reported the
synergistic effect between xanthan gum and surfactant
additives for mild steel corrosion in 1 mol L-1 HCl. They found
that the inhibition efficiency of xanthan gum was enhanced
from 75.03% to 90.79% by the addition of surfactant additives.
makes
a threat to human’s safety issue. The organic
heterocyclic compounds have good adsorption performance,
which can effectively suppress acid corrosion of metals.1-11
Among them, N-heterocyclic compounds with different
substituent groups, such as hydroxy, amino, phenyl and
sulfhydryl, have been studied intensively due to their corrosion
inhibition ability, low toxicity and easy synthesis. Although
progress has been made, these synthetic organic compounds
require toxic organic solvents, expensive catalysts, ligands, and
a long reaction time, which creates potential safety hazards for
industrial production and does not comply with the principles
of green and sustainable chemistry. Recent researches into
corrosion inhibition have turned to use more environmentally
friendly inhibitors, especially those from natural resources. For
instance, the corrosion inhibiting effect of gum arabic has been
reported for mild steel in an acid medium.12 Pectin, guar gum,
and argan oil were considered as corrosion inhibitors for steel
In this context, 1-(2,5-dioxoimidazolidin-4-yl)urea,
a
potential green corrosion inhibitor for mild steel, combined
with Tween-80 was designed here as a probe to evaluate the
possible synergistic inhibition effect. Its corrosion inhibition
properties in hydrochloric acid medium were investigated by
weight-loss methods, electrochemical measurements and
surface characterization methods, from which an obviously
synergistic inhibition effect was observed. Quantum chemical
calculations were employed to correlate the inhibition ability
of DMU with its electronic structural parameters. Moreover,
aSchool of Marine Science and Technology, Harbin Institute of Technology, Weihai
264209, P. R. China. E-mail: lihuijing@iccas.ac.cn, ycwu@iccas.ac.cn
bSchool of Materials Science and Engineering, Harbin Institute of Technology,
Weihai 264209, P. R. China. E-mail: lijuanwanghit@163.com
cWeihai Institute of Marine Biomedical Industrial Technology, Wendeng District,
Weihai 264400, P. R. China
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molecular dynamics simulations were employed to ascertain
the adsorption behaviour of inhibitors on Fe (110) surface.
Tween-80 was obtained from Sigma–Aldrich Ltd., Hungary.
View Article Online
It is a hydrophilic nonionic surfactant wiDthOIg: o10o.1d03s9o/lCu9bNilJi0za33ti3o6nJ
and thermal stability. The main functional group is hydroxyl,
containing a number of units of CH2CH2O.
Experimental
Inhibitors
Materials and electrolytes
The chemical composition of the mild steel used in the
experiment is as follows (wt. %): 0.16 C, 0.35 Mn, 0.20 Si, 0.03
S, 0.01 P, and balance Fe. The samples for electrochemical and
surface analysis experiments were processed to the following
specifications: 1.0 cm × 1.0 cm × 0.05 cm and 3.0 cm × 0.6 cm ×
0.6 cm. The working electrode was embedded in the epoxy
resin in the electrochemical experiment. Epoxy resin has good
bonding strength and chemical resistance, the working
electrode embedded in it is to ensure the exposure area, so
that the current density can be calculated, and most
importantly, to prevent the wire and the working electrode
contact exposure to the solution. The exposed area was 0.36
9
DMU was synthesized in the lab as shown in Figure 1. A
mixture of 50% glyoxylic acid (3 mmol) and urea (9 mmol) in a
250 mL flask was stirred at 60 °C for 4 h, added with an ice-
water mixture, and filtrated. The filter cake was washed three
times with distilled water, and dried at 100 °C to obtain white
crystals. The DMU molecule synthesized in the text has the
advantages of good yields, easy work-up, no catalyst, readily
available starting materials and mild reaction conditions, and
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1
has certain economic benefits. H NMR (400 MHz, d6-DMSO):
δ/ppm (Figure 2a) = 10.51 (s, 1H), 8.05 (s, 1H), 6.90 (d, 2H, J =
7.9Hz), 5.79 (s, 1H), 5.24 (d, 1H, J = 7.9Hz); 13C NMR (100 MHz,
d6-DMSO): δ/ppm (Figure 2b) = 174.0, 157.9, 157.2, 62.9.
cm2. At first, the electrode was polished surface to 1200
♯ with
SiC abrasive papers, then degreased with acetone and washed
with ethanol. Finally, the electrode surface is dried at room
temperature. We used analytical grade 37% HCl and ultrapure
water to prepare 1 mol L-1 HCl corrosion test solutions. The
concentration range of DMU used was 50 ppm to 200 ppm and
Tween-80 was 10 ppm.
Figure 1 Synthetic route of the studied 1-(2,5-Dioxoimidazolidin-4-yl)urea.
(a)
Weight loss experiments
Weight-loss experiments were conducted in the presence of
DMU (50 ppm to 200 ppm) alone and in combination with
Tween-80 (10 ppm) in 1 mol L-1 HCl medium at 30−60 °C for 72
h. At first, the specimen’s surface was rinsed carefully with
ultrapure water. And then the specimens were dried and
weighed after the corrosion test was completed. By calculating
the average of three parallel experimental data, the corrosion
rate (v) was obtained. The corrosion rate v and inhibition
efficiency ηw were calculated as:
푊 − 푊2
1
푣 =
(1)
(2)
푠 × 푡
푣표 − 푣
휂푤
=
× 100%
푣표
where v is the corrosion rate; W1 and W2 (mg) are the steel
sample weight before and after immersion, respectively; s
(cm2) and t (h) are the exposed area of steel specimens and
immersion time; vo and v indicate the corrosion rate without
and with inhibitors, respectively.
(b)
Electrochemical measurements
Electrochemical tests were performed in the electrochemical
workstation. The electrochemical workstation is a traditional
three-electrode system, including working electrode (mild
steel); auxiliary electrode (platinum plate); and reference
electrode (saturated calomel electrode). All potentials are
relative to reference electrodes. In order to obtain a stable
working surface, the electrode was immersed in the solution
for half an hour before the test. In a potential range from −750
to −250 mV (vs. SCE), the potentiodynamic polarization curves
were measured with a scan rate of 1 mV s-1. The EIS
Figure 2 NMR spectra of DMU: (a) 1H NMR and (b) 13C NMR.
2 | J. Name., 2012, 00, 1-3
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measurements were carried out at EOCP, and the frequency
range is from 100 kHz to 10 mHz with an amplitude of 5 mV.
By fitting the experimental results with ZSimpWin software,
the corresponding EIS parameters and the appropriate
equivalent circuit were obtained.
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Surface Morphology: SEM and SECM analysis
The scanning electron microscopy (SEM) and scanning
electrochemical microscopy (SECM) were used to observe the
surface morphologies of steel specimens after immersion in 1
mol L-1 HCl solution without and with studied inhibitors at
30 °C. SEM was conducted study on a SUPRTM55 instrument
(Zeiss, Germany) at 2000 magnification. SECM (Uniscan
Instruments Ltd., UK) was performed with a four-electrode
system including a working electrode (mild steel), a probe (25
μm platinum ultramicroelectrode), a counter electrode
(platinum plate), and a reference electrode (saturated calomel
electrode).
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Figure 3 Potentiodynamic polarization curves of mild steel in 1 mol L-1 HCl solution
without and with different concentrations of DMU.
increased with the increase of inhibitor concentration. The
corrosion potential moves in the positive direction with the
maximum displacement at 19 mV, which was less than 85 mV.
This result indicates that the DMU belongs to a mixed-type
inhibitor based on the anode. Other N/O-heterocyclic
inhibitors have similar results inhibitors for the corrosion of
steel in acidic medium.26-28 Moreover, as the concentration of
corrosion inhibitor increases, the shape of the cathode
polarization curve (which was approximately parallel) did not
change significantly. It shows that the use of the corrosion
inhibitor does not change the hydrogen evolution mechanism.
The charge-transfer mechanism causes the hydrogen ion
reduction reaction on the steel surface. In other words, the
adsorbed inhibitors on the metal surface inhibited only the
active site of hydrogen evolution reaction.6 With the high
concentrations of inhibitors, the anodic current density sharply
increased in the anode region when the potentials were higher
than -270 mV. The corrosion potential is usually called
Quantum chemical calculations
Density function theory (DFT) was used for quantum chemical
calculations and structural parameters using B3LYP/6-31G(d, p)
level. The quantum chemical parameters such as the lowest
unoccupied molecular orbital energy (ELUMO), highest occupied
molecular orbital energy (EHOMO), energy gap (ΔE), and 3D MEP
plot were determined to explain the adsorption of inhibitor
molecules on steel surface.
Molecular dynamics simulation
The interaction of the studied molecules with Fe (110) surface
were investigated by molecular dynamics (MD) simulations
using Materials Studio5.0 software. The steps and parameters
of the simulation system construction have been detailed in
our previous work.5 The interaction energy between the
inhibitor molecule and Fe (110) surface can be calculated using
Eq. (3):
(
)
퐸interaction = 퐸total − 퐸surface+solution + 퐸inhibitor (3) desorption potential, and the dissolution rate of steel is faster
than the formation of protective film. Prabakaran et al.
believed that the phenomenon on the anodic region due to
the desorption of inhibitor molecules from active sites on steel
surface.1
where Etotal is the total energy of the entire system, Esurface+solution
represents the total energy of Fe (110) surface and solution without
the inhibitor, Einhibitor is the energy of free inhibitor molecule. The
negative value of interaction energy gives the value of binding
Table
1 summarized the calculated electrochemical
energy which is expressed as follows:
parameters including corrosion potential (Ecorr), corrosion
current density (icorr), anodic and cathodic Tafel slopes (βa, βc),
and inhibition efficiency (ηi). The inhibition efficiency was
computed as Eq. (5):
퐸binding = −퐸interaction
(4)
Results and discussion
Potentiodynamic polarization curves
푖c0orr − 푖corr
( )
휂푖 % =
( )
5
× 100
푖c0orr
The polarization curves of the working electrode in 1 mol L-1
HCl solution without and with different concentrations of DMU
were shown in Figure 3. It can be seen from Figure 3 that the
cathodic and anodic current density was significantly reduced
when the DMU was added to the corrosive solution, that is,
the DMU suppresses the cathode and anode reactions of the
steel. Specifically, the inhibitor has a stronger effect on the
anode than the cathode. The inhibition efficiency was
where i0
and icorr are the current density values in the
corr
absence and presence of inhibitor. By extrapolating Tafel lines
i0 and icorr can be obtained using CHI760E analysis software.
corr
As shown in Table 1, the corrosion inhibition efficiency (ηi)
increased and the icorr gradually decreased with the increase of
the inhibitor concentration. The maximum efficiency was
75.26% at an inhibitor concentration of 200 ppm. Therefore,
the DMU has a moderate corrosion inhibition performance in 1
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Table 1 Polarization curve parameters for the corrosion of mild steel in 1 mol L-1 HCl at 30 °C.
Inhibitors
c
Ecorr
icorr
βa
βc
η (%)
(ppm)
(mV vs.SCE)
(µA cm-2)
(mV dec-1)
(mV dec-1)
-
Blank
-481
731.45
119.5
-181.5
-
DMU
50
100
-476
-471
-468
-462
-464
-478
301.28
223.46
192.52
180.96
415.68
35.41
110.2
115.7
121.4
117.8
124.7
123.5
-157.8
-152.5
-161.3
-155.7
-170.6
-167.9
58.81
69.45
73.68
75.26
43.17
95.16
150
200
Tween-80
10
DMU+Tween-80
200+10
mol L-1 HCl solution. The changes of βa and βc were not evident
at several inhibitor concentrations, indicating that the
corrosion was slowed down by the surface blocking effect of
inhibitors. A protective film formed on the metal surface
impeded the contact between the corrosive medium and the
metal. As the reactive sites were suppressed, the corrosion
was reduced. Thus, DMU has a moderate behavior of
suppressing on the corrosion of steel in 1 mol L-1 HCl solution.
Electrochemical impedance spectroscopy (EIS)
The corrosion inhibition behavior of DMU on mild steel was
studied by using EIS. The Nyquist plots of steel in 1 mol L-1 HCl
solution without and with different concentrations of DMU
were shown in Figure 4. It was found that a semi-circular
capacitive loop, slightly depressed, was displayed in the
Nyquist plots. It means that there is only a one-time constant
associated with the charge-transfer process. The depressed
capacitive loop is often caused by frequency dispersion due to
the roughness and other inhomogeneities of the electrode
surface.29-32 In addition, the diameter of the capacitive loop in
the presence of the inhibitor was larger than that in the
uninhibited acidic solution. As the concentration of the
inhibitor increases, the diameter of the high-frequency loop
increases significantly. This indicates that the DMU inhibitor
hinders steel corrosion in 1 mol L-1 HCl. It was worth noting
there was no significant change in the shape of impedance at
different inhibitor concentrations, indicating that the addition
of the inhibitor did not change the corrosion mechanism of the
steel electrode. It is in accordance with the curve of the
polarization test. With regard to the bode plots (Figure 5), a
distinct phase peak appeared around 1 kHz, which indicated a
one-time constant was relevant to electrical double layer at
the steel/solution interface. In addition, as the concentration
Figure 4 Nyquist plots for mild steel in 1 mol L-1 HCl solution without and with different
concentrations of DMU.
Figure 5 Bode plots for mild steel in 1 mol L-1 HCl solution without and with different
concentrations of DMU.
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solution. As expected, the Cdl value tended to decrease with an
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increase of inhibitor concentration, resuDltOinI:g10fr.1o0m39/aC9dNeJc0r3e3a3s6eJ
in the local dielectric constant or an increase in the thickness
of the electrical double layer capacitor.1 These results revealed
that the inhibitor adsorbed on steel surface acted at the
interface electrode and solution. Another meaningful finding
was that the values of n were close to unity and there was no
obvious fluctuation. It showed that the interface behavior was
almost capacitive and the dissolution mechanism of steel was
controlled by charge transfer.12 Of particular note was the
changing trend of the ηz value calculated by EIS was the same
as the electrodynamic polarization curve method.
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Figure 6 Equivalent circuit fit the Nyquist plot for mild steel in the presence of DMU at
200 ppm.
Effect of Tween-80 addition on DMU Performance
of the inhibitor increased, the absolute impedance increased
significantly at low frequencies. This is because the inhibitor
adsorbed on the steel surface is in proportion to the
concentration of the inhibitor. The thicker the adsorption film,
the more difficult the process of charge-transfer becomes.29
In order to study the electrochemical impedance
characteristics, ZSimpWin software was used to process the
data. Specifically, the impedance behavior of mild steel can be
modeled with the circuit shown in Figure 4. For example, we
simulated the Nyquist plot of DMU at 200 ppm by an
equivalent circuit and illustrated the fitting curve in Figure 6.
The small values of χ2 (5.04 × 10-4) indicate a better fit.
Electrochemical parameter Rs is the solution resistance; CPE
and Rct represent the constant phase element and the charge-
transfer resistance, respectively. These three parameters
reflect the adsorption characteristics of the inhibitor on the
electrode surface. As the adsorption characteristics of the
inhibitor has an important influence on the charge-transfer
process, CPE was often used to optimize the double layer
capacitance (Cdl) in order to obtain a more accurate fit of
The use of a suitable additive is necessary as DMU has only
moderate inhibition ability. The effect of incorporation of
Tween-80 (10 ppm) into DMU system on the inhibition
efficiency was researched by polarization curve and EIS
methods. In our previous study,33 the optimal corrosion
inhibition concentration of Tween-80 was 10 ppm. Figure 7
shows the (a) polarization curves, (b) Nyquist and (c) Bode
graphs for mild steel unprotected and protected with DMU
and DMU+Tween-80. The electrochemical parameters
obtained from Figure 7a and Figure 7b were respectively
displayed in Tables 1 and 2. As compared with the polarization
curves of steel unprotected and protected with DMU, both
cathodic and anodic current densities were sharply reduced in
the presence of DMU+Tween-80, reflecting a synergistic effect
between DMU and Tween-80. Besides, the use of Tween-80 as
additive led to a slight negative shift in the corrosion potential
of mild steel, and the dissolution process of cathode and
anode was inhibited. Therefore, the mixture of DMU and
Tween-80 was a mixed corrosion inhibitor. The synergistic
effect of DMU and Tween-80 is also very obvious in Figure 7b.
The semicircular diameter of the DMU+Tween-80 composite is
significantly increased compared to the one for DMU, meaning
the charge transfer process was suppressed more in
DMU+Tween-80 system than that in DMU alone. The positive
influence of Tween-80 addition was also seen in the absolute
impedance and Phase angle displacements (Figure 7c). The
presence of Tween-80 in DMU reduced the current density (i.e.,
180.96 μA cm-2 versus 35.41 μA cm-2, Table 1), increased the
charge-transfer resistance (i.e., 201.4 Ω cm2 versus 1250.6 Ω
cm2, Table 2), and thus obviously enhanced the inhibition
efficiency (i.e., 75.26%–77.31% versus 95.16%–96.35%). When
the concentrations of DMU and Tween-80 exceed 200 ppm
and 10 ppm, respectively, the corrosion inhibition efficiency
increases little and tends to be stable. In this case, the best
corrosion inhibition efficiency was obtained. The presence of
multiple polar nitrogen atoms and amino groups of DMU
might get into the gap of Tween-80 to form a complementary
network structure, and thus enhanced the inhibition efficiency.
The synergistic effect of DMU+Tween-80 in acidic solution
was compared with the literature data by electrochemical
method. The results show that DMU+Tween-80 has better
corrosion inhibition performance as shown in Table 3.
experimental data. The impedance of CPE (ZCPE
)
was
represented below in Eq. (6):
n −1
(
푍CPE = [푌 푗푤
0
)
]
(6)
where Y0 represents the proportionality coefficient; j and ω are
the imaginary number and the angular frequency, respectively;
n is the phase shift related to surface inhomogeneity. The
parameters of electrochemical impedance were listed in Table
2, which was obtained from the equivalent circuit. The double
layer capacitance (Cdl) was expressed by Eq. (7) and the
inhibition efficiency (ηz) by Eq. (8):
푛−1
퐶dl = 푌 (푤max
0
)
(7)
푅ct − 푅c0t
푅ct
(
)
( )
휂푧 % =
× 100 (8)
where wmax is the angular frequency when the imaginary part
of impedance reaches its maximum value, R0 and Rct
ct
represent the charge-transfer resistance values of the mild
steel electrode in an aggressive medium without and with an
inhibitor. As shown in Table 2, the values of Rct and ηz
increased significantly with the increase of the inhibitor
concentration. The maximum ηz was 77.31% when the
concentration of DMU reached up to 200 ppm, indicating that
the inhibitor can inhibit the corrosion of mild steel in HCl
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Table 2 EIS parameters for the corrosion of mild steel in 1 mol L-1 HCl at 30 °C.
Inhibitors
c
(ppm)
Blank
50
Rs
Rct
n
CPEdl
(μF cm-2)
471.2
275.4
163.8
149.7
132.5
364.1
49.7
ηz
(Ω cm2)
0.312
0.507
0.396
0.644
0.692
0.844
2.343
(Ω cm2)
(%)
-
45.7
0.92
0.93
0.91
0.90
0.94
0.91
0.89
-
DMU
123.6
161.1
180.2
201.4
84.3
63.03
71.63
74.64
77.31
45.79
96.35
100
150
200
Tween-80
10
DMU+Tween-80
200+10
1250.6
Table 3 Comparison of the inhibition efficiency of DMU+Tween-80 with the literature data as synergistic inhibitor for mild steel in acidic solution.
Inhibitor
1-(2,5-dioxoimidazolidin-4-yl)urea + Tween-80
L-Histidine + Sodium dodecyl sulfate
η (%)
96.35
87.99
90.79
94.21
93.45
93.07
85.11
88.43
90.34
90.38
81.69
89.28
87.60
90.30
Ref.
This paper
[25]
L-Histidine + Cetyltrimethyl ammonium bromide
Dextran 1(α-1,3 linkages) + KI
[25]
[28]
Dextran 2(α-1,4 linkages) + KI
[28]
Dextran 3(α-1,6 linkages) + KI
[28]
3-(2-((4-hydroxybenzylidene)amino)thiazole-4-yl)-2H-chromen-2-one (HBATC) + Enoxacin
HBATC + Pefloxacin
[29]
[29]
HBATC + Gemifloxacin
[29]
Xanthan gum + Sodium dodecyl sulfate
Xanthan gum + Cetyl pyridinium chloride
Xanthan gum + Triton X-100
[34]
[34]
[34]
Imidazoline quaternary salt + Benzotriazole
Imidazoline quaternary salt + Octyl phenol ethoxylates
[35]
[35]
The synergistic inhibition effect of DMU and Tween-80 effect of DMU and Tween-80. Indeed, a very small amount of
can be described by synergistic parameter S which is defined surfactant Tween-80 could significantly improve the
as:
adsorption performance of DMU on steel surface.33
1 − 퐼1+2
1 − 퐼1′+2
Effect of temperature
푆 =
(9)
In order to understand the effect of temperature on corrosion
inhibition, we measured the weight-loss of mild steel in 1 mol
L-1 HCl without and with DMU and DMU+Tween-80 at
30 °C−60 °C, and the calculated v and ηw were listed in Table 5.
The v value of inhibited steel was lower than those of
uninhibited at a specific temperature. Besides, it was observed
that the mixture of DMU and Tween-80 was found to be very
effective in inhibiting the corrosion of mild steel particularly at
where I1+2 = (I1 + I2), I1 is the η of DMU, I2 is the η of Tween-80,
and I'1+2 is the η of the combination DMU and Tween-80.
Generally, it is considered that the S value greater than one is
a synergistic effect between the two species, while the S value
less than one will lead to competitive adsorption.34,35 The
values of
S
were calculated from electrochemical
measurements, and the results were shown in Table 4. The S
values were more than one for all tests, reflecting a synergistic
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Table 4 Synergism parameters of mild steel in 1 M HCl solution at 30 °C.
View Article Online
(a)
Inhibitors
c
Potentiodynamic
EIS
DOI: 10.1039/C9NJ03336J
5
6
7
8
(ppm)
200
10
ηi (%)
75.26
43.17
95.16
S
ηz (%)
77.31
45.79
S
-
DMU
-
-
Tween-80
-
9
DMU+Tween-80 200+10
1.247
96.35 1.281
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Table 5 Weight loss results of mild steel in 1 M HCl with 200 ppm DMU alone and
DMU+Tween-80 mixtures at different temperatures.
T
(°C)
HCl
DMU
v (mg
DMU+Tween-80
v
ηw
v (mg
ηw
(mg cm-2 h-1)
cm-2 h-1)
(%)
cm-2 h-1)
(%)
30
40
50
60
14.58
26.95
50.03
91.15
3.85
6.67
73.59
75.25
76.85
79.12
0.55
0.73
0.99
1.28
96.23
97.29
98.02
98.60
(b)
11.58
16.94
To learn more about the effects of temperature on the
inhibition mechanism, kinetic analysis was performed. The
activation energy (Ea) of the corrosion process was calculated
by Arrhenius equation as:
퐸푎
(
)
ln푣corr = ln퐴 −
10
푅푇
where vcorr is the corrosion rate; A represents the pre-
exponential factor; R and T are the gas constant and the
absolute temperature, respectively. The Ea values in presence
of DMU alone or in combination with Tween-80 were
calculated by the Arrhenius plots line slope (Figure 8), and the
results were shown in Table 6. The values of Ea in the presence
of inhibitor were smaller than that of the uninhibited solution.
It may correlate to chemisorption phenomenon that works
through the transfer or sharing of electron pairs on the O/N
heteroatoms with the empty d-orbital of iron, as reported in
the literature.37,38 Furthermore, the enthalpy and entropy of
the process activation (ΔH*, ΔS*) was calculated from the
transition state Eq. (11), listed in Table 6:
(c)
푣corr
푇
푅
Δ푆∗
−훥퐻∗
=
exp (
) exp (
) (11)
푁ℎ
푅
푅푇
where N and h represent Avogadro’s number and Planck’s
constant, respectively. At different temperatures, the plots of
ln(vcorr/T) vs. 1/T for steel in 1 mol L-1 HCl in the presence and
Figure 7 Polarization curves (a), Nyquist (b) and Bode graphs (c) for mild steel
unprotected and protected with DMU and DMU+Tween-80 in 1 mol L-1 HCl.
Table 6 Activation parameters for the steel dissolution in 1M HCl without and
with inhibitors.
Inhibitors
Blank
Ea (kJ mol-1) ΔH* (kJ mol-1) ΔS* (J mol-1 K-1)
higher temperatures. For instance, the ηw of this mixture in
corrosive acid solutions was 96.23%, 97.29%, 98.02% and
98.60% at 30 °C, 40 °C, 50 °C and 60 °C, respectively. According
to the relation between ηw and temperature, chemisorption is
may be the prevalence adsorption mechanism.36
51.29
41.98
23.81
48.65
39.34
21.17
-62.80
-104.34
-180.65
DMU
DMU+Tween-80
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Figure 8 Arrhenius plots of lnvcorr vs. 1/T for mild steel in 1 mol L-1 HCl in the absence
and presence of inhibitors.
Figure 10 Plots of Langmuir isotherm for the adsorption of inhibitors on the surface of
mild steel in 1 mol L-1 HCl.
Figure 9 Transition state plots of ln(vcorr /T) vs. 1/T for mild steel in 1 mol L-1 HCl in the
absence and presence of inhibitors.
Figure 11 Plot of ln Kads vs. 1/T for mild steel in 1 mol L-1 HCl solution.
absence of inhibitors was shown in Figure 9. The values of ΔH*
from Table 6 were positive, indicating that the adsorption of
the additives on steel surface was an endothermic corrosion
process. The ΔS* increases negatively with the addition of
DMU and DMU+Tween-80, meaning that an increase in
ordering takes place on going from reactants to the activated
In the formula, c represents the inhibitor concentration, θ is the
surface coverage obtained from weight loss method, and Kads is the
adsorption equilibrium constant associated with the standard
adsorption free energy (ΔG0ads), as per following relationship:33
∆푮ퟎ풂풅풔 = −푹푻퐥퐧(ퟏ × ퟏퟎퟔ푲풂풅풔
)
(ퟏퟑ)
complex by the formation of stable adsorption layer of the where 1×106 is the amount of water molecules expressed in ppm, T
inhibitor molecules on mild steel surface.36
is the thermodynamic temperature and R is the universal gas
constant. The plots of c/θ versus c for DMU+ Tween-80 at 30-60 °C
were shown in Figure 10, and the adsorption parameters were
listed in Table 7. A well-recognized judgment that the value of
Adsorption isotherms
The synergistic effect is mainly due to the ability of Tween-80 to
facilitate the adsorption of DMU on mild steel surface. The mode of
adsorption and the adsorption isotherm were performed to better
understand the interactions of the inhibitor on the metal surface. In
this research, the Langmuir isotherm was found to display the finest
fitting results since the linear regression coefficient was almost to 1
(Table 7), which is defined as:
ΔG0
(absolute value) less than 20 kJ·mol-1 is indicated as
ads
physiosorption, which is attributed to the electrostatic attraction
between inhibitor molecules and charged metal surface. Instead,
the ΔG0
absolute value more than 40 kJ·mol-1 indicates a
ads
chemisorption process due to charge sharing or transfer between
the inhibitor molecules and iron atoms. As seen in Table 7, the
ΔG0
values are ranged between –26.84 kJ·mol-1 and –32.12
ads
풄
ퟏ
kJ·mol-1, which indicates the adsorption of DMU+Tween-80 on mild
steel surface is a mixed from physical and chemical adsorption.
=
+ 풄
(ퟏퟐ)
휽
푲풂풅풔
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Table 7 Thermodynamic parameters of inhibitors on mild steel surface in 1 mol L-1 HCl
solution.
(a)
(b)
(c)
View Article Online
DOI: 10.1039/C9NJ03336J
T (°C)
R2
Kads
(ppm/mol)
ΔG0
ΔH0
ΔS0
ads
ads
ads
(J mol-1)
(kJ mol-1)
(kJ mol-1)
30
40
50
60
0.9993
0.9998
0.9998
0.9997
0.0424
-26.84
26.62
176.71
0.0633
0.0879
0.1091
-28.77
-30.84
-32.12
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From the listed values of Kads and ΔG0 in Table 7, ΔH0 (standard
ads
ads
enthalpy) and ΔS0ads (entropy) of adsorption can be calculated using
Eq. (14):33
∆푯ퟎ풂풅풔 ∆푺ퟎ풂풅풔
퐥퐧푲풂풅풔 = −
+
− 퐥퐧(ퟏ × ퟏퟎퟔ)
(ퟏퟒ)
푹푻
푹
The values of ΔH0 and ΔS0 can be determined from the slope
ads
ads
and intercept of the plot of lnKads vs. 1/T (Figure 11). As seen in
Table 7, the positive value of ΔH0ads indicates that the adsorption of
DMU+Tween-80 is an endothermic process. The large positive value
of ΔS0 related to substitutional adsorption process, which can be
ads
attributed to the increase in the solvent entropy and a more
positive water desorption entropy.33
SEM and SECM analysis
The surface morphology of mild steel after immersion in
corrosion solution for 48 h without and with 200 ppm DMU
and combined with 10 ppm Tween-80 were examined by SEM
(Figure 12) and SECM (Figure 13). As can be seen from Figure
12a, the steel sample exposed to the corrosive medium
severely damages the surface, and the products produced
during the corrosion process can be clearly seen deposited on
the surface. In the acid solution containing DMU (Figure 12b),
the steel surface became relatively flat with a small amount of
shallow pitting corrosion, suggesting that the DMU molecules
adsorbed on steel surface. However, the deposited films
exhibit porous properties, and this could be the reason DMU
exhibit moderate inhibition performance. Undoubtedly, the
DMU+Tween-80 mixture (Figure 12c) offered satisfactory
protection to mild steel in HCl solution, and formed a compact
and uniformly film on steel surface.
The results of SEM were further confirmed by SECM,
which could directly characterize the morphology and
electrochemical activity of the matrix surface. Judging from the
values of current peak in Figure 13a, it can be said that there
was serious corrosive attack on steel surface, exhibiting
multiple current peaks, and 4500 nA was the maximum
recording current value. The presence of DMU (Figure 13b) in
HCl solution greatly reduced the samples surface roughness
such that the current density was decreased from 4500 nA to
920 nA. The effectiveness of DMU+Tween-80 mixture was also
seen in Figure 13c, where the scanning surface was
significantly smooth and the maximum current density
recorded was 48 nA.
Figure 12 SEM images of mild steel surfaces: (a) 1 mol L-1 HCl, (b) DMU, (c)
DMU+Tween-80
Density functional theory
The frontier molecular orbitals were used to study the
adsorption activity of inhibitor to the electrons distribution. In
particular, DFT has been used to analyse the characteristics of
inhibitor/surface interaction and hence inhibitor mechanisms
and to describe the effect of the structural nature of an
inhibitor on the corrosion process.39-43 Table 8 showed the
proton affinity (PA) and the energy in protonation process, the
larger the atomic PA value, and the more easily protonated.44
Although the largest PA value exists on N12, the hydrogen
atom is transferred from N12 to O15 during the configuration
optimization, and the O15 also shows a higher PA value,
indicating that O15 is most prone to protonation. Figure 14a
represents the HOMO, LUMO and MEP of neutral DMU,
whereas Figure 14b represents those of protonated DMU. The
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(a)
a higher EHOMO in neutral form and a lower ELUMO iVniepwrAortticolenOantleinde
form. It reveals that the neutral form haDsOaI:s1t0r.o10n3g9e/Cr9aNbJi0li3t3y36toJ
donate electrons compared to the protonated form, but the
ability of accepting electrons is opposite. In addition, the
smaller value of ΔE (5.79 eV) in protonated form indicates that
it is easier to adsorb on steel surface. Therefore, the
protonated inhibitor molecule is physisorbed to steel surface
via electrostatic interactions, and its neutral form is
chemisorbed to steel surface through donor-acceptor
interactions between the free electron pairs of heteroatoms (N,
O) as well as π-electrons and unoccupied d-orbital of iron
atoms. Another noteworthy is that the protonated dipole
moment is higher than the neutral molecule, which may be
due to the increased attraction of superfluous electrons on
metal surface after protonation, resulting in the enhanced
interaction between inhibitor molecule and metal.44
The MEP contains information about a total electronic
constitution of the molecule and provides a visual method for
understanding relative polarity. As shown in Figure 14, the
negative electrostatic potential regions (shades of red) are
mainly located in ketone groups, indicating the possible sites
to be electrophilic attacked.44 However, the positive potential
regions are located around hydrogen atoms, reflecting the
possible sites to be nucleophilic attacked.44 The protonated
DMU was expected to has excess positive potential regions,
which means the protonated form is more susceptible to be
nucleophilic attacked, and has a stronger ability to accept
electrons from the steel surface. This result is similar to that
observed from the frontier molecular orbital and ΔE analyses,
thus, both forms of DMU are good inhibitors.
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(b)
(c)
Molecular dynamics simulation
The adsorption behavior of the studied inhibitor on mild steel
surface can also be corroborated by molecular dynamics
simulation, which has emerged as a powerful tool to study the
adsorption nature of the inhibitor molecule on metallic
surface.5,44 In the present study, MD simulations were
performed to study the adsorption behavior of the neutral
form and protonated form of DMU on Fe (110) surface. In the
geometric optimization process, atomic coordinates were
adjusted based on COMPASS force field until the total energy
of a single structure was minimized,44 and a system containing
500 water molecules, 5H3O+, 5Cl- and one molecule of the
Figure 13 SECM images of mild steel surfaces: (a)
DMU+Tween-80.
1
mol L-1 HCl, (b) DMU, (c)
HOMO and LUMO diagrams of neutral and protonated
inhibitor all showed that the electron densities are mainly
concentrated on the imidazoline ring, reflecting it as both an
electron donor and an acceptor for electrons. While, the urea
group trends to donate electrons in neutral form, and accept
electrons in protonated form. The values of the quantum
chemical parameters (EHOMO, ELUMO, and ΔE) were listed in
Table 9. As reported previously,43-46 molecules with higher
EHOMO values exhibit greater electron-donating capabilities.
Conversely, molecules with lower ELUMO values exhibit stronger
electron-withdrawing abilities. Consequently, lower energy
gap (ΔE = ELUMO − EHOMO) will provide stronger reactivity to a
chemical species. As shown in Table 9, the DMU molecule has
Figure 14 The frontier orbital density distributions and 3D MEP plot of the neutral form
(a) and protonated form (b).
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Table 8 Relative energies of the protonation sites of inhibitor at B3LYP/6-31G(d, p) set level.
Energies
N11
N12
N13
N14
O15
O16
O17
Inh
-
PA (kJ/mol)
H (hartree)
769.237
-600.946
898.706
-600.995
846.243
-600.975
821.955
-600.966
881.630
-600.989
833.704
-600.970
873.882
-600.986
-600.655
Table 9 The calculated quantum chemical parameters for the neutral and protonated
form of DMU.
Compounds
ELUMO (eV)
-0.64
EHOMO (eV)
-7.31
ΔE (eV)
6.67
μ (Debye)
3.636
Inh
InhH+
-5.36
-11.15
5.79
10.045
Table 10 Interaction energy and binding energy values between the inhibitors and Fe
(110) surface.
System
EInteraction
(kJ mol-1)
-780.69
EBinding
(kJ mol-1)
780.69
Fe + Inh + 500 H2O + 5 H3O+ + 5 Cl-
Fe + InhH+ + 5 00H2O + 5 H3O+ + 5Cl-
-770.35
770.35
tested inhibitors was constructed to simulate the actual aggressive
medium. After 500000 steps, the system reaches equilibrium which
explains that both the temperature and energy reach balance.44
Figure 15 represented the top and side views of the neutral form
and protonated form of inhibitor molecule on Fe (110) surface. It is
noticeable that the inhibitor of both forms were adsorbed on Fe
(110) surface with almost parallel or flat when the system reaches
equilibrium, indicating that the inhibitor molecule has a strong
interaction between active sites and iron atoms. Therefore, it is
reasonable that the parallel adsorption model is caused by the
simultaneous interactions with Fe (110) surface through N/O atom
as well as amino groups. Additionally, the Einteraction and Ebinding
values can also be used to determine the adsorption capabilities of
tested inhibitors, which were calculated and listed in Table 10. The
higher positive value of Ebinding, the more stable adsorption of
inhibitor molecules on the Fe surface.43 In the same case, the higher
negative value of Einteraction leads to a strong interaction between the
inhibitor and metallic surface.43 As showed in Table 10, the Einteraction
of the neutral form more negative than those of protonated form,
indicating that the neutral form of tested compound could more
strongly adsorbs on Fe surface than protonated form. These
findings are in good agreement with the experimental evaluation
results, as electronic donation and retrodonation are mainly
through neutral forms of inhibitors. Actually in a hydrochloric acid
medium, the protonated form will be in equilibrium with the
Figure 15 Side and top views of the adsorption of the neutral form (a) and protonated
form (b) inhibitor on Fe (110) surface.
corresponding neutral form, one may draw a conclusion that the
adsorption of DMU is mostly attributed to chemisorption
meanwhile combined with physisorption.
Mechanism of inhibition
Generally speaking, the acid corrosion mechanism of inhibitor
can be interpreted by adsorption theory. In acidic solution
inhibitor is not only present as a neutral molecule, but also has
a protonated form in equilibrium as given in Eq. (15):
+
Inh + xH ↔ [InhH]x+ (15)
[
]
Therefore, the adsorption mechanism cannot be explained by
single physical or chemical adsorption, because the
a
adsorption of inhibitor molecules exists in different forms. An
ideal adsorption mechanism should consider two adsorption
models, as shown in Figure 16.
Initially, a large amount of Cl- ions in the hydrochloric acid
solution adsorbed on the surface of the positively charged
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Acknowledgements
View Article Online
This work was supported by the DKOeI:y10.1R0e39se/Ca9rNchJ033a3n6dJ
Development
Program
of
Shandong
Province
(2019GSF108089), the Natural Science Foundation of
Shandong Province (ZR2019MB009), the National Natural
Science Foundation of China (21672046, 21372054), and the
Found from the Huancui District of Weihai City.
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Notes and references
Figure 16 Schematic illustration of the adsorption mechanism of DMU and Tween-80
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M. Prabakaran, S. H. Kim, V. Hemapriya and I. M. Chung, Res.
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on mild steel surface.
steel to form a negative electric region, and then the
protonated DMU/Tween-80 molecule would have electrostatic
adsorption (physical adsorption) with the negative steel
surface, forming a protective film to inhibit the anodic
dissolution of iron. Also, the negative charge on the carbonyl
group on DMU interacts with the positively charged metal
surface (free space uncovered by Cl-) and forms the DMU
monolayer, and the cationic group of DMU monolayer
interacts with the heteroatoms present in Tween-80 and forms
the bilayer of inhibitor.29 Thus this bilayer inhibitor contributes
more inhibition when it is adsorbed on mild steel than the
inhibition of DMU alone in acidic medium. On other hand, the
lone pair electrons in heteroatoms (N/O) can enter the empty
d-orbital of iron atoms to form coordination bonds for
chemisorption, which slows down the corrosion reaction. In
addition, Tween-80 is a macromolecule and DMU is a small
molecule, When DMU is compounded with Tween-80, the
small molecule of DMU can be inserted into the gap of Tween-
80 to form a complementary double layer network structure,
so that their active groups can best play a synergistic effect.
,
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Conclusions
In this work, 1-(2,5-dioxoimidazolidin-4-yl)urea was
synthesized from urea and glyoxylic acid under mild reaction
conditions, and its corrosion inhibition for mild steel in acidic
solution was studied. Based on the experimental analysis, the
following conclusions are drawn:
(1) DMU is found to behave as an anode-based mixed
inhibitor, which moderately inhibits the corrosion of mild
steel in HCl solution.
(2) The inhibition efficiency of DMU increases with the
increase of concentration and temperature. A synergistic
inhibition effect between DMU and Tween-80 is observed,
and the adsorption on steel surface follows Langmuir
adsorption isotherm.
(3) Surface morphology by SEM and SECM confirmed the
synergistic inhibition effect between DMU and Tween-80.
(4) Density functional theory and MD simulation analysis
supports the fact that both the neutral and protonated
forms of DMU can inhibit the corrosion of mild steel in HCl
solution.
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The inhibition performance and synergistic inhibition effect of DMU with
Tween-80 on mild steel corrosion in 1M HCl was studied for the first time.