The Preparation of a Novel Corrosion Inhibitor and Its Corrosion Inhibition Behavior on 2024 Al-Cu-Mg Alloy in Acid Solution

Article abstract of DOI:10.1002/jsde.12259

A new corrosion inhibitor was synthesized from chloroacetyl chloride, 1,3-propanediamine, and dodecyldimethyl tertiary amine. The structure of the synthesized product was characterized using Fourier transform infrared(FTIR). The critical micelle concentration (CMC) was determined from surface tension measurements. The inhibition behavior of the corrosion inhibitor for 2024 Al-Cu-Mg alloy was studied using the weight-loss method and electrochemical measurements in hydrochloric acid solution. Experimental results show: The CMC is 7.767 × 10^?4 mol L^?1 and IE (C_pro = 1 × 10^?3 mol L^?1) ≈ 87.9%. The adsorption free energy is calculated to be ?33.12 kJ mol^?1. Therefore, the adsorption mode is more inclined to physical adsorption. When the concentration of the synthesized product is lower than the CMC, it satisfies the Langmuir adsorption model; when the concentration is higher than the CMC, it no longer follows the Langmuir adsorption model.

Full text of DOI:10.1002/jsde.12259

J Surfact Deterg (2019)
DOI 10.1002/jsde.12259
ORIGINAL ARTICLE
The Preparation of a Novel Corrosion Inhibitor and Its
Corrosion Inhibition Behavior on 2024 Al-Cu-Mg Alloy in Acid
Solution
Du Juan1 · Jia Shuai1 · Yang Fan1 · Wei Ziming1 · Song Haipeng1 · Yang Xudong1 · Wang Fusheng1
Received: 31 August 2018 / Revised: 14 November 2018 / Accepted: 17 December 2018
© 2019 AOCS
Abstract A new corrosion inhibitor was synthesized from
chloroacetyl chloride, 1,3-propanediamine, and dodecyldi-
methyl tertiary amine. The structure of the synthesized
product was characterized using Fourier transform infrared
(FTIR). The critical micelle concentration (CMC) was deter-
mined from surface tension measurements. The inhibition
behavior of the corrosion inhibitor for 2024 Al-Cu-Mg alloy
was studied using the weight-loss method and electrochemi-
cal measurements in hydrochloric acid solution. Experimen-
tal results show: The CMC is 7.767 × 10−4 mol L−1 and IE
(Cpro = 1 × 10−3 mol L−1) ≈ 87.9%. The adsorption free
energy is calculated to be −33.12 kJ mol−1. Therefore, the
adsorption mode is more inclined to physical adsorption.
When the concentration of the synthesized product is lower
than the CMC, it satisfies the Langmuir adsorption model;
when the concentration is higher than the CMC, it no longer
follows the Langmuir adsorption model.
forming, and are used in a wide range of applications such
as aerospace, shipbuilding, and so on (Yang and Dai,
2005). In chemical cleaning work such as in aircraft pick-
ling, to prevent excessive corrosion of the aluminum alloy
by acidic substances such as hydrochloric acid, it is neces-
sary to add an appropriate amount of corrosion inhibitor to
the acid-washing liquid, and this method can effectively
remove various types of dirt of the aircraft. This will extend
the life of the aircraft and ensure flight safety. Therefore, a
suitable corrosion inhibitor is often added to the pickling
solution to inhibit excessive corrosion of the aluminum
alloy during the pickling process.
Gemini surfactants have the advantages of high surface
activity, a strong tendency to form micelles, and good
water solubility due to their novel structure with double
hydrophilic groups and double hydrophobic chains. There-
fore, it is easier to form a protective film on the surface of
the aluminum alloy, thereby slowing the contact between
the acidic substance and the aluminum alloy to avoid corro-
sion. Therefore, it is of great theoretical and practical sig-
nificance to study high-efficiency, environmentally friendly
corrosion inhibitors, and their compounding that can effec-
tively alleviate the excessive corrosion of aluminum alloys
in acidic solutions.
Keywords Aluminum alloy ꢀ Corrosion inhibitor ꢀ
Isothermal adsorption model ꢀ Infrared spectroscopy ꢀ
Corrosion inhibition
J Surfact Deterg (2019).
In recent years, the study of corrosion inhibition perfor-
mance using Gemini surfactants is more prevalent (Al-
Amiery et al., 2014; Asefi and Mahmoodi, 2010; Cao and
Hou, 2016; Du et al., 2018; Hegazy and Abdallah, 2010;
Qiu et al., 2008; Qiu and Lv, 2016; Song et al., 2011;
Wang et al., 2010; Xia et al., 2015, 2016; Zhang et al.,
2011, 2015). Most studies have focused on the corrosion
inhibition performance of steel. Research on aluminum
alloys for aviation is relatively rare. Song et al. (2011) stud-
ied the corrosion inhibition performance of Q34 steel with
Introduction
Aluminum alloys show many advantages including low
density, good corrosion resistance, and easy fabrication and
* Du Juan
dujuan247@163.com
1
Sino-European Institute of Aviation Engineering, Civil Aviation
University of China, Tianjin, 300300, China
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an imidazoline ring asymmetric double quaternary ammo-
nium salt corrosion inhibitor (DBA) in 1 mol L−1 HCl
medium. The results showed that the inhibition efficiency
(IE) increases with the increase of its concentration. When
the concentration was 2.89 × 10−4 mol L−1 in the experi-
mental temperature range of 25–55ꢁC, the IE was above
90% and the corrosion IE increased with increasing temper-
ature. The polarization curves indicated that DBA was a
mixed corrosion inhibitor, which mainly suppresses the cath-
ode. The adsorption process of the corrosion inhibitor on the
surface of Q235 steel was an endothermic process, and the
adsorption followed the Langmuir isotherm and belonged to
chemical adsorption. Wang et al. (2010) synthesized 1,6-di
(decylmorpholine)hexane (HDMB) as a corrosion inhibitor
and studied its corrosion inhibition performance in 1 mol
L−1 hydrochloric acid. When the concentration reached
1 × 10−3 mol L−1, the IE tended to be constant. The mecha-
nism of corrosion inhibition was obtained by adsorption the-
ory, kinetic and thermodynamic formulae: the entropy effect
was the main driving force of the adsorption process. The
corrosion inhibition process was mainly the activation energy
control mechanism. The polarization curve method showed
that: HDMB was a mixed inhibition-type corrosion inhibitor.
Zhang et al. (2011) synthesized four long alkyl groups as
hydrophobic groups of quaternary ammonium salt bimolecu-
lar surfactants m-6-m (m = 10, 12, 14, 16). The corrosion
inhibition performance of aluminum in 25ꢁC hydrochloric
acid solution was studied by means of the weight loss
method, the hydrogen evolution test, and scanning electron
microscopy (SEM). The results showed that the IE of these
surfactants increases with the increase of concentration and
reached the maximum near the critical micelle concentration
(CMC). At the same time, the IE increased with the increase
of the m value in the specific concentration range. These sur-
factants had strong adsorption at the metal–solution interface,
and the adsorption behavior conforms to the Langmiur
adsorption model.
Most of the corrosion inhibition studies on quaternary
ammonium salt bimolecular surfactants have focused on
the use of linear imino groups as linking groups, while the
use of amide groups as linking groups to link long carbon
chains is relatively rare. The amide group is hydrophilic
and can increase the solubility of the corrosion inhibitor.
In this article, a novel high-efficiency, environmentally
friendly quaternary ammonium Gemini surfactant with sym-
metric double hydrophilic groups was prepared using the
small organic molecule 1,3-propanediamine as the matrix
and the amide group as the linking group. The structure was
characterized using infrared spectroscopy and 1H nuclear
magnetic resonance (NMR); the surface activity was investi-
gated using the surface tension test. The corrosion inhibition
performance and corrosion inhibition type of 2024 aluminum
alloy in an acidic environment were studied using the static
coupon weight loss method, electrochemical method, and
SEM. The corrosion inhibition mechanism of the surfactant
was analyzed in-depth by means of the isothermal adsorption
model and Fourier transform infrared (FTIR) test.
Experimental
Experimental Reagents
Chloroacetyl chloride, 1,3-propanediamine, dichloro-
methane, ethyl acetate, dodecyldimethyl tertiary amine,
hydrochloric acid, acetone, and absolute ethanol with a
mass fraction of 36% to 38% were used. All chemicals
were of analytical grade. The research object of this experi-
ment is 2024 Al-Mg-Cu alloy for aviation. The chemical
composition is shown in Table 1.
Experimental Equipment
A BS 224S electronic analytical balance (Shanghai, China),
a PARSTAT 2273 electrochemical workstation (Beijing,
China), a HITACHI S-3400N scanning electron microscope
(Tokyo, Japan), a magnetic stirrer, a SHB-3T circulating
water multipurpose vacuum pump iron stand (Zhengzhou,
China), a KRUSS DSA 25 drop shape analyzer (Hamburg,
Germany), an Avatar 300 FTIR Fourier infrared spectros-
copy analyzer (Massachusetts, USA), and a RE-52AA rotary
evaporator (Shanghai, China) were used.
Intermediate Product Synthesis
Dichloromethane (50 mL) was added as a solvent in a bea-
ker. Chloroacetyl chloride (14.1 g) was dissolved in
dichloromethane, and 1,3-propanediamine (3.7 g) was dis-
solved in dichloromethane. The mixture was slowly
dropped into the beaker at a rate of 5 s per drop under an
ice bath and stirred. After the dropwise addition was com-
pleted, the temperature was raised and the mixture was
reacted in a water bath at 35ꢁC for 5 h. The mixture was
then evaporated under reduced pressure for 1.5 h in a water
bath at 60ꢁC using a rotary evaporator to remove the sol-
vent and excess chloroacetyl chloride. The residue was dis-
solved in deionized water at 60ꢁC, allowed to stand at
room temperature for 8 h, filtered, and the crystals were
dried to give white crystal intermediate 4.9 g, 1,3-bis
(2-chloroacetamide) propylamine.
Table 1 Chemical composition of 2024 Al-Cu-Mg alloy (mass, %)
Al
Mg
Cu
Mn
Cr
Zn
bal.
1.2–1.8
3.8–4.9
0.3–1.0
0.1
0.25
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Final Product Synthesis
corrosion rate (V) and corrosion IE (%) of the 2024 alumi-
num alloy were obtained by the following equations
(Du et al., 2018):
1,3-bis(2-chloroacetamide)propylamine(4.9 g) was dis-
solved in ethyl acetate (50 mL), and dodecyldimethylamine
(13.3 g) was added to it, and the reaction mixture was stir-
red at room temperature for 16 h. Then, it was distilled
under reduced pressure for 1.5 h in a water bath at 75ꢁC to
remove the solvent and excess dodecyldimethyl tertiary
amine, the residue was dissolved in n-hexane, filtered, and
the filtrate was dried. The residue was recrystallized twice
from ethyl acetate, taken out and dried to dryness. 9.3 g of
a white wax was obtained: bis[2-amido-3-(dodecyldimethy-
lammonium)propyl]-propylamine dichloride. The synthetic
route is shown in Fig. 1.
m0 −m
V ¼
ð1Þ
S × t
V0 −V
IE ¼
× 100%
ð2Þ
V0
where m0/m is the mass before and after corrosion of the
test piece (g); S is the area of the test piece soaking (cm2);
t is the corrosion time (h). V0/V is the average corrosion rate
(gꢀcm −2 ꢀ h −1) of the test piece when the corrosion inhibi-
tor is added/not added.
Electrochemical Test
Calculation of Corrosion Inhibition Efficiency
A three-electrode system was used: a 2024 aluminum alloy
test piece as a working electrode; a graphite electrode as an
auxiliary electrode; and silver-silver chloride as a reference
electrode. A 20-min open-circuit potential test was first per-
formed to stabilize the circuit. The scanning frequency
ranges from 10 mHz to 100 kHz with an amplitude of
10 mV. The corrosion IE is calculated using the following
formula (Al-Amiery et al., 2014):
The size of the 2024 aluminum alloy test pieces was
50 mm × 25 mm × 2 mm. The test pieces were polished
with 400#, 800#, and 1200# sandpaper sequentially. The
polished pieces were rinsed sequentially for 2 min with
distilled water, acetone, and absolute ethanol and again
with distilled water before blown dry with cold air and
weighed. The corrosive solution medium was 1 mol L−1
hydrochloric acid solution. The aluminum alloy test piece
was statically suspended vertically in the etching solution
for 4 h. The test pieces were then taken out, and the corro-
sion product was removed by ultrasonic cleaning in deio-
nized water for 20 min. The pieces were then washed with
acetone and absolute ethanol, dried, and weighed. The
mass before and after corrosion is accurate to 0.1 mg. The
Rp −R’p
IE ¼
× 100%
ð3Þ
Rp
I0 −I
I0
I0 −I
I0
where Rp ¼
× 100%, R0p ¼
× 100% is the charge
transfer resistance (Ωꢀcm2) before and after the addition of
the corrosion inhibitor, respectively.
Fig. 1 Synthetic route for Bis[2-amido-3-(dodecyldimethylammonium)propyl]-propylamine dichloride
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The potentiodynamic polarization curve was plotted K. The relationship between K and the adsorption free
energy ΔG0ads (kJ mol−1) is as follows (Du et al., 2018):
using values obtained by a potentiodynamic polarization
scan with a scan range of ꢂ500 mV and at a scan rate of
0.166 mV s−1. The corrosion IE is calculated using the fol-
lowing formula (Al-Amiery et al., 2014):
ꢀ
ꢁ
1
−ΔG0ads
K ¼
exp
:
ð7Þ
55:5
RT
I0 −I
IE ¼
× 100%
ð4Þ
I0
where I0 and I are the corrosion current density (Aꢀcm−2
)
Results and Discussion
before and after the addition of the corrosion inhibitor.
Structural Characterization of New Corrosion
Inhibitors
Isotherm Adsorption Model
The infrared spectrum of the final product is shown in
Fig. 2. The peak of vibration of the N H bond in the amide
group can be seen at 3385 and 3203 cm−1, the peak of
vibration of the C O bond in the amide group can be seen
at 1662 cm−1, and the peak of vibration of the C N bond
can be seen at 1270 cm−1 (Bartels, 1978). The two strong
The Langmuir adsorption model expression is given in
Eq. (5) (Du et al., 2018):
C
1
¼ C +
ð5Þ
θ
K
where (L mol−1) is the adsorption equilibrium constant,
C is the corrosion inhibitor concentration (mol L−1), and θ absorption peaks at 2920 and 2852 cm−1 are characteristic
is the surface coverage that can be calculated by formula 6:
vibration peaks of long-chain carbon. The characteristic
vibration peak of C H is at 1466 cm−1. The characteristic
peaks of C C, C N, and C Cl between 1050–1200 cm−1
and 500–700 cm−1 (Ge et al., 2010) indicate that the syn-
thesized product is the target product.
IE
θ ¼
ð6Þ
IEm
where IE has a corrosion IE (%) for this corrosion inhibitor
concentration; IEm is the maximum corrosion IE (%) within
the selected concentration range.
1
The H NMR test result of the synthesized product is
shown in Fig. 3. Specific analysis is as follows: 1H NMR:
δ
0.700–0.712 (t,6H,(CH3)2), 1.129–1.193 (m,40H,
Adsorption Thermodynamic Constant
(CH3(CH2)9)2), 1.561–1.624 (m,12H,(N+(CH3)2)2),
1.891–1.903 (m, 4H,(N + CH2)2), 2.931–2.956 (m,1H,
The reciprocal of the intercept of the Langmuir adsorption (COCH2)2), 3.453 (s,2H,(NH)2), 3.075–3.114 (m, 4H,
model fitting curve is the adsorption equilibrium constant (CH2)3), and 3.962–3.985 (d,2H, (CH2)3).
100
80
60
40
20
0
4000
3500
3000
2500
2000
1500
1000
Wavenumbers(cm-1)
Fig. 2 Infrared spectrum of the synthesized product
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10.0
5.0
0.0
ppm (f1)
Fig. 3 1H NMR spectra of the synthesized product
By analyzing the peak positions of relative hydrogen
atoms and the relative areas of absorption peaks, it can be
concluded again that the synthesized product is the target
product.
60
55
50
45
40
35
30
25
20
15
10
Surface Tension Measurements
The surface tension of the solution depends primarily on
the concentration of the surfactant monomer in the solution.
The surface tension of different concentrations of the
synthesized product at room temperature was tested using
the hanging drop method. The surface tension
(γ)-concentration log (logC) curve of the synthesized prod-
uct at an ambient temperature of 25ꢁC is shown in Fig. 4.
When the surfactant concentration is low, the surface ten-
sion of the solution decreases sharply with increasing con-
centration, and after reaching the CMC, the surface tension
of the solution slowly decreases or becomes substantially
constant. The surface tension values in the two states were
linearly fitted, and the obtained inflection point was approx-
imately the CMC of the surfactant (Tiangong and Zheng-
gang, 2001). The CMC of the synthesized product is
calculated by the intersection of two straight lines, and the
cmc
–3.6
–3.4
–3.2
–3.0
–2.8
–2.6
logC (mol L–1)
Fig. 4 Surface tension (γ)-logarithm of the concentration (log C)
curve of new corrosion inhibitors
thereby greatly increasing the number of hydrophobic car-
bon atoms, reducing the CMC. At the same time, the elec-
trostatic repulsion between the hydrophilic groups of
ordinary single-chain surfactants causes them to be loosely
arranged in the adsorption layer on the surface of the solu-
tion, affecting the surface activity; and the hydrophilic
groups of the synthesized products are linked with the link-
ing groups through chemical bonds in the solution. The sur-
face adsorption layer is closely arranged, and the
hydrophobic groups are vertically arranged perpendicular
to the gas/liquid interface, thereby improving surface activ-
ity and being more likely to aggregate to form micelles.
value is about 7.767 × 10−4 mol L−1
.
Kim et al. (2000) proposed that the logarithm of the qua-
ternary ammonium salt surfactant decreases linearly with
the increase of the number of hydrophobic carbon atoms.
Compared with the common single-chain quaternary
ammonium salt surfactant, the synthesized product bimo-
lecular surfactant utilizes a linking group to link two
ammonium cations substituted by long-chain carbons,
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Corrosion Inhibition Performance Study
product is further increased, the corrosion rate and the cor-
rosion IE remain relatively constant.
The effect of different concentrations of synthesized prod-
ucts on the corrosion rate (V) and corrosion IE of 2024 alu-
minum alloy in an acidic medium at 25ꢁC is shown in
Fig. 5. The static weight loss test results are listed in
Table 2. As the concentration of the synthesized product
increases, the corrosion rate of the 2024 aluminum alloy
gradually decreases and the corrosion IE gradually
increases. The corrosion IE is maintained between 80% and
90% over the surfactant concentration range 5 × 10−4 mol
L−1 to 1.75 × 10−3 mol L−1. The corrosion rate decreases
before the concentration reaches the CMC. The reason may
be that the hydrophilic group quaternary ammonium cation
in the surfactant is adsorbed on the surface of the 2024 alu-
minum alloy when the concentration of the synthesized
product is low. The long carbon chain hydrophobic groups
are aligned, forming a protective film on the surface of the
alloy, thereby inhibiting corrosion; when the concentration
of the surfactant reaches the CMC, the adhesion of the sur-
factant molecules on the surface of the alloy is substantially
saturated. Therefore, as the concentration of the synthesized
We also used sodium dodecylbenzene sulfonate (SDBS)
as a comparison. The results of the weight loss test of
SDBS on 2024 Al-Cu-Mg alloy in 1 mol L−1 HCl solution
are given in Fig. 6 and Table 3. When the concentration of
0.010
60
0.009
V
IE
0.008
0.007
0.006
0.005
40
20
0
0
2
4
6
8
10
12
14
16
18
20
C (10-4mol/L)
Fig. 6 Static weight loss curve of 2024 Al-Cu-Mg alloy in 1 Mol L−1
HCl solution with different concentrations of the SDBS
100
Table 3 Test parameters obtained using the static weight loss method
0.0040
of SDBS
0.0035
80
C (10−4 molꢀL−1
)
Δm (g)
V (gꢀmm−2ꢀh−1
)
IE (%)
0.0030
1
1.1123
1.0032
0.9444
0.7564
0.5583
0.5841
0.7032
9.9313 × 10−5
8.9571 × 10−5
8.4321 × 10−5
6.7536 × 10−5
4.9848 × 10−5
5.2152 × 10−5
6.2786 × 10−5
22.5
30.1
34.2
47.3
61.1
59.3
51.0
V
IE
60
40
20
0
0.0025
0.0020
0.0015
0.0010
0.0005
0.0000
3
6
9
12
15
18
0
2
4
6
8
10
12
14
16
18
C(10–4 mol L–1)
0.0
–0.2
–0.4
–0.6
–0.8
–1.0
Fig. 5 Static weight loss curve of 2024 Al-Cu-Mg alloy in 1 Mol L−1
HCl solution with different concentrations of the new corrosion
inhibitor
Table 2 Test parameters obtained using the static weight loss method
C (10−4 molꢀL−1
)
Δm (g)
V (gꢀcm−2ꢀh−1
)
IE (%)
Blank
2.5·10-4mol L–1
0
0.4309
0.1448
0.0671
0.0612
0.0520
0.0645
0.0748
0.0583
3.847 × 10−3
1.293 × 10−3
5.99 × 10−4
5.46 × 10−4
4.64 × 10−4
5.76 × 10−4
6.68 × 10−4
5.21 × 10−4
0
1.0·10-3mol L–1
7.5·10-4mol L–1
5.0·10-4mol L–1
2.5
5
66.390
84.427
85.243
87.943
84.667
82.641
86.491
7.5
10
–1.2
–6
–5
–4
–3
–2
–1
0
-2
12.5
15
LogI/A·cm
Fig. 7 Tafel polarization curves with different concentrations of the
new corrosion inhibitor added
17.5
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Table 4 Test parameters obtained using Tafel polarization curves
C (10−4 molꢀL−1
)
ba
bc
Icorr
Ecorr
(mV)
IE
(%)
(mVꢀdec−1
)
(mVꢀdec−1
)
(10−4Aꢀcm−2
)
0
62.31
78.62
97.28
88.73
100.26
125.05
112.74
134.17
119.52
108.69
15.1705
5.9620
2.6700
2.1997
0.7130
−601
−582
−588
−583
−579
/
2.5
5
60.7
82.4
85.5
95.3
7.5
10
change much, the anodic polarization curve moves
toward the low current direction, and the corrosion cur-
rent gradually decreases. This indicates that the anodic
dissolution of the metal is inhibited, thereby achieving
the purpose of corrosion inhibition, so that the synthe-
sized product is presumed to be an anode-type corrosion
inhibitor.
It can be seen from Table 4 that as the concentration of
the synthesized product increases, the corrosion current
Icorr gradually decreases. When the concentration is
1 × 10−3 mol L−1, the corrosion current is reduced by an
order of magnitude compared with the blank control
group, reaching 1.058 × 10−6 A cm−2, and the IE
increases monotonously, reaching 95.3%. It can be seen
that it has a good inhibitory effect on the corrosion of alu-
minum alloy.
200
180
160
140
120
100
80
Blank
2.5·10-4mol L–1
5.0·10-4mol L–1
7.5·10-4mol L–1
1.0·10-3mol L–1
10HZ
100HZ
1000HZ
60
40
20
0
0
20
40
60
80
100
120
140
160
180
200
Zreal (Ω.cm–2)
Fig. 8 Nyquist diagrams with different concentrations of the new cor-
rosion inhibitor added
SDBS is 1 × 10−4 to 1.2 × 10−3 mol L−1, the corrosion
rate of 2024 Al-Cu-Mg alloy decreases monotonously,
while the IE of SDBS increases monotonously. When the
concentration reaches 1.2 × 10−3 mol L−1, the corrosion IE
is the highest, reaching 61.1%. When the concentration of
the corrosion inhibitor is further increased, the corrosion IE
decreases. The presumed reason may be that 1.2 × 10−3 mol
L−1 is close to the CMC of SDBS. When the concentration
is lower than the CMC, SDBS as an anionic corrosion
inhibitor will adsorb on the metal surface. When the con-
centration is higher than the CMC, the adsorption of SDBS
molecules on the metal surface is saturated, so the corro-
sion IE cannot continue to increase.
Electrochemical Impedance Spectroscopy Test
The Nyquist plot results for the addition of different con-
centrations of the synthesized product are shown in Fig. 8.
The blank solution and the added synthesized product
exhibit a capacitive antiarc. The high-frequency capaci-
tive antiarc is related to the electric double layer capaci-
tance and the transfer resistance (Li et al., 2003).
Through observation, the arcs are oblate, which may be
caused by the rough surface of the aluminum alloy during
corrosion.
It can be seen from Table 5 that as the concentration of
the Gemini surfactant increases, Rp increases, and reaches a
maximum at a concentration of 1 × 10−3 mol L−1. When
the concentration of the synthesized product reaches
1 × 10−3 mol L−1, the anticorrosion effect of the system
becomes significant. Y0 and the charge transfer resistance
Rct basically show opposite trends. When the concentration
of the synthesized product is 1 × 10−3 mol L−1, the IE is
the highest, reaching 94.3%.
By comparing the two results, we found that the corro-
sion rate of the Gemini surfactant is lower than that of
SDBS, and the IE is greater. The synthesized product has
better corrosion inhibition properties than SDBS.
Dynamic Potential Polarization Curve Test
Figure 7 shows the Tafel curves for different concentra-
tions of the Gemini surfactant in an acidic medium at
room temperature. The relevant experimental parameters
are shown in Table 4. As the concentration of the product
increases, the cathodic polarization curve does not
The equivalent circuit diagram is shown in Fig. 9. Rs is
the solution resistance (Ωꢀcm2), which represents the
impedance of the solution electrolyte between the counter
electrode and the working electrode; Rp is the charge trans-
fer resistance (Ωꢀcm2). The capacitive reactance arc is a
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Table 5 Fitting parameters of the equivalent circuit
C
Rs
Rp
Y0
ndl
IE
(%)
(10−4 molꢀL−1
)
(Ωꢀcm2)
(Ωꢀcm2)
(10−4 Ω−1ꢀcm−2ꢀs−1
)
0
5.799
1.261
7.212
5.696
1.864
4.8912
11.7568
30.895
70.654
85.393
49.66
8.64
0.62
/
2.5
5
0.791
0.697
0.89
58.4
84.2
93.1
94.3
17.02
4.63
7.5
10
2.12
0.88
Analysis of Corrosion Products
To further study the corrosion inhibition mechanism, the
corrosion products on the aluminum alloy surface were
determined using infrared spectroscopy. It can be seen from
Fig. 11 that the characteristic vibration peak of AlCl3
occurs at 1626 cm−1 (Shahrabi et al., 2008). The character-
istic vibration peaks of Al2O3 and MgO occur at 1397 and
946 cm−1, respectively (Liu, 2015). The characteristic peak
of Cu2O occurs at 618 cm−1 (Shahrabi et al., 2008). The
comparison shows that there is no obvious characteristic
peak change between the two, indicating that the Gemini
surfactant is physically adsorbed on the surface of the alu-
minum alloy.
Fig. 9 Equivalent circuit of Nyquist diagrams
flattened semicircle, so a constant phase angle element
(CPE) representing an electric double layer capacitor is
added to fit it. The parameters are Y0 (Ω−1ꢀcm−2ꢀs−1) and
ndl, which represent the CPE constant and the CPE index,
respectively.
Isothermal Adsorption Model
Surface Topography Observation
Figure 12 shows the Langmuir fit curve of the synthesized
product in an acidic medium at room temperature. The
slope of the fitted curve is 1.0087, indicating that the
adsorption model of the synthesized product on the surface
of the 2024 aluminum alloy satisfies the Langmuir adsorp-
tion model when the concentration of the synthesized prod-
uct is lower than the CMC. The synthesized product forms
a monomolecular film on the surface of the 2024 aluminum
alloy, which effectively blocks the corrosion ions of Cl−
from approaching the metal surface and prevents further
corrosion of the aluminum alloy.
Figure 10 shows SEM micrographs of 2024 aluminum
alloy in an acidic medium. When the synthesized product is
not added, the surface of 2024 aluminum alloy is very
rough, and there are a lot of corrosion pits. After adding the
synthesized product, the surface of 2024 aluminum alloy is
relatively flat, and there are no large corrosion pits indicat-
ing that the synthesized product can form a protective film
on the surface of 2024 aluminum alloy, which inhibits
corrosion.
Fig. 10 Corrosion surface morphology of 2024 Al-Cu-Mg alloy: (a) blank sample; (b) 1 Mol L−1 HCl; and (c) 1 Mol L−1 HCl + the new corro-
sion inhibitor
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100
Blank
The new inhibitor
80
Fig. 13 Adsorption of the new corrosion inhibitor on the surface of
2024 Al-Mg-Cu alloy
100
IE
95
60
4000
3500
3000
2500
2000
1500
1000
500
90
85
80
75
70
65
60
55
50
Wavenumbers (cm-1)
Fig. 11 FTIR plots of corrosion products on the metal surface in the
absence/presence of the new corrosion inhibitor
8
y = 0.9055·x+0.8685
R2=0.9958
7
6
5
4
3
2
1
0
0
1
2
C(θ)
Fig. 14 IE-C/CMC curve of 2024 Al-Cu-Mg alloy
the concentration of the synthesized product is less than
the CMC. When the concentration of the synthesized
product is close to the CMC, there is an inflection point.
When the concentration of the synthesized product is
greater than the CMC, the tendency of the corrosion IE to
increase with increasing concentration slows down and
remains substantially constant. It can be inferred that
when the concentration of the synthesized product is
higher than the CMC, the adsorption mode on the surface
of the aluminum alloy may change. The reason may be
that the adsorption site on the surface of the aluminum
alloy is saturated, and the adsorption form of the new cor-
rosion inhibitor changes, which no longer conforms to the
Langmuir adsorption model.
0
1
2
3
4
5
6
7
8
C (10-4mol L–1)
Fig. 12 Langmuir fitting curve of the new corrosion inhibitor
Using the Langmuir fitting curve, K is determined to be
1.1514 × 104 L mol−1, and the adsorption free energy of
the synthesized product is determined to be −33.12 kJ
mol−1. ΔGa0ds < 0 indicates that the adsorption process is
ꢂ
ꢂ
ꢂ
ꢂ
spontaneous. Because ΔGads < 40 kJ mol−1, the process
was judged to be physical adsorption (Lin et al., 2017). The
adsorption mode of the synthesized product on the surface
of the 2024 aluminum alloy is more inclined to physical
adsorption, that is, the quaternary ammonium salt cationic
hydrophilic group is attracted to the negative surface of the
metal surface by electrostatic interaction, thereby adsorbing
on the surface of the 2024 aluminum alloy. The adsorption
diagram is shown in Fig. 13.
0
Corrosion Inhibition Mechanism
The adsorption process of the new corrosion inhibitor on
the surface of the aluminum alloy is shown in Fig. 15. First,
the number of adsorption sites on the surface of the alumi-
num alloy is fixed. When the concentration of the synthe-
sized product is low, the two quaternary ammonium groups
(hydrophilic groups) of the synthesized product are
adsorbed on the surface of the aluminum alloy. This means
The corrosion IE values obtained were fitted to the con-
centration curve, and the results are shown in Fig. 14. The
IE increases slowly with increasing concentration when
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Fig. 15 Adsorption principle of the new corrosion inhibitor on 2024 Al-Cu-Mg alloy surface.
that each quaternary ammonium salt group will occupy an
adsorption site (Fig. 15a). As the concentration of the new
corrosion inhibitor increases and the number of adsorption
sites is fixed, one of the quaternary ammonium salt groups
may be adsorbed on the surface of the aluminum alloy, and
the other may not be adsorbed (Fig. 15b).
In the actual situation, the adsorption mode of the metal
surface tends to coexist in two adsorption modes. When the
concentration of the new corrosion inhibitor is above the
CMC, the hydrophilic groups of the new corrosion inhibitor
will occupy each adsorption site on the surface of the alumi-
num alloy. When the concentration is continuously
increased, the long carbon chain of the two molecules forms
a central hydrophobic group, and the hydrophilic group has
one end on the surface of the aluminum alloy and the other
end faces the double-layer adsorption film of the solution
medium, as shown in Fig. 15c. At this time, the adsorption
form no longer conforms to the Langmuir adsorption model,
so in Fig. 14, the curve has an inflection point.
increase, the corrosion IE did not change substantially, and
was maintained between 84% and 88%.
The Langmuir fitting curve of the synthesized product in
an acidic medium at room temperature indicates that the
adsorption of the synthesized product on the surface of 2024
aluminum alloy satisfies the Langmuir adsorption model when
the concentration is lower than the CMC. The adsorption free
energy of the synthesized product is calculated to be −33.12 kJ
mol−1, at this value, the adsorption is spontaneous and the
adsorption mode is more inclined to physical adsorption.
The new corrosion inhibitor is a mixed corrosion inhibitor.
The inhibition mechanism in solution is: when the concen-
tration of the synthesized product is lower than the CMC,
it satisfies the Langmuir adsorption model, i.e., monolayer
adsorption. When the concentration is higher than the
CMC, the adsorption site of the metal surface is saturated,
and the adsorption of the synthesized product no longer
conforms to the Langmuir adsorption model.
Acknowledgements This work was supported by supporting funds
of National Natural Science Fund (NSFC, Grant No.11502285) and
Basic Research Business Expenses of Central University Specialized
Project of Civil Aviation University of China (3122018z006).
Conclusions
Conflict of Interest The authors declare that they have no conflict
of interest.
The static hanging weight loss method, the potentiody-
namic polarization curve test, the and electrochemical
impedance spectroscopy test results show that the synthe-
sized Gemini surfactant has a good corrosion inhibition
effect on the 2024 aluminum alloy in an acidic environ-
ment. When the concentration is 1.0 × 10−3 mol L−1, the
average corrosion IE reaches a maximum of 87.9%. When
the concentration of the synthesized product continued to
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