Indolizine quaternary ammonium salt inhibitors, part III: Insights into the highly effective low-toxicity acid corrosion inhibitor-synthesis and protection performance

Article abstract of DOI:10.1039/c9nj04203b

Three indolizine derivatives (Di-BQC, QM-DiBQC, and PyM-DiBQC), which were prepared facilely in high yield via a 1,3-dipolar cycloaddition reaction, were found to exhibit good corrosion inhibition for steel in concentrated acid without the synergism of propargyl alcohol (PA). The inhibitive derivatives exhibit high protection efficiency and have eco-friendly advantages over the highly toxic PA. The accurate formulas and structures of the three compounds are characterized by high resolution mass spectrometry (HRMS) and nuclear magnetic resonance (NMR) spectroscopy. The corrosion inhibition performance of the indolizine derivatives toward N80 steel was investigated in 15 wt% HCl and 20 wt% HCl by weight loss measurements, electrochemical tests (Tafel and EIS), SEM/EDX analysis and theoretical calculations. The biological toxicity was investigated by using the Microtox toxicity test. At 90 °C, a dosage of 0.1 wt% indolizine derivatives in 15 wt% HCl would decrease the corrosion rate of N80 steel dramatically to less than 10 g m^-2 h^-1. While for PA, a much higher corrosion rate was observed under the same conditions, indicating that the indolizine derivatives are more effective inhibitors in contrast with PA. Results from gravimetric analysis as well as the electrochemical studies and DFT methods are in good agreement, verifying the fine corrosion prevention of the three compounds. The results of biotoxicity tests confirmed the relatively low-toxicity properties of the indolizine derivatives and the suggested inhibition mechanism was also discussed. All the conclusions above indicate that the new indolizine derivatives could be presented as highly effective acidizing inhibitors and the derivatives may offer a new enlightening strategy for corrosion protection under acidizing conditions.

Full text of DOI:10.1039/c9nj04203b

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Indolizine quaternary ammonium salt inhibitors,
part III:† insights into the highly effective low-
toxicity acid corrosion inhibitor – synthesis and
protection performance
Cite this: New J. Chem., 2019,
43, 18461
a
Zhen Yang,
Cheng Qian,
Yefei Wang,*a Fengtao Zhan,b Wuhua Chen,a Mingchen Ding,a
a
Renzhuo Wanga and Baofeng Houc
Three indolizine derivatives (Di-BQC, QM-DiBQC, and PyM-DiBQC), which were prepared facilely in high
yield via a 1,3-dipolar cycloaddition reaction, were found to exhibit good corrosion inhibition for steel in
concentrated acid without the synergism of propargyl alcohol (PA). The inhibitive derivatives exhibit high
protection efficiency and have eco-friendly advantages over the highly toxic PA. The accurate formulas
and structures of the three compounds are characterized by high resolution mass spectrometry (HRMS)
and nuclear magnetic resonance (NMR) spectroscopy. The corrosion inhibition performance of the
indolizine derivatives toward N80 steel was investigated in 15 wt% HCl and 20 wt% HCl by weight loss
measurements, electrochemical tests (Tafel and EIS), SEM/EDX analysis and theoretical calculations. The
biological toxicity was investigated by using the Microtox toxicity test. At 90 1C, a dosage of 0.1 wt%
indolizine derivatives in 15 wt% HCl would decrease the corrosion rate of N80 steel dramatically to less
than 10 g mꢀ2 ꢀ1. While for PA, a much higher corrosion rate was observed under the same conditions,
h
indicating that the indolizine derivatives are more effective inhibitors in contrast with PA. Results from
gravimetric analysis as well as the electrochemical studies and DFT methods are in good agreement,
verifying the fine corrosion prevention of the three compounds. The results of biotoxicity tests confirmed
the relatively low-toxicity properties of the indolizine derivatives and the suggested inhibition mechanism
was also discussed. All the conclusions above indicate that the new indolizine derivatives could be
presented as highly effective acidizing inhibitors and the derivatives may offer a new enlightening strategy
for corrosion protection under acidizing conditions.
Received 13th August 2019,
Accepted 23rd October 2019
DOI: 10.1039/c9nj04203b
rsc.li/njc
As a result, new effective corrosion inhibitors in strong acid
media (especially during acidizing) are in great demand in the
1. Introduction
Considering that acid corrosion problems represent a signifi- crude oil and gas industry. A variety of organic corrosion
cant cost burden in petroleum engineering, the prevention of inhibitors that contain heteroatoms (N, O, P, and S) or aromatic
mild corrosion in acid media is worth investigating. The choice unsaturated functional moieties have been developed to retard the
of an appropriate corrosion control method can help to avoid corrosion of tubulars, casing, and downhole metal components.
the problems of concentrated acid corrosion.1–4 For this reason, Among numerous corrosion inhibitors, quaternary ammonium
corrosion inhibitors are generally used in acid blends to reduce salts have been investigated and used for decades.7–9 Quaternary
corrosion on metallic surfaces when concentrated acid is heterocyclic ammonium compounds such as N-alkyl quinolinium
pumped downhole for damage removal, stimulation treatment, salts,10 N-alkyl pyridinium salts,11 and polymeric quinolinium
and other clean-up operations.5,6
compounds12 have been studied for their protective efficacy.13,14
It has been widely accepted that benzyl quinolinium chloride
(BQC) shows good inhibitive performance toward N80 steel under
acidizing conditions.2,15,16 Many other nitrogen or heteroatom-
containing heterocyclics such as Mannich bases, furfuryl alcohol,
fatty acid triazoles, and benzimidazole have been used to protect
steel from severe corrosion.2 Propargyl alcohol (PA), although
producing toxic vapours during the acidizing process, is widely
used as a highly effective synergistic corrosion inhibitor.
a College of Petroleum Engineering, China University of Petroleum (East China),
66 West Changjiang Rd, Qingdao, Shandong Province, 266580, P. R. China.
E-mail: wangyf@upc.edu.cn; Tel: +86-532-86981709
b College of Science, China University of Petroleum (East China),
66 West Changjiang Rd, Qingdao, Shandong Province, 266580, P. R. China
c School of Petroleum Engineering, Yangtze University, 111 Daxue Rd, Wuhan,
Hubei Province, 430100, P. R. China
† Part II, see ref. 1: New J. Chem. 2018, 42, 12977–12989.
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Since PA is relatively expensive and of high toxicity, this
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excellent inhibitor usually acts as a key synergistic component
in inhibitors and is not recommended to be used in prohibited
areas.8,17 All these characteristics of PA and the other high-
toxicity inhibitors call for the constant exploration of novel
highly efficient, environmentally friendly inhibitive compounds
that might replace the currently used poisonous components.
The objective of this work is to introduce a new method and
mechanism for synthesizing the highly effective corrosion
inhibitor that has been reported previously.15 The other two
Fig. 1 The chemical structure of indolizine.
The highlighted nitrogen containing heterocyclic parts in
inhibitive indolizine derivatives were also obtained through a Fig. 1 and 2 should be assigned to the indolizine structure.19,20
similar reaction at a relatively high yield. All the indolizine Considering that the chemical structures of the three compounds
derivatives in this paper were prepared from BQC (the com- contain an indolizine derived functional group, they can be
monly used quaternary heterocyclic ammonium salt inhibitor classified as indolizine derivatives, or more specifically, defined
for acidizing) and chemical structures of the derivatives were as ‘indolizine quaternary ammonium salts’.1
characterized. Compared with PA, the effectiveness and low-
The chemical structure, molecular weight, and abbreviated
toxic property of the indolizine derivatives as fine corrosion nomenclature of the compounds of interest in this study are
inhibitors for N80 steel in HCl are discussed and confirmed by listed in Table 2. HRMS (electrospray ionization, positive ion
the experimental results.
mode) and NMR (Agilent NMR-400 MHz) were used to determine
the mass of the molecular ion of the synthesized compounds.
Quaternary quinoline salt BQC was obtained easily by the
method shown in Fig. 2. Quinoline and 1.0 equiv. benzyl
chloride were mixed with distilled water in a flask. The reaction
mixture was stirred and heated to 90 1C for 6 hours. After
cooling, the solid product was washed in acetone and recrystallized
three times from ethanol. The purified benzyl quinolinium
chloride (BQC) was obtained as a white crystalline solid. Yield:
2. Experimental section
2.1 Materials
The fundamental raw materials used in the synthesis of the
indolizine derivatives in this study are listed as follows: quino-
line, pyridine, benzyl chloride, chloroacetic acid and Na2CO3.
These chemical reagents, solvent and PA were obtained from
Adamas-betas and used without further purification. API N80
steel was used as the test material: the elemental composition
of the N80 steel is given in Table 1.
N80 steel coupons measuring 50 mm ꢁ 10 mm ꢁ 3 mm were
used for weight loss analysis. While under electrochemical test
conditions, similar coupons with 1 cm2 exposed surface patches
were used as working electrodes. The exposed area of the coupons
was abraded with increasing grades of silicon carbide abrasive
papers and then cleaned with anhydrous ethanol. Before testing,
all coupons were kept and dried at room temperature for at
least 24 h.
1
89%. H NMR (400 MHz, D2O): d 9.18 (d, J = 7.5 Hz, 1H), 8.97
(d, J = 8.8 Hz, 1H), 8.15 (dd, J = 5.6, 4.5 Hz, 2H), 7.92 (m, 2H), 7.72
(t, J = 9.6 Hz, 1H), 7.24 (m, 3H), 7.16 (m, 2H), 6.06 (s, 2H).
13C NMR (101 MHz, CDCl3): d 148.88, 148.32, 137.98, 135.88
132.68, 130.69, 130.18, 129.94, 129.28, 129.17, 127.27, 121.58,
118.51, 60.75. HRMS: calcd for [C16H14N]+: 220.2891; found:
220.2890.
As illustrated in Fig. 2, for the synthesis of Di-BQC: firstly,
BQC and 1.0 equiv. Na2CO3 were dissolved in ethanol as the
solvent and heated at 60 1C for 10 hours, then the pigment-like
mixture containing Di-BQC was obtained. The mixture was recrys-
tallised in acetone/chloroform (1 : 3, v/v) twice and Di-BQC was
purified as a yellow solid. Yield: 33%. 1H NMR (400 MHz, CDCl3):
d 12.26 (s, 1H), 9.38 (d, J = 13.7 Hz, 1H), 8.00 (m, 2H), 7.90 (d, J =
8.6 Hz, 1H), 7.77 (m, 2H), 7.65 (m, 2H), 7.60–7.20 (m, 12H), 6.35
(s, 2H). 13C NMR (101 MHz, CDCl3): d 149.93, 135.53, 134.31,
133.82, 133.59, 133.38, 131.46, 130.98, 130.79, 130.06, 129.31,
129.24, 129.10, 129.05, 128.43, 127.91, 127.48, 126.97, 126.78,
125.12, 124.81, 122.67, 119.82, 119.38, 118.24, 118.03, 109.76,
59.58. HRMS: calcd for [C32H23N2]+: 435.1854; found: 435.1854.
For the synthesis of QM-DiBQC: BQC and 1.0 equiv. acetoxyl
quinolinium chloride (AcQC, obtained by quaternization of
quinoline and chloroacetic acid in advance) were combined
and heated to 80 1C in acetonitrile as the solvent for 5 hours.
The indolizine derivative QM-DiBQC was formed through an
intermolecular 1,3-dipolar cycloaddition reaction. The product
was recrystallised in ethanol/acetone (1 : 1, v/v) twice and finally
QM-DiBQC was purified as a bright yellow powder. Yield: 49%.
Hydrochloric acid, at concentrations of 4.407 M (approx.
15 wt%) and 6.016 M (approx. 20 wt%), was prepared from stock
concentrated HCl (12.08 M) obtained from Sigma Aldrichs by
diluting with double-distilled water.
2.2 Synthesis and characterization
Fig. 1 shows the chemical structure of the conventional indo-
lizine. The preparation of the three indolizine derivatives
through 1,3-dipolar cycloaddition reaction is illustrated in
Fig. 2. BQC was prepared easily according to the literature15
and the synthesis of DiBQC, QM-DiBQC, and PyM-DiBQC was
performed by reference to published methods.18,19
Table 1 Chemical composition of the N80 steel coupon
Element
C
P
Si
S
Mn Cr
V
Al
Fe
In wt% 0.34 0.02 0.25 0.015 1.6 0.15 0.11 0.02 Balance 1H NMR (400 MHz, DMSO-d6): d 10.57 (s, 1H), 9.85 (s, 1H), 8.84
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Fig. 2 Synthesis of the indolizine derivatives Di-BQC, QM-DiBQC, and PyM-DiBQC from quaternary quinolinium BQC salts.
Table 2 Name, structure, and molecular weight of the corrosion inhibitors
Structure
Molecular weight
56.05
Name (Abbr.)
Propargyl alcohol (PA)
Benzyl quinoline chloride (BQC)
255.5
470.6
394.7
344.7
Dimer indolizine derivative of benzyl quinoline chloride (Di-BQC)
Quinolyl-3-methene-dimer indolizine derivative of BQC (QM-DiBQC)
Pyridyl-3-methene-dimer indolizine derivative of BQC (PyM-DiBQC)
(d, J = 8.6 Hz, 1H), 8.71–8.56 (m, 2H), 8.32 (t, J = 10.6, 8.4 Hz, 6.11 (s, 2H). 13C NMR (101 MHz, CDCl3): d 148.87, 135.39, 132.98,
2H), 8.20 (d, J = 8.6 Hz, 1H), 8.08 (t, J = 7.7 Hz, 1H), 7.94–7.83 132.11, 131.60, 130.58, 130.30, 129.27, 128.77, 128.24, 128.15,
(m, 2H), 7.80 (td, J = 7.8, 7.2, 1.6 Hz, 1H), 7.45 (d, J = 7.1 Hz, 2H), 125.88, 125.73, 122.84, 120.16, 117.35, 116.04, 109.94, 109.86,
7.36 (dt, J = 21.7, 7.1 Hz, 3H), 6.14 (s, 2H). 13C NMR (101 MHz, 59.66. HRMS: calcd for [C22H17N2]+: 309.3831; found: 309.3830.
CDCl3): d 148.71, 135.33, 133.77, 133.17, 132.33, 131.62, 130.34,
2.3 Weight loss test
130.27, 129.50, 129.31, 128.88, 128.71, 128.23, 127.30, 126.04,
126.00, 122.91, 122.03, 120.33, 117.71, 116.51, 110.17, 109.79, Weight loss measurements at 90 1C and 120 1C were all conducted
59.42. HRMS: calcd for [C26H19N2]+: 359.1543; found: 359.1545. in a high-temperature, high-pressure apparatus at 1000-psi. The
PyM-DiBQC was synthesised from BQC and acetoxyl pyridi- coupon was immersed in 15 wt% or 20 wt% HCl solutions in the
nium chloride (AcPyC) and the preparation process is similar to absence or presence of a series of concentrations of the five
that of the preparation of QM-DiBQC. Yield: 41%. 1H NMR inhibitors listed in Table 2 for 4 h at 90 1C or 120 1C, respectively.
(400 MHz, DMSO-d6): d 10.22 (s, 1H), 9.67 (s, 1H), 8.58 (d, J = In comparison with the inhibition of the synthesized indolizine
8.7 Hz, 1H), 8.70–8.44 (m, 2H), 8.21 (d, J = 8.9 Hz, 1H), 8.15 derivatives, the synergistic protection performance of the PA mix-
(t, J = 7.6 Hz, 1H), 7.85–7.97 (m, 2H), 7.26 (dt, J = 20.5, 7.3 Hz, 3H), tures with BQC was also investigated in 15 wt% HCl solution at
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90 1C with a dosage ranging from 0.01 wt% to 0.20 wt%. The ratio iR drop. The corrosive medium was kept under quiescent con-
of volume (mL) of corrosive solutions to surface area (cm2) of each ditions throughout the experiment. In each test solution, the
coupon (V/A) was 20 : 1. Before each test, the masses of the coupons three-electrode cell was fixed in the corrosive medium for 3600 s
were determined to ꢂ0.1 mg. Two identical coupons were used in in order to ensure a stable open circuit potential. Both the
each test, and the average corrosion rate thereof was calculated. potentiodynamic polarization and the EIS measurements were
After the test, the coupons were removed, cooled to room tempera- performed at the open circuit potential.
ture, washed using ethanol, dried, and weighed.
The data obtained from the electrochemical test were analysed
The corrosion rate of each coupon was calculated as follows: using the Gamry Instrument Framework version 1.5 software and
all the electrochemical data are presented without iR correction.
m1 ꢀ m2
R ¼
(1)
The electrochemical experiments were done in the presence and
absence of the five inhibitors at the same concentrations as those
in the weight loss tests.
S ꢁ t
where R (g mꢀ2 hꢀ1) is the corrosion rate and m1 (g) and m2 (g)
are the masses of each coupon before and after 4 h immersion.
S (m2) is the exposed surface area, and t (h) is the immersion
time. The corrosion inhibition efficiency (IE) of N80 steel can be
calculated by the following equation:21
The Tafel polarization was carried out from a cathodic
potential of ꢀ250 mV to an anodic potential of +250 mV by
changing the electrode potential versus OCP at a sweep rate of
0.2 mV s .
ꢀ1 25 The corrosion current density (Icorr) and Ecorr were
measured by the Tafel linear extrapolation procedure. The
linear Tafel segments of anodic and cathodic areas were extra-
polated to Ecorr to obtain the value of Icorr. The IE calculated
from the Tafel curves was measured using the following rela-
tionship:
R0 ꢀ Rcorr
IE ð%Þ ¼
ꢁ 100
(2)
R0
where R0 (g mꢀ2
h
ꢀ1) and Rcorr (g mꢀ2
h
ꢀ1) are the corrosion
rates without and with inhibitor, respectively. The mean corro-
sion rate (Rcorr) and the mean inhibition efficiency (IE) are
obtained from weight loss measurements of N80 steel in HCl
with and without various concentrations of the five inhibitors.
The coupons were examined visually for localized corrosion,
observed as pits or edge attack, and the pitting corrosion can be
qualitatively described by assigning a visual pitting index
(PI).16,22 The maximum allowable pitting index during the test
period is PI r 2 (preferably zero), as recommended by internal
procedures. The visual pitting indices ranging from PI = 0
(no pitting) to PI = 9 (severe and extensive pitting) are given
in Table 3.
0
Icorr ꢀ Icorr
IE ð%Þ ¼
ꢁ 100
(3)
Icorr
where Icorr and Icorr0 are the mean values of the uninhibited and
inhibited corrosion current densities, respectively.
The EIS measurements were made at the corrosion potential
(Ecorr) over a frequency range from 100 000 Hz to 0.01 Hz, using
an amplitude signal of 5 mV peak-to-peak and a scan rate of
10 points per decade of frequency.26 The EIS results were
collected and analyzed by ZSimpWin software. The inhibition
efficiencies (IEs) can be calculated from the Nyquist plots by
means of the following equation:
2.4 Electrochemical analysis
0
Rp ꢀ Rp
A Gamry Reference 600 electrochemical workstation was used
to perform the electrochemical test. Both the potentiodynamic
polarization (Tafel curves) and electrochemical impedance
spectroscopy (EIS) tests were carried out in 15 wt% HCl at
25 1C under non-stirred conditions.23,24
To conduct the electrochemical test, N80 steel with a 1 ꢁ 1 cm
exposed surface area was used as the working electrode (WE); a
saturated calomel electrode (SCE, coupled to a fine Luggin
IE ð%Þ ¼
ꢁ 100
(4)
0
Rp
0
where Rp and Rp are the polarization resistance of the steel in
the absence and0presence of the inhibitors, respectively. The
value of Rp or Rp was obtained through the equivalent circuit
model, which was employed to simulate the experimental EIS
results by ZSimpWin software.
capillary) and a platinum electrode were used as the reference 2.5 Surface characterization
electrode and counter electrode (CE), respectively. The Luggin
capillary was kept close to the working electrode to decrease the
For surface morphological investigations of the N80 samples in
the acid solutions with and without the synthesized inhibitors,
scanning electron microscopy and energy dispersive X-ray
spectroscopy analyses (SEM/EDX) were performed by using a
HITACHI S-4800 scanning electron microscope at an accelerat-
ing voltage of 30 kV. The change in steel surface composition
before and after the immersion was investigated by using an
energy dispersive X-ray analyzer (EDAX XM2-60S, HITACHI)
equipped with a scanning electron microscope. The coupons were
pre-treated and immersed in the corrosive medium as per the
same procedure in the weight loss tests. Before the SEM/EDX test,
the coupons were taken out of the corrosive solutions, cleaned
with distilled water and ethanol, and dried with cool air.
Table 3 Definition of different pitting indices (PIs)
Index
Description of pitting
0
1
2
3
4
5
6
7
8
None
Minor cut edge corrosion
No pits on major surfaces. Small shallow pits on cut edges
Pin point pits on surface o25
Pin point pits on surface 425
o10 pits 16–31 mils diameter, 8–16 mils deep
11–25 pits with P.I. of 5
425 pits with P.I. of 5
Large pits 63–126 mils diameter, 431 mils deep
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The micrographs of the corroded N80 samples and the EDX Photobacterium phosphoreum to different concentrations of
spectra of the coupons in the presence and absence of the the five inhibitors at 25 1C.27 The light output of the bacterial
inhibitor were obtained.
suspension before and after its exposure to the sample (lasting
for 15 minutes) is measured by a photomultiplier tube.28,29 The
light emitted by the luminescent bacteria is a product of their
metabolic processes. The light output is proportional to the
toxicity of the sample because the exposure to toxic chemicals
interferes with their metabolic processes and results in a
reduction of the light output. At the endpoint of the test, an
effective concentration for a 50% reduction in light output, that
is, the EC50, is calculated.
2.6 Computational details
2.6.1 Quantum chemical calculations. In this work, density
functional theory (DFT)-based quantum chemical calculations
were performed by the DMol3 module in BOVIA Materials
Studio software (version 8.0, Accelrys, Inc.). Geometrical opti-
mizations and frequency calculations were conducted with the
employed generalized gradient approximation functional (GGA)
with a double numerical basis set with polarization (DNP). The
following quantum chemical indices were taken into consideration:
energy of the highest occupied molecular orbital (EHOMO), energy
of the lowest unoccupied molecular orbital (ELOMO), energy gap:
DE = ELOMO ꢀ EHOMO and dipole moment (m). The HOMO and
LUMO, which reveal the active sites of the studied inhibitive
molecules, were illustrated and discussed.
2.6.2 Molecular dynamics (MD) simulations. The strength
of the interaction between corrosion inhibitors and the Fe(110)
surface was studied by using the Forcite module implemented
in the MS software from BIOVIA Company, USA. The Monte
Carlo simulation methodology used the adsorption locator
module in the MS 8.0 version to model the adsorption process
of the inhibitor molecules onto the Fe(110) surface. The molecular
dynamics processes were performed in a three-dimensional simu-
lation box (22.34 Å ꢁ 22.34 Å ꢁ 38.11 Å) with periodic boundary
conditions. Fe(110) was next enlarged to a (9 ꢁ 9) supercell to
provide enough surface for the interaction of the inhibitors and a
vacuum of 30 Å thickness was built on the Fe(110) model surface
to model a representative part of the interface devoid of any
arbitrary boundary effects. The condensed-phase optimized mole-
cular potentials for atomistic simulation studies (COMPASS) force
field was used to optimize the structures for the systems within
the whole simulation procedures.
3. Results and discussion
3.1 Formation mechanism of the indolizine derivatives
The concept of ‘indolizine quaternary ammonium salts’ was
proposed because the conventional indolizine N-containing
heterocycle (see Fig. 1) and the quaternary N+ ion exist within
the fused molecules.1 Therefore, similar compounds could also
be classified as indolizine derivatives. As shown in Fig. 2, the
homogenous indolizine derivative Di-BQC was prepared from
BQC through 1,3-dipolar cycloaddition.18,30 Actually, Di-BQC
could be considered as a dimer of BQC and, as a result, Di-BQC
was obtained by combining two molecules of BQC.
The other two derivatives QM-DiBQC and PyM-DiBQC were
synthesized from BQC and AcQC as well as AcPyC, respectively
(Fig. 2). The quaternary salts were combined and heated in
solvent, and the dimer derivatives QM-DiBQC and PyM-DiBQC
were formed through an intermolecular 1,3-dipolar cyclo-
addition reaction. The cycloaddition reaction was followed by
the initial decarboxylation step, which produces the active
N-ylide intermediate.1,31–34 Since the 3-position of QM-DiBQC
and PyM-DiBQC is a hydrogen atom, these two compounds
could be named as ‘3-unsubstituted’ indolizine derivatives.32,34
The accurate structures of these indolizine derivatives were
characterized through HRMS and NMR analyses and the
detailed results are shown in Section 2.2.
During the simulations, the Fe atoms were kept ‘frozen’ in
advance so that the inhibitor molecule would be allowed to
reach the metal surface freely. The molecular dynamics stimu-
lations were conducted in the NVT canonical ensemble at 298 K
with a time step of 0.1 fs and a total simulation time of 1000 ps
using the Anderson thermostat. The trajectory was recorded
every 2000 fs. Adsorption energy (Eadsorption) between the inhi-
bitor molecule and Fe(110) surface was calculated as follows:
In the related previous research,15 the formation mechanism
of a similar compound was ambiguous (Michael reaction) and
the structure of the compound was not classified accurately. The
highly effective inhibitive compound was merely separated
from the by-product mixture of the quaternization process for
BQC several years ago. Owing to a new understanding of the
1,3-dipolar cycloaddition route, the same derivative could now
be targeted and synthesized and the yield of the derivatives was
increased dramatically in this work. Furthermore, a facile synth-
esis process of the similar 3-unsubstituted inhibitive indolizine
derivatives was also provided according to the new mechanism.
Eadsorption = Etotal ꢀ (Esurface + Einhinitor
)
(5)
where Etotal is the total energy of the simulation system, and
Esurface and Einhinitor are the energies of Fe crystal and the free
state inhibitor molecule, respectively. The binding energy
(Ebinding) equals the negative value of the adsorption energy:
3.2 Gravimetric measurements
Ebinding = ꢀEadsorption
(6)
The relationship between the dosage and the corrosion rate of
N80 coupons in hydrochloric acid in the absence and presence
of the five inhibitors at 90 1C and 120 1C is shown in Fig. 3(a)
2.7 Microtox toxicity test
The toxicity test was performed using the Microtox method by and (b), respectively. The pitting index of each point in the
exposing a suspension of active marine luminescent bacterium weight loss test is also provided in Table 3.
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Fig. 3 (a) The corrosion rate and the pitting index (numbers) for N80 steel in HCl at 90 1C in the absence and presence of the five inhibitors for 4 h
obtained by the weight loss test. (b) The corrosion rate and the pitting index (numbers) for N80 steel in HCl at 120 1C in the absence and presence of the
five inhibitors for 4 h obtained by the weight loss test.
According to the literature,35,36 the ratio between the acid here is more exciting than those of the corrosion inhibitors
volume and the coupon surface area (V/A) can affect the coupon in the literature.38–41 The inhibition of the three indolizine
weight loss results, and usually a V/A ratio of 21 mL inꢀ2 derivative in Fig. 3 is in good agreement with previous
(3.4 mL cmꢀ2) would stimulate acidizing through a 5.5-in work,1,15 revealing that compared with the original quinolinium
diameter pipe. In this paper, the V/A ratio of 20 mL cmꢀ2 is salts, the indolizine derivatives could exhibit a much stronger
sufficient and reasonable to stimulate acidizing through a 23-in anti-corrosion ability in strong acid media. Also, from Fig. 3, it
diameter pipe.37 From Fig. 3, it can be observed that the can be seen clearly that there is a systematic decrease in the
corrosion rate decreased with increasing dosage of the five visual pitting index with increasing concentration for each of the
inhibitors. For all the concentrations, both PA and the three inhibitors.
novel indolizine derivatives can protect the raw metal more
As reported in the literature,2,17 PA is best used in practice in
effectively than BQC and this result is in accordance with the synergy with many quaternary ammonium salts (including
previous report.15 It is even more exciting to notice in Fig. 3 BQC). The synergistic inhibition performance of PA mixtures
that, in general, the inhibition of the three heterocyclic deriva- with BQC in different molar ratios from PA : BQC = 1 : 20 to
tives is much better than that of propargyl alcohol.
5 : 1 (mol molꢀ1) is shown in Fig. 4. The experiment was
Another important result from the gravimetric tests should conducted at four different total corrosion inhibitor (CI) dosages
not be overlooked: although the quinolinium salt BQC has been from 0.01 wt% to 0.2 wt%.
utilized as the key component of many commercially available
It is obvious from Fig. 4 that when PA or BQC was added
inhibitor products for decades, the primary salts could not solely into the corrosive solution, PA would invariably decrease
retard the corrosion speed effectively in contrast with the three the corrosion rate much more significantly than BQC, showing
dimer derivatives. The result of gravimetric experiments reported that the protection of PA far outweigh that of BQC. When the
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cathodic and anodic Tafel slopes, respectively. When E is far
away from Eeq, eqn (7) gives Tafel’s law:
E = a ꢂ b log|i|
(8)
where a is a constant, and b equals bc or ba. Eqn (8) indicates
that the logarithm of the external current density varies linearly
with the potential at high overpotential. The corrosion current
density can be determined by extrapolating the straight line of
E B log|i| back to the equilibrium potential. The cathodic Tafel
slope of the linear behaviour is:
2:303RT
bc ¼
;
acnF
and the anodic Tafel slope of the linear behaviour is:
2:303RT
ba ¼
Fig. 4 The corrosion rate and the pitting index (numbers) for N80 steel in
15 wt% HCl at 90 1C in the presence of mixtures of PA with BQC.
aanF
where ac and aa are, respectively, the cathodic and anodic charge
transfer coefficients; n is the number of electrons involved in the
electrode reaction; F is the Faraday constant and R is the
universal gas constant. The Tafel parameters including corrosion
current density (Icorr), corrosion potential (Ecorr), cathodic Tafel
slope (bc), anodic Tafel slope (ba) and the inhibition efficiency
(IE) calculated from the Tafel plots according to formula (4) are
given in Table 4.
total CI is lower than 0.1 wt%, the inhibition of any mixture is
inferior to that of pure PA. However, when the total CI dosage
reaches 0.1 wt% or 0.2 wt%, the presence of the mixed inhibitor
(PA : BQC (mol molꢀ1) = 1 : 1, 2 : 1 and 5 : 1) could slow down the
corrosion rate more effectively than PA. From the above
descriptions, no one can deny the fact that PA could show a
fine synergistic effect with BQC.
The results in Table 4 show that the corrosion current
density decreases dramatically while the inhibition efficiency
increases in the presence of the inhibitors, particularly for
the three indolizine derivatives. Obviously, as the indolizine
derivatives were added, the corrosion current density (Icorr) in
Table 4 declined much faster and could reach a much lower
value compared with the other two inhibitors BQC and PA,
indicating that the protection performance of the three indolizine
derivatives Di-BQC, QM-DiBQC, and PyM-DiBQC is far better than
that of the others, especially at lower dosages from 0.01 wt% to
0.1 wt%. Compared with their precursor quaternary ammonium
salts, the active adsorption sites are doubled and reinforced in the
planar indolizine derived dimers, and it is not difficult to explain
their good protection effect.
After a detailed inspection of Table 4, for the indolizine
derivatives, it seems that the inhibition efficiencies hardly grow
further once the dosages are over 0.10 wt% (this may be due to
the relatively restricted solubility at room temperature). As a
result, 0.1 wt% is chosen to be a reasonable dosage for the
comparison of the inhibitors and Tafel plots (polarization
curves) of N80 steel in 15 wt% HCl with or without 0.1 wt%
inhibitors are given in Fig. 5.
Conclusions can be made by comparing Fig. 3(a) and 4 that
when the inhibitor dosage is below 0.1 wt% (the black and red
lines in Fig. 4), the inhibition of any PA/BQC mixture is worse
than those of the indolizine derivatives (see Fig. 3(a)). It is only
when the total CI dosage is beyond 0.1 wt% (the blue and violet
lines in Fig. 4) that the protection performance of the mixture
(PA : BQC (mol molꢀ1) = 5 : 1) might possibly be the same as that
of the indolizine derivatives, as shown in Fig. 3(a). It could be
summarized from the above description that all the derivatives
of BQC (DiBQC, QM-DiBQC, and PyM-DiBQC) could exhibit a
superior inhibition to that of propargyl alcohol.
From Fig. 4, it could be noted that there is an overall
decrease in the corrosion rate and pitting index with increasing
dosage of each inhibitor. For these groups, the assigned pitting
indices lie in the range from 1 to 7. Severe pitting is observed
when BQC is present as the only additive, while the use of PA
would produce a much smoother surface with a relatively low
corrosion rate and pitting indices.
3.3 Tafel plots
The fundamental expression for describing the over potential/
current relationship of the corrosion system, which consisted
of a cathodic reaction and an anodic reaction, is the following
Butler–Volmer equation:42,43
As shown in Fig. 5, a decrease in both cathodic and anodic
currents (log I) was observed when the inhibitor was added to
the solution, suggesting that the inhibitor could either reduce
the dissolution speed in the anodic areas or retard the cathodic
hydrogen evolution process at the same time.41 Also, as shown
in both Table 4 and Fig. 5, compared with those of the control
À
Á
À
Á
ꢂ
ꢀ
ꢁ
ꢀ
ꢁꢃ
2:303 E ꢀ Eeq
2:303 E ꢀ Eeq
i ¼ i0 exp
ꢀ exp
(7)
ba
bc
group, the polarization curves and corrosion potential (Ecorr
)
where E is the electrode potential; Eeq is the equilibrium show minor shifts towards the more anodic regions for all
potential; i0 is the exchange current density; bc and ba are the the inhibitors. The changes in Ecorr value were 28 mV for BQC,
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Table 4 Potentiodynamic parameters and inhibition efficiency (IE) of N80 steel in 15 wt% HCl in the absence and presence of inhibitor
NJC
Inhibitor
Dosage (in wt%)
ba (mV decꢀ1
)
ꢀbc (mV decꢀ1
)
Icorr (mA cmꢀ2
)
ꢀEcorr (mV vs. SCE)
IE (%)
No inhibitor
BQC
0
87.6
96.5
110.8
106.9
121.7
123.3
129.8
110.5
120.6
140.7
138.8
107.2
129.5
138.1
140.8
113.2
118.7
130.0
131.4
109.9
118.1
129.3
134.8
897.7
376.6
209.3
111.0
65.4
168.1
106.9
90.8
42.4
46.5
34.9
28.7
26.9
49.6
37.0
34.1
32.8
51.0
44.4
41.2
40.9
428
417
408
400
396
429
410
403
388
394
432
414
409
425
412
398
389
420
409
402
385
—
0.01
0.05
0.10
0.20
0.01
0.05
0.10
0.20
0.01
0.05
0.10
0.20
0.01
0.05
0.10
0.20
0.01
0.05
0.10
0.20
58.0
76.7
87.6
92.7
81.3
88.1
89.9
95.3
94.8
96.1
96.8
97.0
94.5
95.9
96.2
96.3
94.3
95.0
95.4
95.4
107.8
108.2
113.7
100.3
111.4
113.5
112.9
99.8
114.3
120.1
123.4
98.8
109.1
118.9
116.8
101.0
109.4
105.6
111.7
PA
Di-BQC
QM-DiBQC
PyM-DiBQC
Fig. 6 Nyquist plots for N80 steel in 15 wt% HCl without and with the
same concentration (at 0.1 wt%) of the five inhibitors at 25 1C. (The plot for
blank has been shown in inset.)
Fig. 5 The Tafel curves for N80 steel in 15 wt% HCl without and with
inhibitors.
25 mV for PA, 14 mV for Di-BQC, 30 mV for QM-DiBQC, and
26 mV for PyM-DiBQC towards the anodic region. According to
the literature, the displacement of Ecorr in Table 3 between
blank and any inhibitor groups lies within ꢂ85 mV per SCE,
indicating that the five inhibitors studied here could be
characterised as mixed-type inhibitors.17,44
semi-circular in uninhibited corrosive medium, while in the
presence of the inhibitors, the shape of the semicircle is
depressed. This phenomenon can generally be attributed to
different factors like the frequency dispersion and surface
roughness of the electrode.42 Fig. 6 shows that the imperfect
diameters of the depressed capacitive loops in the inhibited
solutions are increased sharply compared with those in the
3.4 Electrochemical impedance spectroscopy (EIS)
As 0.1 wt% was chosen as the moderate dosage for the five uninhibited group for all five inhibitors (especially the three
inhibitors in the Tafel plots (Fig. 5), the corresponding EIS indolizine derivatives). This implies that all the inhibitors
Nyquist plots, and Bode and phase angle curves are also could exhibit anti-corrosion ability in the corrosive solution.
displayed at the same dosage (0.1 wt%) in Fig. 6 and 7. It is More importantly, it can be summarized from Fig. 6 that after
well known that in Nyquist plots, the diameter of the capacitive being transformed into the indolizine derivative, the inhibition
loop represents the charge transfer resistance of the electrode, performance of BQC in hydrochloric acid shows a marked
and a higher diameter of the impedance semicircle indicates increase; the novel indolizine derivatives show a highly efficient
better protection.17,38 Therefore, it is a convenient and direct protection performance.21–23 It is apparent from Fig. 3 and 6
way to use the capacitive loop diameter to identify a better that the results obtained from weight loss measurements and
corrosion inhibitor. Naturally, the Nyquist plots of the steel are EIS tests are in good agreement; the dimer indolizine derivatives
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Fig. 7 The Bode plots (a) and phase plots (b) for N80 steel in 15 wt% HCl without and with the same concentration (at 0.1 wt%) of the five inhibitors at 25 1C.
are more likely to be used as the key inhibitive component than resistance (Rct), the accumulation resistance (Ra), the diffuse
their precursor BQC and propargyl alcohol.
layer resistance (Rd) and the film resistance (Rf), since the
The Nyquist plots for N80 steel in the blank group as well as presence of inhibitors leads to depressed semi-circles in the
that in the inhibited solutions have a similar appearance, which Nyquist plots in Fig. 6.24
means that these inhibitors are retarding the corrosion process
The fitted spectra obtained by using the equivalent circuit
without changing the corrosive mechanism.22 Evidently, the to fit the experimental data for N80 steel in 15 wt% HCl with
inhibited and uninhibited Nyquist plots both exhibit a single 0.1 wt% concentrations of the five inhibitors are shown in
semicircular capacitive loop, indicating that the corrosion Fig. 9. The calculated EIS parameters are displayed in Table 5
process is under charge-transfer control.5,6
according to the model circuit and calculated by the ZSimpWin
Fig. 7(a) and (b) shows the Bode impedance and phase angle software and they could be employed to study the protective
plots for N80 steel corrosion immersed in 15 wt% HCl solution. performance of the inhibitors.
It can be noticed that only one narrow phase peak is present in
In Table 5, generally, as the dosages increases, the polarization
the studied frequency range with and without the five inhibi- resistance (Rp) increases, while the values of electric double layer
tors, suggesting that there is only one time constant, which can capacitance (Cdl, which is related to the charge storage ability of the
be ascribed to the charge transfer resistance and electric double double layer) keep on decreasing. With the adsorption of the
layer capacitance. Compared with that of BQC and PA, the inhibitive molecules, the reduction of accumulated charges and
presence of the three dimer indolizine derivatives leads to an the increase in the thickness of the electrical double layer lead to
apparent increase in the impedance modulus at low frequency. the reduction of Cdl. This could be due to the possible replacement
Meanwhile, the phase angle shifted to a more negative direc- of water molecules by the adsorbed inhibitive compounds on the
tion, which demonstrated the formation of a more effective steel surface.38,44 With the increase in inhibitor concentration, the
protection layer at the steel surface and a decreased corrosion gradual increase of Rp implies a greater blocking effect of the active
speed upon adding indolizine derivatives into the acid solution. metal surface, and meanwhile, the transfer of charges from the
Fig. 8 shows the equivalent circuit model employed to metal surface to the corrosive medium becomes more and more
simulate the results of the EIS test. The equivalent circuit difficult.42,43 Upon inspection of the data in Table 5, it can be stated
consists of a constant phase element (CPE), a polarization significantly that the inhibitors functioned by adsorption at the
resistance (Rp), and a solution resistance (Rs). The value of Rp steel/solution surface. The difference of Rp between the inhibitors
is considered to represent a combination of the charge transfer in Table 5 is well consistent with the imperfect diameter of the
impedance semicircles shown in Fig. 6. All the discussions above
implied that the indolizine derivative could form a strongly
adsorbed protective layer on the metal surface.
3.5 Surface investigation
The rough N80 steel surface with big cracks after the weight
loss measurement without inhibitor is shown in Fig. 10(a). A
large amount of corrosion particles could be observed when the
coupon was immersed in the uninhibited acid directly.
Fig. 8 Equivalent circuit model used to fit the EIS Nyquist plots.
Fig. 10(c) shows the micrograph of the metal surface in the
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Fig. 9 The fitted spectra of the experimental Nyquist results (1), Bode plots (2) and phase plots (3) according to the equivalent circuit.
presence of Di-BQC at a dosage of 0.1 wt%. It is obvious that the DE (ELOMO ꢀ EHOMO) and dipole moment (m) were obtained in
surface of the specimen was smoother when the indolizine deriva- order to predict the inhibitors’ protection activity toward the
tive was added. This phenomenon reveals that similar inhibitive metal surface.
dimer indolizine derivatives would perform well as a strong acid
The above quantum chemistry parameters were determined
inhibitor and suggests the formation of a protective film on the after geometric optimization with respect to all nuclear coordinates
steel surface. The results of the related EDX spectra are shown in and are listed in Table 6. Fig. 11 shows the optimized structures of
Fig. 10(b and d). The EDX spectra of steel after corrosion in blank the inhibitors and Fig. 12 illustrates the diagrams of the frontier
HCl aqueous solution (Fig. 10b) exhibited the characteristic signals molecular orbital density distributions: the highest occupied mole-
of Fe, C and O elements, which originally existed in the raw cular orbital (HOMO) and the lowest unoccupied molecular orbital
material. The EDX spectra recorded in the presence of the novel (LUMO), which generally represent the regions of a molecule that
inhibitive derivatives (using Di-BQC as an example) showed the can donate or accept electrons, respectively.45
characteristic peaks for N and Cl, which confirmed the presence As can be observed from Fig. 12, for the indolizine derivative
and formation of the protective film of indolizine derivative molecules, the HOMO and LUMO are mostly delocalized
inhibitor on the steel surface in concentrated HCl.
throughout the conjugated plane over the whole molecule,
whereas the frontier orbital of BQC is mainly distributed on
the substituted benzene ring. For this reason, the relatively
3.6 Quantum chemistry calculations
The frontier molecular orbitals provide information about the poor inhibition performance of BQC in strong hydrochloric
inhibitor molecules involved in the donor–acceptor relation- acid in our former research work could be explained reasonably
ship between the inhibitor and the metal atoms. Quantum and a parallel adsorption orientation of the indolizine deriva-
chemical parameters such as EHOMO, ELOMO, the energy gap tives onto the metal surface is expected.15 The distributions of
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Table 5 EIS parameters and inhibition efficiency (IE) of N80 steel in
Table 6 Calculated quantum chemical parameters of BQC and its indo-
15 wt% HCl in the absence and presence of inhibitor
lizine derivatives
Dosage
(in wt%)
Rs
Rp
Cdl
Inhibitor
EHOMO (eV)
ELUMO (eV)
DE (eV)
m (D)
Inhibitor
(O cm2)
(O cm2)
(mF cmꢀ2
)
IE (%)
BQC
Di-BQC
QM-DiBQC
PyM-DiBQC
ꢀ9.304
ꢀ7.872
ꢀ8.143
ꢀ8.462
ꢀ5.896
ꢀ5.804
ꢀ5.994
ꢀ6.137
3.408
2.068
2.149
2.325
5.45
1.28
1.32
5.20
No inhibitor
BQC
0
1.21
1.08
1.13
0.97
1.01
0.99
1.13
1.05
1.04
1.16
0.98
1.10
0.99
1.11
1.13
0.96
1.21
1.08
1.13
0.97
1.01
20.7
141.4
168.0
198.3
249.7
306.6
334.7
364.5
443.8
338.0
479.8
558.5
600.3
345.8
457.9
543.0
582.3
350.7
433.6
550.4
569.1
157.8
143.1
130.5
106.4
88.7
130.1
108.9
95.3
—
0.01
0.05
0.10
0.20
0.01
0.05
0.10
0.20
0.01
0.05
0.10
0.20
0.01
0.05
0.10
0.20
0.01
0.05
0.10
0.20
85.4
87.7
84.2
89.6
93.2
93.8
94.3
95.3
93.9
95.7
96.3
96.5
94.0
95.5
96.3
96.4
94.1
95.2
96.4
96.2
PA
the frontier molecular orbital density also indicate that the
newly formed five-membered indolizine fused group would
play an important role as an active site for the interaction
between these aromatic plane derivatives and the metallic
surface.
The adsorption of inhibitive molecules onto the steel surface
results from ‘donor–acceptor’ interactions between the com-
pounds and the metal surface. The EHOMO is usually associated
with the capacity of a molecule to donate electrons and the
ELUMO represents the ability of the molecule to accept electrons.
High values of EHOMO indicate the tendency of inhibitor
molecules to donate electrons to the acceptor species with
empty molecular orbitals. Conversely, the lower value of ELUMO
suggests the easier acceptance of electrons from the Fe surface
and results in a higher inhibition efficiency.46 The energy gap
83.8
Di-BQC
117.1
105.6
90.4
73.8
QM-DiBQC
PyM-DiBQC
120.0
103.8
93.1
75.5
122.5
104.3
92.9
77.7
Fig. 10 SEM micrograph (a) and EDX spectrum (b) of N80 steel after being immersed in 15 wt% HCl solution for 4 hours; SEM micrograph (c) and EDX
spectrum (d) of N80 steel after immersion in 15% HCl with 0.1 wt% Di-BQC.
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the other two derivatives, which correlates well with the experi-
mental findings in gravimetric tests and electrochemical
studies.
3.7 Molecular dynamics simulations
In order to better understand the adsorption behaviour of the
indolizine derivatives on the steel surface, Metropolis Monte
Carlo simulation was carried out to quantify the adsorption of a
single inhibitor molecule on the Fe surface. Fig. 13 shows the
snapshots of the stable equilibrium configuration of BQC and its
related indolizine derivatives on the Fe(110) surface. It is easy to
notice in Fig. 13 that the dimer inhibitive molecules adsorbed in a
nearly parallel orientation to the model metal surface, which
effectively maximized the contact and improved the surface coverage
on N80 steel. It is documented in the literature that the higher the
binding energy (Ebinding), the stronger the interaction of inhibitor
molecules with metal surfaces.47 The order of Ebinding is Di-BQC
(227.30 kcal molꢀ1) 4 QM-DiBQC (198.44 kcal molꢀ1) 4
PyM-DiBQC (173.62 kcal molꢀ1) 4 BQC (110.28 kcal molꢀ1).
Therefore, it could be concluded that the indolizine derivatives
are expected to inhibit the metal corrosion more effectively
than their precursor BQC. The theoretical results from mole-
cular dynamics simulation speculation are in good agreement
with the results from the experimental measurements dis-
cussed in Sections 3.2 to 3.4 as well as the conclusions from
the quantum chemistry calculations.
Fig. 11 Optimized molecular structures of the inhibitors: (a) BQC, (b) Di-BQC,
(c) QM-DiBQC, and (d) PyM-DiBQC.
Fig. 12 Frontier molecular orbital density distributions of the inhibitors:
BQC, Di-BQC, QM-DiBQC and PyM-DiBQC (left, HOMO; right, LUMO).
DE between the LUMO and HOMO energy levels is another
important factor as a function of reactivity of the inhibitor
molecule towards adsorption onto the metal surface. A mole-
cule with a small value of DE is easily polarized, which facil-
itates adsorption and enhances the efficiency of the inhibitor.
In Table 6, compared with the DE value of BQC, the highly
effective potential protection of the three indolizine derivatives
is apparent. The increasing tendency of DE from Di-BQC (2.068
eV) to PyM-DiBQC (2.325 eV) in Table 6 shows that Di-BQC has
a higher adsorptive ability on the metal surface compared to
Fig. 13 Equilibrium adsorption configurations recorded for the inhibitors
on the Fe(110) surface: BQC, Di-BQC, QM-DiBQC and PyM-DiBQC (left,
side view; right, top view).
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Table 7 EC50 values of the inhibitors
1. The indolizine derivatives Di-BQC, QM-DiBQC, and PyM-
DiBQC, with highly effective corrosion inhibition efficiencies
and eco-friendly properties, were synthesised facilely from the
conventional corrosion inhibitor benzyl quinoline chloride (BQC).
The formation mechanism of the derivatives was proposed
reasonably as the 1,3-dipolar cycloaddition and the three indo-
lizine derivatives are prepared in relatively high yields.
2. The excellent anti-corrosion performance of the indolizine
derivatives for N80 steel was confirmed and evaluated in
concentrated hydrochloric acid. The results obtained from
gravimetric tests, electrochemical methods, surface analysis
and theoretical calculations were in good agreement and con-
firmed the exciting inhibition of the newly synthesized indoli-
zine derivatives. The inhibition of the indolizine derivatives is
even better than that of propargyl alcohol.
Inhibitor
EC50 (mg Lꢀ1
)
PA
BQC
Di-BQC
QM-DiBQC
PyM-DiBQC
3.3 ꢁ 102
1.2 ꢁ 106
2.3 ꢁ 105
1.4 ꢁ 105
9.7 ꢁ 104
3.8 Ecological (toxicity) aspects
The results of the Microtox toxicity test are shown in Table 7.
Usually, when the value of EC50 is higher than 2.0 ꢁ 104 mg Lꢀ1
,
the sample could be classified as ‘practically nontoxic’ or ‘low
toxicity’.27 According to Table 7, it is apparent that the newly
synthesized indolizine derivatives have much better eco-friendly
advantages than the widely used high-toxicity propargyl
alcohol.
3. The biological toxicity assessment showed a considerable
eco-friendly advantage of the indolizine derivatives. The use of
inhibitive indolizine derivatives as acidizing inhibitors may
offer a new strategy for effective corrosion protection in strong
acid media.
3.9 Possible mechanism of inhibition
Electron-rich heteroatoms (nitrogen, sulphur, oxygen, etc.) and
the aromatic conjugated functional groups (usually containing
delocalized p-electrons) are the key inhibitive structures among
most of the organic corrosion inhibitors. The interaction of
organic inhibitors with metallic surfaces involves donor–acceptor
interactions.2 The p electrons and unshared electron pairs in the
organic inhibitors would coordinate with the empty d-orbitals of
the crude metal atoms to form coordination bonds (usually known
as chemical adsorption), and eventually, the inhibitive molecular
groups would become firmly adsorbed onto the steel surface.5 The
adsorbed inhibitors would thereby protect the steel from the
contact of the corrosive H+ ions. It is clear that the quaternary
ammonium cation and the five or six membered aromatic rings,
which are rich in unshared p-electrons, are the two major inhibitive
groups in BQC and its indolizine derivatives.2,15
By comparison of these indolizine derivatives and their
precursor BQC, the condensed dimer derivative of the quaternary
salt could provide much more active adsorption sites.48,49 As a result,
after the protonation of the electron-rich tertiary N atom, the newly
formed adsorption sites and delocalized p-electrons will fasten the
inhibitor to the crude surface and thus prevent the corrosive ions
from approaching the metal, leading to a significant increase in the
inhibition.50–52 In addition, the unsaturated planar hydrophobic
groups among the indolizine derivatives would also prohibit the
corrosive H+ ions from approaching the metallic surface and become
firmly adsorbed on the bare metalic material to form a protective
film.45,53–55 These are possibly the main reasons why the condensed
indolizine derivatives would behave as such good corrosion
inhibitors compared to the simple quinolinium salt and PA.
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
The authors acknowledge the management of ‘‘the Fundamen-
tal Research Funds for the Central Universities’’ (16CX06030A)
for the permission to present this paper. This research is
supported by the National Natural Science Foundation of China
(Youth Fund) (Grant No.: 51704036). Special thanks are given to
Prof. Hu Huayou from Huaiyin Normal University for his
assistance and helpful advice on synthentic works for the
indolizine derivatives. Special thanks are given to vice Prof.
Sun Shungqing and his masters students Bo Yu and Li Fengting
from China University of Petroleum (UPC) for their critical
guidance on theoretical calculations. Song Ni and Ren Sumei
from Ocean University of China (OUC) are acknowledged for
their smart work on instrumental analysis.
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