Indolizine quaternary ammonium salt inhibitors part II: A reinvestigation of an old-fashioned strong acid corrosion inhibitor phenacyl quinolinium bromide and its indolizine derivative

Article abstract of DOI:10.1039/c8nj02505c

Phenacyl quinolinium bromide (PaQBr), an old-fashioned corrosion inhibitor in strong acid medium, was reinvestigated and a new indolizine derivative (DiPaQBr) was synthesized from PaQBr. DiPaQBr is classified as an indolizine quaternary ammonium salt. Bas

Full text of DOI:10.1039/c8nj02505c

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Indolizine Quaternary Ammonium Salt Inhibitors: A
reinvestigation of an Old Fashioned Strong Acid
Corrosion Inhibitor Phenacyl Quinolinium Bromide
and Its Indolizine Derivative
Yefei Wang*,1, Zhen Yang1, Fengtao Zhan2, Zhifeng LYu2, Chengyou Han2, Xiaonuo
Wang2,Wuhua Chen1, Mingchen Ding1, Renzhuo Wang1, Yingnan Jiang3
1College of Petroleum Engineering, China University of Petroleum (East China), 66 West
Changjiang Rd, Qingdao, Shandong Province, 266580, P. R. China
2College of Science, China University of Petroleum (East China), 66 West Changjiang Rd,
Qingdao, Shandong Province, 266580, P. R. China
3College of Computer Engineering, Cinema and Mechatronics, Politecnico di Torino, 24 Corso
duca degli abruzzi, Torino, 10129, Italy
1

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ABSTRACT: Phenacyl Quinolinium Bromide (PaQBr), an oldꢀfashioned corrosion inhibitor in
strong acid medium was reinvestigated and a new indolizine derivative (DiPaQBr) was
synthesized from PaQBr. The DiPaQBr is classified as an indolizine quaternary ammonium salt.
Based on High Resolution Mass Spectrometry (HRMS) and NMR, the formula and chemical
structure of DiPaQBr was accurately confirmed and characterized. The formation mechanism of
DiPaQBr from the condensation of two molecules of PaQBr through 1,3ꢀdipolar cycloaddition
was proposed. The corrosion inhibition of PaQBr and DiPaQBr on N80 steel in 15% HCl was
investigated by weight loss measurement, potentiodynamic polarization and electrochemical
impedance spectroscopy (EIS). Polarization results indicate that the examined two compounds
are of mixedꢀtype inhibitor. The adsorption of the studied inhibitors obeyed the Langmuir
adsorption isotherm. The obtained results from gravimetric research as well as electrochemical
study indicated that compared with the inhibitive PaQBr, its indolizine derivative DiPaQBr could
exhibit a better corrosion prevention performance at a much lower concentration. The achieved
results indicate that the derivative (indolizine quaternary ammonium salt) of heterocyclic
quaternary ammonium salts would inhibit the steel more effectively in acid medium.
KEYWORDS: Acid inhibition; N80 Steel; Phenacyl Quinolinium Bromide; Indolizine
derivative;
1. Introduction
New corrosion inhibitors for acidizing (≥15 wt.% hydrochloric acid as the working fluid at 90℃
or higher temperature) in petroleum industry are in great demand as the EOR technique
develops.1ꢀ4 Among the numerous corrosion inhibitors, nitrogenꢀcontaining heterocyclic
quaternary ammonium salts have been generally used to prevent N80 steel from getting severely
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corroded in acidizing engineering.2ꢀ6 These quaternary ammonium salts are usually synthesized
by nitrogenꢀcontaining heterocyclic bases (pyridine, quinoline or their derivatives) and series of
halogenated compounds through quaterisation reaction.7ꢀ9
For decades, the BQC (Benzyl Quinolinium Chloride) served as a commonly used inhibitor
for acidizing. In our previous work,1,10 we have reported that the BQD (Benzyl Quinolinium
Chloride Derivative), a dimer derivative of BQC, would be formed by the condensation of two
BQC molecules (Fig. 1). Compared with BQC, the protection performance of BQD in 15 wt.%
HCl gets dramatically promoted. However, the formation mechanism of BQD was ambiguous
since the chemical structure of BQD was not fully studied and assigned accurately in previous
work.
As illustrated in Fig. 1, in this paper, the dimer derivative (like BQD) is redefined as the
indolizine derivative (indolizine quaternary ammonium salt) according to literatures.11ꢀ13
Heterocyclic quaternary ammonium salts which possess active methylene group (like BQC)
would easily get converted to their corresponding indolizine derivatives (like BQD) via 1,3ꢀ
dipolar cycloaddition.14ꢀ19
Fig. 1 Structure of Indolizine and the formation of BQD from BQC.
The great difference of corrosion inhibition between BQC and BQD1 implies that similar
dimer derivative of BQD may also exhibit a better inhibition performance than the precursor
heterocyclic quaternary ammonium salts inhibitors. In order to further confirm the hypotheses,
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heterocyclic quaternary ammonium salt with active methylene group is the appropriate target
compound to investigate. R.I. Yurchenko et al. had once studied the inhibitive behavior of
Phenacyl Quinolinium Bromide (PaQBr, Fig. 2) for 08 KP steel in 3 M H2SO4, indicating its
effective inhibition in acid medium.20ꢀ22 Owning to the electronegative effect of quaternary N+
atom as well as the carbonyl group in PaQBr, the active methylene group in PaQBr could be
transformed to its NꢀYlide structure easily.23ꢀ27 The active NꢀYlide intermediate would thereby
react with another molecule of PaQBr under certain condition to form a corresponding indolizine
quaternary ammonium salt (the dimer derivative of PaQBr, DiPaQBr, see Fig. 2), whose
structure is very similar with BQD.1,14,28ꢀ36
Fig. 2 Synthesis of Phenacyl Quinolinium Bromide (PaQBr) and its dimer derivative, DiPaQBr.
In this study, PaQBr was synthesized and its indolizine quaternary ammonium salt DiPaQBr
was obtained with the assistance of triethylamine. The structure of DiPaQBr was confirmed by
High Resolution Mass Spectrometry (HRMS) and NMR. The formation mechanism of DiPaQBr
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from PaQBr was characterized as 1,3ꢀdipolar cycloaddition.11,33 The inhibition ability of PaQBr
and DiPaQBr in 15 wt.% HCl for N80 steel is investigated via weight loss measurement, Tafel
polarization measurements and electrochemical impedance spectroscopy (EIS). The possible
inhibitive mechanism of the two compounds is discussed.
2. Experimental section
2.1 Preparation of PaQBr and DiPaQBr.
0.1 mol quinoline was mixed with 0.1 mol 2ꢀbromoacetophenone in a 250 mL threeꢀnecked
flask. Then approximately 20 mL of acetone was added to the flask as solvent. The reaction
mixture was stirred at room temperature for 6 hours, and then a large amount of light brown
solution with yellow precipitate was obtained.37ꢀ39 The yellow product was washed with acetone
and recrystallized with alcohol. Pure phenacyl quinolinium bromide (PaQBr) was obtained as
light yellow powder. The chemical structure of PaQBr was confirmed by NMR (Agilent NMRꢀ
400 MHz).
°
Phenacyl Quinolinium Bromide (PaQBr): Yield 85.9%, m.p. 166 C (decomposed), solubility:
1
15.8 g/100mL in water. H NMR (400 MHz, DMSOꢀd6):δ9.67 (d, J = 5.9 Hz, 1H), 9.51 (d, J =
8.3 Hz, 1H), 8.59 (d, J = 6.8 Hz, 1H), 8.49 (d, J = 9.0 Hz, 1H), 8.36 (dd, J = 8.4, 5.8 Hz, 1H),
8.22 (dd, J = 18.9, 7.2 Hz, 3H), 8.07 (t, J = 7.6 Hz, 1H), 7.84 (t, J = 7.4 Hz, 1H), 7.71 (t, J = 7.7
Hz, 2H), 7.16 (s, 2H). 13C NMR (101 MHz, DMSOꢀd6):δ191.29, 151.42, 149.03, 139.11, 136.38,
135.29, 134.09, 131.08, 130.43, 129.86, 129.54, 129.17, 122.65, 119.67, 63.80.
5 g PaQBr and 0.5g triethylamine was dissolved with acetonitrile in a threeꢀnecked flask. The
mixture was heated and kept at 70℃ for 1 hour, and then the solution became dark reddish.
Afterwards, the acetonitrile was evaporated and a solid pigmentꢀlike mixture was obtained. A
5

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silica gel column chromatography was used to isolate the pigmentꢀlike mixture and the solvent
was isoꢀpropanol–chloroform (1:9, v/v).
Thin layer chromatography (TLC) with the same solvent as in column chromatography was
used to monitor the eluate of column chromatography. Compared with BQD,1 the DiPaQBr was
also viewed as light fluorescent yellow point at ultraviolet light and the Rf value of DiPaQBr was
approximately 0.65. Finally, the yellow eluates containing DiPaQBr were evaporated at reducing
pressure, and the DiPaQBr was obtained as dark orange solid. The chemical structure of
DiPaQBr was confirmed by NMR (Agilent NMRꢀ400 MHz).
Dimer Derivative of Phenacyl Quinolinium Bromide (DiPaQBr): Yield 23.2%
°
1
(approximately), m.p. 208 C (decomposed), slightly soluble in water. H NMR (400 MHz,
CDCl3):δ11.93 (s, 1H), 9.00 (d, J = 9.0 Hz, 1H), 8.28 (d, J = 7.7 Hz, 2H), 8.19 (d, J = 8.2 Hz,
1H), 8.08 (d, J = 8.9 Hz, 1H), 7.99 (dd, J = 10.9, 7.1 Hz, 2H), 7.76 (d, J = 7.8 Hz, 2H), 7.72ꢀ7.50
(m, 8H), 7.41 (t, J = 7.3 Hz, 3H), 7.16 (s, 2H). 13C NMR (101MHz, CDCl3):δ191.17, 190.95,
150.04, 136.91, 135.82, 135.24, 134.85, 134.56, 133.84, 132.33, 130.93, 130.57, 130.40, 130.09,
129.69, 129.67, 129.26, 128.85, 128.48, 127.46, 126.99, 126.55, 123.20, 120.80, 120.77, 119.29,
118.56, 117.10, 109.99, 61.88.
2.2 Preparation of the acid solution and specimens.
15 wt.% hydrochloric acid solution was chosen as corrosive medium for both the weight loss
measurements and the electrochemical experiments . The acid solution was prepared by dilution
of 36ꢀ38 wt.% commercially available analytical grade hydrochloric acid with double distilled
water. The composition of N80 specimens was as follows: (in wt.%) Cr, 0.20; Mn, 0.92; S,
0.008; P, 0.01; Si, 0.19; C, 0.31; and Fe, balance. The dimension of the N80 steel specimens for
the weight loss analysis was 50 mm×10 mm×3 mm. The same specimens with 10 mm2 exposed
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surface areas were used as working electrodes for electrochemical measurements. The exposed
area of the specimens was mechanically abraded with increasing grades of emery papers (600,
800, 1200 and 2000 grit), followed by cleaning in turn with double distilled water, acetone and
ethanol, and finally dried at room temperature before the inhibition experiments.
2.3 Weight loss measurement.
After the weighting, the specimens were suspended in 15 wt.% HCl solutions with and without
°
different concentrations of PaQBr or DiPaQBr for 4 h at 90 C, in which the ratio of volume
(mL) of acid solutions to surface area (cm2) of the specimen was 20:1. The experiments were
carried out in triplicates in each test and only the average value of corrosion rate was used in
calculations. The corrosion rate (ꢀꢁꢂꢃꢃ) in g m−2 h−1of each specimen was calculated from the
following equation:
ꢄ ꢆ ꢄ
ꢅ
ꢇ
ꢀꢁꢂꢃꢃ
=
(1)
ꢈ × ꢉ
where, ꢀꢁꢂꢃꢃ = corrosion rate (g m−2 h−1), ꢊ (g) and ꢊ (g) =weights of a specimen before and
ꢋ
ꢌ
after the 4h experiment respectively, S = surface area of the specimen (m2), t =immersion time
(h).
2.4 Electrochemical measurements.
Electrochemical analysis was carried out on Gamry Reference 600 electrochemical work
station in 15 wt.% HCl solution at 25°C under nonꢀstirred conditions. Threeꢀelectrode corrosion
cell including N80 steel (1 cm2 exposed surface area) as a working electrode (WE), SCE
(saturated calomel electrode coupled to a fine Luggin capillary) as a reference electrode (RE),
and a platinum counter electrode (CE) was employed to conduct the electrochemical test. The
Luggin capillary was kept close to the working electrode to decrease IR drop. All the
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electrochemical experiments were performed in the absence and presence of PaQBr or DiPaQBr
at the concentrations as same as those for weight loss measurement.
The threeꢀelectrode cell was immersed in each test solution for 20 min in order to ensure a
stable open circuit potential (OCP), and after this, both the potentiodynamic polarization and the
EIS measurements were performed at OCP. All the electrochemical data are presented without
iR correction. The anodic and cathodic polarization curves were obtained in the potential range
from ꢀ200 mV to +200 mV versus OCP at an automatically scanning rate of 0.5 mV sꢀ1. The
corrosion current density (ꢍꢎꢏꢐꢐ) and corrosion potential (ꢑꢎꢏꢐꢐ) were measured by Tafel liner
extrapolation procedure. The EIS measurements were taken for the frequency spectrum from 100
kHz to 10 mHz, using an AC amplitude signal of 5 mV peakꢀtoꢀpeak. The EIS experimental data
was analyzed using ZSimpWin software.
3. Results and discussion
3.1 Formation of DiPaQBr from PaQBr
3.1.1 Structure characterization of PaQBr
The synthesis of PaQBr is carried out according to the reaction in Fig. 2.
From the 13CꢀNMR spectrum of PaQBr (see Fig. S1 in Electronic Supplementary Information,
ESI) in DMSOꢀd6, there is one signal of aliphatic carbon at 63.80 ppm, corresponding to the
active ꢀCH2ꢀ carbon next to the quaternary N+ atom.40 Thirteen signals of aromatic carbons
ranging from 119.67ꢀ151.42 ppm are assigned to the fifteen sp2 aromatic carbon atoms in PaQBr,
two of which at 129.17 and 129.54 ppm are overlapped signals represent two carbon atoms
respectively. The distinctive signal at 191.29 ppm belongs to the carbonyl group in PaQBr,
which emerges at low field area, showing a strong electronegative effect of its chemical
environment.
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3.1.2 Structure characterization of DiPaQBr
The platform of HRMS (High Resolution Mass Spectrometry, electrospray ionization, positive
ion mode) was used to determine the mass of the molecular ion of DiPaQBr.
The HRMS spectrum of DiPaQBr is shown in Fig. S2, ESI. The HRMS result (positive ion
mode) of DiPaQBr shows that its molecular ion peak is m/z 491.1755 and the elemental
composition is [C34H23N2O2]+.
1
The HꢀNMR spectrum of DiPaQBr (Fig. S3, ESI) in CDCl3 shows a signal at 7.16(2H),
corresponding to the protons on ꢀCH2ꢀ group next to the quaternary N+ atom. Signals at 7.41
(3H), 7.57ꢀ7.70 (8H), 7.75 (2H), 7.99 (2H), 8.08 (1H), 8.19 (1H), 8.28 (2H), 9.00 (1H) are
assigned to the aromatic protons in DiPaQBr. The signal at 11.93(1H) is assigned to the aromatic
proton directly attached to the N+ atom. From the 13CꢀNMR spectrum of DiPaQBr (Fig. S4, ESI),
there is one signal of aliphatic carbon at 61.88 ppm which represents the ꢀCH2ꢀ carbon. Twentyꢀ
nine signals from 109.99ꢀ191.17 ppm belong to thirtyꢀone sp2 aromatic carbon atoms and two
carbonyl carbons in DiPaQBr, four of which at 128.85, 129.26, 129.67 and 129.69 ppm are
overlapped signals represent two carbon atoms respectively. It is apparent that the DiPaQBr
molecule contains thirtyꢀfour carbon atoms, which is in good agreement with the result of
HRMS.
The type of carbon atom (primary, secondary and tertiary carbon) could be accurately
13
identified from the DEPT experiment. DEPT spectrum is a kind of special CꢀNMR spectrum.
Quaternary carbon signals could not be found in all DEPT spectrums. The DEPT 90° spectrum
displays tertiary carbon only, while primary/tertiary carbon gives signal upwards and secondary
carbon gives signal downwards in DEPT 135° spectrum. The results of DEPT experiment are
shown in Fig. S5 (a) and Fig. S5 (b), ESI.
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DEPT 135° spectrum indicates seventeen different signals of aromatic tertiary carbons at
150.04, 135.24, 134.85, 130.93, 130.57, 130.40, 130.09, 129.69, 129.67, 129.26, 128.85, 128.48,
127.46, 126.55, 119.29, 118.56, 117.10 ppm and one signal of one aliphatic secondary carbon at
61.88 ppm, four of which at 128.85, 129.26, 129.67 and 129.69 ppm are the overlapped signals
represent two carbon atoms respectively. According to DEPT 90° and DEPT 135° spectrum,
conclusion can be made that there is no primary carbon in DiPaQBr. It could be easily
recognized that signals of the twelve quaternary carbons (191.17, 190.95, 136.91, 135.82,
13
134.56, 133.84, 132.33, 126.99, 123.20, 120.80, 120.77 and 109.99 ppm) in CꢀNMR spectrum
of DiPaQBr could not be observed in both DEPT 90° and DEPT 135° spectrum. The chemical
shifts of these twelve quaternary carbons are marked in Fig. S5 (b), ESI.
As a result, from all the data above it is clear that the DiPaQBr molecule contains thirtyꢀfour
carbon atoms, which comprise two carbonyl carbons, ten aromatic quaternary carbons, a single
carbon atom belongs to aliphatic ꢀCH2ꢀ structure and twentyꢀone aromatic tertiary carbons.
The chemical structure of DiPaQBr (CAS Registry Number was not found) can be concluded
from the spectrum results and related references.22ꢀ32 The formation mechanism of DiPaQBr is
shown in Fig. 3.
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Fig. 3 The formation mechanism of DiPaQBr from PaQBr.
3.1.3 The formation mechanism of DiPaQBr
Fig. 3 illustrates the formation mechanism of DiPaQBr. Owning to the strong electronegativity
of the carbonyl group and the quaternary N+ atom in PaQBr, the PaQBr is easily to get
transformed to its corresponding NꢀYlide (structure A) in alkaline condition.23ꢀ27 Meanwhile,
strong electronꢀwithdrawing conjugated effect engender the partial positive charges at 2ꢀposition
and 4ꢀposition of PaQBr (structure B).14,18 The induced dipole between 3ꢀposition and 4ꢀposition
of the structure B could be attacked by structure A via 1,3ꢀdipolar cycloaddition reaction and
form a cyclized intermediate tetrahydroindolizine derivative12,28ꢀ32 (structure C). The structure C
could easily be deprotonated gradually and the dihydroindolizine derivative (structure D) is
formed first.41ꢀ43 Then structure D will undertake a further deprotonation process44 until the final
product indolizine derivative (DiPaQBr, C34H23N2O2Br) is formed. Considering that the
DiPaQBr and BQD1 both comprises the indolizine structure (Fig. 1) and a quaternary N+ atom,
similar compounds could be classified as the indolizine quaternary ammonium salts.
3.2 Weight loss measurement
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The mean value of corrosion rate (ꢀꢁꢂꢃꢃ ), inhibition efficiency (η) as well as the surface
coverage (ꢒ) are obtained from weight loss measurements of N80 steel in 15 wt.% HCl with and
without various concentrations of PaQBr or DiPaQBr. Table 1 shows the results of weight loss
measurements. The inhibition efficiency (ꢓ) and surface coverage (ꢒ) of each inhibitor was
determined as the following equation45:
ꢒ = ꢓ = ꢔ ꢆꢔꢖꢗꢘꢘ × 100
(2)
ꢕ
ꢔ
ꢕ
where, ꢀꢙ (g m−2 h−1) and ꢀꢎꢏꢐꢐ (g m−2 h−1) stands for the corrosion rates of N80 steel without and
with inhibitor respectively.
With the increasing concentrations for both of the inhibitors, the result in Table 1 shows that
the inhibitors would exhibit a better anticorrosive activity at a higher concentration. For all the
concentrations of PaQBr, the corrosion rate (ꢀꢎꢏꢐꢐ ) of N80 steel decreases to lower values
compared with BQC1 under the same experimental condition, revealing that PaQBr presents a
much better corrosion protection than BQC in strong acid solution. It could be inferred that the
conjugated molecular skeleton and the synergistic effect of the Brꢀ ion are the two main reasons
to explain the considerable protection efficiency of PaQBr.
The inhibition property of inhibitor usually could be enhanced by heteroatoms (N, O or P et al.)
or conjugated organic groups.46 Compared with BQC, the carbonyl double bond structure in
PaQBr provides an extra oxygen item and more πꢀelectrons as adsorbing sites in the inhibition
action. Meanwhile, it is widely accepted that the synergistic effect of the halide anions with the
inhibitive quaternary cations increases in the order Clꢀ＜Brꢀ＜Iꢀ,47 indicating that the presence of
Brꢀ (from PaQBr) in the corrosive medium may further improve the inhibition efficiency of the
quaternary ammonium inhibitors.
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In Table 1, it is worth observing that compared with PaQBr, the corrosion rate does not
decrease apparently as the concentration of DiPaQBr increases. The reason for this could be that
the hydrophobic double carbonyl groups and fusedꢀring aromatic structure in DiPaQBr make it
harder to get dissolved.21ꢀ22 The corrosion rate of N80 steel in Table 1 decreased to 634.96 g mꢀ2
hꢀ1 (inhibition efficiency is 47.5%) when the concentration of DiPaQBr was 0.356 mM. However,
when the concentration of PaQBr was 3.559 mM, the corrosion rate of the steel merely reached
798.94 g mꢀ2 hꢀ1 (inhibition efficiency is 34%). As an indolizine derivative of PaQBr, the
presence of DiPaQBr could reduce the corrosion rate more apparently at a lower concentration
compared with PaQBr, indicating that the DiPaQBr is a relatively more effective corrosion
inhibitor than the precursor quaternary ammonium salt inhibitor (PaQBr).
Table 1 Weight loss results for N80 steel in 15 wt.% HCl at 90 °C in the absence and presence of
inhibitor for 4h
substance Conc. (mM) v (g mꢀ2 hꢀ1)
θ
η (%) Error (%)
15% HCl
PaQBr
0
1210.14
798.94
450.42
213.11
139.53
64.74
0.79
3.559
8.873
17.771
26.619
35.591
0.3398 34.0
0.6278 62.8
0.8239 82.4
0.8847 88.5
0.9465 94.7
1.13
0.97
1.27
1.15
1.98
DiPaQBr
0.036
0.089
0.177
0.267
0.356
966.91
829.68
709.75
667.15
634.96
0.2011 20.1
0.3144 31.4
0.4135 41.4
0.4487 44.9
0.4753 47.5
1.88
2.07
1.62
0.97
1.85
3.3 Open circuit potential measurement
The variation of OCPof the N80 working electrode with time in 15 wt.% HCl in the absence
and presence of PaQBr or DiPaQBr is graphically presented in Fig. 4. Apparently, the value of
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OCP in uninhibited acid solution is more negative than that in inhibited medium and gradually
remained unchanged.
With the addition of the two inhibitors, the steadyꢀstate potential moves to a relatively positive
direction, suggesting that a high resistance of the charge transfer is taking place on the steel
surface. Additionally, it is clear from Fig. 4 that the positive shift of OCP is concentration
dependent, since the steady OCP became less negative with increasing concentrations of PaQBr
or DiPaQBr.
The OCPꢀtime relationships show that it took about 15 min for the OCPs to reach an
unchanged steady state. As a result, during the following electrochemical measurements, the
working electrode was immerged in the test solution for 20 min to obtain the steady OCP.
Fig. 4 OCPꢀtime curves for N80 steel in 15 wt.% HCl without and with different concentrations
of (a) PaQBr, (b) DiPaQBr at 25 °C.
3.4 Potentiodynamic polarization measurement
The inhibition efficiency from the polarization curves for both inhibitors was determined as the
following equation48:
ꢛ
ꢆꢚ
ꢓ = ꢚ
ꢖꢗꢘꢘ × 100
(3)
ꢖꢗꢘꢘ
ꢚ
ꢖꢗꢘꢘ
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where, ꢍꢎꢏꢐꢐ and ꢍꢜ
stands for the mean corrosion current densities in the absence and
ꢎꢏꢐꢐ
presence of PaQBr (or DiPaQBr), respectively.
The polarization curves of N80 steel in 15 wt.% HCl solution at 25°C are shown in Fig. 5. The
corrosion current density (ꢍꢎꢏꢐꢐ), the corrosion potential (ꢑꢎꢏꢐꢐ), the anodic Tafel slopes (ba), the
cathodic Tafel slopes (bc) and the inhibition efficiency (η) calculated from the curves are given in
Table 2.
As shown in Fig. 5(a) and 5(b), the log ꢍ values of both cathodic and anodic parts moved to
more negative area when the inhibitor exists. It clears from Fig. 5 that both the cathodic and
anodic reactions were suppressed by PaQBr and DiPaQBr, which suggests that the two inhibitors
reduces the dissolution process of anodic region and retards the cathodic hydrogen evolution
reaction as well.49
Fig. 5 Potentiodynamic polarization curves for N80 steel in 15 wt.% HCl without and with
different concentrations of (a) PaQBr, (b) DiPaQBr at 25 °C.
Result in Table 2 shows that the corrosion current density (ꢍꢎꢏꢐꢐ) decreases sharply with the
increase in the concentration of both the two studied inhibitors, leading to the increase in the
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inhibition efficiency. The electrochemical reaction rate was reduced due to the formation of a
barrier layer over the steel surface by the inhibitor molecule and both the two compounds act as
good corrosion inhibitor in 15 wt.% HCl solution. Obviously, the inhibitive capability of
DiPaQBr is much better than that of PaQBr, because the ꢍꢎꢏꢐꢐ reaches a much lower value at a
much lower concentration of DiPaQBr. Considering the active adsorption sites (nitrogen atoms,
πꢀelectrons, as well as the carbonyl groups) are reinforced in DiPaQBr, it is not surprising that
this dimer derivative could display such a fine anticorrosive performance.
Table 2 Potentiodynamic parameters and inhibition efficiency for N80 steel in 15 wt.% HCl at
25 °C in the absence and presence of inhibitor
﹣ꢑꢎꢏꢐꢐ (mV
vs. SCE)
substance Conc. (mM) ba (mV decꢀ1)
﹣
bc (mV decꢀ1)
η (%)
ꢍ
ꢎꢏꢐꢐ (ꢁA cmꢀ2)
15% HCl
PaQBr
0
85.9
126.8
151.3
133.7
145.5
152.5
152.3
985.9
72.0
43.4
31.5
27.6
22.7
437
416
411
413
417
419
3.559
8.873
17.771
26.619
35.591
136.0
132.2
147.7
155.2
160.6
92.7
95.6
96.8
97.2
97.7
DiPaQBr
0.036
0.089
0.177
0.267
110.3
111.4
111.5
116.5
120.6
132.0
109.8
124.5
36.5
30.8
28.7
29.1
410
406
419
413
96.3
96.9
97.1
97.0
0.356
117.9
109.8
28.5
414
97.1
Inspection of Fig. 5 reveals that the polarization curves exhibit some shifts in potential towards
more anodic regions relative to the blank solution. After a careful examination of the results in
Table 2, it is easy to recognize that compared with the uninhibited group, the inhibited specimens
did not show any significant change in values of ꢑꢎꢏꢐꢐ as the largest shift in ꢑꢎꢏꢐꢐ values were 27
mV and 31 mV for PaQBr and DiPaQBr respectively. According to literatures of corrosion,50ꢀ51
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the displacement of ꢑꢎꢏꢐꢐ in Table 2 between blank and in presence of PaQBr or DiPaQBr lies
within ±85 mV/SCE, indicating that the two inhibitors belong to mixedꢀtype inhibitor.
From Table 2, it is clear that as the concentration of the two inhibitors changed, the values of
anodic and cathodic Tafel slope constants (ꢝꢞ and ꢝꢎ) increase or decrease slightly. This implies
that the mechanism of the corrosion reaction on both two electrodes was not changed clearly in
the presence of PaQBr and DiPaQBr.52 The irregular variation of the cathodic and anodic slopes
in Table 2 reveal that the inhibition of steel was due to the adsorption of inhibitive molecules
along with the blocking of active sites and the formation of complexes on the steel surface.53
However, for both PaQBr and DiPaQBr, it is clear that the values of ꢝꢞ are greatly affected
compared with the value of ꢝꢎ, which indicate that the two compounds can be considered as
predominantly anodic type inhibitors.
3.5 EIS measurement
The Nyquist and Bode plots for the N80 steel corrosion in the absence and presence of PaQBr
(or DiPaQBr) is shown in Fig. 6(a and b) and Fig. 7(a, b, c and d) respectively. It is evident that
the inhibited and uninhibited specimens exhibit a single semicircular capacitive loop, indicating
that the corrosion process of N80 steel in 15 wt.% HCl is under charge transfer control.54 The
appearance of Nyquist plots for N80 steel is very similar in uninhibited and the inhibited solution.
This indicates that the inhibitors are retarding the corrosion process without changing the
corrosive mechanism.
It is well known that in Nyquist plots the diameter of the capacitive loop represents the charge
transfer resistance of the electrodes. The larger diameter of the impedance semicircle usually
indicates a better corrosion prevention behavior.55 Fig. 6 shows that the diameters of the
capacitive loops in the inhibited solutions are bigger than that in the uninhibited group for both
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of the two inhibitors. This implies that the two studied compounds could exhibit an relatively
good corrosion inhibition in 15 wt.% HCl solution. Besides, the shape and size of impedance
semicircle in Fig. 6 is gradually increased with the addition of inhibitor, showing that the charge
transfer resistance was increasing as the concentration of inhibitor rises. This phenomenon is also
known as the reduction in the dissolution of the electrode, owning to the protective layer of
inhibitors formed on the steel surface.56
Fig. 6 Nyquist plot for N80 steel in 15 wt.% HCl without and with various concentrations of (a)
PaQBr, (b) DiPaQBr as inhibitor at 25 °C.
For PaQBr, it is obvious in Fig. 6(a) that the diameter of the capacitive loop is proportional to
the increasing concentration of the inhibitor. However, since DiPaQBr is slightly soluble in 15
wt.% HCl, the diameter of the impedance loop in Fig. 6(b) did not enlarge apparently in
accordance with the increasing concentration of DiPaQBr. Nevertheless, it is not surprise to find
in Fig. 6 that the diameter of the impedance semicircles of DiPaQBr is larger than that of PaQBr
even when the concentration of DiPaQBr (0.036 mM) is only one percent of that of PaQBr
(3.559 mM), which indicates that the inhibitive capability of DiPaQBr is much better than PaQBr.
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Fig. 7 The Bode plots for N80 steel in 15 wt.% HCl in the absence and presence of different
concentrations of inhibitors at 25 °C: (a and b) PaQBr, (c and d) DiPaQBr.
The Bode impedance and phase angle plots for N80 steel in 15 wt.% HCl with and without
inhibitors are given in Fig. 7. It is noticed that compared with the uninhibited blank group, both
the impedance modulus Zꢟꢏꢠ in Fig. 7(a, c) and Phase angle in Fig. 7(b, d) increased
significantly when inhibitor was added. Both the PaQBr and DiPaQBr present the ability to
prevent the specimen from severely corroded. For the two inhibitors, it can be seen in Fig. 7(b)
and Fig. 7(d) that compared with the uninhibited specimen, the values of phase angles moved to
more negative direction as the concentration of the inhibitors increased. This suggests that more
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inhibitive molecules were adsorbed onto the steel surface and the smoothness of the metalꢀ
solution interface was modified at higher concentration of inhibitors.54ꢀ55
Fig. 8 depicts the equivalent circuit model employed to simulate the experimental EIS results.
The circuit consists of the solution resistance (ꢡꢢ), the polarization resistance (ꢡꢣ) and a constant
phase element (CPE). The depressed semicircular Nyquist plot in the presence of inhibitors is
considered as the polarization resistance between the steel surface and outer Helmholtz plane.57
In that case, the polarization resistance (ꢡꢣ) containing charge transfer resistance (ꢡꢎꢤ), diffuse
layer resistance (ꢡꢠ), accumulation resistance (ꢡꢞ) and film resistance (ꢡꢥ) etc.58 were included.
CPE was used at the place of double layer capacitance (ꢦꢠꢧ) to consider the roughness and other
homogeneities of the electrode, dislocation, adsorption of inhibitors etc.
Fig. 8 Equivalent circuit model used to fit the EIS Nyquist plots.
The inhibition efficiencies (ꢓ) can be calculated by means of the following equation:52
ꢛ
ꢆꢨ
ꢓ =ꢨ
ꢩ × 100
(4)
ꢩ
ꢛ
ꢨ
ꢩ
where, ꢡꢣ and ꢡꢜꢣ are the polarization resistance of the steel in the absence and presence of the
inhibitors, respectively. The representative example of using the equivalent circuit to fit the
experimental data for N80 steel in 15wt. % HCl with 3.559 mM PaQBr and 0.036 mM DiPaQBr
is shown in Fig. 9. The calculated EIS parameters are displayed in Table 3 according to the
model circuit. It is clear that the very small value of the associated chiꢀsquare (ꢪꢌ) in Table 3
indicates the validity of the used circuit. As shown in Table 3, with the increasing concentration
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of the two inhibitors, the values of charge transfer resistance (ꢡꢣ) increases while the electric
double layer capacitance (ꢦꢠꢧ) values decreases. The ꢡꢣ values are observed to be proportional to
the concentration of the two inhibitors and the gradual increase of ꢡꢣ implies a greater blocking
effect of the active area at the surface.51ꢀ52 The inhibitor molecules could form a protective film
on the metal surface by the adsorption and therefore, the transfer of charges from the metal
surface to the corrosive medium becomes more difficult. The value of ꢦꢠꢧ is related to the charge
storage ability of the interface double layer. The decreasing tendency of ꢦꢠꢧ values implies the
reduction of charges accumulated in the double layer due to the adsorption of the inhibitor
molecules.52 Also, the decrease in ꢦꢠꢧ reveals the decrease of the local dielectric constant or an
increase in the thickness of the electrical double layer.56 In addition, it is worth noticing that the
inhibition efficiencies obtained from EIS measurement and the polarization methods are in good
agreement among the two inhibitors.
Table 3. EIS parameters of N80 steel in 15wt. % HCl with and without various concentrations of
PaQBr and DiPaQBr at 25 °C
substance Conc. (mM) Rs (ꢂ cm2) Rp (ꢂ cm2) Cdl (ꢁF cmꢀ2）
η (%)
ꢪꢌ × 10ꢆꢫ
1.13
n
1.13
0.98
1.06
1.09
0.89
1.23
0.75
0.82
0.69
0.67
0.59
0.81
15% HCl
PaQBr
0
21.2
129.5
107.2
89.8
78.2
68.1
62.0
1.89
2.01
1.36
1.58
3.559
8.873
17.771
26.619
35.591
336.4
509.3
602.1
727.9
811.0
93.7
95.8
96.5
97.1
97.4
1.47
0.99
0.95
1.07
1.11
1.09
0.67
0.85
0.72
0.91
0.79
1.32
1.68
2.21
0.99
1.77
DiPaQBr
0.036
0.089
0.177
0.267
0.356
633.2
660.0
674.6
696.2
704.6
92.3
88.1
77.5
64.5
55.8
96.6
96.8
96.8
96.9
97.0
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Fig. 9 The representative of example simulation of Nyquist and Bode plots for PaQBr at 3.559
mM (a and c) and DiPaQBr at 0.036 mM (b and d).
It can be summarized from the results of EIS measurements that after being transformed into
the indolizine derivative, the anticorrosion performance of the heterocyclic quaternary
ammonium inhibitor (PaQBr) gets promoted. Similar result also could be concluded in literature1
where the inhibition of BQC and its indolizine derivative BQD was studied. The experimental
results from previous work and this work indicates that the antiꢀcorrosion ability of the indolizine
derivative is far better than the original quaternary ammonium inhibitor (heterocyclic quaternary
ammonium salts with active ꢀCH2ꢀ structure).
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3.6 Adsorption isotherms
In order to obtain the thermodynamic parameters about the adsorption process of the inhibitors,
the relationship between the surface coverage (ꢒ) and the concentration of the inhibitor (ꢦ) must
be considered. In this work, several attempts (including Langmuir, Temkin, Frumkin and
Freundlich isotherms) were made to find the relationship between ꢒ and ꢦ according to the
weight loss results in Table 1. The Langmuir adsorption isotherm was found to be the best one
that fitted with the results based on the correlation coefficient (ꢡꢌ).
According to the Langmuir adsorption isotherm, the relationship between ꢒ and ꢦ can be
expressed as following:59
ꢬ
ꢭ
ꢋ
=
+ ꢦ
(5)
ꢮ
ꢯꢰꢱ
From the equation, ꢲꢞꢠꢢ is the adsorptionꢀdesorption equilibrium constant for the adsorption
process and ꢦ is the inhibitor concentration, ꢒ represents the coverage area of the surface
obtained from weight loss tests. From the intercept of the isotherm plot, the equilibrium constant
(ꢲꢞꢠꢢ) could be acquired and therefore, the Gibbs standard free energy of adsorption (∆ꢳꢞꢙꢠꢢ) can
be determined by ꢲꢞꢠꢢ using the following equation:
ꢕ
ꢋ
∆ꢵ
ꢯꢰꢱ
lnꢲꢞꢠꢢ = ln ꢴꢴ.ꢴ
−
(6)
ꢨꢶ
where, ꢡ is the general gas constant, ꢷ is the absolute temperature and 55.5 is the molar
concentration of water in solution in mol Lꢀ1.60 The plots of ꢦ/ꢒ against ꢦ for the two inhibitors
are shown in Fig. 10 and the thermodynamic parameters are given in Table 4.
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Fig. 10 Langmuir adsorption isotherm plot of N80 steel obtained in 15 wt.% HCl with the
addition of different concentrations of (a) PaQBr and (b) DiPaQBr as inhibitor at 90 °C.
For PaQBr and DiPaQBr, the obtained plots is found to be perfectly linear with the correlation
coefficient (ꢡꢌ) higher than 0.999. This indicates that both the two inhibitors obey the Langmuir
adsorption isotherm. As shown in Table 4, the standard free energy (∆ꢳꢞꢙꢠꢢ) are ꢀ27.17 kJ molꢀ1
for PaQBr and ꢀ39.42 kJ molꢀ1 for DiPaQBr, the negative sign for ∆ꢳꢞꢙꢠꢢ refers to the
spontaneous adsorption of the inhibitors.
Table 4. Adsorption parameters of the adsorption process of PaQBr and DiPaQBr
∆Gꢞꢠꢢ ꢸkJ mol−1ꢹ
﹣27.17
−1
ꢙ
Rꢌ
substance
⁄
⁄
ꢲꢞꢠꢢ ꢸL mol ꢹ
PaQBr
0.9993
0.9995
145.81
DiPaQBr
8422.47
﹣39.42
In general, when the magnitude of ∆ꢳꢞꢙꢠꢢ for the inhibitor lies in the order of ꢀ20 kJ mol−1 or
more positive, it satisfies the physisorption between the inhibitor and the metal surface. The
∆ꢳꢞꢙꢠꢢ values around ꢀ40 kJ mol−1 or more negative are associated with the chemisorption
process.55 Considering the above description and the calculated ∆ꢳꢞꢙꢠꢢ values in Table 4, it can be
claimed that DiPaQBr (∆ꢳꢞꢙꢠꢢ = ꢀ39.42 kJ molꢀ1) closely exhibits the chemical adsorption with
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the steel surface in 15 wt. % HCl, while the type of interaction between PaQBr (∆ꢳꢞꢙꢠꢢ = ꢀ27.17
kJ molꢀ1) and the metallic surface included physisorption as well as chemisorption.59
3.7 Suggested mechanism of inhibition
Electronꢀrich atoms such as nitrogen, sulfur, oxygen, etc.2,4,60ꢀ62 and conjugated structures
containing πꢀelectrons are main functional groups of inhibitors for metal or alloy. Quaternary N+
cation, the oxygen atom in carbonyl group as well as the aromatic πꢀelectrons are key inhibitive
groups63ꢀ65 in PaQBr and its indolizine derivative. The unshared electron pair and πꢀconjugated
electrons would coordinate with the empty dꢀorbital in metal atoms, which the adsorption is
classified as chemical adsorption. The firmly adsorbed inhibitors would thereby prohibit the
metal surface from the contact of H+ ions. By comparison of PaQBr and BQC,1,10 the extra
adsorbing sites and the beneficial synergistic effect of Brꢀ lead to a great rise in corrosion
prevention. These are the main reasons why PaQBr shows a better inhibitive performance than
that of BQC.
The observed corrosion inhibition of N80 steel in 15 wt. % HCl in the presence of PaQBr or
DiPaQBr can be explained by the adsorption of the inhibitor and the mechanism of the inhibition
process is discussed by Bockris, Drazic and Despic (BDD) mechanism.66
As is proposed in the potentiodynamic polarization result, the hydrogen evolution process is
activationꢀcontrolled and the mechanism of reduction is not affected even in the presence of the
two inhibitors. Probable protocols of the cathodic electrode reaction can be abridged as follows:
Fe + Hꢺ + eꢆ ↔ ꢸFeHꢹꢞꢠꢢ
(7)
(8)
(9)
Fe + ꢸInꢺꢹꢞꢠꢢ + eꢆ → ꢸFe · Inꢹꢞꢠꢢ
ꢸFeHꢹꢞꢠꢢ+ Hꢺ + eꢆ → Fe + Hꢌ
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where, the subscript ‘ads’ represents the adsorption on the surface of steel, ‘Inꢺ’ represents the
inhibitive cation. It is apparent from the Eq. (7) and Eq. (8) that there is a competition between
H+ and Inꢺ to get adsorbed onto the metal surface. Since the first step (Eq. 7) is very likely to be
the rateꢀcontrolling step, the presence of the inhibitor in the acidic solution retards the hydrogen
reduction. The high molecular weight and large size of the studied inhibitor also increases the
inhibition efficiency.
The mechanism for the anodic dissolution of Fe in acidic solution was proposed initially in
literature.66ꢀ67 In the presence of inhibitor, the mechanism for the retardation of Fe anodic
dissolution can be proposed as follows:
Fe + HꢌO ↔ ꢸFe · HꢌOꢹꢞꢠꢢ
(10)
(11)
(12)
ꢸFe · HꢌOꢹꢞꢠꢢ + In ↔ ꢸFeOHꢆꢹꢞꢠꢢ + Hꢺ + In
ꢸFe · HꢌOꢹꢞꢠꢢ + In ↔ ꢸFeInꢹꢞꢠꢢ + HꢌO
ꢐꢠꢢ
ꢸFeOHꢆꢹꢞꢠꢢ ꢻꢼ ꢸFeOHꢹꢞꢠꢢ + eꢆ
(13)
(14)
(15)
(16)
ꢸFeInꢹꢞꢠꢢ ↔ ꢸFeInꢺꢹꢞꢠꢢ + eꢆ
ꢸFeOHꢹꢞꢠꢢ + ꢸFeInꢺꢹꢞꢠꢢ ↔ ꢸFeInꢹꢞꢠꢢ + FeOHꢺ
FeOHꢺ + Hꢺ ↔ Feꢌꢺ + HꢌO
where, ‘In’ represents the inhibitor molecule. According to the above mechanism, the formation
of the adsorbed intermediate FeInꢞꢠꢢ (Eq. 12) reduces the amount of FeOHꢆ
(Eq. 11)
ꢞꢠꢢ
available for the key rate determining step (Eq. 13). Depending on the relative solubility of the
adsorbed intermediate, FeInꢞꢠꢢ could either inhibit or catalyze the further dissolution of the
metal. From the experimental results, it can be inferred that the insoluble complexes of Feꢀ
PaQBr or FeꢀDiPaQBr dominates the intermediates and thus enhances the inhibitive effects.
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As a condensed dimer derivative of PaQBr, the active adsorption sites (nitrogen atoms, πꢀ
electrons, as well as the carbonyl groups) in PaQBr are reinforced in DiPaQBr. As a result, the
indolizine derivative DiPaQBr could provide more adsorption sites than that of PaQBr. The extra
adsorption sites in the dimer derivative would fasten the inhibitor with the crude surface and
finally increases the inhibitive ability. Although a stronger protective interaction would occur
between the DiPaQBr and the steel, the relatively low solubility of DiPaQBr (containing two
hydrophobic carbonyl groups) make it less effective to present its inhibitive potentiality.
3.8 Comparison of inhibitory efficiencies of some heterocyclic quaternary ammonium salts
inhibitors in acidic solution
Table 5 shows the inhibitory efficiencies of the studied inhibitor and some of the reported
similar heterocyclic quaternary ammonium inhibitor for N80 steel in 15 wt.% HCl. It is clear
from the table that the prepared compound could exhibit a relatively better corrosion protection
than those of the analogous inhibitive Nꢀheterocyclic quaternary ammonium salts in literature.
Table 5. Some heterocyclic quaternary ammonium salts investigated as corrosion inhibitors in
literatures and this work for N80 steel in 15 wt.% HCl (by weight loss mesaurment)
Name of the inhibitor
Inhibition efficiency (%) Reference
1ꢀbenzylquinlinium chloride
80.6
91.5
1, 10
2
Nꢀdodecylpyridinium
bromide
N,N′ꢀButaneꢀ1,4ꢀdiylꢀ
bisquinolinium dibromide
N,N′ꢀButaneꢀ1,6ꢀdiylꢀ
bisquinolinium dibromide
N,N′ꢀButaneꢀ1,8ꢀdiylꢀ
bisquinolinium dibromide
1ꢀnaphthylmethylquinolinium
chloride
81.1
86.1
91.0
92.5
4
4
4
68
(4ꢀvinyl)ꢀbenzyl quinoidine
chloride
76.9
94.7
69
Phenacyl Quinolinium Bromide
This work
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4. Conclusion
DiPaQBr, the dimer indolizine derivative of a strong acid corrosion inhibitor Phenacyl
Quinolinium Bromide (PaQBr) was synthesized from PaQBr. Through HRMS and NMR, the
structure of DiPaQBr was characterized and determined as C34H23N2O2Br. The corrosion
inhibition of PaQBr and DiPaQBr in 15 wt.% HCl were evaluated by weight loss measurement,
potentiodynamic polarization and EIS measurement. Based on the experimental results, the
following conclusions can be made:
(1) With the assistance of triethylamine, heterocyclic quaternary ammonium salts (PaQBr) with
active methylene group could get converted to its dimer derivatives (DiPaQBr). The dimer
derivative could be classified as indolizine quaternary ammonium salt. The formation
mechanism of DiPaQBr from the condensation of two molecules of PaQBr through 1,3ꢀdipolar
cycloaddition was proposed and confirmed.
(2) It is evident that the corrosion inhibition of DiPaQBr is better than the PaQBr, since the
DiPaQBr could reduce the corrosion rate to a lower value at a much lower concentration in
gravimetric analysis. EIS results also showed that the DiPaQBr exhibits a greater inhibitive
capability than PaQBr. Either PaQBr or DiPaQBr acted as a mixedꢀtype inhibitor and retards the
corrosion process without changing the corrosive mechanism. The results obtained from weight
loss measurement and electrochemical methods are in agreement and confirm the big inhibition
difference between PaQBr and its indolizine derivative DiPaQBr.
(3) It can be inferred that through 1,3ꢀdipolar cycloaddition, the other heterocyclic quaternary
ammonium salt inhibitors (including pyridine, quinoline and isoꢀquinoline salts etc.) which
contain active methylene group may get converted to the corresponding inhibitive indolizine
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derivatives. In that way, using the dimer indolizine derivative as the novel corrosion inhibitors
may offer a new method for corrosion protection in acid medium.
Conflicts of interests
The authors declare no competing financial interest.
Acknowledgement
Supported by “the Fundamental Research Funds for the Central Universities” (16CX06030A).
Special thanks for Ren Sumei and Song Ni from Ocean University of China (OUC) for their
smart work on HRMS and NMR experiments. Thanks for Yang Menglong, Yang Haiyan from
Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) of the Chinese Academy
of Sciences (CAS) for their assistance and helpful discussion on preparative chromatography.
The author also express thanks to Sun Shuangqing, Tang Shiming and Xi Yanyan from China
University of Petroleum (UPC) for their technical guidance on electrical and instrumental tests.
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corrosion inhibition in 15 wt.% hydrochloric acid, Corros. Sci., 2015, 99, 281ꢀ294.
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