Imidazolines containing single-, twin- and triple-tailed hydrophobes and hydrophilic pendants (CH2CH2NH)nH as inhibitors of mild steel corrosion in CO2-0.5 M NaCl

Article abstract of DOI:10.1039/c5ra21276f

Inhibition of mild steel corrosion in CO₂-0.5 M NaCl (40 °C, 1 atm; 120 °C, 10 bar) by a new series of imidazolines having single-, twin- and triple-tailed phenyl substituents at C(2) and N(1) pendants of CH₂CH₂NH₂ and (CH₂CH₂NH)₂CH₂CH₂NH₂ have been examined. Imidazolines containing twin-tailed (3,5-dioctyloxyphenyl) hydrophobes outperformed their single- and triple-tailed counterparts as well as two commercial imidazolines. The triamine pendant imparted better inhibition at higher temperature and pressure. The XPS study confirmed the presence of an imidazoline film covering the metal surface. The imidazolines prefer to be adsorbed on the metal surface rather than micellization.

Full text of DOI:10.1039/c5ra21276f

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Imidazolines containing single-, twin- and triple-
tailed hydrophobes and hydrophilic pendants
(CH2CH2NH)nH as inhibitors of mild steel corrosion
in CO2–0.5 M NaCl†
Cite this: RSC Adv., 2016, 6, 12348
a
Mohammad A. J. Mazumder, Mazen K. Nazal,a Mohamed Faizb and Shaikh A. Alia
*
Inhibition of mild steel corrosion in CO2–0.5 M NaCl (40 ꢀC, 1 atm; 120 ꢀC, 10 bar) by a new series of
imidazolines having single-, twin- and triple-tailed phenyl substituents at C(2) and N(1) pendants of
CH2CH2NH2 and (CH2CH2NH)2CH2CH2NH2 have been examined. Imidazolines containing twin-tailed
(3,5-dioctyloxyphenyl) hydrophobes outperformed their single- and triple-tailed counterparts as well as
two commercial imidazolines. The triamine pendant imparted better inhibition at higher temperature and
pressure. The XPS study confirmed the presence of an imidazoline film covering the metal surface. The
imidazolines prefer to be adsorbed on the metal surface rather than micellization.
Received 13th October 2015
Accepted 22nd January 2016
DOI: 10.1039/c5ra21276f
www.rsc.org/advances
2H+ (aq) + 2e # H2 (g)
(3)
(4)
(5)
(6)
1. Introduction
2H2O + 2e # 2OHꢁ (aq) + H2
The oil and gas industries, at every stage of production involving
extraction, rening, storage and transportation through pipe-
lines, face severe corrosion problems, which must be alleviated
by the application of corrosion inhibitors.1 CO2 corrosion,
induced by its presence in the produced uids, is one of the
most prevalent forms of attack, which causes failures of carbon
steels.2 Signicant progress has been made in pinpointing
factors and conditions like temperature, pH, CO2 pressure, etc.,
which inuence the complicated mechanism of CO2 corro-
sion.3,4 However, a story unraveling the inhibition mechanism
and its kinetics, which would help design efficient chemical
inhibitors to combat corrosion, still remains incomplete.4–6
In an oxygen-free anaerobic aqueous solution, CO2 is in
ꢁ
2H2CO3 (aq) + 2e # 2HCO3 (aq) + H2 (g)
ꢁ
2ꢁ
2HCO3 (aq) + 2e # 2CO3 (aq) + H2 (g)
and iron dissolution at the anodic sites:
Fe (s) # Fe2+ (aq) + 2e
(7)
Combination of relevant ions then forms several iron(II)
compounds as shown by eqn (8)–(11):7,9,10
Fe2+ (aq) + 2OHꢁ (aq) # Fe(OH)2 (s)
Fe(OH)2 (s) # FeO (s) + H2O
(8)
(9)
ꢁ
equilibria with H2CO3, HCO3 and H+, as described by eqn (1)
and (2):
ꢁ
Fe2+ (aq) + 2HCO3 (aq) / Fe(HCO3)2 (aq)
(10)
(11)
CO2 (g) # CO2 (aq)
(1)
Fe(HCO3)2 (aq) # FeCO3 (s) + H2O (aq) + CO2 (g)
ꢁ
CO2 (aq) + H2O # H2CO3 (aq) # HCO3 (aq) + H+ (aq) (2)
Dry CO2 is not corrosive to mild steel pipelines,11 it is the
carbonic acid (H2CO3) which, at the same pH, is more corrosive
than aqueous HCl.12 The predominant cathodic reduction at pH
< 4 is believed to be reaction (3), while at pH values in the range
5–6, reactions (5) or (6) takes place.13 The pitting corrosion caused
by CO2 leads to the deposition of layers of hydroxides and
carbonates on the metal surface. At higher temperatures, the rate
of cell reactions (3), (5) and (6) decreases as a consequence of
decreased solubility of CO2 in the aqueous layer as represented by
eqn (1), while the rates of reactions (4), (7) and (11) increase.7
Advantageous passivation of steel surface by the lm composed
The corrosive reactions on the metal surface are the results
of the following half-cell reactions at the cathodic sites:7,8
aChemistry Department, King Fahd University of Petroleum and Minerals, Dhahran
31261, Saudi Arabia. E-mail: jafar@kfupm.edu.sa; Web: http://faculty.kfupm.edu.sa/
CHEM/jafar/; Fax: +966-13-860-4277; Tel: +966-13-860-7836
bPhysics Department, King Fahd University of Petroleum and Minerals, Dhahran
31261, Saudi Arabia
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra21276f
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of the corrosion products such as water–insoluble FeCO3 and
Fe(HCO3)2 minimizes the corrosion rate of mild steel.14
Most corrosion inhibitors used in oilelds have motifs of
surfactant molecules having a hydrophilic polar head and
a hydrophobic tail. Adsorption of surfactants onto metal surface
leads to the formation of a protective layer, which decreases the
wettability of steel surface, while their accumulation at the oil–
water decreases the oil–water interfacial tension thereby
making it easier for the oil to entrain the water.15 Imidazolines
(IMs) I, containing sub-structures of a hydrophilic IM ring
having electron-rich amidine (N]C–N) motifs embedded in it,
a hydrophobic alkyl chain at C(2) and a hydrophilic pendant at
N(1), are widely used as inhibitors in CO2-satuarted saline
media (Scheme 1).16
While the nitrogens in amidine motifs are weakly nucleo-
philic, they are strong bases akin to organic bases like 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) (pKb 1.1) and 1,5-
diazabicyclo[4.3.0]non-5-ene (DBN) (pKb 0.5),17 all having the
same amidine motifs. The pendant NH2 is a weaker base (pKb z
4) but a stronger nucleophile enabling it to form carbamate salt
Scheme 1 IMs in an aqueous CO2 system.
Scheme 2 Synthesis of IMs.
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II in the presence of CO2.18–20 As a base, all the nitrogens can
abstract a proton from carbonic acid (i.e. aqueous CO2, H2CO3)
to form protonated III and IV.20–22 Amidine motifs in IMs on
partial hydrolysis is converted into amides V.21 The IM–CO2 (aq)
system thus contains several compounds (I–V), H2CO3, CO2,
H2O and ionic species such as HCO3ꢁ, CO32ꢁ, OHꢁ, and H3O+.
It is indeed an arduous task to unlock the poorly understood
mechanism of corrosion inhibition in such a complex scenario
where the system consists of a complex maze of neutral and
ionic species let alone the effects of the presence of Clꢁ ions,
temperature, partial pressure of CO2, corrosion products among
many other factors.
2. Experimental
2.1. Materials
p-Hydroxybenzoic acid, a-resorcyclic acid, gallic acid, 1-bro-
mooctane, cysteine hydrochloride, silica gel 100 and thionyl
chloride from Fluka Chemie AG, and diethylenetetramine
(DETA) (99.5%) from Aldrich chemical were used as received.
Tetraethylenepentamine (TEPA) (ꢂ60% purity) from Aldrich
chemical was puried as before,32 and IMs 7a and 8a were
synthesized using literature procedures.20 3,5-Dioctyloxy- and
3,4,5-trioctyloxy-benzoic acid 2b and 2c were prepared as
described.33
There are contradictory reports on the relative importance
of each sub-structure in IMs in corrosion inhibitions.23,24 Bis-
IM and IM compounds have been shown to form a compact
inhibitor layer to reduce the cathodic controlled CO2 corro-
sion.25,26 The protonated form of a thioureido IM acts as an
inhibitor of CO2 corrosion by its adsorption on the negatively
charged steel surface thereby shiing the potential of zero
charge (PZC) to positive direction.27 Corrosion inhibition
performance of several 2-undecyl-2-IMs has indicated that the
hydrophilic N-pendants have remarkable inuence on the
inhibition efficiency.28 A study of corrosion inhibition of an
amido-IM on mild steel in CO2–saturated NaCl solution has
revealed that its inhibition efficiency was enhanced by addi-
tion of iodide ions due to synergistic effect.29 The presence of
double bonds in the hydrocarbon chains at C(2) and func-
tional groups in N-pendants30 as well as polycentric adsorption
sites has been shown to increase inhibition efficiency by the
formation of a chemically adsorbed lm on the surface of the
metal.31 For two IMs, both having a heptadecyl hydrophobe at
C(2), the one having the longer N-pendant of (CH2CH2NH)3H
performed better than the other having CH2CH2NH2 pendant;
they are shown to cover the steel surface prior to reaching their
critical micelle concentrations (CMCs).32 A novel series of IMs
having hydrophobes of octyloxy, dodecyloxy- and octadecy-
loxyphenyl substituent at C(2) and N-pendants of (CH2CH2-
NH)nH has been shown to impart very good inhibition of CO2
corrosion of mild steel in CO2–0.5 M NaCl.20 For ‘n’ values of 1
and 3, there was no change in inhibition efficiencies, while
increasing hydrophobe chain lengths imparted increasing
corrosion inhibition.
2.2. Physical methods
Melting points are recorded in an Electrothermal Apparatus.
IR spectra were recorded on a Perkin Elmer 16F PC FTIR spec-
trometer. NMR spectra were taken in CDCl3 (+TMS) on a JEOL
LA 500 MHz NMR spectrometer. Elemental compositions were
determined with an Elemental Analyzer (Carlo-Erba: model
1106).
2.3. Synthesis
2.3.1. General procedure for the synthesis of di-, and tri-
octyloxybenzamides (3b, 3c). The detailed procedure for the
synthesis of octyloxybenzamides 3b and 3c given in the ESI.†
2.3.2. General procedure for the synthesis of di-, and tri-
octyloxybenzonitriles (4b, 4c). The detailed procedure for the
synthesis of di-, and tri-octyloxybenzonitriles 4b and 4c given in
the ESI.†
2.3.3. General procedure for the synthesis of 1-(2-
aminoethyl)-2-(di-, tri-alkoxyphenyl)-2-imidazolines (7b, 7c). A
solution of di- or tri-octyloxybenzonitriles (4b or 4c) (25 mmol)
and DETA 5 (62 mmol) containing cysteine-HCl (100 mg) in a RB
ask tted with an U-tube containing mineral oil was stirred
under N2 at 145 ꢀC for 1 h. Aer adding another portion of
cysteine-HCl (100 mg), the mixture was heated at 145 ꢀC for an
additional 1 h. Aer the ceasing of the evolution of NH3 gas, the
cooled reaction mixture in CH2Cl2 (50 mL) was carefully washed
with water (3 ꢃ 300 mL). Aer drying the organic layer and
removal of the solvent, IM (7b or 7c) was obtained as a pinkish
liquid/semisolid. The detailed characterization of 7b and 7c is
given in ESI.†
In the current work, it is our objective to synthesize
a series of IMs having C(2) single-, twin- and triple-tailed
phenyl substituents and N-pendants of (CH2CH2NH)nH (n ¼
1 and 3) (Scheme 2). The presence of electron-donating
octyloxy tail(s) would increase the electron density in the
amidine motifs and thus retard its hydrolysis to amides
by making the amidine C less susceptible to nucleophilic
attack. The study, assisted by potentiodynamic polarizations,
electrochemical impedance spectroscopy (EIS), gravimetric
weight loss, surface tension and X-ray photoelectron spec-
2.3.4. General procedure for the synthesis of IMs 8b, 8c.
The reaction described under Section 2.3.3 was repeated under
same conditions using TEPA 6 (62 mmol) instead of DETA 5
keeping all other materials unchanged. Similar work up affor-
ded 8b or 8c as a pinkish liquid/semisolid. The detailed char-
acterization of 8b and 8c is provided in the ESI.†
2.4. Specimens
troscopy (XPS), would permit us to examine the effects of the Corrosion tests by electrochemical measurements in CO2–
hydrophobic tails and length of N-pendants on the inhibition 0.5 M NaCl solution were performed using working electrode
in CO2–0.5 M NaCl solutions. We anticipate an interesting prepared from mild steel having the composition (in wt%):
outcome to enrich the current knowledge of the inhibition 0.089% (C), 0.34% (Mn), 0.037% (Cr), 0.022% (Ni), 0.007%
process.
(Mo), 0.005% (Cu), 0.005% (V), 0.010% (P), 99.47% (Fe). Mild
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steel sheet of 1 mm thickness was machined to a ag shape
having a 3 cm stem, which was insulated by Araldite paint
(affixing material). The remaining exposed area of 2 cm2
was abraded with emery papers (100, 400, 600 and 1500 grit
size), washed with acetone, deionized water and nally
acetone. Prior to their use, an ultrasonic bath was used to
treat the specimens for 5 min followed by washing with
distilled water.
2.6.2. Linear polarization resistance (LPR) method. The
LPR measurements were carried out using the same cell
described above to obtain polarization resistance values from
current potential plots by scanning through a potential range of
ꢄ10 mV around Ecorr
.
2.6.3. Electrochemical impedance spectroscopy (EIS). The
impedance measurements were carried out in a 750 mL round-
bottom ask tted with a three electrodes system as described
under Section 2.6.1. A solution of 0.5 M NaCl (250 mL) con-
taining 100 mg Lꢁ1 NaHCO3 were saturated with CO2 (1 atm) at
40 ꢀC was used as a test solution. The impedance plots were
recorded using a computer controlled potentiostat-galvanostat
(Auto Lab, Booster 10A-BST707A, Eco Chemie, Netherlands).
The impedance measurements were carried out at the open
circuit potential with a frequency scan ranging from 100 kHz to
50 mHz and amplitude value of 10 mV. The impedance curves
were tted and the electrochemical equivalent circuit obtained
by NOVA (Version 1.8) soware.
Mild steel coupons A and B measuring z2.5 ꢃ 2.0 ꢃ 0.1 cm3
for the autoclave tests, have the following elemental analyses
(in wt%):
2.4.1. Coupon A. 0.082% (C), 0.016% (Cr), 0.207% (Mn),
0.062% (Ni), 0.029% (Cu), 0.012% (Mo), 0.032% (Si), 0.011%
(Co), 0.045% (Al), <99.3% (Fe).
2.4.2. Coupon B. 0.168% (C), 0.038% (Cr), 0.495% (Mn),
0.034% (Ni), 0.074% (Cu), 0.237% (Si), 0.014% (P), 0.024% (S),
0.011% (Co), 0.080% (Al), <98.6% (Fe).
Mild steel, also known as carbon steel, is the steel in which
the main interstitial alloying constituent is carbon. Low-carbon
steel, containing approximately 0.05–0.25% carbon, is now the
most common form of steel for many applications because its
price is relatively low while it provides acceptable material
properties. We have used three kinds of mild steel: coupons for
electrochemical measurements and coupons A and B for the
autoclave tests having carbon content of 0.089%, 0.082% and
0.168%, respectively, to cover a range of carbon content.
2.7. Autoclave experiments
The weight-loss measurements were performed by immersing
carbon-steel coupons in a 0.5 M NaCl solution (250 mL) at 120
ꢀC in the presence of CO2 (10 bar) was conducted in a R&D
Autoclave Bolted Closure System (Autoclave Engineers, model #
401C-0679) for 48 h. The detailed experimental procedure with
or without inhibitors (200 ppm) is described in our earlier
work.34
2.5. Solutions
Test solutions of CO2–0.5 M NaCl for the inhibition study in the
presence and absence of IMs at 40 ꢀC (1 atm) as well as at
a higher pressure (10 bar) and temperature (120 C) were dea-
2.8. Measurement of surface tension
ꢀ
The surface tension of the IM samples in CO2–0.5 M NaCl
solution at 40 ꢀC were measured by a PHYWE surface tensi-
ometer (Germany) as described.20
erated by purging with 99.999% N2 (20 min) followed by
continuously bubbling with 99.999% pure CO2 (20 min) to avoid
corrosion caused by oxygen. During polarization measure-
ments, the gentle ow of CO2 was maintained above the surface
of the solution without agitating the bulk of the solution. The
addition of NaHCO3 (100 mg Lꢁ1) in the test solutions maintain
their pH between 5.0 and 5.5 to avoid any change in the
corrosion mechanism.
2.9. The standard free energy of micelle formation (DGmꢀ ic
The DGꢀmic of the IM surfactant is given by eqn (12),35
)
DGꢀmic ¼ RT ln(Ccmc
)
(12)
2.6. Electrochemical measurements
where R, T and Ccmc represent the gas constant, temperature
and surfactant concentration in mol Lꢁ1 at the critical micelle
concentration (CMC).
2.6.1. Tafel extrapolations. The experiments were per-
formed in a 750 mL round-bottom ask tted with a mild steel
working electrode, saturated calomel electrode (SCE), and
a counter electrode of graphite (z5 mm diameter). The test
solutions of 0.5 M NaCl (250 mL) were saturated with CO2 (1
2.10. X-ray photoelectron spectroscopy
atm, 40 ꢀC). Aer connecting all three electrode cells to the The metal coupons of dimension of 2.5 ꢃ 2.0 ꢃ 0.1 cm3 having
potentiostat, the polarization curves were recorded by the same composition to those used in the electrochemical tests
a computer controlled potentiostat–galvanostat (Auto Lab, were cleaned (vide supra) and then immersed in a CO2–0.5 M
Booster 10A-BST707A, Eco Chemie, Netherlands) where the NaCl–IM (100 ppm) at 40 ꢀC for 6 h. The coupons were then
cutoff current was set at 10 mA. A computer (Windows 7) loaded dried under N2 aer carefully rinsing with distilled deionized
with NOVA (Version 1.8) soware processed the data. The water. The XPS analysis was carried out as described20 using
working electrode was pre-corroded for 30–60 min to achieve a Thermos Scientic X-ray photoelectron spectrometer (model #
a stable open circuit potential, aer which, a scan of ꢄ250 mV Escalab 250 Xi). The C 1s peak at 285.4 eV was selected to be the
with respect to the open circuit potential Ecorr is conducted at reference peak. Deconvoluted XPS spectra were obtained using
a rate of 0.5 mV sꢁ1
.
a non-linear least squares algorithm.
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their efficacies were compared with that of two commercial
inhibitor samples: QI80-E (R ¼ C12 to C22) from Materials
Performance and ARMOHIB 219 from AKZO NOBEL (Scheme 2).
3. Results
3.1. The IMs' synthesis
As outlined in Scheme 2, mono-, di- and tri-hydroxybenzoic acids
1 was alkylated with octyl bromide to give the corresponding
octyloxybenzoic acids 2, which were converted into nitriles 4 via
benzamide 3 in excellent yields. Heating a mixture of nitrile 4 with
DETA 5 or TEPA 6 in the presence of catalyst cysteine. HCl at
3.2. Electrochemical measurements
3.2.1. OCP vs. time measurements. In this study, the effect
of immersing time on the open circuit potential (OCP) versus
saturated calomel electrode (SCE) in CO2 saturated 0.5 M NaCl
solution was explored for the mild steel coupon in the absence
(blank) and presence of different concentrations (1 and 100
ꢀ
145 C led to the formation of the IMs 7 and 8, respectively, in
1
excellent yields. The IMs were readily identied by H and 13C
NMR spectroscopy. The 1H and 13C NMR spectra of some of the
representative IMs 7c, 8a and 8c are displayed in Fig. 1 and 2,
respectively. The characteristic Hs marked a–c are readily identi-
able in the proton spectra. Aromatic Hs appeared at expected
places with anticipated multiplicities. The carbon spectra
revealed the presence of four signals for the carbons of 7c marked
as a–d, while for the TEPA-derived IMs 8a and 8c, all eight carbons
marked a–h appeared as non-overlapping signals in the spectra.
Thus a series of IMs having single-, twin- and triple-tailed octy-
loxyphenyl groups and different N-substituents has been synthe-
sized to study the effects of C(2) substituents and N(1) pendants;
ꢀ
ppm) of 7b and 8b at 40 C. The results obtained for the vari-
ation of OCP with time are depicted in Fig. 3. In the absence and
presence of inhibitors, the OCP changes very quickly towards
the less negative value and become stable aer 20 min. Hence,
the 30 minutes of mild steel immersion time was used in all
subsequent electrochemical measurements.
3.2.2. Tafel extrapolation. The extrapolation data for the
mild steel corrosion in a CO2–0.5 M NaCl are summarized in
Table 1. Some representative Tafel plots are shown in Fig. 4;
usual analysis of each pair of Tafel plots provided the icorr and
Fig. 1 1H NMR spectra of the IMs 7c, 8a and 8c in CDCl3.
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Fig. 2 13C NMR spectra of the IMs 7c, 8a and 8c in the d 40–55 ppm range in CDCl3.
3.2.3. LPR. The inhibition efficiency (h%) from LPR tech-
nique was calculated using eqn (13):
!
R0p ꢁ Rp
hð%Þ ¼
ꢃ 100
(13)
R0p
where Rp and R0p represent the polarization resistance values in
inhibitor-free and inhibitor-added CO2–0.5 M NaCl at 40 ꢀC,
respectively. The results are summarized in Table 1.
3.2.4. Impedance measurements. The Randles electro-
chemical equivalent circuit shown in Fig. 5 was used to repre-
sent the Nyquist diagrams of solution-mild steel interface in
absence and presence of inhibitors (7b, 8b). The tted equiva-
lent circuit included three elements; a solution resistance (Rs),
polarization resistance (Rp) and constant phase element (CPE).
The Rs resulted from the potential drop between the mild steel
electrode and the reference electrode and its value obtained
from crossing the semicircle with the real part (Z0) axis at high
frequency. The polarization resistance includes a charge
transfer resistance and a diffusion layer resistance at the surface
of the mild steel electrode.36 The polarization resistance
Fig. 3 Variation of OCP of mild steel with time of immersion in CO2–
saturated 0.5 M NaCl containing different concentrations (1 and 100
ppm) of 7b and 8b at 40 ꢀC.
Ecorr values. The cathodic Tafel lines were extrapolated with
respect to free corrosion potential by using an automated linear
curve tting Nova (version 1.8) soware.
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Table 1 Results of Tafel plots and LPR of a mild steel sample in CO2–0.5 M NaCl containing inhibitors 7(a–c) and 8(a–c) at 40 ꢀ
Paper
C
LPRe
Concentration
Tafel plots
ppm
(by wt)
Ecorr vs. SCE
(mV)
ba
(mV decꢁ1
bC
(mV decꢁ1
icorr
Sample
mM
)
)
(mA cmꢁ2
)
ha (%)
ha (%)
Blankb
0
1
5
10
20
50
100
1
0
ꢁ700
ꢁ694
ꢁ683
ꢁ677
ꢁ671
ꢁ667
ꢁ665
ꢁ691
ꢁ678
ꢁ674
ꢁ667
ꢁ654
ꢁ643
ꢁ693
ꢁ686
ꢁ680
ꢁ673
ꢁ661
ꢁ655
ꢁ671
ꢁ660
ꢁ648
ꢁ645
ꢁ641
ꢁ637
ꢁ667
ꢁ658
ꢁ653
ꢁ642
ꢁ632
ꢁ627
ꢁ697
ꢁ689
ꢁ675
ꢁ664
ꢁ659
ꢁ650
41.2
75.2
40.0
44.3
63.9
45.6
36.4
32.5
53.7
48.1
52.1
32.5
55.4
66.5
28.1
46.6
42.3
64.9
64.0
61.1
32.4
57.5
35.8
69.2
66.7
71.9
46.9
50.3
46.6
51.3
52.6
65.8
50.4
58.3
65.9
21.3
45.3
ꢁ258
ꢁ141
ꢁ166
ꢁ172
ꢁ181
ꢁ139
ꢁ123
ꢁ165
ꢁ161
ꢁ149
ꢁ168
ꢁ171
ꢁ157
ꢁ103
ꢁ167
ꢁ80.6
ꢁ81.5
ꢁ177
ꢁ146
ꢁ249
ꢁ146
ꢁ228
ꢁ183
ꢁ124
ꢁ238
ꢁ156
ꢁ184
ꢁ176
ꢁ618
ꢁ169
ꢁ190
ꢁ112
ꢁ133
ꢁ180
ꢁ155
ꢁ148
ꢁ182
103.6
49.9
38.5
30.9
24.6
17.3
8.32
49.5
36.1
26.7
18.3
9.30
4.02
54.2
45.3
31.4
28.8
22.7
13.5
49.2
41.2
29.1
22.5
19.6
9.2
48.5
38.3
26.2
19.6
11.4
3.9
56.9
47.3
31.0
27.9
20.5
14.5
ꢁ
ꢁ
7ac
3.15
15.7
31.5
63.0
157
51.8
62.8
70.1
76.2
83.3
92.0
52.2
65.2
74.2
82.3
91.0
96.1
47.7
56.3
69.7
72.2
78.1
87.0
52.5
60.3
71.9
78.2
81.1
91.1
53.2
63.0
74.7
81.1
89.0
96.2
45.1
54.3
70.1
73.1
80.2
86.0
58.9
63.7
67.5
73.6
84.2
90.8
61.0
68.9
74.1
84.1
92.1
96.7
56.1
62.3
68.1
71.5
79.9
85.4
44.6
58.1
68.1
74.7
82.6
90.2
48.9
67.0
76.3
84.2
93.8
97.1
49.1
56.2
62.3
67.1
78.3
84.1
315
7bc
7cc
8ac
8bc
8cc
2.24
11.2
22.4
44.9
112
5
10
20
50
100
1
224
1.74
8.71
17.4
34.8
87.1
174
2.48
12.4
24.8
49.6
124
5
10
20d
50d
100d
1
5
10
20
50
100
1
248
1.88
9.40
18.8
37.6
94.0
188
1.52
7.58
15.2
30.3
75.8
152
5
10
20
50
100
1
5
10
20d
50d
100d
a
b
c
Inhibition efficiency (i.e., h) ¼ surface coverage q. The blank was a CO2–0.5 M NaCl solution. Inhibitor sample was dissolve in 0.5 cm3 2-
propanol, and added with 249.5 cm3 blank solution. d Cloudy solution. e Polarization resistance for blank: 89.7 U cm2.
resulted at the mild steel electrode surface R0p excluding the
potential drop between the two electrodes can be calculated
using eqn (14).
1
Z ¼
(15)
n
ðiuQÞ
where Z is the imaginary part, u is the frequency (Radian) and i
is the current (A).
R0p ¼ Rp ꢁ Rs
(14)
Fig. 6a and b shows the Nyquist plots in the absence (blank)
and presence of inhibitors 7b or 8b with the concentration
ranging from 1 to 100 ppm. The inset gures (Fig. 6a and b)
depict magnied form of Nyquist plots for blank and in the
presence of 1 ppm inhibitor. The electrochemical equivalent
circuit parameters were obtained by tting the data using Nova
(version 1.8) so-ware. The electrochemical parameters of the
tted equivalent circuit were normalized using the specimen
surface area (2 cm2), are tabulated in Table 2. Bode plots for
magnitude or phase angle versus frequency in Randle equiva-
lent circuit are shown in Fig. 6c–f.
The h% of the inhibitor can be determined from the corro-
sion resistance Rp in inhibitor-free solution and R0p in inhibitor-
added solution using eqn (13).
The double layer capacitance and coating capacitance
modeled with a CPE that represents a non-ideal capacitance,
which depends on a degree of surface homogeneity (n).37 Due to
inadequate homogeneity of the un-inhibited and inhibited mild
steel surface, n value in eqn (15) is lower than 1 and the
capacitance (c) is replaced with symbol Q.
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Fig. 4 Potentiodynamic polarization curves at 40 ꢀC for mild steel in CO2–saturated 0.5 M NaCl containing 1–100 ppm concentrations of (a) 7b
and (b) 8c, and 20 ppm concentration of (c) 7 and (d) 8.
where Kads is the equilibrium constant of the adsorption
process. The data tted the Langmuir isotherm the best as
revealed by the correlation coefficient values (Fig. 7 and
Table 3). Some of the inhibitors demonstrated a good t for
Temkin or Freundluich adsorption isotherms. The molecular
interaction parameter f, which describes interactions of the
inhibitor molecules in the adsorption layer as well as inhomo-
geneities on the electrode surface, was calculated from the
Temkin isotherm (Table 3).5,39 The Kads is related to the free
Fig. 5 Randles Electricalchemical equivalent circuit diagram used to
modeling metal/solution interface. Rs: solution resistance, Rp: polari-
energy of adsorption (DGꢀads), by the eqn (20):
zation resistance, CPE: constant phase element.
ꢀ
ꢁ
1
ꢁDGaꢀds
Kads
¼
exp
(20)
55:5
RT
The values of Kads and DGꢀads obtained from Langmuir
3.3. Adsorption isotherms
Fractional inhibition efficiency h and surface coverage q are
equal at lower concentration range of an inhibitor molecule,
while their relationship does not remain linear at higher
concentrations owing to a change of surface coverage from
a monolayer to a multilayer coverage. The h values obtained by
LPR method (Table 1) in CO2–0.5 M NaCl and C were used to t
the following adsorption isotherms, namely:
isotherms are summarized in Table 3.
3.4. Gravimetric measurements in CO2–0.5 M NaCl at 120 ꢀC
and a pressure of 10 bar
The results of the average of duplicate experiments carried out
for 48 h using coupons of almost identical masses are given in
Table 4. Eqn (21) was used to calculate percent inhibition effi-
ciency (i.e. h%):
Temkin: KadsC ¼ efq
(16)
(17)
(18)
(19)
weight loss ðwithout inhibitorÞ ꢁ weight loss ðinhibitorÞ
Langmuir: q/(1 ꢁ q) ¼ KadsC
h% ¼
weight loss ðwithout inhibitorÞ
38
Frumkin:
K
C ¼ {q/(1 ꢁ q)}eꢁ2aq
ads
ꢃ 100
Freundluich:39 q ¼ KadsCn
(21)
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Fig. 6 Nyquist diagram of (a) 7b and (b) 8b, Bode phase angle plots of (c) 7b and (d) 8b, and impedance plots (e) 7b and (f) 8b of mild steel at 40 ꢀC
in CO2 saturated 0.5 M NaCl containing various concentrations of inhibitors in 30 minutes immersion time. Solid line in the Nyquist plot fitted to
the equivalent circuit; various symbols represent experimental data and solid lines represent fitted data.
In cases where the coupons differed in masses, relative
weight loss of the coupons were used to determine the h%.40,41
4. Discussion
The average h% reported in the Table 4 is found to have
a standard deviation of 2–3%.
A series of new IMs 7 and 8 having 4-mono-3,5-di- and 3,4,5-
trioctyloxyphenyl substituents at C(2) and (CH2CH2NH)nH (n ¼
1 and 3) pendants at N(1) have been synthesized. The structural
variation has permitted us to study the effects of hydrophobic
electron-rich aromatic ring in shiing charge density to the
coplanar N]C–N motifs as shown using 7a (Scheme 2) as well
as the effects of the number of hydrophilic amine groups in the
pendants. A variety of nitrogen centers is expected to provide
lone pair of electrons for the formation of coordinate-type
bonds with the empty d-orbitals of Fe on the anodic sites of
the metal surface. The presence of hydrophobic octyloxy chains
is anticipated to provide ample surface coverage to safeguard
the metal by entraining water layer from its surface into the
hostile aqueous environment.
3.5. Surface tension
The surface tension g and CMC of IMs 7 and 8 are measured in
0.5 M NaCl, 0.5 M NaCl + CO2 and water at 40 ꢀC and the results
are given in Table 5. Fig. 8a and b shows the plot of surface
tension g against the concentration of the IMs while Fig. 8c and
d show the h% versus concentration proles.
3.6. X-ray photoelectron spectroscopy
The intensity (counts) versus binding energy (eV) plots are dis-
played in Fig. 9. The XPS scan compositions of the metal surface
are given in Table 6.
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Table 2 Impedance parameters for the corrosion of a mild steel sample in various solutions containing inhibitors 7b and 8b in CO2–0.5 M NaCl
at 40 ꢀC
Concentration
(ppm by weight)
Sample
Rs (U cm2)
Rp (U cm2)
CPEa (mF cmꢁ2
)
n
R0p (U cm2)
h (%)
Blank
7b
4.19
2.07
2.3
2.62
3.1
2.49
2.89
3.57
3.4
3.08
3.47
3.91
5.88
18.7
36.4
59.1
74.4
152
310
400
42.5
47.2
128
699
501
227
281
307
341
306
393
301
290
268
220
226
0.963
0.969
0.963
0.945
0.926
0.914
0.791
0.959
0.906
0.785
0.653
0.615
0.599
14.5
34.4
56.8
71.7
149
308
397
38.9
43.8
125
1
5
57.8
74.5
79.8
90.3
95.3
96.3
62.8
66.9
88.4
92.1
96.4
97.5
10
20
50
100
1
8b
5
10
20
50
100
186
407
591
183
403
585
a
Double layer capacitance (Cid) and coating capacitance (Cc) are usually modelled with a constant phase element (CPE) in modeling an
electrochemical phenomenon.
Fig. 7 Langmuir adsorption isotherm of (a) 7 and (b) 8 at 40 ꢀC in CO2 saturated 0.5 M NaCl solution.
The IMs in both series 7 and 8 demonstrated very good outperformed their mono- and triple-tailed counterparts 7a, c
corrosion inhibition as determined by Tafel extrapolation and and 8a, c. It is quite possible that twin-tailed hydrophobe in 3,5-
corroborated by LPR study (Table 1). In the presence of 100 ppm dispositions might actually be covering greater surface area of
inhibitors, the LPR study revealed h% values 90.8, 96.7 and 85.4 the metal than the triple-tailed IMs where hydrophobes in 3,4,5-
for the DETA-derived IMs 7a, 7b and 7c, respectively, while for dispositions may associate and occupy smaller space (bottom
the TEPA-derived inhibitors 8a, 8b and 8c, the corresponding box, Scheme 2). In twin-tailed IMs, hydrophobe wings are
values were determined to be 90.2, 97.1 and 84.1 (Table 1). In all spread out since the geometry in the 3,5-dispositions will not
the concentrations studied, twin-tailed IMs 7b and 8b allow intramolecular association between them. It is a plausible
Table 3 Square of coefficient of correlation (R2) and values of the constants in the adsorption isotherms of Temkin, Frumkin, Freundlich and
Langmuir in the presence of inhibitors 7 and 8 in CO2 saturated 0.5 M NaCl solution at 40 ꢀC (LPR data from Table 2 used for the isotherms), and
the equilibrium constant and free energy from the Langmuir isotherm
Langmuir
Compound
Temkin (R2, f)
Frumkin (R2, a)
Freundlich (R2)
(R2)
Kads ꢃ 10ꢁ5 (L molꢁ1
)
DGꢀads (kJ molꢁ1
)
7a
7b
7c
8a
8b
8c
0.9260, 14
0.9705, 12
0.9731, 15
0.9954, 10
0.9901, 9.2
0.9647, 13
0.7313, ꢁ3.5
0.7214, ꢁ2.7
0.9202, ꢁ4.9
0.9542, ꢁ2.4
0.7222, ꢁ1.1
0.9758, ꢁ0.78
0.9503
0.9723
0.9875
0.9864
0.9677
0.9855
0.9984
0.9973
0.9868
0.9919
0.9949
0.9866
27 083
93 424
25 723
32 676
175 082
28 555
ꢁ37.0
ꢁ40.2
ꢁ36.9
ꢁ37.5
ꢁ41.9
ꢁ37.2
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Table
Paper
4
Corrosion rates and inhibition efficiencies of corrosion
inhibitors causes slightly higher reduction in the current
densities (icorr) in the anodic than the cathodic branch. Inhib-
itor action is thus more pronounced in mitigating the iron
dissolution at the anode than the cathodic reaction for
hydrogen evolution. The IMs in the current work are thus
considered as mixed-type inhibitors under the predominance of
anodic control. At higher current densities, the slope of the
cathodic branch is higher in absence of inhibitor. Within the
limitation of an experimental error, the cathodic (bc) and anodic
slopes (ba) are not changed considerably; the inhibitors simply
block the anodic and cathodic reaction sites, and follow the
same the mechanism of inhibition of the electrode reactions as
discussed above. While negative DGꢀads values certify the favor-
ability of the inhibitor adsorption process (Table 4);43 the
ꢁDGꢀads values in the range 37–41.9 kJ molꢁ1 is not large enough
to described the adsorption mechanism by a chemisorption
process. A value greater than 20 kJ molꢁ1 points towards
involvement of both physi- and chemisorption of the IMs on the
metal surface.5,42 A protective lm can be constructed by the
formation of a coordinate type chemical bonds between d-
orbitals of iron and the non-bonding as well as p-electrons in
the electron-rich IM motif and the aromatic ring.44
inhibitors (200 ppm by weight) at 120 ꢀC and 10 bar pressure of CO2 in
0.5 M NaCl
CRb
Inhibition
Average inhibition
Solution
Coupona (mm yꢁ1
) efficiency (%) efficiency (%)
Blank
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
2.19
—
—
2.23
—
7a
0.607
0.638
0.45
0.41
0.41
72.3
71.4
79.7
81.5
81.5
83.1
74.0
71.0
86.7
89.6
88.1
86.7
80.4
81.6
81.9
83.5
71.8
80.6
82.3
72.5
92.4
93.3
81.0
82.7
7b
7c
0.38
8a
0.569
0.647
0.29
0.23
0.26
8b
8c
0.29
Q I 80
0.429
0.410
0.396
0.368
ARMOHIB29
a
b
Two mild steel coupons A and B having compositions. Corrosion
rate.
Most of the anodic polarization curves in the current-versus
potential plots exhibit desorption potentials as indicated by the
presence of current-increasing before reached
a plateau
(Fig. 4).44,45 The existence of desorption potential suggests
a pathway in which the inhibitors initially block the anodic sites
on the electrode and desorb at a higher potential leading to
accelerated steel dissolution.
rationale supported by the experimental ndings; however, at
this stage, we do not have additional experimental evidence.
Note that the variance in N(1) pendants did not impart any
difference in their h%; pentamine derivatives 8 showed inhi-
bition efficiencies similar to those of their triamine counter-
parts 7 (Table 1).
The results of the LPR method corroborated the ndings of
the Tafel extrapolations (Table 1). Some representative Tafel
plots are shown in Fig. 4. As evident from the Table 1 and the
Fig. 4, the IMs shied the Ecorr values towards less negative
values (i.e. noble direction) with increasing inhibitor concen-
trations. In the presence of 100 ppm inhibitor, a shi in the
range 35–73 mV does not qualify these inhibitors to be classied
under the anodic type; a shi of at least 85 mV is suggested to be
a requirement for an inhibitor to be classied as cathodic or
anodic type.42 It is evident from Fig. 4, the presence of the
Electrochemical impedance spectroscopy (EIS) measure-
ments were carried out to obtain better understanding of mild
steel corrosion inhibition using various concentrations of 7b
and 8b in CO2–0.5 M NaCl at 40 ꢀC (Fig. 6a and b). It has been
found that increasing the concentrations of the inhibitors
increases the diameter of the impedance semicircle. This can be
attributed to increase in the adsorption of the inhibitors on the
mild steel surface, thereby increasing the resistivity of the
electrode. The gures show that the Nyquist diagrams do not
adopt ideal semicircle where the capacitance at the inhibitor
solution–metal interface does not exhibit a real capacitor. The
depressed semicircle in the Nyquist plots obtained at high to
medium frequency is characteristic to the solution–solid elec-
trode interface that is associated with the surface roughness
and inhomogeneity of the electrode.46 The deviation from the
semicircle shape in the Nyquist plots observed at low frequency
for higher inhibitor concentration (50 and 100 ppm) solutions,
indicate changing the electron transfer process including
a diffusion-limiting step.47
Table 5 Surface properties of IMs 7 and 8 in 0.5 M NaCl at 40 ꢀ
C
Surface tension
Ccmc
Ccmc
(ppm)
DGꢀmic
Compound
(mN mꢁ1
)
(mmol Lꢁ1
)
(kJ molꢁ1
)
As shown in Table 2, increasing the inhibitor concentration
of 7b continuously increases the value of R0p, consequently the
inhibition efficiency increase to its maximum value of 96.3%
aer 30 minutes of mild steel immersion in 100 ppm. Applying
similar experimental conditions, the inhibition efficiency of 8b
was found to be 97.5%. Table 2 also shows that CPE value
decreases with increasing the inhibitor concentration. This can
be attributed to increase the roughness and inhomogeneity of
the electrode surface because of increase the inhibitor
7a
7b
7c
33.5
30.0
27.1
36.2
27.1
32.0
34.2
20.0
30.2
18.2
12.8
22.4
17.5
23.4
26.3
9.0
9.59
8.11
7.34
8.99
9.27
12.4
13.9
5.92
ꢁ27.1
ꢁ28.4
ꢁ29.3
ꢁ27.9
ꢁ28.5
ꢁ27.7
ꢁ27.4
ꢁ30.2
8a
8b
8ba
8bb
8c
a
0.5 M NaCl saturated with C02. b Salt-free water.
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Fig. 8 Surface tension versus concentration of IM (a) 7 and (b) 8 in 0.5 M NaCl solution; and inhibition efficiency versus concentration of IMs (c) 7
and (d) 8 in CO2 saturated 0.5 M NaCl solution at 40 ꢀC.
Fig. 9 (a) XPS survey spectrum and the XPS deconvoluted profiles of (b) N 1s, and (c) Fe 2p after immersing in CO2 saturated 0.5 M NaCl at 40 ꢀC
for 6 h in the presence of 8b (100 ppm); and (d) XPS deconvoluted profiles of O 1s in the presence of 8c (100 ppm) and (e) N 1s and (f) C 1s in the
presence of 7b (100 ppm).
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Table 6 XPS scan composition of Fe immersed in inhibited solution of
0.5 M NaCl–CO2 (1 atom) at 40 ꢀC for 4 h
and 8 have been determined in order to shed light on the
adsorption process: do the IMs prefer micellization prior to
adsorption or vice versa? As expected, the CMC as well as the
surface tension at the CMC was found to be progressively lower
as the number of alkyl tails increases.34 Twin-tailed IM 8b is
determined to have CMC values of 26.3, 17.5 and 23.4 mM in
salt-free water, 0.5 M NaCl, and CO2/0.5 M NaCl, respectively
(Table 5). The presence of NaCl makes the aqueous system more
hostile environment for the hyhdrophobic tails which are thus
encouraged to micellize resulting in a lower CMC value of 17.5
mM. In the presence of CO2, formation of salts of types II–IV
(Scheme 1) make the IM more water-soluble hence requiring
higher concentration to reach the CMC value of 23.4 mM. For 8b
at a concentration of 10 ppm, the surface coverage (q) of 76%
conrms that the IM covers most of the surface before its
concentration reach the CMC value of 12.4 ppm (i.e. 23.4 mM) in
CO2–saturated 0.5 M NaCl (Fig. 8, Table 5). The above data is
validated by the nding that the adsorption on the metal
surface is favored over the micellization as a result of
Composition (atom%)
Approx. binding
Peak energy (eV)
7a
7b
7c
8a
8b
8c
C 1s
C 1s
O 1s
O 1s
O 1s
O 1s
N 1s
N 1s
Fe 2p 707.2
Fe 2p 711.3
Fe 2p 714.2
Cl 2p 197.6
Cl 2p 198.8
285.4
286.5
530.2
531.3
532.8
533.7
400.2
401.0
24.6
37.8
5.7
35.8
29.4
6.25
1.52
20.3
43.7
19.9
6.21 11.5
0.71 14.4
28.7
22.8
28.5
38.0
5.76
32.7
32.2
6.98
4.72
16.0
16.0
9.54
22.6
1.16
11.4
4.94
20.1
5.98
0.37
1.26
3.33
5.15
2.22
3.23
3.94
0.68
2.08
0.26
2.07
1.42
0.78
0.9
4.17
0.81
1.24
adsorption on the surface. The
n
values decrease with DGꢀmic (zꢁ28 kJ molꢁ1, Table 5) being lesser negative than
increasing concentrations of 7b and 8b. The lower n values at DGꢀads (zꢁ41 kJ molꢁ1, Table 3). Individual inhibitor molecule
higher concentration of 7b or 8b can be attributed to increase is adsorbed on the metal surface to form a monolayer; aer
the adsorption of the inhibitors on the surface of mild steel which adsorption of the micelles may impart further
coupon, thereby suggested to increase the heterogeneity of their protection.52
surface.
The results of XPS survey of the surface of the metal coupons
In Fig. 6c and d the value of Rs can be obtained from the aer immersion in 0.5 M NaCl–CO2 containing inhibitors (100
horizontal plateau region at high frequency, it is almost ppm) are given in Table 6. The high carbon, small Fe, and the
constant and does not affect by increasing the inhibitor signicant amount of N contents on the metal surface conrm
concentration in the solution.48 However, at low frequency, the the presence of a nitrogen- and carbon-aceous IM lm (Fig. 9).
Rp can be determined from the Z magnitude, which increases As expected, the lm of TEPA-derived IMs 8 contributed higher
with increasing the concentration of 7b or 8b; this is attributed amount of N than their DETA-derived counterparts 7. As an
to the increase in the polarization resistance value. Bode plot of example, the XPS spectrum in the presence of inhibitor 8b is
phase angle versus frequency curve (Fig. 6e and f) shows a peak shown in Fig. 9a. The XPS deconvoluted proles of C 1s spec-
shape and maximum angle value at intermediate frequency trum displayed a two-peak prole (e.g. Fig. 9f): the peak at 285.4
increases with increasing the inhibitor concentration, which eV is assigned to the C–C aliphatic bonds, while the other peak
indicates the thickness of the surface increases as a result of suggests the presence of C]C, C]O, and C–N bonds. The O 1s
additional amount of inhibitor adsorbed on the mild steel spectra (e.g. Fig. 9d) revealed the presence of 2 to 4 peaks (Table
surface and consequently decrease the value of the capacitance. 6): while the peaks at 530.2 and 531.3 eV are attributed to the
The results obtained from Nyquist and Bode plots conrmed O2ꢁ in Fe2O3 and hydrous iron oxide FeOOH, peaks at 532.8 and
the ndings from the Tafel and LPR methods.
533.7 eV may account for the presence of the oxygen of adsorbed
The surface coverage data (q) indicate that the adsorption of water.20,53 The presence of Fe3+ (2p) and Fe0 (2p) is revealed by
the IMs are tted best by the Langmuir adsorption isotherm; the appearance of respective peaks at 711.3 and 707.2 eV, their
while some of them followed Temkin as well as Freundlich small concentrations conrm the rather carbonaceous nature of
adsorption isotherms (Table 4). Note that higher values of f the metal surface.
indicate energetic inhomogeneity of the metal surface.49,50
The inhibition efficacy h% in CO2–0.5 M NaCl at 120 ꢀC and
pressure (10 bar) revealed that TEPA-derived IMs 8 performed
5. Conclusions
better than their DETA-derived counterparts 7 in arresting A new series of electron-rich IMs having hydrophobic electron-
corrosion as determined using two types of metal coupons A rich single-, twin-, and triple-tailed phenyl substituents at C(2)
and B having different elemental compositions and carbon and hydrophilic N-pendants of (CH2CH2NH)nH (n ¼ 1 and 3)
content. It is a great satisfaction to note that both twin- and have been synthesized. Twin-tailed IMs 7b and 8b imparted
triple-tailed IMs 8b and 8c performed much better than the two better inhibition of mild steel corrosion in CO2–0.5 M NaCl at
commercial IMs QI80 and ARMOHIB219 (Table 4).
40 ꢀC than their single- and triple-tailed counterparts 7a, c and
The IMs having hydrophilic amidine motifs and hydro- 8a, c, respectively. The changes in the N-pendants from CH2-
phobic alkyl chain pendants are considered as cationic surfac- CH2NH2 in 7 to (CH2CH2NH)2CH2CH2NH2 in 8 seems to offer
tants because of the presence of cationic forms II and IV in no advantage in corrosion at lower temperature and pressure,
equilibrium with I (Scheme 1).51 In this work, CMCs of the IMs 7 however at higher temperature (120 ꢀC) and pressure (10 bar) 8b
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outperformed 7b and two other commercial IM-based inhibi- 15 S. Hernandez, J. Bruzual, F. Lopez-Linares and J. Luzon,
tors. The greater reduction of icorr values in the anodic branch of
Tafel plots and the shi of the Ecorr values in the anodic direc-
tion established that the IMs acted mainly as anodic inhibitors.
Isolation of potential corrosion inhibiting compounds in crude
oils, Proceedings of Corrosion/2003, NACE International,
Houston, Taxes, paper no. 330, 2003.
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