Corrosion inhibition performance of newly synthesized 5-alkoxymethyl-8-hydroxyquinoline derivatives for carbon steel in 1 M HCl solution: Experimental, DFT and Monte Carlo simulation studies

Article abstract of DOI:10.1039/c8cp03226b

Three new organic compounds primarily based on 8-hydroxyquinoline have been successfully synthesized and characterized via different spectroscopic methods (FTIR, ¹H, and ¹³C NMR). The synthesized compounds, namely 5-propoxymethyl-8-hydroxyquinoline (PMHQ), 5-methoxymethyl-8-hydroxyquinoline (MMHQ) and 5-hydroxymethyl-8-hydroxyquinoline (HMHQ), were evaluated as corrosion inhibitors for carbon steel in 1 M HCl solution using electrochemical impedance spectroscopy, potentiodynamic polarization and weight loss measurements at 298 K. Electrochemical measurements confirmed that the newly synthesized 5-alkoxymethyl-8-hydroxyquinoline derivatives are mixed type corrosion inhibitors and confirmed maximum protection efficiencies of 94, 89 and 81% for PMHQ, MMHQ, and HMHQ, respectively, at the optimum concentration of 10^-3 M. The EIS spectra confirmed a slightly depressed semi-circle profile with a single time constant in Bode diagrams for the three organic compounds over the whole concentration and temperature ranges studied. The adsorption of PMHQ, MMHQ, and HMHQ on the carbon steel surface followed the Langmuir adsorption isotherm. In addition, the kinetic and thermodynamic parameters for carbon steel corrosion and inhibitor adsorption, respectively, were determined and discussed. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses supported the formation of a protective film on carbon steel in the presence of PMHQ, MMHQ, and HMHQ. Density functional theory calculations (DFT) showed that the effectiveness of the inhibitive actions of the studied compounds correlates well with their electron donating ability, whilst Monte Carlo simulations revealed that the extent and favourability of adsorption of inhibitor molecules on the carbon steel surface establish their corrosion inhibition performances.

Full text of DOI:10.1039/c8cp03226b

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Chem. Chem. Phys., 2018, DOI: 10.1039/C8CP03226B.
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Page 1 of 49
Physical Chemistry Chemical Physics
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Corrosion inhibition performance of newly synthesized
ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives for carbon
5
steel in 1 M HCl solution: Experimental, DFT and Monte
Carlo simulation studies
a
b,c
b
d
e
M. El Faydy , R. Touir , M. Ebn Touhami , A. Zarrouk , C. Jama ,
a
f,g
f
e,h,
B. Lakhrissi , L.O. Olasunkanmi , E.E. Ebenso , F. Bentiss *
a
Laboratoire d’Agroressources, Polymères et Génie des Procédés, Université Ibn Tofail, Faculté des
Sciences, Kénitra, Morocco
b
Laboratoire d’Electrochimie, de Corrosion et d’Environnement, Faculté des Sciences, Université Ibn
Tofail, Kénitra, Morocco
c
Regional center of education and training (CRMEF), Avenue Allal alꢀFassi, Madinat Al Irfane PO Box
6
210, Rabat, Morocco
d
LC2AME, Faculty of Science, First Mohammed University, PO Box 717, 60 000 Oujda, Morocco.
Univ. Lille, CNRS, INRA, ENSCL, UMR 8207, ꢀ UMET ꢀ Unité Matériaux et Transformations,
e
Fꢀ59000 Lille, France
f
Material Science Innovation & Modelling (MaSIM) Research Focus Area, Faculty of Natural and
Agricultural Sciences, NorthꢀWest University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735,
South Africa
g
Department of Chemistry, Faculty of Science, Obafemi Awolowo University, IleꢀIfe 220005, Nigeria
Laboratory of Catalysis and Corrosion of Materials, Faculty of Sciences, Chouaib Doukkali University,
h
PO Box 20, Mꢀ24000 El Jadida, Morocco
*
Corresponding author. Tel.: +33ꢀ320ꢀ434ꢀ482; fax: +33ꢀ320ꢀ436ꢀ584
Eꢀmail address: fbentiss@enscl.fr (F. BENTISS).
1

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Three new organic compounds primarily based on 8ꢀhydroxyquinoline have been successfully
1
13
synthesized and characterized via different spectroscopic methods (FTIR, H, C NMR). The
synthesized compounds namely 5ꢀpropoxymethylꢀ8ꢀhydroxyquinoline (PMHQ),
ꢀmethoxymethylꢀ8ꢀhydroxyquinoline (MMHQ) and 5ꢀhydroxymethylꢀ8ꢀhydroxyquinoline
HMHQ) were evaluated as corrosion inhibitors for carbon steel in 1 M HCl solution using
5
(
electrochemical impedance spectroscopy, potentiodynamic polarization and weight loss
measurements at 298 K. Electrochemical measurements confirmed that the newly synthesized
5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives are mixed type corrosion inhibitors and
confirmed maximum protection efficiencies of 94, 89 and 81% for PMHQ, MMHQ, and HMHQ
ꢀ3
respectively at the optimum concentration of 10 M. The EIS spectra confirmed slightly depressed
semiꢀcircle profile with a single time constant in Bode diagrams for the three organic compounds
over the whole concentration and temperature ranges studied. The adsorption of PMHQ, MMHQ,
and HMHQ on the carbon steel surface followed the Langmuir adsorption isotherm. In addition,
the kinetic and thermodynamic parameters for carbon steel corrosion and inhibitor adsorption,
respectively, were determined and discussed. Scanning electron microscopy (SEM) and XꢀRay
photoelectron spectroscopy (XPS) analyses supported the formation of a protective film on carbon
steel in the presence of PMHQ, MMHQ, and HMHQ. Density functional theory calculations
(DFT) showed that the effectiveness of the inhibitive actions of the studied compounds correlates
well with their electron donating ability, whilst Monte Carlo simulations revealed that the extent
and favourability of adsorption of inhibitor molecules on carbon steel surface establish their
corrosion inhibition performances.
Keywords: Hydroxyquinoline derivatives; Carbon steel; Acid; Corrosion inhibition; XPS; Theoretical
calculations.
2

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Introduction
Corrosion is an undesirable phenomenon that has become a foe to many industries. It
causes the deterioration of industrial pieces of equipment and installations. Indeed, the resultants
1
yearly industrial losses due to corrosion are estimated at hundreds of billion dollars. Generally,
the experts assess the cost of corrosion from 3 to 5 % of the gross domestic product (GDP),
depending on the country. For example, in Morocco, a study of Moroccan confederation for
anticorrosion certification estimates the cost of corrosion control at 5% of the country’s gross
2
national product (GNP). Moreover, corrosion has an adverse effect on the environment such as
pollution, accidents, and dispersion of corrosion products into the environment. Carbon steel is
one of the main raw materials used in large quantities in industries, unfortunately, it corrodes
3
when exposed to different industrial environments, which could result in large economic loss.
Acidification processes are generally carried out in the industry to eliminate undesirable
precipitates and rust. Moreover, acid pickling of ferrous alloys and steels surfaces employs mostly
inorganic acids. Hydrochloric acid is one of the most utterly used acids for these processes due to
the fact of its relatively low cost, aggressive nature, efficiency in comparison to other mineral
acids. However, the continuous utilization of acids in these repetitive processes may induce
4
corrosion of steel materials and then deterioration of the industrial tools.
In an acidic medium, the cathodic reaction is the reduction of dissolved oxygen and the
release of hydrogen gas produced during the reduction of hydrogen ions, and the anodic reaction
represents the passage of the metal element from the surface into its metallic form to the solution.
Among the best ways to conserve metals towards corrosion is the use of organic inhibitors, whose
molecules have a tendency to adsorb on the metal surface to form a protecting film. These organic
molecules can minimize the corrosion rate of metals by acting on both the cathodic and anodic
5
reactions or by selectively retarding the anodic or cathodic reaction rates. Most of these inhibitors
are organic molecules that mostly contain heteroatoms with a lone pair(s) of electrons, πꢀelectrons,
3

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polar groups, multiple bonds and aromatic rings, which can act as the reactive centers used by the
6
ꢀ13
molecules to accept or donate electrons in the course of adsorption onto metal surface.
The
introduction of alkyl chain or substituent to heterocyclic organic compounds has demonstrated
1
4ꢀ17
improvement of inhibitive efficiency.
8ꢀhydroxyquinoline and its derivatives are used as chelating agents in analytical chemistry
18ꢀ20
and radiochemistry for metal ion extraction and fluorometric analysis.
8ꢀHydroxyquinoline is
an heterocyclic organic compound formed by two juxtaposed aromatic rings; a pyridine ring and a
phenolic ring. Existing data reveal that very limited studies have been carried out on the effect of
2
1
8ꢀhydroxyquinoline and its derivatives on the corrosion behavior of steel in acidic media.
A
recent work in our laboratory on 8ꢀhydroxyquinoline and its derivatives revealed that their
excellent corrosion inhibition potentials as the tested compounds showed more than 90 %
ꢀ3
22
inhibition efficiency at 10 M concentration in 1 M HCl. The interesting observations on the
previously tested 8ꢀhydroxyquinoline derivatives motivated us to expand to the scope of the
research to other similar compounds.
In this work, three compounds primarily based on 8ꢀhydroxyquinoline, namely 5ꢀ
propoxymethylꢀ8ꢀhydroxyquinoline (PMHQ), 5ꢀmethoxymethylꢀ8ꢀhydroxyquinoline (MMHQ)
and 5ꢀhydroxymethylꢀ8ꢀhydroxyquinoline (HMHQ) introduced in Fig. 1, have been efficaciously
1
13
synthesized and characterized by exploiting FTIR, HꢀNMR and CꢀNMR spectroscopies. The
inhibition properties of PMHQ, MMHQ, and HMHQ for carbon steel corrosion in 1 M HCl
medium were executed exploiting weight loss, Tafel polarization, electrochemical impedance
spectroscopy (EIS), quantum chemical and Monte Carlo simulation approaches. The surface
characterization was studied using SEM and XPS analyses, which supported the adsorption of the
5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives on the steel surface.
4

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CH3
O
CH3
OH
O
N
N
OH
OH
N
OH
HMHQ
MMHQ
PMHQ
Fig. 1 Molecular structures of the investigated 5ꢀalkoxymethylꢀ8ꢀquinolinols derivatives.
Experimental and computational details
Materials
All used chemicals in this study were provided from Sigma Aldrich (France) or Acros Organics
Spain). Melting points were determined on an automatic electrothermal IA 9200 digital melting
(
23
point apparatus in capillary tubes. The recording of NMR spectra was performed on a Bruker
Advanced 300 WB at 300 MHz for solutions in Me SOꢀd and chemical shifts are given in δppm
with reference to tetramethylsilane (TMS) as an internal standard. The reaction was followed
2
6
2
3
23
using Thin Layer Chromatography (TLC) of silica 60 F254 (E. Merck). The Infrared spectra of
solid PMHQ, MMHQ and HMHQ have been registered by Bruker Tensor Series FTꢀIR
Spectrometer.
5ꢀAlkoxymethylꢀ8ꢀhydroxyquinoline derivatives were synthesized according to Scheme 1.
In the first step, 8ꢀhydroxyquinoline (1) was converted to 5ꢀchloromethylꢀ8ꢀhydroxyquinoline
23
hydrochloride (2) according to the modified method described elsewhere. The crude product was
washed with concentrated hydrochloric acid (37 %) and acetone to give 5ꢀchloromethylꢀ8ꢀ
hydroxyquinoline hydrochloride (2) with high purity. In the second step, compound (2) was
5

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treated using water and ammonia to give the desired compound (3). The alkyloxyꢀ8ꢀquinolinols
2
4
compounds (4) were synthesized using a method described elsewhere, which consists of the
reaction of 5ꢀchloromethylꢀ8ꢀquinolinol hydrochloride with the appropriate alcohol in the
presence of sodium bicarbonate, the mentioned method in the reference is the same as that used
for the synthesis of two molecules (compound 4), but these two compounds are synthesized and
evaluated for the first time. so, this is new synthetic products. In other words, the mentioned
method was invented by our laboratory. The synthesized compounds structures were characterized
1
13
by HꢀNMR, CꢀNMR and FTIR spectroscopy.
Scheme 1 Synthetic route to 5ꢀalkoxymethylꢀ8ꢀquinolinols derivatives.
1
HMHQ: Yield 86 %; mp 110ꢀ112°C; HꢀNMR (300 MHz, CDCl ): δppm=7.00ꢀ7.64(m, quinoline),
3
1
3
8
.45ꢀ8.86 (m, quinoline), 4.75 (ꢀCH
300MHz, CDCl ), δppm = 63.25 (CH
quinoline), 122.00, 133.41, 147.78, 149.23 (Cꢀquinoline); FTIR (cm ): 3800ꢀ2700(ꢀOH), 1599
2
ꢀ), 3.52 (ꢀOH), 5.40 (s, aromatic (CꢀOH)); CꢀNMR
(
3
2
ꢀaliphatic), 108.87, 109.43, 121.84, 127.81, 132.70 (CHꢀ
ꢀ1
2
and 3026 (aromatic C=C and CꢀH stretching), 1275ꢀ1298 (CꢀN), 2850, 2940, 1450 (ꢀCH ꢀ).
1
MMHQ: Yield 82 %; mp = 78–80 °C; HꢀNMR (300 MHz, DMSOꢀd ): δppm=6.96ꢀ7.56 (m,
6
1
3
quinoline), 8.399ꢀ8.83 (s, phenolicꢀOH), 4.67 (s, ꢀCH
2
ꢀ), 3.24 (s, ꢀOMe); CꢀHMR (300 MHz,
ꢀ1
DMSOꢀd ), δppm = 57.54 (ꢀCH ), 71.91 (ꢀCH ꢀ), 110ꢀ153 (quinoline); FTIR (cm ): 3800ꢀ2700 (ꢀ
6
3
2
6

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OH), 1599 and 3026 (aromatic C=C and CꢀH stretching), 1275ꢀ1298 (CꢀN and C=N), 2850, 2940,
450 (ꢀCH ꢀ).
1
2
1
PMHQ: Yield 70 %; mp = 80–82 °C; HꢀNMR (300 MHz, DMSOꢀd
quinoline), 8.51ꢀ8.83 (s, phenolicꢀOH), 4.82 (s, quinoline ꢀCH ꢀ O), 3.47 (m, OꢀCH
); CꢀHMR (300MHz, DMSOꢀd ): δppm = 10.45 (ꢀCH
ꢀCH ꢀCH ), 70.70 (ꢀCH ꢀCH ꢀO), 71.82 (OꢀCH ꢀquinoline), 108.60ꢀ152.31 (quinoline);
6
): δppm = 7.08ꢀ7.53 (m,
ꢀC), 1.58ꢀ1.65
), 22.97
2
2
13
(
m, ꢀCH
2
ꢀCH
2
ꢀ), 0.91 (m, ꢀCH
3
6
3
(
2
3
2
2
2
ꢀ1
FTIR (cm ): 3800ꢀ2700 (ꢀOH), 1599 and 3026 (aromatic C=C and CꢀH stretching),
1275ꢀ1298 (CꢀN and C=N), 1180ꢀ1080 (CꢀC stretching).
The composition and preparation of steel was mentioned in a previously published article,
2
3
the same thing for the corrosive solutions. The range of concentration for each tested inhibitor
ꢀ3
ꢀ6
was from 10 to 10 M.
Corrosion evaluation tests
Weight loss measurements
Gravimetric experiments have been carried out according to the standard method "A.S.T.M G1ꢀ
2
5
0
3" described beforehand. Weight loss testing used to be measured in specimens with a size of 4
cm×1.5 cm×0.05cm. Experiments were replicated three times for each concentration and the pass
mark weight loss was reported.
Electrochemical measurements
Platinum and silver chloride (Ag/AgCl) were used as an auxiliary and reference electrode,
respectively. All potentials in this study were given against this last electrode. The working
2
electrode was carbon steel electrode, which immersed only a 1 cm of area in the test solution
during 30 min until a steady state open circuit potential (Eocp) was reached. The potentiodynamic
currentꢀpotential curves were recorded using a transfer function analyzer Voltalab PGZ 100
7

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monitored by a personal computer. After measuring the E , the cathodic polarization curve was
ocp
ꢀ1
recorded by polarization from Eocp to negative direction with a scan rate of 0.5 mV s and under
air atmosphere at 298 K. In addition, for the scan of Eocp towards positive direction, the anodic
polarization curve was recorded by polarization in this domain under the same conditions already
mentioned.
The electrochemical impedance spectroscopy measurements (EIS) experiments were conducted in
the frequency range 100 kHz to 100 mHz at open circuit potential, with five points per decade
under potentiostatic conditions, by applying a small amplitude a.c signal (10 mV).
Scanning electron microscopy (SEM)
The surface morphology of the uninhibited and inhibited carbon steel surfaces were
observed using a Scanning Electron Microscope Hitachi S4700 field emission gun. The
experiments were carried out at an accelerating voltage of 5 kV and a current of 15 µA.
Xꢀray photoelectron spectroscopy (XPS)
DLD
XPS technique was carried out using an XPS KRATOS, AXIS Ultra
spectrometer Thermo
Scientific KꢀAlpha XPS system. The XPS experiments were detailed in a previously published
2
6
27
paper. Quantification took into account a nonꢀlinear Shirley background subtraction. The
2
carbon steel disk (1 cm ) was preꢀtreated using the same process as for the weight loss test.
Carbon steel coupon before analysis was rinsed with acetone and ultra pure water after 6 h of
immersion at 298 K.
Density functional theory calculations
Molecular and electronic parameters of PMHQ, MMHQ, and HMHQ were obtained from DFT
calculations. This was done with the view to correlate the theoretical reactivity parameters of the
8

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molecules with the corrosion inhibition potentials. Due to the lone pair of electrons on the pyridine
Nꢀheteroatom, it is susceptible to protonation in aqueous acidic medium utilized in the
experimental studies. Therefore, DFT calculations were extended to the neutral and protonated
forms of PMHQ, MMHQ, and HMHQ, both in the gas and aqueous phases. The respective
+
+
protonated forms of PMHQ, MMHQ, and HMHQ are represented at PMHQꢀH , MMHQꢀH , and
+
HMHQꢀH . Molecular structures were modeled with GaussView 5.0 and optimized to final
2
8
geometry with the aid of Gaussian 09 software suite. All geometry optimization and frequency
29ꢀ31
calculations were accomplished at the B3LYP/6ꢀ31+G(d,p) level of the theory.
selfꢀconsistent (solvent) reaction field in Gaussian 9.0, which is the polarizable continuum model
PCM) using the integral equation formalism variant (IEFPCM) was used for the aqueous phase
The default
(
calculations, while water was defined as the solvent with dielectric constant,
ε = 78.39.
Optimized structures and frontier molecular orbital (FMO) canonical isosurfaces were
viewed with the aid of GaussView 5.0. FMO energy parameters; that is, the highest occupied
molecular orbital energy (EHOMO) and the lowest unoccupied molecular orbital energy (ELUMO
)
were extracted from the output. Dipole moments of the molecules were also recorded. Other
32,33
derivable reactivity indicators were calculated as:
(
Energy gap, ꢁE) :
ꢀE = ELUMO − EHOMO
(1)
(2)
1
(Global hardness, η) :
η
χ
= −
= −
(
ELUMO − EHOMO
)
2
1
2
(Global electronegativity, χ)
(
ELUMO + EHOMO
)
(3)
(4)
(Fraction of electron transferred)
9

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where ΦFe(110) is the work function of the body center cubic (bcc) crystal structure of Fe(110), ηFe
is the hardness of Fe, χinh and ηinh the electronegativity and hardness of the inhibitor respectively.
3
4,35
A value of 4.82 eV was used for ΦFe(110) as reported in literature,
while ηFe was taken to be 0
3
6
eV/mol. The use of ΦFe(110) instead of electronegativity of Fe for the calculation of ꢁN is based
3
4
on the observations of Kokalj that the work functions of low Miller index surfaces of metals give
more reasonable trends of charge transfer compared to electronegativity of the metals.
Monte Carlo simulation
The interactions of neutral and protonated forms of PMHQ, MMHQ and HMHQ with carbon steel
surface were simulated within the Materials studio (6.0 acquired from Accelrys) environment.
Carbon steel being essentially iron was approximated by a cleaved Fe(110) clean crystal surface.
The adoption of Fe(110) for the study is premised on literature reports that adjudged it to be the
3
7
most stable Fe plane with reasonable atomic density. Since the studied inhibitors mitigated
carbon steel corrosion in aqueous solution, it also important to investigate the adsorption of the
inhibitor molecules (both in neutral and protonated forms) in the presence of water molecules,
competing for the adsorption sites on Fe(110) surface. For this purpose, 50 molecules of H
introduced as coꢀadsorbates.
2
O were
Each of the inhibitor molecules and water molecule were first subjected to geometry
optimization using the forcite module. The setꢀup for the forcite calculations comprises the smart
ꢀ5
algorithm with ultrafine convergence tolerance; that is, 8.36 × 10 kJ/mol energy difference,
ꢀ3
ꢀ5
4
.18 × 10 kJ/mol force gradient and 1 × 10 Å displacement limit, within a maximum of 500
iterations. Universal force field was used for the forcite module geometry optimization of PMHQ,
MMHQ and HMHQ molecules. The adsorbent/substrate was built by cleaving the pure Fe metal
into the (110) plane after which forcite module was used to refine the energy with the aid of
COMPASS force field. A 10 × 10 supercell of the Fe(110) crystal was placed in a 10 nm vacuum
1
0

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slab and used to represent carbon steel for the purpose of inhibitorꢀsteel interactions. Simulated
annealing task was invoked in the adsorption locator module and the forcite module optimized
inhibitor molecules were allowed to interact with Fe atom sets within the surface region of
Fe(110) layers. The inhibitorꢀFe(110) system was optimized, heated for 10 cycles at 100,000 steps
5
2
per cycle by ramping the temperature from 10 K to 10 K during each cycle. COMPASS27 force
field was used for the geometry optimization of the complex and a maximum adsorption distance
of 5.0 Å was allowed for the inhibitor on the metallic slab. Adsorption energy (E ) of each
ads
3
7
inhibitor molecule was calculated as:
E
ads = Ecomplex – (Einh + EFe
)
(5)
when water molecules are not included, and as:
Eads = Ecomplex – (Einh + 50 EH2O + EFe)
(6)
in the presence of 50 molecules of H O; where E
Einh, EH2O and EFe represent the total
complex,
2
energy of the optimized inhibitorꢀFe(110) (in equation 5) or inhibitorꢀ(50)H
2
OꢀFe(110) (in
equation 6) complex, isolated inhibitor molecule, H
2
O molecule, and Fe(110) crystal respectively.
Results and discussion
Weight loss measurements
The 5ꢀalkyloxymethylꢀ8ꢀhydroxyquinoline derivatives were tested as corrosion inhibitors in the
concentration interval 10− ≤ C ≤10 M. This boundary interval was determined for each
6
−3

inh

compound by its solubility (for maximum concentration) and the protective effect reached (for the
minimum concentration). The influence of the different 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline
derivatives on the weight loss of carbon steel was followed at 298 K. The corrosion rate (w) in
ꢀ2
ꢀ1
mg cm h was obtained by exploiting the next formula:
ꢀ
St
m
w =
(7)
1
1

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where ꢁm is the mass loss of carbon steel, S is the total area of carbon steel specimen, and t is the
2
5
immersion period (6 h). The inhibition efficiency, η (%), was calculated as:
WL



w
i



W
L
η
(%) = 1−
×100
(8)
w0
where w
0
i
and w are the values of the corrosion rates of carbon steel specimen in the absence and
presence of PMHQ, MMHQ and HMHQ respectively. Corrosion rate (w) and inhibition efficiency
ηWL) values after 6 h of carbon steel immersion in the acidic environment at different
(
concentrations of 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives are illustrated in Table 1.
Table 1 Corrosion parameters obtained from weight loss measurements for carbon steel after 6 h
of immersion in the 1 M HCl containing various concentrations of 5ꢀalkoxymethyꢀ8ꢀ
hydroxyquinoline derivatives at 298 K
Inhibitor
C
inh
w
ηWL
(%)
—
ꢀ2
ꢀ1
(M)
(mg cm h )
1.844
0.194
0.142
0.099
0.074
0.385
0.326
0.221
0.154
0.990
0.529
0.352
0.225
Blank
—
ꢀ6
1
0
89.5
92.3
94.6
96.0
79.1
82.3
88.0
91.6
46.3
71.3
80.9
87.8
ꢀ5
PMHQ
10
ꢀ4
1
1
0
0
ꢀ3
ꢀ6
MMHQ
10
ꢀ5
1
1
1
0
0
0
ꢀ4
ꢀ3
ꢀ6
HMHQ
10
ꢀ5
1
1
1
0
0
0
ꢀ4
ꢀ3
It is clear that the corrosion rate decreases and the inhibition efficiency increase with
increasing concentration of the inhibitors. Based on the values of inhibition efficiency obtained in
the concentration range studied, the 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives can be ranked
as PMHQ ˃ MMHQ ˃ HMHQ. Each inhibitor possesses a quinoline ring with an alkoxymethyl
function in 5ꢀposition. They also contain nitrogen and oxygen atoms in their molecules. The
1
2

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existence of heteroatoms (N and O) in these genre of molecules increases their stability in acidic
environments because they contain active adsorptive centers. The difference in inhibition
efficiencies could probably originate from the presence of aliphatic radical, R linked to the 8ꢀ
hydroxyquinoline unit at position 5 via a ꢀCH
2
ꢀO. PMHQ exhibited the best inhibition
−3
performance, which reached a maximum of 96 % at 10 M, compared to MMHQ and HMHQ
(Table 1). This is justified by the fact that the positive inductive (+I) effect of ꢀCH ꢀCH ꢀCH
2 2 3
(
propyl) in PMHQ is higher than that of ꢀCH (methyl) in MMHQ, which in turn is higher than
3
that of ꢀH (hydrogen) in HMHQ. It well is known that the introduction of an alkyl substituent will
change the electronic structure, thus increasing the delocalization of electron density on the
quinoline ring. Which makes the molecule more stable and therefore gives higher protection
efficiency for the metal. In summary, the results clearly indicate that increasing alkyl chain length
enhances corrosion protection efficiency and facilitates adsorption onto the metal surface. The
obtained data agree well with most of the literature findings on the effects of alkyl side chain on
38ꢀ40
corrosion inhibition potentials.
Potentiodynamic polarization study
Electrochemical polarization curves for PMHQ, MMHQ, and HMHQ are shown in Fig. 2. The
extracted electrochemical parameters namely, corrosion potential (Ecorr), corrosion current density
(i
corr) and both Tafel slopes (cathodic (β
c
) and anodic (β
a
)) are given in Table 2. The icorr values
8
were used to estimate the inhibition efficiency, ηTafel (%) exploiting the next formula:
icorr −icorr(i)
η
Tafel(%) =
× 100
(9)
icorr
where icorr and icorr(i) are the corrosion current densities for steel electrode in the uninhibited and
inhibited solutions, respectively.
1
3

Physical Chemistry Chemical Physics
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Analysis of the potentiodynamic polarization curves shows that the synthesized
compounds decrease both the cathodic and anodic current densities, and their effect becomes more
remarkable with increasing concentration. These results reveal that the synthesized compounds
inhibit both anodic metal dissolution and cathodic hydrogen evolution reactions. In addition, the
increase of the protective effect with the inhibitor concentration reveals that 5ꢀalkoxymethylꢀ8ꢀ
41
hydroxyquinoline derivatives act by adsorption on the metallic surface. Moreover, the shape of
cathodic polarization curves for all inhibitors is similar in comparison to those obtained in the
blank medium. However, anodic polarization curves show some changes for potential values
lower than ꢀ250 mV/Ag/AgCl. For values higher than ꢀ250 mV/Ag/AgCl, the anodic polarization curves
coincide with those obtained for the blank solution, and the presence of the inhibitors in this range
does not produce any significant change in the currentꢀpotential profiles. This phenomenon is
often explained by the dissolution of the carbon steel, which causes desorption of the adsorbed
molecules from the working electrode surface. It might also be due to the fact that the adsorption
rate of inhibitor molecules could be lower than their desorption rate. This potential value can,
42
therefore, be considered as the desorption potential.
1
00
PMHQ
100
10
1
MMHQ
10
1
ꢀ3
0.1
Blank
0
.1
1
1
1
0 M
ꢀ3
ꢀ4
10 M
0 M
0.01
ꢀ4
0
.01
ꢀ5
10 M
0 M
ꢀ
5
ꢀ6
10 M
1
Eꢀ3
Eꢀ4
1
1
Eꢀ3
10 M
Blank
ꢀ6
10 M
1
Eꢀ4
ꢀ
0.9
ꢀ0.8
ꢀ0.7
ꢀ0.6
ꢀ0.5
ꢀ0.4
ꢀ0.3
ꢀ0.2
ꢀ0.1
ꢀ
0.9
ꢀ0.8
ꢀ0.7
ꢀ0.6
ꢀ0.5
ꢀ0.4
ꢀ0.3
ꢀ0.2
ꢀ0.1
E (V/Ag/AgCl)
E (V/Ag/AgCl)
1
4

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1
00
HMHQ
1
0
1
0
.1
Blank
ꢀ
3
0
.01
10 M
ꢀ4
1
1
1
0 M
ꢀ5
1
1
Eꢀ3
Eꢀ4
0 M
ꢀ6
0 M
ꢀ
0.9
ꢀ0.8
ꢀ0.7
ꢀ0.6
ꢀ0.5
ꢀ0.4
ꢀ0.3
ꢀ0.2
ꢀ0.1
E(V/Ag/AgCl)
Fig. 2 Polarization curves for carbon steel in the 1 M HCl in the absence and presence of
5
ꢀalkoxymethyꢀ8ꢀhydroxyquinoline derivatives at various concentrations.
No definite shift was observed in the corrosion potential (Ecorr) (Table 2). According to
4
3
44
Riggs and other authors, for an inhibitor of the cathodic or anodic type, the displacement
should be more than 85 mV/Ecorr while for an inhibitor of the mixed type it is equal to or less than
8
5 mV/Ecorr. In our case, the maximum displacement is < 85 mV/Ecorr, which indicates that
PMHQ, MMHQ, and HMHQ can be considered as a mixedꢀtype inhibitor. Addition of PMHQ,
MMHQ, and HMHQ to the corrosive environment causes a modification of cathodic Tafel slope
(
βc), indicating that the 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives have a direct influence on
the kinetics of hydrogen evolution. Also, when compared corrosive environment, the values of the
) changed irregularly in the presence of 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives, which
(β
a
indicates that the anodic reaction is affected by the presence of PMHQ, MMHQ, and HMHQ.
Table 2 Electrochemical polarization parameters for corrosion of carbon steel in the 1 M HCl
without and with the addition of 5 alkoxymethyꢀ8ꢀhydroxyquinoline derivatives at various
concentrations
Inhibitor
C
inh
ꢀEcorr
i
corr
ꢀβ
c
β
a
ηTafel
(%)
ꢀ1
ꢀ1
(
mV dec )
(
M)
(mV/Ag/AgCl) (ꢂA cm²)
(mV dec )
Blank
PMHQ
—ꢀ
484
491
489
489
479
594.5
68.3
59.4
40.4
29.1
140.3
120.0
128.2
128.2
137.3
116.3
109.1
66.7
66.7
57.6
—
6
4
4
3
6
10
88.5
90.0
93.2
95.1
ꢀ
10
10
10
ꢀ
ꢀ
ꢀ
MMHQ
10
479
123.0
170.2
109.1
79.3
1
5

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ꢀ5
ꢀ4
ꢀ3
10
10
10
476
496
503
105.8
61.8
49.9
158.9
141.3
165.7
76.7
97.8
92.7
82.2
89.6
91.6
ꢀ
ꢀ
ꢀ
ꢀ
6
5
4
3
HMHQ
10
478
481
463
479
515
508
509
500
483
466
461
482
352.5
180.7
121.3
70.7
300.6
228.2
198.6
185.0
299.0
213.0
185.0
163.3
159.5
170.2
161.9
122.4
144.5
134.0
131.3
126.1
131.0
132.0
140.0
130.0
99.0
119.1
84.4
56.5
62.9
69.2
68.4
74.6
182.9
105.2
102.3
117.1
40.7
69.6
79.6
88.1
49.4
61.6
66.6
68.9
49.7
64.2
68.9
72.5
10
10
10
4
5
ꢀ6
8ꢀHQ
10
ꢀ5
ꢀ4
ꢀ3
10
10
10
4
5
ꢀ6
AMHQ
10
ꢀ5
ꢀ4
ꢀ3
10
10
10
Addition of PMHQ, MMHQ, and HMHQ to the corrosive environment causes a
modification of cathodic Tafel slope (β ), indicating that the 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline
c
derivatives have a direct influence on the kinetics of hydrogen evolution. In addition, the change
in β values (Table 2), suggests that the reaction mechanism of the hydrogen reduction is not the
c
same in the absence and in the presence of inhibitor. This may probably be due to a diffusion or
46
barrier effect. Shreir et al. in their book pointed out that a displacement of the polarization curve
without a change in the Tafel slope in the presence of the inhibitor indicates that the adsorbed
inhibitor acts by blocking active sites so that reaction cannot occur, rather than by affecting the
mechanism of the reaction. An increase in the Tafel slope of the polarization curve due to the
47
inhibitor indicates that the inhibitor acts by affecting the mechanism of the reaction. In acidic
+
medium the reduction of H ions at the steel surface takes place mainly through a charge transfer
mechanism. The inhibitor molecules are first adsorbed onto the steel surface and, therefore,
impedes by merely blocking the reaction sites of the steel surface. Also, when compared corrosive
environment, the values of the (β ) changed irregularly in the presence of 5ꢀalkoxymethylꢀ8ꢀ
a
hydroxyquinoline derivatives, which indicates that the anodic reaction is affected by the presence
of PMHQ, MMHQ, and HMHQ. The presence of heteroatoms with a lone pair of electrons, such
as N and O atoms in the quinoline rings of the studied 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline
1
6

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derivatives can facilitate the formation of Fe(II)ꢀPMHQ, Fe(II)ꢀHMHQ and Fe(II)ꢀMMHQ)
48
complexes and thus changes the dissolution mechanism of iron. ηTafel (%) values are higher in
comparison to those obtained using the gravimetric method. Such difference could be due to the
fact that immersion times are different for the two methods. However, the potentiodynamic
polarization measurements give the same order of inhibition efficiencies of the three compounds
as deduced from weight loss measurements. In order to compare the effect of 8ꢀhydroxyquinoline
derivative, we added 8ꢀhydroxyquinoline (8ꢀHQ) and 5ꢀaminomethylꢀ8ꢀhydroxyquinoline
45
(
AMHQ) as an example. Through the table 2, we see that 8ꢀHQ and AMHQ present lower
inhibition efficiency than three of its derivatives, this connected by the introduction of different
substituent in the ring of 8ꢀhydroxyquinoline, leading to increase of the delocalization of electron
density in the molecule, which make the molecule more stable.
Electrochemical impedance spectroscopy study
The electrochemical impedance spectroscopy (EIS) has further been employed to complete the
investigation of corrosion inhibition of steel using HMHQ, MMHQ, and PMHQ in the acidic
environment at 298 K. The corresponding Nyquist plots for carbon steel in the absence and
presence of various concentrations of HMHQ, MMHQ, and PMHQ at 298 K are illustrated in Fig.
3
. It can be noted from the plot that the impedance response of the carbon steel in the corrosive
medium was progressively amended after the addition of the inhibitor. The size of the impedance
semicircles changes depending on both structure and concentration of inhibitor, stating a charge
transfer process mainly controlling the corrosion of carbon steel. The Nyquist plots are in the form
of a capacitive loop corresponding to one time constant in Bode plots as illustrated in Fig. 3. The
capacitive loops are slightly depressed semiꢀcircles due to inhomogeneity and/or roughness of the
carbon steel surfaces and adsorption process of inhibitors defining a phenomenon known as
49
“
dispersing effect”.
1
7

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EIS parameters were obtained by fitting the impedance spectra exploiting the equivalent
circuit model using ZView3.3. software (Scribner Associates) proposed in the Fig. 4, in which the
solution resistance (R
polarization resistance (Rp), where R
inhibitor film (R ) formed on the metal surface in the presence of inhibitor (R
CPE latter described the quasiꢀcapacitive behaviour of the ions adsorbed in the electrical double
s
) is shorted by a constant phase element (CPE) that is in parallel to the
p
includes charge transfer resistance (Rct) and the resistance of
50
f
p f
= Rct + R ). The
50
layer and their use shows an improved of simulating with respect to the capacitance.
4
ꢀ80
3
5
ꢀ3
ꢀ6
10 M
PMHQ
10 M
30
25
Blank solution
ꢀ4
ꢀ5
20
PMHQ
10 M
10 M
ꢀ60
15
ꢀ4
ꢀ
5
3
2
1
0
10
10 M
10 M
800
5
0
ꢀ6
ꢀ3
10 M
0
5
10
15
2
0
25
30
35
10 M
ꢀ40
re
Z (ꢁ.cm )
2
Blank
Blank
ꢀ20
400
0
2
0
0
ꢀ1
0
1
2
3
4
5
0
400
800
1200
Log f (Hz)
2
Z (
ꢁ
cm)
re
5
00
3.0
ꢀ80
ꢀ60
ꢀ40
ꢀ
3
ꢀ
3
1
0 M
MMHQ
10 M
ꢀ
4
ꢀ4
1
0 M
2.5
10 M
ꢀ
5
400
ꢀ5
1
0 M
MMHQ
10 M
ꢀ
6
ꢀ
6
1
0 M
2.0
1
0 M
Blank
Blank
300
1.5
ꢀ20
200
1.0
0
0.5
100
20
0.0
0
0
100
200
300
400
500
ꢀ1
0
1
2
3
4
5
2
Z (
ꢁ
. cm)
Logf (Hz)
re
1
8

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3
.0
2.5
.0
1.5
.0
.5
.0
0.5
ꢀ
3
ꢀ3
1
0 M
HMHQ
10 M ꢀ80
MMHQ
ꢀ4
ꢀ4
300
10 M
10 M
ꢀ60
ꢀ
5
ꢀ5
1
0 M
10 M
2
ꢀ6
ꢀ40
ꢀ
6
1
0 M
10 M
Blank
Blank ꢀ20
200
0
1
20
0
100
40
0
60
ꢀ
80
0
0
100
200
300
400
ꢀ1
0
1
2
3
4
5
2
Log f (Hz)
Z (ꢁcm)
re
Fig. 3 Nyquist and Bode plots of carbon steel in the 1 M HCl containing various concentrations
of different inhibitors 298 K.
Fig. 4 Electrical equivalent circuit used for modeling the interface steel/1 M HCl solution without
and with inhibitors 298 K.
The impedance of the CPE is defined by:
−1
−n
ZCPE = A (i
ω)
(10)
ꢀ1
n
ꢀ2
where A is the CPE constant in (ꢃ s cm ), n is the CPE exponent determining the phase shift
2
ꢀ1
(
ranges from 0 to 1), i = 1 is an imaginary number and w is the angular frequency in (rad s )
(w = 2πf, where f is the frequency) and subsequently the values of double layer capacitance are
determined by the following mathematical formula:
1
/ n
1
− n)
(11)
C = (AR
dl
p
The last parameter is the relaxation time constant (τ), which corresponds to the time
required to achieve the equilibrium of the charge distribution after an electrical perturbation and is
determined by the following equation:
1
9

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τ
= Cdl
R
p
(12)
2
6
The term of inhibition efficiency,
Z
η (%), is expressed via the next formula:
R − R
P(i)
p
Z
η (%) =
× 100
(13)
RP(i)
p
where R and Rp(i) are the ac polarization resistance of carbon steel electrode in the uninhibited and
inhibited solutions, respectively.
Good fit with the the equivalent circuit model was obtained for all experimental data. All
fitted parameters and their margins of error for both in absence and presence of 5ꢀalkoxymethylꢀ8ꢀ
hydroxyquinoline derivatives are summarized in Table 3. It is clear from Table 3 that the values of
p
R can be classed in the following order: PMHQ > MMHQ > HMHQ, while the values of A follow
the opposite order, leading an inhibition efficiency order as follow: PMHQ > MMHQ > HMHQ.
The highest inhibition efficiency values are 97.2 %, 94.0 % and 90.8 % for PMHQ, MMHQ, and
HMHQ, respectively. In addition, a remarkable reduction in the values of the Cdl versus
ꢀ2
concentration of inhibitors is evidenced. Cdl value decreases from 302.2 µF cm for the blank to
ꢀ2
ꢀ3
5
6.8 µF cm in the case of 10 M of PMHQ indicating a reduction in local dielectric constant
and/or an augmentation in the thickness of the electrical double layer. It is worth pointing that the
value of the relaxation time constant (τ) lightly increases with 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline
derivatives concentration. PMHQ shows higher values of τ, which may be due to a lower
51
adsorption process. The rise in the value of n (0.782ꢀ0.893), in comparison with the value
obtained in the corrosive environment (0.76), can be related to a reduction of surface
inhomogeneities caused by the 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives adsorption on
52
carbon steel surface. Also, It should be noted that the highest inhibition efficiency values
corresponding to the highest concentrations.
2
0

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Table 3 Impedance parameters for corrosion of carbon steel in the 1 M HCl in the absence and
presence of different concentrations of HMHQ, MMHQ, and PMHQ at 298 K
4
Inhibitor
C
inh
R
p
10 A
n
C
dl
τ
ηZ
(%)
2
ꢀ2
ꢀ
1
n
2
(M)
(Ω cm )
(µF cm )
(ms)
(
Ω S cm )
Blank
—
32.5 ± 0.5
10.0±0.33
0.760 ± 0.002
0.798 ± 0.003
0.809 ± 0.001
0.819 ± 0.002
0.893 ± 0.003
0.782 ±0.003
0.800 ± 0.002
0.826 ± 0.001
0.853 ± 0.005
0.785 ± 0.003
0.793 ± 0.002
0.808 ± 0.003
0.815 ± 0.001
302.2
88.0
69.6
60.7
56.8
9.82
33.3
35.2
43.9
66.2
23.4
25.3
28.2
33.2
10.4
14.6
15.2
23.6
—
ꢀ
6
5
4
3
6
5
4
3
6
5
4
3
PMHQ
10
378.3 ± 0.4
505.3 ± 0.5
722.7 ± 0.4
1166.0 ± 0.4
207.9 ± 0.5
248.1 ± 0.5
370.0 ± 0.3
542.0 ± 0.5
58.0 ± 0.5
1.75 ± 0.04
1.32 ± 0.02
1.07 ± 0.01
0.76 ± 0.03
2.55 ± 0.05
2.13 ± 0.03
1.42 ± 0.01
1.01 ± 0.01
4.80 ± 0.38
2.67 ± 0.12
1.69 ± 0.06
1.34 ± 0.02
91.4
93.6
95.5
97.2
84.4
86.9
91.2
94.0
44.0
75.2
84.4
90.8
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
10
10
10
MMHQ
HMHQ
10
112.4
102.1
76.3
10
10
10
61.2
10
179.9
111.3
76.3
10
10
10
131.2 ± 0.5
208.6 ± 0.8
352.7 ± 0.5
67.0
The adsorption of 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline molecules on carbon steel surface
reduces its electrical capacity due to the replacement of water molecules and other
5
3
coꢀions at the metal–solution interface. Such decrease when PMHQ, MMHQ, and HMHQ
concentrations increase may be connected to the formation of a protective layer on the metal
surface.
Bode plots (log ǀZǀ vs.log (f) and angle plots (phase angle (α) vs.log (f) for carbon steel in
1
M HCl solution without and at different concentrations of 5ꢀalkoxyomethylꢀ8ꢀhydroxyꢀquinoline
derivatives are given in Fig. 3. Three distinct segments can be observed. At very high frequencies,
log ǀZǀ values tend to approach zero and the phase angle falls above zero degree due to a resistive
behavior and are attributed to the electrolyte resistance. The slopes of Bode plots, s, and the
maximum phase angles, αmax are given in Table 4 for intermediate frequency values. The analyse
of the latter shows that the slope values of the three inhibitors are greater than that of the blank
ꢀ6
solution with the exception of HMHQ at 10 M, While PMHQ received the highest slope value
that MHHQ and HMHQ. and likewise for phase angle values for the three inhibitors. So, the
PMHQ inhibitor has the largest values of these two parameters compared to the other inhibitors,
5
4
indicate that the PMHQ is the closest to behave as a pure capacitor’s. One can observe that the
2
1

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differences that exist between s, αmax to the ideal capacitive values are reduced when the
concentration of inhibitor increase. This can be due to the slowing down of the rate of dissolution
of the metal. In the low frequency region, the resistive behavior of the electrode increases, and log
5
5
ǀZǀ becomes independent of frequency.
Table 4 The slopes of the Bode impedance magnitude plots at intermediate frequencies (s) and
the maximum phase angles αmax in the 1 M HCl in the absence and presence of 5ꢀalkoxymethylꢀ8ꢀ
hydroxyquinoline derivatives
Inhibitor
Cinh
M)
ꢀαmax
(deg.)
55.6
64.3
63.3
64.1
77.0
70.0
70.4
66.7
66.7
50.9
59.2
69.6
71.3
ꢀs
(
Blank
PMHQ
—
10
0.65
0.74
0.74
0.73
0.87
0.82
0.82
0.76
0.76
0.60
0.67
0.81
0.82
ꢀ6
ꢀ5
ꢀ4
ꢀ3
ꢀ6
ꢀ5
ꢀ4
ꢀ3
ꢀ6
ꢀ5
ꢀ4
ꢀ3
1
1
1
0
0
0
MMHQ
HMHQ
10
1
1
1
0
0
0
10
1
1
1
0
0
0
Adsorption isotherm and surface studies
The scanning electron microscopy (SEM) micrographs (Figs. 5 aꢀe) of the surfaces of carbon steel
strips were taken at same magnification in order to see the changes that occurred during corrosion
process in absence and presence of the 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives. Parallel
features on the polished steel surface before exposure to the corrosive solution are clearly visible
in Fig. 5a, which are associated with abrading scratches. Figs. 5b to 6e show the steel surface after
ꢀ3
6
h of immersion in 1 M HCl without and with addition of 10 M. As can be seen, there are
distinct differences between the carbon steel surfaces after immersion in 1 M HCl solution in the
absence and presence of 8ꢀhydroxyquinoline inhibitors, respectively. Indeed, the high resolution
SEM micrograph in Fig. 5b shows that the steel surface was strongly damaged after immersion in
2
2

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uninhibited solution (blank), due to rapid corrosion attack in the absence of inhibitors. However,
relatively smoother surfaces with few small notches are seen in the presence of the
5
ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives (Fig. 5c, 5d or 5e), particularly in the case of the
PMHQ (Fig. 5c). These results may be explained by the formation of an adsorbed protective film
of 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline molecules on the steel surface. The SEM findings are in
good agreement with the corrosion techniques evaluation.
(
a)
(b)
(
c)
(d)
(e)
Fig. 5 SEM micrographs of the carbon steel surface: aꢀ Metallic surface after being polished, bꢀ
metallic surface after 6 h immersion in 1 M HCl and c), d) and e) metallic surface after 6 h of
–3
immersion in 1 M HCl with 1×10 M of PMHQ, of MMHQ and of HMHQ, respectively.
The major behavior of the organic molecules onto the metal/solution interface is generally known
by the adsorption. The nature of the adsorption depends on the interaction of
5
ꢀalkoxymethylꢀ8ꢀhydroxyquinoline molecules and the active sites on the carbon steel surface. It
has been noted that the adsorption process could be affected by several factors such as metal
2
3

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nature, the chemical structure of the studied molecules and the nature of acids. So, with a view to
clarifying the nature and strength of adsorption, different isotherms were tested, but the use of the
Langmuir isotherm provides the best straightꢀline fit. The coverage surface (θ) was determined
from EIS measurements by the following expression:
R − R
P(i)
p
θ
=
(14)
RP(i)
For the different inhibitors, the plots of Cinh/θ as a function of Cinh presents a straight line (Fig. 6)
2
with regression coefficient (R ) higher than 0.999 suggesting that the adsorption of
5
ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives on the carbon steel surface is well adapted to the
5
6
Langmuir adsorption isotherm:
Cinh
1
=
+Cinh
(15)
θ
Kads
where Kads is the equilibrium constant of the adsorption process and its value was calculated from
the intercept of the Langmuir isotherm plot. Kads serves to determine the standard Gibbs energy of
°
ads
57
adsorption ꢀG by the following equation:
ο
ads
1
 − ꢀG




Kads
=
exp
(16)

5
5.55
RT

where R is the universal gas constant, T is the thermodynamic temperature and the value 55.55 in
the above equation is the concentration of water in solution in mol/L. The obtained values of Kads
°
ads
and ꢀG at each organic compound are given in Table 5.
2
4

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Physical Chemistry Chemical Physics
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DOI: 10.1039/C8CP03226B
0.0012
0.0009
0.0006
0.0003
0.0000
HMHQ
MMHQ
PMHQ
0.0000
0.0003
0.0006
0.0009
0.0012
Cinh (mol/L)
Fig. 6 Langmuir adsorption plot for different inhibitors on carbon steel in 1 M HCl solution.
Table 5
Thermodynamic parameters for the adsorption of the 5ꢀalkoxymethylꢀ8ꢀ
hydroxyquinoline derivatives on the carbon steel in the 1 M HCl at 298 K
2
°
ads
Inhibitor
Kads
R
Slope
ꢀ
G
ꢀ1
M
ꢀ1
kJ mol
ꢀ44.89
ꢀ43.35
ꢀ40.80
PMHQ
MMHQ
HMHQ
1331421.4
714622.6
255747.9
1.000
1.000
0.999
1.03
1.06
1.09
°
ads
Applying the equation (9), the values of Kads and ꢀG for each organic compound are
given in Table 5. A slight increase in the slope from unity for the inhibitor may be due to the
interactions between organic molecules and iron atom. Nevertheless, the high values of Kads mean
that the inhibitor is strongly adsorbed on the metal surface, corresponding to a better the inhibition
performance. According to the Table 5, the Kads values can be classed in the following order
Kads (PMHQ) > Kads (MMHQ) > Kads (HMHQ), explaining the best inhibition performance of
PMHQ molecule in comparison to the others quinolinꢀ8ꢀol. Besides, the negative sign of
°
ads
ꢀ
G
suggests that all the 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives were strongly adsorbed
2
5

Physical Chemistry Chemical Physics
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DOI: 10.1039/C8CP03226B
°
ads
on the steel surface. However, values of ꢀG approximate to ꢀ20 kJ/mol or less negative are
coherent with physisorption, while those aroundꢀ40 kJ/mol or more negative correspond with
58
°
ads
chemisorption. The obtained values ꢀG of 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives
suggest that the interaction of PMHQ and MMHQ with carbon steel is typical of chemisorption,
°
ads
while the interaction of HMHQ involves both physisorption and chemisorption. Moreover, | ꢀG
|
of these compounds decreases in the order PMHQ > MMHQ > HMHQ. The large negative value
°
ads
of ꢀG of PMHQ indicates that this inhibitor is strongly adsorbed on the steel surface. This is in
good agreement with the ranking of inhibition efficiency obtained from the different techniques.
Xꢀray photoelectron spectroscopy (XPS) analysis was employed to further study the
adsorption mechanism of 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivatives on the carbon steel
surface and to provide insight into the chemical nature of the adsorbed film on the substrate
surface. The XPS spectra of the C 1s, N 1s, O 1s and Fe 2p core levels were acquired from
carbonꢀsteel sample subsequent to 6h of immersion in 1 M HCl with 1 mM of PMHQ at 298 K
and given in Fig. 7. The relative chemical states and the corresponding quantification (%) of each
peak component are given in Table 6. For comparison purpose, the N 1s XPS spectrum of the pure
PMHQ was also given in Fig. 8.
The Fe 2p spectrum for carbon steel surface covered with PMHQ depicts a double peak
profile located at a binding energy (BE) around 711 eV (Fe 2p3/2) and 725 eV (Fe2p1/2) together
with an associated ghost structure on the high energy side showing the subsequent oxidation of the
steel surface. The deconvolution of the high resolution Fe 2p3/2 XPS spectrum consists in three
main peaks as described in Fig. 7a and Table 6, which are associated to iron in different
59
environments. Indeed, the small first peak located at 707 eV could be attributed to metallic iron,
3
+ 60,61
while that located at 711 and at 712 eV may be assigned to Fe .
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6

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Physical Chemistry Chemical Physics
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DOI: 10.1039/C8CP03226B
The deconvolution O 1s spectrum for carbon steel surface after immersion in 1 M HCl
solution containing PMHQ is shown in Fig. 7b. As described in this figure and Table 6, the O 1s
spectrum is resolved into three main components. The two first components at 530.1 eV and 532.2
2
ꢀ
ꢀ
62,63
eV are attributed to O and OH in inorganic species, respectively.
The latest at 534.0 eV can
6
4
be attributed to oxygen of adsorbed water, which remains on the surface after drying the sample.
However, the second component at 532.2 eV can be also assigned to singly bonded oxygen (ꢀOꢀ)
6
5,66
in CꢀO and in OꢀH which are present in the PMHQ molecules.
Indeed, it is not possible to
67
separate contributions of organic (O in carbonyl groups) and inorganic oxygen since O 1s signal.
Their presence demonstrates that PMHQ molecules are adsorbed on the steel surface. This is in
agreement with the C 1s region, where peaks due to C–H, C–C, C=C, C–N, and C–O bonds were
detected. Indeed, the corresponding C 1s spectrum can be fitted into three components with
different intensities (Fig. 7c, Table 6). The first and most intense, at 284.8 eV is mainly attributed
68
to the presence of contaminant hydrocarbons and to the C–C, C=C, and C–H bonds, in
agreement with the inhibitor chemical structure (Fig. 1). The second component at 285.4 eV may
61
be associated with the presence of C=N and C−O groups in the PMHQ molecules. The last and
less intense component (19 %) at higher binding energy (289.1 eV) can be partly ascribed to
+
aromatic rings (π−π shakeup satellite) and partly associated to the carbon atom of the C=N in the
69,70
8
ꢀquinolinol moiety,
resulting probably from the protonation of the =N− structure and/or the
coordination of nitrogen with the iron of steel surface.
The surveyed spectrum for N 1s of the studied carbon steel surface immersed in the
solution including PMHQ presents one peak (Fig. 7d). However, the investigated bare carbon steel
is nitrogen free, and the XPS analysis confirms the absence of N 1s peaks for this substrate as
71
previously described. Therefore, the adsorption of PMHQ on the steel surface can be explained
based especially on the N 1s signal. Indeed, one chemical environment of N 1s, detected at 399.9
6
1
eV as described in Fig. 7d, can be assigned to =N− structure in the quinoline ring. This finding
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provides evidence that the investigated 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivative is adsorbed
on the steel surface. However, the obtained component appears at with a shift of about 0.7 eV in
high binding energy side compared to the one observed in the case of pure PMHQ (Fig. 8). It can
be related to the =N− structure in the quinoline ring and also can be attributed to the coordinate
nitrogen in quinoline ring with the steel surface (NꢀFe bond complex), which leads to a positive
polarization of the nitrogen atom, and a coreꢀlevel chemical shift to higher binding energy is
7
2
produced. Therefore, the results reflect that the adsorption occurs through chemical chelation of
2
the vacant d orbitals of iron with the lone sp electron pairs present on the N atom of the
hetercocyclic ring of PMHQ.
a
O 1s
b
Fe 2p3/2
7
17
715
713
711
709
707
705
536
534
532
530
528
Binding energy (eV)
Binding energy (eV)
2
8

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Physical Chemistry Chemical Physics
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DOI: 10.1039/C8CP03226B
C 1s
N 1s
c
d
291
290
289
288
287
286
285
284
283
402
401
400
399
398
Binding energy (eV)
Binding energy (eV)
Fig. 7 Highꢀresolution Xꢀray photoelectron deconvoluted profiles of aꢀ Fe 2p , bꢀ O 1s, cꢀ C 1s,
3
/2
dꢀ N 1s for PMHQ treated carbon steel.
The above XPS detail analyses demonstrate that the presence of chemical (chemisorption)
interactions between surface steel ions and 5ꢀalkoxymethylꢀ8ꢀhydroxyquinoline derivative and
confirm the adsorption isotherm findings. Furthermore, the comparison of the Fe2p3/2 XPS results
for carbon steel after immersion in 1 M HCl medium containing PMHQ with that for bare carbon
71
steel, shows that the corrosive solution containing PMHQ molecules induces a significant
0
3+
decrease in Fe amount in favour of oxidized species, specially Fe , indicating therefore that the
oxide layer thickness is increasing. The addition of PMHQ in the corrosive solution promotes the
formation of the stable metalꢀorganic complex (PMHQ/Fe) leading to reducing the attack of acid
ions as well as restraining the corrosion process simultaneously.
2
9