American Journal of Physical Chemistry
2021; 10(1): 6-15
http://www.sciencepublishinggroup.com/j/ajpc
doi: 10.11648/j.ajpc.20211001.12
ISSN: 2327-2430 (Print); ISSN: 2327-2449 (Online)
Comparative Study of S235 Steel Corrosion Inhibition by
Eucalyptus camaldulensis and Cyperus rotundus Essential
Oils in Hydrochloric Acid Solution
Khaly Cisse1, 2, Diadioly Gassama1, *, Abdoulaye Thiam2, 3, El Hadji Barka Ndiaye2, 3,
Momar Talla Gueye3, Modou Fall2
1
Department of Physics Chemistry, UFR Sciences & Technologies, University of Iba Der Thiam, Thies, Senegal
2
Department of Chemistry, Faculty of Sciences and Techniques, Cheikh Anta Diop University, Dakar, Senegal
3
Laboratory of Phytosanitary Analyses, Institute of Food Technology, Dakar, Senegal
Email address:
*
Corresponding author
To cite this article:
Khaly Cisse, Diadioly Gassama, Abdoulaye Thiam, El Hadji Barka Ndiaye, Momar Talla Gueye, Modou Fall. Comparative Study of S235
Steel Corrosion Inhibition by Eucalyptus camaldulensis and Cyperus rotundus Essential Oils in Hydrochloric Acid Solution. American Journal
of Physical Chemistry. Vol. 10, No. 1, 2021, pp. 6-15. doi: 10.11648/j.ajpc.20211001.12
Received: February 23, 2021; Accepted: March 9, 2021; Published: April 12, 2021
Abstract: Synthetic compounds certainly exhibit good anticorrosive activity but also toxicity for humans and their
environment. Because of these concerns, we turned to more environmentally friendly substances such as essential oils and of
course other types of plant extracts. These products are considered green corrosion inhibitors. In this present study, we
propose to make a comparative study of the inhibitory effect of extracts of two different essential oils, Eucalyptus
camaldulensis (EC) and Cyperus rotundus (CR) on the corrosion of structural steel S235 in the medium 1 M hydrochloric
acid. This inhibitory action was studied using potentiodynamic polarization measurements and electrochemical impedance
spectroscopy. The GC-MS analysis of the essential oils extracts showed that EC contains 96.2% oxygenated terpenes (90.6%
monoterpenes and 5.6% sesquiterpenes), whereas CR includes 78.1% oxygenated terpenes (70.6% sesquiterpenes and 7.5%
monoterpenes). Polarization measurements indicate that EC and CR are mixed inhibitors. The experimental results gave an
inhibition efficiency close to 78.9% for EC and 86.7% for CR for an inhibitor concentration of 4 gL-1 at 293 K. The inhibition
performances of these essential oils were correlated with their composition. The adsorption of the molecules of the oils
responsible for the inhibition on the surface of the steel, in the hydrochloric acid solution, obeys the Langmuir adsorption
isotherm. This present work has therefore shown that these two types of essential oils have a good inhibitory effectiveness on
the corrosion of the metal S235 in 1M hydrochloric acid solution.
Keywords: Corrosion, Essential Oil, Eucalyptus Camaldulensis, Cyperus Rotundus, S235 Steel, Langmuir Isotherm
1. Introduction
Corrosion is a recurring phenomenon, difficult to eliminate
completely and which is caused by aggressive environments.
However, corrosion is perceived as an industrial problem which
generates numerous environmental, economic and even human
damages. Economically, for example, corrosion damages and
repairs can be valued in billions of dollars each year [1]. The
use of acids in crude oil refining, acid pickling, industrial
cleaning and acid descaling exposes metals to these corrosive
environments. Hydrochloric acid is extensively used in industry,
but causes degradation of some metals. To deal with this
unwanted and destructive phenomenon, several methods or
techniques of protection have been developed. These methods
of protection include the application of a protective barrier,
electroplating, cathodic protection, etc., or the use of anti-rust
solutions or corrosion inhibitors. The latter remains by far the
most coveted method. Most synthetic compounds exhibit good
corrosion inhibiting action, but the majority of them are highly
toxic to humans and the environment [2]. New environmental
demands encourage the development of new forms of inhibitors
American Journal of Physical Chemistry 2021; 10(1): 6-15
in the scientific world, which are less toxic and inherently safer.
In view of these considerations, intensive research has been
carried out on the substitution of toxic inhibitors for less toxic or
non-toxic products. The use of clays as inhibitors for S235 steel
corrosion has been investigated in our studies [3, 4]. Recently
several extracts from various parts of plants (leaves, fruits,
seeds and flowers) have been used as corrosion inhibitors [5-14].
In the same vein, in recent years a major importance was given
to the use of essential oils as corrosion inhibitors. According to
the literature, many studies have mentioned the good inhibitory
effectiveness of essential oils on the corrosion of steel in acidic
environment (hydrochloric and sulfuric) [15-29].
The objective of the present work is to make a comparative
study of the inhibitory action of two types of essential oils on
the corrosion behavior of S235 steel in a 1 M hydrochloric
acid solution.
2. Experimental
S235 structural steel was provided by Pole de
Développement Industriel, Dakar, Senegal. It is composed of
(mass percentage): C = 0.17, Mn = 1.40, N = 0.012, P = 0.04, S
= 0.04, Cu = 0.55, Fe = balance [30]. It was cut into
rectangular sheets of 3.6 cm2 area. Before each test, it was
sequentially polished using silicon carbide papers, rinsed with
distilled water and dried in the open air.
The corrosion solution consisted of a molar (1 M) solution
of hydrochloric acid prepared from a commercial
(Sigma-Aldrich) solution of hydrochloric acid of 37% purity
and 1.18 density. For each test, a quantity of the essential oils
of Eucalyptus camaldulensis (EC) or Cyperus rotundus (CR)
corresponding to concentrations ranging from 0 to 4 gL-1 was
added to the 1 M HCl solution. This concentration range was
determined after preliminary tests on the solubility of the
inhibitor in the corrosive medium. The corrosion of the steel
and the inhibitory efficacy were studied in these media.
Eucalyptus camaldulensis species were collected in February
2018 in Saint-Louis (northern Senegal), and Cyperus rotundus
was harvested in June 2018 in Mlomp (southern Senegal). The
EC essential oils were extracted from the leaves of the plant
whereas CR essential oils were isolated from the roots. These
extractions were carried out by steam distillation during 90 min
using a "Clevenger" type apparatus and the resulting essential
oils were stored in amber bottles maintained at 4 °C until use.
The Gas Chromatography (GC) experimentations were
carried out using a Thermo Electron TRACE GC Ultra
7
chromatograph (Thermo Electron Corporation, Interscience
Louvain-La-Neuve, Belgium), coupled to an Agilent
Technologies 5973 Network Mass Spectrometry Detector
(transmission quadrupole mass spectrometer) allowing the
detection and quantification of compounds. The chromatograph
was equipped with an Optima 5-accent type, 5%
phenylmethylsiloxane capillary column: 30 mL, 0.25 mm ID,
0.25 µm film thickness (Macherey-Nagel, Düren-Germany).
The detailed procedure for the GC-MS analysis (operating
conditions, temperature adjustment and identification
procedure) is described elsewhere [29].
The electrochemical investigations were performed in a
classical three-electrode cell containing a platinum grid as the
counter electrode, Ag/AgCl reference electrode (+197
mV/NHE), and the working electrode consisting of the S235
steel sample. The measurements were carried out using a
PalmSens 4 Potentiostat (Palmsens BV, NL) controlled by
PSTrace 4.8 software. The corrosion behavior of S235 steel in 1
M HCl solutions with and without essential oils were studied by
Tafel polarization method (0.5 mV/s scan rate and 30 minutes an
equilibrium time) and Electrochemical Impedance Spectroscopy
(EIS) at open-circuit-potential with a signal amplitude of 10 mV
and a frequency domain ranging from 50 kHz to 100 mHz (10
points/decade). Before running the experiments, the working
electrode was maintained at free corrosion potential for 1 hour in
the proper solution with magnetic stirring, thus allowing an
evenly solubilization of the essential oils in the aqueous
electrolyte at the operating temperature.
The electrochemical parameters (icorr, Ecorr, Rp, ba and bc)
were assessed from the Tafel polarization curves, according
to the Stern-Geary equation after ohmic drop compensation.
The inhibitory efficiency was evaluated using Eq. (1) [31]:
IE
× 100
(1)
i
and i
being the corrosion current densities
without and with inhibitor, respectively. The measured values
of the impedance were then fitted to a Randles
electrical-equivalent circuit and the results were used to
calculate the inhibitory efficacy using equation (2) [31].
IE =
× 100
(2)
R and R represent the charge-transfer resistance in the
absence and presence of inhibitor, respectively.
Figure 1. Chromatograms of Eucalyptus camaldulensis (A) and Cyperus rotundus (B) essential oils.
8
Khaly Cisse et al.:
Comparative Study of S235 Steel Corrosion Inhibition by Eucalyptus camaldulensis and
Cyperus rotundus Essential Oils in Hydrochloric Acid Solution
3. Results and Discussion
3.1. GC-MS Analysis of EC and CR Essential Oils
The composition of EC and CR essential oils extracts were
first determined by GC-MS. The chromatograms are shown in
Figures 1 and 2.
Table 1 gives the chemical composition of essential oils of
Eucalyptus camaldulensis (EC). It shows that the main
compounds present in the essential oil (EC) include:
1,8-Cineole (83.2%), α-Terpineol (3.7%), α-Pinene (2.2%).
The total percentage of oxygenated terpenes represent almost
96.2% of the total (90.6% of monoterpenes and 5.6% of
sesquiterpenes). In figure 2, we show the molecular structures
of the major constituents.
Table 1. Chemical composition of essential oils of Eucalyptus camaldulensis.
Compounds
α-Pinene
p-Cyméne
1,8-Cineole
Terpinen-4-ol
Trans-p-Mentha-1(7) dien-2-ol
α-Terpineol
Caryophyllene-oxide
Globulol
β-Eudesmol
α-Eudesmol
Hydrocarbon monoterpenes
Oxygenated monoterpenes
Oxygenated sesquiterpenes
Total compounds identified
Retention
Times
8.24
10.91
12.95
21.72
22.42
42.76
46.62
49.35
50.22
50.39
Retention
Indices
940
1026
1036
1177
1186
1190
1578
1587
1655
1659
% of
Compounds
2.2
1.6
83.2
1.9
1.7
3.7
0.7
0.7
1.9
2.2
3,8
90,6
5,6
100
Figure 2. Molecular structures of the main constituents of Eucalyptus
camaldulensis essential oil.
Isoaromadendrene epoxide (3.7%) and Longiverbenone
(11.3%). Like EC, CR mainly contains oxygenated terpenes
(78.1% of the total). However, we counted in CR more
sesquiterpenes (70.6%) than monoterpenes (7.5%). The
molecular structures of the most important constituents are
depicted in Figure 3.
Table 2. Chemical composition of essential oils of Cyperus rotundus.
Compounds
trans-Pinocarveol
Myrthenal
Verbenone
α-Copaene
α-Cubebene
δ-Selinene
9.10-Dihydro-isologifolene
non identifié
α-Guaiene
γ-Cadinene
β-Chamigrene
Valencene
Not identified
Not identified
Calacorene
cis-Caryophylleneoxide
Caryophylleneoxide
Humuleneoxide II
Viridiflorol
(-)-Caryophylleneoxide
4,4-Dimethyltetracyclo[6,3,2,0(2,5)0(1,8)]tridecan-9-ol
Nerolidol
Isoaromadendreneepoxide
Not identified
Longiverbenone
(+)(-)-Caryophyllèneoxide
Aristolone
Not identified
Not identified
Limonenedioxide
Nootkatone
Oxygenated monoterpenes
Hydrocarbon sesquiterpenes
Oxygenated Sesquiterpenes
Not identified
Total compounds identified
Retention
Times (min)
10.37
11.77
12.11
16.35
16.47
17.15
17.30
18.78
18.90
19.28
19.50
19.74
20.31
20.57
21.13
21.39
22.25
22.97
23.50
23.61
Retention
Indices
1152
1194
1203
1340
1346
1374
1381
1501
1504
1516
1523
1530
1548
1556
1573
1581
1608
1632
1649
1653
%
Compounds
1.8
1.7
1.2
3.5
3.7
2.0
0.8
0.7
0.5
1.4
0.6
1.0
0.4
0.9
0.6
1.5
19.2
26.1
1.2
1.6
23.72
1656
0.5
23.97
24.36
24.74
24.84
24.95
25.29
25.53
26.09
27.28
28.17
1664
1677
1689
1693
1696
1707
1716
1735
1776
1807
0.5
3.7
2.1
11.3
1.9
1.4
0.6
3.4
2.8
1.3
7.5
14.4
70.6
7.5
92.5
The chemical composition of CR essential oils is shown in
Table 2. The extracts are mainly composed of Caryophyllene
oxide
(19.2%),
Humulene
oxide
II
(26.1%),
Figure 3. Molecular structures of some major Cyperus rotundus essential oil constituents.
American Journal of Physical Chemistry 2021; 10(1): 6-15
3.2. Tafel Polarization Curves
The stationary Tafel polarization curves, in the absence and
in the presence of the inhibitors at various concentrations are
shown in Figure 4.
We observed that the cathodic and anodic Tafel lines are
quite parallel, which shows that the process follows a charge
transfer mechanism [32]. The Tafel stationary polarization
curves are characterized by a slight shift in the corrosion
potential as a function of the concentration of the inhibitor.
The intervention of the inhibitor in the transport process of
electroactive species (oxygen, protons, reaction products) in
the solution is unlikely, the effect of an inhibitor usually
unfolds at the close vicinity of the surface. We can thus
conceive the action of the inhibitor as giving rise to formation
and interposition of a barrier between the metal and the
corrosive environment. It is established that an inhibitor can
be classified as of cathodic or anodic type if the displacement
in Ecorr during the addition of the inhibitor is higher than 85
mV compared to the corrosion potential of the steel in an
uninhibited environment [33, 34]. The shift is slightly
cathodic for EC and comprised between 9 and 19 mV (in
absolute value), but slightly anodic for CR (8-11 mV except
for the higher concentration). CR and EC can be therefore be
classified as mixed inhibitors. In this case and essentially in
acidic environments, the role of adsorption at the surface will
be essential [35]. Mechanisms of dissolution of mild steel in
hydrochloric acid medium have been proposed by many
researchers. In general, in hydrochloric acid solution, the
9
anodic oxidation of iron by the Cl- can be represented by the
following reactions [36, 37]:
Fe + Cl → (FeCl )ads
(3)
(FeCl )ads → (FeCl ) + 2e-
(4)
(FeCl ) → Fe + Cl
(5)
2+
The cathodic reduction of hydrogen is given by the
following reactions [36, 37]:
Fe + H
→ (FeH )ads
(6)
(FeH )ads + e- → (FeH)ads
(7)
(FeH)ads + H+ + e- → Fe +H2
(8)
Figure 4. Tafel polarization curves of S235 steel in HCl (1M), at different
concentrations of Eucalyptus camaldulensis (A) and Cyperus rotundus (B) at
293 K.
Table 3. Electrochemical parameters and steel corrosion inhibiting efficiency of EC and CR essential oils in 1 M HCl at different concentrations at 293 K
obtained by Tafel polarization.
EC
CR
C (g/L)
0
0.5
1
2
3
4
0
0.5
1
2
3
4
Ecorr (V/Ag/AgCl)
-0.426
-0.445
-0.435
-0.446
-0.438
-0.444
-0.426
-0.419
-0.418
-0.415
-0.416
-0.432
∆E (mV)
0
-19
-9
-20
-12
-18
0
7
8
11
10
-6
icorr (µA/cm2)
687.4
303.9
224.9
191.6
164.2
154.5
687.4
239.8
165.7
143.7
130.8
97.5
Rp (Ω.cm2)
27.4
73.8
92.4
107.0
118.8
123.9
27.4
80. 3
111. 9
141. 3
152.7
176. 6
The introduction of EC or CR in the hydrochloric acid
solution does not modify the mechanism of evolution and
reduction of hydrogen on the surface of the steel [38], but act
simultaneously on the half-equations of oxidation and
reduction by modifying the adsorption process.
The experimental results obtained in Table 3 indicate a fairly
small variation of bc and ba in the HCl solution with or without
inhibitor (EC or CR). In addition, they are roughly independent
of the nature of the inhibitor. It can therefore be suggested that the
mechanism of corrosion remains unchanged [39].
The strong decrease in current density observed with the
introduction of essential oils of EC and CR shows the inhibitory
action of these natural compounds on the corrosion of S235
ba (mV/dec)
70
87
81
77
68
74
70
79
66
77
76
57
-bc (mV/dec)
114
127
117
122
133
109
114
101
121
119
116
130
Vcorr (mm/year)
4.72
1.77
1. 44
1.22
1. 05
0. 99
4.72
1.53
1. 06
0. 92
0.83
0.62
IE (%)
55.8
67.3
72.1
76.1
77.5
65.1
75.9
79.1
81.0
85.8
steel in 1 M HCl medium. However, the current densities after
inhibitor addition are much lower for EC compared with CR.
This decrease in the corrosion rate is due to the adsorption of the
majority of organic molecules contained in these essential oils
on the interface of the steel. The presence of the inhibitors
induces a reduction of the current on the metal surface. If this
blockage is only partial, it can lead to an increase in the current
density on these surfaces and therefore a localized corrosion
process, more intense than in the absence of inhibitor and
depending on the importance of its content. Inhibitors act by
forming a passive layer and should be used with caution as lead
to a modification of the corrosion nature [40, 41]
The slopes of the parallel cathode Tafel curves (Figure 4)
10
Khaly Cisse et al.:
Comparative Study of S235 Steel Corrosion Inhibition by Eucalyptus camaldulensis and
Cyperus rotundus Essential Oils in Hydrochloric Acid Solution
show that the reduction is not affected by the presence or
absence of the essential oils and the release of hydrogen is
controlled by an activation mechanism [42]. The inhibitor first
adsorbs to the steel surface before acting by simply blocking
its active sites. Cathodic inhibitors induce an increase in the
cathodic overvoltage, and therefore reduce the corrosion
current. While this inhibitor does not completely stop the
corrosion reaction, it does not present the risk of causing a
localized corrosion.
The inhibitory power increases with the concentration of
substances and reaches for a content of 4 gL-1 values of 77.52%
for EC and 85.82% for CR. The differences of inhibition
efficiencies may be related to the content of the essential oils.
As noted in GC-MS study the CR essential oils contain 70.6%
of sesquiterpenes and 7,5% of monoterpenes, whereas EC is
made of 90.6% of monoterpenes and 5.6% of sesquiterpenes. It
is worth to note that sesquiterpenes result from the combination
of three and monoterpenes from two branched, unsaturated C5
units (isoprene) [43]. The higher efficiencies of CR compared
with EC essential oils may be ascribed to the higher
concentrations of the larger molecules of sesquiterpenes.
3.3. EIS Measurements
To better understand the inhibitory action of these two
varieties of essential oils (EC or CR), we have also used EIS to
study the corrosion of the S235 material in a 1 M HCl medium,
in the presence and in the absence of essential oils of. Figures 5
and 6 show the electrochemical impedance Nyquist and Bode
diagrams of steel in the 1 M hydrochloric acid solution at
different concentrations of essential oils. They were recorded
after 2 h of immersion at room temperature at the corrosion
potential in frequencies ranging from 5000 Hz to 100 mHz.
Nyquist impedance diagrams (Figure 5) exhibit depressed
semi-circles with the center under the real axis. This trend,
typical for solid electrodes is often referred to as frequency of
dispersion. It is generally related to different physical
phenomena such as roughness and inhomogeneities of solid
surfaces, impurities, grain boundaries and distribution of
active sites on the surface [44]. The Nyquist curves recorded
in the different solutions show a single capacitive loop. The
shape of these graphs indicate that the metal dissolution
process was mainly controlled by charge transfer. Rather
imperfect semicircles are noticed. They could be result from
dispersing phenomena which may be related to solid surfaces
roughness and inhomogeneity and inhibitors adsorption [45].
Figure 6. Bode diagrams for the corrosion of S235 steel in 1 M HCl at
different EC (A) and CR (B) essential oils concentrations at 293 K.
Figure 7. Phase angle curves for the corrosion of S235 steel in 1 M HCl
different EC (A) and CR (B) essential oils concentrations at 293 K.
The capacitance values for both inhibitors are comprised
between 6.78 and 119.11 µF. They can especially be
interpreted as a Nernstian process rather than as adsorptive
film formation (capacitance values in the order of magnitude
of few nF). We therefore used a constant-phase element (CPE)
instead of a pure capacitance is used to fit of the experimental
measurements. The impedance of a CPE is given by the
following relationship [46, 47]:
ZCPE = A−1 (jω)
(9)
where A refers to the CPE (in Ω−1sn cm−2), j is the imaginary
number (j2 = −1), ω = 2πf corresponds to the angular frequency
of sinusoidal modulation (in rad/s), and n represents an
empirical exponent (0 ≤ n ≤1) which allows to evaluate the
deviation from a pure capacitance [48, 49] for which the n
exponent is equal to 1. The double-layer capacitance (Zdl) can
be obtained using Eq. (10):
Zdl =
(10)
From Figure 5 A and B, it can be seen that the semi-circle
diameters are higher and that the capacitances are lower for
CR essential oil, which suggests lower corrosion process
when this inhibitor is used, in comparison with EC.
The time constant (τ) and the double-layer capacitance (Cdl)
resulting from the CPE were calculated from Eqs. (11) and (12)
[50]:
Cdl = (AR
τ = CdlRct
Figure 5. Nyquist diagrams of S235 steel in 1 M HCl acid at different
concentrations of EC (A) and CR (B) essential oils at 293 K.
Frequency-domain: 50 kHz - 100 mHz..
–n
)
1/n
(11)
(12)
Bode diagrams are shown on Figure 6 (module) and Figure
7 (phase angle) for both inhibitors. Bode phase angle
diagrams exhibit a single peak which demonstrates
prominence of the charge-transfer in the metal corrosion. [51].
It was noticed that the module and phase angle increased
American Journal of Physical Chemistry 2021; 10(1): 6-15
with inhibitor concentration. Also, the peak positions on
phase angle diagrams slightly shifted to lower frequencies.
These features confirm the previous observations made from
the Nyquist diagrams, suggesting a lowering of the corrosion
rate by the EC and CR essential oils [52]. The general aspects
of the figures were not influenced by the nature of the
inhibitor. However, both module and phase angle values were
higher for CR inhibitor.
11
The EIS results (Rct, CPE (A) related to the double-layer
capacitance and the exponent) obtained after fitting of the
measured values of the impedance to the equivalent
electrical-circuit depicted in Figure 8 are shown in Table 4.
The analysis of the EIS results summarized in Table 4,
shows that for both inhibitors, the charge-transfer resistance
value (Rct) increases with concentration and reaches 101,8
Ωcm2 (EC) and 162,1 Ω cm2 (CR) at a concentration of 4 gL-1
for EC and CR. As glimpsed trough the Nyquist and Bode
diagrams, Rct is higher and Cdl is lower for CR inhibited
solutions. Moreover, the inhibition efficiencies are higher for
CR essential oils. This is in accordance with the Tafel
polarization results and confirms the key role played by the
sesquiterpenes which are present at higher concentrations in
EC essential oils.
Figure 8. Equivalent electrical-circuit for the impedance spectra modeling.
Table 4. EIS results and inhibition efficiencies for S235 steel in 1 M HCl containing different concentrations of EC and CR essential oils
Inhibitor
EC
CR
C gL-1
Rs (Ω.cm2)
Rct (Ω.cm2)
CdL (µFcm-2)
n
τ (ms)
0
0.5
1
2
3
4
0
0.5
1
2
3
4
5.8
4.8
4.7
5.2
5.5
4.0
5.8
4.5
5.1
5.0
5.5
5.6
21.5
47.5
78.2
86.6
92.5
101.8
21.5
67.7
93.6
122.4
133.1
162.1
119.1
108.8
75.4
67.7
63.8
57.9
119.1
76.9
54.9
43.4
36.3
29.8
0.77
0.80
0.83
0.85
0.87
0.91
0.77
0.82
0.87
0.92
0.93
0.97
2.56
5.17
5.90
5.86
5.80
5.90
2.56
5.20
5.13
5.31
4.80
4.83
High charge-transfer resistances are correlated with slower
corrosion systems, owing to a decrease in the active surface
required for the corrosion reaction [53, 54]. The empirical
exponent (n) after addition EC and CR increases, which may
support this conclusion. Indeed, the lower n value in the
absence of inhibitors (n = 0.77) are homogeneity defects of the
surface resulting from roughing and / or corrosion products
deposition. Addition of the essential oils increases the value of
the n coefficient indicating a reduction in surface
inhomogeneity due to the adsorption of molecules on the S235
steel surface. In the inhibited solutions, the values of n are
higher for EC (0.80-0.91) compared with CR (0.82-0.97). The
adsorbed layer is therefore more uniform in the latter case
which may be caused by a better protection of the metal.
In addition, adding these two varieties of essential oils to
the corrosive solution decreases the double layer capacitance
(Cdl) and increases the value of the time constant (τ) (Table 4).
Indeed, in the corrosive medium, the interface parameter (τ) is
comprised between 5.17 and 5.90 ms, while Cdl value
decreases from 108.8 to 57.9 µFcm-2 for EC. An increase of τ
and a decrease of Cdl induced by the inhibitor mean that the
charge and discharge rates at the metal-solution interface are
drastically reduced. This shows an adequacy between the
quantity of charge which can be stored, therefore the
capacitance, and the discharge speed in the interface (τ) [55].
The decrease in this capacity with increasing oil
concentrations can be attributed to the formation of a
IE (%)
(SIE)
54.7
72.5
75.2
76.7
78.9
68.2
76.9
82.4
83.8
86.7
(Tafel)
55.8
67.3
72.1
76.1
77.5
64.2
73.1
77.8
79.9
85.8
protective layer on the electrode surface [56]. The time
constants (4.80-5.20 ms) and double-layer capacitances
(76.9-29.8 µFcm-2) are lower in CR than in EC containing
media. If the result was predictable for Cdl, the reason why the
τ values are roughly lower with the CR inhibitor is actually not
clear for us. Moreover, τ varied randomly with the inhibitor
content in each case.
The inhibition efficiency was calculated from Rct values. It
is observed that IE (%) increases with the concentration of
inhibitor (Table 4) reaching a value of 77.52% for oil EC and
85.82% for oil CR for a concentration of 4 g/L. Furthermore,
CR essential oils appear to be more efficient as inhibitor. The
results obtained by EIS perfectly corroborate those obtained
by the Tafel polarization method.
3.4. Adsorption Isotherm and Surface Analysis
In aqueous solution, the adsorption of an organic adsorbate
at the metal / solution interface can be presented as a process
of substitutive adsorption between organic molecules in the
solution (Org(sol)) and water molecules on a metallic surface
(H2O(ad[[(s)) [57]:
Org (sol)+ XH2O (ads) ⇔ Org (ads) + XH2O (sol)
(13)
where Org (sol) and Org (ads) are the organic molecules in the
solution and adsorbed on the metal surface, respectively.
H2O (ads) represents the water molecules on the metal
12
Khaly Cisse et al.:
Comparative Study of S235 Steel Corrosion Inhibition by Eucalyptus camaldulensis and
Cyperus rotundus Essential Oils in Hydrochloric Acid Solution
surface, X is the size ratio representing the number of water
molecules exchanged with an organic adsorbate molecule.
The use of isotherms to describe the adsorption behavior of
corrosion inhibitors is important because they provide
important clues about the nature of the metal-inhibitor
interaction. The surface coverage (IE / 100) of the metal
surface has been fitted to different isotherms, including
Frumkin, Langmuir and Temkin. However, the best results
were obtained with the Langmuir isotherm. In the present
study, the values of θ were obtained using the EIS results
according to Eq. (14) [58]:
θ=
!
=
(14)
where R and R are the charge-transfer resistance values
without and with inhibitor, respectively.
The action of an inhibitor in aggressive acidic media is
presumably due to its adsorption at the metal / solution
interface, which hinders that of intermediate products of
anodic and cathodic corrosion reactions on the same interface.
The adsorption process is controlled by various parameters,
including the electronic characteristics of the inhibitor,
temperature, surface state of the metal, etc. [59]. The values of
the degree of surface coverage (θ) for different concentrations
of EC and CR essential oils (Cinh) were used to study the
adsorption processes according to the Langmuir isotherm.
"#
$
=
%&'(
+ C*
energy change (∆G./0 ) according to Eq. (16) [60]:
∆G./0 = −RTln 4105 K ./0 7
(16)
where 1.103 corresponds to the concentration of water (in g/L),
R represents the perfect gas constant (JK-1mol-1) and T the
temperature (K).
Figure 9 shows the linear plots of Cinh/ θ vs. Cinh, Kads was
obtained from the intercepts and were used in Eq. 9 to access
∆G./0 .
The results are summarized in Table 5. A very good fit was
observed in the studied range of concentrations (0.50 – 4 gL-1),
with good regression coefficients and slopes close to unity.
Therefore, the Langmuir isotherm reasonably fits the
experimental data. It is assumed in this type of isotherm, that
there are no interactions among the species which are
adsorbed on the electrode surface [61]. Kads illustrates the
adsorption forces linking the molecules of the essential oils
and the metal [62]. The negative sign of ∆G./0 values
indicate that the adsorption process is spontaneous.
(15)
+
In Eq. (15), Kads refers to the adsorption equilibrium
constant which is related to the standard adsorption Gibbs
Figure 9. Langmuir adsorption plots for S235 steel in 1 M HCl containing EC
(A) and CR (B) essential oils.
Table 5. Thermodynamic results of the Langmuir isotherm of EC and CR essential oils for S235 steel corrosion inhibition in the hydrochloric acid solution.
Inhibitor
EC
CR
R2
EIS
0.9997
0.9995
Tafel
0.9994
0.9983
Slope
EIS
1.22
1.12
Tafel
1.23
1.11
The type of adsorption is usually considered as a
physisorption if the absolute value of ∆G./0 is of the order of
20 kJmol-1 or less, and as a chemisorption if from 40 kJ.mol−1.
In this process, covalent bonds are formed by the sharing or
charge transfer of inhibitory molecules at the metal surface [63,
64]. The values of ∆G./0 calculated in this work which are
close to -21 kJmol-1 (Table 5) which suggests a physisorption
process of organic molecules for both essential oils on
structural steel S235 1 M HCl [65]. Similar results have
frequently been reported for essential oils inhibitors for steel
in HCl solution [18, 19, 23]. In a previous work, we showed
that a physicochemical adsorption was more probable in the
case of Cyperus articulatus essential oils in hydrochloric acid
solution [29]. It was however revealed by Sathiyanarayanan et
al. [66] that adsorption of inhibitors to anodic sites follows
chemical adsorption and adsorption to anodic sites is due to
electrostatic attraction.
The average of adsorption free energy values obtained by
Tafel polarization and EIS methods are almost identical for
two inhibitors: -20.88 kJmol-1 for EC and -20.85 kJmol-1 for
8 9:; (L/g-1)
EIS
4.34
4.71
Tafel
4.87
4.37
=
∆<9:;
(kJmol-1)
EIS
-20.74
-20.94
Tafel
-21.02
-20.76
CR. Similar observation was made regarding the interaction of
mild steel with the mercaptotriazole compound and 3,
4-dihydropyrimidin-2 (1H)-ones, respectively by Wang et al.
[67] and Yadav et al. [68].
4. Conclusion
In this work, we studied the S235 structural steel corrosion
inhibition effect of Eucalyptus camaldulensis (EC) and
Cyperus rotundus (CR) essential oils in 1 M hydrochloric acid
solution by Tafel polarization and EIS measurements. The
studies showed that the corrosion inhibition efficiency could
reach 78.9% for EC and 86.7% for CR essential oils at a
concentration of 4 gL-1. These inhibitors act as mixed
inhibitors. The highest inhibitory efficacy of CR is ascribed to
the greater presence of sesquiterpenes in these essential oils.
The results confirm the preponderant role of larger organic
molecules in the inhibition phenomena.
The adsorption of EC as well as CR on the steel surface in 1
M hydrochloric acid medium follows the Langmuir isotherm
American Journal of Physical Chemistry 2021; 10(1): 6-15
and is of physical nature.
Acknowledgements
The authors are grateful to TWAS, the World Academy of
Science for the Advancement of Science in developing
countries for financial and material support (TWAS RGA no.
16-499 RG/CHE/AF/AC_G–FR3240293299).
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