Corrosion Science 48 (2006) 2765–2779

www.elsevier.com/locate/corsci

Inhibitive action of some plant extracts

on the corrosion of steel in acidic media
A.M. Abdel-Gaber *, B.A. Abd-El-Nabey, I.M. Sidahmed,
A.M. El-Zayady, M. Saadawy
Department of Chemistry, Faculty of Science, Alexandria University, Ibrahimia, P.O. Box 426,
Alexandria 21321, Egypt

Received 2 March 2005; accepted 10 September 2005

Available online 20 December 2005

Abstract

The effect of extracts of Chamomile (Chamaemelum mixtum L.), Halfabar (Cymbopogon proxi-
mus), Black cumin (Nigella sativa L.), and Kidney bean (Phaseolus vulgaris L.) plants on the corro-
sion of steel in aqueous 1 M sulphuric acid were investigated by electrochemical impedance
spectroscopy (EIS) and potentiodynamic polarization techniques. EIS measurements showed that
the dissolution process of steel occurs under activation control. Potentiodynamic polarization curves
indicated that the plant extracts behave as mixed-type inhibitors. The corrosion rates of steel and the
inhibition efficiencies of the extracts were calculated. The results obtained show that the extract
solution of the plant could serve as an effective inhibitor for the corrosion of steel in sulphuric
acid media. Inhibition was found to increase with increasing concentration of the plant extract
up to a critical concentration. The inhibitive actions of plant extracts are discussed on the basis
of adsorption of stable complex at the steel surface. Theoretical fitting of different isotherms,
Langmuir, Flory–Huggins, and the kinetic–thermodynamic model, were tested to clarify the nature
of adsorption.
 2005 Elsevier Ltd. All rights reserved.

Keywords: Corrosion; Steel; Plant extracts; Critical concentration

*
Corresponding author. Tel.: +20 34288078; fax: +20 33911794.
E-mail address: ashrafmoustafa@yahoo.com (A.M. Abdel-Gaber).

0010-938X/$ - see front matter  2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.corsci.2005.09.017
2766 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779

1. Introduction

The known hazard effects of most synthetic corrosion inhibitors are the motivation for
the use of some natural products. Recently, Plant extracts have again become important as
an environmentally acceptable, readily available and renewable source for a wide range of
needed inhibitors. Plant extracts are viewed as an incredibly rich source of naturally syn-
thesized chemical compounds that can be extracted by simple procedures with low cost.
However, synergistic (and antagonistic) effects are often expected with these mixtures of
inhibitors that may affect their inhibition efficiency. Several investigations have been
reported using such economic plant extracts. El Hosary et al. [1] studied the corrosion
inhibition of aluminium and zinc in 2 N HCl using naturally occurring Hibiscus subdariffa
(Karkode) extract. The inhibition of corrosion of steel, aluminium and copper in HCl,
H2SO4 and citric acid by molasses was also studied [2] and 83% and 13% inhibition effi-
ciencies were obtained for HCl and H2SO4 solutions, respectively, containing 0.75%
molasses. Loto reported the inhibitive action of Vernonia amygdalina (bitter leaf) on the
corrosion of mild steel in 0.5 M HCl at 28 C [3]. Avwiri and coworkers studied the inhib-
itive action of V. amygdalina on the corrosion of aluminium alloys in HCl and HNO3 at
concentrations of 0.2 and 0.4 g/L at 29 C [4]. They showed that the solution extract of the
leaves serves as an excellent inhibitor. The inhibition effect of Zenthoxylum alatum plant
extract on the corrosion of mild steel in 20%, 50% and 88% aqueous orthophosphoric acid
has been investigated by weight loss and electrochemical impedance spectroscopy (EIS).
Plant extract was found to reduce the corrosion of steel more effectively in 88% than in
20% phosphoric acid [5]. An inhibition efficiency of 75.11% was observed with the extract
of the leaves of Nypa fruticans Wurmb [6] for the corrosion of mild steel in hydrochloric
acid solutions. El-Etre et al. examine a some naturally occurring substances as corrosion
inhibitors for different metals in various environments [7–11].
Earlier, Barannik and Putilova [12] showed that the actual inhibitors in the plant
extracts are usually alkaloids and other organic nitrogen bases, as well as carbohydrates,
proteins and their acid hydrolysis products. The existing data show that most organic
inhibitors act by adsorption at the metal/solution interface. This phenomenon could take
place via (i) electrostatic attraction between the charged metal and the charged inhibitor
molecules, (ii) dipole-type interaction between unshared electron pairs in the inhibitor with
the metal, (iii) p electrons-interaction with the metal, and (iv) a combination of all of the
above [13]. The adsorption process depends on the electronic characteristics of the inhib-
itor, the nature of the surface, the temperature and pressure of the reaction, steric effect,
multilayer adsorption and a varying degree of surface site activity.
The aim of the present work is to test extracts of Chamomile, Halfabar, Black cumin,
and Kidney bean as inhibitors for the acidic corrosion of steel and to discuss their inhibi-
tion mechanism.

2. Experimental

2.1. Electrochemical tests

Electrochemical impedance and polarization curve measurements were achieved using

Gill AC instrument. The frequency range for EIS measurements was 0.1–1 · 103 Hz with
applied potential signal amplitude of 10 mV around the rest potential. The data were
 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779 2767

obtained in a three-electrode mode; platinum sheet and saturated calomel electrodes were
used as counter and reference electrodes. The material used for constructing the working
electrode was steel that had the following chemical composition (wt.%): C, 0.21; S, 0.04;
Mn, 2.5; P, 0.04; Si, 0.35; balance Fe. The steel was encapsulated in epoxy resin in such
a way that only one surface was left uncovered. The exposed area (1 cm2) was mechani-
cally abraded with a series of emery papers of variable grades, starting with a coarse
one and proceeding in steps to the finest (800) grade. The samples were then washed thor-
oughly with double distilled water, followed with A.R. ethanol and finally with distilled
water, just before insertion in the cell.
Before polarization and EIS measurements the working electrode was introduced into
the test solution and left for 10 min at the open circuit potential. Polarization curve mea-
surements were obtained at a scan rate of 20 mV/min starting from cathodic potential
(Ecorr  250 mV) going to anodic direction. All the measurements were done at 30.0 ±
0.1 C in solutions open to the atmosphere under unstirred conditions.
To test the reliability and reproducibility of the measurements, duplicate experiments
were performed in each case of the same conditions.

2.2. Solutions preparation

Double distilled water and analytical reagent-grade H2SO4 were used for preparing
solutions. Stock solution of plant extracts was obtained by drying the plant for 2 h in
an oven at 80 C and grinding to powdery form. A 10 g sample of the powder was refluxed
in 100 mL double distilled water for 1 h. The refluxed solution was filtered to remove any
contamination. The concentration of the stock solution was determining by evaporating
10 mL of the filtrate and weighing the residue. Prior each experiment, 4 M H2SO4 is added
to an appropriate volume of the stock solution and double distilled water to obtain a solu-
tion of 1 M H2SO4 and the required concentration of the extract.

2.3. Spectrophotometric IR and UV analysis

For IR studies (KBr pellet method), 5 mL of 1 M sulphuric acid having 1.14 g/L of
black cumin extract was left to dry. The dried material was scratched with a glass rode
and the resultant powder was mixed with KBr and prepared as pellets. The IR spectrum
was performed by using a Perkin–Elmer 1430 spectrophotometer in the spectral region
between 4000 and 500 cm1.
For UV–visible measurements, solutions of 1 M sulphuric acid containing 1 · 103 M
2+
Fe ions, 1 M sulphuric acid having 1.14 g/L of black cumin extract, and 1 M sulphuric
acid having 1 · 103 M Fe2+ ions and1.14 g/L of black cumin extract were used. The UV–
visible spectrum was achieved by using a Jasco V-530 spectrophotometer.

3. Results and discussion

3.1. Potentiodynamic polarization results

Typical potentiodynamic polarization curves for steel in 1 M sulphuric acid in the

absence and presence of different Halfabar extract concentrations are shown in Fig. 1.
As seen, addition of Halfabar extracts affects both anodic dissolution of steel and cathodic
2768 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779

Fig. 1. Potentiodynamic polarization curves for steel in 1 M sulphuric acid in absence and presence of different
Halfabar extract concentrations.

reduction reactions indicating that the extracts could be classified as mixed-type inhibitors.
In general, for acid solutions, when dissolved oxygen is present both hydrogen evolution
and oxygen reduction reactions will be possible. However, in view of the fact that, the sat-
urated solubility of oxygen in pure water at 25 C is only about 103 mol dm3 [14] and
decreases slightly with concentration of dissolved salts. In addition, the concentration of
H3O+ in acid solutions, at pH  0, is high, and since this ion has a high rate of diffusion.
Consequently, the contribution made by the hydrogen evolution reaction on the cathodic
process will predominant that takes place by oxygen reduction reaction. Similar observa-
tions were recorded for other plant extracts.
Fig. 2 shows the Potentiodynamic polarization curves for steel in 1M sulphuric acid in
absence and presence of 1 g/L Halfabar, Chamomile, Kidney bean and Black cumin,
respectively. The plant extracts inhibit both cathodic and anodic parts of the polarization
curves. The cathodic parts of the polarization curves for these plant extracts are matched
indicating that the chemical constituents of the extracts have similar effects on the cathodic
process. However, the plant extracts affect the anodic part of the polarization curves by
different degrees indicating that their inhibitive effect depends on the type of plant extract.
The anodic shifts for steel in presence of different plant extracts is in the order
Halfabar < Chamomile < Kidney bean < Black cumin
The corrosion current density was calculated from the intersection of cathodic and ano-
dic Tafel line. The values of the electrochemical parameters for different plant extracts con-
centrations are given in Table 1. The displayed data show that increasing plant extracts
concentration decreases the corrosion current density (icorr) but slightly affect the values
 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779 2769

Fig. 2. Potentiodynamic polarization curves for steel in 1 M sulphuric acid in absence and presence of 1 g/L
Halfabar, Chamomile, Kidney bean and Black cumin, respectively.

of corrosion potential (Ecorr) indicating that it could act as pickling inhibitor [15]. The per-
centage of inhibition efficiency (%inh) were calculated from polarization measurements
using the relation
%inh ¼ ½ði0  iÞ=i0   100
where i0 and i are the corrosion current density in the absence and presence of plant ex-
tract. However, it is generally observed that each plant has a concentration after which
the corrosion rate (Rate) increases and the % inhibition decreases. The anodic and catho-
dic Tafel slopes (ba and bc) are approximately constant suggesting that the inhibiting ac-
tion occurred by simple blocking of the available cathodic and anodic sites on the metal
surface.

3.2. Electrochemical impedance spectroscopy results

The Nyquist plots for steel in 1 M sulphuric acid in the absence and presence of differ-
ent Halfabar extract concentrations are shown in Fig. 3. These plots indicate that the dis-
solution process occurs under activation control. The impedance response consisted of
characteristic semicircles. These semicircles are of a capacitive type whose size increases
with increasing Halfabar concentration up to 2.52 g/L; a further increase of the Halfabar
concentration leads to a decrease in the size of the semicircle.
The Nyquist plots of impedance data for steel in 1 M sulphuric acid containing 1 g/L of
different plant extracts are shown in Fig. 4. These plots have characteristic semicircles of a
2770 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779

Table 1
Electrochemical parameters for steel in 1 M sulphuric acid containing different plant extracts concentrations
Plant [Conc.] Ecorr ba bc icorr Rate % inh
(g/L) (mV) (mV) (mV) (mA/cm2) (mm/y)
– 0.00 503 92.9 120.6 2.42 28.07 –
Halfabar 0.63 516 62 124 0.63 7.29 73.97
1.05 516 61 124 0.49 5.70 79.75
1.68 510 55 118 0.28 3.22 88.43
2.52 509 54 117 0.23 2.61 90.50
3.36 510 55 122 0.36 4.18 85.12
3.78 513 56 126 0.39 4.55 83.88
Chamomile 0.84 511 58 129 0.45 5.19 81.40
3.36 496 56 134 0.32 3.79 86.78
4.20 499 56 128 0.25 2.92 89.67
7.56 503 70 120 0.17 1.97 92.97
7.98 493 73 134 0.18 2.03 92.56
8.40 500 68 128 0.19 2.17 92.15
Kidney bean 0.48 515 66 130 0.61 7.08 74.79
1.44 514 65 120 0.40 4.58 83.47
1.92 510 59 116 0.34 3.91 85.95
2.40 511 61 113 0.28 3.28 88.43
2.52 492 46 139 0.39 4.52 83.88
2.76 475 57 148 0.43 5.07 82.23
Black cumin 0.15 489 65 127 0.83 9.60 65.70
0.35 493 61 121 0.41 4.72 83.05
0.51 484 57 125 0.33 3.85 86.36
1.14 487 61 118 0.23 2.69 90.50
1.27 495 64 121 0.27 3.17 88.84
1.40 492 67 116 0.28 3.22 88.43

capacitive type whose size depends on the type and chemical constituent of the additive.
The order of increasing the size of the semicircle is:
Halfabar < Chamomile < Kidney bean < Black cumin
The impedance spectra for different Nyquist plots were analyzed by fitting the experi-
mental data to a simple equivalent circuit model (Fig. 5) which includes the solution resis-
tance Rs and the double layer capacitance (Cdl) which is placed in parallel to charge
transfer resistance element, Rct. The Rct value is a measure of electron transfer across
the surface and is inversely proportional to corrosion rate.

3.3. Critical concentration of the extracts

The values of Rct and Cdl for steel in 1 M sulphuric acid containing different Halfabar
concentrations are shown in Fig. 6. The data indicate that increasing charge transfer resis-
tance is associated with a decrease in the double layer capacitance up to a critical concen-
tration (2.52 g/L). This behaviour is thereafter reversed with further increase in the
concentration of the extract. The decrease in the Cdl values could be attributed to the
adsorption of the chemical constituents of Halfabar extracts at the metal surface. It has
been reported that the adsorption process on the metal surface is characterized by a
 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779 2771

Fig. 3. Nyquist plots for steel in 1 M sulphuric acid in the absence and presence of different Halfabar extract
concentrations.

Fig. 4. Nyquist plots for the steel in 1 M sulphuric acid free from and containing 1 g/L of different plant extracts.
2772 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779

Cdl

Rs

Rct

Fig. 5. The equivalent circuit model.

260
60 Rct, Halfabar
Cdl, Halfabar 240
50 220
200
2
Rct, oh m.cm

40 180

Cdl, μF
160
30 140
120
20
100

10 80
60
0 40
0 1 2 3 4
Conc, g/L

Fig. 6. Variations of the Rct, and Cdl values against the concentration of Halfabar extract.

decrease in Cdl [16]. Similar observations are recorded for other plant extracts under study
but at different critical concentrations (CC).
The numerical values for the critical concentration of different plants extracts and the
corresponding percentage inhibition are given in Table 2. The % inh were calculated from
impedance measurements using the relation
% inh ¼ ½ðRct  Rct0 Þ=Rct   100
where Rct0 and Rct are the charge transfer resistances in the absence and presence of plant
extract.
The values of % inh are in quite good agreement with the results obtained previously
from polarization measurements (Table 1). This demonstrates the fact that the corrosion
rate depends on the chemical nature of the electrolyte and the temperature of the medium,
rather than the applied technique.

Table 2
Critical concentration (CC) and percentage inhibition efficiency (% inh) for different plants extracts
Plant Chamomile Black cumin Halfabar Kidney bean
CC (g/L) 7.56 1.14 2.52 2.4
% inh 90.2 87.2 87.1 83.5
 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779 2773

A lot of natural products were previously used as corrosion inhibitors in various acidic
media and their optimum concentrations were reported. For examples, the optimum con-
centration for maximum inhibition efficiency of Z. alatum plant extract on the corrosion of
mild steel exposed to 88% phosphoric acid at 30 C was found to be 3200 ppm of plant
extract. However in case of 20% phosphoric acid, the optimum concentration is found
to be 2400 ppm [5]. The highest inhibition efficiency exhibited by the aqueous extract of
the leaves of henna (Lawsonia) for C–steel corrosion in 0.1 M HCl solutions was
95.78% with additives concentration of 800 ppm at 30 C [11]. These data and our results
for the corrosion of steel in 1 M sulphuric acid in presence of different plant extracts (Table
2) suggests that the plant extracts could serve as effective corrosion inhibitors.
The phenomenon of critical concentration for these extracts can be explained on the
basis that different chemical compounds could be extracted from any plant. Since, it is well
known that the adsorption process depends on the electronic characteristics of the inhib-
itors. The natural substances are very complex and also very active. They can be used as
soothing, stimulant, and poisons. In order to clarify these points, the main chemical com-
positions and folk medicine uses of Kidney bean [17–21], Black cumine [22–30], Chamo-
mile [31–35], Halfabar [36–39] are given in Table 3. The use of These plant extracts in the
folk medicine suggests that it can be classified as environmentally safe inhibitors.

Table 3
The main chemical compositions and folk medicine uses for the Kidney bean, Black Cumin, Chamomile, and
Halfabar plant extracts
Plant Main chemical compositions Folk medicine uses
Kidney bean Anthocyanins (cyanidin Good source of
(Phaseolus vulgaris L.) 3,5-diglucoside, delphinidin cholesterol-lowering fiber,
3-glucoside, cyanidin prevents blood sugar levels
3-glucoside, petunidin from rising too rapidly
3-glucoside, and pelargonidin after a meal, and used for cardiac
3-glucoside), vitamin A,
thiamine, riboflavin, niacin,
ascorbic acid
Black cumin Monoterpenes, alanine, Antitumor, immunopotentiation,
(Nigella sativa L.) p-cymene, thymol, antihistaminic, antidiabetic,
thymoquinone, antihypertensive, antiinflammatory,
dithymoquinone, and antimicrobial activity
and thymohydroquinone
Chamomile Chamazulene, a-bisabolol, Spasmolytic and antiphlogistic properties
(Chamaemelum mixtum L.) bisabololoxides A and B,
polyacetylenes (cis- and
trans-spiroethers), flavonoids
(apigenin-7-glucoside and
luteolin-7-glucoside)
Halfabar Six sesquiterpenes Alleviate the toxic effect of carcinogens
(Cymbopogon proximus) (proximadiol; 5a- in the liver of diabetic, antispasmodic,
hydroperoxy-b-eudesmol; diuretic agent, and antimicrobial activity
7a,11-dihydroxy-cadin-10(14)-ene;
5a-hydroxy-b-eudesmol; 1b-
hydroxy-b-eudesmol; and 1b-
hydroxy-a-eudesmol)
2774 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779

3.4. Spectrophotometric analysis and corrosion inhibition mechanism

IR spectrum (KBr pellet method) of 1 M sulphuric acid having 1.14 g/L of black cumin
extract is displayed in Fig. 7. The broad band at about 3475 cm1 can be assigned to the
presence of intermolecular hydrogen bond, stretching mode of an O–H and/or N–H.
Monosubstituted alkenes, that is, vinyl group and simple open chain, secondary amides,
absorb near 1640 cm1. The C–O stretching vibrations in alcohols and phenols, which
can be obtained from the hydrolysis of quinone, produce a strong band in the 1260–
1000 cm 1 [40]. Therefore, it is evident that the extracted organic compounds are stable
in 1M sulphuric acid.
The large number of different chemical compounds for plant extract (PE) may react
with the iron, which is firstly dissolved from the metal surface, forming organo-metalic
complex such as Fe–plant extract [Fe–PE] according to the following mechanism [5,11].
Fe ! Fe2þ þ 2e
Fe2þ þ PE ! ½Fe–PE2þ
The existence of Fe–plant extract complex is documented from UV–visible measurement
(Fig. 8). As seen, the solution having Fe2+ ions exhibit three absorption bands at 196, 220,
and 301 cm1. On the other hand, the solution containing black cumin extract shows only
two bands at 201, and 258 cm1. Addition of the Fe2+ ions to black cumin extract results
on replacing the 220, and 301 cm1 absorption bands of ferrous ions by intense absorption
band at 266 cm1 indicative of a formation of Fe–plant extract complex.
Fe–plant extract may be as a stable complex adsorbed over the metal surface resulting
in an inhibitive effect or it may be as a soluble complex leading to a catalytic effect. In sys-
tems that contain different chemical species, there is a possibility to form both of these two
types of complexes. Therefore, the effect of plant extract on the corrosion behaviour of
steel in 1 M sulphuric acid could be explained on the basis of two factors: the inhibitive
effect of the stable complex and other catalytic effect of soluble metal complex. It seems
that the inhibitive effect of the adsorbed metal complex is initially predominant until a

Fig. 7. IR spectrum for 1 M sulphuric acid having 1.14 g/L black cumin.
 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779 2775

Fig. 8. UV–visible spectrum for 1 M sulphuric containing different additives.

critical concentration where a certain leveling off value of their inhibition efficiency is

reached. On further increase of the amount of the plant extract, the accelerating effect of
other soluble species appears, resulting in a decreasing of the maximum inhibition efficiency.

3.5. Application of adsorption isotherms

The degree of surface coverage (h) of the metal surface by an adsorbed plant extract is
calculated using the equation
h ¼ ðRct  Rct0 Þ=Rct
The variations of surface coverage with concentration of different plant extracts are shown
in Fig. 9. These curves have S-shaped adsorption isotherms that are characterized by an
initial steeply rising part indicating a formation of a mono-layer adsorbate film on the steel
surface. At high concentration, the inhibitory effect remained constant suggesting com-
plete saturation of the surface by the inhibitor molecules. The appearance of critical con-
centration after which the inhibitive effect of extract decreased is also observed.
The Langmuir isotherm is given by [41]:
½h=ð1  hÞ ¼ K½C
where K is the binding constant representing the interaction of the additives with metal
surface and C is the concentration of the additives.
The Flory–Huggins isotherm is given by [42]:
h=½xð1  hÞx  ¼ K½C
2776 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779

0.9

Degree of surface coverage, θ

0.6

Halfabar
Chamomile
Kidney bean
Black cumin
0.3

0 3 6 9
Conc., g / L

Fig. 9. Variations of surface coverage with concentration of different plant extracts.

where x is the size parameter and is a measure of the number of adsorbed water molecules
substituted by a given inhibitor molecule.
The kinetic–thermodynamic model is given by [43]:
log½h=ð1  hÞ ¼ log K 0 þ y log C
where y is the number of inhibitor molecules occupying one active site. The binding con-
stant K is given by
K ¼ K 0ð1=yÞ
The previously mentioned isotherms were used to fit the corrosion data of the Chamomile,
Halfabar, Black cumin, and Kidney bean extracts. Figs. 10–12 shows the linear fitting of
Chamomile, Halfabar, Black cumin, and Kidney bean according to the mentioned models.
The parameters obtained from these figures are given in Table 4.
It is clear that the Langmuir isotherm is applicable to fit the data of Kidney bean but
unsuitable to fit the data of other plant extracts indicating that there might be non-ideal
behaviour in the adsorption processes [15] of the complex of Black cumin, Halfabar,
and Chamomile extracts on the steel surface. On the other hand, Flory–Huggins isotherm
is found to be applicable only to chamomile extract. The values of the size parameter x
indicates that the adsorbed species of chamomile is bulky since it could displace more than
one water molecule from the steel surface [44]. The kinetic–thermodynamic model is found
to fit the data of all plant extracts. The number of active sites occupied by a single inhibitor
molecule, 1/y, are nearly equal to the size parameter x for Chamomile. The binding con-
stant K values for Kidney been and Chamomile obtained from Langmuir and Flory–
Huggins isotherms, respectively, are in a good agreement to that obtained from the
kinetic–thermodynamic model. Since the efficiency of a given inhibitor is essentially a
function of the magnitude of its binding constant K, large values of K mean better and
stronger interaction, whereas small values of K mean that the interaction between the
inhibitor molecules and the metal is weaker [45]. Hence, according to the numerical values
 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779 2777

Black cumin
6 Chamomile
Kidney bean
Halfabar

4
θ / (1- θ )

0
0.0 0.8 1.6
C, g / L

Fig. 10. Linear fitting of different plant extracts to Langmuir isotherm.

0.3
log (θ / [ c ])

0.0

Black cumin
-0.3 Chamomile
Kidney bean
Halfabar

-0.9 -0.6 -0.3

log ( 1 - θ )

Fig. 11. Linear fitting of different plant extracts to Flory–Huggins isotherm.

of K obtained from the kinetic–thermodynamic model, the inhibition efficiency of different

plant extracts could be arranged in the order
Black cumin > Kidney bean > Chamomile > Halfabar
This trend is in good agreement to that obtained from polarization curves and Nyquist
diagrams (Figs. 2 and 4). Therefore, the inhibitive effect can be explained on the basis
of a mechanism suggesting adsorption of the plant extract-complex on the surface of
2778 A.M. Abdel-Gaber et al. / Corrosion Science 48 (2006) 2765–2779

0.5
log (θ / (1 - θ )

0.0

Black Cumin
Chamomile
Kidney bean
-0.5 Halfabar

-1.0 -0.5 0.0

Log C

Fig. 12. Linear fitting of different plant extracts to kinetic–thermodynamic model.

Table 4
Linear fitting parameters of Chamomile, Halfabar, Black cumin, and Kidney bean according to the used models
Plant extracted Models parameters
Langmuir Flory–Huggins Kinetic–thermodynamic
K x K 1/y K
Black cumin – – – 0.50 4.89
Kidney bean 3.76 – – 0.75 3.26
Chamomile – 1.49 2.27 1.32 2.44
Halfabar – – – 0.76 1.92

the native metal which in turns acts as a film forming species decreasing the active area
available for acid attack.

4. Conclusions

The plant extracts of Black cumin, Kidney bean, Chamomile and Halfabar can be used
as excellent corrosion inhibitors for steel in acidic medium. To obtain the maximum protec-
tion efficiency, critical plant extract concentration should be determined. The inhibition
mechanism depends on the formation of a stable plant extract-complex on the steel surface.

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