UAE_Neem_Extract_as_a_Corrosion_Inhibitor-460154-Published Paper-2010

Hindawi Publishing Corporation
International Journal of Corrosion
Volume 2010, Article ID 460154, 9 pages
doi:10.1155/2010/460154
Research Article
UAE Neem Extract as a Corrosion Inhibitor for Carbon Steel in
HCl Solution
Ayssar Nahlé, Ideisan Abu-Abdoun, Ibrahim Abdel-Rahman, and Maysoon Al-Khayat
Department of Chemistry, College of Sciences, University of Sharjah, P.O. Box 27272, Sharjah, UAE
Correspondence should be addressed to Ayssar Nahlé, anahle@sharjah.ac.ae
Received 3 December 2009; Accepted 9 July 2010
Academic Editor: Vesna B. Miˇsković-Stanković
Copyright © 2010 Ayssar Nahlé et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The effect of aqueous extract of UAE Neem (Azadirachta Indica) on the corrosion inhibition of carbon steel in 1.0 M HCl solution
was investigated electrochemically, and by weight-loss experiments at temperatures ranging from 303 to 343 K. The percentage
inhibition increased with the increase of the concentration of the inhibitor. At a concentration of 2.0 g/L, the percentage inhibition
reached about 87% at room temperature and 80% at 303 K. The percentage inhibition decreased with the increase of temperature.
The thermodynamic parameters for the adsorption of this inhibitor on the metal surface were calculated using the Temkin
adsorption isotherm. The aqueous Neem leaves extract was found to be an excellent potential corrosion inhibitor because of
the high content of tannin content as well as the presence of a series of complex triterpene glycosides.
1. Introduction
The corrosion of metals in many industries, constructions,
installations, and civil services such as electricity, water, and
sewage supplies is a serious problem. In order to prevent or
minimize corrosion, inhibitors are usually used especially in
flow cooling systems. Organic, inorganic, or a mixture of
both inhibitors can inhibit corrosion by either chemisorption
on the metal surface or reacting with metal ions and forming
a barrier-type precipitate on its surface [1].
Because of the toxic nature and/or high cost of some
chemicals currently in use as inhibitors, it is necessary to
develop environmentally acceptable and inexpensive ones.
Natural products can be considered as a good source for
this purpose. The aqueous extracts from different parts
of some plants such as Henna, Lawsonia inermis [1],
Rosmarinous officinalis L. [2], Carica papaya [3], cordia
latifolia and curcumin [4], date palm, phoenix dactylifera,
henna, lawsonia inermis, corn, Zea mays [5], and Nypa
Fruticans Wurmb [6] have been found to be good cor-
rosion inhibitors for many metals and alloys. Recently,
an excellent review about “natural products as corrosion
inhibitors for metals in corrosive media” has been published
[7].
No studies have been reported on the aqueous extract of
Neem leaves planted in UAE, Azadirachta indica, as corrosion
inhibitor used in our present work, in terms of studying
both the electrochemical effect and the temperature effect
on the corrosion inhibition of carbon steel in 1.0 M HCl
solution. Carbon steel was chosen in our studies since high
temperature aggressive acids are widely used in industries in
connection to carbon and low alloy steels.
The aim of this work is to study, using potentiodynamic
and weight-loss measurements, the effect of temperature
on the corrosion inhibition of carbon steel in 1.0 M HCl
solution by the aqueous extract of Neem leaves, and to
calculate the thermodynamic parameters. The output of this
study is intended to be a cheap and an eco-friendly natural
source that can be utilized as a corrosion inhibitor for metals
and alloys.
2. Experimental
The leaves were collected from 10 years old Neem trees
planted on the University of Sharjah premises (Sharjah,
United Arab Emirates), dried at room temperature, and
stored in dark for later use.

2 International Journal of Corrosion
Table 1: The effect of soaking time of Neem leaves in distilled water
on the percentage inhibition.
Time of soaking neem
leaves in water (hours)
Inhibition at 303 K(%)
0.17 50
0.33 55
0.5 60
1 70
3 77
7 80
14 80
24 80
168 80
Several extraction procedures were tested. These extrac-
tion procedures included extraction by distilled water at
80◦
C, extraction by distilled water at room temperature, and
extraction by boiling ethanol (78◦
C). Preliminary corrosion
studies by weight-loss measurements were carried out in
order to find the optimal extraction procedure. These pre-
liminary corrosion measurements showed that the aqueous
extraction at room temperature was the same or far more
efficient than the two other procedures. This extraction
procedure was adopted in our current work.
In order to optimize the time of extraction of the
Neem, 2.0 g of the dry Neem leaves were soaked in 60 mL
distilled water at room temperature (20◦
C) for various time
ranging from 0.17 hours to 168 hours, followed by filtration,
then added to an aqueous HCl solution to make a 1.0 L
solution in 1.0 M HCl. Weight-loss measurements described
in (Section 2.2.3) were carried out on steel specimens
prepared as shown in (Section 2.2.1) in each extract solution.
The percent inhibition was finally calculated according to
(2). It was found that 7 hours was the optimal soaking time
as shown in Table 1 and Figure 1.
In this work, 2.0 g of dry Neem leaves were soaked in
60.0 mL distilled water at room temperature (20◦
C) for 7
hours (the optimal time), and then filtered. The filtrate
was added to an aqueous HCl solution to make a 1.0 L
stock solution in 1.0 M HCl. From the stock solution, a
series of diluted solutions in 1.0 M HCl were prepared with
concentrations ranging from 2.0 g/L to 0.02 g/L.
2.1. Electrochemistry
2.1.1. Electrode Preparation. 5 mm diameter working elec-
trode was prepared from a 5 mm diameter carbon steel
rod (IS 226 containing 0.18% C, 0.6% Mn, and 0.35%
Si) supplied by “Reliable Steel Traders”, Sharjah, UAE;
and mounted, using Araldite epoxy resin, in a glass tube
that fits in the electrochemical cell. The working carbon
steel electrode was abraded using a series of carborundum
papers starting with 600 grades and ending with 1200
grades prior to each experiment. The electrode surface was
then polished with 0.3 μm alumina on cloth, washed with
deionized distilled water, and rinsed with pure ethanol before
100
90
80
70
60
50
40
1E−01 1E+00 1E+01 1E+02
Inhibition(%)
1E+03
Time of soaking Neem leaves in water (hr)
Figure 1: The effect of soaking time of Neem leaves in distilled
water on the percentage inhibition.
1
9
2
5
8 10
6
3
4
7
Figure 2: The electrochemical cell. (1) gas bubbler, (2) B 12 glass
socket, (3) Platinum gauze (counter electrode), (4) glass frit, (5)
inlet for nitrogen gas, (6) luggin capillary (reference electrode), (7)
steel rod (working electrode), (8) epoxy resin, (9) B 24 glass socket,
(10) copper wire.
it was transferred to the electrochemical cell that contained
deaerated fresh electrolyte.
2.1.2. Instrumentation. The same electrochemical glass cell
(made in Southampton University, England) used in our
previous works [8–13] and described in Figure 2 was here
employed. The electrochemical cell consisted of a carbon
steel working electrode (WE), a saturated calomel electrode

International Journal of Corrosion 3
(SCE) as a reference electrode (RE), and platinum gauze
counter electrode (CE). Prior to each experiment, the elec-
trolyte was deaerated by nitrogen bubbling. The cell design
allowed nitrogen to escape into the solution, precluding its
collection at the electrode surface that is fitted horizontally.
The counter-electrode compartment was separated from
the working-electrode compartment with a glass frit. This
way, the working electrode was protected from any substance
that may be produced at the counter electrode during the
electrochemical reactions. The electrochemical equipment
and chemicals used were as follows.
A PC-controlled Sycopel AEW2-1000 electrochemical
workstation (supplied from Sycopel Scientific Limited, Eng-
land) capable of driving currents up to ±1 A with an output
potential across the cell of up to ±10 V.
Analytical-grade hydrochloric acid (Ajax) was used.
2.1.3. Measuring Procedure. Electrochemical corrosion mea-
surements (Tafel plots) were carried out on the carbon steel
electrode, prepared as described above, in 1.0 M HCl and in
1.0 M HCl containing various concentrations of the aqueous
Neem extract. The concentration of the Neem extract ranged
from 2.0 g/L to 0.2 g/L.
60 milliliters of the freshly prepared electrolyte was filled
in the electrochemical cell. The solution was then deaerated
with nitrogen gas; and the working electrode equilibrium
potential versus SCE was recorded until it reached a steady
state. Finally, the electrode potential was scanned from
−700 mV to −100 mV versus SCE at a sweep rate of
1 mV·s−1. The data were collected and plotted as logarithm
of the absolute value of the current (log |I|, mA) versus
the electrode potential (E, mV) versus SCE. With extreme
experimental precautions taken, and with a newly polished
electrode and fresh electrolyte, the experiment was repeated
for at least three times in order to ensure the reproducibility
of the results. Once reproducible plots were obtained, the
corrosion currents were then extrapolated from the log |I|
(logarithm of current, mA) versus electrode potential plots
(Tafel plots).
2.2. Effect of Temperature
2.2.1. Specimen Preparation. Rectangular specimens (1cm ×
2.3cm × 0.3cm) were cut from a 3 mm thick carbon steel
sheet (IS 226 containing 0.18% C, 0.6% Mn, and 0.35% Si)
supplied by “Reliable Steel Traders”, Sharjah, UAE; and used
in the weight loss measurements. Close to the upper edge of
the specimen, a 2 mm diameter hole was drilled and served
to be hooked with a glass rod for immersion purposes. The
specimens were polished with 600 grade emery paper, rinsed
with distilled water, degreased with acetone, dried, and
finally weighed precisely on an accurate analytical balance
prior to each experiment.
2.2.2. Instrumentation. For the weight-loss measurements
[14, 15], a 250 mL round bottom flask fitted with a reflux
condenser and long glass rod which served to hook and
immerse the specimen and in turn immersed in a thermally
controlled water bath was used.
−4
−3
−2
−1
0
1
2
−0.7 −0.6 −0.5 −0.4 −0.3
1
2
3
4
5
LogI(mA)
Potential E (V) versus SCE
Figure 3: Anodic and cathodic polarization curves of carbon steel
in an uninhibited 1.0 M HCl solution and in 1.0 M HCl containing
various concentrations of the aqueous extract of Neem leaves.(1)
1.0 M HCl; (2) 1.0 M HCl + 0.20 g/L extract; (3) 1.0 M HCl +
0.50 g/L extract; (4) 1.0 M HCl + 1.00 g/L extract; (5) 1.0 M HCl
+ 2.00 g/L extract.
2.2.3. Measuring Procedure. 100 mL of 1.0 M HCl solution
either with or without the presence of various concentrations
of aqueous Neem extract were transferred in the flask. The
flask was then placed in water bath. When the required
temperature was reached, the precisely weighed carbon steel
specimen hooked with a glass rod was immersed in the
solution for exactly six hours. After that time, the sample was
removed, rinsed with distilled deionized water, degreased
with acetone, dried, and finally weighed precisely on an
accurate analytical balance. This procedure was repeated for
a variety of inhibitor concentrations ranging from 0.02 g/L to
2.00 g/L; and at temperatures ranging from 303 K to 343 K.
3. Results and Discussion
3.1. Electrochemistry. The anodic and cathodic polarization
curves (Tafel plot) of the carbon steel electrode in deaerated
1.0 M HCl solution with and without the addition of
various concentrations of the aqueous Neem extract are
shown in Figure 3. The aqueous Neem extract acted as a
mixed inhibitor because its presence at all concentrations
affected both, the anodic and the cathodic branches of the
polarization curves. Furthermore, the polarization curves
corresponding to all concentrations of the aqueous Neem
extract were all almost at equal distance from that recorded
with uninhibited 1.0 M HCl. The corrosion current of carbon
steel electrode in each solution was determined by locating
the intersections of extrapolated tangents of the anodic and
cathodic curves of the Tafel plot at the corrosion potential
(Erest). Values of associated electrochemical parameters and
corresponding percent inhibition are given in Table 2. The
corrosion current was found to decrease with the increase of
the concentration of the inhibitor as shown in Table 2. In the
absence of inhibitor (1.0 M HCl), the corrosion current was

Table 2: Tafel corrosion currents and percentage inhibitions of the aqueous extract of Neem leaves at various concentrations in 1.0 M HCl
at room temperature.
1.0 M HCl
1.0 M HCl +
0.20 g/LExtract
of Neem
1.0 M HCl +
0.50 g/LExtract
of Neem
1.0 M HCl +
1.00 g/LExtract
of Neem
1.0 M HCl +
2.00 g/LExtract
of Neem
Erest/V versus SCE −0.465 −0.438 −0.433 −0.472 −0.583
ba/(mV/dec.) 57.5 61.1 94.5 112.0 143.9
ba/(mV/dec.) 63.8 56.1 87.7 77.2 76.1
I“corrosion”/mA 0.76 0.237 0.169 0.151 0.101
PercentInhibition — 68.82 77.76 80.13 86.84
0
20
40
60
80
100
1E−01 1E+00 1E+01
Inhibitor concentration (g/L)
Inhibition(%)
Figure 4: Percentage inhibition of different concentrations of the
aqueous extract of Neem leaves on carbon steel surface in 1.0 M
HCl solution at room temperature based on anodic and cathodic
polarization curves.
0.76 mA; and dropped to 0.101 mA when the concentration
of aqueous Neem leaves extract reached 2.0 g/L in 1.0 M HCl
(Table 2).
The values of percentage inhibition of aqueous Neem
extract at various concentrations in 1.0 M HCl were calcu-
lated according to (1), and are shown in Table 2:
Percentage Inhibition =
(ICorr.)Uninh. − (ICorr.)Inh.
(ICorr.)Uninh.
× 100,
(1)
where (ICorr.)Uninh. = Corrosion current in the uninhibited
solution, and (ICorr.)Inh. = Corrosion current in inhibited
solution.
The plot of the percentage inhibition versus the concen-
tration of the aqueous Neem extract in 1.0 M HCl is shown
in Figure 4. This figure shows that the percentage inhibition
has steeply increased from about 69% with 0.20 g/L inhibitor
to about 87% with 2.0 g/L inhibitor.
There are many factors affecting the corrosion inhibition
efficiency by organic compounds such as the number and
types of adsorbing groups as well as their molecular size,
molecular structure, and mode of interaction with the
corroding metal surface. These molecules have the ability to
inhibit corrosion either by chemisorption or physisorption
on the metal surface. The molecules that physisorbed reduce
corrosion by shifting the cathodic reaction at the sides where
they attached, while the chemisorbed ones that attached
at the anodic areas retard the reactivity of these areas
towards corrosion. Neem is bitter in taste due to the high
content of tannins and a series of complex compounds
called “triterpenes” or more specifically “limonoids”. One
of the most important triterpenes is azadirachtin. Nearly
100 protolimonoids, limonoids or tetranortriterpenoids,
pentanortriterpenoids, exanortriterpenoids, and some pon-
terpenoid constituents have been isolated from various parts
of the Neem tree; still more are being isolated. As tannins
contain polyphenolic moieties and these moieties have the
ability to form tanninate salts with ferric ions, the corrosion
inhibition of tannins is due to the formation of a highly
cross-lined network of ferric tanninate salts that protect
the metal surface. In the same way, triterpenes have some
functional groups such as (OH, C = O, C = C) that can
interact with the metal surface and protect it from the attack
of hydronium (H3O+) ions.
When the electrode was pulled out from the 1.0 M HCl
containing an aqueous extract of 2.0 g/L of Neem leaves (at
the end of the experiment), the carbon steel surface was more
clean (about 87% inhibition) compared with the experiment
with concentration of 0.20 g/L in 1.0 M HCl (about 69%
inhibition).
The high inhibition efficiency may be attributed to the
presence of tannins, triterpenes, and many other organic
compounds that have been extracted by only soaking the dry
Neem leaves in distilled water.
3.2. Effect of Temperature
Weight-loss corrosion tests were carried out on the carbon
steel immersed in 1.0 M HCl in the absence or presence
of aqueous Neem extract over a period of 6 hours. Table 3
represents the corrosion rate [mg·cm−2·h−1] and the per-
centage efficiency for Neem inhibitor with concentrations
varying from 0.02 g/L to 2.00 g/L at 303, 313, 323, 333, and

Table 3: The Effect of concentration of the aqueous extract of Neem leaves on the corrosion rate (mg·cm−2·h−1) and percentage efficiency
of carbon steel in 1.0 M HCl at various temperatures.
Temperaure e/K
343 333 323 313 303
Concentration
of Inhibitor
% Efficieny Corr. Rate % Efficieny Corr. Rate % Efficieny Corr. Rate % Efficieny Corr. Rate Corr. Rate % Efficieny
1.0 M HCl 0.961 — 1.394 — 4.671 — 12.225 — 26.280 —
1.0 M HCl +
0.02 g/L
0.548 43 0.920 34 3.643 22 11.247 8 32.324 −23
1.0 M HCl +
0.20 g/L
0.336 65 0.600 57 2.989 36 11.003 10 31.010 −18
1.0 M HCl +
0.50 g/L
0.250 74 0.544 61 2.382 49 9.169 25 27.594 −5
1.0 M HCl +
1.00 g/L
0.221 77 0.516 63 2.055 56 7.964 35 25.229 4
1.0 M HCl +
2.00 g/L
0.192 80 0.446 68 1.822 61 7.335 40 24.178 8
303K
313K
323K
0
1
2
3
4
1E−02 1E−01 1E+00 1E+01
Concentration (g/L)
Corrosionrate(mg·cm−2·h−1)
0.5
1.5
2.5
3.5
Figure 5: Effect of concentration of the aqueous extract of Neem
leaves on the corrosion rate (mg·cm−2·h−1) of carbon steel in 1.0 M
HCl at various temperatures.
343 K, respectively. The percentage efficiency was calculated
according to (2):
%Inhibition =
WUninh. − WInh.
WUninh.
× 100, (2)
where WUninh. = corrosion rate without inhibitor; and
WInh. = corrosion rate with inhibitor.
The corrosion rate of the carbon steel in 1.0 M HCl as
a function of various concentrations of aqueous extract of
Neem leaves at temperatures between 303 and 343 K were
333K
343K
0
5
10
15
20
25
30
35
1E−02 1E−01 1E+00 1E+01
Concentration (g/L)
Corrosionrate(mg·cm−2·h−1)
leaves on the corrosion rate (mg·cm−2·h−1) of carbon steel in 1.0 M
HCl at various temperatures.
shown in Figures 5 and 6. It was obviously noticed that the
corrosion rate dropped rapidly at lower concentrations of
the extract Neem than at higher ones at 303 K and 313 K,
while the situation was the opposite at 323 K and 333 K. At
343 K, there was no inhibition at all at low concentrations
and instead, acceleration in the rate of corrosion was noticed.
This could be due to the presence of the ions in the Neem
leaves extract (extracted with water), and because at this
elevated temperature the adsorption of the inhibitor (at low
concentration) was low, the dissolved ions had high influence
on the increase of the corrosion rate in addition to the HCl

Table 4: The data obtained from the weight loss measurements for Arrhenius Equation: (I/T) against Ln (Corrosion Rate).
Ln (Corrosion Rate)/mg·cm−2·h−1
(1/T) × 103/K−1 1.0 M HCl 1.0 M HCl+ 0.02 g/L 1.0M HCl+ 0.20 g/L 1.0 M HCl+ 0.50 g/L 1.0 M HCl+ 1.00 g/L 1.0 M HCl+ 2.00 g/L
3.30 −0.03978 −0.60148 −1.09064 −1.38629 −1.50959 −1.65026
3.19 0.33218 −0.08338 −0.51083 −0.59059 −0.66165 −0.80744
3.10 1.54137 1.29281 1.09494 0.86794 0.72028 0.59993
3.00 2.50348 2.4201 2.39817 2.21583 2.07493 1.99266
2.92 3.26881 3.47581 3.43431 3.3176 3.22799 3.18544
303K
313K
323K
333K
343K
−30
−10
10
30
50
70
90
1E−02 1E−01 1E+00 1E+01
Inhibitor concentration (g/L)
Inhibition(%)
leaves on the percent inhibition of carbon steel in 1.0 M HCl at
various temperatures.
solution. When the concentration of the extract increased,
more inhibitor seemed to get adsorbed on the metal surface,
which in turn caused less effect of dissolved ions on the
corrosion rate.
The relative decrease in the corrosion rate was more sig-
nificant at lower temperatures than at higher ones as shown
in Figures 5 and 6. This means that the rate of corrosion
decreased more significantly at lower temperatures.
The plots of the percentage inhibition as a function of
the concentration of the aqueous extract of Neem leaves
at temperatures ranging from 303 K to 343 K were shown
in Figure 7. Similar to the rate of corrosion in Figures 5
and 6, the inhibition was so obvious and more effective at
low temperatures than at higher ones, and even a catalysis
effect (negative inhibition values) was noticed at 343 K and
concentrations of equal or less than 0.50 g/L.
According to Arrhenius Equation (3), the values of
natural logarithms of the rate of corrosion (ln Rate of
corrosion) and the values of the reciprocal of the temperature
0.5g/L
1g/L
2g/L0.2g/L
0.02g/L
1M HCl
−3
−2
−1
0
1
2
3
4
2.8 3 3.2 3.4
1/T ×103 (K−1)
Lncorrosionrate(mg·cm−2·h−1)
Figure 8: The Arrhenius plots of the corrosion rate of carbon steel
in 1.0 M HCl solution with and without the presence of various
concentrations of the aqueous extract of Neem leaves.
in K−1 (1/T) were tabulated in Table 4:
ln rate = −
Ea
RT
+ constant, (3)
where Ea = activation energy [kcal·mol−1], R = gas constant
[kcal·mol−1], T = absolute temperature [K], and const. =
constant
The Arrhenius plots of the corrosion of carbon steel in
1.0 M HCl solution, (ln corrosion rate as a function of 1/T)
with or without the presence of the aqueous extract of Neem
leaves at concentrations ranging from 0.02 g/L to 2.0 g/L are
plotted in Figure 8. From this figure, the slope (−Ea/R) of
each line was determined and used to calculate the activation
energy according to (3), with R = 1.987 × 10−3
kcal·mol−1.
It is clearly noticed that as the concentration of the extract
increases, the activation energy for the corrosion of carbon
steel in 1.0 M HCl is also increase (18.27 kcal·mol−1in 1.0 M
HCl without inhibitor to 25.96 kcal·mol−1with an aqueous
extract of Neem leaves of 2.0 g/L in 1.0 M HCl), (Table 5).
The values of the surface coverage of various concentra-
tions of the aqueous Neem leaves extract (from 0.02 g/L to
2.0 g/L) on carbon steel surface at various temperatures are

Table 5: The activation energy (Ea) for the corrosion of carbon steel
in 1.0 M HCl with and without the aqueous extract of Neem leaves
at various concentrations.
System
Activation Energy, Ea (kcal·mol−1)
2.00 g/L 1.00 g/L 0.50 g/L 0.20 g/L 0.02 g/L
1.0 M HCl 18.27 18.27 18.27 18.27 18.27
1.0 M HCl +
The Aqueous
Extract of
Neem Leaves
25.96 25.42 25.43 24.88 22.15
303K
313K
323K
333K
343K
0
0.2
0.4
0.6
0.8
1
−5 −4 −3 −2 −1 0 1 2
Ln concentration (g/L)
Surfacecoverage
−0.4
−0.2
leaves on the surface coverage of carbon steel in 1.0 M HCl at
various temperatures.
tabulated in Table 6. These values were extracted from the
corresponding percentage eﬃciency values reported earlier
in Table 3. The plot of the values of surface coverage, θ,
against the natural logarithm of the concentration of Neem
leaves aqueous extract; ln C, for carbon steel at various
inhibitor temperatures is shown in Figure 9. After examin-
ing these data and adjusting them to diﬀerent theoretical
adsorption isotherms, it was concluded that the inhibitor was
adsorbed on the carbon steel surface according to Temkin
Adsorption Isotherm:
−2aθ = ln KC, (4)
where a = molecular interaction constant, θ = degree of
coverage, K = equilibrium constant for the adsorption
reaction, and C = concentration of the inhibitor.
The equilibrium constant for the adsorption reaction, K,
is related to the standard free energy of adsorption via (5)
[16]:
K =
1
55.5
exp −
ΔG
RT
, (5)
Where: K = equilibrium constant for the adsorption reac-
tion, 55.5 = concentration of water [mol·L−1], ΔG = standard
free energy [kcal·mol−1], R = gas constant [kcal·mol−1], and
T = absolute temperature [K].
According to (4), the straight lines shown in Figure 9 will
have the following slopes and intercepts:
Slope = −
1
2a
, (6)
Intercept = −
1
2a
lnK. (7)
Combination of (6) and (7) leads to the following
relationships:
Intercept = Slope (ln K), (8)
K = e (Intercept/Slope)
. (9)
Using (9), the equilibrium constant for the adsorption
reaction, K, was calculated.
The free energy of adsorption of the inhibitor, ΔG, was
calculated using (5) at various temperatures (303 K to 343 K)
as shown in Table 7.
The enthalpy of adsorption, ΔH, for the inhibitor was
calculated from the following and is shown in Table 8:
ΔH = Ea − RT. (10)
The entropy, ΔS, was calculated at various temperatures for
the inhibitor using the following and is shown in Table 9:
ΔG = ΔH − TΔS. (11)
The activation energy (Ea) for the corrosion of carbon
steel in the presence of the aqueous extract of neem
leaves at all concentrations (0.02 g/L to 2.0 g/L) are higher
compared to the activation energy in the absence of the
extract (25.96 kcal·mol−1 with the extract of 2.0 g/L in 1.0 M
HCl compared with 18.27 kcal·mol−1 in 1.0 M HCl without
extract). This can be attributed to the fact that higher values
of Ea in the presence of inhibitor compared to its absence are
generally consistent with a physisorption, while unchanged
or lower values of Ea in inhibited solution suggest charge
sharing or transfer from the organic inhibitor to the metal
surface to form coordinate covalent bonds (chemisorption).
The increase in the activation energies for the corrosion
is attributed to a decrease in the adsorption of the inhibitor
on the metal surface as the temperature increased, and
subsequently, an increase in the corrosion rate will result due
to the greater exposed area of the metal surface to the acid. At
temperatures as high as 343 K and concentrations of extract
equal or lower than 0.50 g/L an acceleration of corrosion of
carbon steel was occurred.
The thermodynamic data obtained in the presence of
the Neem extract at various temperatures were tabulated
in Tables 7 to 9. These thermodynamic quantities represent
the algebraic sum of the values for adsorption and desorp-
tion. The negative value of ΔG indicates the spontaneous
adsorption of inhibitor on the surface of the carbon steel.

Table 6: The Effect of concentration of the aqueous extract of Neem leaves on surface coverage for carbon steel in 1.0 M HCl at various
temperatures.
Temperature/K
303 313 323 333 343
Concentration
of Inhibitor
Surface Coverage θ Surface Coverage θ Surface Coverage θ Surface Coverage θ Surface Coverage θ
1.0 M HCl +
0.02 g/L
0.43 0.34 0.22 0.08 −0.23
1.0 M HCl +
0.20 g/L
0.65 0.57 0.36 0.10 −0.18
1.0 M HCl +
0.50 g/L
0.74 0.61 0.49 0.25 −0.05
1.0 M HCl +
1.00 g/L
0.77 0.63 0.56 0.35 0.04
1.0 M HCl +
2.00 g/L
0.80 0.68 0.61 0.40 0.08
Table 7: The Free energy of adsorption (ΔGads) for carbon steel in
1.0 M HCl in the presence of the aqueous extract of Neem leaves at
various temperatures (303 K−343 K).
ΔG, kcal·mol−1
303 K 313 K 323 K 333 K 343 K
−8.01 −8.04 −6.58 −5.52 −2.84
Table 8: The enthalpy of adsorption (ΔH) for carbon steel in 1.0 M
HCl in the presence of the aqueous extract of (2.0 g/L) Neem leaves
at various temperatures (303 K−343 K).
ΔH, kcal·mol−1
303 K 313 K 323 K 333 K 343 K
25.36 25.34 25.32 25.30 25.28
Table 9: The change in entropy (ΔS) for carbon steel in 1.0 M HCl
in the presence of the aqueous extract of (2.0 g/L) Neem leaves at
various temperatures (303 K−343 K).
ΔS, kcal·K−1·mol−1
303 K 313 K 323 K 333 K 343 K
0.110 0.107 0.0988 0.0926 0.0820
The free energy, ΔG, increases from about −8 kcal·mol−1
at 303 K to about −3 kcal·mol−1 at 343 K. The adsorption
process is believed to be exothermic and associated with a
decrease in entropy (ΔS) of solute, while the opposite is true
for the solvent [17]. The gain in entropy which accompanies
the substitutional adsorption process is attributed to the
increase in the solvent entropy (Table 9). This agrees with
the general suggestion that the absolute values of free energy,
|ΔG|, increase with the increase of inhibition efficiency [18–
20] as the adsorption of organic compound is accompanied
by desorption of water molecules off the surface.
The high inhibition efficiency may be attributed to the
presence of tannins, triterpenes, and many other organic
compounds that have been extracted by only soaking the
dry Neem leaves in distilled water. This inhibition may be
also due to synergistic interactions between the adsorbed
compounds.
These results agree with Fouda et al. [21, 22] who sug-
gested that the inhibition efficiency of organic compounds
depends on many factors including their charge density,
number of adsorption sites, heat of hydrogenation, mode of
interaction with the metal surface, and formation of metallic
complexes.
4. Conclusions
The aqueous Neem leaves extract was found to be a highly
efficient inhibitor for carbon steel in 1.0 M HCl solution,
reaching about 87% at 2.0 g/L and room temperature, a
concentration considered to be very moderate. Even at one
half of this of this concentration, 1.0 g/L, an inhibition of
about 80% was obtained at room temperature.
The rate of corrosion of the carbon steel in 1.0 M HCl
is a function of the concentration of the neem extract. This
rate decreased as the concentration of the Neem extract is
increased.
The percentage of inhibition in the presence of this
inhibitor was decreased with temperature which indicates
that physical adsorption was the predominant inhibition
mechanism because the quantity of adsorbed inhibitor
decreases with increasing temperature.
The aqueous Neem leaves extract is an excellent, green,
eco-friendly, and very cheap corrosion inhibitor for carbon
steel in 1.0 M HCl solution, so it can be used to replace toxic
and highly cost chemicals.
Acknowledgments
The authors would like to thank the “College of Graduate
Studies and Research” at the University of Sharjah for
financially supporting this research project as well as their
research group entitled: “Corrosion Prevention & Control.”

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