Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 885-890
Published Online September 2012 (http://www.SciRP.org/journal/jmmce)
Adsorpt ion and Thermo dynamics Study of the Inhibition
of Corrosion of Mild Steel in H2SO4 Medium Using
Vernonia amygdalina
Joseph Tagbo Nwabanne*, Vincent Nwoye Okafor
Department of Chemical Engineering, Nnamdi Azikiwe University, Awka, Nigeria
Received April 19, 2012; revised May 31, 2012; accepted June 19, 2012
ABSTRACT
In this work, adsorption and thermodynamics study of the inhibition of corrosion of mild steel in H2SO4 medium using
Vernonia amygdalina was carried out. The inhibitive and adsorptive properties of ethanol extract of Vernonia amygda-
lina for the corrosion of mild steel in 0.2 M H2SO4 solutions was investigated using weight loss technique. The result
has proved that that the extract is a good inhibitor of corrosion of mild steel in H2SO4. The inhibition efficiencies ranged
from 23.37% to 38.59% and from 22.45% to 35.78% at 303 and 323 K respectively. The inhibition efficiency of the
extract decreased as temperature and time of immersion increased but increased with increase in concentration of ex-
tract. The adsorption of the inhibitor on surface of mild steel was found to be exothermic, spontaneous and consistent
with the mechanism of physical adsorption as the value for heat of adsorption ranged from –2.12 to –4.87 KJ·mol–1. The
adsorption data fitted well to Langmuir, Temkin, Frumkin and Flory-Huggins adsorption isotherms.
Keywords: Adsorption; Corrosion; Mild Steel; Inhibition; Vernonia amygdalina
1. Introduction
Mild steel is the most commonly used engineering mate-
rial [1]. Oftentimes, mild steel is exposed to the attack of
acid solutions during industrial processes such as acid
cleaning, pickling and descaling. This results to easy
corrosion of the mild steel, thereby necessitating the use
of inhibitor [2-5]. There are numerous methods for con-
trolling the corrosion of metals but the use of inhibitors is
still one of the best methods of protecting metals against
corrosion [6-8]. Many organic compounds are used as
corrosion inhibitors for mild steel in acidic environments.
Such compounds usually contain nitrogen, oxygen or
sulphur in a conjugated system and function via adsorp-
tion of the molecules on the metal surface, creating a
barrier to the corrodant attack [3,5,8,9]. However, becau-
se of the Sequel to toxic nature, high cost and increasing
awareness and strict environmental regulations of some
of these compounds, the use of natural product of plant
origin as corrosion inhibitor is receiving attention [10,
11]. Corrosion inhibitors derived from plant extract are
biodegradable and do not contain heavy metals or other
toxic compounds [12].
Several researches have been done on the use of ex-
tract of plant as inhibitor for metals against corrosion in
different aggressive media [13,14]. However, not much
has been reported on the use of the extract of Vernonia
amygdalina plant as inhibitor against corrosion of mild
steel in H2SO4. Vernonia amygdalina is a member of the
Asteraceae family which is a small shrub that grows in
the tropical Africa. Vernonia amygdalina is also called
bitter leaf because of its bitter taste. The leaves are used
either as a vegetable (macerated leaves in soups) or aque-
ous extracts for treatment of various illnesses [15]. There-
fore, the present work is aimed at investigating the po-
tential of ethanol extract of Vernonia amygdalina leaves
as corrosion inhibitor of mild steel in sulphuric acid.
2. Materials and Methods
2.1. Materials Preparation
Mild steel of composition (wt%) Mn (0.6), P (0.36), C
(0.15) and Si (0.03) and Fe (98.86) were used for the
study. The sheet was cut to form different coupons of
dimension, 4 × 3 × 0.1 cm. Each coupon was degreased
by washing with ethanol, rinsed with acetone and al-
lowed to dry in the air before preservation in a dessicator.
All reagents used for the study were of analytical grade
and double distilled water was used in this study.
2.2. Preparation of Plant Extract
*Corresponding author. Stock solutions of the plant extract were prepared by

J. T. NWABANNE, V. N. OKAFOR
886
soaking known amount of the dried and ground leaves of
Vernonia amygdalina in solution of ethanol. The sample
was filtered after 48 hours and the filtrate was heated so
as to remove ethanol from the sample. From this stock
solution, test concentrations of 0.1, 0.2, 0.3, 0.4 and 0.5
g/l were prepared by diluting with 0.2 M H2SO4.
2.3. Chemical Analysis
Phytochemical analysis of the ethanol and aqueous ex-
tracts of Vernonia amygdalina was carried out according
to the method reported by Onyeka and Nwabekwe [16].
2.4. Gravimetric Experiment
Mild steel coupon was weighed and immersed in 100 ml
of the test solution in an open beaker. The beaker was
placed into a water bath maintained at 303 K. Each sam-
ple of mild steel was withdrawn from the test solution
after every 24 hours, washed with washing liquor, rinsed
in acetone and dried in air before reweighing. The dif-
ference in weight after 120 hours was taken as the total
weight loss. The experiment was repeated at 323 K.
From the weight loss results, the inhibition efficiency (%
I) of the inhibitor, degree of surface coverage (θ) and
corrosion rates (CR) were determined using Equations
(1)-(3) respectively.
1
2
% 1100
W
IW

 


(1)
1
2
1W
W
  (2)

21
CRg cmhW
A
t


(3)
where W1 and W2 are the weight losses (g/dm3) for mild
steel in the presence and absence of inhibitor, A is the
area of the mild steel coupon (cm2), t is the time of im-
mersion (hours) and W is the weight loss of mild steel
after time t.
3. Results and Discussion
3.1. Phytochemical Analysis
The phytochemical constituents of aqueous and ethanol
extract of Vernonia amygdalina are shown in Table 1.
The result reveals that saponnin, tannin, alkaloids, car-
diac glycoside, flavanoid, anthraquinone are present in
the ethanol extract of Vernonia amygdaliana but absent
in the aqueous extract of Vernonia amygdalina. This in-
dicates that the inhibition efficiency of the extract is due
to the presence of the phytochemical constituents [17].
3.2. Effects of Ethanol Extract of Vernonia
amygdalina on the Corrosion of Mild Steel
The variation of weight loss of mild steel with time for
the corrosion of mild steel in 0.2 M H2SO4 in the absence
and presence of various concentration of ethanol extract
of Vernonia amygdalina at 303 K is shown in Figure 1.
The weight loss of mild steel in H2SO4 increased with
increase in time of immersion, but decreased with in-
crease in the concentration of ethanol extract of Vernonia
amygdalina. This shows that ethanol extract of Vernonia
amygdalina inhibited the corrosion of mild steel in
H2SO4. Odiongenyi and his co-workers [17], in their
study on corrosion inhibition and adsorption properties of
ethanol extract of Vernonia amygdalina for the corrosion
of mild steel in H2SO4, obtained similar result.
The corrosion rate of mild steel in the absence and
presence of Vernonia amygdalina extract and inhibition
efficiencies of various concentrations of Vernonia amyg-
dalina extract is presented in Table 2. The result shows
that the rate of corrosion of mild steel decreased as the
concentration of ethanol extract of Vernonia amygdalina
increased but increased with increase in temperature,
while the inhibition efficiency of the extract of Vernonia
amygdalina decreased with increase in temperature, in-
dicating that the adsorption of extract of Vernonia amyg-
dalina on mild steel surface is physical adsorption as
reported by other researchers [3,18-21].
3.3. Effect of Temperature
The effect of temperature on the rate of corrosion of mild
steel in H2SO4 containing various concentrations of etha-
nol extract of Vernonia amygdalina was investigated us
ing Arrhenius equation as given in Equation (4) [4,22-
24].
Table 1. Phytochemical constituents of aqueous and ethanol
extracts of Vernonia amygdalina.
Phytochemicals Aqueous extract Ethanol extract
Tannins - +++
Saponins - ++
Alkaloids - ++
Cardiac glycosides - ++
Flavanoid - +
Anthraquinones - +++
Table 2. Values of corrosion rate and inhibition efficiency at
various temperatures in the absence and presence of etha-
nol extract of vernonia amygdalina.
Corrosion rate
(g·h–1·cm–2) ×10–7 Inhibition efficiency (%)
Concentration
(g/l) 303 K 323 K 303 K 323 K
Blank
0.1
0.2
0.3
0.4
0.5
6.0
5.0
4.6
4.4
4.1
3.9
6.8
5.3
4.9
4.7
4.4
4.2
23.37
27.72
31.52
35.33
38.59
22.45
26.53
30.63
33.71
35.78

J. T. NWABANNE, V. N. OKAFOR 887


exp Ea CR ART  (4)
where CR is the corrosion rate of mild steel, A is Ar-
rhenius or pre-exponential factor, Ea is the activation
energy, R is the gas constant and T is the temperature. the
corrosion rates of mild steel at 303 K (T1) and 323 K (T2)
denoted as CR1 and CR2. Equation (4) becomes

2
1
21
log1 1
2.303
CR Ea TT
CR R





(5)
Ea values, calculated from Equation (5), are presented
in Table 3. These values ranged from 2.37 to 3.70 KJ·mol–1
and are lower than the threshold value of 80 KJ·mol–1
required for chemical adsorption. This shows that the
adsorption of ethanol extract of Vernonia amygdalina on
mild steel surface is physical adsorption [2,4,21].
3.4. Thermodynamic/Adsorption Parameters
The heat of adsorption Qads of ethanol extract of Verno-
nia amygdalina on the surface of mild steel was calcu-
lated using Equation (6) [4,22,23,25,26].

21
ads
21
12 1
21
2.303 loglog
11
KJ mol
()
QR
TT
TT






 










(6)
Figure 1. Variation of weight loss with different concentra-
tions of Vernonia amygdalina extract for the corrosion of
mild steel in 0.2 M H2SO4 at 303 K.
Table 3. Thermodynamic parameters for adsorption of eth-
anol extract of Vernonia amygdalina on surface of mild steel.
Concentration (g/l) Activation energy
(KJ·mol–1)
Heat of adsorption
(KJ·mol–1)
Blank 5.20
0.1 2.42 –2.12
0.2 2.63 –2.44
0.3 2.74 –2.69
0.4 2.94 –2.90
0.5 3.70 –4.87
where R is the gas constant, θ1 and θ2 are the degree of
surface coverage at temperatures, T1 and T2 respectively.
Calculated values of activation energy and heat of ad-
sorption are shown in Table 3. The values ranged from
–2.12 to –4.87 KJ·mol–1, indicating that the adsorption of
ethanol extract of Vernonia amygdalina on mild steel
surface is exothermic [3,18,19,26].
Data obtained for the degree of surface coverage were
used for the evaluation of different adsorption isotherms
(Langmuir, Frumkin, Temkin and Flory-Huggins).
3.4.1. Langmuir Isotherm
Langmuir adsorption isotherm is expressed according to
Equation (7) [27,28].
1CC
K 

 (7)
where C is the concentration of the inhibitor, K is the
adsorption equilibrium constant and θ is degree of sur-
face coverage of the inhibitor. Taking logarithm of both
sides of Equation (7) yields Equation (8).
loglog log
CC




 K
(8)
Plotting log C




against gave a linear relation- log C
ship as shown in Figure 2. The parameters of Langmuir
isotherm are presented in Table 4. The R2 values of
0.998 and 0.999 indicate strong adherence to Langmuir
adsorption isotherm [29]. The application of Langmuir
isotherm to the adsorption of extract of Vernonia amyg-
dalina on surface of mild steel indicated that there is no
interaction between the adsorbate and adsorbent [30].
3.4.2. Temkin Isotherm
For Temkin adsorption isotherm, the degree of surface
Figure 2. Langmuir isotherm for adsorption of ethanol ex-
tract of Vernonia amygdalina on mild steel surface.

J. T. NWABANNE, V. N. OKAFOR
888
Table 4. Adsorption parameters for adsorption of ethanol
extract of Vernonia amygdalina on mild steel surface.
Isotherm Temperature R2 log K ΔGads KJ/mol
Langmuir 303 K 0.998 –0.32 –8.25
323 K 0.999 –0.35 –8.64
R
2 log K ΔGads KJ/molα
Frumkin 303 K 0.984 –1.29 –2.67 3.23
323 K 0.995 –1.33 –2.55 3.52
R
2 log K ΔGads KJ/mola
Temkin 303 K 0.973 2.004 –21.75 –5.21
323 K 0.993 2.069 –23.58 –5.7
R
2 log K ΔGads KJ/molx
Flory-Huggins 303 K 0.984 –1.29 –2.67 4.67
323 K 0.995 –1.33 –2.55 5.51
coverage (θ) is related to inhibitor concentration (C) ac-
cording to Equation (9) [27,31,32].

exp 2aK  C (9)
where K is the adsorption equilibrium constant and a, is
the attractive parameter. Rearranging and taking loga-
rithm of both sides of Equation (9) gives Equation (10).
2.303log 2.303log
22
K
C
aa


 (10)
Plots of θ against logC, as presented in Figure 3, gave
linear relationship, which shows that adsorption data
fitted Temkin adsorption isotherm. Adsorption parame-
ters obtained from Temkin adsorption isotherms are re-
corded in Table 4. The values of attractive parameter (a)
are negative in all cases, indicating that repulsion exists
in the adsorption layer.
3.4.3. Flory-Huggins Isotherm
Flory-Huggins adsorption isotherm can be expressed
according to Equation (11) [33].
logloglog(1 )Kx
C



 

 (11)
where x is the size parameter and is a measure of the
number of adsorbed water molecules substituted by a
given inhibitor molecule. The plots of log C




against
log(1)

 are shown in Figure 4 which the data con-
formed to Flory-Huggins isotherm. The values of the size
parameter x are positive as shown in Table 4. This indi-
cates that the adsorbed species of ethanol extract of
Vernonia amygdalina is bulky since it could displace
more than one water molecule from the mild steel sur-
face.
3.4.4. Fr um kin Isotherm
Frumkin adsorption isotherm is given by Equation (12)
[4,27].

log2.303log 2
1
CK






 




 (12)
where K is the adsorption-desorption constant and α is
the lateral interaction term describing the interaction in
adsorbed layer. Plots of




log 1C



  versus θ as
presented in Figure 5 were linear which shows the ap-
plicability of Frumkin isotherm. The values for Frumkin
adsorption parameters were recorded in Table 4. This
shows that values of the adsorption parameter α are posi-
tive suggesting the attractive behavior of the inhibitor on
the surface of mild steel [11,34].
Figure 3. Temkin isotherm for adsorption of ethanol extract
of Vernonia amygdalina on the surface of mild steel.
log(θ/C)
Figure 4. Flory-Huggins isotherm for adsorption of ethanol
extract of Vernonia amgydalina on the mild steel surface.

J. T. NWABANNE, V. N. OKAFOR 889
log[(C)
×
(θ/1
–
θ)]
Figure 5. Frumkin isotherm for adsorption of ethanol ex-
tract of Vernonia amgydalina on the mild steel surface.
3.5. Free Energy of Adsorption
The equilibrium constant of adsorption of ethanol extract
of Vernonia amygdalina on the surface of mild steel is
related to the free energy of adsorption (ΔGads) according
to Equation (13) [26,35-37].

ads 2.303log 55.5KGRT

(13)
where R is the gas constant and T is the temperature. The
free energy of adsorption was calculated from values of
K obtained from Langmuir, Temkin, Flory-Huggins and
Frumkin according to Equation (13) and is recorded in
Table 4. The results show that free energy of adsorption
ΔGads are negative and less than the threshold value of
–40 KJ·mol–1 required for chemical adsorption, indicat-
ing that adsorption of ethanol extract of Vernonia amyg-
dalina on mild steel surface is spontaneous and occurred
according to the mechanism of physical adsorption [26,
38,39].
5. Acknowledgements
We are grateful an anonymous referee for helpful com-
ments. We also wish to thank Kim Humphreys for Eng-
lish editing. All errors are ours.
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