Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 667-670
Published Online July 2012 (http://www.SciRP.org/journal/jmmce)
Effect of Aqueous Extracts of Bitter Leaf Powder on
the Corrosion Inhibition of Al-Si Alloy in 0.5 M
Caustic Soda Solution
F. A. Ayeni1*, I. A. Madugu1, P. Sukop1, A. P. Ihom1, O. O. Alabi1, R. Okara1,
1National Metallurgical Development Centre, Jos, Nigeria
2Department of Metallurgical and Materials Engineering, Ahmadu Bello University, Zaria, Nigeria
Received January 23, 2012; revised February 29, 2012; accepted March 27, 2012
The effect of bitter leaf (Vernonia amygdalina) extract as an inhibitor for aluminium silicon alloy in 0.5 M solution of
caustic soda using weight loss method has been investigated. The alloy of composition 9% Si and 91% Al was sand cast
at the Foundry Shop of the National Metallurgical Development Centre, Jos, Nigeria. The cast alloy was cut and ma-
chined to corrosion coupons and immersed into 0.5 M NaOH solution containing varying inhibitor concentrations (0.1%,
0.2%, 0.3%, 0.5% v/v) within a period of fifteen days. From the result, it was found that the adsorption of Vernonia
amygdalina reduced the corrosion rate of this group of alloy in the alkaline medium. The inhibitive action of this p lant
extract was explained using inhibition efficiency and degree of surface coverage. The most suitable inhibitor concentra-
tion was found to be 0.5% with inhibition efficien cy o f 87%. The mechanism of in hib ition is by ph ysical adsorp tio n and
the adsorbed molecules of the inhibitor lies on the surface of the alloy blocking the active corrosion sites on the alloy,
hence, giving the alloy a higher corrosion resistance in the studied environment.
Keywords: Vernoni a amygdalina; Adsorption; Inhibition Efficiency; Surface Coverage
Corrosion of metals/alloys, which can be defined as the
deterioration or disintegration of materials due to their
reaction with the environment, has continued to receive
attention in the tech nological world. Corrosion Scientists
are relentless in seeking better and more efficient ways
of combating the corrosion of metals/alloys. Aluminium
and its alloys are widely used in engineering applications;
these include, structural, domestic, transportation and elec-
trical transmission lines. Due to the availability, moder-
ate cost and relatively low resistance to corrosion, con-
ductivity and oth er properties of this metal and its alloys,
improving the corrosion resistance of these alloys by in-
hibiting the work ing environment is worth studying .
One of the ways of combating corrosion is the additio n
of inhibitors to the corroding environment. Inhibitors
tend to ameliorate the destructive behavior of an aggres-
sive environment. There are different types of inhibitors,
which are organic and inorganic. Corrosion inhibitors are
widely used in industry to prevent or reduce corrosion
rate of metals in alkaline, acidic media and industrial
processes such as acid pickling and cleaning of refinery
equipment, oil well acidizing and acid descaling . The
action of inhibitors is always associated with changes in
the state of the surface being protected due to adsorption
or formation of poorly soluble compounds with metal ca-
tions. Such compounds decrease the area of active metal
surface and/or increase the corrosion energy . The ad -
sorption of the inhibitors unto the metal/alloy surfaces
retards the cathodic or anodic electrochemical processes
that accompany corrosion of the metal/alloy [2,3].
Studies have shown that the efficiency of inhibition is
related to the amount of adsorbed inhibitor on the metal
surface . The inhibitor after adsorption may form a
surface film that acts as a physical barrier restricting the
diffusion of ions/molecules to or from the metal/alloy
surface and may prevent the metal atoms from partici-
pating in either the an odic or cathodic reactions of corro-
The use of plant extracts as organic inhibitors for the
corrosion of metals/alloys, has gained very wide interest
among researchers in recent time [7-9]. In the search for
more environmentally friendly and readily available in-
hibitors, researchers have reported the use of local plants
Copyright © 2012 SciRes. JMMCE
F. A. AYENI ET AL.
such as Vernonia amygdalina  and neem leaf . In
very many cases, the corrosion inhibitive effect of some
plants extracts has been attributed to the presence of tan-
nin in their chemical constituents . Avwiri and Igho,
(2003) reported the inhibition efficiency of Vernonia
amygdalina (bitterleaf) on the corrosion of aluminium
alloy in 0.1 M HCl and 0.1 M HNO3 to be 49.5% and
The use of naturally occurring plant extracts as inhibi-
tors is particularly interesting and economical because
they are cheap, non-toxic, ecological friendly and poses
little or no threat to the environment [2,13].
Vernonia amygdalina commonly called bitter leaf is
non-toxic plant available in every part of Nigeria; it is
mainly used locally as vegetable in soup because of its
medicinal efficacy. Hence, this research is aimed at the
possibility of using this non-toxic plant as a corrosion
inhibitor of Al-Si alloy in 0.5 Molar caustic soda solu tion,
since it has been reported that aluminium based alloys
have low corrosion resistance in alkaline media [1,14].
2. Materials and Methods
The aluminium alloy used in this research was sand cast
at the foundry shop of the National Metallurgical Devel-
opment Centre, Jos, Nigeria. Its composition was 9% Si
and 91% Al. Other materials used include: extract from
Vernonia amygdalina and ethanol.
The equipment used for the work were, Setra electronic
digital weighing balance model BL-410S, steel brush,
beakers, measuring cylinder, thread, retort stand and easy
way digital PH meter model PHS-25.
2.2.1. Determination of the Chemical Composition of
The chemical composition of the plant extract was de-
termined at the Chemistry Department Laboratory of
Ahmadu Bello University, Zaria, Nigeria.
2.2.2. Corrosi on Tes ti ng
After casting, the aluminium alloy was cut an d machined
to corrosion coupons (cylindrical shape) of dimension
1.5 × 1 cm. fifteen coupons were produced for different
concentration of inhibitor (0.1%, 0.2%, 0.3% and 0.5%)
in the 0.5 M NaOH solution and reference medium
which was 0.5 M NaOH solution without inhibitor. The
coupons were polished and degreased in absolute ethanol,
dried, weighed and stored in desiccators. 0.5 M NaOH
solutions containing the inhibitor of concentrations 0.1%,
0.2%, 0.3% and 0.5% v/v were prepared and the coupons
immersed in these solutions. The weight loss of each
coupon was determined at 5 days interval for 15 days.
Then the rate of corrosion, inhibition efficiency and
degree of surface coverage were determined. The ex-
periment was conducted at room temperature in the range
of 26˚C - 30˚C.
2.2.3. Determination of Corrosion Rate (mpy) and
Inhibition Efficiency (%)
The weight loss was determined by finding the difference
between initial and final weight of coupon after 5 days of
immersion from the relationships [1 ].
where W—weight loss after 5 days,
The standard expression for measurement of corrosion
rate in mils per year (mpy) was used which is given as
where mpy—mils per year, W—weight loss in mg, D—
density of the materials in g/cc, T—time of exposure in
hours, A—area in in2.
The inhibition efficiency was determined using th e re-
where W and Wo are the corrosion r ates with and without
The degree of surface coverage, O at each concentra-
tion of inhibitor was evaluated using the equation .
where Wb and Wi are the weight loss in corrodent without
and with inhibitor respectively.
3. Results and Discussions
The physiochemical screening of the bitter leaf extract
revealed that the plant extract contained 0.9% tannin and
Figure 1 shows the variation of the corrosion rates with
time of exposure for the reference and four different in-
hibitor concentrations in the 0.5 M NaOH solution. Fig-
ure 2 is the inhibition efficiency variation with time of
exposure f or different i nhibitor c oncentrations i n the caus-
Copyright © 2012 SciRes. JMMCE
F. A. AYENI ET AL. 669
tic soda solution, while Figure 3 shows the degree of
surface coverage with inhibitor concentration for the dif-
ferent time of exposure.
3.2. Discussion of Results
3.2.1. Visual Observation of the Coup ons
Visual observation of the coupons in the solution with and
without inhibitor after fifteen days (360 hours) of expo-
sure revealed changes in color of the cou pons fro m initial
bright grayish surfaces to dull ones. Cracks and pits were
observed on the coupons which are indication of severe
corrosion attack by the alkaline media. Albeit, the change
in color and presence of cracks were more noticed on the
coupons in the solutions without inhibitor and with in-
hibitor of 0.2% and 0.3% c oncentrati o n .
3.2.2. Corrosion Rate and Inhibition E fficiency
From the results obtained on the corrosion rate against
exposure time at different inhib itor concentrations plotted
in Figure 1; it is clear that corrosion rate increased with
time for coupons in solution without inh ibitor for the first
ten days due to initial corrosion attack and subsequently
decreased after fifteen days probably due to the deposition
Figure 1. Variation of corrosion rate with time of exposure
at different inhibitor concentrations.
Figure 2. Variation of inhibition efficiency with time of ex-
posure at different inhibitor concentrations.
Figure 3. Variation of degree of surface coverage with dif-
ferent inhibitor concentrations at various times of exposure.
of corrosion products as the corrosion progresses which
tends to shield the corroding surface from further corro-
sion attack, thereby depressing the rate of corrosion .
Corrosion also increased with time for coupons in 0.2%
and 0.3% inhibitor concentration, this may mean that at
these inhibitor concentrations, the protective bond of the
inhibitor was broken down and the corrosion rate in-
creased . For 0.1% inhibitor concentration, the corro-
sion rate was higher than others after five days but de-
creased continuously for the rest of period of the experi-
ment, this may be due to initial exposure of the coupons
surface to attack and subse quent protection of same by the
protective bond of the inhibitor for the remaining period
of the experiment , while corrosion rate in the 0.5%
inhibitor concen tration decreased with time of exposure.
The significant decrease in corrosion rate at 0.5% in-
hibitor conce ntration can be attribute d to the adsorpt ion of
molecule of the inhibitor on the alloy surface, since this
inhibitor contains tannin which acts as physical barrier to
restrict t he diffu sion of i ons to and from the all oy and then
prevent the alloy atoms (ions) from participating in further
anodic or cathodic reactions, hence resulting in decrease
in the corrosion rate . The tannin in the plant extract
can adsorb on the alloy surface and block the active sites
on the surface, thereby reducing the corrosion rate in the
From the plot of inhibition efficiency ag ainst exposure
time (Figure 2), it can be seen that 0.5% inhibitor con-
centration has the highest protection efficiency and this
increase wit h tim e of e xposure , from 0% a fter fi ve day s to
84% after ten days and 87% after fifteen days. This shows
that the inhibitor acts best after fifteen days at 0.5% in-
The plot of the degree of surface coverage against in-
hibitor concentration (see Figure 3) revealed that 0.5%
inhibitor concentration has the highest protection effi-
ciency sinc e the highe st de gre e o f s urfa ce c ove rag e b y the
inhibitor occurred at 0.5% inhibitor efficiency for fifteen
Copyright © 2012 SciRes. JMMCE
F. A. AYENI ET AL.
Copyright © 2012 SciRes. JMMCE
1) Bitter leaf extract (Vernonia amygdalina) decreases
the corrosion of aluminium silicon allo y in 0.5 M NaOH
solution subj ect to a level of 0.5% inhib itor con centration
within fifteen days.
2) The mechanism of interactio n between the inhibitor
and the alloy is by physical adsorption. The adsorbed
inhibitor molecules are attached at the alloy surface there-
by blocking active corrosion sites hence lowering corro-
3) The inhibitor is recommended to be used at 0.5%
concentration in 0.5 M NaOH and for a short period of
The authors acknowledge with thanks the management of
the National Metallurgical Development Centre for the
equipment support and Mr. Polycarp Evarastic (Graduate
Student, Ahmadu Bello University, Zaria, Nigeria) for the
provision of the powdered plant extract and its chemical
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