Ethanol extract of musa species peels as a green corrosion inhibitor for mild steel: Kinetics, adsorption and thermodynamic considerations

Abstract

Inhibition of the corrosion of mild steel by ethanol extract of Musa species peel has been studied using hydrogen evolution and thermometric methods of monitoring corrosion. The result of the study revealed that different concentration of ethanol extract of Musa species peel inhibit mild steel corrosion. Inhibition efficiency of the extract was found to vary with concentration, temperature, period of immersion, pH and electrode potentials. Values of activation energy of the inhibited corrosion reaction of mild steel were greater than the value obtained for the blank. Thermodynamic consideration revealed that adsorption of Musa species extract on mild steel surface was spontaneous and occurred according to Langmuir and Frumkin adsorption isotherms. Physical adsorption mechanism has also been proposed for the adsorption of the inhibitor.

Full text

Advances in Natural and Applied Sciences, 2(1): 35-42, 2008
ISSN 1995-0748
© 2008, American Eurasian Network for Scientific Information
This is a refereed journal and all articles are professionally screened and reviewed
ORIGINAL ARTICLE
35
Corresponding Author: N.O. Eddy,Department of Chemistry, Ahmadu Bello University, Zaria, Nigeria.
E-mail: nabukeddy@yahoo.com
Ethanol Extract of Musa acuminate peel as an eco-friendly inhibitor for the corrosion
24
of mild steel in H SO
N.O. Eddy, S.A, Odoemelam and A.O. Odiongenyi
12 2
Department of Chemistry, Ahmadu Bello University, Zaria, Nigeria.
1
Department of Chemistry, Michael Okpara University of Agriculture, Umudike, P.M.B. 7267, Umahia, Abia
2
State, Nigeria.
N.O. Eddy, S.A, Odoemelam and A.O. Odiongenyi,: Ethanol Extract of Musa acuminate Peels as a
Green Corrosion Inhibitor for M ild Steel: Kinetics, Adsorp tion and Thermodynamic Considerations,:
Adv. in Nat. Appl. Sci., 2(1): 35-42, 2008
ABSTRACT
Inhibition of the corrosion of mild steel by ethanol extract of Musa acuminate peel has been studied using
hydrogen evolution and thermometric methods of monitoring corrosion. The result of the study revealed that
different concentration of ethanol extract of Musa acuminate peel inhibit mild steel corrosion. Inhibition
efficiency of the extract was found to vary with concentration, temperature, period of immersion, pH and
electrode potentials. Values of activation energy of the inhibited corrosion reaction of mild steel were greater
than the value obtained for the blank. Thermodynamic consideration revealed that adsorption of Musa
acum inate extract on mild steel surface was spontaneous and occurred according to Langmuir and Frumkin
adsorption isotherms. Physical adsorption mechanism has also been proposed for the adsorption of the inhibitor.
Key word: Corrosion of mild steel, Inhibition, M usa acuminate peel
Introduction
The use of an inhibitor is one of the best options for protecting metals against corrosion. Several
1-2
inhibitors in use are either synthesized from cheap raw materials or chosen from compounds having hetero
atoms in their aromatic o r long chain carbon system (Eddy. N.O. and Odoemelam, S.A., 2008). However, most
of these inhibitors are toxic to the environment(Umoren, S.A., et al., 2006; Umoren, S.A., et al., 2006).
This has prompted the search for green corrosion inhibitors.
Green corrosion inhibitors are biodegradable and do not contain heavy metals or other toxic compounds
and so they are environmentally friendly. Several studies have been carried out on the inhibition of corrosion
of metals using plant extract as green inhibitors (Anauda, L., et al., 2005; Sethuraman, M .G. and Raja, P.B.,
2005). In most of these and other studies, nothing has been reported on the use of ethanol extract of Musa
acum inate peel for the inhibition of mild steel corrosion. Musa acuminate peel is often discarded as waste after
the inner pulp has been removed implying that successful utilization of this biomass may also provide an
option for resource recovery. The present study seeks to investigate inhibitive properties of Musa acuminate
peel for mild steel corrosion.
Materials and methods
Materials preparation
Materials used for the study were mild steel sheet of composition (wt %) Mn (0.6), P (0.36), C (0.15) and
Si (0.03). The sheet was mechanically press-cut into different coupons, each of dimensions, 5x4x0.11cm. Each

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Adv. in Nat. Appl. Sci., 2(1): 35-42, 2008
coupon was degreased by washing with ethanol, dried in acetone and preserved in a desiccator. All reagents
used for the study were analar grade and double distilled water was used for their preparation.
Extraction and preparation of inhibitor solutions
Musa acuminate peels were dried, ground and soaked in ethanol for 48 h. The extract was filtered and
then freed of ethanol by evaporation at 352 K. T he concentrated stock of extract so obtained was used in
preparing 1000, 2000, 3000, 4000 and 5000 ppm solutions by dissolving 0.1, 0.2, 0.3, 0.4 and 0.5 g of the
24
extract in 1 dm of 2.5 M H SO , respectively.
3
Gasometric method
Gasometric experiments were carried out at 303 and 333 K as described in literature (Oguzie, E.E., 2006;
Umoren, S.A. et al., 2006). From the volume of hydrogen evolved per minute, inhibition efficiency (h) and
degree of surface coverage (q) were calculated using Equations 1 and 2, respectively.
Ht
h = {1 - V’ } x 100 (1)
Ht
V 0
q = ç/100 (2)
Ht Ht
where V’ is the volume of hydrogen evolved at time t in the presence of inhibitor and V is the volume
0
of hydrogen evolved at time t without inhibitor.
Thermometric method
This was also carried out as reported by Umoren et al., From the rise in temperature of the system per
minutes, the reaction number (RN) was calculated using Equation 3:
mi
RN (°C minutes) = T - T (3)
t
mi
where T is the maximum temperature attained by the system, T is the Initial temperature and t is the
time. From the above, the inhibition efficiency (h) of the used inhibitor was computed using Equation 4:
aq wi
h= RN - RN x 100 (4)
aq
RN
Results and discussion
24
Table 1 shows values of corrosion rate (CR) of mild steel in 2.5 M H SO in the presence of extract of
24
Musa acuminate peel. Values of corrosion rate of 1.0 –2.5 M H SO are also recorded in Table 1. Table 2
24
shows reaction numbers (R N) for the corrosion of mild steel in H SO in the absence and presence of Musa
acuminate Peel extract. Table 3 shows values of inhibition efficiency of different concentration of Musa
acuminate Peel extract at 303 and 333 K. Values of inhibition efficiency obtained from thermometric analysis
are also recorded in Table 3.
Discussion
Effect of concentration and temperature
24
From Table 1, it can found that the rate of corrosion of mild steel was affected by concentration of H SO ,
temperature, concentration of inhibitor and period of contact. The rate of mild steel corrosion increased as the
24
concentration of H SO increased and also increased as the temperature was increased. Fig. 1 shows gasometric
24
plot for the corrosion of mild steel in different concentrations of H SO . The volume of hydrogen evolved
during the corrosion of mild steel increased as the concentration of the acid increased confirming that the rate
24
of corrosion of mild steel in H SO increased with concentration.
The vo lume of hydrogen evolved in the presence of different concentrations of ethanol extract of Musa
24
acum inate peel were lower than the volumes evolved in H SO alone, indicating that different concentration
of
the extract inhibited the corrosion of mild steel. Figs. 2 and 3 show gasometric plots for the corrosion of mild
steel in the presence of different concentration of Musa acuminate peel at 303 and 333 K, respectively.

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Adv. in Nat. Appl. Sci., 2(1): 35-42, 2008
Comparing Figs. 2 and 3, it would be found that at fixed concentration of the inhibitor, the volume of
hydrogen evolved
24
Table 1: Corrosion rate(mdd x 10 ) for the corrosion of mild steel in H SO in the absence and presence of M usa acumin ate Peel
-2
extract
Concentration of CR Concentration of Musa CR (mdd) CR (mdd)
24
H SO (mol/L) (303K) acum ina te Peel (ppm) at 303K at 333K
1.0 0.180 1000 0.175 1.945
1.5 0.175 2000 0.140 2.015
2.0 0.22 3000 0.215 1.800
2.5 0.38 4000 0.110 1.790
5000 0.150 1.685
24
Table 2: Reaction number for the corrosion of mild steel in H SO in the absence an d presence of M usa acuminate peel extract
24
Concentration of H SO (mol/L) RN (30 3K) Concentration of Musa acum inate Peel (ppm) RN (mdd) at 303K
1.0 0.0500 1000 0.025
1.5 0.0267 2000 0.025
2.0 0.0167 3000 0.025
2.5 0.050 4000 0.025
5000 0.025
Table 3: Values of inhibition efficiency (%I) and degree of surface coverage from gasometric and thermometric methods
Gaso metric Therm om etric
Concentration of Musa acum inate Peel (ppm) -------------------------------------------------------------------------------------- ------------ ------
%I (303K) %I (333K) è (303K) è (333K) %I (303K)
1000 53.95 14.32 0.5395 0.1432 50.00
2000 63.16 11.23 0.6316 0.1123 50.00
3000 43.42 20.70 0.4342 0.2070 50.00
4000 71.05 21.15 0.7105 0.2125 50.00
5000 60.53 25.77 0.6053 0.2577 50.00
Fig. 1: Gasometric plot for the corrosion of mild steel in different concentrations of acid

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Adv. in Nat. Appl. Sci., 2(1): 35-42, 2008
Fig. 2: Gasometric plot for the corrosion of mild steel in the absence and presence of ethanol extract of Musa
acum inate at 303K.
Fig. 4: Curve fitting for adsorption of Musa acuminate according to Langmuir adsorption isotherm
at 333 K is significantly higher (P³0.05) than that evolved at 303K indicating that the inhibition efficiency of
Musa acuminate peel decreased with temperature. The decrease may be due to competition between forces of
adsorption and desorption (Sathiyanathan, R.A., et al., 2006).
From Table 3, it can also be seen that inhibition efficiency of Musa acuminate peel varied with its
concentration. Optimum value of inhibition efficiency (92.11%) was obtained at extract concentration of 4000
ppm. while the least value was obtained at extract concentration of 1000 ppm. The significant difference
(P³0.05) between values of inhibition efficiency of Musa acuminate peel obtained at 303 and 333 K suggests
that the mechanism of adsorption of inhibitor on mild steel surface was by physical adsorption (Eddy. N.O.
and Odoemelam, S.A. 2008). For a physical adsorption mechanism, inhibition efficiency of an inhibitor
decreases with temperature but for a chemical adsorption mechanism, values of inhibition efficiency are
expected to increase with temperature.
Comparing values of inhibition efficiency obtained from thermometric and gasometric methods, it is seen
that values obtained at 303K from therm ometric method were similar at all concentration of Musa acuminate
peel. T he constancy observed for values of inhibition efficiency may be due to the fact that thermometric
methods monitored increase in temperature with time and these may vary insignificantly with time.
Thermodynamic and adsorption considerations
Values of activation energy for the corrosion reaction of mild steel in the presence and absence of different
concentration of ethanol extract of Musa acuminate peel have been calculated using Arrhenius equation
(Equation 5): (Acharya, S., et al., 2004)
a
CR = Aexp (-E /RT) (5)
Taking logarithm of both sides of Equation 5, Equation 6was obtained:
a
logCR = logA - E /RT (6)
a
where CR is the corrosion rate of mild steel. A is Arrhenius constant or pre-exponential factor, E is the
activation energy of the reaction, R is the gas constant and T is the temperature. Considering a change in
1
temperature from 303 K (T ) to 333 K (333 K), the corresponding values of the corrosion rates at these
12
temperatures are CR and CR respectively. Inserting these parameters into Equation 6, Equation 7was obtained:

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Adv. in Nat. Appl. Sci., 2(1): 35-42, 2008
21 a 1 2
log (CR /CR ) = E /2.303R x (1/T – 1/T ) (7)
a
Values of E for the inhibited corrosion reaction of mild steel have been calculated using Equation 7.
These values (Table 4) ranged from 59.9937 – 78.1010KJ/mol (mean = 69.4820KJ/mol) supporting the
a
mechanism of physical adsorption. For a physical adsorption, it is expected that the value of E should be less
than 80.00KJ/mol. (Ebenso, E., 2003; Sheatty, D.S., 2006).
Table 4: Thermodynam ic parameters for the adsorption of Musa acum inate Peel on mild steel surface
a ads ads ads ads
Con. of Musa acuminate Peel (pp m ) E (KJ/mo l) Q ( KJ /m ol) ÄG (KJ/mol) at 303K ÄG (KJ/mol) at 333K ÄS (J/ m ol)
1000 67.4264 -54.4609 -4.7170 -0.2081 164.1713
2000 74.6640 -72.8986 -7.4228 -0.9401 216.0917
3000 59.9937 -30.1593 -6.4141 -4.0686 78.3670
4000 78.1010 -61.7449 -10.0731 -4.9571 170.5340
5000 67.7247 -41.5473 -9.4506 -6.2727 105.9297
The values of heat of adsorption of Musa acuminate peel on mild steel surface were calculated using
Equation 8: (Umoren, S.A., 2006, Ebenso, E.E., 2003).
ads 22 11 1 22 1
Q = 2.303R[log(è/1-è) - log(è/1-è)] x (T x T )/(T – T ) (8)
ads
Values of Q (Table 4) calculated through Equation 8 were positive and ranged from –30.1593 to -
72.8986K J/mol (mean = 52.1622KJ/mol) indicating that the adsorption of Musa acuminate peel on mild steel
surface is exothermic. These values are relatively large indicating that the heat of adsorption is also large. It
ads
can also be stated that since the reaction was carried out at constant pressure, values of Q should
ads
approximate those of enthalpy of adsorption (ÄH ). (Atkins, P., 2002; Sharm a, K.K .,).
Values of free energy of adsorption of Musa acuminate peel on mild steel surface were calculated using
the following Equation: (Ashassi-Sorkhabi, H., 2004; Ashassi-Sorkhabi, H., et al., 2005)
ads
ÄG = -2.303RTlog(55.5K) (9)
ads
where K = q/(1-q)[C], C is the concentration of the inhibitor. Calculated values of ÄG are recorded in
Table 4. These values are negative and raged from –4.7170 to – 10.0731 KJ/mol and from –0.2081 to -
6.2727KJ/mol at 303 and 333 K, respectively. This indicates that adsorption of ethanol extract of Musa
acuminate peel is spontaneous and occur via physical adsorption mechanism (U moren, S.A., et al., 2006). The
ads
result also revealed that values of ÄG were more negative at 303 K compared to values obtained at 333
K indicating that the spontaneity of adsorption hence stability of the adsorbed layer is higher at 303 K. At 333
K, effect of temperature tends to increase the degree of disorder in the adsorbed molecular layer.
ads ads
Values of entropy of adsorption (ÄS ) were calculated by substituting corresponding values of ÄG and
ads
ÄH into the Gibbs-Helmholtz equation according to Equation 10: (Abdallah, M., 202; Bilgic, L.,2001)
ads ads ads
ÄG = ÄH - TÄS (10)
ads
Values of ÄS calculated through Equation 10 are recorded in Table 4. These values were found to be
relatively large and positive. From thermodynamic consideration, the rate of adsorption of Musa acuminate on
mild steel surface is most likely to be controlled by the activation complex.
Adsorption isotherms are very important in understanding the mechanism of inhibition of corrosion reaction
of zinc. The most frequently used adsorption isotherms are Frumkin, Temkin, Freundlich, Florry Huggins,
Bockris –Swinkel, El-Awardy and Langmuir isotherms. All these isotherms can be represented as follows,
f(è, x) exp (-2aè) = kC (11)
where f(è, x) is the configuration factor which depends upon the physical model and the assumptions
underlying the derivation of the isotherm. q is the degree of surface coverage, C is the inhibitor concentration
in the electrolyte, X is the size ratio, a is molecular interaction parameter and k is the equilibrium constant
of the adsorption process. Adsorption behaviour of Musa acuminate peel is best explained by Langmuir and
Frumkin adsorption isotherms.
Langmuir isotherm is an ideal isotherm for physical or chemical adsorption where there is no interaction
between the adsorbate and the adsorbent (Sheatty, D.S., et al., 2006). Assumptions of Langmuir relate the
concentration of the adsorpbate in the bulk of the electrolyte (C) to the degree of surface coverage (è)
according to Equation 12:

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Adv. in Nat. Appl. Sci., 2(1): 35-42, 2008
C/q = 1/k + C (12)
where k is the equilibrium constant of adsorption. Taking logarithm of both sides of Equation 12, Equation
13 was obtained:
log(C/è) = logC - logK (13)
By plotting values of log(C/è) versus values of logC straight line graphs were obtained (Fig. 4).
Applicability of Langmuir adsorption isotherm to the adsorption of Musa acuminate peel on mild steel confirms
the formation of multimolecular layer of adsorption where there is no interaction between the adsorbate and
the adsorbent.
Fig. 5: Curve fitting for adsorption of Musa acuminate extract on mild steel electrode.
Frumkin isotherm equation (Equation 14) is obeyed when a plot of log (è/(1-è)[C] versus è produce a
straight line with slopes equal to 2a/2.303.
log (è/(1-è)[C] = logK + 2aè/2.303 (14)
where a is lateral interaction term describing the molecular interaction in the adsorbed layer. Fig. 5 shows
Frumkin plots for the used inhibitor (E) at different temperatures. Values of a calculated from the slopes of
lines on the plot were 2.0537 and 1.468 at 303 and 333 K indicating attractive behaviour of the inhibitor.
Also value of a at 303 was greater than the value obtained at 333 K indicating that the strength of the
attractive behaviour of the inhibitor decreased with temperature.
Comparing the degree of linearity of Langmuir and Frumkin adsorption isotherms as measured by values
of R it is seen that Langmuir adsorption isotherm is best applicable at 303K while Frumkin isotherm is best
2,
applicable at 303K. This confirms that the adsorption behaviour of the inhibitor was strongly influence by
temperature.
Effect of pH and electrode potential
24
The dissolution of mild steel in H SO occurs according to Equation 15:
2
Fe + H = Fe + ½H (15)
+2+
From the above equation, it can be seen that for every two moles of mild steel consumed, one mole of
2
hydrogen gas is evo lved. In other word, the dissolution of two mole of mild steel liberates 22400cm of H
3
2
= 1mole of H . T herefore if x volume of hydrogen gas is evolved, the number of moles of hydrogen associated
with the dissolution is equal to x/22400. pH of a solution is defined as follows,
10
pH = -log (x/22400) (16)
Values of pH calculated through equation 15 have been used to plot Fig. 6 which shows variation of pH

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Adv. in Nat. Appl. Sci., 2(1): 35-42, 2008
with time. From Fig. 6, it would be seen that the pH of the corrodent decreased as the immersion time
increased indicating that enhancement in the rate of corrosion as the period of immersion increase was due
Fig. 6: Variation of pH with time for the corosion of mild steel in the presence of M usa acum inate extract
at 303K
to increase in acidity.Corrosion of mild steel is an electrochemical process involving the following cathodic
and anodic half reactions:
Fe = Fe + 2e (anodic half reaction) (17)
2+ -
2
2H + 2e = H (cathodic half reaction) (18)
+-
Adding Equations 17 and 18, the overall cell reaction is obtained as follows,
Fe + H = Fe + 2e (19)
+2+-
Nernst equation for the above reaction can be written as follows,
Fe Fe Fe2+
E = E + RT/nFln(a /a ) (20)
o
Fe
For anodic half reaction, E is independent of pH but for the cathodic half reaction, pH is an important
factor. Applying the Nernst equation, the electrode potential for the cathodic half reaction can be written as
follow,
H2 H2 H+
E = E + RT/Flna (21)
H2
At p = 1atm, Equation 21 is rearranged to Equation 22 as follow,
H2
E = -RT/F x 2.303pH (22)
H2
The implication of Equation 22 is that the variation of E with time during the corrosion of mild steel
H2
is expected to follow trend similar to that observed for pH. Therefore, we state that the E of the system
decreased as the corrosion of mild steel proceeds. Values of electrode potential of the system are expected
to vary proportionally with free energy according to Equation 23. (Atkins, P., 2002; Sharma, K.K .,)
ads
ÄG = -nFE (23)
o
where n is the number of electron associated with the redox reaction, F is the Faraday’s constant (F = 96500C)
o
and E is the electrode potential. Values of E calculated through Equation 23 were 0.024, 0.038, 0.033, 0.05
o

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Adv. in Nat. Appl. Sci., 2(1): 35-42, 2008
and 0.05 V at inhibitor concentration of 1000, 2000, 3000, 4000 and 5000 ppm, respectively. From the results,
it can also be stated that the variation of inhibition efficiency with concentration of Musa acuminate was partly
due to differences in values of the electrode potential.
Conclusion
From the present study, it was found that ethanol extract of Musa acuminate peel can be used as an
inhibitor for mild steel corrosion. The inhibitor acts by adsorption onto mild steel surface according to classical
adsorption models of Langmuir and Frumkin adsorption isotherms. Adsorption characteristics of the inhibitor
follow spontaneous physical adsorption mechanism. The inhibitive action of M usa acum inate peel was basically
controlled by temperature, pH , period of immersion, electrochemical potential and concentration of the
inhibitor.
Acknowledgment
The authors of this article are grateful to Ndifreke Nde and Isanedihi Ating for technical assistance.
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