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Ethanol extract of Musa acuminate peel as an eco-friendly inhibitor for the corrosion of mild steel in [H.sub.2]S[O.sub.4].

Introduction

The use of an inhibitor is one of the best options for protecting metals against corrosion. (1-2) Several inhibitors in use are either synthesized from cheap raw materials or chosen from compounds having hetero atoms in their aromatic or 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 acuminate 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 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. The 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 2 4 extract in 1 d[m.sup.3] of 2.5 M [H.sub.2]S[O.sub.4], respectively.

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.

h = {1 - [V.sup.'.sub.Ht]/[V.sup.[??].sub.Ht]} x 100 (1)

q = [eta]/100 (2)

where [V.sup.'.sub.Ht] is the volume of hydrogen evolved at time t in the presence of inhibitor and [V.sup.0.sub.Ht] is the volume 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:

RN ([degrees]C minutes) = [T.sub.m - T.sub.i]/t (3)

where [T.sub.m] is the maximum temperature attained by the system, [T.sub.i] 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:

h = R[N.sub.aq] - R[N.sub.wi]/R[N.sub.aq] x 100 (4)

Results and discussion

Table 1 shows values of corrosion rate (CR) of mild steel in 2.5 M [H.sub.2]S[O.sub.4] in the presence of extract of Musa acuminate peel. Values of corrosion rate of 1.0 -2.5 M [H.sub.2]S[O.sub.4] are also recorded in Table 1. Table 2 shows reaction numbers (RN) for the corrosion of mild steel in [H.sub.2]S[O.sub.4] 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

From Table 1, it can found that the rate of corrosion of mild steel was affected by concentration of [H.sub.2]S[O.sub.4], temperature, concentration of inhibitor and period of contact. The rate of mild steel corrosion increased as the concentration of [H.sub.2]S[O.sub.4] increased and also increased as the temperature was increased. Fig. 1 shows gasometric plot for the corrosion of mild steel in different concentrations of [H.sub.2]S[O.sub.4]. The volume of hydrogen evolved during the corrosion of mild steel increased as the concentration of the acid increased confirming that the rate of corrosion of mild steel in [H.sub.2]S[O.sub.4] increased with concentration.

The volume of hydrogen evolved in the presence of different concentrations of ethanol extract of Musa acuminate peel were lower than the volumes evolved in [H.sub.2]S[O.sub.4] 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. Comparing Figs. 2 and 3, it would be found that at fixed concentration of the inhibitor, the volume of hydrogen evolved at 333 K is significantly higher (P'0.05) 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).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 4 OMITTED]

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 (P'0.05) 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 thermometric method were similar at all concentration of Musa acuminate peel. The 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)

CR = Aexp (-[E.sub.a]/RT) (5)

Taking logarithm of both sides of Equation 5, Equation 6was obtained:

logCR = logA - [E.sub.a]/RT (6)

where CR is the corrosion rate of mild steel. A is Arrhenius constant or pre-exponential factor, [E.sub.a] is the activation energy of the reaction, R is the gas constant and T is the temperature. Considering a change in temperature from 303 K ([T.sub.1]) to 333 K (333 K), the corresponding values of the corrosion rates at these temperatures are CR and C[R.sub.1] and C[R.sub.2] respectively. Inserting these parameters into Equation 6, Equation 7 was obtained:

log (C[R.sub.2]/C[R.sub.1]) = [E.sub.a]/2.303R x (1/[T.sub.1] - 1/[T.sub.2]) (7)

Values of [E.sub.a] 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.sub.a] should be less than 80.00KJ/mol. (Ebenso, E., 2003; Sheatty, D.S., 2006).

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).

[Q.sub.ads] = 2.303R[log([[theta].sub.2]/1-[[theta].sub.2]) - log([[theta].sub.1]/-[[theta].sub.1])] x ([T.sub.1] x [T.sub.2])/([T.sub.2] - [T.sub.10) (8)

Values of [Q.sub.ads] (Table 4) calculated through Equation 8 were positive and ranged from -30.1593 to -72.8986KJ/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 ([DELTA][H.sub.ads]). (Atkins, P., 2002; Sharma, 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)

[DELTA][G.sub.ads] = -2.303RTlog(55.5K) (9)

where K = q/(1-q)[C], C is the concentration of the inhibitor. Calculated values of [DELTA][G.sub.ads] 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 (Umoren, S.A., et al., 2006). The ads result also revealed that values of [DELTA][G.sub.ads] 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.

Values of entropy of adsorption ([DELTA][S.sub.ads]) were calculated by substituting corresponding values of [DELTA][G.sub.ads] and [DELTA][H.sub.ads] into the Gibbs-Helmholtz equation according to Equation 10: (Abdallah, M., 202; Bilgic, L.,2001)

[DELTA][G.sub.ads] = [DELTA][H.sub.ads] - T[DELTA][S.sub.ads] (10)

Values of [DELTA][S.sub.ads] 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([theta], x) exp (-2a[theta]) = kC (11)

where f([theta], 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 ([theta]) according to Equation 12:

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/[theta]) = logC - logK (13)

By plotting values of log(C/[theta]) 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.

[FIGURE 5 OMITTED]

Frumkin isotherm equation (Equation 14) is obeyed when a plot of log ([theta]/(1-[theta])[C] versus [theta] produce a straight line with slopes equal to 2a/2.303.

log ([theta]/(1-[theta])[C] = logK + 2a[theta]/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.sup.2,] 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

The dissolution of mild steel in H SO occurs according to Equation 15:

Fe + [H.sup.+] = [Fe.sup.2+] + 1/2[H.sub.2] (15)

From the above equation, it can be seen that for every two moles of mild steel consumed, one mole of hydrogen gas is evolved. In other word, the dissolution of two mole of mild steel liberates 22400[cm.sup.3] of [H.sub.2] = 1mole of [H.sub.2]. Therefore 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,

pH = -log (x/22400) (16)

Values of pH calculated through equation 15 have been used to plot Fig. 6 which shows variation of pH 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 to increase in acidity.Corrosion of mild steel is an electrochemical process involving the following cathodic and anodic half reactions:

[FIGURE 6 OMITTED]

Fe = [Fe.sup2+] + 2e (anodic half reaction) (17)

2[H.sup.+] + 2[e.sup.-] = [H.sub.2] (cathodic half reaction)

Equations 17 and 18, the overall cell reaction is obtained as follows,

Fe = [H.sup.+] = [Fe.sup.2+] + 2[e.sup.-] (19)

Nernst equation for the above reaction can be written as follows,

E[Fe.sub.] [E.sub.[degrees]] = E + RT/nFln([a.sub.Fe]/[a.sub.Fe2+]) (20)

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,

[E.sub.H2] = [E.sub.H2] + RT/[Flna.sub.H+] (21)

At [p.sub.H2] = 1atm, Equation 21 is rearranged to Equation 22 as follow,

[E.sub.H2] = -RT/F x 2.303pH (22)

The implication of Equation 22 is that the variation of [E.sub.H2] 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.sub.H2] 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.,)

[DELTA][G.sub.ads] = -nF[E.sup.[degrees]] (23)

where n is the number of electron associated with the redox reaction, F is the Faraday's constant (F = 96500C) and [E.sup.[degrees]] is the electrode potential. Values of [E.sub.[degrees]] calculated through Equation 23 were 0.024, 0.038, 0.033, 0.05 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 Musa acuminate 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.

References

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Corresponding Author: N.O. Eddy,Department of Chemistry, Ahmadu Bello University, Zaria, Nigeria. E-mail: nabukeddy@yahoo.com

(1) N.O. Eddy, (2) S.A. Odoemelam and (2) A.O. Odiongenyi

(1) Department of Chemistry, Ahmadu Bello University, Zaria, Nigeria. (2) Department of Chemistry, Michael Okpara University of Agriculture, Umudike, P.M.B. 7267, Umahia, Abia State, Nigeria.

N.O. Eddy, S.A, Odoemelam and A.O. Odiongenyi,: Ethanol Extract of Musa acuminate Peels as a Green Corrosion Inhibitor for Mild Steel: Kinetics, Adsorption and Thermodynamic Considerations,: Adv. in Nat. Appl. Sci., 2(1): 35-42, 2008
Table 1: Corrosion rate(mdd x [10.sup.-2]) for the corrosion of mild
steel in [H.sub.2]S[O.sub.4] in the absence and presence of Musa
acuminate Peel extract

Concentration of Concentration CR CR
[H.sub.2]S[O.sub.4] CR of Musa acuminate (m dd) (m dd)
(mol/L) (303K) 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

Table 2: Reaction number for the corrosion of mild steel in
[H.sub.2]S[O.sub.4] in the absence and presence of Musa
acuminate peel extract

Concentration of
[H.sub.2]S[O.sub.4] Concentration of Musa RN (m dd)
(mol/L) RN (303K) acuminate Peel (ppm) 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

 Gasometric
Concentration
of Musa acuminate [theta] [theta]
Peel (ppm) %I (303K) % (333K) (303K) (333K)

1000 53.95 14.32 0.5395 0.1432
2000 63.16 11.23 0.6316 0.1123
3000 43.42 20.70 0.4342 0.2070
4000 71.05 21.15 0.7105 0.2125
5000 60.53 25.77 0.6053 0.2577

Concentration Thermometric
of Musa acuminate
Peel (ppm) %I (303K)

1000 50.00
2000 50.00
3000 50.00
4000 50.00
5000 50.00

Table 4: Thermodynamic parameters for the adsorption of
Musa acuminate Peel on mild steel surface

 [DELTA]
Con. of Musa [G.sub.ads]
acuminate [E.sub.a] [Q.sub.ads] (KJ/mol)
Peel (ppm) (KJ/mol) (KJ/mol) at 303K

1000 67.4264 -54.4609 -4.7170
2000 74.6640 -72.8986 -7.4228
3000 59.9937 -30.1593 -6.4141
4000 78.1010 -61.7449 -10.0731
5000 67.7247 -41.5473 -9.4506

 [DELTA]
Con. of Musa [G.sub.ads] [DELTA]
acuminate (KJ/mol) [S.sub.ads]
Peel (ppm) at 333K (J/mol)

1000 -0.2081 164.1713
2000 -0.9401 216.0917
3000 -4.0686 78.3670
4000 -4.9571 170.5340
5000 -6.2727 105.9297
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Title Annotation:Original Article
Author:Eddy, N.O.; Odoemelam, S.A.; Odiongenyi, A.O.
Publication:Advances in Natural and Applied Sciences
Article Type:Report
Geographic Code:6NIGR
Date:Jan 1, 2008
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