Inhibitive Properties and Adsorption Consideration of Ethanol Extract of Manihot esculentum Leaves...

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International Journal of Modern Chemistry, 2013, 4(3): 137-146

International Journal of Modern Chemistry

Journal homepage: www.ModernScientificPress.com/Journals/IJMChem.aspx

ISSN: 2165-0128

Florida, USA

Article

Inhibitive Properties and Adsorption Consideration of Ethanol

Extract of Manihot esculentum Leaves for Corrosion Inhibition

of Aluminium in 2 M H2SO4

Sylvester O. Adejo*, Joseph A. Gbertyo and John U. Ahile

Department of Chemistry, Benue State University, Makurdi-Nigeria

* Author to whom corresponding should be addressed: email: [email protected] Tel:

+2348054557652

Article history: Received 8 April 2013, Received in revised form 30 April 2013, Accepted 6 May 2013,

Published 9 May 2013.

Abstract: The inhibition of aluminium corrosion by the ethanol extract of Manihot

esculentum leaves in 2 M H2SO4 solution was studied by weight loss method at the

temperature range of 303 K to 315 K. The percentage inhibition efficiency, %IE, was found

to increase with increase in both the inhibitor concentration and temperature. The increase

in %IE with rise in temperature is suggestive of chemical adsorption process. Generally,

not high inhibition efficiency observed at the studied temperature range may be an

indication of high rate of desorption processes in the adsorption-desorption equilibrium

process. However, the positive values of adsorption equilibrium constant, K, signify

favourable adsorption of the extract unto the metal surface. Activation energy values in the

presence of the extract were lower than in the blank, supportive chemical adsorption

mechanism. Negative values of free energy of adsorption, ∆Gads, indicate that the

adsorption process was spontaneous. Our data best fitted the Freundlich and El-Awady

adsorption isotherms.

Keywords: Corrosion inhibition, Manihot esculentum, Weight loss method, aluminium,

Adsorption isotherm.

1. Introduction

Acid solutions are often used in industry for cleaning, and pickling of metallic structures. These

processes are normally accompanied by considerable dissolution of the metal. One of the useful

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Int. J. Modern Chem. 2013, 4(3): 137-146


138

methods of protection of metals and their alloys deployed in service in aggressive environments

against corrosion is the addition of organic or inorganic substances to the solution in contact with the

surface.. A number of organic compounds are known for their applicability as corrosion inhibitors for

metals, especially iron and aluminium and their alloys in acidic environments [1]. Such compounds

typically contain nitrogen, oxygen and sulphur and multiple bonds in a conjugated system. The

protection is usually via adsorption of the molecules onto the metal surface, thus creating a barrier

between the metal surface and the corrodant attack. The adsorption bond strength is dependent on a

number of the factors such as the composition of the metal and corrodant, inhibitor structure,

concentration, temperature, among others [2]. Despite the broad spectrum of organic compounds

available, the choice of an appropriate inhibitor for a particular application is restricted by several

factors, which include mainly environmental consciousness and cost. However, the vast variety of

possible corrosion systems coupled with the specificity of action of most acid inhibitors often

necessitates the use of combined additives to provide the multiple services required for effective

corrosion inhibition [3]. Consequently, there is the need to develop a new class of corrosion inhibitors

with low toxicity and greater efficiency.

The exploration of natural products of plant origin as inexpensive eco-friendly inhibitors is an

essential field of study of corrosion science and engineering. In addition to being environmentally

friendly and ecologically acceptable, plant products are of low cost, readily available and renewable

sources of materials. Extracts from their leaves, barks, seeds, fruits and roots contain mixtures of

organic compounds which function as effective inhibitors of metals and their alloys corrosion in

different aggressive media [4].

In this work, we report the use of ethanol extract of leaves of Manihot esculentum as eco-

friendly inhibitor for the corrosion of aluminium in 2 M H2SO4 medium within the temperature range

of 303 K to 315 K.

2. Materials and Methods

2.1. Materials

Aluminium wires were obtained from open market and taken to the Department of Mechanical

Engineering, University of Agriculture, Makurdi-Nigeria, where they were melted to give metal of

very high purity and then re-moulded into desired sheets. The sheets were mechanically press-cut to

form coupons, each of dimension 2.1 cm x2.1 cm and 0.35 cm, with a tiny hole drilled at the edge of

each sample for suspension in the corrodant. The coupons’ surfaces were thoroughly polished to mirror

finish using emery paper of different grade, and then degreased by washing in absolute ethanol, dried

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139

with acetone, and preserved in a desiccator. Subsequently, the initial weight of each coupon was

carefully measured [5], using an ADAM Electric Weighing Balance (model AFP-4100L Max: 4100, d

= 10mg). A 2 M H2SO4 solution was prepared which served as corrodant. All reagents used were of

analytical grade, while doubly distilled water was used for the preparation of the solutions.

200 g of shade-dried Manihot esculentum leaves were soaked in 200 cm3 of absolute ethanol

for 48 hours, with occasional shaking and filtered, thereafter, using a cotton wool, and the ethanol

evaporated from the filtrate using thermostated water bath maintained at 321 K to avoid any possible

structural changes to the components of the extract. The residue obtained, therefrom, was the preserved

in a desiccator. Stock solutions containing 0.1, 0.2, 0.3, 0.4, and 0.5 g/dm3 in the 2 M H2SO4 were

prepared from the extract [5 - 9].

2.2. Weight Loss Measurement Method

50 mL of the 2 M H2SO4, which served as the corrodant, was in taken a 100 mL beaker and

placed in the thermostated water bath maintained at 303 K. A coupon of known initial weight was

wholly immersed in the corrodant suspended with the aid of a thread tied to a retort stand for 8 hours.

At retrieval it was immediately dipped into a saturated solution of ammonium acetate to quench the

corrosion reaction, and carefully washed in absolute ethanol, rinsed in distilled water and then kept the

desiccator and allowed to stand to constant weight and re-weighed [1, 3] . The weight loss of the

coupon was obtained as the difference in the weight before and after the immersion. Triplicate results

were used in each case and the mean values reported. This procedure was repeated at 307 K, 311 K,

and 315 K which served as the blank at each of these temperatures. The same procedure was repeated

after the introduction of various concentrations of the extract into the corrodant maintained at same

temperatures. The weight loss was calculated thus,

(1)

where is the weight loss of the coupon, Wi the initial weight and Wf the weight after retrieval. The

percentage inhibition was the obtained through equation (2) [1, 2].

⁄ (2)

where are the weight losses in grams of the coupon in the presence and absence of

the inhibitor, respectively. The degree of surface coverage, is given by equation (3) [10, 11 12].

⁄ (3)

The corrosion rate of the metal was determined for the immersion period from weight loss

through equation (4).

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140

Corrosion rate (mgcm-2

h-1

) =

(4)

where WL is the weight loss in milligrams (mg), A the coupon surface area in cm2and t the immersion

time in hours [12].

3. Results and Discussion

The Table 1 shows the results of weight loss for the corrosion of aluminium in 2 M H2SO4 in

the absence and presence of various concentrations of Manihot esculentum extract at different

temperatures. The results show that Manihot esculentum inhibited the corrosion of aluminium the acid

medium since there was decrease in weight loss in the presence of the extract comparison with the

blank. The weight loss decreased with increase in the extract concentration, but increased with rise in

temperature.

Table 1. Weight loss (WL) and inhibition efficiency (%IE) for the inhibition of aluminium

corrosion by leaves extract of Manihot esculentum at various temperatures

Concentration WL (g) %IE

(g/dm3) 303 K 307 K 311K 315 K 303 K 307 K 311K 315 K

Blank 0.012 0.018 0.027 0.030

0.1 0.011 0.016 0.023 0.023 8.33 11.11 14.81 23.33

0.2 0.010 0.014 0.021 0.021 16.67 22.22 22.22 23.33

0.3 0.009 0.013 0.018 0.019 25.00 27.28 33.33 36.67

0.4 0.012 0.018 0.018 0.018 25.00 33.33 33.33 40.00

0.5 0.008 0.010 0.014 0.015 33.33 44.44 48.15 50.00

The evaluated inhibition efficiency, %IE, (Table 1) was found to increase with both increase in

the concentration of the inhibitor and temperature. This increase in inhibition efficiency with rise in

temperature could be suggestive of chemical adsorption as the mechanism of adsorption of the

inhibitor onto the surface of the metal [13]. The rate of corrosion that was observed to be high in the

blank came down considerably when the inhibitor was introduced.

To demonstrate that the extract was adsorbed unto the metal surface for the inhibition process,

a plot of inhibition efficiency against the logarithm of concentration was carried out at the two extreme

operation temperatures (300 K and 315 K). The S- shaped curve as depicted by figures 1 and 2 clearly

shows that a protective barrier was actually formed by the inhibitor molecules on surface of the metal

[14].

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141

Figure 1. Relationship between inhibition efficiency and logarithm of

concentration of inhibitor at 303 K

Figure 1. Relationship between inhibition efficiency and logarithm of

concentration of inhibitor at 315

Table 2 shows values of activation energy, Ea, and some thermodynamics parameters. Value of

Ea in the blank was found to be higher than in the inhibited solution and it decreases with increase in

the extract concentration. Decrease (or equal to the blank) in the value of Ea with increase in inhibitor

concentration is a feature of chemical adsorption mechanism [13, 15], and this observation supports

our earlier proposed chemical adsorption.

0

5

10

15

20

25

30

35

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0

%IE

log C

0

10

20

30

40

50

60

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0

%IE

log C

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Values of heat of adsorption, Qads, were all found to positive and smaller in the presence of

inhibitor, but did not change with the increase in inhibitor concentration. The positive sign of Qads is

an indication that the process is endothermic and the degree of surface coverage increases with rise in

temperature [16], supportive of the chemical adsorption process.

Table 2. Values of activation energy (Ea) and thermodynamic parameters for extract of

Manihot esculentum leaves as inhibitor for aluminium corrosion

Conc. Ea +Qads +ΔH*ads - ΔS*ads - ΔGads

(g/dm3) (kJ/mol) (kJ/mol) (kJ/mol) (J/mol) (kJ/mol)

303 K 307 K 311K 315 K

Blank 63.09 60.52 81.16

0.1 51.56 9.48 49.05 119.57 9.88 10.82 11.82 13.43

0.2 52.63 5.48 50.13 117.30 10.12 11.16 11.31 12.52

0.3 51.40 4.67 48.79 122.45 0.38 10.82 11.71 12.24

0.4 49.73 5.00 47.14 127.80 9.66 10.82 10.96 11.86

0.5 44.45 5.38 41.85 146.40 10.12 11.45 11.99 12.33

Average 49.95 6.23 47.39 126.70 10.03 11.01 11.56 12.48

Values of enthalpy of activation, ∆H٭ads, are positive and are lower than that of the blank,

signifying inhibition effectiveness increases with increase in temperature. The positive sign is a

reflection of endothermic nature of the dissolution process [17 – 19]. The average difference in value

of the Ea - ΔH*ads (equation 5) was found to be 2.56 kJ/mol, almost same as the value of RT (2.57

kJ/mol), where T is the average of the temperatures (309 K) at which the work was carried out. This

implies that the corrosion process of this metal in the acid medium is a unimolecular reaction [20, 21].

Ea - ΔH*ads = RT (5)

The negative sign of values of entropy of activation, ΔS*ads, shows that the activated complex

in the rate-determining step represents an association, rather than dissociation, that is a decrease in

disorderliness on going from reactants to the activated complex [18]. And the value was observed to

increase with increase in concentration, showing increase in disorderliness in that direction too.

ΔGads values are all negative and increase with rise in temperature. The increase in free energy

value with rise in the temperature is an indication that spontaneity of the adsorption process and

stability of adsorbed layer increased with rise in temperature. However, the values are in the physical

adsorption mechanism range as they are lower than -20 kJ/mol. This, therefore, can be interpreted to

mean that the adsorption process was a mixture of physical and chemical mechanism [18].

The inhibition of metal corrosion by organic additives has been attributed to their adsorption

onto metal surface. This can be usually confirmed from the fitness of the experimental data to various

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adsorption isotherms, and the most frequently used adsorption isotherms are Temkin, Frumkin,

Langmuir, Freundlich, Hill de Boer, Parsons, Flory - Huggins, Dhar-Flory Huggins, El-Awady and

Bockris-Swinkels’ isotherms. All of these isotherms are of the general form [12],

(6)

where f(θ, x) is a configurational factor, which depends upon the physical model and assumptions

underlying the derivative of the isotherm, θ the surface coverage, C the inhibitor concentration, is

the size ratio, ‘ ’ is the molecular interaction parameter and K the equilibrium constant of adsorption

process [22].

Our data best fitted the only Freundlich and El-Awady adsorption isotherms judging from the

value of the regression coefficient of determination, R2 (Table 3). Freundlich isotherm is given by

equation (7).

Freundlich (7)

A plot of logθ versus logC should give a straight line, with slope equal to nF and intercept logKF. The

positive adsorption equilibrium constant K values are indication of favourable adsorption. The

parameter nF in the Freundlich isotherm is related to intensity of adsorption and it varies with

heterogeneity of the material [23], and the value is always positive, but not an integer, with typical

value is 0.6 [24]. The average value of nF is 0.7108, a value not far from 0.6, an indication that this

adsorption process can reasonably be modeled by this isotherm and is heterogeneous. Values of ΔGads

obtained through this isotherm are all above 20 kJ/mol, evidence in support of chemical adsorption

mechanism.

The adsorption also follows the kinetic-thermodynamic model of El-Awady isotherm (equation

8).

log

= log K

’ + ylog

[C] (8)

For this isotherm a plot of

versus should be straight line with slope equals to y.

It has been argued that y < 1 implies single inhibitor molecule occupies more than one active

sites on the metal surface and for y > 1, it means a formation of multilayer [1, 30, 33]. Values of y

(Table 3) are 1.0170, 1.2579, 1.0275 and 0.7445 at 303 K, 307 K, 311 K and 315 K respectively.

Adsorption is therefore a multilayer, except at 315 K it can be said that a single inhibitor molecule

occupies more than one site.

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Table 3. Values of Parameters of Linearised various Adsorption Isotherms

Isotherm Temperature R2 Slope Kads -ΔGads

(K) (kJ/mol)

Freundlich n

303 0.9692 0.8337 60.1451 0.8337 20.44

307 0.9845 0.8161 75.2489 0.8161 21.28

311 0.9606 0.6955 71.6803 0.6955 21.44

315 0.9419 0.4903 64.0619 0.4903 21.42

Average 0.7089

El-Awady y

303 0.9726 1.0170 0.9888 1.0170 10.09

307 0.9095 1.2579 1.7944 1.2579 11.75

311 0.9100 1.0275 1.6571 1.0275 11.69

315 0.8335 0.7445 1.5683 0.7445 11.70

4. Conclusion

The extract of leaves of Manihot esculentum was found to have inhibitory effect on the

corrosion of aluminium in 2 M H2SO4. The adsorption process which was found to be favourable

followed the Freundlich and El-Awady isotherms.

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Inhibitive Properties and Adsorption Consideration of Ethanol Extract of Manihot esculentum Leaves...

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