Adsorption and Thermodynamics Study of the Inhibition of Corrosion of Mild Steel In H2SO4 Medium Using Vernonia Amygdalina

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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

Email: *[email protected]

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.

% 1100



 





(1)



 (2)



CRg cmhW





(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



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].



ads

12 1

2.303 loglog

KJ mol

()



































(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





 (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









 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

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

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

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







(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





 



 (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











 









 (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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