Effect of Aqueous Extracts of Bitter Leaf Powder on the Corrosion Inhibition of Al-Si Alloy in 0.5 M Caustic Soda Solution

Paper Menu >>

Journal Menu >>

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,

M. Abdulwahab2

1National Metallurgical Development Centre, Jos, Nigeria

2Department of Metallurgical and Materials Engineering, Ahmadu Bello University, Zaria, Nigeria

Email: *[email protected]

Received January 23, 2012; revised February 29, 2012; accepted March 27, 2012

ABSTRACT

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

1. Introduction

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 [1].

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 [1]. 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 [2]. 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 [4]. 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-

sion [5,6].

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

*Corresponding author.

F. A. AYENI ET AL.

668

such as Vernonia amygdalina [10] and neem leaf [11]. 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 [12]. 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

72.5% respectively.

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

2.1. Materials

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.

Equipment

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. Methods

2.2.1. Determination of the Chemical Composition of

Plant Extracts

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 ].

WWW



 (1)

where W—weight loss after 5 days,

Wo—initial weight,

Wf—final weight.

The standard expression for measurement of corrosion

rate in mils per year (mpy) was used which is given as

follows [1,15].

534W

WDAT

 (2)

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-

lationship [2,6]:

Inhibitor Efficiency100%





(3)

where W and Wo are the corrosion r ates with and without

inhibitor respectively.

Adsorption Consideration

The degree of surface coverage, O at each concentra-

tion of inhibitor was evaluated using the equation [16].

 (4)

where Wb and Wi are the weight loss in corrodent without

and with inhibitor respectively.

3. Results and Discussions

3.1. Results

The physiochemical screening of the bitter leaf extract

revealed that the plant extract contained 0.9% tannin and

0.64% Saponnin.

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-

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 [1].

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 [2]. 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 [17], 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 [12]. 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

medium.

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-

hibitor concentration.

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

F. A. AYENI ET AL.

670

days.

4. Conclusions

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-

sion rate.

3) The inhibitor is recommended to be used at 0.5%

concentration in 0.5 M NaOH and for a short period of

time only.

5. Acknowledgements

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

composition.

REFERENCES

[1] M. G. Fontana, “Corrosion Engineering,” 3rd Edition, Mc -

Graw-Hill Book Company, Boston, 1987, pp. 143-160.

[2] Y. I. Kuznetsov, “Physicochemical Aspects of Metal Cor-

rosion Inhibition in Aqueous Solutions,” Russian Chemi-

cal Reviews, Vol. 73, No. 1, 2004, pp. 75-87.

doi:10.1070/RC2004v073n01ABEH000864

[3] E. E. Ebenso, “Corrosion In hibition Studies of Some Plant

Extracts on Aluminium in Acidic Medium,” Materials

Chemistry and Physics, Vol. 79, No. 1, 2003, pp. 58-62.

doi:10.1016/S0254-0584(02)00446-7

[4] O. O. Adeyemi, “Effect of Temperature and Concentra-

tion on Protective Action of 5 Membered Heterocycles on

Acid Corrosion of Brass,” Journal of Correction Science

and Technology, Vol. 3, No. 1, 2006, pp. 40-45.

[5] E. E. Oguize, B. N. Okolue, C. E. Ogukwe and A. I. Onu-

chukwu, “Studies on the Inhibitive Action of Methylene

Blue Dye on Aluminium Corrosion in KOH Solution,”

Journal of Corrosion Science and Technology, Vol. 1, No.

1, 2004, pp. 88-91.

[6] A. Chetouani, B. Hanmouti and M. Benkaddour, “Corro-

sion Inhibition of Iron in Hydrochloric Acid Solution by

Jojoba Oil,” Pigment and Resin Technology, Vol. 33, No.

1, 2004, pp. 26-31. doi:10.1108/03699420410512077

[7] C. A. Loto and A. I. Muhammed, “The Effect of Cashew

Juice Extract on the Corrosion Inhibition of Mild Steel in

HCl,” Corrosion Prevention and Control, Vol. 47, No. 2,

2005, pp. 13-22.

[8] P. C. Okafor, “Eco Friendly Corrosion Inhibitors: Inhibi-

tive Action of Ethanol extracts of Garcinia Kola for the

Corrosion of Mild Steel in H2SO4 Solution,” Pigment

Resin Technology, Vol. 36, No. 5, 2007, pp. 5-8.

doi:10.1108/03699420710820414

[9] G. D. Davis, J. A. Fraunhofer, L. A. Krebs and C. M. Da-

cres, “The Use of Tobacco Extracts as Corrosion Inhibi-

tor,” CORROSION 2001 Paper No. 58, NACE, Houston,

2001.

[10] C. A. Loto, “The Effect of Bitterleaf Extracts on Corro-

sion of Mild Steel in 0.5 M HCl and H2SO4 Solutions,”

Nigeria Corrosion Journal International, Vol. 1, 1998, pp

19-20.

[11] C. A. Loto, R. T. Loto and A. P. I. Popoola, “Effect of

Neem Leaf (Azadirachita indica) Extract on the Corro-

sion Inhibition of Mild Steel in Dilute Acids,” Interna-

tional Journal of the Physical Sciences, Vol. 6, No. 9,

2011, pp. 2249-2257.

[12] C. A. Loto, “The Effect of Bitterleaf on the Inhibition of

Mild Steel in HCl and H2SO4,” Corrosion Prevention and

Control Journal, Vol. 50, No. 1, 2003, pp. 43-49.

[13] G. O. Avwiri and F. O. Igho, “Inhibitive Action of Verno-

nia Amygdalina on the Corrosion of Aluminium Alloys in

Acidic Media,” Materials Letters, Vol. 57, No. 22-23, 2003,

pp. 3705-3711. doi:10.1016/S0167-577X(03)00167-8

[14] N. E. Idenyi and S. I. Neife, “The Corrosion Behaviour of

Recrystallized Mild Steel in Various Tetrao xosulphate (VI)

Acid Concentration,” Journal of Correction Science and

Technology, Vol. 1, No. 1, 2004, pp. 9-16.

[15] F. A. Ay eni, V. S. Aigbodion and S. A. Yaro, “Non-Toxic

Plant Extract as Corrosion Inhibitor for Chill Cast Al-

Zn-Mg Alloy in Caustic Soda Solution,” Eurasian Chem

Tech Journal, Vol. 9, No. 2, 2007, pp. 91-96.

[16] K. A. Olusegun, N. C. Oforka and E. E. Ebenso, “The

Inhibition of Mild Steel Corrosion in an Acidic Medium

by Fruit Juice of Citrus Paradisi,” Journal of Corrosion

Science and Technology, Vol. 1, No. 2, 2004, pp. 75-78.

[17] E. E. Ebenso, U. J. Ekpe and S. Moren, “Corrosion Inhi-

bition Studies of some Plant Extracts on Aluminium in an

Acidic Medium using Carica Papaya and Azadiraachta

Indica,” Journal of Corrosion Science and Technology,

Vol. 1, No. 1, 2004, pp. 96-101.