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Inhibitory action ofPhyllanthus amarus extracts on the corrosion

of mild steel in acidic media

P.C. Okafor a,*, M.E. Ikpi a, I.E. Uwah a, E.E. Ebenso a,b, U.J. Ekpe a, S.A. Umoren c

a Department of Pure and Applied Chemistry, University of Calabar, PMB 1115, Calabar, Nigeriab Department of Chemistry and Chemical Technology, National University of Lesotho, P.O. Roma 180, Lesotho, South Africac Department of Chemistry, University of Uyo, PMB 1017, Uyo, Nigeria

a r t i c l e i n f o

Article history:

Received 30 January 2008

Accepted 15 May 2008

Available online 23 May 2008

Keywords:

Mild steel

Corrosion inhibition

Phyllanthus amarus

a b s t r a c t

The inhibitive action of leaves (LV), seeds (SD) and a combination of leaves and seeds (LVSD) extracts of

Phyllanthus amaruson mild steel corrosion in HCl and H2SO4solutions was studied using weight loss and

gasometric techniques. The results indicate that the extracts functioned as a good inhibitor in both envi-

ronments and inhibition efficiency increased with extracts concentration. Temperature studies revealed

an increase in inhibition efficiency with rise in temperature and activation energies decreased in the

presence of the extract. A mechanism of chemical adsorption of the plants components on the surface

of the metal is proposed for the inhibition behaviour. The adsorption characteristics of the inhibitor were

approximated by Temkin isotherm.

2008 Elsevier Ltd. All rights reserved.

1. Introduction

Some investigations have in recent times been made into the

corrosion inhibiting properties of natural products of plant origin,

and have been found to generally exhibit good inhibition efficien-

cies [115]. The significance of this area of research is primarily

due to the fact that natural products are environmentally friendly

and ecologically acceptable. The yield of these natural products as

well as the corrosion inhibition abilities of the plant extracts vary

widely depending on the part of the plant [13,15,16]and its loca-

tion[17]. Of importance also is the specificity of corrosion inhibit-

ing compounds. One compound effective in a certain medium with

a given metal may be ineffective for the same metal in another

medium[11].

Nevertheless, the known hazardous effects of most synthetic or-

ganic inhibitors and the need to developcheap, non-toxic and envi-

ronmentally benign processes have now made researchers to focus

on the use of natural products. These natural organic compounds

are either synthesized or extracted from aromatic herbs, spices

and medicinal plants. The use of natural products as corrosion

inhibitors have been widely reported by several authors. Saleh

et al. [18]reported that Opuntia extract, Aloe Vera leaves, orange

and mango peels give adequate protection to steel in 5% and 10%

HCl at 25 and 40 C. Srivatsava[19]found that tobacco, black pep-

per, castor oil seeds, acacia gum and lignin can be good inhibitors

for steel in acid medium. In fact, the first patented corrosion inhib-itors used were either natural product such as flour, yeast etc.,[20]

or by products of food industries for restraining iron corrosion in

acid media [21]. Cabrera et al. found that molasses treated in alkali

solution inhibit the corrosion of steel in HCl used in acid cleaning

[22]. Srivatsava and Sanyal studied the performance of caffeine

[22]and nicotine [23]in the inhibition of steel corrosion in neutral

media. Khamis et al.[24]has proved the use of herbs (such as cori-

ander, hibiscus, anis, black cumin and garden cress) as new type of

green inhibitors for acidic corrosion of steel. El-Etre[25]has stud-

ied the application of natural honey as corrosion inhibitor for cop-

per in aqueous solution. Similar study has also been conducted on

carbon steel[26]. Parikh et al.[27]studied the anticorrosion activ-

ity of onion, garlic and bitter gourd for mild steel in HCl media. Eth-

anolic extract of Ricimus communis leaves was studied for the

corrosion inhibition of mild steel in acid media by Sathyanathan

et al. [28]. Aqueous extract of Hibiscus flower andAgaricus has

been studied as corrosion inhibitors for industrial cooling system

by Minhaj et al. [29].The application of extracts of henna, thyme,

bgugaine and inriine was investigated for their anticorrosion activ-

ity[3033]. The effect of addition of bgugaine on steel corrosion in

HCl is patented [34]. Saleh et al. studied the peel ofpomegranate

[35]and beet root[36]as corrosion inhibitor for mild steel in acid

media. Sanghvi et al. have investigated the anticorrosion activity of

Embilica officianilis, Terminalia chebula, Terminalia belivia [37], Sap-

indus trifolianus and Accacia conicianna [38]. Corrosion inhibition

has also been studied for the extracts ofSwertia angustifolia [39],

Eucalyptus leaves [40], Eugenia jambolans [41], Pongamia glabra,

0010-938X/$ - see front matter 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.corsci.2008.05.009

* Corresponding author. Tel./fax: +234 803 429 5604.

E-mail address: [email protected](P.C. Okafor).

Corrosion Science 50 (2008) 23102317

Contents lists available at ScienceDirect

Corrosion Science

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Annona squamosa[42],Accacia Arabica[43] and Vernonia amydalina

[44] for steel in acid media. The anticorrosion effect ofAndrographis

paniculata[45]and tea wastes[46,47]have been reported too. Kli-

skic et al. analyzed aqueous extract ofRosmarinus officinalis[48]as

corrosion inhibitor for aluminium alloy corrosion in chloride solu-

tion. Guar gum was analyzed for its anticorrosion activity by

Abdallah et al.[49]. Martinez and Stern have studied the inhibitory

mechanism of low carbon steel corrosion of Mimosa tannin in

H2SO4 media [50]. Oguzie investigated the efficiency ofTelfaria

occidentalisextract as corrosion inhibitor in both HCl and H2SO4media[51]. The extracts ofChamomile, Halfabar, Black cumin and

kidney bean were analyzed for their inhibitive action of corrosion

of steel in acid media by Abdel-Gaber et al. [52]. El-Hosary et al.

[53] studied the corrosion inhibition of aluminium and zinc in

HCl usingHibiscus subdariffaextract. The inhibition effect ofZenth-

oxylum alatum extract on the corrosion of mild steel in aqueous

orthophosphonic acid was investigated by Gunasekaran et al.

[54].Nypa fructicans wurmb[55]leaves were studied for the corro-

sion inhibition of mild steel in HCl media. Muller [56]investigated

the effect of saccharides [reducing sugarsfructose and mannose]

on the corrosion of aluminium and zinc in alkaline media. Hammo-

uti et al. studied the extracts of Ginger [57], jojoba oil [57], eugenol,

acetyl-eugenol [58], artemisia oil [59,60] and Mentha pulegium

[61,62] for corrosion inhibition of steel in acid media.. El-Etre

et al. investigated Khillah extract[63]for the corrosion inhibition

of SX 316 steel in acid media, Lawsonia extract [64]was studied

for its effect against acid induced corrosion of metals, Opuntiaex-

tract[65]was investigated for the corrosion of aluminium in acid

medium and vanillin [66] for the corrosion of mild steel in acid

media. Berberine, an alkaloid isolated from Captis was studied for

its anticorrosion effect for mild steel corrosion in H2SO4 medium

[67]by Yan Li et al. Zucchi and Omar [68]have found thatPapaia,

Poinciana pulcherrima, Cassia occidentalis andDatura stramonmium

seeds, Calotropis procera, Azydracta indica andAuforpio turkiale sap

are useful as acid corrosion inhibitors. Sethuraman et al. [69]have

studied the acid extract ofDatura metel as corrosion inhibitor for

mild steel in acid medium. Quinine [70] has been studied for itsanticorrosive effect of carbon steel in 1 M HCl by Mohamed Ismail

Awad. Anthony et al. has studied the effect of caffeine against chlo-

ride corrosion of carbon steel [71]. An elaborate review has also

been reported on natural products as corrosion inhibitors for met-

als in corrosive media [72].The corrosion inhibition activity in

many of these plant extracts could be due to the presence of het-

erocyclic constituents like alkaloids, flavonoids etc., Even the pres-

ence of tannins, cellulose and polycyclic compounds normally

enhances the film formation over the metal surface, thus aiding

corrosion. A series of other reports have been highlighted in our

laboratories on studies of other natural products (exudate gums)

as corrosion inhibitors of mild steel and aluminium in acidic and

basic media[7378]. In our continuous quest to explore more nat-

ural products of plant origin as corrosion inhibitors, the presentstudy is onP. amarus.

P. amarus is a plant that belongs to the family Euphorbiraceae

and is widely distributed in tropical and subtropical countries

and has long been used in folk medicine. It is the most prominent

and widely used species of the genus Phyllanthus[79]. It was first

identified in central and southern India in the 18th century but is

now found in many countries including Philippines, Cuba, and

Nigeria etc. It is commonly called carry me seed, stone breaker,

wind breaker, gulf leaf flower or gala of wind. P. amarus is an

erect annual herb of not more than one and half feet tall and has

small leaves and yellow flowers. In folk medicine, it has allegedly

been used to treat jaundice, diabetes, gonorrhoea, irregular men-

struation etc. In Nigeria, the plant grows as weeds and every year

large quantities of the plant are weeded and wasted. The extractsof this plant, which contains numerous naturally environmental

organic compounds, may be utilized as eco-friendly corrosion

inhibitors.

As a contribution to the current interest on environmentally

friendly corrosion inhibitors, the present study investigates the

inhibiting effect of extracts from the leaves (LV), seeds (SD) and a

combination of leaves and seeds (LVSD) ofP. amaruson mild steel

corrosion in acidic solutions using the weight loss and gasometric

techniques.

2. Experimental methods

The mild steel sheets used for this study were obtained from

Ejison Resources (Nigeria) Ltd and of the same composition as

those reported previously [1315]. The test coupons were pre-

pared, degreased and cleaned as previously described [1315].

All chemicals used were of Analar grade.

2.1. Preparation of plant extracts

P. amarus leaves and seeds were collected from plants around

the University of Calabar campus, Nigeria. These were dried in an

N53C-Genlab Laboratory oven at 50 C, and ground to powder form.

Four gram of the powder was digested in 1 l of 2 M and 5 M H2SO4solutions (for weight loss and gasometric measurements, respec-

tively). The resultant solution was kept for 24 h, filtered and stored.

From the stock solution (4 g/l), plant extracts test solutions were

prepared at concentrations of 0.1, 0.2, 0.5, 1.0 and 2.0 g/l.

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6

Time/(days)

Weightloss/(g)

b

0

1

2

3

4

5

6

7

8

9

10

Weightloss/(g)

Blank

0.1 g/l

0.2 g/l

0.5 g/l

1.0 g/l

2.0 g/l

4.0 g/l

a

Fig. 1. Variation of weight loss with time for mild steel coupons (of cross sectional

area of 20 cm2) in 2 M: (a) HCl, and (b) H2SO4 solutions containing Phyllanthusamarus leaves (LV).

P.C. Okafor et al. / Corrosion Science 50 (2008) 23102317 2311


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2.2. Gravimetric and gasometric experiments

The gravimetric (weight loss) and the gasometric technique (via

the gasometric assembly) were determined as previously described

[80,81]. However, experiments were conducted at 30 C for weight

loss and 30 and 40 C for hydrogen evolution measurements.

3. Results and discussion

3.1. Gravimetric results

The weight losses (gravimetric measurements) for the mild

steel in 2 M HCl and 2 M H2SO4containing different concentrations

of the leaves (LV) ofP. amarusextracts (PAE) as a function of time

are presented inFig. 1a and b, respectively. The results show that

weight losses increase with increase in time but decrease with in-

crease in concentration of PAE. Similar trends were obtained for

the seeds (SD) and mixture (LVSD) extracts of the plant. The de-

crease is due to the inhibitive effects of PAE and these effects in-

crease with increase in PAE concentration. From the weight loss

data, the corrosion rates (CR) were calculated from

CRWL

At 1

where WL is weight loss in mg, A is the specimen surface area (of

20cm2) and t, the immersion period in hours (120 h). The results

obtained are presented in Table 1 and show that the corrosion rates

decreased with the increase in PAE concentration indicating that

PAE inhibits the corrosion of mild steel in both 2 M HCl and 2 M

H2SO4. It is also observed from Fig. 1andTable 1that the corrosion

rates in HCl solutions is lower than that in H2SO4and that the cor-

rosion rates at all concentration of PAE used followed the trend:

LV< LVSD < SD.

From the weight loss data obtained, the inhibition efficiencies

(%I) for the corrosion of mild steel in 2 M HCl and 2 M H2SO4con-

taining different concentration of PAE were calculated using

%I CRblank CRinh

CRblank

100 2

where CRblank and CRinh are the corrosion rate in the absence and

presence of the PAE, respectively. The results obtained are shown

in Table 1 and indicate that PAE shows a significant inhibitive effect

on mild steel in HCl and H2SO4solutions.Fig. 2shows the variation

of inhibition efficiency with extracts concentration for mild steel in

2 M HCl and 2 M H2SO4solutions containing PAE and indicates that

the inhibition efficiencies increase with increase in PAE concentra-

tion. Comparing the inhibition efficiencies of the different parts of

PAE shows that the efficiencies followed the trend: LV < LVSD < SD.

A plot of the logarithm of the measured weight (in g) of the mild

steel after post treatment (Wf) against time (t) helps to explain the

kinetics of the corrosion of mild steel in the acid media in the ab-

sence and presence of PAE. Linear plots were obtained (Fig. 3for

H2SO4), which reveal first order kinetics. The values of the rate

Table 1

Calculated values of corrosion rate, inhibition efficiency, rate constant and half-life for mild steel coupons (of cross sectional area of 20 cm 2) in 2 M H2SO4and HCl solutions

containing extracts fromPhyllanthus amarus (using the weight loss technique)

Plants part System Corrosion rate (mg cm2 hr1) Inhibition efficiency (%) Rate constant 101 (day1) Half-life (days)

Blank 2 M H2SO4 3.59 2.90 2.4

LV 2 M H2SO4+ 0.2 g/l extract 2.87 20.0 1.99 3.5

2 M H2SO4+ 0.5 g/l extract 2.14 40.4 1.25 5.6

2 M H2SO4+ 1.0 g/l extract 1.70 52.6 0.94 7.4

2 M H2SO4+ 2.0 g/l extract 1.10 69.3 0.55 12.7

2 M H2SO4+ 4.0 g/l extract 0.41 88.6 0.16 42.4

SD 2 M H2SO4+ 0.2 g/l extract 3.56 0.7 2.92 2.4

2 M H2SO4+ 0.5 g/l extract 3.25 9.5 2.45 2.9

2 M H2SO4+ 1.0 g/l extract 2.83 21.0 1.98 3.5

2 M H2SO4+ 2.0 g/l extract 2.00 44.1 1.17 5.9

2 M H2SO4+ 4.0 g/l extract 1.27 64.6 0.70 9.9

LVSD 2 M H2SO4+ 0.1 g/l extract 4.09 14.0 3.86 1.8

2 M H2SO4+ 0.2 g/l extract 3.79 5.6 3.14 2.2

2 M H2SO4+ 0.5 g/l extract 3.04 15.3 1.79 3.9

2 M H2SO4+ 1.0 g/l extract 2.56 28.7 1.51 4.6

2 M H2SO4+ 2.0 g/l extract 1.68 53.3 0.92 7.6

2 M H2SO4+ 4.0 g/l extract 0.86 76.1 0.49 14.3

Blank 2 M HCl 1.34 0.64 10.9

LV 2 M HCl + 0.1 g/l extract 0.18 86.3 0.07 94.02 M HCl + 0.2 g/l extract 0.13 90.0 0.05 130.8

2 M HCl + 0.5 g/l extract 0.12 91.0 0.05 150.5

2 M HCl + 1.0 g/l extract 0.10 92.5 0.03 214.9

2 M HCl + 2.0 g/l extract 0.10 92.8 0.03 214.9

2 M HCl + 4.0 g/l extract 0.08 94.1 0.03 231.5

SD 2 M HCl + 0.1 g/l extract 0.52 61.1 0.28 24.9

2 M HCl + 0.2 g/l extract 0.26 68.7 0.19 36.3

2 M HCl + 0.5 g/l extract 0.19 73.0 0.17 41.2

2 M HCl + 1.0 g/l extract 0.32 76.3 0.16 43.0

2 M HCl + 2.0 g/l extract 0.27 79.8 0.12 56.8

2 M HCl + 4.0 g/l extract 0.27 80.1 0.11 62.7

LVSD 2 M HCl + 0.1 g/l extract 0.37 72.6 0.11 64.0

2 M HCl + 0.2 g/l extract 0.16 88.2 0.06 111.5

2 M HCl + 0.5 g/l extract 0.11 91.9 0.04 158.4

2 M HCl + 1.0 g/l extract 0.08 93.8 0.03 231.5

2 M HCl + 2.0 g/l extract 0.11 91.9 0.03 250.8

2 M HCl + 4.0 g/l extract 0.17 87.2 0.03 273.6

2312 P.C. Okafor et al. / Corrosion Science 50 (2008) 23102317


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constants, k, obtained from the slope inFig. 3are presented inTa-

ble 1. The results obtained reveal that the rate constant decreases

with increase in PAE concentration. The values of half-life, t1/2,were calculated using the equation:

t1=2 0:693

k 3

where k is the rate constant. t1/2, values are presented in Table 1 and

were observed to increase with increase in concentration of PAE.

3.2. Gasometric results

The acidic corrosion of mild steel is characterised by evolutionof hydrogen and the rate of corrosion is proportional to the amount

of hydrogen evolved [11]. The volume of hydrogen evolved, VH,

during the corrosion of mild steel in 5 M HCl and H2SO4solutions

in the absence and presence of PAE at 30 and 40 C was measured

as a function of time. The results obtained at 30 C are as depicted

inFigs. 4 and 5for HCl and H2SO4 solutions, respectively. Similar

results were obtained at 40 C. The presence of leaves (LV) and

seeds (SD) of PAE decreases the volume of hydrogen evolved com-

pared to the blank. From the volume of hydrogen evolved during

the corrosion reaction, the corrosion rate (CRH) was determined

from

CRHVt Vitt ti

4

where Vt and Vi are the volumes of hydrogen evolved at time ttand ti, respectively. The results obtained are presented in Table

2. The results show that the rate decreased with the increase in

PAE concentration and increase in temperature for all systems.

From the corrosion rate (deduced from the hydrogen evolved),

the inhibition efficiency was determined using Eq. (2). The results

-20

0

20

40

60

80

100

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Extract concentration/(g/l)

Inhibition

efficiency/(%)

LV (HCl)

SD (HCl)

LVSD (HCl)

LV (H2SO4)

(H2SO4)

(H2SO4)

SD

LVSD

Fig. 2. Variation of inhibition efficiency with extract concentration for mild steel

coupons (of cross sectional area of 20 cm2) in 2 M HCl and 2 M H2SO4 solutions

containing the different parts ofPhyllanthus amarus.

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

LogWf

Blank

0.2 g/l

0.5 g/l

1.0 g/l

2.0 g/l

4.0 g/l

a

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

0 1 2 3 4 5 6

Time/(days)

LogWf

Blank

0.2 g/l

0.5 g/l

1.0 g/l

2.0 g/l

4.0 g/l

b

Fig. 3. Plot of log Wfagainst time for mild steel coupons (of cross sectional area of

20cm2) i n 2 M H2SO4 solutions containing: (a) leaves (LV), and (b) seeds (SD)

extracts ofPhyllanthus amarus.

0

5

10

15

20

25

VH/(cm

3)

VH/(cm

3)

Blank

4.0 g/l

2.0 g/l

1.0 g/l

0.5 g/l

0.2 g/l

0.1 g/l

a

0

5

10

15

20

25

0 10 20 30 40 50 60 70

Time/(min.)

b

Fig. 4. Variation of volume of hydrogen evolved (VH) with time for mild steel

coupons (of cross sectional area of 10 cm2) in 5 M HCl solutions containing: (a)leaves (LV), and (b) seeds (SD) extracts ofPhyllanthus amarusat 30 C.



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obtained are as also shown in Table 2and depicted inFig. 6a and b

for corrosion in HCl solutions. It is observed that inhibition effi-

ciency increases with increase in the PAE concentration and with

increase in temperature. This suggests that the phytochemical

components of PAE are adsorbed on the mild steel solution

interface. The trend in temperature suggests chemical adsorption

of the components of the PAE. A decrease in inhibition efficiency

with increasing temperature suggests physical adsorption. It is

also quite clear that from what is known about dependence of

adsorption as temperature decreases, that the quantity of equilib-

rium of adsorption increases and as a result, the plot of higher

temperature is above the lower one [82]. This is clearly demon-

strated in Fig. 6. Comparing the maximum inhibition efficiencies

of PAE (Fig. 7) shows that the efficiencies followed the trend:

LV > LVSD > SD in all systems. Similar trend were observed in

2 M H2SO4 from the weight loss measurements.

3.3. Inhibition mechanism

The observed corrosion inhibition of mild steel in H2SO4 solu-

tion with increase in PAE concentration can be explained by the

adsorption of the components of the PAE on the metal surface. In

order to predict the type of adsorption, the corrosion mechanism

of the iron must be known. According to the mechanism for the

anodic dissolution of Fe in acidic sulphate solutions proposed ini-

tially by Bockris et al.[83], Fe electro-dissolution in acidic sulphate

solutions depends primarily on the adsorbed intermediate FeOHadsas follows:

Fe OH () FeOHads H e 5a

FeOHads!rdsFeOH e 5b

FeOH H () Fe2 H2O 5c

The cathodic hydrogen evolution follows the steps:

FeH () FeHads 6

FeHads e() FeHads 7

FeHads H e! FeH2 8

The corrosion rate of iron in H2SO4 solutions is controlled by both

hydrogen evolution reaction and dissolution reaction of iron.

Another mechanism, involving two adsorbed intermediates has

been used to account for the retardation of Fe anodic dissolution in

the presence of an inhibitor[84]

FeH2O() Fe H2Oads 9a

Fe H2Oads Y() FeOHads H

Y 9b

Fe H2Oads Y() FeYads H2O 9c

FeOHads!rdsFeOHads e 9d

FeYads() FeYads e 9e

FeOHads FeYads() FeYads FeOH

9f

FeOH H () Fe2 H2O 9g

where Y represents the inhibitor species.

According to the detailed mechanism above, displacement of

some adsorbed water molecules on the metal surface by inhibitor

species to yield the adsorbed intermediate FeYads (Eq. (9c)) re-

duces the amount of the species FeOHads available for the rate

determining step. Such adsorbed intermediate could, depending

on its relative solubility, either inhibit or catalyse further metaldissolution. PAE are viewed as an incredible rich source of natu-

rally synthesized chemical compounds. These large numbers of

different chemical compounds may form adsorbed intermediates

(organo-metallic complexes) such as FePAE [5, 52 and 54]

which may either inhibit or catalyse further metal dissolution.

From the observed results it can be inferred that the insoluble

FePAE complexes dominates the adsorbed intermediates and

thus the resultant inhibitive effects. This conclusion is in line

with those of Jaen et al. [85].

PAE is composed of numerous naturally occurring organic com-

pounds[18]. This complex composition makes it rather difficult to

attempt to assign the observed corrosion inhibitive effect to a par-

ticular constituent. Studies on the phytochemical constituents of

PAE shows that it contains saponins (24.05%), tannins (17.05%),oxalates (5.47%), alkaloids (2.56%), cyanogenic glycosides (1.46%),

carbohydrates (45.52%), fibre (24.50%), protein (6.10%) and fat

(6.03%) but the percentages are higher in the seeds than in the

leaves especially of fat (24.80%), protein (34.20%), saponins

(38.40%), tannins (29.40%) and alkaloids (19.42%). The high saponin

and tannin, carbohydrate and fibre contents explain its use in folk

medicine for the treatment of liver problems, oedema etc and its

use as corrosion inhibitors[86].However, further investigation to

isolate the active ingredients and test their inhibition ability is

being carried out in our laboratories. Other studies have reported

the presence of phyllanthin and hypophyllanthin in PAE extracts.

The presence of triterpenoids, steroids, alkaloids, sugar, tannins,

glycosides, flavonoids has also been reported in PAE depending

on the type of extraction carried out [87]. Some of these compo-nents especially alkaloids, flavonoids, tannins, fats, proteins were

0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70

Time/(min.)

0

5

10

15

20

25

30

35

40

Blank

4.0 g/l

2.0 g/l

1.0 g/l

0.5 g/l

0.2 g/l

0.1 g/l

VH/(cm

3)

VH/(cm

3)

a

b

Fig. 5. Variation of volume of hydrogen evolved (VH) with time for mild steel

coupons (of cross sectional area of 10 cm2) in 5M H2SO4 solutions containing: (a)leaves (LV), and (b) seeds (SD) extracts ofPhyllanthus amarusat 30 C.

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higher in percentage in the seeds than in the leaves hence the rea-

son why the seeds give rise to higher inhibition but further inves-

tigations to isolate the active ingredients and test their inhibition

ability is going on in our laboratories.

Increase in temperature increased the inhibition efficiencies of

the PAE suggesting chemisorption of PAE components on the sur-

face of the metals[7,65,88]. In order to confirm this, the apparent

activation energies,Ea, for the dissolution of mild steel in HCl and

H2SO4 in the absence and presence of the PAE were calculated from

the condensed Arrhenius equation as follows:

logR2

R1

Ea

2:303R

1

T1 1

T2

10

whereR1 andR2 are the corrosion rates at temperatures T1andT2,

respectively. The calculated activated energy values are as listed

in Table 2. The results indicate that Ea in the presence of the PAE de-

creases. This behaviour is an indication of chemical adsorption of

the components of the PAE on the surface of the metal.

An estimate of the heat of adsorption (Qad) was obtained from

the trend of surface coverage with temperature as follows:

Qad 2:303R log

h2

1 h2

log

h1

1 h2

x

T1T2

T2 T1 11

where h1and h2 are the degrees of surface coverage at temperatures

T1andT2respectively. The calculated values are presented in Table

2. The positive values of the heat of adsorption are consistent with

the phenomenon of inhibitor chemical adsorption[7].

The results obtained from this study have clearly showed that

the inhibition efficiency increases with extracts concentration

and with temperature which indicated that the inhibition mecha-

nism is due to chemical adsorption of the molecular components

of PAE on the surface of the metal. The experimental data were ap-

plied to different adsorption isotherm equations. It was found thatthe experimental data fitted the Temkin isotherm (Fig. 8) which

may be formulated as:

exp2ah Kc 12

where h is the surface coverage, cthe extracts concentration, Kthe

adsorption coefficient, which represents the adsorption-desorption

equilibrium constant, anda is an interaction parameter. Calculated

values ofKanda are as shown inTable 3. Positive values ofa indi-

cates attraction forces between adsorbed molecules while negative

values indicate repulsive forces between the adsorbed molecules

[89]. It is seen in the table that the values ofa in all cases are neg-

ative indicating that repulsion exists in the adsorption layer[90]. It

is generally known that Kdenotes the strength between the adsor-

bate and adsorbent. Large values ofKimply more efficient adsorp-

Table 2

Calculated values of corrosion rate, inhibition efficiency, activation energy and heat of adsorption for mild steel coupons (of cross sectional area of 10 cm 2) in the acid media

containing extracts from Phyllanthus amarus (using the gasometric technique)

Plants part System Corrosion rate (cm3 min1) Inhibition efficiency (%) Activation energy (KJ mol1) Heat of adsorption (KJ mol1)

30 C 40 C 30 C 40 C

5 M H2SO4 0.39 1.01 61.05

LV 5 M H2SO4+ 0.1 g/l extract 0.52 0.99 32.8 2.0 41.48

5 M H2SO4+ 0.2 g/l extract 0.49 0.92 23.7 8.9 41.38 5 M H2SO4+ 0.5 g/l extract 0.39 0.56 2.0 44.5 24.48 102.14

5 M H2SO4+ 1.0 g/l extract 0.31 0.39 22.4 61.5 15.84 47.91

5 M H2SO4+ 2.0 g/l extract 0.14 0.21 65.1 78.9 28.72 19.39

5 M H2SO4+ 4.0 g/l extract 0.07 0.10 81.4 90.5 17.64 21.81

SD 5 M H2SO4+ 0.1 g/l extract 0.47 1.03 18.6 1.7 51.15

5 M H2SO4+ 0.2 g/l extract 0.46 1.02 16.8 0.8 51.56

5 M H2SO4+ 0.5 g/l extract 0.44 1.00 12.9 1.4 52.30

5 M H2SO4+ 1.0 g/l extract 0.43 0.97 8.1 4.6 52.95

5 M H2SO4+ 2.0 g/l extract 0.39 0.90 1.0 11.0 54.24 69.39

5 M H2SO4+ 4.0 g/l extract 0.30 0.73 24.4 28.0 57.92 5.18

LVSD 5 M H2SO4+ 0.1 g/l extract 0.80 2.13 103.6 109.8 63.01

5 M H2SO4+ 0.2 g/l extract 0.71 1.68 80.7 66.0 55.59

5 M H2SO4+ 0.5 g/l extract 0.66 1.20 69.1 18.3 38.07

5 M H2SO4+ 1.0 g/l extract 0.50 0.99 26.8 2.3 44.29

5 M H2SO4+ 2.0 g/l extract 0.44 0.73 10.8 27.9 33.32

5 M H2SO4+ 4.0 g/l extract 0.24 0.50 39.6 50.7 5.73 12.56

5 M HCl 0.22 0.97 96.17

LV 5 M HCl + 0.1 g/l extract 0.33 0.71 48.9 27.0 50.25

5 M HCl + 0.2 g/l extract 0.28 0.59 28.1 39.9 47.42

5 M HCl + 0.5 g/l extract 0.14 0.47 35.7 51.4 78.12 18.04

5 M HCl + 1.0 g/l extract 0.10 0.37 54.3 62.5 83.37 9.51

5 M HCl + 2.0 g/l extract 0.10 0.28 55.0 71.8 66.14 20.47

5 M HCl + 4.0 g/l extract 0.09 0.24 58.9 74.9 64.39 20.52

SD 5 M HCl + 0.1 g/l extract 0.36 0.80 65.1 17.9 51.15

5 M HCl + 0.2 g/l extract 0.30 0.67 35.7 31.6 52.02

5 M HCl + 0.5 g/l extract 0.22 0.57 0.8 41.4 62.25 125.89

5 M HCl + 1.0 g/l extract 0.19 0.50 12.4 49.0 61.37 53.52

5 M HCl + 2.0 g/l extract 0.17 0.40 21.0 59.1 53.78 47.40

5 M HCl + 4.0 g/l extract 0.15 0.33 29.5 66.1 49.01 43.07

LVSD 5 M HCl + 0.1 g/l extract 0.24 0.40 9.3 59.1 32.89

5 M HCl + 0.2 g/l extract 0.23 0.41 3.9 58.2 37.54

5 M HCl + 0.5 g/l extract 0.22 0.36 2.3 62.9 33.81 119.775 M HCl + 1.0 g/l extract 0.18 0.32 17.2 66.7 37.45 63.41

5 M HCl + 2.0 g/l extract 0.13 0.29 41.1 70.0 52.72 33.77

5 M HCl + 4.0 g/l extract 0.11 0.26 48.8 73.6 53.54 29.98



7/8

tion and hence better inhibition efficiency [91].Kvalues are seen to

increase with increase in temperature suggesting that the inhibitors

are chemically adsorbed onto the mild steel surface.

4. Conclusions

1. P. amarus extracts (PAE) acts as inhibitor for mild steel corro-

sion in HCl and H2SO4 solutions and inhibition efficiencies fol-lowed the trend: LV < LVSD < SD.

2. Inhibition efficiency of PAE increases with increase in concen-

tration of PAE and with increase in temperature suggesting

chemical adsorption.

3. The corrosion process is inhibited by adsorption of PAE on the

mild steel surface following the Temkin isotherm.

4. The presence of the extract decreased the corrosion activation

energy in both media and the adsorption heats gave positive

values.

-80

-60

-40

-20

0

20

40

60

80

0 1 2 3 4 5

%I

30 C

40 C

-60

-40

-20

0

20

40

60

80

100

0 1 2 3 4 5

c/(g/l)

c/(g/l)

%I

30 C

40 C

b

a

Fig. 6. Variation of inhibition efficiency with extract concentration for mild steel

coupons (of cross sectional area of 10 cm2) in 5 M HCl solution containing: (a) LV,

and (b) SD extracts fromPhyllanthus amarus.

0

10

20

30

40

50

60

70

80

90

LV LVSD SD

Plant's part

Inhibitionefficiency/(%) HCl

H2SO4

Fig. 7. Maximum inhibition efficiency for mild steel coupons (of cross sectional

area of 10 cm2) in 5 M acid solutions containing 4 g/l PAE at 30 C.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

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

log c

LVSD

LVSD

a

-1.5

-1

-0.5

0

0.5

1

1.5

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

log c

LV

SD

LVSD

b

Fig. 8. Temkin adsorption isotherm plot as hagainst log cfor mild steel coupons (of

cross sectional area of 10 cm2) in 5 M: (a) HCl, and (b) H2SO4 solutions containing

PAE at 40 C.

Table 3

Calculated values of the adsorption-desorption equilibrium constantK and the

interaction parameter a for mild steel coupons (of cross sectional area of 10 cm 2) in

the acid media containing extracts from Phyllanthus amarus (using the gasometric

technique)

Plants part K a

30 C 40 C 30 C 40 C

5 M H2SO4LV 1.55 2.68 0.66 0.84

SD 1.04 1.86 2.10 3.08

LVSD 1.36 2.01 0.59 0.51

5 M HCl

LV 1.61 7.11 0.69 1.63

SD 1.08 5.44 0.86 1.71

LVSD 1.82 5.35 1.30 1.25



8/8

Acknowledgements

The authors acknowledged Mr. Efremfon Edemidiong and

Ochuko Djebah for assistance in performing some measurements.

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