Comparative Study of Corrosion Inhibition Efficiency of Naturally Occurring Ecofriendly Varieties of Holy Basil (Tulsi) for Tin in HNO<sub>3</sub> Solution

doi:10.4236/ojmetal.2012.23010

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Open Journal of Metal, 2012, 2, 68-73

http://dx.doi.org/10.4236/ojmetal.2012.23010 Published Online September 2012 (http://www.SciRP.org/journal/ojmetal)

Comparative Study of Corrosion Inhibition Efficiency of

Naturally Occurring Ecofriendly Varieties of Holy Basil

(Tulsi) for Tin in HNO3 Solution

Nutan Kumpawat, Alok Chaturvedi*, Rajesh Kumar Upadhyay

Department of Chemistry, Government College, Ajmer (Rajasthan), India

Email: *[email protected]

Received June 22, 2012; revised July 21, 2012; accepted August 10, 2012

ABSTRACT

Weight loss technique has been used to study the corrosion inhibition efficiency of tin in HNO3 solution by using the

leaves and stem extract of different varieties of Holy Basil viz. Ocimum basilicum (EB), Ocimum cannum (EC) and

Ocimum sanctum (ES). The results show that all the varieties under study are good corrosion inhibitors, among which

leaves extract of EB is the most effective. Corrosion inhibition efficiency increases with increasing concentration of in-

hibitor and it also increases with increasing concentration of HNO3 solution. Inhibition efficiency was found maximum

up to 96.19% for tin with 0.6% leaves extract.

Keywords: Acid Solution; Inhibitors; Tin Metal, Weight Loss; Surface Coverage

1. Introduction

Tin and its alloys are found useful for many engineering

applications because of their lightness and strength,

thermal and electrical conductivity, heat and light reflec-

tivity and hygienic and non-toxic qualities. Tin is a reac-

tive metal according to the electrochemical series (Eo =

−0.14V), but it is non reactive in moisture due to the

formation of a stable oxide film on its surface. Tin is not

attacked by pure water but dissolves in aqueous acids

with the liberation of hydrogen gas. Acids like hydro-

chloric acid, sulphuric acid etc. are used for drilling op-

eration, pickling and descaling. Many workers [1-4] have

studied corrosion of tin in HNO3 solution.

Holy basil is a very common plant in India. It is anti-

bacterial, anti-fungal and is used as an air purifier and

anti-malarial from ancient times in Indian homes. Pow-

der of its stem and leaves is used as medicine in balanc-

ing blood glucose management, to maintain a healthy

digestive system, to encourage the efficient use of oxy-

gen, to enhance the efficacy of many therapeutic treat-

ments etc.

The importance of the study lies in the fact that natural

plant products are non-polluting, ecofriendly, economic,

less toxic and easily available than synthetic organic

compounds. They are biodegradable and so can be used

without any side adverse effects.

The chemical composition of Ocimum sanctum is highly

complex, containing many vitamins like A and C, calcium,

zinc, iron, chlorophyll along with many other phytonutri-

ents which are present in the extract of Ocimum sanctum.

The major chemical constituents responsible for phys-

ico-chemical action of Ocimum sanctum are volatile oil

(0.1% to 0.9% ), eugenol (60% - 70%), cavacrol (about

3.0%), eugenol methyl ether (20%) and other minor

chemical constituents of Ocimum sanctum are like alka-

loids, glycoside, saponin, tannin, maleic acid, ursolic

acid, citric acid and tartaric acid.

Eugenol

*Corresponding author. Ursolic Acid

N. KUMPAWAT ET AL. 69

β-bisabolene (13% - 20%), methyl chavicol (3% -

19%), 1-8 cineole (9% - 33%), α-bisabolene (4% - 7%),

α-terpineol (1.7% - 7%), campestrol, cholesterol, stigma

sterol, β-sisterol and methyl ester of common fatty acid

were the main constituents of the oil which are found in

there species.

Generally, the organic compounds containing hetero

atoms like nitrogen, oxygen and sulphur etc. have been

found to be very effective corrosion inhibitors [5-7]. The

efficiency of these compounds depends upon the electron

density of hetero atoms. The inhibition efficiency also

depends upon the number of adsorption active centers in

the molecule, their charge density, molecular size and

mode of adsorption and formation of metallic complexes.

Atoms such as nitrogen, oxygen and sulfur are capable of

forming coordinate covalent bond with metal owing to

their free electron pairs. Compounds with  bonds like

aldehydes, ketones, imines also generally exhibit good

inhibitive properties due to interaction of  orbital with

metal surface.

In addition to the heterogeneous organic compounds

like Schiff’s bases, Mannic bases etc. which are synthe-

sized in laboratory assist in inhibition, there are also

some naturally occurring substances like Ficus virens[8],

Delonix regia [9], Ocimum sanctum [10], Caparis de-

ciduas [11], Sanaevieria trifascinata[12], Phylanthus

amarus [13], Prosopis julifforar [14], Argemone maxi-

cana [15] etc. have also been evaluated as effective cor-

rosion inhibitors. The present study deals with the study

of three varieties of Holy basil i.e. Ocimum basilicum,

Ocimum sanctum and Ocimum canum which are most

common as corrosion inhibitors of Al in the most corro-

sive medium of HCl solution.

2. Experimental

The rectangular specimens of tin of dimensions 2.0 cm ×

2.0 cm × 0.014 cm containing a small hole of about 2

mm diameter near the upper edge were cut from a large

sheet of pure tin. The solutions of HNO3 acid were pre-

pared using double distilled water. All chemical used

were of analytical reagent grade. Different inhibitor solu-

tions were prepared in absolute ethanol. The extracts of

leaves and stem of three varieties were obtained by re-

fluxing the dried leaves and stem in a soxhlet using

ethanol as solvent for sufficient time.

Each specimen was suspended with a V-shaped glass

hook made of fine capillary and plunged into a beaker

containing 50 ml of the test solution (HNO3 acid) at

room temperature. After sufficient exposure, the test

specimens were taken out, washed with running water

and dried with hot air dryer. Experiments were repeated

in each case and the mean value of the weight loss was

calculated. The percentage inhibition efficiency was cal-

culated using the following formula [16].

% 100







 (1)

where Wu and Wi are the weight loss of the metal in

uninhibited acid and in inhibited solution respectively.

The corrosion rate (CR) in mm/y can be calculated by the

following equation [17].



87.6

Corrosion rate mmyu



 (2)

where, W is weight loss in mg, A is area of specimen in

cm2, T is time of exposure in hours and d is density of

metal in g/cm3

The degree of surface coverage



by inhibitor can be

calculated as







(3)

where Wu and Wi are the weight loss of the metal in

uninhibited acid and in inhibited solution, respectively.

3. Results and Discussion

Weight loss, percentage inhibition efficiency, corrosion

rate and surface coverage in 3.0 M HNO3 solution with

different inhibitors of leaves extract are given in Table 1.

It can be seen from the table that the inhibition efficiency

of the inhibitor increases with increasing concentration of

inhibitor. The maximum inhibition efficiency (96.19%)

was obtained for Ocimum basilicum (EB) at an inhibitor

concentration of 0.6% in 3.0 M HNO3 solution for leaves

extract whereas it was 72.98% in 3.0 M HNO3 solution

with same concentration i.e. 0.6% for stem extract as

shown in Table 2. The results show that there is more

inhibition efficiency of Ocimum basilicum than Ocimum

canum and Ocimum sanctum in HNO3 solution. The

variation of percentage inhibition efficiency (



%) with

inhibitor concentration is depicted graphically in Figure

1 for leaves extract and in Figure 2 for stem extract in

3.0 M HNO3 solution. Variation of percentage inhibition

efficiency (



%) with the concentration of inhibitor indi-

cate that the inhibition efficiency increases with increas-

ing inhibitor concentration. From Table 1 it is clear that

the surface coverage (



) increases with increasing con-

centration of inhibitor.

Adsorption plays an important role in the inhibition of

metallic corrosion by organic inhibitors. Many investi-

gators have used the Langmuir adsorption isotherm to

study inhibitor characteristics [18,19]. Assuming that the

inhibitors adsorbed on the metal surface decrease the

surface area available for cathodic and anodic reaction to

take place. Hoar and Holliday [18] have shown that the

Langmuir isotherm,









log1log log2.303

CQ RT



  (4)

N. KUMPAWAT ET AL.

should give a straight line of unit gradient for the plot of



log 1











versus log C, where A is a temperature

independent constant, C is the bulk concentration of the

inhibitor (percentage) and Q is the heat evolved during

adsorption.

The corresponding plots, shown in Figures 3 and 4 for

3.0 M HNO3 for leaves and stem extract are linear but the

gradients are not equal to unity as would be expected for

Table 1. Weight loss data (W) and percentage inhibition efficiency (



%) for Tin in 3.0 M HNO3 solution with given inhibitor

additions of leaves extract.

Area of specimen = 0.056 cm2

Temperature: 25C0.1 C Exposure time: 7 mins

Inhibition conc. (%) W (mg) I.E. (



%) Surface coverage (



)Corrosion rate (mm/yr)





log 1





Uninhibited 210 3153.60

Ocimum basilicum (EB)

0.1 29 86.19 0.8619 435.50 0.7952

0.2 25 88.09 0.8809 375.43 0.8690

0.4 17 91.09 0.9109 255.29 1.0095

0.6 8 96.19 0.9619 120.14 1.4022

Ocimum cannum (EC)

0.1 33 84.28 0.8428 495.57 0.7292

0.2 28 86.66 0.8666 420.48 0.8126

0.4 23 89.04 0.8904 345.39 0.9097

0.6 16 92.38 0.9238 240.27 1.0836

Ocimum sanctum (ES)

0.1 36 82.85 0.8285 540.62 0.6840

0.2 32 84.76 0.8476 480.55 0.7452

0.4 29 86.19 0.8619 435.50 0.7952

0.6 21 90.00 0.9000 315.36 0.9542

Table 2. Weight loss data (W) and percentage inhibition efficiency (



%) for Tin in 3.0N HNO3 solution with given inhibitor

additions of stem extract.

Area of specimen = 0.056 cm2

Temperature: 273 ± 0.1 K Exposure time: 7 min

Inhibitor conc.(%) W (mg) I.E. (



%) Surface coverage (



)Corrosion rate (mm/yr)





log 1





Uninhibited 285 4279.88

Ocimum basilicum (EB)

0.1 114 60.00 0.6000 1711.95 0.1760

0.2 99 65.26 0.6526 1486.69 0.2738

0.4 86 69.82 0.6982 1291.47 0.3642

0.6 77 72.98 0.7298 1156.32 0.4315

Ocimum cannum (EC)

0.1 120 57.89 0.5789 1802.05 0.1382

0.2 108 62.10 0.6210 1621.85 0.2144

0.4 98 65.16 0.6516 1471.68 0.2719

0.6 85 70.17 0.7017 1276.45 0.3714

Ocimum sanctum (ES)

0.1 125 56.14 0.5614 1877.14 0.1072

0.2 112 60.70 0.6070 1681.92 0.1887

0.4 106 62.10 0.6210 1591.81 0.2144

0.6 92 67.71 0.6771 1381.57 0.3215

N. KUMPAWAT ET AL. 71

Figure 1. Variation of inhibition efficiency with concentration of leaves extract for Tin in 3.0 M HNO3.

Figure 2. Variation of inhibition efficiency with concentration of stem extract for Tin in 3.0 M HNO3.

Figure 3. Langmuir adsorption isotherm for Tin in 3.0 M HNO3 with inhibitor concentration for leaves extract.

N. KUMPAWAT ET AL.

Figure 4. Langmuir adsorption isotherm for Tin in 3.0 M HNO3 with inhibitor concentraction for stem extract.

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mutual repulsion or attraction. Thus, it is also possible

for inhibitor molecule those are adsorbed on anodic and

cathodic sites to interact with metallic surface as well as

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