Combating Aluminium Alloy Dissolution by employing Ocimum

Combating Aluminium Alloy Dissolution by employing Ocimum tenuiflorum leaves extract

Alka Sharma, Rekha N Nair, Arpita Sharma, Guddi Choudhury

Department of Chemistry, University of Rajasthan, Jaipur (Rajasthan) INDIA

The inhibition of acid corrosion of Aluminium alloy (AA6063) has been studied using ethanolic

extract of Ocimum tenuiflorum syn. Ocimum sanctum Linn. leaves by chemical and

electrochemical methods. Kinetic and adsorption parameters were evaluated. The results

obtained showed that the extract could serve as an effective corrosion inhibitor for AA6063 in

0.5 N HCl. The inhibition mechanism has been predicted on the basis of adsorption parameters

and was found to physisorbed on the metal surface. The experimental data were fit in to the El-

Awady thermodynamic – kinetic model. The protective film formed on the metal surface was

analyzed by FTIR spectroscopy.

Keywords: Aluminium alloy; Acid inhibition; Kinetic and Adsorption parameters; FTIR

spectroscopy.

INTRODUCTION

Use of inhibitors is one of the most practical methods for protection against corrosion especially

in acid solutions. The hazardous effect of most of the synthetic corrosion inhibitors is the

motivation for the use of natural products. Plant extracts have become important because they

are environmentally acceptable, inexpensive, readily available and renewable sources which are

ecologically acceptable. Combating metal corrosion by “eco-friendly green inhibitors” shows

environmental awareness and energy conservation [1-3]. Plant products are organic in nature and

some of the constituents like tannins, alkaloids, amino acids etc. are known to exhibit inhibiting

action. It can be extracted by simple procedures with low cost. An investigation of anti-corrosive

property of the sacred basil - Tulsi (Ocimum tenuiflorum syn. Ocimum sanctum Linn.) as non-

toxic, green corrosion inhibitor for aluminium and its alloy is a step forward in this direction.

International journal of advanced scientific and technical research Issue 2 volume 6, December 2012

Available online on http://www.rspublication.com/ijst/index.html ISSN 2249-9954

Page 713

Tel: 098290 13969 (M)

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Literature survey reveals that Ocimum tenuiflorum leaves [4-6] are rich in (70%) Eugenol (4200-

4970 ppm) as well as Eugenol-methyl ether (1, 2-dimethoxy-4-prop-2-enylbenzene) (1200-1400

ppm), Ascorbic acid (830 ppm), Chavicol (180-210 ppm), Tannins, β-Carotene (25 ppm) and

most of them also has corrosion inhibiting property. Corrosion inhibition property of extract may

be mainly due to eugenol and partly due the synergistic effect of ascorbic acid, eugenol-methyl

ether, tannin, chavicol etc., as all of these are known to possess corrosion inhibition property [7-

10].

Chemical and electrochemical methods were used to evaluate the protective efficiencies. For

assessing optimum concentration of the inhibitor, different concentrations of the extract were

added in the aggressive test solutions. In order to get more information about the power

performance of the plant extract, the nature of adsorption isotherm and activation processes, the

influence of temperature was also studied. For this purpose, investigations were carried out

gravimetrically in the temperature range of 303–343 ± 1 K. The kinetic, thermodynamic and

adsorption parameters that govern metal corrosion were evaluated. Potentiostatic Polarization

method was employed for electrochemical measurements using a PC controlled AUTOLAB

(AUT302) with FRA2 module. Surface analysis (FTIR – KBr pellet method)) was also carried

out to establish the anti-corrosive property of Ethanolic Extract of Ocimum tenuiflorum Leaves

(EEOtL) in aggressive (HCl) solution.

The results obtained were inspiring and depict that EEOtL may be used as green inhibitor for

acid corrosion of aluminium and its alloys and may also be used to replace conventional organic

inhibitors which are not only expensive but also toxic to living beings.

EXPERIMENTAL

Chemical Method (Weight loss measurements)

The industrially used Aluminium (grade 6063) of 98 % purity (Si-0.2-0.6%; Fe-0.35%; Cu-0.1%;

Mn-0.1%; Mg-0.45-0.9%; Zn-0.1%; Ti-0.1%; Cr-0.1%; Al-balance) was used as test coupons.

The dimensions are 3cm x 2cm x 0.15cm with a hole of about 0.12mm diameter near the upper

edge for the purpose of hanging. The coupons were examined carefully to check for rough edges

and the surface treatment of the coupons was done by polishing with emery papers and washed

with distilled water and finally with acetone and then dried in oven at 60o C for about half an

hour. They were then kept in desiccators until cooled to room temperature and then weighed



Page 714

accurately using a digital balance (Adair Dutt 205 A ACS). They were subjected to further

heating, cooling and weighing till a constant weight was obtained and was stored in desiccators

before use.

The solutions were made of AR grade hydrochloric acid. Appropriate concentrations of acids

were prepared by using triple distilled water. 100mL of electrolyte was used and the

concentration of inhibitor (plant extract) was varied.

For preparing the plant extract, dried and weighed plant leaves were soaked in reagent grade

ethanol for 15 days and refluxed for 24 h. The ethanolic solution was filtered. This extract was

used to study the corrosion inhibition properties. To know the mass of plant extract, it was dried

to remove the ethanol. From the weight of the dried liquid, the concentration of plant extract was

calculated. The mass of EEOtL was also determined and was found 18.04 g/L of plant

compounds.

For the study, 100 ml of the corrosive solution (0.5 N HCl) was measured into 6 different 250 ml

beakers numbering S-0, S-1, S-2, S-3, S-4 and S-5. EEOtL was added in these five beakers (S-1

to S-5) so as to have the concentration as 0.18, 0.36, 0.54, 0.72 and 0.9 g/L respectively. No

extract was added to the 1st beaker (S-0).

ELECTROCHEMICAL METHOD

Potentiostatic Polarization method was employed for electrochemical measurements using a PC

controlled AUTOLAB (AUT302) with FRA2 module. Prior to each experiment, the surface pre-

treatment of the working electrode mentioned in the chemical method was performed. For each

run, a freshly prepared test solution as well a cleaned set of electrodes was used. All experiments

were performed at temperature 303 ± 1K.

Various electrochemical parameters, such as corrosion current density (Icorr) and corrosion

potential (Ecorr), cathodic (βc) and anodic (βa) Tafel slopes, and polarization resistance (Rp), were

recorded for AA6063 coupons in corrosive media (0.5 N HCl). The effect of concentration

change of EEOtL was evaluated and the Icorr values were used to calculate percentage inhibition

efficiency (IE %) of EEOtL in corrosive media



Page 715

Surface Analysis:

Surface analysis was done using FT-IR spectrometer (KBr pellet method), 8400 S Shimadzu,

Japan FT-IR spectrometer in the IR range from 4000 to 400 cm-1

. The surface of the dried

specimens was scratched with a knife and the resultant powder was used for the analysis. In case

of FT-IR studies of plant extract, vacuum dried plant extract liquid was used.

RESULTS AND DISCUSSION

Chemical Method (Weight Loss Measurements):

Effect of Immersion Time and Various Concentrations of EEOtL:

The corrosion behaviour of AA6063 for various exposure time (24 h to 240 h) in corrodent and

corrodent with different concentrations of EEOtL was carried out at 303 ± 1K. The corrosion

parameters like, the corrosion rate (ρcorr), inhibition efficiency (IE %), fractional surface

coverage (θ) and adsorption equilibrium constants (Kad) for the acid corrosion of AA6063 at 303

± 1K for various exposure time with different concentration of EEOtL were evaluated and

tabulated in Table-1.

5

15

25

35

45

55

65

75

24 48 72 96 120 144 168 192 216 240

I E

(%

)

Immersion Time (h)

Fig. 2. Inhibition Efficiency vs Immersion Time (at 303 K)

S1

S2

S3

S4

S540

80

120

160

200

240

24 48 72 96 120 144 168 192 216 240

We

igh

t lo

ss

(m

g)

Immersion Time (h)

Fig.1. Weight loss vs Immersion Time (at 303K)

S0

S1

S2

S3

S4

S5



Page 716

The data obtained from the study showed that the weight loss of AA6063 varied linearly with

immersion period in uninhibited and inhibited acid solutions, indicative of the continuous active

corrosion reactions. The curves obtained in the presence of EEOtL fall significantly below that

of free acid (fig.1.). For all the immersion periods, the effective inhibition of AA6063 was

observed at the optimum concentration of EEOtL (0.360 g/L) (Table-1& fig.2.).

The percentage inhibition efficiency (IE %) was observed to increase with the addition of EEOtL

concentration and reaches maximum at an optimum concentration of EEOtL (i.e. 0.36 g/L) for

all the immersion time periods (fig.3.). The maximum IE (%) (74.063 %) was observed at 0.360

g/L of EEOtL with a minimum corrosion rate (ρcorr=3.7449 mmpy) for 24 h exposure time. With

further increase in concentration of EEOtL, the IE (%) was found to decrease and attains almost

a static value close to 60%. This behavior may be attributed to the increase of the surface area

covered by the adsorbed molecules of EEOtL at its optimum concentration due to the synergistic

action of the active ingredients such as (Eugenol, Ascorbic, Tannins etc.) present in it, as all of

these active ingredients has good inhibiting property [5-16].

55

57

59

61

63

65

67

69

71

73

75

0.18 0.36 0.54 0.72 0.9

I E

(%

)

Concentration (g/L)

Fig.3. Inhibition Efficiency vs Concentration (at 303 K)

303 K



Page 717

Table: 1.Corrosion Parameters for Acid Corrosion of AA6063 without and with different

Concentrations of EEOtL at Various Immersion Times (h) at 303 ±1 K.

Time

(h)

EEOt L

Concentration

(g/L)

Corrosion Parameters

Weight

loss

(mg)

Corrosion

Rate (ρρρρcorr)

(mmpy)

Inhibition

Efficiency

(IE %)

Fractional

Surface

Coverage

(θθθθ)

Adsorption

equilibrium

Constant

(Kad)

24 S-0 0.000 144.2 14.4397 - - -

S-1 0.180 63.1 6.3184 56.241 0.5624 7.1283

S-2 0.360 37.4 3.7449 74.063 0.7406 7.9059

S-3 0.540 45.4 4.5460 68.515 0.6851 4.0270

S-4 0.720 51.6 5.1668 64.216 0.6421 2.4917

S-5 0.900 54.3 5.4372 62.343 0.6234 1.8376

48 S-0 0.000 164.4 8.2309 - - -

S-1 0.180 107.1 5.3621 34.854 0.3485 2.9704

S-2 0.360 55.7 2.7887 66.119 0.6611 5.4138

S-3 0.540 60.5 3.0290 63.199 0.6319 3.1721

S-4 0.720 73.6 3.6849 55.231 0.5523 1.7120

S-5 0.900 73.7 3.6048 55.170 0.5517 1.4227

120 S-0 0.000 182.7 3.6588 - - -

S-1 0.180 139.4 2.7917 23.700 0.2370 1.7256

S-2 0.360 80.4 1.6101 55.993 0.5599 3.5324

S-3 0.540 85.3 1.7082 53.311 0.5331 2.1127

S-4 0.720 87.6 1.7543 52.052 0.5205 1.5058

S-5 0.900 87.6 1.7543 52.052 0.5205 1.2061

192 S-0 0.000 205.8 2.5759 -- - -

S-1 0.180 174.4 2.1829 15.257 0.1525 0.9988

S-2 0.360 108.1 1.3530 47.473 0.4747 2.5091

S-3 0.540 110.1 1.3780 46.501 0.4650 1.6076

S-4 0.720 112.5 1.4081 45.335 0.4533 1.1506

S-5 0.900 119.7 1.4982 41.836 0.4183 0.7980

240 S-0 0.000 227.5 2.2780 -- - -

S-1 0.180 198.8 1.9906 12.615 0.1261 0.8001

S-2 0.360 147.1 1.4729 35.340 0.3534 1.5155

S-3 0.540 162.5 1.6271 28.571 0.2857 0.7399

S-4 0.720 171.4 1.7162 24.659 0.2465 0.4536

S-5 0.900 176.2 1.7643 22.549 0.2254 0.3233



Page 718

Kinetic / Thermodynamic Treatment of Weight Loss Results

The corrosion reactions of aluminium for the corrodent and corrodent-inhibitor systems obey the

kinetic relationship as shown in Fig. 4 [17]:

log ρcorr = log k + B log C

where, k is the rate constant and equals to ρcorr at inhibitor concentration of unity; B is the

reaction constant which, in the present case, is a measure for the inhibitor effectiveness and C is

the concentration of EEOtL in g/L. The plot of log ρcorr vs log C resulted in straight lines, thus

verifying that the kinetic parameters (k and B) can be calculated by using the above equation.

The slopes of the lines were positive (except at higher temperature, 343 ± 1K), depicting that the

rate of corrosion process is proportional to EEOtL concentration, meaning that EEOtL is more

effective at its moderately lower concentration (0.360 g/L).

The log plot of the measured weight of coupons after treatment (log Wf) against exposure time

(t) (Fig. 5) in the temperature range of investigations (293 − 303 ± 1K) facilitate in

understanding the kinetics of the corrosion process of AA6063 in 0.5 N HCl in the uninhibited

and inhibited systems. The formula used for the above is expressed as [17-20]:

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

-0.744 -0.60434 -0.46468 -0.32502 -0.18536 -0.0457

log

ρ

log C

Fig.4. log C vs log ρ

303 K

323 K

333 K

343 K



19

log Wf = log Wo −−−− Kt

where, Wf and Wo are the weights of coupons after treatment and before treatment respectively;

K is the rate constant for corrosion process; and t is the immersion time (h). The log value of Wf

was observed to behave linearly with exposure time, t in corrodent and corrodent- EEOtL

systems with negative slopes, thus confirming a first order kinetics.

Adsorption Isotherms

During corrosion inhibition of metals and alloys, the nature of inhibitor on metal surface is

deduced in terms of adsorption characteristics of the inhibitor [17]. The surface coverage, θ, of

inhibitor are very useful while elucidating the adsorption characteristics of inhibitor. The value

of θ were evaluated as per Damaskin [19] formula.. The adsorption of an inhibitor species, Inh.,

at a metal/solution interface can be expressed as a substitution process between the inhibitor

molecules in the aqueous solution (Inh.sol) and the water molecule on the metallic surface

(H2Oads) [20-21]:

Inh.sol + x H2O ads ⇔ Inh.ads + x H2Osol

where x is the size ratio, representing the number of water molecules displaced by one molecule

of inhibitor during adsorption process. When the equilibrium of the process described in above

equation is reached, it is possible to obtain different expressions of the adsorption isotherm plots,

0.3

0.31

0.32

0.33

0.34

0.35

0.36

0 24 48 72 96 120 144 168 192 216 240

log

Wf

Immersion Time (h)

Fig.5. log Wf vs Immersion Time

S0

S1

S2

S3

S4

S5



Page 720

and thus the degree of surface coverage θ = IE (%)/100 can be plotted as a function of the

concentration of the inhibitor under test.

Bockris et. al [22] have assumed that water molecules dipoles have to be oriented and their

orientation depends on the metal charge adapting the suitable adsorption isotherm. It is widely

acknowledged that adsorption isotherm provides useful insights into the mechanism of corrosion

inhibition. It is necessary to determine empirically which adsorption isotherm fits best to the

surface coverage data in order to use the corrosion rate measurements to calculate the

thermodynamic parameters pertaining to inhibitor adsorption. The obtained values of θ were

tested to fit to different isotherm models including Langmuir, Frumkin, Temkin, Freundluich etc.

It is worth noting that the value of fractional surface coverage, θ, varies with concentration of

EEOtL following Langmuir adsorption isotherm in 0.5 N HCl. It is based on the assumption that

all adsorption sites are equivalent and that particle binding occurs independently from nearby

sites being occupied or not and also that the adsorbed molecule decreases the surface area

available for the cathodic and anodic reactions to occur [23-25]. The Langmuir adsorption

isotherm was tested to fit to the experimental data. It is mathematically expressed as:

[θθθθ / (1−−−− θθθθ)] = Kad C

Or log [θθθθ / (1- θθθθ)] = log Kad + logC

where Kad = binding constant of adsorption process; θ is the fractional surface coverage; and C =

the concentration of EEOtL in g/L.

In general, the Kad values were found to decrease with the temperature, and the Kad was low at

higher concentrations of EEOtL, hence it is again verified that the moderately lower

concentration of EEOtL is essential for maximum adsorption.

On plotting C/θ as a function of C, a straight line was obtained, with a positive slope, and the

value of the intercept is the reciprocal of Kad [23-24]. Curve-fitting of the data according to

Langmuir thermodynamic-kinetic model at 303 ± 1K is shown in Fig. 6. The linear dependence

of log [θ / (1− θ)] as a function of log C was also observed explicitly on plotting a graph (Fig. 7).

The slope value (y) was calculated and found to be 0.7681 (closer to unity) and 1/y = 1.3020.



Page 721

The deviation from the ideal value of slope (for ideal Langmuir isotherm) can be attributed to the

molecular interaction among the adsorbed inhibitor species (a fact which was ignored during the

derivation of the Langmuir equation).

Hence, the experimental data fit well into El-Awady’s thermodynamic-kinetic model, in which

number of active sites y is included. It is expressed as:

log [θθθθ / (1- θθθθ)] = log K + y logC

where Kad = K

(1/y)and 1/y = x = number of inhibitor molecules occupying one active site (or

number of water molecules replaced by one molecule of inhibitor). The straight lines observed in

Fig. 7 represent the El-Awady et. al thermodynamic-kinetic model at 303 ± 1K, with the values

of y and K as slope and intercept respectively. The values of 1/y less than one imply multilayer

adsorption, while 1/y greater than unity, suggest that a given inhibitor molecule occupies more

than one active site [23-25]. The values of y and 1/y obtained in the present study correspond

well to the above said isotherm.

Electrochemical Measurement using Potentiostatic Polarization:

Anodic and cathodic polarization curves corresponding to AA6063 were recorded in 0.5 N HCl

solutions without and with different concentrations of EEOtL. Beginning the experiments at

cathodic potentials ensured that any oxide layer present prior to the start of the experiments was

reduced. The electrochemical parameters, such as Rp, icorr, and Ecorr, associated with polarization

0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

0.18 0.36 0.54 0.72 0.9

C / θθ θθ

C

Fig.6. Langmuir Adsorption Isotherm of EEOtL on AA6063

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

-0.744 -0.60434 -0.46468 -0.32502 -0.18536 -0.0457

log

[θ /

(1

-θ)]

log C

Fig.7. Thermodynamic-kinetic model



Page 722

measurements for the test coupon of AA6063 were simultaneously recorded. The IE (%) was

calculated. The values for electrochemical

corrosion current density (Icorr), R

Table 2. From the results, it can be illustrated that the

acid corrosion of AA6063.

Table: 2 . Electrochemical Parameters of AA6063 in corrodent (0.5 N HCl) and with different

concentrations of EEOtL.

EEOtL

Concentration (g/L)

Ecorr

(mV)

0.000 -749

0.180 -736

0.360 -739

0.720 -744

Figure 8 and Table-2 indicate that on adding

direction (i.e. the positive values). However, the shift is very negligible; illustrating that

act as mixed type of inhibitor and gets adsorbed at both the active sites. The addi

decreases markedly the corrosion current density, I

to inhibitor concentrations. This shows the protective efficacy of

AA6063. The I E (%) was observed maximum (89.30 %


The values for electrochemical parameters, such as, corrosion potential (E

), Rp (Ω/cm2), corrosion rate (mmpy) and IE (%) were tabulated in

. From the results, it can be illustrated that the EEOtL has protective behavior towards

. Electrochemical Parameters of AA6063 in corrodent (0.5 N HCl) and with different

corr

(mV)

Icorr

(mA/cm2)

Rp

(ΩΩΩΩ/cm2)

I E (%) Corrosion rate

x10

749 12.300 79.47 −

736 09.999 61.84 18.70

739 01.313 115.00 89.30

744 05.221 148.30 57.70

indicate that on adding EEOtL, in general, Ecorr shifts to less active

direction (i.e. the positive values). However, the shift is very negligible; illustrating that

act as mixed type of inhibitor and gets adsorbed at both the active sites. The addi

decreases markedly the corrosion current density, Icorr, and increases the Rp values with respect

to inhibitor concentrations. This shows the protective efficacy of EEOtL for acid corrosion of

AA6063. The I E (%) was observed maximum (89.30 %) at an optimum concentration of

Fig.8. Polarization curves

for AA6063 without

curve] and with

(0.360 g/L) [Black curve

in 0.5 N HCl.


parameters, such as, corrosion potential (Ecorr),

IE (%) were tabulated in

has protective behavior towards

. Electrochemical Parameters of AA6063 in corrodent (0.5 N HCl) and with different

Corrosion rate

x103 (mmy

-1)

1.809

1.797

0.3108

0.3234

shifts to less active

direction (i.e. the positive values). However, the shift is very negligible; illustrating that EEOtL

act as mixed type of inhibitor and gets adsorbed at both the active sites. The addition of EEOtL

, and increases the Rp values with respect

for acid corrosion of

) at an optimum concentration of EEOtL

. Polarization curves

for AA6063 without [Red

and with EEOtL

Black curve]



Page 723

(0.360 g/L), which also agrees well with the results obtained with chemical methods. In both

methods, the optimum concentration of EEOtL for its effective inhibition was found to be same,

i.e. 0.360 g/L.

Surface Analysis using FT-IR spectroscopy:

FT-IR spectras of EEOtL alone and post immersion coupon-scratched EEOtL were carried out to

further validate the formation of a protective film by EEOtL on AA6063 surface. On comparing

the peaks on IR spectra, it is revealed that the EEOtL acts as inhibitor for the acid corrosion of

AA6063 by adsorption of its active species, such as Eugenol, Eugenol-methyl ether, Tannins,

Ascorbic acid, etc. over the surface of aluminium.

The broad and strong band ranging from 3200 to 3600 cm−1

observed were due to overlapping of

–OH stretching, which is consistent with the peak at 1100 cm−1

assigned to alcoholic C–O

stretching vibration [26-27], thus showing the presence of hydroxyl group in EEOtL. The strong

peak at 1740 cm−1

can be assigned to a C=O stretching in carbonyl groups. Absorptive peaks in

the range of 1220-1260 and 1240 – 1300 cm−1

can be assigned to a C−O stretching in ethers and

carboxyl groups respectively. The strong bands at 1290, 2850, 2900 cm−1

and a weak band at

3070 cm−1

are attributed to aliphatic and aromatic C–H stretching respectively. This shows that

EEOtL contains mixtures of organic compounds.

The changes in the spectrum of EEOtL after its adsorption on AA6063 were observed. It is clear

that certain peaks were found to disappear completely, while some shifted to higher frequency

region, explicitly indicating that adsorption was taking place over the solid surface. Changes in

the FTIR spectrum may be noted at wave numbers of 3200-3600, 2900–2830, 1220-1300 cm−1

after EEOtL adsorption on AA6063 surface. These regions may be assigned to O–H (well

broad), C–H (variable) and C−O stretching respectively. As aluminium is unlikely to be attached

to a carbon atom, these changes suggest the possibility that the oxygen atoms in the hydroxyl

groups of EEOtL are involved in this adsorption process as well. The FTIR study thus reveals

that functional groups like –OH, –COOH, C=O, C−O present in the constituents of EEOtL are

involved in adsorption on AA6063 via hydrogen bonding &/or weak van der Waals forces.



Page 724

Proposed Mechanism for Inhibition by EE

Many mechanisms have been proposed for the adsorption of organic inhibitors on a m

surface. The primary step in action of inhibitors in acid solutions is generally agreed to be

adsorption on to the metal surface, which is usually oxide free in acid solutions. As discuss

earlier the adsorption of the inhibitor species,

should be considered a place exchanger reaction. Basically, the active organic species of EE

blocks the active corrosion sites present on the metal surface. The adsorption, thus, occurs by the

inhibitor’s free electrons linking with the metal. The process of adsorption of inhibitors are

influenced by the nature of the metal surface, the chemical structure of the organic inhibitor, the

type of aggressive electrolyte and the interaction between organic molecules and th

surface [11-16].

In the present case, with increase in corrosive concentration, the IE (%) was found to decrease;

while with increase in concentration of EE

(74 %) at 0.360 g/L concentration. These

EEOtL for corrosion of AA6063 in HCl. Literature reveal that EE

Ascorbic acid, Eugenol-methyl-e

corrosive property [5-16]. The inhibition process occurs largely via adsorption of the active

constituents in the EEOtL, mainly, Eugenol, Eugenol

enylbenzene), Ascorbic acid, Chavicol, Tannins,

constituents such as: Ursolic acid, Rosmarinic acid, alkaloids, saponins, flavonoids,

Bisabolene, phenylpropane glucosides, Apigenin, Luteolin, Orientin etc.

Adsorption of these active organic species of EE

O−H group, an oxygen atom (a heteroatom),

groups. The hetero-atoms such as oxygen and the hydrogen of O

adsorption center in most of active organic compounds of EE

metal surface. The FT-IR spectra also reveal the same. The adsorption occurs primarily via

hydrogen bonding between a negatively charged aluminium surface and the inhibitor [

The + hydrogen is so strongly attracted to the negatively charged aluminium surface that it is

Proposed Mechanism for Inhibition by EEOtL:

Many mechanisms have been proposed for the adsorption of organic inhibitors on a m


adsorption on to the metal surface, which is usually oxide free in acid solutions. As discuss

the adsorption of the inhibitor species, Inh., on the metal surface in aqueous solutions

should be considered a place exchanger reaction. Basically, the active organic species of EE


ns linking with the metal. The process of adsorption of inhibitors are


type of aggressive electrolyte and the interaction between organic molecules and th


while with increase in concentration of EEOtL, it was increased and attained the maximum IE

(74 %) at 0.360 g/L concentration. These findings clearly indicate the anti-corrosive property of

L for corrosion of AA6063 in HCl. Literature reveal that EEOtL is rich in Eugenol (70 %),

ether, chavicol, Tannis etc. [1-4] and most of them possess anti

]. The inhibition process occurs largely via adsorption of the active

L, mainly, Eugenol, Eugenol-methyl ether (1,2-dimethoxy

enylbenzene), Ascorbic acid, Chavicol, Tannins, β-Carotene and other relatively mi

constituents such as: Ursolic acid, Rosmarinic acid, alkaloids, saponins, flavonoids,

Bisabolene, phenylpropane glucosides, Apigenin, Luteolin, Orientin etc.

Adsorption of these active organic species of EEOtL may be explained by the presence

H group, an oxygen atom (a heteroatom), π- electron of aromatic rings and electron donating

atoms such as oxygen and the hydrogen of O−H group are the major

adsorption center in most of active organic compounds of EEOtL for its interaction with the

IR spectra also reveal the same. The adsorption occurs primarily via

hydrogen bonding between a negatively charged aluminium surface and the inhibitor [

+ hydrogen is so strongly attracted to the negatively charged aluminium surface that it is

Many mechanisms have been proposed for the adsorption of organic inhibitors on a metal


adsorption on to the metal surface, which is usually oxide free in acid solutions. As discussed

metal surface in aqueous solutions

should be considered a place exchanger reaction. Basically, the active organic species of EEOtL


ns linking with the metal. The process of adsorption of inhibitors are


type of aggressive electrolyte and the interaction between organic molecules and the metallic


L, it was increased and attained the maximum IE

corrosive property of

L is rich in Eugenol (70 %),

] and most of them possess anti-

]. The inhibition process occurs largely via adsorption of the active

dimethoxy-4-prop-2-

Carotene and other relatively minor

constituents such as: Ursolic acid, Rosmarinic acid, alkaloids, saponins, flavonoids, α- and β-

L may be explained by the presence of the

electron of aromatic rings and electron donating

H group are the major

interaction with the

IR spectra also reveal the same. The adsorption occurs primarily via

hydrogen bonding between a negatively charged aluminium surface and the inhibitor [11-15].

+ hydrogen is so strongly attracted to the negatively charged aluminium surface that it is



Page 725

almost as if beginning to form a co-ordinate (dative covalent) bond. It doesn't go that far, but the

attraction is significantly stronger than an ordinary dipole-dipole interaction.

Figure 9. Proposed sketch of the adsorption of one of the active organic species (i.e. Eugenol) of

EEOtL on AA6063 surface.

The active organic species of EEOtL were physisorbed on the Cl- bridge (and some water

molecules, B) formed on aluminium surface (Fig. 9). This mechanism may occur obviously at

lower inhibitor concentrations at which desorption for the inhibitor species was facilitated with

increasing temperature. The simultaneous adsorption of oxygen atoms forces the EEOtL

molecule to be horizontally oriented at the metal surface, which led to increase the surface

coverage and consequently protection efficiency even in low inhibitor concentrations. The good

inhibitive properties of the EEOtL in HCl, thus, could be explained by the physical adsorption of

EEOtL species on AA6063.

Conclusions

The results from chemical and electrochemical methods clearly illustrate that EEOtL act

as an inhibitor for corrosion of AA6063 in 0.5 N HCl and the dissolution of AA6063 was

inhibited to a comparative degree.

The maximum IE (%) and fractional surface coverage (θ) were observed at an optimum

concentration of extract (i.e. 0.360 g/L).



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The performance of inhibitors was evaluated by adsorption isotherms. In this study the El-

Awady adsorption isotherm provides a formal description of the adsorption behavior of

the inhibitor on Aluminum alloy.

The values for the thermodynamic parameters for the inhibitor-metal interactions clearly

indicate that the adsorptions are of physical nature.

EIS data reveal that EEOtL forms a protective film on AA6063 surface, which is further

verified by FT-IR spectra.

The results from potentiodynamic polarization curves clearly reveals that EEOtL act as

mixed type of inhibitor for acid corrosion of AA6063, via blocking the active sites on

metal surface.

EEOtL is a non-toxic, eco-friendly corrosion inhibitor & can be used to replace toxic

chemicals hitherto being used as inhibitor to combat the corrosion of Aluminium in HCl

solution.

Tulsi (Ocimum tenuiflorum) leaf is an environmentally friendly, low priced, abundantly

found natural product with good inhibition efficiency (74 %), can be a promising

corrosion inhibitor for corrosion of aluminium and its alloys in HCl.

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Combating Aluminium Alloy Dissolution by employing Ocimum

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