combating aluminium alloy dissolution by employing ocimum
TRANSCRIPT
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.
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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 %
measurements for the test coupon of AA6063 were simultaneously recorded. The IE (%) was
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.
measurements for the test coupon of AA6063 were simultaneously recorded. The IE (%) was
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]
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(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.
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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
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
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
blocks the active corrosion sites present on the metal surface. The adsorption, thus, occurs by the
ns 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
In the present case, with increase in corrosive concentration, the IE (%) was found to decrease;
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
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 discussed
metal surface in aqueous solutions
should be considered a place exchanger reaction. Basically, the active organic species of EEOtL
blocks the active corrosion sites present on the metal surface. The adsorption, thus, occurs by the
ns 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 the metallic
In the present case, with increase in corrosive concentration, the IE (%) was found to decrease;
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
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 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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Page 726
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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