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
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 Tel: 098290 13969 (M)
______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.
Page 713 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
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 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
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 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
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.
Fig.1. Weight loss vs Immersion Time Fig. 2. Inhibition Efficiency vs (at 303K) Immersion Time (at 303 K) 240 75 S0 S1 65 200 S1 55 S2 S2 160 45 S3 S3 120 35 S4 E (%) I
Weight loss(mg)Weight 25 S4 80 S5 15 40 S5 5 24 48 72 96 120 144 168 192 216 240 24 48 72 96 120 144 168 192 216 240 Immersion Time (h) Immersion Time (h)
Page 716 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
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].
Fig.3. Inhibition Efficiency vs Concentration (at 303 K)
75 73 71 303 K 69 67 65 63 I E (%) I 61 59 57 55 0.18 0.36 0.54 0.72 0.9
Concentration (g/L)
Page 717 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
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 EEOt L Corrosion Parameters (h) Concentration Weight Corrosion Inhibition Fractional Adsorption (g/L) loss Rate (ρρρcorr) Efficiency Surface equilibrium (mg) (mmpy) (IE %) Coverage Constant (θθθ) (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 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
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).
Fig.4. log C vs log ρ
1.8
1.6 303 K
1.4 323 K 1.2 ρ
log 1 333 K
0.8 343 K 0.6
0.4 -0.744 -0.60434 -0.46468 -0.32502 -0.18536 -0.0457 log C
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]:
Page 719 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
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.
Fig.5. log Wf vs Immersion Time 0.36
0.35 S0
0.34 S1
f S2 0.33 S3 log W log
0.32 S4
0.31 S5
0.3 0 24 48 72 96 120 144 168 192 216 240 Immersion Time (h)
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,
Page 720 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
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:
[θθθ / (1−−− θθθ)] = Kad C
Or log [θθθ / (1- θθθ)] = 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 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
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).
Fig.6. Langmuir Adsorption Fig.7. Thermodynamic-kinetic model Isotherm of EEOtL on AA6063 0.45 2 0.4 1.75 0.35 1.5 0.3 )]
1.25 θ θ θ
θ θ 0.25
1 (1- /
C / C 0.2 θ 0.75 0.15 log [ log 0.5 0.1
0.25 0.05 0 0 -0.744 -0.60434 -0.46468 -0.32502 -0.18536 -0.0457 0.18 0.36 0.54 0.72 0.9 C log C
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 [θθθ / (1- θθθ)] = log K + y logC
(1/y) where Kad = K 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
Page 722 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
measurements for the test coupon of AA6063 were simultaneously recorded. The IE (%) was
calculated. The values for electrochemical parameters, such as, corrosion potential (Ecorr), 2 corrosion current density (Icorr), Rp (Ω/cm ), corrosion rate (mmpy) and IE (%) were tabulated in Table 2. From the results, it can be illustrated that the EEOtL has protective behavior towards acid corrosion of AA6063.
Fig.8. Polarization curves for AA6063 without [Red curve] and with EEOtL (0.360 g/L) [Black curve]
in 0.5 N HCl.
Table: 2 . Electrochemical Parameters of AA6063 in corrodent (0.5 N HCl) and with different concentrations of EEOtL.
EEOtL Ecorr Icorr Rp I E (%) Corrosion rate Concentration (g/L) (mV) (mA/cm2) (ΩΩΩ/cm2) x103 (mmy-1) 0.000 -749 12.300 79.47 − 1.809 0.180 -736 09.999 61.84 18.70 1.797 0.360 -739 01.313 115.00 89.30 0.3108 0.720 -744 05.221 148.30 57.70 0.3234
Figure 8 and Table-2 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 EEOtL act as mixed type of inhibitor and gets adsorbed at both the active sites. The addition of EEOtL
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 EEOtL
Page 723 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
(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−1assigned 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 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
Proposed Mechanism for Inhibition by EEOtL:
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 earlier 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 EEOtL 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 the metallic surface [11-16].
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 EEOtL for corrosion of AA6063 in HCl. Literature reveal that EEOtL is rich in Eugenol (70 %), Ascorbic acid, Eugenol-methyl-ether, chavicol, Tannis etc. [1-4] and most of them possess anti- corrosive property [5-16]. The inhibition process occurs largely via adsorption of the active constituents in the EEOtL, mainly, Eugenol, Eugenol-methyl ether (1,2-dimethoxy-4-prop-2- enylbenzene), Ascorbic acid, Chavicol, Tannins, β-Carotene and other relatively minor constituents such as: Ursolic acid, Rosmarinic acid, alkaloids, saponins, flavonoids, α- and β- Bisabolene, phenylpropane glucosides, Apigenin, Luteolin, Orientin etc.
Adsorption of these active organic species of EEOtL may be explained by the presence of the O−H group, an oxygen atom (a heteroatom), π- electron of aromatic rings and electron donating groups. The hetero-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 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 [11-15]. The + hydrogen is so strongly attracted to the negatively charged aluminium surface that it is
Page 725 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
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).
Page 726 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
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.
REFERENCES
1. Shashi Sharma, Prassan S Pratihar, Rekha N Nair, P S Verma, Alka Sharma, “Influence of ion-additives on inhibitory action of extract of Trigonella foenum graceums seeds for AA6063 in acid medium”, RASAYAN J. Chem., 5 (1) (2012) 16-23. 2. Rekha N. Nair, Shashi Sharma, I. K. Sharma, P. S. Verma, Alka Sharma, “Inhibitory Efficacy of Piper nigrum linn. Extract on Corrosion of AA1100 in HCl”, RASAYAN J. Chem., 3 (4) (2010) 783-795. 3. Rekha N. Nair, Nemichand Kharia, I. K. Sharma, P. S. Verma and Alka Sharma, “Corrosion Inhibition Study of Aluminium in Acid Media by Mango (Mangifera Indica) Leaves as Eco- Friendly Inhibitor”, J. Electrochem. Soc. India, 56 (1/2) (2007) 41-47. 4. J. F. Dastur, Medicinal Plants of India and Pakistan, D.B. Taraporevala Sons & Co.: Bombay (1962).
Page 727 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
5. L. Laakso, T. Seppanen-Laakso, B. Hermann-Wolf and K. Knobloch; “Constituents of the essential oil from the Holy basil or Tulsi basil”, Ocimum sanctum. Planta Med., 56 (1990), 527 6. S. K. Kothari, A. K. Bhattacharya, C.P. Singh, K. Singh, K.V. Syamasundar, S. Ramesh, S. Kumar, “A New Methyl Eugenol-rich and Short Duration Strain of Ocimum tenuiflorum.”, J. Med. Aram. Pl. Sci., 22 (4A) & 23(1A) (2001) 385-388. 7. E. S. Ferreira, C. Giacomelli, F. C. Giacomelli, A. Spinelli, “Evaluation of the Inhibitor Effect of L-Ascorbic Acid on the Corrosion of Mild Steel”, Mater. Chem. Phys., 83 (1) (2004) 129-134. 8. J. Mabrour, M. Akssira, M. Azzi, M. Zertoubi, N. Saib, A. Messaoudi, A. Albizane, S. Tahiri, “Effect of Vegetal Tannin on Anodic Copper Dissolution in Chloride Solutions”, Corros. Sci., 46 (8) (2004) 1833-1847. 9. K. Hnini, A. Chtaini, “Inhibition of Metallic Corrosion with Eugenol”, Bull. Electrochem., 20 (2004) 481-485. 10. E. Chaieb, A. Bouyanzer, B. Hammouti, M. Benkaddour, “Inhibition of the Corrosion of Steel in 1 M HCl by Eugenol Derivatives”, Appl. Surf. Sci., 246 (1-3) (2005)199-206. 11. Ambrish Singh, V. K. Singh, M. A. Quraishi, “Aqueous Extract of Kalmegh (Andrographis paniculata) Leaves as Green Inhibitor for Mild Steel in Hydrochloric Acid Solutions”, Int. J. of Corrosion, 10.1155/2010/275983, article id 275983, June 2010. 12. Shuduan Deng, Xianghong Li, “Inhibition by Ginkgo leaves extract of the corrosion of steel in HCl and H2SO4 solutions” Corros. Sci., 55 (2012) 407-415. 13. M. Behpour, S.M. Ghoreishi, M. Khayatkashani, N. Soltani, “Green approach to corrosion inhibition of mild steel in two acidic solutions by the extract of Punica granatum peel and main constituents”, Mater. Chem.Phy.,131 (2012) 621-633. 14. M. Hazwan Hussin, M. Jain Kassim, The corrosion Inhibition and adsorption behavior of Uncaria gambir extract on mild steel in 1M HCl” Mater. Chem. Phy., 125 (2011) 461-468. 15. M. Dahmani, A. Et-Touhami, S.S. Al-Deyab, B. Hammouti, A. Bouyanzer, “Corrosion Inhibition of C38 Steel in 1 M HCl: A Comparative Study of Black Pepper Extract and Its Isolated Piperine”Int. J. Electrochem. Sci., 5 (2010) 1060 – 1069. 16. K.F. Khaled, S.S. Abdel-Rehim “Electrochemical Investigation of corrosion and corrosion inhibition of iron in hydrochloric acid solutions”Arab. J. Chem., 4 (2011) 397-402.
Page 728 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
17. P. W. Atkins, J. de Paula, in: Physical Chemistry, Oxford University Press: NY (2002). 18. M. Bouklah, N. Benchat, B. Hammouti, A. Aouniti, S. Kertit, “Thermodynamic Characterization of Steel Corrosion and Inhibitor Adsorption of Pyridazine Compounds in
0.5 M H2SO4”, Mater. Lett., 60 (2006) 1901-1905. 19. B. B. Damaskin, Adsorption of Organic Compounds on Electrodes, Plenum Press: New York (1971) 221. 20. P. W. Atkins, Chemisorbed and Physiosorbed Species, A Textbook of Physical Chemistry, University Press Oxford: NY (1980) 936-938. 21. Pierre R. Roberge, Corrosion Inspection and Monitoring, John Wiley & Sons, New York (2007). 22. J. O' M. Bockris and A. K. N. Reddy, Modern Electrochemistry - Electrodics in Chemistry, Engineering, Biology, and Environmental Science, 2nd ed., Kluwer Academic / Plenum Publishers: New York, 2B (2000). 23. Irving Langmuir, “The Constitution and Fundamental Properties of Solids and Liquids. Part I. Solids”, J. Am. Chem. Soc., 38, (1916) 2221-95. 24. I. Langmuir, “The Constitution and Fundamental Properties of Solids and Liquids. Part II. Liquids” J. Amer. Chem. Soc., 39 (1917) 1848. 25. A. Y. El-Etre, M. Abdallah, Z. E. El-Tantawy, “Corrosion Inhibition of Some Metals Using Lawsonia Extract”, Corros. Sci., 47 (2) (2004) 385-395. 26. Robert M. Silverstein, G. C. Bassler, T. C. Morrill, R. M. Silverstein, Terence C. Morrill, Spectroscopic Identification of Organic Compounds, 5th ed., John Wiley & Sons: New York (1991). 27. R.L. Shriner, C.K.F. Hermann, T.C. Morrill, D.Y. Curtin, R.C. Fuson, The Systematic Identification of Organic Compounds, 7th ed., Wiley: New York (1998).
Page 729