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Corrosion inhibition behaviour of Azardirachta indica (Neem) leaves extract as a green corrosion inhibitor for zinc in hydrochloric acid: a preliminary study.

Introduction

Corrosion is the destruction of material resulting from an exposure and interaction with the environment. It is a major problem that must be confronted for safety, environment and economic reasons (Thompson et al., 2007) in various chemical, mechanical, metallurgical, biochemical and medical engineering applications, and more specifically in the design of a much more varied number of mechanical parts which equally vary in size, functionality and useful lifespan. Several efforts have been made using corrosion preventive practices and the use of green corrosion inhibitors is one of them (Anuradha et al., 2008). In line with the emergent concept of "Green Chemistry" and the related couple of principles of 'Less hazardous synthesis' stating that wherever practicable, synthetic methodologies should be designed to use and generate substances that possess little or no toxicity to human health and the environment, and 'Safer chemicals' whereby chemical products should be designed to preserve efficacy of function while reducing toxicity, the use of green inhibitors for the control of corrosion of metals (Valdez et al., 2003) and alloys which are in contact with aggressive environment is an accepted and growing practice (Taylor et al., 2007; Khaled, 2008). Indeed, a relatively large number of organic compounds are presently under study to probe and optimize their corrosion inhibition potential. These studies have shown that organic compounds containing N, S and O show significant corrosion inhibition efficiency. However, a certain proportion of these compounds is not only expensive but also shows some toxicity to living beings (Bothi Raja and Sethuraman, 2008).

Plant extracts and the derived organic species have therefore become important as an environmentally benign, readily available, renewable and acceptable source for a wide range of inhibitors (Rajendran et al., 2004; Mesbah et al., 2007; Okafor et al., 2007). They are the rich sources of molecules which have appreciably high inhibition efficiency (Bothi Raja and Sethuraman, 2008) and are hence termed 'Green Inhibitors' (Lebrini et al., 2008; Sharma et al., 2009a). Green corrosion inhibitors (Sharma et al., 2009a) are biodegradable and do not contain heavy metals or other toxic compounds. The fruitful use of naturally occurring substances/molecules to inhibit the corrosion of metals in acidic and alkaline environment has been reported by some research groups (Benhadou et al., 2006; Abdel-Gaber et al., 2008; Umoren and Ebenso, 2008) to mention but a few. Research efforts to find naturally organic substances and/or other biodegradable organic materials to be used as effective corrosion inhibitors of a wide number of metals has been in focus in our research group (Sharma et al., 2008). As a matter of fact, the present paper adds itself up to the series of our original research papers which report our research findings in using plants extracts for their respective non-competitive corrosion inhibition potential for zinc in acidic media (Sharma et al., 2009a, 2009b, 2009c).

Azadirachta indica--The Neem plant

Our focus in this paper is on Neem (Azardirachta indica), which is more specifically the extract from green mature Neem leaves from an over 80 years old Neem tree. Several studies have been carried out and have remained focused on the Neem plant parts for their various pharmacological activities (anti-inflammatory, antipyretic, analgesic, immunostimulant, antifertility, anticarcinogenic, antimalarial and hepatoprotective) (Biswas et al., 2002; Dasgupta et al., 2004; Subapriya and Nagini, 2005) and medicinal properties (Biswas et al., 2002). AZI is well known in India and its neighbouring countries for more than 2000 years as one of the most versatile medicinal plants having a wide spectrum of biological activity (Biswas et al., 2002). Neem is an evergreen tree, cultivated in various parts of the Indian subcontinent. Neem has been extensively used in ayurveda (Subapriya and Nagini, 2005), unani and homoeopathic medicine and has become a cynosure of modern medicine. The Sanskrit name of the neem tree is 'Arishtha' meaning 'reliever of sickness' and hence is considered as 'Sarbaroganibarini' (Biswas et al., 2002). The tree is still regarded as 'village dispensary' in India. The importance of the neem tree has been recognized by the US National Academy of Sciences, which published a report in 1992 entitled 'Neem--a tree for solving global problems'.

Chemical investigation on the products of the Neem tree was extensively undertaken in the middle of the twentieth century. Nimbin was the first bitter compound isolated from Neem oil, and thereafter more than 135 compounds have been isolated from different parts of Neem and several reviews have also been published on the chemistry and structural diversity of these compounds which are divided into two major classes: isoprenoids (Roy et al., 2007) and others. The isoprenoids include diterpenoids (namely sugiol, nimbiol, margasone) and triterpenoids containing protomeliacins, liminoids, azadirone and its derivatives, genudin and its derivatives, vilarin type of compounds and C-secomeliacins such as nimbin, salannin and azadirachtin. The first compound to be studied was nimbin. The non-isoprenoids include proteins (amino acids) and carbohydrates (polysaccharides), sulphurous compounds, polyphenolics such as flavonoids and their glycosides, dihydrochalcone, coumarin and tannins, aliphatic compounds, phenolic acids (Girish and Shankara Bhar, 2008). Neem extracts also contain significant amount of water soluble electrochemically active compounds, as well as high concentrations of alkaloids, fatty acids and nitrogen- and oxygen-containing compounds. All the more, Neem is bitter in taste and this is due to an array of complex compounds called "triterpenes" or more specifically "limonoids". Nearly 100 protolimonoids, limonoids or tetranortriterpenoids, pentanortriterpenoids, hexanortriterpenoids and some nonterpenoid constituents have been isolated form various parts of the Neem tree (Koul et al., 1990); still more are being isolated. The most important bioactive principal is azadirachtin; at least 10 other limonoids possess insect growth in regulating activity (Saxena and Kidiavai, 1997). Given the wide spectrum of chemical species present in Neem leaves and their respective multifaceted chemical and biological properties, we therefore articulated that channeling Neem leaf extract for yet another use into green inhibition of corrosion studies may yield some interesting results. All the more, Neem extract has been only very occasionally involved in environmental engineering and environmental chemistry research with the analysis of the adsorption of Pb(II) from aqueous solution by neem leaf powder by Bhattacharyya and Sharma (2004), the equilibrium and kinetic studies biosorption of zinc from aqueous solution using Azadirachta indica bark by King et al. (2008), the removal of chromium (Cr(VI)) from aqueous solution using Azadirachta indica leaf powder as absorbent by Venkateswarlu et al. (2007), the successful scale-up of Azadirachta indica suspension culture for azadirachtin production carried out in stirred tank bioreactor with two different impellers by Prakash and Srivastav (2007), the adsorption and corrosion inhibitive properties of AZI in acid solutions (Oguzie, 2006), the study of copper corrosion inhibition by AZI leaves extract in 0.5M sulphuric acid by (Valek and Martinez, 2007) and the corrosion inhibition of Neem leaves extract as a green inhibitor of zinc corrosion in [H.sub.2]S[O.sub.4] using the gravimetric method by Sharma et al. (2009b).

Research objectives

Zinc has widespread uses in industry and many other areas of manufacturing. Zinc is mainly used in the production of non-corrosive alloys and brass and in galvanizing steel and iron products. Zinc undergoes oxidation on the surface, thus protecting the underlying metal from degradation. Galvanized products are widely used in construction materials, automobile parts, and household appliances. When incorporated with copper compounds or arsenic-lead wettable powders and applied by spraying, zinc can minimize the toxic effects of these metals on fruits such as plums, apples and peaches. As a result of its wide use and hence susceptibility to corrosion (oxidation potential +0.76eV), the study of the corrosion inhibition of zinc has become an important area of research in corrosion science. Many nitrogen containing compounds including quinoline, aniline, brucine, and strychnine have shown good corrosion inhibition for zinc in acidic medium. Quinine sulphate, piperazine, caffine, barbitone and pyridine derivatives have been used as corrosion inhibitors for aluminium and zinc in acidic medium. Inhibition of corrosion of zinc in hydrochloric acid by some carbazide derivatives has also been reported. Foad El-Sherbini et al. (2005) have investigated the inhibition effect of ethoxylated fatty acids as inhibitors for the corrosion of zinc metal in 1.0 M hydrochloric acid solutions at various temperatures ranging from 25 to 55[degrees]C by weight loss measurement and electrochemical methods.

In the present study we are trying to study the corrosion of zinc and the inhibition of the corrosion process by AZI extract. To the best of our knowledge, nothing has been specifically reported on the use of AZI extract for the inhibition of zinc corrosion in hydrochloric acid solutions except for the recently published paper by Sharma et al. (2009b) where sulphuric acid was used instead. AZI leaves are often used in the medicinal and pharmaceutical industry. An additional beneficial use of Neem leaves to curb corrosion of zinc would surely imply the successful utilization of this powerful and versatile natural resource in the metallurgical, materials science and chemical engineering industries. The present study therefore probes the inhibitive properties of Azadirachta indica leaves extracts for zinc corrosion using a gravimetric technique in an acidic media (HCl acid) with and without the extract at two temperatures (30[degrees]C and 60[degrees]C). The thermodynamic parameters characterizing the adsorption process have also been calculated.

Experimental

Stock solution preparation of Azardirachta Indica leaves

The stock solution procedure has been adapted from the procedures given in Arora et al. (2007). 1 kg of Azardirachta Indica leaves was taken in a natural condition and dried for 10-15 days in natural shade, then ground and powdered. 100 g of the finely powdered dried sample was taken in 1000mL of double-distilled water in a round bottom flask (RBF) to cover the powder completely. The RBF was covered with condensation coil and left for slow heating for about 2-3 days, and then only filtered. The reaction process for extraction was repeated for maximum extraction. The stock solution of the extract thus obtained was used in preparing different concentration of the extract by dissolving 5, 10, 15, 20, 25, 30 mL of the extract in various acidic solutions, prepared in double-distilled water.

Zinc specimen preparation

All the zinc strips were rectangular in shape, having dimensions 2.5 x 5.0 x 0.053cm with a small hole of 2mm diameter near the upper edge of the strip for handling. The specimen was cleaned by polishing to produce a smooth finish with the help of emery paper and then clean-washed with absolute alcohol (Ethanol-[C.sub.2][H.sub.5]OH) and dried in natural air. Each specimen was suspended by a plastic thread tied in the hole and immersed in a beaker containing 50 mL of the test solution of AZI in HCL solution at 30[degrees]C and 60[degrees]C and left exposed to air for about half an hour for the determination of the corrosion rate (CR).

Gravimetric method

The mass loss method was employed for the two temperatures 30[degrees]C and 60[degrees]C. In this procedure, the mass loss of the metal in uninhibited (without Azardirachta indica extract) and inhibited solution (with Azardirachta indica extract) were measured and recorded. From the data, the percentage inhibition efficiency (% I) and degree of surface coverage (0) were calculated using Equations 1 and 2 respectively.

%I = (1 - [DELTA][M.sub.i]/[DELTA][M.sub.u]) x 100 (1)

where [DELTA][M.sub.u] and [DELTA][M.sub.i] are the mass loss of the metal in uninhibited and inhibited solutions, respectively.

[theta] = (1 - [DELTA][M.sub.i]/[DELTA][M.sub.u]) (2)

The Corrosion Rate (CR) in mmpy (millimetres per year) has been calculated from Equation 3.

CR = [Loss.sub.mass] x 87.6/Area x Time x [Density.sub.Zn] (3)

where mass loss is expressed grams, area is expressed [cm.sup.2], time is expressed hours, metal density is expressed g/[cm.sup.3] and 87.6 is a conversion factor.

Thermometric method

This was carried out as reported by Ebenso (2003) and Umoren et al. (2006). In this technique the immersion of a single specimen measuring 2.5 x 5.0 x 0.053 cm was analyzed in a reaction vessel containing 50 mL test solution. Temperature changes were measured at an interval of one minute using a thermometer with a precision of [+ or -] 0.5[degrees]C. The temperature increased slowly at first and then rapidly and finally attained a maximum temperature. These temperature rises was recorded and use to calculate the Reaction Number (RN) from the Equation 4;

RN([degrees]C /min) = [T.sub.m] - [T.sub.i]/t (4)

where [T.sub.m] is the maximum temperature attained by the system and [T.sub.i] is the initial temperature and t is the time required to reach the maximum temperature in minutes. From the above, the inhibition efficiency (n) of the used inhibitor was computed using Equation 5.

[eta] = (1 - [RN.sub.i]/[RN.sub.Free]) x 100 (5)

where [RN.sub.i] is the reaction number in the presence of the inhibitor and [RN.sub.Free] is the reaction number in the absence of the inhibitor. pH in all experiments was measured using a digital pH meter (Model MS Electronics India Pvt. Ltd., India, 230V, 50Hz AC, Readability 0.01pH and 1mV, pH range 0 to 14 units continuous, mV range 0 to 1999 mV continuous) before and after equilibrium but the solution was not buffered in order to simulate the real situation. At all pH tested, there was no evidence of precipitation.

Results and Discussion

Table 1 shows values of corrosion rate (CR) of zinc in all the concentrations of HCl studied and it shows that corrosion rate increases with an increase in acid concentration. The same trend of increasing corrosion rate is observed for either temperature studied in the experiments. Table 2 shows the corrosion rate for the corrosion of zinc at 2.0N HCl in the absence and presence of AZI extract at 303K and 333K. It may be observed from the data in Table 2 that an addition of an increased concentration of the inhibitor generally retards the corrosion rate of zinc in both the acid solutions. This is also seen and supported from the decreasing change in mass loss taking place at a particular acid concentration corresponding with an increase in inhibitor concentration (Figure 1 and Figure 2).

Effect of concentration of inhibitor and temperature of test solution

From Table 1 and 2, it is observed that the rate of corrosion of zinc is affected by concentration of HCL, temperature and concentration of the of Azardirachta indica leaves extract. The rate of zinc corrosion increases as the concentration of HCl increases and also increases as the temperature increases and this observation is evidenced by data in Figures 1 and 2. This is supported from the overall decreasing change in mass loss taking place at a particular acid concentration corresponding with an increase in inhibitor concentration. This is supposed to be due to adsorption of AZI extract on the surface of zinc. Comparing Figures 1 and 2, it is found that at a fixed concentration of the inhibitor and a fixed acid concentration, the mass loss taking place at 333 K is in most of the instances higher than that occurring 303 K indicating that the inhibition efficiency of AZI extract decreases with increase in temperature.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The decrease may be due to competition between forces of adsorption and desorption. From Table 3, it can also be seen that inhibition efficiency of AZI extract varies with its concentration. At 333K, the optimum value of inhibition efficiency 68.95% was obtained at an extract concentration of 23.08 mg/mL, while the least value was obtained at an extract concentration of 9.09 mg/mL as 36.66%. The significant difference between values of inhibition efficiency of AZI extract obtained at 303K and 333K for especially the higher concentrations of the extract suggests that the mechanism of adsorption of the inhibitor on the zinc surface is by physical adsorption.

Thermodynamics parameters

A value of activation energy for the corrosion reaction of zinc in the presence and absence of different concentration of Azardirachta indica leaves extract has been calculated using Arrhenius Equation (Equation 6)

CR = Ae([E.sub.a]/RT]) (6)

Taking logarithm on both sides, Equation 7 is obtained;

LogCR = LogA - [E.sub.a]/2.303RT (7)

where A is Arrhenius constant, [E.sub.a] is the activation energy of the reaction, R is the gas constant (8.314J/mol.K.) and T is the temperature (K). Considering a change in temperature from 303 K ([T.sub.1]) to 333K ([T.sub.2]), the corresponding values of the corrosion rates at these temperatures are a1 and a2, respectively. Inserting these parameters into Equation 7, Equation 8 is obtained;

Log [a.sub.2]/[a.sub.1] = [E.sub.a]/2.303R (1/[T.sub.1] - 1/[T.sub.2]) (8)

Values of [E.sub.a] (Table 4) for the inhibited corrosion reaction of zinc have been calculated using Equation 8. The activation energy in the absence of the inhibitor is 2839.86 J/mol and is lower than the values obtained for the inhibited systems. The values in the presence of the inhibitor support the mechanism of physical adsorption. For a physical adsorption, it is expected that the value of [E.sub.a] should be less than 80000 J/mol (Sheatty et al., 2006). The values of heat of adsorption ([Q.sub.ads]) of AZI leaves extract on the zinc surface were calculated using Equation 9 (Umoren et al., 2006)

[Q.sub.ads] = 2.303R [log([[theta].sub.2]/1 - [[theta].sub.2])-log(([[theta].sub.1]/1 - [[theta].sub.1])] x ([T.sub.2][T.sub.1]/[T.sub.2] - [T.sub.1])[Jmol.sup.-1] (9)

Values of [Q.sub.ads] given in Table 4 were negative for the range of concentrations of inhibitor studied indicating that the adsorption of the inhibitor on zinc surface is exothermic.

Conclusion

From the present study, it is concluded that Azadirachta indica leaves extract can be used as an inhibitor for zinc corrosion in HCl solutions. While the green inhibitor molecule most supposedly acts by being adsorbed on zinc surface by physical adsorption, the overall inhibition is believed to be provided by a synergistic effect. It has also been found that the inhibitive action of AZI leaves extract is basically controlled by temperature and the concentration of the inhibitor.

Acknowledgements

The authors are grateful to Dr. V.K. Agarwal (Chairman) of the Institute of Engineering and Technology (IET) Group of Institutions (Alwar, India) for giving them the opportunity to establish a Computational & Green Chemistry Research Laboratory at IET whereat this study was carried out.

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Sanjay K. Sharma (1) *, Gargi Jain (1), Jyoti Sharma (2) and Ackmez Mudhoo (3)

(1) Computational & Green Chemistry Research Laboratory, Institute of Engineering & Technology, MIA, Alwar, India

(2) Centre for Applied Research, Department of Chemistry, R.R. (P.G.) College, Alwar, India

(3) Department of Chemical and Environmental Engineering, University of Mauritius, Reduit, Mauritius

* Email of Corresponding Author: drsanjay1973@gmail.com
Table 1 Corrosion rate (CR) for the corrosion of zinc in all
concentrations of HCl at 303 K.

Concentration of HCl (N)    CR (mmpy)

          0.5 N              0.5047
          1.0 N               1.040
          2.0 N               6.225

Table 2 Corrosion Rate (from gravimetric method) for the corrosion of
Zinc in 2N HCl at two different temperatures (303 K and 333 K).

Concentration of Azardirachta   CR (mmpy)    CR (mmpy)
indica leaves extract (mg/mL)    at 303 K     at 333 K

         Uninhibited              6.2250       6.8966
            9.09                  1.8290       4.6309
            16.66                 1.7887       2.3458
            23.08                 1.0659       2.3340

Table 3 Inhibition efficiency (%I) for both the gravimetric and
thermometric methods for the corrosion of zinc in 2N HCl at 303 K and
333K.

                              Gravimetric            Thermometric

 Concentration of AZI    %I (303 K)   %I (333K)   n (303K)   n (333K)
leaves extract (mg/mL)

         9.09              62.08        36.66       6.66       2.33
        16.66              71.66        60.78      23.33       6.98
        23.08              78.33        68.95      30.00      11.63

Table 4 Thermodynamic parameters for the adsorption of AZI leaves
extract on zinc surface in 2N HCl.

 Concentration of AZI    [E.sub.a]    [Q.sub.ads]
leaves extract (mg/mL)    (J/mol)       (J/mol)

     Uninhibited          2839.86          --
        09.09             25749.63     -28575.84
        16.66             7515.42      -13903.93
        23.08             21724.47     -13020.10
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Author:Sharma, Sanjay K.; Jain, Gargi; Sharma, Jyoti; Mudhoo, Ackmez
Publication:International Journal of Applied Chemistry
Article Type:Report
Geographic Code:1USA
Date:Jan 1, 2010
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