INTRODUCTION:

The use of inhibitors is one of the best options of protecting metals against corrosion. Abdullah et.al¹ suggested that most of the tested inhibitors are organic compounds containing sulphur or nitrogen in their chemical structures. It was found that this kind of compounds is chemically adsorbed on the metal surface forming barrier for mass and charge transfer and consequently decreasing the rate of corrosion. Unfortunately, most of these compounds are harmful for both human being and environments. The known hazardous effect of most synthetic corrosion inhibitors is the motivation for the use of some natural products. The use of chemical inhibitors has been reduced because of the environmental threat, recently, due to environmental regulations. Abdel-Gaber et.al² reported that the plant extracts have become important because they are environmentally acceptable, inexpensive, readily available and renewable sources of materials, and ecologically acceptable. Plant products are organic in nature and some of the constituents including tannins, organic and amino acids, alkaloids and pigments are known to exhibiting action.

Moreover, they can be extracted by simple procedures with low cost studied by Abdel-Gaber et.al³and Raja etal ⁴. Plant parts are used as corrosion inhibitors. The anticorrosion activity of Vitis vinifera seed and skin (Grape), Ocimum tenuiflorum (Tulsi), Punica granatum (Pomegranate) Embilica officinalis (Amla) and Jatropha curcas was investigated by Deepa Rani et.al running through references^5-9. In our present study, we have chosen Azadirachta indica (Neem) leaf. In India, the tree is variously known as "Sacred Tree," "Heal All," "Nature's Drugstore," "Village Pharmacy" and "Panacea for all diseases." Products made from neem tree have been used in India for over two millennia for their medicinal properties: Neem products have been observed to be anthelmintic, antifungal, antidiabetic, antibacterial, antiviral, contraceptive and sedative It is considered a major component in Ayurvedic and Unani medicine and is particularly prescribed for skin disease reported by Zillur Rahman¹⁰. The influence of Azadirachta indica (neem) extract on Brass in Natural sea water environment using mass loss measurements with different time and temperature has been studied. The characterization of corrosion product on Brass in the presence of inhibitor is also reported by UV and IR studies.

MATERIALS AND METHODS:

Stock solution of Azadirachta indica leaf extract:

The Azadirachta indica leaf (AIL) materials was about 1 Kg collected from courtallam kills area. 150g of dried powder with required quantity of ethyl alcohol was added to cover the powder completely in a RB flask and left it for 48 hrs. The resulting paste was refluxed for 48 hrs and boiled with activated charcoal (about 1g) to remove hung and the pure fruit peel extract was collected.

Specimen preparation:

Rectangular specimen of Brass was mechanically pressed cut to form different coupons, each of dimension exactly 5.0x2x2.5 Cm. The specimens were mechanically polished; a hole drilled at one end for free suspension and identification numbered by punching. The specimens were decreased with trichloroethylene, washed with distilled water and well polished with emery paper, cleaned and dried then stored in desiccators for present our study.

Mass Loss method:

In the mass loss measurements, Brass coupon in triplicate was completely immersed in 50ml of the test solution of Natural sea water in the presence and absence of inhibitor. The metal specimens were withdrawn from the test solutions after an hour at 303K to 333K and also measured 24, 48, 72, 96, 120,144 and 168hrs at 303K temperature. The mass loss was taken as the difference in weight of the specimens before and after immersion determined using LP 120 digital balance with sensitivity of ±1 mg. The experiments were performed in triplicate to guarantee the reliability of the results and the mean value of the mass loss is reported. From the mass loss measurements, the corrosion rate was calculated using the following relationship.

(1)

Where, mmpy = millimeter per year, W = Mass loss (mg), D = Density (gm/cm³),

A = Area of specimen (cm²), T = time in hours.

The inhibition efficiency (%IE) and degree of surface coverage (θ) were calculated using Eq.(2) and Eq.(3), respectively.

(2)

Where, W₁ and W₂are the corrosion rate in the absence and presence of the inhibitor respectively.

RESULTS AND DISCUSSION:

The corrosion behavior of Brass in Natural sea water environment containing various concentration of Azadirachta indica leaf (AIL) extract at different time (24 to 168 hrs) and temperature (303K to 333K) are shown in Figures 1 and 2 respectively. In Figure 1 reflect that the percentage of inhibition efficiency (% IE) is increased with increase of inhibitor concentration from 0 to 1000 ppm. The maximum of 83% inhibition efficiency is observed at higher concentration of APL extract and it is gradually decreased with increase of exposure time. There is a gradual increase of inhibition efficiency with increase of inhibitor concentration from 0 to 1000 ppm. This highest inhibition efficiency is due to the presence of the heterocyclic compounds adsorption of the plant constituents on the metal surface facilitated. The inhibition efficiency of APL extract may be the adsorption of the main active compounds on the metal surface thereby blocking the surface and protecting the metal from the aggressive atmosphere. The degree of protection increased with increase of surface fraction occupied by the adsorbed molecules. The concentration of inhibitor versus increase of surface coverage at various temperature from 303K to 333K is shown in Figure 2. It is evident that the percentage of inhibition efficiency is increased with increase of inhibitor concentration and slowly decreased with rise in temperature. The maximum inhibition efficiency of 87 % is attained for 303K and 75% for 333K respectively. The degree of surface coverage is slowly decreased with rise in temperature is due to suggestive of physical adsorption on the metal surface as observed earlier by Ebenso et.al ¹¹.

Fig 1: Variation of percentage of inhibition efficiency with concentration of AIL extract in Natural sea water environment.

Fig 2: Variation of surface coverage with concentration of AIL extract in Natural sea water environment.

Mechanism of inhibition:

Azadirachta indica is composed of numerous naturally occurring organic compounds. Over 300 compounds have been isolated and characterised from the plant. Among these: azadirachtin, nimbin, nimbandiol, nimbinene, nimbolide, nimonol, nimbolin, salannin, margolone, melianol, vilasanin, flavanoids and structurally related compounds reported by Ganguli et.al ¹² . Most of these compounds have complicated molecular structures with high molecular weights and significant number of Oxygen, sulphur and nitrogen atoms incorporated in this molecular structure. These compounds can adsorb on the metal surface via the lone pair of electrons present on their oxygen, sulphur and nitrogen atoms. The adsorption of such compounds on the metal surface creates a barrier for charge and mass transfer leading to decrease in the interaction between the metal and the corrosive environment. As a result, the corrosion rate is decreased considerably. The inhibition properties of plant extracts may be due to the presence of nitrogenous compounds in the extract and tannins. In general organic compounds containing N, S and O atoms are good corrosion inhibitors. They may have been responsible for the formation of an oriented film layer which is essentially blocks the discharge of H⁺ and the dissolution of metal ions also previously suggested by Peter C. Okafor et.al ¹³.

Activation energy:

The Arrhenius equation was used to investigate the effect of temperature on the corrosion of Brass in the presence and absence of AIL inhibitor ¹⁴.

CR= Aexp (-E_a/RT) (4)

log (CR₂/CR₁) = E_a/2.303 R (1/T₁-1/T₂) (5)

Where, CR₁and CR₂ are the corrosion rate at the temperature T₁(303K) and T₂(333K) respectively. The values of Corrosion rate obtained from the mass loss measurements are substituted in Equation 4 and the calculated values of activation energy are presented in Table 1. The activation energy increased from 26.72 to 38.77 KJ/mol with increase of bio-inhibitor concentration. The average value of E_a obtained from the blank (19.18KJ/mol) is lower than that of the values obtained for a system containing various concentrations of AIL extract. This result indicated that the AIL inhibitor is adsorbed on the surface of Brass by physical adsorption.

Table: 1 Calculated values of Activation energy (E_a) and heat of adsorption (Q _ads) of Brass containing different concentration of AIL extract in Natural Sea water.

S. no	Concentration of inhibitor (ppm)	% of I.E		E_a (KJmol^-1)	Q _ads (KJmol^-1)
S. no	Concentration of inhibitor (ppm)	30^o	60^o	E_a (KJmol^-1)	Q _ads (KJmol^-1)
1.	Blank	--	--	19.185	---
2.	10	37.50	18.75	26.722	-27.525
3.	30	50.00	43.75	22.681	-07.882
4.	50	62.50	50.00	27.432	-13.691
5.	70	74.99	65.42	28.458	-11.939
6.	100	87.50	75.00	38.769	-22.436

Adsorption consideration:

The heat of adsorption on Brass in the presence of inhibitor is calculated by the following Equation 6 ¹⁵.

Q _ads=2.303 R [log (θ₂/1- θ₂ )-log (θ₁ /1- θ₁ )] x (T₂T₁/T₂-T₁ (6)

Where R is the gas constant, θ₁and θ₂ are the degree of surface coverage at temperatures T₁ and T₂respectively. The calculated values of Q_ads are reported in Table 1. These values are ranged from -27.52 to -22.44 KJ/mol. Bhajiwala et.al¹⁶ studied that the negative values are indicated that the adsorption of inhibitor on Brass surface is exothermic.

Basic information on the interaction between the inhibitor and the metal surface can be provided by the adsorption isotherm. For this purpose, the value of surface coverage (θ) at different concentrations of AIL extract in Natural sea water environment in the temperature range (303-333K) have been used to explain the best isotherm to determine the adsorption process. Gomma et.al¹⁷ shows that the adsorption of an organic adsorbate onto metal-solution interface was donated by a substitutional adsorption process between the organic molecules in the aqueous solution Org _(sol) and the water molecules on the metallic surface H₂O _(sol),

Org_(sol) + x H₂O → Org_(ads) + x H₂O_(sol)

Where Org _(sol) and Org_(ads) are the organic molecules in the aqueous solutions and adsorbed on the metallic surface, respectively, H₂O_(ads) is the water molecules on the metallic surface, and x is the size ratio representing the number of water molecules replaced by one molecule of organic adsorbate. In our present study Langmuir and Temkin isotherm are investigated.

The Langmuir and Temkin adsorption isotherm can be expressed by the Eq. (7) and Eq. (8) given below

log C/q = log C – log K (7)

q = K ln C (8)

Where, q is the surface coverage, C is the concentration of the inhibitor solution and K is an adsorption coefficient.

From Equation 7, by plotting the values of log( C/ θ) versus log C, linear plots were generated (fig 3). Inspection of this figure reveals that the experimental data fitted the Langmuir adsorption isotherm of AIL extract on Brass surface, meaning that there is no interaction between the adsorbed species.

Fig 3: Langmuir isotherm for the adsorption of Brass containing different concentration of AIL extract in Natural Sea water environment.

A plot of q versus log C give almost a straight line on Brass in sea water environment (Fig. 4). The straight line indicated that the inhibitor obeyed Temkin adsorption isotherm.

Fig 4: Temkin isotherm for the adsorption of Brass containing different concentration of AIL extract in Natural Sea water environment.

The equilibrium constant of adsorption of AIL extract on the surface of Brass is related to the free energy of adsorption ∆G _ads by Equation 9.

∆G_ads= -2.303 RT log (55.5 K) (9)

Where, R is the gas constant, T is the temperature and K is the equilibrium constant of adsorption. The values of K obtained from Langmuir and Temkin adsorption isotherm were substituted in Equation 8 and the calculated values of ∆G_adsare recorded in Table 2. The negative values of ∆G_ads suggested that the adsorption of AIL extract onto Brass surface is a spontaneous process and the adsorbed layer is stable one. Eddy et.al¹⁸ showed that the adsorption of free energy involved in a physisorption process (∆G_ads< 40 KJ/mol).

Table: 2 Langmuir and Temkin adsorption parameters for the adsorption of Brass containing different concentration of AIL extract in Natural Sea water.

Adsorption Isotherms	Tempt. (Kelvin)	Slope	logK	R²	∆G_ads KJ/mol
Langmuir	303	0.6295	1.1911	0.9963	-10.560
	313	0.5742	1.3683	0.9994	-11.270
	323	0.4636	1.6899	0.9892	-12.197
	333	0.4059	1.8874	0.9817	-12.873
Temkin	303	0.4863	0.6465	0.9644	-09.020
	313	0.4914	0.6997	0.9934	-09.524
	323	0.5476	0.8742	0.9982	-10.26
	333	0.5477	0.9223	0.9879	-10.875

UV analysis:

The figure 5 shows that the corrosion products on the surface of Brass containing AIL extract in Natural sea water environment. It reflects that only one broad adsorption peaks around 240 nm is suggested that the complex film formed between the metal surface and the inhibitor.

Fig 5: UV absorption spectrum for the corrosion product of Brass in the presence of AIL extract in Natural sea water environment.

IR analysis:

It is well established that FTIR spectrophotometer is a powerful tool that can be used to identify the type of bonding particularly, functional group (s) present in organic compounds. Since extracts contained organic compounds and it is adsorbed on the metal surface providing protection against corrosion. The analyses of FTIR is used, whether the corrosion product on the metal surface can be useful for protecting organic inhibitors are adsorbed or not on the metal surface. In present study, reflectance of FTIR spectra were used to support the fact that the corrosion inhibition of Brass in Natural sea water environment is due to the adsorption of inhibitor molecules on the metal surface. The prominent peaks (fig 6) are given in Table 3. From this data (Table-3), we conclude that reflectance FTIR spectra support good inhibition performance of Azadirachta indica leaves extract on the metal surface in Natural sea water environment.

Fig 6: IR absorption spectrum for the corrosion product of Brass in the presence of AIL extract in Natural sea water environment.

Table 3 : Prominent peaks obtained from reflectance FTIR spectroscopy.

Frequency (Cm^-1)

Band assignment

3780.20

3695.34

2923.87

1728.09

1600.00

1434.93

1380.93

1242.07

7779.18

O-H _(S) stretching

O-H _(b) stretching

C-H _(S) stretching

C=O _(S) stretching

N-H _(b) stretching

C=C _(S) stretching

C-H _(W) stretching

C-O _(S) stretching

C-Cl _(S) stretching

CONCLUSION:

The following conclusions can be drawn from our present study: The Azadirachta indica inhibitor acts as an effective and efficient inhibitor for the dealloying process of Brass in Natural sea water environment. The inhibition efficiency increased with increase of inhibitor concentration to reach maximum of 83% and it is gradually decreased with raise in temperature and period of contact. It has also been found out that the corrosion of Brass in Natural sea water environment depends on concentration of inhibitor, period of contact and the temperature. Corrosion inhibition may be due to the adsorption of the plant constituents on the surface of the metal. The adsorption of the inhibitor on the surface of Brass is exothermic, and is consistent with the mechanism of physical adsorption. The negative value of the free energy of adsorption is an indication of the spontaneity of the adsorption process. Langmuir and Temkin isotherms are best described the adsorption characteristics of the inhibitor. The corrosion product over the surface of Brass in the presence of AIL extract is characterized by UV and IR studies and may conform the complex film formed between the metal and the active groups present in the inhibitor.

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Received on 12.06.2011 Modified on 29.06.2011

Asian J. Research Chem. 4(9): Sept, 2011; Page 1469-1473