Influence of Ocimum tenuiflorum Extract on Mild Steel in Acid Environment

 

P. Deepa Rani1* and S. Selvaraj2

1Dept. of Chemistry, Post Graduate and Res. Centre, Sri Paramakalyani College, Alwarkurichi-627412, T. N. India.

2Department of Chemistry, Faculty of Chemistry, Post Graduate and Research centre,

Sri Paramakalyani College, Alwarkurichi-627412, Tamil Nadu, India.

*Corresponding Author E-mail: rani.Deeps@yahoo.in

 

ABSTRACT:

The inhibition effect of Ocimum tenuiflorum extract on the corrosion of mild steel in 2M Sulphuric acid has been studied at different temperature as well as concentrations of inhibitor by mass loss studies. The inhibition efficiency is markedly higher in H2SO4 environment with addition of Ocimum tenuiflorum extract compared with those in the inhibitor free solution. The inhibition efficiency increased with increase of inhibitor concentration but decreased with increase in temperature and exposure time. Adsorption of Ocimum tenuiflorum extract on mild steel is found to obey Langmuir, Temkin, Flory-Huggins and Frumkin adsorption isotherm. The characterization of corrosion product on mild steel in the presence of inhibitor is analyzed by UV, IR, and XRD.

 

KEYWORDS: Corrosion inhibition, mild steel, mass loss, Ocimum tenuiflorum.

 


 

INTRODUCTION:

Mild steel finds many applications in industries due to its easy availability, ease of fabrication, low cost and good tensile strength besides various other desirable properties. It suffers from severe corrosion when it comes in contact with acid solutions during acid cleaning, transportation of acid, de-scaling, storage of acids and other chemical processes etc. Saratha et.al1 studied that the heavy loss of metal whenever it contact with acids can be minimized to a great extent by the use of corrosion inhibitors. Most of the well known acid inhibitors are Organic compound containing hetero atoms such as N, S and O atoms investigated by Aytac et.al2 and Babi-Samardzija et.al3. Pandian et.al4 suggested that the organic compounds present in the inhibitor can absorb on the metal surface, block the active sites on the surface and thereby reduce the corrosion rate. Obot et.al5 reported that most of the synthetic organic compounds showed good anticorrosive activity, they are highly toxic to cause severe hazards to both human beings and the environment during its application. The safety and environmental issues of corrosion inhibitors arisen in industries have always been a global concern. The recent trend is save human being by eco- friendly inhibitors.

 

Some investigators namely, Mesbah et.al6, Okafor et.al7 and Rajendran et.al8 indicated that the plant extract 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. Several efforts have been made using corrosion preventive practices and the use of green corrosion inhibitors is reported by Anuradha et.al9. Bothi Raja et.al10 and Lebrini et.al11 suggested that they are the rich sources of molecules which have appreciably high inhibition efficiency and hence termed as “Green Inhibitors”. These inhibitors are biodegradable and do not contain heavy metals or other toxic compounds investigated by Sharma et.al12. The successful use of naturally occurring substances to inhibit the corrosion of metals in acidic and alkaline environment have been reported by some research groups namely Muhamath et.al13, James et.al14, Eddy et.al15, Okafor et.al16, Ehteram et.al17 Valek et.al18, and Sharmila et.al19. In our present study, we have chosen eco-friendly inhibitor, a green approach to prevent environmental pollution by harmful organic chemicals. The influence of Ocimum tenuiflorum (Tulsi) extract on mild steel in 2M Sulphuric acid using Mass loss method with different time and temperature have been studied. The characterization of corrosion product on mild steel in the presence of inhibitor is also analyzed by UV, IR, and XRD.

 

MATERIALS AND METHODS:

Specimen preparation:

Rectangular specimen of mild steel was mechanically pressed cut to form different coupons, each of dimension exactly 5x2.5x2 Cm. Specimens containing a small hole of 2mm diameter near the upper edge were used for the determination of Corrosion rate. Each coupon was degreased by washing with Trichloroethylene then dried in acetone and preserved in desiccators. All reagents for the present study were analar grade and double distilled water was used for their preparation. Each specimen was suspended by a glass hook and immersed in a beaker containing exactly 50ml of the test solution and left exposed to air. Evaporational losses were made up with double distilled water. After the exposure, the test specimens were cleaned with acetone. Triplicate experiments were performed in each case and the mean values of Mass loss were calculated.

 

Chemical properties of Ocimum tenuiflorum:

Staples et.al20 reported that the Ocimum tenuiflorum (Tulsi or Holy Basil) is an aromatic plant belongs to Lamiaceae family, which is native throughout the Old world tropics. Tulsi extracts are used in ayurvedic remedies for common cold, headache, stomach disorders, inflammation, heart disease, various forms of poisoning, and malaria etc. It is mostly used for medicinal purposes and in herbal cosmetics due to its anti-bacterial activity. The chemical composition of Tulasi is very complicated because it contains numerous compounds. Some of the important active compounds are Rosmarinic acid, Carvacrol, terpene, β-Caryophlene, Pegenin, Ursolic, Oleanolic acid, Citral, nerol, greaniol, proline and α-humulene etc. These compounds present in entire plant consist of anti-oxidant, adaptogenic (controls the frequent mood swings and provide the mental peace and clarity), anti-inflammatory, antibacterial and immune-enhancing properties. The polyphenolic Rosmarinic acid present in the Tulsi act as the powerful antioxidant and anti-inflammatory reported by Jyoti Sethi et.al21. The Carvacrol and terpene are the antibacterial agents present remarkable in the plant. The presence of  β-Caryophlene in the plant also serves the same purpose studied by Norr et.al22. Pegenin is one more compound available in the composition serving the anti-inflammatory and antioxidant. Kuhn et.al23 investigated that the Ursolic and Oleanolic acid also perform the same function of adaptogen and are very effectual in dropping the stress levels.

 

Preparation of Ocimum tenuiflorum extract:

The Ocimum tenuiflorum plant materials of about 1 Kg collected from the source and dried in natural air condition for 3 to 5 days in Sunshade. Then it is grained well and finely powdered. Exactly 100g of finely powdered dried material was taken in a 500ml round bottom flask and required quantity of ethyl alcohol was added to cover the powder completely. The RBF was covered with stopper and left for 48 hrs. The resulting paste was refluxed for 48 hrs and then it was filtered. The filtrate was collected and the presence of alcohol was removed with the help of distillation unit. The obtained paste mass was boiled with activated charcoal (about 1g) to remove hung and the pure plant extract is collected.

 

Mass Loss method:

In the Mass loss measurements, mild steel coupons in triplicate were completely immersed in exactly 50ml of the test solution of 2M Sulphuric acid in the presence and absence of the inhibitor. The specimens were withdrawn from the test solutions after 24, 48, 72, 96 and 120 hrs exposure at room temperature. Also the temperature varied at 303K and 333K were measured an hour. 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 tests were performed in triplicate to justify 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)

                                  (3)

Where, W1 and W2 are the corrosion rates in the absence and presence of the inhibitor respectively.

 

RESULTS AND DISCUSSION:

In the present study, the dissolution behavior of mild steel in 2M Sulphuric acid containing different concentration of Ocimum tenuiflorum extract with various time and temperature are investigated and their results are presented in the graphical form of Figures 1-3.

 

Fig 1: Variation of Corrosion rate with time from 24hrs to 120hrs in 2M Sulphuric acid on mild steel.

 

Fig 2: Variation of surface coverage (θ) with concentration of Ocimum tenuiflorum extract  in 2M Sulphuric acid on mild steel.

 

Fig 3: Variation of inhibition efficiency with concentration of Ocimum tenuiflorum extract on mild steel in 2M Sulphuric acid

 

Effect of time duration:

The Fig-1 shows the variation of corrosion rate with exposure time in 2M Sulphuric acid on mild steel in the presence and absence of inhibitor. It found that the corrosion rate gradually decreased with increase of exposure time (i.e., from 43.51 to 13.00mmpy). This is because of the formation of oxide film coated on the metal surface due to the presence of Sulphuric acid at the initial stage. However it is necessity to prevent further dissolution of the metal due to its unstability of the oxide film by using the Ocimum tenuiflorum extract markedly reduce the corrosion rate as shown in Fig-2. It indicates that the surface coverage of the metal increased with increase of inhibitor concentration. The maximum of 80.38% of inhibition efficiency is achieved at higher concentration (5000ppm). The inhibition efficiency is gradually decreased with increase of exposure time. Even after 120hrs exposure we may achieve more than 50% of inhibition efficiency. This result reveals that the active molecules present in Ocimum tenuiflorum extract is interact with Fe present in the mild steel.

 

Effect of temperature:

The variation of inhibition efficiency with concentration of Ocimum tenuiflorum extract on mild steel in 2M H2SO4 at different temperature are investigated and the results are presented in Fig-3. In the absence of inhibitor the corrosion rate is increased from 60.18 to 75.78 mmpy with increase of temperature (i.e. 303K to 333K). But in the presence of Ocimum tenuiflorum extract the corrosion rate is substantially reduced and achieved maximum of 86.67% of inhibition efficiency at 303K. However the percentage of inhibition efficiency is slightly decreased with rise in temperature from 303K to 333K. The decrease of inhibition efficiency is due to some kind of competition between forces of adsorption and desorption on the surface of mild steel by certain specific molecules present in the extract may active the corrosion inhibition reactions in the different layers and cathodic sites on the mild steel surface. This observation suggests that the adsorption mechanism of inhibitor on the surface may be the physical adsorption.

 

Adsorption studies:

Adsorption studies are very important to determine the mechanism of corrosion reaction. The most frequently used isotherms are Langmuir, Temkin, Frumkin and Flory- Huggins and Bockris-Sinkles24-26.

The Langmuir and Temkin adsorption isotherms for mild steel in 2M Sulphuric acid with Ocimum tenuiflorum extract are studied for time duration (24 to 120 hrs). The mass loss measurements are tested graphically for fitting two isotherms like Langmuir and Temkin. It is expressed by the Equation 4 and 5 given below,

               log C/q  = log C – log K     --------------- (4)

               q = K ln C                         --------------- (5)

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

 

By plotting values of logC/q versus logC, linear plots are generated (Fig-4) and conforming that the experimental data fitted with the Langmuir adsorption isotherm for the adsorption of inhibitor on mild steel means that there is no interaction between the adsorbed species.

 

Fig 4: Langmuir isotherm for the adsorption of Ocimum tenuiflorum extract on mild steel in 2M Sulphuric acid.

A plot of q versus logC give almost a straight line on mild steel in ocimum tenuiflorum extract (Fig -5)  indicate that the inhibition action obeyed Temkin adsorption isotherm.

 

Fig 5: Termkin isotherm for the adsorption of Ocimum tenuiflorum extract on mild steel in 2M Sulphuric acid.

 

Frumkin and Flory-Huggins isotherm at various temperature:

The effect of temperature on surface coverage on mild steel in 2M Sulphuric acid with inhibitor and the temperature ranges from 303K to 333K is also investigated.  The experimental data obtained from the mass loss measurements is also tested for best fits to the Flory-Huggins isotherm model given by the Equation- 6.

 

               Log (θ/C) = log K+ x log (1-θ)   ------------ (6)

 

Where, θ is the degree of surface coverage, C is the inhibitor concentration, x is the number of water molecules replaced by one molecules and K is the equilibrium constant for the adsorption process.

 

The Fig-6 shows the plot of log (θ/C) against log (1-θ), Straight lines are obtained with linear correlation coefficients values close to unity, (0.96, 0.99 and 0.99) for 303K, 313K and 333K respectively. This is clearly indicates that the adsorption of inhibitor onto the metal surface at 303K to 333K can be approximated by Flory-Huggins adsorption isotherm. The values of x obtained from Flory-Huggins adsorption isotherm value greater than unity, the values being 1.2918, 4.2920 and 4.6828 at 303K, 313K and 333K respectively. This results indicate that each molecule of the inhibitor occupy more than one active site on the metal surface.

 

Frumkin adsorption isotherm given by the Equation-7

 

               Log {[C](θ/1- θ)}= 2.303 log K+ 2 α θ   -----(7)

 

Where, K is the adsorption-desorption equilibrium constant and α is the lateral interaction term describing the molecular interaction in adsorbed layer. Fig-7 shows that the adsorption plots for the adsorption of Ocimum tenuiflorum extract on mild steel surface. Eddy et.al27 suggested that the values of α were found to be positive indicating that the attractive behaviour of inhibitor on the surface of mild steel.

 

Fig:6  Flory-Huggins isotherm for the adsorption of Ocimum tenuiflorum extract on Mild steel in 2M Sulphuric acid.

 

Fig:7  Frumkin isotherm for the adsorption of Ocimum tenuiflorum extract on Mild steel in 2M Sulphuric acid.

 

Fig 8 : UV spectrum of ethanolic crystals of Ocimum tenuiflorum.

UV Analysis

The Fig- 8 and 9 shows that the UV spectrum of ethanolic crystals of Ocimum tenuiflorum extract and the corrosion product on the surface of mild steel in the presence of inhibitor. The Fig-8 reflects that one absorption band around 308nm and Fig-9 shows that a band around 244nm has been noticed. This suggest the λmax value shifted from bathochromic to hypsochromic (i.e,longer wavelength to shoter wavelength region) conformed that the formation of complex between Iron in mild steel and the active functional group in the inhibitor.

 

Fig 9:  UV absorption spectrum of the corrosion product of mild steel in the presence of  Ocimum tenuiflorum extract in 2M H2SO4 environment.

 

Fig10: IR spectrum of ethanolic crystals of Ocimum tenuiflorum.

 

IR Analysis:

The Fig-10 and 11 shows that the IR spectrum of ethanol extract of Ocimum tenuiflorum and the corrosion product on the surface of mild steel in the presence of inhibitor. On comparing the spectra of pure ethanolic crystal of Ocimum tenuiflorum (Fig-10) with spectra of adsorbed active molecules of Ocimum tenuiflorum extract over mild steel (Fig-11). It is observed that certain peak have been disappeared completely and some have shifted to higher frequency region (Fig-11) providing that some interaction/ adsorption has been taking place over the metal surface. It  is  found that the –OH stretch shifted from  3450 cm-1 to 3470 cm-1 may suggest that there is a interaction between Ocimum tenuiflorum extract and the Fe in the mild steel.

 

Fig.11: IR spectrum of mild steel in the presence of Ocimum tenuiflorum extract in 2M H2SO4 environment.

 

XRD analysis:

The Corrosion product scraped from the mild steel surface after immersion was examined by XRD studies (Fig -12). It may be assigned mainly the combination with the rich amount of Fe2O3, Fe2 (SO4)3 FeOCl and FeCl3. This result reveals that the co-ordination between the active molecules in the inhibitor and Fe in mild steel.

 

Fig 12: XRD spectrum of mild steel in presence of  Ocimum tenuiflorum extract in 2M H2SO4 environment.

 

CONCLUSION:

From our present study, the results concluded that Ocimum tenuiflorum extract can be used as an inhibitor for mild steel in 2M H2SO4 environment at different time and temperature. It has also been found that the corrosion of mild steel in 2M H2SO4 depends on concentration of inhibitor, period of contact and temperature. Corrosion inhibition may be due to the adsorption of the plant constituents on the mild steel surface. The adsorption of the inhibitor onto mild steel surface is found to obey Langmuir, Temkin, Florry-Huggins and Frumkin isotherm. The corrosion product over the surface of mild steel in the presence of Ocimum tenuiflorum extract is characterized by FTIR, UV, and XRD studies. The examination of UV and IR studies may conform that  the complex film formation between the active molecule in the inhibitor and Fe  in the  mild steel .The results of XRD studies may suggest that the complex film mainly composite of Fe2O3, Fe2 (SO4)3 FeOCl and FeCl3  on the surface of the mild steel.

 

ACKNOWLEDGEMENT:

The authors sincerely thanks to Dr, P. T. Perumal, Deputy Director, the Central Leather Research Institute, (CLRI), Adyar, Chennai, for giving the opportunity to take spectral studies. Also I would express our heartful thanks to the management of Sri Paramakalyani College, Alwarkurichi, for providing the lab fecilities.

 

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Received on 17.09.2010        Modified on 26.09.2010

Accepted on 30.09.2010        © AJRC All right reserved

Asian J. Research Chem. 4(2): February 2011; Page 211-216