Inhibition of Corrosion of Mild Steel in 0.5 M H₂SO₄ by Tectona grandis Leaves

 

N. Kasthuri¹, R. Saratha², R. Meenakshi² and H.N. Meenakshi²

¹Department of Chemistry, Sengunthar College of Engineering for Women, Tiruchengode, India.

²Department of Chemistry, Avinashilingam University for Women, Coimbatore, India.

*Corresponding Author E-mail:  Saratha63@gmail.com

ABSTRACT:

The inhibition efficiency of acid extract of leaves of Tectona grandis [TG] on the corrosion of mild steel in 0.5 M H₂SO₄ was investigated by weight loss measurements and polarisation studies. The results obtained show that the extract could serve as an effective inhibitor for the corrosion of mild steel in H₂SO₄ media. Inhibition was found to increase with increasing concentration of the plant extract. Polarization curves revealed that this inhibitor acts as a mixed type inhibitor. The inhibition efficiency of up to 97.46% at 2.5 %( v/v) concentration of the extract was obtained.

 

 

 

INTRODUCTION:

Mild steel finds application in many 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 solution during acid cleaning, transportation of acid, de-scaling, storage of acid and other chemical processes. The heavy loss of metal as a result of its contact with acids can be minimized to a great extent by the use of corrosion inhibitors. Inorganic compounds like chromates, phosphates, molybdates and a variety of organic compounds containing heteroatom like nitrogen, sulphur and/or oxygen are being investigated as corrosion inhibitors^1-7.

 

Pure synthetic chemicals are costly, some of them are toxic and their disposal creates pollution problems. Plant extracts are environment friendly, bio-degradable, non-toxic, easily available and of potentially low cost. Most of the naturally occurring substances are safe and can be extracted by simple procedures. Recent literature is full of researches which test different extracts for corrosion inhibition applications. The examples are numerous such as fenugreek⁸, olive^9-12, jojoba¹³, black pepper¹⁴, Occimum viridis¹⁵, Andrographis paniculata¹⁶, Phyllanthus amarus¹⁷, garlic¹⁸, Eugenia jambolans¹⁹, Pongamia glabra²⁰, Opuntia²¹, Eugenol²² and Citrus aurantifolia²³ etc. Many of these naturally occurring substances proved their ability to act as corrosion inhibitors for the corrosion of different metals and alloys in different aggressive media.

 

The aim of this present study is to investigate the corrosion inhibition effect of leaves extract of Tectona grandis (TG), as a cheap and environment friendly corrosion inhibitor for mild steel in 0.5 M H₂SO₄ medium by Weight loss and Polarisation measurements.

 

 

EXPERIMENTAL:

Sample preparation:

The composition of the mild steel specimen was analyzed using ARL 3460 Metal Analyzer, Optical Emission Spectrometer. The weight percentage composition of the specimen is as follows C=0.0715, Si = 0.092, Mn = 0.1747, P = 0.0169, S = 0.0162,

Cr = 0.0095, Mo = 0.002, Ni =0.0048, V = 0.003, Al = 0.0370, Cu = 0.006, Ti = 0.0008, Nb= 0.0006, W =0.0006, Pb = 0.0004, B = 0.0007, Sb = 0.0001, Bi = 0.002, Ca = 0.0005, Zn = 0.0004, Ce = 0.0001 and Fe = 99.5618.

 

Mild steel Sheets of 2 mm thickness were obtained locally and were mechanically cut in to coupons of 5x1 cm² sizes, at one end of each specimen a hole of uniform diameter were drilled to facilitate suspension of the coupon in the test solution. The coupons were mechanically cleaned followed by polishing with emery sheet of fine quality to expose shining polished surface. To remove any oil and organic impurities coupons were degreased with acetone and finally washed with de-ionized water, dried and stored in a desiccator for further study. Accurate weights of the samples were taken using electronic balance (Denver). For Polarization studies mild steel specimens with an exposed area 1 cm² were used.

Extraction of plant materials:

The leaves were collected from a farm in Thondamuthur, Coimbator, shade dried and powdered. Plant materials are dried in shade so as to enrich the active principles in them by reducing their moisture content. The extract was prepared by refluxing 25 g of powdered dry leaves in 500 mL of 0.5 M H₂SO₄ for 3 h and kept overnight. Then it was filtered and the volume of the filtrate was made up to 500 mL using the same acid and this was taken as stock solution.

 

Weight loss measurements:

Weighed samples are immersed in 100 mL of the acid in the absence and presence of different concentrations of the inhibitor for various intervals of time. They are then taken out and immersed in saturated sodium bicarbonate solution to remove residual acids and then washed thoroughly with tap water, rinsed with distilled water, dried, stored in desiccators and reweighed. The percentage of inhibition efficiency (IE %) for various concentrations of the inhibitor were calculated as

 

Where, W₀ and W are the corrosion rates of mild steel in 0.5 M H₂SO₄ in the absence and presence of definite concentrations of the inhibitor.

 

Polarization studies:

A frequency response analyzer 1280 B (Solatron) and an IBM personal computer which automatically controls linear polarization and Tafel polarization was used for the polarization study. The data were analyzed using computer software. The working electrode was mild steel specimen (1 cm²). The surface of the mild steel specimens (area 1 cm²) were mechanically polished and degreased with detergent. The saturated calomel electrode and the platinum foil were used as reference and counter electrode respectively. The inhibition efficiency (IE %) was evaluated from the potentiodynamic polarization data by the formula,

 

Where, i _corr  and i'_corr are the corrosion current densities in the absence and the presence of the inhibitor.

 

Impedance measurements:

Electrochemical impedance spectroscopy (EIS) was carried out with a frequency response analyzer 1280 B (Solatron) and an IBM personal computer. After the determination of steady-state current at a given potential, sine wave voltage (10 mV) peak to peak, at frequencies between 10 kHZ and 0.02 mHZ were superimposed on the rest potential. Computer programs automatically controlled the measurements performed at rest potential after 2 min of exposure. The impedance diagrams are given as Nyquist plots.

The inhibition efficiency is calculated from the charge-transfer resistance using the formula,

 

R_t and R'_t are the charge-transfer resistances in the presence and absence of the inhibitor respectively.

 

RESULTS AND DISCUSSION:

Effect of Concentration and Immersion period:

Table 1 shows the percentage inhibition efficiency obtained with different concentrations of the plant extract in H₂SO₄ medium by weight loss method. The IE% was found to be increasing with increase in the concentration of the extract with maximum IE % of 97.46% at 7 h immersion time for 2.5% v/v. It is evident that the corrosion inhibition may be due to adsorption of the plant constituents on the metal surface. The adsorption of the phytochemical constituents on the metal surface makes a barrier for mass and charge transfer and thus protects the metal surface from corrosion. The degree of protection increases with the increase in surface fraction occupied by the adsorbed molecules. Further increase in immersion period showed a decrease in efficiency. This may be due to the desorption of the constituents from the metal surface.

 

Table 1.Inhibition efficiency of mild steel in the presence of leaves extract of TG at 303 K

CONC (%) (v/v)	INHIBITION EFFICIENCY (%)
	½ h	1 h	3 h	7 h	24 h	48 h	168 h
Blank 0.005 0.010 0.050 0.100 0.150 0.200 0.500 1.000 1.500 2.500	- 38.26 55.40 57.40 63.66 64.66 65.34 68.14 69.00 70.81 74.04	- 50.53 63.16 77.46 80.69 82.67 85.28 87.99 90.31 93.94 94.53	- 76.43 81.32 88.21 90.05 90.89 91.27 92.19 93.52 94.04 94.56	- 80.63 86.13 92.50 93.49 94.30 94.35 95.44 96.20 96.36 97.46	- 63.10 78.35 91.18 92.26 92.99 93.95 95.07 96.00 96.15 96.20	- 25.04 43.34 83.16 86.02 87.23 89.05 91.17 93.30 93.53 93.85	- 3.21 6.64 17.73 36.33 51.28 58.08 69.31 76.86 79.82 79.95

 

Electrochemical Measurements - Potentiodynamic polarization studies:

Potentiodynamic polarization curves for mild steel in 0.5 M H₂SO₄ containing different concentrations of the extract are shown in figure 1.The respective potentiodynamic parameters are given in Table 2.  The addition of plant extract decreased the values of corrosion potential (E_corr) and corrosion current (I _corr).  It is clear from the potentiodynamic curves that both anodic and cathodic reactions of mild steel in acid extract are controlled and suggests that the inhibitor act as a mixed type.

 

Impedance studies:

Nyquist plots obtained for mild steel in 0.5 M H₂SO₄ in the presence of various concentrations of the inhibitor are depicted in figure 2.  They are perfect semicircles and this is attributed to charge transfer reaction. Impedance parameters derived from Nyquist plots are given in Table 3. It can be seen that as the concentrations of the inhibitor increases, C_dl values decrease. The decrease in the values of C_dl may result from an increase in thickness of electrical double layer, suggests that the inhibitor molecules function by adsorption at the metal-solution interface.

 

 

Table 2. Potentiodynamic polarization parameters for mild steel in the presence of leaves extract of TG.

Conc. (%) (v/v)	-Ecorr mV	I corr µAcm-2	Ba mV/dec	Bc mV/dec	I.E (%)	Rp (Ω cm-2)	I.E (%)
Blank 0.005 0.05 0.15 0.5 2.5	548.59 544.21 524.20 518.86 516.03 517.13	5.0670 4.2290 0.0950 0.0580 0.0360 0.0210	318.90 359.40 185.10 159.53 153.89 175.71	215.10 212.96 110.87 98.10 87.44 79.78	- 16.54 98.13 98.86 99.29 99.59	10.90 13.86 31.87 45.81 67.09 110.12	- 21.36 65.80 76.21 83.75 90.10

 

 

Figure 1. Potentiodynamic polarization curves of mild steel in 0.5 M H₂SO₄ in the presence of leaves extract of TG.

 

Table 3. Impedance parameters for mild steel in the presence of leaves extract of TG.

Conc. (%)(v/v)	Rct (Ωcm-2)	Cdl (µFcm-2)	I.E (%)
Blank 0.005 0.05 0.15 0.50 2.5	13.16 18.11 47.50 61.24 107.26 163.22	88.19 78.31 72.68 71.49 51.54 33.34	- 27.33 72.29 78.51 87.73 91.94

 

Figure 2.  Nyquist plot of mild steel immersed in 0.5 M H₂SO₄ in the absence and presence of leaves extract of TG.

 

CONCLUSION:

Acid extract of leaves of Tectona grandis (TG) leaves acts as a good corrosion inhibitor for mild steel in 0.5 M H₂SO₄ medium. Inhibition efficiency increases with increase in inhibitor concentration and the maximum inhibition efficiency was 97.46 % at the inhibitor concentration 2.5% v/v. Corrosion inhibition may be due to the adsorption of the plant constituents on the mild steel surface. Polarization studies indicate that the inhibitor is of mixed type and inhibiting both cathodic as well as anodic reactions.

 

ACKNOWLEDGEMENTS:

The authors would like to thank the authorities of Avinashilingam University for women for providing necessary facilities to carry out this study.

 

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Received on 13.01.2010        Modified on 09.02.2010

Accepted on 20.03.2010        © AJRC All right reserved