Research Journal of Chemical Sciences ___ ______________________________ ______ ____ ___ ISSN 22 31 - 606X Vol. 3 ( 2 ), 10 - 15 , February (201 3 ) Res. J. Chem. Sci. International Science Congress Association 10 Inhibition of Corrosion of Carbon Steel in Sea Water by an Aqueous Extract of Eclipta alba Leaves โ€“ Zn 2+ system Johnsirani V. 1 , Sathiyabama J. 1 , Prabhakar P. 2 and Susai Rajendran 1,3 1 PG and Research Department of Chemistry, GTN Arts College, Dindigul - 6 24005, Tamil Nadu, INDIA 2 PG and Research Department of Chemistry, APA College of Arts and Culture Palani, Tamil Nadu, INDIA 3 Department of Chemistry, RVS School of Engineering and Technology, Dindigul - 624005, INDIA Available online at: www.isca.in Received 22 nd September 201 2 , revised 2 nd October 201 2 , accepted 2 nd November 2012 Abstract The Inhibition efficiency [IE] of an aqueous extract of eclipta alba leaves in controlling corrosion of carbon steel in sea water [ Thondi, Tamil Nadu, India] has been evaluated by weight loss method. The weight loss study reveals that the formulation consisting of 6mL of EAE (Eclipta alba extract ) and 25 ppm of Zn 2+ has 92% inhibition efficiency in controlling corrosion of carbon st eel in sea water. Polarization study reveals that EAE and Zn 2+ system functions as mixed type inhibitor. The nature of the metal surface has been analysed by FTIR spectra and SEM analysis. Keywords: Corrosion inhibition, Eclipta alba, c arbon steel, sea w ater, SEM, FTIR. Introduction Seawater is one of the most corroded and most abundant naturally occurring electrolytes. The corrosivity of the seawater is reflected by the fact that most of the common structural metals and alloys are attacked by this liq uid or its surrounding environments. The seawater environments can be divided into five zones namely: subsoil, continuously submerged, tidal, splash zone above high tidal and atmospheric zone 1 . Plant extracts are low - cost and biodegradable, and so the st udy of plant extracts as corrosion inhibitors is an important scientific research field due to both economic and environmental benefits. As early as in 1930, plant extracts (dried stems, leaves and seeds) of Chelidonium majus and other plants were used as corrosion inhibitors for steel in H 2 SO 4 pickling baths 2 . In 1972, El Hosary et al . 3 studied the extract of Hibiscus subdariffa (Karkode) as the corrosion inhibitor for Al and Zn in HCl and NaOH solutions. In 1980s, Saleh et al . 4,5 reported the inhibition effect of aqueous extracts of some plant leaves ( Opuntia , Aleo eru ) and fruit peels (orange, mango) on the corrosion of steel, aluminum, zinc and copper in acids and aluminum in NaOH solution. In1990s, Azadirachta 6 and Vernonia amygdalina (bitter leaf) 7 leaves extracts were reported as good corrosion inhibitors for steel in HC l and H 2 SO 4 solutions. In the present research work the extract of Eclipta alba leaves is taken as it is a good corrosion inhibitor for carbon steel in marine media collected from Bay of Bengal at Thondi,a small town located in Ramnad District, Tamil Nadu, India. Eclipta alba, a medicinal herb is grown in many parts of India. The parts of the whole plant are used to cure many diseases. In ayirvedic medicine, the leaf extract is considered a powerful liver tonic, rejuvenative, and especially good eclipta prostrate is used for dyeing hair and tattooing 8 - 9 . Scheme - 1 Leaves of Eclipta alba The present work is undertaken: i. To evaluate the inhibition efficiency (IE) of Eclipta alba extract (EAE) - Zn 2+ system in controlling corrosion of carbon steel immersed in sea water in the absence and presence of Zn 2+ by weight loss method. ii. To study the mechanism of corrosion inhibition by polarization study iii. To analyse the protective film by FTIR spectra, scanning electron microscope (SEM), iv. To propose the mechanism of corrosion inhibition based on the above results. Material and Methods Prepa ration of plant extract : The leaves of Eclipta alba were dried and ground to powder and 10gm of the powdered leaves Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 10 - 15 , February (201 3 ) Res. J. Chem. Sci. International Scien ce Congress Association 11 weighed and boiled with double distilled water, filtering the suspending impurities, and making up to 100mL. The extract was used as corrosi on inhibitor in the present study. Preparation of Specimen: Carbon steel specimens (0.02 6% S, 0.06% P, 0.4 % Mn, 0.1% C and rest iron) of the dimensions 1.0 x 4.0 x 0.2 cm were polished to a mirror finish, degreased with trichloroethylene, and used for the weight - loss method and surface examination studies. Weight โ€“ Loss Method: Carbon steel specimens were immersed in 100 ml of the medium containing various concentrations of the inhibitor in the absence and presence of Zn 2+ for one day. The weights of the specimens before and after immersion were determined using a Digital Balance (Model AUY 220 SHIMADZU). The corrosion products were cleaned with Clarkeโ€™s solution 10 . The corrosion IE was then calculated using the equation. IE = 100 [1 - (W 2 /W 1 )] % Where W 1 is the weight loss value in the absence of inhibitor , W 2 is the weight loss value in the presence of inhibitor , Corrosion rate was calculated using the formula 11 , Corrosion rate (mm/year) = 87.6 W/ DAT . Where W = weight loss in milligram, D = density o f specimen g/cm 3 , A = area of specimen in square cm,T=exposure time in hours. Potentiodynamic Polarization Study: Polarization studies were carried out in a CHI - electrochemical work station with impedance model 660A. It was provided with iR compensation facility. A three electrode cell assembly was used. The working electrode was carbon steel. A SCE was the reference electrode. Platinum was the counter electrode. From polarization study, corrosion parameters such as corrosion potential (E corr ), corrosion current (I corr ), Tafel slopes anodic = b a and cathodic = b c were calculated and linear polarization study (LPR) was done. The scan rate (V/S0.01. Hold time at (E fcs ) was zero and quiet time (s) was two. Surface examination study: The carbon steel specime ns were immersed in various test solutions for a period of one day. After one day, the specimens were taken out and dried. The nature of the film formed on the surface of the metal specimen was analyzed for surface analysis technique by FTIR spectra and s canning electron microscopy. Fourier transform infrared spectra: These spectra were recorded in Perkin - Elmer - 1600 spectrophotometer using KBr pellet. The FTIR spectrum of the protective flim was recorded by carefully removing the flim, mixing it with KBr a nd making the pellet. Results and Discussion The physicochemical parameters of sea water are given in table 1. Table - 1 Water analysis (Thondi sea water, Tamil Nadu, India) Parameters Result Total dissolved salts (mg/L) 41881 ppm Electrical conductivity (micro mhos/cm) 61589 pH 7.86 Total Hardness (CaCO 3 equivalent) 6100 ppm Calcium as Ca (mg/L) 800 ppm Magnesium as Mg (mg/L) 984 ppm Sodium as Na (mg/L) 9800 ppm Chloride as Cl (mg/L) 18256 ppm Potassium as K (mg/L) 1300 ppm Sulphate as SO 4 (mg/L ) 1493 ppm The calculated inhibition efficiencies (IE) of Eclipta alba Extract (EAE) in controlling the corrosion of carbon steel immersed in sea water both in the absence and presence of zinc ion have been tabulated in table 2. The calculated values ind icate the ability of Eclipta alba Extract to be a good corrosion inhibitor. The inhibition efficiency is found to be enhanced in the presence of zinc ion. The formulation consisting of 6mL of EAE and 25 ppm of Zn 2+ offers 92% inhibition efficiency. That is mixture of inhibitors shows better IE than the individual inhibitors 12 . Potentiodynamic Polarization Study : Polarization study has been used to detect the formation of protective film on the metal surface. When a protective film is formed on the metal su rface, the linear polarization resistance (LPR) increases and the corrosion current (I corr ) decreases. The potentiodynamic polarization curves of carbon steel immersed in various test solutions are shown in figure - 1. The corrosion parameters namely, corros ion potential (E corr ), Tafel slopes (b c =cathodic; b a =anodic), linear polarization resistance (LPR) and corrosion current (I corr ) are given in table 3. When carbon steel is immersed in sea water the corrosion potential is - 816 mV vs SCE. The formulation c onsisting of 6mL of EAE solution and 25 ppm of Zn 2+ shifts the corrosion potential to - 820 mV vs SCE. The corrosion potential shift is very small. This suggests that the EAE - Zn 2+ formulation functions as a mixed inhibitor controlling the anodic reaction an d cathodic reaction, to the same extent. The corrosion current value and LPR value for sea water are 6.354ื10 - 6 A/cm 2 and 6.500 x 10 3 ohm cm 2 . For the formulation of EAE (6mL) and Zn 2+ (25ppm), the corrosion current value has decreased to 5.863 x 10 - 6 A/ cm 2 and the LPR value has increased to 6.909 x 10 3 ohm cm 2 . This indicates that a protective film is formed on the metal surface. When a protective film is formed on the metal surface LPR value increases and corrosion current value decreases 13 - 17 . Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 10 - 15 , February (201 3 ) Res. J. Chem. Sci. International Scien ce Congress Association 12 Table - 2 Corrosion inhibition efficiencies and the corresponding corrosion rates (millimeter per year) of EAE โ€“ Zn 2+ system Inhibitor HE (mL) Zn 2+ (ppm) 0 25 50 IE% CR(mm/y) IE% CR(mm/y) IE% CR(mm/y) 0 - 0.1576 16 0.1323 47 0.0835 2 50 0.0788 65 0.0555 70 0 .0472 4 55 0.0354 71 0.0457 76 0.0378 6 60 0.0630 92 0.0126 80 0.0315 8 63 0.0583 88 0.0189 82 0.0283 10 65 0.0555 80 0.0315 85 0.0236 Table - 3 Potentiodynamic polarization curves of carbon steel immersed in various test solution System E corr mV vs SCE b c mV/decade b a mV/decade LPR ohm cm 2 I corr A/cm 2 Sea water - 816 157 239 6.500 x 10 3 6.354 x 10 - 6 Sea water + (6mL) EAE+ (25ppm) Zn 2+ - 820 151 240 6.909 x 10 3 5.863 x 10 - 6 Figure - 1 Polarization curves of carbon steel immersed in various tes t solutions Analysis of FTIR spectra: The active principle in an aqueous extract of eclipta alba extract is wedelolactone. The green colour of the extract is due to wedelolactone.The main constituent of eclipta alba is wedelolactone. The structure of wedelolactone is shown in scheme 2. It contains 1,8,9 - trihydroxy - 3 - methoxy - 6H - [1] benzofuro [3,2 - c] chromen - 6 - one 18,19 . The wedelolactone extract was evaporated to dryness to set a solid mass. Its FTIR spectrum is shown in figure - 2a. The โ€“ OH stretching frequency appears at 3413cm - 1 . The C=O stretching frequency appears at 1634cm - 1 . The FTIR spectrum of the protective film formed on the surface of the metal after immersed in the solution containing 25 ppm of Zn 2+ and 6 mL of EAE shown in figure - .2b. It i s found that the - OH has shifted from 3413cm - 1 to 3375cm - 1 . The C=O stretching frequency has decreased from 1634cm - 1 to 1596cm - 1 . The ring oxygen appeared at 1090cm - 1 . It has coordinated Fe 2+ to form protective film on metal surface. The peak at 1365cm - 1 is due to Zn - O stretching. So, it is concluded that Zn(OH) 2 is formed on cathodic sites of the metal surface 20 . Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 10 - 15 , February (201 3 ) Res. J. Chem. Sci. International Scien ce Congress Association 13 Scheme - 2 structure of wedelolactone (extract of eclipta alba) 1,8,9 - trihydroxy - 3 - methoxy - 6H - [1] benzofuro [3,2 - c] chromen - 6 - one Figure - 2 a FTIR spectrum of pure eclipta alba extract Figure - 2b FTIR spectrum of film formed on metal surface after immersion in sea water containing 6mL of EAE โ€“ 25 ppm Zn 2+ Research Journal of Chemical Sciences ___ _ _ _______________________________ ______________ _ ________ ISSN 22 31 - 606X Vol. 3 ( 2 ), 10 - 15 , February (201 3 ) Res. J. Chem. Sci. International Scien ce Congress Association 14 Figure - 3 SEM micrographs of (a) polished Carbon steel (control) - Magnification - X1000, (b) Carbon steel immersed in sea water - Magnification - X1000, (c) Carbon steel immersed in sea water containing 6ml of EAE and 25 ppm of Zn 2+ - Magnification - X1000 Scanning Electron Microscopy(SEM): The SEM images of magnification (X1000) of carbon stee l specimen immersed in sea water for 1 day in the absence and presence of inhibitor system are shown in figure - 3 image (b) and (c) respectively.The SEM micrographs of polished carbon steel surface (control) in f igure - 3 image (a) shows the smooth surface of the metal. This shows the absence of any corrosion products formed on the metal surface. The SEM micrographs of carbon steel surface immersed in sea water in figure - 3 image (b) shows the roughness of the metal surface which indicates the corrosion of ca rbon steel in sea water. Figure - 3 image (c) indicates that in presence of 6mL of EAE - Zn 2+ (25ppm) mixture in sea water, the surface coverage increases which in turn results in the formation of insoluble complex on the surface of the metal EAE - Zn 2+ inhib itor complex) and the surface is covered by a thin layer of inhibitors which control the dissolution of carbon steel. Such results have been earlier 21 - 24 . Mechanism of corrosion inhibition: Weight loss method reveals that the formulation consisting of 6mL of EAE and 25 ppm of Zn 2+ offers 92% IE to carbon steel immersed in sea water. Polarization study reveals that EAE โ€“ Zn 2+ system functions as a mixed inhibitor. FTIR spectra reveal that the protective film consists of Fe 2+ - wedelolactone complex and Zn( OH) 2 . In order to explain the above facts in a holistic way, the following mechanism of corrosion inhibition is proposed. i. When the formulation consisting of sea water, eclipta alba extract and Zn 2+ is prepared, there is formation of Zn 2+ - wedelolact one complex in solution. ii. When carbon steel is immersed in the solution, the Zn 2+ - wedelolactone complex diffuses from the bulk of the solution towards the metal surface. iii. On the metal surface, Zn 2+ - wedelolactone complex is converted into Fe 2+ - wedelolactone complex. Zn 2+ is released Zn 2+ - wedelolactone + Fe 2+ ๏‚ฎ Fe 2+ - wedelolactone + Zn 2+, The released Zn 2+ combines with OH - to form Zn(OH) 2 on the cathodic sites. Zn 2+ + 2OH - ---------- �Zn(OH) 2 Thus the protective film consists of Fe 2+ - wedelolactone complex and Zn(OH) 2 . Conclusion The present study leads to the following conclusions: The formulation consisting of 6 mL EAE and 25 ppm Zn 2+ has 92% inhibition efficiency to carbon steel immersed in sea water. Polarization study rev eals that EAE โ€“ Zn 2+ system functions as a mixed inhibitor. 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