Research Journal of Chemical Sciences ______ ______________________________ ______ ____ ISSN 2231 - 606X Vol. 2 ( 4 ), 24 - 34 , April (201 2 ) Res.J.Chem.Sci. International Science Congress Association 24 Corrosion Protection of Zinc in Natural Sea Water using Citrullus Vulgaris peel as an Inhibitor Petchiammal A . , Deepa Rani P., Selvaraj S. and Kalirajan K. PG and Research Department of Chemistry, Sri Paramakalyani College, Alwarkurichi - 627412, Tamil Nadu, India Available online at: www.isca.in (Received 31 st January 201 2 , revised 6 th February 201 2 , accepted 9 th February 201 2 ) Abstract The effect of Citrullus Vulgaris peel on the corrosion of z inc in n atural s ea w ater has been studied by mass loss measurements at different time and temperature. The present investigation revealed that the percentage of inhibition efficiency is increas ed with increase of inhibitor concentration and decreased with rise in period of contact . The temperature studies reflect that the percentage of inhibition efficiency is decreased with increase of temperature and it indicates that the mechanism of physica l adsorption. The calculated values of activation energy (E a ) also support the physisorp tion process. The thermodynamic parameters such as heat of adsorption (Q ads ) and free energy of adsorption (∆G ads ) are suggested that the adsorption of inhibitor on the z inc metal surface is exothermic and followed by spontaneous process. Experimental data has fitted with the Langmuir and Temkin adsorption isotherm. The corrosion products on the metal surface in the presence and absence of inhibitor is analysed by FTIR a nd XRD studies. The rich of main compounds namely z inc oxide (ZnO) and Pottasium diaquatrichlorozincate(II) (K [ZnCl 3 (H 2 O) 2 ]) are formed on the metal surface may be confirmed by XRD spectrum. The observed results concluded that the Citrullus Vulgaris pee l could serve as an effective inhibitor on z inc in n atural s ea w ater environment. Keywords: M ass loss, z inc , c itrullus v ulgaris peel , i sotherm, i nhibition. Introduction Corrsoion is an economic problem in world wide. Since c orrosion can damage the materials which are used to construct automobiles, pipe line systems (water,oil), bridges and buildings, petroleum refineries etc 1 . Zinc metal has a numerous industrial applications and is mainly used for the corrosion protection of steel. Because steel exhibits a wide range of useful forms and mechanical properties. Due to these fact steel is almost used in all industries 2 . The z inc - coated steel materials provide a greater resistance to corrosion, but they undergo rapid corrosion, when exposed to humid atmosphere leading to the formation of a corrosion pro duct known as white dust 3 . Zinc is an industrially important metal and is corroded by many agents, of which aqueous acids are the most dangerous one 4 . Due to the increasing usage of zinc, the study of corrosion inhibition is most important one. Every year, billions of dollars are spent on capital replacement and control methods for corrosion infrastructure 5 . The inhibitor must be eco - friendly to replace the older, which is more toxic and harmful to the environment. The earlier literature reveals that the study of corrosion inhibition of different metals with various green inhibitor have been reported. A few investigations are Red Peanut Skin 6 , Musa species peels 7 , Vernonia Amygdalina 8 , Piper guinensis 9 , Henna extract 10 , Delonix regia extracts 11 , Rosemary leaves 12 , natural honey 13 , opuntia extract 14 , khillah ( Ammi visnaga ) seeds 15 , Carica Papaya and Camellia Sinensis Leaves 16 , Ricinus communis Leaves 17 , Justicia gendarussa 18 , Vitis vinifera 19 , Punica granatum peel 20 , Leaves of Genus Musa , Genus Saccharum and Citrullus Lanatus 21 have been studied on various metals and alloys. However only a limited number of literature is available for the corrosion inhibition by green inhibitor with zinc metal surface. Some investigators have been reported with zinc met al is Ocimum tenuiflorum 22 , Red onion skin 23 , Nypa fruticans Wurmb 24 , Aloe vera 25 , henna ( lawsonia ) Leaves 26 . Thus our present attention is to study the effect of adsorption and corrosion inhibition of citrullus Vulgaris peel on zinc metal surface with n atural s ea w ater environment. Material and Methods Preparation of Citrullus Vulgaris peel extract : Freshly available Citrullus Vulgaris peel were collected from the fruit stalls and dried under sun shade and then grinded in to fine powder. Approximately 100gm of this powder was soaked in 400ml alcohol in a beaker and kept it for 48hrs. It was filtered followed by evaporation in order to r emove the alcohol and then the pure fruit peel extract was collected. The stock solution was prepared from this extract and used throughout the present investigations. Specimen preparation : Rectangular specimen of Zinc were mechanically pressed cut to fo rm different coupons, each of dimension exactly 5x2x2cm. The specimens were mechanically polished, a hole drilled at one end for free suspension and numbered by punching. After pickling the specimens were degreased with trichloroethylene, washed with disti lled water Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 4 ), 24 - 34 , April (201 2 ) Res.J.Chem.Sci International Science Congress Association 25 and well polished with emery wheel of 80 and 120, cleaned, dried and then stored in desiccators for present study. Properties and Applications of Citrullus vulgaris peel : Citrullus vulgaris (CVP) belongs to Cucurbitaceae family and its originally from s outhern Africa . CVP has a phytochemical compound mainly Citrulline (Fig.1) an amino acid 27 , flavanoids and polyphenols 28 . These compounds have an antioxidant property that can p rotect the human from free radical damage 27 . The CVP can medicinally used for cell division, healing of wounds, excretion of ammonia in urine, and create the amino acid namely arginine which may help to regulate blood sugar levels and improve the function of our arteries and other blood vessels 29 . The peel can also used to cure the sickle - cell anemia - related afflictions 30 and plays an important role to protect hypothyroidism. It can also stimulate the thyroid activity in PTU - induced hypothyroid animals and lipid peroxidation inhibition 31 . Figure 1 The structure of Citrulline Mass Loss method : In Mass loss measurements, z inc specimen in triplicate were immersed in 50ml of the test solution in the presence and absence of the inhibitor. The Zinc specimens were withdrawn from the test solutions after an hour at the temperature range of 303K to 333K and after 24 to 480hrs at r oom temperature. The Mass loss was measured as the difference in weight of the specimens before and after immersion using LP 120 digital balance with sensitivity of ±1 mg. The experiments were performed in triplicate to guarantee the reliability of the res ults and the mean value of the m ass loss is reported. From the m ass loss measurements, the corrosion rate (mmpy) was calculated using the following relationship. (1) w here, mmpy = millimeter per year, W = m ass loss (mg), D = Density (gm/cm 3 ), A = a rea of specimen (cm 2 ), T = time in hours. The inhibition efficiency (%IE) and degree of surface coverage (θ) were calculated using the following e quation (2) and equation (3), respectively. (2) (3) Where W 1 and W 2 are the corrosion rates in the absence and presence of the inhibitor respectively. Results and Discussion The value of corrosion rate (mmpy) Vs various concentration of inhibitor (ppm) on Zinc electrode in Natural sea water environment is shown in f ig ure 2. It reveals that the loss of mass gradually increased from 38mg to70mg with increase of exposure time from 24 hrs to 480 hrs in the absence of inhibitor concentration. In the presence of CVP extract, the value of corrosion rate significantly reduced from 0.9725 to 0.2048 (mmpy) for 24hrs, and 0.0896 to 0.0154 (mmpy) for 480 hrs ( f ig ure 3) respectively. The maximum of 82.81% inhibition efficiency is achieved at 1000 ppm of inhibitor concentration even after 480 hrs exposure time. This is mainly due to the active phyto chemical constituent of CVP extract viz, Ï€ bonds, hetero atoms (O and N). The almost greater than 82% of surface coverage (θ) is du e to the co - ordination between the Zinc metal and the hetero atom present in the inhibitor. Figure - 2 Variation of Mass loss of Zinc at different time in Natural Sea Water environment 0 20 40 60 80 0 100 200 300 400 500 600 Mass loss in mg Time in hrs Blank Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 4 ), 24 - 34 , April (201 2 ) Res.J.Chem.Sci International Science Congress Association 26 Figure - 3 Variation of Corrosion rate with different Concentration of CVP extract on Zinc in Natural sea water environment Figure - 4 Variation of % Inhibition efficiency with Concentration of CVP extract on Zinc in Natural sea water environment 0 0.2 0.4 0.6 0.8 1 1.2 0 200 400 600 800 1000 1200 Corrosion rate(mmpy) Conc.of inhibitor(ppm) 24hrs 72hrs 120hrs 240hrs 360hrs 480hrs 0 10 20 30 40 50 60 70 80 0 200 400 600 800 1000 1200 %inhibition efficiency Con.of inhibitor(ppm) 303K 313K 323K 333K Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 4 ), 24 - 34 , April (201 2 ) Res.J.Chem.Sci International Science Congress Association 27 The f ig ure 4 reflects that the concentration of inhibitor versus the percentage of inhibition efficiency (% IE) at various temperature from 303K to 333K. The maximum of 72.72% , 58.33 % inhibition efficiency is achieved at 303K, 333K respectively. The value of inhibition efficiency is decreased with rise in temperature is suggested that physical adsorption mechanism. This results indicate that the adsorption of main active components present in the inhibitor shield the metal surface at room temperature. However it may be deshielded from the surface wi th rise in temperature. Effect of temperature : The values of E a for the corrosion of z inc in the presence and absence of CVP extract is calculated using the following Arrhenius equations (4) and its derived form equation (5). CR= Aexp ( - E a /RT) (4) log (CR 2 /CR 1 ) = E a /2.303 R (1/T 1 - 1/T 2 ) (5) Where CR 1 and CR 2 are the corrosion rates of z inc at temperatures, T 1 and T 2 respectively , E a is the activation energy and R is the universal gas constant. The value of activation en ergy for blank (7.025 kJ/mol) is lower than in the presence of inhibitors ( t able 1), which is clearly indicates that adsorption process is physisorption 32 - 33 . Adsorption Studies : The values of Q ads on z inc specimen in the presence of inhibitor is arrived by the following equation (6) Q ads =2.303 R [log (θ 2 /1 - θ 2 ) - log (θ 1 /1 - θ 1 )] x (T 2 T 1 /T 2 - T 1 ) (6) Where R is the gas constant, θ 1 and θ 2 are the degree of surface coverage at temperatures T 1 and T 2 respectively. Table - 1 Calculated values of Activation energy, % inhibition efficiency and Heat of adsorption of CVP extract in Natural sea water environment at 303K and 333 S. No Concentration of Inhibitor(ppm) Inhibition Efficiency (%) E a kJ/mol Q ads kJ/mol 303K 333K 1 0 …… ……. 7.025 …….. 2 10 18.18 8.33 36.337 - 24.999 3 50 27.27 19.44 36.018 - 12.397 4 100 36.36 27.78 36.698 - 11.065 5 500 54.54 38.89 41.437 - 17.733 6 1000 72.72 58.33 45.012 - 18.014 The calculated Q ads values ( t able 1) are ranged from - 24.999 to - 18.014kJ/mol. This negative value indicates that the adsorption of CVP extract on the surface of Zinc metal is exothermic 34 . This adsorption isotherm of CVP extract on Zinc surface proceeded according to the following Equation (7) log (C/ θ) = log C - log K (7) Figure 5 Langmuir isotherm for adsorption of CVP extract on Zinc in Natural sea water environment Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 4 ), 24 - 34 , April (201 2 ) Res.J.Chem.Sci International Science Congress Association 28 Figure 6 Temkin isotherm for the corrosion inhibition of CVP extract on Zinc in Natural sea water environment By plotting the values of log (C/ θ) Vs log C, linear plots are generated ( f ig ure 5). Inspection of this Fig. reveals that the experimental data fitted with the Langmuir adsorption isotherm, means that there is no interaction between the adsorbed species. The inhibitor also obeys Temkin adsorption isotherm which is represented in f ig ure 6. The equilibrium constant of adsorption of CVP extract on the surface of the metal is related to the free energy of adsorption (∆G ads ) by the following equation (8) ∆G ads = - 2.303 RT log (55.5 K) (8) Where R is the gas constant, T is the temperature and K is the equilibrium constant of adsorption. The values of intercept (K) obtained from Langmuir and Temkin adsorption isotherm is substituted in e quation (8) and the calculated val ues of ∆G ads are placed in t able 2. The negative values of ∆G ads suggested that the adsorption of CVP extract onto metal surface is a spontaneous process and the adsorbed layer is more stable one. Table - 2 Langmuir and Temkin adsorptio n parameters for the adsorption of CVP extract on Zinc in Natural sea water environment Isotherm Temperature (K) logK R 2 ∆G ads (kJ/mol) Langmuir 303 - 0.9034 0.9981 - 0.1249 313 - 0.9543 0.9975 - 0.1111 323 - 0.0727 0.9874 - 0.8457 333 - 0.0174 0.9859 - 0.9607 Temkin 303 - 0.8723 0.9436 - 5.058 313 - 1.0118 0.9266 - 4.389 323 - 0.7211 0.9574 - 6.328 333 - 0.7351 0.9436 - 6.434 IR Analysis : Figures 7, 8 and 9 reflect that the IR spectrum of the alcoholic crystals of the inhibitor, the corrosion products on the z inc surface in the absence and presence of CVP extract in n atural sea water environment and that of stretching frequency, the corresponding band assignment is shown in t able 3. On comparing the above IR spectras, it found that the stretching frequency was shifted from 3359.77 to 3388.70 Cm - 1 for – OH, 1668.31 to 1633.59 Cm - 1 for amide – C = O and 1448.44 to 1458.08 Cm - 1 for aromatic – C=C. In a ddition, the – CN stretching frequency of amine, 1193.85 Cm - 1 ( f ig ure 7) is disappeared in f ig ure 9 (corrosion product in the presence of inhibitor) may also confirm that the complex film is formed on the anodic sites of the metal surface. Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 4 ), 24 - 34 , April (201 2 ) Res.J.Chem.Sci International Science Congress Association 29 Figure - 7 IR spec trum of the Alcoholic extract of CVP Figure - 8 IR spectrum of the corrosion product on Zinc in the Natural Sea water environment Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 4 ), 24 - 34 , April (201 2 ) Res.J.Chem.Sci International Science Congress Association 30 Figure - 9 IR spectrum of the corrosion product on Zinc in the presence of CVP extract in Natural Sea water environment Table - 3 FTIR Stretching frequency and Band Assignment Frequency (cm - 1 ) Band Assignment Alcoholic crystals of inhibitor 3359.77 - OH Stretch (alcohol) 2891.74 - C - H Stretch (Alkane) 1726.17 - C=O Stretch 1668.31 - C=O Stretch (Amide) 1448.44 - C=C Stretch (Aromatic) 1286.43 - C - O Stretch (Ether) 1193.85 - CN Stretch (Amine) Absence of inhibitor 3406.05 – O - H Stretch (alcohol). 1458.08 and 1400.22 - C=C - Stretch (Aromatic) 1116.71 – CO Stretch 684.68 and 649.97 – C - H deformation (Alkyne) 603.68 – C - Cl (Chloro alkane) Presence of Inhibitor 3388.70 – O - H Stretch (alcohol). 1633.59 – C=O Stretch 1458.08 and 1402.15 – C=C - Stretch(Aromatic) 1336.58 - CN stretch (Amine) 742.54 and 676.97 - C - H deformation (Alkyne) 601.75 - C - Cl (Chloro alkane) XRD analysis : The c orrosion products are scrapped from the Zinc surface in the presence and absence of CVP extract inhibitor is examined by XRD studies are shown in Fig. 10 and 11 respectively. The base peak ( f ig ure 10) is matched with the standard peak of z inc chloride (ZnCl 2 ) which has a crystal structure of monoclinic with the lattice parameter values of a= 6.500,b=11.30, c= 12.300AU which is taken from the JCPDS File no. PDF 740518. Another one peak is matched with the Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 4 ), 24 - 34 , April (201 2 ) Res.J.Chem.Sci International Science Congress Association 31 standard peak of Zinc oxide (ZnO) has a h exagonal structure with the lattice parameter of a= 3.253, c= 5.213AU which is taken from the JCPDS file no. PDF 891397 and the third one is matched with the standard peak of Po tassium Zinc chloride hydrate (K[ZnCl 3 (H 2 O) 2 ])which has a crystal structure of monoclinic in accordance with the JCPDS file no. PDF 760658 with lattice parameter values a = 12.03 b = 10.09 and c = 6.260 AU. The peak obtained from f ig ure 11 is coincided wit h the standard peaks for z inc n itride (Zn 3 N 2 ) with the d value of 31.703. This compound has a crystal structure of simple cubic with the lattice parameter of a=b=c=9.769 AU which is taken from the JCPDS file no. PDF 880618. Hence the present XRD pattern i s indeed a K [ZnCl 3 (H 2 O) 2 ] and Zn 3 N 2 compounds. It reflect that the thin film may be mainly combine with a rich amount of K[ZnCl 3 (H 2 O) 2 ], ZnO and ZnCl 2 etc with the bio - inhibitor. Figure - 10 XRD spectrum of Zinc in the presence of CVP extract in Natural Sea water environment Figure - 11 XRD spectrum of Zinc in Natural Sea water environment Conclusion The alcoholic extract of CVP is used as a good corrosion inhibitor on z inc metal in n atural s ea w ater environment. The corrosion rate is gradually increased with increase of exposure time in the absence of inhibitor. However in the presence of CVP the value corrosion rate is significantly reduced and the maximum of 82% Inhibi tion efficiency is achieved even after 480hrs exposure time. In temperature studies, the inhibitive effect is slightly decreased with rise in temperature is suggestive Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 4 ), 24 - 34 , April (201 2 ) Res.J.Chem.Sci International Science Congress Association 32 of physisorption process. The calculated values of Ea, Q ads , ∆G ads indicates that the a dsorption of inhibitor on the metal surface is physisorption, exothermic and followed by spontaneous process. The inhibitor obeys Langmuir and Temkin adsorption isotherm. The corrosion products over the surface of metal is characterized FTIR and XRD. The s hifting frequency of FTIR spectra confirms that the formation of protective film on Zinc metal surface. The formation of complexes such as K [ZnCl 3 (H 2 O) 2 ], ZnO, ZnCl 2 is also confirmed by XRD studies. Mechanism : The main active phyto constituents in the CVP extract is a Citrulline. From the chemical structure of Citrulline ( f ig ure 1), the molecules contains heteroatoms such as oxygen, nitrogen. The presence of the electronegative oxygen, nitrogen in citrulline might have enhanced the electron donating ability of this compound and hence its inhibition efficiency. Also, the functional groups ( - C=O, - NH 2 and – COOH) presence in citrulline might have provided sites for the adsorption of the inhibitor on the surface of Zinc. Therefore the fo llowing mechanism have been proposed for the adsorption of citrulline on the surface of z inc. i. In aqueous medium, the oxygen atom present in each citrulline molecule can donate electron to vacant d - orbital of the z inc and formed a coordinate covalent bon d ( s cheme 1). ii. Another mechanism is shown by scheme 2. In this scheme, the electron donated for the formation of Zn - citrulline complex is provided by the amide functional group instead of the carbonxyl group as shown in Scheme 1 + + Zn 2+ Scheme - 1 Mechanism of adsorption of Citrulline on the surface of Zinc metal Research Journal of Chemical Sciences ______ _ _ _______________________________ ______________ _ ____ ISSN 2231 - 606X Vol. 2 ( 4 ), 24 - 34 , April (201 2 ) Res.J.Chem.Sci International Science Congress Association 33 Scheme - 2 Mechanism of adsorption of Citrulline on the surface of Zinc metal Reference s 1. US Report on Corrosion Costs and Preventive Strategies in the United States, www.corrosioncost.com, July (2002) 2. Popoola A.P.I. and Fayomi O.S.I., Electrochemical Study of Zinc Plate in Acid Medium, Inhibitory Effect of Bitter Leaf (Vernonia Amygdalina), Int. J. Electrochem. Sci., 6(8), 4581 – 4592 (2011) 3. 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