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Effect of Temperature on Corrosion Inhibition Efficiency of Withania Somnifera (Ashwagandha) on Aluminium in HCl Solution J. Dubey, P. Sharma, A. Chaturvedi, R.K.Upadhyay Synthetic and Surface Science Laboratory Department of Chemistry, Government College, Ajmer (Raj.) India alwayz.rajesh@gmail.com Abstract Weight loss and thermometric methods have been used to study the corrosion inhibition efficiency of leaves extract of Withania somnifera on aluminium in HCl solution at two temperatures i.e. 308 K and 313 K. Both methods show good agreement with each other. Inhibition efficiency of Withania somnifera has been found more at 303 K than at 318 K. Efficiency of the inhibitor increases with increasing concentration of inhibitor as well as with that of HCl. Study reveals that inhibitor works excellently at its concentration of 0.8% for 2N HCl where its inhibition efficiency is maximum i.e.99.28% Keywords : Corrosion inhibition efficiency, corrosion rate, reaction number, Withania somnifera, alkaloid, surface coverage. 1 Introduction Aluminium and its alloys are very important in many industrial as well as household applications due to their corrosion passivity in neutral media and atmospheric conditions due to formation of passive oxide layer on them. Although it is very reactive in emf series but becomes passive on exposure to water and atmosphere but it dissolves in hydrochloric acid liberating H2 gas. 2Al + 6 HCl + 12 H2O 2 [Al (H2O)6 ]Cl3 + 3H2 Compounds containing N, S and O have been found as good inhibitors due to high basicity and electron density and thus assist corrosion inhibition [1] . O, N and S are the active center for the process of adsorption on the metal surface. The size, orientation, shape and electric charges of the molecule also play a part in the effectiveness of inhibition. In addition to the heterogenous organic compounds synthesized in laboratory some naturally occurring substances like Ficus virens[2], Delonix regia[3], Ocimum basilicum[4], Caparis deciduas[5], Sansevieria trifascinata[6], Phylanthus amarus[7], Prosopis julifforar[8], Argemone maxicana[9] have been evaluated as effective corrosion inhibitors. The naturally occurring plant products are eco-friendly, compatible, nonpolluting, less toxic, easily available, biodegradable and economic to be used as corrosion inhibitors. These inhibitors have many N,O and S containing alkaloids which are get adsorbed on metal surface which essentially block the discharge of H+ and dissolution of metal ions. Extract of different parts of plant like seeds, leaves, stem, bark can be used as inhibitor to reduce the corrosion rate of metal like aluminium in acidic media. In the present investigation the inhibition efficiencies of Withania somnifera have been studied in different concentrations of HCl solution at two temperature i.e. 303 K (300 C) and 318K (450 C). Withania somnifera is a very common plant in India and other regions having almost same climatic conditions. Alkaloids and steroidal lactones are the main constituents of Ashwagandha. Plant contains many alkaloids like anaferin, anahygrine, betasisterol, chlorogenic acid, cystein, cuscohygrine, pesudotropine, scopoletin, somniferiene tropanol, withanine, withananine etc. Withaferin A and withanolides A-Y 2 are the main steroidal lactones. The withanolides are a group of naturally occurring C-28 steroidal lactones built on an intact or rearranged ergostane framework in which C-22 and C-26 are appropriately oxidized to form a six membered lactone ring. O O Structure of Withanolides Experimental Commercially available aluminium was used for specimen preparation. The specimens were prepared by cutting the aluminium sheet into square shaped pieces having dimension2.0 × 2.0 × 0.3cm containing a small hole of about 2mm diameter near the upper edge. Specimens were polished to mirror finish by using emery paper. Solutions of HCl were prepared by using double distilled water. All chemicals used were of AR grade. Extract of leaves of Withania somnifera was obtained by refluxing the dried leaves in soxhlet in ethanol. Solutions of different concentrations of extract were prepared in ethanol. Each specimen was suspended by a V-shaped glass hook made of capillary and plunge into a beaker containing 50mL of the test solution (HCl) at 303K and at 318K. After the sufficient exposure, test specimens were washed with running water and dried by hot air dryer. Duplicate experiments were performed in each case and mean value of weight loss was determined. The percentage inhibition efficiency was calculated as[10]. % = 100(Wu - Wi ) Wu Where Wu and Wi are the weight loss of the specimen in uninhibited and in inhibited solution respectively. Degree of surface coverage () was calculated as [11]. = (Wu - Wi ) Wu 3 Inhibition efficiency was also determined by thermometric method. In this method specimen was suspended into test solution in a thermal insulated chamber and initial temperature was noted. As soon as the reaction started temperature increased slowly at first then rapidly and achieved a maximum value before falling. The maximum temperature was noted. All observations were repeated also at higher temperature i.e.318K. Percentage inhibition efficiencies were calculated as[12]. % = 100(RN f - RNi ) RN f Where RNf and RNi are the reaction number in the free (uninhabited) solution and in presence of inhibitor. RN is defined as ­ RN = (Tm - Ti ) t Where Tm and Ti are maximum and initial temperature respectively and t is the time in minutes required to attain maximum temperature. The corrosion rate (CR) in mm/yr can be obtained by the following equation [13]. W × 87.6 A×T×D Corrosion Rate (mm/yr) = Where, W is weight loss in mg, A is area of specimen in cm2, T is time of exposure in hours, D is density of metal in gm/cm3. Results And Discussion Weight loss data and corresponding values of inhibition efficiency and corrosion rate are given in Table-1. It is obvious from the table 1 that inhibition efficiency increases with increasing acid strength and it also increases with increasing concentration of inhibitor. The maximum efficiency (i.e.99.28%) has been observed in 2N HCl at highest inhibitor concentration (i.e.0.8%) at 303K. The corrosion rate has been found maximum in blank solution which decreases with increasing concentration of inhibitor. Observations of inhibition efficiency corresponding to same concentrations of acid and 4 inhibitor at 318K show that efficiency of the inhibitor is less at 318K than that at 303K although the trends are same at 318K.Maximum efficiency at 318K is 98.53% for the same concentration of inhibitor (i.e. 0.8%) for 2N HCl. Variation of inhibition efficiency with concentration of inhibitor for 2N HCl are shown in fig 1. The degree of surface coverage of metal () covered by the adsorption of inhibitor to block the active sites on the surface at various concentrations of inhibitor for different HCl concentrations are shown in Table-2. It is clear from the table that degree of surface coverage increases with increasing concentration of inhibitor at both temperatures i.e.303K and 318K, however at 318K the coverage is less than that at 303K. Thermometric data in the form of reaction number at various concentrations of inhibitor for 1N, 2N, 3N and 4N HCl are shown in Table-3. Table shows that reaction number decreases with increasing concentration of inhibitor although values of RN are lower at 318K than at 303K for the same concentration of inhibitor. Since no significant temperature changes were observed at lower concentration of HCl so higher concentration of acids have been taken for thermometric study. Maximum efficiency (i.e. 99%) has been shown by 0.8% inhibitor in 4N HCl at 303K whereas the maximum efficiency at 318K is 81% for the same acid solution at same concentration of inhibitor. Here also inhibitor shows more efficiency at lower temperature. Variation of RN with concentration of inhibitor for 2N HCl are shown in fig.2. The possible reason for lower efficiency of inhibitor at higher temperature may be due to the reason that at higher temperature the adsorbed molecule of inhibitor start to desorbed from the metallic surface. Hoar and Holiday gave the Langmuir adsorption isotherm [14] Log /(1-) =log A + log C - Q /2.3 RT Where = Surface coverage A = Temperature dependent constant C = Bulk concentration of inhibitor (m/l) Q = Heat liberated in reaction According to which a straight line should be obtained if a graph is plotted between log (/ 1-) versus logC with gradient equal to one. In our investigation the graph is linear 5 but gradient is not equal to unity. This deviation from unit behaviour can be explained on the basis of interaction of the adsorbed molecules on the metal surface. According to langmuir the adsorbed layer is unimolecular i.e. there is no interaction between adsorbed molecules themselves and between adsorbate and adsorbent molecules. Only then the gradient is unity but in actual practice there is an interaction between adsorbed molecules themselves and between adsorbate and adsorbent molecules that is why the gradient is not unity.Variation of log (/ 1-) with concentration of inhibitor for 2N HCl are shown in fig.3. Conclusion The studies on Withania somnifera as corrosion inhibitor for aluminium in HCl have shown that this widely available plant is a very good corrosion inhibitor for aluminium. Studies have shown that both weight loss and thermometric metods show similar trends for different concentrations of acid as well as those of inhibitor. Further it can be concluded from the studies that Withania somnifera is a better corrosion inhibitor at 303K than at 318K. Acknowledgment One of the author's Jitendra Dubey is grateful to Department of Chemistry, Government College, Ajmer for laboratory Assistance. References 1. I.N.Putilova, S.A.Balizin, V.P.Baranik, Metallic corrosion inhibitor, Pergaman Press London 1960. 2. T.Jain, R.Choudhary, S.P.Mathur, Materials and Corrosion 57, 422, 2006. 3. O.K.Abiola, N.C.Oforka, E.E.Ebenso, N.M.Nwinuka, Anticorrosion Methods and Material 54, 219, 2007. 4. E.E.Oguzie, A.L.Onuchukwu, P.C.Okafor, E.E.Ebenso, Pigment and Resin Technology 35, 63, 2006. 5. P.Arora, S.Kumar, M.K.Sharma, S.P.Mathur, E-Journal of Chemistry 4, 450, 2007. 6. E.E.Oguzei, Corrosion Science 49, 1527, 2007. 7. P.C.Okafor, M.E.Ikpi, I.E.Uwah, E.E.Ebenso, U.J.Elcpe, S.A.Umoren, Corrosion Science 50, 2310, 2008. 6 8. R.Choudhary, T.Jain, S.P.Mathur, Bulletin of Electrochem 20. 67, 2004. 9. P.Sharma, R.K.Upadhyay, A.Chaturvedi, R.Parashar, J.T.R. Chem 15(1), 21. 2008. 10. J.D Talati, D.K.Gandhi, Ind. J. Tech. 29, 277, 1991. 11. R.S.Dubey, S.N Upadhyay, J. Electrochem. Soc. India 74. 143, 1944. 12. F.Mylius, Metallik Z. 14, 239, 1992. 13. M.G.Fontana, Corrosion Engineering 3rd Edition Mcgraw Hill Book Company, 173, 1987. 14. I.P.hoar, R.D.Holiday, J.App. chem. 3, 582, 1953. 7 Table 1 Inhibition efficiency (%) for aluminium in HCl with leave extract of Withania somnifera at 303K and 318K Area of specimen: 8.0cm2 Conc. of inhibitor 0.5N HCl (173 hrs.) W % C.R. W 1N HCl (225 hrs.) % C.R. W 2N HCl ( 25 min) % C.R. At 303K uninhibited 0.1 0.2 0.4 0.8 0.207 0.163 0.157 0.154 0.152 21.27 24.15 25.6 26.57 0.7148 0.5629 0.5421 0.5321 0.5249 0.117 0.043 0.015 0.003 0.002 63.24 87.17 97.43 98.29 31.06 11.41 3.98 0.796 0.531 0.1279 0.007 0.005 0.004 0.002 97.49 98.2 98.56 99.28 40.680 10.053 7.185 5.744 2.872 At 318K uninhibited 0.1 0.2 0.4 0.8 0.324 0.271 0.257 0.254 0.251 16.35 20.67 21.6 22.53 1.118 0.9358 0.8875 0.8771 0.8668 0.364 0.136 0.085 0.039 0.037 62.63 76.64 89.28 89.83 96.65 17.525 11.68 0.355 9.824 0.273 0.008 0.006 0.005 0.004 97.06 97.8 98.16 98.53 652.51 19.121 14.341 11.95 9.56 8 Table 2 Surface coverage and log (/ 1-) for aluminium in HCl with leave extract of Withania somnifera at 303K and 313K Conc. Of inhibitor 0.5N HCl(173 hrs.) % 1N HCl(225 hrs.) % 2N HCl(25min..) % log ( / 1-) log ( / 1-) log ( / 1-) At 303K uninhibited 0.1 0.2 0.4 0.8 21.27 0.2127 24.15 0.2415 25.60 0.2560 26.57 0.2657 -0.5684 -0.4970 -0.4633 -0.4415 63.24 0.6324 87.17 0.8717 97.43 0.9743 98.29 0.9829 0.2356 0.8321 1.5788 1.7595 97.49 0.9749 98.20 0.9820 98.56 0.9856 99.28 0.9928 1.5893 1.7368 1.8353 2.1395 At 318K uninhibited 0.1 0.2 0.4 0.8 16.35 0.1635 20.67 0.2067 21.60 0.2160 22.53 0.2253 -0.7089 -0.5841 -0.5599 -0.5364 62.63 0.6263 76.64 0.7664 89.28 0.8928 89.83 0.8983 0.6544 0.8567 0.9206 0.9461 97.06 0.9706 97.80 0.9780 98.16 0.9816 98.53 0.9853 1.5187 1.6479 1.7271 1.8263 9 Table 3: Thermometric data for aluminium in 1.0N, 2.0N, 3.0N and 4.0N HCl in presence of leave extract of Withania somnifera at 303K and 318K Conc of inhibitor 1N HCl (135 min.) 2N HCl (25 min.) 3N HCl (7 min.) n% 4N HCl (3 min.) RN (K min-1) n% RN (K min-1) n% RN (K min-1) n% RN (K min-1) At 303K Free solution 0.1 0.2 0.4 0.8 0.0185 0.0133 0.0118 0.0111 0.0080 31.57 36.84 42.11 57.89 7.5000 3.8000 2.8000 2.0000 1.5000 49.33 62.66 73.33 80.00 1.5000 0.6250 0.2500 0.1250 0.1000 58.33 83.33 91.66 93.33 1.3333 0.2000 0.0666 0.0333 0.0133 84.99 95.00 97.50 99.00 At 318K Free solution 0.1 0.2 0.4 0.8 0.019 0.015 0.013 0.012 0.011 21.05 31.57 35.13 42.11 5.1201 3.0511 2.5421 1.8200 1.2301 40.40 50.35 64.45 75.97 0.120 0.061 0.047 0.033 0.026 50.00 60.83 72.50 73.33 0.100 0.036 0.028 0.021 0.018 66.00 72.00 75.00 81.00 10 Fig: 1 Variation of Inhibition Efficiency With Concentration of Inhibitor For 2NHCl At 303K And 318K 99.5 99 Inhibition Efficiency 98.5 98 97.5 97 96.5 96 95.5 0.1 0.2 0.4 0.8 Concentration (%) At 303K At 318K Fig: 2 Variation of Reaction Number With Concentration of Inhibitor For 2NHCl At 303K And 318K 4 3.5 3 Reaction Number 2.5 2 1.5 1 0.5 0 0.1 0.2 0.4 0.8 Concentration (%) At 303K At 318K 11 Fig: 3 Langmuir Adsorption Isotherm For 2NHCl At 303K And 318K 2.5 2 1.5 At 303K 1 0.5 0 -1 -0.6989 Log C -0.3979 -0.0969 At 318K 12