M. Hazwan Hussin and M. Jain Kassim
Keywords: Uncaria gambir, catechin, corrosion inhibitor, adsorption.
Abstract:
Uncaria gambir, a native Southeast Asia herbal plant has been widely used as an astringent medicine for treatment of spongy gums, tooth acne, diarrheas and sore throat. Previous studies have shown that more than 80 % of gambir extract consist mostly flavan monomer, which is catechin and epicatechin. The anti-oxidant properties exhibit in catechin attracts people to study further of its applications. The effect of both Uncaria gambir and catechin as corrosion inhibitors for mild steel in acidic solution was done using weight loss measurement at various temperatures. The free energy of adsorption ΔGads for both Uncaria gambir and catechin, indicates that the process was spontaneous and physically adsorbed (physiosorption) onto the mild steel surface. The comparison of inhibition efficiency revealed that Uncaria gambir performed more significant than catechin.
Because you are not logged-in to the journal, it is now our policy to display a 'text-only' version of the preprint. This version is obtained by extracting the text from the PDF or HTML file, and it is not guaranteed that the text will be a true image of the text of the paper. The text-only version is intended to act as a reference for search engines when they index the site, and it is not designed to be read by humans!
If you wish to view the human-readable version of the preprint, then please Register (if you have not already done so) and Login. Registration is completely free.
ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 1 Thermodynamic characterisation of mild steel corrosion and inhibition adsorption of Uncaria gambir and catechin in 1 M HCl M. Hazwan Hussin* and M. Jain Kassim Material Chemistry and Corrosion Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800, Penang, Malaysia. *Email: mhh.km08@student.usm.my ABSTRACT Uncaria gambir, a native Southeast Asia herbal plant has been widely used as an astringent medicine for treatment of spongy gums, to oth acne, diarrheas and sore throat. Previous studies have shown that more than 80 % of gambir extract consist mostly flavan monomer, which is catechin and epicatechin. The anti-oxidant properties exhibit in catechin attracts people to study further of its applications. The effect of both Uncaria gambir and catechin as corrosion inhibitors for mild steel in acidic solution was done using weight loss measurement at various temperatures. The free energy of adsǻGads for both Uncaria gambir and catechin, indicates that the process was spontaneous and physically adsorbed (physiosorption) onto the mild steel surface. The comparison of inhibition efficiency revealed that Uncaria gambir performed more significant than catechin. Keywords: Uncaria gambir, catechin, corrosion inhibitor, adsorption . INTRODUCTION Uncaria gambir, a native of Southeast Asia herbal plant, can be found mostly in countries such as Malaysia, Singapore and Indonesia. Some might call it as Gambir, Gou Teng, Asen'yaku, Cat's Claw, Una de Gato, and Pale Catechu [1-3]. Gambir plant can grow about eight feet high and has oval shape of leave around 8 to 14 cm in length with 4 to 5 pairs or nerves [4]. The flowers also originate at the base of the leaves with each pair of leaves may have a pair of globular inflorescences. According to Hadad et al., (2009), gambir plant can be grown only at certain condition, which is the plant must be grown at 200 to 800 meter above sea level with rainfall around ±3.3 mm per year and humidity around 70 to 85 %. Any types of soil can be used for gambir plantation with the pH range from 4.8 to 5.5 [5]. Quantitative analysis of gambir done by Taniguchi et al. (2007a), has shown that the total flavan content b y using the vanillin -acid estimation method ranged from 24 to 79 % while analysis using RP-HPLC techniques reveal that catechin content is around 76 %, epicatechin content is 1.5 %, and 1 % each for the content of gambiriin B1, B3 and A1 . The analysis indicated that catechin was the most abundant constituent in gambir [1]. Besides RP- HPLC, Hayani (2003) has studied the catechin content in three different extraction method ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 2 using spectrophotometer. From the study, she conclude that the extraction of gambir u sing a hot plate shows higher percentage of catechin content with the range around 81 to 88 % [6]. Catechin (Figure 1) is polyphenolic antioxidant plant metabolites. They belong to the family of flavonoids (C6 -C3-C6 skeletal) and to be more specific, flavan-3-ols [7-8]. These compounds are abundant in teas [7, 9], sea buckthorn [10] and other vegetable and plant. Figure 1.13 shows the chemical structure of (+)-catechin. Catechin compounds present in various natural foods are known to show some health -enhancing effects such as carcinogenesis reduction in the external and internal organs [11-13] and reduction in atherosclerotic plaques [14]. Green tea catechins have also shown to possess antibiotic properties due to their role in disrupting a specific stage of the bacterial DNA replication process [15]. Corrosion affects most of industrial sector and may cost billions of dollars each year for preventing and replacement of maintenance [16]. Thus, modern world today made an investigation to overcome this problem by doing enrichment study of corrosion inhibitors. Corrosion inhibitors will reduce the rate of either anodic oxidation or cathodic reduction or both. This will give us anodic, cathodic or mixed type of inhibition. Most of the potential corrosion inhibitor posses an active functional group such a nitro (-NO 2 ) and hydroxyl (-OH) [17-18]. Despite all these, none of previous studies done to test the performance of Uncaria gambir or catechin as corrosion inhibitor. Thus, the aim of this study is to determine the anticorrosive performance of Uncaria gambir extract and catechin standard using various tec hniques. EXPERIMENTAL Raw materials of gambir cube were purchased from Medan Province, Indonesia were ground into fine powder aȝ. The ground gambir were kept in closed container at room temperature prior for extraction. All chemicals and solvents used in this study were analytical reagent (AR) grade and have been used without purification. Standard catechin hydrate were supplied from Sigma Aldrich. 2.1. Preparation of gambir extract Parts from the ground gambir (5.0 g) were dissolved in 100 mL of distilled water (~90 °C). The aqueous extract of gambir was shaken with the rotational speed of 200 rpm for 1 hour. Then it was transferred to a centrifuge tube and centrifuged for 5 minutes to separate the undissolve gambir and the mother liquors. The mother liquors were then treated with 50 mL of n-Hexane three times in separation funnel to remove lipid and oil from the extracts. The aqueous-phase (lower phase) of from this separation were collected and kept in a refrigerator for further freeze-drying purposes. These to ensure that the aqueous extract can be last for a long time as it will polymerized in liquid form. The resulting aqueous extract powder (1.0 g) was then dissolv ed in 50 mL of ethyl acetate. Then, the ethyl acetate extract was concentrated at 50 °C under reduced pressure in a rotary evaporator, and dried in an oven at 50 °C. ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 3 2.2 Preparation of specimens Mild steel coupons having chemical composition (wt %) of 0.17 C, 0.20 Si, 0.37 Mn, 0.03 S, 0.01 P and remaining Fe were used. The specimens were polished successively using 400, 600 and 800 gritted emery paper, degreased with methanol and washed with distilled water before experiment. 2.3 Electrolyte The solutions used were made of AR grade hydrochloric acid. Appropriate concentrations of acids (1 M HCl) were prepared by using distilled water. The concentration range of inhibitor (gambir extract and catechin) employed was varied from 250 ppm to 1000 ppm in non -deaerated solution. 2.4 Weight loss method Weight loss of rectangular mild steel specimens of dimension 1 mm x 3 cm x 4 cm were immersed in 100 mL of electrolyte with and without the addition of d ifferent concentrations of gambir extract and catechin at different temperature (303, 313, 323, 333 K) was determined after 24 hr. The percentage inhibition efficiency (IE) was calculated from: (1) where W o and W i are the weight loss values in absence and in presence of inhibitor. RESULTS AND DISCUSSION 3.1 Effect of temperature In an attempt to get more information about the performance of the inhibitor (gambir and catechin), and the nature of adsorption isotherm which can be used to explain the adsorption and activation processes, the effect of temperature is therefore been studied. Hence, the weight loss measurements are used in the range of temperature 303-333 K, in the absence and presence of inhibitor at various concentrations during 24 h of immersion. The degree of surface coverage ș for different concentrations of gambir extract has been evaluated using the equation: ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 4 (2) where Wm is the smallest corrosion rate.The respective data are shown in Table 1 and 2. Here, as the temperature increases the corrosion rate of acid and inhibitor are also increases. As gambir extract and catechin has been added into the solution, the degree of surface coverage values decreases slightly with increasing temperature in which it could be caused by the desorption of inhibitor from the mild steel surface. In order to calculate activation thermodynamic parameters of the corrosion reaction such as activation energy E a , activated entropy ǻS ǻH, the Arrhenius equation and its alternative formulation called transition state equation were employed [19]: (3) (4) T is the absolute temperature, K is a constant and R is the universal gas constant, h is Plank's constant, N is Avogadro's number. Plotting the natural logarithm of corrosion rate versus 1/T, the activation energy can be calculated from the slope (-Ea/R) (Figure 2 and 3). Plot of log (W/T) versus 1/T give a straight line with a slope of ǻ and an intercept of log (R/Nh) + ǻ as shown in Figure 4 and 5. The value of ǻ and ǻ can be calculated from this relation (Table 3 and 4). The activation energies in the presence of gambir extract and catechin were observed higher than those in absence of gambir extract and catechin. This explains that the energy barrier of corrosion reaction increases with the concentration of gambir extract and catechin. Physiosorption is often related with this phenomenon, where an adsorptive film of electrostatic character is formed on the mild steel surface [20]. Thermodynamic parameters (ǻ and ǻ) of the dissolution reaction of mild steel in the presence of inhibitor are higher than those of the non -inhibited solution. Positive value of ǻH* means that the process is an exothermic process and it needs more energy to achieve the activated state or equilibrium state [19, 21]. Also, the positive of ǻS* of solution containing gambir extract and catechin indicates that the system passes from less orderly to a more random arrangement [22]. 3.2 Adsorption Isotherm In Langmuir adsorption isotherm, the surface of adsorbent is uniform that is all the adsorption sites are equivalent. It was revealed that Langmuir adsorption isotherm explain about the monolayer formation on the metal surface [21, 23]. According to Morad and Kamal ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 5 El-Dean (2006), Langmuir adsorption isotherm is attributing to physisorption and chemisorption phenomenon [22]. Langmuir is given by: (5) where C is the concenș ads is the adsorption equilibrium constant. ǻG ads , also can be calculated using the following equation: ǻG ads = -RT ln(K ads x 55.5) (6) where 55.5 is the water concentration, R is the universal gas constant and T is the temperature in K. Figure 6 and 7 shows the Langmuir adsorption isotherm plots for gambir extract and catechin at 303 K. The calculated value of free energy of adsorption for gambir was ǻG ads = - 21.9570 kJ mol -1 ǻG ads = -13.457 kJ mol -1 , where adsorption -desorption equilibrium constant K value was obtained from the linear regression of Langmuir isotherm (109.89 M -1 for gambir and 3.7651 M -1 for catechin). The negative ǻG ads indicates that the inhibitor, in this case gambir extract and catechin is ǻG ads around -20 kJ mol -1 or lower are associated with the physiosorption phenomenon where the electrostatic interaction assemble between the charged molecule and the charged metal, while those around -40 kJ mol -1 or higher are associated with the chemiosorption phenomenon where the sharing or transfer of organic molecules charge with the metal surface occurs [24, 25 ]. Hence, it is clear that gambir extract and catechin is physically adsorbed onto the mild steel surface. Moreover, the decrease of inhibition efficiency with the increased in temperature may supports that the adsorption of gambir extract and catechin on the mild steel surface is physical in nature. As the temperature increases, the number of adsorbed molecules decreases, leading to a decrease in the inhibition efficiency. The adsorption is enhanced by the presence of electron donor atom of O, with lone pair electron and ʌ in the catechin molecules that create electrostatic adsortion with the mild steel surface. As a result, insoluble stable films formed on the mild steel surface thus decrease the metal dissolution. ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 6 CONCLUSION Gambir extract and catechin exhibit corrosion inhibition properties with the maximum inhibition at 1000 ppm. Temperature study shows that as the temperature becoming high the inhibition efficiency of gambir extract and catechin is decreases. Both gambir extract and catechin follows the Langmuir adsorption isotherm. The negative value of free energy of adsorption ǻG ads , indicates that the process was spontaneous and physically adsorbed (physiosorption) onto the mild steel surface. The above results show that gambir extract performed more significant than standard catechin. REFERENCES [1] S. Taniguchi, K. Kuroda, K. Doi, K. Inada, N. Yoshikado, Y. Yoneda, M. Tanabe, T. Shibata, T. Yoshida, T. Hatano,. Yakugaku Zasshi, 127(8) (2007), 1291-1300 [2] J. P. Remington, H. C. Wood, Gambir, The Dispensatory of the United States of America, 1918. http://www.henriettesherbal.com, Retrived 23 Jan 2009 [3] L. L. Chong, Gambir, www.herbpalace.com/herbs/gambir.html, Retrieved 27 Dec 2009. [4] T. Kim Suan, (2009), Transformation of Rust by Uncaria gambir, Master dissertation, Universiti Sains Malaysia. [5] M. Hadad, N. R. Ahmadi, M. Herman, H. Supriadi, A. M. Hasibuan, http://balittri.litbang.deptan.go.id, Retrieved 27 Dec 2009. [6] E. Hayani, Buletin Teknik Pertanian 8 (1) (2003), 31-33 [7] W. J. Cheong, M. H. Park, G. W. Kang, J. H. Ko, Y. J. Seo,. Bull. Korean Chem. Soc. 26 (5) (2005), 747-754. [8] M. M. Ramos-Tejada, J. D. G. Duran, A. Ontiveros-Ortega, M. Espinosa-Jimenez, R. Perea -Carpio, E. Chibowski,. Colloids and Surfaces B: Biointerfaces, 24 (2002), 297-308. [9] K. H. Row, Y. Jin,. Bioresourse Technology, 97 (2006), 790-793. [10] Y. Zu, C. Li, Y. Fu, C. Zhao,. Journal of Pharmaceutical and Biomedical Analysis 41 (2006), 714 -719. [11] J. D. Laskin, H. Newmark, C. S. Yang,. Carcinogenesis 13 (1992), pg 947-954. [12] J. Cao, Y. Xu, J. Chen, J. Klaunig,. Fundamental of Applied Toxicology 29 (1996), pg 244 -250. [13] T. Majima, M. Tsutsumi, H. Nishino, T. Tsunoda, Y. Konishi,. Pancreas 16 (1998), pg 13 -18. [14] K. Y. Chyu, S. M. Babbidge, X. Zhao, R. Dandillaya, A. G. Rietveld, J. Yano, P. Dimayuga, B. Cercek,. Circulation 109 (20) (2004), 2448-53. [15] H. Gradisar P. Pristovsek A. Plaper R. Jerala. J. Med. Chem. 50 (2) (2007), 264-71. [16] P. R. Roberge, (2008). Corrosion Engineering Principles and Practice. Mc Graw Hill, 19. ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 7 [17] J. R. Davis, (2000). Corrosion: Understanding The Basic, ASM International, The Materials Information Society, pp 402-405. [18] P. A. Schweitzer, Corrosion of Linings and Coatings, Cathodic and Inhibition Protection and Corrosion Monitering, Taylor & Francis Group pp 55-59. [19] M. Bouklah, N. Benchat, B. Hammouti, A. Aouniti, S. Kertit, Mater. Lett. 60 (2006) 1904. [20] A. Popova, E. Sokolova, S. Raicheva, M. Christov, Corros. Sci. 45 (2003) 33. [21] D. Wahyuningrum, S. Achmad, Y.M. Syah, Buchari, B. Bundjali, B. Ariwahjoedi, Int. J. Electrochem. Sci. 3 (2008) 164. [22] M.S. Morad, A.M. Kamal El-Dean, Corros. Sci. 48 (2006) 3409. [23] S. Cheng, S. Chen, T. Liu, X. Chang, Y. Yin, Mater. Lett. 61 (2007) 3279. [24] F.M. Donahue, K. Nobe, J. Electrochem. Soc. 112 (1965) 886. [25] E. Kamis, F. Bellucci, R.M. Latanision, E.S.H. El-r, Corrosion 47 (1991) 677. ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 8 FIGURES AND TABLES Figure 1: The chemical structure of (+) -catechin. Figure 2: Arrhenius plots for mild steel in 1 M HCl at different concentration of gambir extract. o HO OH OH OH OH AB C8 7 6 52 341' 3 ' 5 ' ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 9 Figure 3: Arrhenius plots for mild steel in 1 M HCl at different concentration of catechin. ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 10 Figure 4: The relation between log (W/T) vs. 1/T for mild steel at different concentration of gambir extract. ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 11 Figure 5: The relation between log (W/T) vs. 1/T for mild steel at different concentration of catechin. ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 12 Figure 6: Langmuir adsorption isotherm model for gambir extract in 1 M HCl at 303K. Figure 7: Langmuir adsorption isotherm model for catechin in 1 M HCl at 303K. ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 13 Table 1: Effect of temperature on the corrosion rate of mild steel in 1 M HCl at different concentrations of gambir extract with its efficiency. Temperature (K) Concentration (ppm) WL W (mg/cm 2 h) Ewt (%) ș ș-ș 303 1 M HCl 0.5013 1.7406 - - - 250 0.0244 0.0847 95.13 0.9513 19.5338 500 0.0200 0.0694 96.00 0.9600 24 750 0.0156 0.0542 96.88 0.9688 31.0512 1000 0.0138 0.0479 97.25 0.9725 35.3636 313 1 M HCl 0.8873 3.0809 - - - 250 0.3713 1.2892 58.15 0.5815 1.3894 500 0.0757 0.2628 91.46 0.9146 10.7096 750 0.0352 0.1222 96.03 0.9603 24.1889 1000 0.0255 0.0885 97.12 0.9712 33.7222 323 1 M HCl 2.2417 7.7837 - - - 250 1.2171 4.2260 45.71 0.4571 0.8419 500 0.4086 1.4188 81.77 0.8177 4.4867 750 0.2087 0.7247 90.69 0.9069 9.7411 1000 0.1392 0.4833 93.79 0.9379 15.103 333 1 M HCl 2.7561 9.5697 - - - 250 2.267 7.8715 17.75 0.1775 0.2158 500 2.1893 7.6017 55.72 0.5572 1.2588 750 0.7331 2.5455 73.40 0.7340 2.7593 1000 0.4858 1.6868 82.37 0.8237 4.6721 Table 2: Effect of temperature on the corrosion rate of mild steel in 1 M HCl at different concentrations of catechin with its efficiency. Temperature (K) Concentration (ppm) WL W (mg/cm2 h) Ewt (%) ș ș-ș 303 1 M HCl 0.2279 0.7913 - - - 250 0.1137 0.3948 50.11 0.5011 1.0044 500 0.0535 0.1858 76.52 0.7652 3.2589 750 0.0349 0.1212 84.69 0.8469 5.5317 1000 0.0244 0.0847 89.28 0.8928 8.3284 313 1 M HCl 0.6053 2.1017 - - - 250 0.3912 1.3583 35.37 0.3537 0.5473 500 0.1738 0.6035 71.29 0.7129 2.4831 750 0.1027 0.3566 83.03 0.8303 4.8928 1000 0.0687 0.2385 88.64 0.8864 7.8028 323 1 M HCl 1.925 6.6840 - - - 250 1.3338 4.6313 30.71 0.3071 0.4432 500 0.9137 3.1726 52.54 0.5254 1.1070 ISSN 1466-8858 Volume 13, Preprint 18 submitted 28 March 2010 © 2010 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject to the reviewers" comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. 14 750 0.791 2.7465 58.91 0.5891 1.4337 1000 0.5842 2.0285 69.65 0.6965 2.2948 333 1 M HCl 2.7169 9.4337 - - - 250 2.7147 9.4260 0.809 0.0008 0.0008 500 2.2873 7.9420 15.81 0.1581 0.1878 750 2.1681 7.5281 20.19 0.2019 0.2529 1000 1.6614 5.7688 38.85 0.3885 0.6353 Table 3: Activation parameters of the dissolution reaction of mild steel in 1 M HCl in the absence and presence of gambir extract. Conc. (ppm) Ea (kJ/mol) ǻH* (kJ/mol) ǻS* (kJ/mol.K) 1 M HCl 22.0902 20.9429 -35.24 250 54.3403 53.2013 62.03 500 57.4082 56.2608 69.02 750 48.4706 47.3233 38.42 1000 44.9787 43.8314 26.28 Table 4: Activation parameters of the dissolution reaction of mild steel in 1 M HCl in the absence and presence of catechin. Conc. (ppm) Ea (kJ/mol) ǻ (kJ/mol) ǻ (kJ/mol.K) 1 M HCl 31.4569 30.3062 -6.9754 250 39.2769 38.1313 16.3062 500 47.1612 46.0172 39.2279 750 52.5994 51.4537 55.3072 1000 53.9495 52.8030 58.3501