Studies using extracts of green tea as a corrosion inhibitor for CT38 steel in 0.01M HCl solution - Truong Thi Thao

Trương Thị Thảo và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 135(05): 179 - 183 179 STUDIES USING EXTRACTS OF GREEN TEA AS A CORROSION INHIBITOR FOR CT38 STEEL IN 0.01M HCL SOLUTION Truong Thi Thao 1* , Pham Thi Hien Luong 1 , Tran Quoc Toan 2 1College of Sciences – TNU, 1College of Education - TNU SUMMARY The corrosion inhibition of mild steel in 0.01M HCl acid solution by water extract of Green tea has been studied using electrochemical method. The results of the study reveal that the inhibition efficiency of the extract is found to vary with concentration and period of immersion. Thermodynamic consideration reveals that adsorption of Green tea extract on mild steel surface is spontaneous and occurs according to Langmuir adsorption isotherm. Physical adsorption mechanism has been proposed for the adsorption of the inhibitor from the values of some kinetic and thermodynamic parameters. Key words: Corrosion inhibition, mild steel, Langmuir adsorption isotherm, physical adsorption, green tea INTRODUCTION * Using inhibitors to minimize the corrosion rate for metals is a quite simple method but effectively. Searching Green corrosion inhibitors to replace inhibiting substances which are toxic to humans and the environment is one of particular concern. Green corrosion inhibitors are biodegradable and do not contain heavy metals or other toxic compounds. The successful uses of naturally occurring substances to inhibit the corrosion of metals in acidic environment have been reported by some research groups [1-7] but the application of green tea for this purpose has not been investigated much yet. Green tea contains many kinds of polyphenol promising electrochemical performance as well as the ability to inhibit corrosion of metal. This research concentrates on the inhibitive of water extract of green tea for the corrosion of mild steel in 0.01M HCl solution. EXPERIMENTAL Extraction of plant Leaves of Green tea were collected in Phuc Xuan ward, Thai Nguyen city. The clean air- dried leaves were grounded and extracted 3 times with distilled water; filtered water were * Tel: 0915 216469, Email: thao.truong671@gmail.com evaporated at 353 K. The obtained residue of the extract were washed with n-hexane, dichloromethane, ethylacetate, n-buthanol, respectively. The final product called water extract W(G) is used as corrosion inhibitor at concentrations: 0.1; 0.5; 1.0;5.0 and 10.0g/l in 0.01M HCl solution. Preparation of working electrode Working electrode was made from CT38 Steel (produced in Thai Nguyen) with exposure area S = 0.785cm 2 . Non-working area was isolated by using epoxy resin. Prior to the test, the electrodes were mechanically polished with successively finer grades of emery papers until their surfaces become smooth. Afterward, they were degreased with acetone, washed thoroughly with distilled water, dried and immersed in study solution. Chemicals and Apparatus Chemicals used are analytical grade. All electrochemical measurements were performed in the three-electrode mode using a homemade multifunctional potentiostat connected to a computer (Manufactured in Lab of Computer Application to Chemical Research, Institute of Chemistry, Viet Nam Academy of Science and Technology). A silver/silver chloride electrode and a piece of stainless steel with large area were employed Nitro PDF Software 100 Portable Document Lane Wonderland Trương Thị Thảo và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 135(05): 179 - 183 180 as pseudo-reference and counter electrode, respectively. All experiments were done in unstirred and nondeaerated solutions at room temperature. All experiments were measured after immersion for 60 min in 0.01M solution of HCl solution with and without addition of inhibitor. RESULTS AND DISCUSSION Effect of concentrations of W(G) extract and Effect of Acid Concentration Polarization curves for mild steel in 0.01M HCl solution with various concentrations of W(G) are shown in Figure 1. The extrapolation of Tafel straight line allows the calculation of the corrosion rate (VT) and inhibition efficiency (ET%).The linear polarizations determining the polarization resistance (Rp) and inhibition efficiency (ER%) are given in Table 1. Figure 1. Polarization curves in absence and presence of different concentrations of W(G) in 0.01M solution of HCl 1 – 0,0g/l 2 – 0,1g/l 3 – 0,5g/l 4 – 1,0g/l 5 – 5,0g/l 6 – 10,0g/l Both of two methods show that: as the concentration of W(G) increase, the inhibition efficiency increase. Following the polarization resistance method, the maximum inhibition efficiency of W(G) is 74.74%; and following the Tafel extrapolation method, it is 79.30% at concentration of 10.0g/l. The results of the two methods is quite suitable. The Figure 1 indicate that the presence of W(G) does not modify the mechanism of anodic dissolution as well as cathodic hydrogen evolution: Anodic and cathodic current densities of steel in acidic solution were reduced but the shape of anodic and cathodic curves does not change. It is clear that both cathodic and anodic reactions are inhibited and the inhibition increases as the inhibitor concentration rises. This result indicated that water extract of green tea can be considered as mixed inhibitor in 0.01M HCl solution. Table 1: Potentiodynamic polarization parameters for mild steel without and with different concentrations of W(G) extract in 0.01M HCl solution solution C(g/l) EC(V) RP(Ω) ER% ES (V) VT(mm/year) ET% 0.01M HCl -0.574 204.83 -0.571 1.51 0.01M HCl + W(G) 0.1 -0.548 321.30 36.25 -0.545 7.74.10 -1 48.74 0.5 -0.542 394.75 48.11 -0.539 6.48.10 -1 57.09 1.0 -0.535 547.28 62.57 -0.571 5.14.10 -1 65.92 5.0 -0.593 756.29 72.92 -0.599 3.27.10 -1 78.31 10.0 -0.574 810.74 74.74 -0.605 3.12.10 -1 79.30 Figure 2. Polarization curves Thai Nguyen steel in presence of different concentrations of W(G) 1.0g/l in 0.01M HCl with different immersion time 1 – blank 2 – 10’ 3 – 30’ 4 – 60’ 5 – 120’ 6 – 180’ 7 – 240’ 7 – 240’ Nitro PDF Software 100 Portable Document Lane Wonderland Trương Thị Thảo và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 135(05): 179 - 183 181 Table 2: Potentiodynamic polarization parameters for mild steel presence 1.0g/l W(G) extract in 0.01M HCl solution with different immersion time Solution t (minutes) VS(V) R (Ω) ER (%) VC (V) VT (mm/year) ET (%) 0.01M HCl 10 -0.593 118.76 -0,591 1.75 30 -0.586 185.77 -0,58 1.58 60 -0.582 204.83 -0.571 1.51 120 -0.575 229.49 -0.574 1.43 180 -0.573 233.72 -0.578 1.35 240 -0.576 240.59 -0.577 1.22 0.01M HCl + W 1g/l 10 -0.556 145.33 18.28 -0.571 1,47 15.90 30 -0.559 274.95 32.19 -0,557 9.37.10 -1 40.15 60 -0.535 547.28 62.57 -0.571 5.14.10 -1 65.92 120 -0.543 547.03 58.05 -0.539 5.15.10 -1 63.90 180 -0.535 546.95 57.27 -0.536 5.17.10 -1 61.77 240 -0.535 555.94 56.72 -0.537 5.13.10 -1 57.98 Effect of immersion time on inhibition efficiency Fig. 2 and table 2 show the effect of changing immersion time (0 – 240min) at 25oC on the corrosive inhibition efficiency of 1.0g/l W(G) extracts. It can be seen from the figure that inhibition efficiency increases from ~18% at 10 min to 62% at 60 min and then slightly decreases when the immersion time longer, which indicates that the equilibrium of W(G) extracts adsorbed on the CT38 steel surface reached at 60min. After 60 min, resistance and steel corrosion rate is approximately constant. This demonstrates that the absorption is quite durable and stable. Ecorr of mild steel shifted significantly to more positive values, mechanism of cathodic hydrogen evolution unchanges but mechanism of anodic dissolution changes. Adsorption Isotherm According to some researchers [3,4,5], the corrosion inhibition mechanism of inhibitors which derived from green plants are the adsorption mechanism. It can be seen that the values of surface coverage increases with the rise in inhibitor concentration (Table 1) as a result of more inhibitor molecules adsorption on the steel surface. Assuming that the adsorption of W(G) extracts belongs to monolayer adsorption and the lateral interaction between the inhibitor molecules is ignored, then the Langmuir adsorption isotherm applied to investigate the adsorption mechanism is [4.5]: 1 KC KC    (4) Where C is the inhibitor concentration in the electrolyte, K is the equilibrium constant of the adsorption process.  is the degree of surface coverage of W(G) extracts ( = E(%) /100).By plotting values of C/ versus C(table 1), straight line graphs were obtained (Fig. 3) which proves that Langmuir adsorption isotherm is obeyed over the range of studied concentrations. Figure 3. Langmuir isotherm for the adsorption of W(G) extracts on the surface of CT38 steel in 0.01M HCl The degree of linearity of Langmuir adsorption isotherm as measured by values of R 2 is nearly equal to 1 (Table 3) which indicates that the assumption and the deduction were correct. In other words, the adsorption of W(G) extracts on steel surface in 0.01 M HCl solution is well described by Nitro PDF Software 100 Portable Document Lane Wonderland Trương Thị Thảo và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 135(05): 179 - 183 182 the Langmuir adsorption isotherm [4]. The considerable deviation of the slopes from unity shows that the isotherm cannot be strictly applied. This deviation is attributable to interaction between adsorbate species on the metal surface [4,5]. A modified Langmuir adsorption isotherm [5] could be applied to this phenomenon, which is given by the corrected equation: C n nC K   (5) (n = the deviation of the slopes) The relationship between the standard free energy of adsorption and The adsorption equilibrium constant according to the following equation [3]: Go = -2.303RTlog(55.5xK) Where R is the molar gas constant, T is the absolute temperature and 55.5 is the concentration of water in solution expressed in molar. The result is : K = 9.02 and Go = -15.402 (kJ/mol) The negative values of Goads suggest that the adsorption of W(G) extract on steel surface is spontaneous and the absolute value is less than the threshold value of 40 KJ/mol required for chemical adsorption [5], hence the adsorption occurs via physical adsorption mechanism. CONCLUSIONS 1. Green tea extracts were found to be an efficient ‘green’ inhibitor for Thai Nguyen steel in 0.01M HCl solution. 2. Inhibition efficiency increases with the rise in W(G) concentration, but slight decreases with immersion time after adsorption reached equilibrium at 60min. 3. The corrosion process is inhibited by adsorption of the W(G) extracts onto the steel surface in 0.01M HCl solution following the Langmuir adsorption isotherm. 4. The values of the free energy of adsorption calculated indicate strong, spontaneous and physical adsorption of the extracts on the CT38 steel surface in 0.01M HCl solution. REFERENCES 1. Abdallah M (2002), “Rhodanine azosulpha drugs as corrosion inhibitors for corrosion of 304 stainless steel in HCl solution”, Corrosion Sc. V 44, p 717-728. 2. Abdallah M (2004), ”Antibacterial drugs as corrosion inhibitors for corrosion of aluminium in HCl solution” .Corrosion Sc, V 46, p 1981-1996. 3. Ambrish Singh, V. K. Singh, andM. A. Quraishi(2010), “Water Extract of Kalmegh (Andrographis paniculata ) Leaves as Green Inhibitor forMild Steel in Hydrochloric Acid Solution”, International Journal of Corrosion, V 2010. 4. Ebenso. E. E., Eddy N. O. and Odiongenyi A. O(2008), “Corrosion inhibitive properties and adsorption behaviour of ethanol extract of Piper guinensis as a green corrosion inhibitor for mild steel in H2SO4”, African Journal of Pure and Applied Chemistry, V. 2 (11), p107-115 5. Obot I.B., Obi-Egbedi N.O., Umoren S.A., Ebenso E.E.(2010), “Synergistic and Antagonistic Effects of Anions and Ipomoea invulcrata as Green Corrosion Inhibitor for Aluminium Dissolution in Acidic Medium”, Int. J. Electrochem. Sci.,V 5, p 994 – 1007. 6. Umoren S.A., Obot I.B., Ebenso E.E., Obi- Egbedi N. (2008), “Studies on the corrosion inhibition of Dacroydes edulis exudates gum for aluminium in acidic medium”, Port.Electrochimica Acta. V 6(2), p 199 - 209. 7. Umoren S.A., Ogbobe O., Igwe I.E., Ebenso E.E. (2008), “Inhibition of mild steel corrosion in acidic medium using synthetic and naturally occurring polymers and synergistic halide additives”’ Corros.Sci.V 50, p 1998 – 2006. Nitro PDF Software 100 Portable Document Lane Wonderland Trương Thị Thảo và Đtg Tạp chí KHOA HỌC & CÔNG NGHỆ 135(05): 179 - 183 183 TÓM TẮT NGHIÊN CỨU SỬ DỤNG DỊCH CHIẾT CHÈ XANH LÀM CHẤT ỨC CHẾ ĂN MÒN THÉP CT3 TRONG DUNG DỊCH HCL 0.01M Trương Thị Thảo1*, Phạm Thị Hiền Lương1, Trần Quốc Toàn2 1Trường Đại học Khoa học - ĐH Thái Nguyên 2Trường Đại học Sư phạm - ĐH Thái Nguyên Khả năng ức chế ăn mòn cho thép CT38 trong dung dịch axit HCl 0,01M của dịch chiết nước lá chè xanh bằng phương pháp điện hóa. Kết quả nghiên cứu cho thấy: nồng độ dịch chiết tăng thì khả năng ức chế ăn mòn tăng; thời gian ngâm mẫu tăng thì khả năng ức chế ăn mòn tăng nhanh trong vòng 60 phút đầu tiên sau đó khá ổn định đến 240 phút. Các nghiên cứu nhiệt động lực học cho thấy sự hấp phụ của dịch chiết chè lên bề mặt thép trong dung dịch axit là quá trình tự xảy ra theo cơ chế hấp phụ vật lý và tuân theo mô hình hấp phụ đẳng nhiệt Langmuir. Từ khóa: ức chế ăn mòn, thép CT38, hấp phụ đẳng nhiệt Langmuir, hấp phụ vật lý, chè xanh Ngày nhận bài:31/5/2015; Ngày phản biện:18/6/2015; Ngày duyệt đăng: 31/5/2015 Phản biện khoa học: PGS.TS Đỗ Trà Hương – Trường Đại học Sư phạm - ĐHTN * Tel: 0915 216469, Email: thao.truong671@gmail.com Nitro PDF Software 100 Portable Document Lane Wonderland