N. I. Prajapati and R. T. Vashi
Keywords: Aluminium, Hydrochloric acid, Delonix regia , Corrosion, Inhibitor
Abstract:
The inhibition of the corrosion of aluminium in hydrochloric acid solution by the leaves extract of Delonix regia has been studied using weight loss, Potentiodynamic Polarization, Electrochemical Impedance Spectroscopic (EIS) techniques. Corrosion rate increases with the increase in acid concentration and temperature. As inhibitor concentration increases corrosion rate decreases while percentage of inhibition efficiency (I.E.) increases. The value of free energy of adsorption (ï„G0ads), heat of adsorption (Qads), energy of activation (Ea), enthalpy of adsorption (ï„H0ads) and entropy of adsorption (ï„S0ads) were calculated. The inhibition effect is discussed in view of Delonix regia molecules adsorbed on the metal surface and it obeys Langmuir adsorption isotherm. Polarization curve indicates that inhibitor act as mixed type and the I.E. was found up to 93.48 . The results obtained showed that the leaves extract could serve as an effective green inhibitor for corrosion of aluminium in hydrochloric acid.
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1 Inhibitory Effects of Delonix regia(Gulmohor) extract on Corrosion of Aluminium in Hydrochloric Acid N. I. Prajapati and R. T. Vashi* *Department of Chemistry, Navyug Science College, Rander Road, Surat-395005, Gujarat, India. For correspondence: Email: 6haprajapati@gmail.com; Tel. No.: 09998572828. Abstract The inhibition of the corrosion of aluminium in hydrochloric acid solution by the leaves extract of Delonix regia has been studied using weight loss, Potentiodynamic Polarization, Electrochemical Impedance Spectroscopic (EIS) techniques. Corrosion rate increases with the increase in acid concentration and temperature. As inhibitor concentration increases corrosion rate decreases while percentage of inhibition efficiency (I.E.) increases. The value of free energy of adsorption (DG0ads), heat of adsorption (Qads), energy of activation (Ea), enthalpy of adsorption (DH0ads) and entropy of adsorption (DS0ads) were calculated. The inhibition effect is discussed in view of Delonix regia molecules adsorbed on the metal surface and it obeys Langmuir adsorption isotherm. Polarization curve indicates that inhibitor act as mixed type and the I.E. was found up to 93.48 %. The results obtained showed that the leaves extract could serve as an effective green inhibitor for corrosion of aluminium in hydrochloric acid. Key words: Aluminium, Hydrochloric acid, Delonix regia , Corrosion, Inhibitor. 1. Introduction Aluminium is one of the metals which is used in different human activities and many of important application. Aluminum is widely used in various industrial operations due to light weight, high thermal and electrical conductivity, relative high mechanical strength. Aluminium and its alloy are widely used in construction, vessels, pipes, machinery and packing. Aluminium is used in electronics due to it is super purity [1]. Hydrochloric acid solution is one of the most currently used acids in the pickling and electrochemical etching of aluminium capacitor foil [2]. In the recent years, there is an increasing awareness of environment and green chemistry. Therefore, many works were conducted to use the environmentally safe, readily available and cheap substances, as corrosion inhibitors, instead of the harmful synthetic chemicals [3-7]. Delonix regia contains large amount of terpenoids, polyphenolic compounds, tannins, cardiac glycosides and anthroquinones [8]. The aim of the present study is to investigate the corrosion inhibition effect of Delonix regia as a cheap and environment friendly corrosion inhibitor for aluminium in various concentration of HCl medium by weight loss, effect of temperature, polarization and EIS techniques. 2 2. Experimental section 2.1 Preparation of sample and solution The aluminium specimens with a chemical composition of 99.54 % Al, 0.090 % Si, 0.320 % Fe, 0.0012 % Cu, 0.0034 % Mn, 0.0014 % Mg, 0.0042 % Cr, 0.0046 % Ni, 0.0020 % Zn, 0.0079 % Ti, 0.0005 % Pb, and 0.0026 % Sn were used in the present study. The metal sheet, test specimens of size 5.0 x 2.50 x 0.198cm having an effective area of 0.279 dm2were used. The specimens were cleaned by washing with distilled water, degreased by acetone, washed once more with doubled distilled water and finally dried and weighted by using electronic balance. Hydrochloric acid was used as corrosive solution having concentration of 0.75, 1.0 and 1.25 M prepared by diluting analytical grade of HCl purchased from Merck using double distilled water. 2.2 Preparation of extract The extract of leaves of Delonix regia was prepared as follows: fresh leaves of Delonix regia were dried and ground into powder, then 0.2g of powder was put into a 200ml flat bottom flask containing 100ml of 2 M HCl. The resulting solution was boiled for 30 min and left overnight before filtering. The filtrate was diluted with the appropriate quantity of 2 M HCl to obtain 0.6, 0.8, 1.0 and 1.2g/L concentrations. The above process was repeated for the preparation of the acid leaves extract [5]. 2.3 Weight loss measurements For weight-loss measurement, the aluminium specimen having an area of 0.2797 dm2 were each completely suspended in 230 ml of 0.75, 1.0 and 1.25 M HCl solution with and without different Delonix regia leaves extract concentrations using glass hooks at 301± 1 K for 24h. The coupons were retrieved after 24h, washed with distilled water, dried well and reweighed. From the weight loss data, corrosion rate (CR) was calculated. 2.4 Temperature effect To study the effect of temperature on corrosion rate, aluminium coupons were completely immersed in 230 ml of 0.75M HCl solution without and with different concentrations of Delonix regia leaves extract at 313, 323 and 333K for 2h. From the data corrosion rate, inhibition efficiency (I.E.), activation energy (Ea) and heat of adsorption (Qads) and free energy of adsorption (∆G0ads) were calculated. 2.5 Potentiodynamic polarization measurements Both the potentiodynamic and EIS measurement were carried out using CHI608C –series, U.S. Model with CH- instrument. For polarization study, metal specimens were immersed with and without Delonix regia leaves extract in 0.75 M HCl solution. In the electrochemical cell aluminium specimens having an area of 1 cm2was used as a working electrode, Ag/AgCl electrode as a reference electrode and platinum electrode as an auxiliary electrode and allowed to establish a steady-state open 3 circuit potential (OCP) for approximately 65 min. The polarization curves were plotted with current Vs potential. An anodic and cathodic polarization curve gives corresponding anodic and cathodic Tafel lines.The intersect point of Tafel lines gives the corrosion potential(Ecorr) and corrosion current (icorr) [9]. 2.6 Electrochemical Impedance Spectroscopy (EIS) measurements EIS measurements were made at corrosion potentials over a frequency range of 1 to 105 Hz by a sine wave with potential perturbation amplitude of 5 mV. A graph was drawn by plotting real impedance (Z’) versus imaginary impedance (-Z’’). From the Nyquest plots of Z’ Vs-Z’’ the charge transfer resistance (Rct) and double layer capacitance (Cdl) were calculated. An experiment was carried out in absence and presence of inhibitor. 3. Results and discussion 3.1 Weight loss experiments The weight loss experiments was carried out in 0.75, 1.0 and 1.25 M HCl solution containing 0.6, 0.8, 1.0 and 1.2 g/L concentration of Delonix regia leaves extract at 301±1 K for a exposure period of 24h was investigated. Corrosion rate (C.R.) was calculated using following equation: C.R. (mg/dm 2d) = Weight loss (gm.) x 1000 / Area in dm2 x day (1) Inhibition efficiency (I.E.) was calculated by using following equation, I. E. ሺ%ሻ = ቄ ൫౫ ି ൯ ౫ ቅ × 100 (2) Where: Wu = Weight loss in absence of inhibitor, Wi = Weight loss in presence of inhibitor. The degree of surface coverage (θ) of the aluminium specimen for different concentration of HCl solution have been evaluated by weight loss experiments using the following equation, θ= ሺ౫ ି ሻ (3) Results showed in Table-1 indicate that as the acid concentration increases corrosion rate increase while I.E. decreases. Corrosion rate was 2195.20, 6163.74 and 8283.87 mg/dm 2d corresponding to 0.75, 1.0 and 1.25 M HCl concentrations respectively for an immersion period of 24 h at 301± 1 K (Figure-1). At constant acid concentration, as the inhibitor concentration increases corrosion rate decreases while I.E. increases. For example, in 0.75 M HCl solution, the I.E. was found to be 56.02, 66.61, 83.55 and 93.48 % corresponding to 0.6, 0.8, 1.0 and 1.2 g/L inhibitor concentration respectively (Table-1 and Figure-2). 4 Table-1. Effect of HCl concentration on corrosion rate (C.R.) and inhibition efficiency (IE) of aluminium having different concentration of Delonix regia leaves extract. Inhibitior concentration ( g/L) Blank 0.6 0.8 1.0 1.2 Acid concentration 0.75 M CR (mg/dm2d) 2195.20 965.31 732.92 361.10 143.01 1.0 M 1.25 M I.E. (%) CR (mg/dm2d) I.E. (%) CR (mg/dm2d) I.E. (%) - 6163.74 - 8283.87 - 56.02 2931.71 52.43 4290.31 48.20 66.61 2398.99 61.07 3578.83 56.79 83.55 1337.14 78.30 2506.25 69.74 93.48 829.46 86.54 1433.67 82.69 Table-2: Inhibition efficiency (I.E.), Corrosion rate (ρ) and Surface coverage (θ) of Delonix regia leaves extract on aluminium in 0.75 M HCl for an immersion period of 24h at 301 ± 1 K. Inhibitor Inhibitor Concentration (g/L) C.R. (ρ) (mg/dm2d) Blank - Delonix regia (Gulmohor) log ρ I.E. (%) Surface coverage θ C/θ 2195.20 3.34 - - - 0.6 965.31 2.98 56.02 0.56 1.07 0.8 732.92 2.86 66.61 0.66 1.21 1.0 361.10 2.55 83.55 0.83 1.20 1.2 143.01 2.15 93.48 0.93 1.29 5 Corrosion rate (mg/dm2d) 2500 2000 1500 1000 500 0 Blank 0.6 0.8 1 1.2 Inhibitor concentration (g/L) Fig.1: Corrosion rate of aluminium in 0.75 M HCl solution in absence and presence of different concentration Delonix regia leaves extract for 24 h. Inhibition efficiency (I.E.) 100 80 60 40 20 0.6 0.8 1 1.2 Inhibitor concentration (g/L) Fig.2: Inhibition efficiency of aluminium corrosion in 0.75 M HCl solution in presence of different concentration Delonix regia extract for an immersion period of 24h. 3.2 Temperature effect To investigate the influence of temperature on corrosion of aluminium, the weight loss experiments were also carried out at 313, 323 and 333K in 0.75 M HNO3in absence and presence of Delonix regia for an immersion period of 2h. The results in Table-3 shows that corrosion rate increases with rise in temperature. Corrosion rate was 21451.44, 26642.76 and 43417.92 mg/dm 2d corresponding to 313, 323 and 333K respectively. The value of energy of activation (Ea) has been calculated with the help of Arrhenius equation [10]. log మ భ = ଵ ଵ ቀ − ቁ ଶ.ଷଷୖ భ మ (4) 6 Where ρ1 and ρ2 are the corrosion rate at temperature T1 and T2 respectively. Table-3. Temperature effect on corrosion rate (CR), activation energy (Ea) and heat of adsorption (Qads) for aluminium in 0.75 M HCl in absence and presence of Delonix regia extract for an immersion period of 2 h. Inhibitor concentr -ation (g/L) Temperature 313 K CR 323 K I. E. 2 CR 333 K I. E. 2 CR I. E. 2 Mean (Ea) From Equation (4) (kJ/mol) 313-323 K 323-333 K - 71.54 -48.16 -65.36 76.54 -53.15 -67.85 77.25 -66.49 -46.64 92.83 -119.10 -18.84 mg/dm d (%) mg/dm d (%) mg/dm d (%) Blank 21451.44 - 26642.76 - 43417.92 - 30.92 0.6 3560.88 83.4 6950.28 73.9 18362.52 57.70 0.8 1.0 2874.48 86.6 2102.16 90.2 943.80 1.2 95.6 6006.36 5148.36 4247.40 77.4 16646.40 80.6 12484.80 84.0 8237.28 61.66 71.24 81.02 Qads ( kJ/ mol) log ρ (Corrosion rate) Results given in Table-3, indicates that the values of Ea were higher in inhibited acid ranging from 71.54to 92.83 kJmol-1 than Ea value for uninhibited acid (30.92 kJ/mol) which indicates physical adsorption of the inhibitor on metal surface and the adsorption of inhibitor causes an increase in the Ea value of the process [11]. Results of Table-3 indicates that as temperature increases, rate of corrosion increase while percentage of I.E. decreases. The value of Ea were also calculated from the slope of the Arrhenius plot of log ρ versus 1/T x 1000 (Figure-3) shows good agreement with the calculated values. 5 4.5 4 3.5 3 2.5 2 1.5 1 Blank 0.6 g/L 0.8 g/L 1 g/L 1.2 g/L 310 315 320 325 1/T × 1000 (K) 330 335 Fig.3 : Arrhenius plots for aluminium in 0.75 M HCl in absence and presence of the different concentration of Delonix regia extract. The values of heat of adsorption (Qads) were calculated by using the following equation [12]. Qୟୢୱ = 2.303R ቂlog ቀ మ ଵିమ ቁ − log ቀଵିభ ቁቃ ቂ భିమ ቃ భ మ భ (5) 7 Where, θ1 and θ2 are the fraction of the metal surface covered by the inhibitor at temperature T1 and T2respectively. It is evident that in all cases, the value of Qୟୢୱ were negative and ranging from -18.84 to -119.10 kJ/mol. The negative values shows that the adsorption and hence the I.E. decreases with rise in temperature. The surface coverage ‘θ’ value was calculated by using equation- 3. A plot of inhibitor concentration Cinh versus Cinh/θ was presented in Figure-3 which gives straight line with slope values equal to unity indicates that the system follows Langmuir adsorption isotherm [13]. This isotherm can be represented as, େ = ଵ ౚ౩ +C (6) Where, Kads is the equilibrium constant and Cinh is the inhibitor concentration. 1.6 1.4 C/ϴ 1.2 1 0.8 0.6 0.4 0.6 0.8 1 1.2 C (g/L) Fig.4: Langmuir adsorption isotherm plot for aluminium in 0.75 M HCl containing Delonix regia as green inhibitor at 301 K. Free energy of adsorption (ΔGºads) was determined by the Langmuir isotherm was given by a plot of C/θ versus C (Figure-4) [14]. From the intercepts of the straight line on the C/θ axis, Kads can be calculated which was related to ΔGºads as given by the following equation [15,16]. ΔGºads = -RT ln (55.5 Kads ) (7) Where , R is the gas constant, T is the concentration of water in solution in Molar [17 ], Kads is the equilibrium constant of the adsorption/desorption process. The mean ΔGºads value was negative (-13.82 kJ/mol) indicating that the adsorption mechanism of Delonix regia on aluminium in 0.75 M HCl at the studied temperatures is physisorption with adsorptive layer having electrostatic character [18]. This is concluded on the fact that the values of ΔGºads -20 kJ/mol are consistant with physisorption, while those around -40 kJ/mol or higher are associated with chemisorption [19]. 8 The enthalpy of adsorption (ΔHºa) and entropy of adsorption (ΔSº a ) were calculated using the equations (8) and (9), ΔHºa = Ea –RT (8) ΔSº a = ΔHºa - ΔGºa /T (9) Results indicates that values of ΔHºa were positive and increase in presence of inhibitor indicating a higher degree of surface coverage and higher protection efficiency attained due to raising the energy barrier for the aluminium corrosion reaction. The enthalpy change ΔHºa was positive and ranging between 56.18 to 126.41 kJ/mol indicating the endothermic nature of the reaction suggests that higher temperature favours the corrosion process. Positive values of ΔSºa ranging from 0.225 to 0.450 kJ /mol indicates that corrosion process is entropically favourable. 3.3 Potentiodynamic polarization study Figure-5 represents the potentiodynamic polarization curves of aluminium in 0.75 M HCl in absence and presence of Delonix regia leaves extract. Electrochemical parameters such as corrosion potential (Ecorr),corrosion current density (icorr), anodic Tafel slope(βa), cathodicTafel slope (βc) and percentage inhibition efficiency (I.E.) were given in Table-4. Table -4. Potentiodynamic polarization data and Inhibition efficiency (I.E.) of Delonix regia leaves extract as green inhibitor for aluminium in 0.75 M HCl. System Ecorr (V) Icorr (µA / cm2) Blank -0.895 Delonix regia (Gulmohor) -0.891 Tafel slope (mV / decade) IE(%) Calculated from Anodic (+βa) Cathodic (-βc) β (mV) Polarization method Weight loss method 9156 6.706 5.621 1.329 - - 1085 8.884 6.080 1.569 88.14 93.48 9 Fig.5(a):Polarization curve for corrosion of aluminium in 0.75 M HCl in absence of inhibitor. Fig.5(b): Polarization curves for aluminium in 0.75 M HCl in presence of 1.2 g/L Delonix regia extract. From Table-4, it was observed that the addition of Delonix regia leaves extract in acid solution indicates the significant decrease in corrosion current density (icorr) and decrease in corrosion rate with respect to blank. There is significant change in the anodic and cathodic slopes after the addition of the inhibitor. This Tafel curves indicate that Delonix regia function as a mixed type inhibitor with the predominant cathode effectiveness (Figure-5 and Figure-6). Inhibition efficiency (I.E.) from polarization study was calculated using following equation [20]. I. E. ሺ%ሻ = ೝೝሺೠሻ ିೝೝ() ೝೝ(ೠ) (10) 10 3.4 Electrochemical impedance spectroscopy (EIS) measurements Nyquist plots for the corrosion of aluminium in 0.75MHCl solution in absence and presence of Delonix regia leavesextract wasexamined by EIS method at room temperature was shown in Figure-7 and EIS parameters in Table-5. Table-5. EIS parameters for corrosion of Aluminium in 0.75 M HCl containing Delonix regia leaves extract. System Blank Delonix regia (Gulmohor) Rct (Ω cm2) Cdl (µF / cm2) 84 45.130 240 5.529 IE(%) Calculated from EIS method Weight loss method 87.74 A A 93.48 B Fig.6 : Nyquist plot for aluminium in (A) in 0.75 M HCl (Blank) (B) in 0.75 M HCl in presence of 1.2 g/L Delonix regia extract. It is observed from Figure-6 that the impedance diagram is almost semi circular. The difference has been attributed to frequency dispersion. The semi circular nature of the plots indicates that the corrosion of aluminium is mainly controlled by charge transfer process. The diameter of capacitive loop in the presence of inhibitor is bigger than that in the absence of inhibitor. The high frequency capacitive loop is related to the charge transfer resistance (Rct). To calculate the double layer capacitance (Cdl), the frequency at which the imaginary component of the impedance is maximum was found as presented in the following equation [21]. Cୢ୪ = ଵ ଶౣ౮ୖౙ౪ (11) Where’ f’ is the frequency at the maximum height of the semicircle on the imaginary axis and Rct is the charge transfer resistance [22]. 11 Inhibition efficiency (I.E.) from EIS method was calculated using following equation: ܫ. ܧ. ሺ%ሻ = ௗሺ௨.ሻିௗ(.) ௗ (௨.) ݔ100 (12) The addition of inhibitor increase Rct value while decreases in Cdl values which is due to the adsorption of inhibitor on the metal surface. The results suggest that the inhibitor acts by the formation of a physical protective layer on the surface that retards the charge transfer process and therefore inhibit the corrosion reaction, leading to increase in Rct values. Moreover, the adsorbed inhibitor species decrease the electrical capacity of electrical double layer values at the electrode/solution interface and therefore decrease the value of Cdl [23]. 3.5 Mechanism of corrosion Generally, aluminium dissolves in acid solutions due to hydrogen evolution type of attack, the reactions taking place at the microelectrode of the corrosion cell being represented as [24,25]: Al → Al+3 + 3 e− (anodic reaction) …. (13) H+ + e- → Hads (cathodic reaction) …. (14) Followed by the reactions: H + H → H2(g) …. (15) The following secondary reactons can also take place in acid solutions [26]: 2M + 2H+ → H2 + 2M+ (anodic) …. (16) O2 + 4H+ + 4e- → 2H2O (cathodic) …. (17) 3.6 Mechanism of inhibition Chou and Leu [27] noted that Delonix regia leaves extract contains some phytochemicals with hetero atoms (N,S,O) having higher molecular weight such as 4-hydroxybenzoic acid, gallic acid, 3,4dihydroxycinnunmic acid, 3,5-dinitrobenzoic acid, alkaloids, L-azetidine-2-carboxylic acid, amine oxide base and 3,4-dihdroxybenzaldehyde etc. Structure of some of these compounds are shown in Figure-7. 13 adsorbed on the surface of metal. Since, a physisorption mechanism is indicative of a weak adsorption bond , it may be possible that at higher temperatures, the decrease in I.E. may be a result of increased agitation of the solution resulting from higher rates of hydrogen evolution, thereby reducing the ability of the molecules to be adsorbed on the surface of the metal. 4. Conclusions On the basis of the study the following conclusions can be drawn: 1. As acid concentration increases corrosion rate increases while I.E. decreases. 2. At constant acid concentration, as inhibitor concentration increases corrosion rate decreases while I.E. increases. 3. As temperature increase corrosion rate increases while I.E. decreases. 4. The leaves extract of Delonix regia showed maximum I.E. of 93.48% at an optimum concentration of 1.2 g/L. 5. The values of Ea obtained in the presence of the extract were higher compared to the blank acid solution which indicates that inhibitor was more effective at lower temperature. 6. The values of ΔGºads were negative, which reveals the spontaneous adsorption of inhibitor onto metal surface. 7. Plot of C/θ versus C shows straight line with almost unit slope, which suggest that the inhibitor cover both anodic and cathodic regions through general adsorption following Langmuir isotherm. 8. Polarization curves indicates that the Delonix regia extract act as mixed type of inhibitor. 5. 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