K. K. Patel and R. T. Vashi
Keywords: Corrosion, Copper, Lawsonia extract, Nitric acid, Inhibitor
Abstract:
The inhibition effect of henna leaves (Lawsonia inermis) extract for the corrosion of copper in nitric acid solution has been studied by using weight loss, potentiodynamic polarization and electrochemical impedance spectroscopic methods at different temperature. The effect of inhibitor concentrations on different acid concentrations was investigated. The present study revealed that the percentage of inhibition efficiency is enhanced with increase of inhibitor concentration and decrease with increase in temperature. The inhibitive action of the extract is discussed in view of adsorption of Lawsonia molecules on the metal surface. It was found that this adsorption follows Langmuir adsorption isotherm in all tested systems. Tafel plot of polarization study indicates that the extract acts as a mixed type inhibitor. It was found that the extract acts as a good corrosion inhibitor for all the tested systems and the inhibition efficiency was obtained up to 98%.
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Corrosion Inhibition of Copper in Nitric Acid Using Lawsonia Extract as Green Inhibitor K. K. Patel, R. T. Vashi* Department of Chemistry, Navyug Science College, Surat, Gujarat-395009, India For correspondence: Email: ketan140187@gmail.com (K.K.Patel) Abstract The inhibitive action of henna leaves (Lawsonia Inermis) extract on corrosion of copper in nitric acid solution was investigated through weight loss, potentiodynamic polarization and electrochemical impedance spectroscopic methods at different temperature. The effect of inhibitor concentrations on different acid concentrations was investigated. The present study revealed that the percentage of inhibition efficiency is enhanced with increase of inhibitor concentration and decrease with increase in temperature. The inhibitive action of the extract is discussed in view of adsorption of Lawsonia molecules on the metal surface. It was found that the adsorption follows Langmuir adsorption isotherm. Tafel plot of polarization study indicate that the Lawsonia extract acts as a mixed type inhibitor. Inhibition efficiency of Lawsonia extract was found up to 98%. Keywords: Corrosion; Copper; Lawsonia extract; Nitric acid; Inhibitor. 1. Introduction Corrosion of metal can be defined as the deterioration of materials due to their reaction with the environment. Copper and its alloys are widely used material for its excellent electrical and thermal conductivities in many industrial applications such as electronics and in the manufacture of integrated circuits. Copper is a relatively noble metal, requiring strong oxidants for its corrosion or dissolution; its corrosion resistance becomes less while the aggressive solution concentration increases [1, 2]. Copper corrosion in nitric acid solution induced a great deal of research [3, 4]. It is noticed that presence of heteroatom such as nitrogen [5, 6], oxygen [7, 8], phosphorus [9] and sulphur [10, 11] in the organic compound molecule improves its action as corrosion inhibition. However, as a result of their high cost, toxicity and increasing awareness of health and ecological risks, attention is being drawn towards finding highly efficient, cheaper and non-toxic inhibitors. The present trend in research on environmental friendly corrosion inhibitors is concentrating on products of natural origin due principally to their low cost and eco-friendliness. The aim of the present work is to develop eco-friendly corrosion inhibitors, with good inhibition efficiency (IE) at low risk of environmental pollution [12]. From many decades, plant extracts have attracted attention in the field of corrosion inhibition. As natural products, they are a source of non-toxic, eco-friendly, bio-degradable and of potentially low cost inhibitors for preventing metal corrosion [13]. Most of the naturally occurring substances are safe and can be extracted by simple procedure. The inhibitive action of leaves extract of Azadirachta indica on copper corrosion in HNO3 solutions was studied using weight loss and electrochemical techniques [14]. The results obtained indicated that the extracts functioned as good inhibitors in HNO3 solutions. Henna (Lawsonia Inermis), a herb which has interesting dyeing properties used for centuries in Asia and North Africa for traditional decoration of the skin and hair. Lawsonia Inermis has anti-inflammatory, antipyretic and analgesic effect [15-16]. Henna has been used as corrosion inhibitor for aluminium and steel in aggressive solution [17], iron in hydrochloric acid [18] and aluminium in hydrochloric acid [19-20]. El- Etre A.Y. et al. [21] studied the inhibiting action of Lawsonia extract on the corrosion of different metals. The corrosion inhibition efficacy of these extracts is normally ascribed to the presence, in their composition, of complex organic species such as tannins, alkaloids and nitrogen bases, carbohydrates and proteins as well as their acid hydrolysis products. In the present work, inhibitive action of Lawsonia leaves extract as a cheap, eco friendly and naturally occurring substance on corrosion behavior of copper in HNO3 solution has been investigated through weight loss, polarization measurements and electrochemical impedance spectroscopy methods. 2. Experimental section 2.1. Sample and Solution preparation The copper specimens of the size 4.5 × 2.0 × 0.178 cm having an effective area 0.1988 dm2 with a chemical composition (99.99 % Cu and 0.01% S) was used. The specimens mechanically abraded using different grades of emery papers, cleaned by washing with distilled water, degreased with acetone, washed once more with distilled water and finally dried and weighted by using electronic balance. The corrosive solutions were prepared by dilution of analytical grade of 69% HNO3 (Merck) using distilled water. 2.2. Extract preparation Lawsonia inermis leaves were dried, crushed and extracted in boiled water for 2 h. The extracted solution was then filtered and concentrated until the water from the extract evaporates [22]. This extract was used to study the corrosion inhibition properties and to prepare the required concentrations of Lawsonia inermis varied from 0.6, 0.8, 1.0 and 1.2 g/L. 2.3. Weight loss measurement For weight-loss experiment, the copper coupons were each suspended completely in 0.5, 0.75 and 1.0 M HNO3 solutions without and with different concentrations of Lawsonia extract with the help of glass hooks at 301±1 K for 24h (1 day). The volume of solution kept 230 ml. The coupons were retrieved after 24 h, washed by distilled water, dried well and reweighed. From the weight loss data, corrosion rate in mg/dm2d was calculated. 2.4. Temperature effect To study the effect of temperature on corrosion rate, the copper coupons were completely immersed in 230 ml of 1.0 M HNO3 solution without and with different concentrations of Lawsonia extract at 313, 323 and 333 K for 2h. From the data, inhibition efficiency, energy of activation (Ea) and heat of adsorption (Qads) were calculated. 2.5. Electrochemical measurements Electrochemical measurements were carried out by using an electrochemical work station (CHI608C-series, U.S. Model with CH instrument). In electrochemical experiment Ag/AgCl was used as a reference electrode, platinum as an auxiliary electrode and copper metal was used as a working electrode. For polarization study, copper specimens having an area of 1 cm2 exposed to 230 ml 0.5 M HNO3 in absence and presence of Lawsonia extract and allowed to establish a steady-state open circuit potential (OCP) for about 30 minutes. Test coupons were then polarized by the application of potential drift of −250 mV cathodically and +250 mV anodically with respect to the OCP at a scan rate of 5.0 mV/s. The potentiodynamic polarization plots (Tafel curves) were developed simultaneously. Anodic and cathodic polarization curves give anodic and cathodic Tafel lines correspondingly. The intersect point of Tafel lines gives the corrosion potential (Ecorr) and corrosion current (icorr) [21]. The electrochemical impedance studies were carried out in the same setup using potentiodynamic polarization studies described above. Impedance studies were carried out at steady-state open circuit potential (OCP). A small amplitude (5.0 mV) sinusoidal ac Voltage, in wide frequency range 1 to 100 KHz was applied over the system. A graph was drawn by plotting real impedance (Z’) versus imaginary impedance (Z″). From the Nyquist plots the charge transfer resistance (Rct), and double layer capacitance (Cdl) were calculated. Impedance measurements were carried out both in the absence and presence of Lawsonia extract. 3. Results and discussion 3.1. Weight loss experiment The corrosion rate of copper in 0.5, 0.75 and 1.0 M of HNO3 solution without and with different concentration of Lawsonia extract 0.6, 0.8, 1.0 and 1.2 g/L at 301±1 K for an exposure period of 24h (1 day) was calculated from the weight loss data using following equation: CR (mg/dm2 d) = Weight loss (gm ) x 1000 (metal surface area ) dm 2 x day (1) The Inhibition efficiency (IE) was calculated by using following formula, IE(%) = W uninh −W inh W uninhi (2) x 100 where, Wu = Weight loss without inhibitor, Wi = Weight loss with inhibitor. The degree of surface coverage (θ) for different concentration of the inhibitor in acidic media have been evaluated from weight loss experiments using this equation, θ= (3) W uninh −W inh W uninhi Table 1. Corrosion rate for copper in various HNO3 concentrations in the absence and presence of different concentrations of Lawsonia extract from weight loss measurements at 301±1 K. Inhibitor concentration (g/L) Acid concentration 0.5 M CR 0.75 M IE CR 1.0 M IE CR IE (mg/dm2d) (%) (mg/dm2d) (%) (mg/dm2d) (%) Blank 90.52 - 281.55 - 1030.67 - 0.6 20.11 77.78 80.45 71.43 477.65 53.66 0.8 15.08 83.34 60.33 78.57 236.31 77.07 1.0 10.06 88.89 45.25 83.93 181.00 82.44 1.2 4.02 95.56 20.11 92.86 105.59 89.76 Results showed in Table 1 indicate that as the concentration of acid increases corrosion rate was increases. The corrosion rate was 90.52, 281.55 and 1030.67 mg/dm2d for 0.5, 0.75 and 1.0 M HNO3 concentrations respectively for an immersion period of 24 h at 301±1 K. At constant acid concentration, the IE increases with increase Lawsonia extract concentrations, e.g. Lawsonia extract in 0.5 M HNO3 solution, the IE found to be 77.78, 83.34, 88.89 and 95.56 % with respect to 0.6, 0.8, 1.0 and 1.2 g/L inhibitor concentrations (Table 1). At constant inhibitor concentration, the IE was decreases as the acid concentration increases, e.g. for 0.6 g/L Lawsonia extract, the IE was found to be 77.78, 71.43 and 53.66 % with respect to 0.5, 0.75 and 1.0 M HNO3 solution (Table 1). Table 2. Corrosion rate (log ρ) of copper in 0.5 M HNO3 in absence and presence of Lawsonia extract for an immersion period of 24 h at 301 ± 1K. Inhibitor CR concentration (C) (ρ) (g/L) surface IE log ρ (%) coverage 1-θ (θ) Blank 90.52 1.9567 - - - 0.6 20.11 1.3034 77.78 0.7778 0.2222 0.8 15.08 1.1784 83.34 0.8334 0.1666 1.0 10.06 1.0026 88.89 0.8889 0.1111 1.2 4.02 0.6042 95.56 0.9556 0.0444 The results obtained were presented in Table 2 and in Fig. 1&2, which indicates that with increase in inhibitor concentration from 0.6 to 1.2 g/L the corrosion rate was decreased from 20.11 to 4.02 mg/dm2d while IE increases from 77.78 to 95.56 %. It can be concluded that IE corrosion rate (mg/dm2 d) is directly proportional to the inhibitor concentration. 100 80 60 40 20 0 Blank 0.6 0.8 1 1.2 Inhibitor concentration (g/L) Fig.1. Corrosion rate of copper corrosion in 0.5 M HNO3 solution in absence and presence of different concentration of Lawsonia extract for an immersion period of 24h (1 day). IE (%) 100 90 80 70 0.6 0.8 1 1.2 Inhibitor concentration (g/L) Fig.2. IE of copper corrosion in 0.5 M HNO3 solution in presence of different concentration of Lawsonia extract for an immersion period of 24h (1 day). 3.2. Temperature effect To investigate the influence of temperature on corrosion of copper, the weight-loss experiments were carried out at temperature 313, 323 and 333K in 1.0 M HNO3 without and with Lawsonia extract for an immersion period of 2 h. The results in Table 3 shows that corrosion rate increase with rise in temperature, the corrosion rate was 5309.52, 9050.22 and 23530.58 mg/dm2d with respect to 313, 323 and 333 K. The IE was decrease with increase in temperature, e.g. for 1.2 g/L Lawsonia extract the IE was 95.45, 90.00 and 82.05 % for 313, 323 and 333 K temperature respectively (Fig. 5). The value of energy of activation (Ea) has been calculated with the help of following Arrhenius equation [23]. ρ E 1 1 a log ρ2 = 2.303R �T − T � 1 1 (4) 2 where, ρ1 and ρ2 are the corrosion rate at temperature T1 and T2 respectively. Table 3. Temperature effect on corrosion rate (CR), inhibition efficiency (IE) and activation energy (Ea) for copper in 1.0 M HNO3 in absence and presence of Lawsonia extract for an immersion period of 2 h. Inhibitor 313 K concentration (g/L) Energy of activation (Ea) Temperature (kJ/mol) 323 K 333 K (Ea) from Arrhenius plot 313- 323- 323 K 333K 44.83 85.46 26.15 76.58 102.07 89.32 88.82 38.21 110.50 114.49 112.49 112.45 6516.12 72.31 123.27 127.36 125.32 125.27 4223.40 82.05 111.13 137.78 124.45 123.96 CR IE CR IE CR IE (mg/dm2d) (%) (mg/dm2d) (%) (mg/dm2d) (%) Blank 5309.52 - 9050.22 - 23530.58 - 0.6 2232.36 57.95 5550.84 38.67 17376.48 0.8 1086.00 79.55 4042.44 55.33 14540.76 1.0 362.04 93.18 1568.76 82.67 1.2 241.32 95.45 905.04 90.00 Mean 65.15 (kJ/mol) 64.33 The value of Ea were also calculated from the slope of the Arrhenius plot (Fig. 3), i.e. log ρ Versus 1/T ×103 [23]. (ρ=corrosion rate, T=absolute temperature) It was found that, the values of Ea were found higher in inhibited acid ranging from 89.32 to 125.32 kJ/mol than Ea values for uninhibited acid 65.15 kJ/mol. The higher values of Ea indicate physical adsorption of the inhibitor on the metal surface and the adsorption of inhibitor causes an increase in the Ea of the process [24]. log ρ (CR) 5.5 0.6 g/L 0.8 g/L 1.0 g/L 1.2 g/L 4.5 3.5 2.5 1.5 3.00 3.1 1/T x 1000 K-1 3.2 Fig.3. Arrhenius plot for corrosion of copper in 1.0 M HNO3 in absence and presence of different concentration of Lawsonia extract for an immersion period of 2 h. The mean value of Ea was 65.15 kJ/mol in uninhibited acid and the value calculated from the slop of the Arrhenius plot was found 64.33 kJ/mol, which was found almost similar (± 1.0 kJ/mol). 0.6 g/L 0.8 g/L 1.0 g/L 1.2 g/L 110 IE (%) 90 70 50 30 10 313 323 333 Temperature (K) Fig.4. Effect of temperature on IE for copper corrosion in 1.0 M HNO3 at different concentration of Lawsonia extract for immersion period of 2 h. The values of heat of adsorption (Qads) were calculated by following equation [25]. θ θ T T Qads = 2.303R �log �1−θ2 � − log �1−θ1 �� × �T 1−T2 � 2 1 2 1 (5) where, θ1 and θ2 are the fraction of the metal surface covered by the inhibitor at temperature T1 and T2 respectively. From Table 4, it is evident that in all cases, the Qads values are negative and ranging from -51.61 to -96.21 kJ/mol. The negative values shows that the adsorption and hence the IE decreases with rise in temperature. The IE was correlated to surface coverage (θ) and a 100 % efficiency suggesting to full coverage (θ=1). The degree of surface coverage values were used to determine its adsorption characteristics in HNO3 solution. The plot of Cinh/θ versus Cinh gives straight lines with slope values equal to unity (Fig. 5). All the regression coefficients are very close to one which indicates that the inhibitors cover both the anodic and cathodic region through general adsorption following Langmuir isotherm [26]. This isotherm can be represented as, C inh θ =K 1 ads + cinh (6) where, Kads = equilibrium constant of the adsorption process and Cinh = inhibitor concentration. 1.5 Cinh/ θ 1.3 1.1 0.9 0.7 0.5 0.6 0.8 1 1.2 Cinh (g/L) Fig.5. Langmuir adsorption isotherm for corrosion of copper in 0.5 M HNO3 solution containing different concentration of Lawsonia extract for an immersion period of 24 h. Free energy of adsorption (ΔGao) was determined by the Langmuir isotherm was given by a plot of Cinh/θ Vs Cinh [27] (Fig. 5). From the intercepts of the straight lines on the Cinh/θ axis, Kads can be calculated which was related to ΔGao, as given by following equation. The ΔGao value of the inhibitors on copper surface can be calculated from the following equation [28,29]. ΔGao = -RT ln (55.5 Kads) (7) where, R is the gas constant, T is the absolute temperature (K), and the value 55.5 in the above equation is the concentration of water in solution in Molar [30], Kads is the equilibrium constant of the adsorption/desorption process. The ΔGa0 values were almost negative in all cases indicated the spontaneous adsorption of Lawsonia extract on copper surface and strong interactions between inhibitor molecules and the metal surface. The enthalpy of adsorption (ΔHao) and entropy of adsorption (ΔSao) were calculated using the equations (8) & (9). ΔHa0 = Ea – RT (8) ΔSa0 = ΔHa0 - ∆Ga0/T (9) Table 4. The values of physical parameters Heat of adsorption (Qads), Free energy of adsorption (∆Ga0), Enthalpy of adsorption (ΔHa0) and Entropy of adsorption (ΔSa0) for copper in 1.0 M HNO3 in the absence and presence of different concentration of Lawsonia extract for an immersion period of 2 h. Inhibitor Qads ∆Ga0 ΔHa0 ΔSa0 concentration (kJ/mol) (kJ/mol) (kJ/mol) (kJ/mol K) (g/L) 313-323 K 323-333 K 313 K 323 K 333 K 313 K 323 K 313 K 323 K Blank - - - - - 42.22 82.78 - - 0.6 -65.74 -51.61 73.98 99.39 0.27 0.34 0.8 -96.21 -62.13 107.90 111.81 0.38 0.38 1.0 -88.46 -53.89 120.67 124.68 0.42 0.42 1.2 -71.14 -60.59 108.52 135.10 0.38 0.45 -10.68 -9.51 -8.46 The results revealed that ΔHao values 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 copper corrosion reaction. The enthalpy change ΔHao was positive and ranging between 73.98 to 135.10 kJ/mol indicating the endothermic nature of the reaction suggests that higher temperature favors the corrosion process. Positive value of ΔSao, lie between 0.27 to 0.45 kJ/mol K indicate the affinity of the adsorbent for the inhibitor and the corrosion process is thermodynamically favorable. 3.3. Potentiodynamic polarization measurements Figure 6 represents the Potentiodynamic polarization curves for copper in 0.5 M HNO3 in the absence and presence of Lawsonia extract. Associated electrochemical parameters such as corrosion potential (Ecorr), corrosion current (icorr), anodic Tafel slope (βa), cathodic Tafel slope (βc) and percentage inhibition efficiency were given in Table 5. From Fig. 6 and Table 5, it was observed that the addition of Lawsonia extract in HNO3 solution, the significant decrease in the corrosion current density (icorr) and decrease in the corrosion rate with respect to the blank. There is significant change in the anodic and cathodic slopes after the addition of the inhibitor and slightly shifted towards cathodic region. This Tafel curves indicate that Lawsonia function as a mixed-type inhibitor with the predominant cathode effectiveness. Table 5. Potentiodynamic polarization parameters for copper in 0.5 M HNO3 and in absence and presence of 1.2 g/L Lawsonia extract. System Ecorr icorr (mV) (μA/cm2) Tafel Slope IE (%) mV/decade calculated from Anodic Cathodic Polarization Weight loss βa βc method Method Blank -28.2 2062 169.87 429.74 - - Lawsonia extract -33.3 219.3 103.83 200.56 89.36 95.56 Inhibition efficiency (IE) from (icorr) was calculated using following equation [31]. IE(%) = i corr (uninh ) −i corr (inh ) i corr (uninhi ) x 100 (10) Fig.6. Potentiodynamic polarization curves for copper in (a) 0.5 M HNO3 and (b) 0.5 M HNO3 containing 1.2 g/L Lawsonia extract. 3.4. Electrochemical impedance spectroscopy (EIS) measurements Corrosion of copper in 0.5 M HNO3 solution in the presence of Lawsonia extract was investigated by EIS at room temperature. Nyquist curves for copper obtained in the absence and presence of extract were shown in Figure 7 and EIS parameters were shown in Table 6. It was observed from Figure 7 that the impedance diagram is almost semicircular in appearance, but not perfect semicircle. The difference has been attributed to frequency dispersion. The semicircular nature of the plots indicates that the corrosion of copper is mainly controlled by charge transfer process. Table 6. EIS parameters for the corrosion of copper in 0.5 M HNO3 in absence and presence of 1.2 g/L Lawsonia extract. Rct Cdl (Ω cm2) (μF/cm2) Blank 85 Lawsonia extract 539 System IE (%) calculated from EIS Weight loss Method method 53.52 - - 2.53 84.23 95.56 The charge transfer resistance (Rct) values were calculated from the difference in impedance at lower and higher frequencies. To obtain 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 [32]. Cdl = 2πf 1 max R ct (11) Where fmax is the frequency at maximum height of the semicircle on the imaginary axis [33]. Fig.7. Nyquist plot for copper in (a) 0.5 M HNO3 alone and (b) 0.5 M HNO3 containing 1.2 g/L Lawsonia extract. Inhibition efficiency from Rct values was calculated by using the following equation [32]. IE(%) = R ct (inh ) −R ct (uninh ) R ct (inh ) (12) x 100 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 above results can be explained on the basis that the electrostatic adsorption of inhibitor species at the metal surface leads to form a physical protective film that retards the charge transfer process and therefore inhibits the corrosion reactions, leading to increase 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 values of Cdl [34]. From the result, IE of Lawsonia extract for copper in HNO3 solution obtained by weightloss, polarization and EIS methods were almost similar. 120 100 IE (%) 80 60 40 20 0 Weight loss method Polarization method EIS method Fig. 8. Comparison of inhibition efficiency (IE) values obtained by weight loss, polarization and EIS method for copper in 0.5 M HNO3 solution containing 1.2 g/L Lawsonia extract. 3.5. Mechanism of corrosion Being a strong oxidizing agent, HNO3 is capable of attacking copper. Copper is corroded to Cu+2 in HNO3 solution and no oxide film is formed to protect the surface from the attack of the corrosive medium. The electrochemical reaction for copper in HNO3 solution may be described as follows. Cu ( s ) + 4HNO 3 ( a q ) → Cu(NO 3 ) 2 ( a q ) + 2NO 2 ( g ) + 2H 2 O ( l ) Anodic reaction: Cu → Cu + 2 + 2e - (13) (14) Cathodic reaction: NO 3 - + 3H + + 2e - → HNO 2 + H 2 O (15) NO 3 - + 4H + + 3e - → NO + 2 H 2 O O2 (16) + 4H + + 4e - → 2 H 2 O (17) 3.6. Mechanism of inhibition It was reported that Lawsonia inermis leaves extract contain soluble matter, lawsone (2- Hydroxy-1, 4-naphthoquinone), resin and tannin, coumarins, gallic acid and sterols [25]. The main components of Lawsonia inermis extract are hydroxy aromatic compounds such as tannin and lawsone. The main constituent of the extract is lawsone (Fig. 9) which is present in a relatively higher amount. Lawsone amounts to 1.02 % in the leaves [35]. The coloring matter is quinone. Lawsone molecule is a ligand that can chelate with various metal cations forming complex compounds. Therefore, the formation of insoluble complex compounds, by combination of the metal cations and the lawsone molecules adsorbed on the metal surface [22], is a probable interpretation of the observed inhibition action of lawsone. In the acidic medium, delocalization of the lone pair of electrons on hydroxyl group takes place resulting in the rearrangement shown in Fig. 11. Such a rearrangement, in the presence of metal cations, enhances the complex formation reaction Fig. 10. This could be the reason for the observed high inhibition efficiencies in the acidic medium. O OH O Fig.9. Structure of Lawsone. Fig.10. Forms of M-Lawsonia complexes. M is Cu. Fig.11. Process of delocalization on Lawsone molecule. The inhibitive action of tannin was attributed to the formation of a passivating layer of tannates on the metal surface [36,37]. Tannins are also known to form complex compounds with different metal cations, especially in the basic media. It can be concluded then, due to the higher amount of lawsone in Lawsonia inermis extract, it can be attributed to the main constituent is responsible for inhibition. Moreover, in the presence of henna extract the values of corrosion potential Ecorr are nearly constant; therefore, henna extract could be classified as a mixed type inhibitor with the predominant cathode effectiveness. 4. 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