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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, Gujarat395009, IndiaFor correspondence:Email: firstname.lastname@example.org (K.K.Patel) Abstract The inhibitive actionof hennaleavesLawsoniaInermis extract oncorrosion of copper in nitric acid solution wasinvestigatedthroughweight losspotentiodynamicpolarizationand electrochemical impedance spectroscopic methodsat different temperatureThe effect of inhibitor concentrations on different acid concentrations wasinvestigated. The presentstudy revealed that the percentage of inhibition efficiency is enhanced with increase of inhibitor ncentration and decrease with increase in temperature. The inhibitive action of the extract is discussed in view of adsorption of Lawsoniamolecules on the metal surface. It was found that adsorption follows Langmuir adsorption isothermTafel plotof polarization study indicatethat the Lawsoniaextract acts asmixed type inhibitor. nhibition efficiency of Lawsoniaextract was found up to 98%. Keywords:Corrosion;CopperLawsoniaextractNitric acidInhibitor 1. I ntroduction Corrosion of metalan 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 applicationssuch as electronics and in thmanufacture 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  and sulphur [10, 11] in the organic compound molecule improves its action as corrosion inhibitionHowever, as a result of their high costtoxicityand increasing awareness of health and ecological risks, attention is being drawn towards finding highly efficient, cheaper and nontoxic 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 ecofriendliness.The aim of the present work is to develop ecofriendlycorrosion inhibitors, with good inhibition efficiency(IEat low risk of environmental pollution . From many decades, lant extracts have attracted attention in the field of corrosion inhibition. As natural products, they are a source of nontoxic, ecofriendly, biodegradable and of potentially low cost inhibitors for preventing metal corrosion . Most of the naturally occurring substances are safe and can be extracted by simple procedure. The inhibitive action of leaves extract of Azadirachta indicaon copper corrosion in HNOsolutions was studied using weight loss and ectrochemical techniques 4]. The results obtained indicated that the extracts functioned as good inhibitors in HNOsolutions.HennaLawsoniaInermis, a herb which has interesting dyeing properties used for centuries in Asia and North Africa for traditional decoration of the skin and hair. LawsoniaInermishas antiinflammatory, antipyretic and analgesic effectt5-16]. Hennahas been used as corrosion inhibitor for aluminiumand steel inaggressive solution , iron in hydrochloricacid  and aluminium in hydrochloric acid [Etre A.Y. et al..1] studied the inhibiting action of Lawsoniaextract 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, inhibitiveaction of Lawsonialeavesextract as a cheap, eco friendly and naturally occurring substanceon corrosionbehavior of copperin HNOsolutionhas been investigatedthrough weight loss, polarization measurements and electrochemical impedance spectroscopy methods. 2. E xperimental section 2.1Sample and Solution preparationThe 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 withdistilled water and finally dried and weighted by using electronic balance. The corrosive solutions were prepared by dilution of analytical grade of(Merck) using distilled water. 2.2Extract preparationLawsoniainermisleaves 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 evaporateses22]. This extract was used to study the corrosion inhibition properties and to prepare the required concentrations of Lawsoniainermisvaried from 0.6, 0.8, 1.0 and 1.2 g/L2.3Weight loss measurementFor weightloss experiment, the copper coupons were each suspended completely in 0.5, 0.75 and 1.0 M HNOsolutionwithoutand with different concentrations of Lawsoniaextract 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 24h, washed by distilled water, dried well and reweighed. Fromthe weight loss data, orrosion ratein mg/dmwas calculated.2.4Temperature effectTo study the effect of temperatureon corrosion rate, the copper coupons were completely immersed in 230 ml of 1M HNOsolution withoutand with different concentrations of Lawsoniaextract at 313, 323 and 333 K for 2h. From the data, inhibition efficiency, energy of activation) and heat of adsorption (Qads) were calculated.2.5. Electrochemical measurementsElectrochemical measurements were carried out by using an electrochemical work station (CHI608Cseries, U.S. Model with CH instrument)In electrochemical experiment Ag/AgClwas used as a reference electrode, platinum as an auxiliary electrode and copper metal was used as a working electrode. For polarization study,opperspecimenshaving an area of 1 cmexposed to 230 ml 0.5 M HNOin absence and presence of Lawsoniaextractand allowed to establish a steadystate 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 mVs. 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[21The electrochemical impedance studies were carried out in the same setup using potentiodynamic polarization studies described above. Impedance studies were carried out at steadystate open circuit potential (OCP). A small amplitude (5.0 mV) sinusoidal ac oltage, in wide frequency range 1 to 100 Kwas applied over the system. A graph was drawn by plotting real impedance (Zversus imaginary impedance (Z″). From the Nyquist plots the charge transfer resistance (R), and double layer capacitance (Cdl) were calculated. Impedance measurements were carried out both in the absence and presence of Lawsoniaextract. 3. R esults and discussion 3.1Weight loss experimentThe corrosion rate of copper in 0.5, 0.75 and 1.0 M of HNO3 solution without and with different concentration of Lawsoniaextract 0.6, 0.8, 1.0 and 1.2 g/L at 3011 K for an exposure period of 24h (1 day) was calculated from the weight lossdata usingfollowingequation CRdmWeightloss1000metalsurfacedmday The Inhibition efficiency (IE) was calculated by using followingformula, uninhuninhi 100(2) here, WWeight loss without inhibitor, WWeight 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, uninhuninhi (3) Table 1 . Corrosion ratefor copper in various HNOconcentrationsin the absence and presence of different concentrations of Lawsoniaextract from weight loss measurements at 301±1 K. Inhibitor c oncentration g/L Acid c oncentration 0.5 M 0.75 M 1.0 M CR (mg/ dm 2 d) IE (%) CR (mg/ dm 2 d) IE (%) CR (mg/ dm 2 d) IE (%) Blank 90.52 - 281.55 - 1030.67 - 0.6 2 0.11 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 indicate that as the concentration of acid increases corrosion ratewasincreases. The corrosion rate was 90.52, 281.55 and 1030.67 mg/for 0.5, 0.75 and 1.0 M HNOconcentrations respectively for an immersion period of 24 h at 301±1 K.At constant acid concentration, the IE increases with increase Lawsoniaextract concentrations, e.g. Lawsoniaextract in 0.5 M HNOsolution, 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 At constant inhibitor concentration, the IE was decreases as the acid concentration increases, e.g. for 0.6 g/L Lawsoniaextract, the IE was found to be 77.78, 71.43 and 53.66 % with respect to 0.5, 0.75 and 1.0 M HNOsolution (Table 1). Table 2 . Corrosion rate (log ρ) of copper in 0.5 M HNOin absence and presence of Lawsoniaextract for an immersion period of 24h at 301₱ 1K Inhibitor concentration (C) (g/L) CR (ρ) log ρ IE (%) surface coverage (θ) 1 - 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 Figwhich 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/dmwhile increases from 77.78 to 95.56 %. It can be concluded that is directly proportional to the inhibitor concentration. Fig . 1 . Corrosionrate of copper corrosion in 0.5 M HNOsolution in absence and presence of different concentration of Lawsoniaextract for an immersion period of 24h (1 day). 020406080100 Blank 0.6 0.8 1 1.2 corrosion rate (mg/dm 2 d Inhibitor concentration (g/L) Fig. 2 . IE of copper corrosion in 0.5 M HNOsolution in presence of different concentration of Lawsoniaextract for an immersion period of 24h (1 day). 3.2Temperature effectTo investigate the influence of temperature on corrosion of copper, theeightloss experiments were carried out at temperature 313, 323 and 333K in 1.0 M HNOwithout and with Lawsoniaextract for an immersion period oThe 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/dmwith respect to 313, 323 and 333 K. The IE was decrease with increase in temperature, e.g. for 1.2 g/L awsoniaextract 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 (E) has been calculated with the help of following Arrhenius equation log21 303 1T1 \rF1T2 (4) here,and ρare the corrosion rate at temperature Tand respectively. Table 3 . Temperature effect on corrosion rate (CR), inhibition efficiency (IE) and activation energy (Efor copper in 1M HNOin absence anpresence of Lawsoniaextract for an immersion period of 2 Inhibitor concentration (g/L) Temperature Energy of activation (E a (kJ/mol) (E a from Arrhenius plot (kJ/mol) 313 K 323 K 333 K 313 - 323 K 323 - 333K Mean CR (mg/ dm 2 d IE (%) CR (mg/ dm 2 d IE (%) CR (mg/ dm 2 d IE (%) Blank 5309.529050.2223530.5844.8385.4665.1564.33 0.6 2232.3657.955550.8438.6717376.4826.1576.58102.0789.3288.82 0.8 1086.0079.554042.4455.3314540.768.21110.50114.49112.49112.45 1.0 362.0493.181568.7682.676516.1272.31123.27127.36125.32125.27 1.2 241.3295.45905.0490.004223.4082.05111.13137.78124.45123.96 708090100 0.6 0.8 1 1.2 IE (%) Inhibitor concentration (g/L) The value of Ewere also calculated from the slope of the Arrhenius plot (Fig. ), i.e. log ρ Versus 1/T . (ρ=corrosion rate, T=absolute temperature)It was found that, the values of Ewere found higher in inhibited acid ranging from 89.32 to 125.32 kJ/mol than Evalues for uninhibited acid 65.15 kJ/mol. The higher values oindicate physical adsorption of the inhibitor on the metal surface and the adsorption of inhibitor causes an increase in the Eof the process . Fig . 3 . Arrhenius plot for corrosionof copperin 1M HNOin absence and presenceof different concentration ofLawsoniaextract for an immersion period of 2 The mean value of Ewas 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). Fig . 4 . Effect of temperature on IE for copper corrosion in 1M HNO3 at different concentration of Lawsoniaextract for immersion period of 2 The values of heat of adsorption (Qads) were calculated by following equation . ads \rFlog11\rF1 T1T2T2\rFT1 (5) where, and are the fraction of the metal surface covered by the inhibitor at temperature T1 and Trespectively. From Table 4, it is evident that in all cases, the Qads values are negative and 220.127.116.11.5 5.5 3.00 3.1 3.2 log ρ ( CR) 1/T x 1000 K - 1 0.6 g/L 0.8 g/L 1.0 g/L 1.2 g/L 1030507090110 313 323 333 IE (%) Temperature (K) 0.6 g/L 0.8 g/L 1.0 g/L 1.2 g/L 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 s correlated to surface coverage(θ)and a 100 % efficiency suggestingto full coverage (θ=1). The degree of surface coverage values re used to determine its adsorption characteristics in HNOsolution. The plot of Cinh/θ versus Cinhgives straight lines with slope values equal to unity (Fig. ). All the regression coefficients are very close to one which indicates that the inhibitors cover both the anodic and cathodic region through general adsorptionfollowing Langmuir isotherm . This isotherm can be represented as, inh ads inh here, Kads= equilibrium constant of the adsorption process and Cinh= inhibitor concentration. Fig . 5 . Langmuir adsorption isotherm for corrosion of copper in 0.5 M HNOsolution containing different concentration of Lawsoniaextract for an immersion period of 24h.ree energy of adsorption (ΔG) was determined by the Langmuir isotherm was given by a plot of inhinhnh27] (Fig. From the intercepts of the straight lines on the Cinh/θ axis, Kadscan be calculated which was related to ΔG, as given by following equation. The ΔGvalue of the inhibitors on copper surface can be calculated from the following equation [28,29ΔGo = RT ln (55.5 Kadshere, R is the gas constant, T is the absolutetemperature (K), and the value 55.5 in the above equation is the concentration of water in solution in Molar , Kadsis the equilibrium constant of the adsorption/desorption process.The ΔGvalues were almost negative in all cases indicated the spontaneous adsorption of Lawsoniaextract on copper surface and strong interactions between inhibitor molecules and the metal surface.The enthalpy of adsorption (ΔH) and entropy of adsorption (ΔS) were calculated using the equations (8) & (9). 0.50.70.91.11.31.5 0.6 0.8 1 1.2 C inh / θ C inh (g/L) ΔH= E(8)ΔS= ΔH/T(9) Table 4 . The values of physical parameters Heat of adsorption (adsFree energy of adsorptionEnthalpy of adsorption (and Entropy of adsorption (for copper in.0 HNO3 in the absence and presence of different concentration ofLawsoniaextract for an immersion period of 2 Inhibitor c oncentration (g/L) Q ads (kJ/mol) G a 0 (kJ/mol) a 0 (kJ/mol) a 0 (kJ/ mol K 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 10.689.518.46 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 The results revealed that ΔHvalues 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 ΔHpositive and ranging between 73.98to 135.10molindicating the endothermic nature of the reaction suggeststhat higher temperature favors the corrosion process. Positive value of ΔSlie 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 Figurerepresents the Potentiodynamic polarization curves forcopper in 0.5 M HNOin the absence and presenceof Lawsoniaextract. Associated electrochemical parameters such as corrosion potential (Ecorr), corrosion current (icorr), anodic Tafel slope, cathodic Tafel slope ) and percentageinhibition efficiencyweregiven in Table 5From Fig. and Table 5, it was observed that the addition of Lawsoniaextract in HNOsolution, 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 Lawsoniafunction a mixedtype inhibitor with the predominant cathode effectiveness. Table 5 . Potentiodynamic polarization parameters forcopper in 0.5 M HNO3 and in absence and presence of 1.2 g/LLawsoniaextract System E corr mV i corr μA/cm 2 Tafel Slope mV/decade IE (%) calculated from Anodic a Cathodic c Polarization method Weight loss M ethod Blank 28.22062169.87429.74 Lawsonia extract - 33.3 219.3 103.83 200.56 89.36 95.56 Inhibition efficiency (IEfrom (icorrwascalculated using following equation corruninhcorrinhcorruninhi 100 Fig . 6 . Potentiodynamic polarization curves for copper in (a) 0.5 M HNOand (b) 0.5 M HNO3 containing1.2 g/LLawsoniaextract 3.4 . Electrochemical impedance spectroscopy (EIS) measurements Corrosion of copper in 0.5 M HNOsolution in the presence of Lawsoniaextract was investigated by EIS at room temperature. Nyquist curves for copper obtained in the absence and presence of extractre shown in Figureand EIS parameters were shown in Table 6It s observed from Figurethat the impedance diagram is almost semicircular in appearance, but not perfect semicircle. The difference has been attributed to frequency dispersionThe 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 presenceof 1.2 g/LLawsoniaextract System R ct (Ω cm 2 C dl (μF/cm 2 IE (%) calculated from EIS M ethod Weight loss method Blank 85 53.52 - - Lawsonia extract 539 2. 53 84.23 95.56 The charge transfer resistance (R) 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 dlmax 1 Where fmaxis the frequency atmaximum height of the semicircle on the imaginary axis. Fig. 7 . Nyquist plot for copper in (a) 0.5 M HNOalone and (b) 0.5 M HNO3 containing 1.2 g/LLawsoniaextract Inhibition efficiencyfrom Rvalues was calculated by using the following equation . ctinhctuninhctinh 100(12) The addition of inhibitor, increase Rvalue while decreases in Cvalues 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 Rvalues. 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 .From the result, IE of Lawsoniaextract for copper in HNOsolution obtained by weightlosspolarization and EIS methods were almost similar. Fig. 8 . Comparison of inhibition efficiency (IE) values obtained by weight loss, polarization and EIS method for copper in 0.5 M HNOsolution containing 1.2 g/L Lawsoniaextract. 3.5. Mechanism of corrosion Being a strong oxidizing agent, HNOis capable of attacking copper. Copper is corroded to in HNOsolution and no oxide film is formed to protect the surface from the attack of the corrosive medium.The electrochemical reaction for copper in HNOsolution may be described as follows.(s)+ 4HNO3(aq)Cu(NO2(aq)+ 2NO2(g)+ 2H(l)Anodic reactionCu +2 + 2e 020406080100 120 Weight loss method Polarization method EIS method IE (%) Cathodic reactionNO+ 3H+ + 2eHNOO NO+ 4H+ 3eNO + 2O 2 + 4H+ 4e 3.6. Mechanism of inhibition It was reported that Lawsoniainermisleaves extract contain soluble matter, lawsone (2Hydroxy1, 4naphthoquinone), resin andtannin, coumarins, gallic acid and sterols 25]. The main components of Lawsoniainermisextract are hydroxy aromatic compounds such as tannin and lawsone. The main constituent of the extract is lawsone (Fig. ) which is present in a relatively higher mount. 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 combinationof the metal cations and the lawsone molecules adsorbed on the metal surface , 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. Such a rearrangement, in the presence of metal cations, enhances the complex formation reaction Fig. . This could be the reason for the observed high inhibition efficiencies in the acidic medium. Fig. 9 . Structure of Lawsone. Fig. 10 . Forms ofLawsoniacomplexes. M is Cu. Fig. 11 . Process of delocalization on Lawsonemolecule. O O OH The inhibitive action of tannin was attributed to the formation of a passivating layer of tannates on the metal surface 36,3Tannins 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 Lawsoniainermis extract, it can be attributed to the main constituentis responsible for inhibition. Moreover, in the presence of henna extract the values of corrosion potential Ecorrare nearly constant; therefore, henna extract could be classified as a mixed type inhibitor with the predominant cathode effectiveness. 4. C onclusi on The present study shows thatLawsoniawas found to be a good ecofriendly inhibitor for the corrosion control of copper in HNOsolution.Corrosion rate increaseas HNOconcentration increase.The inhibition efficiency increases with increase in Lawsoniaextract concentration.Lawsoniaadsorbed on metal surface follows Langmuir adsorption isotherm. Tafel plot indicates Lawsoniaacts as a mixed type inhibitor.AC impedance spectra reveal that a protective film is formed on the metal surfaceAll three techniques give almost identical values of inhibition efficiency or copper in HNOsolution 5. 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