Strychnos nuxvomica, Piper longum and Mucuna pruriens seed extracts as eco-friendly corrosion inhibitors for copper in nitric acid

Abstract

The inhibitive effect of the Strychnos nuxvomica (SN), Piper longum (PL) and Mucuna pruriens (MP) seeds extract on the corrosion of copper in 3 M HNO₃ solution was studied using gravimetric, potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) studies. Inhibition efficiency increased with increasing concentration of the extract and maximum inhibition efficiency was 91.60%, 80.00% and 71.6% obtained from gravimetric studies at 0.2 g L⁻¹ of SN, PL and MP respectively. Above this concentration no noticeable change was observed. The adsorption of the inhibitor on copper surface was in accordance with the Langmuir adsorption isotherm. Potentiodynamic polarization study showed predominantly cathodic type inhibition. SEM and AFM study was carried out to support the inhibition data.

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1. Introduction

The protection of metallic objects against corrosion is a major industrial problem. The use of inhibitors is one of the best options for protecting metals against corrosion in acidic media. Acid solutions are used in many industries for chemical cleaning, pickling, etc. and thus the chances of copper corrosion are particularly high in such industrial processes.^1–3 Organic compounds, especially those with N, O and S atoms^4,5 have frequently been used to prevent corrosion in acid solution. Although most of these compounds are not expensive, they are toxic for living beings and very hazardous to the environment.

Therefore, the present trend in research on eco-friendly corrosion inhibitors has been concentrating on products of natural origin owing to their low cast and biodegradability. Plant extracts have become important as inexpensive, non toxic and renewable sources of wide range of corrosion inhibitors. The yield of these natural products as well as the corrosion inhibition abilities of plant extracts highly depends on the part of the plant and the location in which it is found. The different parts of plant like seeds,^6–9 leaves,^10–12 flowers,^13–16 and fruits¹⁷ had been extracted and used as corrosion inhibitors.

An inhibition efficiency of 90.7% has earlier been reported with 0.1 g L⁻¹ of Myrtus comminis (seed oil) for copper in sulfuric acid at 298 K by Mansoor et al.¹⁸ The peel extract of Gnetum fricana and Musa acuminate exhibit 80.5 and 76.1% inhibition efficiencies at 3 g L⁻¹ on the corrosion of copper in HNO₃ solution at 303 K.¹⁹ The effect of fruit extract of Gingko biloba on corrosion inhibition of J55 steel in 3.5% NaCl solution saturated with CO₂ was studied and the maximum efficiency was reported to be 95% at 1000 ppm by polarization measurements.²⁰ The inhibition efficiencies as high as 91.0 and 65.3% at optimum concentration (500 ppm) of Zygophllum coccineum L. extract on the corrosion of copper in 1 M HNO₃ was reported at 298 and 318 K, respectively.²¹ Earlier Vitex nigundo, Adhatoda vasica and Saraca, asoka leave extracts was successfully used as inhibitor of copper corrosion in nitric acid.²²

In the present investigation, seed extract of Strychnos nuxvomica (SN), Piper longum (PL) and Mucuna pruriens (MP) have been used for the study on copper corrosion inhibition in nitric acid. The inhibition study was conducted by gravimetric, potentiodynamic polarization and electrochemical impedance spectroscopy measurements. The experimental inhibition results were complemented with UV-Visible spectroscopy, SEM and AFM investigation.

2. Experimental

2.1. Coupon preparation

The coupons of size 2 × 2 × 0.1 cm and 3 × 1 × 0.1 cm cut from copper were used for gravimetric and electrochemical measurements, respectively. The specimens were mechanically abraded with 600, 1000, 1500 and 2000 grade of emery papers, washed successively with double distilled water and acetone. The samples were dried at room temperature in a desiccator then used for the experiments.

2.2. Electrolytic solution

The corrosive solution 3 M HNO₃ was prepared by dilution of analytical grade HNO₃ of predetermined normality with double distilled water. The concentration range of the extract used was 0.050–0.20 g L⁻¹ and the volume of electrolyte used was 150 mL for each experiment.

2.3. Inhibitor

Strychnos nuxvomica (SN), Piper longum (PL) and Mucuna pruriens (MP) seeds were dried and ground to powder form. 10 g of the powder was soaked for 24 h in 500 mL double distilled water and then refluxed in a round bottom flask for 4 h. After that, it was filtered and the resulting filtrate was concentrated to 100 mL by evaporation using rotavapor at temperature 328 K. The obtained extract was used for the measurements of inhibitive properties. The residue was air dried and weighed. The amount of extract in the filtrate was determined by subtracting the weight of the residue obtained from the total weight of the seeds taken.

A plant extract is a complex mixture of various phytochemical compounds and the major component present in the extracts of SN, PL and MP are brucine,²³ piperine²⁴ and L-DOPA,²⁵ respectively. There are a number of minor constituents present in each of the three extracts. Some of them may have synergistic effects while others may have antagonistic effects. However, in the present case only major constituents are being considered as the species mainly responsible for corrosion inhibition and their structures are given in Table 1.

Table 1 Name and molecular structure of active constituents present in seed extracts

Leaf extract	Major component	Structure
SN	Brucine
PL	Piperine
MP	L-DOPA

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2.4. Gravimetric measurement

The abraded Cu specimens of size 2 × 2 × 0.1 cm were weighed on a balance with 1 mg sensitivity and immersed for 4 h in 3 M HNO₃ in absence and the presence of 0.05, 0.08, 0.1 and 0.2 g L⁻¹ at 308 K. A hole was made in the upper corner of the sample and was hanged in the electrolyte by tying it with the help of a thread. Study of temperature effect on inhibition efficiency was performed in the temperature range from 308 to 323 K for the highest concentration of inhibitor (0.20 g L⁻¹). To compensate the volume change due to addition of inhibitor, volume correction was done while preparing 3 M HNO₃ acid. The corrosion rate (C_R) in mg cm⁻² h⁻¹ was calculated from the following equationwhere, ΔW is the average weight loss, S the total area of the specimen and t is the immersion time. From the corrosion rate thus obtained, the inhibition efficiency (η%) was calculated as below:where, C⁰ and C_R are the corrosion rates of copper in the absence and presence of inhibitors, respectively.

2.5. Electrochemical measurements

Electrochemical tests were carried out in a conventional three electrode cell consisting of a flat bottom Pyrex glass flask with three inlets for working, reference and counter electrodes. A rectangular size working electrode of copper metal with an exposed surface area of 1 cm² was attached to a self-designed holder and the rest of the surface was covered with lacquer. Silver–silver chloride electrode (3 M KCl) and platinum wire were used as a reference and counter electrodes, respectively. The working electrode was first immersed in test solution for establishing a steady state open circuit potential (OCP) and then the cathodic and anodic polarization studies with repeated to OCP were conducted using electrochemical analyzer (CHI604C).

Linear polarization resistance (LPR) measurements were potentiodynamically performed in the potential range +10 to −10 mV on both sides of OCP at a scan rate of 0.01 mV s⁻¹. Polarization resistance (R_P) values were obtained from slope of the resulting linear plot between current density and potential plot. From the measured R_P values, the η% has been calculated using the relation.

where, R⁰_P and R_P are the polarization resistance values in absence and the presence of inhibitor.

In potentiodynamic polarization study the potential was scanned from −250 to +250 mV with respect to OCP at a rate of 0.5 mV s⁻¹. The inhibition efficiency has been calculated by substituting respective corrosion current densities (i⁰_corr and i_corr) in place of corrosion rates (C⁰_R and C_R) in eqn (2).

AC impedance measurements were carried out using signals of 5 mV amplitude in the frequency range from 100 kHz to 0.01 Hz after 4 h of immersion. The obtained impedance data were analyzed with Zsimpwin software. The charge transfer resistance (R_ct), was obtained from the diameter of the semicircle of the Nyquist plot. The inhibition efficiencies of these inhibitors have been determined by replacing R_P and R⁰_P with R_ct and R⁰_ct respectively, in eqn (3).

2.6. Spectral analysis

The composition of electrolyte after corrosion inhibition study at different time intervals from 4 to 24 h was determined by the UV spectroscopy. UV-Visible spectra were recorded in the range 200–1100 nm on a Shimadzu spectrophotometer, Pharmaspec. UV-1700 model.

2.7. Density functional theory (DFT) study

Quantum chemical calculations were performed using DFT method and structural parameters were geometrically optimized using functional hybrid RB3LYP with electron basis set DGDZVP for all atoms. All the calculations were performed with Gaussian 03, E.01 as reported in literature.²⁶ The quantum chemical parameters obtained were E_HOMO, E_LUMO, ΔE (E_LUMO − E_HOMO), I (ionization energy), A (electron affinity), μ (dipole moment) and ΔN (fraction of electron transferred).

2.8. Surface analysis

The surface morphology of the inhibited and uninhibited copper surfaces in 3 M HNO₃ was analyzed by scanning electron microscope. An electron microscope, FEI Qunanta 200f, was used for Scanning Electron Microscopy (SEM). Further characterization of the corroded and inhibited copper surface in term of roughness was done by Atomic Force Microscopy. The experiments were performed by contact mode AFM (Model no. BT 02218, nanosurfeasyscan 2 basic AFM, (Switzerland)) supported by Si₃N₄ cantilever (Nanosensor, CONTR Type) having spring constant of 0.1 Nm⁻¹ and tip radius more than 10 nm. The measurements were done over a square area with an image size of 50.0 μm. The resolution was 256 points per line; the image were obtained at scan forward mode and scanning was carried out from down to up in static regime.

3. Results and discussion

3.1. Gravimetric measurements

3.1.1. Effect of inhibitor concentration. The corrosion inhibition studies of copper in 3 M HNO₃ solution at 308 K containing 0.05, 0.08, 0.1 and 0.2 g L⁻¹ of the SN, PL and MP seed extracts have been conducted using weight loss measurements and the data obtained after 4 h of immersion time are recorded in Table 2. It is apparent from the data that weight loss is substantially decreased on the addition of inhibitors and on increasing its concentration there is an exponential decrease in the corrosion rate. The inhibition efficiency with different concentration of three seeds is illustrated in Fig. 1a. It can be seen from the figure that gradually increased inhibitor concentration increases the percentage inhibition efficiency (η%). The inhibitive action of the inhibitor is due to the presence of hetero atoms such as oxygen, nitrogen and aromatic rings with pi bonds in their constituents, which act as active centers for adsorption and create a protective film on metal surface. The maximum inhibition efficiency (91.6%) was obtained for SN at 0.2 g L⁻¹. The orders of inhibition efficiency are as follows:

SN (91.6%) > PL (80.0%) > MP (71.6%)

Table 2 Corrosion parameters for copper in 3 M HNO₃ in absence and presence of different concentrations (0.05–0.2 g L⁻¹) of SN, PL and MP gravimetric measurement at 308 K for 4 h

Inhibitor	Concentration (g L^−1)	Weight loss (mg cm^−2 h^−1)	CR (mg cm^−2 h^−1)	η (%)
Blank SN	—	60	1.666	—
	0.05	21.0	0.583	65.0
	0.08	18.0	0.500	70.0
	0.1	15.0	0.416	75.0
	0.2	5.0	0.138	91.6
	0.3	5.3	0.147	91.2
	0.4	5.1	0.141	91.5
PL	0.05	23.0	0.638	61.6
	0.08	19.0	0.527	68.3
	0.1	16.0	0.444	73.3
	0.2	12.0	0.333	80.0
	0.3	12.0	0.333	80.0
	0.4	12.3	0.341	79.5
MP	0.05	26.0	0.722	56.6
	0.08	23.0	0.638	61.6
	0.1	19.0	0.527	68.3
	0.2	17.0	0.472	71.6
	0.3	17.6	0.488	70.6
	0.4	17.0	0.472	71.6

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Fig. 1 Variation of inhibition efficiency (η%) of seed extract for copper in 3 M HNO₃ (a) with concentration and (b) with immersion time.

The variation of inhibition efficiency with immersion time at optimum concentration (0.2 g L⁻¹) of SN, PL and MP at 308 K is shown in Fig. 1b. It is found that the inhibition efficiency decreases with increase in immersion time from 4 to 24 h in all the cases of three seed extracts.

3.1.2. Effect of temperature. To evaluate the stability of adsorbed layer/film of inhibitor on copper surface as well as activation parameters of the corrosion process of copper in acidic medium, weight loss measurements were carried out in the range of temperature 308–323 K in the absence and presence of extracts at their optimum concentration (0.20 g L⁻¹). The results obtained are listed in Table 3. It is evident from this table that corrosion rate increases in the absence as well as the presence of inhibitor with the increases in temperature. Thus, because of the increase in corrosion rate at higher temperatures the inhibition efficiency decreases with temperature.²⁷ The inhibition efficiency decreases from 92–75%, 80–65% and 78–48% for three inhibitors. The decreases in inhibition efficiency is due to physisorption of inhibitor where in the desorption process of inhibitor molecules is enhanced at higher temperatures.²⁸ It is further noted that the extent to which the corrosion rate increases with temperature is much larger than the extent to which η% decreases, thus, it can be argued that a large part of the film still remains intact even at higher temperatures.

Table 3 Various corrosion parameters for copper in 3 M HNO₃ in absence and presence of highest concentration (0.2 g L⁻¹) of SN, PL and MP at different temperature

Inhibitor	Temperature (K)	CR (mg cm^−2 h^−1)	θ	η (%)
Blank	308	1.66	—	—
	313	2.38	—	—
	318	3.36	—	—
	323	4.58	—	—
SN	308	0.138	0.92	92
	313	0.222	0.89	89
	318	0.555	0.83	83
	323	1.138	0.75	75
PL	308	0.333	0.80	80
	313	0.583	0.75	75
	318	0.972	0.71	71
	323	1.583	0.65	65
MP	308	0.500	0.78	78
	313	0.861	0.71	71
	318	1.361	0.68	68
	323	2.194	0.48	48

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The apparent activation energy for corrosion reaction of copper in 3 M HNO₃ is calculated using Arrhenius equation:

where, E_a is the apparent activation energy, A is the Arrhenius pre-exponential factor and R is the universal gas constant. The E_a values were determined by linear regression between log C_R and 1/T Fig. 2. The E_a value in absence of inhibitor is 56.13 kJ mol⁻¹ and in the presence of SN, PL and MP, these values are 120.05 kJ mol⁻¹, 86.16 kJ mol⁻¹ and 81.18 kJ mol⁻¹ respectively. Higher E_a values for inhibitor molecules indicate retardation in mass/charge transfer from the surface to the electrolyte due to surface barrier.²⁹

Fig. 2 Arrhenius plots of log C_R versus 1/T at optimum concentration (0.2 g L⁻¹) of SN, PL and MP.

The values of the enthalpy of activation (ΔH⁰_a) and entropy of activation (ΔS⁰_a) were calculated using the following equation.³⁰

A plot of log C_R/T verses 1/T shown in Fig. 3 gave a straight line with a slope of ΔH_a/2.303R and an intercept of log(R/Nh) + ΔS_a/2.303. The values of ΔH⁰_a and ΔS⁰_a were calculated from three parameters and are listed in Table 4. The standard enthalpy of activation energy ΔH⁰_a showed similar trend like the activation energy values. The entropy of activation ΔS⁰_a values is negative in the absence and positive in presence of inhibitors. The negative ΔS⁰_a value indicates the activated complex is more ordered in the blank solution. The positive values of ΔS⁰_a suggest that an increase in disordering takes place in going from reactants to the activated complex on metal/solution interface²² which is driving force for the adsorption.

Fig. 3 Arrhenius plots of log C_R/T versus 1/T at optimum concentration (0.2 g L⁻¹) of SN, PL and MP.

Table 4 Activation parameters E_a, ΔH⁰_a and ΔS⁰_a for the copper dissolution in 3 M HNO₃ in the absence and presence of SN, PL and MP at highest concentration

Inhibitor (g L^−1)	Linear regression coefficient (R^2)	Ea (kJ mol^−1)	ΔH^0a (kJ mol^−1)	ΔS^0a (J mol^−1 K^−1)
Blank	0.999	56.13	53.45	−67.69
SN	0.999	120.05	117.56	118.48
PL	0.998	86.16	83.48	16.75
MP	0.999	81.18	78.80	4.51

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3.2. Electrochemical measurements

3.2.1. Effect of inhibitor on polarization behavior. The effects of seed extracts at highest concentration on the anodic and cathodic polarization behavior of copper in 3 M HNO₃ solution are shown in Fig. 4. The visual examination of polarization curves depicted in the figure reveals that the cathodic curves are shifted towards lower current density side with increase in inhibitor concentration while the shape remains unchanged. However, in case of anodic polarization there is little deviation in the curves. This indicates that the inhibitor affects the rate of cathodic process more prominently than anodic process.

Fig. 4 Potentiodynamic polarization behavior of copper in 3 M HNO₃ with the addition of optimum concentration of SN, PL and MP at 308 K.

HNO₃ is a strong oxidizer capable of attacking copper. The potentiodynamic polarization curves exhibit no steep slope in the anodic range which proves that no passive films are formed on the copper surface. The Pourbaix diagram for copper–water system^31,32 is drawn in Fig. 5. It shows that copper is corroded to Cu²⁺ in HNO₃ solutions and no oxide film is formed to protect the surface from the attack of the corrosive medium. Copper dissolution is thus expected to be the dominant reaction in HNO₃ solution. The electrochemical reactions³³ for copper in HNO₃ solution can be described as following equation:

Fig. 5 Potential–pH equilibrium diagram for the system, copper–water, at 25 ± 1 °C.

Anodic reaction:

Cu → Cu²⁺ + 2e⁻ (6)

Cathodic reaction:

NO₃⁻ + 3H⁺ + 2e⁻ → HNO₂ + H₂O (7)

NO₃⁻ + 4H⁺ + 3e⁻ → NO + 2H₂O (8)

O₂ + 4H⁺ + 4e⁻ → 2H₂O (9)

The corrosion parameters, viz., corrosion potential (E_corr), current density (i_corr), anodic Tafel slope (β_a), cathodic Tafel slope (β_c), polarization resistance (R_P) and inhibition efficiency (η%) calculated from polarization curves are presented in Table 5. It is observed from the table that i_corr is lowered on the addition of inhibitors indicating their retarding effect. The minimum value of i_corr is obtained for SN followed by PL and MP. It is further noted from the table that the values of E_corr shifts slightly towards more negative direction for all the inhibitors. The shift in E_corr values was lower than 85 mV in each case suggesting that these extracts are mixed type inhibitors.³⁰ In present study the displacement in the range (51–70 mV) of E_corr due to the presence of these inhibitors suggests that all the inhibitors are predominantly cathodic mixed inhibitor. The values of β_a in the presence of inhibitor did not show any significant change with respect to those observed in the blank solution. However, β_c values increase drastically in the presence of inhibitor further indicating retardation in cathodic process. Thus, these extracts behave as mixed inhibitors and are predominantly cathodic. The values of R_P obtained from LPR studies are listed in Table 5. It can be noted that R_P increased on the addition inhibitors which increased further on increasing their concentrations. Apparently, all the extracts retard copper corrosion and their inhibition efficiency regularly increases with increase in concentration.

Table 5 Electrochemical parameters obtained by Tafel polarization for copper in 3 M HNO₃ solution in without and with of SN, PL and MP at 308 K

Inhibitor (g L^−1)	Ecorr (mV)	icorr (μA cm^−2)	βa (mV dec^−1)	−βc (mV dec^−1)	η%	RP (Ω cm^2)	η%
Blank	−31	1305	96.1	307.3	—	31.1	—
SN	−70	121.9	66.2	269.1	90.6	298.3	89.2
PL	−58	276.5	78.6	289.7	78.7	158.7	80.4
MP	−51	395.1	95.3	302.6	69.7	101.3	69.2

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3.2.2. Effect of inhibitor on electrochemical impedance. Corrosion behaviour of copper in nitric acid solution in the absence and presence of optimum concentrations of SN, PL, and MP was investigated by EIS at 308 K, and corresponding Nyquist plots are shown in Fig. 6. It is observed that the Nyquist plots consist of capacitive loop in high frequency region and a straight line in low frequency region. The slightly depressed capacitive loop which has the centre below the real axis depends on the roughness and other inhomogeneity of copper electrode surface. The straight line, representing the Warburg impedance, is attributed to diffusion from the surface. The addition of all three extracts leads to change of the Nyquist plot both in shape and size. The size of capacitive loop at a fixed optimized inhibitor concentration, in the sequence: SN > PL > MP, is in the conformity with the highest inhibitive influence of SN.

Fig. 6 Impedance diagram of copper with optimum concentration of SN, PL, MP.

The impedance data have been analyzed using an electrochemical equivalent circuit represented in Fig. 7 where R_s, R_ct, W, and CPE stand for solution resistance, charge transfer resistance, Warburg impedance and constant phase element respectively. The impedance of the CPE is defined as follows:³⁴

Z_CPE = Y₀⁻¹(iω)⁻ⁿ (10)

where Y₀⁻¹ is a proportionality factor and ‘n’ has the meaning of phase shift. The value of ‘n’ represents the deviation from the ideal behavior³⁵ and it lies between 0 and 1. The calculated values of R_ct, CPE and chi square (goodness of fit) were obtained from the above mentioned equivalent circuit and are listed in Table 6. The value of R_ct increases while CPE decreases with the addition of inhibitor in nitric acid solution. The major effect was observed at 0.2 g L⁻¹ of seed extract which provided R_ct value to 272.82 Ω cm² and CPE value is 12.44 μΩ sⁿ cm⁻². The increase in R_ct can be attributed to the formation of an insulating protective film at the metal–solution interface. The values of n are 0.85 in blank and 0.89, 0.87 and 0.86 in presence of SN, PL and MP, respectively. The increase in n values in presence of seed extracts indicating reduction of surface inhomogeneity due to the adsorption of inhibitor molecules on the active adsorption sites at copper surface.³⁶ The diffusion process is controlled by the transport of dissolved oxygen from the bulk to the electrode surface and this accounts for W observed in low frequency region.^33,37 The value of Warburg impedance increases after addition of inhibitor with respect to blank and maximum value is observed for SN.

Fig. 7 The best fitted equivalent circuit for Cu metal in the electrolyte.

Table 6 Impedance parameter obtained fitting the Nyquist plots with the equivalent circuit in absence and presence of highest concentration of SN, MP and PL at 308 K

Inhibitor	Rs (Ω cm^2)	Rct (Ω cm^2)	CPE		Chi square	W (Ω^−1 cm^2 s^1/2)	η%
			Y0 (μΩ^−1 s^n cm^−2)	n
Blank	4.26	23.05	160.2	0.85	0.0016	0.0045	—
SN	7.29	272.82	124.4	0.89	0.0082	0.0146	91.6
PL	5.91	115.64	97.1	0.87	0.0264	0.0384	80.0
MP	3.83	77.17	96.6	0.86	0.0287	0.0927	70.1

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3.3. Adsorption isotherms

The nature of corrosion inhibition can be deduced in terms of adsorption isotherms by fitting the data in various isotherms like Freundlich, Temkin, Langmuir and Frumkin. Langmuir isotherm expressed by the following equation is found to be the best fitted.^38,39where K_ads is adsorption equilibrium constant, C_inh denotes the concentration of inhibitor and θ represents the surface coverage.

The plots of C_inh/θ and C_inh at different concentrations of SN, PL and MP gave straight lines (Fig. 8) suggesting the adsorption of inhibitors on the metal surface follows the Langmuir adsorption isotherm. It was found that R² and slope value obtained from Langmuir plots are close to 1, a necessary condition for the validity of Langmuir isotherm. The adsorption equilibrium constant (K_ads) is associated with standard free energy of adsorption ΔG⁰_ads by the following equation:

ΔG⁰_ads = RT ln (C_H2OK_ads) (12)

where R is universal gas constant, T is the absolute temperature and C is the concentration of water (1000 g L⁻¹). Since the values of K_ads is represented here is in g⁻¹ L, so, the concentration of water is also taken in g L⁻¹.

Fig. 8 Langmuir adsorption isotherm plot for copper in 3 M HNO₃ in presence of optimum concentration of (0.2 g L⁻¹) SN, PL and MP.

The values of K_ads and ΔG⁰_ads were calculated and recorded in Table 7. It is seen that the negative value of ΔG⁰_ads is found in all cases indicating the spontaneity of adsorption processes.

Table 7 Thermodynamics parameters for adsorption of SN, PL, and MP inhibitors on copper in 3 M HNO₃ solution at 308 K

Inhibitor	Kads (L g^−1)	R^2	−ΔGads (kJ mol^−1)
SN	10.54	0.998	27.21
PL	10.35	0.999	27.00
MP	10.20	0.998	26.14

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3.4. Effect of time on the stability of adsorbed extract

The stability of adsorbed layer of inhibitors (SN, PL, MP) with respect to time was analyzed by UV-Visible spectral analysis of the resulting electrolyte. Absorbance of electrolyte were measured after corrosion studies at 4, 8, 12 and 24 h immersion time shown in Fig. 9a–e. The maximum absorbance found at 795, 346, 337 and 336 nm for blank, SN, PL and MP, respectively. These peaks presented copper¹ in blank and also brucine,⁴⁰ piperine^41,42 and L-DOPA^43,44 which are major component present in SN, PL and MP, respectively. The peak observed in blank because of copper ions introduced in the electrolyte as result of corrosion. Whereas, no peak was observed with SN, PL and MP indicate inhibition of copper corrosion due to these extracts. From Fig. 9b–d it was observed that peak intensity of each leaf extract has decreased and correspondingly copper peak intensity has increased with immersion time. This shows that the adsorbed layer of these extracts destabilizes with time indicating physical adsorption. The peak intensity of characteristic peak of each seed extract is greater than corresponding copper which indicates that these inhibitors were still effective after 24 h of immersion time (Fig. 9e) but the inhibition efficiency decreased.

Fig. 9 UV-Visible spectra of seed extracts in 3 M HNO₃ after corrosion study (a) SN, PL, MP after 4 h, (b) SN after 4–24 h, (c) PL after 4–24 h, (d) MP after 4–24 h and (e) SN, PL, MP after 24 h.

3.5. Quantum chemical calculation

The inhibitive property of an inhibitor depends upon the electronic structure of the molecules.⁴⁵ The quantum chemical calculation gives an idea about the mechanism of inhibitor action and recently a large number of articles have been published which are focusing on the corrosion inhibition.⁴⁶ Major component present in the extracts of SN, PL and MP (Table 1) are considered for DFT study. The HOMO and LUMO population of the studied major components in both molecular and protonated forms are shown in Fig. 10 and 11, respectively. The quantum chemical parameters i.e. E_HOMO, E_LUMO, ΔE, ionization potential (I), electron affinity (A) and the fraction of electron transferred (ΔN) of each major constituent are listed in Table 8.

Fig. 10 Energy level diagrams for the frontier MOs of molecular form of major components brucine (SN), piperine (PL) and L-DOPA (MP).

Fig. 11 Energy level diagrams for the frontier MOs of protonated form of major components brucine (SN), piperine (PL) and L-DOPA (MP).

Table 8 Quantum chemical parameters calculated for SN (brucine), PL (piperine), MP (L-DOPA) in both molecular and protonated forms

Inhibitor	Total energy (a.u.)	HOMO (kJ mol^−1)	LUMO (kJ mol^−1)	ΔE (kJ mol^−1)	μ (Debye)	I	A	χ	γ	ΔN
SN	−1302.3	−484.1	−273.0	211.0	3.54	484.1	273.0	378.5	105.5	0.824
Protonated SN	−940.3	−1297.8	−1246.0	51.8	5.12	1297.8	1246.0	1271.9	25.9	−15.61
PL	−941.0	−566.5	−82.1	484.4	4.50	566.5	82.1	324.3	242.2	0.572
Protonated PL	−703.1	−2449.8	−2380.7	69.1	6.06	2449.8	2380.7	2415.2	34.5	−28.29
MP	−705.4	−606.7	−66.1	540.5	2.79	606.7	66.1	336.4	270.2	0.468
Protonated MP	−1112.1	−1316.9	−1230.7	86.2	5.00	1316.9	1230.7	1273.8	43.1	−9.40

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According to the frontier molecular orbital (FMO) theory, the ability of an inhibitor to donate their electrons to metal surface is associated with the E_HOMO values i.e. higher the E_HOMO values easier will be the donation of electrons from inhibitor to empty metal d-orbital. On the other hand, the ability of inhibitor molecules to accept the electrons is associated with the E_LUMO values i.e. lower its value easier will be the acceptance of additional negative charges by the inhibitor molecules. Also the stability index of any inhibitor is related to the energy difference (ΔE) between E_HOMO and E_LUMO. Thus, the lower ΔE values, will correspond to higher stability of inhibitor on the metal surface.⁴⁷ From Table 8, it is evident that E_HOMO values are as follows SN (−484.1 kJ mol⁻¹) > PL (−566.5 kJ mol⁻¹) > MP (−606.7 kJ mol⁻¹), which suggests that electron donation ability of SN is the highest and it will adsorb to a greater extent over copper surface. In the same way E_LUMO values are in the order: SN (−273.0) > PL (−82.1) > MP (−66.1), lower value of SN indicates its ability to accept electrons and thereby reducing corrosion of copper to a greater extent.⁴⁸

In brucine, the presence of two methoxy groups directly attached to the benzene nucleus enhances the electron density of benzene ring which favors its greater adsorption on the copper surface. On other hand, piperidine has better inhibition efficiency as compared to L-DOPA because heteroatoms, O, and N are fused in ring structure. The presence of COOH (electron withdrawing) group which decreases the electron density on nitrogen atom of NH₂ and OH moiety attached to benzene ring also decreases electron density of the ring in case of PL.

The adsorption/interaction between major component of an extract and copper–electrolyte interface can be described as follows: (i) charge-transfer-type interactions between unshared electron pairs (present on oxygen and nitrogen) or delocalized π electrons on aromatic ring(s) and the vacant low-energy d orbital's of surface copper atoms (chemisorptions); (ii) electrostatic interactions between protonated inhibitor and negatively charged Cu surface (physisorption) and (iii) a combination of the two (mixed type). The energy gap ΔE (E_LUMO − E_HOMO) is a function which quantifies the chemical reactivity of an inhibitor molecule for the adsorption on the metallic surface.⁴⁹ Lower value of ΔE gives good inhibition efficiency because the energy to remove an electron from the last occupied orbital will be low.⁵⁰ The order of energy gap in protonated inhibitor is similar to that obtained for molecular inhibitor. The observed trend of ΔE is the same for both molecular as well as protonated species. The inspection of the energy gap data in Table 8 reveals that protonated molecules show better inhibition than the molecular one. This may be due to increase in electrostatic interaction between protonated inhibitor and negatively charged Cu metal surface.

According to the Hartree–Fock theorem, the frontier orbital energies given by −E_HOMO = I, −E_LUMO = A, electronegativity (χ) and global hardness (γ) have proved to be very useful quantities. During a reaction between copper and inhibitor molecules, electrons move from the molecules with lower χ (inhibitor compound) to higher χ (copper) until their chemical potentials become equal. The fraction of electron transferred, ΔN is given by:

where, γ_Cu and γ_inh are global hardness of copper and inhibitor, respectively.⁵¹ In order to calculate the fraction of electron transferred, a theoretical value for the absolute electronegativity of copper (χ_Cu) was taken as 463.1 kJ mol⁻¹ as proposed by Pearson.⁵² The global hardness (γ_Cu = 0) for copper was assigned a value equal to zero based on the assumption⁵³ that for metallic bulk I = A because they are softer than the neutral metallic atoms. The number of electrons transferred (ΔN) has been tabulated in Table 8. The value of ΔN < 3.6 indicates the tendency of a molecule to donate electrons to the metal surface. Hence, the inhibition efficiency increases with increasing electron donating ability of these inhibitors to the metal surface.³⁰ The electron donating ability of inhibitor molecules follows the order brucine (0.824) > piperine (0.572) > L-DOPA (0.468). The highest fraction of electrons transferred is associated with the best inhibitor, while the lower fraction is associated with lesser inhibition efficiency. After protonation electron may transfer from filled orbital of metal to vacant orbitals of heteroatoms of inhibitor molecules through back donation of electron. The theoretically calculated results are in good agreement with the experimentally determined data.

3.6. Scanning Electron Microscopy (SEM)

Surface morphology of the copper in 3 M HNO₃ in absence and presence of SN, PL, and MP was investigated by SEM studies and illustrated in Fig. 12a–e. It can be seen that the surface is highly corroded in the case of copper surface exposed to 3 M HNO₃ (Fig. 12b). Fig. 12c–e indicate that on the addition of best inhibitor (SN) to the solution, the damage has been reduced and the copper surface has been relatively smooth owing to the formation of a protective film on the surface, which prevents the dissolution of copper. These confirm that the presence of adsorbed layer of SN can protect copper from corrosion effectively.

Fig. 12 SEM micrograph of copper surface (a) abraded, (b) in 3 M HNO₃, (c–e) in presence of 0.2 g L⁻¹ SN, PL and MP.

3.7. Atomic Force Microscopy (AFM)

The surface morphology of the copper specimens was further studied by Atomic Force Microscopy, which predicted the same trend as in SEM micrographs. Fig. 13a–e shows the 3D AFM micrographs of the corroded and inhibited copper samples in 3 M HNO₃ and in the same electrolyte containing 0.2 g L⁻¹ of SN. The average roughness factors calculated values are 162.3 and 108.5 nm, 130.2 nm and 115.4 nm for both blank and inhibitor SN, PL and MP are listed in Table 9. The smoothening of the surface was caused by the deposition of the inhibitor molecules on the surface which protected the surface from the attack of corrosive medium. Hence, both SEM and AFM studies of the metal surface coverage support the results obtained from other techniques.

Fig. 13 AFM micrograph of copper surface (a) abraded, (b) in 3 M HNO₃, (c–e) in presence of 0.2 g L⁻¹ SN, PL and MP.

Table 9 The area and line roughness obtained from AFM of Cu surface in 3 M HNO₃ solution absence and presence of SN, PL and MP at 308 K

AFM data	Abraded Cu	Blank	3 M HNO3
			SN	PL	MP
Area roughness (nm)	131.5	162.3	108.0	130.2	115.4
Line roughness (nm)	107.2	152.7	90.0	144.7	116.5

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4. Conclusions

SN, PL and MP extracts were found to act as good inhibitors for copper corrosion in 3 M HNO₃ solution. Potentiodynamic polarization measurements have shown that the inhibitors affect both anodic and cathodic reactions and act as mixed inhibitors which are predominantly cathodic. The inhibition efficiency increases with increasing concentrations of extracts. The value of charge transfer resistance was found to be much larger in presence of these extracts than that in their absence. The adsorption process obeys the Langmuir adsorption isotherm. The theoretical study demonstrated that the inhibition efficiency increases with increase in E_HOMO and decrease in ΔE. The results calculated from DFT study are in good agreement with the data obtained experimentally.

Acknowledgements

Authors are thankful to Prof. R. K. Mandal, Head, Department of metallurgical Engineering, I.I.T. (B.H.U.) Varanasi for providing SEM facility. Authors are also thankful to Prof. R. B. Rastogi, Department of Chemistry, I.I.T. (B.H.U.) Varanasi for providing AFM facility. They are also thankful to the Head, Chemistry Department, Faculty of Science, Banaras Hindu University, Varanasi, India, for carrying out theoretical calculations.

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