Experimental and theoretical insights into copper corrosion inhibition by protonated amino-acids

Abstract

The effects of cysteine (Cys) and L-methionine (L-Met) on copper corrosion inhibition were examined in 1 M HNO₃ solution for short and long exposure times. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) were used. The EIS determined the potential for zero charges of copper (PZC) in the inhibitor solution. SEM and AFM have been used to study material surfaces. Energy-dispersive X-ray spectroscopy (EDS) was used to identify surface elemental composition. DFT and molecular dynamics simulations explored the interaction between protonated amino acids and aggressive media anions on a copper (111) surface.

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1. Introduction

Copper, with a reddish-orange color, is the fifth most common metal in the earth's crust which allows it to be used in a wide variety of industrial applications and also in everyday life, such as industrial equipment, marine industries, power station, coinages, electricity, desalination plants, fabrication of heat exchangers, tubes, cooling towers, water treatment, and electronics and many more.^1–9 This metal is preferred in all these applications because of favorable properties like high electrical and thermal conductivity,¹⁰ mechanical workability, malleability, and resistance to atmospheric and chemical agents. In addition to all these properties, copper has antimicrobial activity. Copper is a relatively noble metal. Copper dissolves in severe environments regardless of the application location. Although passive protective oxide film formation provides the appearance that copper is resistant to corrosion under some circumstances or in the atmosphere,¹¹ copper oxidizes rapidly in certain situations and generates a layer of copper oxides and/or hydroxides on its surface. The nature of this film depends on the medium's composition. Despite this self-protective film, this metal may undergo corrosion in acid medium or industrial environments containing chloride, sulfate, or nitrate ions. Nitric acid is an oxidizing acid whose oxidizing capacity depends on its concentration. Oxygen in nitric acid adds to the acceleration of copper surface corrosion.¹² The nitric acid reaction of copper gives hydrogen used to reduce nitric acid to different products.^13–16 Most metals are known to react with nitric acid and form compounds rich in hydrogen (NH₃ or NH₂OH). Likewise, noble metals like silver and copper generate oxygen-rich molecules (NO₂, NO, or HNO₂).^1,17–20

Understanding the corrosion process aims to discover techniques for limiting or preventing the metal's reactivity to its surroundings. Organic inhibitor chemicals have recently been shown to be the most efficient and feasible strategy for achieving this objective. Organic inhibitors,^21,22 comprising nitrogen, oxygen, and/or sulfur atoms and aromatic rings, are potent and commonly employed to preserve copper and its alloys.^23–28 Due to their toxicity, however, the use of some inhibitors has been banned in many nations and regions. In this context, some of our research examines the applicability of non-toxic, inexpensive, healthy, ecologically friendly, and efficient corrosion inhibitors. Amino acids are non-toxic, biodegradable, and environmentally benign molecules, relatively inexpensive, and simple to make in high purity. They are involved in many pharmacological and biological activities.^26,29,30 Such characteristics would encourage their usage as corrosion inhibitors.

Twenty amino acids include at least one carboxyl (–COOH) and one amino (–NH₂) group, while the other group of a-carbon ligands consists of –H and R of various lengths, shapes, and chemical (side chain) characteristics. They may be categorized based on (i) lateral chain structure (aliphatic, cyclic); and (ii) the polarity of the side chain (polar, non-polar). It is important to note that amino acids are widely used in a variety of fields; these molecules are the basis of all vital processes because they are essential to the integrity of metabolic processes, and they are used in pharmaceutical and cosmetic applications (intermediates for the chemical industry), as well as in animal feed and food.^31,32 Since amino acids are the fundamental components of proteins and the source of many other compounds required by the body's organs, their presence is of utmost significance and value. Because of this, there is a rising need for amino acids.^33–35

Some studies on corrosion involving amino acids have been published. Xu et al.³⁶ studied the anticorrosion performance of three derivatives of pyrazolo-pyrimidine (APP), 4-hydroxy pyrazolo-pyrimidine (HPP), and 4-mercapto pyrazolo-pyrimidine (MPP). Electrochemical, surface analysis, and theoretical techniques examined copper in a 0.5 M H₂SO₄ solution. The results indicated that these inhibitors sufficiently inhibit copper corrosion, and the inhibition efficiency increases with concentration. The sequence of the inhibition efficiencies follows MPP > HPP > APP. Zhang et al.,³⁷ tested 5-(4-methoxyphenyl)-3h-1,2-dithiole-3-thione (ATT) for copper protection against corrosion in 0.5 M H₂SO₄ solution. The authors reported that ATT shows excellent performance for copper protection. The inhibition efficiency of ATT increases with the increasing temperature of the solution. The adsorption of ATT molecules on the copper surface is compatible with the Langmuir adsorption process. Lastly, theoretical calculations show that the ATT molecule has small energy gaps, greater dipole moment values, and binding energy. SEM and AFM were also reported to be highly consistent with surface morphology analyses and electrochemical test results. In addition to these studies, various amino acids have been reported for the protection of different metals against corrosion in acidic media, such as mild steel,^9,33,38–43 copper,^44–52 or aluminum.^53–57

Compared to previous work,^58,59 the experimental and theoretical research we conducted for this study is more in-depth. The processes behind the adsorption and inhibition of the chemicals have been explored in great detail. Many electrochemical tests, in addition to surface characterizations, have been conducted. After the copper surface had been subjected to the aggressive solution, SEM and AFM techniques were used to investigate the surface structure. It was discovered that EDX might be used to ascertain the typical chemical composition and the dispersion of an inhibitor layer that had developed on a metal surface. A comprehensive theoretical and computational investigation was carried out to understand better how Cu interacts with inhibitors.⁶⁰

2. Experimental

2.1. Preparation of electrodes

The working electrode was cut from a cylindrical copper rod (98.9%) and covered with epoxy resin (excluding the bottom exposed surface area of 0.35 cm²). The electrode's surface was shortened using several grades of emery paper (evolution ranging from 600, 800, 1000, 1200, 2400) and polished with 3 μm diamond paste. After degreasing them with acetone, rinsing them with distilled water, and drying them with filter paper, they were used in the tests.

2.2. Test solution

In order to make an HNO₃ solution, a 65 percent analytical grade HNO₃ solution was first diluted with distilled water. The supply of amino acids came from Sigma-Aldrich. Fig. 1 illustrates the atomic and molecular structures of these compounds. These compounds include nitrogen, oxygen, and sulfur atoms, which act as adsorption centers. In a solution of 1 M HNO₃, the concentration of Cys and L-Met was 1 mM. A thermostat was used to maintain the temperature of the solution at 298 K throughout the experiment.

Fig. 1 Molecular structure of L-Met (a) and Cys (b).

2.3. Electrochemical measurements

Electrochemical analysis was performed utilizing a computer-controlled CHI 660D AC electrochemical analyzer. The experiments were conducted using a three-electrode electrochemical cell. A platinum sheet with a 1 × 1 cm² surface area was used as a counter electrode, and the reference electrode was a commercially purchased Ag/AgCl (3.0 M KCl). A scan rate of 1 mV s⁻¹ was used to produce potentiodynamic polarization curves throughout the potential range of −8 to +0.6 V. At an open circuit potential, EIS tests were performed at frequencies ranging from 100 kHz to 10 mHz. It had a 5 mV amplitude. Additionally, EIS experiments were performed at various anodic and cathodic overpotentials in 1 M HNO₃ with 1 mM L-Met or 1 mM Cys. The copper's excess surface charge was measured to further investigate the inhibitors' adsorption mechanism.

2.4. Characterization of a metal surface by SEM, EDX, AFM

A variety of surface characterization methods were used to analyze the metal surface. The electrodes were submerged in 1 M HNO₃ solutions containing 1 mM inhibitors for 2 hours throughout these experiments. After removing them from the cell, they were cleaned with distilled water and dried. SEM (JEOL6510) in conjunction with an EDX (model 550) and AFM were used to analyze the surface of copper (model Park SYSTEMS SPM controller).

2.5. Molecular simulation and chemical calculations

2.5.1. DFT calculations. Turbomole 7.4 commercial software was used to optimize the geometric properties of the inhibitors examined and perform quantum chemical calculations.^61,62 The def2-TZVP basis set was utilized to optimize the structures of L-Met and Cys using the B3LYP density-functional theory (DFT). Chemical descriptors such as the highest occupied molecular orbital energy (E_HOMO), the lowest occupied molecular orbital energy (E_LUMO), the energy gap (ΔE = E_LUMO − E_HOMO), the dipole moment (μ), the global hardness (γ), the electronegativity (χ), and the maximum number of electrons transferred (ΔN) were calculated and discussed.

The Mulliken atomic charges of L-Met and Cys molecules were determined to provide further information on the location of the active adsorption sites.

2.5.2. Monte Carlo simulations. Materials Studio 2017 was used with molecular mechanics and Monte Carlo simulations to elucidate the interactions between amino acids and the copper surface. In corrosion, these have evolved into highly effective mathematical methods. During the simulation, two amino acid molecules were present on the Cu (111) crystal surface. Cu (111) was chosen because it is the most stable copper surface with a low Miller index and the most prevalent.

3. Results and discussion

3.1. Potentiodynamic polarization studies

According to the following processes, HNO₃ is a powerful copper oxidant capable of quickly oxidizing copper. The dissolution of copper in nitric acid is known to proceed in two stages (reaction 1 and 2). During the dissolution of copper, forming Cu⁺ ion species is a fast step that doesn't diffuse into the bulk solution. The diffusion of soluble Cu²⁺ species controls the dissolution of copper from the outer Helmholtz plane to the bulk solution.^1,63

Anodic reaction

Cu → Cu_ads⁺ + e⁻ (1)

Cu_ads⁺ → Cu_ads²⁺ + e⁻ (2)

In all tests, the corrosion potential is much greater than the redox potential of hydrogen evolution in this medium, proving that the cathodic reaction is not an evolution of hydrogen reaction. The cathodic reaction's primary mechanism has been identified. It is feasible to examine the oxygen reduction in an acidic medium; however, NO₃⁻ ions at cathodic sites dominate the process.⁶⁴

Cathodic reactions

NO₃⁻ + 3H⁺ + 2e⁻ → HNO₂ + H₂O (3)

NO₃⁻ + 4H⁺ + 3e⁻ → NO + 2H₂O (4)

O₂ + 4H⁺ + 4e⁻ → 2H₂O (5)

According to the literature,^65–67 electrochemical methods are frequently used to assess different alloys' corrosion rate and corrosion resistance directly. One of the key techniques for calculating the corrosion current density involves the analysis of polarization curves, specifically the extrapolation of the linear section of the polarization curve on the semilogarithmic coordinates to the corrosion potential E_corr. The method of Tafel extrapolation can be used only in the presence of a well-pronounced Tafel region in even one (cathodic or anodic) branch of the polarization curve.

The corrosion current density i_corr is frequently calculated by using an equation, which is based on the Butler–Volmer equation of electrochemical kinetics:

with: , and ; n, n′: number of electrons involved in the reactions, respectively anodic and cathodic; α: electronic transfer coefficient (0< α < 1); η: overvoltage applied to the electrode η = E − E_corr (V); R: perfect gas constant (J mol⁻¹ K⁻¹); T: temperature (K); F: Faraday number (96 500 C mol⁻¹).

Here, b_a and b_c are the coefficients that characterize the corrosion process's anodic and cathodic components. This equation may be used provided that the charge transfer limits the rates of both cathodic and anodic reactions. The concentrations of reactive substances near the electrodes and in the bulk solution are equal.

The Butler–Volmer equation may analyze both pre-Tafel and Tafel sections of polarization curves. To minimize a significant change in the surface conditions of the test sample during polarization, measuring the polarization curves within a small range next to the corrosion potential (E_corr ± 10 mV) was recommended.

To achieve this objective, a polarization resistance approach⁶⁶ was devised. By extending the exponents in eqn (6) into series limited to the first terms, we can get a simple equation for computing the corrosion current in the region of the corrosion potential.

To estimate the polarization resistance and the Tafel coefficients in the same tests, it was advised to use polarization curves having both linear and relatively modest nonlinear sections. This may be achieved using computer programs and the fitting technique.

The polarization resistance (R_p), can be easily determined by a slope of the polarization curve in the linear range near the corrosion potential.

In this study, the potentiodynamic polarization curves in Fig. 2 exhibit no steep slope in the anodic range, meaning that no passive films are formed on the copper surface. Consequently, copper may directly dissolve in 1.0 M HNO₃ solutions. It was evident that the presence of amino acids in the electrolyte (Fig. 2) causes the Cu electrode's open circuit potential (E_ocp) to move to more negative values, which may be ascribed to the dominance of the cathodic reaction inhibition. L-Met and Cys additions to corrosive media alter the appearance of the anodic and cathodic branches of the curves, indicating their effect on both anodic and cathodic processes. The cathodic reaction is more inhibited than the anodic reaction; the amino acid-adsorbed layers serve as a cathodic barrier, inhibiting the transport of O₂ or NO₃⁻ to the metal's cathodic sites. In addition, this barrier layer delays the metal's decomposition. Therefore, corrosion rates decrease proportionately with the number of adsorbed inhibitor molecules that obstruct the electrochemically active sites.

Fig. 2 Potentiodynamic polarization curves of Cu after 2 h immersion in 1 M HNO₃ solution in the absence and presence of 1 mM Cys and 1 mM L-Met.

L-Met is more protective than Cys as a corrosion inhibitor for a short duration of exposure. Multiple functional groups have often resulted in variations in the electron density of a molecule, which may alter its adsorption behavior.⁶³ These compounds most likely adsorption through C=O, –OH, –NH₂, or –S– groups, regarded as active adsorption centers. The longer alkyl chain and more significant mass of L-Met provide a modest advantage on its inhibitory efficacy. This hypothesis will be confirmed in the following sections.

The corrosion potential (E_corr), corrosion current density (i_corr), and inhibition efficiency (IE%) values associated with these curves were computed and are presented in Table 1. The inhibition effectiveness IE% was determined using the following equation:

i_corr⁰ and i_corr are the uninhibited and inhibited corrosion current densities, respectively, determined by extrapolation of the linear parts of related polarization plots to their respective corrosion potentials.

Table 1 Electrochemical parameters of Cu were determined for polarization plots obtained in 1 M HNO₃ solution in the absence and containing 1 mM Cys and 1 mM L-Met after 2 h immersion

Cinb	Ecorr (mV)	icorr (μA cm^−2)	IE (%)
Blank	−11.31	710.1	—
1 mM Cys	−86.97	90.45	87.3
1 mM L-Met	−69.24	50.25	92.3

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As shown in Fig. 2 and Table 1:

(I) The E_corr was slightly shifted to greater cathodic potentials, and when the inhibitors were applied, both the anodic and cathodic current densities were decreased. Consequently, the inhibitors are classified as mixed-type corrosion inhibitors with a mostly cathodic component.^68,69

(II) The lack of Tafel regions prevented the determination of the anodic and cathodic Tafel slopes from the obtained curves. In contrast, the lines corresponding to these regions were almost parallel, demonstrating that the inhibitors prevent corrosion without interfering with the processes.

(III) The corrosion current density reduced from 710.1 to 50.25 and 90.45 μA cm⁻², respectively, with the addition of 1 mM Cys or 1 mM L-Met. Thus, amino acid adsorption inhibits anodic and cathodic processes by inhibiting active sites.

(IV) The highest IE values for Cys and L-Met were 87.3% and 92.3%, respectively, demonstrating the protective role of the two amino acids against Cu corrosion in 1 M HNO₃.

3.2. Electrochemical impedance spectroscopy measurements

To investigate the processes that occur at the copper/solution interface, the production of films, and their protective efficacy, electrochemical impedance spectroscopy (EIS) was used in 1 M HNO₃ solution without and with the addition of Cys and L-Met at a concentration of 1 mM. Fig. 3 depicts the electrode's Nyquist graphs in the absence and presence of inhibitors.

Fig. 3 Nyquist plots (a) with fitting and Bode (b and c) of Cu after 2 h immersion in 1 M HNO₃ solution in the absence and presence of 1 mM Cys and 1 mM L-Met.

Cu's Nyquist plot in the absence of inhibitors (Fig. 3) exhibits a capacitive loop at high frequencies, followed by a second one at low frequencies that appears as a straight line and is usually characterized as Warburg impedance. The initial capacitive study at high frequencies is linked to charge transfer resistance (R_ct) and double-layer capacitance (C_dl).⁶³ The second may be ascribed to the diffusion process at the surface (i.e., dissolved oxygen).⁷⁰ Cys induced the appearance of comparable Nyquist and Bode graphs. The charge transfer and diffusion resistances rise when this inhibitor is added to the corrosive solution, indicating that copper dissolution is dominated.⁷¹ Corrosion rates are decreased due to developing a protective inhibitor layer at the metal/solution contact.

After adding L-Met to the corrosive media, the appearance of Nyquist plots altered. Although the Nyquist plots do not show it well, two capacitive loops were not well isolated. However, the low-frequency straight line vanished, indicating the formation of an adhering and protective L-Met layer on the copper surface.

The appearance of Nyquist plots changed after the presence of L-Met. Although it cannot be seen clearly from the Nyquist plots, the two capacitive loops were not well separated. But, the low-frequency straight line disappeared, which shows that an adherent and protective L-Met film forms over the copper surface.

Although it is not evident from the Nyquist plots, two well-defined loops have emerged on the Bode plots (Fig. 3c and d). The low phase angle at low frequencies relates to nitrate ions attacking the metal surface. The elevated phase results from either the production of corrosion products at the metal/solution interface or the adsorption of amino acid molecules on the copper surface. This avoids forming further corrosion products at the contact between the metal and the solution through diffusion.

As seen in Fig. 3c, the amplitude (log|Z|) rises in the presence of inhibitors, reaching a maximum of 3.9 cm² for 1 mM L-Met and 3.3 cm² for 1 mM Cys, compared to 2.30 cm² in the absence of inhibitors. The improved log(Z) demonstrates the protective effect of the amino acids.

When a new chemical (corrosion inhibitor) is introduced into the metal–electrolyte interface, its electrical double layer undergoes composition and structural modifications. Consequently, the inhibitor adsorption could be monitored by measuring the double layer capacitance before and after the addition of the corrosion inhibitor.⁷² Using the EC-lab simulation program, the electrical equivalent circuit (EEC) models shown in Fig. 4a and b were utilized to match the experimental results for 1 M HNO₃ solutions without and with Cys and L-Met. The electrochemical parameters derived from the fitting results are summarized in Table 2. R_s represents the solution resistance, R_ct the charge-transfer resistance, R_f the film resistance, and CPE_f the constant phase element of the surface film. CPE_dl signifies the constant phase element of the double layer. The following relationship was used to test the inhibitory efficiency of amino acids:

Fig. 4 Electrical equivalent circuit models for Cu exposed to 1 M HNO₃ without and with 1 mM Cys (a) and 1 mM L-Met (b) during a 2 h exposure period.

Table 2 Electrochemical parameters of Cu obtained from EIS studies in 1 M HNO₃ solution without and with the addition of 1 mM L-Met and Cys for 2 h immersion

C(inh)	Rs (Ω cm^2)	Rf (Ω cm^2)	CPEf (μF cm^−2)	n1	Rct (Ω cm^2)	CPEdl (μF cm^−2)	n2	w	IE (%)
Blank	1.1	52.3	27.37 × 10^−3	0.91	211.6	2.74 × 10^−3	0.77	9132	—
1 mM Cys	3.01	936.7	4.37 × 10^−5	0.95	1116.0	4.7 × 10^−4	0.73	711.8	81.03
1 mM L-Met	2.4	406.3	1.33 × 10^−5	0.94	4075	7.85 × 10^−4	0.68	—	94.80

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R_ct0 and R_ct are the charge transfer resistance in the absence and presence of the inhibitors, respectively.

As demonstrated in Table 2, the R_ct values in the inhibited media are greater than those in the un-inhibited media. The experimental results are due to the formation of an inhibitor layer at the copper/solution contact. At 1 mM L-Met, the inhibition efficiency approaches 90,9%, resulting in enhanced electrode surface coverage.

3.3. Potential of zero charges and inhibition mechanism

In general, organic inhibitors work via adsorption on metal surfaces. Their adsorption on metal surfaces is influenced by various variables, including excess surface charge, surface structure, electrolyte composition, and the molecule's chemical structure. The extra surface charge of metals is a critical characteristic when examining the adsorption process of organic molecules. It is determined by the metal's open circuit potential (E_ocp) position about the corresponding zero charge potential (PZC). As is widely known, all types of adsorptions begin with electrostatic contact between the adsorbent and the metal surface. So, the zero charge potential is a significant variable associated with the concept of adsorption. Experimentally, measuring the charge on the surface metal is difficult.⁷³

Consequently, an indirect method, such as extrapolating EPZC from EIS measurements, is a viable alternative. The potential is shown when the surface has a zero charge comparable to the maximum polarization resistance or the lowest capacitance. At this voltage, there is no ionic double layer on the surface of the electrode. At zero charge potential, the electrodes may absorb electrolyte molecules.^73–78

For this reason, electrochemical impedance measurements analysis was performed, and a plot of polarization resistance, R_p (total resistance estimated at the metal surface contact), against applied potential was created.^79–81

Fig. 5 shows the EIS curves generated after two hours of exposure. The PZC open-circuit voltage determines the copper surface charge.

E_r = E_ocp − E_PZC (10)

Fig. 5 R_p vs. applied potential in 1 M nitrate acid solution.

E_r is Antropov's “rational” corrosion potential; E_ocp is the open circuit potential. When E_r is negative, the surface of the working electrode has a net negative charge, which promotes cation adsorption. When E_r is positive, however, anions prefer to adsorb over cation exchange.^79–81 This shows that the surface of the metal is positively charged, enabling the inhibitor molecules to be adsorbed as negatively charged nitrate ions, which bind the metal surface to the protonated amino acid molecules.

Fig. 5 is a diagram of a parabolic curve with maximum R_p values of −0.137 V for Cys and −0.09 V for L-Met (the maximum R_p value corresponds to the minimum C_dl value). Copper's E_ocp (Ag/AgCl) in 1 M HNO₃ under similar conditions is −0.73 V (Ag/AgCl). The E_r value (−0.641 V or −0.593 V) determined from eqn (3) indicates that the copper surface is negatively charged after 2 hours of exposure to the presence of amino acids. Anions (NO₃⁻) in an aqueous solution of nitric acid are attracted to the positively charged copper surface. Electrostatic interactions with NO₃⁻ produce bridges between the cationic amino acid form and the positively charged copper surface, resulting in the protonated positively charged inhibitors, Cys+ or L-Met+, adhering to the metal surface as adsorbed Cys+ or L-Met+. Because the inhibitor molecules attach to the copper surface through electrostatic interactions with positively charged Cys or L-Met molecules under acidic conditions, this outcome is possible. Multiple bonds or adsorption through heteroatoms such as N, S, or O may also be used to bind inhibitor compounds.

In addition, molecules may be adsorbed through donor–acceptor interactions involving the d electrons of copper atoms and the electrons of organic compounds. However, further research is necessary to validate this theory. The surface coating resists rusting well.

3.4. Scanning electron microscopy and energy-dispersive X-ray spectroscopy studies

The SEM picture of Cu subjected to 1 M HNO₃ for 2 hours without inhibitors is given in Fig. 6a. The micrograph of the Cu surface in 1 M HNO₃ (Fig. 6a) shows that the metal surface is severely damaged due to copper dissolution in this aggressive medium; many fractures and pits are visible on the copper surface. The interaction between the copper surface with the NO₃⁻ ions results in the formation of copper-oxide. The EDX spectrum of the outer layer of copper after 2 hours of immersion in 1 M nitric acid (Fig. 6a′), The spectrum reveals the appearance of an oxygen peak (62.8%), which corresponds to the formation of copper oxides CuO and/or the cuprites Cu₂O characterizing the corrosion products formed at the copper surface. However, adding 1 mM of L-Met and Cys to the corrosive medium individually (Fig. 7a and b) reduced its aggressiveness significantly and prevented corrosion on the copper surface, as seen by the much smoother copper surface was almost entirely covered with the inhibitor molecules. The use of inhibitors such as L-Met and Cys effectively prevents Cu oxidation.

Fig. 6 SEM images of Cu exposed to 1 M HNO₃ solution without inhibitor after exposing for 2 h (a) and the EDX spectrum of the same surface (a′).

Fig. 7 The surface SEM images of Cu in 1 M HNO₃ in the presence of 1 mM L-Met (a) and 1 mM Cyc (b) after 2 h immersion; the distribution (EDX mapping images) of S and N over the metal surface exposed to L-Met and Cys respectively.

Maximum protection was obtained when the copper sample was treated with L-Met. As indicated by the EDS spectra exhibited in the SEM pictures, it is believed that the adsorption of the inhibitor on the copper surface protects it against corrosion. EDS, an analytical method used for the elemental analysis or chemical characterization of materials, was used to explain the adsorption of amino acids. In the presence of two amino acids, the EDS spectra reveal the existence of sulfur, nitrogen peaks, and other essential constituents of inhibitors, with sulfur predominating. L-Met and Cys are composed of the elements sulfur and nitrogen.

Therefore, the protection of copper is associated with the adsorption of Sulphur atoms on metal surfaces. Both L-Met and Cys molecules include sulfur atoms. However, L-Met compounds are much more inhibitive than Cys. This phenomenon may be explained by the variation in the number of carbon atoms in the carboxylic chain. Increasing the amount of CH₂ groups in a molecule's radical “R” modifies its inhibitory efficacy. L-Met works as a soft base and rapidly gives an electron to metal due to its increased reactivity.

3.5. Atomic force microscopic analysis

AFM is a highly effective method for examining nano to microscale surface morphologies. It has emerged as a new option for reviewing the inhibitor's effect on the initiation and continuation of corrosion at the metal/solution interface.^19,44,82,83 AFM studies may be used to show the local corrosion behavior of metals in aggressive electrolytes at this level.

Fig. 8 illustrates the surface morphologies of copper submerged in a 1 M HNO₃ solution in the absence and presence of 1 mM L-Met or 1 mM Cys. As seen in Fig. 8a, the surface of copper was very rough and pitted owing to the metal's active breakdown in the absence of an inhibitor. Following the amino acid addition, surface layers develop and completely cover the metal surface (Fig. 8b and c).

Fig. 8 The 3D AFM images of Cu exposed to 1 M HNO₃ without (a) and with the addition L-Met (b) and Cys (c) for 2 h.

The average roughness R_a (the average departure of all points' roughness profiles from a mean line throughout the assessment length) and root mean square roughness, R_q, were determined using AFM image analysis (the average of the measured height deviations taken within the evolution length and measured from the mean line). Ra was 49.07 nm in an unrestricted 1 M HNO₃ solution. Adding 1 mM L-Met or 1 mM Cys to the aggressive solution reduces the R_a to 27.50 or 47.64 nm, respectively. Compared to the surface of copper exposed to a free solution, the smoother surface may be attributed to the development of protective coatings on the metal surface. On the other hand, it should be noted that developing an organic layer over the surface increases surface roughness.

4. Effect of immersion time on the inhibition performance

For practical applications, it is expected that inhibitors would exhibit corrosion protection. Long-term studies were conducted to determine the inhibitor's potential to protect against oxidative stress due to exposure duration. The copper electrodes were subjected to 1 M HNO₃ for 120 hours, followed by electrochemical and surface characterization investigations.

4.1. Potentiodynamic measurements

Copper polarization graphs after 120 hours of immersion are shown in Fig. 9.

Fig. 9 Potentiodynamic polarization curves of Cu after exposing to 1 M HNO₃ solution in the absence and presence of 1 mM Cys or 1 mM L-Met for 120 h immersion.

The current density rises substantially in the absence of the inhibitor when the exposure period is short (2 h), owing to the increased surface area created as Cu dissolves. However, the presence of both inhibitors significantly decreases the rate of Cu dissolution in the corrosive media. As seen in Table 3, the current densities are substantially reduced. In the case of inhibitors, the current densities are lower after 120 hours than during the shorter exposure period. Another critical aspect is that Cys grew more protective with prolonged exposure. The excellent inhibitory effect of the copper surface may be attributed to the development of a stable and protective layer. Therefore, the amino acids could act as a conventional barrier and prevent the penetration of corrosive media (H₂O, O₂, NO₃⁻).⁸⁴

Table 3 Electrochemical parameters of Cu determined from polarization plots performed in 1 M HNO₃ solution in the absence and presence of 1 mM Cys and 1 mM L-Met after 120 h exposure

Cinb	Ecorr (mV)	icorr (μA cm^−2)	IE (%)
1 M HNO3	−24.481	1654	—
1 mM Cys	−127.454	145.81	91,8
1 mM L-Met	−91.606	8.63	99.2

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4.2. Electrochemical impedance measurements

Fig. 10 illustrates the Nyquist plots of Cu after 120 hours of immersion in 1 M HNO₃ solution in the absence and presence of amino acids. As shown in Fig. 10, a main capacitive loop is found at high frequencies, while another appears in the low-frequency range. The first loop was ascribed to the charge transfer resistance between the metal and the OHP (outer Helmholtz plane) and the double-layer capacitance. At low frequencies, the second loop corresponds to the development of films on the metal surface and all other types collected at the metal/solution contact (inhibitor molecules, corrosion products, etc.). The sum of all resistance is polarization resistance (R_p).

Fig. 10 Nyquist plots of the Cu obtained in 1 M HNO₃ solution in the absence and containing 1 mMCys or 1 mM L-Met after 120 h.

After 120 hours of exposure in the absence of inhibitors, the polarization resistance and width of Nyquist plots of Cu were substantially decreased compared to 2 hours (see Fig. 2). On the other hand, resistances are considerably enhanced in the case of inhibitors, indicating that the surface inhibitor film is growing continuously and is approaching complete covering of the metal surface with the film.

Table 4 contains some electrochemical data deduced from Nyquist plots. Data acquired after a short exposure time (2 h, Table 1) are compared to those obtained after a longer exposure time (120 h, Table 4).

Table 4 Electrochemical parameters of Cu determined from EIS studies performed in 1 M HNO₃ solution in the absence and presence of 1 mM Cys or 1 mM L-Met after 120 h

C(inh)	Rs (Ω cm^2)	Rf (Ω cm^2)	CPEf (F cm^−2)	Rct (Ω cm^2)	CPEdl (F cm^−2)	E (%)
1 M HNO3	4.11	87.37	0.23 × 10^−3	120	1.61 × 10^−3	—
1 mM Cys	1.77	278.7	45 × 10^−5	1051	2.03 × 10^−6	95.40
1 mM L-Met	2.41	1378	0.22 × 10^−6	16 155	0.102.3 × 10^−3	97.34

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The values of R_f and R_ct rise with a longer immersion duration (120 h) when compared to data obtained after a short exposure time (2 h). This finding is most likely owing to an increase in the inhibitors' surface coverage and the thickness of the protective layer, which results in greater inhibitory efficiency and enhanced protection capacity. After prolonged immersion, Cys offer superior protection. The EIS results and the polarization measurements match well.

4.3. Surface morphologies after long immersion

Fig. 11 shows the surface SEM images of copper after 120 hours of immersion in 1 M HNO₃ solution without and with 1 mM Cys and 1 mM L-Met. Without inhibitors (Fig. 11a), the surface of copper was severely degraded; the number and depth of pits grew as copper breakdown was accelerated in the aggressive media. After the inhibitors were added, the metal surface was almost completely coated by inhibitor molecules, and protective inhibitor films were developed (Fig. 11b and c). The surface inhibitor coatings are compact and cling to the surface, providing high protection against corrosion. This shows that an effective protective layer of the tested inhibitor was adsorbed on the metal surface and that it effectively prevented surface steel corrosion.⁸⁵

Fig. 11 The SEM images of Cu after exposure to 1 M HNO₃ solution in the absence (a) and presence of 1 mM Cys (b) or 1 mM L-Met (c) after 120 h.

AFM studies were also used to inspect the surfaces of the same samples. Fig. 12 illustrates the electrode's 3D AFM pictures after exposure to Cu samples in test solutions. The data clearly show that copper develops high-quality amino acid films on its surface when exposed to an inhibitor-containing solution. These films provide a degree of protection that is unmatched in the industry.

Fig. 12 3D AFM images of Cu after exposing for 120 h to 1 M nitrate acid solution (a) without amino acids (b) Cys (1 M) (c) L-Met (1 M).

5. Theoretical chemical calculation

The interaction between the studied amino acids and the copper surface was investigated and carried out in a simulation box (25.55 × 22.55 × 34.86 ų) with periodic boundary conditions. COMPASS (Condensed Phase Optimized Molecular Potentials for Atomistic Simulation Studies) force field was used to optimize the structures of the components of the system.⁸⁶ The quantum chemical parameters and geometric optimization results are given in Table 5.

Table 5 Quantum chemical parameters

		EHOMO (eV)	ELUMO (eV)	GAP (eV)	χ	η	ω	ΔN
Cys	Neutral	−6.729	−0.324	6.406	3.527	3.203	1.942	0.202
	Protonated	−7.214	−0.912	6.302	4.063	3.151	2.619	0.120
L-Met	Neutral	−6.101	−0.068	6.033	3.084	3.016	1.577	0.288
	Protonated	−6.261	−0.797	5.464	3.529	2.732	2.280	0.236

---

E_HOMO, the highest occupied molecular orbital, is often linked to a molecule's ability to donate electrons to other molecules. The high value of E_HOMO indicates that inhibitor molecules seem to donate electrons to relevant low-energy receptor molecules with empty molecular orbitals. In contrast, the energy value of the lowest unoccupied molecular orbitals, E_LUMO, is directly linked to the molecule's capacity to receive electrons. Reactivity of an inhibitor is heavily influenced by the HOMO and LUMO electron distributions, EHOMO and ELUMO, and the energy gap (ΔE).⁸⁷

HOMO and LUMO orbitals of the neutral and protonated forms of both amino acids are illustrated in Fig. 13. The low values of E_LUMO make it easier to accommodate additional negative charges from the surface of the metals. The value of gap (ΔE) is the difference between the E_HOMO and E_LUMO values. Therefore, the smaller value of ΔE would correspond to the inhibitor's higher reactivity and inhibition efficiency.

Fig. 13 The optimized geometry, HOMO, LUMO of the neutral and protonated structures.

As shown in Table 5, the trend of E_HOMO is L-Met > Cys (for the neutral and protonated forms). The higher E_HOMO value of L-Met (neutral: −6.101 eV; protonated: −6.261 eV) indicates that the inhibition effect of this molecule is better, and the molecules are more likely to donate electrons to those molecules whose empty molecular orbital energy is lower. Furthermore, Cys E_LUMO (neutral: −0.324 eV; protonated: −0.912 eV) < L-Met E_LUMO (neutral: −0.068 eV; protonated: −0.797 eV) indicates that the Cys molecules tend to form a back-donating bond. Therefore, it will be easier for the inhibitor to accept electrons from the orbital “d”.^88–90

The gap energy ΔE has a strong influence on the adsorption reactivity of the inhibitor molecules on the metal surface. When the ΔE value decreases, the molecule's reactivity increases, and the inhibition effect increases. As seen in Table 5, the ΔE value of the L-Met (neutral: 6.033 eV; protonated: 6.085 eV) is much lower than the Cys (neutral: 6.406 eV; protonated: 6.302 eV). This corresponds to a higher reactivity and inhibition efficiency of L-Met molecules, which agrees with the experimental results. According to Sanderson's principle of equalizing electronegativity, absolute electronegativity (χ) is the chemical characteristic that describes a molecule's ability to attract electrons. According to Lukovits,⁹¹ the effectiveness of inhibition is increased by enhancing the inhibitor's capacity to donate electrons to the metal surface. This implies that a higher ΔN value indicates a more significant inhibitory effect. Thus, positive values of ΔN represent electron transport from molecules to metal surfaces, while negative values represent electron transfer from metal surfaces to molecules.^{23,74,88,92–94}

The results indicate that the ΔN value correlates substantially with experimental inhibition efficiencies (Table 5). Thus, the highest proportion of electron transfer (neutral: 0.288 eV; protonated: 0.236 eV) is associated with the most potent inhibitor, L-Met, which is experimentally confirmed.

Mulliken analysis has been extensively used to estimate inhibitor adsorption sites.⁹⁵ The Mulliken charge is proportional to the vibrational characteristics of the molecule and measures how the electronic structure is charged. It is mainly used to determine the charge distribution inside the molecule's skeleton, which provides information about the reactive sites. Thus, the more negatively charged the heteroatom, the higher the capacity for donor–acceptor absorption on the metal surface.

Fig. 14 illustrates the Mulliken charges for the two chemicals examined. These findings show all heteroatoms (nitrogen, oxygen, and sulfur) have negative charges and a high electron density. When these atoms contact the copper surface, they function as nucleophilic centers.

Fig. 14 Mulliken charges: (a) Cys and (b) L-Met.

The COSMO-RS research is primarily concerned with the chemical molecular and comparative atomic reactivity sites. Various colors correspond to different values of the charge distribution at the surface (Fig. 15). Red denotes areas with the highest concentration of negative charges (high electron density), blue denotes regions with the most increased positive charges (low electron density), and green indicates areas with no charges. Charge levels rise in the following order: red, orange, yellow, green, blue, and purple (purple is only present for the cationic part of the protonated molecule).

Fig. 15 Charge density distribution (COSMO surface) for both amino acids.

As shown in Fig. 15, the electron-rich areas (red to orange color) are mainly concentrated around heteroatoms (N, O, and S) and conjugated double bonds, which favors an electrophilic attack. Hydrogen atoms are positively charged (blue) and serve as a target for nucleophilic assaults.

6. Monte Carlo

Monte Carlo simulations investigated the adsorption properties of the two amino acids examined (L-Met and Cys) on a copper surface (111). Cu(111)/Cys and Cu(111)/L-Met in water as solvent and HNO₃ (acidic medium as H⁺ and NO₃⁻) are determined via system optimization (Fig. 16). The collected data are shown in Table 6.

Fig. 16 Equilibrium adsorption configurations of studied inhibitors on copper (111) surface obtained by molecular dynamics simulations Cys has relatively higher inhibition energy than L-Met.

Table 6 Monte Carlo simulation of Cys and L-Met adsorption on Cu (111) surface in acidic medium (energies are given in kcal mol⁻¹)

Amino acid	Form	Total energy	Adsorption energy	Rigid adsorption energy	Deformation energy	dEad/dNi
						Inhibitor	H2O	H3O^+	Cl^−
L-Met	Neutral	−53.24	−1780.79	−60.57	−1720.21	−82.58	−16.51
	Protonated	−60.29	−2090.96	−69.78	−2021.17	−22.58	−16.52	−36.10	−0.53
Cys	Neutral	−48.79	−1771.79	−54.01	−1717.78	−75.95	−16.51
	Protonated	−66.64	−2098.37	−74.98	−2023.39	−24.00	−16.51	−36.47	−0.31

---

Theoretically, the chemical inhibitor with the lowest adsorption energy value should have the highest inhibitory energy. The adsorption energies of two inhibitors in water are negative, as illustrated in Table 6. The significant negative result suggests inhibitor adsorption on Cu (111) happens spontaneously, becomes more stable, and is stronger.^96–98

These results are attributable to an increase in active adsorption sites in the molecular structure of amino acids.^99–101 Stiff adsorption energy is needed when the unrelaxed molecules (L-Met and Cys) are adsorbed on the copper (111) surface before the geometry optimization process. When the molecules of the adsorbed component are relaxed on the copper surface, the deformation energy is released.

As shown in Fig. 16, the studied inhibitor molecules were parallel adsorbed on the copper surface, promoting the interaction of the amino acid molecules' π-electrons (N atoms) with the copper surface.^94–96

7. Mechanism of action of Cys and L-Met on the metal surface Cu

Experiments and theoretical investigations provide a comprehensive understanding of the corrosion inhibition mechanism and, as a result, the nature of inhibitor–Cu interactions.¹⁰²

The presence of nitric acid in the medium allows the protonation of the nitrogen of the amino acids to form the cationic derivatives. However, the variation in inhibition effectiveness between Cys and L-Met comes from the nature of the R substituents. Therefore, the nucleophilicity of sulfur is responsible for the inhibition. In general, the efficiency of an organic substance as an inhibitor for metallic corrosion depends on its molecule structure, the characteristics of the environment, and the mode of interaction with the metal surface. The presence of the long chain in L-Met decreases its polarity. This allows blocking the action of the corrosive solution. The molecule is absorbed on the metal surface by electron donation to the vacant d-orbital. Cys is more polar than L-Met, which explains the excellent inhibition of L-Met. In addition, the donor effect of the methyl group increases the electronic density of the sulfur group (Fig. 17).

Fig. 17 Schematic representation of the interaction between Cys and L-Met with copper surface in the presence of HNO₃.

Following the theoretical study performed on protonated Cys and L-Met, it was determined that the inhibition effect of the L-Met compound is more effective than the Cys. In the present investigations, the energy gap of L-Met is 5464 eV. A small energy gap of HOMO–LUMO means more chemically active, low kinetic stability, and shows that the compound can be easily excited, favoring the compound inhibition.

This table shows that all the Mulliken charges carried by the heteroatoms of L-Met are greater than the charges of Cys (Table 7).

Table 7 Mulliken Charges

L-Met		Cys
S–CH3	0.089	SH	0.026
C=O	−0.391	C=O	−0.382
C–O	−0.507	C–O	−0.503
NH	−0.528	NH	−0.521

---

8. Conclusions

• The obtained results showed that Cys and L-Met perform good inhibition efficiency for corrosion of Cu in 1 M HNO₃ and affect by acting on both anodic and cathodic mechanisms.

• The inhibition efficiency of the amino acids increases with the increasing exposure time due to thickening or better surface coverage of the surface films.

• The variation in inhibition effectiveness between Cys and L-Met comes from the nature of the R substituents. Therefore, the nucleophilicity of sulfur is responsible for the inhibition.

• The presence of the long-chain in L-Met decreases its polarity. This allows blocking the action of the corrosive solution. The molecule is absorbed on the metal surface by electron donation to the vacant d-orbital of copper. Cys is more polar than L-Met, which explains the excellent inhibition of L-Met. In addition, the donor effect of the methyl group increases the electronic density of the sulfur group.

• The surface analysis by SEM and AFM studies showed that the corrosion of copper in nitric acid occurs mainly through pitting. Adding inhibitors to the aggressive solution forms a protective film on a copper surface.

• As experimental and theoretical studies have shown, chemical interactions may occur through reactive areas inside molecules. The inhibitor molecules adsorb on the copper surface, protecting the hostile medium. Quantum chemical analysis corroborated the experimental results and established that the inhibitors under study have a strong tendency for adsorption onto the metallic surface.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are immensely thankful to Bingöl University Central Laboratory for characterization measurements and Chemistry Department for electrochemical measurements. The authors are grateful for the support of the researchers supporting project number (RSP-2021/113), King Saud University, Riyadh, Saudi Arabia.

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