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Influence of 5-Chlorobenzotriazole on Inhibition of Copper Corrosion in Acid Rain Solution

This paper aims to examine the efficiency of 5-chlorobenzotriazole (5Cl-BTA) as a copper corrosion inhibitor in acidic rain solutions with a pH value of 2.42 by the electrochemical polarization method. 5-Chlorobenzotriazole acts similar to a mixed type inhibitor, according to the potentiodynamic polarization measurements. Results obtained in this research suggest that 5Cl-BTA is a good inhibitor; it decreases anodic and cathodic reaction rates, and the highest inhibition efficiency was 91.2%. The inhibitory effect of 5-chlorobenzotriazole is explained by the formation of the layer on the copper surface. Stability of the protective layer increased with inhibitor concentration. Scanning electron microscopy and energy-dispersive analysis of X-rays analysis confirmed that on the electrode surface, a protective layer was formed. Adsorption of 5Cl-BTA obeys the Langmuir adsorption isotherm. 5Cl-BTA showed good inhibitory characteristics even when the Cl– ions were present in examined solutions.


■ INTRODUCTION
Copper is different from most of the other metals in that it combines corrosion resistance and electrical and heat conductivity. It is a relatively noble element and that is the reason it does not corrode readily in acids, unless some oxidizing agents or oxygen is present. Copper owes its nobility to the formation of a passive oxide film on its surface or other insoluble corrosion products. 1 Several studies deal with copper and copper alloy decay under different climatic conditions, both natural and artificial. To understand the degradation processes is very important for restoration purposes. 2 To protect metals and alloy surfaces from corrosion caused by the attack of atmospheric corrosion (acid rains), suitable inhibitors can be applied.
Copper corrosion is highly dependent on the composition of the electrolyte, which is in contact with the metal surface. The process of copper corrosion includes copper dissolution at local anodic sites and the electrochemical reduction of some species, for example, oxygen at the cathodic area. 3 When Cl − ions are present in the experimental solution, it leads to creating sites that are more liable for the corrosion.
In order to protect the metal surface in aggressive environments, various, mainly organic, substances are used, and they block reactive sites on the copper surface. Based on previous research, the most effective corrosion inhibitors are the organic molecules consisting of a π-system and/or containing atoms such as nitrogen, oxygen, or sulfur in their molecule structures as well as molecules with high molecular mass. 4−7 Many investigations showed that especially good inhibitory effect on copper and its alloys had amino acids, 8,9 imidazoles, 10,11 benzotriazole and its derivatives 12 and many others. Among them, benzotriazole is particularly distinguished. Using the organic inhibitors to protect metals from corrosion is one of the most important methods in the corrosion protection. The inhibition mechanism can take place through two processes: formation of a protective thin layer via inhibitor adsorption or the formation of a precipitate of the inhibitor on the metal surface. 13 The mechanism of action of the organic corrosion inhibitors is usually not known. However, it is generally accepted that the inhibition of corrosion is achieved thanks to the interaction between corrosion inhibitor molecules and the metal surface. This leads to the formation of the inhibitive film on the metal surface. 7 Although benzotriazole has been shown to have excellent inhibitory properties in alkaline and neutral environments, in acidic environments, the efficiency has been shown to decline. 8 However, BTA still showed great results in various media and on different metals and alloys even at different temperatures. 14−16 Benzotriazole and its derivatives were investigated as inhibitors of metal corrosion and are the most commonly used N-containing compound. From the literature data it can be observed that benzotriazole is more efficient for copper and its alloys than for other metals. This was the reason to examine 5-chlorobenzotriazole in various concentrations in acid rain solution pH 2.42 and in the presence of chloride ions, based on electrochemical methods, quantum chemical calculations, and scanning electron microscopy (SEM) and energy dispersive analysis (EDS) of X-rays.

■ RESULT AND DISCUSSION
The open circuit potential (OCP) values of the copper electrode in acid rain solution and in solutions with different concentrations of the inhibitor were recorded for 30 min. Results are shown in Figure 1a,b. Obtained results showed that the curve recorded in basic solution shifts toward more positive values at the end of the immersion period. In the presence of 5-chlorobenzotriazole, the values of OCP were shifted to the positive direction at the end of measurement in comparison to the blank solution. This behavior could be due to the fact that the adsorption of 5Cl-BTA occurs on the active sites of the copper electrode, which leads to the inhibition of copper electrode corrosion. 17 According to the results, the shift of the OCP is lower than 85 mV, and it can be proposed that 5Cl-BTA acts as a mixed-type inhibitor. 18 Results shown in Figure 1b showed evident dissolution of the copper electrode in the presence of Cl − ions in basic acid rain solution. Further, in the presence of the highest concentration of the 5-chlorobenzotriazole value of OCP shifted to positive values because of the adsorption of 5Cl-BTA on the active sites of the copper electrode.
Cyclic voltammograms were recorded in order to obtain more information about the corrosion of copper in acid rain solution and in the presence of 5Cl-BTA. Voltammograms are shown in Figure 2a and the results indicate that the presence of different concentrations of the used inhibitor has the effect of reducing the current density that is particularly pronounced at higher concentrations. This phenomenon can be explained by the formation of a protective film on the copper surface.
According to the literature, the initial step of the copper corrosion is the charge transfer reaction, and this leads to the formation of an adsorbed Cu + species (Cu ads + ) 19 Therefore, from the reaction mechanism (eq 1), it can be concluded that the copper electrode can produce cuprous ions in the H 2 SO 4 solution containing air, on the basis of which it can be assumed that 5Cl-BTA molecules form a coordination compound with cuprous ion. 20 Cu ads + associate with an anion The cathodic reaction in acid solutions is the reduction of oxygen, it can be described as oxygen diffusion from the solution and adsorption on the surface of the electrode In the presence of oxygen, the cathodic reaction is enhanced because of oxygen reduction, which leads to copper corrosion rapidly and forming of a porous oxide film, which is in good electrical contact with the underlying metal 19,21,22 Anodic dissolution of copper in acid media follows the proposed mechanism Unlike the two-electron electrodissolution mechanism, copper in the presence of 5Cl-BTA electrooxidized primarily to Cu + and is able to form slightly soluble [Cu(I)−(Inh)] ads complexes as the main electrooxidation products in the presence of a clean surface. Equations 8 and 9 present the dissolution of copper in acid rain solution, but in the presence of 5Cl-BTA, this may participate in the forming of adsorbed [Cu(I)−(Inh)] ads complexes, according to the eq 10. 23 It was proposed that the growth of the [Cu(I)−(Inh)] ads film is controlled by the transport of Cu + ions from the matrix copper metal through the surface where the inhibitor molecules are adsorbed. Initial adsorption of the 5Cl-BTA molecules and the positively charged surface, followed by the formation of an insoluble polymeric complex on the electrode surface. Change in the parameters such as the exposure time, pH value, temperature, or 5Cl-BTA concentration did not affect the structure of the [Cu(I)−(Inh)] ads complex. Taking into account that when pH < 3.5 Cu 2 O is unstable than thick acicular [Cu(I)−Inh] ads , crystallites grew on the surface. The formed surface film act as a physical barrier to the aggressive ions. 24 The inhibition mechanism can be presented by the following reaction Taking into account that 5-clorobenzotriazole contains N atoms in its structure, they could be proposed as centers of adsorption on the copper electrode surface, allowing the formation of N−Cu chemical bond. 25 Literature survey showed that when the Cu(I)−BTA complex was forming, some discrepancies existed between the authors about the nature of the interaction and the possibility of π bonding. Some authors proposed a polymeric structure in a σ-bonded Cu(I)triazole model, but some other authors suggested a model involving d−p bonding between Cu(I) and the triazole ring. Recent studies showed that d−p bonding probably represents only a minor contribution to the bonding between Cu(I) and the benzotriazole anion because Cu is chemically more noble and have d-bent completely full. Kovacěvićand Kokalj 26 had reported that benzotriazole bonds weakly on the copper surface with unsaturated N atom(s) through σ-molecular orbitals. Considering other compounds that act similar to copper corrosion inhibitors and which are characterized by a chemical structure similar to benzotriazole (5-chlorobenzotriazole), it can be assumed that they also protect copper by forming complexes as well as benzotriazole. 27 Because the 5Cl-BTA is neutral in slightly acidic solution, according to the literature, it could be said that the formed complex on the Cu surface is polymeric with a linear structure, in which copper was bonded by coordination involving a lone pair of electrons from one nitrogen atom and a covalent link formed by the replacement of the H atom from the N−H group.
Moreover, substituted BTAH derivatives were found to be effective Cu corrosion inhibitors when functional groups are present on the benzene ring, but not the triazole ring such as 5Cl-BTA. 24 According to reaction 10, the protective complex was formed, but at potential values more positive than 0.25 V versus SCE, the current density increases. This is probably due to the oxidation of the formed layer on the electrode surface. 28 In the reverse scan direction, the cathodic peak at potential ∼−0.220 V versus SCE is noticed and represents the reduction of copper ions. 29 Figure 2a also showed that in the positive sweep, pitting corrosion occurs. Literature data indicate that pitting corrosion occurs when there is a sudden increase in the current density and when the reverse current density is greater in the reverse sweep than in the positive sweep in the anodic region. The continuous growth of pits on the electrode surface that occurs in the blank solution and in solutions with inhibitor, but on more positive potential values, can be the reason for this noticed current increase. 30 When chloride ions in the concentration of 0.05 mol/dm 3 NaCl (Figure 2b) were present in the acid rain solution, a sharp increase of the current density at lower values of potentials is evident. Also, when the inhibitor with concentration of 1 × 10 −3 mol/dm 3 was present in the solution, the current density had the same trend, but the inhibitory effect of benzotriazole derivate was evident. The presence of the chloride ions in the examined solutions lead to the formation of the CuCl species layer on the copper surface. 31 These species are unstable in an acidic environment. Also, CuCl species in the presence of Cu + ions are easily converted into a soluble CuCl 2 − complex, and this is evident on cyclic voltammograms as a sharp increase in current density values. The following reactions fit the described mechanism 32 The reduction that occurs in the reverse sweep can be ascribed to the reduction of the soluble CuCl 2 − complex and CuCl layer reduction on the copper surface. 34,35 The presence of the inhibitor decreases the reduction peak intensity, which indicates the inhibitory effect of 5Cl-benzotriazole.
Electrochemical corrosion parameters were obtained by potentiodynamic polarization curves, which are shown in Figure 3a,b. All potentiodynamic measurements are carried out after the OCP measurements. From Figure 3a, it could be noted that the addition of 5-chlorobenzotriazole leads to the decrease of the current density at all applied concentrations compared to the blank solution. Increase of the inhibitor concentration decreases the current density value. This leads to the conclusion that 5Cl-BTA is adsorbed on the electrode surface and hinders both cathodic and anodic reactions. The displacement of E corr is less than 85 mV, and the inhibitor can be characterized as a mixed type of inhibitor, 18 although the displacement is toward more positive values with the addition of the inhibitor.
From Figure 3a, it can be seen that in the blank solution, a small cathodic current peak appeared at approximately −0.06 V versus SCE. This peak can be ascribed to the process of reduction of cupric corrosion products formed during the waiting time at E corr and remaining at the electrode surface. 36,37 Polarization curves of the anodic branch showed that the slope of the anodic polarization curve increases sharply on more positive potential values. The gradual desorption of 5Cl-BTA molecules occurs and the anodic current density remarkably increases. Therefore, the formation of the anchored adsorption film of the used inhibitor exhibits an active blocking effect. 20,38 When Cl − ions were present in the acid rain solution (Figure  3b), the anodic curves for copper are in agreement with the Tafelian behavior and simultaneous inhibition of both the anodic and cathodic reactions occurs. In Figure 3b, the cathodic current peak, which was appeared at approximately −0.217 V versus SCE is evident and could be explained by the reduction of cupric species, which were formed during the waiting time at E corr and remained at the copper surface. 37 The electrochemical parameters of copper corrosion in acid rain solution: corrosion potential (E corr ), corrosion current density (j corr ), anodic (b a ) and cathodic (b c ) Tafel slopes, and inhibition efficiency (IE) are calculated from potentiodynamic curves and presented in Table 1. The j corr and E corr parameters were calculated from anodic and cathodic Tafel lines in the vicinity of the linearized current regions. 39 The inhibition efficiency was calculated according to the following equation where j corr and j inh are corrosion current densities for basic solution of acid rains and with the addition of 5chlorobenzotriazole. Electrochemical Impedance Spetroscopy. In order to examine in more detail the influence of the 5Cl-benzotriazole on the electrochemical behavior of copper in acid rain solution, electrochemical impedance spectroscopy (EIS) was applied. The obtained results are shown in Figure 4. According to data shown in Figure 4, EIS parameters were obtained by fitting and can be seen in Table 2. It is observed by analyzing the Nyquist diagram that the semicircle diameter increases when in blank solution was added 1 × 10 −3 mol/dm 3 5Cl-BTA, indicating a   decrease in corrosion. 40 Additionally, the appearance of Warburg impedance at a low frequency region, points to that the corrosion process is controlled by mixed charge-transfer and diffusion in solution, 40 that is, the diffusion of dissolved oxygen or some other corrosive products on to the electrode surface 41 or the diffusion of soluble copper species. 42 The IVIUM soft program was used for fitting experimental data, and the equivalent circuit is shown in Figure 5. From this equivalent circuite, R s represents solution resistance, R f is the resistance of the protective inhibitor film formed on the copper surface, and R ct is the charge transfer resistance. Q f and Q dl represent the constant phase elements, C f is film capacitance while C dl is double-layer capacitance, W stands for Warburg impedance, and n for the deviation parameter. 43 The values of C f and C dl are calculated according to the following equations According to the data showed in the Table 2, it can be seen that the introduction of an inhibitor in acid rain solution leads to decrease of C f and C dl values and R ct and R f values increase with the addition of 5Cl-BTA. C f decreases because of the adsorption of the inhibitor on the copper surface, 44 while decrease of C dl corresponds to the increase of the electrical double layer thickness and the decrease of the local dielectric constant because of the adsorption of inhibitor molecules. 45 The R f values are higher when the inhibitor is present in solution indicating the formation of the protective film and/or corrosion products on the copper surface.
It is very important to highlight that the value of n increases in the presence of 5Cl-BTA, which implys a decrease of the surface inhomogeneity as a result of the inhibitor adsorption. 23 The inhibition efficiency is calculated via equation 46 where R 0 p and R p (Ω cm 2 ) stand for the resistance (R p = R f + R ct ) of copper in acid rain solution without and with the addition of 5Cl-benzotriazole.
Quantum Chemical Calculations. In order to determine the relationship between some quantum chemical parameters gathered from the structure of the inhibitor molecule ( Figure  6) and the inhibition efficiency of corrosion obtained by electrochemical methods, the theoretical calculations were applied (Figure 7). The inhibition property of the inhibitor has been often correlated with the energy of HOMO and LUMO and the HOMO−LUMO gap. 47 E HOMO (the highest occupied molecular orbital energy), E LUMO (the lowest unoccupied molecular orbital energy), ΔE (energy gap), η (global hardness), σ (softness), μ (dipole moment), ionization potential (I), electron affinity (A), χ (electronegativity), and ΔN (function of electron transferred from the inhibitor molecule to the metal surface) were calculated. 48 All calculated parameters are given in Table 3. Calculated quantum chemical parameters were obtained by the following equations The absolute chemical hardness is given by 18) and absolute electronegativity is given by The fraction of electrons transferred between the inhibitor molecule and the metal surface can be presented as 49     (20) where χ matal and χ molecule are absolute electronegativity of copper (metal) and the inhibitor molecule, respectively; η metal and η molecule are absolute hardness of metal and the inhibitor molecule, respectively. Theoretical values of χ matal and η metal for copper are 4.48 and 0 eV/mol, respectively. 47 The value of E LUMO indicate the ability of a molecule to accept electrons, and hence, to be adsorbed on the metal surface, the values of E HOMO represent the ability of molecule to donate electrons and subsequently have better adsorption and inhibition efficiency. 50 Increasing values of E HOMO facilitate adsorption and therefore enhance the inhibition efficiency by influencing the transport process through the adsorbed layer. 47 The low values of ΔE corresponds to a higher corrosion inhibition efficiency, 51 also small value of η shows that it reacts with the surface more readily and the corrosion effect decreases, and on the other hand great μ facilitates interaction with the metal surface. Similar observations are already presented in the literature. 52,53 Parameter ΔN, also known as electron-donating ability, evaluates the tendency of a molecule to donate electrons to the metal surface, and the inhibition efficiency increases by increasing the electron-donating ability of the inhibitor to donate electrons to the metal. 48 SEM and EDS of Copper. In order to observe the morphology of the copper surface in acid rain solution pH 2.42 and in the presence of 1 × 10 −3 mol/dm 3 5-chlorobenzotriazole with the addition of 0.05 mol/dm 3 NaCl, the SEM was applied. In addition, for the determination of the elements present on the copper surface, in uninhibited solution, and in inhibited solution, EDS technique was applied. According to Figure 8, it is evident that copper in acid rain solution was degradedbecause of metal dissolution in aggressive media. However, when the inhibitor was present in the blank solution (Figure 9), the surface of copper was relatively smoother. This behavior can be explained by the adsorption of 5chlorobenzotriazole on the copper surface and inhibition of the copper corrosion. From Figure 10 it can be observed that the copper surface was strongly damaged as a result of the addition of Cl − ions in the blank solution of acid rains. Nevertheless, with the addition of the inhibitor ( Figure 11) the copper surface was much smoother, which led to a conclusion that the inhibitor was adsorbed on the metal surface.
Conducted EDS analysis of the copper surface after immersion in acid rain solution (pH 2.42) detected Cu and O peaks (Figure 8), so it could be concluded that copper corrosion products, such as Cu 2 O and CuO, were formed. 54 When the inhibitor was added to the basic solution, in concentration of 1 × 10 −3 mol/dm 3 (Figure 9), at the EDS spectrum C peak, which was derived from the organic inhibitor, was also detected beside the Cu and O peaks. Upon the addition of NaCl in acid rain solution, except Cu and O peaks, the EDS spectrum ( Figure 10) showed Cl peak as a consequence of the soluble Cl 2 − complex and CuCl layer on the copper surface. Figure 11 shows the EDS analysis for copper in acid rain solution with the presence of the inhibitor and NaCl and beside Cu, O, and Cl peaks, N peak, which was derived from 5-chlorobenzotriazole, is also detected. SEM figures and EDS spectra proved that 5-chlorobenzotriazole could be used as a copper corrosion inhibitor in acid rain solution, pH 2.42 and when Cl − ions are present in solution as well. These results are in agreement with the electrochemical measurements.
Adsorption Isotherm. The adsorption of the inhibitor on the electrode surface could be observed as the adsorption of 5chlorobenzotriazole at the copper solution interface and the substitution process between the organic compound (org sol ) from the aqueous medium and the water molecules associated with the metal surface (H 2 O ads ) where "x" is the number of water molecules replaced by the adsorption of one 5-chlorobenzotriazole molecule. 55 According to the type of the forces, adsorption can be physisorption, chemisorption, or a combination of both. 29 Literature data showed that if the values of −ΔG are 20 kJ/mol or lower physisorption occurs, but when the values are above 40 kJ/mol charge, sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of bond is involved. 56 The Langmuir adsorption isotherm was tested as an adsorption model, which can be presented in the following way  (22) where K ads is the equilibrium constant for the adsorption/ desorption process and C inh is the inhibitor concentration. 57 Gibbs free energy of adsorption was calculated according to the equation where R stands for the universal gas constant (J/Kmol) and T is the thermodynamic temperature (295 K). A linear relation (R 2 = 0.9999) between the C inh /θ and C inh ( Figure 12) points to the adsorption of 5Cl-BTA on the copper surface obeys the Langmuir adsorption isotherm. Based on data obtained from Figure 12 (K ads = 7.67 × 10 −6 ) and according to the previous equation, Gibbs free energy of adsorption was calculated −38.5 kJ/mol and leads to the conclusion that strong adsorption of inhibitor molecules on the electrode surface appeared. 58 ■ CONCLUSIONS 5-Chlorobenzotriazole acts as a good copper corrosion inhibitor in acid rain solution pH 2.42, with the highest inhibition efficiency of 91.2%. The potentiodynamic polarization measurements indicated that this organic molecule acts as a mixed type of inhibitor. The inhibitory effect and adsorption of 5Cl-BTA were confirmed by SEM and EDS. The adsorption of 5-chlorobenzotriazole obeys the Langmuir adsorption isotherm. Gibbs free energy of adsorption was calculated −38.5 kJ/mol and leads to the conclusion that adsorption of inhibitor molecules on the electrode surface was spontaneous.

■ EXPERIMENTAL SECTION
All electrochemical tests of the copper behavior were conducted in acid rain solution (AR) pH 2.42, with or without the addition of the 5Cl-benzotriazole (Aldrich) (concentration 1 × 10 −3 , 5 × 10 −4 , 1 × 10 −4 , 5 × 10 −5 mol/dm 3 ). The basic acid rains solution consisted of: 0.2 g/L Na 2 SO 4 (Alkaloid Skopje), 0.2 g/L NaHCO 3 (Hemos), 0.2 g/L NaNO 3   (Merck), and distillated water. The pH value of the basic solution was adjusted by adding H 2 SO 4 . Chloride ions in the form NaCl were added to the blank acid rain solution and solution containing 1 × 10 −3 mol/dm 3 Cl-BTA. Potentiostat (Ivium XRE, Ivium Technologies) with corresponding software was used for testing. The working electrode was made of copper with an area of 0.49 cm 2 . This electrode was prepared from a copper wire, which was cut and sealed with epoxy resin. Before each measurement, the copper electrode was polished with emery paper (Al 2 O 3 with SiO 4 ) and 0.3 μm grit alumina paste (Buehler USA). Measurements were made in a three-electrode system with the saturated calomel electrode (SCE) as reference and the platinum electrode was the auxiliary one. The following methods were used: measuring of the OCP for 30 min, linear voltammetry, cyclic voltammetry, SEM and EDS, and quantum chemical calculations. Linear voltammetry was recorded from the OCP to ±0.300 V versus SCE in both cathodic and anodic directions at a scan rate of 1 mV/s. Cyclic voltammetry was conducted in the potential range from −1.000 to −1.000 V versus SCE with a scan rate of 10 mV/s. All measurements were performed at room temperature (298 K) in naturally aerated solutions. The potential is expressed referring to a saturated calomel electrode. The SEM−EDS measurements were conducted using the Tescan VEGA 3 LM scanning electron microscope equipped with the Oxford EDS X-act Inca 350 system. Quantum chemical calculations and molecule geometry optimization were performed using ArgusLab 4.0, software. This software was already proven useful for similar investigations. The PM3-SCF method was applied. The geometry of the inhibitor in its ground state, as well as the nature of their molecular orbital, the HOMO, and the LUMO are the properties influencing the activity of inhibitors.
EIS measurements were performed at OCP. The frequency range was of 100 kHz to 10 mHz with an amplitude of 10 mV peak to peak using IVIUM soft.