Copper Corrosion Inhibition using BTAH Inhibitor in Sodium Chloride Medium : Experimental and Theoretical Studies

The effect of 1H-benzotriazole (BTAH) with ppm (part per million) grade concentrations on copper corrosion in aerated 0.5 M NaCl solution is studied using chemical method (weight loss) and electrochemical methods (Potentiodynamic Polarization and Electrochemical Impedance Spectroscopy (EIS)). The present study confirm that the BTAH acts as a mixedtype inhibitor of copper corrosion in 0.5 M NaCl. The optimum inhibition efficiency is at 30 ppm of BTAH. The surface characterization performed using Scanning Electron Microscopy (SEM) to confirm the adsorption of the inhibitor molecules after 21 days of immersion time in aerated 0.5 M NaCl. The results obtained from different techniques used in this research are in very good agreement and revealed that the BTAH is a very good inhibitor of copper corrosion in sodium chloride medium. Computer Simulation techniques confirm that the BTAH molecules adsorbed on the Cu (111) Surface.


INTRODUCTION
From a wide range of metals used in industries, copper extensively used owing to its remarkable thermal and electric properties.It is usually employed in heating and cooling systems because of its excellent thermal conductivity [1][2][3][4][5][6][7].Copper also exclusively used for piping and delivery of water in marine industry.These pipes used in a medium rich of Cl- [8].It is known that the corrosion products caused by chloride ions Cl-leads to a reduction in the efficiency of copper, which causes huge economic loss [9,10].
To overcome this problem, an electrochemical monitoring was done by studying the behaviour of copper in 0.5 M NaCl in presence and absence of the inhibitor BTAH.
In this context, several studies realized, BTAH known for six decades as a corrosion inhibitor for copper.1.Cotton and al. [16] are the first contributors in the field of BTAH as copper corrosion inhibitor.They demonstrated that the pre-treatment of the copper surface by BTAH induce a long lasting prevention of staining, they elucidate the BTAH inhibitory action in the terms of physical barrier.Wall and Davies [17] showed that, in the presence of BTAH dissolution of copper and pick up of copper ions from the solution reduced in closed circuit systems containing copper.They claimed that BTAH forms an insoluble and invisible chelate on the copper surface.That is responsible for reducing corrosion attack.
Poling [18] confirmed the linear polymeric Cu (I) BTA structure proposed by Cotton [16] and stated more decisively that the structure contains Cu(I) ions, the formation of Cu (I) BTA was not limited to a monolayer, but could grow further to from films up to several thousand Å thick.

Chemicals and preparation of the simples
BTAH (self-prepared 97%), NaCl electrolyte prepared with deionized water.A threeelectrode electrochemical cell was used which contain counter electrode of Platinum (Pt. 1 cm ²) and saturated calomel electrode (SCE) as reference electrode.The working electrodes made using pure copper 99.99 % cylinder.
The samples were mechanically cut into cylinders (D1= 1.1 cm & D2= 0.8 cm) x 1cm dimensions.The samples used for the electrochemical study were welded with electric cables for easier use, then coated with epoxy resin and finally polished with abrasive papers (1200,1500,2000 and 2500) followed by a finishing polishing (Felt) with Diamond Polishing Paste (0.1 µm).The samples used for weight loss experiment polished with same way.All samples has cleaned successively with acetone, distilled water and deionized water.

Electrochemical measurements
The potentiodynamic polarization and EIS measurements had performed using an Auto lab (PGZ-402) electrochemical workstation and an electrochemical cell (100 ml) with three electrodes; the solution was not stirred or deaerated.Before the potentiodynamic polarization measurements, an open circuit measurement for 30 min performed to stabilize the potential.The potential was scanned from -400 to 400 mV at a scan rate of 1mV.min -1 .The EIS measurements were performed at open circuit potential for 30 min, in a frequency range from 100 KHz to 100 MHz.

Weight loss and SEM analyses
Samples used for weight loss measurement were prepared by the same method mentioned previously (Paragraph 2.1.).In a cylindrical shape (d = 0.8 cm & h = 0.3 cm) with an exposed total area (A=1.76 cm²).After polishing and weighing (m1), the samples introduced in 100 ml of 0.5 M NaCl solution with and without inhibitor used for (2-21) days.Subsequently, the tested samples were removed, cleaned and weighed (m2).
In order to see if the BTAH molecules effectively adsorbed on the copper surface executed the SEM analysis, SEM is widely used to detect the morphological features of metal surface.The SEM micrograph obtained for copper samples used in weight loss part.The surface morphology of these copper samples investigated by using SEM analysis (VEGA 3, TESCAN) at 5, 10 and 20.0 KV.

Theoretical study
Molecular simulation studies carried out using Materials Studio 7 software from accelrys Inc. to find the correlation between theoretically calculated properties and experimentally determined inhibition efficiency for copper corrosion in 0.5 M NaCl solution by BTAH organic inhibitor.
The DFT+ semi-empirical tight binding method was used for building and to optimize BTAH molecule, determine the electronic properties of BTAH, effect of the frontier molecular orbital energies The energy of the highest occupied molecular orbital (EHOMO), the energy of the unoccupied molecular orbital (ELUMO), electronic charges on reactive centres, dipole moment and the energy of the gap, Equation (1).
Interaction between BTAH molecules and Cu (111) surface carried out in a simulating box (14.45 Å×10.22Å × 29.99 Å) with periodic boundary conditions.The Cu (111) surface firstly built and relaxed by minimizing its energy using molecule mechanics then the surface of Cu (111) increased by constructing a supercell, a vacuum slab of 30 Å thickness built on the Cu (111) surface.The number of layers in the structure chosen so that the depth of surface is greater than the non-bond cutoff used in the calculation; we choose six as a number of layers which sufficient depth that the inhibitor molecules will only be involved in non-bond interactions with Cu (111) surface.After minimizing Cu (111) surface and BTAH molecules, the corrosion system will be built by layer builder to place the inhibitor molecule on Cu (111) surface using a forcefield COMPASS (Condensed phase Optimized Molecular Potentials for Atomistic Simulation Studies).The adsorption locator module in Materials Studio 7 software from accelrys Inc. [19] allows selecting thermodynamic ensemble and associated parameters, temperature and pressuring and initiating a dynamic calculation.The dynamic simulations procedures have been described elsewhere [20].

Potentiodynamic Polarization Results
Figure 1 represent the behavior of pure copper electrodes in aerated 0.5 M NaCl solution at room temperature, in the absence and in the presence of different concentrations from 0.5 ×10 -4 M to 3.5 ×10 -4 M grade of BTAH after an immersion time of 30 min.as an open circuit potential measurement.
The cathodic corrosion reaction of copper in NaCl solution is the reaction of oxygen usually, the dissolution of copper (anodic corrosion reactions) is: moreover, Cu + ions can undergo disproportionation according to Equation (4) [21-24] When we use an aerated corrosive aqueous medium in near neutral pH, which contained complexing agents such as Cl -, we have to consider the formation of copper complex such as    2 − as indicated by following anodic reactions: Compared with the solution without inhibitor the corrosion potential (Ecorr) shifted to the more positive values and both the anodic and cathodic currents (icorr) decreased.This indicates that the BTAH inhibitor acts as a mixed-type corrosion inhibitor.The cathodic and the anodic currents progressively diminish with an increment in BTAH concentration that is clearer in anodic current.The electrochemical parameters shown in Table 1 extracted from polarization curves shown in Figure 1 obtained after an electrochemical follow of the behavior of pure copper in 0.5 M NaCl medium in absence and presence of different concentrations of BTAH at room temperature.The results obtained using Tafel extrapolation method.Figure 1 shows clearly that the cathodic polarisation curves does not display an extensive Tafel region which confirm a limiting diffusion current return to the reduction of dissolved oxygen, the Tafel extrapolation method was used for both anodic and cathodic Tafel region using Voltamaster 4.0 program.The kinetics of electron transfer at the metal-solution interface can be shown using Butler-Volmer equation [25].The Butler -Volmer equation given by Equation (7).
Where icorr is the corrosion current density at the corrosion potential Ecorr, α is the transfer coefficient (α = 0.5), and n the number of electrons transferred.When the rate of the back reaction is negligible, Equation (7) gives: where a and b constants.In Equation ( 8), when E=Ecorr and when i=icorr this is the basis of Tafel exploitation.The inhibition efficiency (ηi (%)) shown in Table 1 was calculated from values of (icorr) using the following equation: Where   0 and   are the corrosion current densities for Cu electrode in aerated 0.5 M NaCl in absence and presence of different concentrations of BTAH.The inhibition effect of BTAH on the polarization behaviors of copper in chloride medium, it known that the corrosion current density of copper usually calculated by the use of the Stern-Geary equation as: where Rp is the polarization resistance and B in a constant, which varies with the expression with the expression that: where   ,   are the anodic and the cathodic Tafel slopes obtained from anodic and cathodic polarization curves of copper [26].
Generally, the calculation of the determination of Rp and rarely gives attention on the value of B [27].The value of B commonly considered being between 10 and 30 mV for almost metals.Table 2 shows the calculated parameters obtained using Stern-Geary method.
It can be conclude that the corrosion current density decreased and the inhibition efficiency increased with the increase of BTAH concentration, The BTAH adsorbed on copper surface acted as a barrier layer to block corrosion process.The addition of 0.5 ×10 -4 M of BTAH to the electrolyte can reduces to great importance of inhibition efficiency.The optimal inhibition efficiency of 99.45% was obtains for 2.5 ×10 -4 M of BTAH.The parameters calculated using linear polarization method or Stern-Geary method confirmed the results from Tafel method.

EIS results
In this experimental part of electrochemical measurements the Electrochemical Impedance Spectroscopy (EIS) used to confirm results of the potentiodynamic polarization step and to get further information of the inhibition process with the same concept with potentiodynamic measurement, The EIS is an excellent tool to investigate the corrosion and the adsorption phenomena.[25] Several experiences done using copper electrode in different electrolytes in the absence and in the presence of four different concentrations of the inhibitor  In the presence of BTAH the impedance spectra for the Nyquist plots Figure 2 shows a depressed semicircle in the high frequency region.This high frequency semicircle attributed to the charge transfer and double layer capacitance [28].
The lowest frequency area generally known as Warburg impedance related to the diffusion of soluble copper species from electrode surface to bulk solution [28].The diameter of semicircles in extent with the increasing of the inhibitor concentrations.The Bode plots Figure 3 show that the impedance values over the whole frequency range increased with increasing the BTAH concentration.It can be obtains from Bode phase plots Figure 4 that the corrosion process-taking place at the electrode surface has one relaxation time constant related to the relaxation of the electrical double layer capacitor.We can also observe that the increasing of BTAH concentrations results an increase in the maximum phase angle, which confirm the inhibiting action of BTAH on copper in the study medium.
The equivalent circuit model used to construe impedance characteristics is shown in this circuit was reported in several studies for copper/solution interface [28,30].The parameters obtained by fitting the equivalent circuit and the inhibition efficiency represented in Table 3.
Here Rs represented the solution resistance. represents the constant phase element (CPE), Rt represent the charge transfer resistances and W is the Warburg impedance.
The impedance of CPE represented by the following equation: where  0 is the modulus, j is the imaginary root,  is the angular frequency and n is the phase.
In the practical electrode system, the impedance spectra are offer depressed semicircles with their centres below the real axis.This phenomenon known as the dispersing effect [31].The inhibition efficiency (ηi) is calculated using charge transfer resistance as follow:

Weight loss and SEM analyses results
In this part the variation of the weight loss of copper at different immersion times in aerated 0.5 M NaCl solution, at room temperature 25°C for (2, 4, 7, 10, 14 and 21) days, without inhibitor and with 2.5 10 -4 M of BTAH results shown in Figure 6.The concentration of inhibitor used in this part has chosen as the optimal concentration confirmed in the electrochemical study part.
The loss of weight mentioned (∆m: mg.cm -²).The corrosion rate (Rcorr: mg.cm -1 .day - ) and the inhibition efficiency (ηw %) were calculated as follow [32,33] : Here,  is the total area exposed to the solution,  is the time of immersion,    is the corrosion rate without inhibitor and    is the corrosion rate with inhibitor.
We found that the inhibition efficiency of BTAH on copper immersed in aerated solution of 0.5 M NaCl, varied from 50% after two days of immersion time to 84.84 % after 21 days immersion time Figure 7, it's clear that the BTAH has a very good effect against copper corrosion in the study solution, also it stays effective after 21 days of immersion.Without forget the low concentration of inhibitor used in this part.The SEM micrograph for the copper samples immersed in aerated 0.5 M NaCl in absence and presence of BTAH with concentration equals to 2.5 ×10 -4 M for 21 days shown in Figure 8 and 9 .It is obvious that the BTAH molecules partially distributed on the copper surface.The surface coverages obtained from: where ,  ℎ are weight loss obtained from previous measurements.The corrosion rates obtained from equation (15).
The inhibition efficiency, coverages and corrosion rates tabulate in Table 4.
The Figure 8 represents the copper sample before immersion, the Figure 9(a) shows the surface morphology of the copper sample immersed in solution without inhibitor, it is clear that the surface strongly corroded by the Sodium Chloride solution.The Figure 9(b) shows the morphology of the copper sample, immersed in the presence of 2.5 ×10 -4 M of BTAH.Protection layers formed on the copper surface, which indicate that the BTAH adsorbed on the copper surface.
In addition, we conclude that the BTAH has a good inhibiting effect on copper corrosion which confirmed in weight loss part, at 2.5 ×10 -4 M of BTAH an after 21 days the inhibition efficiency attain 84.84 %.

DFT simulation results
The present part focus on the geometry optimization step of the BTAH molecule using DFT+ module, this optimization step aim to calculate the Mullikan charge distributions of BTAH as well as HOMO and LUMO were calculated and represented in Figure 10.We find that the HOMO is located on the Benzene ring, which indicate that the preferred active sites for an electronic attack and the favourite sites for interactions with the metal surface are located within the region around the Nitrogen (azole function) atoms belonging to the benzene ring [34].
According to DFT-Koopmans' theorem [35], the ionization potential I written as follow: Then the negative of the energy of the LUMO represent the electron affinity A Equation ( 17): Other quantum chemical parameters has been correlated recently using DFT modules [36], these calculated parameters such as dipole moment µ which given as follow: Where  represents the charge and  is the distance.
The value of the electronegativity  and the chemical potential [37] given by Equation ( 19): Other parameters [38,39] calculated such as the global hardness  and the global softness where the global hardness given by Equation ( 20): The global softness S or the absolute hardness defined by the inverse of the global hardness where: The propensity of chemical species to accept electrons defined as the global electrophilicity  it given by Par et Al.[40] as follow: The Equation ( 22) becomes as follow: Finally and according to Person [41] the fraction of electrons transferred from the inhibitor molecule to the metallic surface gives by:  =   −   / (  +   ) (26) where   and  ℎ denote the absolute electronegativity of metal and inhibitor molecule,   and  ℎ are the absolute hardness of the metal and the inhibitor.
Obtained results for BTAH molecule and interaction of BTAH molecule with copper surface calculated with DFT+ module shows in Table 5.According to Lukovits [42], if ΔN < 3.6 (our case Δ = 0.479) the inhibition efficiency of organic inhibitor increase with increasing electron donating ability at the metal surface.We concluded the BTAH could adsorbed on the copper surface by donating the unshared pair of electrons from the N atoms to the vacant d orbitals of copper.The BTAH molecule placed on the surface of copper, optimized then quench molecular dynamics run. Figure 11 shows the optimization energy step for BTAH molecule, before putting it on the Cu (111) surface.
Total energy, average energy, Van der Waals energy, electrostatic energy and intermolecular energy in interaction of BTH/Cu (111) surface figured in Figure 12.The adsorption locator process tries to get to the lowest energy for the system in comprising BTAH/Cu (111).
The possibility of BTAH adsorption on Cu (111) surface simulated in Figure 13(a).We can see that BTAH molecule moves near to the copper surface, indicating that the BTAH adsorbed at copper surface [43].The parameters tabulated in Table 6 include total energy of the BTAH-Cu (111) configuration.The total energy is defined is the sum of the energies of the adsorbate components, the rigid adsorption energy and the deformation energy.In the present study, the energy of the substrate (Cu (111) surface) taken as zero.Then adsorption energy reports energy required when the relaxed adsorbate BTAH adsorbed on the substrate surface Cu (111).The adsorption energy defined as the sum of rigid adsorption energy and the deformation energy for BTAH molecule.The rigid adsorption energy released when the unrelaxed BTAH molecule (before geometry optimization step) adsorbed on Cu (111) surface.The deformation energy required when the BTAH molecule is relaxed on the Cu (111) surface.The report (dEads/ dNi) of BTAH-Cu (111) configurations where one of the BTAH molecule removed is also shows in Table 6.

CONCLUSIONS
The BTAH known as a very good inhibitor for copper corrosion in aerated 0.5 M NaCl solution.The inhibition mechanism is attributable to the adsorption of the inhibitor on the copper surface and blocking its active sites.All results obtained from electrochemical measurements and chemical measurement are reasonably in good accord.
To go so far and follow the stability of the inhibition efficiency, BTAH stays stable and it has a very good inhibition efficiency 84.84 % after 21 days of immersion time in aerated 0.5 M NaCl solution.The molecular modelling as well as quantum chemical simulation precisely  the calculation of the both energies EHOMO and ELUMO indicate that the preferred active sites for an electronic attack and the favourite sites for interaction with the copper surface are located within the region around the Nitrogen atoms, which confirm that the BTAH molecule adsorb on the Cu (111) surface.

Figure 1 .
Figure 1.Polarisation curves of copper electrode in the absence and in the presence of various concentrations of BTAH in aerated 0.5 M NaCl solution

281(
BTAH) in aerated 0.5 M NaCl medium at room temperature.The results obtained at an open circuit potential immersed for 30 min represented as typical Nyquist and Bode plots, shown in (Figures 2, 3 and 4).

Figure 2 .
Figure 2. Nyquist plots of copper electrode at an open-circuit potential after 30 min in aerated 0.5 M NaCl solution without and with various concentrations of BTAH

Figure 3 .Figure 4 .
Figure 3. Bode plot for copper electrode in aerated 0.5 M NaCl solution without and with different concentrations of BTAH

Figure 5 . 3 .
Figure 5. Equivalent circuit used to fit experimental EIS data in Figure 2, symbols in the circuit indicated in the text

Figure 6 .
Figure 6.Variation of the weight loss as function of time for copper coupons in in aerated solution of 0.5 M NaCl without and with 2.5 ×10 -4 M of BTAH

Figure 7 .
Figure 7. Variation of the inhibition efficiency as function of time for copper coupons in aerated solution of 0.5 M NaCl containing 2.5 ×10 -4 M of BTAH

Figure 9 .
Figure 9. Molecular structure, Charge distribution, Electron density and frontier molecular orbitals for the optimized BTAH by DFT+ module

Figure 10 .
Figure 10.DFT+ geometry optimization and energy step of BTAH

Figure 13 (
Figure 13(a) shows that the adsorption occurred through the Nitrogen atoms.The adsorption density of BTAH on Cu (111) surface shown in Figure 13(b).Therefore, the studied molecules are likely to the copper surface to form a stable adsorption layer and protect copper from corrosion.

1 StepFigure 11 .
Figure 11.Total energy distribution for BTAH/Copper system during energy optimization process

Figure 12 .
Figure 12.(a) Most suitable configuration for adsorption of BTAH on the Cu (111) surface obtained by Adsorption locator module; (b) Adsorption density of BTAH on the Cu (111) substrate

Table 1 .
Corrosion inhibition parameters of copper in aerated 0.5 M NaCl solution in the absence and presence of various concentrations of BTAH using Tafel method

Table 2 .
Corrosion inhibition parameters of copper in aerated 0.5 M NaCl solution in the absence and presence of various concentrations of BTAH using Stern-Geary method

Table 4 .
The inhibition efficiency (w %), changes of the degree of copper surface coverages () and corrosion rates (   : with 2.5 ×10 -4 M of BTAH and    solution without inhibitor) obtained from

Table 5 .
Quantum chemical and molecular dynamics parameters for BTAH molecule calculated with DFT+ module in aqueous phase

Table 6 .
Outputs and descriptors calculated with adsorption locator for BTAH on Cu (111) surface