The cylindrical samples of copper with a purity of 99.9% are mounted in glass tubes of suitable diameter to provide an exposed active geometrical surface area of 3.14 cm² to the corrosive medium. Prior to each test, copper substrates were grinded with abrasive papers of decreasing particle size (400, 800, 1000, 1200 and

2000), rinsed with Milli-Q water (18.2 MΩ・cm), degreased with ethanol and then rinsed again with Milli-Q water and dried in air.

The corrosive medium consists of a 3.5 weight % sodium chloride solution which is obtained by dissolving Sigma-Aldrich sodium chloride (99.5%) in deionized water. The analytical nicotinamide was purchased from Sisco Research Laboratories Pvt. Ltd and solutions of concentrations ranging from 1 to 11 mM were prepared. All tests are carried out in aerated solution stirred with magnetic. A Thermo-cryostat Lauda model E100 permits to keep the electrolyte at fixed temperature.

The electrochemical measurements were performed in a three-electrode cell with a volume of 0.5 L. The working electrode (WE) was copper samples, the counterelectrode (CE) was a platinum wire and the reference (Ref) electrode was a saturated calomel electrode (SCE: 0.241 V vs SHE). The electrochemical study (Electrochemical impedance spectroscopy (EIS), potentiodynamictest and linear polarization) of the behavior of copper with contact to the corrosive medium without or with inhibitor is carried out using an experimental device composed of a Potentiostat-Galvanostat MODULAB, a DELL computer equipped with MODULAB XM ECS software allowing data processing. The potentiodynamic current-potential curves were recorded by varying the electrode potential automatically from −300 to 150 mV from the OCP with a scanning rate of 0.2 mV・s⁻¹. The polarization resistance measurements were performed by applying a controlled potential scan over a small range, typically ± 20 mV with respect to E_corr. The resulting current is linearly plotted versus potential; the slope of this plot at E_corr being the polarization resistance (Rp).

The molecular structure and reactivity parameters of NAM, were calculated by DFT methods using B3LYP (Becke 3 parameters Lee Yang Parr) functional with the 6-31 + G (d, p) basic set for all atoms using Gaussian 03 program package [11] . The molecular structure of NAM molecules was built and visualized by Gauss View.

The electronic structure properties such as lowest unoccupied molecular orbital (LUMO), highest occupied molecular orbital (HOMO), energy gap (DE (E_LUMO - E_HOMO)), ionization potential (I), electron affinity (A), electronegativity (χ), molecular dipole moment (µ), global hardness (η), softness (s), electron transfer fraction (DN) have been calculated.

The open circuit potential is the most immediately measurable electrochemical parameter. This simple technique provides preliminary informations on the nature of current processes at the metal/electrolyte interface: corrosion, passivation. Figure 2 presents the evolution of open circuit potential of the copper electrode in a 3.5% NaCl solution with and without inhibitor.

In blank solution, the open circuit potential of the copper electrode slightly evolves from −220 to −255 mV/SCE during the first hours of immersion due to the dissolution of copper and the formation of the corrosion product film. This latter film limits the dioxygen access towards the metallic interface that leads to a slight shift of the open circuit potential towards more negative values. We can observe some small potential fluctuations during the immersion that can be attributed to the evolution of the nature and/or surface coverage of the corrosion product film on the copper surface.

In presence of NAM, it is observed from the figure that in all studied systemethat OCP values start from a more positive value and decreases steadily before rising again to reacha steady state value. This rise in corrosion potential after the initial decrease is associated with the formation and thickening of the film at the copper surface, indicating the improvement of the corrosion resistance. For 1 mM and 11 mM concentrations, the corrosion potential increases up to 20 minutes and then decreases. This behavior can characterize a beginning of inhibition. For 11 mM the OCP value increased dramatically at beginning indicating the formation of a very thick protective layer on copper surface. Nevertheless, the film formed at these concentrations would not be of a nature (composition or morphology) to protect the copper surface durably.

On the other hand, the displacement of the corrosion potential towards negative values is considered as an indicator of surface activation [12] . In chloride-containing media such as seawater, the decrease in the value of the open circuit potential is associated with the dissolution of the oxide native layer formed upon air exposure and the formation of CuCl complexes on the surface which does not adequately protect it [13] .

Generally, the cathodic reaction on copper in the aerated NaCl solution is a reduction reaction of dioxygen according to the following Equations [14] :

O 2 + 2 H 2 O + 4 e − → 4 OH − (1)

The process of anodic dissolution of copper in NaCl medium can be described by the following steps [14] :

Cu + Cl − → CuCl+e − (2)

CuCl + Cl − → CuCl 2 − (3)

CuCl 2 − → Cu 2 + + 2 Cl − + e − (4)

Followed by the oxide formation:

2 CuCl 2 − + 2 OH − → Cu 2 O + H 2 O + 4 Cl − (5)

The inhibitor addition leads to the evolution of the open circuit potential towards more positive values suggesting that the NAM protects the metal against corrosion by adsorption and the formation of a protective layer on its surface.

Figure 3 shows the polarization curves of the copper in an aerated 3.5% NaCl

solution without and with addition of the NAM at different concentrations. The polarization curves were plotted after one hour of immersion in saline solution.

In the blank solution, the corrosion potential is around −220 mV/SCE. In the cathodic domain, it can be observed a first regime up to −340 mV/SCE where a linear variation of the current density versus potential is noticed. This evolution corresponds to the dioxygen reduction reaction on the copper surface. For higher cathodic overpotentials, a reduction peak is observed, this could be associated to the reduction of the copper oxides that were formed during the immersion period. A linear variation of the current density versus potential is observed in the anodic domain. With the addition of inhibitor, the shape of the polarization curves is quite similar.

An increase in NAM concentration leads to a shift of the corrosion potential towards more cathodic potentials for 10 mM and no significant displacement for other concentrations. We can also observe a remarkable drop of the current density for 10 mM compared to that obtained in a blank solution with a modification of the cathodic Tafel slope values.

The anodic polarization curves are not drastically affected by the presence of the NAM nevertheless it can be noted slight anodic current drops for 5 and 11 mM. The blank and 1 mM curves are confounded, this indicates that the adsorbed molecules of NAM at that concentration have no effect on the mechanism of copper dissolution. A slight modification of the anodic slope is observed with the presence of the inhibitor species. The electrochemical features suggest a mixed type character of this inhibitor [15] . The Electrochemical parameters such as corrosion potential (E_corr), anodic (b_a), and cathodic (b_c) Tafel slopes and the corrosion current density (j_corr) were determined by Tafel extrapolation method on the potentiodynamic polarization curves. The values are gathered in Table 1. The current density was used to calculate the copper corrosion rate (CR) and the inhibitor efficiency (h) using the Equations (6) and (7):

C R ( mm / an ) = 3.280 × j c o r r × M Cu n × ρ Cu (6)

η = j c o r r − j c o r r i n h j c o r r × 100 (7)

j c o r r et j c o r r i n h (mA/cm²) are the corrosion current densities of the copper after immersion in 3.5% NaCl medium without and with addition of inhibitor respectively. M Cu (g/mol) is the molar mass of copper, ρ Cu (g/cm³) the copper density and n is the number of exchanged electrons during the oxidation. The polarization resistance was calculated from the Stern-Geary Equation:

R P = b a × b c 2.303 ( b a + b c ) j c o r r (8)

b_a and b_c are the Tafel slopes.

From the results obtained in Table 1, we can conclude that the polarization resistance and the inhibitory efficiency of NAM increase with the concentration of inhibitor and reach respectively maximum values of 6431 Ω and 80% for a inhibitor concentration of 10 mM and decrease thereafter. L. Elhousni et al., [16] . have reported a similar behaviour in their work on copper in the cooling liquids of industrial systems in the presence of Cetyltrimethyl Ammonium Bromide (CTAB). These authors have justified the decrease of the efficiency in the case of high inhibitor concentrations by the formation of an oxide and/or hydroxide layer on the surface of the material.

Others authors [17] attribute this behavior to an adsorbed layer saturation and to the formation of micelles which should decrease the number of transported inhibitory molecules on the metal surface, thus causing a decrease in the inhibition efficiency. The Tafel cathode slope (b_c) drastically changes with inhibitor addition while the anodic Tafel slope (b_a) variation is very small, thus indicating that the NAM changesonly the dioxygen reduction mechanisms. It is widely believed that a compound could be recognized as an anodic or cathodic inhibitor if the corrosion potential variation is greater than 85 mV [18] . In our study, the largest displacement of E_corr was about 50 mV toward cathodic direction. Based on these results, NAM is considered as mixed-type inhibitors for copper in the 3.5% NaCl solution.

The concentration of 10 mM seems to be the optimal concentration of NAM accordind to the potentiodynamic polarization study. Long-term immersion measurements at the open circuit potential were performed and polarization resistance test was also scheduled in order to follow the dissolution kinetic. In Figure 4 it is possible to observe the polarization resistance of copper as a function of time in the corrosive solution without and with 10 mM of NAM.

Figure 4 presents the evolution of the polarization resistance during the extended immersion test. Without inhibitor, the polarization resistance increases up to 25 hours. This increase suggests the ennoblement of the metal. After 25 h of immersion, an important decrease of the R_p is observed and maybe associated to a change in the nature of the corrosion product film during the immersion. With inhibitor, after the two first hours a continuous decrease of polarization resistance is observed during the immersion time. It is found that the inhibition efficiency of the inhibitor (10 mM) decreases with increasing immersion time. This effect can be due to desorption of inhibitor molecule with immersion time [10] [19] .

It is generally accepted that organic inhibitor inhibits metal corrosion by getting adsorb on the surface and the process is mere substitution in which water molecules on the metal surface are replaced by inhibitor molecules [20] . Thus, useful information on metal corrosion inhibition by an organic inhibitor can be obtained by employing adsorption isotherms. Experimental data obtained from PDP measurements for the corrosion inhibition of copper in NaCl 3.5 % solution by NAM were fitted into various adsorption isotherms including Langmuir, Temkin, El-Awady thermodynamic, Frumkin, and Frundlich adsorption isotherms. By far, the best fit is obtained for Langmuir adsorption isotherm. Langmuir adsorption isotherm is defined according to the following equation [20] :

C i n h θ = 1 K a d s + C i n h (9)

The curve of C_inh/q as a function of the C_inh (inhibitor concentration) is shown in Figure 5.

However, there is a slight deviation from the value of the slope of the unit (Table 2). Therefore, the corrected Langmuir model is more suitable for NAM compound adsorption. The deviation of the value of the slope to unit is due to the interactions between the adsorbed species on the metal surface. The modified Langmuir model takes into account the interactions between the adsorbed species. The modified Langmuir equation is as follow [21] :

C i n h θ = n K ′ a d s + n C i n h (10)

where n is the slope of the straight line corresponding to the Langmuir isotherm.

θ = j c o r r − j c o r r ( i n h ) j c o r r is the coverage rate and K a d s is the equilibrium constant

of the adsorption process related to the free enthalpy of adsorption by the Equation (11) [22] :

Δ G a d s 0 = − R T ln 55.5 K a d s (11)

The estimated values of K a d s and Δ G a d s 0 have been recorded in Table 2.

These K a d s value is higher. This high value indicates that the adsorbed layer on the copper surface is stable [23] . It is reported in the literature [24] that values of Δ G a d s 0 greater than −20 kJ・mol⁻¹ are generally related to interactions between charged molecules and metal charges (physisorption). On the other hand, the values less than −40 kJ・mol⁻¹ is attributed to a transfer of charges between the molecules of the inhibitor and the metal surface (chemisorption) with the formation of covalent or coordination bonds. In the present work, the value of Δ G a d s 0 = −36.20 kJ・mol⁻¹suggested that the adsorption of NAM on copper surface involve physical as well as chemical adsorption.

Temperature is one of the factors that can modify the behavior of materials in a given corrosive medium as well as the inhibitory efficiency of a compound. Considering the importance of this factor, we have carried out potentiodynamic polarization tests of copper in the 3.5% NaCl study medium without and with addition of 10 mM of inhibitor in a temperature varying from 20˚C to 50˚C. The obtained curves are shown in Figure 6. The E_corr and j_corr values obtained from these curves are shown in Table 3.

According to the Figure 6, the increase in temperature leads to an increase in the current density both without and with inhibitor. The decrease in inhibition efficiency with increasing temperature as observed in Table 3 is suggestive of physisorption mechanism generally attributed to electrostatic interaction between charged molecules and charged metal. The decrease in inhibition efficiencies might be due to the weakening of adsorbed inhibitor film on the copper surface. According to Obot et al., as temperature increases, Gibbs free energy and change in enthalpy rise to a higher value, so that some of the chemical bonds joining the molecules onto the metallic surface are impaired and the film stability reduced. It may further be attributed to a possible shift of the adsorption/desorption equilibrium towards desorption of adsorbed inhibitor due to increasing solution agitation. Thus, as the temperature increases the number of adsorbed molecules decreases, leading to a decrease in the inhibition efficiency [25] .

We also note that the study of the effect of temperature can inform us about the type of adsorption of the inhibitor on the surface of the metal (chemisorption or physisorption) by the determination of the activation energy ( E a ), the enthalpy ( Δ H a ) and entropy ( Δ S a ) of activation of the corrosion process. The dependence of corrosion current density at temperature can be expressed by Arrhenius Equation (12) and transition state Equation (13):

j c o r r = A exp ( − E a R T ) (12)

log ( j c o r r T ) = [ log ( R Ν h ) + Δ S a 2.303 R ] − Δ H a 2.303 R T (13)

where E_a is the apparent effective activation energyR (J・mol⁻¹ K⁻¹) is the perfect gas constant, h (J.s) the Planck constant, T (K) the absolute temperature, N (mol⁻¹) the number of Avogadro and A the pre-exponential factor of Arrhenius.

Figure 7 shows the plots ln j c o r r = f ( 1000 T ) and log ( j c o r r T ) = f ( 1000 T ) . The

activation parameters recorded in Table 4 were determined.

In our investigations, the activation energy found in presence of NAM is higher than that obtained without this molecule, so the adsorption of the studied molecule onto the copper surface is found to be physical in nature [26] . The positive sign Δ H a shows the endothermic nature of the copper dissolution process. The increase in the enthalpy value after the addition of NAM indicates that in the presence of inhibitory molecules, the dissolution of the metal is more difficult [27] . For the negative sign of the activation entropy both in the solution with and without an inhibitor, it can be attributed to the disorder created by the inhibitory molecules on the surface of the metal .The adsorption of adsorbate on the metal surface is regarded as a substitutional adsorption process between the organic compound in the aqueous phase and water molecule adsorbed on the metal surface [20] . The adsorption of inhibitors on the copper surface would be in equilibrium with the desorption of surface water molecules:

O r g ( s o l ) + n H 2 O ( a d s ) → O r g ( a d s ) + n H 2 O ( s o l ) (14)

where O r g ( s o l ) and O r g ( a d s ) are the adsorbates in the bulk solution and adsorbed on the surface respectively; H 2 O ( a d s ) and H 2 O ( s o l ) are water molecules adsorbed on the surface and in the solution respectively.

Quantum chemical calculations were carried out to study the mechanism of adsorption and inhibition of the molecule studied in functional theory of density by the hybrid functional B3LYP (Becke 3 parameters Lee Yang Parr) with the base of orbitals 6-31 + G (d, p) using the Gaussian 03 W software.