cid solutions are widely used in different industries. The most important fields of application are acid pickling, industrial acid cleaning, acid descaling [1]. copper is a metal that has been widely used in different types of industries, including electronics due to its relatively high corrosion resistance. Its protection against corrosion saves large costs on repairs and equipment replacements. In spite of copper being a relatively noble metal, it reacts easily in a medium containing oxygen. It has long been known that when copper corrodes in aerated, chloride containing solutions, a layer of Cu 2 O is formed on the surface of the metal. In addition, copper is dissolved by reactions which can be summarized as follows [2] :

Cathodic :

Then in solution:

At chloride concentrations lower than 1 M, the copper dissolution occurs through the formation of CuCl, which is not protective enough and is converted to soluble CuCl 2 ??? by reacting with excess chloride [3]. According to Bacarella and Griess [4], the anodic dissolution of copper is under mixed control by the electrodissolution of copper and the diffusion of soluble CuCl -2 from the Helmholtz plane into the bulk solution. On the other hand-higher chloride complexes such as CuCl 3

2and CuCl 4 3are formed in addition to CuCl and CuCl 2 at chloride concentrations higher than 1M [5]. One of the most important methods in the protection of copper against corrosion is the use of organic inhibitors. Organic compound containing polar groups including nitrogen, sulfur and oxygen [6][7][8][9][10][11][12][13][14][15][16][17][18] have been reported to inhibit copper corrosion. The inhibiting action of these organic compounds is usually attributed to their interactions with the copper surface via their adsorption. Polar functions groups are regarded as the reaction center that stabilizes the adsorption process [19]. In general, the adsorption of an inhibitor on a metal surface depends on the nature and the surface charge of the metal, the adsorption mode, its chemical structure and the type of the electrolyte solution [20]. Also, heterocyclic compounds containing nitrogen are known to be good inhibitors of copper and alloys [21]. As the awareness of environment protection, the research is oriented to the development of environmental-friendly corrosion inhibitors [22][23][24][25]. Seeking for alternatives of toxic inhibitors is under consideration. Some studies used drugs as green corrosion inhibitors for various metals and alloys [26][27][28][29][30].The use of environmental friendly pharmaceutical compounds (streptoquin and septazole) as corrosion inhibitors for copper has not been reported before. So, our aim is to study the inhibiting effect of these drugs on copper corrosion in HCl solution using various electrochemical techniques.

Copper was used as the metal electrode for corrosion studies and its chemical composition As follows (weight %): Ni 0. Polarization experiments were carried out in a conventional three-electrode cell with a platinum counter electrode and a saturated calomel electrode (SCE) coupled to a fine Luggin capillary as the reference electrode. The working electrode was in the form of disc cut from copper with dimension 1x1 cm which embedded in epoxy with resin of polytetrafluoroethylene. Before measurement, the exposed surface was wetpolished with silicon carbide abrasive paper (800, 1000, 1200 grit), then the electrode was immersed in solution at natural potential until a steady state was reached. All experiments were carried out in freshly prepared solutions and results were always repeated at least three times to check the reproducibility.

Polarization measurements performed at potentials in the range from -450 mV to -150 mV (SCE) at a scan rate of 0.33 mVs -1 . All measurements were carried out in aerated solution of 0.1 M of HCl in the absence and presence of different concentrations of inhibitors. The Tafel plots of the anodic and cathodic curves were extrapolated to obtain the corrosion potential (E corr. ) and corrosion current density (i corr. ).

Electrochemical impedance spectroscopy were performed at corrosion potentials, E corr , over a frequency range of 10 5 Hz to 0.5 Hz with a signal amplitude perturbation of 10 mV, using potentiostat/galvanostat (Gamry PCI 300/4) and personal computer with EIS300 software for calculations. Data were presented as Nyqusit and Bode plots. Experiments were always repeated at least three times.

The electrochemical frequency modulation technique (EFM) provides a new tool for electrochemical corrosion monitoring. EFM experiments were performed with applying potential perturbation signal with amplitude 10 mV with two sine waves of 2 and 5 Hz. The choice for the frequencies of 2 and 5 Hz was based on three arguments [31]. The larger peaks were used to calculate the corrosion current density (i corr ), the Tafel slopes (? c and ? a ) and the causality factors CF2 and CF3 [32&33].

All electrochemical measurements were performed using Gamry Instrument (PCI 300/4) Potentiostat / Galvanostat /ZRA. This includes a Gamry framework system based on the ESA 400. Gamry applications include DC105 software for potentiodynamic polarization measurements, EIS300 software for electrochemical impedance spectroscopy and EFM 140 software for electrochemical frequency modulation measurements along with a computer for collecting data. Echem Analyst 6.03 software was used for plotting, graphing, and fitting data.

Potentiodynamic polarization curves of copper electrode in 0.1 M HCl solution without and with various concentrations (100, 300, 500, 700, 900 ppm) of streptoquin and septazole at 25 °C were recorded. The polarization curves of copper in 0.1 M HCl at 25°C in the presence and absence of different concentrations of streptoquin and septazole are shown in Figs. 1 & 2. Electrochemical parameters such as corrosion current density (i corr ), corrosion potential (E corr ), anodic (? a ), cathodic (? c ) Tafel slopes and inhibition efficiency (?%) were calculated and are listed in Table 2. The inhibition efficiency (?%) and the corrosion rate (R corr. ) were calculated using Eqs.1&2:

where i corr. is the corrosion current density (A cm -2 ), EW the equivalent weight of copper (g) and D the density of Cu (g cm -3 ). The presence of increasing amounts of septazole and streptoquin led to a decrease in both the cathodic and anodic current density. It is clear from these data that the inhibitors may affect either the anodic or the cathodic reaction, or both [34]. Since the anodic (? a ) and cathodic (? c ) Tafel slopes of streptoquin and septazole were found to change slightly with inhibitor concentration, this indicates that these inhibitors affected both of these reactions i.e. these drugs act as mixed type inhibitors [35].

Table 3 : Electrochemical parameters obtained from potentiodynamic polarization measurements of copper in 0.1 M HCl in the absence and presence of different concentrations of streptoquine and septazole at 25°C

The addition of streptoquine and septazole shift the E corr values towards the negative. A compound can be classified as an anodic-or cathodic-type inhibitor when the change in E corr value is larger than 85 mV [36]. Since the largest displacement exhibited by streptoquine and septazole was about 71 mV as shown in Table 3, it may be concluded that these molecules should considered as a mixed-type inhibitor, meaning that the addition of these drugs to 0.1 M HCl solution both reduces the anodic dissolution of copper and also retards the cathodic hydrogen evolution reactions. From the calculated values of (?%) as shown in Table 3, the order of decreasing inhibition efficiency is as follows: streptoquin > septazole.

From Table 4 we found that the corrosion rate increases with the rise of temperature and the inhibition efficiency decreases with increasing temperature. This can be attributed to the shift of the adsorptiondesorption equilibrium towards desorption. Such behaviour suggests that investigated compounds were physically adsorbed on copper surface. The activation energy (E * a ) of the corrosion process was calculated using Arrhenius eq. ( 3):

Where A is Arrhenius constant, R is the gas constant and T is the absolute temperature. Figures (3&4) show the Arrhenius plot (log i corr vs. 1/T) in the presence and absence of streptoquin and septazole. The values of activation energies E * a can be obtained from the slopes of the straight lines and are given in Table (5). Table 4 : Electrochemical parameters obtained from potentiodynamic polarization measurements of copper in 0.1 M HCl in presence of 500 ppm from streptoquine and septazole at different temperature R corr. mmy -1 ?% ? (1/T) curves for the corrosion of copper in 0.1 M HCl in the absence and presence of different concentrations of streptoquine at different temperatures It is noted that the values of activation energy is higher in the presence of drugs than in their absence indicating that inhibitors exhibit low ?% at elevated temperatures [37] and also, due to the film formation on copper surface. An alternative formulation of the Arrhenius equation is the transition state equation [38]:

where h is Planck's constant, N is Avogadro's number, Î?"S * is the entropy of activation and Î?"H * is the enthalpy of activation. Table 5 : Kinetic-thermodynamic corrosion parameters for copper corrosion in the absence and presence of various concentrations of streptoquin and septazole

The values of Î?"H * and Î?"S * were calculated and listed in Table (5). The negative values of Î?"H * reflect that the process of corrosion is an exothermic one. The negative values of Î?"S * implies that the activation complex is the rate determining step that represents an association rather than dissociation step [39]. This means that the activated molecules are in higher order state than that the initial state.

In order to get a better understanding of the electrochemical process on the metal surface, adsorption was studied. For determining the related mechanism, adsorption isotherms were drawn. A number of mathematical relationships for the adsorption isotherms have been suggested to fit the experimental data of the present work. The simplest equation is that due to Langmuir which is given by the general relation:

where K ads is the equilibrium constant for the adsorption/desorption process, C is the inhibitor concentration in the bulk of the solution in mol L -1 . From the intercepts of the straight lines on the C/?"¨ axis, one can calculate K ads values that relate the standard free energy of adsorption, (?G°a ds ), as given by this Eq. :

K ads =1/55.5 exp (-?G°a ds ) / RT (6) The value of 55.5 is the concentration of water in the bulk solution in mol L -1 . Free energy, ?G°a ds , values were calculated and are given in Table (6), the negative values of ?G°a ds indicate spontaneous adsorption of inhibitors on copper surface [42]. Generally, values of ?G°a ds up to -20 kJ mol -1 are consistent with physisorption, while those around -40 kJ mol -1 or higher are associated with chemisorptions as a result of the sharing or transfer of electrons from inhibitor molecules to the metal surface to form co-ordinate bond [43]. the calculated ?G°a ds values are 22-36 kJ mol -1 indicating that the adsorption mechanism of septazole and streptoquin in 0.1 M HCl solution at the studied temperatures is mixed one (physisorption and chemisorption). Figs. (7&8) show that the plot of C/? vs. C for streptoquin and septazole are linear plots which obtained for the different temperatures and inhibitors, indicating that the adsorption of these compounds followed Langmuir adsorption isotherms. The various adsorption parameters obtained from this isotherm are listed in Table (6).

-Î?" S * J mol -1 K -1

Figure 9 shows the plot of ln K ads. Vs. 1/T which gives straight lines with slopes (-?H °ads /2.303R) and intercepts [(?S °ads /2.303R) + ln (1/55. 5)]. Calculated values of ?H °ads and ?S °ads using the Van't Hoff equation are listed in Table 6. (1/T) curves for the corrosion of copper in 0.1 M HCl in the absence and presence of different concentrations of septazole and streptoquine at different temperatures Negative sign of ?H °ads indicates that the adsorption of septazole and streptoquine on copper surface is an exothermic process. ?H°a ds is about 31.2-45 kJ mol -1 which is larger than the common physical adsorption heat (40 kJ mol -1 ), but smaller than the common chemical adsorption heat (100 kJ mol -1 ) [45], probably meaning that both physical and chemical adsorption take place (i.e. comprehensive adsorption). The negative Î?"S°a ds values are accompanied with exothermic adsorption process. This is agrees with the expected, when the adsorption is an exothermic process, it must be accompanied by a decrease in the entropy change and vice versa [46].

Electrochemical impedance is a powerful tool in the investigation of the corrosion and adsorption phenomena [47 ] . The correlation of the experimental -Î?"S º ads J mol impedance plot to an equivalent circuit enables the verification of the mechanistic model for the system. Such a correlation leads to the calculation of the numerical values corresponding to physical and/or chemical properties of the electrochemical system [48,49]The impedance data of Cu, recorded after 30 min immersion in 0.1 M HCl are presented as Nyquist and Bode plots in presence and absence of various concentrations of streptoquin and septazole are shown in Figures (10&11), respectively. The Nyquist plots show a single semi-circle shifted along the real impedance (Z r ) axis, single capacitive loop, which is attributed to charge transfer of the corrosion process, and the diameters of the loops increase with the increase in the concentration of the drugs. The Bode plots show resistive region at high frequencies and capacitive region at intermediate frequencies but do not show a clear resistive region (horizontal line and a phase angle = 0) at low frequencies. These plots show two overlapped phase maxima at intermediate and low frequencies. The impedance data of the copper electrode in the presence of different inhibitors concentrations were analyzed using the equivalent circuit shown in Figure 12. This circuit includes constant phase elements (CPE) in place of capacitors to represent various types of non homogeneities typical of corroding electrodes such as surface roughness, insufficient polishing, grain boundaries and surface impurities [50]. The impedance of this element is frequency dependent, and can be mathematically expressed using two parameters, Y ? and n as:

where Y ? is the CPE coefficient, ? the angular frequency (? = 2?f , where f is the AC fr According to the value of n, Eq. ( 8) accounts for an inductance (n = -1), a resistance (n = 0), a Warburg impedance (n = 0.5) or a capacitance (n = 1). A Warburg element occurs when charge carriers diffuse through a material. A rough or porous surface can cause a double layer capacitance to appear as a constant phase element with n varying between 0.5 and 1. This circuit is typical of oxide-coated metals and has been used before by other authors in relation to copper corrosion in potable water [51]. R s represents the solution resistance between the electrode surface and the tip of the Luggin capillary, Z CPEo a constant related to the surface oxide, R o the resistance to current flow through defects in the surface oxide, Z CPEdl a constant related to the double layer and R dl the double layer resistance. From these Nyquist plots, the difference in real impedance at lower and higher frequencies is generally considered as chargetransfer resistance. The resistances between the metal and outer Helmholtz plane (OHP) must be equal to the R ct . The adsorption of inhibitor molecules on the metal surface decreases its electrical capacity because they displace the water molecules and other ions originally adsorbed on the metal surface. This modification results in an increase of charge-transfer resistance. The R ct values increased with inhibitors concentrations may suggest the formation of a protective layer on the copper surface. This layer makes a barrier for mass and charge-transfer. The values of the charge-transfer resistance (R ct ) were obtained from the difference in real component (Z \ ) of impedance at lower frequencies. Also the double layer capacitances (C dl ) were calculated by the Eq. ( 9) [31].

where f max is the frequency value at which the imaginary component (Z") of impedance is maximum. The calculated values of R ct and C dl are listed in Tables (7). The data obtained from fitted spectra are listed in Table 7.The degree of surface coverage (?) and %? were calculated from the EIS data using following equation:

where R ct and R ct * are the charge-transfer resistances with and without the inhibitors, respectively. ? and ? are also listed in Table 7, and also show that the R s values are very small compared to the R ct values. By increasing the inhibitor concentrations, the R ct values increase and the calculated C dl values decrease, which causes an increase of ? and ?%, the R ct are obtained by septazole at all concentrations is higher than that obtained by streptoquin. The order of the IE obtained from the R ct values is: septazole > streptoquin.

As it can be seen from Tables 7, the C dl values tend to decrease with the increase of the concentration of the surfactants in 0.1 M HCl solution. The decrease in the C dl , which can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that surfactants molecules function by adsorption at the metal/solution Electrochemical frequency modulation (EFM) technique is a new tool for monitoring the electrochemical corrosion. EFM is non-destructive technique, rapid test, gives directly values of the corrosion current without a prior knowledge of Tafel constants and has also great strength due to the causality factor, which serve as an internal check on the validity of the EFM measurements Table 8 shows the corrosion kinetic parameters such as inhibition efficiency (E EFM %), corrosion current density ( ?? Acm -2 ), Tafel constants (? a & ? c ) and causality factors (CF-2, CF-3) at different concentrations of drugs at 25°C. Figures 13-15 represent the EFM intermodulation spectra of copper in 0.1 M HCl devoid of and containing 500 ppm concentrations of streptoquin and septazole, respectively. The inhibition efficiency, E EFM % of streptoquin and septazole was calculated using eq. ( 1). As can be seen from and 3.0, there is a causal relationship between the perturbation signal and the response signal. Then the data are assumed to be reliable [32]. The calculated inhibition efficiency E EFM % enhances with increasing streptoquin and septazole concentration. Over the past decades the semi empirical molecular orbital methods have been widely used in computational studies. Semi empirical methods serve as efficient computational tools which can yield fast quantitative estimates for a number of properties [52]. MNDO (modified neglect of differential overlap) is a semiempirical methods based on the NDDO (neglect of diatomic differential overlap) approximation. The quantum chemical calculations were employed to give further insight into the mechanism of inhibition action of streptoquin and septazole. For this purpose the highest occupied molecular orbital E HOMO , energy of the lowest unoccupied molecular orbital (E LUMO ), energy gap (?E) between LUMO and HOMO and Mulliken charges on charge we can observe the presence of excess of negative charge on nitrogen, oxygen and sulfur atoms can be adsorbed on the copper surface using these active centres leading to the corrosion inhibition action.

The calculated parameters such as E HOMO , E LUMO and the dipole moment (µ) of the two inhibitors were shown in Table 9, the dipole moment (µ) is an indicator of the electronic distribution in a 1molecule and is one of the properties used to discuss and to rationalize the structure. Low value of the dipole moment (µ) will favour the accumulation of inhibitor molecules on the metallic surface. The comparison between the calculated dipole moments of inhibitors reveals that the lower septazole has better inhibition efficiency. streptoquin and septazole were plotted in Figure 15. There is a general consensus by several authors that the more negatively charged heteroatom, the more it can be adsorbed on the metal surface through donor-acceptor type reaction [53]. From the values of Mulliken charge we can observe the presence of excess of negative charge on nitrogen, oxygen and sulphur atoms can be adsorbed on the copper surface using these active centres leading to the corrosion inhibition action. Table 9 shows the quantum chemical calculation parameters (E HOMO, E LUMO , Î?"E= (E LUMO -E HOMO ), dipole moment (µ), heat of formation and ionization potential) for streptoquin and septazole. From these tables the less negative E HOMO and the smaller Î?"E reflect a great inhibition efficiency of the inhibitors [54]. From the energy of highest occupied molecular orbital (E HOMO ), the order of inhibiting efficiency is as follow: septazole > streptoquin. This is a good agreement with the experimental data obtained by potentiodynamic polarization, EIS and EFM techniques.

The corrosion resistance of copper increases with the increasing concentration of streptoquin and septazole and decreases with increasing temperature indicating that the inhibition occurs through mainly physical adsorption of the investigated drugs on copper surface. These drugs act as mixed-type inhibitors, suppressing the corrosion reactions by forming a protective adsorption film on copper surface. Adsorption