Employing electrochemical frequency modulation for studying corrosion and corrosion inhibition of copper in sodium chloride solutions

3.1 Electrochemical frequency modulation, EFM

Electrochemical frequency modulation is a nondestructive corrosion measurement technique that can directly give values of the corrosion current without prior knowledge of Tafel constants. Like EIS, it is a small ac signal. Unlike EIS, however, two sine waves (at different frequencies) are applied to the cell simultaneously. Because current is a non-linear function of potential, the system responds in a non-linear way to the potential excitation. The current response contains not only the input frequencies, but also contains frequency components which are the sum, difference, and multiples of the two input frequencies. The two frequencies may not be chosen at random. They must both be small, integer multiples of a base frequency that determines the length of the experiment.

Intermodulation spectra obtained from EFM measurements are presented in Fig. 1. Each spectrum is a current response as a function of frequency. The two large peaks are the response to the 2 Hz and 5 Hz excitation frequencies. These peaks are used by the EFM140© software package to calculate the corrosion current and the Tafel constants. It is important to note that between the peaks the current response is very small. There is nearly no response (<100 nA) at 4.5 Hz, for example, the frequencies and amplitudes of the peaks are not coincidences. They are direct consequences of the EFM theory.

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Figure 1 Intermodulation spectra recorded for copper electrode in 3.5% NaCl solutions in the absence and presence of various concentrations of CPD at 25 ± 1 °C.

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Corrosion kinetic parameters are calculated from EFM techniques using the following equations (Khaled, 2008a):

Table 1 shows the corrosion kinetic parameters such as inhibition efficiency, corrosion current density (μA/cm²), Tafel constants (β_a, β_c) and causality factors (CF-2, CF-3) at different concentration of CPD derivative in 3.5% NaCl at 25 °C.

From Table 1, the corrosion current densities decrease by increasing the concentrations of CPD. The inhibition efficiencies increase by increasing CPD concentrations. The causality factors in Table 1 are very close to theoretical values which according to the EFM theory (Bosch et al., 2001) should guarantee the validity of Tafel slopes and corrosion current densities. Inhibition efficiency ( ${\rho }_{\text{EFM}}\%$ ) depicted in Table 1 calculated from the following equation:

The great strength of the EFM is the causality factors which serve as an internal check on the validity of the EFM measurement (Bosch and Bogaerts, 1996). With the causality factors the experimental EFM data can be verified. The causality factors in Table 1 indicate that the measured data are of good quality. The standard values for CF-2 and CF-3 are 2.0 and 3.0, respectively. To evaluate the EFM technique as an effective corrosion monitoring technique, several traditional corrosion techniques implied to study the corrosion inhibition of copper by CPD derivative in 3.5% NaCl solutions. EIS, Tafel extrapolation and weight loss measurements are used to calculate the inhibition efficiency as well as other corrosion kinetic parameters.

3.2 Electrochemical impedance spectroscopy (EIS)

Nyquist plots recorded for copper electrode in 3.5% NaCl solutions and containing various concentrations of CPD derivative are shown in Fig. 2. As can be seen from Fig. 2, the Nyquist plots do not yield perfect semicircles as expected from the theory of EIS. The deviation from ideal semicircle was generally attributed to the frequency dispersion (El Achouri et al., 2001) as well as to the inhomogenities of the surface and mass transport resistant (Khaled and Hackerman, 2004). The shape of the impedance diagrams of copper in 3.5% NaCl is similar to those found in the literature (Khaled, 2008b). The presence of CPD increases the impedance but does not change the other aspects of corrosion mechanism occurred due to its addition. Symbols in Fig. 2 represent the measured data and solid lines represent the fitting data obtained using the equivalent circuit (Sherif and Park, 2006) presented in Fig. 3.

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Figure 2 Nyquist plots for copper in 3.5% NaCl solutions in the absence and presence of various concentrations of CPD at 25 ± 1 °C.

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Figure 3 Equivalent circuit used to model impedance data for copper in 3.5% NaCl solutions in the absence and presence of various concentrations of CPD at 25 ± 1 °C.

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An equivalent circuit, such as shown in Fig. 3, was used to consider all the process involved in the electrical response of the system. R_s represents the solution resistance, R_p is the polarization resistance and can be defined also as the charge-transfer resistance, CPE₁ and CPE₂ are constant phase elements (CPEs), $R_p^{\prime }$ is another polarization resistance and W, is a Warburg impedance. The different elements were evaluated by a fitting procedure.

A constant phase element (CPE) instead of a capacitive element is used in Fig. 3 to get a more accurate fit of experimental data sets using generally more complicated equivalent circuits. The impedance, Z, of CPE has the form (Abdel-Rahim et al., 2006):

The parameters obtained by fitting the experimental data using the equivalent circuit, and the calculated inhibition efficiencies are listed in Table 2. The Nyquist plots presented in Fig. 2 clearly demonstrate that the shapes of these plots for inhibited copper electrode are not substantially different from those of uninhibited electrode. Addition of CPD molecules increases the impedance but does not change other aspects of the electrode behaviour.

Parameters derived from EIS measurements and inhibition efficiency is given in Table 2. Addition of CPD increases the values of R_p and $R_p^{\prime }$ , and lowers constants of CPE₁ and CPE₂ and this effect is seen to be increased as the concentrations of CPD increase. The constant phase elements (CPEs) with their n values $1>n>0$ represent double layer capacitors with some pores (Sherif and Park, 2006). The CPEs decrease upon increase in CPD concentrations, which are expected to cover the charged surfaces and reducing the capacitive effects. This decrease in (CPE) results from a decrease in local dielectric constant and/or an increase in the thickness of the double layer, suggested that CPD molecules inhibit the copper corrosion by adsorption at the copper/NaCl interface The semicircles at high frequencies in Fig. 2 are generally associated with the relaxation of electrical double layer capacitors and the diameters of the high frequency semicircles can be considered as the charge-transfer resistance (R_ct = R_p) Ma et al., 2002. Therefore, the inhibition efficiency, ${\tau }_{\text{EIS}}\%$ of CPD for the copper electrode can be calculated from the charge-transfer resistance as follows (Ma et al., 2002):

3.3 Potentiodynamic polarization measurements

Fig. 4 shows typical polarization curves for copper in chloride media. The three distinct regions that appeared were the active dissolution region (apparent Tafel region), the active-to-passive transition region, and the limiting current region. In the inhibitor-free solution, the anodic polarization curve of copper (blank-curve in Fig. 4) showed a monotonic increase of current with potential until the current reached the maximum value. After this maximum current density value, the current density declined rapidly with potential increase, forming an anodic current peak that was related to CuCl film formation.

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Figure 4 Anodic and cathodic polarization curves for copper in 3.5% NaCl solutions in the absence and presence of various concentrations of CPD at 25 ± 1 °C.

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In the presence of CPD, both the cathodic and anodic current densities were greatly decreased over a wide potential range from −650 to +300 mV_SCE. It was found from Fig. 4 that CPD had a much greater influence on the anodic current in 3.5%NaCl solution than that on the cathodic current.

It is also seen that increasing the CPD concentrations, decreases the cathodic, anodic and corrosion currents (i_corr) and consequently the corrosion rates.

Inspection of polarization curves in Fig. 4 shows that the anodic dissolution of copper in aerated 3.5% NaCl solutions obeys Tafel’s law. The anodic curve is, therefore preferred over the cathodic one for evaluation of corrosion currents, $i_{\text{corr}}$ , by the Tafel extrapolation method. However, the cathodic polarization curve deviate from Tafel behaviour and exhibiting a limiting diffusion current which may be due to the reduction of dissolved oxygen. This is the reason why values of the cathodic Tafel slopes, calculated from the software, are not included here.

Addition of 10⁻³ M of CPD reduces to a great extent the cathodic and anodic currents, $i_{\text{corr}}$ . The corresponding electrochemical kinetics parameters such as corrosion potential (E_corr), anodic Tafel slopes (β_a) and corrosion current density (i_corr), obtained by extrapolation of the Tafel lines are presented in Table 3. The inhibitor efficiency was evaluated from dc measurements using Eq. (13) (Hack and Pickering, 1991):

Inspection of Table 3 shows that the addition of different concentration of CPD decreases corrosion current densities and increases the inhibition efficiencies ${\delta }_p\%$ .

3.4 Adsorption considerations

Important information about the interaction between the inhibitors and metal surface can be provided by adsorption isotherm. During corrosion inhibition of metals, the nature of inhibitor on the corroding surface has been deduced in terms of adsorption characteristics of inhibitor.

Organic molecules inhibit the corrosion by adsorption at the metal surface (Gil et al., 2004). Theoretically, the adsorption process has been regarded as a simple substitution process, in which an inhibitor molecule ${\mathit{Inh}}_{(\text{aq}.)}$ in the aqueous phase substitutes an “x” number of water molecules adsorbed on the metal surface vis,

A direct relationship between inhibition efficiency of the heterocyclic pyrimidine derivative CPD and the degree of surface coverage (θ)[Inhibition efficiency = 100 × θ] can be assumed for the different concentrations of the CPD. The degree of surface coverage (θ) for the different concentrations of CPD has been evaluated from the potentiodynamic polarization measurements in 3.5% NaCl at 25 ± 1 °C. The data were tested graphically by fitting to various adsorption isotherms including Freundlich, Temkin, Flory–Huggins, Bockris–Swinkles, Langmuir and Frumkin isotherms. The correlation coefficient (R²) was used to determine the best fit isotherm which was obtained for Langmuir. According to this isotherm, θ is related to the inhibitor concentration by the following Eq. (15) (Morad and Kamal El-Dean, 2006).

The data was obtained from Fig. 5. The intercept of the lines in Fig. 5 yielded K_ads in (dm³ mol⁻¹), and the corresponding standard Gibbs free energy of adsorption in (kJ mol⁻¹) was then calculated from Eq. (16) (Sethi et al., 2007):

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Figure 5 Langmuir adsorption isotherm of Cu in 3.5% NaCl containing CPD at 25 ± 1 °C.

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Results presented in Table 4, indicate that the values of $\mathrm{\Delta }{G}_{\text{ads}}^{\text{o}}$ are negative and equals −26 kJ mol⁻¹.The negative values signify adsorption of the CPD molecules via mixed adsorption mechanism. It is also seen that the values of $\mathrm{\Delta }{G}_{\text{ads}}^{\text{o}}$ increased with an increase in inhibitor concentrations, a phenomenon which indicates that the adsorption of the inhibitor onto copper surface was favorable with increasing CPD concentrations.

Literature demonstrates that values of standard Gibbs free energy of adsorption in aqueous solution around −20 kJ mol⁻¹ or lower (more positive) indicate adsorption with electrostatic interaction between the adsorbent and adsorbate (physisorption), while those around or higher (more negative) than −40 kJ mol⁻¹ involve charge sharing between the molecules and the metal (chemisorption) Bilgic and Sahin, 2001. However, this should be taken with caution since the enthalpy of adsorption is the parameter that actually reflects the adsorption bond strength, rather than the standard Gibbs free energy of adsorption (Gharebaa and Omanovic, 2010).

3.5 Inhibition mechanism

Adsorption of CPD molecules can be described by two main types of interactions: physical adsorption and chemisorption. In general, physical adsorption requires the presence of both the electrically charged surface of the metal and charged species in solution.

A chemisorptions process, on the other hand, involves charge sharing or charge-transfer from the inhibitor molecules to the metal surface to form a coordinate type of a bond. This is possible in case of a positive as well as a negative charge of the surface. The presence of a transition metal, having vacant, low-energy electron orbitals (Cu⁺ and Cu²⁺) and of an inhibitor with molecules having relatively loosely bound electrons or hetero-atoms with a lone pair of electrons is necessary (Thomas, 1980).

In neutral NaCl solutions, pyrimidine heterocyclic derivative, CPD may be adsorbed on the copper surface in the form of neutral molecules involving the displacement of water molecules from the metal surface and sharing of electrons between the nitrogen atoms and the metal surface (chemical adsorption).

Also, the presence of pyrimidine heterocyclic derivative, CPD molecules may induce the formation of semi conductive copper oxides as in case of benzotriazoles (Baba and Kodama, 1999). This was possibly responsible for the improvement of corrosion resistance.

The type of intermediates that formed on Cu surface in 3.5% NaCl can be explained according to the potential-pH diagram. The presence of Cu₂O may facilitate adsorption via H-bond formation. Another possible mechanism, therefore may be adsorption assisted by hydrogen bond formation between unprotonated N, O and S atoms in pyrimidine heterocyclic derivatives molecule and the oxidized surface (Cu₂O) species. Unprotonated N, O and S atoms may adsorb by direct chemical adsorption or by hydrogen bonding to a surface oxidized species. The extent of adsorption by the respective modes depends on the nature of the metal surface. The adsorption layer acts as an additional barrier to the corrosive attack and enhances the performance of the passive layer as a result.

The criteria for inhibitor selection can also be inferred from above considerations. A good inhibitor must have strong affinity for the bare metal atoms. The requirement is different in the presence of the oxide species, a passive oxide film is formed on the electrode surface, where hydrogen bond formation accounts for most of the inhibition action. An effective inhibitor is one that forms hydrogen bonds easily with the oxidized surface. These findings could be further explained on the basis that in presence of dissolved oxygen (Amin and Khaled, 2010), where the metal surface is oxidized, the ability of a pyrimidine heterocyclic derivative, CPD to provide corrosion inhibition is related to its tendency to form hydrogen bonds with the oxide species on the metal surface. It is quite evident from the chemical structure of pyrimidine heterocyclic derivative molecule, CPD that it has NH bonds. These results confirmed the importance of hydrogen bonding in effective corrosion inhibition in presence of oxide species.