Formation, characterization and corrosion protection efficiency of self-assembled 1-octadecyl-1H-imidazole films on copper for corrosion protection

2.4.1 Electrochemical impedance studies

Impedance studies of bare copper and OI film covered copper electrodes were carried out in aqueous NaCl solution. The electrode potential was allowed to stabilize for 1 h, since the open circuit potential became stable within 1 h. These studies were carried out in the frequency range from 60 kHz to 10 mHz under excitation of a sinusoidal wave of 5 mV amplitude. The impedance data were analysed with THALES software and fitted to the appropriate equivalent circuits. The impedance studies were first carried out to develop the optimum conditions for the formation of protective film on copper surface. These studies were also carried out in order to evaluate the corrosion protection ability of OI film in an aggressive environment viz. aqueous NaCl at different concentrations (0.02–0.20 M) and at different immersion periods (1–24 h).

2.4.2 Potentiodynamic polarization studies

Potentiodynamic polarization studies were performed in the potential range of −0.700 V to +0.700 V vs. Ag/AgCl (3 M KCl) at a scan rate of 2 mV s⁻¹. These studies were carried out in aqueous NaCl environment at different concentrations (0.02–0.20 M).

2.4.3 Cyclic voltammetric studies (CV)

Cyclic voltammetric studies were carried out in 0.02 M aqueous NaCl solution in the potential range of −0.400 to +0.350 V vs. Ag/AgCl (3 M KCl) at three different sweep rates of 30, 60 and 120 mV s⁻¹ for two cycles at each sweep rate. The CV experiments were also carried out for 15 cycles at a constant sweep rate of 30 mV s⁻¹ in the same potential range in order to study the stability of the OI film.

2.6.1 X-ray photoelectron spectral studies

The XPS spectra of the bare copper and OI film covered copper were taken using the X-ray photoelectron spectrometer, ESCA Kratos model AXIS-165, with monochromatic Mg Kα source (1253.6 eV) with a resolution of 0.1 eV. Computer deconvolution was applied to identify the elemental peaks of copper, oxygen, carbon and nitrogen present in the film.

2.6.2 Reflection absorption FTIR spectral studies

Reflection absorption FTIR spectra for bare copper and copper covered with OI film were recorded in single reflection mode using FTIR spectrometer, Perkin Elmer Spectrum 100S in the spectral range of 400–4000 cm⁻¹ with a resolution of 4 cm⁻¹. Bare copper and OI film covered copper specimens were mounted on the reflection accessory and the plane polarized light was incident at a grazing angle of 80° from the surface normal. The sample compartment was continuously purged with nitrogen during the measurement.

2.6.3 Contact angle measurements

The contact angle measurements for bare copper and for copper covered with OI film were made by the sessile water drop method using a contact angle measuring system, model G10, Kruss, Germany.

2.6.4 Atomic force microscopy (AFM) studies

Veeco Nanoscope IV multimode AFM was used to study the surface morphologies of bare copper and OI modified copper. The AFM was used in contact mode between a silicon nitride tip attached to a micro cantilever and the surface of the sample.

2.6.5 Scanning electron microscopy (SEM) studies

SEM studies for bare copper and copper modified with OI film were carried out after immersing the specimens in 0.02 M aqueous NaCl environment for a period of 10 days. These studies were carried out by using TESCAN VEGA 3 scanning electron microscope.

3.1.1 Effect of concentration of OI

Quan et al. studied the self-assembled film of Schiff base on copper surface. They reported that the best immersion time was more than 20 h in order to form a protective film (Quan et al., 2001). Our results showed that an immersion period of 48 h facilitates formation of a protective film. Therefore, in the present study an immersion period of 48 h was chosen initially and OI films were formed on copper surface at various concentrations of OI (5.0–15.0 mM). Impedance studies were carried out in aqueous 300 ppm chloride environment. Nyquist and Bode plots for the OI modified copper electrode are obtained after an immersion period of 48 h and are shown in Fig. 1A and B, respectively. It is found that with an increase in concentration of OI from 5 to 15 mM the diameter of the semicircle is increased in the Nyquist plots and phase angle vs. frequency Bode plots show an increase in phase angle maximum from −72° to −82° and it is broadened in a wide frequency range. The corresponding impedance parameters are obtained by the best fit of the impedance plots using the equivalent circuit models shown in Fig. 2A and B. The circuit shown in Fig. 2A, with the Warburg element, is used in the case of OI film covered copper, where the Warburg impedance is seen in the Nyquist plots. In Fig. 1A, at 5, 10 mM concentrations, the Nyquist plot shows slightly Warburg behaviour. This indicates that at these concentrations, there is diffusion of chloride ions from bulk of the solution to copper surface and diffusion of Cu⁺ ions from the surface to bulk of solution. That means, the OI film is not protective. Whereas, at 15 mM concentration of OI, the Warburg behaviour is completely absent. This is an indication that protective film is formed at this concentration. The impedance parameters are shown in Table 1. With an increase in concentration from 5 to 15 mM, the charge transfer resistance (R_ct) value increased from 1411 to 7443 kΩ cm². The C_dl value is decreased from 42.00 to 7.55 nF cm⁻² and the n value is increased from 0.84 to 0.93. The high R_ct, n values and lower C_dl value at 15 mM concentration reveal the dense and protective film formation on copper surface. Therefore, 15 mM concentration of OI was chosen as the optimum concentration to form the OI film.

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Figure 1 (A) Nyquist, (B) Bode plots of copper covered with OI film formed in different concentrations of OI [environment: 300 ppm chloride, temperature: 30 °C].

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Figure 2 Equivalent circuits used in impedance measurements of (A) copper covered with OI film with Warburg, (B) Copper covered with OI film.

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3.1.2 Effect of immersion period

By fixing the concentration of OI as 15 mM, self-assembled films were formed on copper surface at different immersion periods in the range 12–48 h. Impedance studies were carried out in aqueous 300 ppm chloride environment and the corresponding Nyquist and Bode plots are shown in Fig. 3A and B. The corresponding impedance parameters are shown in Table 2. The results indicate that with an increase in immersion period from 12 to 48 h, the R_ct value is increased from 1310 to 7443 kΩ cm². However, the increase in R_ct value from 36 to 48 h is relatively less. The results of impedance studies infer that there is a formation of a protective film after an immersion period of 48 h.

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Figure 3 (A) Nyquist, (B) Bode plots of copper covered with OI film formed in 15 mM OI at different immersion periods. [environment: 300 ppm chloride, temperature: 30 °C].

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From the above studies it is inferred that the optimum concentration of OI and immersion period for the formation of OI films on copper surface are (i) 15 mM concentration of OI in methanol and (ii) 48 h immersion period. These conditions were used for the formation of OI films on copper surface throughout the studies.

3.2.1 X-ray photo electron spectroscopic studies

The peaks due to Cu 2p, C 1s and O 1s electrons are detected in the XPS survey spectrum of bare copper and the corresponding computer deconvolution spectra are shown in Fig. 4(A–C), respectively. The Cu 2p_3/2 peak at a binding energy of 932.6 eV and the Cu 2p_1/2 peak at 952.4 eV can be attributed to Cu (I) (Wang et al., 2002). The binding energy of C 1s electron at 285 eV corresponds to contaminant carbon, which is likely due to cracking of vacuum oil used in the XPS instrument (Cicileo et al., 1999). The O 1s peak at 531.1 eV is due to the formation of Cu₂O on the copper surface (Ye et al., 1998; Petkova et al., 1998), which is formed during the interval between polishing of the copper surface and the XPS analysis.

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Figure 4 XPS deconvolution spectra of different elements present on the surface of bare copper. (A, Cu 2p; B, C 1s; C, O 1s).

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The XPS survey spectrum of copper surface covered with OI film shows peaks due to Cu 2p, O 1s, C 1s and N 1s and their computer deconvolution spectra are shown in Fig. 5(A–D) respectively. Cu 2p spectrum shows peaks due to Cu 2p_3/2 and Cu 2p_1/2 at 933.2 and 953.4 eV, respectively, which are due to the initial oxidation of copper surface to Cu₂O during film formation. Kamdem et al. reported in their studies that the Cu 2p_3/2 peak at 935 eV and the presence of shake up satellites indicate the presence of cupric copper, while the peak around 933 eV and the absence of shake-up satellites indicate the presence of cuprous copper (Kamdem et al., 2001). Therefore, it can be inferred that the peak observed in our studies at 933.2 eV without any shake-up satellites is due to the presence of cuprous copper. O 1s spectrum shows a peak at 531.9 eV, which corresponds to oxygen of Cu₂O formed on copper surface. The C 1s shows three peaks one each at 284.6, 286.3 and 287.7 eV. The intense peak at 284.6 eV is due to the presence of 18 carbon atoms in the alkyl chain of OI film (Taneichi et al., 2001; Hutt and Liu, 2005). The C 1s peak at 286.3 eV arises due to the contaminated carbon, which is likely due to cracking of vacuum oil used in the XPS instrument (Cicileo et al., 1999). Another C 1s peak at 287.7 eV corresponds to the carbons present in the imidazole ring. The N 1s spectrum shows two peaks, which are due to the presence of nitrogen atoms in OI molecule in different chemical environments. The peak at 399.6 eV is due to nitrogen in the first position and the peak at 401.7 eV is due to nitrogen in the third position of OI molecule. This can be explained as follows. Nitrogen in the third position can donate the electron pair easily to form a bond with copper surface. Therefore, the electron density on this nitrogen is reduced and the binding energy is shifted towards a higher value of 401.7 from 398 eV. Whereas, the nitrogen in the first position is attached with long alkyl chain, which being an electron releasing group, adds to the electron density at this nitrogen. Therefore, in comparison to the nitrogen in the third position, nitrogen in the first position has more electron density and hence the observed less shift in binding energy to 399.6 from 398 eV. The characteristic binding energy of the elemental nitrogen was reported at 398.0 eV in the literature (Beccaria and Bertolotto, 1997). The shift in elemental binding energies of N1s reveals that the nitrogen atoms present in OI molecule are involved in the complex formation with cuprous ions (Cicileo et al., 1999).

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Figure 5 XPS deconvolution spectra of different elements present on the surface of copper covered with OI film. (A, Cu 2p; B, O 1s; C, C 1s; D, N 1s).

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3.2.2 Reflection absorption FTIR spectral studies

Fig. 6A and B shows the reflection absorption FTIR spectra for bare copper and the copper covered with OI film respectively. For bare copper, the spectrum shows a peak at 445 cm⁻¹, which is assigned to cuprous oxide on the surface (Papadimitropoulos et al., 2005). This peak is absent in the spectrum of the copper surface covered with OI film. On the other hand, the spectrum of copper covered with OI film exhibits two bands, one at 2856 cm⁻¹ and the other at 2922 cm⁻¹, which are due to CH₂ symmetric stretch and asymmetric stretch modes respectively. Yoshida and Ishida studied FTIR reflection absorption spectrum of undecylimidazole on copper surface. They obtained two bands at 2853 and 2925 cm⁻¹ and interpreted them to CH₂ symmetric stretching and asymmetric stretching modes respectively (Yoshida and Ishida, 1995). Thus, there is a clear evidence of the presence of aliphatic hydrocarbon chain in the film. The FTIR reflection absorption spectrum of OI film covered copper also shows C⚌N stretching band at 1436 cm⁻¹. It infers the formation of a film of OI on the copper surface.

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Figure 6 Reflection absorption FTIR spectra of (A) bare copper (B) OI modified copper.

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3.2.3 Atomic force microscopy

The surface topography of polished copper and OI modified copper is studied by AFM. Fig. 7A and B shows the AFM images namely 3D-Topography, flatten and section analysis of the polished and OI film covered copper respectively. The root mean square (rms) roughness measurements were carried out for both polished and OI film protected copper. The vertical lines in the polished copper sample are due to the fine scratches obtained during the polishing process (Kleber et al., 2007). These polishing scratches result in a rms roughness of 17.27 nm. The AFM image of copper surface covered with OI film clearly shows that there is an ordered film of OI formed and there is a reduction in the rms roughness to 13.92 nm. A uniform variation of the thickness in section analysis also indicates homogeneity of the OI film.

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Figure 7 AFM images of (A) bare copper (B) OI film covered copper.

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3.2.4 Contact angle measurements

Fig. 8A and B shows the images of the sessile water drop on the bare and OI modified copper surfaces respectively. These measurements were carried out to determine the wetting properties of the bare and OI modified copper surfaces. The contact angle values for bare copper and OI modified copper are found to be 78° and 104°, respectively. Contact angle measurement data show a dramatic improvement in the hydrophobicity of the OI film covered copper when it is compared to unmodified copper. The hydrophobic part plays an important role in corrosion inhibition by the inhibitor molecules. Non-polar interactions between the long alkyl chains of the molecules due to van der Waals forces also are responsible for protective quality of the film. The hydrophobicity of the OI film is greater than that of the film formed by 4-aminobenzenthiol (Tan et al., 2006), 5-methoxy-2-(octadecylthio)benzimidazole (Appa Rao et al., 2010) and 2-(octadecylthio)benzothiazole (Appa Rao et al., 2009).

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Figure 8 Contact angle images of (A) bare copper (B) OI film covered copper.

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3.3.1 Electrochemical impedance studies

Fig. 9A and B shows the Nyquist plots of bare copper and OI modified copper after 1 h immersion in different concentrations (0.02–0.20 M) of aqueous NaCl solution. The corresponding Bode plots are shown in Fig. 10A and B. The impedance parameters obtained from these results are shown in Table 3. EIS data of copper covered with OI film are best fitted by using the equivalent circuit shown in Fig. 2B whereas, for the bare copper in NaCl solution, the plots are best fitted by using the equivalent circuit shown in Fig. 11.

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Figure 9 Nyquist plots of (A) bare copper (B) OI film covered copper in different concentrations of NaCl [immersion period: 1 h, temperature: 30 °C].

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Figure 10 Bode plots of (A) bare copper (B) OI film covered copper in different concentrations of NaCl [immersion period: 1 h, temperature: 30 °C].

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Figure 11 Equivalent circuits used in impedance measurements of bare copper.

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The small high frequency semicircle, which is observed in the Nyquist plots for bare copper electrode in NaCl solution is attributable to the time constant of charge transfer resistance and double layer capacitance (C_d1) (Feng et al., 1996). In the high frequency region, the electrode reaction is controlled by a charge transfer process and the diameter of the semicircle represents the charge transfer resistance. The Warburg impedance in the low frequency region is ascribed to the diffusion process of soluble copper species from the electrode surface to the bulk solution and the diffusion of dissolved oxygen from the bulk solution to the electrode surface.

The Nyquist plots of OI film covered copper are quite different from those of the bare copper electrodes. For the OI film covered copper electrode, the Warburg impedance almost disappeared at lower frequencies, indicating that the film is sufficiently densely packed to prevent the diffusion of oxygen or Cl⁻ ion to the copper substrate and thus inhibits corrosion of copper. The Bode plots of phase angle vs. frequency and impedance vs. frequency show characteristic differences between the bare copper and copper covered with OI film. In 0.02 M NaCl solution the Bode plot of bare copper exhibits a phase angle of −45°, whereas for the OI film modified copper, the phase angle maximum is −84° and is shifted towards higher frequency side. R_ct value for bare copper in 0.02 M NaCl is 2.92 kΩ cm², which is increased to 6754 kΩ cm² for copper covered with OI film in the same environment. The C_dl value at the copper/0.02 M NaCl interface is decreased from 6162 nF cm⁻² in the case of bare copper to a very small value of 15.0 nF cm⁻² in the case of copper covered with OI film. The value of n is increased from 0.50 to 0.94 in the presence of OI film. All these results indicate that the film is sufficiently densely packed to prevent diffusion of oxygen and other chloride ions onto the copper surface and thus protects copper from corrosion. The inhibition efficiencies were calculated from the charge transfer resistance data using the Eq. (2).

Fig. 12A and B shows the Nyquist plots of bare copper and OI film covered copper in 0.02 M NaCl solution at different immersion periods (1–24 h). The corresponding Bode plots are shown in Fig. 13A and B. The impedance parameters obtained from these results are shown in Table 4. The R_ct values of OI film covered copper are much higher than those of bare copper at all the immersion periods. The phase angle vs. frequency Bode plot of OI modified copper show a clear merger of the plots at the phase maxima. The decrease in R_ct values is very less from 1 h immersion time to 24 h. C_dl, C_film and n values remain almost constant throughout the immersion period. These results reveal that the OI film is highly homogeneous and stable even after 24 h immersion time. The inhibition efficiencies are found to be 99.9% in 0.02 M NaCl environment at all the immersion periods.

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Figure 12 Nyquist plots of (A) bare copper (B) OI film covered copper in 0.02 M NaCl at different immersion periods [temperature: 30 °C].

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Figure 13 Bode plots of (A) bare copper (B) OI film covered copper in 0.02 M NaCl at different immersion periods. [temperature: 30 °C].

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3.3.2 Potentiodynamic polarization studies

Fig. 14A and B shows the potentiodynamic polarization curves of bare copper and OI film covered copper respectively. These studies were carried out in aqueous NaCl solution over a concentration range of 0.02–0.20 M after an immersion period of 1 h. The corrosion parameters are determined from the polarization curves by the Tafel extrapolation method. These values are shown in Table 5. Lower corrosion current density (j_corr) values are obtained for copper covered with OI film in comparison with the bare copper at all concentrations. The decrease in j_corr value is mainly due to the decrease of attack of chloride ions on the copper surface. With an increase in NaCl concentration, j_corr values are increased for both bare copper and OI film covered copper. The OI film shifts the corrosion potential of the copper electrode marginally to the cathodic side. The cathodic current for the OI film covered copper is much lower than the cathodic current of bare copper. The same is true at all the concentrations of the corrosive environment. These results infer that the OI film effectively retards the cathodic reduction of oxygen and thus protects copper from corrosion. The anodic polarization curve of bare copper shows a current hump around 150 mV. This is due to the formation of CuCl, which is an insoluble adsorbed species. With increasing the potential further towards anodic direction, current increases due to the formation of soluble Cu (II) species (Kear et al., 2004). The OI film covered copper shows a small current hump around −50 mV. It indicates less attack of chloride ions against copper in the presence of OI film. This result indicates the protective nature of OI film on copper surface. The inhibition efficiency was calculated by using the Eq. (3)

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Figure 14 Potentiodynamic polarization curves of (A) bare copper (B) OI film covered copper in different concentrations of NaCl. a, 0.02 M; b, 0.05 M; c, 0.10 M; d, 0.15 M; e, 0.20 M [immersion period: 1 h, temperature: 30 °C].

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3.3.3 Cyclic voltammetric studies

Fig. 15A and B shows the cyclic voltammograms of bare copper and OI film covered copper for 2 cycles at different sweep rates of 30, 60 and 120 mV s⁻¹. Bare copper electrode exhibits an oxidation peak at 0.150 V vs. Ag/AgCl (3 M KCl) in forward scan and a reduction peak at 0.160 V in the reverse scan. The anodic peak corresponds to the formation of CuCl adsorbed layer as reported under Section 3.3.2. With an increase in potential further in the anodic direction, current increases exponentially due to the formation of soluble ${\text{CuCl}}_2^{-}$ complex (Wang et al., 2003). The large reduction peak corresponds to the reduction of soluble ${\text{CuCl}}_2^{-}$ complex and the CuCl layer formed on the copper surface to Cu. In case of OI modified copper no oxidation peak is observed in the forward scan. This result reveals the protective nature of OI film. However, there is limiting current at 0.350 V due to the formation of soluble ${\text{CuCl}}_2^{-}$ complex. Reduction peak is observed at 0.006 V in the reverse scan, which can be understood because of reduction of ${\text{CuCl}}_2^{-}$ complex to Cu. Fig. 16A and B shows the cyclic voltammograms of bare copper and OI film covered copper for 15 cycles at a constant sweep rate of 30 mV s⁻¹. There is no oxidation peak even after 15 cycles in case of OI modified copper. That means OI film is quite stable. Thus, CV studies also provide the evidence for the protection of copper surface against corrosion by OI film in aqueous NaCl environment.

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Figure 15 Cyclic voltammograms in 0.02 M NaCl after 1 h immersion at different sweep rates (a, 30; b, 60; c, 120 mV s⁻¹) (A) bare copper (B) OI modified copper.

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Figure 16 Cyclic voltammograms in 0.02 M NaCl after 1 h immersion for 15 cycles at 30 mV s⁻¹ (A) bare copper (B) OI film covered copper.

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3.3.4 Weight-loss studies

Results of weight-loss studies for bare copper and OI covered copper after an immersion period of 10 days in 0.02 M aqueous NaCl show corrosion rates of 0.0641 mmpy for former and of 0.0029 mmpy for the latter. These data result in an inhibition efficiency of 95.5%, which shows that OI film protects copper quite effectively even after 10 day immersion time.

3.3.5 Scanning electron microscopy (SEM) studies

Scanning electron micrographs of bare copper and copper covered with OI film after immersion in 0.02 M aqueous NaCl for a period of 10 days are shown in Fig. 17A and B, respectively. The SEM image of bare copper shows pits as a consequence of corrosion process. However, in the case of OI film covered copper the surface morphology shows dense and ordered layer like structure without any corrosion products.

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Figure 17 SEM images after 10 day immersion in 0.02 M NaCl (A) bare copper (B) OI film covered copper.

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