*3.4. Electrochemical—Open Circuit Potential Measurements*

The results of the open circuit potential measurements (OCP) of all types of bare or coated specimens after 1 or 4 days of exposure are shown in Figure 9.

**Figure 9.** Open circuit potential measurements of all types of bare or coated specimens after 1 day (**a**) and 4 days (**b**) of exposure in a corrosive environment of 3.5 wt % NaCl solution.

Open circuit potential measurements were used for a preliminary and indicative prediction of the corrosion behavior. As it is known, the general criterion for the estimation of corrosion behavior, based on open circuit potential values, is that the more anodic (lower negative value) the OCP, the lower the corrosion susceptibility. However, this criterion is not absolute. The estimation depends on the variations of the potential during the monitoring period and an indication of better corrosion behavior is the stability of the corrosion potential with time [2,16,23,29]. From the open circuit potential with time diagram, after 1 day of exposure (Figure 9), it can be seen that the more anodic value of OCP was obtained for the I.30E nanocomposite coating (−595 mV) and the corresponding more cathodic value was obtained for bare steel (−670 mV). All curves moved linearly towards the cathodic direction and oscillations were observed mainly in the case of the I.28E nanocomposite coating. This decrease of OCP indicated a continuous dissolution of the surface layers of the steel specimens. From the open circuit potential with time diagram, after 4 days of exposure (Figure 9), it can be seen that the more anodic value of OCP was obtained for both the I.30 and I.28 nanocomposite coatings (−625 mV) and the corresponding more cathodic value was obtained for bare steel (−680 mV). OCP curves of bare and resin coated specimens move initially in the anodic direction and after that continuously towards more cathodic values, indicating the formation of a non-passive oxide layer (in the case of the bare specimen) or the non-passive behavior of the coating (in the case of the resin coated specimen) that cracks and so the potential decreases and corrosion increases. The I.30 nanocomposite coating curve only decreased slightly (from −605 to −625 mV), indicating an increased stability of the coating and better corrosion behavior.

The electrochemical impedance response is a fundamental characteristic of an electrochemical system. Knowledge of the frequency dependency of impedance for a corroding system enables a model equivalent electrical circuit describing that system to be created [29–32]. Processing of the experimental data and fitting of the electrochemical impedance measurements was based on the equivalent circuit shown in Figure 10 for bare and coated steels, using a non-linear regression analysis. The results are presented in Figures 11–16 (Nyquist diagrams) and Table 4 (Resistance values).

**Figure 10.** Electrical equivalent circuit model simulating a corroding system metal/coating/electrolyte, *R*<sup>s</sup> = solution resistance, *R*1, *R*<sup>2</sup> = ohmic resistances, CPE1, CPE2 = constant phase elements with admittance *Y*0(*j*ω) *n*.

**Figure 11.** Nyquist plots of impedance spectra, *R*e (Z), real part of impedance, and *I*m (Z), imaginary part of impedance, for bare specimens after 0, 1, 2, or 4 days of exposure in a corrosive environment of 3.5 wt % NaCl solution.

**Figure 12.** Nyquist plots of impedance spectra, *R*e (Z), real part of impedance, and *I*m (Z), imaginary part of impedance, for epoxy coated specimens after 0, 1, 2, or 4 days of exposure in a corrosive environment of 3.5 wt % NaCl solution.

**Figure 13.** Nyquist plots of impedance spectra, *R*e (Z), real part of impedance, and *I*m (Z), imaginary part of impedance, for I.28 nanocomposite coated specimens after 0, 1, 2, or 4 days of exposure in a corrosive environment of 3.5 wt % NaCl solution.

**Figure 14.** Nyquist plots of impedance spectra, *R*e (Z), real part of impedance, and *I*m (Z), imaginary part of impedance, for I.30 nanocomposite coated specimens after 0, 1, 2, or 4 days of exposure in a corrosive environment of 3.5 wt % NaCl solution.

**Figure 15.** Nyquist plots of impedance spectra, *R*e (Z), real part of impedance, and *I*m (Z), imaginary part of impedance, for bare and coated specimens after 1 day of exposure in a corrosive environment of 3.5 wt % NaCl solution.

**Figure 16.** Nyquist plots of impedance spectra, *R*e (Z), real part of impedance, and *I*m (Z), imaginary part of impedance, for bare and coated specimens after 4 days of exposure in a corrosive environment of 3.5 wt % NaCl solution.


**Table 4.** Resistance values (Ω cm2) for bare and coated specimens.

From the results of the electrochemical impedance measurements (Figures 11–16 and Table 4), it follows that all the coatings used increased the total resistance and both nanocomposites had greater total resistance values than the pristine glassy epoxy polymer, indicating improved protection properties. Thus, the total resistance value, Rtot, after 4 days exposure in the corrosive environment, increased from 1.03 × <sup>10</sup><sup>2</sup> of bare steel, to 5.34 × 103 in the case of pristine resin, to 7.40 × <sup>10</sup><sup>3</sup> in the case of I.28E, and to 2.96 × <sup>10</sup><sup>4</sup> (<sup>Ω</sup> cm2) in the case of I.30E coated specimens. In the case of bare steel, the total resistance values decrease continuously with exposure time and the relation between these two factors was observed to be linear, indicating a constant corrosion rate. A high total resistance value in the case of pristine resin coated steel in conditions of no prior exposure in the corrosive environment (0 days) was observed, 6.80 × 105 <sup>Ω</sup> cm2, that decreased quickly in any conditions of exposure, 5.34 × <sup>10</sup><sup>3</sup> <sup>Ω</sup> cm<sup>2</sup> after 4 days, indicating high corrosion rate values and the fast evolution of corrosion. In the case of nanocomposite coated steel, the total resistance values decreased after four days of exposure, from 7.99 × 104 to 7.40 × 103 <sup>Ω</sup> cm2 in the case of I.28E and from 2.03 × 105 to 2.96 × <sup>10</sup><sup>4</sup> <sup>Ω</sup> cm2 in the case of I.30E, as it was expected due to corrosion initiation. However, the decrease is much smaller and so the corrosion evolution is also lower.

The superior protection performance, indicated from the higher total resistance value, of the epoxy nanocomposite with the I.30E organo-clay compared to that with the I.28E organic-clay, can be attributed to the higher dispersion of the clay nanolayers in the former nanocomposite (XRD and TEM results), which also induced slightly improved barrier properties.

#### **4. Conclusions**

The mechanical, thermomechanical, and barrier properties of all the epoxy—organoclay nanocomposites were improved compared to those of the pristine epoxy polymer. Both the pristine epoxy and the epoxy nanocomposite coatings offered substantial protection to steel from corrosion. The protective properties of the nanocomposite coatings were superior compared to those of the pristine epoxy polymer, as it was revealed from the weight loss results, the optical and microscopy examination of the specimens after the exposure in the corrosive environment, the open circuit potential measurements, and the electrochemical impedance spectroscopy measurements. The protective properties of the nanocomposite coatings varied with the organo-clay used. The epoxy—montmorillonite clay modified with primary octadecylammonium ions, Nanomer I.30E, had a better behaviour than the clay modified with quaternary octadecylammonium ions, Nanomer I.28E. This was attributed to the higher dispersion of the nanolayers in the nanocomposite formed

with the I.30E organoclay compared to that formed with I.28E. The enhanced mechanical properties and thermal stability of both epoxy—clay nanocomposites, in combination with their high protection efficiency, renders them as attractive candidates for various demanding coating applications.

**Author Contributions:** Panagiotis Giannakoudakis, Konstantinos Triantafyllidis and Panagiotis Spathis conceived and designed the experiments; Domna Merachtsaki and Panagiotis Xidas performed the experiments; Domna Merachtsaki, and Panagiotis Xidas analyzed the data; Panagiotis Giannakoudakis, Konstantinos Triantafyllidis and Panagiotis Spathis contributed reagents/materials/analysis tools; Domna Merachtsaki, Panagiotis Xidas and Panagiotis Spathis wrote the paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
