*3.2. Electrochemical Methods*

3.2.1. Electrochemical Impedance Spectroscopy (EIS)

The coating efficiencies of pure PS and all PS nanocomposite coatings on C-steel rods were investigated using electrochemical impedance spectroscopy (EIS), which is a vital tool in electrochemistry research. It is a strong nondestructive tool for examining and evaluating various electrical characteristics of materials [2–4]. Figures 12 and 13 show Nyquist plots of pure PS and all samples of commercial and local clay PCNs, as well as similar circuits. Table 1 shows the EIS parameters as corrosion resistance (RCorr or Rct), electrical double-layer capacitance (Cdl or CCorr), coating resistance (Rpo) (pore resistance), and coating capacitance (CC). As illustrated in Figures 12 and 13, these parameters were derived by fitting the Nyquist plots of all the coated C-steel samples that had undergone testing to an analogous circuit using Gamry software.

**Figure 11.** TEM micrographs of pure OC and 1–5 wt.% PCN for RCKh at high magnifications.

**Table 1.** EIS data of bare C-steel, pure PS, and 1–5 wt.% PCN coating from commercial Indian and local Khulays clay in 3.5% NaCl solution at 30 ◦C.


**Figure 13.** EIS plots of coated C-steel by pure PS and all prepared (1–5% PCNs for RCKh in 3.5% NaCl solution at 30 ◦C.

The fitted EIS data of bare C-steel and PS and the 1–5% PCN data for both types of clay are presented in Table 1. It is noticed that the coating of C-steel with PS greatly increased the RCorr values and decreased the CCorr values relative to bare C-steel. As reported in the literature for the coated substrates, the first semicircle in the high-frequency region was related to the resistance and capacitance of the protective coating and its properties [13–15,22]. The second semicircle in the low-frequency region was attributed to the electrochemical reactions on the C-steel surface. The findings demonstrated that, in comparison with pure PS coating, the introduction of a tiny amount of OC to the PS polymer enhanced the corrosion resistance (RCorr) and pore resistance (Rpo) of the prepared PCNs. It was realized that, as the diameter of the second semicircle increased, so did the values of the RCorr and Rpo. By adding the small percentage of OC, the values of the corrosion and coating capacitance (CCorr, CC) decreased. These observations for the RCorr, Rpo, CCorr, and CC values indicate that the PCN coatings have higher corrosion protection

than pure PS coating. Generally, this indicates that the protective properties of pure PS coating were improved by adding OC content. This behavior was observed for all the prepared PCNs from commercial Indian clay and Khulays clay. In the present study, our impedance results are in agreemen<sup>t</sup> with many previous studies [2,3,19,23,24]. Moreover, adding the organic form of the commercial Indian clay at 1% PCN increased the corrosion resistance value (RCorr) of the coating by about three times the value of the local Khulays clay at 1% PCN. The value of the RCorr in the case of the commercial Indian clay was 6.26 M <sup>Ω</sup>·cm2, while in the case of the local Khulays clay, the value was 2.46 M Ω·cm<sup>2</sup> This was probably due to the high montmorillonite content in the CCIn, which could be easily exfoliated and allows for maximum protection.

## 3.2.2. EFM Method

The electrochemical frequency modulation (EFM) method is considered a new nondestructive way of electrochemical corrosion monitoring. Consequently, it is a good choice for many metals and metal alloys in different aqueous corrosion systems. The corrosion parameters of the EFM test are good for comparison with the results from the Tafel method and linear polarization resistance [40,41]. The results from the EFM are included in Table 2, with the potentiodynamic polarization results prepared from 1–5 wt.% PCNs for commercial Indian (CCIn) and local Khulays (RCKh) clay in 3.5% NaCl solution at 30 ◦C. The percentage of coating, or the protection efficiency, was calculated from the following equation:

$$\% \text{PE} = \frac{(\text{Icorr}(\text{PS}) - \text{Icorr}(\text{PCN}))}{\text{Icorr}(\text{PS})} \tag{2}$$

**Table 2.** EFM data of bare C-steel, pure PS, and 1–5% PCN coating from commercial Indian and local Khulays clay in 3.5% NaCl solution at 30 ◦C, and relative protection efficiencies (%PE) calculated using ICorr.


The EFM parameters (the corrosion current (ICorr), corrosion potential (ECorr), corrosion rate (CR), and calculated relative coating efficiency from the ICorr values) are summarized in Tables 2 and 3. It can be noticed that the coating of C-steel with PS greatly decreased the CR and ICorr values relative to the bare C-steel. Moreover, the PCN coating further decreased the corrosion rate and increased the coating efficiency for both clay types. For the EFM in Table 2, the parameters are the current density in μA/cm<sup>2</sup> and the corrosion rate in MPY (milli-inch per year). The goodness of fit for the EFM method is presented in causality factors 2 and 3. Almost all of our results fit this criterion. It is shown in Table 2 that for the 1% PCN of CCIn, the CR is 5.920 × 10−<sup>4</sup> mpy, which is the lowest CR of all the formulations.

#### 3.2.3. Potentiodynamic Polarization (Tafel Plots)

Tafel plots are represented in Figures 14 and 15 for the coated C-steel samples prepared from PCNs prepared from CCIn and RCKh, respectively, in 3.5 wt.% NaCl solution at 30 ◦C. The cathodic and anodic curves were analyzed directly from a Gamry potentiostat/galvanostat to determine the Tafel corrosion parameters.

**Figure 14.** Tafel plots of coated C-steel by pure PS and from 1–5 wt.% PCN prepared from commercial Indian clay (CCIn) in 3.5% NaCl solution at 30 ◦C.

**Figure 15.** Tafel plots of coated C-steel by pure PS and 1–5 wt.% PCN prepared from local Khulays clay (RCKh) in 3.5% NaCl solution at 30 ◦C.

**Table 3.** Tafel data of bare C-steel, pure PS, and 1–5% PCN coating from CCIn and RCKh in 3.5% NaCl solution at 30 ◦C, and the relative protection efficiencies (PE%) calculated using ICorr.


In Table 3 for the Tafel plots, the parameters are the current density in μA/cm<sup>2</sup> and the corrosion rate in mpy. For Tafel, the values of the chi-square values are included, which reflect the goodness of fit of the present data, which lies in the accepted range. As in the EFM, the best coating efficiency is shown for the 1% PCN of CCIn.

The Tafel plot results showed that the PCN-coated C-steel samples had smaller ICorr and CR values than the PS-coated sample. This indicates that an incorporation of OC in the PS matrix enhances the protective properties of this polymer as a coating for C-steel. This behavior is consistent with previous studies that evaluated the effect of adding small amounts of clay to different types of polymers, such as polyurethane [13], polyaniline [16,23], polypyrrole [22], polystyrene [17,18], and epoxy coating [9,24]. Moreover, the trend in the Tafel results was in agreemen<sup>t</sup> with the EFM investigation. According to the EIS, EFM, and Tafel results, the PCN-coated C-steel samples exhibited better corrosion resistance and coating efficiency than the pure PS coating. The best electrochemical protection efficacy was observed at 1% PCN for both types of clay. This is most likely due to the good dispersion of the 1 wt.% organoclay layers in the PS chains, as demonstrated by the XRD and TEM results. Other research groups [13,17] have obtained greater protection efficiencies at 3 wt.% organoclay with polymers. In the case of 1% PCN, however, the fully exfoliated structure was obtained by comparing the electrochemical results with the characterization method results (XRD, TEM). However, the corrosion protection performances of the 3% and 5% PCNs were worse than that of the 1% PCN. This suggests that the results of the characterization methods are consistent with the electrochemical investigations (EIS, EFM, and Tafel). The order of the coating efficiency for both clay types from Tables 2 and 3 is as follows:

$$1\% \text{ PCN} > 3\% \text{ PCN} > 5\% \text{ PCN}$$
