# Recent Trends and Progress in Corrosion Inhibitors and Electrochemical Evaluation
## 5 Corrosion Monitoring Techniques
### Electrochemical Impedance Method  
Since the EIS approach can explain the electrochemical mechanisms taking place at an electrified interface in just one measurement, it is widely applicable. The invention of certain devices, such as the first potentiostat and the revolving disc electrode, has benefited EIS. It also allows for a better understanding of intermediate adsorption on electrode surfaces. In a multistep mechanism involving electrochemical reactions coupled with adsorbed reaction intermediates, and coupled with homogeneous or heterogeneous reactions, the intermediate adsorption on an electrode surface is more thoroughly analyzed with EIS [129]. The ability to conduct measurements at various potentials is a special benefit of EIS [130]. The measurement system contains the sample, electrodes, corrosive media, and a potentiostat, as shown in Figure 12. Recently, [PERSON] et al. [131] reported the anticorrosion behavior of pure acrylic and inhibitor-loaded dendrimer-like mesoporous silica nanocontaniers (DMSN) with the help of EIS (PARAST 2273 software). The obtained impedance spectra were depicted in the form of Bode plots. The Bode plot's single sloped line indicated the presence of a single time constant. The second slope appeared after prolonged exposure to NaCl solution on the coating, which suggests propagation of the corrosion process. They observed capacitance and resistance values with immersion time. Based on these results, they concluded that the charge transfer reaction's two-time constants were on the impedance curve. The lowered resistance values discovered during this evaluation were associated with corrosion-induced deterioration of the acrylic cover [131]. [PERSON] et al. [132]. carried out EIS studies after immersion of substrates (matrix sol-coated and HN-sol-coated and self-healing (SH)-coated substrates) in 3.5 wt% NaCl over various time periods, resulting in different Nyquist graphs for all the samples. As the exposure time increases, the impedance values of SH-coated substrates increase more than those of the other substrates, and these findings suggest that the applied coating is acting as a good passivate for the metal. While the EIS value of the other two coatings decreased on a high duration of exposure to corrosive media, this shows that the barrier protection lasts only for a shorter time [133]. The efficiency of epoxy and sphere-like PANI-SiO\({}_{2}\) reservoirs (PSR) coating is evaluated by EIS by [PERSON] et al. [134] They studied EIS on a MS surface in 3% NaCl after coating epoxy resin and with/without PSR. As the time of exposure to NaCl media increased, there was a decrease in the coating resistance: initially it was 6.03 \(\times\) 109 \(\Omega\) cm\({}^{2}\), while after 30 days it reached 1.13 \(\times\) 105 \(\Omega\) cm\({}^{2}\), indicating the corrosion rate. Furthermore, after ten days of evaluation,  
Figure 12: Schematic diagram of the electrochemical impedance measurement system. the impedance showed that a charge transfer reaction had occurred, which is caused by iron breakdown and the presence of two-time constants [135]. Some of the recent developments in the corrosion inhibitor-related literature are mentioned below. The carbon steel Nyquist plot displays, as shown in Figure 13, a slightly depressed semi-circular curve, indicating that the charge transport mechanism controls carbon steel corrosion in the 1 M HCl solution primarily. The capacitance loop of the oxide layer that covers the surface of carbon steel is coupled to the first time constant, which appears in the mid-frequency region. The second time constant, which is low frequency, was attributed to an inductive loop that was produced either by the surface dissolution of the carbon steel oxide layer or relaxation of the relaxing of the adsorbed extract Eruca sativa seeds. After the extract was included into the mixture, the diameter of the semi-circle increased, and the corrosion of carbon steel was delayed by decreasing the constant phase element (CPE) values and raising the polarization resistance, Rp.