# Recent Trends and Progress in Corrosion Inhibitors and Electrochemical Evaluation
## 5 Corrosion Monitoring Techniques  
Corrosion monitoring programs are now being used more and more in a variety of huge infrastructures, including pipelines, refinery facilities, offshore structures, bridges, buildings, etc., as well as in electronic devices, instrument panels, big computers, and biological implants. To keep track of corrosion activity, a variety of corrosion monitoring approaches can be used. These methods range in complexity from simple to complex, relying on a variety of sensing concepts, from the most basic corrosion coupons to sophisticated, computer-controlled electrochemical probes [126]. The corrosion coupons, such as electrical resistance (ER), linear polarization resistance (LPR), potential monitoring (PM), zero resistance ammeter (ZRA), and ultrasonics are the techniques for corrosion monitoring that are most frequently employed in industry. New electrochemical methods are beginning to emerge, including electrochemical noise (EN), harmonic impedance spectroscopy (HIS), and electrochemical impedance spectroscopy (EIS). All of the abovementioned techniques are used to monitor the corrosion activity. However, even though these techniques have advantages, there are also some disadvantages.  
The prolonged reaction time of the ER sensors is their primary flaw. The thickness of a sensor element affects the sensitivity of an ER sensor; a thicker sensor element has a slower reaction time. A significant change in resistance can only take place after enough corrosion has occurred [127]. In order to acquire reliable (useful) data, the linear polarization resistance monitoring method must not be used with gasses or water/oil emulsions since electrode fouling will occur [128]. In particular, a measuring resistor rather than an ammeter with zero resistance is used to connect the two corroding electrodes. Each electrode experiences corrosion events that cause current variations, which could cause a dip across the measurement resistor. Consequently, while utilizing the ZRA approach, the potentials  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Nanostructured** & **Encapsulated CI** & \begin{tabular}{c} **Metal** \\ **Sample** \\ \end{tabular} & \begin{tabular}{c} **Corrosive** \\ **Media** \\ \end{tabular} & **Reference** \\ \hline Carbon hollow spheres & 2-mercaptoethanol & MS & 5 wt.\% HCl & [56] \\ \hline Polyaniline microcapsule & 2-mercaptoethanol & Carbon steel & 3.5 wt.\% NaCl & [113] \\ \hline Mesoporous polyaniline (mPANI) hollow nanospheres & 1H-benzotriazole & MS & 3.5 wt.\% NaCl & [117] \\ \hline f-MWCNT & \begin{tabular}{c} Poly (o-anthranilic acid) and GO \\ \end{tabular} & \begin{tabular}{c} Stainless steel 316 \\ \end{tabular} & 2 M HCl & [120] \\ \hline Hollow mesoporous organosilica & 2-mercaptoethanol & Copper alloy & 3.5 wt.\% NaCl & [121] \\ \hline Carbon & Polycarbazole & \begin{tabular}{c} Plain \\ CS specimen \\ \end{tabular} & 5 wt.\% NaCl & [122] \\ \hline Mesoporous polyaniline hollow spheres & Benzotriazole & Carbon steel & 3.5 wt.\% NaCl & [123] \\ \hline Polyaniline (PANI) microcapsule & 2-mercaptoethanol & MS & 3.5 wt.\% NaCl & [124] \\ \hline Acid-modified haloysite nanotubes & \
\begin{tabular}{c} Praseodymium \\ ions (Pr) \\ \end{tabular} &
\begin{tabular}{c} AZ31 \\ magnesium alloy \\ \end{tabular} & 3.5\% NaCl & [125] \\ \hline \hline \end{tabular}
\end{table}
Table 1: List of encapsulated corrosion inhibitors with nanostructured material.  
of the two electrodes differ. The electrochemical impedance method is found to be the most effective technique because it is advantageous among all the abovementioned techniques.