## 7 Corrosion Control and Monitoring  
Corrosion, one type of age-related damage experienced by most marine structures, is caused by long-term chemical and electrochemical reactions due to the harsh marine environment. Marine structures with a long-term operating life, such as a ship, offshore facility, submarine pipeline, wind turbine, etc., are vulnerable to this common type of damage. It may lead to failures of mechanical systems and thereby reduce their safety level and structural integrity. Therefore, continuous corrosion control and monitoring of marine structures are essential for ensuring their safe operation. A corrosion control and protection system can delay and mitigate corrosion initiation in marine structures and allow them to reach the expected service life. The most common corrosion control technologies used in marine structures are coating (PA), corrosion allowance (CA), and galvanic anode cathodic protection (GACP). Corrosion monitoring techniques can be divided into two  
Figure 24: (**a**) An experimental setup with the damage location shown (red circle). (**b**) Locations of accelerometers [120]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Accuracy (\%)** & **Validation Error** & **Training Error** & **Training Time** & **\# of Images** \\ \hline Without data augmentation & 93.81 & 0.1692 & 0.1167 & 11 min & 6400 \\ With data augmentation & 99.90 & 0.0044 & 0.0026 & 1196 min & 1,612,800 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Comparison of accuracy, validation error, training error, and training time when using data augmentation or when using the original dataset [120]. major categories: offline monitoring and online monitoring. Offline monitoring measures the corrosion status within a specific time interval, while online measurement involves continuous real-time monitoring of the corrosion status of marine structures. Offline monitoring methods, including magnetic leakage and ultrasonic methods, are usually used. In contrast, electrochemical measurement, resistance probes, underwater robot probes, and imaging technology are being used in recent developments. This section presents a comprehensive review of the development of corrosion monitoring in the marine environment over the past decade. The monitoring scopes, measurement methods and principles, and related advanced sensing technology and instruments will be highlighted. In 2010, [PERSON] et al. [122] conducted a laboratory test to evaluate corrosion on a submerged plate using the Lamb wave-based technique and probability-based diagnostic imaging using time-of-flight (ToF). A pair of transducers were compiled at the fluid-solid interface, acting as the wave transmitter and receiver to generate Lamb wave signals. The signals were then processed with a signal digitizer (Agilent(r) E1438A). It was found that corrosion can be identified precisely with the support of a probability-based diagnostic imaging approach. In 2013, two sensors (Anode-Ladder-System and Corrowatch System) were developed to monitor the corrosion state in the steel rebar of coastal concrete by [PERSON] et al. [123]. The sensor design was based on a three-electrode electrochemical test architecture, where the sensors' performance was evaluated using a dry-wet cycle test on the upper reinforcement of the concrete block. In this work, Q235 steel bars were used as working electrodes, titanium bars were used as reference electrodes, and stainless steel bars were used as counter electrodes. The investigation revealed that corrosion sensors can efficiently monitor chloride penetration into concrete. In the same year, [PERSON] et al. [124] conducted laboratory-based corrosion behavior monitoring of steel structures in a marine tidal zone using electrochemical impedance spectroscopy (EIS). Comb-shaped electrodes were used for the electrochemical monitor, and a ZKW881 rapid corrosion tester was used to monitor frequency impedance.