# Ultrasound-Based Smart Corrosion Monitoring System for Offshore Wind Turbines
## 1 Introduction
### Corrosion Inspection and Monitoring Techniques  
There are different types of corrosion inspection and monitoring techniques to find out the corrosion condition of metals, as is discussed and reviewed in Reference [20; 21]. Figure 1 shows a detailed classification of these techniques based on the corrosion detection method and sensing parameters.  
In general, corrosion detection measures can be direct measuring metal loss due to corrosion or corrosion rate or indirect measuring any parameter that may be a cause or consequence of the metal loss or corrosion. Some of these techniques need direct contact/exposure to the same corrosive fluids or to access the internal environment where the corrosion process takes place. They are known as intrusive techniques and can alter the corrosion process and create disturbances to the operation during installation and re-installation of measuring probes or inspection processes. Therefore, it is beneficial if corrosion testing can be performed from the outside of the test object. This is possible with non-intrusive techniques through detecting physical and geometry changes due to corrosion (mass loss, crack, or surface discontinuities), and there is no need for any direct contact to the same corrosive fluids or access to the internal environment. On the other hand, it is very important to perform corrosion detection activities in the field applications without destroying the engineering properties of the material and without affecting its long-term future performance during the inspection process. This kind of evaluation can be achieved by using Non-Destructive Testing (NDT) methods for material testing [22]. Among the techniques listed in Figure 1, if this condition is satisfied, they are known as an NDT method. Therefore, we can propose that the most appropriate corrosion detection technique for offshore wind turbines shall be both non-intrusive and non-destructive.  
The main objective of this research work is to develop a smart ultrasound solution for corrosion monitoring in offshore wind turbines. Due to the harsh and distant environment they have been installed in, it is a challenge to find the most suitable corrosion detection technique. Based on the expected solution, we have identified the following design specifications to be achieved and feasible with the selected corrosion detection method.  
* SHM requires a reliable and permanent acquisition of monitoring data with minimum interruptions.
* Corrosion detection technique shall be non-intrusive and non-destructive.
* The monitoring system is able to detect uniform corrosion.  
Figure 1: Corrosion detection methods.  
* The solution is able to detect corrosion in both splash zone and atmospheric zone of the wind turbine tower.
* Data acquisition must run for several years in order to obtain meaningful results about the tower's health.
* Robust wireless communications are needed to rely on the sensor data sent to the tower's central computer in a difficult environment for electromagnetic signals.
* Corrosion detection components must have the capability of integrating with Unmanned Aerial Vehicles (UAVs) as they offer significant potential benefits to the monitoring of large-scale facilities, e.g., atmospheric zone.
* Tower locations which are not easily reached by a mobile platform must be covered by battery powered sensor nodes likely installed in those areas, e.g., splash zone.  
The paper is structured as follows. In Section 2, a review of different ultrasound solutions for corrosion monitoring is introduced. Section 3 describes the concept of the presented approach for corrosion monitoring in offshore wind turbines. Section 4 provides the description of the ultrasound testbed and the design methodology followed to miniaturize the solution. In Section 5, the samples preparation and the experimental setup are presented. Section 6 shows the most relevant results we have obtained through our experiments. Finally, the conclusions are summarized in Section 7.