###### Abstract  
Permanently installed sensors are a cost-effective solution for corrosion monitoring due to their advantages, such as less human interference and continuous data acquisition. Some of the most widely used permanently installed corrosion sensors are ultrasonic thickness (UT) gauges. However, UT sensors are limited by the need for coupling agents between pipe surfaces and sensors. The magnetic eddy current (MEC) method, on the other hand, does not require couplant and can be used over insulations. With the development of powerful rare earth magnets, MEC sensors with low power consumption are possible, and there is the prospect of using them as permanently installed sensors. A novel wireless magnetic eddy current sensor has been designed and optimized using finite element simulation. Sensitivity studies of the sensors reveal that the excitation frequency is a critical parameter for the detection of corrosion defects. An in-depth explanation of the relationship between the sensitivity of the sensor and the excitation frequency is presented in this paper. The results of an accelerated corrosion test, conducted to simulate the service environment of the sensor, are also discussed. It was observed that the sensor signals are very sensitive to corrosion defects and show no subtle differences due to temperature and humidity changes. corrosion monitoring; magnetic eddy current; permanently installed sensor; wireless sensor; oil and gas pipelines; magnetic flux +
Footnote †: journal: Applied sciences  
in long pipes with a certain pressure and no sharp bends, valves, or branches [5; 6; 7]. Despite the extensive inspections, in-line tools are, therefore, run periodically after 5-7 years [8]. On-line inspection techniques, such as manual UT tools, are reliable and have high resolution for corrosion defects [9]. Online inspection requires access to the pipes, and there are high costs associated with scaffolding and rope access [10]. Additionally, the manual UT inspection is influenced by surface roughness, and the cost of surface preparation is a major drawback. [11]. In recent years, permanently installed wireless sensors have become popular in the industry because of their advantages such as low cost, continuous data acquisition, less human interference, and reliable data since the transducers are fixed at the inspection points [12]. A number of wireless corrosion sensors have been developed and discussed in the literature. Radiofrequency identifier (RFID) sensors and interdigitated capacitance-based sensors are based on the relative movement of two components of the sensors [13]. They need access to both sides of the pipe and have an indirect mechanism for corrosion detection [14; 15; 16]. Surface acoustic wave (SAW) sensors and guided wave permanently installed sensors are sensitive to corrosion defects, but they are highly affected by environmental parameters such as temperature [17; 18]. Ultrasonic thickness gauges are commonly used permanently installed sensors [19]. They can detect buried corrosion defects, and the confidence level of UT tools is around 95% [11]. The major disadvantage of the UT sensors is that they require coupling agents to be installed between the pipe surface and the transducers. The magnetic methods, on the other hand, do not require coupling agents. Magnetic sensors, for instance, anisotropic magneto-resistive (AMR) sensors to detect corrosion by measuring the deflection of the low-frequency alternating current (AC) injected into the pipeline at different locations, have been reported [20]. Techniques such as alternating current field method and frequency-dependent magnetic field induced due to AC current injected at remote contact points are also discussed [21]. These approaches require the pipes to be injected at different locations and also may have limited sensitivity. Magnetic NDE techniques such as MFL and MEC are considered reliable for identifying corrosion defects in ferromagnetic pipelines.