## 5 Discussion  
The MEC technique is commercially used for the detection of buried defects such as corrosion and cracks. The efficacy of the MEC sensors in in-line and online inspection tools has been well established. The prospects of using a wireless magnetic eddy current sensor as a permanently installed sensor for continuous corrosion monitoring were tested using an accelerated corrosion test. The sensor design was optimized using finite element modeling to achieve higher sensitivity without compromising on the small size.  
The MEC method relies on sensing the perturbations in the permeability of the test sample due to defects. The inductive reactance of the coil-excited sinusoid-ally is affected by the changes in permeability. It was observed that a wide range of excitation frequencies are discussed in the literature for the detection of surface and buried defects in ferromagnetic structures. Most of the research on MEC sensors is focused on the optimization  
Figure 16: (**a**) The relative humidity (%RH) values, and (**b**) temperature (°C) recorded throughout the procedure.  
Figure 17: The graph showing the rate of change of corrosion per hour throughout the test.  
of the magnetization level and the defect characterization and quantification, by using different signal parameters. Through FE simulations and experiments, it was found that the sensor's excitation frequency has a major effect on its detection capabilities. The finite element modeling revealed that the relationship between sensitivity and frequency is not monotonous. The reactance values increase positively at lower frequencies and decrease considerably with the increasing frequency. At a certain frequency called zero-crossing frequency, the reactance becomes zero and then starts to increase negatively.  
The theoretical studies show that the reactance of the coil is dependent on two sets of magnetic fields set up by the coil. The primary magnetic field is due to the excitation current and secondary magnetic fields because of the induced eddy currents. Both of the fields are magnified by the high permeability of the ferromagnetic sample and are affected by the variation in permeability. They oppose each other due to the opposite nature of the currents producing the fields. At lower frequencies, the primary magnetic field dominates the other and vice versa. The study indicates that two optimal frequencies can be used for inspection and the frequencies near the zero-crossing range need to be avoided.  
Furthermore, the FE model and experimental results were found to disagree at some points. The zero-crossing frequencies and the optimal frequencies for FE models and test results were not the same. The results had some similarities for thick plates with less magnetization level, but the error increased when the magnetic saturation was achieved in the plates due to increased corrosion. This can be explained by the limited capability of the simulation software for modeling magnetic saturation phenomena.  
The excitation frequency of 100 kHz was used for the design of the wireless sensor due to the following reasons: (1) the sensitivity was maximum at 100 kHz for both the FE model and experimental results for thick plates, and (2) the sensitivity near the 100 kHz range did not experience significant fluctuations.  
The wireless sensor designed by interfacing AD 5933 with a microcontroller unit was tested by conducting accelerated corrosion on a mild steel plate. Along with sensor signals, the environmental data was also recorded. The comparison of the signals revealed that the sensor was sensitive to corrosion and the signals were not affected by temperature or humidity level changes.