## 1 Introduction  
Ferromagnetic plates are commonly used materials in petrochemical and power generation industries. Wall thinning is one critical threat to ferromagnetic plates. It is caused by corrosion under insulation (CUI), flow accelerated corrosion (FAC), or liquid droplet impingement (LDI), and severely affects the structural strength and integrity of ferromagnetic plates [1, 2]. Therefore, wall thinning assessment is important. The thickness measurement is an essential early warning method. Whereas the ferromagnetic plates in petrochemical and power generation applications are always wrapped with insulations and externally protected metal claddings. Therefore, it is challenging for the commonly used methods, such as ultrasonic testing (UT) and eddy current testing (ECT), to determine the plate thickness without removing the insulations and claddings [3]. Pulsed eddy current testing (PECT) provides a possible solution. PECT involves excitation by a square-wave pulse rather than a sinusoidal waveform. So, it contains a variety of frequency components and large driving electric currents, which allows for non-contact remote sensing [4, 5]. Therefore, it could be used to measure the thickness of ferromagnetic plates covered with insulations and claddings. The PECT signal is a complicated coupling response related to many factors [6, 7], including ferromagnetic plate thicknesses, insulations, claddings, and lift-offs (distance from the sensor to the cladding), etc. Thus, decoupling the ferromagnetic plate thickness from other factors is a key problem in PECT. Some features have been proposed to evaluate the plate thickness. [PERSON] et al. [8] proved that the peak value, time to zero-crossing(TZC), and lift-off intersection (LOI) of the differential PECT signal could be used to evaluate the thickness. [PERSON] et al. [9] verified these features experimentally and further proposed that the peak value was proportional to the metal loss extent, and the TZC contained information regarding the flaw depth. [PERSON] et al. [10] showed that the LOI point could be adjusted by varying the rising time of pulse excitation, which was beneficial to extend the measurement range and to increase the measurement sensitivity. [PERSON] and [PERSON] [11] used the time-to-peak to characterize defects and structural variations in aging aircraft structures, showing that the time-to-peak and defect depth exhibited a quadratic relationship. [PERSON] et al. [12] compared the time-to-peak and peak value for wall thinning assessment, and found that the time-to-peak was superior to the peak value due to its linear relationship with wall thickness. [PERSON] et al. [13] proposed the rising point to identify and quantify the defects, and proved that the rising point was related to the propagation time of electromagnetic waves in metallic plates. However, these features [8; 9; 10; 11; 12; 13] are all obtained by analyzing special points in PECT signals, they are easily affected by the confounding factors. For example, the peak value [8; 9] and the time-to-peak [11; 12] are related to the cladding thicknesses [12], and the rising point [13] relates to the sensor lift-off [14]. [PERSON] [15] discussed magnetic field variation by using an anisotropic magneto-resistive (AMR) sensor embedded differential detector, showing that the signal decay behavior was only relevant to the pipe wall thickness over a limited time after switching off the excitation current. [PERSON] et al. [16] studied the magnetic flux change with variable pulsed width excitation, demonstrating that the slope of the relative increase in magnetic flux and pulse width could be used to evaluate the ferromagnetic plate thickness. However, the feature proposed by [PERSON] [15] cannot be obtained when a coil is used as a detector, which limits its application. The slope of the relative increase in magnetic flux and pulse width [16] are complicated. Therefore, an efficient and easy-to-use signal feature, which is sensitive only to the ferromagnetic plate thickness, is still needed.