## 1 Introduction  
Moisture separator reheater (MSR) tubes are used to connect the high-pressure and low-pressure steam turbines in a pressurized-water power plant to increase the efficiency of the heat transfer rate [1]. The MSR tubes have fins on the outer surface to increase the contact surface area with the steam by about three times, thus improving the efficiency. As the steam flows through the MSR tube, the moisture contained in the steam is removed, and the steam is reheated; therefore, the MSR is not only improving the thermodynamic efficiency, but also helping to prevent the erosion and corrosion of the turbines' blades [1]. However, the fins are sensitive areas where corrosion could appear after a long period of operation. Thus, nondestructive testing (NDT) is usually applied for early inspection of the corrosion in the MSR tubes to prevent the failure of the MSR tubes and the stop the operation of the nuclear power plant [1, 2]. Ultrasonic testing (UT) has been developed for the inspection of the corrosion in the MSR tubes [3; 4]. The UT system used a guided wave for efficient detection of the corrosion. However, it requires coupling material between the UT probe and the surface of the MSR tubes. Moreover, it is difficult to maintain the coupling material during the inspection, and it could bring side effects because of the possibility of coupling material remaining in the MSR tubing system. Electromagnetic testing (ET) is a preferred method for inspection of the corrosion in the MSR tubes because of non-contact inspection. Various ET methods, including magnetic flux leakage testing (MFLT) and eddy current testing (ECT), have been researched and developed [5; 6; 7]. The MSR tubes are usually made of ferritic stainless steel (SS439), which is an alternative material for 90-10 CuNi material because of higher strength and high capability of corrosion resistance [8]. MFLT is a simple method that supplies DC magnetic field (e.g., produced from a magnet) to the MSR tubes, and the magnetic sensors will measure the magnetic leakage at the corrosion; thus, the corrosion could be inspected [5]. However, the fins in the outer surface of the MSR tubes also produce magnetic leakages into the magnetic sensor that makes noise to the corrosion signal. ECT operates by supplying an AC magnetic field into the inner tube surface, and an eddy current will be induced in the tube thickness [6; 7]. The eddy current is followed by the skin effect that confined in the near-surface that could minimize the MSR fins' effects. However, the high permeability of the MSR tube limits the penetration of the eddy current; thus, the detection of the outer surface corrosion is limited. Full saturation eddy current testing (FSECT) is an improvement of the ECT for inspection of high permeability material [9; 10]. FSECT has an additional strong DC magnetic source (e.g., a magnet) to fully saturate the MSR tube's material that makes the relative permeability of the tube become 1. Then, the eddy current could deeper penetrate the tube thickness that improves the detectability of the outer surface corrosion. However, the strong DC magnetic field could saturate the magnetic sensor, and it is also challenging to produce in a real-life application. Thus, partial saturation eddy current testing (PSECT) was an alternative solution [11; 12; 13]. The PSECT has the same configuration with the FSECT, but the DC magnetic field has a lower intensity that only partially saturates the MSR tube's material. Unfortunately, all the developments could improve the detectability of the outer surface corrosion, but still get affected by the noise produced from the fins because the fins could be considered as multiple corrosion [11; 12; 13]. Therefore, this study proposed the incorporation of a multivariate singular spectral analysis (MSSA) with the ET systems for improvement of signal in the inspection of the outer surface corrosion in the MSR tube.