Application of a HTS Coil with a Magnetic Sensor to Nondestructive Testing Using a Low-Frequency Magnetic Field

Detecting corrosion at an early stage on the backside of a thick steel plate using a magnetic field is difficult because of the high permeability of steel. Therefore, we used a low-frequency magnetic field to enable the penetration of magnetic flux deep into the steel plate. According to Faraday's law, to achieve a sufficiently intense signal at low frequencies, the number of turns of a normal conducting coil must be increased. However, a high-temperature superconducting (HTS) coil has a very low resistance; thus, the shielding current of a HTS coil can flow under magnetic fields with a wide-frequency range. Although HTS coils exhibit good shielding characteristics at low frequencies, their voltage signals are very weak. To solve this problem, a magnetic sensor was used to detect the shielding current of a HTS coil. The detection unit included the HTS coil, and the magnetic sensor was optimized; moreover, the possibility of operating the magnetic sensor at the liquid nitrogen temperature (77 K) was investigated. As a demonstration of nondestructive testing using the magnetic response of the HTS coil and magnetic sensor, we measured the change in the thickness of a steel plate in which corrosion was assumed to have occurred. Consequently, we obtained a good response even with a low-frequency magnetic field when using a HTS coil with a few turns. In the thickness measurements of steel plates, thicknesses ranging from 6 to 19 mm were obtained in the frequency range of 1-5 Hz.