In recent years, corrosion under insulation of steel pipes has become a problem in chemical plants. There is a method using ultrasonic guided waves to inspect the corrosion of pipes over a long distance at a time, but it is costly and time-consuming because the heat insulator must be removed. We are developing a completely non-contacting inspection technique using magnetostrictive guided wave transceive technique and high-temperature superconductor SQUID for STPG370 pipe. A pair of C-shaped electromagnets were used to magnetize two parts of the STPG370 pipe while the pair sandwiching the parts and rotating the pipe, in order to give the respective parts uniform circumferential remnant fields as bias magnetic fields. An excitation coil was wound around one of the magnetized parts, which is used as a transmitter, and a burst wave current of 50 kHz and 9 App was repeatedly supplied to the coil to generate an excitation fields in order to transmit guided waves in the T(0,1) mode on the pipe. The other magnetized part, which is used as a receiver, was combined with a high-temperature superconductor SQUID gradiometer as a high-sensitive magnetic sensor, which center was located above the center of the pipe with a lift-off of 3 mm. In measurements of propagating guided waves in the T(0, 1) mode on the pipe, the pipe was rotated step-wisely to perform all-round measurements of magnetic signal derived from the guided wave with the SQUID gradiometer above the receiver. From the measured signals, a “B-scan-like” contour map of the signal was obtained with the angle and time as vertical and horizontal axes. In the contour map, the guided wave signals in the T(0, 1) mode were detected at the entire circumference of the pipe at approximately predicted times, which were calculated using the group velocity of the T(0, 1) mode waves on the STPG370 pipe. However, background noise probably due to non-uniformity of the remnant fields in the transceiver parts were also measured. As a result, it is concluded that more uniform remnant fields should be given to the transceiver parts with some improved magnetization method.
Keywords: SQUID, magnetostrictive guided wave, electromagnets, STPG370