1. Introduction
Polyethylene pipes are adopted for gas transmission lines and to replace damaged steel pipes. They are lightweight yet resistant to corrosion and cracking. However, steel pipes are still commonly used in many countries and corrosion remains a major problem. In spite of a protective coating, corrosion forms on the outer surfaces of buried pipes. There has been great interest in developing a reliable and easy-to-use inspection technique to ensure the structural integrity of steel pipelines. One wishes to inspect underground pipelines from the inside without excavation work and service shutdown. To meet this objective, the whole apparatus should be compact to be fitted in the pipe and be mounted on a ‘pig’ moving platform. The system should stand alone or operate under a remote control unit and consume minimum electricity. Quick inspection is a top priority, since the pipelines extend over kilometers. Detailed defect characterization is, however, not always necessary for screening purposes.

A possible solution is to use the low-frequency guided ultrasound excited by electromagnetic acoustic transducers (EMATs). A wave source is directly established on the metal surface without an intimate contact. Such ultrasound can propagate axially or circumferentially over a long distance and bring the pertinent information back to the remote sensor. This is feasible because of reduced propagation loss and relatively high transduction efficiency at lower frequencies as well as limited space of the waveguide. The noncontact nature of EMAT is the key to establishing a robust implementation, accommodating unfavorable surface conditions. The permitted liftoff is limited to a couple of millimeters even for frequencies of less than 1 MHz. This will not matter in practice because pipes of fixed dimensions are to be inspected. A number of EMAT techniques have been investigated and tested for on-site applications [1–3] (and Alers RB, private communication). Most of these used the fundamental Lamb wave (S0 mode) propagating in the axial or circumferential direction. They were, however, often found impractical for coated steel pipes, since the polymer coating layer absorbed much wave energy. Alers (private communication) circumferentially moved a pair of EMATs to use SH waves propagating in the axial direction of the pipe. This technique is suitable for weldment inspection, but not for corrosion detection over a long distance.

In the present paper, we propose the use of a periodicpermanent-magnet (PPM) EMAT [3,4], which is moved along a steel pipe to generate and receive SH guided waves propagating in the circumferential direction. Results of a preliminary study demonstrate high-speed operation, sensitivity to small artificial corrosion, and the potential to evaluate remaining wall thickness. Discussion is provided on the thickness dependence of the measured amplitude and phase of circumferential round-trip signals. There are several benefits in the use of SH guided elastic waves such as a simpler dispersion character than that of the PSV families of Lamb waves, leading to easier interpretation of measurements. Also, the individual guided mode can be selected by the driving frequency of the PPM-EMAT. This is in contrast to the laser-generated ultrasonic technique, which excites all possible modes at the same time due to the inherent broadband nature. Finally, the SH waves tolerate damping from the protective coating of lossy polymers, partly because they are independent of the stress-continuity boundary condition at the interface.