Pulsed eddy currents (PEC) are a versatile nondestructive testing (NDT) technique uniquely suited to detect corrosion under insulation (CUI). PEC is an excellent screening tool because of its ability to inspect in-service components through insulation and jackets. By identifying corroded areas outside shutdown periods, PEC enables expanding the scope and frequency of inspections without affecting schedules, which allows the focused application of quantitative methods such as radiography and ultrasonic testing during shutdowns.
PEC’s working principle is this: a coil placed at some elevation (referred to as liftoff) from the surface under test (ferromagnetic and conductive) fires a magnetic pulse. First, the pulse remains active long enough for the magnetic field to penetrate the full thickness of the surface under test (referred to as the excitation phase). The pulse then abruptly stops. Eddy currents appear in the metal mass which induce a magnetic field that decays over time and can be picked up by a magnetic sensor. During the reception phase, the sensor generates a voltage signal (referred to as an A-scan) whose shape and decay rate are directly related to the thickness of the surface under test.