Detection Of Inner Corrosion In Ferromagnetic Metal Pipe Under Insulation With Pulsed Eddy Current Testing

Ferromagnetic pipe under insulation are widely used in petroleum and other industries. High pressure, high humidity and corrosive liquid medium transported inside the pipe, and moist air and rain outside the pipe often cause pipe corrosion. So it is vital to carry out nondestructive testing(NDT). However, owing to the presence of thermal insulation outside the pipe, it is difficult to conduct testing. Pulsed eddy current testing developed in recent years can solve this problem relatively well.Effect of the detection sensitivity caused by the location change of the receiving coil under exciting coil is studied by finite element simulation and experiments. The simulation results show that the sensitivity will reach maximum when receiving coil is right under the outer edge of the exciting coil either in axial or circumferential direction. The experimental results agree well with the simulations and show that the detection sensitivity will reach maximum when receiving coil is right under the outer edge of the exciting coil.Detection sensitivity of focusing sensor is compared with circular sensor. Use circular sensor and focusing sensor which have been optimized respectively to detect the same specimen. The experimental results show that focusing sensor can identify 40 mm ? 40 mm ? 3mm internal corrosion under 90 mm thick insulation in ? 159mm?10mm pipe while circular coil cannot. The focusing sensor detects better than the circular sensor.A new structure of the sensor--U shape sensor is proposed. Use U shape sensor to test internal corrosion and the experimental results show that U shape sensor can identify 20 mm ? 20 mm ? 3mm internal corrosion under 90 mm thick insulation in ? 159?10mm pipe while focusing sensor can no. The U shape sensor detects better than the focus sensor.U shape sensor is also used to detect large area corrosion(outside and inside the pipe) and internal groove-like defects. U shape sensor can detect 150 mm ? 10 mm ? 2mm internal groove type defects under 90 mm thick insulation in ? 159 mm ? 10 mm pipe. It can also detect large areas of uniform corrosion(outer and inner) with length 600 mm and depth 1.0mm(10% thickness of the pipe) very well. The test results show that U-shape sensor can detect local internal corrosion and the detection sensitivity is high enough to meet the needs of industrial standards. Large area corrosion detection is relatively easier than localized corrosion even if the corrosion is shallow.This paper is focused on the internal corrosion detection of ferromagnetic pipe under insulation with pulsed eddy current(PEC) testing, and U shape sensor can get a better sensitivity for smaller internal corrosion defects.