Immersion Type Multi-channel Ae Sensors for Corrosion Monitoring of Underground Fuel Tank

Immersion-type AE sensor is needed for corrosion monitoring of underground gasoline tanks. We are developing a multi-directional optical fiber AE sensor to monitor the longitudinal AE signal through liquid (liquid-borne AE). The developed AE system is an optical-fiber-based Mach-Zender-type laser interferometer using a weak laser and is intrinsically safe. Multi-sensing function was facilitated by winding an optical fiber on four small-size short cup-shaped pipes with different diameters, which were installed on the inside walls of a box of 22.5-mm square and 230-mm height. Each sensor element was screwed on a different sidewall. The four fiber sensors were cascaded in series on a single fiber and monitored the liquid-borne AE signals from four directions simultaneously via frequency discrimination. As the each fiber-wound sensor element monitors at the resonant frequency of the pipe, we can estimate the direction of AE signals using their frequency spectra and wavelet transform. The functionality of the sensor was demonstrated by two month monitoring of liquid-borne AE signals from rust fractures on corroded four carbon steel sidewalls of a water tank. The immersion-type sensor successfully monitored liquid-borne AE signals from the four corroded plates. AE hit rates were considered to relate the corrosion of metallic pieces included in the rust, but not the corrosion of the substrate steel.