Abstract
The first flux leakage type rotary scanning unit for testing 1/4" (6.35 mm) and 3/8" (9.53 mm) flat carbon steel plate and detecting corrosion in tank floor plates was developed in 1989. This paper presents a newly developed rotary scanning technique which combines eddy current and flux leakage inspection methods into one test probe design in a manner that allows each method to inspect the same location simultaneously. The eddy current component of the probe senses primarily the top surface conditions, while the flux leakage portion detects conditions on the top and bottom surfaces. By processing these signals and displaying them in a simultaneous dual trace display, defects on both sides of the tank floor can be visually observed and a determination made as to whether the signal originated from the top or back side of the floor plate. Back surface corrosion can be identified, even in the presence of top surface conditions such as pitting, gouges and corrosion. Several additional techniques for improving test results, such as adjustable magnetization power to maintain proper saturation of the floor plate, and surface ride test probes to retain a constant "lift-off" are also discussed.
