Improving the detection of flaws in steel pipes using SQUID planar gradiometers

The detection by magnetic techniques of corrosion pits on pipes covered with thermal insulation is critical. Without removing the insulating layer, SQUID technology is the only one available, because the sensor must be placed at a distance from the pipe surface. In this paper we compare the performance of different SQUID gradiometer designs, varying coil area and gradiometer baseline, to determine the best configuration for this task. The gradiometer response was simulated and analyzed by using a spatial frequency model that incorporates the design parameters mentioned before. A three-dimensional finite element model was developed to simulate a typical flaw geometry in small diameter steel pipes due to corrosion pitting. It is shown that planar gradiometers enhance dramatically flaw signal visualization when compared with a typical axial gradiometer design.