Eddy current testing for external defects in tubes where external
access is not possible (e.g. buried pipelines), is conducted
using internal probes. When testing thick-walled ferromagnetic
metal pipes with conventional internal probes, very low frequencies
(e.g. 30 Hz for a steel pipe 10 mm thick) are necessary to achieve
the through-penetration of the eddy currents. This situation produces
a very low sensitivity of flaw detection. The degree of penetration
can, in principle, be increased by the application of a saturation
magnetic field. However, because of the large volume of metal
present, a large saturation unit carrying a heavy direct current
may be required to produce an adequate saturating field.
The difficulties encountered in the internal testing of ferromagnetic
tubes can be greatly alleviated with the use of the remote field
eddy current method. This method provides measurable through penetration
of the walls at three times the maximum frequency possible with
the conventional direct field method. This technique was introduced
by Schmidt in 1958. Although it has been used by the petroleum
industry for detecting corrosion in their installations since
the early 1960s, it has only recently evoked general interest.
This interest is largely because the method is highly sensitive to
variations in wall thickness, but relatively insensitive to fill-factor
changes. The method has the added advantage of allowing equal
sensitivities of detection at both the inner and outer surfaces of
a ferromagnetic tube. It cannot, however, differentiate between
signals from these respective surfaces.
For more information on Remote Field Testing can
be found in the Specialized
NDT Methods section of this site.