In the field of Non-Destructive Testing, Potential Drop (PD) techniques have been
used for decades, especially in the petrochemical and power generation industries,
for monitoring crack growth and wall thickness variations due to corrosion and/or
erosion in pipes, pressure vessels and other structures.
Inspection is carried out by injecting currents in the specimen to be tested and
measuring the arising electrical potential di erence between two or more electrodes
placed on its surface. The presence of a defect generally increases the resistance and
hence the measured voltage drop; inversion of these data can give information on
the size and shape of the defect.
However, while the principle underlying these techniques is relatively simple, some
di culties have been encountered in their practical applications. Many commercial
systems based on PD methods, for instance, require the injection of very large
currents in order to obtain su ciently large signals; doubts have been raised on
the stability of these methods to variations in the contact resistance between the
electrodes and the inspected material. The present work aims to show that some
of these problems can be easily overcome, and to evaluate the capabilities of PD
techniques for crack sizing and corrosion mapping.
After a brief review of the advantages, disadvantages and applications of the main
electromagnetic methods for Non-Destructive Testing, an experimental setup for
Potential Drop measurements which was developed for this work and which uses
small alternating currents (AC) is described. The setup is benchmarked against existing
PD systems and then used to validate a model that allows AC PD simulations
to be run with a commercial Finite Element code. The results of both numerical
simulations and experimental measurements are used to investigate the possibility
of sizing defects of complex geometry by repeating the analysis at several di erent
frequencies over a broad range, and of reconstructing the depth pro le of surfacebreaking
defects without the need for assumptions on their shape. Subsequently, the accuracy to which it is possible to obtain maps of corrosion/erosion on the far surface
of an inspected structure is discussed, and results obtained with an array probe
that employs a novel arrangement of electrodes are presented. Finally, conclusions
are drawn and suggestions for further research are made
