Precision measurement of coating thickness on ferromagnetic tube using pulsed eddy current technique

The anti-corrosive metal coatings on ferromagnetic tubes are the important components in power plants and need non-destructive evaluation (NDE) techniques to measure their thickness precisely. The objective of this research is to develop pulsed eddy current (PEC) technique for measuring the coating thickness accurately. PEC signals are investigated for the evaluation of coating thickness using a differential PEC probe, which consists of a driving coil and two pickup coils. The characteristics of PEC signals for coating thickness variations show that the related features, such as the time-to-peak and peak value, change linearly with thickness change. A new feature, called as time to the rising point, related to the propagation time of electromagnetic field in conductive metals is adopted for coating thickness measurement. These features can be used to inspect the coating thickness within the accuracy of 0.02 mm.