The aim of this paper is to present a new design of eddy current probe able to detect defects under a metallic coating of turbine and compressor blades. Defects like pitting , crevice , cracks which are typical defects induced by service may grow , undiscovered under the coating giving up to the failure of the components with catastrophic consequence for the power generation system. The early discover of these defects has motivated a research program to set up a technique for supporting a condition monitoring program in the general maintenance plan. Probe design, samples with simulated defects to reproduce the conditions when inspecting the blade, statistical based tests to establish an acceptable POD have been the focal points of the research program. The feature of the new design of the probe is the double differential coils ; these generate four sensitivity area points and empower the penetration of the magnetic field through the coating. The results after two year experience has been highly satisfactory. The performance of in situ examinations of compressor of GE gas turbine has validated the technique ; the ratio, signal to noise level, is over 3 with small phase angle. These results allow GE to realize a new condition monitoring program with a higher level of reliability. The research program is proceeding versus array of coils, multichannel, signal processing.

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