Abstract
EPRI is involved in a multi-year program to test, evaluate, and develop a field-deployable eddy current NDE system for life assessment of blade coatings for advanced gas turbines. An eddy current system was assembled and tested on service-aged 7FA first stage blade buckets, which were coated with GT 33+ duplex metallic coatings. Testing of transverse cutout sections showed presence of normal coating, coating degradation due to β-phase depletion, and cracked coatings with some cracks penetrating into the GTD-111 substrate. By plotting normalized multiple-frequency eddy current coil impedance values, the eddy current technique was able to assess and provide the following coating and airfoil conditions: 1) normal coating; 2) degraded coating; 3) cracked coating; and 4) possible corrosion of GTD-111 substrate. Using a built-in three-layer inversion program, conductivity values of topcoat, bond coat, and GTD-111 substrate were also estimated using aged blade buckets.
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