Corrosion in multi-layered structures cannot be effectively monitored by most Non-Destructive Testing methods. However, combining a multi-frequency eddy current technique with sliding probe technology, external amplification, digitization, and robotics provides the ability to quantify small amounts (3% of one plate thickness) of material loss in
sub-layers not otherwise detectable using an industrial eddy current scope. The technique developed can be used in the field for aircraft inspection. Keywords: Non-Destructive Testing, eddy current, sliding probe, reflection probe, skin depth, multi-frequency