Eddy-current {NDE} of combustion turbine blade coatings. Determination of conductivity profiles in the presence of a diffusion process

This paper deals with the estimation of conductivity profiles using Eddy- Current measurements over combustion turbine blade coatings affected by depletion of aluminium. First, we model the response of an Eddy-Current coil over a layered metallic structure with a top over-aluminized coating by extending the analytical Uzal-Rose's model for one hyperbolic tangent conductivity profile to a conductivity profile using two hyperbolic tangents for taking inward and outward depletion of aluminum inside the coating into account. Results obtained with this model are similar to those obtained with a numerical multi-layer model with a reduced computing time