Frequency scanning eddy current testing for assessment of remaining lifetime of industrial gas turbine blade high temperature coating

Broddegård, Mattias; Siemens AB; Sweden

Broddegård, M.; Siemens Industrial Turbomachinery; Sweden

ID: ECNDT-0336-2018
Download: PDF
Session: Oil & Gas 5
Room: H2
Date: 2018-06-12
Time: 14:30 - 14:50

Industrial gas turbine blades are exposed to high temperatures and aggressive environmental conditions during operation. The blades are typically manufactured as precision castings in nickel-base superalloys, with internal cooling channels in order to reduce the metal temperature. In order to protect the blades from oxidation, coatings containing aluminium are applied. During operation, depletion of the aluminium in the coating takes place until eventually no protective oxide can be formed. In order to estimate the remaining lifetime of the coating, blades can be cut for metallographic examination and measurement of the aluminium content in the coating. This will however not take into account individual variations between the blades, and furthermore requires removal of the blades from the turbine disc. The use of frequency scanning eddy current testing for measurement of the electrical conductivity of the coating, which is related to the aluminium content has been investigated. Measurements were carried out on test coupons before and after furnace exposure and also on turbine blades that had been removed after operation in order to carry out metallographic examination of the coating status. The results show that there is a useable correlation between the measured electrical conductivity and the aluminium content in the coating.