During the normal operation of a land-based gas turbine, attack will occur of the gas-washed surfaces of the rotating stationary blades in the turbine hot section. This attack and its intensity can be variable depending on the blade’s position in the turbine hot section. This attack will progressively degrade these gas-washed surfaces even if these surfaces have been previously coated with a protective layer. During the service period of the turbine, it will be necessary to refurbish the blades from the hotter section of the turbine. One of the refurbishment steps will be to provide the blades with a suitable replacement coating to afford protection until the next service period. Conventional refurbishment techniques used to clean the blade surface rely on abrasive cleaning and / or chemical pickling. These processes may be capable of removing superficial oxidation and any residual coating but are not able consistently to remove the substrate material that has suffered from corrosive attack. It is important that this attacked substrate layer be removed completely, otherwise any residual corrosion products, particularly the presence of deeply penetrated sulfides in grain boundaries, could significantly reduce the life of any subsequent coating. The technique described in this paper essentially activates the surface layer of the substrate that is corroded, thus rendering it more easily removed by chemical and physical means. It is possible by this method to remove up to 400 μm of the substrate material and provided that all the corrosion products are contained within this zone, it is demonstrated how this produces a clean unattacked surface that is necessary for any subsequent welding, brazing, or recoating operation.
IMAGE OF THE SURFACE OF GAS TURBINE BLADE AS A DIAGNOSTIC SIGNAL