Coatings Thickness Measurement
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Coatings Thickness Measurement
F-SECT:NON DESTRUCTIVE ASSESSMENT OF INTEGRITY AND RESIDUAL LIFE OF GAS TURBINE BLADES PROTECTIVE COATINGS
Competition and deregulation in the electricity market require the optimisation of fundamental aspects of power production, like continuity of operation and availability, also in order to exploit market opportunities and avoid the potentially severe profit penalties for prime time outages. In combined cycle units, the principal causes of forced and scheduled outages are traceable to the gas turbine hot-section components, particularly the degradation of blades/vanes subjected to severe environmental attack. The protection of these components from high-temperature oxidation and corrosion relies on high-efficiency coatings, whose unavoidable service degradation is recognised as the main factor that determines blades refurbishment cycle.
Therefore, initial quality control of the “as applied” coatings is considered as the first protection measure against premature failure of the highly expensive turbine stages. On the other hand, condition assessment of service-run coatings allows optimisation of the components life-cycle (with related savings), as well as early recognition of unexpected problems for timely reaction.
THE F-SECT SYSTEM HAS BEEN THEN DESIGNED FOR QUALITY CONTROL AND ACCEPTANCE TEST OF NEW BLADES AND FOR CONDITION ASSESSMENT OF SERVICE-RUN BLADES, THROUGH FREQUENCY SCANNING EDDY CURRENT.
HOW F-SECT WORKS
The F-SECT system has been developed for nondestructive condition assessment of new and servicerun high-temperature coatings applied on the hot gas-path components of modern gas turbines. The system is capable of providing quantitative data on the following coating parameters:
- Metallic Coating Thickness (bond-coat in case of TBC’s).
- Effective Beta Thickness, the thickness of the coating that is protective after a certain period of operation, since it still contains beta-phase (Ni,Al), the aluminium reservoir available to withstand high-temperature oxidation.
- Ceramic Thickness
- Airfoil Thickness
System Specifications
F-SECT is a portable system consisting of the following components:
Notebook PC equipped with RS232 Serial interface or equivalent PCMCIA board
Frequency scanning eddy current unit
Auto-ranging power supply unit 110/220 VAC, 50-60 Hz
Probes
Software
Examples of Inspection Results
QUALITY ACCEPTANCE TEST ON NEW/REFURBISHED PARTS:
Bond-coat and ceramic thickness measurement on blade airfoil (see Fig.1);
Detection of structural anomalies in the metallic coating, i.e. external layer of non compacted coating (over-spraying) or presence of inadequate bond-coat to base metal interface due to presence of oxidation/polishing residuals or disbanding (often localized in the airfoil to platform fillet).
CONDITION ASSESSMENT OF SERVICERUN COATINGS:
Estimating the remaining protective coating thickness (EBT) and ceramic thickness of service run coatings (Fig. 2-3), in order to localize the most critical areas and build a knowledge on coating degradation that can be used to optimise maintenance actions (Fig. 4);
MEASURING AIRFOIL THICKNESS:
Airfoil thickness in the range 0.3 – 3 mm can be performed even on TBC coated blades/vanes (Fig. 5).
How It Works
The instrument adopts a frequency scanning eddy current technique to characterise a multiple-layer test material (coating and substrate) on the basis of the small difference in electrical conductivity and magnetic permeability of the different layers. The data collected by the system are analysed by a proprietary software, based on a model of the interaction between an electromagnetic wave and a layered electrically conductive material. The analysis simultaneously yields the thickness and the electrical conductivity of each layer. This model-based methodology allows calibrationfree measurement of the thickness of new coatings to perform quality / acceptance tests on new or refurbished blades. Condition assessment of service-run coatings requires the knowledge of the relation between βaluminide depletion and the electrical conductivity for the specific coating-substrate system under examination, which can be readily obtained by cross-checking FSECT data and micrographic analysis results on a scrap blade. Up to now the system has been used both for shop testing on new/service-run blades and for in-field testing of blades mounted on the rotor (turbine case is to be removed). The system can also be integrated with robots for automated inspection of large batches of components.
EXAMPLE OF INSPECTION RESULTS
Fig.1 Estimated bond-coat thickness (blue curves) and ceramic thickness (red curves) on a sample of 5 blades, used as acceptance test of a refurbished set.
Fig. 2 - Definition of EBT (Effective Beta thickness) for a service-run coating.
Fig. 3 – Color-coded maps of EBT and ceramic thickness of Sicoat 2231 after 24,000 EOH.
Fig. 4 - Estimating the rate of degradation of the bondcoat (Effective Beta Thickness) vs. equivalent operating hours (EOH), for the most critical area of a 1st stage blade airfoil. FSECT data from different engines of the same type, operated at different combination of operating hours/start-ups are considered jointly. Complete coating depletion corresponds to equivalent β β β β thickness = 0, from which coating life-cycle can be estimated close to 24,000 EOH
Fig.5 – Airfoil thickness variations along the trailing edge of a 2nd stage blade, pointing correlated over/under thickness (respectively on pressure/suction sides) traceable to casting core displacement.
