Plant engineers are well aware that the rotor is the engine of a turbine. It experiences excessive heat, stress, and tough chemistry, with every start and end. Protective coatings create a controlled barrier between the metal and its environment to minimize attack and extend life.
Why Turbine Rotors Corrode
Turbine rotors are subject to a variety of different threats depending on service. In gas turbines the relatively hot gases contain oxygen, sulfur, and salts which can trigger oxidation and hot corrosion. In steam turbines the wet stages may experience droplet erosion, pitting, and flow-accelerated corrosion mainly during cycling or low-load operation. On both types of turbines, the combination of poor air filtration, fuel impurity, and carryover from boilers or HRSGs (https://en.wikipedia.org/wiki/Heat_recovery_steam_generator) present additional corrosion risks.
Corrosion roughens up the rotor, raises stress concentrations, and can initiate cracks under repeated thermal cycles. If the time between the rotting event and inspection are too far apart, damage could go undetected until it rapidly spreads to critical fit areas and blade attachments.
Types of Protective Coatings
Different coating systems are designed to address different failure modes based on temperature, chemistry, and rotor location in the machine. Coating selection involves operating data, inspection history, and repair options.
- Diffusion aluminides – These coatings created a stable aluminum-rich layer which resists oxidation and basic hot corrosion on gas turbine high-temperature areas.
- MCrAlY overlays (Ni/Co-based) – These coatings sprayed on and provide chromium and aluminum to form a scale formation; however, they typically require a ceramic topcoat. Follow this page for more.
- Thermal barrier coatings (TBCs) – consist of a bond coat plus a ceramic layer to reduce the metal temperature on hot gas-path components.
- A HVOF carbide overlay – consists of tungsten or chromium carbides that protect against erosion and wear in steam turbine last stages as well as in, seal areas.
- Stainless or nickel-based alloy (weld, laser) cladding – to build back material, add corrosion resistance to all journals, seal strips, and coupling fits.
High-Temperature Oxidation Explained
When temperatures are high enough, oxygen will react with base metals to form scales. If the scale cracks or spalls, fresh metal will be exposed to oxygen, and the reaction will continue, resulting in thinner rotors as time goes on. The use of aluminide and MCrAlY systems will temper these processes, but only by providing continuous and adherent oxide layers, and even tolerate minor damage because they can heal during operation.
Hot corrosion is much more severe and aggressive than high-temperature oxidation because salts (e.g. sodium sulfate, vanadium compounds) will melt the protective scales and destroys their protective capabilities. Coatings that provide chromium and aluminum reservoirs can help resist these types of attacks, especially during cycling periods. If the damage is localized, or the geometry is tight, it is possible to perform targeted rotor repair and then apply compatible overlay material to restore the profile and delivered protection without complete rotor replacement.
Long-Term Benefits of Coatings
Coatings will not cure all ailments in repair, but the cumulative benefits of coatings are more likely to occur through sound maintenance. The easiest way of capturing value for your plant is by documenting the above listed service hours, fuel quality, and starts.
- Extended service intervals: With preparing fit surfaces and seal lands, slower wear means less anticipated time between major inspections and fewer emergent associated with forced outage planning.
- Lower life-cycle cost: shrinking scrap rates because you are preserving key critical machine tolerances, are an often times more expensive operation than machining.
- Improved reliability: predictably higher machine stability as stabilizing protective scales, slow crack growth initiation, are less applied stress on the shaft or discs during operation, and they should reflect in in-situ balancing results after outages.
- Improved repairable: with consistent substrates from overlay and cladding systems, there will be less variable in the future non-destructive testing and touch-up work completed post-operationally receiving an NDT work scope.
Case Studies from Industry
A combined cycle plant running mid-frame gas turbine was suffering reproducible oxidation at the rotor compressor discharge end because of high inlet dust and too frequent starts. After implementing better filtration and then overlaying the rotor with MCrAlY after an appropriate controlled grit-blast surface preparation, the following operator noted a significant reduction in the rate of oxide spallation.
In another case, a coastal steam turbine found local pitting and droplet erosion from chlorides on the last-stage rotor sections. The engineers specified HVOF tungsten carbide overlays at the seal lands, and then applied a stainless-steel cladding near the coupling fits. Again, from a practical viewpoint, the follow up borescope inspections and NDE tests confirmed stable and consistent surface conditions after two operational seasons, and a noticeable report of a smoother run-up, attributed to operator experience of preserving machine’s balance features and disciplined hot section inspections.
In both cases, coatings worked because they were part of a broader program that included root cause data analysis, surface preparation, controlled post-coat heat treatment, and data-based inspection intervals. When a coating program is able to be aligned against a vendor’s direction and process procedures within the plant, there is opportunity to promote coatings as a durable line of defense which supports turbines for production under real world conditions.