Hot Corrosion of Coated Single Crystal Superalloys

摘要

Gas turbines are at the heart of many modern power systems, with combined cycle powerrngeneration utilising natural gas being an effective way of reducing environmental emissionsrncompared to conventional pulverised coal fired plants. The development of gas turbinerntechnology has been focused on increasing its efficiency. However, the lives of the hot gasrnpath components within these gas turbines are also critical to the viability of the powerrnsystems. Single crystal superalloys have been developed for use with clean fuel/air but arernnow being used in industrial gas turbines that may need to run with dirtier fuel/air. Indeed,rngas turbine based power systems are being evaluated in which solid fuels (e.g. coal and/orrnbiomass) are gasified to produce fuel gases, which introduces the potential for significantrncorrosive and erosive damage to gas turbine blades and vanes. The performance of thesernmaterials, with coatings, has to be determined before they can be used with confidence inrndirtier fuel environments.rnThis paper reports results from a series of laboratory tests carried out using thern'deposit replenishment' technique to investigate the sensitivity of candidate materials tornexposure conditions anticipated in such gas turbines. The materials investigated havernincluded CMSX-4 and SC2-B (both bare and with Pt-Al and Amdry 997 coatings) as well asrnconventional nickel based superalloys such as IN738LC for comparison. The exposurernconditions within the laboratory tests have covered ranges of Sox (50 and 500 vpm) and HClrn(0 and 500 vpm) in air, as well as 4/1 (Na/K)2SO4 deposits, with deposition fluxes of 1.5, 5rnand 15 μg/cm2/h, for periods of up to 500 hours at 700 and 900℃. Data on the performancernof materials has been obtained using dimensional metrology: pre-exposure contactrnmeasurements and post-exposure measurements of features on polished cross-sections. Thesernmeasurement methods allow distributions of damage data to be determined for use in therndevelopment of materials performance modelling. In addition, the types of damage observedrnhave been characterised using standard optical and SEM/EDX techniques.rnThe damage rates of the single crystal materials without coatings are too high for themrnto be used with confidence in gas turbines fired with gases derived from dirty fuels. Underrnthe more severe combinations of gas composition, deposition flux and metal temperature, therncorrosion rates of these materials with Pt-Al coatings are also excessive. The data producedrnfrom these tests has allowed the sensitivity of hot corrosion damage to changes in thernexposure environment to be determined for the single crystal alloys and coating systemsrnexamined. These data are compared to the behaviour of conventional nickel basedrnsuperalloys, with higher corrosion resistance but lower high temperature strength, that havernbeen evaluated in previous studies.