Modelling the Development of Type I Hot Corrosion on Coated and Uncoated Single Crystal Superalloys

Abstract

Gas turbines are critical components in the combined cycle power systems being developed to generate electricity from solid fuels, such as coal and biomass. The use of such fuels to produce fuel gases introduces the potential for significant corrosive and erosive damage to gas turbine blades and vanes. Single crystal superalloys have been developed for use with clean fuels but are now being deployed in industrial gas turbines. The performance of these materials, with coatings, has to be determined before they can be used with confidence in dirtier fuel environments. This paper reports results from a series of laboratory tests carried out using the ‘deposit replenishment’ technique to investigate the sensitivity of candidate materials to exposure conditions anticipated to cause type I hot corrosion in such gas turbines. The materials investigated have included the single crystal nickel-based superalloys CMSX-4 and SC2-B, both bare and with Pt-Al coatings. The exposure conditions within the laboratory tests have covered ranges of SOx (50 and 500 volume parts per million, vpm) and HCl (0 and 500 vpm) in air, as well as 4/1 (Na/K)2SO4 deposits, with deposition fluxes of 1.5, 5 and 15 5g/cm2/h, for periods of up to 500 hours at 900°C. Data on the performance of materials has been obtained using dimensional metrology: pre-exposure contact measurements and post-exposure measurements of features on polished cross-sections. These measurement methods allow distributions of damage data to be determined for use in the development of materials performance modelling. In addition, the types of damage observed have been characterised using standard optical and SEM/EDX techniques. The damage rates of the single crystal materials without coatings are too high for them to be used with confidence in gas turbines fired with gases derived from ‘dirty fuels’. Under the more severe combinations of gas composition, deposition flux and metal temperature, the corrosion rates of these materials with Pt-Al coatings are also excessive. The data produced from these tests has allowed the sensitivity of hot corrosion damage to changes in the exposure environment to be determined for the single crystal alloys and coating systems examined.