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.
Effects of Different Surface Native Pre-Oxides on the Hot Corrosion Properties of Nickel-Based Single Crystal Superalloys