A. Encinas-Oropesa

Modelling hot corrosion in industrial gas turbines

Abstract Gas turbines are a critical component within combined cycle power systems that are being developed to generate electricity more cleanly and efficiently from solid fuel sources, that include coal and biomass. The use of such fuels, to produce fuel gases, increases the potential for significant corrosion and erosion damage to gas turbine blades and vanes. This paper addresses the modelling and prediction of type II hot corrosion in industrial gas turbines within the aim of given acceptable and predictable lifetimes. A matrix of corrosion tests have been undertaken using the ‘deposit recoat’ test procedure, with samples cooled periodically to re-apply controlled amounts of salt deposit. Deposited salt was 4/1 mole fraction of Na2SO4 and K2SO4, with deposited fluxes of 0, 1.5, 5.0 and 15.0 μg/cm2/h. Samples of polycrystalline (IN738 and IN792) and single crystal superalloys (CMSX4 and SC2B) were exposed for test durations of 500 and 1000 h at 700°C in a variety of gas compositions, consisting of air+50–500 vppm SO2+0–500 vppm HCl+0–5 vol% H2O. Section loss data has been measured, using precision optical metrology and analysed statistically. Models have been developed that predict section loss as a function of salt deposition rate and gas composition to precisions of ±20 mm loss, with 95% confidence (2×standard deviation).

Evaluation of oxidation related damage caused to a gas turbine disc alloy between 700 and 800°C

Abstract This paper presents the results of a study targeted at characterising the oxidation behaviour of a new nickel based disc alloy (RR1000) at intermediate temperatures. Isothermal exposures were carried out using a thermo-microbalance at temperatures in the range 700 – 800°C for exposures up to 200 hours. Cyclic oxidation exposures were carried out at, 700 and 750°C for up to 1000 hours with 100 hour cycles, using mass change to monitor the materials performance. The mass gain data obtained have been used to derive oxidation reaction rate parameters, using established methodologies, with parabolic rate constants varying between 1.4×10–5 mg2/cm4/h at 700°C and 8.4×10–4 mg2/cm4/h at 800°C. Surface oxides were initially analysed using scanning electron microscope/energy dispersive X-ray analysis (SEM/EDX) and X-ray diffraction (XRD) techniques. The results showed that slowly-growing chromium-rich oxide scales had formed on the surface of the samples during these exposures. A more detailed study of cross-sections through the oxide layers and underlying alloy was undertaken using a focused ion beam (FIB) system. Measurements of the thin oxides observed showed ranges of thicknesses from 0.08μm up to 1.9μm, that were consistent with the mass change data gathered. However, the FIB examinations also revealed significant sub-surface damage that contained a mixture of grain boundary pores, a second phase depletion zone, and grain recrystallisation. The depth of these sub-surface damage zones was greater than the thickness of the oxide layers and is believed to have a major impact on the fatigue performance of this disc alloy. The use of the FIB system has enabled characterisation of the development of both the oxide layers and sub-surface damage zones as a function of exposure time and temperature.

High temperature oxidation and corrosion of gas turbine materials in burner rig exposures

Abstract With the introduction of novel fuels, which may contain high levels of trace impurities including sulphur and alkali metals, industrial gas turbines are operating with increasingly corrosive combustion environments. To investigate the effect that this, coupled with the higher combustion gas temperatures needed to increase power plant efficiency, has on current state-of-the-art gas turbine component materials, three burner rig exposure tests have been run. The tests evaluated the effects of fly ash, gas moisture and gas temperatures on the alkali sulphate induced hot corrosion of CM247LC, Haynes 230, IN939 and IN728LC. Type I (sulphidation and internal damage), type II (pitting) and mixed mode hot corrosion were observed under different test conditions; however, the presence of fly ash appeared to reduce the levels of hot corrosion. CM247LC, with its high Al content improving oxidation resistance, showed less resistance to hot corrosion than the other, higher Cr content, alloys.

High temperature oxidation and corrosion of gas turbine component materials in burner rig exposures

Abstract To meet environmental, legislative and commercial targets, gas turbines must operate with increasingly high gas temperatures and fuels with increased contaminant levels. A series of three burner rig tests have been used to evaluate the effects of fly ash, gas moisture and gas temperatures on alkali metal induced hot corrosion in the metal temperature range of 700 – 960°C on three uncoated materials (Haynes 230, IN939, and IN738LC) and one coated system (IN738LC/HVOF SV21). It has been found that the specific burner rig test conditions impact upon the severity of samples’ corrosion and oxidation, with samples exposed to impacting fly ash demonstrating reduced hot corrosion. However, type II hot corrosion (pitting) and type I hot corrosion (internal damage and sulfidation) have been observed under all test conditions. Generally IN939 was more resistant to hot corrosion over both high and low temperatures than IN738LC or Haynes 230. SV21-coatings on IN738LC provided improved resistance to both type I and II hot corrosion.

An Evaluation of the Performance of Candidate Gas Turbine Abradeable Seal Materials Exposed to a High Temperature Combustion Atmosphere

The performance of metallic honeycomb sections has been studied under thermal cycling conditions involving their exposure to a simulated natural gas combustion environment for total exposure times of up to 2,520 hours. Samples manufactured from PM2000 and Haynes 214 were tested at temperatures of 950, 1050, 1100 and 1150 o C and cycled on a weekly basis with 15 thermal cycles between room temperature and the test temperature. Samples of Nimonic 86 were included in the test at 950°C for comparison. The performance of the materials during the tests was monitored using weight change data. More useful dimensional metrology data were derived from surface and cross-sectional oxide thickness and metal-loss measurements on specimens examined after 15 thermal cycles (and after fewer cycles at higher temperatures if significantly enhanced oxidation attack had been experienced by the alloys). After 2,520 hours at 1100 and 1150°C, both PM2000 and Haynes 214 were either totally oxidised or showed very significant damage. At 1050°C, however, the materials performed much better, with little to distinguish them, except Haynes 214 had more internal corrosion damage in places. Damage levels were lower still at 950°C, with Nimonic 86 having higher weight gains and thicker surface oxides than the other two materials. Overall, this study has shown that there is still a lot of development work required in order to move to higher temperature sealing systems utilizing honeycomb structures in gas turbine applications.

Development of Hot Corrosion on Coated Single Crystal Superalloys

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 in such gas turbines. The materials investigated have included CMSX-4 and SC 2 -B, both bare and with Pt-Al coatings. The exposure conditions within the laboratory tests have covered ranges of SOx (50 and 500 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 μg/cm 2 /h, for periods of up to 500 hours at 700 and 900°C. Data on the performance of materials has been obtained using dimensional metrology: preexposure contact measurements and post-exposure measurements of features on polished crosssections. 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.

Degradation of Fe–Cr–Al–RE and Ni–Cr–Al–RE foils in air and combustion gas atmospheres

Abstract In the continuing drive to increase gas turbine operating efficiencies (and reduce environmental emissions), it is necessary to consider ways of improving the temperature capabilities of hot gas path sealing materials. One potential route is to investigate the possibility of using alternative materials within the traditional honeycomb structure. This paper presents the results of investigations into the high temperature oxidation performance of a range of commercial Fe–20wt%Cr–5wt%Al–RE and Ni–16wt%Cr–5wt%Al–RE foil materials in air and simulated combusted natural gas environments. The effects of exposures for periods of up to 1500 hours have been studied in the temperature range 950–1300°C. During each series of tests the foils were subjected to regular thermal cycles (to room temperature) with dwell periods at the target exposure temperatures ranging from 20 hours at the higher temperatures to 100 hours at the lower temperatures. The degradation kinetics of each foil sample were monitored using mass change measurements at each thermal cycle. In addition, samples were periodically removed for destructive examinations to enable more meaningful metal loss measurements to be made and degradation mechanisms to be established. In this way the principal parameters governing the oxidation performance were established, as well as times to the onset of breakaway oxidation (when these fell within the exposure periods studied at each temperature). Earlier models for the performance of Fe–Cr–Al–RE materials have been adapted to describe the performances of the foils observed in this study.

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

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.