Material Characterization for Power Generation Applications

Most of the power generated in the United States is generated with turbines using either a Rankine cycle (with steam created by nuclear reactors or by burning coal or biomass) or a Brayton cycle (with natural or synthesis gas-firing). In either case, the main strategy for increasing efficiency of these turbines is to increase the peak cycle temperature.

The goal of clean coal research is to increase the steam temperature from the current U.S. plant average of ~550°C to 700°-760°C, which would increase efficiency (and decrease emissions) by >30%. Similar to jet engines on aircraft, the peak temperature in the highest efficiency land-based gas turbines is 1500°-1700°C, which is higher than the melting temperature of most conventional alloys. At these conditions, Ni-base superalloy blades and vanes in the turbine are protected by sophisticated film cooling systems and by thermal barrier coatings.

These coatings consist of a ceramic layer (low thermal conductivity zirconia) on top of an oxidation-resistant metallic coating.¹ The high temperatures and corrosive environments (steam, combustion gas) can degrade materials quickly. However, power generation for the commercial electrical grid requires a high degree of reliability with extended periods (years) of operation with minimal shutdowns.

Thus, a key materials research topic is studying the high-temperature degradation of candidate materials (including protective coatings) and quantifying the rate of degradation using various characterization techniques. Degradation can be due to the mechanical load or due to the corrosion of the material due to the reactive environment.

The primary characterization tool for conventional structural alloys (such as ferritic or austenitic steels or Ni-base alloys) used in most power generation applications is metallographic cross-sections. Material is cut, mounted in epoxy or other media and polished to examine the alloy microstructure and depth of attack. Examinations are by light microscopy and, for higher magnifications, electron microscopy.

Using electron microscopy, chemical compositions can be determined by measuring the characteristic x-rays emitted from the material due to excitation by the impinging electron beam. Using energy dispersive x-ray analysis (typical on scanning electron microscopes) or wave length dispersive x-ray analysis (typical on electron microprobes), chemical composition of the surface reaction product and underlying alloy can be determined, especially as a function of depth to determine changes.

Examples are shown in Figures 1 and 2 for a commercial Ni-base alloy. As more Cr-rich oxide forms on the surface at longer time and higher temperature exposures, Cr becomes depleted from the underlying alloy (Figure 2), which can affect the mechanical properties. Eventually, such depletion will lead to an accelerated corrosion rate and component failure. This type of characterization is essential to modeling the long-term (50-250kh) performance of this and other materials and predicting lifetimes and maximum use temperatures.

Figure 1. Secondary electron scanning electron microscope cross-section image of commercial alloy 230 (Ni-22wt.%Cr-14W) after 5,000h at 1000°C in laboratory air. The specimen was Cu-plated before mounting to protect the surface oxide. The data from the scan line is one of the profiles in Figure 2.

More sophisticated characterization techniques are used to study the mechanisms of degradation. For example, ¹⁸O tracers can be used to determine the growth mechanism of the surface oxide or scale.^2,3 X-ray diffraction (XRD) is commonly used to identify phase composition. In-situ XRD can help to understand phase transformations and dynamic phenomena that occur during service at high temperature.^4,5

Figure 2. Electron microprobe Cr composition profiles from samples of alloy 230 exposed at three different conditions in laboratory air. Near the surface, the Cr content is higher due to the formation of a Cr-rich oxide. Beneath the external scale, the alloy is depleted in Cr depending on the exposure time and temperature.

Analytical transmission electron microscopy is used to study the thermally grown scale at even higher magnifications and can identify the location of dopants.⁶ Surface analytical techniques (e.g., auger or x-ray photoelectron spectroscopy) can identify the composition and chemical state of thin surface reaction products and interface segregants.⁷ Currently, new characterization techniques are being explored to study the proposed role of hydrogen in degrading materials exposed to steam or exhaust gas (containing H₂O).³

References

1. B. Gleeson, "Thermal Barrier Coatings for Aeroengine Applications," J. Prop. Power, 22 (2006) 375-383.
2. B. A. Pint, J. R. Martin and L. W. Hobbs, "18O/SIMS Characterization of the Growth Mechanism of Doped and Undoped ?-Al2O3," Oxid. Met. 39 (1993) 167-95.
3. W. J. Quadakkers, J. Zurek, and M. Hänsel, "Effect of water vapor on high temperature oxidation of FeCr alloys," JOM 61(7) (2009) 44-50.
4. B. A. Pint, S. A. Speakman, C. J. Rawn and Y. Zhang, "Deformation and Phase Transformations During Cyclic Oxidation of Ni-Al and Ni-Pt-Al," JOM 58(1) (2006) 47-52.
5. P. Y. Hou, A. P. Paulikas, and B. W. Veal, "Growth Strains in Thermally Grown Al2O3 Scales Studied Using Synchrotron Radiation," JOM 61(7) (2009) 51-55.
6. B. A. Pint and K. L. More, "Characterization of Alumina Interfaces in TBC Systems," J. Mater. Sci. 44 (2009) 1676-86.
7. P. Y. Hou, "Segregation Phenomena at Thermally Grown Al2O3 Alloy Interfaces," Annu. Rev. Mater. Res., 38 (2008) 275-98.