A Novel Low-Temperature Fiffusion Aluminide Coating for Ultrasupercritical Coal-Fried Boiler Applications
An ultrasupercritical (USC) boiler with higher steam temperature and pressure is expected to increase the efficiency of the coal-fired power plant and also decrease emissions of air pollutants. Ferritic/martensitic alloys have been developed with good creep strength for the key components in coal-fired USC plants. However, they typically suffer excessive steam-side oxidation, which contributes to one of main degradation mechanisms along with the fire-side corrosion in coal-fired boilers. As the steam temperature further increases in USC boilers, oxidation of the tube internals becomes an increasing concern, and protective coatings such as aluminide-based diffusion coatings need to be considered. However, conventional aluminizing processes via pack cementation or chemical vapor deposition are typically carried out at elevated temperatures (1000-1150 C). Thermochemical treatment of ferritic/martensitic alloys at such high temperatures could severely degrade their mechanical properties, particularly the alloy's creep resistance. The research focus of this project was to develop an aluminide coating with good oxidation resistance at temperatures {le} 700 C so that the coating processing would not detrimentally alter the creep performance of the ferritic/martensitic alloys. Nevertheless, when the aluminizing temperature is lowered, brittle Al-rich intermetallic phases, such as Fe{sub 2}Al{sub 5} and FeAl{sub 3}, tend to form in the coating, which may reduce the resistance to fatigue cracking. Al-containing binary masteralloys were selected based on thermodynamic calculations to reduce the Al activity in the pack cementation process and thus to prevent the formation of brittle Al-rich intermetallic phases. Thermodynamic computations were carried out using commercial software HSC 5.0 for a series of packs containing various Cr-Al binary masteralloys. The calculation results indicate that the equilibrium partial pressures of Al halides at 700 C were a function of Al content in the Cr-Al alloys. Cr-25Al and Cr-15Al were chosen as the masteralloys in the pack cementation process. In contrast to pure Al masteralloy which led to the formation of Fe{sub 2}Al{sub 5} coatings at 650 C, a coating consisting of a thin Fe{sub 2}Al{sub 5} outer layer and an FeAl inner layer was formed at 700 C with the Cr-25Al masteralloy. By switching to the Cr-15Al masteralloy, thin FeAl coatings ({approx}12 {micro}m) containing < 50 at.% Al were achieved at 700 C. The effect of the amount of masteralloys on coating growth was also studied by employing packs containing 2NH{sub 4}Cl-x(Cr-15Al)-(98-x)Al{sub 2}O{sub 3}, where x = 10, 20, 30, 40, 50, 60, and 70 wt.%. It was noticed that when the Cr-15Al masteralloy was increased from 10 to 40 wt.% in the pack, both coating thickness and surface Al content increased, suggesting that gas phase kinetics played an important role in Al deposition. However, with further increase of the masteralloy, solid state diffusion became the rate-limiting factor. The long-term oxidation performance of the aluminide coatings synthesized at 700 C with Cr-25Al and Cr-15Al masteralloys was evaluated in the water vapor environment at 650-700 C. The low-temperature pack coatings demonstrated excellent oxidation resistance at 650 C in humid air after {approx}1.2 yr testing. Longer lifetimes can be expected for these thin coatings due to minimal interdiffusion at this testing temperature. Exposure at 700 C was conducted to accelerate coating failure via increased interdiffusion of Al with the substrate alloy. The coatings also exhibited good oxidation protection up to 6,000-8,000 h at 700 C, with longer testing needed for coating failure to occur. Furthermore, the oxidation results indicate that in addition to the Al reservoir (as determined by the Al content and coating thickness), the initial coating surface quality had a significant impact on the oxidation behavior. In addition, the effect of various pack aluminide coatings on the creep resistance of coated T91 was investigated. Three representative types of coatings with different thicknesses, Al concentrations and phase constituents were included in the creep test. The creep experiments were performed at 650 C in air at uniaxial stress levels of 100-120 MPa. The thick aluminide coating made at 1050 C showed a considerable reduction ({approx} 42%) in the creep rupture life of the coated alloy, due to the decrease of the load-bearing section. In contrast, the low-temperature thin coatings developed in this project did not significantly reduce the creep resistance of the coated T91 alloy, particularly the thin FeAl coating synthesized at 700 C using the Cr-15Al masteralloy.
- Research Organization:
- Tennessee Technological Univ., Cookeville, TN (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FG26-06NT42674
- OSTI ID:
- 1000505
- Country of Publication:
- United States
- Language:
- English
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