Development of Nanocoatings for Improved Corrosion Resistance of Boiler
Waterwalls
EPRI is conducting research to explore the use of nanotechnology to develop coatings for improved corrosion resistance in the waterwalls of conventional and advanced coal-fired boilers. Begun in 2007, the research is part of a three-year, $2.4 million project, sponsored by the U.S. Department of Energy (DOE), which was awarded under an initiative targeting cost-effective technologies to improve performance and economics of advanced coal-based plants offering near-zero emissions.
The work focuses on the design of nanocoatings optimized for power plant applications and on coating application methods. The project builds on initial strategic research conducted by EPRI in 2006 that highlighted the promise of nanocoatings in fossil plant applications.
Corrosion of Boiler Waterwalls
Fireside corrosion of waterwall tubing is the primary cause of forced outages and availability losses in conventional coal plants, costing U.S. power producers alone almost
$150 million each year. Existing mitigation measures—including weld overlays and thermal spray coatings—offer some protection, especially in subcritical boilers. However, field experience indicates that weld overlays can create additional problems, while conventional coating techniques cannot provide long-term protection in supercritical units. In ultrasupercritical (USC) and advanced boilers, much higher operating temperatures and pressures are anticipated to create more severe fireside tube corrosion challenges.
Nanocoatings
Nanostructured coatings can be defined as ultra-fine microstructures where all the constituents (crystallites, phases) are on the scale of ≤100 nm. In another sense, ultra-fine microstructures contain such a high density of defects (grain boundaries, interphase boundaries, dislocations, etc.) that the spacings between neighboring defects approach interatomic distances.
As a consequence, nanostructured materials can exhibit different properties from their larger-grained, conventional counterparts with the same nominal compositions. These properties include higher hardness and strength, superplasticity, thermal expansion coefficients, magnetic susceptibility and saturation magnetization, superconductivity, giant magnetoresistance and magnetocaloric effects, enhanced thermoelectric power, catalytic effects, etc.
EPRI State-of-Knowledge Review
In 2006, EPRI initiated Technology Innovation work to explore the use of nanotechnology to develop coatings with protective capabilities beyond those achievable with conventional materials. An expert team was convened to review information from the published literature, vendor surveys, university research, and industry experience.
1

The study sought to explore what is known about the abilities of nanocoatings to resist oxidation, corrosion, and erosion in boiler environments. It summarized information on the composition, structure, properties, processes, surface preparation methods, and application procedures associated with conventional and advanced corrosion- and erosion-resistant coatings. Results were published in an EPRI report entitled Program of
Technology Innovation: State of Knowledge Review of Nanostructured Coatings for
Boiler Tube Applications (1014805).
Resistance to Oxidation and Corrosion
As noted in the EPRI report, research to date indicates that nanostructured coatings can also have special oxidation/corrosion properties. Certain nanostructures oxidize selectively to form protective scales with superior adhesion to the substrate coatings. This feature makes nanostructured coatings of special interest for the protection of boiler components.
For example, results of laboratory experiments, to date, indicate that coatings integrating chromium and/or aluminum additives at the nanoscale offer a number of benefits over conventional coatings. Owing to selective oxidation, nanostructured coatings require about one-third the aluminum or one-half the chromium content to establish protective, thin, and continuous thermally grown oxides. The oxides are more adherent and more resistant to thermal cycling and spalling than protective scales forming on the conventional coatings, and they are much more resistant to oxidation and corrosion.
“Laboratory work suggests that nanocoatings could mitigate fireside corrosion in subcritical and supercritical boilers and provide the protective capabilities needed in ultrasupercritical environments,” says David Gandy, Senior Project Manager, EPRI
Technology Innovation. “The challenge now is to create coatings suitable for real-world applications.”
DOE Study
In the DOE project, the EPRI project team is applying computational modeling techniques to evaluate alternative coating compositions and to predict their performance and lifetime. The effectiveness of the application process and the metallurgical and mechanical properties of the nanocoatings are being evaluated in simulated boiler environments using coals from three different regions.
The DOE project consists of six tasks. To date, two have been completed. The first task involved computational modeling of MCrAl nanostructured coating compositions. (M is iron, nickel. or both; Cr is chromium; and Al is aluminum.) Based on prior field and laboratory results published in the literature, MCrAl coatings appeared to be good candidates for USC boiler applications. However, it is necessary to optimize the chemical composition of MCrAl and develop nanocoating process technology for the optimized compositions. The most cost-effective and time-efficient means of optimization is computational modeling.
2

Computational modeling efforts were undertaken to assess the microstructural stability and durability of several potential nano-crystalline coatings. The results identified a new series of Fe-Cr-Ni-Al nano-crystalline coatings that maintain long-term stability by forming a diffusion barrier layer at the coating/substrate interface.
The second task involved testing the coatings identified in the first task. To assess the long-term oxidation behavior of nano-crystalline coatings, cyclic oxidation tests were performed on two coated samples and as well as uncoated samples. The results demonstrated that nano-crystalline coatings showed improvement in cyclic oxidation resistance. The protective oxide scale on the external surface of the nano-crystalline coatings is highly resistant to spallation.
Also, in the second task, metallurgical analysis was performed on the exposed samples to characterize the microstructure of the coating and oxide scales. The results showed the presence of a uniform and continuous protective Al- or Cr-rich oxide layer on the external surface of the coated samples. Though the coating containing 4% Al was internally oxidized, the coatings containing 7 or 10% Al were free from any oxidation and were in excellent condition after long-term testing. As a result, it appears that, for long-term durability, the nano-crystalline coatings should contain at least 7% Al.
These two tasks and their results are described in a recently published EPRI report entitled Program on Technology Innovation: Computational Modeling and Assessment of Nanocoatings for Ultra Supercritical Boilers (1016181).
Related Work—Nanocoatings for Turbines
In a related study, EPRI is also studying the potential of nanocoatings to protect steam and gas turbines from erosion. In initial phases of the study, the project team has assessed the metallurgical performance of these coatings through a series of evaluation methods including scanning electron microscopy, nano-indentation hardness testing, scratch testing, x-ray diffraction, energy dispersive spectroscopy, solid particle erosion, and liquid droplet erosion testing. Results of the initial phase were published in the EPRI report entitled Program on Technology Innovation: Erosion Resistant Coating
Development and Vendor Coating Evaluation for Turbine Components (1014277).
For more information contact David Gandy, 704-595-2198, davgandy@epri.com
.
View or download Program of Technology Innovation: State of Knowledge Review of
Nanostructured Coatings for Boiler Tube Applications (1014805).
View or download Program on Technology Innovation: Computational Modeling and
Assessment of Nanocoatings for Ultra Supercritical Boilers (1016181).
3