Development of Nanocoatings for Improved Corrosion Resistance of

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					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.




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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.




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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).




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posted:9/1/2011
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