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Development of Corrosion-Resistant Cr-Rich Alloys for Gasifier and Kraft Recovery Boiler Applications. J.R. Keiser, M.P. Brady, V.K. Sikka and C.T. Liu Oak Ridge National Laboratory Preet Singh Institute of Paper Science and Technology.

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Development of Corrosion-Resistant

Cr-Rich Alloys for Gasifier and Kraft Recovery Boiler Applications

J.R. Keiser, M.P. Brady, V.K. Sikka and C.T. Liu

Oak Ridge National Laboratory

Preet Singh

Institute of Paper Science and Technology

Objectives Of “Development Of Corrosion-Resistant Cr-Rich Alloys For Gasifier And Kraft Recovery Boiler Applications” Project
  • Develop chromium-rich alloy/coating that has significantly better resistance to molten smelt corrosion than any commercially available alloy
  • Improve the ductility of chromium-rich alloys so that the developed alloy has practical applications in high-temperature gasifiers and recovery boilers

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Schematic of Smelt Immersion Rig

Sample Holder

Ar Gas for Agitation

Graphite Lid





Screening Tests At ORNL Are Conducted In Molten Smelt From Weyerhaeuser’s New Bern, NC Mill

950-1000°C/8-500 h screenings

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Before/After Macros

Optical cross-section

(region of least attack)


Unaffected Metal

200 mm

Alloy 671 (Ni-48Cr wt.% base) Rapidly

Corroded In Molten Smelt (100 h, 1000°C)

Alloy 671 generally considered one of most smelt-resistant

alloys available yet still aggressively attacked

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Cr-6MgO-0.75Ti wt.% Shows Promising

Behavior In 500 h, 1000°C Smelt Immersion

Low mag optical

of 0.6” diameter coupon

SEM cross-section

(region of greatest attack)


Hardened Metal


200 mm

  • SEVEN times slower than alloy 671 corrosion rate (estimated from 1000°C, 100 h smelt immersion)

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- 0.2


Surface Recession (mm)

- 0.4

Alloy 671

Ni-50Cr base


- 0.6

Time (h)

Smelt Corrosion Studies At ORNL Suggest

Cr-MgO Surface Recession Less Than Linear

Surface recession in 950-1000°C molten smelt

Microalloying with 0.3 wt.% La2O3 reduces rate of attack (slows growth of Cr2O3 scale)

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Effort Initiated For Development Of Cr-Rich Alloys/Coatings Specifically Designed For Molten-Smelt Environments

Two-pronged development approach

  • Powder Metallurgy MgO-Dispersed Cr (Cr-6MgO wt.% base)
    • >10% ductility at room-temperature already demonstrated
    • Impact toughness, strain rate/notch sensitivity, high-temperature strength, smelt resistance need to be fully characterized and optimized
  • Cast Cr-based (> 50 wt.%) Alloys
    • Fe, Ni, Mo, ... additions
    • Co-optimize smelt resistance/mechanical properties
    • Goal is improved 671 type alloy optimized for smelt
  • Initially targeted components range from coatings, thermowells and brackets to spouts and structural components

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Cr-X And Cr-MgO Coupons Provided To Institute Of Paper Science And Technology For Corrosion Screening In Molten Smelt

(Collaboration with P. Singh and J. Sugalski)

  • Cast Cr-(50, 65, 85) X wt.% where X = Fe, Ni
    • Level of Cr needed for good smelt resistance will determine whether reasonable chance to successfully co-develop useful mechanical properties
  • Cr-6MgO-X where X = Ti, La2O3
    • Benchmark comparison for cast alloys
    • Assess benefits of microalloying additions

800, 900, 1000°C / 8, 24, 100 h corrosion matrix

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Cr-6 MgO-0.1 La2O3 Rod Delivered To Weyerhaeuser Columbus, Mississippi Mill For Corrosion Exposure

  • Smelt/mixed gas environment at black liquor nozzle/gun port
  • Initial feedback suggests rapid corrosion-analysis of sample planned-results will be incorporated into alloy design effort

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what about mechanical properties
What About Mechanical Properties?
  • Cr usually not useable because brittle at room-temperature
  • Cr-MgO alloys exhibit ductility at room-temperature!

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Chromium Is Usually Brittle At Room-Temperature

  • Impurities a major contributor to room-temperature brittleness in Cr (raises brittle to ductile transition temperature, BDTT)
  • Nitrogen particularly deleterious
    • high elevated temperature solubility, near zero at room temp
    • precipitates as fine, acicular grain boundary phase
    • as little as 5-10 wppm nitrogen can be embrittling
  • Issues regarding dislocation initiation and motion also important in high BDTT of Cr

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bendix corp scruggs et al mid 1960 s ductilized cr via additions of mgo
Bendix Corp./Scruggs et al. (Mid 1960’s)Ductilized Cr Via Additions Of MgO
  • Additions of MgO to commercial-purity Cr powder partially convert to MgCr2O4 spinel during sintering consolidation
  • MgCr2O4 spinel postulated to getter nitrogen: 10-20% tensile elongation at room temperature reported
    • Cr-(3-6)MgO-0.5Ti wt.% typical alloy
  • Cr-MgO alloys used in limited applications but “beaten out” by superalloys- essentially forgotten since then
    • Excellent smelt resistance is driver to revisit Cr

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Microstructure (SEM)

Tensile Behavior



unalloyed Cr

10.7% Elongation

80 mm


Cr matrix (light), MgO (dark)

MgCr2O4 Spinel (gray)

Ambient Ductility Of Scruggs’ Sintered/

Extruded Cr-6MgO-0.5Ti wt.% Confirmed

  • Original alloy supplied by Scruggs (300 wppm nitrogen): sintered 1600°C, 2h, extruded 9:1 reduction ratio (typical)
  • Average room-temperature plastic tensile elongation of 8% (3.33 X 10-3/s strain rate, 600 grit surface)

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Cr2O3 (Cyan)



40 mm



Cr matrix (Black)

Chromium Carbides And Nitrides On Grain

Boundaries In Hot-Pressed Unalloyed Cr

SEM EDX Spectrum Image Phase Map

  • Brittle: room-temperature tensile ductility 1%
  • 400 wppm carbon and 250 wppm nitrogen

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Deleterious Tramp Impurities Found

At Cr/Oxide Dispersion Interfaces

Auger maps of in-situ fractured Cr-6MgO-0.5Ti

(*Ti peak overlap issues)

Secondary Mode SEM

Oxygen Map

5 mm

Sulfur Map

*Nitrogen Map

20 nanometer thick co-segregated impurity layer

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  • Although this project is only in its first year, it builds on a Fossil Energy-funded project
  • Development of an understanding of the mechanism by which MgO additions ductilize chromium provides a basis for further alloy development
  • Alloy modifications have been found that increase the alloy ductility to at least 10%
  • Samples have been provided for exposure in recovery boilers and gasifiers
  • This project should result in practical alloys and/or coatings that are highly resistant to molten smelt

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