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Development of Corrosion-Resistant

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

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  1. 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

  2. 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 G020095.Euro.08.2002 2

  3. Schematic of Smelt Immersion Rig Sample Holder Ar Gas for Agitation Graphite Lid Test Sample Molten Smelt Screening Tests At ORNL Are Conducted In Molten Smelt From Weyerhaeuser’s New Bern, NC Mill 950-1000°C/8-500 h screenings G020095.Euro.08.2002 3

  4. Before/After Macros Optical cross-section (region of least attack) Attacked 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 G020095.Euro.08.2002 4

  5. 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) Cr-Na-O Hardened Metal Unaffected 200 mm • SEVEN times slower than alloy 671 corrosion rate (estimated from 1000°C, 100 h smelt immersion) G020095.Euro.08.2002 5

  6. 200 400 600 0 (Cr-6MgO-0.5Ti) - 0.2 Cr-6MgO-0.5Ti-0.3La2O3 Surface Recession (mm) - 0.4 Alloy 671 Ni-50Cr base (-0.69mm) - 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) G020095.Euro.08.2002 6

  7. 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 G020095.Euro.08.2002 7

  8. 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 G020095.Euro.08.2002 8

  9. 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 G020095.Euro.08.2002 9

  10. What About Mechanical Properties? • Cr usually not useable because brittle at room-temperature • Cr-MgO alloys exhibit ductility at room-temperature! G020095.Euro.08.2002 10

  11. 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 G020095.Euro.08.2002 11

  12. 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 G020095.Euro.08.2002 12

  13. Microstructure (SEM) Tensile Behavior Cr-6MgO-0.5Ti Load unalloyed Cr 10.7% Elongation 80 mm Displacement 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) G020095.Euro.08.2002 13

  14. Cr2O3 (Cyan) Cr-Carbide (Yellow) 40 mm Cr-Nitride (Red) 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 G020095.Euro.08.2002 14

  15. 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 G020095.Euro.08.2002 15

  16. Summary • 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 G020095.Euro.08.2002 16

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