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. G020095.Euro.08.2002 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

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

4. G020095.Euro.08.2002 4 Alloy 671 (Ni-48Cr wt.% base) Rapidly Corroded In Molten Smelt (100 h, 1000°C) Before/After Macros Alloy 671 generally considered one of most smelt-resistant alloys available yet still aggressively attacked Optical cross-section (region of least attack) Attacked Unaffected Metal 200  m

5. G020095.Euro.08.2002 5 Cr-6MgO-0.75Ti wt.% Shows Promising Behavior In 500 h, 1000°C Smelt Immersion 200  m Cr-Na-O Hardened Metal Unaffected SEM cross-section (region of greatest attack) Low mag optical of 0.6” diameter coupon  SEVEN times slower than alloy 671 corrosion rate (estimated from 1000°C, 100 h smelt immersion)

6. G020095.Euro.08.2002 6 Surface recession in  950-1000°C molten smelt - 0.6 - 0.4 - 0.2 0 200400600 Surface Recession (mm) Time (h) Cr-6MgO-0.5Ti-0.3La 2 O 3 Alloy 671 Ni-50Cr base (-0.69mm) (Cr-6MgO-0.5Ti) Smelt Corrosion Studies At ORNL Suggest Cr-MgO Surface Recession Less Than Linear Microalloying with 0.3 wt.% La 2 O 3 reduces rate of attack (slows growth of Cr 2 O 3 scale)

7. G020095.Euro.08.2002 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

8. G020095.Euro.08.2002 8 Cr-X And Cr-MgO Coupons Provided To Institute Of Paper Science And Technology For Corrosion Screening In Molten Smelt 800, 900, 1000°C / 8, 24, 100 h corrosion matrix (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, La 2 O 3 Benchmark comparison for cast alloys Assess benefits of microalloying additions

9. G020095.Euro.08.2002 9 Cr-6 MgO-0.1 La 2 O 3 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

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

11. G020095.Euro.08.2002 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

12. G020095.Euro.08.2002 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 MgCr 2 O 4 spinel during sintering consolidation  MgCr 2 O 4 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

13. G020095.Euro.08.2002 13 Ambient Ductility Of Scruggs’ Sintered/ Extruded Cr-6MgO-0.5Ti wt.% Confirmed 80  m Microstructure (SEM) Cr matrix (light), MgO (dark) MgCr 2 O 4 Spinel (gray) Displacement 10.7% Elongation Tensile Behavior unalloyed Cr Cr-6MgO-0.5Ti Load  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)

14. G020095.Euro.08.2002 14 SEM EDX Spectrum Image Phase Map 40  m Cr matrix (Black) Cr 2 O 3 (Cyan) Cr-Carbide (Yellow) Cr-Nitride (Red) Chromium Carbides And Nitrides On Grain Boundaries In Hot-Pressed Unalloyed Cr  Brittle: room-temperature tensile ductility 1%   400 wppm carbon and 250 wppm nitrogen

15. G020095.Euro.08.2002 15 Deleterious Tramp Impurities Found At Cr/Oxide Dispersion Interfaces Oxygen MapSecondary Mode SEM *Nitrogen Map Sulfur Map 5 mm Auger maps of in-situ fractured Cr-6MgO-0.5Ti (*Ti peak overlap issues) 20 nanometer thick co-segregated impurity layer

16. G020095.Euro.08.2002 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