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

G Euro 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

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

G Euro 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

G Euro 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)

G Euro Surface recession in  °C molten smelt 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)

G Euro 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

G Euro 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

G Euro 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

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

G Euro 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

G Euro 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

G Euro 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 /s strain rate, 600 grit surface)

G Euro 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

G Euro 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

G Euro 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