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. LiuOak Ridge National Laboratory

Preet SinghInstitute of Paper Science and Technology

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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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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

TestSample

MoltenSmelt

Graphite Lid

Schematic of Smelt Immersion Rig

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Alloy 671 (Ni-48Cr wt.% base) RapidlyCorroded In Molten Smelt (100 h, 1000°C)

Before/After Macros

Alloy 671 generally considered one of most smelt-resistantalloys available yet still aggressively attacked

Optical cross-section(region of least attack)

Attacked

Unaffected Metal200 m

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Cr-6MgO-0.75Ti wt.% Shows PromisingBehavior In 500 h, 1000°C Smelt

Immersion

200 m

Cr-Na-OCr-Na-O

Hardened MetalHardened Metal

UnaffectedUnaffected

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)

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Surface recession in 950-1000°C molten smelt

- 0.6

- 0.4

- 0.2

0 200 400 600S

urf

ace

Rec

essi

on (

mm

)

Time (h)

Cr-6MgO-0.5Ti-0.3La2O3

Alloy 671Ni-50Cr base (-0.69mm)

(Cr-6MgO-0.5Ti)

Smelt Corrosion Studies At ORNL SuggestCr-MgO Surface Recession Less Than Linear

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

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, La2O3

• Benchmark comparison for cast alloys• Assess benefits of microalloying additions

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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?

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

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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Ambient Ductility Of Scruggs’ Sintered/Extruded Cr-6MgO-0.5Ti wt.% Confirmed

80 m

Microstructure (SEM)

Cr matrix (light), MgO (dark)MgCr2O4 Spinel (gray)

Displacement

10.7% Elongation10.7% Elongation

Tensile Behavior

unalloyed Cr

Cr-6MgO-0.5TiCr-6MgO-0.5Ti

Loa

d

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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SEM EDX Spectrum Image Phase Map

40

m

Cr matrix (Black)

Cr2O3 (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

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Deleterious Tramp Impurities FoundAt Cr/Oxide Dispersion Interfaces

Oxygen MapSecondary Mode SEM

*Nitrogen MapSulfur Map5 mm

Auger maps of in-situ fractured Cr-6MgO-0.5Ti(*Ti peak overlap issues)

20 nanometer thick co-segregated impurity layer

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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