This paper describes the ceramographic preparation of Cr{sub 2}O{sub 3}-Al{sub 2}O{sub 3} refractory bricks and subsequent microstructural analysis to determine the corrosive effects of molten slag. The porous and friable nature of the brick, especially after exposure to the slag or its individual components, presented some problems in the preparation

Chinn, Richard E.

Hunt, Alton H.

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6played, ready for use.The preparation methods in e-Metalog are taken from Struersmethods database IMAD, andthey have been developed byspecialists in the Struers applica-tion laboratories in the USA, UK,D, F, J, and in the main labora-tory in Copenhagen.Those original methods havebeen developed for a specificmaterial e.g. a ceramic like Al2O3,the material has a defined condi-tion and the method takes thespecial requirements, such asperfect edge retention, into ac-count. The work has been doneon different equipment but hasbeen converted for e-Metalog tothe same generic equipmentconfiguration as the MetalogGuide methods, a RotoSystemwith 300 mm dia disc, 160 mmdia. specimen holder, 6 x 30 mmdia. mounted specimens. After selecting the main groupceramics and the sub-group oxides themethod for Al2O3 is displayede-Metalog is to be found onStruers homepagewww.Struers.com.To enter you must have a username and a password. Fill out ashort form to register and theuser name and password will besend to you by mail.A Ceramographic Evaluation of ChromiaRefractories Corroded by SlagAlton H. Hunt & Richard E. ChinnUnited States Department ofEnergyi, Albany Research CenterAlbany, OregonUSA  97321-2198http://www.alrc.doe.govAbstractThis paper describes the cera-mographic preparation of Cr2O3–Al2O3 refractory bricks and sub-sequent microstructural analysisto determine the corrosive effectsof molten slag. The porous andfriable nature of the brick, espe-cially after exposure to the slagor its individual components, pre-sented some problems in thepreparation.IntroductionChromia-alumina (Cr2O3–Al2O3)refractory bricks lining the com-bustion chambers of gasifiers areexposed to molten slag at tem-peratures as high as 1600°C andpressures as high as 2.8 MPa (27atm) gauge, for as much as twoyears between shutdowns. Theslag penetrates and reacts withthe refractory, ultimately causingan expensive failure resultingfrom a combination of mechani-cal stresses related to engineer-ing design and thermal expan-sion mismatches as well as cor-rosion. In this study, refractorybricks were exposed to individualcoal slag components in a labo-ratory, and compared to bricksthat were removed from a coalgasifier after nearly two years ofcontinuous operation, in order toclarify the mechanisms of refrac-tory corrosion in a gasifier.17Most gasifiers convert coal,steam and oxygen into syngas—CO, H2, CO2 and byproducts—asfuel for combined-cycle turbinepower plants. Gasification, in useworldwide, is a more efficient andlower-emission process thanconventional coal-fired powergeneration. Gasifiers have beendeveloped that use petroleumcoke, black liquor from papermills, biomass, and other organ-ics as feedstocks. Some gasifiersproduce other fuels such asmethanol and methane, orchemicals such as ammonia.2 ,3 ,4The feedstock typically containsoxide mineral components suchas SiO2, FeO and CaO, whichform an amorphous slag thatflows slowly down the refractorywalls.High-chromia compositions ofrefractory bricks consistently out-last other compositions in gasifierapplications.5 ,6  Cr2O3 is generallyless soluble than Al2O3 or MgO inCaO–SiO2 slags.7  Aurex® 90bricks,ii nominally 90 mass%Cr2O3 and 10% Al2O3, were usedin this study. The chromia con-tent is roughly 86% of the molefraction in this composition, re-sulting in a microstructure that ispredominately a solid solution of(Cr, Al)2O3. Chromia-aluminabricks are also available in 75,60, 40 and other mass percent-ages Cr2O3, and most haveabout 15 volume% porosity.Chromia-zirconia (Zirchrom™)iiiand magnesia-chromia(“magchrome”)iv bricks are alsoused in similar applications.Experimental ProcedureHeat TreatmentSections of virgin refractorybricks were drilled to create anarray of cavities, 15 mm diameterby 15 mm deep, as shown inFigure 1a. The cavities were filledwith various combinations ofSiO2, FeO and CaO powders,and an actual slag powder froma gasifier.  The slag contained 46mass% SiO2, 18% FeO, 19%Al2O3, 10% (Ca, Mg)O, and otheroxides as the balance. Thespecimens were heated in asealed tube furnace to 1550°Cfor 24 hours, in a flowing reduc-ing gas of 50:50 Ar-CO that hadbeen bubbled through water.The reducing atmosphere pre-vented the metastable FeO fromoxidizing to a more stable form,i.e. Fe2O3.All the powders penetrated andreacted with the refractory duringthe furnace cycle.8The spent refractory bricks re-moved from the gasifier were ex-amined as received, without anyfurther thermal processing. Thevirgin refractory brick was alsoexamined without any oxide orslag treatment, as an experimen-tal control.CeramographyThe virgin refractory, the reactioncavities in the refractory section -cups - and the spent bricks wereceramographically sawed,mounted and polished by meth-ods similar to those described byAsakura and Takahara,9Podwórny,10  Chinn11  and manyothers. The specific details aregiven below.Cutting and MountingThe cups were first filled with anepoxy resin to stabilize the slag-refractory interface regions andprevent loss during cutting, andthen cross-sectioned with a wa-ter-cooled diamond waferingblade. Oil coolant is difficult toremove completely from materi-als that have a continuous net-work of pores, and therefore wasnot used. The spent refractorysamples were encapsulated withepoxy resin to reinforce and sta-bilize the fragile slag-refractoryzone before sectioning, asshown in Figure 1b.Figure 1a: Refractory brickdrilled for reaction withslag componentsFigure 1b: Cross-section of the drilledrefractory brick after thermal treatment andbackfill with epoxyFigure 1c: Polished cross-section of a 90%Cr2O3 refractory brick. The primary phase is asolid solution of (Cr, Al)2O3i Reference herein to any specific commercialproduct, process or service by trade name,trademark, manufacturer or otherwise, doesnot necessarily constitute or imply itsendorsement, recommendation or favoring bythe United States Government or any agencythereof or its contractors or subcontractorsii RHI Refractories America, Inc., Pittsburgh,Pennsylvania, USA 15219iii Compagnie de Saint-Gobain, Paris, Franceiv Exolon-ESK Company, Tonawanda, NewYork, USA  141508The cup cross-sections were cutto approximately 20 x 20 x 45mm, dried in an oven, anddegassed at 74 kPa negativepressure. Sections of 60 mm(outside diameter) polyvinyl chlo-ride (PVC) plastic pipe were usedfor ring molds. The large moldsize was selected to contain theentire reaction region of eachcup, and to match the dimen-sions of the specimen holder.The inside walls of the moldswere knurled and coated withPVC primer (purple dye, methylethyl ketone and other organicsolvents) to strengthen the bondwith the epoxy resin. Maskingtape was used to seal the bot-tom of the mold and form a cup.The spent refractory specimenswere cold-mounted in epoxyresin and vacuum impregnated at74 kPa negative pressure for tenminutes at ambient temperature.The virgin refractory sectionswere compression-mounted in ametallographic mounting presswith glass-filled diallylphthalate,as shown in Figure 1c.Grinding and PolishingPlanar grinding was carried outon a permanently bonded, 220-grit MD-Pianov diamond disc onan automatic machine.  Finegrinding was performed withhard composite MD-Allegrovdiscs with 15 and 9 µm diamondsuspensions. A hard naplesscloth with 6, 3 and 1 µmpolycrystalline diamond suspen-sion was used for polishing.  Allthe diamond suspensions werewater-based. The specimenswere carefully cleaned betweeneach step of grinding and polish-ing to prevent coarser particlesfrom being carried over to thenext step. Oxide polishing com-pounds were not used for finalpolishing due to the adverse ef-fect of the residue on the x-raymicroanalysis. Because the hard-ness and matrix of each samplediffered significantly and themounted surfaces were unusuallylarge, the grinding and polishingtimes ranged from 1 to 20 min-utes and the applied force variedfrom 100 to 200 N.The specimens were cleaned in asolution of Micro-90®vi in waterin an ultrasonic bath. Cleaningtimes were limited to about 1minute to prevent pullout of thegrains. The specimens were re-moved from the ultrasoniccleaner, rinsed with distilled wa-ter, rinsed with ethanol, and driedunder a heat gun.MicroscopyAll specimens were examined bylight microscope and scanningelectron microscope (SEM).The energy-dispersive x-rayspectrometer of the SEM wasused to generate an elementmap around each cup to deter-mine the depth of penetration bythe slag or its components.Results and DiscussionPreparation DifficultiesPreparation of refractory ceram-ics presents several difficultiesnot normally encountered withdensified ceramics and metals,due mostly to the high porosityand weak bonds between grains.Penetration of the refractory bythe oxides and slag can furtherweaken the bonds. The resultwas that the refractories had nu-merous pullouts, and a very dullsurface after the final polishingstep. Vacuum impregnation withcastable epoxy resin minimizedthe pullouts, by outgassing thepores and filling the voids withresin, but had limited effective-ness. The PVC ring minimizedthe edge-rounding effect.Slag Penetration and ReactionOxidesIn Figure 2, the silica powder didnot react with the chromia refrac-tory to form a new phase, as ex-pected from the SiO2–Cr2O3phase diagram. From the x-raymap, the silicon did penetratesome 5 mm into the refractorymatrix. An x-ray diffraction analy-sis showed that only the original(Cr, Al)2O3 sesquioxide phasewas present in the refractory.The silica-refractory interface re-mained intact.The calcia reacted with the re-fractory to form a low-meltingCa([Cr, Al]O2)2 phase at the ox-v Struers A/S, DK-2610 Rødovre, Denmarkvi International Products Corporation,Burlington, New Jersey, USA  08016Table I: Automatic preparation steps forchromia-alumina refractories on the Abrapolautomatic grinder/polisherGrinding PolishingStep 1 2 3 4 1 2Surface MD-Piano MD-Allegro MD-Allegro DP-Plan DP-Dur DP-Dur220Abrasive Diamond DP- DP- DP- DP- DP-Suspension Suspension Suspension Suspension SuspensionGrain size 15 µm 9 µm 6 µm 3 µm 1 µmLubricant Water DP-Blue DP-Blue DP-Blue DP-Blue DP-BlueSpeed [rpm] 150 150 150 150 150 150Force [N] 100 100 200 200 200 100Time [min] Until plane 5 5 10 5 5937Figure 2: Silica (Si penetration in red) attackon the 90% Cr2O3 refractory cupFigure 3: CaO (Ca penetration in blue) attackon the refractory cupFigure 5:Silica (penetration in red) and CaO (blue)attack on the refractory cupFigure 4:FeO (Fe penetration in green) attack on therefractory cupFigure 9:Actual slag (Si penetration in red, Fe in green)attack on the spent refractory from thegasifierFigure 6: Silica and FeO (Fe penetration ingreen, Si in red) attack on the refractory cupFigure 7: Silica, CaO and FeO (Fe penetrationin green, Si in red, and Ca in blue) attack onthe refractory cupide-refractory interface, in Figure3. The reaction was evident fromXRD results and the discolorationand texture differences in themicrostructure around the cup.The calcium also penetrated therefractory, but not as much asthe silica did. The Ca([Cr, Al]O2)2phase beneath the porous toplayer appears dense and poten-tially could prevent other speciesfrom penetrating into or reactingwith the refractory.The FeO (wüstite) reacted withthe refractory to form a spinelphase, Fe(Cr, Al)2O4, which is ap-parent in the top layer of themicrostructure in Figure 4. Theiron did not penetrate the refrac-tory significantly, but the oxide-refractory interface changed dra-matically. The original drilled cupprofile surfaces were obliteratedby the spinel reaction.In the dual oxide test whereequal masses of silica and calciapowders were heated in the cup,the silicon and calcium togetherpenetrated deep into the refrac-tory in Figure 5, but did not reactwith the refractory to form a newphase.  The wall of the cup wasbarely affected.The silicon and iron had deeppenetration in the silica-FeO testshown in Figure 6, similar to thesilica-calcia dual oxide test.The FeO reacted with the refrac-tory to form the Fe(Cr, Al)2O4spinel phase, and the originalcup surface changed significantlyin both color and texture.  Thesilica and FeO initially reacted atabout 1250°C to form a low-melting phase.In the multiple oxide test whereequal masses of silica, FeO andcalcia powders were used, the Siand Ca again penetrated deep 4Figure 8:Actual slag (Si penetration in red, Ca in blue,Fe in green) attack on the refractory cupA Ceramographic Evaluation of ChromiaRefractories Corroded by Slag10into the refractory while most ofthe FeO reacted to form theFe(Cr, Al)2O4 spinel phase at theinterface. The color, texture anddimensions of the interfacechanged, although not as muchas in the two previous tests thatemployed larger amounts of FeO.The microstructure is shown inFigure 7.SlagThe actual slag penetrated andreacted with the cup in Figure 8.The result was very similar to themultiple oxide test in terms ofpenetration.  The Si and Ca pen-etrated deep into the refractorywhile the FeO reacted to form theFe(Cr, Al)2O4 spinel phase at theinterface.  The interface regionmelted during the reaction, andsolidified into a porous top layerand a dense layer underneath.The spent refractory was highlyeroded on the surface and alsohad a 1–2 mm thick slag layeradhering to the surface, in Figure9.  Similar to the multiple oxideand actual slag cup tests, the Siand Ca again penetrated deeplyinto the refractory while the FeOreacted to form the Fe(Cr, Al)2O4spinel phase near the surface.ConclusionsCeramographic analysis showedthat the corrosive effect of slagon chromia-alumina refractorybrick was not necessarily equalto the sum of the effects of thecomponents of the slag.  Silica,the primary component, pen-etrated the open pores easily butdid not react with the original (Cr,Al)2O3 phase.  The depth of pen-etration by calcium and iron, andthe extent of the reaction be-tween the calcia and the refrac-tory, appeared to depend uponthe presence of silica in the slag.Calcia and especially FeO tendedto react with the refractory at theslag-refractory interface to formcompounds that melted at lowertemperatures than the oxide orthe refractory. The new phaseappeared to lessen further pen-etration by the slag componentsinto the refractory.AcknowledgementsThe authors thankS.T. Anderson, W. K. Collins,R.A. Krabbe, P.K. Palmer, L.M.Peck, A.V. Petty and J. C.Rawers, all of the Albany Re-search Center, for their technicalassistance.References1 W.K. Collins, J.C. Rawers, L.M.Peck & A.H. Hunt, “Coal SlagReaction with High Chrome Re-fractories,” Proceedings of theMicroscopy Society of America58th Annual Meeting in Philadel-phia, PA, USA, 13-17 August2000.  Edited by G.W. Bailey, S.McKernan, R.L. Price, S.D.Walck, P.-M. Charest and R.Gauvin, Springer, 2000.2 “Clean Coal Technology: TheWabash River Coal GasificationRepowering Project, An Update,”US Dept. of Energy and WabashRiver Coal Gasification ProjectJoint Venture, Topical Report No.20, September 2000.3 “Clean Coal Technology: TampaElectric Integrated GasificationCombined-Cycle Project, An Up-date,” US Dept. of Energy andTampa Electric Company, TopicalReport No. 19, July 2000.4 W. Liebner & D. Ulber, “MPG—Lurgi Multi-Purpose Gasification:Application in ‘Gas-Gasifica-tion,’” Proceedings of the 2000Gasification Technologies Confer-ence in San Francisco, CA, USA,8-11 October 2000.5 C. R. Kennedy and R. B.Poeppel, “Corrosion Resistanceof Refractories Exposed to Mol-ten Acidic Coal-Ash Slags,”Interceram, 3 (1978) pp. 221–226.6 W. T. Bakker, “Refractories forPresent and Future ElectricPower Plants,” Key EngineeringMaterials, 88 (1993) pp. 41–70.7  S. Zhang & W.E. Lee, “Use ofPhase Diagrams in Studies ofRefractories Corrosion,” Interna-tional Materials Reviews, 45 [2]41–58 (2000).8 “Refractories,” Engineered Ma-terials Handbook®, Vol. 4: Ce-ramics and Glasses (MaterialsPark, OH: ASM International,1991) pp. 895–909.9 H. Asakura and M. Takahara,“Refractories,” The Best ofStructure 1 – 12 (Copenhagen:Struers A/S, 1988) pp. 54–57.10 J. Podwórny, “AutomaticPreparation of Refractory Materi-als,” Structure 31 (1997) pp. 7–9.11 R. E. Chinn, “Preparation ofMicrostructures of Alumina Ce-ramics,” Structure 33 (1998) pp.16–20.1111Johanna Löttinger,Lambda Instrument AB1.000.000.000 revolutionsThat is how many times a rollingbearing may rotate during its life-time. Did you also know that anaverage household in the west-ern world today contains morethan 150 rolling bearings? Theyare everywhere, in your kitchenappliances, in your washing ma-chine, in your computer and inyour car. There are even mini-ature rolling bearings in the dartsthat professional dart playersuse! Most of these rolling bear-ings probably come from SKFAB, and the quality control of thesteel in the bearings has beencarried out on an Abra-systemfrom Struers for almost thirtyyears. What you probably did notknow, is that SKF was deeply in-volved in the development of theAbra-machines, and that the firstever Abrapol and Abraplan wasdelivered here in the early 70’s!The Struers symposiumin 1970It all started back in February1970. Struers then invited anumber of Swedish customers toa symposium, named “Metallo-graphic sample preparation forthe 70’s”. The participants camefrom a number of well-knownSwedish industries, such asVolvo AB, SKF AB, Sandvik ABand Avesta Sheffield AB. At thisdate, most metallographic labo-ratories were doing manualpreparation of the samples,which was difficult, time-con-suming and boring.The topics for the symposiumwere:· What type of machines do youneed in the future?· Can we use existing equipmentor do we need to develop newmachines?· How can we transform theseideas to serial made machines?Many wild ideas were discussed,and one of the wildest was a ma-chine for automatic samplepreparation. The idea came fromSKF Steel AB (now Ovako SteelAB) in Hofors, who were willingto invest time and money on de-velopment in cooperation withStruers.The original idea was to fix thesamples in one position, andthen have the different surfacesfor grinding and polishing on ro-tating arms and let them work onthe samples in consecutivesteps. This idea was quicklyabandoned, and Struers startedthe design of a semi-automaticpolishing machine instead. Thefirst drawings were sent to MrHans Söderström in Hofors inNovember 1970. After this an ex-tensive correspondence tookplace, and in March 1971 thefirst quotation for the machinewas sent to SKF Steel AB. Theprice quoted was 12 100 SEK(10 SEK » 1 USD). The first pro-totype of an Abrapol was deliv-ered in July 1971. Specimenholders were ordered from a lo-cal mechanical firm at a cost ofapproximately 400 SEK a piece.The development of theAbraplanFrom the very beginning SKFSteel AB claimed that a coarsegrinding machine was necessaryto increase the speed of theplane grinding, but Struers in-sisted that plane grinding on pa-per would be sufficient. In Sep-tember 1971 SKF Steel AB ex-pressed their satisfaction withthe Abrapol, but once againstressed the importance of aseparate machine for fast planegrinding. Struers then decided tocontinue the design of the exist-ing polishing machine, but toalso develop a new plane grind-ing machine.The first plane grinding machine,better known as Abraplan, wasThe first Abrapol prototype delivered to SKFOne of the first prototypes in Struers labin CopenhagenThe original specimen holders,especially made for SKF Steels samplesThe story of the firstAbra-machines

A ceramographic evaluation of chromia refractories corroded by slag

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A ceramographic evaluation of chromia refractories corroded by slag

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