JOM • August 200650

Figure 2. A schematic of limiting cases for thermal nitridation based on the classic Wagner Oxidation Theory. (a) Continuous, external layer formation, (b) internal pre-cipitate formation.

a

b

Fuel CellsResearch Summary

The Formation of Protective Nitride Surfaces for PEM Fuel Cell Metallic Bipolar Plates

M.P. Brady, B. Yang, H. Wang, J.A. Turner, K.L. More, M. Wilson, and F. Garzon

Figure 1. A schematic of a three-cell fuel cell stack showing bipolar plates (artwork from Reference 6, courtesy of Los Alamos National Laboratory and the U.S. Department of Energy). Each fuel cell delivers on the order of only 0.6–0.9 V under typical operating conditions.1 In practice, tens to hundreds of fuel cells can be stacked together to achieve useful voltage levels.

Figure 3. A schematic Ellingham-Richardson diagram for nitride stability (based on and redrawn from diagrams reported in References 47 and 48). Nitrides of nickel are typically not stable on reaction with N2 at atmospheric pressures, and would lie above Fe4N on this diagram. Note that AlN was not pursued as a protective layer for metallic bipolar plates due to its poor electrical conductivity.

Theselectivegasnitridationofmodelnickel-based alloys was used to formdense,electricallyconductiveandcor-rosion-resistantnitridesurface layers,includingTiN,VN,CrN,Cr

2N,aswellas

acomplexNiNbVNphase.Evaluationforuseasaprotectivesurfaceformetallicbipolarplatesinprotonexchangemem-branefuelcells(PEMFC)indicatedthatCrN/Cr

2Nbasedsurfacesholdpromiseto

meetU.S.DepartmentofEnergy(DOE)performancegoalsforautomotiveappli-cations.ThethermallygrownCrN/Cr

2N

surfaceformedonmodelNi-Crbasedalloysexhibitedgoodstabilityandlowelectricalresistanceinsingle-cell fuel

celltestingundersimulateddrive-cycleconditions.Recentresultsindicatethatsimilar protective chromium nitridesurfacescanbeformedonlessexpensiveFe-CrbasedalloyspotentiallycapableofmeetingDOEcostgoals.

INTRoduCTIoN

Protonexchangemembranefuelcells(PEMFCs) are of interest for powergenerationduetotheirhighefficiencyand near-zero emissions.1–5 They aretypically based on an ion-conductivesulphonated fluoropolymer membranesuch as Nafion® and operate in the60–80°Ctemperaturerange.1Applica-

tions range from portable power (cellphones, laptops, etc.) to automobilesandon-sitepower-generationsystems.Costanddurabilityconcernsarethekeybarrierstotheirwidespreaduse.1–5

PEM FuEl CEllS aNd BIPolaR PlaTES

Among themostexpensivecompo-nents in PEMFCs, and the dominantweightandvolumeofthefuelcellstack,arethebipolarplates(Figure1).1–5Thebipolarplatesservetoelectricallycon-necttheanodeofonecelltothecathodeofanotherinastacktoachieveausefulvoltage.Theyalsoseparateanddistrib-

2006 August • JOM 51

Figure 4. Cross sections of Ni-10Ti (wt.%) after 48 h at 1,100°C in N2.43,50 (a) Scanning-

electron microscopy (SEM) image; (b) scanning-transmission-electron microscopy (STEM) image of nitride layer.

a b

a b

Figure 5. Cross sections of Ni-10Nb-5V (wt.%) after 24 h at 1,100°C in N2. (a) An SEM image; (b) STEM image of nitride layer.50

Figure 6. Polarization data in aerated pH 3 sulfuric acid at 80°C (scan rate of 0.1 mV/s) vs. SHE.43,55,56

800

600

400

200

0

1,000

10−9 10−8 10−7 10−6 10−5 10−4 10−3

Current Density (A/cm2)

Pote

ntia

l (m

V vs

. SH

E)

a bFigure 8. Cross sections of nitrided Ni-50Cr (wt.%) (1,100°C, 2 h, N2) after 4,100 h exposure in a bipolar corrosion test cell.54–56 (a) An SEM image of the cathode-exposed face56 and (b) TEM image of the nitride.55 The fine black lines on the columnar Cr2N in the TEM image are bend contours caused by electron scattering through the buckled sample.

Figure 7. Interfacial contact resistance data.55,56

ute reactant and product streams. Toaccomplishthis,flow-fieldgroovesaremanufacturedintothefacesoftheplates(Figure1).Graphite is thebenchmarkmaterialforbipolarplatesbecauseitiselectrically conductive and corrosionresistantinthehighlyaggressiveanodeand cathode PEMFC environments(60–80°Cacidicconditions,containingF–leachedfromthemembrane).How-ever,thebrittlenessandrelativelyhighgaspermeabilityofgraphitenecessitatestheuseofthickplates(>2–5mm),whichlowersthepowerdensityofthefuelcellstack.1–5Machiningofflowfields intographiteplatesisalsoexpensive,makinggraphiteimpracticalformostwide-scalecommercialuses.Developmentalbipo-lar plate materials under investigationincludegraphite/carbon-basedcompos-ites,7–10polymer-basedcompositeswithconductive graphite/carbon fillers,11–16andmetals.17–38

TheU.S.DepartmentofEnergy(DOE)hassettechnicaltargetsforbipolarplatesforautomotiveapplications(TableI).39Thus far, no material has definitivelyestablisheditselfascapableofmeetingallthetargetpropertiesforthedesiredlifetimesof>5,000hunderdrive-cycleconditions. The graphite/carbon andpolymer-based composites generallyexhibit excellent corrosion resistancein PEMFC environments. However,the graphite/carbon composites3–5,7–10typicallyhavetobesealedtoreducegaspermeability,havebrittlenessissues,andcanbedifficulttoproduceatthicknesses<1mmtoachievethehighpowerdensi-tiesdesiredforautomotiveapplications.The manufacture of graphite/carboncompositescanalsobecostly,especiallywhenmeasuresaretakentomitigatetheirpropertyshortcomings.Tomeetautomo-tivecostgoals,bipolarplatesneedtobelessthan∼$1–$2fora∼500cm2plate.Thepolymer-basedcompositesarethecurrentstateoftheartforbipolarplatesand are available commercially.3–5,11–16Costtargetsappearachievable,butthusfarthrough-thicknessconductivitiesareinadequateforautomotiveapplications.Currently available plates typicallyexhibit conductivities of only ∼20–30S/cm, whereas the DOE goal is >100S/cm.Betterconductivitiesappeartobeachievablewithveryhigh loadingsofconductivephaseadditions(graphiteorcarbonparticles,fibers,nanotubes,etc.),

JOM • August 200652

TableI.DOETechnicalTargets:BipolarPlates39

Status

Characteristic Units 2004 2010

Cost $/kW 10 6Weight kg/kW 0.36 <1H

2PermeationFlux cm3sec–1cm–2@80°C, <2×10–6 <2×10–6

3atm(equivalentto<0.1mA/cm2)Corrosion μA/cm2 <1a <1b

ElectricalConductivity S/cm >600 >100Area-SpecificResistancec Ohmcm2 <0.02 0.01FlexuralStrength MPa >34 >4(crush)Flexibility %deflectionatmid-span 1.5to3.5 3to5

a–Basedoncoatedmetalplates;b–mayhavetobeaslowas1nA/cm2ifallcorrosionproductionsremaininionomer;c–includescontactresistance.

althoughthiscanmaketheplatesmoredifficulttomanufacture.Thehighload-ingsalsotendtomaketheplatesbrittle,especiallywhenmakingthinplatesontheorderof0.5mmto1mmthick.Despitetheircurrentlimitations,graphite/carbonand polymer-based composite bipolarplatesremainpromisingcandidates,andanumberofpublic andprivate sectordevelopmenteffortsarecurrentlyunderwaytoaddresstheirshortcomings. Seethesidebaronpage54fordetailsonmetallicbipolarplates.

ThERMally GRowN NITRIdE CoaTINGS

A possible solution to forming adefect-freeprotectivesurfaceonmetal-licbipolarplatesistogrowaprotectivenitridelayerfromthebipolarplatealloybyhigh-temperaturenitridation.43Tran-sitionmetalnitrides44offeranattractivecombinationofhighelectricalconductiv-ityandgoodcorrosionresistance,whichmakes them of interest for protectivecoatingsformetallicbipolarplates.2Inthermalnitridation,thegoalistodesignthealloyandsetthereactionconditionssuch that the desired nitride-formingelementdiffusesoutwardfromthealloytoreactwiththenitridinggastoformacontinuoussurfacelayer(Figure2a).Pin-holedefectsarenotanissuebecause,atelevatedtemperatures,thermodynamicandkineticfactorsfavorthereactionofallmetalsurfacesexposedtothenitridinggas.43Assuch,itisalsonotline-of-sightlimitedandisamenabletousewithcom-plex-shapedsurfacessuchastheedgesandcornersoftheflowfieldgroovesinbipolarplates(Figures1andA). Thedrawbackofthethermalnitrida-tionapproachisthatitisverysubstrate-alloydependentanditcanbedifficulttocontrolthecompositionandmorphology

ofthenitridethatisformed.Forexample,if thealloycompositionornitridationconditions are not selected properly,discrete internal nitride precipitates(Figure2b),whichwouldnotprovidecorrosionprotection,canforminsteadofthedesiredexternal,continuousnitridesurfacelayer(Figure2a).Further,evenifacontinuousnitridesurfacelayercanbe formed, cracking of the layer canoccur on cooling due to the thermalexpansionmismatchbetweenthenitrideandthesubstratealloy.Controlofsuchphenomena is the basis for protectiveoxidescaleformationbyheat-resistantalloys(e.g.,References45and46),buthasnotpreviouslybeenwellexploredasasynthesismethod to formprotec-tivenitridesurfacelayers.Theproposedthermalnitridationapproachisdifferentfromthemorefamiliarconventionalfer-rousalloynitridationtoachievesurfacehardening,wherebyconditionsaresettodiffusenitrogenextensivelyintothealloytoformironnitridesand/ornitro-gen-saturatediron-basedphasestenstohundredsofmicrometersdeep.Thegoalofbipolarplateprotectionistoformanexternal, continuous nitride layer nomorethanafewmicrometersthick. Exploratory work was performedwith the nitridation of model Ni-Xalloys, where X = Cr, Nb, Ti, V, andZrtoformCrN,Cr

2N,NbN,TiN,VN,

ZrN,etc.,phases.43Nickelwaschosenastheinitialalloybasebecauseitdoesnotformastablenitrideundertypicalnitridationconditions(Figure3)andhasalowpermeabilitytonitrogen,which,based on classic Wagner OxidationTheory (reviewed in References 45,46,and49), favors theformationofacontinuous, external nitride layer onnitridationratherthandiscreteinternalnitride precipitates (Figure 2). At the

timethisworkwasinitiated(2000),thecost of nickel was also relatively low(∼$2–3/lb),andpotentiallyabletomeetDOEautomotivecostgoals. In recentyears,thepriceofnickelhassignificantlyincreased (∼$6–10/lb), leavingnickel-basedalloystooexpensiveforPEMFCbipolar plate automotive applications.However,theprimarypurposeofthesestudieswasproof-of-principleevaluationofthethermalnitridationapproach,andto identify which nitride compoundshadthemostpromiseforprotectioninPEMFCenvironments. Nitrideformationwasexploredinthetemperaturerangeof800–1,200°CinN

2

and N2-4H

2 environments, rather than

conventionalsurface-hardeningnitridingconditionsof∼500–600°CinNH

3,which

produces a highly effective nitrogenactivityandfavorsinternalpenetrationofnitrogen.SomerepresentativenitridedmicrostructuresareshowninFigures4and5.43,50Acontinuous,external∼10µmthicklayerofTiNwasreadilyformedon Ni-10Ti (wt.%) (Ni-12Ti [at.%])afterexposureat1,100°Cfor48hinN

2

(Figure4a).Themicrostructureconsistedofahighlyplanarnitride/alloyinterface,withsubmicrometer,equiaxedgrainsofTiNofcompositionTi-(45–50)N(at.%)(Figure 4b). Such a microstructure isindicativeofconditionsfavoringnitridenucleationovergrowth,andisconsistentwith the expected high driving forcefornucleationduetothehighthermo-dynamicstabilityofTi/TiNrelativetonickel (Figure 3). Detailed studies ofnitride formation in Ni-Ti alloys bySavvaetal.51indicatethatthetransitionfrominternal toexternalTiNinNi-Tialloysoccursattitaniumlevelsaslowas∼Ni-(1–2)Ti(wt.%)(1–2.5at.%Ti)at1,020°C. Attheoppositeendofthespectrum,acoarse-grained,columnarmicrostructure(Figure5)wasformedonNi-10Nb-5V(wt.%)(Ni-6.5Nb-6V[at.%])afterexpo-sureundersimilarconditions(1,100°C,24 h, N

2) to Ni-10Ti (wt.%).50 Both

continuousexternalnitridelayerforma-tionandinternalnitrideformationwereevident.EffortstoformanNbNlayeronbinaryNi-Nballoyswerenotsuccessful,with very little reaction with nitrogenobserved.Additionsofvanadiumweremadeinanattempttoincreasereactiv-itywithnitrogen,asNi-Valloysreadilyformed VN surface layers. Vanadium

2006 August • JOM 53

was also selected because VN is ofintermediate thermodynamic stabilitybetween nickel and NbN (Figure 3),via the so-called third-element effect,whereby ternaryadditionsof interme-diate stability can help promote theformationofaprotectivesurfacelayerat alloy concentrations lower than inthe corresponding binary alloys. (TheclassicexampleofthisphenomenonischromiumadditionstoNi-AlandFe-Alalloys,whichlowerthelevelofaluminumneededtoformaprotectiveAl

2O

3scale

onthermaloxidationrelativetobinaryNi-Al andFe-Al alloys; the third ele-ment effect is reviewed inReferences45and46).Interestingly,ratherthananNbNsurfacelayer,acomplexnitrideofcomposition Ni-(12–15)V-(15–20)Nb-(20–30)N(at.%)wasformedonnitridedNi-10Nb-5V(wt.%).50X-raydiffractionanalysisindicatedthatboththealloyandthecomplexnitridewereface-centeredcubic structures (Fm3-m), suggestingthat the Ni-Nb-V-N composition andcolumnar structure may be more theresultofasolid-solutiontypereactionpaththananexclusiveselectivenitrida-tionprocess. Thecorrosionresistanceofthenitridesurface layers was assessed by polar-izationinpH3sulfuricacidsolutionsat80°CtosimulatePEMFCoperatingconditions.43Corrosioncurrentdensitiesoflessthan1× 10–6A/cm2upto∼0.9Vvs.standardhydrogenelectrode(SHE)under aerated conditions (to simulatethecathode-sideenvironment)and–0.2to +0.1 V vs. SHE under H

2-purged

conditions(tosimulate theanode-sideenvironment) were considered suf-ficiently promising to warrant furtherinvestigation.RelativelylowcorrosioncurrentdensitieswereobservedfortheTiN surfaces formed on Ni-Ti basedalloys, but post-test analysis revealedlocalareasofthrough-thicknessattackinsomecoupons.43Thisseemedtocor-relatewithTiNthickness,andwaslessprevalentwiththinnerTiNlayers(<1–2µm).TheVNandNi-Nb-V-Nsurfaceswerelesscorrosionresistantundertheseconditions,withcorrosioncurrentdensi-tiesinthe10–5to10–4A/cm2rangeat<0.9Vvs.SHEunderaeratedconditions.43Among the nitrided surfaces/nitridesevaluated,onlyCrN/Cr

2Nconsistently

exhibited the target corrosion currentdensities.43

PRoTECTIvE Cr-NITRIdE SuRFaCES oN ModEl Ni-Cr-BaSEd alloyS

Nitridation studies of binary Ni-Cralloys have indicated little reactivitywithnitrogenatlowlevelsofchromium;however, as the chromium level isincreased,nitrogenpermeability(whichfavorsinternalratherthanexternalnitrideformation) also increases,52 such thatlevelsofchromiuminexcessof∼30–35wt.%53,54areneededtoformanexternal,continuousnitride.Therefore,Ni-50Crwasusedasamodelmaterialtoevaluatethebehaviorofthermallygrownchro-mium-nitridesurfaces.PolarizationdatafornitridedNi-50Cr(1,100°C,2h,N

2)

inaeratedpH3sulfuricacidat80°CisshowninFigure6,andinterfacialcontactresistance(ICR)datainFigure7.43,55,56ThecorrosionresistanceofnitridedNi-50CrwassignificantlybetterthanthatofNi-50Crmetalandpurechromium,purenickel,andtype316stainlesssteel(∼18Cr-10Ni [wt.%] base) shown forcomparativepurposes.NitridationalsoloweredtheICRofNi-50Cr∼fivefold,totheDOEtargetvalueof10mohm-cm2atcontactpressuresof150–200N/cm2,which isanorderofmagnitude lowerthanthatof316stainlesssteel. Figure8showsthemicrostructureofnitridedNi-50Craftera4,100hexposureinabipolarcorrosiontestcellusingpH3sulfuricacidwith2ppmF–at80°C,withonecouponfaceexposedtoasolutionpurgedwithairandtheotherwithH

2.55

Thenitridedstructureconsistedofathin,equiaxedCrNlayeroverlyingadense,columnarCr

2Nlayer.Significantinternal

nitridationalsooccurred.Noattackofthenitridesurfacewasevident,despitethelong-termcorrosionexposure.TherewasalsonoincreaseinICRcomparedwiththeas-nitridedsurface. Basedonthesepromisingcorrosionand ICR screening results, single-cell fuel cell testing was pursued fornitridedNi-50Cr.A simple serpentine50 cm2 active area plate architecturewasadopted,usingthickNi-50Cranodeandcathodeplatesdesignedtoduplicategraphitetestplatehardware(Figure9a).Fullnitridesurfacecoveragewasreadilyachievedacrossalledge,corner,andflowfieldfeatures,withnoevidenceofCrN/Cr

2Nlayercrackingorlossofadherence.

Theseplateswereinitiallyrunfor500h

underconstant0.7Vconditions(80°C).Therewasnoplateresistanceincreaseorlossofperformance.Analysisofthemembraneelectrodeassembly (MEA)byx-rayfluorescence(XRF)indicatednometallicioncontaminationwithinthedetectionlimitsofthemeasurement,oftheorderof∼0.1–1×10–6g/cm2.Thisresult provides evidence that thermalnitridationcanprotectirregularsurfacefeaturessuchascorners,edges,holes,andflowfieldgrooves. ThenitridedNi-50Crplateswerethensubjectedtoanadditional∼1,160hofdrive-cycletestingusingacycleof0.94Vfor1min.,0.60Vfor30min.,0.70Vfor20min.,and0.50Vfor20min.Anadditional24fullshutdowns(cellcooledoff,gasesremoved,andopenedtoairatconnections)weresuperimposedinanattempttoinduceevenmoreaggressiveconditions.Nolossofperformancewasobserved;infact,performanceslightlyincreased during drive cycle testing(Figure9b).AsmallamountofnickelcontaminationoftheMEA,2×10–6g/cm2,was,however,detected.Thissmallamountofnickelwaslikelytheresultofleachingfromoccasional,localregionsoftheCrNiNπphase57–59thatwereini-tiallyformedduringnitridation,insteadof the more protective CrN/Cr

2N sur-

face.54NoattackoftheCrN/Cr2Nsurface

wasevident.(Suchlocalπphaseforma-tioncanoccuratregionsofsignificantchromium depletion in cast Ni-50Cr.)Overall,proof-of-principleexploratorycorrosion,ICR,andsingle-cellfuelcellstudiesindicatedthatthermallygrownCrN/Cr

2Nsurfacesshowgoodpotential

tomeetDOEbipolarplateperformanceanddurabilitygoals.

ExTENSIoN oF PRoTECTIvE Cr-NITRIdE

FoRMaTIoN To Fe-Cr-BaSEd alloyS

Withthecurrenthighcostofnickel(∼$6–10/lb),Ni-Crbasedalloyscannotmeet DOE automotive cost goals.Therefore,effortswereinitiatedtoformsimilarprotectiveCrN/Cr

2Nsurfaceson

Fe-Crbasedalloysbythermalnitridation.Thisisdifficultbecauseatcommerciallyviablelevelsofchromium,lessthan∼30wt.%chromiumduetotheσphasefield,the permeability of nitrogen in Fe-Cralloys issufficientlyhighthat internalchromium-nitrideprecipitatesaregen-

JOM • August 200654

10 cma

b

Figure 9. Single-cell fuel cell testing of nitrided Ni-50Cr (1,100°C, 2 h, N2-4H2) anode and cathode plates. (a) Typical nitrided plate; (b) performance curves. METallIC BIPolaR PlaTES

Metallicalloyssuchasstainlesssteelswouldbeidealasbipolarplates(e.g.References26–30)becausetheyareamenabletolow-cost/high-volumemanufacturingmethodssuchasstamping,offerhighthermalandelectricalconductivities,havelowgaspermeabilityandexcellentmechanicalproperties,andcanbereadilymadeinfoilform(∼≤0.1mmthick) to achieve high power densities (Figure A). The primary limitations are highcontactresistanceandborderlinecorrosionresistanceandcost.DespitebulkelectricalconductivitieswhichareordersofmagnitudegreaterthantheU.S.DepartmentofEnergy(DOE)goalof>100S/cm,stainlesssteelsgenerallyexhibitinterfacialcontactresistance(ICR)valuesanorderofmagnitudehigher(worse)thantheDOEgoalof10mohm-cm2(e.g.,References28–30).Thisisduetothepassiveoxidelayerpresentonstainlesssteels,whichisthesourceoftheircorrosionresistance.Onexposuretothehighlyaggressiveprotonexchangemembranefuelcell(PEMFC)environments,especiallyontheaeratedcathodeside,furthergrowthoftheoxidelayerandevenhigherICRvaluescanresult.28–30DissolutionofmetallicionsfromstainlesssteelscanalsooccurunderPEMFCoperatingconditions, especially in the reducinganode sideenvironmentwhereprotectiveoxidefilmformationcanbedifficult,andundercyclicoperatingconditionsathighvoltagessuchasthoseencounteredinautomotiveapplications(e.g.,References2–5,25,31,and32).Sulphonatedfluoropolymermembranesareverysensitivetopoisoningbymetallicions,andcellperformancecanbesignificantlydegraded.2–5,25,31,32

The degradation in fuel cell performance from metallic ion dissolution andcontaminationdependsonthecomplexinteractionofanumberoffactors,includingthedegreeofcaptureofthemetallicspeciesinthemembrane,itsspecificinteractionwiththemembrane,thechemistryofthemembrane,andthefuelcellstackcelldesignandoperating conditions.2–5,25,31,32 A rough estimate would be that contamination levels oftheorderofonly10–100ppmofmetallicionsareasignificantconcernforperformancedegradation. For some PEMFC applications and operating conditions/durabilityrequirements,untreatedstainlesssteelalloysmayprovideacceptableperformanceanddurability.However,forautomotiveapplications,thehighICRandborderlinecorrosionresistancearenotacceptable,atleastwithconventionalfuelcelldesigns.Therisingcostsof stainless-steel alloy components, especially nickel, also leaves many compositionsunabletomeetcostgoals.Othermetallicmaterialshavealsobeeninvestigatedasbipolarplatematerials,particularlyNi-Cr,titanium,andrefractorymetalssuchasniobiumandtantalum (e.g.,References2,22,37, and43).However, thecostof thesematerials isgenerallyinexcessofDOEautomotivegoals,andICRvaluesand/orcorrosionresistancearestillborderlinewithrespect to theDOEtargets.Theyremaincandidatesforsome

Figure 10. Scanning-electron microscopy cross sections of Fe-27Cr (wt.%) exposed for 4 h at 900°C in flowing, purified N2-4H2. (a) Low-magnification view showing internal chromium-nitride formation (dark phase); (b) high-magnification view of alloy surface region showing discontinuous chromium-nitride surface (dark phase).

b

a

erally favored rather than the desiredexternal,continuouschromium-nitridelayer(Figures2band10).60,61NitridationinmodelFe-27Cr(wt.%)basedalloysasafunctionofternaryalloyingadditionsandnitridationenvironment(N

2partial

pressure,H2level,O

2-impuritylevel,and

temperature)wasstudiedinanattempttoformadense,externalchromium-nitridesurface.Aferriticbasedalloywaschosenratherthananausteniticbasedalloytoeliminatetheneedforcostlynickeladdi-tions,with27wt.%chromiumchosenbecauseitisrepresentativeofthetype446 family of high-chromium, com-mercialstainless-steelalloys. Under some conditions, oxygenimpuritiespresentinanN

2-4H

2nitriding

environment resulted in the formationofa thin (submicrometer)Cr

2O

3 layer

onheat-uptothenitridingtemperature(850–900°C). This occurred becauseCr

2O

3 ismuchmore stable thermody-

namicallythanthechromiumnitrides,with only 5–10 ppm of O

2 impurities

in N2-4H

2needed to form oxide. The

initiallyformedCr2O

3layerwasfound

tobehighlyeffectiveinlimitinginter-nalnitridationofbinaryFe-27Cr,withsubsequent conversion of the externalchromiumoxidesurfacetoexternalchro-miumnitrideastheoxygenimpuritiesinthenitridingenvironmentwerecon-sumed.61 The resulting microstructurewasasemi-continuouschromium-nitrideoverlying the remaining chromiumoxide,butexhibitedborderlinetounac-ceptablyhighcorrosionrates. However,theuseofinitialoxidefor-mationtolimitinternalnitridationprovedhighlyeffectiveinFe-27Cralloysmodi-fiedwith2wt.%and6wt.%vanadium.61In the Fe-27Cr-2V and 6V alloys, O

2

impuritiesinthenitridingenvironmentresulted in the initial formation of a(V,Cr)

2O

3surfacelayerduringheat-up,

whichwassubsequentlyconvertedtoadense,fullycontinuousvanadium-dopedchromium-nitridesurfacelayer,overly-inganintermixedoxide/nitrideregion.Corrosion resistance and ICR valuescomparable to the nitrided Ni-50Cr

0.90.80.70.60.50.40.30.20.1

1

0 0 0.2 0.4 0.6Current (A/cm2)

Volta

ge

2006 August • JOM 55

Figure A. Metallic bipolar plate flow field features can be manufactured by high-volume manufacturing methods such as stamping. (a) Stamping of metallic bipolar plates; (b) stamped flow-field features. (Photographs courtesy of GenCell Corporation, Southbury, Connecticut.)

portableorstationaryapplications,wherecosttargetsand/oroperatingconditionsmaybelesssevere. To meet DOE bipolar plate targets for automotive applications, metallic bipolarplates will require conductive and corrosion-resistant coatings or surface treatments.Unfortunately,coatingsformetallicbipolarplateshavethusfarnotprovensufficientlyviableduetolocalareasofinadequatesurfacecoverage(i.e.,pin-holedefects),40whichresultinlocalcorrosionandmetallicioncontaminationofthemembrane.Formanynon-fuel-cellapplications,suchcoatingdefectsarenotasignificantissue.However,duetothesensitivityofthesulphonatedfluoropolymermembranestopoisoningbymetallicionsand theaggressivenessof thePEMFCoperatingenvironment,bipolarplates requireafullydense,essentiallydefect-freeprotectivecoating.Thisisespeciallytrueforlow-costbutlesscorrosion-resistantmetalsubstratessuchaslow-alloysteelsoraluminum,whichcanberapidlyattackedinPEMFCenvironments.2–5,24,41Methodstomitigatethepresenceofpin-holedefects(i.e.,theuseofinterlayers)arebeingpursued(e.g.,Reference33),but can significantly increasecosts.Difficultiesarealsoencountered inobtaining fullcoverage of complex flow field corner and edge geometries. Emerging approachesincludetheuseofcladdingsonlow-costsubstrates42andtheuseoflow-ICRcoatingsorsurface treatments thatareused incombinationwithstainless-steelalloysubstrates toprovidecorrosionresistance.20,21,23,60

a b

Figure 12. The ICR data for Fe-27Cr-6V preoxidized at 900°C for 8 h in flowing Ar-10N2 + O2 impurities followed by 900°C for 24 h in flowing, purified N2-4H2. As-nitrided and after a 7.5 h hold in aerated pH 0 sulfuric acid + 2 ppm F.– at 0.84 V vs. SHE. Note that the plotted ICR values include resistance contributions from both major coupon faces. The data are typically divided by 2 to obtain ICR; in this case that was not possible as only one major coupon face was exposed to the sulfuric acid.

800

600

400

200

0

1,000

10−9 10−8 10−7 10−6 10−5 10−4

Current Density (A/cm2)

Pote

ntia

l (m

V vs

. SH

E)

Figure 11. Polarization data in aerated pH 3 sulfuric acid at 80°C (scan rate of 0.1 mV/s) vs. SHE for preoxidized and nitrided Fe-27Cr-6V vs. Ni-50Cr. 1Ni-50Cr: 1,100°C, 2 h, N2.

2Fe-27Cr-6V: preoxidized at 900°C for 8 h in flowing Ar-10N2 + O2 impurities followed by 900°C for 24 h in flowing, purified N2-4H2;

3Fe-27Cr-6V: preoxidized at 800°C for 20 min. in flowing Ar-2O2 followed by 900°C for 24 h in flowing, purified N2-4H2.

wereachieved.VanadiumwaseffectivebecauseVNhasgreaterstabilityrelativetoV

2O

3thandoesCrNrelativetoCr

2O

3

(approximately100ppmO2isrequired

toformoxideonvanadiuminN2-4H

2at

900°Cvs.∼10ppmforchromium),whichaidedconversionoftheinitiallyformedoxidetonitride.VanadiumisalsoreadilysolubleinFe-Cralloys,andV

2O

3-Cr

2O

3,

CrN-VN,andCr2N-V

2Nareall,respec-

tively,mutuallysoluble.(FormationofV

2O

5,whichistoxicandexhibitsalow

meltingpoint,wasnotobservedundertheconditionsstudied.)

Thepromisingresultsobtainedwithnitrided Fe-27Cr-2V and Fe-27Cr-6V(wt.%)wereaccomplishedinasealednitridingenvironment,dependentonanarrowrangeofO

2impuritylevels,and

proved difficult to reproduce consis-tently.61 Preoxidation studies are cur-rentlyunder investigation tosolve thereproducibility issues by controllablyintroducing a defined level of oxideformation prior to nitridation, and tofurtherinvestigatetheproposedreaction

mechanism.Figure11showspolariza-tiondatainaeratedpH3sulfuricacidat80°CforFe-27Cr-6V(wt.%)nitridedafterpreoxidationinflowingAr-2O

2or

Ar-10N2+O

2 impurities, followed by

nitridation in flowing O2-gettered N

2-

4H2. Corrosion current densities were

in the range of the nitrided Ni-50Cr.TheICRvaluesforpreoxidized/nitridedFe-27Cr-6V met the DOE goal of 10mohm-cm2 at a contact pressure ofonly∼100N/cm2(Figure12),whichissomewhatbetter(lower)thanthosefornitridedNi-50Cr(Figure7).CorrosionscreeningswerealsoconductedunderaggressiveconditionsofaeratedpH0H

2SO

4+2ppmF–at70°Cheldfor7.5

hat0.84Vvs.SHE.Interfacialcontactresistance values increased slightly(Figure12),butstillmettheDOEgoalatcontactpressuresofonly∼100N/cm2.Ascanning-electronmicroscopy(SEM)crosssectionofthenitridedFe-27Cr-6ValloypreoxidizedinAr-10N

2+O

2impu-

ritiesisshowninFigure13.Itconsistedofexternalvanadium-dopedchromium

nitridewithfinger-likeprojectionsintoaninnerregionofV-Croxide,consistentwith (V,Cr)

2O

3 (identification based

solely on SEM and energy-dispersivex-ray analysis in this sample). Littleinternalnitridationwasevident. The use of oxide to aid in externalnitride formation is a concern withregards to through-thicknesselectricalconductivity.TheICRdatareportedinthispaperweredeterminedbymeasur-ing the voltage drop from a couponplacedbetweencopperplatesandcarboncontactpaperasafunctionofcompac-tion pressure28–30,55,56,60 and showed noindicationsoflimitedthrough-thicknessconductivity.Theintermixednatureof

400

300

200

100

0 0 20 40Contact Pressure (N/cm2)

ICR

(moh

m-c

m2 )

60 100

— As-Nitrided� — After Polarization

Goal�

�

�

�

�� � � � � �

� � ��������

�

�

� �

80xx

JOM • August 200656

Figure 13. Scanning-electron microscopy cross sections of Fe-27Cr-6V preoxidized at 900°C for 8 h in flowing Ar-10N2 + O2 Impurities followed by 900°C for 24 h in flowing, purified N2-4H2 from Figure 12. (a) Low-magnification view showing little internal chromium-nitride formation (dark phase); (b) high-magnification view of surface layer showing mixed nitride/oxide structure.

b

a

thenitride/oxideandN-dopingand/orpartialconversionoftheoxidestillpres-enttonitrideoroxy-nitridelikelyresultedinhigh-conductivitythrough-thicknesspaths. V

2O

3 is also highly conductive

for an oxide.62 Above room tempera-tureand/orondopingwithchromium,thereisatransitiontomoreinsulatingbehavior.However,resistivityvaluesarestilloftheorderofonly∼2ohm-cmatchromiumlevelsof10at.%substitutionfor vanadium in (V,Cr)

2O

3,62 although

chromiumlevelsinsomeregionsofthe(V,Cr)

2O

3 formed on the Fe-27Cr-6V

exceeded 10 at.%. It should be notedthat preoxidized and nitrided binaryFe-27CrcouponsalsoshowedlowICRvaluesdespiteinnerCr

2O

3-basedlayers

ontheorderof0.5µmthick61(Cr2O

3is

not conductive at room temperature).Thisresultindicatesthateithertherewerealsohigh-conductivitypathsthroughtheinnerCr

2O

3layer,duetonitrogendoping

orlocalareasofnitride,orthatconduc-tionalongthesurfacenitrideacrossthecoupon edges contributed to the lowICR values. Calculations suggest thatthelimitededgesurfaceareawouldhave

resultedinhigherICRvaluesthanthosemeasured; however, further study andevaluationofthisissueareplanned. Anadvantageofusingoxidation toinitiallyenrichthesurfaceinchromiumand vanadium prior to converting tonitride is thatsuchselectiveoxidationmaybeaccomplishedatrelativelylowlevelsofchromiumandvanadiumduetothelowsolubilityofoxygeniniron-basedalloysandthehighthermodynamicstabilityofCr

2O

3andV

2O

3.Although

ICR and corrosion studies have onlybeenpursuedfornitridedFe-27Cr-6VandFe-27Cr-2Vtodate,itisestimatedthat the composition range amenabletothisapproachcanlikelybeextendeddownward to∼15–20wt.%chromiumand0.5–2wt.%vanadium,whichwillloweralloycost.

ChallENGES aNd FuTuRE dIRECTIoNS

Thechromium-basednitridesurfacesformedonFe-27Cr-6Vhavenotyetbeenevaluatedinfuelcelltesting.However,basedonthesingle-celltestbehaviorofthethermallygrownCrN/Cr

2Nsurfaces

onthenitridedNi-50Crmaterial,similargoodperformanceisexpected.SeveralkeyissuesremaintobeaddressedtofullyestablishthepotentialofnitridedFe-Cr-ValloystomeetDOEperformanceanddurabilitygoals. First, to meet cost goals, metallicbipolarplatesmustbeintheformofthinfoil(0.1mmrange),andthermalnitrida-tioncouldpotentiallywarporotherwisecompromisethepropertiesofstampedalloy foil bipolar plates. Preliminaryassessmentwith stamped andnitridedNi-Cralloyfoilsuggestedthatsignificantwarpingdoesnotoccur;63however,theslight warping observed could causeissueswithregardstosealingtheedgesof thefuelcellstack.Nitridationwas,however,observedtoembrittlethinNi-Cralloyfoilunderconditionswhereinternalnitridation accompanied the externalnitridelayerformation.63Theuseofaninitialoxidelayertolimitinternalnitro-genpenetrationandaidexternalnitrideformationonvanadium-modifiedFe-Crbasedalloysshouldhelpamelioratethisissue. Second,althoughsingle-cellfuelcelltesting has indicated good durabilityfor thermally grown CrN/Cr

2N sur-

faces, more aggressive conditions can

be encountered in fuel cell stacks. Inparticular,short-termlocalvoltagesinexcessof1V64,65mayoccurunderstart-up conditions if oxygen gains accesstotheanodesideofthecell.CorrosioncurrentdensitiesincreasedsignificantlyforthermallygrownCrN/Cr

2Nsurfaces

above0.8–0.9Vvs.SHE inpolariza-tion testing in sulfuric acid solutions(Figures 6 and 11). On nitrided Ni-50Cr,thisincreasewaspartiallylinkedtooxygenincorporationintheformofagraded,mixedCr-O/Cr-N-Ospeciessolidsolution,54whichdoesnotappearto increase contact resistance, at leastaftershort-termexposures.TaguchiandKurihara66 report increased corrosioncurrentdensities and transpassivedis-solutionofchromiummetalandnitridedpurechromiumasCr

2O

72-in1kmol-m–3

sulfuricacidat40°Catpotentialshigherthan ~1.2V vs. SHE, which suggeststhat degradation of thermally grownchromium-nitridesurfacesonFe-Cr-Valloysmayoccurabove1V.Althoughsuchexcursionsgreaterthan1Vmaybeforonlyforafewseconds,theireffectscanaccumulatepershut-down/start-upovertimeunderautomotivedrive-cycleconditions. Approaches to reduce oreliminate transient high voltages onstart-uparebeingpursued(e.g.,Refer-ence65),anditshouldbenotedthattheMEAcarboncatalystsupportsarealsoattacked under these conditions. Fuelcell stack testing, including frequentshut-down/start-upcycles,isplannedtoevaluatetheseeffectsfornitridedFe-Cr-Vbipolarplates.

aCkNowlEdGEMENTS

The authors thank B.A. Pint, J.H.Schneibel, G.J. Tatlock, and P.F. Tor-torelliforhelpfulcommentsinreviewingthismanuscript.FundingfromtheU.S.DepartmentofEnergy(DOE)Hydrogen,FuelCells,andInfrastructureprogramisgratefullyacknowledged.OakRidgeNationalLaboratoryismanagedbyUT-Battelle,LLCfortheU.S.DOEundercontractDE-AC05-00OR22725.

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(1991), pp. 259–278. 54. I. Paulauskas et al., “Corrosion Behavior of CrN, Cr2N and PI Phase Surfaces Formed on Nitrided Ni-50Cr with Application to Proton Exchange Membrane Fuel Cell Bipolar Plates,” Corrosion Science (in press). 55. M.P. Brady et al., Scripta Mat., 50 (7) (2004), pp. 10–17.56. H. Wang et al., J. Power Sources, 138 (2004), pp. 86–93.57. A.A. Kodentsov et al., Metal. and Mater. Trans. A, 27 (1996), pp. 59–69.58. U. Krupp, S.Y. Chang, and H.-J. Christ, Z. Metallkunde, 91 (2000), pp. 1006–1012.59. N. Ono, M. Kajihara, and M. Kikuchi, Met. Trans. A, 23 (1992), pp. 1389–1393.60. H. Wang et al., J. Power Sources, 138 (2004), p. 79. 61 B. Yang et al., to be submitted to Acta Materialia.62. H. Kuwamoto, J.M. Honig, and J. Appel, Phys. Rev. B, 22 (1980), p. 2626. 63. M.P. Brady et al., “Cost-Effective Surface Modification for Metallic Bipolar Plates,” 2005 Annual Progress Report, DOE Hydrogen, Fuel Cells, and Infrastructure Program, www.hydrogen.energy.gov/annual_progress05_fuelcells.html#d.64. C.A. Reiser et al., Electrochemical and Solid State Letters, 8 (6) (2005), pp. A273–A276.65. T.A. Bekkedahl et al., “Reducing Fuel Cell Cathode Potential during Startup and Shutdown,” U.S. patent application 20,040,081,866 (2004). 66. M. Taguchi and J. Kurihara, Materials Transactions, 32 (1991), pp. 1170–1176.

M.P.Brady,B.Yang,andK.L.MorearewithOakRidgeNationalLaboratoryinTennessee;H.Wangand J.A. Turner are with National RenewableEnergyLaboratoryinGolden,Colorado;andM.WilsonandF.GarzonarewithLosAlamosNationalLaboratoryinNewMexico.

For more information, contact M.P. Brady, Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, TN 37831-6115; (865) 574-5153: fax (865) 241-0215; e-mail bradymp@ornl.gov.

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