MATER ALS SELECTI ON & DES C N

Corrosion Resistance of a New,

Wrought Ni-Cr-Mo Alloy

PaulCrook,M.L. Caruso,andD.A. KingseedHaynesInternational,1020 WestParkAve.,P.O. Box 9013,Kokomo,IN 46904-9013

Anew,nickel-basedalloy resistanttoa verywiderangeofcorrosivemediaisdescribed, shipbetweenchromiumcontentandThemain alloyingadditionsare chromiumand molybdenum;however,thealloy also resistancetocorrosiondo notholdincontains—1.6% copper,which significantlyenhancesits resistanceto dilutesulfuric thecaseof concentratedsulfuric.and hydrochloricacids.Thesafeoperatingregimesfor thealloy in thesetwo acidsare Solubility constraintsat the so-defined,asare itscurrentand potentialapplicationsin thechemicalprocessindustries. lutionannealingtemperaturesdefine

howmuchchromium,molybdenum,—~ f the materialsavailable to a fuocilori of the levels of moiybde- andtungstencanberetainedin solu-

chemical processindustry numandtungsten,which rangefrom tion, assumingthat thematerialsareengineers,the wrought Ni- —13 wt% to 16.5 wt% and 0 wt% to quenchedafterannealing.Fora given

Cr-Moalloysareamongthemostver- 4 wt%. Reducingacids include hy- amount of chromium, only certainsatile.Not only dotheyresistuniform drochioric,hydrofluoric,phosphoric, quantitiesof molybdenumandtung-attackin a wide rangeof acidicand and dilute sulfuric; however,when stencanbe added;furtheradditionsalkalineenvironments,but theyalso theseacidscontainsufficientquanti- thenpartitionto primaryprecipitates.withstandstresscorrosioncracking, tiesof ferric ions, cupricions, or dis- The sameis trueif the molybdenumpitting, andcrevicecorrosion.In ad- solvedoxygentheybecomeoxidizing, and tungstenlevels are fixed, anddition, they can be formed and Nitric acidandconcentratedsulfuric chromium is added.Thus,only cer-welded,withoutdifficulty, into com- acidarenaturallyoxidizingtonickel- tam levelsof resistanceto bothoxi-plex components. basedalloys, although the relation- dizing and reducingacids can be

Many of the attractive proper -_____________________________________________________________________tiesof the wroughtNi-Cr-Mo alloys TABLE ‘1stemfrom the physicalpropertiesof . ..

Typical Compositionsof New and Existing Wrought Ni-Cr-Mo Alloys (wt%)nickel. First, it is more noble than ____________________________________________________________________—iron;second,it exhibitsa ductile,face- Alloy

centeredcubicstructureat all tern- Designation Ni Cr Mo W Fe Mn Si C Others

peratures;andthird,it isverytolerantof usefulsolutes,such aschromium ~ Bal 16 16 4 5 0.5 0.02 0.002 V-o.15

and molybdenum,which enhance Ai-O.25

passivityandnobility. C-4 BaI 16 16 — 0.5 0.2 0.02 0.002 Ti-0.2

The chromium contentsof the Ai-0.2

wroughtNi-Cr-Mo alloysrangefrom C22 BaT 22 13 3 3 0.3 0.02 0.002 V-0.15

~r16wt% to 23 wt% (Table 1). The Ai-0 2559 Bai 23 15.75 — 0.3 0.2 0.02 0.002 Ai-O.25

higher the chromium content, theB~i 20.5 1~.b 4 I o.2 0.02 0.002 Ai-0.25

oetter is theperrormancein oxiaiz-ing acids. The performanceof the New

wrought Ni-Cr-Mo alloys in reduc- C.2000 Bai 23 16 — 0.5 0.2 0.02 0.002 Cu-1.6

ingacids,ontheotherhand,islargely

F\AfJ/~A rch 19 7 Repnnted .~rnM.d-orois Performance, Vc.’ 3~’No. 3, p 1°T3” ‘~33~March/ ~ .. Copyright 1998 by NACE International, P.O. Box 218340, Ho s:on Texas 77218-8340/1— ~ic9

MATERIALS SELECTION & DESIGN

indicatesthe“very safe,”“moderately

safe,”and“unsafe”regimes,wascon-structedfrom80 datapoints,i.e.,twotestresults at eachof 40 concentra-tion and temperaturecombinations.The tops of the bars representtheboiling points. This diagramshowsthat alloy C-2000 is usablein puresulfuricacidupto ~-100°Cat concen-trations up to 70 wt%, this being asignificant advanceover the mostwidely used Ni-Cr-Mo material,al-

loy C-276 (UNS N10276). The newalloy alsopossessesadvantagesoverotherexistingNi-Cr-Mo alloysinsul-furic acid.1

The performance of alloyC-2000 in hydrochloric acid is de-

FIGURE 1 picted by the iso-corrosiondiagramApplication ranges ofexisting and new Ni-Cr-Mo alloys. (Figure 3). This chart was con-

structed from 90 data points, i.e.,

attainedusingonly chromium,mo- Resistance to Corrosion two test resultsat eachof 45 con-lybdenum,and tungstenadditions. In common with the existing, centration and temperaturecombi-The compositionsof the existingal- wrought Ni-Ti-Cr-Mo materials,al- nations.Fromthischart, it is evidentloys reflect the type of media for ioy C-2000is veryresistantto pitting, that the alloy canbeusedin boilingwhichtheyweredesigned,sincenone crevice corrosion,and stresscorro- solutionsup to a concentrationof 3spanthe entire Ni-Cr-Mo alloys ca- sion cracking, in the presenceof wt%, which is beyondthe capabil-pability range,as illustrated using chlorides. ity of existingNi-Cr-Mo alloys,1andcorrosion rate reciprocalsin strong As part of the assessmentof its up to a temperatureof ~~~60°Cin theoxidizing and reducing solutions uniformcorrosionresistance,alloyC- concentration range 7 wt% to(Figure 1). 2000wastestedextensivelyin sulfu- 20 wt%. Concentrationsin excessof

A new,wroughtNi-Cr-Mo alloy ncandhydrochloricacids,sincethese 20 wt% werenotstudiedbecauseofwasdesignedalongslightly different are among the most corrosive and the volatility of hydrochloric acid,lines. The technical objectiveduring commoncompoundsencounteredin i.e., it is not possibleto maintainadevelopmentwas a wider applica- the chemicalprocessindustries.The boiling solution at concentrationsHon range,i.e., increasedversatility, resultsof the testsin sulfuricacidare >20 wt% in a flask/condensersys-The developmentgoal wasto equal summarizedin theiso-corrosiondia- tem, dueto the evolution of hydro-the performanceof theexistinghigh gram(Figure2).Thisdiagram,which gen chloridegas.chromium alloys in oxidizing acids,andto equalthehigh molybdenumand tungstenalloys in reducingac-ids. To achievethis,a combinationofmolybdenumandcopperwasused.Copper was found not only to bemore effectivethan tungstenin en-hancingnobility underactivecorro-sion conditions,but also it allowedtheuseof a highchromiumcontent.1

The successof this approachis evi-dent in comparingthe applicationrangeof thenewmaterialwith thoseof the existingalloys, as definedbythe sametwo media (Figure 1). Thecompositionof the new material,which is known commercially asal-loy C-2000, and designatedUNSN06200is givenin Table1.

FIGURE 2Iso-corrosion diagram for alloy C-2000 in sulfuric acid.

50 MP/March 1997

wi/’\i0Ml/’kL~ ~tLLLIIUN & DESIGN

AlIqy C-2000 is also resistanttohydrofluoric acid, phosphoricacid,nitric acid,organicacids,mixed inor-ganicacids,andsodiumhydroxide.

Physical MetallurgyThe wrought Ni-Cr-Mo alloys

are typically usedin the solutionan-nealedand water quenchedcondi-tion. At the solution annealingtemperature,whichisnormallyin therange1,120°Cto 1,150°C,secondaryphasesdissolvein the fcc structure.The effectof the quenchis to “freezein” the high temperaturestructure.Only when the alloys are subse-quentlyraisedtoa temperaturesuffi-dent to causeappreciablediffusion,e.g.,duringwelding is thereneedforconcern.

Severalprecipitatescanoccurinalloy C-276,for example,whenit isexposedto elevatedtemperatures.Intherange300°Cto 650°C,anorderedphaseof thetypeNi2(Cr,Mo)canformhomogeneouslythroughoutthe mi-cro- ‘“tough thereactionki-

v. At ~“rcs

~eprecip tes,~t

phaseisthemostabundant,andM6Cthesecondmostabundant.Theyareboth rich in molybdenumand canquickly form continuous grainboundarynetworks,which renderthealloy proneto intergranularattack,sincetheypossessdifferentcomposi-tions from the alloy solid solution,andin forming,depletethesurround-ing matrix of molybdenum.To re-duce the tendencyof the wroughtNi-Cr-Mo alloys to form such pre-cipitates,specialmelting proceduresareusedto minimizethe contentsofcarbonandsilicon(aknownpromoterof intermetallics).

The issuesof thermal stabilityandintergranularattackarecomplex.Not only are the kinetics of the pre-cipitation reactionsimportant, butalso the natureof the corrosiveenvi-ronment,andthe electrochemicalef-fectsof theprecipitatesmustbetakeninto account.

To determinethe effects of el-evatedtemperatureprecipitationonalloy C-2000, samples were held(aged)for threeminutesat tempera-tures rangingfrom 760°Cto 982°C,thentestedaccordingtotheASTMC28A.3 Theseprocedureswere de-signedspecificallyto detectthe sus-ceptibility of wrought nickel-basedchromium-bearingalloys to inter-granularcorrosion,which havebeenusedbeforeto establishthetime-tem-perature-sensitizationcharacteristicsof the wrought Ni-Cr-Mo alloys.~Theheatingcycle in eachcasewas13mm (10 mm to reachthe precipita-

Hon temperature,and 3 mm at thistemperature,followed by waterquenching).To assessthe level ofgrain boundaryattackat eachtem-perature,the sampleswerestudiedmetallographically,in section,andthemaximumdepthsof attackrecorded.For comparison,similar testswereperformed on alloys C-276, 59(N06059),and686 (N06686).The re-sultsof thesetestsare shownin Fig-ure 4. Thosealloyswith the highestchromium contents (alloys C-2000and59) exhibit thehighestresistanceto intergranular corrosion afteraging.

FIGURE 3Iso-corrosion diagram for alloy C-2000 in hydrochloric acid.

FIGURE 4ASTM G 28A penetration depth as a function of aging temperature (3 mm age).

MP/March 1997 51

MATER IALSSELECTION & DESIGN

ApplicationsAs a resultof its easeof forming

andwelding,alloy C-2000is suitablefor manytypesof hardware,includ-ing reactionvessels,heatexchangers,piping and fittings, valves, andpumps.ASME approval, for use inpressurizedsystems,has beenap-plied for. Its superiorityoveralloy C-276 has so far led to two chemicalprocessindustry applications,oneinvolvingthehandlingof sulfuricacid

overawiderangeof acid concentra-tions,the other involving a mixtureof acids,includinghydrofluoric.Fieldtestsalso indicatealloy C-2000pos-sessesadvantagesin brominatedwa-ter and in chloride-containingacidmixtures.

SummaryThe wrought Ni-Cr-Mo alloys

areextremelyversatilematerials,re-sistantto many forms of corrosiveattack. By using copperin alloy C-2000, this versatility has beenex-

tendedsignificantly.Copperprovidesgreatlyenhancednobility in certainreducingacids,andallowstheuseof

a high chromium content,for opti-mumpassivityin oxidizingmedia.

References1. P.crook,CORR0SION/96,paperno. 412(Hous-ton,TX: NAcE, 1996).2. M. Raghavan,B.). Berkowitz,J.C. Scanlon,Met-allurgicalTransactionsA, 13A(1982):p,

979.

3. ASTM G 28, “StandardTest Methodsof Detect-ing Susceptibility to IntergranularCorrosion inWrought,Nickel-Rich,ChromiumBearingAlloys,”AnnualBook ofASTMStandards,Vol.03.02(Phila-delphia,PA: ASTM).4. Material Data SheetNo. 4030, NICROFER5923hMo - Alloy 59, (Werdohi,Germany:Krupp VDM,1993).

Paul Crook is Managerof CommercialTechni-cal Services (Corrosion)with Haynes Interna-tional. He hasmorethan23years’experiencewith corrosionresistantalloysandis a memberof NACE andASM International.

Martin Caruso is Market DevelopmentMan-agerwith HaynesInternational.He hasmorethan10 years’experiencewith corrosionresis-tant materialsand is a memberof NACE andASTM.

Dale Kingseedis a Sales/MarketingConsultantwith HaynesInternational.He has40 years’

experienceworking in metals related fieldsand hasheld positionsin engineeringmanu-facturing,sales,and marketing.

52 Printed in U.S.A. MR/March 1997