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A new, nickel-based alloy resistant to a very wide range of corrosive media is described. The main alloying additions are chromium and molybdenum; however, the alloy also contains ~1.6% copper, which significantly enhances its resistance to dilute sulfuric and hydrochloric acids. The safe operating regimes for the alloy in these two acids are defined, as are its current and potential applications in the chemical process industries.
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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