Columbium in alloy 625 provides resistance to sensitization to alloy. Few specimens of the material, although found to be less resistant as compare to others

1. Influence ofmetallurgical reactionsin Inconel 625 on corrosion behaviourin nitric acid
HoweverInconel alloy625offerssupreme resistance tosensitizationcausedbycarbides
precipitation,fewsamplesof the material have beenfoundtoprovide lowerresistance thanothers.
Lab studiesof the precipitationreactionsinthe alloygive nosign thatanyspecificphase causesan
increasedcorrosionof alloy.The extentof sensitizationisassociatedwiththe chromium
concentrationinprecipitatesandthe processingcontrol,specificallyannealingprocesses,can
developmaterialofferingthe highestresistance tosensitization.
Sensitizationtointergranularcorrosioninalloysthathave chromiumtoresistcorrosioninoxidizing
conditionshasfoundtobe resultedbythe developmentof chromiumcarbidesduringspecificheat
treatments.Areasneartothe carbidesare depletedinchromiumandlose theirresistance to
oxidizingmedia.Mechanismstopreventsensitizationare control of processingtolimitthe
applicationmode,inclusionof othercarbide developerslike columbiumandtitaniumandlimitation
of residual elements,carboninspecific.
Columbiuminalloy625 providesresistance tosensitizationtoalloy.Few specimensof the material,
althoughfoundtobe lessresistantascompare to others.Asit wasassumedthat the variations
resultedfromdifferencesinprocessinghistory,particularlyheatprocessing,astudyperformedto
specifythe relationsbetweenheatprocessing,precipitationandcorrosionresistance.
Resultscomparable tothose receivedforInconel 625 are reportedforotheralloys.Varioushigh
nickel alloysare studiedsuchasstainlesssteel 304l,316l and others.A relationbetweencorrosion
resistance andthe magnitude andtype of precipitatesdevelopedinalloys.Theyalsonoticedthat
howevermethodsof stabilizationforalloyswere effectiveonaregularbasis,prolongexposure to
sensitizingtemperaturescouldstill developsensitization.
The initial phase of studywasto determine the heatprocessingcausingsensitizationof alloyand
whateffects,the temperature of apreviousanneal hadonthe magnitude of sensitization.Samples
were annealedat1700oF or 927oC to 2200oF for1 hour andwater cooled.The annealedsamples
were subjectedtosensitizingheatprocessingof 1300oF to 1600oF for 60 minutesandairquenched.
The annealedandsensitizedsampleswereexposedto65% boilingnitricacidtestafterthe ASTM
A262 procedure followed.
The preliminarytestsshowedthatthe heatprocessingresultedintothe maximumsensitizationwas
1400oF or 760oC for 1 hour for all samplesandthe material annealedinthe range of 2000oF to
2200oF attainedthe highestcorrosion.Samplesannealedupto1800oF showedthe minimum
corrosionrates.Followingthe preliminarytests,time-temperature-sensitizationdiagramswere
preparedforannealsof 1800oF and 2100oF. The diagramswere developedbysubjectingannealed
material toextraheatprocessingsinthe range 1100oF- 1800oF fortimesof 0.1-1000 hour. Duplicate
samplesfrommaterial statedeveryheatprocessingof Inconelwasanalyzedin 65% nitricacid and
the resultingcorrosionrateswere plottedasfunctionsof temperatureandtime attemperature.
The final phase of the studycomprisedof associatingtime-temperature-transformationdiagramsfor
carbide precipitationinInconel 625.Carbidesthusdevelopedwereextractedelectrolyticallyin10%
HCl-methanol solution,recognizedbyx-raydiffractionandevaluatedbyx-rayspectrography.

2. The influence of annealingtemperatureisavariationinthe magnitude of sensitization.The results
showthat solutionannealingcausesgreatersensitizationthannon-solutioninganneals.