Nickel base superalloys such as Inconel and Hastelloy grades outperform in the vigorous chemical conditions where other alloys usually fail

1. WhichNickel alloysare used against corrosion in chemical plants?
Nickel alloysshowanadvancedstepfromtraditional stainlesssteelsandsuperausteniticironbased
alloysinpreventingcorrosionbyalarge varietyof acids,alkalisandsalts.A greatattribute of nickel
alloysisoutstandingresistance toaqueoussolutionscomprisingof halide ions.Nickelsuperalloys
are far superiortoausteniticstainlesssteelsthatare notoriouslysensitivetocorrosionbywet
chloridesandfluorides.
Thisexcellentcorrosionbehaviourof nickel alloysmanifestsitselfnotjust consideringthe lower
metal lossevenalsothe potential tobetterpreventlocalizedcorrosion,preferablypittingand
crevice corrosion,intergranularcorrosionandstresscorrosioncracking.These typesof localized
corrosion,more general thinning,responsibleformostof corrosion-inducedfailureinthe chemical
plant.Nickel alloysofferexcellentcorrosionresistance tothe inherentlowerreactivityof nickel
relative toiron,asevaluatedbyitsmore noble oxidationpotential inthe EMFseries.Asof stainless
steels,chromiumbasednickel alloyshave the potentialtopassivate.Anadditional benefitof nickel
overironis the potential toacceptlarge fractionsof alloyingelementswithoutdevelopingbrittle
phases.Commonlyusedalloysforimproved corrosionresistance are chromium, molybdenumand
copper.
Weldingof highperformance nickel superalloys
Most of nickel alloyweldsare developedbyshieldedmetal arcwelding,gastungstenarcwelding
and gas metal arc welding.Nickel alloyweldmentsare ductileandtheirnominal thermalexpansion
propertiesdecreaseresidual stressandwarpage.Postweldheatprocessingisnormallyneededfor
precipitationhardenable grades.
Weldingproceduresfornickelalloysare similartothose utilizedforausteniticstainlesssteels.
Althoughdue tothe extensive sluggishnessof Nickel richweldpuddlesandlowerpenetration
propertiesof nickel allos,the developmentof full penetrationweldsmayneedmodificationof joint
configurationsandweldingmethods.Nickel alloysare lesswithstandtocontaminationascompare
to ferrousmaterialsthatcouldleadtoembrittlement.

2. The combinationof highductility,nominal thermal expansionandthe potential totoleratedilute
throughvariousmetallicelementshasmade nickel-richweldingconsumablesuniversallyaccepted
for weldingof differentmetals.Itnotjustincludesweldsof nickel basedalloystoironbasedalloys,
evenalsoforweldsof stainlesssteelstocarbonand alloysteels.Identically,nickel alloyscan be weld
depositedoncarbonsteel withoutthe fearof cracking.
Nickel alloytypes
Nickel alloysare widelyavailable indifferentproductformsandare alsodevelopedascastings.They
usuallyhave differentpropertiesfromtheirwroughtcounterparts.Nickel alloysare categorized
followingtheirbasiccomposition.Nickelalloysthatare commonlyusedinchemical industryare:
Pure Nickel
Monel alloy 400
Inconel 600
Inconel 625
Incoloy 825
HastelloyC alloys- HastelloyC276 is one of the popularalloysused inthe chemical plantsfor
vigorous corrosive conditions wheregeneral stainlesssteelscannotbe used.
HastelloyB alloys- Inthisfamily,HastelloyB2isknownforitsoutstandingresistance tosulfuric,
phosphoricandhydrochloricacidsunderreducing conditions.Itisparticularlysuitableforuse in
equipmenthandlinghydrochloricacidatall concentrationsandtemperaturesuptothe boiling
temperature.