materials corrosion prevention and maintenance（下）

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1、Chapter5Corrosionindifferentenvironmentscorrosionbysulfuricacidispossible,andUNSN08904(904LAlloy)typealloyswith4%−4.5%minimumMomaybenecessary36.TheevaporationtraincommonlyusesAISI316Lor317Lstainlesssteel.Pittingandunder-depositcorrosionarethemostcommonlyreportedproblems26.Magnesiumbasedsolutionsarem

2、orecorrosivethancalciumorsodiumbasedsolutions.Ammoniabasedsolutionsareleastcorrosive36.Sulfiterecoveryboilersareprimarilycarbonsteelconstructionwithcorrosionallowances.Thermallysprayedstainlesssteelhassometimespreventedcorrosionofcarbonsteelwaterwalls.Compositetubingisbecomingmorepopular.Fluegasesco

3、ntainmoresulfurtrioxide,hydrogensulfide,andpossiblyhydrochloricacid.TheAISI316Land317Lstainlesssteelsandglassreinforcedplasticshaveuseforscrubbers.Pittingandcrevicecorrosionmayrequireuseofhigheralloyedstainlesssteelsornickelalloys26.5.5CorrosionunderthermalinsulationMostprocessesrequireuseofthermali

4、nsulationtoreduceenergylossandassociatedcostsinheatingandcooling.Insulationcoversthemetalsurfaceallowingacorrosiveenvironmentanddegradationtodevelopandproceedunnoticed.Thecriticalfactorsforcorrosionunderinsulationareavailabilityofoxygen,hightemperature,andconcentrationofdissolvedspecies.Asthetempera

5、tureincreases,thesolubilityofoxygennormallydecreasesresultinginreducedcorrosionrates.Underinsulation,themoistureremainsinaclosedsystem.Figure17showstheschematicbehaviorofcorrosioncausedbydissolvedoxygeninopenandclosedvessels.Whenthetemperatureincreases,thecorrosionrateincreases.Inanopensystem,thecor

6、rosionratebeginstodecreaseabovethetemperaturewheresolutionvaporpressureincreasesrapidlyreducingoxygenpartialpressure.Inaclosedsystem,theoxygencannotescape,andcorrosionratecontinuestoincreaseastemperatureincreases.Fielddatahasconfirmedthatcorrosionofsteelunderinsulationincreasessteadilywithincreasing

7、temperature37.Thecorrosionofsteelunderinsulationisidenticaltocorrosioninaclosedhotwatersystem.Thethermalinsulationdoesnotcorrodesteelbutprovidessitesformoistureandimpuritycollection.Itpreventstheescap