金属在某些环境中的腐蚀ppt课件

.,1,第三章金属在某些环境中的腐蚀,.,2,第三节金属在土壤中的腐蚀特点CorrosioninSoils,.,3,3.3.1Introduction,Soilcorrosionisacomplexphenomenon,withamultitudeofvariablesinvolved.Chemicalreactionsinvolvingalmosteachoftheexistingelementsareknowntotakeplaceinsoils,manyofwhicharenotyetfullyunderstood.,.,4,Variationsinsoilpropertiesandcharacteristicsacrossthreedimensionscanhaveamajorimpactoncorrosionofburiedstructures.,.,5,Varioustypesofsoildifferconsiderablyinstructureandcomposition,andthereforealsoincorrosivitytomaterials.Alsoclimaticinfluences(precipitation,temperature)andtheirfluctuations,airmovements(erosion,aeration),andsunlightmayverymuchinfluencethepropertiesofthesoilandthereforethecorrosionbehaviorofmaterialsinsoil.,.,6,Somegeneralrules:Soilswithhighmoisturecontent,highelectricalconductivity,highacidity,andhighdissolvedsaltswillbemostcorrosive.Theeffectofaeration(通风)onsoilsissomewhatdifferentfromtheeffectofaerationinwaterbecausepoorlyaeratedconditionsinsoilscanleadtoacceleratedattackbysulfate-reducing(硫酸盐还原)anaerobicbacteria(厌氧细菌).,.,7,3.3.2SoilVariables,a.WaterWater(liquid)-theessentialelectrolytesaturated/unsaturatedwaterflowunsaturated-representsmovementofwaterfromwetareastowardsdrysoilareas.Saturatedwaterflowisdependentonpore(孔)sizeanddistribution,texture,structure,andorganicmatter.,.,8,b.Degreeofaeration,Theoxygenconcentrationdecreaseswithincreasingdepthofsoil.Inneutraloralkalinesoils-importanteffectoncorrosionrate-participationinthecathodicreaction.However,certainmicrobes(suchassulfatereducingbacteria)corrosionratescanbeveryhigh,evenunderanaerobicconditions.,.,9,c.pH,SoilsusuallyhaveapHrangeof5-8.Moreacidicsoilsobviouslyrepresentaseriouscorrosionrisktocommonconstructionmaterialssuchassteel,castironandzinccoatings.,.,10,d.Resistivity,Resistivityhashistoricallybeenusedasabroadindicatorofsoilcorrosivity.Sinceioniccurrentflowisassociatedwithsoilcorrosionreactions,highsoilresistivitywillarguableslowdowncorrosionreactions.,.,11,.,12,e.Redoxpotential,Theredoxpotentialessentiallyisameasureofthedegreeofaerationinasoil.Anhighredoxpotentialindicatesahighoxygenlevel.Lowredoxvaluesmayprovideanindicationthatconditionsareconducivetoanaerobicmicrobiologicalactivity.,.,13,f.Chloridelevel,Chlorideionsaregenerallyharmful,astheyparticipatedirectlyinanodicdissolutionreactionsofmetalsandtheirpresencetendstodecreasethesoilresistivity.,.,14,h.Sulfatelevel,Sulfatesaregenerallyconsideredtobemorebenignintheircorrosiveactiontowardsmetallicmaterials.However,concretemaybeattackedasaresultofhighsulfatelevels.Thepresenceofsulfatesdoesposeamajorriskformetallicmaterialsinthesensethatsulfatescanbeconvertedtohighlycorrosivesulfidesbyanaerobicsulfatereducingbacteria.,.,15,3.3.3FormsofCorrosioninSoils,Smallmetalwork-generalcorrosionEquipmentsinlargesize/longpipelines-localizedcorrosion,.,16,A.CorrosionbyConcentrationCells,theextentofaeration:differentiallyaeratedsoils;proportionimmersedinwater;coveragebymoistporousconstructionmaterials;biologicalgrowthanddeposits;thecompositionandaggressivityofthesoil(pH,saltcontent,resistivity,etc.).,.,17,Anodicregionsareproducedunderthefollowingconditions:noaerations,highsaltcontent,lowpHvalue,highmoisture.Cathodicregionsareproducedunderthefollowingconditions:goodaeration,lowsaltcontent,higherpHvalue,moderatelywetareas.,.,18,Inthecaseofcorrosioncausedbyconcentrationcells,anodicregionshaveamorenegativepotentialthancathodicregions.Thepotentialdifferencemaybeupto300mV.Thesolutionfortheseproblemsiscathodicprotection.,.,19,土壤腐蚀,Aerationinducedcorrosion,Differential-aerationcellscanbesetupbetweenthetopandbottomsurfacesofpipes,ordownburiedpilesandwellcasingsowingtothegenerallyloweroxygencontentatdeepersoildepths.Thesameeffectcanhappenwheremetalsystemspassunderroads,whicharerelativelyimpervioustooxygen.,.,20,B.Microbiologicallyinfluencedcorrosion(MIC),Microbiologicallyinfluencedcorrosion(MIC)referstocorrosionthatisinfluencedbythepresenceandactivitiesofmicroorganisms(微生物)and/ortheirmetabolites(theproductsproducedintheirmetabolism(新陈代谢).,.,21,TheformsofMicrobiologicallyInfluencedCorrosionarenotunique.Biocorrosionresultsinpitting,crevicecorrosion,selectivedealloying,stresscorrosioncracking,andunder-depositcorrosion.,.,22,Tubercles节瘤Termreferstoamound隆起onthemetalsurfacewhichcontainbiologicalspecies,probablyinanadherent,slimy粘性的material.Eventuallythemoundscontaincorrosionproductsresultingfromtheirelectrochemicalactivity.,.,23,Thiobacillus-硫杆菌Gallionclla-铁氧化菌,.,24,Tuberculationcanresultfromnonbiomassmaterialse.g.,carbonates,silicates,phosphates,greases(油脂),mud,roaddebris(碎片).Theresultispittingorevencrevicecorrosionunderneath.,.,25,Microbiologicallyinfluencedcorrosionina316stainlesssteelweld,Iron-oxidizingbacteriacanperforatea5mmthick316stainlesssteeltankinjustoveramonth!,.,26,PreventionofMIC,Regularmechanicalcleaningifpossible.Chemicaltreatmentwithbiocidestocontrolthepopulationofbacteria.Completedrainageanddry-storage.,.,27,C.StrayCurrentCorrosion杂散电流腐蚀,杂散电流是指直流电源设备漏电进入土壤产生的电流，对地下管道、贮罐、电缆等金属设施，造成严重的腐蚀破坏。杂散电流流出的部位成为腐蚀电池的阳极区，金属发生氧化反应转变为离子进入土壤。腐蚀掉的金属量和流过的杂散电流的电量成正比，可以按法拉第电解定律进行估算。,.,28,从电车轨道漏出的杂散电流对土壤中钢铁管道的腐蚀作用图解根据Akumob引自金属腐蚀理论及其研究方法P133,电车,电流流出,轨道,阳极区域（严重腐蚀）,阴极区域,架空线,.,29,控制杂散电流的方法：,直流电源要加强绝缘，不使电流流入土壤。改善管道绝缘质量。将受