Capillarity and Corrosion毛细与腐蚀

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1、Chapter2CapillarityandCorrosionIntroductionThesurfaceofaliquidorsolidexhibitsthepropertyofsurfacetensionbecausemoleculesatthesurfaceoftheliquidorsolidhavefewerneighborsthanthoseinthebulk.Thissituationleadstoanetforceperpendiculartothesurfaceplanetowardtheinterioroftheliquid(orsolid).Inthecas

2、eofsolids,thepropertyiscalleditssurfacefreeenergyandmaydependontheorientationofthesolidsurface.Thesurfacetensionofwaterat20°Cis72.8mN/m(millinewtons/m).Thefollowingunitsareequivalent:72:8dynes=cm¼72:8erg=cm2¼72:8mN=m¼72:8mJ=m2SomevaluesofsurfacetensionforvariousliquidsaregiveninTable2.1[1,2]

3、.CapillarityWhenasmall-diameterglasstubeisimmersedinaliquid,theliquidwillriseinthecapillarytubing,asshowninFig.2.1.(Iftheliquidismercurytherewillbeadepressionratherthanariseinthecapillarycolumn,butwewillbedealingwithonlyaqueousfluidshere.)Theheightofthecapillaryrise(h)isgivenby:2ch¼ð2:1Þqgr©T

4、heAuthor(s)201555E.McCafferty,SurfaceChemistryofAqueousCorrosionProcesses,SpringerBriefsinMaterials,DOI10.1007/978-3-319-15648-4_2562CapillarityandCorrosionTable2.1SomesurfaceWater72.94tensionvaluesinmN/mat20°C[2]Methyleneiodide67.00Dimethylsulfoxide43.54Benzene28.88Methanol22.50Perfluorohept

5、ane13.19Fig.2.1Capillaryriseofwaterinasmall-diameterglasstubewhereγisthesurfacetensionoftheliquid,ρisthedensityofthewettingliquid,gistheaccelerationduetogravity,andristheradiusofthecapillary.Thisequationarisesasfollows.Theforceupwardinthecapillaryliquidis:FðupÞ¼cð2prÞcoshð2:2Þwhereθisthecont

6、actangleandisusuallyzero(seeFig.2.1).Thedownwardforceisduetotheweightoftheliquidandisgivenby:FðdownÞ¼pr2qghð2:3ÞAtequilibrium,theforceupandtheforcedownareequal:FðupÞ¼FðdownÞð2:4ÞThisgivesEq.(2.1).Figure2.2showssomecalculatedvaluesofthecapillaryrisehforvariouscapillaryradiir.Thesmallerthecapi

7、llaryradius,thehigherthecapillaryrise.ApplicationtoOrganicCoatings57Fig.2.2Calculatedcapillaryriseh(inmm)forvariousglasstubingradiir(inmm)ApplicationtoOrganicCoatingsItiswell-knownthatorganiccoatingsdonotformperfectbarrierstocorrosion,butinsteadthe