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rust chemistry

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 3/2/2015 Rust chemistry http://www.corrosion-doctors.org/Experiments/rust-chemistry.htm 1/3 GLOSSARIES corrosion electrochemistry monitoring water RESOURCES  biographies  batteries  books course modules fuel cells global warming modules  periodic table  pictures quizzes ti  pping   points tr aining center ty  pes of corros ion WEBMASTER Rust Chemistry Rusting of iron consists of the formation of hydrated oxide, Fe(OH) 3 , FeO(OH), or even Fe 2 O 3 .H 2 O. It is an electrochemical process  which requires the presence o f water , oxygen and an electrolyte. In the absence of any one of these rusting does not occur to any significant extent. In air, a relative humidity of over 50% provides the necessary amount of water and at 80% or above corrosion of bare steel is worse. When a droplet of water containing a little dissolved oxygen falls on an steel pipe, the solid iron or Fe(s) under the droplet oxidizes: Fe(s) --> Fe 2+ (aq) + 2e - The electrons are quickly consumed by hydrogen ions from water (H 2 O) and dissolved oxygen or O 2 (aq) at the edge of the droplet to produce water:

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  • 3/2/2015 Rustchemistry

    http://www.corrosiondoctors.org/Experiments/rustchemistry.htm 1/3

    GLOSSARIEScorrosionelectrochemistrymonitoringwater

    RESOURCESbiographiesbatteriesbookscoursemodulesfuelcellsglobalwarmingmodulesperiodictablepicturesquizzestippingpointstrainingcentertypesofcorrosion

    WEBMASTER

    RustChemistryRustingofironconsistsoftheformationofhydratedoxide,Fe(OH)3,FeO(OH),orevenFe2O3.H2O.Itisanelectrochemicalprocesswhichrequiresthepresenceofwater,oxygenandanelectrolyte.Intheabsenceofanyoneoftheserustingdoesnotoccurtoanysignificantextent.Inair,arelativehumidityofover50%providesthenecessaryamountofwaterandat80%orabovecorrosionofbaresteelisworse.

    Whenadropletofwatercontainingalittledissolvedoxygenfallsonansteelpipe,thesolidironorFe(s)underthedropletoxidizes:

    Fe(s)>Fe2+(aq)+2e

    Theelectronsarequicklyconsumedbyhydrogenionsfromwater(H2O)anddissolvedoxygenorO2(aq)attheedgeofthedroplettoproducewater:

  • 3/2/2015 Rustchemistry

    http://www.corrosiondoctors.org/Experiments/rustchemistry.htm 2/3

    4e+4H+(aq)+O2(aq)>2H2O(l)

    Moreacidicwaterincreasescorrosion.IfthepHisverylowthehydrogenionswillconsumetheelectronsanyway,makinghydrogengasinsteadofwater:

    2H+(aq)+2e>H2(g)

    Butwhere'stherust?Theequationsabovetellonlyasmallpartofthestory.

    Hydrogenionsarebeingconsumedbytheprocess.Astheironcorrodes,thepHinthedropletrises.Hydroxideions(OH)appearinwaterasthehydrogenionconcentrationfalls.Theyreactwiththeiron(II)ionstoproduceinsolubleiron(II)hydroxidesorgreenrust:

    Fe2+(aq)+2OH(aq)>Fe(OH)2(s)

    Theiron(II)ionsalsoreactwithhydrogenionsandoxygentoproduceiron(III)ions:

    4Fe2+(aq)+4H+(aq)+O2(aq)>4Fe3+(aq)+2H2O(l)

    Theiron(III)ionsreactwithhydroxideionstoproducehydratediron(III)oxides(alsoknownasiron(III)hydroxides):

    Fe3+(aq)+3OH(aq)>Fe(OH)3(s)

    ThelooseporousrustorFe(OH)3canslowlytransformintoacrystallizedformwrittenasFe2O3.H2Othefamiliarredbrownstuffthatiscalled"rust"formingtuberclesasshownhere.Sincetheseprocessesinvolvehydrogenionsorhydroxideions,theywillbeaffectedbychangesinpH.WithlimitedO2,magnetiteisformed(Fe3O4).

  • 3/2/2015 Rustchemistry

    http://www.corrosiondoctors.org/Experiments/rustchemistry.htm 3/3

    (picturecourtesy)

    Ifotherionslikecalciumorcarbonatearepresent,theymakeavarietyofprecipitatesthatmixinwiththeironhydroxidetoproduceacrusty,twistedcoatingwhichcaneitherslowcorrosionbycuttingtheironofffromtheacid,water,andairsupplyorgrowintoconvolutedshapescalledtubercles.Thegrowthofthesetuberclescangreatlyaffecttheflowofwaterthroughwatermainsasshownhere.

    Seealso:Chemistryofcorrosion,Equilibriumreactionsofironinwater,Ironcorrosionproducts,Ironspeciesandtheirthermodynamicdata,Pourbaixdiagramofiron


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