Rust Chemistry

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GLOSSARIEScorrosionelectrochemistrymonitoringwater

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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:

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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).

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(picturecourtesy)

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

Seealso:Chemistryofcorrosion,Equilibriumreactionsofironinwater,Ironcorrosionproducts,Ironspeciesandtheirthermodynamicdata,Pourbaixdiagramofiron