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rust
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3/2/2015 Rustchemistry
http://www.corrosiondoctors.org/Experiments/rustchemistry.htm 1/3
GLOSSARIEScorrosionelectrochemistrymonitoringwater
RESOURCESbiographiesbatteriesbookscoursemodulesfuelcellsglobalwarmingmodulesperiodictablepicturesquizzestippingpointstrainingcentertypesofcorrosion
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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:
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