2010, Corrosion of Aluminum Metal in Concrete - A Case Study, 32nd ICMA

7/29/2019 2010, Corrosion of Aluminum Metal in Concrete - A Case Study, 32nd ICMA

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CORROSIONOFALUMINUMMETALINCONCRETEACASESTUDY

DipayanJana1andDavidG.Tepke

2

1ConstructionMaterialsConsultants,Inc.4727Rt.30,Greensburg,PA15601USA,and

2SuttonKennerly&Associates,Inc.300PomonaDrive,Greensboro,NC27407USA.

Abstract

Galvanic corrosion of steelanchored aluminum nosing in outdoor portland cement concrete stairs

caused debonding and cracking of the nosing, and, cracking and spalling of the adjacent concrete.

Chlorideinduced corrosion of embedded steel anchors in concrete has created iron oxide corrosion

productsandassociatedcrackinginconcrete. Chlorideinducedgalvaniccorrosionofaluminumnosing

at the locations of steel anchors has created more severe distress due to corrosion (reduction in

thickness)ofthenosing. Aluminumcorrosion inthepresenceofmoisture,hydroxylandchloride ions

from concrete formed a gelatinous mass of hydrated aluminum corrosion products, and associated

aluminumhydroxidecrystals(boehmite,bayerite,andgibbsite). Acharacteristicbandedmicrostructure

of sheetlike friable corrosion products of aluminum metal is detected, due to layerbylayer

advancementofcorrosion,whichconsistsofalternatinglayersofsofthydratedaluminumdepositsand

hard siliconrich phases (the latter from silicon carbide grains embedded in nosing during

manufacturing).

The

bottom

1

mm

zone

of

nosing

shows

a

corrosion

microstructure

where

silicon

rich

blades were distributed in a hydrated aluminum mass, which eventually grades into the sheetlike

severelycorrodedmetal,asdescribed.Formationofagelatinousmassofhydratedaluminumproducts

andassociatedexpansion isresponsibleforthenosingdebondingandassociatedcrackingofconcrete,

which is similar to concrete cracking by formation of alkalisilica reaction gel. Possibility of galvanic

corrosionofaluminum inconcreteandcorrosionrelateddistress,especially incontactwithsteeland

chloride, therefore, should be minimized by using a protective insulating coating on the surface of

aluminumnosingincontactwithconcreteandembeddedsteel.


2/33

Introduction

Corrosionofaluminummetal in aportland cement concrete iswellknown formore than fifty years

causing deterioration or collapse of aluminum conduit, and cracking and spalling of concrete [16].

Studiesdoneinthe1950sand1960s[1,2,6]establishedthatseverecorrosionofaluminumcommonly

occurswhen

it

is

embedded

in

concrete

that

contains

calcium

chloride

and

steel

that

is

electrically

connectedtothealuminum(galvaniccorrosionofaluminumcoupledtosteel). Corrosionofaluminum

canstilloccurinthealkaline(highpH)environmentofportlandcementconcrete,fromalkalihydroxide

and moisture in the pore solution (alkali hydroxide attack of aluminum), without the presence of

chloride,orcouplingtosteel,althoughsuchcorrosionmaynotbeassevereas inthecasesofgalvanic

corrosion of aluminum from coupling with steel in the presence of chloride [2, 5]. The presence of

chloridehasalsobeen reported tocausealuminumcorrosion inthealkalineenvironmentofportland

cement concrete, even in the absence of steel (chloride attack of aluminum), which, again, is not

generally reportedasbeingassevereas thecaseofchlorideinducedgalvaniccorrosionofaluminum

coupled to steel (some other studies reported no apparent corrosion of aluminum in concrete

containingcalcium

chloride

and

no

coupling

to

steel,

[7].

McGeary

[6]

concluded

that

stray

currents

can

sustain corrosion and associated damage, especially in the presence of chlorides. The literature

suggests that the presence of moisture, level of alkalinity, presence of chlorides, galvanic coupling

betweensteelandaluminumandassociatedratiosofconnectedmetalareas,andstraycurrentsareall

factors that may affect aluminum corrosion, with the associated corrosion rates and damage being

dependenton the interactionsof these factors. Aluminumcorrosionevolveshydrogengas,produces

varioushydratedaluminumcorrosionproductswithsubsequentreductioninstrengthofthemetal,and

causesexpansion,cracking,andspallingofadjacentconcretefromtheformationofexpansivealuminum

corrosionproducts. Thismay reduce the structural capacityandpermit additionalmoisture into the

concretetofacilitatefurthercorrosion. Toprotectagainstcorrosion,somehaverecommendedtheuse

ofprotective

inert

or

insulating

coatings

when

concrete

may

be

exposed

to

chloride

(e.g.,

chloride

containing deicing agents or calcium chloride admixture) [2, 3, 6, 8]. ACI 31808 [9] Building Code

Requirements for Structural Concrete prohibits the use of uncoated or uncovered aluminum in

structuralconcreteandtheuseofwateroradmixtureswithdeleteriousamountsofchlorideionswhen

aluminumisembeddedinconcrete.

Background

The present study involves corrosion of aluminum nosing anchored on concrete stairs in a parking

facility in North Carolina, where the stairs, though partially covered, are open to the outdoor

environmentofmoisture,cyclicfreezingandthawing,anddeicingsaltsononeormoresides inNorth

Carolina. Aluminumnosingswerereportedly:(a)manufacturedwithsiliconcarbidegranulesembedded

intothewalkingsurfacewhilethemetalmatrixwasinamoltenstate;and(b)approximately96in.long,

23/4in.wide,and

1/4in.thickwithantisliptreadsanchoredtotheconcretestairnoseswithembedded

steelwinganchorandsteelnutsandboltsatapproximately12in.anchorintervals,intheplasticstateof

concrete. Figure 1 shows the configuration of aluminum nosing anchoring, along with elemental

analysesoffourdifferentmetals(aluminumnosing,steelanchor,nutandbolt) inasampledonebyx

rayelementalanalysesinascanningelectronmicroscope.


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Figure1:Schematicdiagramofaluminumnosingand itsattachment toconcretestairbysteel

winganchor,bolt,andnut.ThetableshowsSEMEDSanalysesofcompositionsofmetals.

As canbe seen from theelementalanalyses: (a) thealuminumnosing contains amajor amount (i.e.

>75%)ofaluminum,andsubordinateamount(15to20%)ofsiliconfromtheembeddedsiliconcarbide

granules; (b) the steel wing anchor contains a major amount (i.e. >85%) of iron and subordinate

amounts (i.e. 410%) of aluminum and silicon; (c) the bolt used to attach the nosing to the anchor

containsamajor

amount

(i.e.

45

to

50%)

of

iron

and

subordinate

amounts

(i.e.

10

to

25%)

of

aluminum

andchrome;and,(d)thenutcompletelyembeddedinconcretecontainsamajoramount(i.e.>85%)of

ironandminoramounts(i.e.3to9%)ofaluminumandsilicon.


4/33

Theanchoringprocess itself, therefore, createdanexcellentaluminumironalloysteel coupling inan

outdoorenvironmentofmoistureandsalts intheconcretestairs,whichhassetthestageto initiatea

galvanicactionofdissimilarmetals inthepresenceofchloridebaseddeicersreportedlyappliedtothe

stairs.

FieldEvidence

of

Distress

Adetailedfieldinvestigationwasdonetoexaminethenosingfailuresatseverallocationsofthestairs.

The investigation included evaluation of three stair towers. In many stairs, aluminum nosing show

debondingorseparationfromtheconcretesurface(bowing),andfracturing(thelatterespeciallyatthe

locationsofsteelanchors,Figure2). Separationswerehairlineupto3/16in.ormoreinwidth. Extentof

distressvariedfromminortoextensivewithcrackednosingsandsignificantspalls.

Undersidesofdebondednosingplatesshow(preferentiallylocatedattheplacesofsteelanchors):

(a) Severecorrosionofmetal in termsof lossof thecrosssectional thickness (e.g., from7.9mm

maximumto

6.1

mm

at

the

location

of

steel

anchor

in

one

nosing

sample),

and

(b) Ringtextured corrosion products (see Figure 2) concentric around the corroded steel nuts,

consistingofreddishbrownironoxidecorrosionproductsofsteelnut/bolt/anchoratthecenter,

dark greenish graymassofhighly corroded, fragile, sheetlikemetal consistingof alternating

bandsofwhitecorrosionproductsandgraymetalhydrates,andanouterringofsoft,powdery,

whitecorrosionproductsfromthenosing.

No protective coating between the aluminum and concrete or aluminum and ironbased anchoring

componentswasobserved.

Concreteat

the

locations

of

steel

anchors

show

cracking

and

spalling.

Cracks

sometimes

are

filled

with

white corrosion products from the nosing and/or reddish brown corrosion products from the steel

anchor/nut/bolt. Portionsofthenosingunderside inbetweenthesteelanchorsarepresentmostly in

soundconditionwithnonoticeablecorrosioncomparedtotheareasadjacenttotheanchorlocations.


5/33

Figure2:Fieldphotographsofcorrosionanddebondingofaluminumnosingandassociated

concretedistress.

Samples

Basedonthefieldsurvey,samplesweretakenfromtworepresentativelocationsofthestairs,

where thealuminumnosing show severe corrosion, fracturing, formationofwhite corrosion

productsattheundersides,andcrackingofconcreteallpreferentiallylocatedattheareasof

steelanchors.

Figure3showsthetwosamplesreceivedfordetailedlaboratorystudies. Thefirstsample,SKA

31,was takenbydry sawcuttinga24 in. long51/4 in.wide5 in. thicksectionofa stair

containinga30in.long3in.wide1/4in.thickaluminumnosing(showninthetopleftfield

photo),where

the

aluminum

nosing

shows

separation

(bowing)

from

concrete,

and

severe

corrosionat itsunderside,at thetwo locationsofsteelanchor. Asshown inthebottom left

columnphoto,theundersideofthenosingshowssevereandpreferentialcorrosionofmetalat

thelocationsofsteelanchor,whitecorrosionproductsformingaroundtheholeinthemetalfor

steelbolt,andcrackingofconcreteatthecorrodedlocations.


6/33

Thesecondsample,SKA32,consistsofaportionofacorroded,debondedaluminumnosing

(shown in the top rightcomposite fieldphoto),andseveralplasticbagsof loosepowdery to

fragmentedpiecesofcorrosionproductsofnosingcollectedfromaroundthesteelanchors.Of

particular importance inthissampleare:(a)thewhitecorrosionproducts frommetalnosing,

(b)

loose

pieces

of

sheet

like

masses

of

severely

corroded

metal

consisting

of

alternating

bands

ofwhitecorrosionproductsanddarkcorrodedmetal,and (c) theundersideof thecorroded

aluminumpiece.

Figure3:Two samples, SKA31and32 from twodifferent locationsof stairs that show

aluminumnosingcorrosionandassociatedconcretecrackingandspalling.

The right composite photos in Figure 3 show field locations of debonding and associated

corrosionproductsofnosingforSampleSKA32. Themiddlephotointhebottomrowofthis

rightcompositesetshowsatypicalringstructureofcorrosionconsistingofdarkgraycolored

somewhathardcorrosionproductsformedaroundasteelnut,whichissurroundedbyalayer

of soft, powdery white corrosion products. The underside of nosing at this location shows

severecorrosion

and

remains

of

the

white

deposits.

Notice

the

nosing

was

fractured

at

this

location due to significant thickness reduction of metal by corrosion. Small pieces of these

corrosionproducts in the fieldwerecarefullycollected inplasticbags fordetailed laboratory

examinations.


7/33

Reduction inMetal Thickness by Corrosion ofAluminumNosing at the Locations of Steel

Anchors

Ameasurementofnominalmaximumthicknessofthedebondedaluminumnosing inSKA31

byadigitalmicrometershowsseverepreferentialcorrosionofnosingatthelocationsofsteel

anchors,which

is

measured

to

be

18to

23percent

loss

of

thickness

by

corrosion

of

the

metal,asshown inthegraph inFigure4. Suchathicknessreductionbycorrosionhascaused

lossofstrengthofthemetal,andcrackinganddebondingofthenosingatmanystairs.

Figure4:Reductionincrosssectionalthicknessofaluminumnosingatthelocationsofsteel

anchors.


8/33

Methodology

ThesampleswereexaminedbyusingthemethodsandproceduresofASTMC856"Standard

PracticeforPetrographicExaminationofHardenedConcrete,"which includes:(a)preparation

ofpulverized,

fractured,

saw

cut,

lapped,

and

thin

sections

of

nosing,

concrete,

and

corrosion

productsforopticalandscanningelectronmicroscopy,andxraydiffractionstudies,(b)optical

microscopyofasreceived,fractured,sectioned, lapped,polished,andthinsectionsofnosing,

concrete,andcorrosionproductsinlowpowerstereomicroscopeandhighpowerpetrographic

microscope, (c) scanning electron microscopy (backscatter and secondary electron imaging)

with energydispersive xray elemental microanalyses (SEMEDS) of metals and corrosion

products,and(d)xraydiffractionstudiesofcorrosionproducts.

Additionally, acidsoluble chloride analyses were done from the top and bottom ends of

concrete stair in one sample, which detected 0.655 and 0.006 percent chloride by mass of

concreteatthetopandbottomendsofconcrete,respectively,wheremorethanonehundred

timeshigherchloridecontentatthetopofthestair(directlyadjacenttothenosing)compared

tothatatbottomisindicativeofexposuretochloridecontainingdeicingsaltsatthesurfaceof

concrete,whichhascreatedanidealenvironmentforgalvaniccorrosionofaluminumnosingin

contactwithsteelanchorandnut.

SampleSKA31:ASectionThroughSteelBolt,Anchor,Nut,andAluminumNosing

Figure5showsaclearepoxyimpregnatedlappedcrosssection(topphoto,carefullysectioned

through

a

steel

bolt

and

nut),

and

a

blue

dye

mixed

epoxy

impregnated

polished

cross

section(bottomphoto)ofsmallportionssawcutfromaroundacorrodednosing/anchorareain

thesample. Importantfeaturesinthesefiguresinclude:

(a) Completeseparationofnosingfromconcrete,

(b) Reddishbrown ironoxidecorrosionproductsandassociatedconcretecrackingaround

thecorrodedsteelcomponent,

(c) Layeredtype or banded corrosion products of aluminum nosing (described in detail,

later determined to be amorphous hydrated aluminum) situated around the steel

anchoraswellasattheinterfacebetweenthenosingandconcrete,and

(d) Aluminumironalloysteelcouplingofnosingandanchor/bolt/nutwherecompositions

ofmetalsaregivenbefore.


9/33

Figure 5: Lapped (top) and blue dyemixed epoxy impregnated polished cross section

(bottom) of SKA 31 through aluminum nosing, steel anchor, nut, and bolt showing

corrosionofnosingandanchormaterials.

CorrosionMicrostructureofAluminumNosingattheNosingConcreteInterfaceinSKA31

Figure6shows

backscatter

electron

(BSE)

images

of

cross

section

of

aluminum

nosing

in

Sample

SKA31,wherethetopleftphotoshowsthebottomhalfofthemetalincludingthecorrosion

zoneattheverybase(e.g.,boxed)thatwasincontactwithportlandcementconcretestairs.

The overall microstructure of metal body above the bottom corrosion zone is more or less

homogeneouswithsmallscaleheterogeneitiesofveryfineareasorspotsofsiliconrichareas

(from the silicon carbide grains embedded in the molten aluminum matrix) uniformly


10/33

distributedintheoverallaluminummatrix.Apartfromthissmallscalecompositionalvariations

between the siliconrich spots and aluminum matrix, approximately 100 to 200 micron size

rastermodecompositionalanalysesofthemetalbySEMEDS,atthetopexposed location, in

middepth location, and at the bottom sound location away from the corrosion zone show

more

or

less

similar

compositions

(in

atomic

weight

percent)

of

75%

Al,

18%

Si

and


11/33

sixtosevendifferentspotswithphasesofdifferentcompositionsaredetected. Thefollowing

Figure7isanenlargedviewofthebottomleftphoto,wheresixdifferentphasesaredetected.

Themajorphase (thedarkestphase inBSE image) isaluminumhydroxide(AlandOasmajor

peaksinSEMEDSspectra,e.g.,SpotNos.4and7)withvariousincorporationsofCa,Mg,Cl,S,

Cu,

Si,

etc.

The

brightest

areas

(e.g.,

Spot#

1)

are

of

Al

Si

Fe

composition

(e.g.,

>70%

Al,

10

15%

Si,10%Fe),where incorporationof iron (possibly fromthesteelnut/bolt/anchor)hascaused

increasedbrightnessinBSEimage. Thebladeshapedphases(e.g.,SpotNos.6and6a)arerich

insilicon,fromtheoriginalsiliconcarbidegrains,with85to90%Siand5to10%Al.Thebright

dendritictexturedandstoutrodshapedphasesincorporatedcopper(e.g.,>60%Al,>25%Cu).

Theoverallgrayareasinthematrix(e.g.,Spot#2)areveryrichinaluminum(e.g.,>90%Al,2.5%

Si,1%Cu).Therefore,thecorrosionzoneshows:(a)anoverallshapelesshydratedaluminum

matrix (thedarkestareas)with (b)majoramountsof irregularlyshapedaluminumrichareas

(gray areas), (c) minor amounts of bladeshaped siliconrich phases from embedded silicon

carbidegrainsandbrightAlSiFeareas,and(d)traceamountsofdendriticorstoutrodshaped

AlCualloyphases.

Figure7:Differentphasesfromcorrosionofaluminumnosing.


12/33

The following Table and SEMEDS spectra show detailed compositional analyses of all these

phasesinthebottom1mmthickcorrosionzoneofthealuminumnosing.


13/33

TwoTypesofCorrosionProductsinSKA31

PetrographicexaminationsofbluedyemixedepoxyimpregnatedthinsectionofSKA31(from

the top right polished block in Figure 8) detected two different types of corrosion products

(fromtwodifferentmetals)intheconcreteandatconcretenosinginterface.

Thefirst

one

is

reddish

brown

iron

oxide

based

corrosion

products

(the

top,

middle,

and

bottomrightcolumnphotos inFigure8)fromcorrosionofsteelanchorandnutembedded in

concrete,inthepresenceofchloride,oxygen,andmoistureinconcretethecommonchloride

inducedcorrosionofsteelinconcrete,whichhascausedsomelocalmacroandmicrocracking

in concrete adjacent to the anchor and nut, and migration of corrosion products along the

cracks. Apartfromsomelocalcrackinginconcretefromexpansionsassociatedwithformation

of steel anchor/nut corrosion products in concrete, such a steel corrosion process itself is

judgednottohavecausedthereportedseparationofnosingplatesfromtheconcretestairs.

Thesecondoneiswhitedepositsofaluminumnosingcorrosionproducts,whichoccurassoft,

powdery to flakelikedeposits,oftenpresentasouter ringsaround reddishbrowncorrosion

productsofsteelnuts,(e.g.,seeFigure2),atandaroundthelocationsofcorrodedsteel,where

corrosion(thicknessreduction)ofaluminumnosingismaximum.Thebottomsurfaceofnosing

at the locationof steel anchor shows severe corrosion,and, formationof flakyor sheetlike

massesofsoft, friablecorrodedmetalhydrateswithalternatingbandsofwhitedepositsand

darkhydratedaluminumlayers(Figures8and9).

Thewhitedeposits(appearblueinFigure8dueimpregnationwithabluedyemixedepoxy)are

opticallyisotropic,indicatingthegelatinousoramorphoustocryptoormicrocrystallinenature

(see the left column photos in Figure 8). The microstructure of these white deposits of

aluminumcorrosionproductsarefascinating,showingadistinctbandedtextureofalternating

layersofsmall,stout,prismaticdarkgray,browntoblackcrystalsalignedinadense,dark,hard

layer(whicharedeterminedtobesiliconrichphasesfromtheembeddedhardsiliconcarbide

grainsinthesoftaluminummatrix)andalternatingcolorlessorlightcoloredisotropiclayersof

gelatinoushydratedaluminummass.

The white deposits are present both on the surface of concrete directly adjacent to the

corrodedsteelanchor/nutandaluminumnosing interface,andalsoatthe interfacebetween

thenosing and concrete surface,as soft layereddeposits, significantly softer than themetal

itself.

The gelatinous nature of the white deposits and their presence at the nosing concrete

debonded interface indicate theirpotential expansion in the presence of moisture, which is

judgedtohavecausedthenosingdebonding.

Similarwhitecorrosionproductsweredescribedincorrosionandcollapseofaluminumconduit

embedded in concrete in the studies by Monroe and Ost [Ref. 2], who determined their


14/33

compositionsfromXrayspectrographicanalysestobeapproximately40percentaluminum,20

percentchlorine,andlessamountsofotherelements.Thepresentstudydetectedaround20to

30atomicpercentaluminum,60to65atomicpercentoxygen,andminoramountsofsilicon,

calcium,magnesium,chlorine,etc.

Figure8:PhotomicrographsofthinsectionofSKA31,sectionedthroughironbasedanchor,

nut,bolt,andaluminumnosingshowingtwotypesofcorrosionproductsareddishbrown

productassociatedwithcorrodedsteelanchor,andawhite(appearsblueinthephotofrom

blueepoxy)bandedtypeproductfromcorrodedaluminumnosing.


15/33

Banded&GelatinousMicrostructureofAluminumCorrosionProductsinSKA31

Figure9showstransmittedlightphotomicrographsofbluedyemixedepoxyimpregnatedthin

sectionofSKA31(oftheblockshowninthebottomphotoofFigure5andtoprightphotoin

Figure8). Thephotosweretakenfromthealuminumnosingcorrosionproductssituatedinthe

debondedinterface

between

the

nosing

and

concrete

surface.

Due

to

the

soft,

friable

nature

of

the white deposits that formed from nosing corrosion, an epoxyimpregnation step was

essential to improve the integrityprior to thin sectioning.Thephotoswere takenbyusinga

stereomicroscopewherewhitelightwastransmittedfromthebasethroughthethinsection.

The characteristic texture of nosing corrosion products, as mentioned before, is its banded

nature with alternating dark and lightcolored bands (ranging in thickness from 0.05 to 0.5

mm),wherethedarkbandsareenrichedinfine,stoutprismaticorirregularlyshaped,isotropic

massesofsiliconrichphases(lessthan10to50micronsinlength)fromtheembeddedsilicon

carbidegrains inthemetal,andthe lightbandsareenriched inhydratedaluminumcorrosion

products.The

microstructure

is

indicative

of

preferential

corrosion

of

the

overall

aluminum

matrixleavingsiliconrichresiduesasbandsalternatingwithhydratedaluminaproducts.

Figure 9: Photomicrographs of thin section of corroded nosingpieces showing characteristic

bandednatureofcorrosionproductsconsistingofalternatingdarkandlightbands.


16/33

Figure10showsbackscatterelectronimageandcorrespondingxrayelementalmapsforsilicon

and aluminum, where alternate bands of siliconrich areas (consisting of fine, prismatic or

irregularshapedsiliconrichphases)andsiliconpooraluminumrichareasaredistinct.

Figure10:Xrayelementalmapsandelementalanalysesofareas fromsiliconrichandsilicon

pooraluminumrichbands.


17/33

PreferentialCorrosion&CrackingofAluminumNosingattheSteelAnchorsinSKA32

Figure 11 shows a debonded nosing piece from Sample SKA 32 (the top photo shows the

undersideofthenosingandthebottomphotoshowscorrespondingexposedwalkingsurfaceof

thenosing).Twofeaturesareapparentfromthisfigure(a)preferentiallocationofcorrosion

ofnosing

underside

at

the

steel

anchors

locations,

where

white

corrosion

products

were

formedaround thehole innosing in the right sideofphotos,where steelcomponentshave

anchoredthenosingtothestair;and(b)radialcrackingofthenosingplateitselffromreduction

incrosssectionalthicknessofnosingbygalvaniccorrosionofnosingatthesteel interface, in

thepresenceofchloridefromtheapplicationofdeicingsaltsonconcretestairs.

Figure11:Preferentialcorrosionofnosingundersideatthelocationofasteelanchorbolt,and

crackingbyreductioninthicknessfromcorrosion.

Banded&GelatinousNatureoftheNosingCorrosionProductsinSKA32

Figures12 through14 show the characteristicbandedmicrostructureof individual sheetlike

piecesofcorrosionproductsfromnosinginSampleSKA32,whichwasalsoevidentinSample

SKA31. Plasticbagsofloosesamplesreceivedwiththeabovecorrodednosingplatecontained

many small pieces of nosing corrosion products, which are soft, friable, sheetlike layered

massesofcorrodedmetal.

Crosssectionsofthosecorrodedmetalpiecesshowuniquebandedmicrostructure(Figure12)

of alternating layers of white deposits and dark corroded metal. The light (white) and dark

(gray)alternatingbandsareverydistinctandsimilartogneissosestructureseeningneiss. The


18/33

dark bands are rich in silicon phases, whereas the light white bands are rich in hydrated

alumninacorrosionproducts.

Blue dyemixed epoxyimpregnated thin section of those corroded metal pieces reveal the

bandedmicrostructureingreatdetailshowingthealternatingdarkandlightbands,asfound

inthe

corrosion

products

from

Sample

SKA

31(Figure

9).

Figures

13

and

14

show

this

banded

microstructureinthinsectionasseenthroughapetrographicmicroscopeathighmagnification

andastereomicroscopeatrelatively lowmagnification,respectively inbothcases lightwas

transmitted through the thin section. The tables following the figures show compositional

variationsofwhitedepositsthatwerecollectedfromthesampleaspowdersorsmallflakes.

Figure12:Bandednatureofindividualpiecesofnosingcorrosionproductsshowingalternating

bands of white deposits and dark hydrated aluminum layers on the fresh fractured cross

sections.


19/33

Figure 13: Transmitted light photomicrographs of banded microstructure of nosing

corrosionproductsshowingalternatingbandsofsiliconandaluminumrichphases.

Figure 14: Transmitted light photomicrographs of banded microstructure of nosing

corrosionproductsshowingalternatingbandsofsiliconandaluminumrichphases.


20/33

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ture.

asses

wing

osing

rous

poor

32

icon

rous

)and

rated

,and

and

arate


21/33

Figure1

andalte

product

SEMED

hydrate

Figure1

layersan

wheret

element

:An

enlar

natingban

faluminu

elemental

aluminum

Are

SiRi

SiRi

SiP

showsde

dsiliconpo

e leftphot

lmapping

edview

of

dsof silicon

nosing.

analysesof

layersshow

schLayers

chPhase

orLayers

tailedcomp

orhydrated

showsth

fdistributi

theright

B

rich (arro

iliconrichl

thefollowi

Al18.1

0.5

22.1

ositionalan

aluminum

BSE image

nofsilicon

Eimage

in

s)andsilic

ayers,asilic

ggeneralc

1

3

1

alysesand

layersinth

,andmiddl

andalumin

Figure15

onpoor lay

onrichpha

omposition

i.9

.9

.9

xrayelem

corrosion

eandright

m,respect

howingpar

ers ina she

se,andsilic

(inatomic

O63.5

66.6

63.6

ntalmappi

roductofn

photossho

ively:

allelmicroc

etlikecorr

npoor

eightperc

ngof silico

osinginSK

wcorrespo

racks

sion

ent):

rich

32,

ding


22/33

Figure17:BSE imageandcorrespondingsiliconandaluminumdistributionmapsshowing the

bandednatureofsiliconrichandsiliconpoorlayers.

The siliconrich layers are due to the use of silicon carbide grains embedded in the molten

metalmatrixduring themanufacturingofnosing,whicharedenser,darker,and significantly

harder than the alternating siliconpoor hydrated aluminum layers (which are soft, powdery

andwhite). Suchbanded arrangementof siliconrich and siliconpoor layers in the corroded

productsduetoseverecorrosionofmetalinthealkalihydroxidesolutionfromconcrete.Inthe

sound metal body no such banding is observed, where siliconrich areas are uniformly

distributed over a metal matrix. This banded arrangement of layers is distinct in corrosion

productsinbothSamplesSKA31(Figures9,10)andSKA32(Figures1217).

Thefollowingtablelistsvariationsinchemicalcompositionsofwhitedepositscollectedassmall

powdersor fine flakes from variouspiecesof corrodednosing in Sample SKA32 thatwere

analyzedbySEMEDS. Inallcases,overallabundanceofaluminumandoxygen, indicatingthe

basic hydrated aluminum composition of the corrosion product is evident, in which various

incorporationsofsilicon,calcium,magnesium,chlorine,etc.aredetected.


23/33

XRDEvidenceofAluminumHydroxidePhasesinGelatinousMassofNosingCorrosion

Products

Despite the overall optically amorphous nature of white corrosion products of aluminum

nosing,xraydiffractionstudiesofthecorrosionproductsdetectedafewlowintensitypeaksof

crystallinephases

of

aluminum

hydroxide

[Bayerite

and

Gibbsite

Al(OH)3, and Boehmite

AlO(OH)],alongwithminor to traceamountsofcalcite [CaCO3]andNantokite [CuCl] (Figure

18). Detectionofbayerite inconcretefromaluminumcorrosion isnotuncommon[4],where

bayerite formsasmetastablephasebetweenamorphoushydratedaluminumandmonoclinic

gibbsite.Occurrencesofaluminumhydroxidephases,i.e.bayerite,gibbsite,andboehmitewere

previouslydescribedinotherstudies[4],wherealuminumhydroxideinitiallyformasbayerite,

which is metastable and is reportedly an intermediate stage between amorphous hydrated

aluminumandmonoclinicgibbsite.Bothbayeriteandgibbsiteformofaluminumhydroxideare

found(wheregibbsiteispreviouslyreportedinRef.4toformatadvancedstageattheexpense

ofbayerite),

as

well

as

ametastable

phase

boehmite,

which,

reportedly,

forms

at

an

initial

stage from solublehydroxyl aluminate ions [Al(OH)4

]prior to the formationofbayerite and

gibbsite[4].DetectionofaminorNantokite(CuCl)phase inthepattern isnotsurprisingfrom

thedetectionofAlCualloyphasesinthecorrodedbaseofnosing(Figure7)andthepresence

ofchlorideintheconcrete.


24/33


25/33


26/33

Figure19:Photomicrographsof lappedcrosssection(left),andthinsection(middleandright)

of concrete (topphotos) and repair mortar (bottomphotos) showing airentrained concrete

containing sand and crushed stone aggregates,andportland cement and fly ashparticles in

paste, and finequartz sand,portland cement,and limestone fineparticles inadense repair

mortarcoatappliedonconcrete.

Discussion

CorrosionofAluminumNosingandCorrosionProducts

Siliconcarbidegrainsembeddedonthewalkingsurfaceofaluminumnosingcreatedauniform

distribution of siliconrich phases in the aluminum matrix over the entire thickness of the

nosinguptothedepthofthecorrodedbase,adjacenttotheconcrete.

Corrosion of aluminum nosing along the base, in contact with alkali hydroxide and chloride

solutions inconcretecreatedbladeshaped siliconrichphases inahydratedaluminummetal

matrixwith

occasional

phases

of

Al

Si

Fe

and

Al

Cu

compositions.

Furthercorrosionofmetalhascausedasignificantlossofstrength,cracking,andcreatedsoft,

friable,sheetlike layeredmassofhighlycorrodedmetal,withalternatingbandsofdenseand

darksiliconrichlayersfrompreferentialalignmentofsiliconrichphasesinalayer,andlayersof

soft, powdery white deposits of gelatinous hydrated aluminum layers with finely crystalline

phasesofaluminumhydroxide(gibbsite,bayerite).Theseprogressivechangesinmicrostructure


27/33

fromthesoundmetaltosheetlikecorrodedmetalcorrosionproductcanbeshownbytheBSE

imageandcorrespondingAlSidistributionmapsinFigure20,andintheschematicdiagramin

Figure 21 illustrating possible process of formation of sheetlike corrosion products through

metaldissolutioninsolution(thethroughsolutionprocessofaluminumcorrosion).

Formationof

an

overall

gelatinous

(amorphous)

mass

of

hydrated

aluminum

corrosion

products

hascreatedexpansionandassociatedcrackingofconcreteanddebondingofaluminumnosing

from the concrete. Corrosion process of aluminum initially forms the gelatinous hydrated

aluminummass, fromwhichmicrocrystallinemassesofboehmite,bayerite,andgibbsitecan

formwithadvancedstagesofcorrosion. Formationofthegelatinousmassitselfisresponsible

fortheexpansionandcracking inadjacentconcrete. Areduction inthicknessofnosingfrom

corrosion has caused cracking of the nosing itself. Evolution of hydrogen gas due to alkali

hydroxideattackofaluminumfromconcretecanalsocreateaporouszoneatthealuminum

concreteinterface,aswasreportedbyRef.4.

Preferential occurrences of aluminum corrosion and formation of corrosion products at the

locations of steel anchors indicate galvanic corrosion of aluminum in contact with steel,

especiallyinthepresenceofhighchlorides,detectedatthetopportionofconcretestairsfrom

thereportedapplicationsofchloridecontainingdeicingsalts.

A typical ringstructured corrosion product around the steel nuts, bolts, and anchors were

detected after removing the nosing plates. The product consists of: (a) reddish brown

corrosionproductsofsteelanchorandnut/boltinconcreteinthepresenceofchloride,oxygen,

and moisture,which are surrounded by (b) white corrosionproducts of hydrated aluminum

fromnosing

at

the

locations

of

coupling

to

steel

anchor,

where

aluminum

corrosion

was

enhanced by the presence of chloride. It is hypothesized that corrosion of the steel

componentsmighthavestartedafterthealuminumcorrosionproductscreatedabarrierinthe

direct vicinity of the connection, or once the steel became atmospherically exposed in the

presenceofchloridesfromassociatedaluminumdistress.


28/33

Figure20:Progressivechangeinmicrostructureofaluminumsilicondistributionfromuniform

distributionofAlandSiinthesoundmetalnosing(top)tobladeshapedsiliconrichphasesat

itscorrodedbase(middle)tosheetlikecorrosionproductshowingbandedmicrostructurewith

layeredarrangementofsiliconrich(dashedlines)andsiliconpoorphases(bottom).


29/33

MechanismofFormationofSheetlikeBandedMicrostructureofNosingCorrosionProducts

ThefollowingschematicdiagraminFigure21illustratesapossiblemechanismofformationofa

sheetlikebandedmicrostructureof severelycorrodednosingby layerbylayerdissolutionof

theoriginalaluminumnosingbyalkalihydroxidesolutionfromtheconcrete,followedbylayer

bylayer

density

settlement

of

hard

silicon

rich

phases

(from

original

embedded

SiC

granules)

andsofthydratedaluminumcorrosionproductsontopofsiliconrichphases:

Figure21:Schematicdiagramofformationofbandedmicrostructureandsheetlikecorrosion

productofnosing.

Suchabanded

microstructure

is

the

result

of

embedding

avery

hard

material

(silicon

carbide

granuleshavingMohshardnessof9.5)inaverysoftaluminummetalmatrix(Mohshardnessof

2.75),andisindicativeofformationofcorrosionproductsbydissolutionoftheoriginalmetalin

alkali hydroxide solution, followed by density settlement of hard siliconrich phases in the

solution to form a siliconrich layer beneath the relatively less dense layer of hydrated


30/33

aluminumcorrosionproducts.Thisthroughsolutionmechanismofthecorrosionprocessisbest

evidencedbythisbandedmicrostructureoftwomaterialshavingdifferenthardness.

MechanismofCorrosionofAluminum inConcretenot inElectricalConnectivity to Steel,and

GalvanicCorrosionofAluminuminConcretethatisCoupledtoSteel

The following two schematic diagrams (Figures 22 and 23) illustrate the mechanism of

corrosion of aluminum in a portland cement concrete in a case where the metal is not

electricallyconnected (coupled) tosteel (Figure22),and inacasesimilar to thepresentone

where aluminum is coupled to steel and exposed to moisture and chloride for galvanic

corrosion(Figure23):

Figure 22: Mechanism of corrosion of aluminum metal in concrete without electrical

connectivityto

steel.

Steelembeddedinconcretedoesnottypicallycorrodeduetotheformationofapassivelayer

over the steel surface in high pH environments. When the passive layer is destroyed, for

instances,fromaloweringofthepHfromcarbonationorthepresenceofsufficientquantities

ofchlorides,corrosionofsteelmayoccur,with the ratebeingdependenton several factors.


31/33

Corrosionofsteelinconcretereleaseselectronsattheanode,whichflowtothecathode,and

combine with moisture and oxygen to form hydroxyl ions. Hydroxyl ion and/or chloride

combines with oxidized iron at the anode to form iron hydroxide and chloride corrosion

products, further oxidation of hydroxide forms various iron oxide corrosion products with a

volume

expansion

of

2

to

6

times

the

original

volume

of

iron

metal

(depending

on

the

corrosion

productthatforms),andassociatedcrackingofconcrete.Corrosioncanoccurinthepresence

ofchlorideevenatahighpHwherechloride ionspenetratethroughtheprotectiveoxidefilm

aroundironandcausespittingtypecorrosion.

Unlike steel, which is generally protected at the high pH environment of portland cement

concretebyan ironoxide film,corrosionofaluminum israpid inthehighpHenvironmentof

moist concrete,wherehydroxyl ions from concretespore solutiondissolvealuminummetal

(including its protective oxide passive layer) and form soluble hydroxyl aluminum ions and

evolvehydrogengas.Thehydroxylaluminumionformsagelatinoushydratedaluminummass,

fromwhich

various

aluminum

hydroxide

crystals

(initially

boehmite,

followed

by

bayerite,

and

thengibbsite)canformwithadvancedstagesofcorrosion. Thistypeofcorrosioniscommonin

an amphoteric metal, such as aluminum, which shows corrosion at the cathodic portion of

metal surface because of the build up of alkalis caused by the cathodic electrochemical

reactions.

Ingalvaniccorrosionofaluminumcoupled tosteel,hydroxyl ion fordissolutionofaluminum

comes not only from concrete but also from corrosion of steel in concrete (from cathode),

which aggravates aluminum corrosion process compared to the case with no steel or no

coupling

with

steel.

The

galvanic

reactions

between

aluminum

and

iron

cause

preferential

loss

ofmoreanodicaluminumandpreservationofthe morecathodicsteel,wherechlorideionsact

ascatalysttosustaintheprocessandaluminumcorrosion. Thisissimilartotheuseofsacrificial

zinc or aluminum anodes connected to reinforcing steel to preserve reinforcing steel and

extendtheservicelife. Aluminumreleaseselectronsattheanode(electronsconductthrough

metals to iron at cathode), receives hydroxyl ions from iron cathode by ionic conductivity

through concrete, and formsaluminumhydroxide.This reaction is similar to typical galvanic

corrosionofaluminumcoupledtocopperinburiedpowerortelephonecables. Thepresence

ofmoistureandchloridearebothessentialtosustainthegalvanicprocess(bothgalvanicand

straycurrentcorrosion),whichareindeedpresentinamoistoutdoorenvironmentofconcrete

exposedtochloridecontainingdeicingsalts.


32/33

Figure23:Mechanismofgalvaniccorrosionofaluminummetalinconcretecoupledtosteel.

Conclusion

Fieldevidence,coupledwithdetailedpetrographicexaminationsofaluminumnosingcorrosion,

productsofcorrosion,andrelateddistress innosingandconcreteillustratethe importanceof

proper protection of aluminum metal with an inert or insulating (e.g., bituminous) coating

when used in a highly alkaline environment of portland cement concrete, especially in an

outdoorenvironment in thepresenceofchloride (e.g., fromchloridecontainingdeicers)and

steel electrically connected to aluminum. Nosing surfaces directly attach to concrete stairs

generallydidnotshowanycorrosionorcorrosionrelateddistress.Itisonlyatthelocationsof

steel anchors to the nosing, where aluminum nosing show severe corrosion, thickness

reduction,metal fracturing, and formationof corrosionrelatedhydratedaluminumproducts

withconcomitantdebonding(bowing)ofmetalfromconcrete,mainlyatlocations inbetween

twosteelanchors.Formationofagelatinous (amorphous)corrosionproductofnosing,along

with some crystallinealuminumhydroxidephasesarejudged tohave thepotential to cause

expansionbymoistureabsorptionandassociatedliftingofthenosingfromthenosingconcrete

interface.


33/33

Acknowledgments

SincerethanksareduetoDr.ChrisBagnall,editorinchiefofMaterialsCharacterizationanda

Fellow of ASM International for detailed review and various comments on the article, Mitzi

Casper (CMC) for sample preparation and photomicrography, and Jennifer L. Hall (CMC) for

reviewand

formatting

of

the

final

manuscript.

References

1. Wright, T.E., An unusual case of corrosion of aluminum conduit in concrete, The

EngineeringJournal,V38,No.10,13571362,1955.

2. Monfore, G.E., and Ost, Borje, Corrosion of aluminum conduit in concrete,Journal

PortlandCementAssoc.ResearchandDevelopmentLaboratories,Vol7,No.1,January,

pp.10

22,

1965.

3. Woods, Hubert, Corrosion of Embedded Material Other than Reinforcing Steel,

SignificanceofTestsandPropertiesofConcreteandConcreteMakingMaterials,STP

No.169A,230238,AmericanSocietyforTestingandMaterials,1966.

4. Setiadi, A., Milestone, N.B., Hill, J., and Hayes, M., Corrosion of aluminum and

magnesium in BFS composite cements, Institute ofMaterials,Minerals andMining,

2006.

5. Nurnberger,Ulf,Corrosionofmetalsincontactwithmineralbuildingmaterials,Otto

GrafJournal,Vol12,pp6980,2001.

6. McGeary,F.,

Performance

of

Aluminum

in

Concrete

Containing

Chlorides,

Journal

of

theAmericanConcreteInstitute,V63,No.9,247265,1966.

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AberdeenGroup,1965.

8. ACI Committee 222, Design and Construction Practices to Mitigate Corrosion of

Reinforcement in Concrete Structures (ACI222.3R03),ManualofConcretePractice,

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9. ACICommittee318,BuildingCodeRequirementsforStructuralConcrete(ACI31808),

ManualofConcretePractice,TheAmericanConcreteInstitute,FarmingtonHills,MI,465

pp,2008.

2010, Corrosion of Aluminum Metal in Concrete - A Case Study, 32nd ICMA

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