Corrosion of Aluminum and Aluminum Alloys (#06787G)

262 Pages·1999·12.38 MB·English

by  J. R. Davis

Corrosion of Aluminum and Aluminum Alloys Copyright © 1999 ASM International®
J.R. Davis, editor, p 1-24 All rights reserved.
DOI: 10.1361/caaa1999p001 www.asminternational.org
Chapter 1
Introduction
ALUMINUM became an economic competitor in bedirectly orindirectly affectedbyuse.Today,alumi
engineering applications toward the end ofthe 19th numissurpassedonlybysteelinitsuseasastructural
century. Thereason aluminum was not used earlier material.
was the difficulty ofextractingit from its ore. When
theelectrolyticreductionofaluminumoxide (Al2O:3) Key Characteristics ofAluminum
dissolvedinmolten cryolite was independentlydevel
oped by Charles Martin Hall in theUnitedStates and
PaulT.HeroultinFrance, thealuminumindustry was Aluminumoffersawiderangeofpropertiesthatcan
bom. beengineered precisely to the demands of specific
The emergenceofthree importantindustrial devel applications through the choice ofalloy, temper, and
opments inthe late 18008and early 1900swould, by fabrication process. The properties ofaluminum and
demandingmaterialcharacteristicsconsistentwith the its alloys which give rise to their widespread usage
unique qualities of aluminum and its alloys, greatly include thefollowing:
benefit growth in the production and use ofthe new
• Aluminumislight;itsdensity isonlyone-thirdthat
metal. The first ofthese was the introduction ofthe
ofsteel.
first internal-combustion-engine-powered vehicles.
• Aluminumand aluminumalloys are available ina
Aluminum would play a role as an automotivemate widerangeofstrengthvalues-fromhighlyductile
rialofincreasingengineeringvalue. Secondly,elec low-strengthcommerciallypure aluminumtovery
trification would require immense quantities of tough high-strength alloys with ultimate tensile
lightweightconductivemetal for long-distancetrans strengths approaching690MPa (100ksi).
mission and for constructionofthe towers neededto • Aluminum alloys have a high strength-to-weight
support the overhead network ofcables that deliver ratio.
electrical energy from sites of power generation. • Aluminumretains its strength at low temperatures
Within a few decades, a third important application andisoftenused forcryogenicapplications.
area was made possible by the invention ofthe air • Aluminum has high resistance to corrosion under
plane by the Wright brothers. This gave birth to an the majority ofservice conditions, and no colored
entirely new industry which grew in partnershipwith salts are formed to stain adjacent surfaces or dis
thealuminumindustrydevelopmentofstructurallyre colorproductswithwhichitcomesintocontact.
liable,strong,andfracture-resistantpartsforairframes, • Aluminum is an excellent conductor of heat and
engines, and ultimately, for missile bodies, fuel cells, electricity.
andsatellitecomponents. • Aluminumishighlyreflective.
However,.the aluminum industry growth was not • Aluminumis nonferromagnetic, aproperty of im
limited to these developments. The first commercial portanceintheelectricalandelectronicsindustries.
applications ofaluminum were novelty items such as • Aluminumisnonpyrophoric,whichisimportantin
mirror frames, house (address) numbers, and serving applications involving inflammable or explosive
trays.Cookingutensils werealsoamajorearlymarket materials handling orexposure.
Intime,aluminumapplicationsgrewindiversitytothe • Aluminumisnontoxic andisroutinelyusedincon
extent thatvirtually everyaspectofmodemlifewould tainersforfood andbeverages.
2 I Corrosion of Aluminum and Aluminum Alloys
• Aluminumhasanattractiveappearanceinitsnatu Strength. Commerciallypurealuminumhasaten
ralfinish, whichcanbesoftandlustrousorbright silestrengthofabout90MPa(13ksi).Thusitsuseful
andshiny.Itcanbevirtuallyanycolorortexture. nessasastructuralmaterialinthisformissomewhat
• Aluminumisrecyclable.Aluminumhassubstantial limited Byworkingthemetal,asbycoldrolling,its
scrapvalueandawell-establishedmarketforrecy strengthcanbe approximately doubled.Muchlarger
cling,providingbotheconomicandenvironmental increasesinstrengthcanbeobtainedbyalloyingalu
benefits. minumwithsmallpercentages of one or moreother
• Aluminumis easilyfabricated. Aluminumcanbe elementssuchasmanganese, silicon,copper,magne
formedandfabricatedbyallcommonmetalwork sium,orzinc.Likepurealuminum,thealloysarealso
ingandjoiningmethods. madestrongerbycoldworking.Someofthealloysare
furtherstrengthenedandhardenedbyheattreatments.
Table1liststhe importantphysicalpropertiesof
Figure1showstherangeofstrengthlevelsofrepre
purealuminum.Table2 showsthecharacteristics of
sentativealuminumandaluminumalloys.
aluminumandtheirimportancefordifferentenduses.
High Strength-to-Weight Ratio. The strength
LowDensity. Aluminumhasadensityofonly2.7
to-weightratioofaluminumismuchhigherthanthat
glcm3, approximately 35%thatofsteel(7.83glcm3)
of many commongrades of constructionalsteels
and30%ofcopper(8.93g/cm')orbrass(8.53glcm3).
oftendoubleormore(Fig.1).Thispropertypermits
Onecubicfootofsteelweighsabout222kg(490lb);a
designandconstruction of strong,lightweightstruc
cubicfootofaluminumweighsonlyabout77kg(170
turesthatareparticularlyadvantageousforanythingthat
lb).
moves-spacevehiclesandaircraftaswellasalltypes
ofland-andwater-bornevehicles.
Corrosion Resistance.Whenaluminumsurfaces
Table 1 Summaryoftheimportantphysical
areexposedtotheatmosphere, a thininvisibleoxide
propertiesofhigh-purity(~.95%AI)
skinformsimmediately,whichprotectsthemetalfrom
aluminum
further oxidation. This self-protecting characteristic
Property Va'" givesaluminumitshighresistanceto corrosion. Un
lessexposedtosomesubstanceorconditionthatde
Thermalneutroncrosssection 0.232±0.003bams
Latticeconstant(lengthof 4.0496x1O-lOmat298K stroysthisprotectiveoxidecoating,themetalremains
unitcube) fullyprotectedagainstcorrosion.Aluminumishighly
Density(solid) 2699kg/m3(theoreticaldensity resistanttoweathering,eveninindustrialatmospheres
basedonlatticespacing)
2697-2699kg/m3 thatoften corrodeother metals.It is also corrosion
(polycrystallinematerial) resistantto manyacids. Alkalisare among the few
Density(liquid) 2357kg/m3at973K substancesthatattacktheoxideskinandthereforeare
2304kg/m3at1173K corrosivetoaluminumAlthoughthemetalcansafely
Coefficientofexpansion 23xIo-<>IKat293K
beusedinthepresenceofcertainmildalkaliswiththe
Thermalconductivity 2.37Wlcm· Kat298K
Volumeresistivity 2.655x10-8Q. m aidofinhibitors, ingeneral,directcontactwithalka
Magneticsusceptibility 16x1O-3/m3g/atomat298K linesubstancesshouldbeavoided.
Surfacetension 868dyne/cmatthemelting The high thermal conductivity of aluminum
point (about50 to 60%thatof copper)cameprominently
Viscosity 0.012poiseatthemeltingpoint
intoplayintheveryfirstlarge-scalecommercialappli
Meltingpoint 933.5K
Boilingpoint 2767K cation of the metalin cooking utensils. Thischarac
Heatoffusion 397Jig teristic isimportantwheneverthe transferof thermal
Heatofvaporation 1.08x10-4Jig' K energyfromonemediumtoanotherisinvolved, either
Heatcapacity 0.90Jig.K
heatingorcooling.Thusaluminumheatexchangersare
Table2 Propertycombinationsimportantfortheuseofaluminum invariousapplicationareas
Characteristics TypeofsemiCabrieatedproducts
Goodbeat Deeoratiseaspeds Castings W"U'e
andelectrical Resioltanee (withorwithout or .Formed Impart Extruded aDd
Fieldofuse Lightness cooductivity tocorrosion surfacetreatment) forgings sheet extmsions sedions cable Foil
Transport 1 2 2 2 2 2
Building 2 2 1 2 2
Packaging 3 3 I 1 2 2
Electrical 3 1 2 2 2 2 2
Household 2 1 1 2 2 2
Machines, 1 2 2 2 2 2 2
appliances
Chemicalsandfood 2 2 2 3 2 2 2
1,veryimportant;2,important;3,desirable
Introduction I 3
commonlyused inthe food, chemical, petroleum, air The high reflectivity gives aluminumadecorative ap
craft,andotherindustries. pearance; italsomakes aluminumaveryeffectivebar
High Electrical Conductivity. Aluminum is one rier against thermal radiation, suitable for such appli
ofthe two common metals having an electrical con cationsasautomotiveheat shields.
ductivity highenoughforuseasanelectric conductor. Nontoxic Characteristica. The fact that alumi
The conductivityofelectric conductorgrade (1350) is num is nontoxic was discovered in the early days of
about 62% that ofthe International AnnealedCopper the industry. It is this characteristic that permits the
Standard (lACS). Because aluminum has less than metaltobeusedincookingutensils withoutanyharm
one-third the specific gravity ofcopper, however, a fuleffectonthebody.Todayagreatdealofaluminum
pound of aluminum will go about twice as far as a equipment is used in the food processing industry.
poundofcopperwhenusedforthispurpose. Nontoxicity permits aluminum foil wrapping to be
Reflectivity.Smoothaluminumishighly reflective usedsafelyindirectcontactwithfoodproducts.
of the electromagnetic spectrum, from radio waves Finishability.Forthemajorityofapplications,alumi
through visible lightand on intotheinfrared and ther numneeds no protectivecoating. Mechanicalfinishes
mal range. Itbouncesaway about 80% ofthe visible suchaspolishing,sandblasting,orwirebrushingmeet
light and 90% ofthe radiant heat striking its surface. the majority ofneeds. In many instances, the surface
2500r-------------------,
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Fig 1
Comparisonofaluminumalloyswithcompetingstructuralalloyson
• thebasisoflal tensilestrengthand~blspecifictensilestrength lten-
silestrength,inksl,dividedbydensity,ing/cm1
4 I Corrosion ofAluminum andAluminum Alloys
finishsuppliedisentirelyadequatewithoutfurtherfin CompetingMetalsforLightweightConsbvction.
ishing. Where the plain aluminumsurfacedoes not Thelight(lowdensity)metalsandalloysofcommer
sufficeorwhereadditionalprotectionisrequired,any cialimportancearebasedonaluminum,magnesium,
of awidevarietyof surfacefinishes maybeapplied. andtitanium. Eachofthesemetalshasdistinctquali
Chemical, electrochemical, and paintfinishes are all ties that makethemsuitableor preferredforcertain
used.Manycolorsareavailableinbothchemicaland applications.
electrochemicalfinishes. Ifpaint, lacquer, orenamelis Withadensityof 1.8g/cm3, magnesiumalloysare
used, any color possiblewith these finishescan be amongthelightestknown structuralalloys.Thisistheir
applied. Vitreous enamelshave been developedfor chiefadvantage whencomparedwithaluminumand
aluminum,andthemetalcanalsobeelectroplated. titanium. However, alowyieldstrengthandmodulus
EaseofFabrication.Theeasewithwhichalumi ofelasticitycombinedwithpoorthermalandelectrical
numcanbefabricatedintoanyformisoneofitsmost conductivitylimittheirrangeofapplication. Figure1
importantassets. Often it can compete successfully comparesthepropertiesofmagnesiumandaluminum
withcheapermaterialshavingalowerdegreeofwork alloys.
ability.Themetalcanbecastbyanymethodknownto Thecombinationoflowdensity(-4.5 g/cm3), out
foundrymen. It canberolledtoanydesiredthickness standingcorrosionresistance,andhighstrengthmake
downtofoilthinnerthanpaper;aluminumsheetcan titaniumandtitaniumalloyspopularintheaerospace,
bestamped,drawn,spun or roll-formed. The metal chemicalprocessing,andmedical(prostheses) indus
also can be hammered or forged. Aluminumwire, tries.However,itshighprice(duetoprocessingdiffi
drawnfromrolledrod,maybestrandedintocableor culties)haslimitedtheuseoftitaniumtonichemar
anydesiredsizeandtype.Thereisalmostnolimitto kets.Figure1comparesthepropertiesoftitaniumand
thedifferentprofiles(shapes)inwhichthemetalcan aluminumalloys.
beextruded.
The easeandspeedwithwhichaluminumcanbe
The Aluminum Industry
machinedisoneoftheimportantfactorscontributing
tothelowcostoffinishedaluminumparts.Themetal
PrimaryAluminumProduction
can be turned, milled, bored, or machinedin other
mannersatthemaximumspeedsofwhichmostma
Occurrence. Aluminumcomprisesabout 8% of
chinesarecapable.Anotheradvantage ofitsflexible
the earth's crust, making it second only to silicon
machiningcharacteristicsisthataluminumrodandbar
(-28%).Ironisthirdatabout5%.Theprincipaloreof
canreadilybe employedinthehigh-speedmanufac aluminum,bauxite,usuallyconsistsofmixturesofhy
tureofpartsbyautomaticscrewmachines. dratedaluminumoxide,eitherAlO(OH)orAl(OHh.
Almostanymethodofjoiningisapplicabletoalu
Besidesthesecompounds,bauxitecontainsironoxide
minum: riveting, welding, brazing,or soldering. A
(whichgivesitareddish-browncolor),aswellassili
widevarietyofmechanicalaluminumfastenerssimpli
cates(clayandquartz),andtitaniumoxide.Thebaux
fiestheassemblyofmanyproducts.Adhesivebonding
ites used for the productionof aluminumtypically
ofaluminumpartsiswidelyemployed,particularlyin contain35to60%totalaluminumoxide.
joiningaircraftcomponents. Extraction or Refining Methods. The most
Table3listsfabricationcharacteristicsofcommonly widelyusedtechnologyforproducingaluminuminvolves
usedwroughtaluminumandaluminumalloys. twosteps:extractionandpurificationofaluminumox
Property Combinations Needed for Specific ide (alumina) from ores (primarily bauxitealthough
End Uses. In most applications, two or more key alternaterawmaterialscanbeused),andelectrolysisof
characteristicsof aluminumcome prominentlyinto theoxideafterithasbeendissolvedinfusedcryolite.
play-for example, light weight combined with TheBayerprocessisalmostuniversallyemployed
strengthin airplanes, railroadcars,trucks,andother forthepurificationofbauxite.Inthisprocess,which
transportation equipment. High resistance to corro wasdevelopedbyAustrianKarlJosephBayerin1892,
sion and high thermalconductivity areimportantin thecrushedandgroundbauxiteisdigestedwithcaus
equipmentforthechemicalandpetroleumindustries; tic sodasolution,at elevatedtemperature and under
thesepropertiescombine withnontoxicity for food pressure,andthealuminaisdissolvedoutasasolution
processingequipment. of sodium aluminate. The residue, known as "red
Attractiveappearancetogetherwithhighresistance mud,"containstheoxidesofiron,silicon,andtitanium
toweatheringandlowmaintenancerequirementshave andisseparatedbysettlingandfiltration. Aluminum
led to extensiveuse in buildingsof all types.High hydrateisseparatedfromthesolutionofsodiumalu
reflectivity, excellentweathering characteristics, and minatebyseedingandprecipitationandisconvertedto
light weight are all importantin roofing materials. theoxide,AlZ03,bycalcination.
Lightweightcontributestolowhandlingandshipping Presentpracticeforaluminumelectrolysisinvolves
costs, whateverthe application. Table2 reviewsthe theuseoftheHall-HeroultcellaspicturedinFig.2.
materialcharacteristics requiredfordifferentmarkets Thecellislinedwithcarbon,whichactsasthecath
andapplications. Additionalinformationcanalsobe ode;steelbarsareembeddedinthecathodeliningto
foundinthesection"Applications"inthischapter. provideapathforcurrentflow.Theanodesarealsoof
Introduction I 5
Table3 Comparativefabricationcharacteristicsofwroughtaluminumalloys
Weldabilily(b)
Cold Resistance
AHoy Temper workabilily(a) Machinability(a) Gas An: spolIIIIdseam Brazeability(b) Solderability(c)
1050 0 A E A A B A A
H12 A E A A A A A
H14 A D A A A A A
H16 B D A A A A A
H18 B D A A A A A
1060 0 A E A A B A A
H12 A E A A A A A
HI4 A D A A A A A
H16 B D A A A A A
H18 B D A A A A A
1100 0 A E A A B A A
H12 A E A A A A A
H14 A D A A A A A
H16 B D A A A A A
H18 C D A A A A A
1145 0 A E A A B A A
H12 A E A A A A A
H14 A D A A A A A
H16 B D A A A A A
H18 B D A A A A A
1199 0 A E A A B A A
H12 A E A A A A A
HI4 A D A A A A A
H16 B D A A A A A
H18 B D A A A A A
1350 0 A E A A B A A
H12,Hlll A E A A A A A
H14,H24 A D A A A A A
H16,H26 B D A A A A A
HI8 B D A A A A A
2011 T3 C A D D D D C
T4,T451 B A D D D D C
T8 D A D D D D C
2014 0 D D D B D C
T3,T4,T451 C B D B B D C
T6,T651,T651O,T6511 D B D B B D C
2024 0 D D D D D C
T4,T3,T351,T351O,T3511 C B C B B D C
T361 D B D C B D C
T6 C B D C B D C
T861,T81,T851,T851O, D B D C B D C
T8511
172 B
2036 T4 B C B B D
2124 T851 D B D C B D C
2218 T61 C C
172 B D C B D C
2219 0 D A B D
T31,T351,T351O,T3511 C B A A A D NA
T37 D B A A A D
T81,T851,T8510,T8511 D B A A A D
T87 D B A A A D
2618 T61 B D C B D NA
3003 0 A E A A B A A
H12 A E A A A A A
H14 B D A A A A A
H16 C D A A A A A
H18 C D A A A A A
H25 B D A A A A A
(continued)
(a)RatingsAthroughDforcoldworkabilityandAthroughEformachinabilityarerelativeratingsindecreasingorderofmerit.(b)RatingsAthrough
Dforweldabilityandbrazeabilityarerelativeratingsdefinedasfollows:A,generallyweldablebyallconunercialproceduresandmethods;B,weld-
ablewithspecialtechniquesorforspecificapplicationsandrequiringpreliminarytrialsortestingtodevelopweldingprocedureandweldperfor-
mance;C,limitedweldabilitybecauseofcracksensitivityorlossinresistancetocorrosionandmechanicalproperties;D,nocommonlyusedwelding
methodshavebeendeveloped(c)RatingsAthroughDandNAforsolderabilityarerelativeratingsdefinedasfollows:A,excellent;B,gond;C,fair;
D,poor;NA,notapplicable
6 I Corrosion ofAluminum andAluminum Alloys
Table3 (continued)
Weldability(b)
Cold IlesIstaoce
AHoy Temper workability(a) MathiDability(a) Gas Arc spotaodseam Brazeability(b) Solderability(c)
3004 0 A 0 B A B B B
H32 B 0 B A A B B
H34 B C B A A B B
H36 C C B A A B B
H38 C C B A A B B
3105 0 A E B A B B B
HI2 B E B A B B B
HI4 B 0 B A A B B
HI6 C 0 B A A B B
HI8 C 0 B A A B B
H25 B D B A A B B
4032 T6 B 0 B C 0 NA
4043 NA C NA NA NA NA NA
5005 0 A E A A B B B
HI2 A E A A A B B
HI4 B 0 A A A B B
HI6 C 0 A A A B B
HI8 C 0 A A A B B
H32 B E A A A B B
H34 C 0 A A A B B
H36 C 0 A A A B B
H38 0 A A A B B
5050 0 A E A A B B C
H32 A 0 A A A B C
H34 B 0 A A A B C
H36 C C A A A B C
H38 C C A A A B C
5052 0 A 0 A A B C 0
H32 B 0 A A A C 0
H34 B C A A A C D
H36 C C A A A C D
H38 C C A A A C 0
5056 0 A 0 C A B 0 D
Hll1 A 0 C A A D D
H12,H32 B 0 C A A 0 0
HI4,H34 B C C A A 0 D
H18,H38 C C C A A 0 D
HI92 0 B C A A 0 0
H392 0 B C A A 0 0
5083 0 B D C A B D 0
H321,H116 C 0 C A A 0 0
Hll1 C 0 C A A 0 D
5086 0 A 0 C A B 0 D
H32,H1116 B 0 C A A 0 0
H34 B C C A A 0 0
H36 C C C A A 0 0
H38 C C C A A 0 0
H1l1 B 0 C A A 0 0
5154 0 A 0 C A B 0 D
H32 B 0 C A A 0 0
H34 B C C A A 0 0
H36 C C C A A 0 0
H38 C C C A A 0 D
5182 0 A 0 C A B D D
HI9 0 B C A A 0 0
5252 H24 B 0 A A A C 0
H25 B C A A A C 0
H28 C C A A A C D
5254 0 A 0 C A B 0 0
H32 B 0 C A A 0 D
H34 B C C A A 0 0
H36 C C C A A 0 0
H38 C C C A A 0 0
5356 NA B NA NA NA NA NA
5454 0 A 0 C A B 0
(continued)
Introduction I 7
Table3 (con'nuecl)
Weldability(b)
Cold Resistance
AHoy Temper workability(a) Machinability(a) Gas An: spotandseam Brazeability(b) Solderability(c)
5454(continued)
H32 B D C A A D
H34 B C C A A D NA
Hlll B D C A A D
5456 0 B D C A B D
Hll1 C D C A A D
H321,H115 C D C A A D NA
5457 0 A E A A B B B
5652 0 A D A A B C D
H32 B D A A A C D
H34 B C A A A C D
H36 C C A A A C D
H38 C C A A A C D
5657 H241 A D A A A B
H25 B D A A A B NA
H26 B D A A A B
H28 C D A A A B
6005 T5 C C A A A A NA
6009 T4 A C A A A A B
6010 T4 B C A A A A B
6061 0 A D A A B A B
T4,T451,T451O,T4511 B C A A A A B
T6,T651,T652,T651O, C C A A A A B
T6511
6063 TI B D A A A A B
T4 B D A A A A B
T5,T52 B C A A A A B
T6 C C A A A A B
T83,T831,T832 C C A A A A B
6066 0 B D D B B D
T4,T451O,T451I C C D B B D NA
T6,T651O,T6511 C B D B B D
6070 T4,T4511 B C A A A B NA
T6 C C A A A B
6101 T6,T63 C C A A A A NA
T61,T64 B D A A A A
6151 T6,T652 B
6201 T81 C A A A A NA
6262 T6,T651,T651O,T651I C B A A A A NA
T9 D B A A A A
6351 T5,T6 C C A A A A B
6463 T1 B D A A A A
T5 B C A A A A NA
T6 C C A A A A
7005 T53 C A B B B B B
7049 T73,T7351,T7352 D B D C B D D
T76,T7651 D B D C B D D
7050 T74,T7451,T7452 D B D C B D D
T76,T761 D B D C B D D
7072 A D A A A A A
7075 0 D D C B D D
T6,T651,T652,T651O, D B D C B D D
T6511
T73,T7351 D B D C B D D
7175 T74,T7452 D B D C B D D
7178 0 D C B D D
T6,T651,T6510,T6511 D B D C B D D
7475 T6,T651 D B D C B D D
T73,T7351,T7352 D B D C B D D
T76,T765I D B D C B D D
(a)RatingsAthroughDforcoldworkabilityandAthroughEformachinabilityarerelativeratingsindecreasingorderofmerit.(b)RatingsAthrough
Dforweldabilityandbrazeabilityarerelativeratingsdefinedasfollows:A,generallyweldablebyallcommercialproceduresandmethods;B,weld-
ablewithspecialtechniquesorforspecificapplicationsandrequiringpreliminarytrialsortestingtodevelopweldingprocedureandweldperfor-
mance;C,limitedweldabilitybecauseofcracksensitivityorlossinresistancetocorrosionandmechanicalproperties;D,nocommonlyusedwelding
methodshavebeendeveloped.(c)RatingsAthroughDandNAforsolderabilityarerelativeratingsdefinedasfollows:A,excellent;B,good;C,fair;
D,poor;NA,notapplicable
8 I CorTosionofAluminum andAluminum Alloys
carbonandaregraduallyfedintothetopofthecell withthealuminum. Examplesoftwocommonmetals
becausethe anodesare continually consumed during associatedwithaluminumoresthatfitthisdescription
electrolysis.Agroupofcellsareconnectedinseriesto areironandsilicon.Itis, therefore, veryimportant
obtainthevoltagerequiredbytheparticulardirectcur thatrawmaterialsbe asfreeofthesemetaloxidesas
rentpowersourcethatisbeingused. possible.By carefulcontrolof rawmaterials,alumi
For aluminum, the electrolyte used is cryolite numwithapurityof99%orhighermaybeproduced.
(Na3AlF6)with8to 10%Al203dissolvedinit.Other Generally,thepurityofaluminumasitcomesfromthe
additives,suchasCaF2andAlF3,areaddedtoobtain electrolysiscell(i.e.,upto99.9%)isadequate.High
desirablephysicalproperties.Themeltingpointofthe purityaluminumofatleast99.97%AIcontentisnec
electrolyte isapproximately 940 °C(1725"F), and essaryforcertain specialpurposes(e.g.,reflectorsor
the Hall-Heroultcell operatesat temperatures of ap electrolyticcapacitors).Forsuchapplications,second
proximately960to1000°C(1760to1830oF). stagerefmingoperations(Hoopescellelectrolysis)are
At thecathodeofthealuminumcell,aluminumis necessary.Aluminumproducedinthiswayis99.99%
reducedfrom an ionic state to a metallicstate-for pure.Higherpuritiesofupto99.9999%("six-nines"
example: aluminum)canbeobtainedwithzone-refiningoperations.
SecondaryAluminum Production
Thisisaverysimplifiedrepresentationofthecomplex
Advantages. Aluminum recovered from scrap
reactionsthattakeplaceatthecathode.However,itdoes
(secondaryaluminum)hasbeenanimportantcontribu
represent theoverallproduction of moltenaluminum,
tortothetotalmetalsupplysincethe1950s.Theeco
whichforms a moltenpoolinthebottomof thecell.
nomicsofrecycling,togetherwithimprovedtechniques
Periodically, the molten pool of aluminummetal is ofscrappreparationandmelting,whichprovidehigher
drainedorsiphonedfromthebottomofthecellandcast yields,ledtothedevelopmentofthesecondaryalumi
Attheanode,oxygenisoxidizedfromitsionicstate numindustry. The increasedconcem with, andeco
tooxygengas.Theoxygengasintumreactswiththe nomicimplications of,energysupplyinrecentyears
carbon anode to form carbon dioxide gas, which havefocusedevenmoreattentiononrecyclingofalu
graduallyconsumestheanodematerial.Twotypesof minumbecauseofitsenergy-intensivenature.Theen
anodesareinuse:prebakedandself-baking.Prebaked ergyrequiredtoremeltsecondaryaluminumprepara
anodesareindividualcarbonblocksthatarereplaced torytofabricationforreuseisonly5%ofthatrequired
one afteranotheras theyareconsumed.Self-baking toproducenew(primary)aluminumTodaysecondary
anodes,as shownin Fig. 2, aremadeup of carbon aluminumaccountsfor about35% ofthe aluminum
pastethatisfedintoasteelframeabovethecell.Asthe supplyinboththeUnitedStatesandEurope.
anodedescendsinthecell,ithardens,andnewcarbon The Recycling Loop.Thereclamationofaluminum
pasteisfedcontinuallyintothetopofthesteelframe. scrapisacomplexinteractiveprocessinvolvingcollec
ImpuritiesintheAl203rawmaterialwhicharemore tioncenters,primaryproducers,secondarysmelters,metal
noblethanaluminumarereducedatthecathodealong processorsandconsumers.Figure3depictstheflowof
Anodeleads
Steelstuds
Cathodecart>oo
Cathodeleads
Fig. 2
Hall-Heraultaluminumproductioncellwithself-bakinganode
Imports
Imports Exports Imports Exports
Enduse
Containersand
packaging,21.7%
Buildingand
construction, 12.9%
UBC Total Transportation,29.2%
processingfacility I I I •I aluminum Newscrap
supply generated Electrical,6.9%
Consumer
durables,6.8%
Machineryand
equipment,6.1%
Scraprecycling Secondary Other,3.1%
aluminum
industry
(moltenoringot)
t
Imports Exports
Fig. 3 Flowdiagramforaluminum intheUnitedStates,showingtheroleofrecyclingintheindustry.Scraprecycling (lowerleft)includesscrapcollectors,processors, i
dealersandbrokers,sweatfurnaceoperators,anddrossreclaimers.
8
.......
'0
10I Corrosion ofAluminum and Aluminum Alloys
metal originating in primary smelting operations and zinc; other elements are also added in smaller
throughvariousrecyclingactivities.Theinitialreproc amounts for grain refmement and to develop special
essing ofscrap takes place in thefacilities ofprimary properties. The total amount of these elements can
producers. In-process scrap, generatedboth incasting constitute up to 10% ofthe alloy composition (per
and fabricating, isreprocessedby melting and recast centages given in weight percent unless otherwise
ing. Increasingly, primary producers are purchasing noted). Impurityelementsare also present, buttheir
scraptosupplementprimarymetalsupply;anexample total percentageis usually less than 0.15% in alumi
ofsuch activity is the purchase oftoll conversion of numalloys.
used beveragecans (UBC) by primary producers en
gaged intheproductionofrigidcontainerstock. ClassificationsandDesignations
Scrap incurred in the processing or fabrication of
semifabricatedaluminumproductsrepresents anaddi It isconvenienttodivide aluminumalloys into two
tional source of recyclable aluminum. Traditionally, major categories: wrought compositionandcast com
this form ofnew scrap has been returned to the sup positions. Afurtherdifferentiationforeachcategoryis
plier for recycling, or ithas been disposedofthrough basedontheprimarymechanismofproperty develop
saleon thebasis ofcompetitivebiddingbymetaltrad ment.Manyalloysrespondtothermaltreatmentbased
ers,primaryproducersandsecondarysmelters. on phase solubilities. These treatments include solu
Finished aluminum products, which include such tion heat treatment, quenching, and precipitation (or
items as consumer durable and nondurable goods; age) hardening. For either casting or wroughtalloys,
automotive, aerospace, andmilitaryproducts;machin such alloys are described as heat treatable. A large
ery; miscellaneous transportation parts; and building number ofotherwroughtcompositionsrelyinsteadon
and construction materials, have finite lives. In time, workhardeningthrough mechanicalreduction,usually
discarded aluminumbecomes available for collection incombinationwith various annealingprocedures for
and recovery. So-calledold scrap (metal productthat property development. These alloys arereferred to as
has been discarded after use) can be segregated into work hardening or non-heat-treatable. Some casting
classificationsthatfacilitaterecyclingand recovery. alloys are essentially not heat treatable and are used
ProcessTechnologies.Scrappedaluminumprod only in as-east or in thermally modified conditions
ucts are brokeninto small pieces and separatedfrom unrelatedtosolutions orprecipitationeffects.
dirtandforeign materials soastoyield feedstock suit Cast and wrought alloy nomenclatures have been
ableforremelting. This isdone usingbreakers, shred developed. TheAluminumAssociationsystemismost
ders, magnetic, and settlement/flotation separators. widely recognized in the United States. Their alloy
Such scraptypically contains alloysofmanytypes,all identificationsystememploys differentnomenclatures
mixedtogether.Amoresophisticatedkindofrecycling for wrought and cast alloys but divides alloys into
was developed in the 1970s and 1980s for process familiesforsimplification.
scrap and UBCs. By selectively collecting scrap in Wrought Alloy Families. For wrought alloys, a
targeted alloy categories, the goal was to recycle the four-digit systemis used to producealist of wrought
material back into products similar to those from compositionfamilies asfollows:
which itoriginated.Thus, thecasthousesofextrusion
plantsproduceextrusionbilletsfromprocessscrapand • lxx-x: Controlled unalloyed (pure) composition,
fromrecycledscrapextrusions.Similarly,thehighrate usedprimarilyintheelectricalandchemicalindus
of recovery of UBCs from the consumer enables a tries
large proportion ofcanstock coils to be made from • 2xxx:Alloysinwhichcopperistheprincipalalloy
UBCs. Recovery ofUBCs has multiplied repeatedly ingelement,althoughotherelements, notably mag
since the early 1970s. In 1997, some 2,052 million nesium, can be specified. 2xxxseries alloys are
pounds ofUBCs were collectedin the United States. widely used in aircraft where their high strengths
This constitutes 66.8% of can shipments. In some (yield strengths ashigh as455 MPa, or66ksi) are
countries, for example Sweden, recycling rates ex valued.
ceeding80%areachieved. • 3xxx: Alloys in which manganese is the principal
alloying element, used as general-purpose alloys
forarchitectural applicationsandvariousproducts
Aluminum Alloys • 4xxx: Alloysinwhich silicon istheprincipalalloy
ingelement,usedinweldingrodsandbrazingsheet
• 5xxx: Alloys in which magnesium is the principal
The mechanical, physical, and chemical properties alloying element, used in boat hulls, gangplanks,
of aluminum alloys depend on composition and andotherproductsexposedtomarineenvironments
microstructure. The addition ofselected elements to • &xx: Alloys in which magnesium and silicon are
pure aluminum greatly enhances its properties and theprincipal alloyingelements, commonlyusedfor
usefulness. Becauseofthis,mostapplications for alu architecturalextrusions.
minum utilize alloys having one or more elemental • Txxx: Alloysinwhichzincistheprincipal alloying
additions. The major alloying additions usedwith alu element (although other elements, such as copper,
minum are copper, manganese, silicon, magnesium, magnesium, chromium, and zirconium, can be