Corrosion of Aluminum and Aluminum Alloys (#06787G) Copyright © 1999 ASM International ® Editor(S): J.R

Chapter 1

Introduction

ALUMINUM became an economic competitor in bedirectly or indirectly affected by use. Today, alumi engineering applications toward the end of the 19th num is surpassed only by steel in its use as a structural century. The reason aluminum was not used earlier material. was the difficulty of extracting it from its ore. When the electrolytic reduction of aluminum oxide (Al2O:3) dissolved in molten cryolite was independently devel Key Characteristics of Aluminum oped by Charles Martin Hall in the United States and Paul T. Heroult in France, the aluminum industry was Aluminum offers a wide range ofproperties that can bom. be engineered precisely to the demands of specific The emergence of three important industrial devel applications through the choice of alloy, temper, and opments in the late 18008 and early 1900swould, by fabrication process. The properties of aluminum and demanding material characteristics consistent with the its alloys which give rise to their widespread usage unique qualities of aluminum and its alloys, greatly include the following: benefit growth in the production and use of the new • Aluminum is light; its density is only one-third that metal. The first of these was the introduction of the ofsteel. first internal-combustion-engine-powered vehicles. • Aluminum and aluminum alloys are available in a Aluminum would play a role as an automotive mate wide range ofstrength values-from highly ductile rial of increasing engineering value. Secondly, elec low-strength commercially pure aluminum to very trification would require immense quantities of tough high-strength alloys with ultimate tensile lightweight conductive metal for long-distance trans strengths approaching 690 MPa (100 ksi). mission and for construction of the towers needed to • Aluminum alloys have a high strength-to-weight support the overhead network of cables that deliver ratio. electrical energy from sites of power generation. • Aluminum retains its strength at low temperatures Within a few decades, a third important application and is often used for cryogenic applications. area was made possible by the invention of the air • Aluminum has high resistance to corrosion under plane by the Wright brothers. This gave birth to an the majority of service conditions, and no colored entirely new industry which grew in partnership with salts are formed to stain adjacent surfaces or dis the aluminum industry development ofstructurally re color products with which it comes into contact. liable, strong, and fracture-resistant parts for airframes, • Aluminum is an excellent conductor of heat and engines, and ultimately, for missile bodies, fuel cells, electricity . and satellite components. • Aluminum is highly reflective. However, .the aluminum industry growth was not • Aluminum is nonferromagnetic, a property of im limited to these developments. The first commercial portance in the electrical and electronics industries. applications of aluminum were novelty items such as • Aluminum is nonpyrophoric, which is important in mirror frames, house (address) numbers, and serving applications involving inflammable or explosive trays. Cooking utensils were also a major early market materials handling or exposure. In time, aluminum applications grew in diversity to the • Aluminum is nontoxic and is routinely used in con extent that virtually every aspect ofmodem life would tainers for food and beverages. Corrosion of Aluminum and Aluminum Alloys (#06787G) Copyright © 1999 ASM International ® Editor(s): J.R. Davis All rights reserved. www.asminternational.org 2 I Corrosion of Aluminum and Aluminum Alloys

• Aluminumhas an attractive appearancein its natu Strength. Commercially pure aluminumhas a ten ral finish, which can be soft and lustrousor bright silestrengthof about90 MPa (13 ksi). Thus its useful and shiny.It can be virtuallyany color or texture. ness as a structuralmaterialin this form is somewhat • Aluminumis recyclable. Aluminumhas substantial limited By working the metal, as by cold rolling, its scrap value and a well-established marketfor recy strength can be approximately doubled. Much larger cling, providingboth economicand environmental increases in strengthcan be obtained by alloying alu benefits. minum with small percentages of one or more other • Aluminum is easily fabricated. Aluminum can be elements such as manganese, silicon, copper, magne formed and fabricated by all common metalwork sium, or zinc. Like pure aluminum, the alloys are also ing andjoining methods. madestrongerby cold working. Some of the alloysare further strengthened and hardenedby heat treatments. Table 1 lists the important physical properties of Figure 1 shows the range of strength levels of repre pure aluminum. Table 2 shows the characteristics of sentativealuminumand aluminumalloys. aluminumand their importancefor differentend uses. High Strength-to-Weight Ratio. The strength Low Density. Aluminumhas a densityof only 2.7 to-weightratio of aluminum is much higher than that glcm3, approximately 35% that of steel (7.83 glcm3) of many common grades of constructional steels and 30% of copper (8.93 g/cm') or brass (8.53 glcm3). often double or more (Fig. 1). This property permits One cubicfoot of steel weighsabout 222 kg (490lb);a design and construction of strong, lightweight struc cubic foot of aluminumweighs only about77 kg (170 tures that areparticularly advantageous for anything that lb). moves-space vehiclesand aircraftas wellas all types ofland- and water-borne vehicles. Corrosion Resistance. When aluminum surfaces Table 1 Summaryof the important physical are exposed to the atmosphere, a thin invisible oxide (~.95% properties of high-purity AI) skin formsimmediately, whichprotectsthe metalfrom aluminum further oxidation. This self-protecting characteristic Property Va'" gives aluminum its high resistance to corrosion. Un Thermalneutroncross section 0.232± 0.003bams less exposed to some substance or condition that de Latticeconstant(lengthof 4.0496x 1O-lO m at 298 K stroysthis protectiveoxide coating, the metal remains unitcube) fully protectedagainst corrosion.Aluminumis highly 3 Density(solid) 2699kg/m (theoretical density resistantto weathering, even in industrialatmospheres basedonlatticespacing) 2697-2699kg/m3 that often corrode other metals. It is also corrosion (polycrystalline material) resistant to many acids. Alkalis are among the few Density(liquid) 2357kg/m3at 973K substances that attack the oxide skin and thereforeare 3 2304kg/m at 1173K corrosiveto aluminum Although the metal can safely Coefficientofexpansion 23x Io-<>IK at 293K Thermalconductivity 2.37Wlcm· Kat 298K be used in the presenceof certain mild alkalis with the Volume resistivity 2.655x 10-8Q. m aid of inhibitors, in general, direct contact with alka Magneticsusceptibility 16x 1O-3/m3 g/atomat 298K line substances shouldbe avoided. Surfacetension 868dyne/cmat themelting The high thermal conductivity of aluminum point (about 50 to 60% that of copper) came prominently Viscosity 0.012poiseat themeltingpoint Meltingpoint 933.5K intoplay in the very firstlarge-scalecommercialappli Boilingpoint 2767K cation of the metal in cooking utensils. This charac Heatof fusion 397Jig teristic is important whenever the transfer of thermal Heatof vaporation 1.08 x 10-4 Jig' K energy from one medium to anotheris involved, either 0.90 Jig . K Heatcapacity heating or cooling.Thus aluminumheat exchangers are

Table2 Propertycombinations important for the use ofaluminum in various application areas Characteristics TypeofsemiCabrieated products Goodbeat Deeoratiseaspeds Castings W"U'e and electrical Resioltanee (with orwithout or .Formed Impart Extruded aDd Field ofuse Lightness cooductivity tocorrosion surface treatment) 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 Chemicals andfood 2 2 2 3 2 2 2

1,veryimportant;2,important;3, desirable Corrosion of Aluminum and Aluminum Alloys (#06787G) Copyright © 1999 ASM International ® Editor(s): J.R. Davis All rights reserved. Introductionwww.asminternational.orgI 3

commonly used in the food, chemical, petroleum, air The high reflectivity gives aluminum a decorative ap craft, and other industries. pearance; it also makes aluminum a very effective bar High Electrical Conductivity. Aluminum is one rier against thermal radiation, suitable for such appli of the two common metals having an electrical con cations as automotive heat shields. ductivity high enough for use as an electric conductor. Nontoxic Characteristica. The fact that alumi The conductivity of electric conductor grade (1350) is num is nontoxic was discovered in the early days of about 62% that of the International Annealed Copper the industry. It is this characteristic that permits the Standard (lACS). Because aluminum has less than metal to be used in cooking utensils without any harm one-third the specific gravity of copper, however, a ful effect on the body. Today a great deal ofaluminum pound of aluminum will go about twice as far as a equipment is used in the food processing industry. pound ofcopper when used for this purpose. Nontoxicity permits aluminum foil wrapping to be Reflectivity. Smooth aluminum is highly reflective used safely in direct contact with food products. of the electromagnetic spectrum, from radio waves Finishability. For the majority ofapplications, alumi through visible light and on into the infrared and ther num needs no protective coating. Mechanical finishes mal range. It bounces away about 80% of the visible such as polishing, sand blasting, or wire brushing meet light and 90% of the radiant heat striking its surface. the majority of needs. In many instances, the surface

2500 r------, 300M

300

2000 AISI4340 250 AISI8640

OJ Q. 'iii ::;; 3A1-8V-6Cr-4Mo-4Zr ~ 1500 200 ~ ~ 15> 10V-2Fe-3A1 c c ~ ~ .9! .9! 150 'iii 'iii AISI c c 6AI-4V Ql Ql 1000 1095 f- f-

3AI-2.5V 100 ASTM A 715 ASTM [~~2014-T6 500 ASTM grade4 A 242 6061-T6 t HK31A-H24 50 AISI ZK40A-T5 1015 3OO3-H18 ASTM AZ31B-0 grade1 2014-0 1060-H18 0 1060-0 Steels, Titanium Aluminum Magnesium -7.9 glcm3 alloys, alloys, alloys, (8) -4.5 glcm3 -2.75 glcm3 -1.8 glcm3 50 ~ c ~ .9! r~~'~ ~ 'iii :,~ ::,~~ [ro~~ HK31A-H24 c 25 [AISI8640 ~ 3AI-2.5V 2014-T6 ZK40A-T5 0 AZ31B-0 s AISI1095 ASTM 6061-T6 Ql a. ASTMA 715 grade4 3003-H18 (/) AISI ASTM 1060-H18 0 1015 grade1 1060.0 Steels Titanium Aluminum Magnesium (b) alloys alloys alloys

Fig 1 Comparison ofaluminum alloys with competing structural alloys on • the basis of lal tensile strength and ~bl specific tensile strength lten- sile strength, in ksl, divided by density, in g/cm 1 Corrosion of Aluminum and Aluminum Alloys (#06787G) Copyright © 1999 ASM International ® Editor(s): J.R. Davis All rights reserved. www.asminternational.org 4 I Corrosion of Aluminum and Aluminum Alloys

finish suppliedis entirelyadequatewithoutfurtherfin CompetingMetals forLightweight Consbvction. ishing. Where the plain aluminum surface does not The light (low density)metals and alloys of commer suffice or where additionalprotectionis required, any cial importanceare based on aluminum, magnesium, of a wide variety of surface finishes may be applied. and titanium. Each of these metals has distinct quali Chemical, electrochemical, and paint finishes are all ties that make them suitable or preferred for certain used. Many colors are availablein both chemical and applications. electrochemical finishes. If paint, lacquer, or enamel is With a density of 1.8 g/cm3, magnesiumalloys are used, any color possible with these finishes can be amongthe lightest known structural alloys. Thisis their applied. Vitreous enamels have been developed for chief advantage when compared with aluminum and aluminum, and the metal can also be electroplated. titanium. However, a low yield strength and modulus Ease of Fabrication. The ease with which alumi of elasticity combinedwithpoor thermaland electrical num can be fabricatedinto any form is one of its most conductivity limit their range of application. Figure 1 important assets. Often it can compete successfully comparesthe propertiesof magnesiumand aluminum with cheapermaterialshaving a lowerdegreeof work alloys. ability.The metal can be cast by any methodknownto The combinationof low density (-4.5 g/cm3), out foundrymen. It can be rolled to any desired thickness standingcorrosionresistance, and high strengthmake down to foil thinner than paper; aluminum sheet can titaniumand titaniumalloys popular in the aerospace, be stamped, drawn, spun or roll-formed. The metal chemical processing, and medical (prostheses) indus also can be hammered or forged. Aluminum wire, tries. However, its high price (due to processingdiffi drawn from rolled rod, may be stranded into cable or culties) has limited the use of titanium to niche mar any desired size and type. There is almost no limit to kets. Figure 1 comparesthe propertiesof titaniumand the different profiles (shapes) in which the metal can aluminumalloys. beextruded. The ease and speed with which aluminum can be machined is one of the importantfactors contributing The Aluminum Industry to the low cost of finished aluminumparts.The metal can be turned, milled, bored, or machined in other Primary Aluminum Production manners at the maximum speeds of which most ma Occurrence. Aluminum comprises about 8% of chines are capable. Another advantage of its flexible the earth's crust, making it second only to silicon machiningcharacteristics is that aluminumrod and bar (-28%). Iron is third at about5%. The principalore of can readily be employed in the high-speed manufac aluminum, bauxite,usuallyconsistsof mixturesofhy ture of parts by automaticscrewmachines. drated aluminum oxide, either AlO(OH) or Al(OHh. Almost any method of joining is applicableto alu Besidesthese compounds, bauxitecontains iron oxide minum: riveting, welding, brazing, or soldering. A (whichgives it a reddish-brown color), as well as sili widevarietyof mechanicalaluminumfasteners simpli cates (clay and quartz),and titaniumoxide. The baux fies the assemblyof manyproducts.Adhesivebonding ites used for the production of aluminum typically of aluminumparts is widelyemployed,particularly in contain35 to 60% total aluminumoxide. joining aircraft components. Extraction or Refining Methods. The most Table3 lists fabricationcharacteristics of commonly widely usedtechnology forproducing aluminum involves used wroughtaluminumand aluminumalloys. two steps:extractionand purificationof aluminumox Property Combinations Needed for Specific ide (alumina) from ores (primarily bauxite although End Uses. In most applications, two or more key alternateraw materialscan beused),andelectrolysisof characteristics of aluminum come prominently into theoxideafterithasbeendissolved in fusedcryolite. play-for example, light weight combined with The Bayer process is almost universallyemployed strength in airplanes, railroad cars, trucks, and other for the purificationof bauxite. In this process, which transportation equipment. High resistance to corro wasdevelopedby AustrianKarlJosephBayerin 1892, sion and high thermal conductivity are important in the crushed and ground bauxite is digested with caus equipmentfor the chemical and petroleum industries; tic soda solution, at elevated temperature and under these properties combine with nontoxicity for food pressure,and the aluminais dissolvedout as a solution processing equipment. of sodium aluminate. The residue, known as "red Attractiveappearancetogether with high resistance mud,"containsthe oxidesof iron, silicon,and titanium to weatheringand low maintenancerequirements have and is separatedby settling and filtration. Aluminum led to extensive use in buildings of all types. High hydrate is separated from the solution of sodium alu reflectivity, excellent weathering characteristics, and minateby seedingandprecipitationand is convertedto light weight are all important in roofing materials. the oxide,AlZ03, by calcination. Light weight contributes to low handlingand shipping Present practice for aluminumelectrolysis involves costs, whatever the application. Table 2 reviews the the use of the Hall-Heroult cell as pictured in Fig. 2. material characteristics required for different markets The cell is lined with carbon, which acts as the cath and applications. Additional information can also be ode; steel bars are embedded in the cathode lining to found in the section"Applications" in this chapter. providea path for currentflow.The anodes are also of Corrosion of Aluminum and Aluminum Alloys (#06787G) Copyright © 1999 ASM International ® Editor(s): J.R. Davis All rights reserved. www.asminternational.org Introduction I 5

Table 3 Comparative fabrication characteristics of wrought aluminum alloys Weldabilily(b) Cold Resistance AHoy Temper workabilily(a) Machinability(a) Gas An: spolIIIId seam Brazeability(b) Solderability(c)

1050 0 A E AAB A A H12 A E AA A A A H14 A D AAA A A H16 B DAAA A A H18 B DAAA A A 1060 0 A E AAB A A H12 A E AAA A A HI4 A DA AA A A H16 B DAAA A A H18 B DAAA A A 1100 0 A EAAB A A H12 A EAAA A A H14 A DA A A A A H16 B DAAA A A H18 C DA AA A A 1145 0 A EAAB A A H12 A EAAA A A H14 A DAAA A A H16 B DAAA A A H18 B D AAA A A 1199 0 A E AAB A A H12 A E AA A A A HI4 A DAAA A A H16 B DAAA A A H18 B DAAA A A 1350 0 A EAAB A A H12,Hlll A EAAA A A H14,H24 A DAAA A A H16,H26 B D AAA A A HI8 B DAAA A A 2011 T3 C ADDD D C T4,T451 B ADDD D C T8 D ADDD D C 2014 0 DDDB D C T3, T4, T451 C BDBB D C T6, T651, T651O,T6511 D BDBB D C 2024 0 DDDD D C T4, T3, T351, T351O,T3511 C BCBB D C T361 D BD C B D C T6 C BDCB D C T861, T81, T851, T851O, D BD C B D C T8511 172 B 2036 T4 B C BB D 2124 T851 D BD C B D C 2218 T61 C C 172 BDCB D C 2219 0 DAB D T31, T351, T351O, T3511 C BAA A D NA T37 D BAAA D T81, T851, T8510, T8511 D BAAA D T87 D BAA A D 2618 T61 BDC B D NA 3003 0 A E AA B A A H12 A E AA A A A H14 B D AAA A A H16 C DAA A A A H18 C DAAA A A H25 B DAAA A A

(continued) (a)RatingsA throughDforcoldworkabilityandA throughEformachinability arerelativeratingsindecreasingorderofmerit.(b)RatingsA through Dforweldability andbrazeabilityarerelativeratingsdefinedasfollows:A, generallyweldableby allconunercialproceduresandmethods;B,weld- able with specialtechniquesor for specificapplications and requiringpreliminarytrials or testingto developweldingprocedureand weld perfor- mance;C, limitedweldabilitybecauseofcracksensitivityorlossinresistancetocorrosionandmechanicalproperties;D, nocommonlyusedwelding methodshavebeendeveloped (c)RatingsA throughDandNA forsolderabilityarerelativeratingsdefinedasfollows:A,excellent;B, gond;C, fair; D,poor;NA, notapplicable Corrosion of Aluminum and Aluminum Alloys (#06787G) Copyright © 1999 ASM International ® Editor(s): J.R. Davis All rights reserved. www.asminternational.org 6 I Corrosion of Aluminum and Aluminum Alloys

Table 3 (continued)

Weldability(b) Cold IlesIstaoce AHoy Temper workability(a) MathiDability(a) Gas Arc spot aod seam Brazeability(b) Solderability(c)

3004 0 A 0 B A B B B H32 B 0 B AA B B H34 B C B AA B B H36 C C B A A B B H38 C C B AA B B 3105 0 A E B A B B B HI2 B E B A B B B HI4 B 0 B AA B B HI6 C 0 B AA B B HI8 C 0 B AA B B H25 B D B AA B B 4032 T6 B 0 B C 0 NA 4043 NA C NA NA NA NA NA 5005 0 A E AA B B B HI2 A E AAA B B HI4 B 0 AAA B B HI6 C 0 AAA B B HI8 C 0 AAA B B H32 B E AAA B B H34 C 0 AAA B B H36 C 0 AAA B B H38 0 AAA B B 5050 0 A E AA B B C H32 A 0 AAA B C H34 B 0 A AA B C H36 C C AAA B C H38 C C AAA B C 5052 0 A 0 AA B C 0 H32 B 0 AAA C 0 H34 B C AAA C D H36 C C AAA C D H38 C C A AA C 0 5056 0 A 0 C A B 0 D Hll1 A 0 C AA D D H12,H32 B 0 C AA 0 0 HI4,H34 B CC AA 0 D H18,H38 C CC AA 0 D HI92 0 B C AA 0 0 H392 0 B C A A 0 0 5083 0 B DC A B D 0 H321,H116 C 0 C A A 0 0 Hll1 C 0 C AA 0 D 5086 0 A 0 C A B 0 D H32,H1116 B 0 C AA 0 0 H34 B CC AA 0 0 H36 C CC AA 0 0 H38 C CC AA 0 0 H1l1 B 0 C AA 0 0 5154 0 A 0 C A B 0 D H32 B 0 C AA 0 0 H34 B C C AA 0 0 H36 C C C AA 0 0 H38 C CC AA 0 D 5182 0 A 0 C A B D D HI9 0 B C AA 0 0 5252 H24 B 0 AAA C 0 H25 B C AAA C 0 H28 C C AA A C D 5254 0 A 0 C A B 0 0 H32 B 0 C AA 0 D H34 B CC AA 0 0 H36 C CC AA 0 0 H38 C C C AA 0 0 5356 NA B NA NA NA NA NA 5454 0 A 0 C A B 0

Table3 (con'nuecl)

Weldability(b) Cold Resistance AHoy Temper workability(a) Machinability(a) Gas An: spotand seam Brazeability(b) Solderability(c)

5454(continued) H32 B D C A A D H34 B CCAA D NA Hlll B D C A A D 5456 0 B D CAB D Hll1 C D CAA D H321,H115 C D C A A D NA 5457 0 A E A AB B B 5652 0 A D A AB C D H32 B D A A A C D H34 B C A AA C D H36 C CA A A C D H38 C CA A A C D 5657 H241 A DA AA B H25 B DA A A B NA H26 B D A AA B H28 C D A A A B 6005 T5 C C A AA A NA 6009 T4 A CA A A A B 6010 T4 B CA A A A B 6061 0 A DA AB 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 DAAA A B T4 B D A AA 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 BB D T4,T451O, T451I C C D B B D NA T6, T651O, T6511 C B D BB D 6070 T4, T4511 B C A A A B NA T6 C CAAA B 6101 T6, T63 C CAAA A NA T61,T64 B D AAA A 6151 T6,T652 B 6201 T81 CAA A A NA 6262 T6, T651,T651O, T651I C BAAA A NA T9 D BAAA A 6351 T5,T6 C CAA A A B 6463 T1 B D AA A A T5 B CA AA A NA T6 C C A AA A 7005 T53 C A B BB 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 BD C B D D T76,T761 D BD C B D D 7072 A DAAA A A 7075 0 DD C B D D T6,T651, T652,T651O, D B D C B D D T6511 T73,T7351 D BD C B D D 7175 T74,T7452 D BD 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 BD C B D D T76,T765I D B D CB D D

(a)RatingsA throughDforcold workabilityand AthroughEformachinabilityarerelativeratingsindecreasingorderof merit.(b)RatingsA through D for weldabilityand brazeabilityarerelativeratingsdefinedasfollows:A, generallyweldableby all commercialproceduresand methods;B, weld- able with specialtechniques or for specificapplicationsand requiringpreliminarytrials or testing to develop welding procedureand weld perfor- mance;C, limitedweldabilitybecauseofcracksensitivityor lossin resistancetocorrosionandmechanicalproperties;D, nocommonlyusedwelding methodshavebeendeveloped.(c)RatingsA throughDandNA forsolderabilityarerelativeratingsdefinedas follows:A,excellent;B,good;C, fair; D, poor;NA, not applicable Corrosion of Aluminum and Aluminum Alloys (#06787G) Copyright © 1999 ASM International ® Editor(s): J.R. Davis All rights reserved. 8 I CorTosion of Aluminum and Aluminum Alloys www.asminternational.org

carbon and are gradually fed into the top of the cell with the aluminum. Examplesof two common metals because the anodes are continually consumed during associated with aluminumores that fit this description electrolysis.A group of cellsare connected in series to are iron and silicon. It is, therefore, very important obtain the voltagerequired by the particular direct cur that raw materials be as free of these metal oxides as rentpowersource thatis being used. possible. By careful control of raw materials, alumi For aluminum, the electrolyte used is cryolite num with a purity of 99% or higher may beproduced. (Na3AlF6) with 8 to 10% Al20 3dissolvedin it. Other Generally, the purityof aluminumas it comesfrom the additives, such as CaF2and AlF3, are addedto obtain electrolysis cell (i.e., up to 99.9%) is adequate. High desirable physical properties.The meltingpoint of the purity aluminumof at least 99.97% AI content is nec electrolyte is approximately 940 °C (1725 "F), and essary for certain special purposes (e.g., reflectorsor the Hall-Heroultcell operates at temperatures of ap electrolytic capacitors). For such applications, second proximately 960 to 1000 °C (1760to 1830oF). stagerefmingoperations(Hoopescell electrolysis)are At the cathode of the aluminum cell, aluminum is necessary. Aluminumproduced in this way is 99.99% reduced from an ionic state to a metallic state-for pure. Higher purities of up to 99.9999% ("six-nines" example: aluminum) canbe obtained withzone-refining operations.

Secondary Aluminum Production

This is a very simplified representation of the complex Advantages. Aluminum recovered from scrap reactions that takeplaceat thecathode. However, it does (secondary aluminum) has beenan importantcontribu represent the overall production of molten aluminum, tor to the total metal supply since the 1950s. The eco which forms a molten pool in the bottom of the cell. nomics of recycling, together with improved techniques Periodically, the molten pool of aluminum metal is of scrappreparation andmelting,whichprovidehigher drainedor siphoned fromthe bottomof thecellandcast yields, led to the development of the secondaryalumi At the anode,oxygenis oxidizedfromits ionic state num industry. The increased concem with, and eco to oxygen gas. The oxygen gas in tum reacts with the nomic implications of, energy supply in recent years carbon anode to form carbon dioxide gas, which have focusedeven more attentionon recyclingof alu gradually consumes the anode material.Two types of minum becauseof its energy-intensive nature.The en anodes are in use: prebakedand self-baking. Prebaked ergy required to remelt secondary aluminum prepara anodes are individual carbon blocks that are replaced tory to fabrication for reuseis only 5% of that required one after another as they are consumed. Self-baking to producenew (primary)aluminum Todaysecondary anodes, as shown in Fig. 2, are made up of carbon aluminum accounts for about 35% of the aluminum paste that is fed into a steelframeabovethecell. As the supply in both the UnitedStatesand Europe. anode descendsin the cell, it hardens,and new carbon The Recycling Loop. The reclamation of aluminum paste is fed continuallyinto the top of the steelframe. scrapis a complexinteractive processinvolvingcollec Impurities in the Al20 3raw materialwhichare more tioncenters,primary producers, secondary smelters, metal noble than aluminumare reduced at the cathodealong processors and consumers. Figure 3 depictsthe flow of

Anode leads

Steel studs

Cathode cart>oo

Cathodeleads

Fig. 2 Hall-Herault aluminum production cellwithself-baking anode Corrosion of Aluminum and Aluminum Alloys (#06787G) Copyright © 1999 ASM International ® Editor(s): J.R. Davis All rights reserved. www.asminternational.org

Imports Imports Exports Imports Exports

Enduse Containers and packaging, 21.7%

Building and construction, 12.9%

UBC Total Transportation, 29.2% New scrap processing facility I II•I aluminum generated supply Electrical, 6.9%

Consumer durables, 6.8%

Machinery and equipment, 6.1% Secondary Other, 3.1% Scrap recycling aluminum industry (molten or ingot) t Imports Exports

Fig. 3 Flow diagram for aluminum in the United States,showing the role of recycling in the industry. Scrap recycling (lower left) includes scrap collectors, processors, dealers and brokers, sweat furnace operators, and drossreclaimers. i 8 ...... '0 Corrosion of Aluminum and Aluminum Alloys (#06787G) Copyright © 1999 ASM International ® Editor(s): J.R. Davis All rights reserved. www.asminternational.org 10 I Corrosion of Aluminum and Aluminum Alloys

metal originating in primary smelting operations and zinc; other elements are also added in smaller through various recycling activities. The initial reproc amounts for grain refmement and to develop special essing of scrap takes place in the facilities of primary properties. The total amount of these elements can producers. In-process scrap, generated both in casting constitute up to 10% of the alloy composition (per and fabricating, is reprocessed by melting and recast centages given in weight percent unless otherwise ing. Increasingly, primary producers are purchasing noted). Impurity elements are also present, but their scrap to supplement primary metal supply; an example total percentage is usually less than 0.15% in alumi of such activity is the purchase of toll conversion of numalloys. used beverage cans (UBC) by primary producers en gaged in the production ofrigid container stock. Classifications and Designations Scrap incurred in the processing or fabrication of semifabricated aluminum products represents an addi It is convenient to divide aluminum alloys into two tional source of recyclable aluminum. Traditionally, major categories: wrought composition and cast com this form of new scrap has been returned to the sup positions. A further differentiation for each category is plier for recycling, or it has been disposed ofthrough based on the primary mechanism ofproperty develop sale on the basis ofcompetitive bidding by metal trad ment. Many alloys respond to thermal treatment based ers, primary producers and secondary smelters. 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 wrought alloys, automotive, aerospace, and military products; machin such alloys are described as heat treatable. A large ery; miscellaneous transportation parts; and building number of other wrought compositions rely instead on and construction materials, have finite lives. In time, work hardening through mechanical reduction, usually discarded aluminum becomes available for collection in combination with various annealing procedures for and recovery. So-called old scrap (metal product that property development. These alloys are referred to as has been discarded after use) can be segregated into work hardening or non-heat-treatable. Some casting classifications that facilitate recycling and recovery. alloys are essentially not heat treatable and are used Process Technologies. Scrapped aluminum prod only in as-east or in thermally modified conditions ucts are broken into small pieces and separated from unrelated to solutions or precipitation effects. dirt and foreign materials so as to yield feedstock suit Cast and wrought alloy nomenclatures have been able for remelting. This is done using breakers, shred developed. The Aluminum Association system is most ders, magnetic, and settlement/flotation separators. widely recognized in the United States. Their alloy Such scrap typically contains alloys ofmany types, all identification system employs different nomenclatures mixed together. A more sophisticated kind ofrecycling for wrought and cast alloys but divides alloys into was developed in the 1970s and 1980s for process families for simplification. 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 system is used to produce a list of wrought material back into products similar to those from composition families as follows: which it originated. Thus, the casthouses of extrusion plants produce extrusion billets from process scrap and • lxx-x: Controlled unalloyed (pure) composition, from recycled scrap extrusions. Similarly, the high rate used primarily in the electrical and chemical indus of recovery of UBCs from the consumer enables a tries large proportion of canstock coils to be made from • 2xxx: Alloys in which copper is the principal alloy UBCs. Recovery of UBCs has multiplied repeatedly ing element, although other elements, notably mag since the early 1970s. In 1997, some 2,052 million nesium, can be specified. 2xxx series alloys are pounds of UBCs were collected in the United States. widely used in aircraft where their high strengths This constitutes 66.8% of can shipments. In some (yield strengths as high as 455 MPa, or 66 ksi) are countries, for example Sweden, recycling rates ex valued. ceeding 80% are achieved. • 3xxx: Alloys in which manganese is the principal alloying element, used as general-purpose alloys for architectural applications and various products • 4xxx: Alloys in which silicon is the principal alloy Aluminum Alloys ing element, used in welding rods and brazing sheet • 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 and other products exposed to marine environments microstructure. The addition of selected elements to • &xx: Alloys in which magnesium and silicon are pure aluminum greatly enhances its properties and the principal alloying elements, commonly used for usefulness. Because of this, most applications for alu architectural extrusions. minum utilize alloys having one or more elemental • Txxx: Alloys in which zinc is the principal alloying additions. The major alloying additions used with alu element (although other elements, such as copper, minum are copper, manganese, silicon, magnesium, magnesium, chromium, and zirconium, can be