Alu­minium and its al­loys

Vyv Cox ex­plains all about the prop­er­ties of alu­minium and cast and wrought alu­minium al­loys – and out­lines their var­ied uses in a marine ca­pac­ity

Practical Boat Owner - - Contents -

Var­ied uses in a marine ca­pac­ity

Some­times you have to feel sorry for alu­minium. As the third most abun­dant el­e­ment in the Earth’s crust, after oxy­gen and sil­i­con, it might be thought that peo­ple would use its name cor­rectly, but they don’t.

To most, the word ‘alu­minium’ is syn­ony­mous with ‘al­loy’, widely and incorrectly used to de­scribe car wheels; but this is a mis­use of the word. The re­al­ity is that al­most every me­tal used by the hu­man race is formed by the com­bi­na­tion of two or more met­als (and some non-met­als) such as cast iron, steel, brass, bronze and hun­dreds of oth­ers. All of these are al­loys.

Un­usu­ally for such an abun­dant el­e­ment, alu­minium me­tal was not pro­duced un­til the mid-1800s due to its ex­treme re­ac­tiv­ity, par­tic­u­larly with oxy­gen. This re­ac­tiv­ity means that it re­quires huge en­ergy to sep­a­rate it, these days car­ried out elec­tri­cally on the molten ore at around 950°C. It also means that alu­minium has a strong ten­dency to re­turn to its low­est en­ergy state by re­ac­tion with air, wa­ter, acids and al­ka­lis. For­tu­nately for us, as with stain­less steel the ox­ide that forms on its sur­face is pas­sive, re­sist­ing fur­ther ox­i­da­tion and help­ing the me­tal to re­main bright and dec­o­ra­tive. One of its prin­ci­pal prop­er­ties is its low den­sity, pro­vid­ing light weight with ad­e­quate strength for many uses.

Items in alu­minium are pro­duced in two prin­ci­pal forms – ei­ther as cast­ings, or wrought, ie formed into shape by rolling, ex­tru­sion or forg­ing. In com­mon with most pure met­als the en­gi­neer­ing prop­er­ties of un­al­loyed alu­minium are limited, re­quir­ing the ad­di­tion of var­i­ous other met­als to give it the de­sired strength, sta­bil­ity, cor­ro­sion re­sis­tance or what­ever is needed for the ap­pli­ca­tion. How­ever, pure (99.9%) alu­minium can be hard rolled (work hard­ened) to pro­vide use­ful prop­er­ties in sheet and strip form.

Al­loys for marine use

When it comes to se­lect­ing alu­minium al­loys for use on the boat, our choices are fairly limited. Cor­ro­sion re­sis­tance is by far the most crit­i­cal fac­tor, with me­chan­i­cal strength some­what less im­por­tant as sec­tion sizes can be in­creased where nec­es­sary. The strong­est al­loys may suf­fer se­lec­tive cor­ro­sion be­tween the met­als in the al­loy, which is es­pe­cially the case with alu­mini­um­cop­per in sea­wa­ter but also prob­lem­atic in fresh.


AA514 (LM5) is prob­a­bly the most widely used al­loy for cast­ing marine hard­ware such as cleats, fair­leads, steer­ing pedestals, saildrives, mast fit­tings and sim­i­lar. All of the 5xx se­ries al­loys are non­heat-treat­able, with ex­cel­lent cor­ro­sion re­sis­tance, machin­abil­ity and sur­face ap­pear­ance, ideal for highly pol­ished fit­tings.

The alu­minium-sil­i­con al­loy A413, equiv­a­lent to LM6, is the most widely used alu­minium cast­ing al­loy for gen­eral pur­poses. It is duc­tile and has lower cor­ro­sion re­sis­tance than the 5xx se­ries, but it casts

well into in­tri­cate shapes and can be used for marine parts.

Wrought al­loys

Wrought alu­minium al­loys for boatbuilding and marine on­shore ap­pli­ca­tions are ex­clu­sively from the 5xxx and 6xxx se­ries, as shown in the next ta­ble (below). The val­ues pre­ceded by a letter H in­di­cate the ex­tent of work hard­en­ing in these non-heat-treat­able al­loys, whereas those pre­ceded by a T re­fer to lev­els of heat treat­ment (tem­per).

In­ci­den­tally, 6061 is the al­loy gen­er­ally used for hot-forged au­to­mo­tive wheels. Many of us will be aware that this al­loy can suf­fer quite se­vere pit­ting cor­ro­sion, even in the rel­a­tively non-saline con­di­tions of UK roads.


An­o­dis­ing is the process by which the pro­tec­tive ox­ide film on the sur­face of alu­minium is ar­ti­fi­cially thick­ened. It is thus not a coat­ing but is a con­ver­sion of the par­ent me­tal. An­odised films pro­tect the me­tal from cor­ro­sion and pro­vide a durable and at­trac­tive fin­ish that can be coloured to fur­ther im­prove the ap­pear­ance of the fin­ished prod­uct.

The an­o­dis­ing process is car­ried out in an elec­tric cell. The ob­ject to be treated forms the an­ode of the cell, with cath­odes at­tached to the cell walls. The elec­trolyte in the cell is an acid, typ­i­cally sul­phuric acid. A di­rect elec­tric cur­rent is passed be­tween the an­ode and cath­ode, caus­ing oxy­gen to be formed at the an­ode, which com­bines with the alu­minium to form alu­minium ox­ide. The ox­ide film formed is por­ous and needs to be sealed to harden and so­lid­ify the sur­face. In some cases the sealant in­cor­po­rates a coloured dye. Var­i­ous meth­ods are used for these fi­nal stages. It is pos­si­ble for the owner to carry out DIY an­o­dis­ing, as de­tailed in David Berry and Amanda Potts' ar­ti­cle ‘DIY an­o­dis­ing’ (PBO March 2012).

The an­odised film thickness de­pends upon the length of time for which the ob­ject is ex­posed to the an­o­dis­ing process. Film thickness is spec­i­fied to cope with the in­tended ap­pli­ca­tion:

■ 25 mi­crons for marine and heavy-duty ar­chi­tec­tural du­ties

■ 15 mi­crons for most gen­eral pur­pose ex­te­rior du­ties

■ 10 mi­crons for in­te­rior du­ties and some dec­o­ra­tive ex­te­rior du­ties where fre­quent clean­ing is likely.

Alu­minium an­odes

Zinc is prob­a­bly the most widely used an­ode me­tal for marine leisure ves­sels, but this me­tal de­vel­ops a white layer of zinc hy­drox­ide in fresh and brack­ish wa­ter, pre­vent­ing its ef­fi­cient func­tion. In brack­ish wa­ter, or vary­ing fresh/sea wa­ter, it is com­mon to use an alu­minium al­loy as the an­ode me­tal. Alu­minium is al­loyed with zinc and some mi­nor met­als in a va­ri­ety of ways, a typ­i­cal com­po­si­tion be­ing shown in the ta­ble above.

Alu­mini­umhulled boats

Alu­minium is in­creas­ingly be­ing de­ployed for the con­struc­tion of yachts, in gen­eral us­ing all-welded con­struc­tion in 5086. This al­loy grade pro­vides great strength and re­sis­tance to hol­ing in com­bi­na­tion with weight sav­ings of 10% over GRP and 35% over tim­ber. Alu­minium itself has good re­sis­tance to cor­ro­sion in both fresh and sea­wa­ter but it is highly sus­cep­ti­ble to gal­vanic cor­ro­sion when in con­tact with al­most every me­tal in the gal­vanic se­ries, with the ex­cep­tion of zinc. There is a grain of truth in the story of a cop­per coin dropped into the bilge of an alu­minium boat that sub­se­quently cor­roded its way right through, although I am not sure I have ever seen the pho­to­graphs! The two ar­eas that need to be con­sid­ered very care­fully are the at­tach­ment of stain­less steel or other fit­tings and elec­tri­cal paths.

No me­tal other than stain­less steel should be at­tached to the hull, with the ex­cep­tion of

sac­ri­fi­cial an­odes. Fas­ten­ers should be plas­tic or stain­less steel, avoid­ing any cop­per al­loys. Me­tal ob­jects such as fish­ing weights, fish hooks, coins and tools should not be left in con­tact with the hull for any ap­pre­cia­ble time. Stain­less steel fit­tings should be bed­ded on an iso­lat­ing film such as Du­ralac, Te­fgel or plas­tic film.

Elec­tri­cally, the main re­quire­ment is that all wiring is made us­ing twin leads, not us­ing the hull for the neg­a­tive re­turn. Shore power should al­ways be checked for po­lar­ity. Elec­tri­cal de­vices such as iso­la­tion trans­form­ers and leak de­tec­tors are good pre­cau­tion­ary mea­sures and should be fit­ted.

Con­ven­tional cop­per-based an­tifoul­ing paints if ap­plied to alu­minium hulls would re­sult in gal­vanic cor­ro­sion and should never be used. Most paint man­u­fac­tur­ers produce cop­per-free paints specif­i­cally for alu­minium hulls. Some foul­ing or­gan­isms are known to cre­ate con­di­tions that ren­der alu­minium more sus­cep­ti­ble to cor­ro­sion.

Saildrives are in­creas­ingly com­mon on the yachts of to­day, and the same rules ap­ply. Cop­per-based an­tifoul­ing paints should not be used on them. The rec­om­mended paint sys­tem should be ap­plied reg­u­larly to pre­vent lo­cal at­tack. Volvo saildrives are iso­lated from the en­gine and the re­main­der of the boat by rub­ber and plas­tic in­su­la­tors and these should not be com­pro­mised by ad­di­tional wiring, or even dirty oil on the gas­ket. Yan­mar saildrives are not iso­lated in the same way. Both rely heav­ily on zinc an­odes at­tached ahead of the pro­pel­ler and these must be re­placed in good time.

Adding alu­minium fit­tings

When it comes to adding fit­tings by the owner, there can be prob­lems. Cast­ings such as cleats and fair­leads bought in a chan­dlery are al­most cer­tainly made in the ap­pro­pri­ate 5xx al­loy. How­ever, there is a very good chance that wrought ma­te­rial, sheet or strip, bought at your lo­cal DIY store, will be of 1xxx man­u­fac­ture and there­fore sus­cep­ti­ble to cor­ro­sion. Grades 6063 and 6082 are the most widely ex­truded al­loys, sold by many good-qual­ity sup­pli­ers in a wide range of shapes. Gen­eral ex­tru­sions such as an­gles and hol­low sec­tions may not have the an­o­dis­ing thickness rec­om­mended for marine use. For above-wa­ter use these are gen­er­ally ac­cept­able, but for any­thing un­der­wa­ter – es­pe­cially keel bands and rub­bing strips that are likely to lose any an­odised fin­ish – only 5xxx al­loys will have a long life. I have been un­able to source any ex­tru­sions in this ma­te­rial us­ing in­ter­net searches.

Fit­tings added by the owner are most com­monly at­tached to alu­minium plate, sheet and ex­tru­sions us­ing ei­ther pop riv­ets in alu­minium, stain­less steel or Monel, or stain­less steel bolt­ing. Alu­minium pop riv­ets are avail­able in two grades, ei­ther ‘soft set’ in 1xxx alu­minium or ‘al­loy’ in 5xxx grade. The lat­ter is rec­om­mended for all marine ap­pli­ca­tions and is avail­able from good qual­ity sup­pli­ers. In all cases it is ad­vis­able to bed the fas­tener in a sealant to limit gal­vanic cor­ro­sion. Fit­tings at­tached by the boat­builder are likely to be in 5xxx ma­te­rial.

Some­times own­ers are tempted to use re­cy­cled alu­minium for marine pur­poses without know­ing their com­po­si­tion. This can have se­ri­ous con­se­quences, as shown in ‘The jury’s out!’ (Mak­ing a jury rud­der, PBO Jan­uary 2016). A length of alu­minium ob­tained from a scrap­yard was used to make a rud­der. De­spite hav­ing been tested for strength, after three or four years the rud­der snapped off at the stock (see photo above). Although most alu­minium al­loys are duc­tile, this frac­ture face ap­pears brit­tle, with no ev­i­dence of duc­til­ity. Alu­minium-cop­per al­loys of the 2xxx se­ries are highly sus­cep­ti­ble to stress­cor­ro­sion frac­tures, par­tic­u­larly in chlo­ride en­vi­ron­ments. The ap­pear­ance of this type of fail­ure in­di­cates brit­tle­ness, and it is my guess that this al­loy was alu­minium-cop­per.


Alu­minium is highly sus­cep­ti­ble to gal­vanic cor­ro­sion, be­ing at the an­odic end of the gal­vanic se­ries with only mag­ne­sium and zinc above it. How­ever, although its po­si­tion gives a good in­di­ca­tion of the likely re­sult of com­bi­na­tions of met­als, there is another fac­tor: rel­a­tive size. A sim­ple anal­ogy ex­plains this, a sin­gle rivet in a sheet of another me­tal. If an alu­minium rivet is placed in a steel sheet and im­mersed it will cor­rode very rapidly as the re­ac­tion is be­ing driven by a large cath­ode (the steel). In the re­verse case of a steel rivet in an alu­minium sheet, the large sur­face area of the an­ode pro­tects the small cath­ode and the re­ac­tion will be slow. In re­al­ity, not all of the alu­minium can pro­tect the rivet, and cor­ro­sion will take place in a fairly small area around it. ■ Vyv Cox’s book Met­als In Boats (Crowood Press) is avail­able now.

A fair­lead for the leisure marine in­dus­try, cast in a 514 al­loy. Fit­tings for yachts need ex­cel­lent cor­ro­sion re­sis­tance and long-term at­trac­tive ap­pear­ance

A zinc an­ode sus­pended from the tran­som of a berthed alu­minium boat, pro­vid­ing ca­thodic pro­tec­tion. This 42ft boat had four such an­odes dis­trib­uted around the hull. Fixed zinc an­odes should be re­newed reg­u­larly to over­come the pas­si­va­tion that oc­curs due to ox­i­da­tion

After three or four years, this re­cy­cled alu­minium rud­der snapped off at the stock

Here is another ex­am­ple, this time around a rivet se­cur­ing the foot of a mast to its base. In this case the rivet ap­pears to be an alu­minium one fit­ted pro­fes­sion­ally, in which case al­most cer­tainly in 5xxx ma­te­rial. It would seem that some sur­face dam­age re­moved the an­o­dis­ing locally, per­haps when the rivet hole was be­ing drilled. In both these cases it would seem that there is no iso­lat­ing sealant be­tween the rivet and alu­minium

Gal­vanic cor­ro­sion is shown in this photo of riv­ets in a boom. The light cor­ro­sion dam­age is quite lo­calised. These riv­ets ap­pear to be in a stain­less steel ma­te­rial

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