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1. Characteristics of marine environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471.1 Sea water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471.2 Sea atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

2. Corrosion behaviour of aluminium under control of the natural film of oxide . . . . . . . . . 148

3. Influence of the pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

4. Influence of alloying elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

5. Forms of aluminium corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495.1 Uniform corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495.2 Pitting corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495.3 Transcrystalline corrosion and intercrystalline corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505.4 Exfoliation corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1515.5 Waterline corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1515.6 Crevice corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

6. Bimetallic corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1526.1 The galvanic cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1526.2 Conditions of bimetallic corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1536.3 Concept of potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1546.4 Practical aspects of bimetallic corrosion of aluminium in marine environments . . . . . . . . . . . . . . . . . . . 1556.5 Submerged mixed contacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1556.6 Emerged mixed contacts in open air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1556.7 Influence of the type of metal in contact with aluminium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

7. Aluminium tarnishing and blackening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

8. Role and prevention of marine fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

9. Effects of welding and design arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1599.1 Effect of welding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1599.2 Effect of design arrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

10. Sensitivity to corrosion of aluminium alloys in marine applications . . . . . . . . . . . . . . . . 15910.1 Choice of alloys for marine applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15910.2 Sensitivity of 5000 series alloys to intercrystalline corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

11. Corrosion tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16211.1 Exfoliation corrosion testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16211.2 Test for intercrystalline corrosion of the 5000 alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16311.3 Test for intercrystalline corrosion of the 6000 alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

C h a p t e r 1 0C O R R O S I O N B E H AV I O U R O F A L U M I N I U M

I N M A R I N E E N V I R O N M E N T S

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THE MARINE ENVIRONMENT ishighly aggressive towards

most materials, as amply demons-trated by the dilapidated state ofold ships and wrecks (whethermade from wood or steel) that canbe seen lying abandoned aroundour coasts.

There are few metals capable ofwithstanding this hostile environ-ment unprotected. Bronze is oneof these metals, and in museumsaround the Mediterranean we mayadmire statues recovered from theancient wrecks which they onceadorned and which have lain sub-merged in the sea for centuries,even millennia.

Based on our very long experienceof aluminium’s corrosion behaviourin marine atmospheres and in seawater [1], we can now assert thatthe service life of aluminium inmarine environments will be excep-tionally long and can be measuredin decades!!! We can truly claimthat aluminium is the “metal of thesea” of the modern age.

Although this evidence is nowwidely accepted, the issue of theresistance of aluminium to corro-sion in marine applications isimportant enough to merit adigression of some length anddetail to review a number ofbasic principles and in doing so

perhaps to prevent disappoint-ments and avoid unnecessaryprotection.

Even though, around the middleof the 20th century, metallurgistsand corrosion specialists per-fected alloys that are “intrinsi-cally” resistant to corrosion inmarine environments – the alu-minium-magnesium alloys thatbelong to the 5000 series –casesof corrosion still occur now andthen. An examination of thesecases shows that very often thecause of the corrosion lies in thedesign of the structure and inservice conditions to which alu-minium is not suited.

10. CORROSION BEHAVIOUR OF

146

TRICAT 50

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1.CHARACTERISTICSOF MARINE ENVI-RONMENTS

A distinction must be madebetween sea water and seaatmosphere.

1.1Sea water

The sea is a liquid medium thatcontains the following substancesin equilibrium:■ dissolved mineral salts, some30 to 35 g per litre,■ dissolved gases, including 5 to 8ppm (1) oxygen,■ living organisms,■ decomposing organic matter,■ mineral matter in suspension.

This mix forms a very complexmedium in which the influence ofeach individual factor, whetherchemical (composition), physical(temperature, pressure) or biologi-cal (flora and fauna), on the resist-ance to corrosion of metals can beneither truly isolated nor sepa-rately quantified.

The great oceans – the Atlantic,the Indian and the Pacific – whichconnect with one another in the

southern hemisphere have a virtu-ally uniform total salinity, 32 to 37g.l-1, including 30 g.l-1 of sodiumchloride or “salt”.The salinity of enclosed or isolatedseas can be very different fromthat of the great oceans, and mayvary seasonally down the year. TheBaltic for example has a total salin-ity of 8 g.l-1, the Black Sea 22 g.l-1,the Mediterranean 41 g.l-1 and thePersian Gulf 57 g.l-1 [2].

The hostility of sea water to met-als and other materials is due to itsabundance of chlorides “Cl-”.

Experience and the results of cor-rosion tests show that alu-minium’s resistance to corrosion isthe same whatever the sea orocean. Comparative tests carriedout on the same alloys submergedin the North Sea (16 to 17 g.l-1 chlo-rides, annual temperature variation0 to 18 °C) and the Arabian Gulf (26 to 34 g.l-1 chlorides, annualtemperature variation 17 to 30 °C)show that there is no significantdifference between the two sitesdespite their difference in salinity(ratio of 1:2) [3].

The corrosion behaviour of alu-minium may change when thewater in a port is polluted by urbanor industrial waste, provided of

course the polluting elementsmodify the corrosivity of sea watervis à vis aluminium (2).

1.2Sea atmosphere

The aggressive nature of sea air isaggravated by moisture and seaspray consisting of very fine dro-plets of sea water borne on thewind. The effect of sea air dependson the direction and strength ofthe prevailing winds, and is greatlydiminished at a distance of a fewkilometres inland.

ALUMINIUM IN MARINE ENVIRONMENTS

147

(1) ppm = parts per million or mg.l-1.(2) The diluted sea water in riverestuaries is usually more aggressive tomaterials, including aluminium, thanwater off shore. This paradox can beexplained in various ways: theprecipitations of calcium and magnesiumcarbonate which deposit a more or lessprotective film on the metal do not occurin diluted sea water. Biological activity isslower. Domestic or industrial wastesmodify physical-chemical balances andmay themselves contain corrosiveagents.

LIVING QUARTER

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2.CORROSION BEHA-VIOUROF ALUMINIUMUNDER CONTROLOF THE NATURALFILM OF OXIDE

Aluminium owes its excellent cor-rosion resistance to the presenceon the metal of a permanent filmof natural oxide that consists ofalumina (Al2O3) and makes themetal “passive” to the environ-ment.

Although it is extremely thin,between 5 and 10 nanometers (3),the oxide film forms a protectivebarrier between the metal and thesurrounding environment as soonas the metal comes into contactwith an oxidizing medium such asatmospheric oxygen or water. Thefilm forms within about one thou-sandth of a second and can evenform under oxygen pressures aslow as 1 millibar.

The physical-chemical stability ofthe oxide film is therefore key toaluminium’s resistance to corro-sion, and depends on the charac-teristics of the environment, suchas its pH, and on the compositionof the alloy itself.

3.INFLUENCEOF THE PH

The rate of dissolution of the oxidefilm is determined by the pH of theenvironment (figure 123). It is veryfast in acid and alkaline media andslow in media that are close to pH-neutrality (pH 5 to 9). The oxidefilm is therefore very stable in seawater whose pH is 8 – 8.2.

Contrary to widespread belief, thepH is not the only criterion thatmust be taken into account whenpredicting the behaviour of alu-minium in an aqueous environ-ment – the nature of the acids orbases also plays an essential part.This is very important whenselecting a cleaning or picklingproduct for aluminium.

Thus while hydrogen acids such ashydrochloric and sulphuric are veryaggressive towards aluminium(the more so in a strong solution),concentrated nitric acid has noeffect on aluminium (4) and can beused in concentrations strongerthan 50% to pickle aluminium andits alloys. Organic acids only havea very moderate action on alu-minium.

This is equally true in alkaline envi-ronments – caustic soda andpotassium attack aluminiumfiercely, whereas the effect of con-centrated ammonia is much lesssevere. The same applies toorganic bases.

4.INFLUENCEOF ALLOYINGELEMENTS

Some alloying elements of alu-minium alloys actually reinforcethe protective character of the nat-ural oxide film, while othersweaken it. The former includemagnesium, whose oxide (magne-sia) combines with the alumina toenhance the protective propertiesof the natural oxide film, account-ing for the remarkable corrosionperformance of magnesium alloysin the 5000 series: 5754, 5083,5383, 5086 etc.

Copper on the other hand is one ofthe elements that weaken the pro-tection provided by the oxide film,which is why the use of copperaluminium alloys in the 2000series and those belonging to the7000 series with added copper isnot advisable in marine applica-tions unless special protection isprovided.

148

SOLUBILITY OF ALUMINA

0 2 4 6 8 11 12 14

10-1-2-3

pH of sea waterLog (V)mg.dm-2.h-1

Alkalinedissolution

in AlO2-Acid

dissolutionin Al3+

➤

Figure 123

(3) 1 nanometer = one thousandmillionth of a metre (10-9 m).(4) Its oxidizing function even reinforcesthe oxide layer very slightly.

GANGWAY

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5.FORMS OF ALUMI-NIUM CORROSION

This section only deals with theforms of corrosion that might befound in marine environments onwrought alloys of the 1000, 3000,5000 and 6000 series, and on thesilicon casting alloys (40000) ormagnesium casting alloys (50000).

These are:■ uniform corrosion,■ pitting corrosion,■ transcrystalline corrosion,■ intercrystalline corrosion,■ exfoliation corrosion,■ waterline corrosion,■ crevice corrosion,■ bimetallic corrosion.

5.1Uniform corrosion

This type of corrosion is accompa-nied by a regular, uniformdecrease in thickness over thewhole surface area of the metal.The rate of dissolution can varyfrom a few microns per year in anon-aggressive environment toseveral microns per hour depend-ing on the type of acid or base insolution in the water.

In marine environments, whetherimmersion in sea water or expo-sure to sea air, uniform corrosionis minuscule, of the order of onemicron per year, and is not meas-urable.

We can therefore say that theservice life of aluminium alloyequipment properly designed andbuilt for use in a marine environ-ment will not be limited by thistype of corrosion.

5.2Pitting corrosion

This very localised form of corro-sion is common to many metals(figure 124) and is characterised bythe formation of cavities in thematerial. The diameter and depthof the cavities vary depending on anumber of parameters inherent ineither the metal itself (type ofalloy, manufacturing process etc.)or its environment, e.g. concentra-tion of mineral salts.

Aluminium is sensitive to pittingcorrosion in environments wherethe pH is close to neutral, whichessentially means all naturalenvironments such as surfacewater, sea water and atmos-pheric moisture.

Unlike in other common metals,this type of corrosion is worthy ofour attention because the corro-sion pits of aluminium are alwayscovered by very large white blis-ters of gelatinous hydrated alu-mina Al(OH)3. The blister is alwaysmuch larger than the underlyingcavity.

Pitting corrosion occurs at siteswhere the natural film of oxide isimperfect due to thinning, localrupture or gaps caused by variousfactors associated with manufac-turing conditions, additives and soforth. Experience shows thatareas which are ground or scored

during sheet metal working, fold-ing or welding operations provideideal pockets where pits candevelop during the first few weeksof immersion in sea water.

What is important for the user is toknow the rate at which the pitsdeepen once they are initiated.Unlike other metals, e.g. zinc,whose corrosion products are sol-uble, alumina Al(OH)3 is insolublein water so that, once formed, itadheres to the surface of themetal inside the pit cavities. In thisway, hydrated alumina consider-ably inhibits exchanges betweenthe metal and sea water or atmos-pheric moisture.

The rate of pitting corrosion in alu-minium and its alloys thereforedecreases very rapidly in mostmedia including sea water, andmeasurements of pit depths takenat regular intervals have shownthat the rate of pitting “v” is a fac-tor of time “t” by equations of thetype v = Kt1/3.

Many decades of experience withthe use of unprotected aluminiumin coastal construction (roofs, wallpanels) and in shipbuilding corrob-orate the results obtained in thelaboratory, from natural exposureor at corrosion testing stationsover long periods of time: thedepth of pits hardly changes oncethey have formed during the initialmonths of service.

10. CORROSION BEHAVIOUR OF ALUMINIUM IN MARINE ENVIRONMENTS

149

PITTING CORROSION OF ALUMINIUM

AI(OH)3

3e- 3 e-

➤

➤

➤

➤

➤

➤

➤H2Cl-

➤

➤

➤➤

6 OH-

Al3Fe

3/2 O23/2 H2

3 H+

CuOxide

➤Al3+➤

Figure 124

3H2O

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This slowdown in the rate of pit-ting corrosion explains the factthat aluminium equipment can beused for decades in certain naturalenvironments (country air, sea air,sea water) without any protection.This applies as much to sea air (fig-ure 125) as it does to immersion insea water (figure 126). In bothcases the pits are rarely deeperthan one millimetre even after sev-eral years.

5.3Transcrystallinecorrosion andintercrystallinecorrosion

Corrosion inside the metal, atgrain level, can spread in two ways(figure 127):■ in all directions: corrosionaffects all metallurgical consti-tuents indiscriminately, it is notselective. This is transgranular ortranscrystalline corrosion, so cal-led because it spreads inside thegrains,■ along preferential paths: corro-sion spreads along the grain boun-daries. Unlike transgranular corro-sion, this type of corrosion consu-mes very little metal.

It is invisible to the naked eye, andmicrographic examination isneeded to detect it, usually with amagnification of 50. When it pene-trates deep into the metal itaffects its mechanical properties,especially elongation, and caneven cause component failure.

Intercrystalline corrosion spreadsfrom pits. There is no correlationbetween the depth to which inter-crystalline corrosion penetratesand the diameter of the corrosionpits. In other words, intercrystallinecorrosion can spread in depth fromvery small superficial pits.

Intercrystalline corrosion is causedby the difference in electrochemi-cal potential that can existbetween the actual grain and thegrain boundary zone where inter-metallic compounds, such as theβAl3Mg2 phase for magnesiumalloys, can precipitate. The dissolu-tion potential of this intermetallicis very electronegative: -1150 mVSCE (saturated calomel electrode)compared with the grain: -750 mV.

Intercrystalline corrosion canoccur when three conditionscome together:■ presence of a corrosive aqueousmedium,■ difference in potential of at least100 mV between the intermetal-lics and the solid solution,■ continuous precipitation of inter-metallics in the grain boundaries.

Given the 400 mV difference inpotential between the βAl3Mg2phase and the grain, aluminium-magnesium alloys are sensitive tothis form of corrosion under welldefined and well known condi-tions, as reviewed in Section 10.2.They depend on the conditions ofworking and the conditions ofservice.

It is normal to quantify the inten-sity of intercrystalline corrosion bythe number of grain layers that are150

SEA SHORE EXPOSURE

5 10 15 20Length of exposure (years)

Pechiney, Centre de Recherche de Voreppe

800

600

400

200

0

Depth of pits

maximum

mean

➤

➤

(microns)

IMMERSION IN SEA WATER

1 5Length of immersion (years)

Pechiney, Centre de Recherche de Voreppe

800

600

400

200

0

Maximum depth of pits(microns)

Figure 125

Figure 126

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10. CORROSION BEHAVIOUR OF ALUMINIUM IN MARINE ENVIRONMENTS

151

affected by it, and it is acceptedthat intercrystalline corrosion issuperficial and harmless when itdoes not spread beyond 3 or 4 lay-ers, as is observed in the alloys ofthe 6000 series.

Selective tests that can be used todetect the sensitivity of aluminiumalloys to intercrystalline corrosionare given in Section 11.

5.4Exfoliation corrosion

Exfoliation corrosion is a form ofcorrosion that spreads alongplanes parallel to the direction ofrolling (or extrusion). Betweenthese planes are very thin sheetsof sound metal. The build-up ofcorrosion products causes the cor-roded zone to swell, peeling awayleaves of metal like the layers ofan onion, hence the name “exfolia-tion corrosion”.

The sensitivity of aluminium alloysto this very characteristic form ofcorrosion (figure 136, p. 162)depends on the production condi-tions (the rolling or extrusionprocess, rates of work hardening

and the elongated texture of grains). In the aluminium-magnesiumalloys (5083, 5383, 5086 etc.), theH116 temper, examined with theASSET test (5), is immune to thistype of corrosion.

5.5Waterline corrosion

This form of corrosion affectssemi-submerged metal structures,especially steel, in which the sub-merged zone close to the air/waterboundary can suffer preferentialcorrosion that is sometimessevere (6).

In aluminium, this corrosion is dueto the difference in the concentra-tion of chlorides caused by evapo-ration in the thinnest part of thefilm of water wetting the metal(figure 128). The result is a differ-ence in dissolution potential whichcan be significant between thebottom and top of the meniscuswhich is anodic and therefore cor-roded [4].

Aluminium and its alloys arelargely immune from this form ofcorrosion in sea water, so the

waterline of barges and boats withunpainted hulls is not particularlycorroded by this medium. Thesame applies to foundation pilesand the floats of landing-stagesetc. in marinas.

In flowing water this effect is veryweak as the meniscus is continu-ally being renewed.

In stagnant water, the area eitherside of the air/water boundary linemust be painted to avoid the risksof waterline corrosion.

TRANSCRYSTALLINE AND INTERCRYSTALLINE CORROSION

Transcrystalline Corrosion Intercrystalline Corrosion

WATERLINE CORROSION

Water

Air- 800 mV

- 980 mV

Aluminium

Figure 128

Figure 127

(5) Cf. Section 11. (6) In steel, this form of corrosion,known as “corrosion by differentialaeration”, is due to the difference in theconcentration of oxygen between thelayer of water that is in direct contactwith air – which is therefore richer inoxygen – and the layer beneath.Corrosion occurs below the waterline.

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5.6Crevice corrosion

Crevice corrosion, also known asdeposit attack, occurs in tinyrecesses beneath deposits wherewater penetrates but is notrefreshed (figure 129).

The spread of crevice corrosion inaluminium is usually minimal, dueno doubt to the precipitation ofalumina – a product of corrosion –which quickly seals off access tothe crevices. When a bolted or riv-eted joint that has been immersedfor a very long time in sea water isdismantled, a continuous depositof alumina is very often foundbetween the two sheets.When joining components how-ever it is essential to avoid leavingrecesses that might provide afoothold for corrosion in the longterm. Intermittent welds on struc-tures that are permanently or evenonly occasionally immersedshould therefore be avoided.

6.BIMETALLIC CORRO-SION

For a long time, the often exagger-ated fear of bimetallic corrosionimpeded growth in the use of alu-minium, both in naval constructionand in coastal equipment.

Today, after many years of experi-ence with the use of aluminium inmarine environments, we canproperly assess and quantify therisks of bimetallic corrosion of alu-minium in mixed assembliesbetween aluminium and othercommon metals of the type foundin shipbuilding (or other applica-tions).

Such an assessment is a neces-sary exercise, as it is neither pos-sible nor even desirable for com-mercial and technical reasons toconstruct ships and equipmentthat are made entirely of alu-minium. On pleasure craft forexample, transmission shafts,screws, valves and pipes arealmost never made from alu-minium.

It is therefore the task of thedesigner and operator to ensurethat contact between aluminiumand other metals in marine envi-ronments cannot cause bimetalliccorrosion. This is a straightforwardmatter provided they follow anumber of elementary rules basedon fundamental principles, in par-ticular the concepts of the galvaniccell and the potential scale, and ofcourse on experience.

6.1The galvanic cell

When two dissimilar metals oralloys such as copper and zinc areplaced in direct contact (or areelectrically connected) in a wetand electrically conductive envi-ronment, e.g. a sulphuric acidsolution, one of the two metals, inthis case zinc, will dissolve whilethe other, copper, retains itsintegrity and appearance.

This is a battery (7) (or galvanic cell)consisting of two electrodes, eachof which is a metal (figure 130):■ the one that is consumed – cal-led the anode – undergoes an oxi-dation reaction:

M → Mn + + ne-or

Al → Al3 + + 3e-

when it is aluminium,■ the other – the cathode – under-goes a reduction reaction, usuallyto the H+ ions present in thewater (8):

H+ + e- → H2

The amount of hydrogen that isgiven off bears no direct relation-ship to the mass of metal that isdissolved at the anode. Understandard conditions – 25 °C at apressure of 1013 mbar – it is 33.6litres for 27 grams of aluminium.

The reactions at the anode and thecathode occur simultaneously andare balanced in electrical charges“e-”. The complete reaction for thebimetallic corrosion of aluminiumis as follows:

152

CREVICECORROSION

➤

H+

H+

Al3+

Al3+Al3+

Al3+

Al3+

H+H+H+

➤

➤

➤

e-Cl-

e-Al3+

Al3+

Figure 129

(7) So called because the first currentgenerator invented by Alessandro Voltain 1800 was a “pile“of zinc and copperdiscs insulated from one another by feltand immersed in a diluted solution ofsulphuric acid.(8) The H+ ions are produced either bythe dissociation of the water itself or byan acid dissolved in the water.

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10. CORROSION BEHAVIOUR OF ALUMINIUM IN MARINE ENVIRONMENTS

Hydrogen is given off (at the cath-ode) and aluminium (the anode)dissolves, forming alumina Al(OH)3.

6.2Conditions ofbimetallic corrosion

These simplified equations showthat for bimetallic corrosion to takeplace, 3 conditions must cometogether simultaneously:- presence of an electrolyte,- electrical continuity,- dissimilar metals.

■ Presence of an electrolyte: thecontact area between the metalsmust be wetted by fresh water orsea water. The more conductivethe medium, the stronger thebimetallic corrosion, and it is the-refore much more intense in seawater whose resistance is of theorder of 10 to 25 W.cm-1 than infresh water (or rainwater) whoseresistance attains several thou-sand W.cm-1, depending on thesource of the water.

■ Electrical continuity betweenthe two metals, either throughdirect contact or made by meansof a fastener such as clampscrews.

One simple way of avoidingbimetallic corrosion therefore is toinsulate the two metals from oneanother as carefully as possible,and this is easily done with a highohmic resistance, i.e. an insulatingmaterial (figure 131) such as neo-prene (9).

As with any battery (or galvaniccell), anything that retards orinhibits the electrochemical reac-tions on the electrodes reduces itsoutput – this is referred to as“polarisation”. Applied to the caseof bimetallic corrosion, the accu-mulation of corrosion products inthe contact zone between the twometals slows down the process.

When hybrid joints made fromplates of steel and aluminium alloythat are bolted together withoutany insulation are taken apart after

years of immersion in sea water, avery dense alumina “poultice” istherefore found in the contactarea. The extent of the corrosionwill be limited because the alu-mina has significantly slowed theexchange of ions (10).

■ Dissimilar metals. This raisesthe concept of potential which weshall now discuss.

153

PRINCIPLE OF A GALVANIC CELL

➤

➤ e- e-

➤

➤

➤

Cathodecopper

Electrical connection

Electrons flow

Ion connection

Dilute acid solution

➤

Anodezinc

2H+ + 2e- → H2↑

Zn → Zn2 + 2e-➤

➤

Figure 130

INSULATION OF ALUMINIUM

➤

➤

➤

Aluminium

Gasket ( PVC,elastomer)

Other métal(Steel…)Bolt

➤

➤

➤

Figure 131

(9) Once a much-used technique inshipbuilding, it has been superseded bycathodic protection for immersedstructures (cf. Chapter 11). (10) It goes without saying that thisinitial corrosion cannot be expected toprovide reliable long term protection.

Sleeveand

insulatingwashers

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6.3Concept of potential

This is a thermodynamic phenom-enon that measures a metal’s ten-dency to oxidize. The more elec-tronegative the potential, themore the metal tends to oxidize.

Potential is measured in millivolts(mV) against a reference electrodeand in a defined medium.Corrosion specialists usually usethe saturated calomel electrode(SCE) as the reference.

The result is a classification or‘potential scale‘ of metals andalloys; table 67 shows a part ofthis scale for the common metalsin natural sea water in motion.

The rankings on this scale can beused to predict which of the twometals in contact with each otherin an aqueous medium, or morespecifically in sea water, will beattacked:■ if both metals have electrone-gative potential, it will be themore electronegative,■ if one metal is electropositive itwill be the electronegative.

In this combination for example:Zinc Ed = – 1130 mVIron Ed = – 610 mV

it is the zinc that corrodes (11).

Experience shows that bimetalliccorrosion only occurs when thetwo metals in contact have a dif-ference in potential of at least100 mV.

The aluminium alloys used inmarine applications have dissolu-tion potentials that are very closeto one another (table 66).

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DISSOLUTION POTENTIAL OF ALUMINIUM ALLOYS (NACL SOLUTION, H2O2, ASTM G 69)

Alloy Potential mV SCE 1050A – 750 3003 – 740 5052 – 760 5056 – 780 5083 – 780 5086 – 760 5154 – 770 5182 – 780 5454 – 770 5456 – 780 6005A – 710 6060 – 710 6061 – 710 6063 – 740 42000 (A-S7G03) – 820 51300 (A-G5) – 870

Table 66

DISSOLUTION POTENTIALS MEASURED IN NATURAL SEA WATER IN MOTION AT 25°C

Alloy Dissolution Potential (mV SCE) (*) Graphite + 90 Stainless steel – 100 Titanium- – 150 Inconel – 170 Cupronickel 70-30 – 250 Cupro-nickel 90-10 – 280 Bronze – 360 Brass – 360 Copper – 360 Lead – 510 Mild steel – 610 Cast iron – 610 Cadmium – 700 Aluminium – 750 Zinc – 1 130 Magnésium – 1 600

(*) mV SCE = millivolts, saturated calomel electrode. Table 67

(11) This accounts for the effectivenessof coating steel with zinc, and hence theuse of galvanised steel in manyapplications where it must be protectedfrom corrosion.

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In a marine environment thereforethere is no risk of bimetallic corro-sion between these alloys. This isalso true of casting alloys, and isthe reason why stiffeners in 6000alloys are frequently used weldedto plate in 5000.

6.4Practical aspects ofbimetallic corrosionof aluminium inmarine environments

Bimetallic corrosion is a verylocalised phenomenon and veryoften confined to the contactzone. In theory, the current den-sity that determines the rate ofdissolution of the anodic metal is afunction of the relationship:

cathodic surfaceanodic surface

Experience shows that this arearelationship is of no great rele-vance, as bimetallic corrosionoccurs primarily in or near the con-tact zone, and so the anode areamust be judged to be more or lessequal to the cathode area.

Expressed in practical terms, asmall component such as a screwor probe made from copper orcopper alloy and attached to alarge aluminium alloy structurewithout any insulation – and per-manently submerged – will causesevere bimetallic corrosion at thepoint of attachment.

Aluminium’s position on the scaleof potential (table 67) means thatin virtually every instance in whichit is joined to another commonmetal (12), it will be the anode ofthe resulting battery and so likelyto suffer bimetallic corrosion if theconditions are right.

To fully appreciate the risks of thebimetallic corrosion of aluminiumin a mixed joint, we must distin-guish between:

■ a submerged structure (quickworks)■ an emerged structure (deadworks) – the type of metal mustalso be considered in this case.

Note: Anodising affords no protec-tion from bimetallic corrosion.

6.5Submerged mixedcontacts

As we have already shown, whencontacts are submerged, bimetalliccorrosion of aluminium is unavoid-able when in contact with mostcommon metals (13). The intensityof this corrosion will depend on thetype of metal, on the compositionof the environment and on thelength of time if the contact area issubmerged occasionally ratherthan continuously.

The aluminium structure must beprotected, and there are a numberof ways of achieving this:■ Insulation, by inserting plasticseals between the two metals.Taking pipework as an example, agasket of sufficient thickness is fit-ted between the aluminium flan-ges of the pipe and the flange of astainless steel or brass valve.■ Painting: the surface of thecathode, i.e. of the other metal,must first be masked. Painting thealuminium only is far less effec-tive.■ Cathodic protection is the mostpractical solution (14) and the oneused most often to neutralise thegalvanic couples between a ship’saluminium hull and the propulsionsystem (shaft and screw) andother items attached to the hull(intake strainers, various accesso-ries).

Note. A steel tool such as a span-ner left behind in the bilge of avessel can cause bimetallic corro-sion if water is allowed to stagnatein the same area as the item madeof steel (or any other metal).

6.6Emerged mixedcontacts in open air

Many decades of experience withaluminium applications in marineenvironments, both on board shipsand in on-shore coastal installa-tions, show that contacts withmild steel, whether bare or coated(galvanised, cadmium plated), andstainless steel cause no significantbimetallic corrosion of aluminiumand its alloys (15) in the 1000,3000, 5000 and 6000 series (16).

10. CORROSION BEHAVIOUR OF ALUMINIUM IN MARINE ENVIRONMENTS

155

(12) Except cadmium, zinc andmagnesium. There will be no bimetalliccorrosion of aluminium in contact withfasteners made from galvanised orcadmium-plated steel. Instead, as longas there is zinc or cadmium on the steel,these metals will dissolve to protect it. (13) This will also apply to mixedcontacts in the bilges of a ship if waterstagnates there. (14) Cf. Chapter 11. (15) For a number of reasons: a layer ofcorrosion products (rust, alumina) formson the contact faces, inhibitingelectrochemical reactions as we haveseen.(16) In some cases copper alloys in the2000 and 7000 series are sensitised tocorrosion on contact with steel.

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156

As a result, such contacts are nowhardly ever protected on ships (17)or in coastal installations exceptobviously in cases where rust dis-coloration must be prevented onaluminium structures (in the caseof contact with steel). Today,steel/aluminium transition joints(18) are used to make welded con-nections between a hull and alu-minium alloys.

6.7Influence of the typeof metal in contactwith aluminium

The risk of bimetallic corrosionmay be greater when aluminium isin contact with certain metals andalloys.

Copper and its alloysWhile contact between aluminiumalloys and copper and its alloys(brass, bronze) causes no appre-ciable bimetallic corrosion, whenemerged structures are in a wetmarine environment it is advisablewhere possible to provide insula-tion between the two metals toavoid local deterioration in theappearance of the aluminium.

Verdigris however (the corrosionproduct of copper and its alloys)is aggressive towards aluminiumand its alloys and is reduced totiny particles of copper when incontact with them. These parti-cles in turn cause local pittingcorrosion in the aluminium, and itis therefore preferable to protectstructures that are beneath anaccessory that is made of cop-per, bronze or brass. The opti-mum solution is to paint them toprevent verdigris forming in thefirst place.

LeadLead is often used as ballast forpleasure craft. The combination ofaluminium and lead reacts verystrongly in sea water, and it istherefore essential to preventwater entering the keel if this isballasted with lead shot or piglead. The following procedure isusually used to prevent the risk ofbimetallic corrosion which can bevery severe (19):■ Carefully degrease the inside ofthe keel space that is accessiblefrom the bottom of the boat,■ Apply a layer of resin 15 to20 mm thick in the bottom of thekeel,

■ Once the resin has set, placethe shot (or pig lead) in position,carefully avoiding contact with thealuminium edge of the keel,■ Now pack the spaces aroundthe lead with resin up to a level 20to 25 mm above the ballast,■ Finally, access to the keelshould be sealed off by suitablemeans to prevent any ingress ofmoisture into the keel.

Lead ballast should never be laidon the bare hull, e.g. on the floorof a hold, but should be sealed in awatertight covering to prevent thebimetallic corrosion of the alu-minium and the entrainment oflead salts generated by that corro-sion in the presence of moisture.Lead salts will cause pitting corro-sion in those areas of the hullwhere they are entrained by mois-ture.

If the lead ballast is fastened tothe hull directly, it must be care-fully insulated from it, as indicatedin figure 132.

Lead-based paints should never beused to protect steel componentson board an aluminium vesselbecause lead oxides, like copperoxides, cause severe corrosion tothe aluminium in the presence ofmoisture.

MercuryAluminium in contact with mer-cury suffers very severe corrosionwhich takes the form of flakywhite ‘blooms’ that spread widelywhere the quantity of mercury issignificant or finely divided. Theeffect of mercury is all the moreinsidious as it tends to occur atlow points where fine droplets ofmercury accumulate.

156

ATTACHMENT OF LEAD OR IRON BALLAST

Aluminiumhull

Insulatinggaskets and

sleeves

Galvanised steel bolts placed priorto casting ballast

Ballast (leador cast iron)

Insulatingwashers➤

➤

➤

➤

➤

Figure 132

(19) All the careful precautions taken toinsulate the lead from the walls of thekeel will be wasted if the keel weldmentsare porous! Gradual penetration of seawater through cracks due to poor weldingwill cause irreparable corrosion to thekeel in the medium term.

(16) In some cases copper alloys in the2000 and 7000 series are sensitised tocorrosion on contact with steel.(17) For safety reasons (cf. Chapter 9),all metal masses on board a ship or on astructure intended to house electricalinstallations must be equipotentiallybonded. Even if contacts are insulatedfor reasons of corrosion or convenience,an electrical connection by means of acable or braided wire must still beprovided at one point.(18) Cf. Chapter 9.

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It is therefore most inadvisable tocarry mercury instruments such asbarometers or thermometers onboard a ship when spillage orbreakage might cause severe cor-rosion of the hull.

GraphiteAlthough graphite is not a metal(but a metalloid), contact with it ina wet environment causes bimetal-lic corrosion that is more or lessintense depending on the porosityof the graphite. It is therefore inad-visable to use graphite based lubri-cants on aluminium boats.

There is as yet insufficient experi-ence with the use of carbon fibreequipment on board aluminiumships to be able to draw conclusionsabout contact between aluminiumand carbon fibre composites.Laboratory tests with immersionin synthetic sea water show thataluminium suffers a mild and verysuperficial attack when in contactwith carbon fibre parts.

Note: When in port, craft shouldbe moored/anchored using ropesmade from hemp or syntheticfibres as this creates an ‘open cir-cuit’ between the aluminium craftand the metal anchor or bollardson the quayside.

7.ALUMINIUM TARNIS-HING AND BLACKE-NING

Aluminium in a humid atmosphereor in contact with sea water orfresh water tarnishes to a degreethat depends on the medium. Thisphenomenon, known as blacken-ing, is not a type of corrosion butmerely an alteration of the visualproperties of the film of naturaloxide. It does not produce anysubsequent sensitivity to corro-sion.

In a marine environment, blacken-ing can be prevented on certainitems by anodising or polishingthem. Anodising is by far the morelasting method, and is used onboat masts.

8.ROLE AND PREVEN-TION OF MARINEFOULING

Unlike salts of copper (or mer-cury), aluminium salts are not toxicto living organisms and they cantherefore easily attach themselvesto aluminium surfaces where theywill grow unchecked unless dis-lodged.

Such marine encrustations includebarnacles, corals, algae, spongesetc., but cause no appreciable cor-rosion to the underlying metalapart from a very superficial attackto a depth of a few hundredths ofa millimetre. Shells leave a virtuallyindelible impression on the metal.

As with other metals, antifoulingpaints can be used on aluminiumto prevent the growth of marineencrustations (20).

However there is one very impor-tant restriction: antifouling paintsbased on salts of copper, mercuryor lead cause severe corrosion (21)and should be avoided. Thisbimetallic corrosion is due to thereduction of copper, mercury andlead salts in contact with the alu-minium surface and can doirreparable damage to the hull of avessel.

10. CORROSION BEHAVIOUR OF ALUMINIUM IN MARINE ENVIRONMENTS

157

(20) Cf. Chapter 11.(21) The use of antifouling paints basedon minerals salts is actually banned.

WELDMENT ON A GANGWAY

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158

GOODBAD GOODBAD

➤Moisture

Moisture

➤

Watertight seal

➤

Watertight seal

➤

➤

Watertight seal

Exter

ior

Exter

ior

Area easy to cleanGood paintadhesion

➤

Area difficult toclean

➤

➤

Poor paint adhesion Continuous weldDiscontinuous weld

EFFECT OF DESIGN ARRANGEMENTS

Figure 133

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10. CORROSION BEHAVIOUR OF ALUMINIUM IN MARINE ENVIRONMENTS

9.EFFECTSOF WELDING AND DESIGNARRANGEMENTS

Design layouts can significantlyinfluence the corrosion perform-ance of aluminium alloy structuresin service.

9.1Effect of welding

Experience shows that the corro-sion behaviour of welded struc-tures made from alloys belongingto the 1000, 3000, 5000 and 6000series is similar to that of non-welded surfaces. In other words,weld seams and heat affectedzones will not be the site of pref-erential corrosion provided weld-ing is done according with the rel-evant codes of practice and withthe right filler alloys (22).

The same applies to adhesivebonding which does not affect thecorrosion behaviour of compo-nents joined by this process.

9.2Effect of designarrangements

Despite the excellent resistance tocorrosion of the alloys used inshipbuilding (and in marine appli-cations), instances of corrosionare occasionally seen in service.

An examination of these casesshows that it is most often thedesign layout that causes theseproblems. Figure 133 illustratesclassic cases of design layoutsthat are detrimental to good corro-sion performance.

Experience shows that areas wherewater stagnates or where dust (orsoot) can accumulate can suffervery severe corrosion in the form ofdeep pits and even perforations.

It is therefore essential to assistthe drainage of water and conden-sation on divisions and partitionssubject to temperature variations,and to provide as much ventilationas possible for spaces that aremore or less accessible (ballasttanks, holds etc.).These areas, as well as watertightbuoyancy chambers, must be reg-ularly inspected and may requiremore frequent maintenance thanthe rest of the structure.

Finally, anything that encouragesthe permanent presence of mois-ture, such as certain types of lin-ing, must be carefully avoided. Forexample, expanded foam (or insu-lation blankets made from rock-wool or similar materials) placedagainst an aluminium division forthermal insulation must be posi-tioned so that it cannot trap con-densation or attract moisture.

Floor coverings should also beadhesive bonded to prevent theingress of moisture between thecovering and the aluminium“floor”.

Note: Stray electrical currents canbe a determining factor leading tocorrosion of the hull surface incontact with sea water. As isstated in Chapter 10 (23), the hull(and any other metal structure onthe boat) must never act as areturn conductor for the current,whether this is d.c. or a.c.

10.SENSITIVITY TOCORROSION OFALUMINIUM ALLOYSIN MARINE APPLI-CATIONS

Many decades of experience withmarine applications of aluminiumhave shown that its resistance tocorrosion is remarkable.

Nevertheless: ■ some series of alloys are notsuited to this type of application, ■ production processes must bedesigned to avoid accidentally sen-sitising 5000 series alloys to struc-tural corrosion (intercrystalline cor-rosion, exfoliation corrosion orstress corrosion).

Finally, service conditions must alsobe controlled to ensure that certainlimits of temperature and time arenot exceeded. The H116 temper hasthe advantage of undergoingmandatory inspection for sensitivityto exfoliation corrosion.

10.1Choice of alloys formarine applications

While the 5000 and 6000 seriesalloys will be preferred for theirresistance to corrosion, their weld-ability and their level of mechanicalcharacteristics, other wroughtalloys belonging to the 1000 and3000 series are suitable for use innon-structural applications for dec-oration, interior fittings etc.

The use of 2000 and 7000 seriesalloys on the other hand must bethe exception in view of their poorresistance to corrosion, and theyrequire special protection whenused in a marine environment.

The 2000 and 7000 series copperalloys cannot be welded with theclassical arc processes. The 7000series alloys without copper, e.g. 159

(22) Cf. Chapter 6.(23) Chapter 9.

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7020 or 7108, are easy to weldhowever, and present a high levelof mechanical characteristics afterwelding, an attractive attribute thatcould be an advantage for sheetmetal working in general and ship-building in particular.

Even so, the heat affected zone ishighly sensitive to exfoliation cor-rosion in all environments, somuch so that the development ofthese alloys has had to be aban-doned except in very specificapplications out normal fields.

In the absence of any significantprogress in the control of sensitiv-ity to exfoliation corrosion, it is notpossible with our present level ofknowledge (24) of the metallurgyof these alloys to use them innaval construction without specialprotection [5].

The casting alloys commonly usedin marine applications are the42100 (A-S7G03), the 42200 (A-S7G06) and the 51100 (A-G3).

10.2Sensitivity of 5000series alloys to intercrystallinecorrosion

It has been well known since theearly 1950s that aluminium-mag-nesium alloys can have a tendencyto intercrystalline corrosion, themore so the higher the level ofmagnesium, which is why indus-trial alloys do not normally containmore than 5.5 % magnesium.

This sensitivity is due to thechanges in the distribution of themagnesium in the metal in thesolid state. The solubility of mag-nesium in aluminium is very highat high temperatures – 15 % at450 °C – but is no more than 1 %at ambient.

When the temperature falls below450 °C, the magnesium precipi-tates in the form of intermetallicsAl3Mg2 (or Al8Mg5), commonlyreferred to as “β phase”. But whencertain conditions occur theseprecipitates can collect on thegrain boundaries, and the metal isthen “sensitised”. A micrographexamination soon reveals whetherthe alloy is sensitised or not (fig-ure 134).

The precipitation of the β phase onthe grain boundaries has a ten-dency to be continuous, and willbe the more rapid and moredense:■ the higher the level of magne-sium,■ the higher the service tempera-ture, and■ the greater the degree of strainhardening.

As figure 135 shows, the tempera-ture range between 125 and 225 °Cis one of high sensitisation, andsensitisation can start at even lowertemperatures for the alloys with themost magnesium, over 5%.

As we have already seen, theseintermetallics are anodic relativeto the mass of the grain, theirpotential is –1150 mV SCE, a dif-ference of some 400 mV from thesolid solution, which is consider-able. There is therefore a risk ofintercrystalline corrosion if theirprecipitation is continuous on thegrain boundaries and if the envi-ronment is corrosive.

160 (24) June 2003.

MICROGRAPHIC APPEARANCE OF SENSITISED ALLOY 5083

5083 Sensitised State 5083 Non-sensitised StateFigure 134

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Temperature is not the onlyparameter that governs the precip-itation of the β phase, as is oftenand mistakenly forgotten (25).Time, i.e. the period spent at tem-perature, is also a factor whenevaluating the intensity of sensiti-sation. This should be obvious, asthe rate at which the magnesiumatoms migrate towards the grainboundaries obeys the laws of dif-fusion in the solid state (the alu-minium matrix in this case). It isdependent on temperatureaccording to the classic equation:

τ = t exp – (Q/RT)where:τ = the distance coveredt = timeT = absolute temperatureQ is a constant that depends onthe element.

Too often, the limit at which 5000alloys with more than 3.5 % mag-nesium can be used has been setat 65°C (26). It is in fact the productof “Time xTemperature” that mustbe taken into consideration. On a5086 for example, it will take2 years at 65°C or several monthsat 100°C to cause continuous pre-cipitation on the grain boundaries.The effect of time is cumulative.

The sensitisation of these alloys –which is always avoidable – can bedue to manufacturing conditionsor service conditions when theyare held at temperature for longperiods.

Much research has been done onall of these parameters since the1950s [7]. Sensitisation can beinhibited by thermal treatmentswhich cause discontinuous “pearlnecklace” precipitation of the βphase on the grain boundaries [8].For many years now, the produc-tion processes for alloys 5083,5086 and Sealium® – includingthe H116 temper – have been spe-cially adapted for shipbuilding [9] toavoid the supply of semi-finished

products that are susceptible tocorrosion. These alloys are sys-tematically controlled by the selec-tive tests described below.

We should remember howeverthat the H116 temper will not pre-vent precipitation of the β phaseon the grain boundaries when theservice conditions can cause it.This temper is used very widely inshipbuilding.

10. CORROSION BEHAVIOUR OF ALUMINIUM IN MARINE ENVIRONMENTS

SENSITISATION OF 5000 ALLOYSAND MAGNESIUM CONTENT [6]

0,5

075 100 125 150 175 200 225 250 (°C)

After 8 hours immersion inNaCl 3 %, HCI 1 %

Loss

ofma

ss

g.dm-2

Mg-5,59 %Mg-5,15 %Mg-4,60 %

➤

➤

➤

(25) It is not because a metal issensitised by precipitation at the grainboundaries that makes corrosioninevitable, it depends on theenvironment. Experience has confirmedthis at temperatures well above 65°C,including in heat exchangers operating insea water. There are road tankerscarrying heavy fuel oil, loaded at 65°C,that have been in service for 20 years ormore, with 8 to 10 hours of rotation aday, i.e. at least 50,000 hours of totaltime at 65-70°C. But obviouslysensitisation must be avoided.

(26) In the document “AD-Merkblatt W6/1” of May 1982, published by theVereinigung der technischenÜberwachungsvereine e.V., D 4300Essen 1, entitled “Aluminium andaluminium alloys malleable materials”,the limit is set at 80°C for alloyAlMg4.5Mn, equivalent to the 5083, withtolerances of 150°C for periods notexceeding 8 hours, provided the servicepressure is reduced by half and 24 hoursif the service pressure is reduced toatmospheric.

Figure 135

MARINA PONTOON AND FLOATS MADE OF 5754

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11.CORROSION TESTS

European standard EN 13195-1 (27)states that alloys 5083, 5383 and5086 supplied in the H116 temper“in the form of sheet, strip andplate must be tested to assesstheir resistance to intergranular cor-rosion and exfoliation corrosion.”

This test is either the ASSET test(ASTM G66-99) which determinessusceptibility to exfoliation corro-sion, or any other method agreedbetween supplier and buyer.

Section 9 of draft standard ASTMB924 04 relates to the 5059,5083, 5086, 5383 and 5456 alloysin the H116 and H321 tempers.Susceptibility to exfoliation corro-

sion is measured with the ASSETtest (ASTM G66-99) and suscepti-bility to intercrystalline corrosionwith the ASTM G67 test (NAMLT).

11.1Exfoliation corrosiontesting

This is the ASSET test (28) whichis conducted according to ISO11881 (29) and ASTM G66 (30) onsamples whose dimensions(length and width) are at least 40 x100 mm. The conditions of thistest are summarised below:

After surface preparation bydegreasing, alkaline pickling andnitric neutralising, the specimensare immersed for 24 hours at65 °C in the specific reagent of the5000 series (31):

NH4Cl - 1 M : 53 g.l-1NH4NO3 - 0,25 M : 20 g.l-1(NH4) 2C4H4O6 - 0,01 M : 1,84 g.l-1H2O2 - at 30 % : 10 ml. l-1

Susceptibility to exfoliation corro-sion is assessed against typicalimages (figure 136) and result in agrading as shown in table 68.

CLASSIFICATION OF EXFOLIATION CORROSION WITH THE ASSET TEST

From figure 6 of NF ISO 11881 Standard, June 2002 Figure 136

(27) European standard EN 13195-1,December 2002, “Aluminium andaluminium alloys – Wrought and castproducts for marine applications(shipbuilding, marine and offshore).(28) ASSET = Ammonium Salt SolutionExfoliation Test (29) ISO 11881, June 2002, Corrosion ofmetals and alloys – Exfoliation corrosiontesting of aluminium alloys. (30) Standard test method for visualassessment of exfoliation corrosionsusceptibility of 5XXX series aluminumalloys (ASSET test).(31)NH4Cl = ammonium chloride, NH4NO3 = ammonium nitrate,(NH4)2C4H4O6 = ammonium tartrate,H2O2 = oxygenated water.

EVALUATION OF RESULTS OF THE ASSET TESTEvaluation Grading

No significant attack N Pitting corrosion P Generalised corrosion G Exfoliation corrosion EA, EB, EC, ED

Table 68

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11.2Test forintercrystallinecorrosion of the 5000 alloys

Two tests can be used to deter-mine the susceptibility of 5000 alu-minium alloys to intercrystallinecorrosion:

■The NAMLT tests (32) accordingto ASTM G67 (33).

The samples are immersed in a 70to 72 % solution of nitric acidHNO3 at 30 °C for 24 hours, andweighed before and after the test.

According to standard B928-04(34):- the batch is passed when themass loss is < 15 mg.cm-2,- the batch is rejected when themass loss is > 25 mg.cm-2,- for mass losses between 15 and25 mg.cm-2, the form of corrosionmust be determined by micro-graph examination after phos-phoric attack. The batch is rejectedif the corrosion proves to be inter-crystalline.

■ The INTERACID test accordingto the protocol published in theJournal of the EuropeanCommunities on 13/09/1974, Nos.C104/84 to 89.

Here the reagent consists of asolution made from 30 g.l-1 of NaCland 5 g.l-1 of 37 % HCl (d =1.19).The samples are immersed in thereagent for 24 hours at 23 °C.After determining the mass loss(weighed before and after thetest), the samples are examinedunder a microscope with a magni-fication of x200.

These tests can be conducted onsamples heated to 100 °C for 7days (35).

11.3Test forintercrystallinecorrosion of the 6000 alloys

These alloys are not susceptible toexfoliation corrosion. Any suscep-tibility to intercrystalline corrosioncan be determined by the testaccording to ASTM G110 (36) inthe reagent with 57 g. l-1 de NaClet 0,3 % de H2O2.

Bibliography[1] Corrosion de l’aluminium, C. VARGEL,502 pages, Dunod, 1999.[2] “Chemical aspects of physicaloceanography”, J. LYMAN, R. B. ABEL,Journal Chemical Education, Vol. 35,1958, pp. 113-115.[3] “Effect of the temperature and saltcontent of sea water on the corrosionbehavior of aluminium”, W. HUPPATZ, H.MEISSNER, Werkstoffe und Korrosion, Vol.38, 1987, pp. 709-710.[4] “Corrosion de l’aluminium souscouche mince électrolytique, corrosion àla ligne d’eau et corrosion au stockage”,C. FIAUD, G. AZERAD, E. GROSGOGEAT,Journées “Durabilité de l’aluminium etde ses alliages dans les industriesélectriques”, Cefracor Paris, December1986, pp. 33-37.[5] “Experience from the constructionand operation of the Stena HSS”, H.NORDHAMMAR, STENA REDERI, The ThirdInternational Forum on Aluminium Ships,Hauguesund, Norway, May 1998. [6] “Influence de la teneur enmagnésium sur la résistance à lacorrosion sous tension des alliages Al-Mg”, A. GUILHAUDIS, Pechiney SREPCReport, September 1959.[7] “Development of Wrought Aluminium-Magnesium Alloy”, E. H. DIX, W. A.ANDERSON, M. B. SHUMAKER, TechnicalPaper, No. 14, Alcoa, 1958.[8] “Traitements thermiques destabilisation des alliages aluminium-magnésium à 5% contre les effets dechauffage à basse temperature”, A.GUILHAUDIS, Revue de l’Aluminium, 1955,Nos. 223 and 224.[9] “Aluminium-Magnesium alloys 5086 and5456 H116”, C. L. BROOKS, Naval EngineersJournal, August 1970, pp. 29-32.

10. CORROSION BEHAVIOUR OF ALUMINIUM IN MARINE ENVIRONMENTS

(32) NAMLT = Nitric Acid Mass LossTest.(33) ASTM G67: Standard test methodfor determining the susceptibility tointergranular corrosion of 5XXX seriesaluminum alloys by mass loss afterexposure to nitric acid. (34) B 928 - 04: Standard Specificationfor High Magnesium Aluminum AlloySheet and Plate for Marine Service,February 2004.(35) According to the formula τ = t exp–(Q/RT), 7 days at 100 °C is equivalentto 6 months at 60 °C. (36) ASTM G110: Standard practice forevaluating the intergranular corrosionresistance of heat treatable aluminumalloys by immersion in sodium chloride +hydrogen peroxide solution.

NO ALUMINIUM CORROSION

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CHEVY TOY