The Corrosion of Archaeological Iron During Burial and Treatment

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The Corrosion of Archaeological Iron during Burial and TreatmentAuthor(s): S. TurgooseSource: Studies in Conservation, Vol. 30, No. 1 (Feb., 1985), pp. 13-18Published by: Maney Publishing on behalf of the International Institute for Conservation ofHistoric and Artistic WorksStable URL: http://www.jstor.org/stable/1506129 .

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THE CORROSION OF ARCHAEOLOGICAL IRON DURINGBURIAL AND TREATMENT

S. Turgoose

Abstract-The corrosion processes occurringinarchaeologicalron rtifactsre discussedn the ight fpublishedmeasurementsfelectrodeotentialsndpH,and therelevancef this o the election fconservationproceduress considered.nparticulart sconcludedhatwet anaerobictorage f ronworkhould e successful,and hatorrosion ay ontinuen odium ydroxideolu-tions.

1 Introduction

In recentyears

therehas been alarge

amount ofpublishedwork oncerninghe tructuref corrodedarchaeological ronwork1-6] and gradually con-siderabledegree of agreementhas been reached.However,this s onlya preludeto thedevelopmentofmoreeffectiveonservation rocedures, ince forthisto occurwe need to understand he processesthatgiverise to these structuresnd theeffects fvarious treatments.Much less attentionhas beengiven o this ield nd,perhapsfor hisreason, hereisnot the ame consensus fopinion. n particular,recentpublicationbyNorth 3] has suggested hathydrogen volution s themajor cathodicreactionoccurringduringthe corrosion of iron in marineenvironments,nd that corrosionceases immedi-atelyupon immersion f ironworkn sodiumhyd-roxide olutions.Both of theseconclusions re con-traryo thosereachedpreviously ythe author 1, 7]and ifcorrectmay considerablyffect he choice ofstorage nvironment r treatmentmethod.

Ifhydrogenvolutionsthepredominantathodicreactionduringburial t follows hatstorage n ananaerobic environment illallow corrosion o con-tinue at the same rateafter xcavation.However, foxygen reductionis the cathodic reaction thenremovalofoxygenwillmarkedly educe the corro-sion rate, enablinganaerobic environments o be

used safely or ong-term torageofironwork.Whether orrosioncontinuesduringwashing n

sodiumhydroxideolutions sobviously fconsider-able importancewhenconsideringhis s a possibletreatmentmethod.Notonlywouldcorrosion ead tometal oss but tmay lso reducethe ease of chlorideremoval 7] and lead to the existence fair-unstableferrous ompounds t the end of treatment.

Thus both of these conclusionshave importantconsequences nconservation nd theargumentsor

Received 7August983

and against,hem need to be examined. Since theproposals of Northarise frompotentialand pHmeasurementsn corrodedmarine ron, t sworth-while summarizinghe relevantpoints,especiallysincethevalidity f the results s not ndispute.

2 Potential and pH measurements n corrodedmarine ron

North 4] measuredpH profiles cross sectionsoffreshlyxcavated concreted annonballs nd found

that hepH rosefrom -8toapproximately-0mov-ing awayfrom he metal urface o the outer dgeofthe ron-stained oncretion.At the nnerregions fthe corrosionproduct, n contactwith the pH 4-8solution, he solid phases presentwere magnetite,FeO,4, and siderite, eCO,.

Macleod [2] found imilar H variations orbothcast and wroughtron; in all cases the pH at themetal surfacewas 4-8. Potentialsmeasuredat themetal surfacewere -0-412, -0-316 and -0-247vfor the threeobjects. The potential, n all cases,increasedwith istance wayfrom hemetal urface,reaching alues of0-016to0-040vat the outer on-

cretionwhere hepH was that fsea-water, -2. Healso reported hloride concentrationst the metalsurface f 1-1-5M.

After mmersionn sodiumhydroxide olutionsthemeasuredpotentials fdeconcreted rtifactsregiven s -0-607 (at pH 12-9) [2] andbetween 0-6and -0-72v (in 0-5MNaOH) [3].

The potential measurements of Macleod [2]confirm heapplicabilityfthermodynamiconsid-erations o a study farchaeological roncorrosionproducts.twaspreviouslyuggested 1] that hepHof 4-8 found n the Batavia cannonballs 4] rep-resented heequilibrium etweenferrous arbonate

and approximatelyM ferrousons.In

this ase thechloride, rtotal nion, oncentration ouldbe 2M,close to thatreported, nd thepotential hould bewithin he range -0-409 to -0-242v, in excellentagreementwith thatmeasured.AlthoughMacleod[2] didnotanalyzethe solidspresent, ll theobjectswere concreted and it seems likely that FeCO3would form romreactionof ferrousons withtheoriginal alcium arbonatematrix ftheconcretion.

It is agreed [1-3] that the solution withinthegraphitized egionof corrodedmarinecast iron isessentially ferrous hloride solution whose con-

Studiesn Conservation0 1985) 13-18 13

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S. Turgoose

centration s 1M or greaterwithpH 4-8, and thatboth thepotential ndpH increasewith istance utintotheconcretion. his evidenceprovides goodbasis for discussion f theprocessesoccurring.

3 The cathodicreaction uringburialCorrosionis an electrochemical rocesswiththeanodic and cathodicreactions ccurringt spatiallyseparate points. n ironcorrosion he inodic reac-tion 1) mustobviously ccurat themetalsurface.The cathodichalf-reaction an be eitherhydrogenevolution 2) or oxygenreduction3) or occasion-allythereduction fanother pecies.

Fe ---- Fe2 ++ 2e- (1)2H+ 2e- H2(2)

or 2H20 + 2e- ---- H2+ 20H (2)

02 + 4H + 4e- ---- 2H20(3)or 02 + 2H20 + 4e- 40H- (3)

Northbases hispresent uggestionhat 2) is themaincathodicreaction n threefactors.1 No correlation etween dissolvedoxygen evels

and corrosion ateshasbeenobserved, utthereis also no evidencethat such a correlation oesnot exist.

2 A gas is releasedwhen concretions re brokenunderwater,n situ,although henatureof thegas isnotmentioned. earson 8] has shown hatimmersion f corroded cannon in sodiumhy-droxide solutions caused release of nitrogen,methane and hydrogen, nd whilstthere aremechanisms vailable to explaintheformationof hydrogenn the alkaline solution, heothergases,at least,must resumably ave beenpres-ent at excavation.

3 Calculationsof corrosionrates based on equa-tions 1) and (2) givereasonable greementwiththose observed [2]. AlthoughMacleod foundgood agreementfor two objects out of threestudied,detailsofhow hiscalculated rateswerearrived t were notpublished.He quotescalcu-lated rates f10-30 /Acm-2, depending nhyd-rogenpartial pressure, ora cannon potential-0-412v, pH 4-8) correspondingo a corrosiondepthof 2-6cm for he timeof mmersion. heactual corrosion depth was 2-5cm. Similaragreementwas obtained for a wrought-ironanchor but no calculations re quoted for thethird amplewherenone of the measured oten-tials was low enoughforhydrogenvolution obe a possiblereaction.This in itself asts doubtontheconclusions, ut themajorproblemsthatthe ratesdiffer onsiderably rom hosecalcu-latedbelow.

There have been manykinetic nd mechanistic

studiesof thehydrogenvolution eaction nd theyhave been reviewedelsewhere 9, 10]. This workindicatesthat,except at very highoverpotentials,the reaction n ironproceedsvia twosteps, 4) and(5), where Hads ndicates an adsorbed hydrogen

atom.H++ e- -- Hads (4)

Hads+ Hads - H2 (5)The firsttep,hydrogenon discharge4), is rate-determiningnd thereaction roceedswithowsur-face coverageof adsorbedhydrogen. n thisbasisthe calculated dependence of hydrogen volutioncurrent ensity,ic A cm-2), s givenby 6) and thisrelationshipasbeendemonstratedyKelly 11],in0-5M sulphate olution verthepH range0-4.

ic= kCH+ exp - EF/2RT) (6)

Equation (6) and Kelly'sresults ivenequation 7)

for c,whereE is the electrodepotential elative othe standardhydrogenlectrode.

log ic = -5-12-pH-E/0-118 (7)ThusforthecannonstudiedbyMacleod thecalcu-lated hydrogenvolution urrents 0-37 A cm-2,and is notdependent n thehydrogen artialpres-sure,since theelectrodepotential s sufficientlyarbelowthereversible otential. hisratecorrespondsto a penetration ateof less than1mmduring hetimeof burial,farbelow thatobserved,althoughroughness f the metal urfacemayraisetheactualsurfacearea, the corrosion urrent nd the calcu-lated

enetrationate o omextent.

Thus,whilehydrogenvolution t themetal ur-face will contribute owardsthe cathodicreaction,the evidence vailableatpresent oes notappeartojustify he conclusionthat t is the maincathodicreaction.

The possibilityof sulphate-reducingbacteriacatalyzinghisreactionhas also been suggested, utthe locationof the reducedsulphur pecies in theconcretionends osuggest hat his snotoccurring.North 4] found ulphides ndsulphurn the oncre-tion,where thepH was 5 or more,but not at themetal surface.At thispart of the concretion heconditions re suchthathydrogen volution s not

thermodynamicallyossible, o thesulphide annotresult rom irectnvolvement fthebacteria nthecathodicreaction. t is possiblethat hebacteria reusinghydrogen, roduced at the metalsurface, oreduce ulphate, utthiswillnot ncrease herateofhydrogenvolution. incethere s a plentifulupplyoforganicmaterial rom heconcretionrganisms,tseemsmore ikely hat hebacteria reusing rganicelectrondonors in the reduction f sulphate.Theformationf ferrous ulphidecan be explainedbyprecipitationf thesesulphide ons byferrousonsdiffusingwayfrom hemetalsurface nd does not

14 StudiesnConservation0 (1985) 13-18







The orrosionf rchaeologicalron uringurialndtreatment

necessarily ndicate that 'microbial corrosion' isoccurring.

Althoughthe argumentsn favour of hydrogenevolution s themaincathodicreaction re incon-clusive,the experimental vidence on whichthey

werebased does enable some conclusions s to thenatureof theprocesses occurringo be drawn.Boththepossiblecathodicreactions ause a local

rise in pH, byproduction f hydroxideons or byremoval fprotons.At the anodic areas thepH mayfalldue tohydrolysiseactions uch as (8) and pre-cipitation fhydroxidesuch as (9).

Fe2+ + H20 = FeOH+ + H+ (8)FeOH+ + H20 = Fe(OH)2 + H+ (9)

A fall npH to a value below thatofthebulksolu-tion can only occur if the solution that becomesacidic is in contactwith a predominantlynodicarea. Thus North 3, 4] and Turgoose 1] have con-

cludedthat thepH of 4-8 across the metal surfaceindicates hat heentiremetal urfacesacting s theanode, the cathodicreactionoccurringomewherewithin heconcretion emote rom he urface.Mac-leod's calculations f reactionrateswerebased onhydrogenvolution ccurringn the metalsurface,since t sonly here hat hepotential s lowenoughfor he reaction o bepossible.On the otherhand, foxygen reduction were the cathodic reaction, twould be expectedto occur remotefrom he metalsurface.Oxygendiffusingn from utsidethe con-cretion an be reduced s soon as itreaches levelatwhich there s sufficientlectronic onductivityn

thesolidphase for lectrons oflow o the cathodicsites. Since magnetites a majorcomponent f thecorrosionproducts, nd possesses good electronicconductivity, e would expect oxygenreduction ooccur some distance from he metal,and this willgiverise to the observedpH variations,ncorrodedobjects.

Thus whilst hepotential nd pH measurementsdo not appear to be consistentwith the idea thathydrogen volution s the major cathodic reactiontheyare exactlythose that would be expected ifoxygenreductionwere.As a consequence,storageof excavated ronworknoxygen-freeolutionswill

reduce the rate of corrosionto well below thatoccurringduringburial and, since the damagingoxidative hangeswill lso be prevented,onstitutesa safemethod.

4 The corrosion f ron n sodiumhydroxideolu-tions

It has oftenbeen claimed thatarchaeological ronartifacts ill notcorrode further8-12] or that thecorrosionratewillbe very ow [13] if mmersednsodium hydroxide olutions,but these argumentshave been justified y reference o the undoubted

inhibitive roperties f aerated sodiumhydroxidesolutions owards'clean' ron, .e. that overedby, tmost, an air-formed xide film.The validityofapplying hese results o artifacts overedby thickcorrosion roducts s notobvious,sinceto function

as a corrosion nhibitor ydroxideonsmustbe pre-sent n sufficientoncentrationt all pointson themetal surface. The major problemwithwashingmethodsforchlorideremoval from orroded ron-work s thatequilibration etweenthe solution atthe metal surface,the location of much of thechloride t excavation, nd that n the bulk of thewash-batholution s slow, nd there s no reasontosuppose thatdiffusion f hydroxide ons into thecorrosion roductswillbe anyfaster handiffusionofchloride ut. It canthusbe arguedthat nhibitionof corrosion yhydroxideons s not to be expectedbefore he state s reached t which oththesurface

hydroxide nd chlorideconcentrationsqual thosein thebulkand washing s complete.Macleod has claimedon the basis of the results

quoted above thatinspection fthe Pourbaixdiag-ram for ronshows that implewashinghas shiftedthe cannonfrom regionof active corrosion o apassivezone'. Thisargumentssumes hat hepH atthe metal urface s the same as inthe bulk solution(12-9) and can thuseasilybe rejected.

RecentlyNorth [3], acceptingfor the reasonsabove that corrosion ould be expected during heinitial stages of immersion n NaOH solutions,statedthat thisdoes nothappen npractice nd thereason for

fhiss found in the E values for the

artifact'.The reportedpotentialsfordeconcretedmarine

cast ironobjectsin sodiumhydroxideolutions rebetween -0-6 and -0-72v (vs. S.H.E.). Northargues hat ecause themeasured lectronic esistiv-ity fgraphitized amples s low

(.01Ofcm)all areas

of the artifact,ncluding he residualmetal,are atthe samepotential' nd thus hat he metalpotential(- 0-6vatpH 4-8) is intheregion f mmunity,ithhydrogenvolution ccurringn the metalsurface.The consequencesof this re that:

1 theresidual ron willnotcorrode;2 hydrogenwillbe evolved;and

3 hydroxideons will be generateddeep insidethe corrosion roducts.

It is claimedthat1) and 2) areobserved nprac-tice but, as regardsthe statement hat it can beobservedthat rondoes not corrode nNaOH solu-tions,tshouldbe stressed hat ack of visible hangedoes notnecessarilymean thatno change s occur-ring. n thiscase, where we wouldexpectdiffusionofhydroxideons intothe corrosionproducts, nyfresh orrosionproductswillbe precipitatednsidetheobjectand so notbe visible. n view of thefactthat corrosion will result n iron compounds that

StudiesnConservation0 1985) 13-18 15







S. Turgoose

maybe rapidly xidizedon exposureto air, evennon-visible orrosionmay markedlydecrease thepost-treatmenttabilityfartifacts. he evolution fhydrogen an be, and has been forarchaeologicalmaterial 81, ttributed o thereduction fwaterby

ferrous ydroxide,he Schikkor eaction, quation(10), althoughthe mechanism s apparentlymorecomplicated hansuggested ythis quation[14].

3Fe(OH), = Fe3O4 + 2H,O + H2 (10)The generation fhydroxideonswithin he cor-

rosionproductss stated o continueuntil hepH ofthe entrappedsolutionreaches approximately 0.At this H levelthe ron snow na region fpassiv-ity nd,even if E subsequentlyncreases, orrosionwill be minimal'.The suggestions therefore hatcorrosion of ironworkceases immediatelyuponimmersionn sodiumhydroxide olutions, nd willnotrestartwhilst he mmersionontinues.

If thiswereso it would be a considerable dvan-tageofthis reatmentmethodbut North'sproposalsconcerningheprocesses ccurringeem to be basedon a misconceptions to the nature of electrodepotentials, nd there s no justificationorthecon-clusion hat he ronpotential alls ntotheregion fimmunity.

The essential artof the rguments that incetheelectronicresistivityf the graphitized amples islow,all areas of the artifact re at the samepoten-tial. Whatthe owresistivityctuallyndicatessthatthere s negligible otentialdifferenceetweenthegraphitend themetal nthe olidphase. Itdoes not

indicate hat he electrodepotentials fthe ron ndthegraphite re thesame, and it is these electrodepotentials, .e. potentialdifferencescross solid-solutionnterfaces,hatdeterminehenature f theelectrodereactions hatmayoccur.

In any system f this sort wherethere are twoelectrodes in solution there are four sources ofpotentialdifference:1 in the solid phase between the iron and the

graphite, Fe-Eg, although, s is usual with woconductinglectrodes, his snegligible wing othe ow resistance fthe solidphase;

2 between the graphiteand the bulk solution,

Eg-Eb, which n this case is themeasuredelec-trodepotentialof the graphitized bject, sincethe referencelectrodewillbe placedin thebulksolution, lose to thegraphite urface;

3 betweenthe metallic ron core and thesolutionat themetal urface,EFe-Es,

theelectrode oten-tial of themetallic ronabout whichwe wouldlike some information;nd

4 between he olution t themetal urface nd thebulk olution,Es-Eb,

which ependsontheresis-tanceofthesolution nd the current lowing.

Since thepotentialdropbetweenthemetalcoreand the bulk solution mustbe independent f the

pathtaken we getequation 11)

(EFe- Eg) + (Eg - Eb) = (EFe - Es) + (ES -Eb)(1l)

or

E'= E'

-iR (12)g FewhereE'g and E'Fe are theelectrodepotentials fthe graphite nd ironrespectively, is the resis-tance per unit area of the solutionpath betweensurface nd bulk,and i is theconventional urrentdensity lowingn solution towardsthemetalsur-face. Positive meansa flow fcationstowards hemetalsurfaceor of anions away from t; thusthemetal urfacescathodic othegraphite.) hemetalelectrodepotential, 'Fe, only equals the measuredpotential, 'g, ifthere s no solutionresistance, utin view of the low diffusionatesof chloride onsthrough raphitized egions 15] and theextrabar-

riers o onicmovementhatwillbe imposedbypre-cipitationf ronhydroxides ithinheseregions,nextremely ighresistance s to be expected.

The lowpotentialmeasured ftermmersionsthemixedpotential dopted bythesystemnwhich hecathodicreactionsoxygen eduction nd the nodicreaction s oxidation f ferrous ydroxide r mixedoxidation tategreenrusts, othreactions ccurringat orveryneartheouter urface f thegraphite. heinitially igherpotential t the metalsurface ndi-cate that thissurfacewill be a net cathode (i inequation 12) is positive)but the rateat which hecathodic eaction ccurswilldependon the solution

resistance nd thedifference etween he two elec-trodepotentials.In the absence of solutionresistance, quation

(12) shows the assumption f Northto be correct,but in this ase therewouldbe otherconsequencesthatdo notseem tobe borne out inpractice.A fallin the metal electrodepotentialmustbe accom-paniedbyreductioni.e. plating ut) of ferrousonsinitially resent n solution. The requirement orchargeneutralitynthe olution t the metal urfacemeans that this must be accompanied by eitheringressof anothercation,Na+ in thiscase, or byoutwardmovement f chloride ons, or both.The

highmobilityf chloride ons indicates that their

movementwillmake themajorcontributiono thisionicmovement,o a rapidremoval f chloride romthe object should be achieved. The long washingtimesused in sodiumhydroxidereatmentsuggestthat his s not o. On thecontrary, e woulddeducethatthere s considerablehindrance o ionicmove-ment n solution, nd that he solutionpath posses-ses appreciableresistance.

In thepresenceofresistance,he metalelectrodepotentialwillbe higher han hatmeasured, nd themetalwillbe a net cathode. However, the effectsthat thiswill have on the artifact epend on the

16 StudiesnConservation0 (1985) 13-18







The orrosionf rchaeologicalron uringurialndtreatment

magnitude f the current lowingn solution bet-ween metal ndgraphite.At the metal urface hereare threepossibleelectrodereactions hatwillcon-tribute o the netcurrent,ne anodic:

Fe--- Fe2 + + 2e- (13)

and twocathodic:Fe2+ + 2e- ---Fe (14)2H+ + 2e----+ H2 (15)

and if thepartialcurrent ensities ue to these arecalled i13, i14 and i15 the requirement or a netcathodicreactionmeans that:

i15+ i14- i13 > 0 (16)or:

is5 >i13

-i14 (17)

wherethe right-handide of equation (17) is the

corrosionrate of the metal core. The rate of hy-drogen onreduction,is,

has alreadybeen shown obe extremelyow, and the corrosionrate of theresidualmetal, althoughnot necessarily ero, willalso be extremelyowinthe nitial tagesof mmer-sion. This low corrosionrate does not, however,arise fromany inhibitive ropertiesof hydroxideions,orfrom epression f themetalpotentialntothe regionof immunity,ut merelyfrom he factthattheoxygenbeingreducedat thegraphite ur-face is oxidizingferroushydroxide r greenrustsrather hanmetallic ron.

Eventually hestagewill be reachedat which heferrous ons

presentat excavationhave all been

oxidized,and themeasuredpotential f theobjectwill rise. Northreports hisrise forwroughtron

objectsafter fewweeks' immersion,ut notafterseveralyearsfor argecast ironartifacts,eflectingthemuch arger mount f ferrousons nitially re-sent n the atter ase. The potential ise meansthatthe outer surfacebecomes cathodicto the metalcore,whichcan now corrodeat an increasedrate,controlledby the rate of oxygenreduction n theoutersurface.

Northhas claimed that if the pH at the metalsurfacerisesabove 10 the iron willbe passiveandwill not corrode ven fthepotential ises.Whether

thepH at themetal urface oes riseto 10 is uncer-tain since hydroxideons diffusingn towardsthesurfacewillbe precipitatedythe ron onspresent,and hydrogenvolution t themetalwill notoccurfast noughto affect hepH noticeably. ven ifthis

pH rise does occur, t is unlikely hatpassivitywillresult, or everalreasons.Firstly,xperimentalas-sivationpotentials re muchhigher approximatelyOvat pH 10 [16], than those measured.Secondly,inhibitionyhydroxides due totheformationt aferric xidefilm n themetal urfaces, nd thisusu-allyrequiresthepresenceofoxygen t the surface

which s not ikelynthis ase. Thirdly, ydroxidesonly neffectivenhibitorf he olution H isabove12-6,presumably ecause thissolutionconcentra-tion is needed to maintain the local surfacepHabove the critical alue of about 9.

The processes occurring after immersionofdeconcreted ronwork n sodium hydroxide olu-tions can thus be summarized. n the initial tagestheoxygen eaching heouter surface ftheobjectwillbe usedfor he oxidation fferrous ompoundsderivedfrom heferrousonspresent n solution tthe timeof excavation.These ferrous ompounds,ferrous ydroxidergreenrusts, an easilybe oxid-ized byair,so cessation f treatment hilethey restillpresentwill yield an object thatmay sufferdamaging xidative hanges fexposed tomoist ir.The durationof this nitialstage depends on theamount f ferrousonpresent t the timeofexcava-tionand in the case ofmarine ast iron ow

poten-tials have been measured, nd greenrust dentifiedinthecorrosion roducts,fter evenyears' mmer-sion in 0-5M NaOH. Duringthisstage the rateofcorrosionwill be low, but once the ferrous om-poundshave been oxidized theoxygen eaching heartifactwillonce again be used to oxidizemetalliciron, .e. as partof theusual corrosion ell. The largespatial separationof the anodic and cathodic ites,and the difficultccess of hydroxide o the metalsurface,willprevent nyinhibitive ctionby thesesolutions.Ferrouscompoundswillthuscontinue obe produced nside hecorrosion roduct ayers, ndtheir

presenceat the end of treatment

mayead to

instability.

5 Conclusions

Potentialand pH measurements an be helpful nunderstandinghereactions ccurringnarchaeolog-ical material, utonly fthepossible imitations fthemeasurements re recognized. n particularheeffect hat thickcorrosionproduct ayershave onbothpH andelectrodepotentialsmakes the pplica-tion of the deas ofimmunitynd passivity ifficultsincethe relevant onditions re notthose that anbe measured outside the artifact.

However,an

understandingfthenature fthefundamentalro-cesses does enable some informationobe deducedas tothereactions ccurringt themetal urface ndthelikely ffect ftreatmentmethods.

Acknowledgements

Thisworkwas carriedout while theauthorwas atUniversityollege,Cardiff,ndhe isgratefulo theSERC forfinancial upport nd to ProfessorR. D.Gillardand Dr D. Leighfortheir ncouragement.

StudiesnConservation0 1985) 13-18 17







S. Turgoose

References

1 TURGOOSE, ., The natureofsurvivingronobjects'in Conservation of Iron, National MaritimeMuseum Monograph No.53, National MaritimeMuseum,Greenwich1982) 1-7.

2 MACLEOD, I. D., 'Shipwrecks and applied elec-trochemistry', .Electroanal.Chem. 118 (1981)291-303.

3 NORTH,N. A., 'Corrosionproducts n marine ron',Studies n Conservation 7 (1982) 75-83.

4 NORTH,N. A., 'Formationof coral concretions nmarine ron', nt.J.Naut. rchaeol. 5 (1976) 253-258.

5 ARGO,J.,On thenature f ferrous orrosion ro-ductson marine ron',Studies n Conservation 6(1981) 42-44.

6 GILBERG,M. R., and SEELEY,N. J., The identityfcompounds ontaininghloride ons in marine roncorrosionproducts--a criticalreview',Studies in

Conservation 6 (1981) 50-56.7 TURGOOSE,., Post-excavation hanges n ironanti-quities', tudies n Conservation 7 (1982) 97-101.

8 PEARSON, ., 'The preservationf cannon after 00years under the sea', Studies n Conservation17(1972) 91-110.

9 SHREIR, . L., inCorrosionVol.I (ed. L. L. SHREIR),2nd edn,Newnes-Butterworth1976) 9:49-9:52.

10 BOCKRIS,J. O' M., 'Electrode kinetics' n ModernAspects f Electrochemistryed. J.O'M. BOCKRIS),Butterworths1954) 180-277.

11 KELLY, E., 'The active iron electrode',J. Electro-chemical ociety112 (1965) 124-131.

12 PEARSON, ., 'On the conservation equirementsormarine archaeological excavations', Int.J.Naut.

Archaeol. 6 (1977) 37-46.13 NORTH,N. A., and PEARSON, ., Investigationsnto

methodsforconservingron relicsrecoveredfromthe sea' in Conservationn Archaeology nd theAppliedArts, IC, London (1975) 173-181.

14 SCHRAUZE,G. N.,andGUTH, . D.,'Hydrogenevolv-

ing systems : the formation f H2 from queoussuspensionsof Fe(OH)2 and reactionswith re-ducible substrates ncludingmolecular nitrogen',J.Am.Chem.Soc. 8 (1976) 3508-3513.

15 NORTH,N. A., and PEARSON, ., 'Washingmethodsfor chlorideremoval frommarine ron artifacts',Studies n Conservation 3 (1978) 174-186.

16 SHREIR, . L., inCorrosionVol.I (ed. L. L. SHREIR),2nd edn, Newnes-Butterworth1976) 1:103-1: 110.

S. TURGOOSE raduated nmetallurgyt CambridgeUni-versity,wherehe also obtained a PhD on the subjectofcorrosion nhibitors. rom 1978 to 1984 he was a SERCpost-doctoralresearchassistant n the DepartmentsofChemistryndArchaeologytUniversityollege,Cardiff,and afterworkingn theConservation ivisionoftheBrit-ishMuseumhe is now a lecturern corrosion cience andengineeringn the Corrosion nd Protection entre,Uni-versityfManchesternstitute fScienceandTechnology.Author's ddress; UMIST, PO Box 88, ManchesterM601QD, UK.

Resume--Les processus de corrosion des objetsarcheologiques n fersont discutes ci, h a lumieredespublications ur les mesuresdes potentiels 'electrodeetdes pH, en relation vec le choixdesmethodes e conser-vation.On conclut n particulierue le stockaged'objetsen fer dans un milieuhumide et anaerobie est possible,alorsque la corrosion eutse mainteniransdes solutionsd'hydroxyde e sodium.

Auszug--Es werden die in archiologischenEisenwerk-zeugen auftretenden orrosionsprozesse nterBeriick-sichtigung eriffentlichter essungen von Elektroden-

spannungen ndpH und der Relevanz derselbenfiirdieAuswahl von ErhaltungsprozedurenrSrtert. nsbeson-dere wirdgefolgert, ab die feuchte, naerobe Lagerungvon Eisenwaren erfolgreich ein miiBte,und daB dieKorrosion in Natriumhydroxyd-L6sungenortgesetztwerdenkann.

18 Studies n Conservation 0 (1985) 13-18

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The Corrosion of Archaeological Iron During Burial and Treatment