The Mechanism and Kinetics of In Situ Conservation of Iron Cannon on Shipwreck Sites

The Mechanism and Kinetics of In Situ Conservation of IronCannon on Shipwreck Sites

Ian D. MacLeodWestern Australian Maritime Museum, Victoria Quay, Fremantle, WA 6160, [email protected]

Analysis of the pre-disturbance values of the in situ corrosion parameters on historic iron shipwrecks and artefacts hasestablished that the arithmetic product of the pH and corrosion potential is dependent on the burial environment and providesa unique insight into the objects’ state of decay. The value of the product changes during in situ conservation treatment withsacrificial anodes, and reaches a minimum at which point the treatment is completed. Treatment times vary with water-depth,being faster on shallower sites and shorter for more extensively corroded artefacts. The model was developed using data fromthe Duart Point wreck (1653), the Monitor-styled warship HMVS Cerberus (1926) and a series of wrecks in Australia and theUSA.

© 2012 The Author

Key words: Duart Point wreck, HMVS Cerberus, predicted treatment times, sacrificial anodes.

The use of sacrificial anodes for the in situstabilization of corroded iron artefacts on his-toric shipwrecks is well established, but there

has been a dearth of information assessing the effec-tiveness of the process. A recent review of the treat-ment of guns from the breastwork of Monitor-styledHMVS Cerberus (1926) alongside the wreck-site inPort Phillip Bay, Australia, provided the catalyst toassess the outcomes of treatments from a number ofsites in Scotland, the USA and Australia. Experiencehas shown that owing to the normal operationalimperatives of recovering historically significantobjects, having them conserved and placed on publicexhibition, many of the objects have been recoveredbefore effective completion. Until this point the lack ofdata on completed in situ conservation projects hastended to result in the methodology being undervaluedby heritage managers. This work provides a newinsight into the chemical and electrochemical processescontrolling the corrosion and conservation of ironobjects from historic shipwrecks. The review has pro-duced a series of relationships that allows for the use ofa predictive model that will provide maritime archae-ologists and heritage managers with a tool for moni-toring the progress of in situ treatments and how todetermine when the objects have been stabilized.

Cannon at the Duart Point wreck (1653)The Cromwellian shipwreck at Duart Point, Isle ofMull, Scotland, has turned out to be one of the mostintriguing 17th-century shipwrecks to have been

subject to any excavation and site-managementprogrammes in recent times (Fig. 1). Colin Martin hasdemonstrated a remarkable capacity for detailed docu-mentation and analysis of shipwrecks and the reader isreferred to the many publications regarding the natureof the complex matrix of wooden, metallic and ceramicobjects which have revealed the true character of thevessel and its crew (Martin, 1995; 1998; 1999; 2004).

The first in situ corrosion measurements oniron artefacts on the site were conducted in 1994at the request of Colin Martin, the archaeologicaldirector. The scope of the in situ assessment was the

Figure 1. Maps showing the location of the Duart Pointwreck, Sound of Mull, Scotland. (Colin Martin)

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International Journal of Nautical Archaeology (2012) ••.••: ••–••doi: 10.1111/1095-9270.12002

© 2012 The Author. International Journal of Nautical Archaeology © 2012 The Nautical Archaeology Society.Published by Blackwell Publishing Ltd. 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA.

determination of the depths of graphitization of the ironguns, the pH of the corroding metal interface and thecorrosion potential (Ecorr) of the artefacts in conjunctionwith the water-depth. The long-term corrosion-rates ofcast-iron objects can be gauged by drilling into thegraphitized cast iron and measuring the depth of pen-etration (mm) and dividing that figure by the number ofyears since the vessel was wrecked. The initial datademonstrated high corrosion-rates associated with theinherently aggressive nature of the wreck-site owing to acombination of the high flux of dissolved oxygen asso-ciated with the strong ebb tide which at times runs at 3knots (Smith, 1866; MacLeod, 1995; Robertson, 2007).A site plan with the corrosion rates of the cannon,expressed as dg in mm/year, is shown in Figure 2.

One of the recommendations arising from the firstsurvey was that some degree of stabilization could bebrought about through passive methods, such as sand-bagging, and this was done before the second siteassessment in 1995. The in situ values of the corrosion

potential (Ecorr) and the pH of the guns and anchor intheir pre-disturbance state and during conservationtreatment with sacrificial anodes are listed in Table 1.Inspection of the data from the 1994 and 1995 pre-treatment conditions shows that the voltages of theiron objects varied significantly. Using the site-specificcorrosion equation for the Duart Point wreck,

log dg Ecorr= +3 70 0 228. . (1)

it is possible to calculate the difference in corrosion ratebetween the most and least aggressive parts of the wrecksite (MacLeod, 1995). Thus the separation in voltagesshown in Table 1 equates to a 67% difference betweenthe corrosion rates in 1994 which was reduced to adifference of 55% after the sandbagging experiment.

The 1994 survey recorded data on five guns andthe anchor while two more cannon were assessedin the following year after the sandbagging opera-tions had been conducted. Analysis of the depths of

Figure 2. Site-plan of the small Cromwellian warship off Duart Point, Isle of Mull, Scotland. Cannon numbers and dg in red.Cannon 7 lies 3m NE of the crown of the anchor and had a dg of 0.124. (Colin Martin)

Table 1. In situ corrosion parameters for the Duart Point wreck cannon and anchor before and during treatment

1994Ecorr vs.Ag/AgCl



1998Ecorr vs. Ag/AgCl

1994pH

1995pH

1997pH

1998pH

cannon 1 -0.452 -0.495 -0.578 -0.602 5.42 5.69 6.90 7.96cannon 2 -0.505 -0.521 -0.678 -0.636 5.90 5.86 7.10 8.36cannon 3 -0.503 -0.506 -0.672 -0.517 5.63 6.56 7.40 6.88cannon 4 -0.532 -0.507 -0.574 -0.640 6.65 7.62 7.40 8.40cannon 5 -0.512 -0.511 -0.711 -0.711 6.85 7.51 8.00 8.58cannon 6 -0.518 -0.626 6.85 8.48cannon 7 -0.509 -0.632 6.92 8.29anchor -0.479 -0.470 -0.530 -0.566 4.61 4.61 6.90 7.83

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graphitization of the cast iron indicated that thecomplexities of the wreck-site were reflected in threedifferent corrosion micro-environments; three guns hada median corrosion-rate of 0.184 � 0.022 mm.y-1, twocannon had a median value of 0.122 � 0.002 mm.y-1andtwo had a median value of 0.042 � 0.033 mm.y-1. Whenthe guns were examined in 1997 by David Gregory here-measured the depths of graphitization and based onhis data the extent of corrosion was significantly less,with the three groups giving corrosion-rates of 0.113 �0.014 mm.y-1, 0.075 � 0.006 and 0.044 � 0.004 mm.y-1

(Gregory, 1999). The objects in the lowest corrosiveenvironment have the same rate of decay as determinedby MacLeod in 1994, but the other data-sets indicate anapparently lower corrosion-rate. The apparent conun-drum is resolved by looking at comparative materialfrom the wreck of the Batavia (1629), wrecked onMorning Reef in the Abrolhos Islands in Western Aus-tralia. Data collected during conservation of the exca-vated cannon showed that in many cases the large gunshad significantly different values for the corrosiondepths at the muzzle compared with the cascabel end,which is simply due to the gun experiencing differentcorrosion micro-environments (Carpenter andMacLeod, 1993). The reported corrosion profiles relateto the depths of graphitization at the muzzle and thecascabel, and three Batavia cannon gave BAT 8720 (23and 48 mm), BAT 8726 (32 and 16 mm) and BAT 8723(33 and 54 mm) as the respective corrosion profiles.Thus the 60% variation in the re-measured data-pointson the Duart Point wreck is normal and much less thanthe ratio of two times for some of the Batavia guns.

One advantage of using the depths of graphitizationof the iron cannon as an indicator of corrosion-rates isthat it averages out the variations over the centuries ofimmersion since the time of wrecking. The disadvan-tage of this approach is that it does not provide atimely indicator of the present rate of deterioration,which can be dramatically influenced by activities suchas rock-falls from underwater cliffs or the presence offast-moving ferries and the wash from their propellers.Since the pH is directly related to the steady-state con-centration of iron corrosion-products under the con-cretion layer, the pH readings provide a highly linkedindicator of the present corrosion-rate (MacLeod,2006) (Fig. 3). Thus it is found that while the historicgraphitization data indicated that there were three cor-rosion micro-environments on the site, the pH datafrom 1995 showed only two distinct groups, which is areflection of the more recent changes that haveoccurred on the site (Martin, 1995). When the pH datawas plotted as a function of the annualized depth ofgraphitization dg it was found to follow the generalrelationship shown in Equation 2,

log d a c pHg = − (2)

where a is a constant dependent on the averagecorrosion-rate which is interdependent on the water

depth and the value of c indicates the degree ofsensitivity of the corrosion-rate to the pH which is alsodependent on the water flux over the corroding con-creted object. In the initial site survey cannon 1 and 2were among the most actively corroding objects whilecannon 5 was the least, but one year later, after sand-bagging had significantly reduced the corrosion-rate ofthese two guns, the scouring in and around cannon 5had elevated its long-term corrosion-rate to make thethree cannon into a similarly related data set (seeTable 2). In a similar fashion the other four cannonwere found to be corroding at a slightly faster rate andthis is consistent with the previous groupings wherecannon 6 and 7 were in the moderate long-termcorrosion-rate group and cannon 3 was in the highercorroding category. The site-plan shows that cannon 3is physically close to cannon 1 and 2 and so its formerlyvery high corrosion-rate has been ameliorated by thesandbagging but not to the same extent as withcannons 1 and 2 (Fig. 2). The difference between thetwo sets of data is best seen by calculating the apparentcorrosion-rate at the median 1995pH of 5.77 whichshows that the least-reactive cannon are corroding at arate of 0.18 mm.y-1 while the other four guns are cor-roding at an estimated rate of 0.39 mm.y-1. This calcu-lated rate is much higher than the mean value of 0.12 �0.05 mm.y-1 based on the long-term data from the cor-rosion profiles, and this indicated that the then rate ofcorrosion was unacceptable and that some form ofintervention was needed to stabilize the site.

Site intervention consisted of placing sandbags nearthe most rapidly corroding cannon in July 1995, whichsaw the corrosion-rate fall by 20%, but in areas bycannon 4, where there was no sandbagging activity, thecorrosion-rate had increased by 43% as a result of sitescouring (see Table 1) (MacLeod, 1998). Although the

Figure 3. Diver measuring the pH of a cannon on the DuartPoint wreck-site in 1997. (Colin Martin)

I. D. MACLEOD: MECHANISM AND KINETICS OF IN SITU CONSERVATION OF IRON CANNON

© 2012 The Author. International Journal of Nautical Archaeology © 2012 The Nautical Archaeology Society 3

pH measurements reflect the localized corrosion-rateand the variations in the dg have already been dis-cussed, the steeper slope of the relationship forcannons 1, 2 and 5 is probably a reflection of theirlocation, closer to the foot of the underwater cliff,which is going to be more responsive to changes inwater-movement. The differences in the interceptvalues for the two equations are less than the sum ofthe standard deviations associated with the linearregression analyses, and so they are not statisticallysignificant. All that this means is that the long-termcorrosion-rate is reasonably consistent across the siteand is in line with the variations in the pH readings.The main point to note from the above relationships isthat accurately recorded pH data provides a very clearindicator of the amount of corrosion that is occurringat a particular area on the wreck-site at that time. Thusiron artefacts move from being regarded as ‘wretchedobjects’ requiring so much careful conservation tobeing the champion of the maritime archaeologists asthey provide so much supporting data about thewrecking processes and the present rate of deteriora-tion and the long-term historical data on deterioration.Although the underwater determination of degrada-tion in timbers using a pilodyn can provide historicaldata on the rates of decay on an archaeological site(Gregory et al., 2007) there is no instrument thatallows for an instantaneous reading of the decay-rateof wood, leather, glass or ceramics.

Being mindful of the successful deployment of sac-rificial anodes on the carronade and anchor on thewreck of HMS Sirius (1790) on Norfolk Island(MacLeod, 1996b), the project-leader decided to installa series of zinc sacrificial anodes to minimize seriouscorrosion of the guns and to begin the in situ conser-vation programme. The results of the initial measure-ments have been reported by David Gregory (1999),which showed that some of the anode attachments hadbeen fully effective but that others needed adjustmentsince they had lost good electrical contact with theartefacts (Fig. 4). The data reported in Table 1 includesthe measurements on the protected cannon in 1998,from which a detailed analysis of the rate at which theguns were stabilized can be developed. However it isacknowledged that the addition of more detailed timestudies on a number of sites would assist in the valida-tion of the model. It is proposed to conduct trials oncast-iron artefacts both in protected areas of shipwrecksites and among materials recovered and undergoingtreatment in the laboratory to compare the effective-

ness of anodes versus storage in sodium hydroxidesolutions (pers. comm. Paul Mardikian, 22/03/2011).

Kinetics of stabilization, Duart PointWhen an anode of zinc or an aluminium alloy isattached to an historic iron object the anode begins tocorrode at an accelerated rate, and the driving force forthis reaction is due to the differences in the electro-chemical reactivity of the cast or wrought iron and thebase metal of the anode. The current density flowingthrough the insulated copper wire from the anode,through the concreted artefact and out to the dissolvedoxygen in the surrounding seawater will depend in parton the ratio of the surface area of the anode to the gun.Maritime archaeologists are referred to cathodic pro-tection manuals for more detailed advice on determin-ing the size of anode relative to the surface area of the

Table 2. Analysis of the corrosion-rate as a function of surface pH (1995) as per Equation 2

Artefact Slope∂∂log ipH Slope error

Interceptvalue

Intercepterror R2 value

Cannon 1, 2 & 5 -0.55 �0.04 2.42 �0.24 0.9798Cannon 3, 4, 6 & 7 -0.41 �0.05 1.95 �0.33 0.9741

Figure 4. Diver reconnecting the anode bracket after dis-covering poor electrical contact in 1997. (Colin Martin)



cannon or other iron object they are seeking to protect(Morgan, 1993). Because all the zinc anodes were ofthe same size and configuration the experimental datareported in Table 1 provides good examples of howeffective the anodes can be, and the relatively commonratio of the surface areas of the guns to the anodesenables the responses of the artefacts to be directlycompared with one another. As electrons flow into thecannon and anchors a combination reaction occurs inwhich initially free hydrogen ions are reduced tohydrogen gas, with a concomitant increase in the pH,and any iron (III) compounds are reduced to magne-tite, Fe3O4 according to a formalized Equation 3,

3 3 3 23 4 2 2 FeO OH H e Fe O H O H. + + → + + ↑+ − (3)

When all the cannon and anchor data are reviewed(see Table 1) it can be seen that there was a progressiveremoval of the acidity built up from the underlyingcorrosion process so long as a good electrical connec-tion between the anode and the object remained. Whenthe pH of the iron artefacts was plotted against thelength of time that the anodes had been attached, eachartefact showed a linear increase in pH and the slopesare dependent on the dg values (the long-termcorrosion-rate). The exceptions were cannons 3 and 4which had connector problems between the anodebracket attachment and the gun, which were correctedafter the measurements were taken. The rate at whichthe pH increased as the value of dg increased, as shownin Equation 4;

∂∂

= ( ) − ( )pHt

dg4 47 0 54 0 00 0 0. . . .7 86 (4)

The linear regression for Equation 4 had an R2 of0.9315, and the standard errors associated with theslope and the intercept are shown in parenthesis. Theunits of the rate of change in the pH are pH.y-1 wherey is the number of years of the in situ treatment. Duringdetailed site recording and some urgent recovery workswhen wooden and other delicate artefacts were at risk,an eighth cannon was unexpectedly found. Routineexamination of the gun was conducted by drilling intoit to obtain a corrosion profile, and it was found to beremarkably little corroded. This is shown dramaticallyin Figure 5 which shows the backscattered secondaryelectron image of a seaward surface of the gun—thescale bar is one millimetre and the dark patches relateto corrosion loss. After consultation with HistoricScotland it was decided that this gun should be recov-ered without any pre-treatment and conserved in thelaboratories of National Museums Scotland in Edin-burgh and prepared for an exhibition of artefacts fromthe site in time for the 360th anniversary of the wreckin 2013. This smaller gun turned out to be a drake, alightweight tapered-chamber design dating from the1620s made by John Browne, gunfounder to Charles I(Martin, 2004). A metallurgical and chemical analysis

of the gun confirmed its atypical microstructure whichhas essentially no ferritic phase, which is partly respon-sible for the very low corrosion-rate. The microstruc-ture is dominated by the pearlite I and II phases withlarge concentrations of iron phosphide and manganesephosphide inclusions (Preblinger et al., 2012).

If the pH and Ecorr data measured in 1998 after thesuccessful application of anodes to the cannon andanchor are plotted on a Pourbaix diagram (Pourbaix,1974), the slopes of the Ecorr vs. pH graph have anaverage value of -0.087 which is consistent with thefollowing corrosion reaction taking place, viz.,

Fe H O HFeO H e+ → + +− + −2 3 22 2 (5)

Equation 5 has a predicted slope of − ∗32 0 0568. or

-0.085 at a seawater temperature of 13°C, and so theobserved value of the slope is the same as the theoreti-cal value for Equation 5, which strongly indicates thatthe low passive state corrosion process taking placewhile the cannon are being treated is oxidation of ironto produce the hydrogen-ferrate ion HFeO2

- and notthe normal ferrous ions associated with corrosion inthe absence of anodes. The corrosion process is nolonger controlled by the oxidation of iron to produceFeCl+ species (Man-Seung, 2004) but it now has a veryslow corrosion-rate under alkaline conditions.

Treatment of guns from HMVS CerberusCorrosion of the hull of the former HMVS Cerberus(1926) in Port Phillip Bay, Australia, saw the vesselundergo a major collapse in 1994. Constructed in 1870and once the pride of the Victorian colonial navy, shewas Australia’s first capital ship, a twin-turreted float-ing gun-battery, which was sunk as a breakwater in1926. The wreck now rests on its 8″–10″ armour belt onthe sea-bed following a second stage of collapse in 1999(Nicholls, 2001; Steyne and MacLeod, 2011) (Fig. 6).Owing to the advanced state of decay of the relativelyfinely constructed hull, the two sets of 10″ Armstrong

Figure 5. Scanning electron micrograph of a section fromthe John Browne foundry showing minimal corrosion. Scalebar is 1 mm. The roseate black material is graphite and thedark grey areas represent corroded iron.



rifled gun-barrels had been removed from the forwardand aft gun-turrets of the vessel which had sufferedadditional collapse in 2005. This intervention was nec-essary to minimize the load on the decaying, buoyanthull, as each gun weighed 16 tonnes.

Prior to removing the guns with a floating crane andplacing them in the sea alongside the vessel, the barrelswere drilled and tapped near the cascabel end in orderto receive the 316 stainless-steel bolts that connectedthe insulated cables from the zinc anodes. Insulatingheat-shrink butyl mastic material had been placed overthe exposed iron core of the anodes prior to them beingdeployed on the sea-bed to avoid current leakagethrough the exposed iron core rod. The Ecorr and pH ofthe guns were periodically monitored over a period of27 months, at the end of which the gun-barrelsappeared to be stable (Fig. 7). During the following sixmonths there was no substantial change in pH or thevoltage, and it was thought that the treatment of theatmospherically corroded guns might have been com-pleted (MacLeod and Steyne, 2011). However, there isno published criterion that demonstrates how suchdeterminations are effected, other than the very inter-ventive process of recovery of the artefact followed bywet chemical analysis of the drilled sample core. Thefollowing approach provides a tool that will enablefuture site directors to make informed decisions on themanagement of artefacts undergoing in situ cathodictreatment on their wreck-sites. A summary of the datapertaining to the in situ treatment of the guns is foundin Table 3.

Since the gun-barrels had only suffered from 60 yearsof atmospheric corrosion there were no measurablecorrosion profiles, unlike those found on the cast-ironguns of the Duart Point wreck. Similarly as there was noexperimental data on the way in which the corrosion-rate varied with voltage and pH for the type of steel usedin the construction of the guns, some alternative form ofassessment of the progress of the treatment had to bedetermined. Plotting the Ecorr and pH data on tradi-tional Pourbaix diagrams gave relationships that werecharacterized by steep slopes that did not have anychemical or electrochemically valid interpretation. Thisis in part due to the artefacts undergoing treatment notbeing in a state of equilibrium. The site-specific equa-tion for the HMVS Cerberus, which was developed froma series of experimental data (MacLeod, 1996a) is givenby Equation 6:

Figure 6. View of Cerberus awash at high tide, lying on itsarmour belt. (Heritage Victoria)

Figure 7. Divers taking pH measurements on the anodesattached to one of the Cerberus guns. (Heritage Victoria)

Table 3. In situ corrosion parameters for Cerberus guns during treatment on the sea-bed

ObjectsEcorr 15/03/06vs. Ag/AgCl

Ecorr 03/06/09vs. Ag/AgCl

Ecorr 14/01/10vs. Ag/AgCl

pH15/03/2006

pH03/06/09

pH14/01/10

gun 1 -0.810 -0.863 -0.976 7.68 8.14 n.d.gun 2 -0.808 -0.920 -0.820 7.87 8.15 n.d.gun 3 -0.821 -0.633 -0.934 7.66 8.04 8.18gun 4 -0.754 -0.933 -0.861 7.81 8.14 8.29anode 1 -0.958 -0.946 -0.980 5.89 7.67 7.24anode 2 -0.893 -0.960 -0.820 6.79 7.78 6.87anode 3 -0.839 buried -0.920 6.59 buried 6.43anode 4 -0.724 buried -0.912 5.98 buried 6.62



logdg corr= +3 29 E 286. .0 (6)

Thus Equation 6 illustrates that the corrosion-rate,measured in mm.y-1, is dependent on a power serieswhich contains the value of the corrosion potential.Thus plots of the logarithm of the reversed polarity ofthe Ecorr could be used as an indicator of how thecorrosion-rate was changing as a function of the surfacepH measured on the gun-barrels undergoing treatment.The polarity of the Ecorr is reversed to enable determina-tion of its value, since logarithms are only possible forpositive numbers. The data in Table 3 was analysed bylinear regression and it was found that the voltage andthe pH of the gun-barrels were linked according toEquation 7:

log E pHcorr−( ) = − +2 00 0 24. . (7)

The intercept value of -2.00 � 0.17 and the slopevalue of 0.24 � 0.02 shows that the data-set has a verygood fit for the linear regression with an R2 of 0.9763for the regression Equation 7. As the corrosionpotential falls the logarithm of the value of (-Ecorr)increases and the corrosion-rate decreases with theincreasing pH, thus the linkage between the two in situvariables is established.

Determining the end of in situ treatmentPre-disturbance surveys on many iron artefacts showthat shallow wreck-sites have lower pH and lesscathodic (less negative) values of Ecorr while more benignsites have higher pH and smaller Ecorr values. The solu-tion micro-environment underneath the concretionconsists of FeCl+ in equilibrium with a small percentageof FeCl2 and a trace of FeOH+ (Man-Seung, 2004).When the arithmetic product of these two in situ param-eters is evaluated it is found that it is remarkablyuniform, as seen in Table 4. The coherence of the data isdue to the fact that all the iron artefacts are corrodingwith a common mechanism and this arithmetic producthas been named the ‘Corrosion Indicator’ or CI for

short. A summary of the CI values for iron artefactsmeasured on a series of historic shipwrecks on which theauthor has worked is given in Table 4.

The observation that the median pre-disturbancevalue of the artefacts is -3.6 � 0.4 points to the value ofusing the CI as an indicator of the effectiveness of theapproach, since all the objects have a commonstarting-point, regardless of the water-depth, thewater-temperature or the amount of dissolved oxygenin the seawater. It is these variables which normallydefine the differences in the rates of deterioration ofartefacts on historic shipwreck sites. By monitoring theCI value for artefacts undergoing treatment it is pos-sible to develop a set of data that provides a practicalguide in determining the end-point when the bulk ofthe chlorides have been removed from the object, thesurface pH has become more alkaline than the sur-rounding seawater, and the overall corrosion-rate hasfallen to quite low values.

Data from the Sirius wreck-site showed that whenthe carronade was recovered after 31/2 years onthe sea-bed approximately 85% of the chloride ionshad been removed from the gun, with the balance beingremoved under standard laboratory conditions(MacLeod, 1996b). When the as recovered CI value of-6.0 is normalized to 100% chloride removal, the finalCI value is -7.1 which is within the range of the valueof -7.4 � 0.3 for the four guns from the Cerberus wreckat the apparent end of its treatment programme. Whenthe Sirius anchor was recovered after one year of in situtreatment the CI value was -5.1, and it was subse-quently treated using conventional electrolysis in asodium hydroxide solution which removed the remain-ing chloride ions. A series of analyses enabled theauthor to estimate that the first stage had removedabout 70% of the chloride ions thus normalization ofthe Sirius CI anchor gave a final value -7.4 for theequivalent of a fully conserved object (MacLeod,1987). While the data-set is limited to a few valueswhere artefacts have been quantitatively analysed forchloride following treatment with anodes, there is asatisfactorily small spread of values for the CI value atthe beginning and the ‘end’ of a treatment, which

Table 4. Corrosion Indicator (CI) values for iron artefacts undergoing in situ treatment

Treatmentstage

Duart Pointcannon

1653

HMS Siriuscarronade

1770

HMS Siriusanchor1770

JamesMatthews

knees1841

USSMonitor

turret1862

H. L.Hunley

hull1864

City ofLaunceston

hull1865

SS Xanthoengine1872

HMVSCerberus

guns1926

Before -3.2 -2.9 -3.1 -3.7 -3.3 -3.6 -3.9 -3.3 -4.1During -4.8 -6.0 -5.1 -6.9 — -4.9 -5.5 -6.3End -5.3 -7.1# -7.4* n.d. — — n.d. -7.4

# HMS Sirius carronade was 85% conserved during 21/2 years on the sea-bed at a CI of -6.0.* HMS Sirius anchor was 69% conserved during 1 year on the sea-bed at a CI of -5.1.n.d. not determined.



indicates a commonality of approach. It is apparentthat the Cerberus gun-barrels are now completed afterapproximately three years of stabilization, so nowthe task of Heritage Victoria is to either keep theanodes in functional order on the sea-bed or recoverthe guns and place them in their stabilized condition ina land-based facility, with appropriate protective coat-ings on them.

When iron artefacts corrode in a buried or partlyburied marine micro-environment there is a differentcorrosion mechanism with the anodic or oxidation stepproducing magnetite Fe3O4, as shown in Equation 8,and the cathodic process involves reduction of eitherhydrogen or water. The different mechanism is reflectedin different pre-disturbance values of the CI or corro-sion indicator. Examples of this type of micro-environment are found for the WWI submarine HMASAE2 (1915) in the Sea of Marmara, and the submarineResurgam (1880) off the coast of North Wales where themedian CI value for the two submarines was -4.6 � 0.1,which is typical of a partly stabilized, aerobically cor-roded iron artefact. This supposition is supported bythe proposed Equation 8 which has magnetite as areaction product from the corrosion of marine ironunder a low-oxygen environment:

3 4 8 82 3 4Fe H O Fe O H e+ → + ++ − (8)

The hydrogen ions produced in the reactiondescribed in equation 8 will be concomitantly reducedto molecular hydrogen as the electrons flowing throughthe protected cannon are consumed (Gregory, 2000;MacLeod, 2010). Although the SS Xantho was periodi-cally buried and exposed, all the pre-disturbance valuesrelate to the extended aerobic micro-environment witha CI value of -3.3 for the historic Penn of Greenwichengine (MacLeod, 1986; McCarthy, 1988). Data listedin Table 4 from the H. L. Hunley submarine wasprovided courtesy of Paul Mardikian, conservator incharge of the project at the Clemson Universityconservation laboratory in Charleston, South Carolina(Mardikian, 2004). Information pertaining to theconditions on the USS Monitor (1862) site camefrom a combination of studies by the NOAA teamand from laboratory-based measurements on recov-ered artefacts (Arnold et al., 1991; MacLeod et al.,2008). All the other experimental data in Table 4 hasbeen collected by the author during various fieldworkoperations.

The effect of depth on treatment timeWhen the CI values for concreted corroded marineiron objects undergoing in situ treatment were plottedagainst treatment time there was a steady decreasetowards more negative values as the treatments pro-gressed. Using the data from the Duart Point wreckin Table 1, it was found that the CI value became

increasingly negative as time increased according toEquation 9:

CIDuart = − −3 35 0 72. . t (9)

where t is the treatment-time in years and the R2 value of0.9602 was associated with the linear regression shownin Equation 9. If the amount of time for completing thein situ conservation of the object(s) is required thensubstitution of the end CI value of 7.4 into the aboveequation gives a predicted treatment time of 5.65 years.Through periodic measurements of the normal in situcorrosion parameters it is possible to determine thepercentage of the conservation programme that hasbeen achieved by the artefact(s). Since it is now well pastthe predicted treatment-time for the guns and anchor atthe Duart Point wreck-site, the in situ conservationtreatment will have been completed and the site kept ina state of suspended animation until there is a compel-ling need for additional excavation and a home for theconserved artefacts is established as part of a properlycurated exhibition in a venue that is fit for the purpose.Alternatively a dedicated museum might be built onMull to tell the remarkable story of this vessel and theill-fated attempts by Cromwellian troops to quell therebellious Scots. The archaeological director reportsthat the anodes are now corroding away at a muchreduced rate which also supports the understandingthat the conservation process has been completed (pers.comm. Colin Martin, 31/03/2011).

Similar analyses of data on the speed at which the CImoved towards the end-value of -7.4 � 0.3 was con-ducted for the Sirius, Xantho, Cerberus and other sitesshowed that the corrosion indicator moved fastertowards the completion value as the depth (d) becameshallower in accordance with Equation 10:

∂ ⋅∂

= − +−CI y

dd

1

1 39 0 18. . (10)

This relationship means that in situ conservationtreatments will work fastest at shallower depths sinceincreased wave action, brought about by wind-drivenevents, will increase the flux of dissolved oxygen to thecorroding anodes and make them work more effi-ciently. This relationship enables conservators, site-managers and maritime archaeologists to predict howfast the artefacts can be treated based on the water-depth in metres. It should be noted that the DuartPoint wreck-site does not follow this general equationsince its rate of decrease in the corrosion index had aslope of -0.72 V.pH.y-1 which is equivalent to an Aus-tralian water-depth of 3.7 m and not the observeddepth of 11.5 m. The wreck at Duart Point is charac-terized by strong diurnal currents as the tide flows andebbs. Historically this increased flux of dissolvedoxygen resulted in unexpectedly high corrosion-rates(MacLeod, 1995) of the cannon and anchor on the site.The high energy of the site, which led to the high



historic iron corrosion-rates, assists in the in situconservation process by corroding the anodes moreextensively and thus making them more effective.

ConclusionAnalysis of the pre-disturbance in situ corrosionsurveys can reveal a wealth of information that pro-vides site-managers, maritime archaeologists and con-servators with an array of interpretive data which canbe correlated with site-formation processes as well asthe ultimate fate of the artefacts scattered across ashipwreck site. The data consisting of periodic sets of insitu measurements on a wreck-site has proved tobe both sensitive to subtle changes in the micro-environment of the wreck and also able to quantifythe nature of the interactions of archaeologists on awreck (MacLeod, 2010; MacLeod and Steyne, 2011;MacLeod and Richards, 2011). Measurements on theseven cannon and the anchor on the Duart Point wreckhas shown that the impact of site-stabilization exercisessuch as laying out sandbags is readily quantified. Theapplication of sacrificial anodes to the treatment ofhistoric iron objects on the wreck-site has shown thatthe rate of change in the micro-environment of theobjects is directly linked with the extent of corrosion orgraphitization of the object. The more heavily cor-roded a cannon, the faster the treatment will be sincethe voltage created when the anodes are joined with thecorroding object is greatest for the most degraded andrapidly corroding guns.

Comparison of pre-disturbance values of the corro-sion potential and the pH of the interfacial solutionunderneath the concretion layer shows up a commonvalue of the arithmetic product of these two para-meters, the Corrosion Indicator or CI, which respondsto the changes in the micro-environment of the metalduring the treatment with sacrificial anodes. A com-parison of CI values at the pre-disturbance stage,during treatment and at the end of the conservationprogramme (either all in situ or as a combined in situpre-treatment followed by traditional electrolytic treat-ment in a conservation laboratory) has providedpractitioners with a useful guide for determining whena conservation treatment is finished. The rate at whichguns and anchors are stabilized increases with dimin-ishing water-depth, which is due to the combinedaction of the flux of dissolved oxygen on the corrodinganode and the pre-disturbance corrosion potential thatthe object had. Through the use of these newly estab-lished relationships it is now possible to estimate thelength of time it will take to treat a concreted ironobject on the sea-bed. When half-buried submarinessuch as the WWI HMAS AE2 in the Sea of Marmaraand the Resurgam are examined, their pre-disturbanceCI reflects values of aerobically corroded vessels at thepart-way stage of their treatment with sacrificialanodes. This data is consistent with the anodes bring-ing about a change in the corrosion mechanism as wellas reducing the overall rate of corrosion of the ironobjects.

AcknowledgmentsI am indebted to Colin Martin and his family and the MacLean of Duart for their continuing support and encouragement formy work on the Duart Point wreck. Peter Harvey at Heritage Victoria and his team have provided long-term support andcommitment. Assistance from the J Paul Getty Trust enabled the author to prepare this work during a fellowship at the GettyConservation Institute in Los Angeles.

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Documents

The Mechanism and Kinetics of In Situ Conservation of Iron Cannon on Shipwreck Sites