Corrosion and Conservation Management of the Submarine HMAS AE2 (1915) in the Sea of Marmara, Turkey

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Article Corrosion and Conservation Management of the Submarine HMAS AE2 (1915) in the Sea of Marmara, Turkey

Ian D. MacLeod

Western Australian Maritime Museum, Victoria Quay Drive, Fremantle, WA 6160, Australia; [email protected]

Received: 12 February 2019; Accepted: 5 March 2019; Published: 14 March 2019

Abstract: The wreck site of the Australian First World War submarine HMAS AE2 in the Sea of Marmara had a salinity of 26‰ (parts per thousand) for the first 13 m, which increased to 41‰ at 21 m, after which it remained constant to the bottom at 72 m, where the dissolved oxygen was three parts per million. The vessel is protected by a very dense anaerobic concretion and lies half above a silt mound. Cross-sections of a concretion sample revealed the original surface, associated paint films and a series of burial-exposure episodes that reflected periodic changes in the silt levels, which are likely associated with major storms. Core samples of sediment have established the impact of the vessel on the site. Corrosion simulation experiments have established the direct linkage between chloride levels underneath the concretion layer and the pH of the entrapped solution. Following the initial drop camera survey, an ROV (Remote Observation Vehicle) examination of the interior of the boat showed a remarkable degree of preservation. A network of ten-tonnes of zinc anodes were distributed at the stern, amidships and the bow to bring about in-situ conservation of the historic submarine.

Keywords: AE2 submarine; Turkey; First World War; in-situ conservation; sediment cores; corrosion; simulation; sacriﬁcial anodes

1. Introduction The Australian submarine AE2 was built in 1912 by Vickers Armstrong at Barrow-in-Furness, England, launched in 1913 and it arrived in Australia on 24th May in 1914 under the command of Lieutenant Commander Henry Stoker. The AE2 was on patrol in New Guinea when her sister ship AE1 was lost, including all hands. The latter wreck was discovered in December 2017 at a depth greater than 300 m. the Duke of York Islands in Papua New Guinea. The vessel had imploded when it exceeded its crush depth. The AE2 returned to Australia and escorted the second ANZAC convoy to the island of Tenedos in the Aegean Sea. After penetrating the heavily mined Dardanelles, AE2 successfully reached the Sea of Marmara on the 25 April 1915, providing a boost for the ANZAC Gallipoli Campaign. The submarine created havoc for five days, before it was holed by the Turkish torpedo boat Sultanhisar when it broached after mechanical problems. Stoker ordered his crew to abandon ship and then scuttled the vessel, which now lies in 73 m. of water on a silt mound at the bottom of the Sea of Marmara at position 47◦320 N, 27◦170 E. The wreck site was discovered by Selçuk Kolay, Director of the Rami Koçh museum in Istanbul in 1995, and the identity confirmed by a combined Turkish and Australian team in 1998. Owing to its cultural significance to both Turkey and Australia, the 55-m long submarine was considered for possible relocation to a dry dock, where it could be conserved at a purpose-built museum. Alternative options included moving it to depths of 20–25 m, where normal scuba supply diving would enable full maritime archaeological inspection, documentation and in-situ

Heritage 2019, 2, 868–883; doi:10.3390/heritage2010058 www.mdpi.com/journal/heritage Heritage 2019, 2 869 conservation. At a similar time, maritime archaeological investigations, at the Dardanelles battleﬁelds, by the Australian team, led by Tim Smith, found signiﬁcant numbers of sunk barges and support vessels in remarkably good condition [1]. Previous experience with in-situ conservation of historic iron artefacts has demonstrated that the monitoring and treatment of anchors, cannon and the Xantho engine was practical, so long as the sites were accessible by scuba [2,3]. Since the submarine lies in deep water, all diving involves use of mixed gases (helium, oxygen and nitrogen), and this necessitates two hours of decompression bottom time for each 30-min dive. An additional complication is the presence of one unexploded torpedo in the stern of the boat. Both the explosive charge and the mercury fulminate detonator become increasingly sensitive to shock over time, so the vibrations associated with a relocation were likely to result in a devastating detonation. Until the interior of the submarine could be analysed for any traces of high explosives, which would indicate the warhead is wet and; therefore, safe, it had to be regarded as being very sensitive to shock and vibrations. Thus, in 2014, a more detailed examination took place, which involved opening the main hatch to allow a remote observation vehicle (ROV) inside the boat to assist in the development of future management plans, which included in-situ treatment. During this trip the anodes were attached to the submarine.

2. Materials and Methods During the 2004 assessment the water column above the wreck was measured for salinity, dissolved oxygen (YSI probe) and temperature at 0.5-m intervals, using a TPS 90DC. Measurements on the profiles from the sediment cores collected data on the dissolved oxygen, pH and redox potentials, using 0.3 mm diameter rhodium wire. The microelectrodes were inserted through pre-drilled 3 mm diameter holes in the polycarbonate core holders, and the measurements were made with a Unisense Microsensor Multimeter. This instrument has amperometric channels for measurement of dissolved oxygen and one for potentiometric measurements of redox potentials and pH, and direct communication to PC via a USB connection. The holes were sealed with PVC tape until measurement access was required. The hand-held readings from the Cygnus® ultrasonic metal thickness gauge had an accuracy of ±0.1 mm of metal. In-situ pH measurements on the submarine were made from a 100-m-rated polycarbonate box with a BDH (British Drug House) Gelplas flat surface (pressure compensating) electrode connected to a Cyberscan 200 pH metre. The voltage was measured using a Fluke digital multimeter with the positive connection to a platinum electrode (2 mm outside diameter) encased in non-conducting epoxy resin, and the negative terminal connected to a flow-through Ag/AgCl electrode, which was calibrated in the field station against a known standard reference electrode. All electrode wires leading into the metre box had double “O” rings to guard against possible leakage under pressure at 70+ m [4,5]. The surface pH was measured after drilling through rock-hard anaerobic concretion with a masonry-tipped drill bit.

3. Results

3.1. Physical Oceanography of the Wreck Site in the Sea of Marmara During the 2004 assessment, the water column above the wreck was measured for salinity, dissolved oxygen (YSI probe) and temperature at 0.5-m intervals, using a TPS 90DC meter that logged the data, to a maximum depth of 73 m. The first 13 m had a mean salinity of 26.1‰ ± 0.1‰, which reflects evaporative concentration of the 18‰ water inflowing from the Black Sea. Over the next eight metres, the salinity rapidly increased to 41.3‰ ± 0.8‰ at the bottom of the halocline. The higher salinity is typical of the hypersaline Aegean Sea, which enters the Sea of Marmara via the Dardanelles. Captain Stoker had used this counter current to help him penetrate the mined straits. At the 2004 halocline there was a corresponding thermocline (change in temperature) where the temperature fell from 26 to 18 ◦C and a second thermocline, at 40 m, saw the temperature fall from 18 to 16 ◦C, which was due to the heat capacity of the higher salinity water (Figure1). Heritage 2019, 2 870 Heritage 2019, 2 FOR PEER REVIEW 3

Figure 1. Salinity on the AE2 site in September 2007 (orange triangles) and June 2014 (blue diamonds). Figure 1. Salinity on the AE2 site in September 2007 (orange triangles) and June 2014 (blue During the 2014 expedition, a more sophisticated Sonde recorded the dissolved oxygen, diamonds). temperature, salinity and redox potential every 20 s. Graphical comparative analysis of the data from theDuring two timesthe 2014 of the expedition, summer and a more spring, sophisticated seven years apart,Sonde showedrecorded similar the dissolved trends. This oxygen, data recordedtemperature, the greatest salinity falland in redox temperature potential inevery the lower20 s. Graphical end of the comparative less salty water, analysis between of the data 10–20 from m, ± ◦ andthe two above times this of depth the summer the temperature and spring, was seven relatively years apart, constant showed at 15 similar0.9 trends.C between This data 20 and recorded 72 m, thethe depth greatest of thefall wreck.in temperature in the lower end of the less salty water, between 10–20 m, and above this Thedepth surface the temperature waters in the was Sea relatively of Marmara constant were well at 15 oxygenated ± 0.9 °C between between 20 7 andand 5.572 ppm,m, the as depth shown of inthe the wreck. 2007 and 2014 proﬁles shown in Figure2. The data from 2007 shows the dissolved oxygen fallingThe from surface 5 to 3 ppmwaters as in the the depth Sea increases of Marmara from were 35 to well 50 m. oxygenated At the upper between sections 7 of and the 5.5 submarine, ppm, as theshown dissolved in the oxygen 2007 and was 2014 recorded profiles at 2.8 shown± 0.2 in ppm, Figure which 2. representsThe data from 36% saturation2007 shows of the the dissolved seawater ◦ foroxygen a salinity falling of 41.3from‰ 5 atto 163 ppmC[6 as]. the depth increases from 35 to 50 m. At the upper sections of the submarine,The data the from dissolved June 2014 oxygen showed was a recorded dissolved at oxygen 2.8 ± 0.2 that ppm, fell which with increasing represents salinity, 36% saturation to reach of a minimumthe seawater of 4.5 for ppm, a salinity and then of 41.3‰ increased at 16 in °C response [6]. to currents ﬂowing in the hypersaline waters at depths until the submarine environment was reached, at 67 m, where it fell to the 3-ppm level seen in 2007.

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Figure 2. Dissolved oxygen vs. depth for the AE2 site; the zero-ppm data in 2007 (orange triangles) is when the weighted probe penetrated the silt mound. June 2014 data blue diamond shapes.

3.2. CorrosionFigure 2. Dissolved of the Submarine oxygen vs. in thedepth Sea for of the Marmara AE2 site; the zero‐ppm data in 2007 (orange triangles) is whenThe submarine the weighted lies probe on thepenetrated bottom the of thesilt mound. Sea of Marmara June 2014 data with blue a silt diamond mound shapes. coming half-way up the hull. Images of the wreck at www.submarineinstitute.com show that a combination of corrosion The data from June 2014 showed a dissolved oxygen that fell with increasing salinity, to reach a and trawler net damage has holed the thin plate sections at the bow and on the combing (structure that minimum of 4.5 ppm, and then increased in response to currents flowing in the hypersaline waters covers a range of fittings to streamline the water flow) behind the conning tower, leaving the structural at depths until the submarine environment was reached, at 67 m, where it fell to the 3‐ppm level seen frames exposed (Figures3 and4). The drop camera used in 2007 showed that the non-ferrous metal in 2007. fixtures on the interior of the conning tower were covered in a thin layer of concretion from the galvanic protection afforded by the surrounding iron objects [7]. The interior of the conning tower was made 3.2. Corrosion of the Submarine in the Sea of Marmara of bronze to make sure that the magnetic compass would not be affected by the proximity of ferrous metals.The Thesubmarine hatch cover, lies on ladder, the bottom control of the valves Sea andof Marmara steering with gear a were silt mound all made coming of brass half or‐way bronze. up theOwing hull. to Images the confined of the wreck nature at of www.submarineinstitute.com the seawater inside the vessel show it is that likely a combination that the massive of corrosion banks of andbatteries trawler will net be damage involved has in holed long-range the thin or plate proximity sections corrosion at the bow that and will on have the affectedcombing the (structure electric thatmotors covers and a the range diesel of enginesfittings to used streamline for recharging the water the batteryflow) behind banks [the8]. conning tower, leaving the structuralProximity frames corrosion exposed refers(Figures to 3 long-distance and 4). The drop galvanic camera coupling used in found 2007 onshowed historic that shipwreck the non‐ ferroussites and metal on fixtures submerged on the off-shore interior modern of the conning structures, tower such were as covered production in a thin gas layer and of oil concretion platforms. fromWhen the dissimilar galvanic metals protection are covered afforded with by the a concretion surrounding layer, iron they objects lose their [7]. The electrical interior isolation, of the conning and the towermore reactivewas made metals of bronze corrode to while make the sure more that noble the magnetic alloys are compass protected. would not be affected by the proximityBased of on ferrous the drop metals. camera The study, hatch it cover, was deemed ladder, likely control that valves the hatch and steering cover could gear be were opened, all made since ofcathodic brass or protection bronze. Owing from the to hullthe confined will have nature prevented of the major seawater corrosion inside attack the vessel on the it hinge is likely mechanism. that the massiveThe opening banks of of the batteries main hatch will tobe allow involved the ROVin long to‐ enterrange the or submarineproximity corrosion was delayed that underwater will have affectedwhen divers the electric found thatmotors there and was the 25–35-mm-thick diesel engines used concretion for recharging on the hatch the battery mechanism, banks some [8]. 15 times thicker than expected. Because the higher salinity environments have higher total mineral content, the alkalization of the hinge resulted in a massive concretion deposit (Figure5). The bond around the spindle and hinge brackets was broken using a combination of de-concreting tools and a hydraulic jack. A chain block was needed to gently ease the hatch open for the first time in over 100 years. Cathodic current from reduction of oxygen on the non-ferrous metals makes the surface more alkaline (Equation (1)). In the case of the AE2 the outcome was the precipitation of massive calcareous deposits.

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− − O2 + 2 H2O + 4 e → 4 OH (1) By consuming the soluble bicarbonate ion present in seawater, the hydroxide ions produced in Equation (1) increase the carbonate concentration and so the calcareous deposits are formed (Equation (2)). 2+ − − Ca + HCO3 + OH → CaCO3 ↓ + H2O (2) A barnacle from the base of the concretion was a deep charcoal grey to black, which is likely toHeritage be associated 2019, 2 FOR PEER with REVIEW magnetite, the corrosion product associated with iron corrosion in a low5 oxygenHeritage 2019 environment., 2 FOR PEER REVIEW 5

Figure 3. Trawler net damage on the AE2 submarine. Figure 3. Trawler net damage on the AE2 submarine.

Figure 4. Corrosion damage to the casing of the AE2; yellow shows active corrosion. Figure 4. Corrosion damage to the casing of the AE2; yellow shows active corrosion. Proximity corrosion refers to long‐distance galvanic coupling found on historic shipwreck sites Proximity corrosion refers to long‐distance galvanic coupling found on historic shipwreck sites and on submerged off‐shore modern structures, such as production gas and oil platforms. When and on submerged off‐shore modern structures, such as production gas and oil platforms. When dissimilar metals are covered with a concretion layer, they lose their electrical isolation, and the more dissimilar metals are covered with a concretion layer, they lose their electrical isolation, and the more reactive metals corrode while the more noble alloys are protected. reactive metals corrode while the more noble alloys are protected. Based on the drop camera study, it was deemed likely that the hatch cover could be opened, Based on the drop camera study, it was deemed likely that the hatch cover could be opened, since cathodic protection from the hull will have prevented major corrosion attack on the hinge since cathodic protection from the hull will have prevented major corrosion attack on the hinge mechanism. The opening of the main hatch to allow the ROV to enter the submarine was delayed mechanism. The opening of the main hatch to allow the ROV to enter the submarine was delayed underwater when divers found that there was 25–35‐mm‐thick concretion on the hatch mechanism, underwater when divers found that there was 25–35‐mm‐thick concretion on the hatch mechanism, some 15 times thicker than expected. Because the higher salinity environments have higher total some 15 times thicker than expected. Because the higher salinity environments have higher total mineral content, the alkalization of the hinge resulted in a massive concretion deposit (Figure 5). The mineral content, the alkalization of the hinge resulted in a massive concretion deposit (Figure 5). The bond around the spindle and hinge brackets was broken using a combination of de‐concreting tools bond around the spindle and hinge brackets was broken using a combination of de‐concreting tools and a hydraulic jack. A chain block was needed to gently ease the hatch open for the first time in over and a hydraulic jack. A chain block was needed to gently ease the hatch open for the first time in over 100 years. Cathodic current from reduction of oxygen on the non‐ferrous metals makes the surface 100 years. Cathodic current from reduction of oxygen on the non‐ferrous metals makes the surface

Heritage 2019, 2 FOR PEER REVIEW 6 more alkaline (Equation (1)). In the case of the AE2 the outcome was the precipitation of massive calcareous deposits.

O2 + 2 H2O + 4 e−→ 4 OH− (1) By consuming the soluble bicarbonate ion present in seawater, the hydroxide ions produced in Equation 1 increase the carbonate concentration and so the calcareous deposits are formed (Equation (2)).

Ca2+ + HCO3− + OH−→ CaCO3 ↓+ H2O (2) A barnacle from the base of the concretion was a deep charcoal grey to black, which is likely to Heritagebe associated2019, 2 with magnetite, the corrosion product associated with iron corrosion in a low oxygen873 environment.

Figure 5. Concreted main hatch way showing the 10 cm gap for insertion of the drop camera and the very thick marine concretion deposits.

3.3. In In-situ‐situ Corrosion Assessment Operational constraints limited data collection in 2007 to one location. The measurements were made ≈≈66 m m aft aft of of the the leading leading edge edge of of the casing, behind the conning tower. The corrosion potential (Ecorr)) value value was was −−0.6190.619 volts volts vs. vs. Ag/AgCl Ag/AgClseasea, which, which had had a a calibrated calibrated voltage voltage of of +0.228 +0.228 volts volts vs. the Standard Hydrogen Electrode (SHE). Comparison with another historic shipwreck at the same depth showed thatthat the theAE2 AE2corrosion corrosion potential potential was was the the same same as those as those found found on the on turret the andturret armour and armour plating platingsections sections of the USS of theMonitor USS Monitor(1862), (1862), which hadwhich a mean had a valuemean ofvalue−0.600 of −0.600± 0.016 ± 0.016 volts volts vs. Ag/AgCl vs. Ag/AgCl [9]. The[9]. The pH waspH was 7.27, 7.27, which which is the is same the same as the as mean the mean pH of pH 7.11 of ±7.110.54 ± 0.54 for the for WWI the WWI “J5” “J5” submarine submarine in Bass in BassStrait, Strait, Victoria, Victoria, Australia. Australia.

3.4. Concretion and Site History History The accidental impact of a two two-tonne‐tonne concrete anchor during an intense storm removed a small 2 AE2 section (50(50 cmcm2)) ofof concretion concretion from from the the AE2hull, hull, which which facilitated facilitated direct direct measurement measurement of the of hull the plate hull platethickness. thickness. Samples Samples of the of concretion the concretion were examinedwere examined by materials by materials and corrosion and corrosion scientists. scientists. An ROV An ROVinspection inspection showed showed that that the barethe bare metal metal had had been been covered covered by by a a new new concretion concretion layer layer withinwithin three years [[10,11].10,11]. The concretionconcretion sample sample showed showed a a typical typical anaerobic anaerobic concretion, concretion, with with a verya very dense dense milieu milieu of sharp shell debris and black iron corrosion products, which consisted of magnetite (Fe3O4 determined by petrographic analysis) and a number of iron sulphides, which included FeS, based on atomic ratios from the Energy Dispersive X-Ray Spectrum (EDS) analysis from the electron microscope. As expected, there were several other species, containing iron, sulphur, chloride and oxygen, present [12]. Samples of the priming coat of red lead oxide, Pb3O4, were found at the original metal surface, which was covered by a primary concretion layer ≈1.6 mm thick, a secondary 8.8 mm layer, and the outer layer was 3.9 mm thick. Sections of concretion showed pitting corrosion beneath the original surface, to a depth of 3 mm. The differences in colonization of the conning tower and the upper sections of the combing indicates that the sediment levels have not always been the same as those found when the site was inspected in 2007. The concretion model indicates an initial four–ﬁve years of relatively intense Heritage 2019, 2 FOR PEER REVIEW 7 of sharp shell debris and black iron corrosion products, which consisted of magnetite (Fe3O4 determined by petrographic analysis) and a number of iron sulphides, which included FeS, based on atomic ratios from the Energy Dispersive X‐Ray Spectrum (EDS) analysis from the electron microscope. As expected, there were several other species, containing iron, sulphur, chloride and oxygen, present [12]. Samples of the priming coat of red lead oxide, Pb3O4, were found at the original metal surface, which was covered by a primary concretion layer ≈1.6 mm thick, a secondary 8.8 mm layer, and the outer layer was 3.9 mm thick. Sections of concretion showed pitting corrosion beneath the original surface, to a depth of 3 mm. The differences in colonization of the conning tower and the upper sections of the combing indicates that the sediment levels have not always been the same as those found when the site was Heritage 2019, 2 874 inspected in 2007. The concretion model indicates an initial four–five years of relatively intense corrosion, as paint layers are penetrated and a steady corrosion rate is established [13]. The primary corrosioncorrosion, layer as paint in the layers concretion are penetrated appears and to correspond a steady corrosion to the first rate 13 is ± established two years of [13 immersion]. The primary (i.e., fromcorrosion 1915 layer to 1928). in the Changing concretion conditions appears todue correspond to massive to storms the ﬁrst disturbing 13 ± two years the site of immersionresulted in (i.e.,the secondfrom 1915 and to third 1928). layers. Changing conditions due to massive storms disturbing the site resulted in the secondSamples and third of oil layers. from inside the boat were collected from the surface of the ROV following its successfulSamples navigation of oil from of inside the internal the boat werespaces collected of the fromengine the surfaceroom. Viscosity of the ROV data following and gas its chromatographicsuccessful navigation analysis of the of internal the volatile spaces organic of the engine compounds room. coming Viscosity from data the and oily gas residue chromatographic indicated thatanalysis it was of diesel the volatile fuel, which organic had compounds been leaking coming from fromthe forward the oily tanks. residue The indicated analysis that showed it was that diesel the oilfuel, was which biologically had been leakingfractionated from the through forward removal tanks. The of analysisthe sour showed and acidic that the smelling oil was biologicallythiols and thiophenes.fractionated The through bacteria removal preferentially of the sour consumed and acidic much smelling of the thiols long andchain thiophenes. alkanes of The the bacteriageneral formulapreferentially CnH2n+2 consumed (Figure 6). much The biodegradation of the long chain of alkanesthe AE2 of oil the mixture general was formula indicated CnH by2n+2 the(Figure presence6). ofThe the biodegradation branched alkanes, of the norpristane AE2 oil mixture (C18H38), was pristane indicated (C19H by40) theand presence phytane (C of20 theH42 branched), which are alkanes, much morenorpristane stable than (C18H the38 ),branched pristane hydrocarbons. (C19H40) and phytane It is interesting (C20H42 that), which pristane are muchis derived more from stable shark than liver the oilbranched and so hydrocarbons. this may be one It is of interesting the principal that pristane components is derived of lubricating from shark oils liver and oil diesel and so fuel this mayfor the be generatorsone of the principal that was components recovered offrom lubricating the surface oils andof the diesel ROV. fuel The for thevideo generators inside the that submarine was recovered has evidencedfrom the surface movement of the of ROV. large The black video mats inside of thebiological submarine growth, has evidenced which are movement the by‐products of large of black the microbiologicalmats of biological attack growth, on the which diesel are fuel. the by-products of the microbiological attack on the diesel fuel.

Figure 6. GasGas chromatographs chromatographs of AE2 oils compared with undegraded diesel fuel. 3.5. Assessment of the Corrosion Rate on AE2

The hand-held readings from the Cygnus® ultrasonic metal thickness gauge gave 4.6 ± 0.9 mm for the bare metal of the ballast tank, which was originally 6.35 mm thick. The apparent loss of 1.55 mm of metal over the 92.4 years of immersion equates to a corrosion rate equivalent to 0.017 ± 0.003 mm/year. Comparative data from the Monitor showed that the interior of the turret, which was in an anaerobic microenvironment like the AE2, had the same corrosion rate of 0.016 mm/year [14]. It should be recalled that the Ecorr values of the two wrecks were the same. The corrosion rate for iron shipwrecks in open-ocean waters can be calculated using an empirical equation (Equation (3)) relating the log of Heritage 2019, 2 FOR PEER REVIEW 8

3.5. Assessment of the Corrosion Rate on AE2

The hand‐held readings from the Cygnus® ultrasonic metal thickness gauge gave 4.6 ± 0.9 mm for the bare metal of the ballast tank, which was originally 6.35 mm thick. The apparent loss of 1.55 mm of metal over the 92.4 years of immersion equates to a corrosion rate equivalent to 0.017 ± 0.003 mm/year. Comparative data from the Monitor showed that the interior of the turret, which was in an Heritageanaerobic2019, 2 microenvironment like the AE2, had the same corrosion rate of 0.016 mm/year [14]. It should875 be recalled that the Ecorr values of the two wrecks were the same. The corrosion rate for iron shipwrecks in open‐ocean waters can be calculated using an empirical equation (Equation (3)) therelating corrosion the rate,log of measured the corrosion in mm/year rate, measured as dg or depthin mm/year of graphitization as dg or depth of cast of graphitization iron objects on of the cast wreckiron toobjects water on depth the wreckd in metres to water [3] viz.,depth d in metres [3] viz.,

Open-oceanOpen‐ocean log log dg d=g −= −0.6300.630− − 0.01560.0156 (3)(3) With a depth of 71 m, Equation (3) predicts a corrosion rate of 0.018(3) mm/year, which is the With a depth of 71 m, Equation (3) predicts a corrosion rate of 0.018(3) mm/year, which is the same as the observed rate of 0.017 ± 0.003 mm/year. The silty microenvironment of the submarine same as the observed rate of 0.017 ± 0.003 mm/year. The silty microenvironment of the submarine would naturally result in an overall lower corrosion rate than that for wrecks lying proud of the would naturally result in an overall lower corrosion rate than that for wrecks lying proud of the seabed in well oxygenated conditions. More simply, the lower sections of the submarine may be in a seabed in well oxygenated conditions. More simply, the lower sections of the submarine may be in a better state of preservation, but testing would be required to assess the impact of microbially‐induced better state of preservation, but testing would be required to assess the impact of microbially-induced corrosion (MIC) in the lower section of the pressure hull. corrosion (MIC) in the lower section of the pressure hull. 3.6. Sediment Cores 3.6. Sediment Cores

3.6.1.3.6.1. Redox Redox Potentials Potentials AA fundamental fundamental archaeological archaeological principal principal is is to to establish establish what what impact impact a givena given deposit deposit has has on on its its immediateimmediate microenvironment microenvironment and and vice vice versa. versa. Thus, Thus, core core samples samples were were recovered recovered in in the the immediate immediate vicinityvicinity of of and and at at 20 20 m m distance distance from fromAE2 AE2. The. The cores cores were were chemically chemically characterized characterized through through measurementsmeasurements of of the the dissolved dissolved oxygen, oxygen, pH pH and and redox redox potentials. potentials. The The mean mean redox redox potential potential of of the the sedimentsediment core core adjacent adjacent to to the the vessel vessel was was +0.119 +0.119± ±0.044 0.044 volts volts vs. vs. NHE NHE (Normal (Normal Hydrogen Hydrogen Electrode), Electrode), whichwhich was was statistically statistically indistinguishable indistinguishable from from the the off-site off‐site core, core, +0.157 +0.157± ±0.038 0.038 volts volts (Figure (Figure7). 7).

Figure 7. Voltage E readings on the two cores relative to the Normal Hydrogen Electrode. Figure 7. Voltageh Eh readings on the two cores relative to the Normal Hydrogen Electrode.

AE2 The Eh fell by 180 mV, from 0.260, in the ﬁrst ﬁve cm of sediment, while the off-site sample The AE2 Eh fell by 180 mV, from 0.260, in the first five cm of sediment, while the off‐site sample took 30 cm to fall to the same minimum value of 0.082 ± 0.003 volts. The more rapid fall in the took 30 cm to fall to the same minimum value of 0.082 ± 0.003 volts. The more rapid fall in the redox redox potential for the AE2 core indicates that the microenvironment near the submarine rapidly potential for the AE2 core indicates that the microenvironment near the submarine rapidly becomes becomes anaerobic as a direct result of the corrosion process consuming oxygen trapped in the silt. anaerobic as a direct result of the corrosion process consuming oxygen trapped in the silt. This is This is supported by the observation that as the core depth increased the AE2 core remained relatively constant at 0.107 ± 0.014 volts, while the off-site redox potential had a mean value of 0.142 ± 0.020 or some 35 mV more oxidizing than that by the submarine (Figure7). This indicates that the distance of the impact of the submarine on the microenvironment is less than 20 m. Heritage 2019, 2 876

3.6.2. pH Proﬁles

HeritageAnalysis 2019, 2 FOR of thePEER redox REVIEW and the pH data of the AE2 core indicated that the reduction of dissolved9 oxygen present in the sediment was the dominant electrochemical process viz, supported by the observation that as the core depth increased the AE2 core remained relatively 1 constant at 0.107 ± 0.014 volts, while theO off+‐ 2Hsite+ redox+ 2 e− potential→ H O had a mean value of 0.142 ± 0.020 (4) or 2 2 2 some 35 mV more oxidizing than that by the submarine (Figure 7). This indicates that the distance of the impactThe regression of the submarine analysis of on the the E hmicroenvironmentand pH data conformed is less to than the 20 equation m. Eh = 0.759 − 0.084 pH vs. NHE which had an R2 of 0.909. The intercept value of 0.759 ± 0.069 volts is the same as the standard reduction3.6.2. pH Profiles potential of 0.680 volts for the above oxygen reduction reaction; however, the slope for the ± pH atAnalysis 0.084 of0.009 the voltsredox is and higher the pH than data the of theoretical the AE2 core 0.059 indicated volts. It that is likely the reduction that the high of dissolved salinity ofoxygen the Sea present of Marmara in the sediment resulted inwas the the non-ideal dominant behaviour electrochemical of the electrolytes, process viz, which were based on behaviour at infinite dilution. The off-site core data did not follow any systematic trend. AE2 ½ O2 + 2H+ + 2 e− → H2O (4) The pHcore values showed a steady alkaline microenvironment for the initial 25 cm at pH ± − 7.80 The0.02, regression before rapidly analysis falling of the overEh and the pH last data 7 cm conformed at a rate ofto the0.05 equation pH/cm Eh distance = 0.759 − down0.084 thepH AE2 corevs. NHE (Figure which8). The had changean R2 of in 0.909. pHThecore interceptmay reflect value a of limiting 0.759 ± impact0.069 volts of the is the vessel same on as the the sediment, standard sincereduction the pH potential values of for 0.680 both volts cores for coincided the above at oxygen 7.3 ± 0.1reduction after a corereaction; depth however, of ≈30 cmthe ofslope sediment. for the off-site ThepH at 0.084pH ±core 0.009rapidly volts fellis higher from than its initial the theoretical value of 7.7 0.059 to 7.1volts.± 0.1It is in likely the first that 25the cm high of salinity sediment. of Thethe Sea dissolved of Marmara oxygen resulted measurements in the non in‐ theidealAE2 behaviourcore recorded of the sevenelectrolytes, zero values, which with were a based mean on of 0.018behaviour± 0.023 at infinite ppm (i.e., dilution. essentially The zero),off‐site while core thedata off-site did not core follow mean any was systematic 0.067 ± 0.068 trend. ppm with five zero values.

Figure 8. pH of AE2 core proﬁles and off-site values. Figure 8. pH of AE2 core profiles and off‐site values. 3.7. Modelling Corrosion Processes on AE2 AE2 ItThe has beenpHcore previously values showed reported a steady that alkaline the concretion microenvironment sample was for used the initial to study 25 cm how at fastpH 7.80 the corrosion± 0.02, before reaction rapidly of steelfalling plate over equilibrates the last 7 cm in at seawater a rate of [ −150.05]. The pH/cm concretion distance sample down wasthe core attached (Figure to AE2 a8). section The change from anin 1890’spHcore paddle may reflect steamer a limiting boiler, which impact had of the openable vessel spaceson the tosediment, allow monitoring since the pH of values for both cores coincided at 7.3 ± 0.1 after a core depth of ≈30 cm of sediment. The off‐sitepHcore both the pH and the chloride ions, and an insulated copper wire permitted monitoring of the Ecorr ofrapidly the metal. fell from During its initial the 11-monthvalue of 7.7 experiment to 7.1 ± 0.1 therein the wasfirst a25 systematic cm of sediment. increase The in dissolved the acidity oxygen and themeasurements chlorinity of in the the solutions AE2 core trapped recorded underneath seven zero the values, concretion. with a The mean pH of of 0.018 the seawater ± 0.023 ppm fell from (i.e., essentially zero), while the off‐site core mean was 0.067 ± 0.068 ppm with five zero values.

3.7. Modelling Corrosion Processes on AE2 It has been previously reported that the concretion sample was used to study how fast the corrosion reaction of steel plate equilibrates in seawater [15]. The concretion sample was attached to a section from an 1890’s paddle steamer boiler, which had openable spaces to allow monitoring of

Heritage 2019, 2 877

7.9 to 4.3, while the chloride increased from 16,300 to 34,600 ppm. The increased acidity was due to hydrolysis reactions of the primary corrosion product FeCl2 with water (Equation (5)).

+ − FeCl2 + 2 H2O → Fe(OH)2 + 2 H + 2 Cl (5)

The corrosion cell took 10 months to reach equilibrium. On this basis, it is reasonable to assume that recorded in-situ values of the pH and the Ecorr, of this data accurately reflect the dynamic equilibrium between the wreck and the local marine environment. Plots of the concentration of chloride and pH best fitted square root of time plots, which support the diffusion-controlled nature of the corrosion process. The AE2 concretion corrosion cell showed that the chloride ion concentration increased by a factor of 2.1 times after sitting for 11 months in an unstirred solution. By comparison, the increase in chlorinity with the high energy surging environment of the Zuytdorp (1712) wreck, off the cell Western Australian coast, was 2.9 times after 270 years [3]. Monitoring of the Ecorr value showed that it reached the on-site value in just eight months, but it took another few months of measurements to confirm that the plateau had been reached.

3.8. Cathodic Protection System Given that relocation of the submarine was not a possibility, owing to the combination of the high risk of detonation of an unexpended torpedo and the prohibitively expensive creation of a dry-dock and museum conservation facility, an alternative approach to the conservation management had to be developed. In the 18th century, Volta found that the difference in reactivity of various elements could be harvested through the development of a voltaic pile, consisting of alternating layers of copper and zinc sheets. These studies were the first major experiments in differential corrosion rates of metals. The use of sacrificial reactions of a more reactive metal to provide a cathodic (negative) current, to stop the removal of electrons from a more noble metal, has been the subject of electrochemical experiments since the time of Sir Humphrey Davy, on Royal Navy copper-sheathed wooden vessels in 1761. In the case of AE2, the difference in reactivity of zinc and iron provides the driving force of the cathodic protection system for the wreck. There is approximately 400 mV difference between the Ecorr of the iron on the AE2 and the zinc anodes, and this is enough to stop corrosion of the submarine. As cathodic current flows through the insulated copper cables connecting the anode pods to the boat, the change of polarity causes outward migration of chloride ions and consumption of the acidity that had resulted from the accumulation of acid during the hydrolysis reactions of the corrosion products. Design calculations considered a 5–10-year protection program and it was understood that the rate of anode consumption would be initially much higher than the long-term rate, as readily consumed hydrogen ions had built up to a moderate level underneath the concretion. Another consideration impacting on the design life was the high cost of mounting sea-borne diving operations. The final configuration consisted of three anode pods (Figure9). The cathodic protection of the submarine is the largest in-situ conservation project ever attempted on an historic iron shipwreck. Previous work by the author has resulted in successful in-situ treatment of guns, anchors and a marine steam engine, which resulted in a more rapid processing of the artefacts from recovery to exhibition status [2,3,16]. The anode pods were assembled on a 2.6 m sided square base, which was filled with concrete to prevent them sinking into the silt. The supporting structure weighed 0.5 tonnes, giving an aggregate weight of around 1.5 tonnes to each pod. Each anode (150 × 7.5 × 7.5 cm) had a central iron core, to which insulated current-carrying cables were attached (Figure 10). The distal end of the cable was attached to the submarine using an industrial three bolt system, as used in the cathodic protection of gas and oil production facilities. The 17 anodes on each pod had a geometric surface area of 7.84 m2. In order to achieve a good spread of protection through the length of the boat, one pod was attached aft near the rear hydroplane, one amidships at the conning tower and one up forward on the windlass (Figure 10). With an estimated geometric surface area of approximately 660 m2 for the submarine and with the area of the anodes at 23.5 m2, the industry standard ratio of approximately 30 times object to Heritage 2019, 2 FOR PEER REVIEW 11

Heritage 2019, 2 878 (Figure 10). With an estimated geometric surface area of approximately 660 m2 for the submarine and with the area of the anodes at 23.5 m2, the industry standard ratio of approximately 30 times object anodeto anode ratio ratio was was achieved. achieved. This This ratio ratio helps helps to avoid to damageavoid damage to the submarine to the submarine if it was over-protected,if it was over‐ asprotected, this could as producethis could too produce much hydrogen too much pressure,hydrogen which pressure, would which blast would off the blast protective off the concretionprotective andconcretion lead to and excessive lead to consumption excessive consumption of anodes. of anodes.

Figure 9. View of assembled anode pods prior to attachment to AE2. Figure 9. View of assembled anode pods prior to attachment to AE2. This ratio was developed by John McCoy of McCoy and Associates to comply with industry This ratio was developed by John McCoy of McCoy and Associates to comply with industry standards. Each of the locations was selected on the basis that there was signiﬁcant residual metal standards. Each of the locations was selected on the basis that there was significant residual metal present in that part of the wreck, to enable the safe and long-lasting attachment of the cables from present in that part of the wreck, to enable the safe and long‐lasting attachment of the cables from the the anode pods onto the submarine. The data listed in Table1 reports the variability of the E corr anode pods onto the submarine. The data listed in Table 1 reports the variability of the Ecorr measurements, which were reproducible to within a few millivolts. measurements, which were reproducible to within a few millivolts.

Table 1. Corrosion potential (Ecorr) measurements as a function of time (hours). Table 1. Corrosion potential (Ecorr) measurements as a function of time (hours). Ecorr Time, Date Time Location Ecorr Time, Date Time Location vs. Ag/AgCl hours vs. Ag/AgCl hours Adjacent to CT* upper hatch, 10/06/14 12:45 Adjacent to CT* upper hatch, naturally −0.632 ± 0.002 0.1 10/06/14 12:45 naturally cathodically protected −0.632 ± 0.002 0.1 13/06/14 10:00 Aftcathodically site on port, protected aft of the hydroplane −0.672 ± 0.008 96 13/06/1416/06/14 10:00 10:30Aft site on Fwd. port, site, aft windlass of the hydroplane shaft, 4 h later −−0.6720.654 ± 0.008 4 96 − 16/06/1416/06/14 10:30 14:30Fwd. site, Fwd. windlass site, windlass shaft, shaft, 4 h later 8 h later 0.656−0.654 8 4 18/06/14 09:15 Anodes attached midships to CT* −0.678 ± 0.003 141 16/06/14 14:30 Fwd. site, windlass shaft, 8 h later −0.656 8 CT* = Conning Tower. 18/06/14 09:15 Anodes attached midships to CT* −0.678 ± 0.003 141 Prior to attaching the anode cables withCT* clamps= Conning to Tower. the submarine, the surfaces were hydro-blasted to removePrior smallto attaching areas of the concretion anode cables from the with metal clamps surface to tothe get submarine, ohmic connections. the surfaces The were elapsed hydro time‐ atblasted each attachmentto remove small point wasareas measured of concretion from from the time the atmetal which surface the pods to get were ohmic attached connections. and the timeThe whenelapsed the time Ecorr atmeasurements each attachment were point taken. was The measured observed from voltages the time were at dependentwhich the pods on the were logarithm attached of theand elapsed the time time when between the Ecorr anode measurements attachment were and taken. voltage The measurement. observed voltages The linear were regression dependent analysis on the 2 giveslogarithm Equation of the (6), elapsed which hadtime an between R of 0.9648 anode with attachment an associated and equation, voltage measurement. The linear regression analysis gives Equation 6, which had an R2 of 0.9648 with an associated equation, Ecorr = −0.646 (0.002) − 0.014 (0.001) log t hours (6) Ecorr = −0.646 (0.002) − 0.014 (0.001) log t hours (6)

Heritage 2019, 2 879

The numbers in parentheses are the errors associated with the intercept and the rate at which the submarine was being cathodically polarised. Since all the measurements fall along the same line, this indicated that the submarine was acting as an interconnected electrical unit, which was an unexpected bonus as it indicated that the overall condition of the submarine was structurally sound. The data in Table1 shows that the greatest voltage drop between the natural cathodic protected EHeritagecorr of 2019 the, 2 conning FOR PEER tower REVIEW was 50 mV after 141 h or close to eight days. This voltage drop is the12 equivalentHeritage 2019 of, 2 FOR a 42% PEER drop REVIEW in the corrosion rate of the submarine, which is remarkable given the size12 of the boat!The Thenumbers Ecorr valuesin parentheses are diagrammatically are the errors illustrated associated in with Figure the 11intercept. This calculation and the rate is basedat which on a combinationthe submarineThe numbers of was theoretical inbeing parentheses cathodically and practical are polarised.the determination errors Sinceassociated all of the the with measurements rate the at whichintercept the fall and corrosion along the therate changessame at which line, for concretedthisthe submarineindicated materials. thatwas beingthe For submarine electrochemically cathodically was polarised. acting controlled as Since an interconnected all reactions, the measurements such electrical as the fall in-situ unit, along treatmentwhich the same was ofline, an the AE2unexpectedthis submarine,indicated bonus that the as logthe it of indicatedsubmarine the corrosion that was the rate acting overall can beas condition calculatedan interconnected of accordingthe submarine electrical to the was changes unit, structurally which in Ecorr wassound.values, an withunexpected a drop of bonus approximately as it indicated 330 that mV neededthe overall for condition a ten-fold of decrease the submarine in the rate was of structurally corrosion. sound.

Figure 10. Clamp and cable arrangements for central anode pod connection to the conning tower. Figure 10. NoteFigure the 10. replicaClamp Clamp hatch and and cable is cable in the arrangements arrangements “locked down” for for central position. central anode anode pod pod connection connection to the to conningthe conning tower. tower. Note theNote replica the replica hatch ishatch in the is in “locked the “locked down” down” position. position.

Figure 11. Plot of the E values vs. log time (hours) after application of anodes. corr Figure 11. Plot of the Ecorr values vs. log time (hours) after application of anodes.

Figure 11. Plot of the Ecorr values vs. log time (hours) after application of anodes. The data in Table 1 shows that the greatest voltage drop between the natural cathodic protected Ecorr ofThe the data conning in Table tower 1 shows was that50 mV the greatestafter 141 voltage h or close drop to between eight days. the natural This voltage cathodic drop protected is the equivalentEcorr of the ofconning a 42% droptower in was the corrosion50 mV after rate 141 of theh or submarine, close to eight which days. is remarkable This voltage given drop the is size the ofequivalent the boat! ofThe a 42%Ecorr valuesdrop in are the diagrammatically corrosion rate of illustrated the submarine, in Figure which 11. is This remarkable calculation given is based the size on aof combination the boat! The of E corrtheoretical values are and diagrammatically practical determination illustrated of inthe Figure rate at 11. which This calculationthe corrosion is basedchanges on fora combination concreted materials. of theoretical For electrochemicallyand practical determination controlled ofreactions, the rate suchat which as the the in corrosion‐situ treatment changes of for concreted materials. For electrochemically controlled reactions, such as the in‐situ treatment of

Heritage 2019, 2 FOR PEER REVIEW 13

the AE2 submarine, the log of the corrosion rate can be calculated according to the changes in Ecorr Heritagevalues,2019 with, 2 a drop of approximately 330 mV needed for a ten‐fold decrease in the rate of corrosion.880

3.8.1. Chemical Environment in the Water Column and Inside AE2 3.8.1. Chemical Environment in the Water Column and Inside AE2 During diving operations on the 2014 expedition, the ROV and the attached Sonde recording the in‐situDuring data diving became operations stuck inside on the the submarine 2014 expedition, for just over the ROV a day, and recording the attached information Sonde every recording 20 s. theA in-situ total of data nearly became 657,900 stuck sets insideof 12 variables the submarine were recorded for just before over a the day, ROV recording was recovered, information but before every 20the s. data A total could of nearlybe analysed 657,900 it had sets to of be 12 averaged variables over were every recorded two minutes, before the as there ROV was was insufficient recovered, butmemory before theon the data museum could be computers analysed itto had resolve to be nearly averaged eight over million every data two points. minutes, One as of there the wasfirst insufficientoutcomes memoryof this analysis on the museumwas that computersthe apparent to resolvehalocline nearly observed eight in million 2007, datawith points. the drop One camera, of the firstwas outcomes an artefact of this due analysis to poor was lighting that the and apparent specular halocline reflection observed from suspended in 2007, with material the drop inside camera, the wasvessel. an artefact due to poor lighting and specular reflection from suspended material inside the vessel. TheThe pH pH data data recorded recorded by by the the Sonde Sonde is is shown shown versus versus time time inside inside thethe conningconning tower (Figure 12).12). TheThe red-brown red‐brown line line shows shows a a fallfall fromfrom the surface surface value value that that is is within within the the normal normal range range of pH of pHfor the for theSea Sea of of Marmara, Marmara, which which has has an anaverage average pH pHof 8.26 of 8.26 ± 0.18.± 0.18. The green The green line represents line represents the period the period when whenthe Sonde the Sonde was wasinside inside the submarine the submarine and operating and operating as normal, as normal, which which shows shows an average an average pH of pH7.15 of ± 7.150.03,± 0.03,which which is typical is typical of marine of marine sediments sediments that have that bacteria have bacteria active activein them. in them.

Figure 12. Plot of the pH of the water column and interior of the submarine. Figure 12. Plot of the pH of the water column and interior of the submarine. The extended blue line represents the period when the oxygen and salinity sensors were “poisoned”The extended by sulphur blue species line represents in the stirred-up the period sediment. when Whenthe oxygen the divers and salinity began to sensors recover were the Sonde,“poisoned” the purple by sulphur line and species the plateau in the value stirred of‐ 7.2up representssediment. theWhen solution the divers value began in the to water recover column the aboveSonde, the the sediment. purple line As and the the instrument plateau value recovery of 7.2 operations represents increased,the solution the value plateau in the readings water column of 7.4 andabove 7.6 represent the sediment. the different As the instrument chemical microenvironmentsrecovery operations insideincreased, the submarine.the plateau readings The value of of 7.4 7.4 and is for7.6 the represent water 3.1 the m different up from chemical the floor microenvironments of the control room inside and 7.6 the is submarine. for water at The 3.8 value m. The of latter 7.4 is pHfor correspondsthe water 3.1 to them up mean from pH the of floor the sediment of the control cores room from outsideand 7.6 theis for submarine. water at 3.8 The m. values The latter between pH 7.2corresponds and 7.4, those to the closer mean to the pH internal of the sediment spaces of cores the lower from controloutside room, the submarine. correspond The with values the mean between pH 7.2 and 7.4, those closer to the internal spaces of the lower control room, correspond with the mean (7.26 ± 0.08) from the external core adjacent to the submarine. The equivalent value at the first plateau pH (7.26 ± 0.08) from the external core adjacent to the submarine. The equivalent value at the first region of the pH with the sediment core close to the submarine is a strong indication that the chemical plateau region of the pH with the sediment core close to the submarine is a strong indication that the processes of diagenesis inside the submarine were the same as those externally, but near the hull. chemical processes of diagenesis inside the submarine were the same as those externally, but near 3.8.2.the hull. Analysis of the Dissolved Oxygen profiles inside AE2

3.8.2.The Analysis dissolved of oxygenthe Dissolved inside theOxygen conning profiles tower inside was generally AE2 around 3.2 ± 0.1 ppm but there were a series of rapidly rising and falling spikes, which saw an upward excursion to approximately 4.0 ± 0.2 ppm. At times these elevated levels were restored to their former values with a sigmoidal decrease in the concentration of oxygen. This is probably due to bacteria that consumed oxygen. More generally, Heritage 2019, 2 FOR PEER REVIEW 14

The dissolved oxygen inside the conning tower was generally around 3.2 ± 0.1 ppm but there

Heritagewere a2019 series, 2 of rapidly rising and falling spikes, which saw an upward excursion to approximately881 4.0 ± 0.2 ppm. At times these elevated levels were restored to their former values with a sigmoidal decrease in the concentration of oxygen. This is probably due to bacteria that consumed oxygen. More theregenerally, was athere rapid was return a rapid to the return steady to the state steady value, state as shownvalue, as in shown Figure in13 Figure. These 13. phenomena These phenomena indicate thatindicate there that is mixing there is of mixing different of bodiesdifferent of bodies water inside of water the inside conning the tower conning and tower that there and isthat a signiﬁcant there is a levelsignificant of microbiological level of microbiological activity inside activity the submarine. inside the submarine.

Figure 13. Plot of dissolved oxygen vs. time inside the conning tower. Figure 13. Plot of dissolved oxygen vs. time inside the conning tower. The specific time intervals have no meaning other that providing an insight into the fluid dynamics insideThe the specific submarine. time There intervals was have an interval no meaning between other 400 andthat 700 providing min when an thereinsight was into no apparentthe fluid ± mixingdynamics of theinside layers the ofsubmarine. solution, butThere the was time an between interval oxygen between spikes 400 and was 700 typically min when 75 therefive minutes. was no Theapparent varying mixing speeds of the at which layers theof solution, elevated but dissolved the time oxygen between levels oxygen resumed spikes their was stasis typically values 75 ± were five dueminutes. to which The ofvarying the three speeds processes at which were the dominant: elevated Firstly, dissolved there oxygen is natural levels diffusion resumed from their a high stasis to avalues low concentration were due to which of oxygen, of the the three second processes is bacterial were consumption, dominant: Firstly, and thethere third is natural method diffusion is water movementfrom a high around to a low the concentration sensing heads of of oxygen, the Sonde. the Imagingsecond is recovered bacterial fromconsumption, the ROV showedand the largethird blackmethod algal is water matts. movement The simplest around explanation the sensing of heads the oscillating of the Sonde. oxygen Imaging levels recovered is that the from Sonde the ROV was positionedshowed large at ablack point algal where matts. there The was simplest movement explanation of different of the bodies oscillating of water oxygen inside levels the submarine, is that the andSonde that, was at positioned times, a more at oxygenateda point where solution there flowedwas movement into the boat of different from an externalbodies of source. water Inspectioninside the ofsubmarine, the dissolved and oxygenthat, at vs.times, depth a more plots, oxygenated in Figure2, showsolution that flowed just above into thethe submarineboat from thean external oxygen concentrationsource. Inspection was of around the dissolved 4 ppm, and oxygen so mixing vs. depth of these plots, waters in Figure with 2, the show internally that just equilibrated above the solutionssubmarine would the oxygen result in concentration the fluctuations. was Observations around 4 ppm, from and the so ROV mixing operators of these of the waters cameras with inside the theinternally submarine equilibrated confirm thatsolutions there waswould a small result but in measurable the fluctuations. flow of waterObservations inside the from control the roomROV andoperators the waterborne of the cameras sediments inside the fall submarine from suspension confirm when that there the current was a small meets but the measurable bulkhead. flow At this of pointwater theinside changed the control direction room of and water the flow waterborne caused thesediments sediment fall to from collect suspension in that area when of the current control room.meets the The bulkhead. entrapment At of this the point Sonde the inside changed the submarinedirection of was water extremely flow caused fortunate the sediment for it provided to collect an extendedin that area period of the of control passive room. observation The entrapment of how the of currents the Sonde were inside changing the submarine the chemical was environment extremely insidefortunate the submarine.for it provided The dataan extended from the Sondeperiod indicates of passive that observation there are a range of how of environments the currents insidewere thechanging submarine the chemical that range environment from low oxygen inside tothe anaerobic, submarine. and The that data these from different the Sonde environments indicates willthat havethere varyingare a range effects of environments on the corrosion inside of artefacts the submarine that are that naturally range from retained low withinoxygen the to anaerobic, confines of and the hullthat oftheseAE2 different. environments will have varying effects on the corrosion of artefacts that are naturally retained within the confines of the hull of AE2. 4. Conclusions 4. ConclusionsThe data gathered from in-situ corrosion measurements on the hull of the submarine AE2 and the core profiles close to and 20 m from the vessel has shown that the dense marine concretion and the great depth of the wreck site is providing a relatively benign storage environment for the vessel. Heritage 2019, 2 882

The rate at which the dissolved oxygen profile fell in the two core samples indicated that the submarine corrosion is consuming oxygen from the surrounding silty sediments. Analysis of the pH and Ecorr data from the cores shows that the sample near the submarine is controlled by the reduction of oxygen in the sediments, whereas this is not the case for the core some 20 m distance from the submarine. It can be concluded that the submarine has a real but limited impact on the silt and sediments. Data from the corrosion simulation experiment showed that it takes approximately one year for a concreted lump of iron to establish the long-term microenvironment that is routinely assessed when making in-situ measurements on historic shipwrecks. Not only does this data of direct measurement of chloride ion activity and the pH of the interstitial solution confirm the nature of the corrosion mechanism for marine iron, but it also provides opportunities to test the efficacy of changing the environment as a means of moderating the rate of decay of historic iron shipwrecks. It is possible to use such corrosion cells to directly gauge the impact of cathodic currents on the interface of the metal and the concretion. As a result of this work, the AE2 is now being actively conserved while remaining in-situ at the bottom of the Sea of Marmara. Not only will this cathodic protection system stop corrosion of AE2, it will actively remove chloride ions and so stabilize the vessel and preserve it for future generations. Measurements inside the conning tower and in the operational spaces of the interior of the submarine show that bacterial consumption of fractions of the diesel fuel, and other lubricants that have escaped from storage tanks inside the boat, has significantly reduced the chemical hazards associated with leaking diesel fuel. Accidental trapping of the Sonde inside the conning tower for one day determined that the interior of the submarine is much the same as the exterior, with around 3 parts per million dissolved oxygen and a salinity of 40 parts per thousand. There appear to be breaches in the submarine that allow a small amount of external seawater to flow into the open spaces. Silt layers within the boat are like the mound on which the AE2 sits, in that they are totally anaerobic, with active sulphate-reducing bacteria producing sulphide rich metabolites. Owing to the low level of dissolved oxygen inside the submarine, the corrosion implications of the “contamination” are not of major significance.

Funding: This research received no external funding. Acknowledgments: Vicki Richards was very helpful in assembling the field equipment, the AE2 Commemorative Foundation provided logistical and financial support for the Turkish operations, Roger Neill and Peter Graham from the Defence Science and Technology Organization gave vital assistance in the field. Special thanks go to Rear Admiral (retired) Peter Briggs AO, CSC, for his inspirational leadership. Conflicts of Interest: The author declares no conflict of interest.

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