CORROSION CONDITION ASSESSMENT, MITIGATION, AND PRESERVATION OF USS MONITOR ARTIFACTS

C.S. Brossia, M. Yunovich, D. Hill, K.M. Lawson, R. Denzine, J.T. Schmidt, and E. Klechka

CC Technologies 5777 Frantz Road Dublin, OH 43017

E. Schindelholz, E. Nordgren, and D. Krop

The Mariners’ Museum 100 Museum Drive

Newport News, VA 23606

R. Baboian RB Corrosion Service

84 Ruff Stone Rd Greenville, RI 02828

H. Hack Northrop Grumman Corp. P.O. Box 1488, MS 9105

Annapolis, MD 21404

J.D. Flessas Pond and Company

2635 Century Parkway, STE 800 Atlanta, GA 30345

D.C. Cook Department of Physics

Old Dominion University Norfolk, VA 23529

ABSTRACT

Artifacts from The USS Monitor, a U.S. Civil War era warship, have been recovered from the site where the Monitor sank and are presently undergoing archeological study and preservation at The Mariners’ Museum. Of concern is the continued degradation of the artifacts due to corrosion. The present condition, corrosion rate, and other characteristics of the artifacts were assessed. Based on these assessments, the corrosion rate of the artifacts is generally higher in the museum than was likely experienced when the artifacts were still submerged in the ocean, due primarily to the increased availability of oxygen and concentration of any salts present during dry-out periods. In order to help mitigate further corrosion, cathodic protection and corrosion inhibitors have been evaluated for possible use. Keywords: USS Monitor, conservation, preservation, archeology

INTRODUCTION

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The USS Monitor (Figure 1) was the first successful iron-clad battleship that was equipped with a revolving gun turret and was constructed under the supervision of John Ericsson in 1861. The turret, which was located amidships was 21 feet 8 inches (~ 6.6 m) in diameter and 9 feet (~2.7m) high and was constructed from 8 layers of 1-inch (2.54 cm) iron plates. The turret contained two 11 inch (~28 cm) Dahlgren smooth-bore shell guns that were capable of firing shot weighing up to 180 lbs (~81.6 kg). The main advantage that the Monitor had over its contemporary warships was that despite having only two guns it could sail under steam power and fire the cannons in any direction rather than the traditional broadside arrangement that was in use at the time. Another advantage that the Monitor had over its contemporaries was an extremely low hull profile of 18” (~45.7 cm) above the waterline. This then presented opponent vessels with an extremely small silhouette to hit.

Following its official launch in January 1862 from Greenpoint, New York the Monitor engaged

in several naval battles only to sink in a storm in December 1862 off the coast of Cape Hatteras, North Carolina. Discovered in 1973, the Monitor wreck site is now a National Marine Sanctuary under the auspices of the National Oceanographic and Atmospheric Administration (NOAA). In 1998, salvage work began and artifacts from the Monitor have been brought up to the surface for further study and preservation. NOAA has designated The Mariners’ Museum in Newport News, Virginia as the caretakers of the artifacts and has charged them with the preservation work. These artifacts include the original gun turret, two gun carriages, two Dahlgren shell guns, the engine, and condenser as well as numerous other smaller artifacts.

The main mission of The Mariners’ Museum with respect to the Monitor is to excavate and

preserve the artifacts as they exist presently. In contrast to restoration where new components are installed to replace damaged areas, the focus of preservation is to study and eventually display the artifacts in their present condition. Thus, preservation efforts are cannot result in any permanent damage or for that matter permanent repairs. For example, one possible solution to arresting possible corrosion of the artifacts is to apply a clear coat coating and essentially seal the artifact. However, this would involve making a modification to the artifact and thus would go against the principles of preservation. In support of the Mariners’ Museum’s mission to analyze, study, preserve, and ultimately display these artifacts, the present condition of the artifacts was assessed and the corrosion conditions characterized. In addition, work has also been performed for formulating approaches to mitigate on-going corrosion processes on the artifacts as the Museum conducts its archeological mission.

ARTIFACT ASSESSMENT APPROACH

The condition and corrosion assessment of USS Monitor artifacts was conducted following a multi-pronged approach that included:

General visual and electrical conduction assessments Measurement of corrosion potentials Determination of corrosion rate Evaluation of corrosion mechanisms pH measurements Remaining metal thickness measurements

The general visual evaluation of the artifacts included obtaining appropriate representative

photographs detailing overall condition, the level of corrosion damage observable, and the degree of concretion. Electrical continuity measurements consisted of verifying electrical connectivity between

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different points of an artifact using a digital multimeter (DMM). Verification of electrical connection also proved valuable in conducting corrosion potential and corrosion rate measurements.

Corrosion potential measurements were performed using a Cu/CuSO4 pencil reference electrode pressed against the artifact using a sponge wetted with the storage solution and a DMM utilizing an electrochemical interface designed to increase the input impedance of the meter. The corrosion rate at specific locations on the artifacts was measured by the linear polarization resistance (LPR) method using a “barnacle” cell. The “barnacle” cell consisted of a centrally located platinum pseudo reference electrode, a stainless steel loop counter electrode, and a stainless steel loop guard electrode (Figure 2). Similar to the corrosion potential measurements, electrochemical contact between the barnacle cell and the artifact was made utilizing a sponge wetted with the artifact storage solution (Figure 3).

At specific locations on each artifact and in some cases the bulk solution in which the artifacts were stored, pH measurements were obtained using pH paper. In some locations, solution and biofilm/slime samples were obtained and taken back to the laboratory to determine if corrosion-influencing bacteria (primarily acid producers, sulfate reducers, but in limited cases other bacteria as well) were present. Lastly, where applicable, a Panametrics 36DL Plus ultrasonic tester was used to measure remaining metal thickness.

ARTIFACT ASSESSEMENT FINDINGS

The assessment mainly focused on the condition of the gun turret, the Dahlgren guns, the gun carriages, the engine, and the condenser with some additional smaller artifacts (16 in all) also part of the investigation. Due to space limitations, only the results pertaining to the turret and the engine are presented here. The corrosion assessment of the other artifacts revealed similar results and the mitigation methods being employed for the turret and engine are also being applied elsewhere. Thus, the turret and engine can serve as illustrations of the overall condition of the artifacts. Turret

The gun turret is approximately 22 feet (~6.7m) in diameter and 9 feet (~2.7 m) tall weighing approximately 120 tons (~108.9 metric tons) and composed primarily of wrought iron, cast iron, copper, and wood and is composed of some 3,000 components. A schematic of the gun turret is shown in Figure 4. It was raised in 2002 and has been stored in a freshwater tank as shown in Figure 5.

The overall appearance of the turret exterior wall, interior wall, inner roof, and external roof

surfaces were found to be very similar with some areas still concreted with minerals (Figure 6 and Figure 7) with a summary of the assessment findings in Table 1. The turret displayed limited evidence of significant corrosion damage (e.g., some heavily mineralized stanchions and rivets and mineralized nut guards). A thorough evaluation of the electrical connectivity showed that once the outer corrosion product layer or concretion layer was breached and intimate metal contact achieved, the turret and all associated components were electrically shorted together, including mineralized areas. The nominal resistance between different areas typically was on the order of a few tenths of an ohm to perhaps a few ohms at most. Ultrasonic inspection of the outer turret plates at eight deconcreted locations near the gun openings showed remaining thickness of between 790 and 810 mils (~2 cm), which translates to approximately 200 mils (~ 5mm) of metal loss or an average corrosion rate of 1.4 mpy (0.04 mm/y) [200 mils/144 yrs]. The inner turret plates showed less corrosion loss with approximate remaining thickness of 900 mils (100 mils of metal loss or a corrosion rate of 0.7 mpy) based on four measurements in regions comparable to the outer areas measured. These measurements were verified with a few spot

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checks at other locations around the turret and had a nominal accuracy of approximately ± 10-15%. These low corrosion rates seem reasonable as averages, given the well preserved appearance of some components (see for example the bolt threads shown in Figure 7 which were exposed in 2004 when the nut was removed).

A total of 194 potential measurements were made on the outer and inner turret plate surfaces, the turret roof interior and exterior, and the turret roof interior structural beams. The median and standard deviation in the potentials measured for all areas of the turret was -520 ± 59 mV vs. Cu/CuSO4. In addition to the predominantly wrought iron construction of the turret, a few copper/brass pins/fittings were also found. These exhibited corrosion potentials of -280 and -490 mV respectively. In addition, these fittings were confirmed to be in electrical contact with the rest of the turret structure. The relatively small spread in the potential data gives further evidence of good electrical connectivity throughout the turret. The differences in potential that were observed are related to differences in the conditions at the measurement locations. For example, removal of the surface deposits and corrosion products creating freshly exposed areas tended to result in higher measured potentials, whereas areas of heavy concretion often had lower potentials. Potential contour plots along with appropriate reference photographs and drawings for the outer and inner turret plates are shown in Figure 8 and Figure 9.

The corrosion rates measured for the turret ranged from 1.8 to 13.9 mpy (0.046 – 0.35 mm/y), with higher rates typically observed on the turret exterior plates and roof, and lower rates on the interior areas of the turret. The corrosion rate of three stanchions that originally held up the roof cover was also measured and was found to be comparable to the external areas (4.6 to 6.8 mpy or 0.12 to 0.17 mm/y). Furthermore, the rates measured in still-concreted areas were generally lower than non-concreted areas and is consistent with the lower (more negative) corrosion potentials measured in those areas. In the case of the roof exterior, a comparison was made between areas that had been deconcreted over one year ago and areas that had been deconcreted as recently as one week prior to the assessment. The older deconcreted area exhibited a corrosion rates of only 1.8 to 2.2 mpy (0.046 – 0.056 mm/y) based on 3 measurements whereas the newly exposed areas showed corrosion rates 3 to 7 times higher (5.8 to 12.7 mpy or 0.15 – 0.32 mm/y) indicating that the corrosion rate of newly exposed metal will tend to be initially high and will likely decay over time (as is often observed in corrosion). By way of comparison, the copper alloy components exhibited corrosion rates of 3.3 and 4.2 mpy (0.083 – 0.11).

The corrosion rates measured using LPR were generally higher than those estimated based on the UT remaining metal thickness measurements. This could be the result of uncertainties in the UT measurements and LPR measurements. An alternative and perhaps a more likely explanation is that the corrosion rates measured using LPR represent the corrosion rate of the turret (and components) at present which is higher than that prevailing during the majority of the turret’s life. That is, the corrosion rate of the artifact in the museum is higher than while the turret was at the ocean floor. This could be caused by greater availability of oxygen and environment corrosivity changes during episodic dry (or quasi-dry) periods wherein the concentration of any salts or aggressive species on the turret surface increases dramatically as water evaporates during various evaluation studies.

Corrosion mitigation while submerged will take the form of an impressed current cathodic

protection system using mixed metal oxide ribbon anodes. The cathodic protection system that has been designed will utilize four 6 foot (1.8m) long anodes in the turret interior and two 70 foot (21.3 m) loop anodes to protect the exterior surface walls and the roof exterior (underneath the turret since the turret is presently stored upside down on its roof). During conservation studies when the storage tank is drained and the cathodic protection system is de-energized, two potential spray applied inhibitors are under investigation. These inhibitors are biodegradable and do not leave a permanent surface film. It is

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important to recognize that the formation of a permanent (or even semi-permanent, hard to remove) surface film by the inhibitor is counter to the overall preservation/conservation mission and cannot be tolerated. Thus, the inhibitors presently under investigation can easily be washed away using water spray but still provide a short term (hours to a day is usually sufficient) barrier for oxygen transport. Since personnel will be in intimate contact with the inhibited surface, it is also important that the inhibitors not be toxic.

Engine The engine, a side lever steam design (Figure 10), is approximately 12’ x 8’ x 4’ (3.7x2.4x1.2 m) and weighs 32 tons (29 metric tons) and is composed of wrought and cast iron, copper, rubber, lead, wood, and glass. The engine was raised in 2001 and has been stored in a pH 12 NaOH solution since. This solution was selected as a good candidate to minimize the overall degradation of the engine but the development of a blue tinge due to copper dissolution provides some evidence that galvanic corrosion is occurring. In addition, lead, wood, and other organic materials (e.g., leather) are unstable in highly alkaline environments.

The overall appearance of the engine is shown in Figure 11, Figure 12, and Figure 13 with a summary of the assessment findings in Table 2. As the engine has basically been stored in a pH 12 alkaline solution since being raised from the ocean floor, it is still heavily concreted in many areas (~ 90%). Attempts to utilize UT to determine remaining metal thickness proved unsuccessful due to the large areas of concretion and curvature issues on deconcreted areas that prevented good coupling of the probe to the engine. It was determined, however, that once the concretion layer was penetrated and native metal contact made that electrical connectivity throughout the engine seemed to be reasonable (only a few mV difference between different locations was noted when comparing and less than 10 ohms resistance was measured).

An example of the measurement locations and values of corrosion potentials and corrosion rates measured on the front side of the engine are shown in Figure 14. Similar measurement values were noted on the other faces and sides of the engine with a total of 104 potential and 15 corrosion rate measurements. As was noted on the turret, areas that were concreted generally exhibited lower corrosion potentials and lower corrosion rates than those that were deconcreted. This was further demonstrated when two sequential corrosion rate measurements were taken in one location on a wrought iron component with the concretion present and immediately after its removal with the corrosion rate increasing by a factor of over 150. It is evident from these observations that particular care is needed during deconcretion activities, as the corrosion rate of the artifact may increase to high values during these operations. Corrosion mitigation in the form of inhibitors is being evaluated for storage of the engine (and condenser) as a possible replacement for the pH 12 NaOH solution. The candidates that are being evaluated include molybdate, nitrite, and carbonate. Molybdate is a well known effective inhibitor that will not likely leave a long-term film on the surface but could however present environmental concerns should the storage water be disposed of through the municipal sewer system. Carbonate, though environmentally friendly, could form a tenacious film that could be hard to remove. Of the candidates, nitrite might prove to be the best, however the inhibition offered by nitrate to such diverse materials as iron, copper, and lead is unclear and it is uncertain if galvanic corrosion can be reduced. Evaluation of these alternatives is on-going.

SUMMARY AND CONCLUSIONS

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USS Monitor artifacts, including the gun turret, gun carriages, Dahlgren guns, engine, condenser, and several smaller artifacts, were examined and assessed from a corrosion perspective. The assessments consisted of overall visual evaluations, electrical connectivity, corrosion potential, corrosion rate, pH, hardness, and remaining metal thickness measurements. Based on these assessments, the corrosion rate of the artifacts is generally higher in the museum than was likely experienced when the artifacts were still submerged in the ocean, due primarily to the increased availability of oxygen and concentration of any salts present during dry-out periods. It was also observed that the corrosion rate in concreted areas tended to be lower than deconcreted areas and was often quite rapid in freshly exposed areas. Cathodic protection systems have been designed and will be installed on several of the artifacts. For others where simple storage in alkaline environments is presently used, alternative inhibitor solutions are being evaluated.

Table 1: Summary of Turret findings. Meas. Number Outer Inner Roof Interior Roof Exterior Beams

OCP 181 -406 to -620 avg: -528

-360 to -590 avg: -468

-460 to -670 avg: -542

-480 to -520 avg: -496

-460 to -530 avg: -490

CR (mpy) 19 3.0 to 13.9 avg: 6.4

2.4 to 6.6 avg: 5.1

3.6 to 9.4 avg: 4.2

Newly exposed: 5.8 to 12.7

Older: 1.8 3.3 to 3.6

pH 16 5.5 to 7 avg: 6.5

6.5 to 7 avg: 6.8

6.5 to 7 avg: 6.8 n/a 7

Thick.(mils) 16 790 to 810 880 to 920 n/a n/a n/a

OCP – open circuit or corrosion potential in mV vs. Cu/CuSO4 CR – Corrosion Rate (Note 1 mpy = 0.0254 mm/y).

Table 2: Summary of Engine findings. Measurement Number Range Average

Corrosion Potential (mV vs. Cu/CuSO4) 104 Fe: -136 to -635 Cu: -136 to -542

Fe: -469 Cu: -233

Corrosion Rate (mpy) 16 Fe: 0.03 to 16.1 Cu: 0.4 to 2.8

Fe: 3.4 Cu: 1.6

pH 12 7 to 9 n/a

Note: Copper components tended to have higher (less negative) corrosion potentials and lower corrosion rates.

Table 3: Summary of Condenser findings. Measurement Range Average

Corrosion Potential (mV vs. Cu/CuSO4) -124 to -599 -442

Corrosion Rate (mpy) 0.05 to 61 9.2

pH 7 to 9 n/a

Note: Copper-based components tended to have higher (less negative) corrosion potentials and lower corrosion rates.

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Figure 1: Drawing of USS Monitor.

Figure 2: Photographs of barnacle cell used to conduct corrosion rate measurements.

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Figure 3: Photograph of corrosion rate measurement process.

Figure 4: Schematic of Monitor gun turret (courtesy of NOAA).

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Figure 5: Photograph of Monitor gun turret in freshwater storage tank. Note that the turret is upside down.

Figure 6: Appearance of turret roof exterior area showing regions of concretion and areas that had been

deconcreted.

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Figure 7: Photograph at the turret main roof beam (note good condition of treads on bolt).

Bolt Threads

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1 3 5 7 9 11 13 15 17 19 21 23OD plate number

Figure 8: Outer turret plate corrosion potential contour plot (note x’s represent locations where measurements were taken). Potentials are vs. a copper/copper sulfate reference electrode.

400

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11 10 9 8 7 6 5 4 3 2 1 24 23 22 21 20 19 18 17 16 15 14 13 12

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Figure 9: Inner turret plate corrosion potential (vs. a copper/copper sulfate reference electrode).

12 13 14 15 16 17 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 350

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Figure 10: 1862 diagram of Monitor engine (courtesy of NOAA)

Figure 11: appearance of forward side of engine.

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Figure 12: Close up view of cast valve and copper tubing.

Figure 13: Close up view of shaft showing evidence of extensive preferential dissolution of iron

(note wood-like texture).

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Figure 14: Corrosion Potential and Corrosion Rate measurement locations for the engine forward side. Corrosion potentials are in units of millivolts vs. copper-copper sulfate reference electrode. (Note 1 mpy = 0.0254 mm/y).

-574

-593

-568-564 -558

-206

-136 -146

-519

-321 -498 -499

-434

-388

-486

-500

-517

-514

-398

-450

-160

-498

-438

-504

-260 -233-402

-476

-474

-531

-550

-440

-499

-200 -308 -413

0.85 mpy

4.2 mpy

0.25 mpy

1.4 mpy 0.48 mpy

6.5 mpy

0.08 mpy w/ concretion 12.1 mpy deconcreted

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