PREPARATION PRIOR TO THE RECOVERY:
SITE ANALYSIS

H.L. Hunley scientist working on the submarine.
In order to gain a better understanding of the microenvironment of the H.L. Hunley submarine and clarify its complex interaction with the surrounding sediment, a series of pre-disturbance surveys were conducted. In an interdisciplinary approach, archaeologists, conservators, geologists, electrochemists, corrosion engineers, and microbiologists worked as a team and collected as much in situ information as possible prior to the raising of the submarine. A YSI multi-parameter probe was used for the collection of on site measurements such as pH, Eh, salinity, conductivity, oxygen, and water temperature. These readings helped assess the true environment of the H.L. Hunley and confirmed the anoxic nature of the site. Many studies are helping to explain the process of the submarine's burial (i.e. the geology and site formation processes) and the interaction between the sub and its environment. An extensive microbiological study headed by Dr. Pam Morris from the Fort Johnson Marine Biomedicine & Environmental Sciences Program is currently being conducted at the Medical University of South Carolina. Dr. Morris's study should provide significant information regarding the bacterial communities in and around the submarine.

In site corrosion potential measurements were carried out by the H.L. Hunley team under the guidance of Senior Conservator Paul Mardikian and Steve West of Orion Research Inc.


Senior conservator Paul Mardikian and Steve West of Orion Research discussing measurements in the H.L. Hunley tank.
The data collected demonstrated that the H.L. Hunley had reached a state of "rest" after 136 years underwater and that any attempt to raise the vessel and change the submarine's environment might have serious consequences on the rate of corrosion. The very low level of dissolved oxygen in the sediment around the submarine compared to the 8 ppm of oxygen in a fresh water tank would theoretically represent an increase in the corrosion rate by 2500 times at 48F (~ 9C) and 7500 times at 90F (~ 32C). In addition, the presence of surface chlorides could further increase the rate of corrosion. The presence of chlorides at the metal's surface increases the corrosive effect of oxygen by destabilizing the resulting oxide film. In the worse case scenario, in less than 6 months, the hull could experience more corrosion related metal loss than in the last 136 years if not properly stabilized.

Fortunately, the chemical process of surface oxidation also produces a cement-like shell, which actually reduces further oxidation. The hardened layer encapsulating the submarine is the result of a complex interaction with the environment involving microbial and electrochemical reactions. In order to lessen the catalytic effect of the oxygen on the surface of the submarine, the first goal was to minimize any adverse effects of the lifting process on the encapsulating concretion that covers the entire surface of the submarine. The concretion, which is a matrix of iron corrosion products and carbonates from seawater, acts as a semi permeable barrier against the oxygen, thereby reducing corrosion. Keeping the concretion intact was critical since chemical corrosion inhibitors cannot be used in the storage tank. Chemical inhibitors would have had deleterious effects on the human remains, other organic artifacts contained in the submarine, and last but not least, on the health of the H.L. Hunley staff during the excavation. In simple words, this natural concretion is the best protection of the sub and its fragile contents until interior excavations are complete. Fortunately, an inspection of the vessel after the raising showed not a single crack on the concretion.


 
The H.L. Hunley submarine in its storage/treatment tank in the Warren Lasch Conservation Center. Close-up of the forward hatch area of the H.L. Hunley submarine.



Related Pages:

Conservation
Site Analysis
Conservation Lab
Research
Photo Gallery


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