The Titanic: from metals to minerals.
For more than 80 years the wreck of HMS Titanic has lain on the Atlantic ocean bottom, 3800 m deep, off the Grand Banks of Newfoundland. Besides being the object of much public interest, the sunken ship has scientific value as it provides an opportunity to assess chemical reactions affecting metals in a deep sea environment.
In 1991, six years after the ship's remains were first located by a scientific team lead by R.D. Ballard |1~, the IMAX Corporation chartered the Russian Academy of Science vessel Akademik Keldysh to film the Titanic wreck. During one dive of the manned submersible MIR-2, dedicated to scientific purposes, nine fragments of metal (debris), as well as samples of corrosion products, and 16 sediment cores were recovered near or on the wreck.
The structure, chemistry and mineralogy of corrosion products were studied to gain insight into the geochemistry of iron and other metals that had been accidentally introduced in a deep-sea environment. In this case, it was significant that the Titanic was a new ship, on its maiden voyage in 1912. Our research used a scanning electron microscope equipped with an x-ray energy dispersive spectrometer. Because elements lighter than sodium could not be detected, mineralogical determinations were confirmed or refined using a powder x-ray diffractometer. Given the very small amounts of samples, XRD identification was not always possible for some of the phases. In these cases, the chemical name of identified crystals has been used rather than mineral name.
One of the most conspicuous corrosion products are iron rusticles resembling stalactites that can reach lengths of several tens of centimetres. These features do not have a physical metal precursor, Fig. 1. Rusticles are formed of a brittle iron oxy-hydroxide shell approximately 100 to 200 ||micro~meter~ thick, with a smooth dark red outer surface (toward seawater) and an orange rough inner surface (toward the centre of the rusticle). The core of the rusticle and the inner surface of the shell are made of a reticular framework of spherical aggregates.
The small needle-like crystals forming these aggregates, Fig. 2, are goethite ||Alpha~-FeO(OH)~. The outer surface of the shell is covered with microspheres resembling bacteria, Fig. 3. This surface has the same chemical composition as the inner surface, but different crystal structure |lepidocrocite: |Gamma~-FeO(OH)~.
The Biological Activity Test (BART |TM~) was used to check for the presence of bacteria in rusticles. A variety of bacteria grew in the culture media and the test indicated a predominance of sulphate reducing species that multiplied particularly rapidly in anaerobic conditions.
Corrosion of the Titanic has taken unusual forms such as rust "flows" which cover the deck of the Titanic, or adjacent sediment in stream-line patterns. Comparison of similar photographs taken on the first and latest expeditions to the Titanic site, five years apart, suggest that these flows spread at a rate of 10 cm/y. Moreover, anomalously high iron concentrations were found in sediment near the hull to depths of 15 cm, suggesting penetration at a rate faster than can be predicted by ionic diffusion in interstitial water alone. These observations further support the hypothesis that biological activity plays a major role in the mobilization and deposition of iron from the Titanic wreck.
The rust flakes
Rust flakes from a cast iron beam were also examined. Most of the flake is a web of iron-rich microspheres, Fig. 4, attached to thin massive layers of iron oxy-hydroxide. The microspheres are similar to those on the outer surface of the rusticles, Fig. 3, and their common chain-like arrangement further suggests bacterial colonies.
The bulk mineralogy of the rust flakes is identical to that of the rusticles: a mixture of goethite and lepidocrocite. However, a variety of other minerals is also present in the rust flakes, mostly as coatings on the massive iron layers. SEM examination revealed a very well crystallized iron mineral, believed, on the basis of its hexagonal crystal shape, to be hematite (|Fe.sub.2~|O.sub.3~). This mineral was always associated with silicon-rich iron mineral (iron silicate?), Fig. 5. Black patches of siderite (FeC|O.sub.3~) and iron-rich cubes, Fig. 6, most likely magnetite, are also present.
More unexpectedly, a thin coating of lead carbonate and small cubes of galena (PbS) were common, Fig. 6. To investigate a possible source for this lead, paint residue found on a piece of hull plate was analyzed. X-ray diffraction showed that the paint contains minimum (|Pb.sub.3~|O.sub.4~) in a matrix of barite (BaS|O.sub.4~) and calcite (CaC|O.sub.3~).
Some of the minerals found in the rust flakes (e.g., siderite, galena) are indicative of reducing conditions that are unexpected in the well-oxygenated deep seawater in which the Titanic rests. Reducing conditions are confirmed by low Eh values measured at the contact between rust and metal by Pimenov and Savvichev |2~. It is most likely that bacteria such as sulphate reducing species are able to maintain reducing conditions in microenvironments within the rust flakes. A shift toward more reducing conditions is suggested by Fig. 6: the iron-rich cubic crystals believed to be magnetite show signs of dissolution whereas the galena exhibits fresh, euhedral shapes.
By contrast, there is no evidence for extreme reducing conditions in the rusticles. Lepidocrocite is obviously the stable iron form in contact with seawater. Inside the rusticles, water exchange must be sufficiently limited for iron concentration to exceed the solubility product of geothite, as witnessed by the abundance of the typical euhedral needle-like crystals, Fig. 3.
This study shows that biological activity plays a major role in promoting corrosion of the Titanic and results in fast-growing structures such as rust flows and rusticles. There is evidence that bacterial activity can create and maintain microenvironments with physico-chemical characteristics very different from the ambient well-oxygenated deep seawater. Because the resulting chemical reactions occur at the microbial scale, the analytical scanning electron microscope is the ideal instrument to document such complex biogeochemical environments.
1. R.D. Ballard, The Discovery of the Titanic, Madison Publishing, Toronto, Canada, 248 p.
2. N.V. Pimenov and A.S. Savvichev, in S.M. Blasco, et. al., Akademik Keldysh/Titanic Expedition, in press (1993).
We thank the IMAX Corporation and S.M. Blasco for providing the Titanic samples and Fig. 1. Dr. D.R. Cullimore kindly provided the BART |TM~ tests. This work is part of Energy, Mines and Resources Canada Contract No. 23420-1-M023/01-OSC.
Patricia Stoffyn-Egli is with Microchem, Geochemistry Consultants, East Jeddore Road, East Jeddore, NS; Dale E. Buckley is with the Geological Survey of Canada, Bedford Institute of Oceanography, Dartmouth, NS.
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|Title Annotation:||HMS Titanic; structure, chemistry and mineralogy of corrosion products|
|Author:||Stoffyn-Egli, Patricia; Buckley, Dale E.|
|Publication:||Canadian Chemical News|
|Article Type:||Cover Story|
|Date:||Oct 1, 1993|
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