A rusticle is a recently-discovered corrosion phenomenon, found underwater, where a community of microbes has eaten away at an iron ship and created things resembling stalactites or icicles. Some rusticles on the wreck of the Titanic have been raised and analysed since 1996. They are various colours up to the bright orange-red of iron ochre, and can be tens of centimetres long.

They are composed partly of iron products such as goethite or iron oxyhydroxide, FeO(OH), and partly of clusters of living things, such as bacteria and fungi. They mostly cluster around various points on the wreck where water flows more freely. Inside the rusticle are channels and ribs, and the outer surface is layered like tree rings. The outside is rigid like concrete but shatters easily, releasing the contents. Up to about 35% of the mass is iron compounds.

It is estimated that the organisms are "mining" over 100 kg of iron from the Titanic every day, so the ship won't last more than a few hundred years. They also mine phosphorus and sulfur, and for this reason prefer wrought iron to steel. Since the rivets are of wrought iron, this makes them particularly weak, and this may have implications for modern structures such as oil rigs.

Fluff: New Scientist, 26 July 2003
Details: www.encyclopedia-titanica.org/articles/biodeterioration_cullimore_01.shtml

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