An Introduction to Rusticles, Accumulated Iron Oxides/Hydroxides, on Shipwrecks

Abstract

Rusticles on shipwrecks are iron oxides/hydroxides with accumulated metal ions. Bacteria, particularly iron-related and sulfate-reducing bacteria, have been identified in some rusticles. The spatial relationship between bacteria and rusticles has been interpreted as cause and effect, i.e., biodeterioration or microbiologically influenced corrosion (MIC). However, specific mechanisms for rusticle formation have not been proposed. The following is an introduction to the properties of rusticles, e.g., structure and mineralogy, microorganisms, and metal ion adsorption and a discussion of the problems in their interpretation as MIC.

Introduction

Accumulations of oxidized iron, especially iron corrosion products, are typically referred to by shape, e.g., tubercles (mounds), whiskers, chimneys and combinations of shapes, e.g., tubercles with chimneys. Ballard¹ observed rust-colored, icicle-like formations on the wreck of the (Royal Mail Ship) RMS Titanic in the North Atlantic Ocean and coined the term rusticle. Since that observation, rusticles have been reported on shipwrecks and other iron substrata in seawater environments. ^2-4 The properties of rusticles have been used to assume corrosion,4 especially the potential role of microorganisms in their formation.

Structure and Mineralogy

Rusticles have been located on ironclad ships and on iron-containing materials on wooden shipwrecks, e.g. chains, machinery, typically associated with carbon steel and wrought iron in marine environments. Carbon steel is commercial iron that contains carbon as an essential alloying constituent in amounts up to about 1.7 weight percent (wt%). Wrought iron is an iron alloy with a very low carbon content (0.1 to 0.25 wt%) that has up to 2.0 wt% fibrous inclusions (slag). The most extensively investigated rusticles are those associated with the RMS Titanic, sunk in 1912 and discovered in 1987. Steel portions of the wreckage are covered with “massive, braided, or ropelike” rusticles ranging in size up to 3-4 meters in length. ⁵ In describing accumulations of iron corrosion products on the RMS Titanic, Cullimore, et al. ⁵ reported hanging, plate-like, tubercle, and whorled formations. Stoffyn-Egli and Buckley⁶ used environmental scanning electron microscopy coupled with energy dispersive x-ray spectrometry and powder X-ray diffraction (XRD) to examine rusticles from the RMS Titanic. They reported, “Rusticles are formed of a brittle iron oxy-hydroxide shell…with a smooth dark red outer surface (toward seawater) and an orange rough inner surface (toward the center of the rusticle). The core of the rusticle and the inner surface of the shell is (sic) made of a reticular framework of spherical aggregates.’’ Stoffyn-Egli and Buckley⁶ described the aggregates as a-FeOOH (goethite) and the surface as ?-FeOOH (lepidocrocite). They also examined flakes of iron corrosion products from the RMS Titanic and demonstrated that the flakes had the same bulk mineralogy as the rusticles, i.e., a mixture of a-FeOOH and ?-FeOOH. However, Stoffyn-Egli and Buckley⁶ also reported Fe₂O₃ (hematite), FeCO₃ (siderite), PbCO₃ (lead carbonate) and PbS (galena) in association with the flakes. The source of Pb was confirmed to be from a Pb-based paint. Based on the mineralogical data, Stoffyn- Egli and Buckley⁶ concluded that, despite the presence of minerals indicative of differing redox potentials within rusticles, there was no evidence of extreme (undefined) reducing conditions in the rusticles from the RMS Titanic.