Archaeological wood-iron assemblies often testify of an intense sulfide-generating microbial activity. When remains such as shipwrecks stay in waterlogged environments during centuries, buried in sediments or immersed, they age in deaerated conditions. Combined with an abundant organic matter, these conditions are favorable for the development of sulfide-generating bacteria and the persistence of their activity. The produced sulfides (H2S, HS-) precipitate with Fe2+ ions resulting from the corrosion of hundreds (sometimes thousands) of nails used to assemble the pieces of wood. This phenomenon is well known in the field of cultural heritage, as archaeologists or restorers could sometimes observe pyrite (FeS2) inside wood at the scale of an entire wreck. Thus as a system, archaeological wood-iron assemblies prove to be very efficient bioreactors for iron sulfides production since the sulfide-promoted corrosion process of the iron reinforcements can reach a generalized level.

Studying archaeological waterlogged wood-iron assemblies can then contribute to a better understanding of the long-term sulfide-promoted corrosion process occurring on buried or immersed steel structures. Wood samples and nails extracted from several shipwrecks, of various ages and aged in various media (waterlogged soils, seawater, river) were analyzed, the investigations being focused on the iron corrosion products spread inside the wood and those still present in the rust layers. An original combination of characterization methods combining magnetic measurement methods, elemental and structural analysis methods was used.

The first results allowed highlighting a tendency concerning the influence of crucial parameters such as the aerated vs anoxic feature of the burial context and the duration of exposure to anoxic conditions. FeS mackinawite is known as the first iron sulfide to precipitate in case of sulfide-promoted corrosion of iron. However, it can be oxidized either in anoxic or in aerated conditions. In anoxic conditions and in case of a sulfide-generating activity, mackinawite transforms into pyrite via greigite (Fe3S4) as an intermediate compound. This pathway, proposed in literature, proved to be illustrated by our analyses showing the presence of mackinawite and greigite in recent wrecks (18th-19th centuries) and the presence of greigite and pyrite in the oldest ones (2nd-5th centuries). Besides, remains of nails, showing different degradation states although dating from the same period (2nd-5th centuries), testify for the reactivity of iron-carbonate corrosion products (siderite FeCO3 and chukanovite Fe2(OH)2CO3) towards sulfides. This demonstrates a post-corrosion evolution of the system in case of a long-term sulfide-generating activity even when there is no metal anymore.