The corrosion of iron, a major issue in the conservation of archaeological waterlogged composite wood-iron assemblies extracted from sulfide-containing environments

摘要

Waterlogged archaeological woods contaminated by iron and extracted from anoxic and sulfidecontainingmedia are exposed to dramatic damages. Sulfide species (H_2S, HS-), possibly biogenicdue to the presence of organic matter as source of nutrients for sulfide-generating bacteria, tend toprecipitate with Fe2+ ions resulting from the corrosion of the ferrous parts of the composite systemto produce iron sulfides. FeS mackinawite is known as the first precipitate but, as a metastablephase, it can be transformed through redox mechanisms occurring either in anoxic or in aeratedconditions. Parameters such as oxic/anoxic character of the environment, sulfur supply via theproduction of sulfide species, temperature, moisture, pH, etc. determine the transformationpathway. In anoxic conditions and in case of an active sulfide-generating activity during the burial,mackinawite transforms into FeS_2 pyrite via Fe_3S_4 greigite. After excavation, aerial oxidation ofiron sulfides leads to the occurrence of voluminous iron/sulfate-containing crystals responsible ofunsightly efflorescence, cracks and crumbs. Moreover, iron sulfides oxidation (aerial or anoxic) isconcomitant to a high acidification, which is dramatic for holocellulose preservation.Iron and/or sulfur-containing phases proved to have a strong influence in the degradationmechanisms but the nature of iron sulfides present inside the wood is not clearly determined yet.Wood samples and nails extracted from several archaeological shipwrecks, of various ages andaged in various media (waterlogged soils, seawater, river) were analyzed, the investigations beingfocused on iron reinforcements corrosion products spread inside the wood and those still present inthe rust layers of nails. An original combination of characterization methods including elemental,structural analysis methods and magnetic measurement methods was used.The results showed the presence of mackinawite, greigite and pyrite but suggest that theirpresence is linked to the duration of exposure to anoxic conditions. The remains of nails hadvarious degradation levels, showing mostly carbonated ferrous corrosion products (FeCO_3 siderite,Fe_2(OH)_2CO_3 chukanovite) associated with iron sulfides. They also testify for the reactivity of thecarbonate phases towards dissolved sulfides and bring interesting evidences about the long-termevolution of the whole system.