Raman study of a deuterated iron hydroxycarbonate to assess long‐term corrosion mechanisms in anoxic soils

Abstract

International audienceIn several contexts such as cultural heritage, oil and gas or nuclear waste disposal, the long-term corrosion mechanisms of iron in anoxic soils are studied. For this purpose, corrosion layers formed on ferrous archaeological artefacts from the site of Glinet (16th century, Normandy, France) were characterised. The main phases identified are siderite (FeCO3), chukanovite (iron hydroxycarbonate: Fe2(OH)2CO3 and magnetite (Fe3O4). In order to provide reliable Raman references for further studies on carbonated systems, the iron hydroxycarbonate (chukanovite) was synthesised on iron discs. The corrosion mechanisms were investigated by re-corroding the archaeological samples in a deuterated solution. Raman characterisation on cross sections inside the layer revealed the presence of deuterated chukanovite, allowing the deuterium tracing of the spreading of the corrosion. A set of chukanovite samples was synthesised with various D/H ratios. Using these reference data, the proportion of deuterated chukanovite in re-corroded artefacts was evaluated, and the corrosion rate was estimated as less than 1.6 µm/year

Long-term anoxic corrosion of iron

“…Moreover, based on a normalisation of the NRA signal [38], a signifi cant deuterium enrichment on the internal part of the corrosion layer, with a mean thickness of between 16 and 28 μm (±5 μm), is highlighted. In this zone, analyses performed by Raman microspectrometry [37] confi rm the integration of deuterium in the corrosion products detected by NRA. Consequently, we propose that Fe 2+ (formed from the anodic process) and DCO 3 − (from the deuterated electrolyte) precipitate together in the vicinity of the metallic interface within the corrosion layer.…”

“…In an anoxic carbonated environment, the cathodic reaction is a water reduction reaction, which, depending on pH and carbonate concentrations [37][38][39], is commonly written as: The products of these equations react with oxidised Fe(II) formed during the anodic process, which leads to the precipitation of carbonated phases, such as siderite (FeCO 3 ) and chukanovite (Fe 2 (OH) 2 CO 3 ). The location of these phases inside the corrosion layer gives information on the mechanisms, as it is linked to the permeability and the electronic properties of the corrosion layer.…”

“…As the duration of the experiment (6 months) is low compared to the entire duration of the corrosion of the artefacts, we consider that this method provides an instantaneous corrosion rate. It is possible to estimate the amount of deuterium in the corrosion layer based on Raman microspectrometry [37] and nuclear reaction [38] measurements. Whatever the characterisation technique, the instantaneous corrosion rate is between 0.6 and 2 μm/year.…”

An Analytical Methodology for the Study of the Corrosion of Ferrous Archaeological Remains in Soils

“…Isotopic marking (Saheb et al, 2010b;2012) In an anoxic carbonated environment, the cathodic reaction is water reduction following equations (I), (II) and (III) depending on pH and carbonate concentrations:…”

“…Consequently, we have decided here to give a complete overview of the methodology that has been followed to study the corrosion in this kind of environment. The analytical tools, the dedicated devices and the detailed operating modes have been presented in detail in previous articles (Saheb et al, 2010a;2010b;2012), and we chose to focus here only on the main results that have been obtained in order to propose a relevant corrosion mechanism. This kind of mechanism can be a helpful tool in the future for estimating the alteration state of the archaeological remains and for conservation diagnoses.…”