P. Dillmann

Characterization of protective rust on ancient Indian iron using microprobe analyses

Abstract Local compositional and structural information was obtained from an ancient 1500-year-old Indian iron and its protective scale utilizing microprobe techniques (μXRD and μPIXE). Different locations in the iron matrix and in the entrapped slag inclusions were also analyzed for P contents. The P content of the metallic iron matrix was very heterogeneous. Lower P contents were observed in the regions near slag inclusions. The surface oxide scales was layered. Enrichment of P in the metal–scale interface and in the scale adjacent to the interface was determined. The P content in the scale decreased on moving away from the interface. Microdiffraction patterns obtained at different locations in the oxide scale indicated that at locations where the P content was high (i.e. nearer the interface), the oxide was amorphous while at locations where P was low, crystalline phases were identified. The presence of crystalline phosphates was also confirmed at some regions in the scale, where the P content was relatively very high. The probable reasons for the presence of the identified phases in the atmospheric corrosion product have been discussed.

Characterisation of corrosion layers formed on ferrous archaeological artefacts buried in anoxic media

Abstract In the context of the in situ conservation and preservation of archaeological artefacts, the long term corrosion mechanisms of iron in anoxic soils are studied. To this purpose, a first step is the characterisation of the corrosion layers formed on archaeological artefacts provided from the archaeological site of Glinet (the sixteenth, Normandy, France). On all the corrosion systems formed on artefacts, the main phases constitutive of the corrosion layer are siderite (FeCO3), an iron carbonate containing hydroxide groups [probably chukanovite Fe2(OH)2CO3] and magnetite (Fe3O4). Furthermore, the arrangement of these phases reveals three corrosion distribution types with corresponding corrosion pattern diagrams.

First examination of slag inclusions in medieval armours by confocal SR-µ-XRF and LA-ICP-MS

To elucidate the origin of armours supposed to be from Lombardy according to art-historians, the very small Slag Inclusions entrapped in the metallic matrix were analysed on the samples taken on armours. This paper presents the analytical protocol, based on the coupling of LA-ICP-MS and confocal SR-µ-XRF (confocal Micro-X-ray fluorescence under Synchrotron Radiation) developed to obtain trace element information from this kind of inclusion. Confocal SR-µ-XRF has been applied to inclusions of ancient iron artefacts for the first time. The reliability and reproducibility of the procedure developed for the trace elements quantifying have been checked by a comparison of the results obtained by LA-ICP-MS and by confocal SR-µ-XRF. Our first results allow us to draw some preliminary observations on the origin of the samples.

Archaeological analogues and corrosion prediction: from past to future. A review

Abstract A new approach including the use of archaeological analogues is needed to predict corrosion phenomena over extended time periods lasting centuries to several millennia. The corrosion rates observed on analogues generally range from 0·1 to 10 μm/year, depending on the medium. Isotopic markers (deuterium or oxygen-18) can be used on archaeological objects to determine and localise the anodic and/or cathodic mechanisms. Modelling and simulation take into account statistical aspects (modelling by cellular automata) and kinetics, including localised corrosion phenomena (pitting factor).

In situ structural characterisation of nonstable phases involved in atmospheric corrosion of ferrous heritage artefacts

Abstract The prediction of very long term corrosion of iron and low alloy steel in atmospheric conditions or in hydraulic binder media is a crucial issue for the conservation and restoration of heritage artefacts. For both media, the typical iron corrosion product layers (CPL) can be described as a matrix of goethite (α-FeOOH) crossed by marblings of reactive phases: maghemite (γ-Fe2O3), ferrihydrite (Fe5HO8.4H2O), feroxyhyte (δ-FeOOH), etc. The aim of the experiments presented here is to bring new insights on the role that the maghemite could potentially play in the mechanisms of corrosion. For that purpose, electrochemical reductions have been coupled with in situ Raman microspectroscopy. These experiments enable the authors to propose a hypothesis of local mechanisms in the specific case of marblings of maghemite connected to the metallic substrate. These local mechanisms could drastically influence the global corrosion rate.