Delphine Neff

Raman Studies of Corrosion Layers Formed on Archaeological Irons in Various Media

The description and identification of corrosion products formed on archaeological iron artefacts need various approaches at different observation scales. Among analytical techniques available to document phase structure at the microscopic range, Raman spectroscopy offers sensitivity and discrimination between iron corrosion products with an easy implementation. Results obtained for iron artefacts corrosion in soils and atmosphere are presented. Corrosion forms observed for anoxic and aerated soils on one hand and indoor atmosphere on the other are documented. Beyond the identification and organisation of corrosion products through hyperspectral imaging, Raman micro-spectroscopy could also provide quantitative phase proportions which will be needed in the proposition of reactivity diagnosis indicators.

Investigation at the nanometre scale on the corrosion mechanisms of archaeological ferrous artefacts by STXM

For the first time, corrosion products of a 450 year old archaeological iron nail were investigated at the nanometer level using STXM. NEXAFS acquisitions at the Fe L-edge were performed on a thin film taken of the metal–corrosion products including the interface. Comparison with Fe L-edge reference spectra gathered on maghemite (Fe2O3), magnetite (Fe3O4), siderite (FeCO3), chukanovite (Fe2(OH)2CO3) and metallic iron (Fe) showed the presence of an interfacial layer of about 100 nm at the metal–corrosion product interface consisting of maghemite and magnetite. Further from this interface, corrosion products are mainly constituted of Fe-carbonates, as well as smaller quantities of iron oxides, probably maghemite. These results support the hypothesis of the presence of a nanolayer controlling the corrosion processes at the metal–corrosion product interface, proposed during former studies at the macroscopic and microscopic levels. They also bring important new insights for the prediction of very long term corrosion of steels, especially into the fields of cultural heritage conservation and storage of nuclear wastes.

Silicate glass alteration enhanced by iron: origin and long-term implications.

Silicate glasses are used as containment matrices for deep geological disposal of nuclear waste arising from spent fuel reprocessing. Understanding the dissolution mechanisms of glasses in contact with iron, an element present in large amounts in the immediate environment (overpack, claystone, etc.) would be a major breakthrough toward predicting radionuclide release in the geosphere after disposal. Two different reacted glass-iron interfaces-a short-term nuclear system and a long-term archeological system-were examined using a multiscale and multianalytical approach including, for the first time on samples of this type, STXM under synchrotron radiation. Comparisons revealed remarkable similarities between the two systems and shed light on Fe-Si interactions, including migration of iron within a porous gel layer and precipitation of Fe-silicates that locally increase short-term glass alteration and are sustainable over the long-term.

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.

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).

Microbiologically influenced corrosion process of archaeological iron nails from the sixteenth century

Abstract The presence of Fe and S containing compounds inside rust layers covering iron archaeological nails was suspected but their real nature was not clearly determined. However, this finding suggested that sulphate reducing bacteria (SRB) could be involved in the corrosion processes. A thorough study focused on SRB and FeS compounds potentially present inside the rust layers was achieved on other nails recently excavated. For microbial investigations, the authors used a probe targeting SRB and performed fluorescence in situ hybridisation for cells identification. This procedure revealed that SRB were present in all analysed samples. Analysis of rust revealed that FeCO3 was the major component, indicating that the nails remained in anaerobic conditions, but FeS compounds were detected on each sample. Iron sulphides were localised on a few spots in the outer part of rust layers. This shows that the presence and activity of SRB had little influence upon the corrosion system.

Influence of crucial parameters on the dechlorination treatments of ferrous objects from seawater

Abstract This article compares chemical dechlorination treatments (immersion in sodium hydroxide or alkaline sulphite) and electrochemical treatments of iron bars from the Gallo-Roman period excavated from a marine environment. Some important parameters, such as storage before treatment, temperature, solution composition, and drying after treatment, were varied during the study to assess their influence on the chloride extraction process. The kinetics of these treatments depend mainly on chloride diffusion through the corrosion layers. The kinetics are promoted by high temperatures and, in the case of electrolysis, by the electric field effect. The reduction of corrosion products during electrolysis occurs only for objects previously stored in air. In fact, the manner in which the objects are stored before treatment is critical in the dechlorination processes. The sooner the objects are treated after excavation (with water storage), the better the removal of chloride ions, in both chemical immersion and electrolysis treatments. But if the object is stored in air, material losses occur, and only electrolysis results in complete extraction of the chlorides. These differences are due to modifications in the corrosion products during storage. Drying after treatment also has a significant impact on the composition of the corrosion layers. If the objects are dried too quickly, Fe(OH)2 oxidizes into FeOOH, which thus forms a layer with low cohesion.

Influence of corrosion products nature on dechlorination treatment: case of wrought iron archaeological ingots stored 2 years in air before NaOH treatment

Abstract Three wrought iron ingots immersed during 2000 years at 12 m deep in Mediterranean Sea were stored after excavation for 2 years without specific protection in air. After that period, two of them were treated by immersion in a NaOH solution, while the third was used to describe the corrosion system resulting from the storage conditions. This characterisation was achieved by a combination of microanalytical techniques. It could be concluded that though ferrous hydroxychloride β-Fe2(OH)3Cl was the main Cl containing phase at the time of excavation, akaganeite [β-FeO1–x(OH)1+xClx] was the only one present in the rust layers after storage. In order to determine the influence of corrosion products nature on dechlorination treatment, the evolution of a corrosion system composed of both Cl containing phases β-FeOOH and β-Fe2(OH)3Cl has been followed during in situ NaOH experimental treatment. Specific behaviours of each phase to the dechlorination treatment have been revealed.

In situ monitoring of corrosion processes by coupled micro-XRF/micro-XRD mapping to understand the degradation mechanisms of reinforcing bars in hydraulic binders from historic monuments

Historic monuments have been partly built since antiquity with iron or steel reinforcements sealed in mortars or hydraulic binders. But the presence of chloride in the environment can weaken the structures due to the corrosion of these metallic parts, leading to the cracking of the binder. In this context, in order to better understand the first steps of these corrosion mechanisms a chemical cell was designed to operate in situ analyses of the phases precipitated when a chlorinated solution is introduced in the vicinity of the bar. The chemical and structural characterization (micro-XRF and micro-XRD respectively) was performed under synchrotron radiation at the SOLEIL-DiffAbs beamline. Moreover, complementary SEM-EDS analyses were carried out before and after the in situ cell experiment in order to determine the final localisation of the corrosion products inside the crack network. The results show that iron can spread up to 1 mm away from the metallic bar inside the pores of the binder after 44 h of corrosion. Moreover, in accordance with laboratory experiments conducted in solution in the presence of Fe2+ and Cl− ions the reaction pathways conduct to the successive formation of an intermediate Fe(II)–Fe(III) chlorinated green rust which transforms into ferric oxyhydroxides such as akaganeite or goethite depending on the local concentration of iron.

Fluctuation of redox conditions in radioactive waste disposal cell: characterisation of corrosion layers formed on archaeological analogues

Abstract Oxygen trapped during the operational phase in disposal cells of an underground radioactive waste repository is often considered to be quickly consumed, notably by corrosion of metallic materials or reducing microorganisms. This would lead to anoxic conditions in most of each disposal cell. In addition to this, a shift from anoxic to oxic conditions could not be excluded locally in disposal cells after their closure due to the ventilation in handling drifts that could contribute to the regeneration of oxygen at the head of each disposal cell. The impact of these transient phases on corrosion processes may affect the confinement properties of metallic components and should thus be assessed. To this end, ferrous archaeological analogues are studied. The present paper focuses on the characterisation of nails that have undergone such transient phases. First, a sample exposed for several hundred years to an aerated environment and then to an anoxic carbonated environment in laboratory for 3 years has been analysed. Corrosion layers after the oxic phase contain mostly oxihydroxides (such as goethite) and some layers perpendicular or parallel to the interface, which is in good agreement with an aerated corrosion. After 3 years under anoxic conditions, the main phase of the corrosion layer is siderite. Second, a sample exposed to an anoxic environment for several hundred years has been immersed in an aerated solution for 5 weeks. The main phases of the corrosion layer after the anoxic phase are iron carbonates and a goethite layer formed on the outer part of the corrosion layer after the immersion in an aerated solution. In both cases, the formation of new phases in the corrosion layer is in good agreement with predictions of thermodynamic modelling based on the respective environmental conditions.