Structural characterisation of corrosion products on archaeological iron. An integrated analytical approach to establish corrosion forms

International audienceThe description and identification of corrosion products formed on archaeological iron artefacts needs various approaches at different observation scales. For this study, samples of five different sites were prepared using two techniques. The first one consists in cutting cross sections perpendicular to corrosion layers. This allows local observations and analysis of the corrosion layer stratigraphy at different levels. The second one consists in performing manual grinding or abrading of the corrosion layers starting from the current surface of the excavated artefact to the metal core. It allows the description of the successive layers and is well adapted for the analysis on a larger scale. In addition to these two observation scales, the identification of the iron oxides formed needs the coupling of several complementary techniques. Elementary compositions were determined by SEM-EDX and Electron Micro-Probe Analysis (EMPA). Structural identification was performed by X-ray micro Diffraction under synchrotron radiation (µXRD) and micro Raman spectroscopy. These analyses were performed on the same samples both with X-ray diffraction and Raman spectroscopy in order to ensure a reliable characterisation. In some cases there are some ambiguities or overlapping between signatures of different phases by micro X-rays diffraction (as maghemite/magnetite) or Raman spectroscopy (as goethite/magnetite) which can be raised by the association of the two methods. The final aim is to set up an analytical methodology that will be the best for the study of ancient iron corrosion products. It is the first step of the study of long term mechanisms of iron in soil

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

International audienceHistoric 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

Laser cleaning of Cu-based artefacts: laser/corrosion products interaction

This study aims to develop a low invasive and selective laser cleaning procedure for the removal of reactive corrosion products on Cu-based artefacts without damage the substrate. In a preliminary step, laser cleaning was performed on two typologies of artificially corroded copper reference samples. The effect of the variation of laser parameters as pulse duration and output power, was thus evaluated on an oxide layer, simulating a protective patina, and a hydroxychloride layer, simulating a reactive corrosion products layer to be removed. The optimized cleaning procedure was validated on an archaeological artefact, a bronze coin. Morphological, microchemical and microstructural characterizations were performed by means of optical microscopy, confocal microscopy, field emission scanning electron microscopy, X-Ray diffraction and Raman spectroscopy, before and after laser cleaning. The experimental findings show that laser cleaning, in optimized conditions, can reduce the thickness of the hydroxychloride layers slightly affecting the oxide layers. The difference in the interaction with laser radiation of these two layers seems to be mainly related to the difference in grain size and porosity. Notwithstanding these encouraging results, in order to define the real feasibility of the laser cleaning procedure, a further validation on real artefacts is mandatory due to the variation in thickness and composition of the corrosion products formed during long-lasting uncontrolled degradation processes.</p

Application of ToF-SIMS for sulfur isotopic fractionation in sulfide phases of iron corrosion layers: determination of bacterial impact on the formation of these phases

International audienceTo a better understanding of the iron corrosion mechanisms, it is crucial to elucidate the role of bacteria. Thus, in order to have a diagnosis of the action of bacteria in the iron corrosion, the objective of this study is to determine by ToF-SIMS the isotopic sulfur fractionation δ 34 S in iron sulfides of (sub)micrometric size located in the corrosion product layers of the ferrous objects corroded in anoxic soils and marine environment. These iron sulfides may have two origins: an abiotic origin resulting in the dissolution/reprecipitation of sulfides from the surrounding medium (for example from pyrite in a soil) or a biotic origin by the action of the sulfate-reducing bacteria that reduce sulfate ions into sulfides leading to the formation of iron sulfides with ferrous ions issued from the aqueous dissolution of metal iron. A difference in the isotopic sulfur fractionation δ 34 S in the sulfur phases according to the abiotic or biotic origin of these phases is reported in the literature. Thus in this study, the isotopic sulfur fractionation δ 34 S in the iron sulfides formed in the corrosion product layers of iron was determined to identify the formation origin of these phases and the role of bacteria in the iron corrosion. After characterization of the distribution of the different iron sulfides within the layers (optical microscopy, SEM-EDS, μRaman spectroscopy), the local isotopic fractionation of sulfur is measured by the imaging and spectroscopy ToF-SIMS technique. Some important results have been obtained. First of all, the reproducibility and the accuracy of the method were established by the choice and the validation of a hydrothermal pyrite sample calibrated relatively to the international sulfur isotopic standard Canon Diablo Troilite. Secondly, it was observed that there was no or little variation in the isotopic sulfur fractionation δ 34 S with the degree of oxidation of sulfur. Finally, a noticeable difference in the isotopic sulfur fractionation δ 34 S between iron sulfides formed in laboratory under abiotic or biotic conditions was evidenced: δ 34 S is close to 0 in abiotic conditions whereas in biotic conditions it is shifted from 0 with values up to up to some tens per mil. These first results being promising, the study is actually in progress to determine the formation origin (abiotic/biotic) of the Fe-S present in the corrosion product layers of natural systems of iron corrosion, from soils, subaquatic and marine environments

The James Webb Space Telescope Mission

Twenty-six years ago a small committee report, building on earlier studies, expounded a compelling and poetic vision for the future of astronomy, calling for an infrared-optimized space telescope with an aperture of at least $4m$. With the support of their governments in the US, Europe, and Canada, 20,000 people realized that vision as the $6.5m$ James Webb Space Telescope. A generation of astronomers will celebrate their accomplishments for the life of the mission, potentially as long as 20 years, and beyond. This report and the scientific discoveries that follow are extended thank-you notes to the 20,000 team members. The telescope is working perfectly, with much better image quality than expected. In this and accompanying papers, we give a brief history, describe the observatory, outline its objectives and current observing program, and discuss the inventions and people who made it possible. We cite detailed reports on the design and the measured performance on orbit.Comment: Accepted by PASP for the special issue on The James Webb Space Telescope Overview, 29 pages, 4 figure

Du nucléaire à la conservation du patrimoine - Compréhension des mécanismes de corrosion des alliages ferreux sur le long terme grâce à la caractérisation multi-échelles

The corrosion of metals such as steel presents a high variability depending on the environments of alteration (aqueous aerated and deaerated atmosphere, binders and concrete ...) and durations considered. As part of my research, in order to understand these mechanisms over long periods the study of archaeological artefacts corroded over tens to hundreds of years has been chosen. This research was crucial in many application areas, ranging from the preservation of objects of cultural heritage and studies for the storage and disposal of radioactive waste in deep geological environment. In this area the archaeological artefacts constitute unparalleled analogues for the study of corrosion of steel elements embedded in the multi-barrier to separate the radionuclides from the biosphere. Characterization of corrosion systems was conducted through a methodology based on the coupling of multiscale techniques for materials characterization (SEM-EDS, Raman microspectroscopy, microXRD, XAS synchrotron radiation ...) providing information on the location, morphology, composition and structure of the crystalline phases present in a layer of corrosion products. Moreover, studies of the reaction processes and of the chemical or electrochemical reactivity of the systems by recorrosion experiments of archaeological objects in labeled medium (D2O, O18) or under external stimuli (chemical, electrochemical) have highlighted phenomena controlling the corrosion process at the micrometric scale. Perspectives of my research are the study of further constraints of the corrosion processes such as the presence of bacteria in the environment, the study of processes at the nanoscale (TEM, STXM) but also the methodological development of new analytical approaches based on a combination of morphological imaging methods (FEG-SEM), composition (EDS), structure (μXRD under synchrotron radiation) or hyperspectral (Raman microspectroscopy) to determine the synergy of the nano and microscale phenomena at the scale of an object. Taken together, these studies should lead to develop further protocols for diagnosis, treatment and conservation of cultural objects and reinforce the mechanistic models developed for the design of ferrous elements included in the storage facilities of radioactive waste.Les phénomènes de corrosion des métaux tels que les aciers présentent une grande variabilité en fonction des milieux d'altération (aqueux aéré et désaéré, atmosphère, liants et béton...) et des durées prises en compte. Dans le cadre de mes activités de recherche, afin d'appréhender ces mécanismes sur le très long terme il a été choisi d'étudier des objets archéologiques corrodés sur plusieurs dizaines à centaines d'années. Cet axe de recherche est en effet crucial dans plusieurs domaines d'applications, allant de la conservation des objets du patrimoine culturel aux études menées pour l'entreposage et le stockage des déchets radioactifs en milieu géologique profond. Dans ce domaine les objets archéologiques constituent des analogues sans équivalents pour la corrosion des éléments en acier constitutifs de la multibarrière destinée à séparer les radioéléments de la biosphère. La caractérisation des systèmes de corrosion a été menée grâce à une méthodologie d'étude basée dans un premier temps sur le couplage de techniques de caractérisation des matériaux multi-échelles (MEB-EDS, , microspectroscopie Raman, microXRD, XAS sous rayonnement synchrotron...) donnant des informations sur la localisation, la morphologie, la composition et la structure des phases cristallines présentes dans une couche de produits de corrosion. Par ailleurs les études des processus réactionnels et de la réactivité chimique ou électrochimique des systèmes à l'aide de remise en corrosion de ces objets archéologiques en milieu marqué (D2O, O18) ou sous sollicitations externes (chimique, électrochimique) ont mis en évidence des phénomènes à l'échelle micrométrique contrôlant les processus de corrosion d'objets archéologiques. Les perspectives de mes recherches sont d'une part l'étude de nouvelles perturbations des processus de corrosion telles que la présence de bactéries dans le milieu, l'étude des processus à l'échelle nanométrique (MET, STXM) mais également sur le plan méthodologique, le développement de nouvelles approches analytiques basées sur la combinaison de méthodes d'imagerie de morphologie (MEB-FEG), de composition (EDS), de structure (µXRD sous rayonnement synchrotron) ou hyperspectrale (microspectroscopie Raman) afin de déterminer la synergie des phénomènes nano et microscopiques à l'échelle d'un objet. L'ensemble de ces études devra conduire à d'une part développer les protocoles de diagnostic, de conservation et de traitement des objets culturels ferreux et d'autre part conforter les modèles mécanistiques mis en place pour le dimensionnement des éléments ferreux inclus dans les dispositifs de stockage des déchets radioactifs

Preservation of metallic artefacts: understanding the long term degradation processes thanks to multi-scale characterisation

International audiencePreservation of metallic artefacts: understanding the long term degradation processes thanks to multi-scale characterisation Metallic artefacts undergo corrosion over long periods. This conducts to a loss of readability or shape of the artefacts prejudicial for the preservation of metallic cultural heritage. Stabilisation and protection treatments are conducted to preserve them from destruction. They are based on immersion or coating protocols who can lack of efficiency. That is why the development of scientific studies to understand the physico-chemical processes involved is necessary. This presentation will explained how the NIMBE/LAPA has developed a specific approach to achieve this aim. It will be shown how this scientific researches are conducted in close collaboration with curators. Case studies of artefacts coming from the archaeological terrestrial site of Glinet (16 th c, Normandy, France) and the marine site of Les Saintes Maries de la mer (1 st BC, Bouches du Rhône, France) will be presented

Apport des analogues archéologiques à l'estimation des vitesses moyennes et à l'étude des mécanismes de corrosion à très long terme des aciers non alliés dans les sols

Marc Aucouturier Rapporteur Gérard Béranger Directeur de thèse Didier Crusset Invité Philippe Dillmann Codirecteur de thèse Jean-Marie Gras Invité Gérard Moulin Président Antoine Pourbaix Invité Philippe Refait Examinateur Gérard Santarini RapporteurIn the context of the French nuclear waste storage, a multi-barriers disposal is envisaged. Wastes could be put in metallic overpacks disposed in a clay soil. As these overpacks could be made of low carbon steel, it is important to understand the corrosion behaviour of this material in soil during period of several centuries. Indeed, it is necessary to consolidate the empirical data by a phenomenological approach. This includes laboratory experiments and modelling of the phenomenon which have to be validated and completed by the study of archaeological artefacts. This was the aim of this PhD-work. To this purpose, an analytical protocol has been elaborated : about forty archaeological artefacts coming from five dated sites (2nd to 16th centuries) have been studied on cross section in order to observe on the same sample all the constituents of the system : metallic substrate/corrosion products/environment. The corrosion products are divided into two zones : the Dense Product Layer (DPL) in contact with the metal, and the Transformed Medium (TM) which are the corrosion products formed around soil minerals (quartz grains…). The metallic substrate has been studied by the classical methods of materials science (optical and scanning electron microscope, energy and wavelength dispersive spectroscopies). It has been verified that despite their heterogeneity of structure and composition, they are all hypoeutectoïdes steels that can contain phosphorous until 0.5 wt%. The corrosion products have been analysed by local structural analytical methods as microdiffraction under synchrotron radiation (µXRD) and Raman microspectroscopy. These two complementary techniques and also the elemental composition analysis conducted to the characterisation of the corrosion forms. On the majority of the samples coming from four sites, the DPL are constituted by goethite including marbles of magnetite/maghemite. On the artefacts from the fifth site, a particular corrosion form has been identified. This corrosion form, constituted among others by a siderite layer is due to a particular environment : waterlogged soil containing wood. In the whole, analyses conducted in the TM show that it is composed of goethite badly crystallised in comparison with those of the DPL. Moreover, in this zone, the average elemental iron amount decreases progressively from the metal to the soil in which it stabilises. In order to know the behaviour of the identified phases in soil water, some thermodynamic data have been involved to calculate their solubility in function of pH, potential and various water composition. The first conclusion concerns the influence of the composition and the structure of the material which is not important for the corrosion behaviour. From the results, some hypothesis have been formulated on the long term corrosion mechanisms of hypoeutectoïdes steels in the considered environment. The role of the cracks formed in the DPL during the burial was evidenced. Moreover, these corrosion products undertake a dissolution in the soil water and a reprecipitation, explaining the progressive decrease of the iron amount in the TM. Lastly, some average corrosion rates have been measured with the help of the analytical and thermodynamic results : they do not exceed 4 µm/year.Dans le cadre du programme français de stockage des déchets radioactifs, mis en place suite à la loi du 30 décembre 1991, un dispositif multi-barrières est envisagé. Les déchets seraient inclus dans des surconteneurs métalliques, eux-mêmes déposés dans une barrière ouvragée en argile. Comme ces surconteneurs pourraient être en acier non allié, il est important de connaître le comportement en corrosion de ce matériau dans les sols pour des périodes de plusieurs centaines d'années. Pour ce faire, il est nécessaire de consolider les données empiriques par une approche phénoménologique. Celle-ci comprend des expérimentations de simulation en laboratoire et une modélisation des phénomènes qui se doivent d'être validées et complétées par l'étude d'objets archéologiques servant d'analogue. Ceci a été l'objectif de ce travail de thèse. Dans ce but, un protocole d'analyse a été mis au point : une quarantaine d'objets prélevés en mottes et provenant de cinq sites datés (2ème-16ème siècle) ont été étudiés sur coupe transversale afin d'observer sur le même échantillon l'ensemble du système matériau/produits de corrosion/milieu. Les produits de corrosion sont subdivisés en deux domaines : les Couches de Produits Denses de corrosion (CPD) en contact avec le métal, et le Milieu Transformé (MT) qui sont les produits de corrosion formés autour des minéraux du milieu (grains de quartz…). Le substrat métallique a été étudié par les méthodes classiques d'analyse des matériaux (microscope optique et électronique, spectroscopies dispersive en énergie et en longueur d'onde). Il a été vérifié que les substrats métalliques, malgré leur hétérogénéité de structure et de composition sont des aciers hypoeutectoïdes pouvant contenir jusqu'à 0,5 %mas de phosphore. Les produits de corrosion ont fait l'objet d'analyses structurales locales telles que la microdiffraction sous rayonnement synchrotron et la microspectroscopie Raman. Ces deux techniques utilisées de manière complémentaire, ainsi que la mise en œuvre d'analyses de composition élémentaires ont conduit à la caractérisation de faciès de corrosion. Sur la majorité des objets provenant de quatre sites, les CPD sont principalement composées de goethite marbrée de liserés de magnétite/maghémite. Sur les objets du cinquième site, un faciès particulier de corrosion a été caractérisé. Ce faciès, présentant notamment de la sidérite, est dû à un environnement d'enfouissement particulier : sol détrempé contenant du bois. Sur l'ensemble des échantillons, les analyses du MT montrent que celui-ci est composé de goethite moins bien cristallisée que celle formant les CPD. De plus, la teneur moyenne élémentaire en fer dans cette zone décroît progressivement du métal vers le sol, dans lequel elle se stabilise. Afin de cerner le comportement des phases identifiées en milieu aqueux, des calculs thermodynamiques ont été mis en œuvre pour déterminer leur solubilité en fonction du pH, du potentiel et de différentes compositions en eau. Une première conclusion de ce travail concerne l'influence de la composition et de la structure du matériau qui n'est pas prépondérante sur le comportement en corrosion. A partir de l'ensemble des résultats, des hypothèses ont pu être formulées sur les mécanismes de corrosion à long terme des aciers hypoeutectoïdes dans les milieux étudiés. Elles mettent en évidence le rôle des fissures dans les transports d'espèces ioniques dans les produits de corrosion. De plus, ces produits de corrosion subissent une dissolution puis une reprécipitation dans le milieu environnant ce qui entraîne le décroissance de la teneur moyenne en fer observée dans le MT. Enfin, à l'aide de ces données, des vitesses moyennes de corrosion ont été évaluées : elles sont au maximum de 4 µm/an sur l'ensemble du corpus