Thermal Reconversion of Oxidised Lead White in Mural Paintings via a Massicot Intermediate

Lead white is the most ancient and common white pigment used in mural paintings. However, it tends to blacken with time due to its oxidation to plattnerite (\b{eta}-PbO2). Chemical treatments were used but they can put the pictorial layers supports at risks. Hereby we address the possibility of thermally reconverting black plattnerite to white lead carbonates via a massicot (\b{eta}-PbO) intermediate, with a view to developing a restoration procedure using continuous wave laser heating. We first investigated the conditions (temperature, time, and environment) in which pure powders react, before studying mural painting samples. Experiments were made in ovens and TGA and XRD and SEM characterization were achieved. Litharge ({\alpha}-PbO) and massicot were obtained from plattnerite respectively between 564 and 567 {\deg}C and at 650 {\deg}C. Lead carbonates (cerussite, hydrocerussite and plumbonacrite) formed from massicot in wet CO2 below 100 {\deg}C in a few hours. Lastly, when heating plattnerite based mural painting samples, lead species reacted with binders and mortar, yielding massicot, plumbonacrite but also lead silicate and calcium lead oxides. This demonstrates the viability of thermal reconversion of darkened lead in mural, while raising concerns about the formation of several lead species by reaction with mural painting constituents

Formation and characterization of hydride blisters in Zircaloy 4 cladding tubes

International audienceThis article is focused on the formation of hydride blisters in zirconium alloys an experimental and theoretical standpoint, and their characterization in terms of morphology, hydrides crystallographic phases, hardness and hydrogen concentration. An experimental setup was developed to grow hydride blisters on pre-hydrided Zircaloy-4 cladding tubes by thermo-diffusion. The thermal conditions were optimized based on thermo-diffusion calculations, that take into account the hysteresis in the hydrogen solubility limit, to obtain a high blister growth rate. Micro-X-ray Diffraction (XRD), nano-hardness and Elastic Recoil Detection Analysis (ERDA) showed that the blisters contain a hydrogen gradient, with pure δδ-hydride phase close to the external surface over one third of the blister depth. Thermo-diffusion calculations showed these half thickness blisters should grow in only a few days in PWR conditions. Eventually, the Diffusion Equilibrium Threshold (DET) was defined as a criterion that limits the blister growth, and emphasizes that the hysteresis in the hydrogen solubility limit in zirconium must be taken into account to model hydrogen thermo-diffusion in zirconium alloys

Macroscopic and microscopic determinations of residual stresses in thin Oxide Dispersion Strengthened steel tubes

To improve the efficiency of components operating at high temperatures, many efforts are deployed to develop new materials. Oxide Dispersion Strengthened (ODS) materials could be used for heat exchangers or cladding tubes for the new GENIV nuclear reactors. This type of materials are composed with a metallic matrix (usually iron base alloy for nuclear applications or nickel base alloy for heat exchangers) reinforced by a distribution of nano-oxides. They are obtained by powder metallurgy and mechanical alloying. The creep resistance of these materials is excellent, and they usually exhibit a high tensile strength at room temperature. Depending on the cold working and/or the heat treatments, several types of microstructure can be obtained: recrystallised, stress relieved. One of the key challenges is to transform ODS materials into thin tubes (up to 500 microns thick) within a robust fabrication route while keeping the excellent mechanical properties. To prevent cracking during the process or to obtain a final product with low residual stresses, it is important to quantify the effect of the heat treatments on the release of internal stresses. The aim of this study is to show how residual stresses can be determined on different thin tubes using two complementary approaches: (i) macroscopic stresses determination in the tube using beam theory (small cuts along the longitudinal and circumferential directions and measurements of the deflection), (ii) stress determination from X-ray diffraction analyses (surface analyses, using "sin 2 Ψ" method with different hypothesis). Depending on the material and the heat treatment, residual stresses vary dramatically and can reach 800 MPa which is not far from the yield stress; comparisons between both methods are performed and suggestions are given in order to optimize the thermo-mechanical treatment of thin ODS tubes

Growth Kinetics and Characterization of Chromia Scales Formed on Ni–30Cr Alloy in Impure Argon at 700 °C

The oxidation of a Ni–30Cr alloy at 700 °C in impure argon was studied in order to provide new elements of understanding on chromia scale growth in low oxygen partial pressure atmosphere (10−5 atm). Oxidation tests were carried out during 30 min to 50 h in a thermogravimetric analysis system using a symmetrical balance with in situ monitoring of the oxygen partial pressure. The oxidation kinetics were determined as parabolic with an estimated stationary parabolic constant value of 10−15 cm2 s−1, after a transient stage of about 3 h. The oxide scale was identified as a pure chromia layer by TEM and XPS characterisations. After 50 h at 700 °C, the scale thickness estimated by TEM cross section observation was about 100 nm. A slightly thicker and more porous oxide scale was observed above the alloy’s grain boundaries. The metal/oxide interface also exhibited a deeper recession towards the substrate above the alloy’s grain boundaries. The orientation of chromia grains was determined by TKD (transmission Kikuchi diffraction). A strong preference was noted for the orientation perpendicular to the surface, along the direction of the corundum structure. Finally, the type of semiconduction was determined for the scales formed after 7 h and 50 h of oxidation. For the shorter oxidation time, the chromia scale exhibited an n-type semiconduction, whereas for the longer exposure, both n-type and insulating semiconduction were identified

DFT Study of Defects in Paramagnetic Cr2O3

International audienceNi-based alloys with high Cr concentrations tend to show good corrosion resistance at high temperatures thanks to the formation of a protective chromia scale on the alloy surface. Growth kinetics of the scale is generally controlled by a diffusion process. This factor depends on the nature, concentration, and diffusivity of the dominant type of point defects in the chromia scale. However, the nature of the dominant point defects, which depends on alloy nature, temperature, and oxidizing atmosphere, remains unclear. DFT calculation is a powerful technique to predict the nature of dominant point defects by calculating defect formation energies. In the literature, the magnetic state of chromia used for calculating defect formation energies is always the fundamental state at low temperature, which is antiferromagnetic. But chromia formed during oxidations above the Néel temperature (318 K) exhibits the paramagnetic state. The transition of magnetic state near Néel temperature may affect the defect formation in chromia.The aim of this work is to determine the kind of dominant 0D-defects in chromia scale during high temperature oxidation. For this purpose, defect formation energies were calculated with ab initio calculations in antiferromagnetic and paramagnetic chromia. Brower diagrams at different temperatures were drawn based on the calculated defect formation energies with thermodynamic approximations, in order to study the dependence of temperature and oxygen partial pressure on the nature of dominant point defects.A previous experimental study suggests that O vacancies are the favourable point defects at low temperature/or low oxygen partial pressure p(O2) while Cr vacancies are dominant at high temperature/or high p(O2) for Ni-30Cr oxidations. In order to compare experimental and simulation results, oxidation tests are carried out in Rhines Pack controlled atmosphere. The grown oxides are characterized by photoelectrochemistry in order to determine their electronic properties and to deduce the dominant type of point defects

Effect of a pre-oxidation on the hydrogen desorption from Zircaloy-4

International audienceDuring the transport of nuclear spent fuel, part of the tritium formed by ternary fission in core of nuclear reactors is susceptible to desorb from the oxidized cladding. This study aimed at identifying the rate-limiting step in the hydrogen desorption process from pre-oxidized Zircaloy-4 specimens. Controlled-thickness oxide scales shifted the hydrogen desorption from the alloy towards higher temperatures during a temperature ramp under vacuum. Scanning electron microscopy observations and finite elements modelling of the oxide layer dissolution led to the conclusion that, in such conditions, hydrogen desorption from the alloy was controlled by the oxide dissolution kinetics

Behavior of three cast chromium-rich cobalt-based alloys in oxidation at 900°C in a 100% water vapour atmosphere

International audienceBeside the famous single-crystalline γ/γ' superalloys other metallic alloys retain high interest for some high temperature applications. This is for example the case of the foundry-made cobalt-based superalloys family. These alloys may offer to the users high mechanical strength in service at elevated temperature as well as correct resistance against oxidation by gases and corrosion by molten salts. A typical example of use is as the constitutive material of glass working tool for shaping molten glass. The oxidative gaseous milieu in which cast cobalt-based superalloys work the most often are dry or wet air, or mixtures of burner gases. However there are applications, notably in the nuclear power field, in which hot gases may contain very high quantities of water vapour and for which cobalt-based superalloys may be considered. Their behaviour in such water-rich gaseous environment seems to be never investigated and this is the reason why testing a little series of cast cobalt-based alloys in pure water vapour was decided here

Corrosion of Two Iron-Based Aluminaforming Alloys in NaCl-MgCl₂ Molten Salts at 600 °C

Molten salts have been used as heat transfer fluids since the middle of the 20th century. More recently, molten chloride salts have been studied for use in concentrated solar power plants or molten salt reactors. However, none of the materials studied to date has been able to withstand this highly corrosive environment without controlling the salt’s redox potential. The alumina-forming alloy was a promising option, as it has not yet been widely studied. To investigate this possibility, two iron-based alumina-forming alloys were corroded in NaCl-MgCl2 eutectic at 600 °C for 500 h after being pre-oxidised to grow a protective layer of α-alumina on each alloy. A salt purification protocol based on salt electrolysis was implemented to ensure comparable and reproducible results. During immersion, alumina was transformed into MgAl2O4, as shown by FIB-SEM observation. Inter and intragranular corrosion were observed, with the formation of MgAl2O4 in the corroded zones. The nature of the oxides was explained by the predominance diagram. Intragranular corrosion was 2 µm deep, and intergranular corrosion 10 µm deep. Alumina formed at the bottom of the intergranular corrosion zones. The depth of intergranular corrosion is consistent with O diffusion control at the grain boundary

Macroscopic and microscopic determinations of residual stresses in thin Oxide Dispersion Strengthened steel tubes

International audienceTo improve the efficiency of components operating at high temperatures, many efforts are deployed to develop new materials. Oxide Dispersion Strengthened (ODS) materials could be used for heat exchangers or cladding tubes for the new GENIV nuclear reactors. This type of materials are composed with a metallic matrix (usually iron base alloy for nuclear applications or nickel base alloy for heat exchangers) reinforced by a distribution of nano-oxides. They are obtained by powder metallurgy and mechanical alloying. The creep resistance of these materials is excellent, and they usually exhibit a high tensile strength at room temperature. Depending on the cold working and/or the heat treatments, several types of microstructure can be obtained: recrystallised, stress relieved. One of the key challenges is to transform ODS materials into thin tubes (up to 500 microns thick) within a robust fabrication route while keeping the excellent mechanical properties. To prevent cracking during the process or to obtain a final product with low residual stresses, it is important to quantify the effect of the heat treatments on the release of internal stresses. The aim of this study is to show how residual stresses can be determined on different thin tubes using two complementary approaches: (i) macroscopic stresses determination in the tube using beam theory (small cuts along the longitudinal and circumferential directions and measurements of the deflection), (ii) stress determination from X-ray diffraction analyses (surface analyses, using "sin 2 Ψ" method with different hypothesis). Depending on the material and the heat treatment, residual stresses vary dramatically and can reach 800 MPa which is not far from the yield stress; comparisons between both methods are performed and suggestions are given in order to optimize the thermo-mechanical treatment of thin ODS tubes

Low oxygen partial pressure oxidation in a model Ni-30Cr alloy between 500 and 900°C : point defects and oxidation mechanisms in chromia.

International audienceNi-based alloys containing high Cr concentrations are considered as structural materials for present and future nuclear reactors [1,2]. Indeed, their good corrosion resistance is provided, at intermediate and high temperature (300 to 900 °C), by the formation of a protective chromia scale on the alloy surface. Growth kinetics of chromia scale follows a parabolic law with a parabolic constant kp which depends on the nature, the concentration, and the diffusivity of the predominant point defects in the chromia scale [3]. The growth of this layer can be limited by either anionic (O2-) or cationic (Cr3+) diffusion. However the nature of the diffusing element (cation or anion) and of the point defects involved in its diffusion process in chromia is still not clear. Indeed, various defects (oxygen and chromium vacancies, chromium interstitials) were identified for chromia scale growing on Ni-Cr [4_6] and other chromia-forming alloys [7_13]. Both (diffusing element and its defects) can depend on the nature of the alloy and on oxidation conditions such as temperature and atmosphere. The oxidation kinetics of chromia-forming alloys, depending on the diffusion defects and then on the alloy nature and the oxidizing environment are then very scattered. The aim of this work is to clarify what happens during the oxidation of a Ni-Cr model alloy containing 30 wt% of Cr. The objective is to study the effect of oxidation temperature on the nature of the predominant point defects responsible for the chromia scale growth and to propose a growth mechanism. To this intent, thermogravimetric analysis (TGA), photoelectrochemistry (PEC), 18O and 54Cr tracers experiments and SIMS and nanoSIMS analysis have been carried out