Corrosion sous contrainte de l'alliage 600 en milieu primaire des réacteurs à eau sous pression : apport à la compréhension des mécanismes

Alloy 600, a nickel base alloy (containing 15 wt% Cr), is susceptible to intergranular stress corrosion cracking in hydrogenated PWR primary water. Despite the fact that many laboratory studies have been performed and that many models have been proposed in the literature, the SCC mechanisms involved are still not well understood. Among others models, the internal oxidation model and models based on hydrogen/material interactions appear likely to explain this phenomenon. Therefore, the aim of this study is to improve our knowledge of Alloy 600 SCC mechanisms in PWR primary water. Experimental tests and calculations were performed in order to select models. Imposed strain specimens (“U-bend”) and without imposed strain specimens (coupon) were tested in simulated PWR primary water containing tracers (18 oxygen and deuterium). After testing, crack tips and intergranular oxide penetrations were characterized by SEM, TEM, SIMS, Nano-SIMS, and hydrogen titration permit to discuss oxide thermodynamic stability, oxide geometry and nature in crack tip, hydrogen role, intergranular chromium carbide role and cracking mode. Thus, a new model of cracking has been proposed. This model is based upon chromium oxide formation in the grain boundary. Material parameters (defect rate, strain, grain boundary type) and environmental parameters (hydrogen content, temperature) would affect oxygen diffusion kinetic across the oxide formed in the grain boundary and chromium diffusion kinetic in the grain boundary of the alloy.De nombreuses études ont été réalisées par la communauté scientifique internationale sur la Corrosion Sous Contrainte (CSC) de l'Alliage 600 en milieu primaire des Réacteurs à Eau Pressurisée (REP). Néanmoins, les mécanismes à l'origine de cette fissuration sont encore mal compris. Un certain nombre de modèles ont été développés et proposés, cependant peu d'entre eux arrivent à intégrer l'influence de tous les paramètres. Le mécanisme d'oxydation interne et la famille de modèles basés sur l'action de l'hydrogène ont paru les plus prometteurs. L'objectif de l'étude était d'apporter des connaissances nouvelles sur le/les mécanisme/s de CSC. Il a été choisi de cibler les expérimentations de façon à sélectionner les modèles les plus pertinents. Pour cela des essais dans un milieu simulant le milieu primaire avec des marqueurs isotopiques (oxygène 18 et deutérium) ont été menés sur deux types d'éprouvettes. Des éprouvettes à déformation imposée de type « U-bend » ont été utilisées dans un premier temps et des éprouvettes sous forme de plaquette ayant des microstructures contrôlées ont été utilisées dans un second temps. Les caractérisations des fissures et des pénétrations d'oxyde intergranulaire par MEB, MET, SIMS, Nano-SIMS et des dosages d'hydrogène, ont permis une nouvelle analyse de la CSC de l'Alliage 600 en milieu REP, concernant plus particulièrement les points suivants : la stabilité thermodynamique des phases, la répartition et la morphologie des oxydes observés dans les fissures, le rôle de l'hydrogène, la nature des carbures de chrome et le caractère continu/discontinu du mécanisme de fissuration.A partir des résultats obtenus à l'aide du traçage isotopique de l'hydrogène et de l'oxygène couplé à l'utilisation de différentes techniques d'analyse complémentaires, il a été proposé un nouveau modèle de mécanisme de fissuration basé sur la formation d'oxyde de chrome aux joints de grains de l'alliage. Ce modèle permet de prendre en compte le rôle des paramètres liés au matériau (taux de défauts, déformation, type de joint de grain) et au milieu (teneur en hydrogène, température) qui influeraient sur les cinétiques de diffusion de l'oxygène dans l'oxyde aux joints de grains de l'alliage et du chrome aux joints de grains de l'alliage

Corrosion sous contrainte de l'Alliage 600 en milieu primaire des Réacteurs à Eau Pressurisée (apport à la compréhension des mécanismes)

PARIS-MINES ParisTech (751062310) / SudocSudocFranceF

Mécanisme de la corrosion sous contrainte de l'alliage 600 en milieu REP : apport à la compréhension des mécanismes

National audienc

Advanced TEM characterization of stress corrosion cracking of Alloy 600 in pressurized water reactor primary water environment

International audienceAdvanced transmission electron microscopy techniques were carried out in order to investigate stress corrosion cracking in Alloy 600 U-bend samples exposed in simulated PWR primary water at 330 °C. Using high-resolution imaging and fine-probe chemical analysis methods, ultrafine size oxides present inside cracks and intergranular attacks were nanoscale characterized. Results revealed predominance of Cr2O3 oxide and Ni-rich metal zones at the majority of encountered crack tip areas and at leading edge of intergranular attacks. However, NiO-structure oxide was predominant far from crack tip zones and within cracks propagating along twin boundaries and inside grains. These observations permit to suggest a mechanism for intergranular stress corrosion cracking of Alloy 600 in PWR primary water. Indeed, the results suggest that stress corrosion cracking is depending on chromium oxide growth in the grain boundary. Oxide growth seems to be dependent on oxygen diffusion in porous oxide and chromium diffusion in strained alloy and in grain boundary beyond crack tip. Strain could promote transport kinetic and oxide formation by increasing defaults rate like dislocations

Corrosion and decommissioning

International audienceMore than 110 commercial nuclear reactors, 48 prototypes and 250 research nuclear reactors have been retired from operation. Only 17 of them had full decommissioning. For all the others, corrosion of key components for the confinement is been faced linked to the exposure to more or less corrosive environments. For the 12 reactors closed following an accident, including partial core melt for 8 reactors (for instance the last 4 damaged reactors of Fukushima), the cooling of the core melt is needed and has to be assumed over long period of time which leads to specific corrosion issues. The importance of corrosion management during decommissioning is outlined regarding (i) structural materials which have been chosen for reactor standard operation, but not for decommissioning conditions during which functions like tightness or mechanical resistance have still to be maintained; (ii) long term issues for deferred dismantling, (iii) and after core melt the behavior and the influence of corium interactions and fuel debris has to be investigated.The corrosion issues during decommissioning lead to increase the knowledge in several areas including thermodynamics (for the corium), material behavior and irradiation damages coupled with long term issues. Examples will be given and will illustrate also the importance of the modeling and simulation for coupling multi-corrosion issues faced during decommissioning

A step towards a better understanding of corrosion of zirconium in nitric acid with additions of fluorine: focus on the role of the presence of an initial oxide film

International audienceThe presence of fluorine is known to drastically reduce the corrosion resistance of zirconium in nitric acid. Nevertheless corrosion processes involved for this system appear controversial, especially concerning the role played by the initial oxide film (ZrO2) that can naturally be formed in nitric acid. To study the contribution of the initial oxide film, zirconium samples were pre-oxidized in several ways so as to obtain different thicknesses of the initial oxide film. Then, the corrosion evolution of these samples was studied in nitric acid with fluorine additions. It appears that the initial oxide film present on surface plays a protective role for corrosion. But as the oxide film slowly dissolves, this protective role may only be transitory (depending on the thickness of the initial oxide film). When the oxide film has been sufficiently dissolved, it breaks by a process of initiation, growth and coalescence of pits. Zirconium is then directly attacked by fluorine and its corrosion rate increases largely. After the complete vanishing of the initial oxide film, corrosion becomes uniform and the corrosion rate decreases slowly: the corrosion process is limited by the diffusion and the consumption of fluorine

Stable isotopes used in the definition of corrosion mechanisms

International audienceThe ability of secondary-ion mass spectrometry (SIMS) to separate isotopes and to analyze thin layers by sputtering provides unique tools for studying corrosion mechanisms. The methodology with oxygen 18 has been tested with success at the beginning of the 70s for the oxidation of tantalum (Ta) by water in two steps: first oxidation by H216O and subsequent one by H218O [1]. Nevertheless, very few corrosion studies have taken the advantage of isotope substitution for the investigation of corrosion mechanisms. The objective of the paper is to show that the use of stable isotopes has been a major step in the understanding and the modeling of several corrosion phenomena. The first illustrations will be linked to the localization of the anodic and cathodic reactions on archeological analogues. Then the use of isotopic tracers will be shown in more complex environments.In liquid lead-bismuth, sequential experiments with dissolved 18O and 16O have been performed to determine the mechanisms of growth of the duplex structure oxide layer: It was found that the magnetite layer grows at the Pb–Bi/oxide interface whereas the Fe–Cr spinel layer grows at the metal/oxide interface. The modeling of the growth mechanisms of the duplex layer lead to the evaluation of corrosion damages, in accordance with available data. It should be underlined that the same type of growth of duplex layer has been observed in supercritical water with the subsequent used of H216O followed by an exposure to H218O.Stress corrosion cracking of Alloy 600 in water at 300-350°C has been investigated with Alloy 600 (nickel base alloy with 15% Cr and more than 72% Ni) samples exposed to heavy water with dissolved hydrogen (D2O / H2, diss) or to natural water with dissolved deuterium (H2O / D2, diss). SIMS analysis of D (2H) and 16O were done to determine the deuterium concentration profiles together with the oxide film thickness on the alloy surface. Almost no deuterium is observed for samples exposed in the H2O/D2 environment and only in the oxide layer, whereas the intensity of the deuterium profile is much larger in D2O/H2 with deuterium observed not only in the oxide layer but also in the alloy. Clearly, the main source of hydrogen is the cathodic reaction (water dissociation). Two mechanisms may be proposed for modelling the hydrogen transport associated with the oxide growth during alloy passivation: (i) diffusion of hydrogen as an interstitial proton through the oxide lattice, or (ii) diffusion as a hydroxide ion towards the oxide in the anionic sub-lattice. The latter hypothesis implies the oxygen and hydrogen diffusivities through the oxide layer to be the same. To check which hypothesis is correct, Alloy 600 specimens have been exposed in PWR primary conditions using 2H and 18O as markers. The values obtained for diffusion coefficient of 2H and 18O are very close (around 5 10-17 cm2/s) which supports the idea of a hydrogen transport mechanism through the oxide layer as hydroxide ions. The strong correlation between hydrogen absorption and oxidation occurs not only for the formation of the oxide layer on the surface of the alloy, but also during intergranular oxidation of grain boundaries. The question here is to assess whether the oxide grown at the grain boundaries in the case of intergranular corrosion would act as a barrier to hydrogen arrival to the oxide/crack tip or not. After a primary oxidation in nominal primary water followed by a short period under the same conditions but with D and 18O isotopes, deuterium and oxygen 18 are found at the tip of the intergranular oxidation, even for short exposure times. The results lead to the conclusion that oxygen and hydrogen transport in the oxidized grain boundary are not the rate-controlling step for SCC initiation in PWR nominal conditions. To check if chromium diffusion is the limiting step, diffusion experiments with 54Cr in Ni-Cr alloys have been performed.The conclusive remarks will include some recommendations and some interests for the use of radioactive tracers to determine corrosion kinetics

The role of intergranular chromium carbides on intergranular oxidation of nickel based alloys in pressurized water reactors primary water

International audienceAlloy 600 is used in pressurized water reactors (PWRs) but is susceptible to primary water stress corrosion cracking (PWSCC). Intergranular chromium carbides have been found beneficial to reduce PWSCC. Focussed ion beam coupled with scanning electron microscopy (FIB/SEM) 3D tomography has been used to reconstruct the morphology of grain boundary oxide penetrations and their interaction with intergranular Cr carbides in Alloy 600 subjected to a PWR environment. In presence of intergranular Cr carbides, the intergranular oxide penetrations are less deep but larger than without carbide. However, the intergranular oxide volumes normalized by the grain boundary length for both samples are similar, which suggest that intergranular oxidation growth rate is not affected by carbides. Analytical transmission electron microscopy (TEM) shows that the intergranular oxide consists mainly in a spinel-type oxide containing nickel and chromium, except in the vicinity of Cr carbides where Cr 2 O 3 was evidenced. The formation of chromium oxide may explain the lower intergranular oxide depth observed in grain boundaries containing Cr carbides

Mécanismes de la corrosion sous contrainte de l'alliage 600 en milieu primaire REP : mise au point de dépôt du chrome

National audienc

Influence of laser powder bed fusion processing parameters on corrosion behaviour of 316L stainless steel in nitric acid

International audienceThe effect of process parameters on the microstructure and corrosion behavior of additively manufactured 316L stainless steel was reported. Immersion tests were performed in nitric acid solution at boiling temperature and the corrosion behaviour was correlated to microstructure of 316L stainless steel specimens produced by laser powder bed fusion (L-PBF) as a function of the process parameters such as scanning strategy, laser power and hatching distance. These parameters were found to influence the porosity, the grain size and the cellular microstructure. The corrosion tests revealed a higher impact of hatching distance on corrosion behaviour. Post-mortem microstructural examination revealed that the corrosion preferentially occurred at cellular structures and at grain boundaries and melt pools when the scanning strategy and hatching distance were respectively modified. The solute segregation at boundaries cells, the grain size distribution and the porosity could explain this corrosion behaviour. The results were compared with those of solution annealed counterpart, which would provide a factual basis for future applications of L-PBF 316L stainless steel