Haut PDF Stress corrosion cracking of Ni-base alloys in PWR primary water

Stress corrosion cracking of Ni-base alloys in PWR primary water

Stress corrosion cracking of Ni-base alloys in PWR primary water

26 MAI 2016 CEA | 16es Journées Scientifiques de la DANS | PAGE 1 Den-Service de la Corrosion et du Comportement des Matériaux dans leur Environnement (SCCME), CEA, Université Paris-Saclay, F-91191, Gif-sur-Yvette, France CEA and MINES ParisTech : E. Chaumun, P. Laghoutaris CEA : O. Raquet

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Influence of δ phase precipitation on the stress corrosion cracking resistance of alloy 718 in PWR primary water

Influence of δ phase precipitation on the stress corrosion cracking resistance of alloy 718 in PWR primary water

1. Introduction Stress corrosion cracking (SCC) is a damaging mode in number of structural alloys used in pressurized water reactors (PWR) of nu- clear power plants, particularly of nickel based alloys [1–5] . Even if alloy 718 is not the most commonly used structural alloy in term of mass when compared to alloy 690 constituting the vapour genera- tor tubes, it is yet constitutive of highly stressed structures of fuel assemblies, such as springs, hold-down system screws, etc. The use of alloy 718 for such applications is both justified by its high mechanical properties and its excellent resistance to crack initia- tion in the very severe service conditions of the PWR. That excel- lent behaviour of alloy 718 is due to the formation during the aging heat treatment of coherent metastable Ni 3 Nb c 00 precipitates
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Influence of ageing heat treatment on A-286 microstructure and stress corrosion cracking behaviour in PWR primary water

Influence of ageing heat treatment on A-286 microstructure and stress corrosion cracking behaviour in PWR primary water

* Corresponding author. Tel.: +33-4-77-42-02-98; fax: +33-4-77-42-01-57; E-mail address: david.delafosse@emse.fr 1. Introduction Precipitation-hardening austenitic stainless steel A-286 is widely used in the gas turbine industry for its high strength and corrosion resistance. Since it also has a thermal coefficient of expansion comparable to type 304 SS, it is also used for reactor vessel internals bolting applications in the nuclear industry [1]. Several A-286 reactor vessel internals bolts failures were reported during PWR in-service inspections in the early 80’s [1-2]. The subsequent extensive evaluation program identified Intergranular Stress Corrosion Cracking (IGSCC) as the degradation mechanism and recommended to operate bolts at stress levels below the material’s yield stress [1-2]. However, the effect of heat treatment and metallurgical structure on IGSCC as received little attention for A-286 in comparison to other precipitation- hardening NiFeCr alloys 718 and X-750 also used in the nuclear industry for bolts, springs, beams and pins applications [3-4]. The aim of this study was therefore to gain insights into the influence of microstructure on A-286 SCC behaviour in simulated PWR primary water.
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Evaluation of stress corrosion cracking of irradiated 304L stainless steel in PWR environment using heavy ion irradiation

Evaluation of stress corrosion cracking of irradiated 304L stainless steel in PWR environment using heavy ion irradiation

CERT tests were conducted in a tensile testing device CORMET C137 in the Service de Corrosion et du Comportement des Mat! eriaux dans leur Environnement (SCCME) at CEA Saclay in a simulated PWR primary water chemistry environment. The setup consisted of an autoclave with a capacity of 5 L, a load frame, and a computer-driven 30 kN load train for straining the samples. The sample was mounted on heat-treated Inconel sample holder and into the load frame of the autoclave. The autoclave was filled with primary water (25e35 cc/kg H2 STP, 1000 ppm B, 2 ppm Li), sealed, and pressurized with argon gas to detect any leakage. The tem- perature of the system was increased to reach the test temperature of 340 ! C, and the pressure was maintained at 155 bars. Pressure and temperature were monitored using a PT (Pressure-Tempera- ture) sensor located in the center of the autoclave. Prior to straining, environmental conditions were allowed to stabilize for a few hours. Pressure, dissolved oxygen content, and water conductivity were measured by sampling the water after the test. The displacements were measured by a Linear Variable Displacement Transducer (LVDT) located on the traction line of the autoclave. Load and displacement data was collected by a computerized data acquisi- tion system and recorded every 10 s. After achieving stable condi- tions, the tensile specimen was strained at a rate of 5 $ 10 #8 s #1 up to 4% plastic strain.
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The role of intergranular chromium carbides on intergranular oxidation of nickel based alloys in pressurized water reactors primary water

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

intergranular oxide depth observed in grain boundaries containing Cr carbides. 1. Introduction Currently, there are plans to extend the lifetime of pressurized water reactors (PWRs) of French nuclear power plants up to 60 years. Among the materials used in PWRs, nickel based alloys such as Alloy 600 (Ni-15Cr-10Fe) are used as structural materials in the primary circuit. Alloy 600 provides a good overall corrosion resistance but is susceptible to primary water stress corrosion cracking (PWSCC) [1, 2]. As a consequence, PWSCC of Alloy 600 and parent weld metals (Alloys 182/82) is a significant cause of failure in the primary circuit of PWRs [3]. This mode of damage leads to a brittle-type intergranular cracking of the material under the combined action of the environment and a tensile stress state. The SCC phenomenon is composed of several stages: development of oxide penetrations at the grain boundaries, fracture of the oxide leading to crack initiation, and then propagation of an intergranular crack [4]. However, SCC is a complex process and even though it has been studied extensively over the last thirty-five years, the cracking process is still under question [5-11]. To prevent PWSCC of Alloy 600, the presence of chromium carbides at the grain boundaries
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Evaluation of stress corrosion cracking of irradiated 304L stainless steel in PWR environment using heavy ion irradiation

Evaluation of stress corrosion cracking of irradiated 304L stainless steel in PWR environment using heavy ion irradiation

CERT tests were conducted in a tensile testing device CORMET C137 in the Service de Corrosion et du Comportement des Mat! eriaux dans leur Environnement (SCCME) at CEA Saclay in a simulated PWR primary water chemistry environment. The setup consisted of an autoclave with a capacity of 5 L, a load frame, and a computer-driven 30 kN load train for straining the samples. The sample was mounted on heat-treated Inconel sample holder and into the load frame of the autoclave. The autoclave was filled with primary water (25e35 cc/kg H2 STP, 1000 ppm B, 2 ppm Li), sealed, and pressurized with argon gas to detect any leakage. The tem- perature of the system was increased to reach the test temperature of 340 ! C, and the pressure was maintained at 155 bars. Pressure and temperature were monitored using a PT (Pressure-Tempera- ture) sensor located in the center of the autoclave. Prior to straining, environmental conditions were allowed to stabilize for a few hours. Pressure, dissolved oxygen content, and water conductivity were measured by sampling the water after the test. The displacements were measured by a Linear Variable Displacement Transducer (LVDT) located on the traction line of the autoclave. Load and displacement data was collected by a computerized data acquisi- tion system and recorded every 10 s. After achieving stable condi- tions, the tensile specimen was strained at a rate of 5 $ 10 #8 s #1 up to 4% plastic strain.
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Oxidation kinetics and hydrogen uptake of titanium alloys in pressurized water reactors (PWR) primary water

Oxidation kinetics and hydrogen uptake of titanium alloys in pressurized water reactors (PWR) primary water

1 DEN, DANS, DPC, SCCME, Laboratoire d’Etude de la Corrosion Aqueuse, CEA Saclay F-91191 Gif sur Yvette cedex, France 2 DEN, DANS, DPC, SEARS, Laboratoire d’Ingénierie des Surfaces et des Lasers, CEA Saclay F-91191 Gif sur Yvette cedex, France

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Effects of the Cr-depletion on the stress state of the sublayer of ni-base alloys oxidized in high temperature water

Effects of the Cr-depletion on the stress state of the sublayer of ni-base alloys oxidized in high temperature water

the Cr-depletion effect on a structure cooled down is shown to be a compressive stress gradient. If the macroscopic scale is considered, this compressive stress gradient may be involved in the durability of the structure. Firstly, the most stressed part of the Cr-depleted zone is at the interface with the oxide layer. Moreover, at this interface, oxide growth also generates stresses. Therefore, taking into account all the previous features added with the oxide scale deformation with temperature, different situations seem possible. If the oxide scale remains in a tensile stress state larger than fracture strength, spallation or cracking of the oxide layer may occur. The occurrence of cracks into the oxide layer will involve, for further exposure (for example: cyclic test…) a localised corrosion phenomenon. Conversely, if the loading stress is lower than such a critical value, the oxide scale remains uncracked. Secondly, the compressive state of the surface of the metal after the cooling down may prevent the propagation of cracks providing from the Cr preferential oxidation. If some cracks had been generated during the high temperature exposure, their propagation will be slowed or even stopped by such compressive stress.
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Corrosion mechanism of a Ni-based alloy in supercritical water: Impact of surface plastic deformation

Corrosion mechanism of a Ni-based alloy in supercritical water: Impact of surface plastic deformation

Fig. 1. (a) Scheme of the experimental set-up and (b) scheme of a mini-autoclave. consisted of mixed spinel oxides Ni x Fe y Cr z O 4 whose compositions depended on the alloy composition. They identified, especially on Alloy 690, NiFe 2 O 4 and assumed the presence of chromia. Maslar et al. [18] similarly used Raman spectroscopy to identify the sole presence of NiFe 2 O 4 within the oxide scale that formed on a polished (alumina gel finish) alloy 600 (Ni–15Cr–8Fe–Mn–Ti–Si) sample after testing at 543 ◦ C and 25.4 MPa. Maslar et al. [18] also investigated the influence of surface finish on the phases present with the oxide layer. It was determined that polishing alloy 600 with a coarse grain SiC paper induced the formation of both NiFe 2 O 4 and traces of ␣-chromia. In the same way, in different conditions of temperature and pressure, the oxide morphology or the corrosion kinetics [19–21] has been shown to be dependent on the sur- face finish of the alloy sample. In subcritical condition (325 ◦ C and 15 MPa), i.e., in simulated primary media of a pressurised water- cooled reactor (PWR), Lefaix-Jeuland et al. [20] showed that the chromia morphology was dependent on the crystallographic defect structure present near the surface of the sample prior to testing. A high content of subsurface defects leads to a high density of chro- mia nodules at the oxide/alloy interface. In contrast, a low defect density induces fewer chromia nodules at the same interface. More- over, Lefaix-Jeuland et al. reported that the evolution of oxidation kinetics exhibited by these two types of samples (high and low defect densities) differed strongly. Here, the corrosion behavior of Ni base alloys in supercritical water has been studied taking into account the alloy surface finish.
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XPS study of Ni-base alloys oxide films formed in primary conditions of pressurized water reactor

XPS study of Ni-base alloys oxide films formed in primary conditions of pressurized water reactor

The XPS analysis of the oxide scale formed on alloy 690 corroded in PWR primary simulated water have been led for all exposition durations investigated (24 h, 48 h, 66 h, 164 h, 304 h, 406 h and 858 h). Through XPS spectrum deconvolutions, it has been found that the set of corrosion durations can be separated into three classes; one duration from each class will be further discussed in detail. The results obtained for three characteristic exposure durations (24 h, 164 h and 858 h) on the Cr-2p 3/2 , Ni-2p 3/2 and O-1s core level spectra are presented in subsection 3-2-1. In the subsection 3-2-2, which deals with the interest of XPS analysis for studying nickel ferrite precipitation phenomena, some results obtained on Ni-2p 3/2 and Fe- 2p 1/2 core levels are discussed for the entire set of exposure times.
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Effects of the Cr-depletion on the stress state of the sublayer of ni-base alloys oxidized in high temperature water

Effects of the Cr-depletion on the stress state of the sublayer of ni-base alloys oxidized in high temperature water

the Cr-depletion effect on a structure cooled down is shown to be a compressive stress gradient. If the macroscopic scale is considered, this compressive stress gradient may be involved in the durability of the structure. Firstly, the most stressed part of the Cr-depleted zone is at the interface with the oxide layer. Moreover, at this interface, oxide growth also generates stresses. Therefore, taking into account all the previous features added with the oxide scale deformation with temperature, different situations seem possible. If the oxide scale remains in a tensile stress state larger than fracture strength, spallation or cracking of the oxide layer may occur. The occurrence of cracks into the oxide layer will involve, for further exposure (for example: cyclic test…) a localised corrosion phenomenon. Conversely, if the loading stress is lower than such a critical value, the oxide scale remains uncracked. Secondly, the compressive state of the surface of the metal after the cooling down may prevent the propagation of cracks providing from the Cr preferential oxidation. If some cracks had been generated during the high temperature exposure, their propagation will be slowed or even stopped by such compressive stress.
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Kinetic study of the low temperature internal oxidation of nickel based model alloys exposed to PWR primary water

Kinetic study of the low temperature internal oxidation of nickel based model alloys exposed to PWR primary water

duplex description of oxide scales. However, such a structure has already been observed on a similar alloy [16]. Furthermore, it should also be kept in mind that exposure to PWR primary water at 360°C affects the underlying metal; a chromium depleted layer is present over a few tens nanometers under the inner chromium rich oxide scale. This phenomenon could be linked with what is well known at higher temperatures (800°C – 1200°C): nickel-chromium alloys exhibit sensitivity to intergranular and internal oxidation for chromium contents lower than 10% [17]. Moreover, the formation of the chromium depleted zone in the metal is related to that of the chromium rich inner oxide layer. As a gradient of chromium concentration is observed, it is commonly assumed that a chromium rich oxide layer is formed by the diffusion and selective oxidation of chromium atoms. Such a mechanism has been studied [15,18] and implies the injection of vacancies into the material beneath the oxide scale [18]. Up to now, the oxide layers grown on nickel-base alloys after exposure to PWR primary medium have been described as having a duplex structure formed by an inner Cr-rich layer and an outer layer consisting of two well-defined families of crystallites. A third area can be identified as a consequence of the corrosion phenomena, namely the chromium depleted layer beneath the oxide. On average, the overall thickness of the corrosion scales was about a micrometer.
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Electrochemical and fractographic analysis of Microbiologically Assisted Stress Corrosion Cracking of carbon steel

Electrochemical and fractographic analysis of Microbiologically Assisted Stress Corrosion Cracking of carbon steel

the presence of iron sulfides. Even though they are different micro­ bial population, it is possible to observe similar behavior and cor­ rosion rates between the two sulfidogenic systems, PG and DAL. According to the available literature, EFM may show contradic­ tory results when applied to corrosion rate determination as it was shown in particular examples presented above. Obviously, there is an apparent discrepancy between the corrosion rates obtained by weight Joss and EFM technique, however, deviations varies from system to system. Sorne discrepancies are enormous, for example in the case of the DDS system, while in other cases most of the deviations are acceptable. The reasoning why these differences are considered as acceptable is the very nature of the corrosion techniques that were used; EFM provides instantaneous corrosion rate white weight loss provide absolute corrosion rate. From other point of view, reliability ofEFM corrosion rates can be questionable depending on the situation and may be the cause of the observed discrepancies. When EFM is applied for measurement of high cor­ rosion rates, the technique provides valid and reasonable values, for example for mild steel in a 0.05 M H 2 SO 4 solution where it can reach value of 2.1 mm y- 1 [19] ; on the other hand, when ap­ plied for measuring low corrosion rates, it provides non-reliable values [19] . In order to validate EFM measurements, causality fac­ tors were used. Causality Factor 2 (CF2) and Causality Factor 3 (CF3) are in general used to validate the data obtained by EFM in case of uniform corrosion. Usually, if the CF2 values are close to 2 and the CF3 values are close to 3, it is considered that measure­ ments are conducted correctly and obtained corrosion rates are va­ lid [24] . These assumptions are valid only when uniform corrosion mechanism is present and care must be taken when localized at­ tack is dominant; in this latter case, a deeper analysis regarding corrosion rate and the values for CF must be carried out. For exam­ ple, CF2 and CF3 can be applied as a valuable tool for detection of localized corrosion as it is shown by Rauf and Bogaerts [24] who proposed a causality factor evaluation mode! for localized corro­ sion. The values for CF2 and CF3 were analyzed following the main observations and conclusions extracted from the literature but having on mind the possible impact of tensile stress on the ac­ quired data.
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Electrochemical and fractographic analysis of Microbiologically Assisted Stress Corrosion Cracking of carbon steel

Electrochemical and fractographic analysis of Microbiologically Assisted Stress Corrosion Cracking of carbon steel

Open Circuit Potential (OCP) has been used in corrosion studies for long time, especially for detecting potential ennoblement [15] which, is the tendency of the electrode potential to become more positive. Ennoblement of ferrous metals in presence of biofilms most often leads to reaching the breakdown potential leading to crevice or pitting corrosion initiation. On the other hand, Electro­ chemical Frequency Modulation (EFM) [16,17] is a rather new technique used for corrosion monitoring in various environments and conditions [18,19] ; however, there are just few references for EFM used for monitoring MIC [20] and none for carbon steel spec­ imens exposed to biotic environments while being under tensile stress. An advantage of using EFM resides in the small polarizing signais and its ability to provide promptly corrosion currents and causality factors ( considered to be a factor determining measure­ ment reliability), ail in a single experiment. In addition, prelimin­ ary knowledge of kinetic parameters, e.g. Tafel slopes, are not required. Since EFM possess a non-aggressive nature (minor system perturbation), it could be a promising method to monitor corrosion rates in environments supporting biological activity and therefore development of MIC. The current study comprises a combination of mechanical, topographical and electrochemical descriptions for S235JR carbon steel, when subjected to a constant Joad, in presence and absence of bacteria. Pure cultures of SRB, Des­ ulfovibrio alaskensis ALl and Desulfovibrio desulfuricans ATCC
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Simulation of inter- and transgranular crack propagation in polycrystalline aggregates due to stress corrosion cracking

Simulation of inter- and transgranular crack propagation in polycrystalline aggregates due to stress corrosion cracking

The results on different aggregate morphologies (71gr2DIG, 2DIG2 and 2DIG3) in Fig.10) show the various possible crack configurations. The gen- eral direction of crack propagation is normal to the tensile axis, but some noticeable deviations can be produced by the local crystallography. This is observed on both types of shapes (Vorono¨ı tessellation and hexagonal grains). After having checked that the qualitative aspect of the cracks is similar in all the analyses, the crack propagation rate is also evaluated, to provide a quantitative view of the phenomenon. Following the procedure presented for Fig.7b, the new results are now shown in Fig.11. Despite the differ- ent crack geometries, the result obtained with the regular hexagonal grain shape (71gr2DIGhexa) does not differ too much from the curve obtained with the various random Vorono¨ı polyhedra generations, the rectangle shape 71gr2DIG, and the three different Vorono¨ı tessellations of a 100 grain 2D aggregate, 2DIG, 2DIG2 and 2DIG3. The parameters of the model are also in agreement with the results previously shown in Fig.7b. This is an encour- aging result, that confirms the validity of the whole approach.
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Study of the Oxidation Assisted Intergranular Cracking Mechanism on a Ni-Base Superalloy

Study of the Oxidation Assisted Intergranular Cracking Mechanism on a Ni-Base Superalloy

Keywords Alloy 718 ⋅ Oxidation ⋅ Intergranular cracking ⋅ Deformation mode Introduction The superalloy 718 is well known for possessing good mechanical properties and corrosion/oxidation resistance over a wide range of temperatures [1 – 3]. However, the alloy 718 is widely reported to be subject to an intergranular brittle fracture when loaded in particularly harsh environments [4 – 6]. The effect of the environ- ment is essential in the damaging process of the superalloy as no intergranular brittle fracture was observed on samples tested under vacuum or inert gas [7, 8]. Whether in the water of Pressurized Water Reactors (PWRs) or in air at 650 °C, the resulting intergranular damage of the alloy (assimilated to an InterGranular Stress Corrosion Cracking “IGSCC” mechanism on one side and to an Oxidation Assisted Intergranular Cracking “OAIC” mechanism on the other side) depends on the de- formation mode of the alloy. The transition from an unserrated flow typically observed in the Dynamic Strain Aging (DSA) domain to a serrated flow observed when the Portevin-Le Chatelier (PLC) effect occurs in a DSA subdomain lead to a transition in the fracture mode from intergranular fragile to fully transgranular ductile [7 – 9]. Even if several still-debated mechanisms for the IGSCC or OAIC of the alloy 718 have been reported in literature [10 – 18 ], the particular flow induced by the plastic instabilities and the resultant modification of the interaction between dislocations and solutes inhibits the damaging mechanisms of the alloy in which a preferential loading of grain boundaries or segregating elements at grain boundaries are possibly involved [7 – 9, 19].
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Microstructural evolution of Q12$^{TM}$ alloy irradiated in PWR and comparison with other Zr base alloys

Microstructural evolution of Q12$^{TM}$ alloy irradiated in PWR and comparison with other Zr base alloys

26 temperature of 60°C [23]. According to ATEM analyses on extraction replica , small particles up to 150 nm are depleted in iron to a greater extent than the coarse ones [10]. The ATEM analyses on thin foils did not permit determination whether smaller particles are more depleted in iron than the bigger ones. But micro-crystallized particles were observed as soon as two 18-month cycles of irradiation. The core of the SPPs has a homogeneous composition and contains only a few w% iron versus 20 w% before irradiation. The ratio Nb/Fe in the core is 10 versus 2.2 before irradiation. On the periphery of the SPPs, the observed micro- crystallites contain neither iron nor chromium. After four 18-month cycles the precipitates have lost all the iron and chromium they contained. These observations completely agree with the literature with a rapid loss of iron from the Laves phases.
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Radiolysis effect on oxide film of 316L stainless steel formed in PWR primary water under irradiation

Radiolysis effect on oxide film of 316L stainless steel formed in PWR primary water under irradiation

No main difference between the thickness of the oxide layers with or without irradiation (electrons) Mi Wang & al., Corrosion of 316L stainless steel under radiation and exposed to representative PWR chemistry, paper O-1030, EUROCORR2012, Estoril, Portugal, 2012 STRUCTURE OF OXIDE FILM - UNIRRADIATED

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From network depolymerization to stress corrosion cracking in sodium-borosilicate glasses: Effect of the chemical composition

From network depolymerization to stress corrosion cracking in sodium-borosilicate glasses: Effect of the chemical composition

Available online 12 August 2016 The study herein examines how chemical composition impacts sub-critical stress corrosion cracking (SCC) in so- dium borosilicate glasses. The crack speed versus stress intensity factor (v vs. K I ) curves were obtained for seven ternary SiO 2 -Na 2 O-B 2 O 3 (SBN) glasses of selected chemical compositions. Na 2 O plays an interesting role in the SCC behavior. First, increasing the Na 2 O concentration yields an increase in the environmental limit (K e ). Second, increasing the Na 2 O concentration affects how fast SCC occurs as K I increases (i.e. the slope in region I SCC). This second effect is highly nonlinear: it is insignificant for Na 2 O b20% but it becomes increasingly important above 20%, when sodium acts as a network modi fier. Raman spectroscopy and Molecular Dynamics (MD) simulations aid in revealing the structural variations which arise from increasing concentrations of Na 2 O. Na 2 O causes the rel- ative proportions of the different chemical bonds accessible in SBN glasses to vary. For this series of glasses, the Si –O–Si bond does not dominate the SCC properties. SCC variations originate in the mesoscale structure where sodium ions act as network modifiers on both the silica and borate units, thus yielding a partial depolymerization (i.e. a decrease in the reticulation level) of the network. This second effect reveals itself to be the one responsible for the SCC chemical dependency. Poisson's ratio increases approximately linearly with increasing Na 2 O concen- tration, and thus, it is also not simply proportional to the slope in region I SCC. Partial depolymerization of the glass provides a novel prospective on the controlling factors in the sub-critical crack growth.
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Cinétique et mécanismes de corrosion sèche d'un cermet à base Cu-Ni

Cinétique et mécanismes de corrosion sèche d'un cermet à base Cu-Ni

toutes les pressions d’oxygène. Ensuite, tel que le prédisait la thermodynamique (Annexe 2 : Thermodynamique du système Fe-Ni-Cu-O), le cermet n’est pas dans un état stable lorsqu’il est soumis à une atmosphère oxydante. La phase métallique disparaît progressivement depuis le bord vers le centre de la pastille. A l’échelle d’un grain d’alliage, le nickel est le premier élément à s’oxyder en formant une couronne de monoxyde à la périphérie du grain métallique et en consommant la phase spinelle à proximité (côté bord de la pastille). Puis le cuivre s’oxyde et migre en surface du matériau ou à l’intérieur des autres oxydes présents dans le matériau. La migration du cuivre laisse momentanément une bande poreuse dans le matériau qui sera comblée par du monoxyde (NiFeCu)O au fur et à mesure de l’avancement de la réaction. Ceci donne l’impression que la porosité migre vers le centre de la pastille.
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