No description available.

Most stress corrosion cracking (SCC) mechanisms initiate from localised corrosion (pitting, intergranular attack, de-alloying), which provides local stress concentration. Alloys are generally more susceptible to SCC than pure metals because selective dissolution or oxidation is possible. De-alloying involves the selective dissolution of the less noble (LN) component from an alloy. The more noble (MN) component enriches on the surface forming a brittle, metallic, nanoporous layer. In noble metal alloys and brass, SCC shows correlation with the threshold LN content below which de-alloying stops (the parting limit). In Fe-Ni-Cr engineering alloys de-alloying may be responsible for Cl-SCC, although this has not been proven explicitly. Initial indications show that de-alloying causes SCC in hot, caustic environments. In some cases, Ni enrichment and porosity are associated with cracks in stainless steel after long-term service in nuclear high temperature water environments, but it is unclear if this plays a causal role in cracking. Here the de-alloying mechanism (primarily the effect of Ni (MN) content) and its relationship to SCC in Fe-Ni-Cr materials (Fe10Ni, 310SS and Alloy 800) is examined using a hot caustic environment, and compared to classical de-alloying systems. De-alloyed layers formed on all materials, although Alloy 800 required a higher temperature. Increasing Ni content improved de-alloying resistance according to classical theory. Unlike classical systems, de-alloying occurred with concurrent MN dissolution and, at open circuit potential (OCP), the layers retained significant Fe and Cr (LN) instead of being ‘almost pure’ MN. Layers formed with applied anodic potential were friable and highly LN depleted. This behaviour was successfully modelled in Kinetic Monte Carlo simulations. Recently, it has been shown that SCC in noble element alloys depends on the mechanical integrity (quality) of the de-alloyed layer; a finding that was reflected here. At 140 °C at OCP the layer on 310SS was too thin to promote SCC and Alloy 800 did not de-alloy significantly. Layers formed with anodic potential did not result in SCC. In 50% NaOH at 280 °C, severely stressed 310SS cracked where thick de-alloyed layers formed. However, the thin layer formed on Alloy 800 was associated with SCC, even with low residual stress.

Most stress corrosion cracking (SCC) mechanisms initiate from localised corrosion (pitting, intergranular attack, de-alloying), which provides local stress concentration. Alloys are generally more susceptible to SCC than pure metals because selective dissolution or oxidation is possible. De-alloying involves the selective dissolution of the less noble (LN) component from an alloy. The more noble (MN) component enriches on the surface forming a brittle, metallic, nanoporous layer. In noble metal alloys and brass, SCC shows correlation with the threshold LN content below which de-alloying stops (the parting limit). In Fe-Ni-Cr engineering alloys de-alloying may be responsible for Cl-SCC, although this has not been proven explicitly. Initial indications show that de-alloying causes SCC in hot, caustic environments. In some cases, Ni enrichment and porosity are associated with cracks in stainless steel after long-term service in nuclear high temperature water environments, but it is unclear if this plays a causal role in cracking. Here the de-alloying mechanism (primarily the effect of Ni (MN) content) and its relationship to SCC in Fe-Ni-Cr materials (Fe10Ni, 310SS and Alloy 800) is examined using a hot caustic environment, and compared to classical de-alloying systems. De-alloyed layers formed on all materials, although Alloy 800 required a higher temperature. Increasing Ni content improved de-alloying resistance according to classical theory. Unlike classical systems, de-alloying occurred with concurrent MN dissolution and, at open circuit potential (OCP), the layers retained significant Fe and Cr (LN) instead of being ‘almost pure’ MN. Layers formed with applied anodic potential were friable and highly LN depleted. This behaviour was successfully modelled in Kinetic Monte Carlo simulations. Recently, it has been shown that SCC in noble element alloys depends on the mechanical integrity (quality) of the de-alloyed layer; a finding that was reflected here. At 140 °C at OCP the layer on 310SS was too thin to promote SCC and Alloy 800 did not de-alloy significantly. Layers formed with anodic potential did not result in SCC. In 50% NaOH at 280 °C, severely stressed 310SS cracked where thick de-alloyed layers formed. However, the thin layer formed on Alloy 800 was associated with SCC, even with low residual stress.

The stress corrosion cracking (SCC) behaviour of Mg alloy AZ31B was investigated with respect to surface condition. Salt fog U-bend testing was used to identify changes in SCC as a result of surface conditioning pre-treatments. Six surface conditions were investigated: as-received, mechanically-polished, sulphuric acid (H2SO4)-cleaned, mechanically-polished then H2SO4-cleaned, aged H2SO4-cleaned, and acetic acid (C2H4O2)-cleaned. Results showed that the rate of SCC was accelerated and the SCC mode was intergranular for all surface conditioning treatments involving H2SO4-cleaning. It was found that the accelerated intergranular SCC was a result of three contributing factors: a low pH, the presence of aggressive ions, and a porous film which allowed direct contact between the metal surface and the electrolyte. Characterization of the surfaces using potentiodynamic polarization and cross-sectional images of sample surfaces showed that in the absence of one of these three contributing factors intergranular SCC would not occur. / Thesis / Master of Applied Science (MASc)

No description available.

No description available.

No description available.

No description available.

Ce travail de thèse s’intéresse aux alliages à base nickel exposés au milieu primaire des réacteurs à eau pressurisée : ceux-ci, et en particulier, l’alliage 600, contenant environ 16% de chrome, montrent, en service, une sensibilité à un phénomène de corrosion localisée appelé corrosion sous contrainte (CSC). La corrosion sous contrainte aboutit, à terme, au développement de fissures intergranulaires nécessitant le remplacement des matériaux de structure. La compréhension de ces phénomènes constitue donc un enjeu majeur dans le cadre de la sûreté et du prolongement de la durée de vie des réacteurs, avec, également, des aspects économiques évidents. Le rôle de cette étude est d’apporter des éléments de compréhension quant au rôle de l’hydrogène dans ces phénomènes de corrosion sous contrainte. L’objectif de ce travail était double : d’une part, déterminer la source principale de l’hydrogène absorbé par l’alliage lors de exposition au milieu primaire, et d’apporter des éléments permettant de caractériser le mécanisme responsable de son absorption. D’autre part, un second objectif consistait à évaluer dans quelle mesure l’hydrogène absorbé par l’alliage pouvait jouer un rôle dans ces phénomènes de CSC, notamment, en regard de ses interactions possibles avec les défauts de structure du matériau. À cette fin, des techniques de traçage isotopique mises en œuvre lors de la corrosion de ces alliages en milieu primaire ont été utilisées, la pénétration des traceurs étant ensuite analysées par spectrométrie de masse d’ions secondaires. Ces analyses ont permis de montrer que l’hydrogène absorbé provenait principalement de la dissociation de la molécule d’eau lors de l’édification du film d’oxyde passif. Par ailleurs, la création de défauts de structure dans le matériau, et leur étude par annihilation de positons et microscopie électronique en transmission, après création ou après interaction avec l’hydrogène introduit par chargement cathodique, ont permis de caractériser les interactions de cet élément avec les défauts. Ces interactions sont importantes, et mènent à une réorganisation des défauts (coalescence, migration), mais sont transitoires, leur intensité dépendant de l’activité locale de l’hydrogène en solution. Ces résultats ont permis la proposition d’un nouveau modèle d’amorçage et de propagation des fissures de CSC. / Since the late 1960s, a special form of stress corrosion cracking (SCC) has been identified for Alloy 600 exposed to pressurized water reactors (PWR) primary water: intergranular cracks develop during the alloy exposure, leading, progressively, to the complete ruin of the structure, and to its replacement. The main goal of this study is therefore to evaluate in which proportions the hydrogen absorbed by the alloy during its exposure to the primary medium can be responsible for SCC crack initiation and propagation. This study is aimed at better understanding of the hydrogen absorption mechanism when a metallic surface is exposed to a passivating PWR primary medium. A second objective is to characterize the interactions of the absorbed hydrogen with the structural defects of the alloy (dislocations, vacancies…) and evaluate to what extent these interactions can have an embrittling effect in relation with SCC phenomenon. Alloy 600-like single-crystals were exposed to a simulated PWR medium where the hydrogen atoms of water or of the pressuring hydrogen gas were isotopically substituted with deuterium, used as a tracer. Secondary ion mass spectrometry depth-profiling of deuterium was performed to characterize the deuterium absorption and localization in the passivated alloy. The results show that the hydrogen absorption during the exposure of the alloy to primary water is associated with the water molecules dissociation during the oxide film build-up. In an other series of experiments, structural defects were created in recrystallized samples, and finely characterized by positron annihilation spectroscopy and transmission electron microscopy, before or after the introduction of cathodic hydrogen. These analyses exhibited a strong hydrogen/defects interaction, evidenced by their structural reorganization under hydrogenation (coalescence, migrations). However, thermal desorption spectroscopy analyses indicated that these interactions are transitory, and dependent on the local hydrogen activity in the bulk material. Finally, these results allowed a new model describing SCC crack initiation and propagation to be formulated.

Stress corrosion cracking (SCC) susceptibility of the phosphorus de-oxidized copper has been evaluated in synthetic seawater polluted by sulfides using slow strain rate test (SSRT). The effect of concentration of sulfide, temperature, and applied cathodic and anodic potentials on the final strain values and maximum stress were also studied. No cracks were found under the tested conditions. The final strain and maximum stress values decreased but not significantly, with increase in the temperature, applied anodic potential and sulfide concentration. The observed effect is due to the section reduction by uniform corrosion. Lateral cross section and microscopic examination of the fractured specimen ruled out the existence of the localized corrosion. Electrochemical measurements showed that the Cu2S film is not a protective film and also exhibits a mass transfer limitation to the inward diffusion of the sulfides. Based on these results the reasons for the absence of cracking are also discussed.