<p>  </p> <p>Stress corrosion cracking (SCC) is a longstanding critical materials challenge in austenitic stainless steels (AuSS). Recently, there has been mounting concern regarding the potential for Chloride-induced stress corrosion cracking (CISCC) along arc weld seams on austenitic stainless-steel canisters used as spent nuclear fuel (SNF) dry storage containers, due to the residual stress from the welding process and exposure to chloride-rich coastal air at storage sites. To ensure the safety of the SNF storage, fundamental understanding and mitigation methods of CISCC are critical in both engineering design and maintenance of the storage canisters before and after their deployment. With the recent development of high-resolution characterization and analysis techniques, a more robust and comprehensive understanding of the fundamental TGCISCC mechanism starts to be more accessible. In this thesis, comprehensive state-of-the-art techniques, including SEM, EBSD, HREBSD, FIB, ATEM, TKD, potential dynamic measurement, XRD, and nanoindentation will be used to further understand the mechanistic mechanism of TGCISCC in AuSS from macroscopic scale down to atomistic scale. </p>

Friction stir processing (FSP) has been utilized to locally process regions of arc weldments in 304L stainless steel to improve the microstructure and mechanical performance. The cast microstructure and coarse delta-ferrite has been replaced with a fine-grained wrought microstructure. Furthermore, twins were introduced throughout the friction stir processed region. Although sub-surface sigma and carbides were introduced during FSP, their presence is not expected to adversely affect the resulting mechanical or corrosion properties of friction stir processed 304L arc welds. The resulting mechanical properties of FS processed weldments were also an improvement over as-welded arc welds. FSP resulted in an increase of 6% for both yield and ultimate strength. It is expected that the improved microstructure will lead to improved stress corrosion cracking and general corrosion properties.

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<p>Heat exchanger tubes are commonly made from stainless steel.In a heat exchanger both the process fluid and the cooling orheating media can be corrosive. It is therefore important to beable to select materials that do not suffer from corrosion.Current methods in this area, however, suffer from limitationsand shortcomings. This thesis concerns corrosion tests forgeneral corrosion, dewpoint corrosion and stress corrosioncracking, respectively. For evaluation of general corrosion ofsuperduplex stainless steels in hydrochloric acid and sulphuricacid the importance of activation was studied. The results showthat activation has a great influence on the test result.Further the results indicate that experimental differences canbe the explanation for previously reported differences incorrosion resistance of superduplex stainless steel that havebeen attributed to the alloying with Cu and W. Furthermore, asimple test loop for testing stainless steels under dew formingconditions with a formed condensate of 1 % hydrochloric acidhas been developed. In the work constant strain and constantload test result have been compared and the observationsindicate that the differences can be explained by differencesin the relaxation properties of the materials.</p><p><b>Key words:</b>corrosion testing, heat exchanger, stainlesssteel, general corrosion, immersion tests, activation, stresscorrosion cracking, constant load, u-bends, relaxation,dewpoint corrosion</p>

Austenitic stainless steels are frequently used in the cooling circuits of nuclear reactors. It has been found that cold-worked 304 stainless steels can be particularly susceptible to stress corrosion cracking at the operating conditions of such reactors. Despite more than 130 years of research underlying mechanisms are still not properly understood. For this reason, the effects of cold-work and applied stress on the oxidation behaviour of 304SS have been studied in this thesis. A set of samples with/without prior cold-work, and with/without stress applied during oxidation, were oxidized in autoclaves under simulated pressurised water reactor primary circuit conditions. Atom-probe tomography and analytical transmission electron microscopy were used to investigate the local chemistry and microstructure in the different samples tested. Regions containing grain boundaries, deformation bands, and matrix material in contact with the environment, were extracted from the coupon specimens with a focused ion beam machine. Cross-sections of crack tips were studied with secondary ion mass spectrometry and electron backscatter diffraction. The compositions of oxides grown along the surface and the different microstructural features were analysed. Fe-rich spinels were found at the surface and Cr-rich spinels were observed along fast diffusion paths. Ni-enrichment was found at the metal/oxide interfaces and a Ni-rich phase was detected in precipitates ahead of grain boundary oxides. Li was observed in all oxidised regions and B segregation, originating from impurities in the alloy, was observed in grain boundaries and crack tip oxides. Cavities and hydrogen associated with Ni-rich regions were found ahead of the bulk Cr-rich oxide in some of the samples. The implications of these findings for the understanding of SCC mechanisms are discussed. It is suggested that Ni precipitation as well as the presence of deformation bands may play an important role in controlling SCC susceptibility in 304 stainless steel. A modification of the film-rupture model including internal oxidation and fast diffusion along H-stabilised vacancies in strain fields at the crack front is proposed.

Pour réduire le poids des structures aéronautiques, plusieurs voies ont été explorées. Parmi elles, l'utilisation des alliages d'aluminium légers et le remplacement des structures rivetées par des structures soudées par Friction Stir Welding (FSW) sont envisagées. La question de la durée de vie de ces structures préoccupe les industriels. Dans ce cadre, cette étude porte sur la sensibilité à la corrosion sous contrainte intergranulaire (CSC-IG) du noyau de soudure par FSW de l'alliage Al-Cu-Li 2050. Ce matériau est composé de grains équiaxes dont la taille diminue de 17 à 4 µm à mesure que l'on s'éloigne de la surface de soudage. Une variation de texture est révélée grâce à des cartographies EBSD formant la microstructure des « onion rings ». La périodicité de ces « onions rings » est égale à l'avancée du pion FSW sur un tour (500 µm pour notre matériau). Ces hétérogénéités microstructurales entraînent des gradients de champs mécaniques locaux quantifiés par corrélation d'images lors des essais mécaniques. Ces hétérogénéités microstructurales et mécaniques favorisent les phénomènes de corrosion localisée lorsque le matériau est soumis à un environnement agressif. Les effets des contraintes et de la microstructure sur la CSC-IG sont mis en évidence par des essais de corrosion et des essais de corrosion sous contrainte (CSC). Les essais de corrosion montrent une sensibilité du matériau à la piqûration alors que les essais de CSC révèlent l'amorçage de fissures intergranulaires. Les plus grosses fissures s'amorcent préférentiellement à la frontière des « onion rings ». Un modèle par éléments finis a été développé dans le but de simuler la propagation des fissures intergranulaires sur des agrégats réels générés par des cartographies EBSD. / To reduce the aircraft components weight, several solutions were explored. Among them, the using of light aluminum alloys and the substitution of riveting by friction stir welding (FSW) are investigated. Industry is concerned by the question of the life of such structures. For this, this study is focused on intergranular stress corrosion cracking (IGSCC) sensitivity of the 2050 Al-Li-Cu alloy friction stir weld nugget. This material consists of equiaxed grains whose size is decreasing with the distance from the weld surface between 17 µm at the top and 4 µm at the bottom. The “onion rings” microstructure is revealed by EBSD cartographies as a texture variation. They appear with a periodicity of 500 µm corresponding to the advance per revolution of the tool. Those microstructural heterogeneities cause local mechanical field gradients quantified by digital image correlation measurements during mechanical tests. Those microstructural and mechanical heterogeneities promote localized corrosion when the material is submitted to an aggressive environnement. Microstructure and stress effects on IGSCC are shown by corrosion tests and stress corrosion tests. The first ones show a sensitivity to pitting corrosion and a stress application reveal initiation of intergranular cracks. The biggest ones preferentially initiate at “onion rings” boundaries. A finite element model was developed in order to simulate intergranular cracks propagation on real aggregates obtained by EBSD cartographies.

Um dos principais mecanismos de falha que causam riscos de fratura a reatores de água pressurizada é a corrosão sob tensão de ligas metálicas em água do circuito primário (CSTAP). É causada por uma combinação das tensões de tração, meio ambiente em temperatura e microestruturas metalúrgicas susceptíveis. Ela pode ocorrer, dentre outros locais, nos bocais do mecanismo de acionamento das barras de controle. Essa fratura pode causar acidentes que comprometem a segurança nuclear através do bloqueio das barras de controle e vazamentos de água do circuito primário reduzindo a confiabilidade e a vida útil do reator. O objetivo desta Tese de Doutorado é o estudo de modelos e uma proposta de modelagem para fraturas por corrosão sob tensão em liga 75Ni15Cr9Fe (liga 600), em água de circuito primário de reator de água pressurizada nesses bocais. São superpostos modelos eletroquímicos e de mecânica da fratura e validados com dados obtidos em experimentos e na literatura. Na parte experimental foram utilizados resultados obtidos pelo CDTN no equipamento recém-instalado de ensaio por taxa de deformação lenta. Na literatura está proposto um diagrama que exprime a condição termodinâmica de ocorrerem diversos modos de CSTAP na liga 600: partiu-se de diagramas de potencial x pH (diagramas de Pourbaix), para a liga 600 imersa em água primária à alta temperatura (3000C a 3500C). Sobre ele, determinaram-se os submodos de corrosão, a partir de dados experimentais. Em seguida acrescentou-se uma dimensão adicional ao diagrama, correlacionando uma variável a que se denominou fração de resistência à corrosão sob tensão. No entanto, é possível acrescentar-se outras variáveis que exprimem a cinética de iniciação e/ou crescimento de trinca, provenientes de outras modelagens de CSTAP. A contribuição original deste trabalho se insere nessa fase: partindo-se de uma condição de ensaio de potencial versus pH, foram iniciadas as modelagens de um modelo empírico-comparativo, um semi-empírico-probabilístico, um de tempo de iniciação e um de taxa de deformação, a partir dos ensaios experimentais e superpostas a essa condição. Esses exprimem respectivamente a susceptibilidade à CSTAP, o tempo de falha, e nos dois últimos o tempo de iniciação de falha por corrosão sob tensão. Os resultados foram comparados com os da literatura e se mostraram coerentes. Através desse trabalho, obteve-se uma metodologia de modelagem a partir de dados experimentais. / One of the main failure mechanisms that cause risks to pressurized water reactors is the primary water stress corrosion cracking (PWSCC) occurring in alloys. It can occurs, besides another places, at the control reactor displacement mechanism nozzles. It is caused by the joint effect of tensile stress, temperature, susceptible metallurgical microstructure and environmental conditions of the primary water. These cracks can cause accidents that reduce nuclear safety by blocking the rods displacement and may cause leakage of primary water, reducing the reactors life. In this work it is proposed a study of the existing models and a modeling proposal to primary water stress corrosion cracking in these nozzles in a nickelbased Alloy 600. It is been superposed electrochemical and fracture mechanics models, and validated using experimental and literature data. The experimental data were obtained at CDTN-Brazilian Nuclear Technology Development Center, in a recent installed slow strain rate testing equipment. In the literature it is found a diagram that indicates a thermodynamic condition for the occurrence of some PWSCC submodes in Alloy 600: it was used potential x pH diagrams (Pourbaix diagrams), for Alloy 600 in high temperature primary water (3000C till 3500C). Over it, were located the PWSCC submodes, using experimental data. It was added a third parameter called stress corrosion strength fraction. However, it is possible to superpose to this diagram, other parameters expressing PWSCC initiation or growth kinetics from other models. Here is the proposition of the original contribution of this work: from an original experimental condition of potencial versus pH, it was superposed, an empiric-comparative, a semi-empiric-probabilistic, an initiation time, and a strain rate damage models, to quantify respectively the PWSCC susceptibility, the failure time, and in the two lasts, the initiation time of stress corrosion cracking. It was modeling from our experimental data. The results were compared with the literature and it showed to be coherent. From this work was obtained a modeling methodology from experimental data.

No presente trabalho foi estudado o comportamento de um aço inoxidável austenítico do sistema Fe-Cr-Mn-N, nas condições como recebido, solubilizado e sensitizado, quando submetidos à meios contendo cloretos. Para analisar a suscetibilidade à corrosão sob tensão foram utilizados corpos de prova dos tipos DCB (\"Double Cantilever Beam\") e C(T) (\"Compact- Tension\"), pré-trincados em fadiga, com entalhes laterais e carregados com cunhas. Os meios empregados foram as soluções aquosas 45% em peso de MgCl2 na temperatura de ebulição de 154°C, água do mar sintética na temperatura ambiente e de ebulição de 100°C e 3,5% em peso de NaCI na temperatura ambiente. A suscetibilidade à corrosão sob tensão foi avaliada em termos do fator limite de intensidade de tensão, KISCC, e foram caracterizados os aspectos fractográficos dos corpos de prova em que ocorreram propagação de trinca por corrosão sob tensão. Foi verificado que apenas os corpos de prova do aço E3949 nas condições como recebido e sensitizado, foram suscetíveis à corrosão sob tensão em solução aquosa de 45% em peso de MgCl2 na temperatura de ebulição. Também foi determinado o comportamento eletroquímico do material nas condições citadas acima com relação aos meios empregados. Os resultados das curvas de polarização obtidas mostraram que o material apresenta baixa resistência à corrosão, principalmente em meios de MgCl2. / The present work studies the stress corrosion cracking behavior in austenitic Fe-Cr-Mn-N stainless steel, in as received, solubilized and sensitized conditions, when submited to several chlorides environments. To evalued the stress corrosion cracking susceptibility, DCB (\"Double Cantilcver Beam\") and C(T) (\"Compact- Tension\") specimens, fatigue precracked, side grooved and wedge loaded were used. The environments employed were boiling solution of 45 wt% of MgCl2 at 154ºC, synthetic sea water at room and 100°C temperatures, and a solution with 3,5 wt% of NaCI at room temperature. The susceptibility to stress corrosion cracking has been evaluated in terms of the threshold stress intensity factor, KISCC, and the fracture surface appearance of those specimens whose the crack propagation took place was characterized. The results showed that only the specimens in the as received and sensitized conditions, were suceptible to the stress corrosion cracking effect in the solution with 45 wt% of MgCl2 at the boilling temperature. Also, it has been verified the electrochemical behavior of this steel when submited in the above environments. The results of polarization curves showed that the material presents low corrosion strength, mainly in MgCl2 environments.

In recent years, the demand for alternatives to fossil fuels has risen dramatically, and ethanol fuel has become an important liquid fuel alternative globally. The most efficient mode of transportation of petroleum-based fuel is via pipelines, and due to the 300% increase in ethanol use in the U.S. in the past decade, a similar method of conveyance must be adopted for ethanol. Low-carbon, low-alloy pipeline steels like X52, X60, and X65 comprise the existing fuel transmission pipeline infrastructure. However, similar carbon steels, used in the ethanol processing and production industry, were found to exhibit stress corrosion cracking (SCC) in ethanol service. Prior work has shown that contaminants absorbed by the ethanol during distillation, processing or transport could be the possible determinants of SCC susceptibility; 200 proof ethanol alone was shown not to cause SCC in laboratory studies. To ensure the safety and integrity of the pipeline system, it was necessary to perform a mechanistic study of SCC of pipeline steel in fuel grade ethanol (FGE). The objective of this work was to determine the environmental factors relating to SCC of X65 steel in fuel grade ethanol (FGE) environments. To accomplish this, a systematic study was done to test effects of FGE feedstock and common contaminants and constituents such as water, chloride, dissolved oxygen, and organic acids on SCC behavior of an X65 pipeline steel. Slow strain rate tests (SSRT) were employed to evaluate and compare specific constituents' effects on crack density, morphology, and severity of SCC of X65 in FGE. SCC did not occur in commercial FGE environments, regardless of the ethanol feedstock. In both FGE and simulated fuel grade ethanol (SFGE), SCC of carbon steel was found to occur at low water contents (below 5 vol%) when chloride was present above a specific threshold quantity. Cl- threshold for SCC varied from 10ppm in FGE to approximately 1 ppm in SFGE. SCC of carbon steel was inhibited when oxygen was removed from solution via N2 purge or pHe was increased by addition of NaOH. During SSRT, in-situ¬ electrochemical measurements showed a significant role of film rupture in the SCC mechanism. Analysis of repassivation kinetics in mechanical scratch tests revealed a large initial anodic dissolution current spike in SCC-causing environments, followed by repassivation indicated by current transient decay. In the deaerated environments, repassivation did not occur, while in alkaline SFGE repassivation was significantly more rapid than in SCC-inducing SFGE. Composition and morphology of the passive film on X65 during static exposure tests was studied using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results showed stability of an air-formed native oxide under static immersion in neutral (pHe = 5.4) SFGE, and dissolution of the film when pHe was decreased to 4.3. XPS spectra indicated changes in film composition at high pHe (near 13) and in environments lacking sufficient water. In light of all results, a film-rupture anodic-dissolution mechanism is proposed in which local plastic strains facilitates local breakdown of the air-formed oxide film, causing iron to dissolve anodically. During crack propagation anodic dissolution occurs at the crack tip while crack walls repassivate preserving crack geometry and local stress concentration at the tip. It is also proposed that SCC can be mitigated by use of alkaline inhibitors that speed repassivation and promotes formation of a more protective Fe(OH)3 film.

Las condiciones ambientales de trabajo de los aceros estructurales y de conducciones en instalaciones energéticas y petrolíferas han puesto en evidencia la necesidad de encontrar una metodología de caracterización de su resistencia a la fisuración, tanto en régimen de dominio elástico como elastoplástico. Este trabajo recoge una metodología experimental y analítica adecuada para ser aplicada a los procesos de fisuración estudiados sobre probetas y estructuras, en particular a aquéllos asociados a la presencia de ambientes agresivos, como corrosión bajo tensión o fisuración inducida por hidrógeno de aceros microaleados. Una vez aplicada y validada, la metodología ofrece resultados de gran interés en la caracterización cuantitativa del comportamiento de fisuración y su correlación con los micromecanismos de rotura presentes. Esta tesis se ha mostrado eficaz en la caracterizacióny establecimiento de un modelo de comportamiento de aceros microaleados utilizados en condiciones de fisuración inducida por hidrógeno, como es el caso de las plataformas petrolíferas en alta mar (estructuras off-shore). / The environmental conditions of employment of structural steel and energy pipelines and oil facilities have highlighted the need to find a methodology for characterizing their resistance to cracking, both in the elastic and the elastoplastic regime. This work includes experimental and analytical methodologies that are appropriate to be applied to cracking processes studied on specimens and structures, in particular to those associated with the presence of aggressive environments, such as stress corrosion cracking or hydrogen induced cracking of microalloyed steels. Once applied and validated, the methodology provides results of great interest in the quantitative characterization of the cracking behavior and its correlation with the fracture micromechanisms taking place. This thesis has been shown to be effective in the characterization and establishment of a behavioral model of microalloyed steels used in conditions of hydrogen-induced cracking, such as oil platforms at sea (off-shore structures).