[pt] As tubulações de aços utilizadas pela indústria do petróleo estão constantemente expostas à ação do ácido sulfídrico (H2S) que é o veículo para a difusão do hidrogênio no aço, provocando o surgimento de trincas induzidas pela fragilização por hidrogênio e/ou trincas de corrosão sob tensão. A junta soldada, que é considerada a região crítica dos dutos, foi o objeto de avaliação neste trabalho mediante ensaios de tração sob baixa taxa de deformação (BTD) e através da norma NACE TM0177/96 - Método A (Standard Tensile Test), verificando o seu comportamento quanto à fragilização por hidrogênio e à corrosão sob tensão em aços da classe API grau X70 e X80. Uma solução de tiossulfato de sódio foi utilizada no ensaio BTD para avaliação da fragilização por hidrogênio e corrosão sob tensão, reduzindo custos e atendendo aos ítens de segurança. Os resultados mostraram que os aços API 5L X70 e X80 são susceptíveis à fragilização por hidrogênio e à corrosão sob tensão. / [en] The steel tubes used in the Oil Industry are constantly exposed to hydrogen sulphide (H2S) which leads to the diffusion of hydrogen into the steel, provoking hydrogen induced embrittlement cracks and/or stress corrosion cracking. Welded joints, generally considered to be the critical region in pipelines, were evaluated in this study, using slow strain rate tensile testing (SSRT) and NACE TMO177/96 - Method A (standard tensile test) norm, verifying the behaviour of joints in API X-70 and X-80 grade steels, with regard to hydrogen embrittlement and stress corrosion cracking. Sodium thiosulphate was used in the slow strain rate tests permit the evaluation of hydrogen embrittlement and stress corrosion cracking while reducing testing costs and maintaining safety standards. The results show that the API X-70 and X-80 grade steels are susceptible to hydrogen embrittlement and stress corrosion cracking.

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The time reversal (TR) process manipulates a system's impulse response in order to focus a peak of acoustic energy at a specific location in space and time. This technique has been implemented in both fluid and solid media for purposes ranging from communications to source localization. This thesis will examine both the implementation and processing of TR for nondestructive evaluation in steel, specializing in nonlinear detection methods. A series of steel samples are inspected for stress corrosion cracking (SCC) using TR focusing to excite nonlinearities inherent in cracks. It is determined that SCC exists in the expected regions of the steel samples and that an induced increase in SCC corresponds to an increase in detected nonlinearity. In addition to this, a study is shown wherein TR signal processing is optimized for the detection of cracks. The TR impulse response is modified in a number of ways with the primary goal of increasing the amplitude of the TR focus. Each of these modifications is experimentally scrutinized for characteristics necessary for application to nondestructive evaluation, and ultimately one is chosen that amplifies TR focusing without increasing system nonlinearity. The optimized technique, decay compensation TR, is employed in the detection of SCC and is found to be as or perhaps even more successful than typical TR nondestructive evaluation methods.

Pitting corrosion is a prevalent form of corrosive damage that can weaken, damage, and initiate failure in corrosion-resistant metallic materials. For instance, 304 stainless steel is commonly utilized in various structures (e.g., miter gates, heat exchangers, and storage tanks), but is prone to failure through pitting corrosion and stress corrosion cracking under mechanical loading, regardless of its high corrosion resistance. In this study, to better understand the pitting corrosion damage development, controlled corrosion experiments were conducted to generate pits on 304 stainless steel specimens with and without mechanical loading. The pit development over time was characterized using a high-resolution laser scanner. In addition, to achieve scalable and automatic assessment of pitting corrosion conditions, two convolutional neural network-based computer vision algorithms were adopted and implemented to evaluate the efficacy of networks to identify existence of pitting damage. One was a newly trained convolutional neural network (CNN) using MATLAB software, while the other one was a retrained version of GoogLeNet. Overall, the experimental results showed that time is the dependent variable in predicting pit depth. Meanwhile, loading conditions significantly influence pit morphology. Under compression loading, pits form with larger surface opening areas, while under tension loading, pits have smaller surface opening areas. Deep pits of smaller areas are dangerous for structural members, as they can lead to high stress concentrations and early stress corrosion cracking (SCC). Furthermore, while the training library was limited and consisted of low-resolution images, the retrained GoogLeNet CNN showed promising potential for identifying pitting corrosion based on the evaluation of its performance parameters, including the accuracy, loss, recall, precision, and F1-measure.

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Nickel and iron-based alloys are important structure and cladding materials for modern nuclear reactors due to their high mechanical properties and high corrosion resistance. To understand the radiative and corrosive environment influence on the mechanical strength, computer simulation works are conducted. In particular, this dissertation is focused on multiscale modeling of the effects of radiation-induced solute segregation and oxidation on grain boundary (GB) strength in nickel-based and iron-based alloys. Besides the atomistic scale density functional theory (DFT) based calculations of GB strength, continuum-scale cohesive zone model (CZM) is also used to simulate intergranular fracture at the microstructure scale. First, the effects of solute or impurity segregation at GBs on the GB strength are studied. Thermal annealing or radiation induced segregation of solute and impurity elements to GBs in metallic alloys changes GB chemistry and thus can alter the GB cohesive strength. To understand the underlying mechanisms, first principles based DFT calculations are conducted to study how the segregation of substitutional solute and impurity elements (Al, C, Cr, Cu, P, Si, Ti, Fe, which are present in Ni-based X-750 alloys) influences the cohesive strength of Σ3(111),Σ3(112),Σ5(210) and Σ5(310) GBs in Ni. It is found that C and P show strong embrittlement potencies while Cr and Ti can strengthen GBs in most cases. Other solute elements, including Si, have mixed but insignificant effects on GB strength. In terms of GB character effect, these solute and impurity elements modify the GB strength of the Σ5(210) GB most and that of the Σ3(111) least. Detailed analyses of solute-GB chemical interactions are conducted using electron localization function, charge density map, partial density of states, and Bader charge analysis. The results suggest that the bond type and charge transfer between solutes and Ni atoms at GBs may play important roles on affecting the GB strength. For non-metallic solute elements (C, P, Si), their interstitial forms are also studied but the effects are weaker than their substitutional counterparts. Nickel-base alloys are also susceptible to stress corrosion cracking (SCC), in which the fracture mainly propagates along oxidized grain boundaries (GBs). To understand how oxidation degrades GB strength, the next step is to use density functional theory (DFT) calculations to study three types of oxidized interfaces: partially oxidized GBs, fully oxidized GBs, and oxide/metal interface, using Ni as a model system. For partially oxidized GBs, both substitutional and interstitial oxygen atoms of different concentrations are inserted at three Ni GBs: Σ3(111) coherent twin, Σ3(112) incoherent twin, and Σ5(210). Simulation results show that the GB strength decreases almost linearly with the increasing oxygen coverage at all GBs. Typically, substitutional oxygen causes a stronger embrittlement effect than interstitial oxygen, except at the Σ3(111). In addition, the oxygen-induced mechanical distortion has a much smaller contribution to the embrittlement than its chemical effect, except for oxygen interstitials at the Σ3(111). For the fully oxidized GBs, three NiO GBs of the same types are studied. Although the strengths of Σ3(112) and Σ5(210) NiO GBs are much weaker than the Ni counterparts, the Σ3(111) NiO GB has a higher strength than that in Ni, indicating that Σ3(111) GB may be difficult to fracture during SCC. Finally, the strength of a Ni/NiO interface is found to be the weakest among all interfaces studied, suggesting the metal/oxide interface could be a favorable crack initiation site when the tensile stress is low. Furthermore, the effects of co-segregation of oxygen and solute/impurity elements on GB strength are studied by DFT, using the 5(210) GB in an face-centered-cubic (FCC) Fe as a model system. Four elements (Cr, Ni, P, Si) that are commonly present in stainless steels are selected. Regarding the effects of single elements on GB strength, Ni and Cr are found to the increase the GB strength, while both P and Si have embrittlement effects. When each of them is combined with oxygen at the GB, the synergetic effect can be different from the linear sum of individual contributions. The synergetic effect also depends on the spatial arrangement of solute elements and oxygen. If they are aligned on the same plane at the GB, the synergetic effect is similar to the linear sum, although P and Si show stronger embrittlement potencies when they combine with both interstitial and substitutional oxygen. When they are arranged on a trans-plane structure, only nickel combined with oxygen show larger embrittlement potencies than the linear sum in all cases. Crystal Orbital Hamilton Populations analysis of bonding and anti-bonding states is conducted to interpret how the interaction between solutes and oxygen impacts GB strength. Finally, the continuum-scale CZM method, which is based on the bilinear mixed mode traction separation law, is used to model SCC-induced intergranular fracture in polycrystalline Ni and Fe based alloys in the MOOSE framework. The previous DFT results are used to justify the input parameters for the oxidation-induced GB strength degradation. In this study, it is found that the crack path does not always propagate along the weak GBs. As expected, the fracture prefers to occur at the GB orientations perpendicular to the loading direction. In addition, triple junctions can arrest or deflect fracture propagation, which is consistent with experimental observations. Simulation results also indicate that percolated weak GBs will lead to a much lower fracture stress compared to the discontinuous ones. / Doctor of Philosophy / Iron and Nickel based alloys are important structural materials for nuclear reactors due to their good mechanical properties, corrosion resistance, and radiation resistance. Under radiation and corrosive conditions, those alloys are susceptible to radiation induced segregation (RIS) and stress corrosion cracking (SCC). This dissertation is mainly focused on understanding the influence of the two effects on grain boundary (GB) strength. Systematic atomistic scale density functional theory (DFT) simulations are applied for the nickel and iron systems. Based on the DFT results, cohesive zone model is utilized for the continuum scale fracture simulation in nickel and iron polycrystal. First, DFT calculations are conducted for studying the RIS effect on the GB strength in nickel. Al, Cr, Cu, C, Si, P, Fe, and Ti are chosen as segregated element. Σ3(111), Σ3(112), Σ5(210), Σ5(310) four types of GBs are built for GB strength calculations. It is found that substitutional C and P always embrittle the GB, while substitutional Ti and Cr can strengthen the GB in most cases. Partial density of states (PDOS) analysis indicates the formation of C-Ni and P-Ni covalent bonds is the possible reason for their embrittlement effects. Bader charge analysis shows negatively charged elements likely reduce the GB strength. Interstitial element segregation is applied for non-metal elements (C, P, and Si). The results indicate interstitial elements have weaker effects than substitution ones. On the next stage to study the SCC effect, DFT calculations are performed for nickel Σ3(111), Σ3(112), and Σ5(210) GBs with difference oxygen concentration and oxygen incorporation types. Besides partially oxidized GBs, fully oxidized GBs (NiO GBs) and metal-oxide interface are also constructed for comparison. Simulation results show that the GB strength decreases nearly monotonically as oxygen concentration goes up. Typically, substitution oxygen causes a larger embrittlement effect than interstitial oxygen except at Σ3(111). It is found that the large mechanical distortion in this coherent twin GB contributes significantly to the GB strength drop. NiO GBs can be weak (Σ3(112),Σ5(210)) or strong (Σ3(111)). NiO/Ni interface shows lowest strength compared with partially and fully oxidized GBs, indicating the importance of the metal-oxide interface in the SCC process. Furthermore, the combined effects between segregated elements and oxygen are studied in face center cubic (FCC) iron system. In this part only Σ5(210) GB is selected with substitutional Cr, Ni, P, and Si as segregated elements. The results of single element effects show Cr can strength the GB while P has an opposite effect. Other two elements show little effect. For the co-segregation effects, the trans-plane structures have larger embrittlement potencies than in-plane ones for Ni, suggesting the GB strength can also be affected by the spatial arrangement of segregated elements. Finally, cohesive zone model is applied for fracture simulations in polycrystalline nickel and iron under tensile loading condition. It is found that intergranular fracture depends on both GB strength and orientation. GBs perpendicular to the loading direction have higher chances to crack. It is also found the percolated weak GBs induce larger strength drop than the discontinuous ones.

A series of experiments was performed in an attempt to measure the effects of surface residual stresses on the stress corrosion cracking/hydrogen embrittlement (SCC/HE) behavior of AISI 4340 steel. Stress corrosion tests were performed under load control on cylindrical and notched tensile specimens in acidified 3.5% NaCl solution. The electrochemical potential of the specimens was maintained at -0.7 V versus a saturated calomel reference electrode. Time to failure for specimens tested at various applied and residual stress levels was measured. Stress relieved specimens as well as specimens containing mechanically induced residual stresses were tested. Residual stresses were estimated using Neuber's rule and were measured using an x-ray diffraction technique. In all cases, the sum of the applied and residual stresses was greater than zero. Test results showed the initiation of SCC/HE cracks to be insensitive to the effects of surface residual stresses under the conditions evaluated. This is probably a result of the total time to failure criterion used to evaluate the SCC/HE tests. The extremely aggressive environment used in these experiments apparently led to rapid crack initiation, even in specimens containing compressive residual stresses. Another possible explanation of the insensitivity of this series of tests is crack initiation in the interior of the specimens below the depth of the mechanically induced residual stresses. / Master of Science