Stress corrosion cracking (SCC) in austenitic stainless steel (SS) welds remains a complex challenge due to the interplay of metallurgical heterogeneities and environmental parameters. This study proposes a new accelerated SCC test designed to evaluate the SCC susceptibility of 316L SS welds under conditions that better simulate in-service environments. The newly designed test is a uniaxial tensile test which incorporates incremental loading combined with anodic polarization cycles in a 1 M NaCl solution acidified to pH 4 at 60 degrees C, i.e., a solution more representative of in-service applications than the usual boiling MgCl2 often used in the literature. Tensile specimens were machined to align the weld axis with the loading direction, and 1-mm diameter stress concentrators (i.e., holes) were strategically located between the fusion zone (FZ), heat-affected zone (HAZ), and base metal. When the tensile loading was incrementally increased to 450 MPa, fracture of the tensile specimen was observed. Fractographic analysis confirmed crack initiation in the HAZ and unstable propagation toward the FZ. Two reference tests were performed for comparison. For one reference test, the same methodology was used, but without stress concentrators: only microcracks were observed, primarily intergranular in nature and concentrated in the HAZ, validating this zone as the most susceptible to SCC. Another reference test used the tensile specimen with stress concentrators, but it was conducted at open-circuit potential (OCP) for one month. This long-term test demonstrated similar failure patterns and corrosion mechanisms as the newly designed test, confirming its representativeness despite anodic polarization. The newly designed test successfully accelerates SCC while preserving the relevance of observed damage modes, bridging the gap between traditional accelerated methods and actual service conditions. The findings emphasize the critical role of the HAZ in SCC initiation, as well as the influence of microstructural features and stress and deformation gradients. This methodology offers a robust framework for screening SCC resistance in welded SS components and for further mechanistic studies under controlled but representative conditions.

The embrittlement of alloy 82 welds exposed to a simulated pressurised water reactor (PWR) environment was analysed and compared with that observed for cathodically hydrogen-precharged specimens. Similarities in mechanical behaviour, i.e. the evolution of the constitutive equations with a decrease in flow stress compared with non-precharged specimens, and in the fracture modes, with brittle zones in the outer layer, enabled us to conclude that the embrittlement observed in simulated PWR environment corresponded to a two-step hydrogen-assisted cracking mechanism. The first step corresponded to hydrogen absorption into the surface layer leading to crack initiation, and the second step to hydrogen localisation ahead of the crack tip promoting crack propagation.

The susceptibility to stress corrosion cracking of 316 L welds was investigated in a moderately aggressive electrolyte, performing long-term U-Bend tests at the OCP. Micro-cracks were observed in the heat-affected zone (HAZ), initiating at pits. This was due to the increased susceptibility to pit initiation under tensile stress of the HAZ, which was also the most deformed zone in terms of plastic deformation. Accelerated U-Bend tests, carried out applying a fixed current density, made it possible to reproduce in a short time the main damage modes observed after 9 months at OCP, but led to the growth of large cavities.

Corrosion of Ni-based alloy steam generator tubes in simulated primary water leads to the formation of oxides on the surface of the tubes and to the release of corrosion products. The oxides formed on two industrial samples were studied using X-ray photoemission spectroscopy and transmission electron microscopy. The morphology, structure and composition of the oxide layers were determined, and the differences observed were discussed in relation to previous analyses on the as-received inner surfaces. A fine grains layer, alumina on the surface of native oxides could influence the behavior of the surface in primary water.

The steam generator (SG) tubes of pressurized water reactors are manufactured with Ni-based alloys; their corrosion behavior in the simulated primary water environment largely depends on the properties of the oxides formed during the exposure to this environment. Ex-situ experiments, in particular electrochemical impedance spectroscopy (EIS) measurements at room temperature, were performed to characterize the electrical properties of the oxides. The analyses were conducted on two industrial samples, in their as-received state to study the native oxide films, and after exposure to the simulated primary water environment to analyze the oxide layers formed in this environment. A physical model was proposed based on the EIS data and previous TEM and XPS characterizations. For both conditions, EIS showed a capacitive behavior of the oxide films in relation with their passive properties: the passive contribution of the film was modelled using the power law model. The data acquired for the oxide films formed in simulated primary water environment also showed a porous behavior that was considered using a De Levie impedance. The data were successfully fitted and the parameters of the model determined. The values of these parameters were discussed according to previous TEM observations and XPS chemical analyses of the films formed on the two SG tubes. A good correlation was evidenced between the results obtained from the electrochemical measurements of this work and the chemical and structural analyses of the oxide films formed in the simulated primary water environment.

This study focused on the low-temperature embrittlement of alloy 82 welds in a simulated PWR primary water environment, and in particular on assessing the influence of pre-exposure on the embrittlement in this environment. In most cases, the hydrogen amount inserted during pre-exposure in the simulated PWR primary water without mechanical loading is low due to the barrier effect of the oxide layer, so it does not significantly influence the tensile behaviour of the welds. On the contrary, hydrogen absorption in the outer layer of the tensile specimens is promoted by the breakdown of the oxide layer when the tensile tests are carried out in the simulated PWR primary water environment. This weakens the outer layer, leading to crack initiation; then, crack propagation is controlled by the hydrogen amount at the crack tip. Hydrogen transport by mobile dislocations and hydrogen diffusion promoted by the dislocation network help to establish the critical amount of hydrogen at the crack tip. In addition, crack propagation is also probably enhanced by the effect of hydrogen on the corrosion behaviour of the weld, i.e., destabilisation of the passive film and increased dissolution processes.

The stress corrosion cracking (SCC) susceptibility of a non-polished 316 L austenitic stainless steel (ASS) welded joint, i.e., covered by a thermal oxide film, was studied by carrying out uniaxial tensile tests in a 1 M NaCl solution (pH 4, 60 degrees C). Experiments were also performed on a polished weld, i.e., covered by a native oxide film, for comparison. The thermal oxide film presented a bi-layer structure similar to that of the native oxide film, but the ratio Fe/Cr in the outer layer of the thermal oxide film was higher. Moreover, the thermal oxide film was thicker than the native oxide film, but it was more porous and showed a gradient in chemical composition and morphology as a function of the distance from the fusion line of the weld. The defective structure of the thermal oxide film was associated with a degradation in the corrosion behaviour of the non-polished weld, as well a change in the crack propagation mode during SCC tests, from intergranular to transgranular even though the cracks formed preferentially in the heat affected zone (HAZ) for both polished and non-polished samples. Furthermore, crack propagation was promoted, with cracks extending across multiple grains, in the HAZ of the non-polished sample near the fusion line, where the thermal oxide was more porous and richer in Fe compared to the HAZ far from the fusion line. The results therefore highlighted the major influence of the thermal oxide properties on the corrosion behaviour and SCC susceptibility of the 316 L weld.

This work focuses on the microstructural characterization of an industrial low carbon steel which was submitted to high temperature hydrogen attack for decades in petroleum refineries. Scanning electron microscopy (SEM) was used to make an initial assessment of the attack stage by identifying the defects. Cavities filled with methane are located at ferrite-ferrite or ferrite-pearlite grain boundaries as well as at the carbide/matrix interfaces. Cracks are also observed next to zones showing less advanced damage which reflects a heterogeneity of attack. In order to establish a link between the fine-scale microstructure, which may be responsible for this heterogeneity, and the formation of cavities, a 3D visualization of a zone displaying cavities was performed thanks to a 3D focused ion beam (FIB) -SEM analysis. The main result is that cavities mostly formed at the junction of a transgranular inclusion-enriched plane with a grain boundary or carbides in pearlite grain. These inclusion-enriched planes that pass through ferrite or pearlite grains may correspond to former austenitic grain boundaries that have been overtopped by subsequent ferrite and pearlite grain boundaries during the steel manufacturing. A transmission electron microscopy (TEM) observation of an attacked pearlite reveals that in most cavities located at carbide/matrix interface, an inclusion of AlN remains stuck. The potential catalytic effect of inclusions as AlN on the trigger of attack is not yet fully understood and will be the subject of further investigations on brand new C-steels.

Surface effects were investigated using ultrathin specimens with thicknesses in the order of the grain size of the material. The candidate material was a polycrystalline Ni-based superalloy (Alloy 718) purposely heat treated to document both the effects of the grain size and the metallurgical state, i.e., solid solution and precipitation hardened state, on the polycrystalline-tomulticrystalline behavior. Ultrathin tensile specimens were prepared with a dedicated technique to obtain specimens with thicknesses ranging between 20 and 550 mu m, then tensile tested at room temperature. The polycrystalline-to-multicrystalline transition (PMT) was found to depend on the material grain size relative to the specimen thickness and to impair severely the tensile strength of the material. The yield strength, ultimate tensile strength (maximal stress on the stress-strain curve) and strain-to-failure severely dropped for specimens thinner than approximately two times the grain size of the material regardless of the metallurgical state. Such a decrease in tensile properties is mainly attributed to free-surface effects acting as an escape sink of dislocations, thus leading to a significant decrease of the primary dislocations density within the surface grains in comparison with the core grains. Interestingly, difference in work-hardening behavior with size reduction was found between both precipitation states, the solid solution state being more sensitive with the size reduction. The decrease in tensile properties was not found as expected from the commonly reported "thickness/grain size (t/D)"ratio. Therefore, a numerical approach using a modified Berveiller-Zaoui self-consistent model based on a continuum crystal plasticity approach was conducted in the present paper to distinguish microstructural features acting as strengthening (dislocation accumulation) and softening (dislocation escape at the free-surface) features. 3D numerical materials were produced using Voronoi tessellation methods to represent the fraction of "core grains" versus "surface grains". These fractions were then used as microstructural parameters for the identification of a crystal plasticity model using mean-field homogenization with different populations of grains, i.e., core versus surface features. The present work aimed at distinguishing the mechanical behavior of surface grains from core grains in Alloy 718 Ni-based superalloys using various thicknesses of specimens and different microstructure and metallurgical state variants.

This article focuses on the fine characterization of steels commonly used in the petrochemical industry damaged by the phenomenon of high temperature hydrogen attack (HTHA). The study was conducted in two steps. To begin with, a damaged 0.5-Mo pearlitic steel from the petroleum refineries, submitted to HTHA for decades, was characterized in detail using multiscale electron microscopy techniques. As part of an upstream study to better understand the onset and the growth of cavities, a brand new SA516 grade 60 low carbon–manganese steel was subsequently exposed to accelerated HTHA conditions through interrupted cycles carried out in autoclaves and then examined. Numerous cavities, plausibly filled with methane, were noticed in both materials. These cavities were mostly located at ferrite–pearlite grain boundaries along carbides and at triple grain boundaries near large carbides. The 0.5-Mo pearlitic steel showed cavities reaching significant sizes, up to 1 µm, but surprisingly no cracks were observed in the depth of the pipe. The major outcome is that 3D focused ion beam–scanning electron microscopy combined with transmission electron microscopy (TEM) analyses unveiled different natures of precipitates as well as in and nearby HTHA cavities for both 0.5-Mo and low carbon–manganese steels. Inclusions, likely AlN, but also Mo- and Cu-rich precipitates were observed in cavities of the industrial steel. These results confirmed a previous study performed on a similar industrial steel that drew a possible correlation between cavities nucleation and the intersection of transgranular inclusion-enriched plane with a grain boundary or carbides in pearlite grains (Flament in Microscopy and Microanalysis 28:1602–1604, 2022).

Ni based alloys steam generator (SG) tubes in pressurized water reactors may suffer generalized corrosion and Ni release in primary water. The processes of corrosion and oxidation are strongly dependent on the surface properties. In particular, the chemical composition and structure of the native oxide films present at the surface of the SG tubes are crucial and improving the passive properties of the surface could contribute to reduce the corrosion damage and Ni release. The native oxides on the inner surface of two as-received Ni base alloy SG tubes were investigated using two main techniques, namely X-ray photoemission spectroscopy and transmission electron microscopy. The chemical elements present in the native film for the two tubes were identified and the structure and composition of the native film at the nanoscale was determined. A duplex oxide structure was identified for both samples, but with differences in the chemical composition of the inner and outer oxide layer. Chemical heterogeneities such as alumina, Ti oxides and Cr oxides were also detected; they are considered to play a role in the evolution of the surface exposed to primary water. The two techniques used allowed a multi-scale description of the surface and led to complementary results.

Irreversible deformation in relation to the microstructure was investigated for a polycrystalline Ni-based superalloy (Alloy 718) from room temperature to 650 °C using high-resolution digital image correlation (HR-DIC) techniques. Interrupted tensile tests were performed under a protective atmosphere to ensure the stability of the speckle pattern to track kinematics fields from surface analyses. In-plane strain localization was captured using HR-DIC on scanning electron microscopy (SEM) images. A statistical analysis of different strain localization events in relation to the microstructural features was conducted, i.e., intragranular slip localization, slip localization parallel to and near Σ3-twin boundaries (Σ3-TB), and grain boundary sliding (GBS). Alloy 718 exhibited slip localization at room temperature and 350 °C. Intense strain localization develops parallel and in the vicinity of Σ3-TB from the onset of the microplasticity. Few intense slip stimulated-grain boundary sliding events were found due to slip localization on both grains adjacent to the grain boundary. At 650 °C, Alloy 718 experienced grain boundary sliding at the onset of the yield without particular slip localization in adjacent grains. At lower temperatures, strain localization parallel to and near Σ3-TB was intense, and intragranular slip localization intensified with increasing macroscopic deformation. Particular microstructural configurations were found at 650 °C leading to premature damage: (i) sub-surface cavitation at grain boundaries, and (ii) grain boundary cracking due to intense shearing near a Σ3-TB.

Micromechanical characterization of the oxygen-rich layer (ORL) of a Ti-6Al-4V alloy due to high-temperature oxidation was investigated at room temperature. The tensile strength of the pre-oxidized specimens linearly decreased as a function of the surface fraction of ORL in relation to the gage section, demonstrating a competition between oxygen strengthening and embrittlement. Electron-probe microanalyses and nanoindentation testing aimed at locally assessing the elastic and hardness response of the material as a function of the oxygen content. These properties were used in finite element simulations to quantify stress profiles within the oxygen-graded material for different ORL thickness/specimen thickness couples.

Hydrogen is a crucial element in refinery operations and has historically been employed under different conditions, often involving high pressure and elevated temperatures. However, when hydrogen levels surpass 3,5 bar (50Psi) and temperatures exceed 207 degrees C (400F), a potentially catastrophic damage called High-Temperature Hydrogen Attack (HTHA) can occur on ferritic steels. HTHA leads to irreversible damage in steels, posing a significant risk within the Oil & Gas industry. Despite its pervasive impact, HTHA remains inadequately understood, making it a persistent challenge for Oil & Gas industry. In response to the threat posed by HTHA, the TotalEnergies Company undertook research to address this issue. Recognizing the need for comprehensive insights, TotalEnergies collaborated with scientists from the French Atomic Energy and Alternative Energy Commission (CEA) to conduct meticulous evaluations of HTHA damaged samples. These expert assessments were aimed to better understand the intricate processes underlying HTHA degradation, particularly during its early stages. Moreover, the collaborative effort extended to research involving artificial samples, allowing for controlled experimentation and analysis. This approach tries to provide a nuanced understanding of HTHA mechanisms. This paper presents current and ongoing work. Additionally, this paper outlines plausible hypotheses that delve into the intricate interplay of atomic hydrogen within a metal matrix of ferritic steel under HTHA conditions. These hypotheses not only enhance our comprehension of HTHA but also try to open new approaches for advancements in the mitigating risks coming from this formidable challenge of the use of hydrogen.

Although considered a 1st-class option to meet the drastic requirements of Reactor Pressure Vessels (RPV) environments, Mn-Mo-Ni bainitic alloys also have segregated microstructures that can lead to heterogeneous mechanical properties. The latter should explain the scatter and the brittle fracture sometimes observed along the ductile-to-brittle transition during qualification tests. The present study focuses on a highly segregated SA 508 Gr.3 (aka 20MND5) steel and investigates the evolutions of local mechanical properties with temperature. Microstructure analysis revealed positive and negative segregations mainly composed of lower and upper bainite, respectively. An experimental device was developed and combined with a systematic procedure to enable local microhardness measurements from room temperature to - 40 degrees C. Analysis of hardness maps revealed two critical parameters assumed to be involved in the macroscopic brittle fracture as the temperature decreases. The first one is a significant increase in the hardness of both phases, particularly a continuous increase in the lower bainite's hardness, suggesting that this phase could preferentially lead to brittle fracture. A wide range of hardness values in selected positive segregation reinforced the idea of the possible existence of very local weak points that could explain the dispersion of the results of the macroscopic Charpy test. The second parameter is an extension in the brittle surfaces, leading to a greater probability of testing a brittle area due to an extension surface area of the weak points.

Three methodologies, i.e., optical microscope (OM) observations, analyses using tomography without synchrotron radiations, and mechanical tests, were used, and their relevancy compared to provide a quantitative description of the intergranular corrosion (IGC) damage. The study was performed on three plates of 2024 aluminum alloy, to take into account variations in the microstructure of the material, using two types of exposure conditions, i.e. continuous immersions and cyclic tests. The resolution of tomography was too low to allow an analysis at the scale of the elementary IGC defects. For thin plates with recrystallized grains, the corrosion damage corresponded mostly to intragranular corrosion, and OM observations were the most relevant technique to quantify correctly this damage. Thicker plates were mainly affected by IGC. Continuous immersion tests led to slightly branched IGC defects and low hydrogen uptake, and OM observations provided an accurate description of the IGC damage that corresponded mainly to a geometrical one. However, combining OM observations and mechanical tests was required for a full description of the IGC damage after cyclic tests that were associated with a strong branching of the IGC defects, i.e. geometrical damage, and significant hydrogen uptake, i.e., a non-negligible volume damage.

The inner surface of two as-received steam generator (SG) tubes made of nickel base alloy 690 was analyzed to explain the differences in nickel release obtained for the tubes in a loop simulating the primary environment of pressurized water reactors. The roughness, microstructure and chemical heterogeneities were characterized; differences that could explain the specific behavior of the two tubes in primary water were noted. In particular, the presence of alumina particles at the surface of SG tubes influences the nickel release: their dissolution in the primary coolant induces a change in roughness and a local breakdown of the passive film.

This study provides some clarifications about the influence of microstructural parameters on the susceptibility of low-copper Al-Zn-Mg alloy to stress corrosion cracking (SCC). Tensile tests in air were carried out on AA7046 in T4 and T4 aged at 150 degrees C (named 150/20) metallurgical states after pre-exposure of the specimens to a chloride solution under mechanical loading. The results showed the predominant role of the corrosion-induced hydrogen during SCC process on the loss of elongation to failure. Scanning kelvin probe force microscopy (SKPFM) measurements were performed for the T4 specimens as well as for a 530 degrees C heat-treated T4 specimen with a coarse-grained microstructure; this allowed the contribution of hydrogen diffusion at the grain boundaries on the hydrogen distribution to be highlighted. The analysis of the fracture modes after tensile tests and hydrogen diffusion profiles obtained by SKPFM in the framework of previous studies investigating the microstructure-hydrogen and plasticity-hydrogen relationships allowed to propose a qualitative model to describe SCC phenomena. The detrimental role of hydrogen at the grain boundaries on the mechanical behaviour was highlighted; the outcome of the evaluation of results from the present study in combination with our previous studies and literature data suggested that it can be limited by hydrogen trapping on intragranular eta-MgZn2 precipitates. (C) 2021 Elsevier B.V. All rights reserved.