Al-Zn-Mg-Cu (7xxx series) aluminium alloys are widely used for aircraft structures. It is difficult to obtain a combination of optimal strength and stress corrosion cracking (SCC) resistance for these alloys. It appears that SCC resistance of these alloys is related to grain boundary precipitate morphology. One technique to control the grain boundary precipitate morphology is to introduce a controlled cooling procedure referred to as High Temperature Pre-precipitation (HTPP) treatment following the solution heat treatment. There is need for a detailed study of the effect of HTPP on the properties of commercial Al-Zn-Mg-Cu alloys using different intermediate temperatures. In this thesis research, the results of ten HTPP processes applied to 7075 and 7050 commercial 7xxx series alloys are presented in terms of hardness, electrical conductivity, corrosion resistance, TEM analysis of grain boundary precipitate morphology and EDS analysis of solute concentration profile at the grain boundary. Results indicate that subsequent to HTPP processing, the 7050 alloy can be precipitation aged to a higher hardness compared to 7075; this result is associated with the modification of 7050 alloy by zirconium versus chromium in 7075 alloy. HTPP heat treatment achieves better SCC resistance compared to standard T6 temper. However, it does not appear that HTPP can achieve a combination of hardness, electrical conductivity and corrosion resistance superior to standard T6 and T7X tempers. / October 2014

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This research uses nonlinear Rayleigh surface waves to characterize stress corrosion cracking (SCC) damage in sensitized 304 Stainless Steel (304 SS). 304 SS is widely used in reactor pressure vessels and fuel pipelines, where a corrosive environment in combination with applied stress due to high internal pressures can cause SCC. SCC poses great risk to these structures as it initiates cracks late in the lifetime and often unexpectedly. The initiated microcracks grow and accumulate very quickly to form macroscopic cracks that lead to material failure. Welds and the nearby heat affected zones (HAZ) in the vessels and pipework are particularly affected by SCC as welding induces sensitization in the material. SCC damage results in microstructural changes such as dislocation movement and microcrack initiation that in the long term lead to reduced structural integrity and material failure. Therefore, the early detection of SCC is crucial to ensure safe operation. It has been shown that the microstructural changes caused by SCC can generate higher harmonic waves when excited harmonically. This research considers different levels of SCC damage induced in samples of sensitized 304 SS by applying stress to a specimen held in a corrosive medium (Sodium Thiosulfate). Nonlinear Rayleigh surface waves are introduced in the material and the fundamental and the second harmonic waves are measured. The nonlinearity parameter that relates the fundamental and the second harmonic amplitudes, is computed to quantify the SCC damage in each sample. The results obtained are used to demonstrate the feasibility of using nonlinear Rayleigh waves to characterize SCC damage.

Duplex stainless steels (DSS) with roughly equal amount of austenite and ferrite phases are being used in industries such as petrochemical and pulp and paper mills. However, many DSS grades have been reported to undergo corrosion and stress corrosion cracking in some aggressive environments such as chlorides and sulfide-containing caustic solutions. Although stress corrosion cracking of duplex stainless steels in chloride solution has been investigated and well documented in the literature, SCC mechanisms for DSS in caustic solutions were unknown. Microstructural changes and environmental factors, such as pH of the solution, temperature, and resulting electrochemical potential also influence the SCC susceptibility of duplex stainless steels. In this study, the role of material and environmental parameters on corrosion and stress corrosion cracking of duplex stainless steels in caustic solutions were investigated. Results showed that the austenite phase in the DSS is more susceptible to crack initiation and propagation in caustic environment, which is different from that in the low pH chloride environment where the ferrite phase is the more susceptible phase. This study also showed that alloy composition and microstructural changes in duplex stainless steels due to different heat treatments could affect their SCC susceptibility. Moreover, corrosion rates and SCC susceptibility of DSS was found to increase with addition of sulfide to caustic solutions. Corrosion films on DSS indicated that the metal sulfide compounds formed along with oxides at the metal surface in the presence of sulfide containing caustic environments made the steel susceptible to SCC initiations. The overall results from this study helped in understanding the mechanism of SCC in caustic solutions. Favorable slip systems in the austenite phase of DSS favors slip-induced local film damage thereby initiating a stress corrosion crack. Repeated film repassivation and breaking, followed by crack tip dissolution results in crack propagation in the austenite phase of DSS alloys. Result from this study will have a significant impact in terms of identifying the alloy compositions, fabrication processes, microstructures, and environmental conditions that may be avoided to mitigate corrosion and stress corrosion cracking of DSS in caustic solutions.

This research investigates the mechanism of transgranular stress corrosion cracking (TGSCC) on fuel transmission pipelines. This research proposes that in near-neutral pH environments, hydrogen can be generated by the dissociation of carbonic acid and the reaction of metal ions with bicarbonate solutions, significantly increasing the available hydrogen for diffusion into the pipeline steel. This research has shown that TGSCC of pipeline steels is possible in simple groundwater solutions containing bicarbonate ions and carbon dioxide. Microstructural characterization coupled with hydrogen permeation indicates that the level of strain in the microstructure has the most influence on hydrogen diffusivity. Hydrogen accumulation occurs preferentially in at high energy discontinuous interfaces such as inclusion interfaces. It was determined that a stress concentration is required to facilitate sufficient hydrogen accumulation in the pipeline steel in order to initiate TGSCC. It was discovered that these stress concentrations develop from inclusions falling out of the pipeline surface. Slow strain rate tests found that TGSCC occurred in a wide range of compositions and temperatures as long as near-neutral conditions were maintained. Microcracks ahead of the crack tip provide evidence of hydrogen in these cracking processes. Morphology of these microcracks indicates that cracks propagate by the coalescence of microcracks with the main crack tip. Further research findings, scientific impact, and potential future work are also discussed.

The influence of steel metallurgy on rock bolt SCC was studied using a series of commercial carbon and low-alloyed steels. The chemical composition, their mechanical properties and the microstructures of these steels varied considerably in order to gather information for the discussion of the metallurgical influences under Hydrogen Embrittlement (HE) and Stress Corrosion Cracking (SCC) conditions. In order to understand the metallurgical influences on Rock Bolt SCC, an evaluation was carried out to fifteen commercial steels. The experiments reproduced the Stress Corrosion Cracking condition at which commercial rock bolts had failed in Australians mines. Due to the selected materials, stress and electrolyte condition it is expected that Hydrogen Embrittlement (HE) will affect the steel failure. The approach was to use the Linearly Increasing Stress Test (LIST) and exposing the sample to a dilute pH 2.1-sulphate solution, in accordance with prior studies. Stress Corrosion Cracking was evaluated by analysing the decrease in tensile strength, loss of ductility and fractography observed using Scanning Electron Microscopy (SEM). The initial series of test to the fifteen steels were performed at the free corrosion potential (f.c.p.) vs. Ag/AgCl. From this initial test only five steels (AISI 1008, AISI 4140, AISI 4145H, pipeline X-65 and X-70) did not show Stress Corrosion Cracking features. These five steel were tested in accordance with the Linearly Increased Stress Test (LIST) in the dilute pH 2.1 sulphate solution at different electronegative applied potential to minimum value of -1500mV. The experimental procedure tried to reproduce the Stress Corrosion Cracking condition to identify the most aggressive condition the steel is able to support before failing due to Stress Corrosion Cracking to then compare the theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. The investigation compared the well-known theory of SCC and HE in low carbon and low alloy steel with the obtained experimental results. Surprisingly, the experimental result did not always agree with the theory.

According to the National Institute of Occupational Safety and Health (NIOSH), about 100 million rock anchors were installed in the USA mining industry during 1999 (Dolinar, 2000). The rock bolt usage in US coal mining industry fell from 85 million in the year 1988 to 68 million by 2005 (Tadolini, 2006), and is assumed to be close to that number of rock anchors consumed currently since, the tonnage from underground is almost the same. Most underground coal mines have conditions such as moisture in the atmosphere, ground water with different chemical contents that are conducive for corrosion of rock anchors and ancillaries (such as plates), and the effects of this on the performance of the anchors had been researched in the US to an extent from the past research at Southern Illinois University Carbondale (SIUC). In addition to the general corrosion like pitting and crevice, stress corrosion adds to the process a potentially serious threat and results in material failure underground due to stress corrosion cracking (SCC) yet the effects are not fully understood in the USA. The results of this research therefore will have a positive and direct effect on rock related safety. During this research project in situ specific tests were conducted with bolts to try and determine the corrosion potential in a specific coal-mining region. The coal mining areas were divided into three regions and were named as East, Mid-West and West respectively. To enhance the value/importance of the field data collected from the mines, a metal mine and a salt mine (two non-coal mines) were included in the plan and the data analysis proved that the methodology developed for determining the corrosion potential underground is applicable to any underground mines. The Insitu studies include water samples collection and analysis and open circuit potential (OCP/Eoc) testing and analysis. Open Circuit Potential (OCP) data were recorded to estimate probability of active corrosion. Hypothetically, probability of active corrosion is lower if the actual OCP of roof bolts in the mine is less than the characteristic OCP of the steel grade, and vice versa. The effects of certain factors such as the roof condition, reference distance (distance between bolt and reference electrode) on the open circuit potential data during the measurements were studied to ensure its impact on the corrosion potential determination technique developed. The findings from this research helps standardize the corrosion potential determination methodology. The preliminary study of stress corrosion cracking of the subject test sample (Grade 60 rebar roof bolt) was conducted in this research work. The experimental study invloves testing a complete roof bolt in the mine simulated environment. The mine simulated environment in the test cell consists of the roof strata material collected from the mine site with continuous flow of water at slower and varaible flow rate (0 to 3 ml/minute) with pH in the range of 7.5 to 9.0. The results showed that stress corrosion could be very serious problem when it comes to long term mining applications. The stress corrosion test cell developed and tested was proved to be significant in conducting the long term stress corrosion tests. The strength results of the Grade 60 rebar roof bolt tested had a significant strength loss after 3 months of testing in the stress corrosion cell. Hence, more SCC studies are deemed necessary to evaluate the seriousness of the problem and if possible eliminate it.

abstract: This research examines several critical aspects of the so-called "film induced cleavage" model of stress corrosion cracking using silver-gold alloys as the parent-phase material. The model hypothesizes that the corrosion generates a brittle nanoporous film, which subsequently fractures forming a high-speed crack that is injected into the uncorroded parent-phase alloy. This high speed crack owing to its kinetic energy can penetrate beyond the corroded layer into the parent phase and thus effectively reducing strength of the parent phase. Silver-gold alloys provide an ideal system to study this effect, as hydrogen effect can be ruled out on thermodynamic basis. During corrosion of the silver-gold alloy, the less noble metal i.e. silver is removed from the system leaving behind a nanoporous gold (NPG) layer. In the case of polycrystalline material, this corrosion process proceeds deeper along the grain boundary than the matrix grain. All of the cracks with apparent penetration beyond the corroded (dealloyed) layer are intergranular. Our aim was to study the crack penetration depth along the grain boundary to ascertain whether the penetration occurs past the grain-boundary dealloyed depth. EDS and imaging in high-resolution aberration corrected scanning transmission electron microscope (STEM) and atom probe tomography (APT) have been used to evaluate the grain boundary corrosion depth. The mechanical properties of monolithic NPG are also studied. The motivation behind this is two-fold. The crack injection depth depends on the speed of the crack formed in the nanoporous layer, which in turn depends on the mechanical properties of the NPG. Also NPG has potential applications in actuation, sensing and catalysis. The measured value of the Young's modulus of NPG with 40 nm ligament size and 28% density was ~ 2.5 GPa and the Poisson's ratio was ~ 0.20. The fracture stress was observed to be ~ 11-13 MPa. There was no significant change observed between these mechanical properties on oxidation of NPG at 1.4 V. The fracture toughness value for the NPG was ~ 10 J/m2. Also dynamic fracture tests showed that the NPG is capable of supporting crack velocities ~ 100 - 180 m/s. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2014

abstract: Dealloying induced stress corrosion cracking is particularly relevant in energy conversion systems (both nuclear and fossil fuel) as many failures in alloys such as austenitic stainless steel and nickel-based systems result directly from dealloying. This study provides evidence of the role of unstable dynamic fracture processes in dealloying induced stress-corrosion cracking of face-centered cubic alloys. Corrosion of such alloys often results in the formation of a brittle nanoporous layer which we hypothesize serves to nucleate a crack that owing to dynamic effects penetrates into the un-dealloyed parent phase alloy. Thus, since there is essentially a purely mechanical component of cracking, stress corrosion crack propagation rates can be significantly larger than that predicted from electrochemical parameters. The main objective of this work is to examine and test this hypothesis under conditions relevant to stress corrosion cracking. Silver-gold alloys serve as a model system for this study since hydrogen effects can be neglected on a thermodynamic basis, which allows us to focus on a single cracking mechanism. In order to study various aspects of this problem, the dynamic fracture properties of monolithic nanoporous gold (NPG) were examined in air and under electrochemical conditions relevant to stress corrosion cracking. The detailed processes associated with the crack injection phenomenon were also examined by forming dealloyed nanoporous layers of prescribed properties on un-dealloyed parent phase structures and measuring crack penetration distances. Dynamic fracture in monolithic NPG and in crack injection experiments was examined using high-speed (106 frames s-1) digital photography. The tunable set of experimental parameters included the NPG length scale (20-40 nm), thickness of the dealloyed layer (10-3000 nm) and the electrochemical potential (0.5-1.5 V). The results of crack injection experiments were characterized using the dual-beam focused ion beam/scanning electron microscopy. Together these tools allow us to very accurately examine the detailed structure and composition of dealloyed grain boundaries and compare crack injection distances to the depth of dealloying. The results of this work should provide a basis for new mathematical modeling of dealloying induced stress corrosion cracking while providing a sound physical basis for the design of new alloys that may not be susceptible to this form of cracking. Additionally, the obtained results should be of broad interest to researchers interested in the fracture properties of nano-structured materials. The findings will open up new avenues of research apart from any implications the study may have for stress corrosion cracking. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2012

La Corrosion Sous Contrainte (CSC) est un des principaux modes de dégradation des composants assemblés par soudage dans Réacteurs à Eau Pressurisée (REP). Le retour d’expérience de 2007 sur les soudures en alliage à base de nickel a présenté 3 cas de fissuration par CSC sur l’Alliage 82 sur les 300 cas recensés dans le circuit primaire. L’objectif de cette étude est alors d’identifier des paramètres microstructuraux et mécaniques à l’échelle de la microstructure impliqués dans l’amorçage de fissures. Les caractérisations du matériau pour identifier ces paramètres sont composées d’une part, d’analyses de la composition chimique et d’analyses EBSD (Electron Back-Scattered Diffraction) pour la morphologie et les orientations cristallographiques des grains pour la microstructure et, d’autre part, de mesures expérimentales de déformation en surface et de calculs numériques de champs de contrainte autour des joints de grains par éléments finis pour le comportement mécanique. La corrélation de ces informations avec les sites d’amorçage de fissures de CSC obtenus avec les essais d’amorçage entrepris sur des éprouvettes U-bend en milieu vapeur d’eau hydrogénée à 400°C, 188 bar a confirmé la sensibilité de l’Alliage 82 en CSC avec une fissuration intergranulaire des joints de grains dont la particularité première est d’être perpendiculaire à la sollicitation (mode I). Les autres paramètres concernent celui de la chimie locale au niveau des joints de grains, de leur nature (généraux ou spéciaux) et du chargement mécanique appliqué à ces derniers (contrainte et différence de déformation). Cette méthodologie, applicable à d’autre matériau, a permis de mieux comprendre quels sont les paramètres microscopiques sensibilisent la cohésion du joint de grains et à quels degré d’importance doivent-il être pris en compte dans le mécanisme d’amorçage de fissures de CSC. / In Pressurize Water Reactors (PWR), Stress Corrosion Cracking (SCC) is the mean degradation mode of components pieced together by welding. Nickel based alloys are, among others, used in dissimilar metal welding (DMW). International report showed only 3 cracking cases in Alloy 82 out of 300 cracking cases concerned on nickel based alloys DMW in primary water circuit. The aim of this study is to identify which microstructural and local mechanism parameters at microstructure scale provide the initiation of SCC cracks. Characterizations performed on specimen surface to identify those parameters are composed ofchemical composition analysis and EBSD analysis (Electron Back-Scattered Diffraction) to know the morphology and the crystallography of grains for microstructure features on one hand, and experimental strain fields measured by Digital Imaging Correlation (DIC) of gold microgrids deposed by electronic lithography on U-bend specimen surface and stress fields calculated along grains boundaries by finite element for local mechanical features on the other hand. The correlation between those characterizations and localization of initiation sites of SCC cracks, obtained on U-bend specimens tested in autoclave in hydrogen steam water at 400°C and 188 bar for 3500 hours, confirmed the susceptibility of the Alloy 82 in SCC conditions with intergranular SCC cracks. The perpendicular position to the loading direction (mode I) is the worst conditions for grains boundary in SCC. The others points concern the chemical composition (precipitation, impurities) around grain boundary and the grain boundary type which is more susceptible when it is a High Angle Grain Boundary. It is following by the mechanical characterization (stress and strain gradient) along grain boundary. This methodology can be used to other material and helped to define which microstructural and mechanical parameter can be define the initiation of SCC cracks.