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This thesis examines the use of nonlinear ultrasound to evaluate sensitization, a precursor to stress corrosion cracking in austenitic stainless steel. Ultrasonic Rayleigh surface waves are generated on a specimen; as these waves pass through sensitized material, second harmonic generation (SHG) increases. In austenitic stainless steel with oven-induced sensitization, this increase is due only to the formation of chromium carbide precipitates, key products of the sensitization process. Weld-induced sensitization specimens demonstrate additional increases in SHG, likely caused by increased residual stress and dislocation density as a result of uneven heating. Experimental data are used to calculate the acoustic nonlinearity parameter, which provides a single value directly related to the quantity of micro- and nano-scale damage present within any given sample. Using this procedure, the effects of weld- and oven-induced sensitization are compared. Results demonstrate the feasibility of using nonlinear Rayleigh waves to detect and monitor stress corrosion susceptibility of welded material.

Austenitic stainless steels are frequently used for engineering applications in aggressive environments. Typical sources of component failures are associated with localized attack at grain boundaries, such as intergranular corrosion and stress corrosion cracking. To prevent premature failures, structural integrity assessments are carried out, with the aim of predicting the maximum likelihood of cracking that may develop. For accurate predictions it is of great importance to know the interaction of parameters involved in life-determining processes. This PhD thesis investigates the effect of microstructure and stress on intergranular stress corrosion cracking in Type 302 / Type 304 austenitic stainless steels. High-resolution X-ray tomography has been successfully applied to examine, for the first time in 3-dimensions, in-situ, the interaction between microstructure and crack propagation. The development and subsequent failure of crack bridging ligaments has been observed and correlated with regions of ductile tearing persistent on the fracture surface. These ductile regions were consistent with the morphology of low-energy, twin-type grain boundaries, and are believed to possess the capability of shielding the crack tip. Following this observation, a new grain bridging model has been developed, in order to quantify the effect of static stress and crack bridging on the maximum likely crack length. The model was compared and evaluated with in the literature available percolation-like models. Intergranular stress corrosion tests in tetrathionate solutions have been designed and carried out to validate the new model. The assessment comprised,(i) a thorough examination of the microstructure and analysis parameters employed,(ii) the determination of the degree of sensitisation with subsequent crack path investigations,(iii) the identification of a suitable test system with associated grain boundary susceptibility criteria,(iv) the application of Grain Boundary Engineering (GBE) for microstructure control,(v) statistical crack length assessments of calibrated IGSCC test specimens. The results of these tests showed that the new model successfully predicts the magnitude of stress and the effect of grain boundary engineering on the maximum crack lengths.

Titanium alloys are highly resistant to all types of corrosion due to their excellent ability to form an oxide film on the surface. However, under certain circumstances, these alloys may experience an environmental degradation which could potentially, under the application of mechanical stress, lead to a complete failure of the material. One of these cracking processes is stress-corrosion cracking (SCC). SCC has an embrittling effect on otherwise ductile materials under tensile stress. Since titanium alloys are frequently used in the aerospace industry and it is therefore of interest to test these alloys in different environment in order to prevent any future accidents. SCC testing is frequently tested at GKN Aerospace and a new testing method is of interest. The main objective with this work was to gain knowledge of the testing method. Bent-beam testing method has been used to investigate stress-corrosion cracking (SCC) of titanium alloys in a laboratory based experiment. The bent-beam testing method was of type 2-point bent beam test, where a saline solution was applied at the apex of the specimen. The specimens were loaded to a range of stresses from 40%, to 95% of the materials yield strength and the salt concentration in the saline solution was 1wt% and 3wt%. By doing so, a relative susceptibility of the different alloys could be established. Three different titanium alloys were tested: Ti-6Al-4V, Ti-8Al-1Mo-1V, and Ti-6Al-2Sn-4Zr-2Mo. The testing method was able to cause cracking on all titanium alloys, where Ti-6Al-4V was found to be the least susceptible to SCC. Ti-8Al-1Mo-1V, and Ti-6Al-2Sn-4Zr-2Mo showed an overall high susceptibility to SCC as cracking occurred in all testing configurations. Cracking was observed on both the surface of the specimen as well as in the cross sections, where the cracks grew perpendicular to the surface. SEM was also used to evaluate the crack propagation in Ti-8Al-1Mo-1V, and Ti-6Al-2Sn-4Zr-2Mo, and it was found that the cracks grew mostly along the grain boundaries.

O presente estudo objetivou caracterizar as juntas soldadas de uma chapa de aço inoxidável duplex UNS S32205 soldada utilizando diferentes energias (0,5 kJ/mm, 1,0 kJ/mm e 3,5 kJ/mm) e avaliar a susceptibilidade dessas juntas à corrosão sob tensão (CST) em ambiente contendo cloreto. O ensaio por gotejamento \"drop evaporation test\" (DET) de água do mar sintética foi utilizado para avaliar a susceptibilidade à CST das juntas soldadas e investigar os efeitos da temperatura (70 ºC, 90 ºC e 110 ºC) e da tensão de tração (70%, 90% e 100 % do limite de escoamento do metal base) na resistência à CST. Os resultados da caracterização mecânica, suscetibilidade à corrosão intergranular e caracterização microestrutural das juntas soldadas não evidenciaram a presença de fases deletérias, como as fases ? e ?. A proporção da fase ferrita na zona fundida e na zona afetada pelo calor (ZAC) foi inversamente proporcional à energia de soldagem. Por exemplo, a ferrita na ZAC variou de 68% para 0,5 kJ/mm a 54% para 3,5 kJ/mm. Os resultados dos ensaios de CST mostraram houve trincamento e fratura para todos os corpos de prova ensaiados a 110 ºC (mesmo com 70% do limite de escoamento), enquanto que nenhuma trinca foi observada nos corpos de prova ensaiados a 70 ºC e 90 ºC. Os ensaios de CST realizados a 90 ºC apresentaram corrosão localizada e preferencial da fase de ferrita longe do cordão de solda, enquanto os ensaios realizados a 70 ºC não apresentaram sinais significativos de corrosão. Todos os corpos de prova ensaiados a 110 ºC apresentaram fratura abaixo do deposito de sal com aspecto de vulcão formado pela evaporação da solução gotejada. As trincas propagaram pela interface ?/? ou clivagem transgranular nas fases ferrita e austenita. Para menores tensões ensaiadas, a proporção de clivagem transgranular da fase de ferrita foi mais significativa que a fase de austenita. O ensaio de DET não foi eficiente para investigar o efeito das alterações microestruturais na ZAC na susceptibilidade à CST. A análise de distribuição de temperatura na superfície dos corpos de prova indicou que o gotejamento da água do mar sintética teve um efeito de resfriamento e a temperatura na região de gotejamento foi pelo menos 10 ºC mais baixa quando comparada a outras regiões do corpo de prova. Além disso, o pH na região gotejante foi mais básico (pH = 11) do que a solução de água do mar sintética (pH = 8,2). / The present investigation characterized the welded joints of a UNS S32205 duplex stainless steel plate using different welding energies (0.5 kJ/mm, 1.0 kJ/mm and 3.5 kJ/mm) in order to evaluate the susceptibility of these joints to stress corrosion cracking (SCC) in Cl- environments. Drop evaporation test (DET) of synthetic seawater was used to assess the SCC behavior of the welded joints and to investigate the effects of the temperature (70 ºC, 90 ºC and 110 ºC) and the tensile strength (70%, 90% and 100% of the base metal\'s yield strength) on their SCC resistance. The results of the mechanical, intergranular corrosion susceptibility and microstructural characterization of the welded joints did not show the presence of deleterious phases, such as ? and ? phases. Additionally, the proportion of ferrite phase in the molten zone and in the heat-affected zone was inversely proportional to the welding energy. For instance, the ferrite in the HAZ varied from 68% for 0.5 kJ/mm to 54% for 3.5 kJ/mm. The results of SCC testing showed the cracking and fracture for all specimens tested at 110 ºC (even at 70% of the yield strength), while no cracking was observed at 70 ºC and 90 ºC. DET performed at 90 ºC showed that there was a localized and preferential corrosion of the ferrite phase far away from the weld bead, while DET performed at 70 ºC did not show any significant sign of corrosion. All samples tested at 110 ºC presented a noteworthy salt deposition in the shape of a volcano and their cracks were formed underneath the salt layer by the pit corrosion of the ferrite phase. The cracks propagated by either ? / ? interfacial or transgranular cleavage at ferrite and austenite phases. For lower stresses, the proportion of transgranular cleavage of the ferrite phase was more significant than the austenite phase. DET was not an efficient test to investigate the effect of the microstructural changes in the HAZ on the SCC susceptibly of the welded joints as the crack took place below the salt deposit and away from the weld bead. The temperature distribution analysis on the surface of the DET samples indicated that the seawater drip has a significant cooling effect and the temperature of this region was at least 10 ºC lower when compared to other regions of the samples. Additionally, the pH on the drip region was more basic (pH = 11) than the synthetic seawater solution (pH = 8.2). Keywords: Duplex stainless steels; stress corrosion cracking; synthetic seawater; welding.

Environmentally Assisted cracking (EAC) is a very critical materials science problem that concerns many technological areas such as petrochemical engineering, aerospace operations and nuclear power generation, in which cracking or sudden failure of materials may happen at stress far below the tensile strength. This type of corrosion is initiated at the microscopic level and is complicated due to the combination of chemistry (reaction caused by corrosive agents) and mechanics (varying load). As EAC is generally related to the segregation of impurity elements to defects (mainly grain boundaries), the symptoms of risk may not be apparent from the exterior of the metal components: hence EAC remains latent and gives no sign of warning until the failure occurs. Due to its intricate nature, conducting experiments on this phenomenon involves difficulties and requires much effort. In this work, we employed advanced molecular simulation techniques to study EAC in order to give insight into its atomistic behavior. First, Density-Functional Theory (DFT) method was used to investigate the fundamental processes and mechanism of EAC-related issues at the nanoscale level, with two case studies concerning the stress corrosion in iron and hydrogen embrittlement in palladium. When segregating to the grain boundary (GB) of iron, different impurity elements such as sulfur, phosphorus and nitrogen raise corrosion failures in a variety of ways. Hydrogen atoms, due to their mobility and small atomic size, are able to form high occupation at crystal defects, but show different interactions to vacancy and GB. Then, we used the classical Molecular Dynamics (MD) method to gain an understanding of the dynamic response of materials to mechanical load and the effects of temperature, strain and extreme conditions (high pressure shock compression) on structural properties. The MD simulations show that hydrogen maintains the highest localization at grain boundaries in the vicinity of ambient temperatures, and grain boundaries are the preferred nucleation sites for dislocations and voids. This computational work, using DFT and MD techniques, is expected to contribute to the better understanding on chemistry and mechanisms of complex environment-assisted cracking phenomenon at a fundamental level in order to beneficially complement conventional laboratory approaches.

Corrosion and stress corrosion cracking of structural components cost an estimated $300 billion annually in the United States alone and are a safety concern for a number of industries using hot alkaline environments. These process environments may contain different amounts of sulfide and chloride; however, the combined role of these ions on the stress corrosion cracking of duplex stainless steels, which are widely used because of their generally reliable performance, had never been studied. This study shows that chlorides in sulfide-containing caustic environments actually have a significant influence on the performance of these alloys. A mechanism for stress corrosion cracking of duplex stainless steels in hot alkaline environments in the presence of sulfide and/or chloride was proposed. Microstructural and environmental aspects were studied using mechanical, electrochemical, and film characterization techniques. The results showed that selective corrosion of the austenite phase depended on percent sulfidity, alkalinity, and chloride content. Chlorides enhanced crack initiation and coalescence along the austenite/ferrite phase boundaries. Unstable passivity of duplex stainless steels in hot alkaline-sulfide environments was due to anion adsorption on the surface leading to defective film formation. Chlorides and sulfide available at the electrolyte/film surface reduced the charge transfer resistance and shifted the response of the films to lower frequencies indicating the films became more defective. The surface films consisted of an outer, discontinuous layer, and an inner, barrier layer. Fe, Mo, and Mn were selectively dissolved in alkaline and alkaline-sulfide environments. The onset of stress corrosion cracking was related to the extent of selective dissolution and was consistent with a film breakdown and repair mechanism similar to slip-step dissolution. Recommendations for reducing the susceptibility of duplex stainless steels to stress corrosion cracking in sulfide-containing caustic environments include reducing the chloride to hydroxide ratio and alloying with less Mo and Mn. The results will impact the petrochemical, pulp and paper, and other process industries as new duplex grades can be developed with optimal compositions and environments can be controlled to extend equipment life.

Duplex stainless steel (DSS) is a dual-phase material with approximately equal volume amount of austenite and ferrite. It has both great mechanical properties (good ductility and high tensile/fatigue strength) and excellent corrosion resistance due to the mixture of the two phases. Cyclic loadings with high stress level and low frequency are experienced by many structures. However, the existing study on corrosion fatigue (CF) study of various metallic materials has mainly concentrated on relatively high frequency range. No systematic study has been done to understand the ultra-low frequency (10-5 Hz) cyclic loading effect on stress corrosion cracking (SCC) of DSSs. In this study, the ultra-low frequency cyclic loading effect on SCC of DSS 2205 was studied in acidified sodium chloride and caustic white liquor (WL) solutions. The research work focused on the environmental effect on SCC of DSS 2205, the cyclic stress effect on strain accumulation behavior of DSS 2205, and the combined environmental and cyclic stress effect on the stress corrosion crack initiation of DSS 2205 in the above environments. Potentiodynamic polarization tests were performed to investigate the electrochemical behavior of DSS 2205 in acidic NaCl solution. Series of slow strain rate tests (SSRTs) at different applied potential values were conducted to reveal the optimum applied potential value for SCC to happen. Room temperature static and cyclic creep tests were performed in air to illustrate the strain accumulation effect of cyclic stresses. Test results showed that cyclic loading could enhance strain accumulation in DSS 2205 compared to static loading. Moreover, the strain accumulation behavior of DSS 2205 was found to be controlled by the two phases of DSS 2205 with different crystal structures. The B.C.C. ferrite phase enhanced strain accumulation due to extensive cross-slips of the dislocations, whereas the F.C.C. austenite phase resisted strain accumulation due to cyclic strain hardening. Cyclic SSRTs were performed under the conditions that SCC occurs in sodium chloride and WL solutions. Test results show that cyclic stress facilitated crack initiations in DSS 2205. Stress corrosion cracks initiated from the intermetallic precipitates in acidic chloride environment, and the cracks initiated from austenite phase in WL environment. Cold-working has been found to retard the crack initiations induced by cyclic stresses.

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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