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.

There has been an increase in the production of bio-fuels. Organosolv delignification, high temperature ethanol/water environments, can be used to separate lignin, cellulose, and hemicelluloses in the bio-mass for bio-fuel production. These environments have been shown to induce stress corrosion cracking (SCC) in 316L stainless steel. Previous research has been done in mixed solvent environments at room temperature to understand SCC for stainless steels, but little is known about the behavior in high temperature environments. Simulated organosolv delignification environments were studied, varying water content, temperature, pHe, and Cl- content to understand how these constituents impact SCC. In order for SCC to occur in 316L, there needs to be between 10 and 90 volume % water and the environment needs to be at a temperature around 200°C. Once these two conditions are met, the environment needs to either have pHe < 4 or have more than 10 ppm Cl-. These threshold conditions are based on the organosolv delignification simulated environments tested. SCC severity was seen to increase as water content, temperature, and Cl- content increased and as pHe decreased. To prevent failure of industrial vessels encountering organosolv delignification environments, care needs to be taken to monitor and adjust the constituents to prevent SCC.

Stress corrosion, SC, in some cases gives rise to stress corrosion cracking, SCC, which differs from purely stress intensity driven cracks in many aspects. They initiate and grow under the influence of an aggressive environment in a stressed substrate. They grow at low load and may branch. The phenomenon of SCC is very complex, both the initiation phase and crack extension itself of SCC is seemingly associated with arbitrariness due to the many unknown factors controlling the process. Such factors could be concentration of species in the environment, stress, stress concentration, electrical conditions, mass transport, and so on.In the present thesis, chemically assisted crack initiation and growth is studied with special focus on the initiation and branching of cracks. Polycarbonate plates are used as substrates subjected to an acetone environment. Experimental procedures for examining initiation and branching in polycarbonate are presented. An optical microscope is employed to study the substrate.The attack at initiation is quantified from pits found on the surface, and pits that act as origin for cracks is identified and the distribution is analysed. A growth criterion for surface cracks is formulated from the observations, and it is used to numerically simulate crack growth. The cracks are seen to coalesce, and this phenomenon is studied in detail. Branching sites of cracks growing in the bulk of polycarbonate are inspected at the sample surface. It is found that the total width of the crack branches are approximately the same as the width of the original crack. Also, angles of the branches are studied. Further, for comparison the crack growth in the bulk is simulated using a moving boundary problem based algorithm and similar behaviour of crack branching is found. / <p>Both papers in thesis as manuscript, paper II with title "Branching cracks in a layered material - Dissolution driven crack growth in polycarbonate"</p>

The effect of a specific type of pre-exposure heat treatment on the susceptibility of AlSI type 304 stainless steel to stress corrosion cracking was studied in terms of time for crack nucleation and rate of crack propagation. U-bend specimens were exposed to 42 weight percent magnesium chloride aqueous solution after pre-exposure heat treatments at 140°C and 154°C. The straight-line relationship between maximum crack depth and the logarithm of exposure time expressed by the empirical equation log t = log t_o + D/M was obtained. The stress corrosion constants derived from the empirical equation indicate that this type of pre-exposure heat treatment promotes crack nucleation because of the formation of less protective surface films, and retards the rate of crack propagation because of effects on internal structural changes within the alloy. / Master of Science

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.

No description available.

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.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE / Vita. / Includes bibliographical references. / by Arnaldo Homobono Paes de Andrade. / Ph.D.