Matching Items (12)
Description
Al 7075 alloys are used in a variety of structural applications, such as aircraft wings, automotive components, fuselage, spacecraft, missiles, etc. The mechanical and corrosion behavior of these alloys are dependent on their microstructure and the environment. Therefore, a comprehensive study on microstructural characterization and stress-environment interaction is necessary. Traditionally, 2D techniques have been used to characterize microstructure, which are inaccurate and inadequate since the research has shown that the results obtained in the bulk are different from those obtained on the surface. There now exist several techniques in 3D, which can be used to characterize the microstructure. Al 7075 alloys contain second phase particles which can be classified as Fe-bearing inclusions, Si-bearing inclusions and precipitates. The variation in mechanical and corrosion properties of aluminum alloys has been attributed to the size, shape, distribution, corrosion properties and mechanical behavior of these precipitates and constituent particles. Therefore, in order to understand the performance of Al 7075 alloys, it is critical to investigate the size and distribution of inclusions and precipitates in the alloys along with their mechanical properties, such as Young's modulus, hardness and stress-strain behavior. X-ray tomography and FIB tomography were used to visualize and quantify the microstructure of constituent particles (inclusions) and precipitates, respectively. Microscale mechanical characterization techniques, such as nanoindentation and micropillar compression, were used to obtain mechanical properties of inclusions. Over the years, studies have used surface measurements to understand corrosion behavior of materials. More recently, in situ mechanical testing has become more attractive and advantageous, as it enables visualization and quantification of microstructural changes as a function of time (4D). In this study, in situ X-ray synchrotron tomography was used to study the SCC behavior of Al 7075 alloys in moisture and deionized water. Furthermore, experiments were performed in EXCO solution to study the effect of applied stress on exfoliation behavior in 3D. Contrary to 2D measurements made at the surface which suggest non-uniform crack growth rates, three dimensional measurements of the crack length led to a much more accurate measurement of crack growth rates.
ContributorsSingh, Sudhanshu Shekhar (Author) / Chawla, Nikhilesh (Thesis advisor) / Alford, Terry (Committee member) / Solanki, Kiran (Committee member) / Oswald, Jay (Committee member) / Arizona State University (Publisher)
Created2015
Description
In this dissertation, micro-galvanic corrosion effects and passivation behavior of single-phase binary alloys have been studied in order to formulate new insights towards the development of “stainless-like” lightweight alloys. As a lightweight material of interest, Mg-xAl alloys were studied using aqueous free corrosion, atmospheric corrosion, dissolution rate kinetics, and ionic liquid dissolution. Polarization and “accelerated” free corrosion studies in aqueous chloride were used to characterize the corrosion behavior and morphology of alloys. Atmospheric corrosion experiments revealed surface roughness and pH evolution behavior in aqueous environment. Dissolution in absence of water using choline-chloride:urea ionic liquid allowed for a simpler dissolution mechanism to be observed, providing additional insights regarding surface mobility of Al. These results were compared with commercial alloy (AZ31B, AM60, and AZ91D) behavior to better elucidate effects associated with secondary phases and intermetallic particles often present in Mg alloys. Aqueous free corrosion, “accelerated” free corrosion and ionic liquid dissolution studies have confirmed Al surface enrichment in a variety of morphologies, including Al-rich platelet and Al nanowire formation. This behavior is attributed to the preferential dissolution of Al as the more “noble” element in the matrix. Inductively-coupled mass spectroscopy was used to measure first-order rate reaction constants for elemental Mg and Al dissolution in aqueous chloride environment to be kMg= 9.419 x 10-6 and kAl = 2.103 x 10-6 for future implementation in kinetic Monte Carlo simulations. To better understand how “stainless-like” passivation may be achieved, Ni-xCr alloys were studied using polarization and potential pulse experiments. The passivation potential, critical current density, and passivation current density were found to decay with increasing Cr composition. The measured average number of monolayers dissolved during passivation was found to be in good agreement with percolation theory, with a fitted 3-D percolation threshold of p_c^3D=0.118 compared with the theoretical value of 0.137. Using these results, possible approaches towards achieving passivation in other systems, including Mg-Al, are discussed.
ContributorsAiello, Ashlee (Author) / Sieradzki, Karl (Thesis advisor) / Buttry, Daniel (Committee member) / Chan, Candace (Committee member) / Crozier, Peter (Committee member) / Arizona State University (Publisher)
Created2018
Description
Solar photovoltaic (PV) industry is tipped to be one of the front-runners in the renewable industry. Typically, PV module manufacturers provide a linear or step warranty of 80% of original power over 25 years. This power loss during the field exposure is primarily attributed to the development of performance affecting defects in the PV modules. As many as 86 different defects can occur in a PV module. One of the major defects that can cause significant power loss is the interconnect metallization system (IMS) degradation which is the focus of this thesis. The IMS is composed of cell-interconnect (cell-ribbon interconnect) and string-interconnect (ribbon-ribbon interconnect). The cell interconnect is in turn composed of silver metallization (fingers and busbars) and solder bonds between silver busbar and copper ribbon. Weak solder bonding between copper ribbon and busbar of a cell results in increase of series resistance that in turn affects the fill factor causing a power drop. In this thesis work, the results obtained from various non-destructive and destructive experiments performed on modules exposed in three different climates (Arizona - Hot and Dry, Mexico - Warm and Humid, and California - Temperate) are presented. These experiments include light I-V measurements, dark I-V measurements, infrared imaging, extraction of test samples from the modules, peel strength measurements and four-point resistance measurements. The extraction of test samples was performed using a mechanical method and a chemical method. The merits and demerits of these two methods are presented. A drop of 10.33% in fill factor was observed for a 0.05Ω increase in the series resistance of the modules investigated in this work. Different combinations in a cell that can cause series resistance increase were considered and their effect on fill factor were observed using four-point probe experiments. Peel test experiments were conducted to correlate the effect of series resistance on the ribbon peel strength. Finally, climate specific thermal modelling was performed for 4 different sites over 20 years in order to calculate the accumulated thermal fatigue and also to evaluate its correlation, if any, with the increase of series resistance.
ContributorsTummala, Abhishiktha (Author) / Tamizhmani, Govindasamy (Thesis advisor) / Phelan, Patrick (Thesis advisor) / Wang, Liping (Committee member) / Arizona State University (Publisher)
Created2016
Description
The solar energy sector has been growing rapidly over the past decade. Growth in renewable electricity generation using photovoltaic (PV) systems is accompanied by an increased awareness of the fault conditions developing during the operational lifetime of these systems. While the annual energy losses caused by faults in PV systems could reach up to 18.9% of their total capacity, emerging technologies and models are driving for greater efficiency to assure the reliability of a product under its actual application. The objectives of this dissertation consist of (1) reviewing the state of the art and practice of prognostics and health management for the Direct Current (DC) side of photovoltaic systems; (2) assessing the corrosion of the driven posts supporting PV structures in utility scale plants; and (3) assessing the probabilistic risk associated with the failure of polymeric materials that are used in tracker and fixed tilt systems.
As photovoltaic systems age under relatively harsh and changing environmental conditions, several potential fault conditions can develop during the operational lifetime including corrosion of supporting structures and failures of polymeric materials. The ability to accurately predict the remaining useful life of photovoltaic systems is critical for plants ‘continuous operation. This research contributes to the body of knowledge of PV systems reliability by: (1) developing a meta-model of the expected service life of mounting structures; (2) creating decision frameworks and tools to support practitioners in mitigating risks; (3) and supporting material selection for fielded and future photovoltaic systems. The newly developed frameworks were validated by a global solar company.
As photovoltaic systems age under relatively harsh and changing environmental conditions, several potential fault conditions can develop during the operational lifetime including corrosion of supporting structures and failures of polymeric materials. The ability to accurately predict the remaining useful life of photovoltaic systems is critical for plants ‘continuous operation. This research contributes to the body of knowledge of PV systems reliability by: (1) developing a meta-model of the expected service life of mounting structures; (2) creating decision frameworks and tools to support practitioners in mitigating risks; (3) and supporting material selection for fielded and future photovoltaic systems. The newly developed frameworks were validated by a global solar company.
ContributorsChokor, Abbas (Author) / El Asmar, Mounir (Thesis advisor) / Chong, Oswald (Committee member) / Ernzen, James (Committee member) / Arizona State University (Publisher)
Created2017
Description
In 2015, the United States consumed about 140.43 billion gallons of gasoline, resulting in the emission of over 1 billion metric tons of carbon dioxide, according to the U.S. Energy Information Administration. Despite continued efforts to develop more efficient engines and cleaner fuels, a major barrier to reducing energy consumption and CO2 production is the mass of the vehicle. Replacing traditional automotive materials such as iron and steel with lighter-weight materials is a big step toward improving fuel economy. Magnesium has great potential for use in the automotive industry because of its low density, about 78% less than the density of steel, and high strength-to-weight ratio. Using cast magnesium instead of steel can reduce the overall weight of a vehicle, improving performance and increasing fuel efficiency. However, magnesium’s high susceptibility to corrosion limits its feasibility as a substitute for traditional materials.
This project aimed to understand the effects of composition and phase distribution on the corrosion behavior of magnesium-aluminum (Mg-Al) alloys in an ionic liquid electrolyte. The purpose of studying corrosion in nonaqueous ILs is to determine the anodic dissolution behaviors of the alloy phases without the interference of side reactions that occur in aqueous electrolytes, such as di-oxygen or water reduction. Three commercial Mg-Al alloys were studied: AZ91D (9% Al), AM60 (6% Al), and AZ31B (3% Al). An annealed alloy containing solid-solution α-phase Mg-Al with 5 at% aluminum content (Mg5Al) was also used. The ionic liquid chosen for this project was 1:2 molar ratio choline-chloride:urea (cc-urea), a deep eutectic solvent. After potentiostatic corrosion in cc-urea, the magnesium alloys were found to form a high surface area porous morphology as corrosion duration increased. This morphology consists of aluminum-rich ridges formed by Al nanowires surrounding an aluminum-poor base area, but with an overall increase in surface Al composition, indicating selective dealloying of the Mg in cc-urea and redistribution of the Al on the surface. Further work will focus on the development of hydrophobic coatings using ionic liquids.
This project aimed to understand the effects of composition and phase distribution on the corrosion behavior of magnesium-aluminum (Mg-Al) alloys in an ionic liquid electrolyte. The purpose of studying corrosion in nonaqueous ILs is to determine the anodic dissolution behaviors of the alloy phases without the interference of side reactions that occur in aqueous electrolytes, such as di-oxygen or water reduction. Three commercial Mg-Al alloys were studied: AZ91D (9% Al), AM60 (6% Al), and AZ31B (3% Al). An annealed alloy containing solid-solution α-phase Mg-Al with 5 at% aluminum content (Mg5Al) was also used. The ionic liquid chosen for this project was 1:2 molar ratio choline-chloride:urea (cc-urea), a deep eutectic solvent. After potentiostatic corrosion in cc-urea, the magnesium alloys were found to form a high surface area porous morphology as corrosion duration increased. This morphology consists of aluminum-rich ridges formed by Al nanowires surrounding an aluminum-poor base area, but with an overall increase in surface Al composition, indicating selective dealloying of the Mg in cc-urea and redistribution of the Al on the surface. Further work will focus on the development of hydrophobic coatings using ionic liquids.
ContributorsWeiss, Anna Caroline (Author) / Sieradzki, Karl (Thesis director) / Chan, Candace (Committee member) / Materials Science and Engineering Program (Contributor) / Barrett, The Honors College (Contributor)
Created2016-05
Description
Solid-state and non-equilibrium processings are of great interest to researchers due to their ability to control and refine bulk and/or surface microstructure of metallic alloys and push them to surpass their conventional properties limit. In this dissertation, solid-state processing i.e., Shear Assisted Processing and Extrusion (ShAPE), and non-equilibrium processes i.e., surface mechanical attrition (SMAT) and additive manufacturing (AM) techniques were used to process the magnesium and aluminum alloys respectively. A synergistic investigation of processing-induced microstructural modification and its effect on corrosion resistance was performed using various ex-situ, quasi in-situ, and in-situ electrochemical, microscopy, and spectroscopy characterization techniques. To evaluate the effect of the same processing condition on a range of microstructures, a variety of magnesium alloys such as AZ31B, Mg-3Si, ZK60, and Pure Mg were processed using a novel solid-state processing method, namely ShAPE. It induced a significant grain refinement, homogenized distribution of second phases, and low residual strain in AZ31B alloy, which contributed toward a noble breakdown potential, stable protective film, and hence better corrosion resistance compared to the parent extruded counterpart. However, with variations in composition, volume fraction, and distribution of second phases with Mg-3Si and ZK60 magnesium alloy an opposite response was inferred indicating a strong dependence of corrosion on underlying microstructure compared to a processing condition. Non-equilibrium processes, i.e. SMAT and AM were utilized to process high-strength 7xxx series aluminum alloys. Continuous high energy impacts of hard balls in room temperature (RT SMAT) and liquid nitrogen (LN2 SMAT) flow environment generated a gradient nanocrystalline surface layer with the dissolution of inherent second phase and precipitation of new phases in aluminum 7075 alloys. RT SMAT showed a reduced anodic dissolution rate and improved film resistance, which was attributed to the thicker and composite oxide layer along with new nanoscale precipitates. Lastly, reactive AM was used to process aluminum 7075 and 7050 alloys which resulted in a refined and textureless microstructure. A reduction in corrosion resistance was observed with precipitation of excessive reactive particles (Ti and B4C) in AM alloys compared to wrought counterparts.
ContributorsBeura, Vikrant Kumar (Author) / Solanki, Kiran N (Thesis advisor) / Peralta, Pedro (Committee member) / Alford, Terry (Committee member) / Ankit, Kumar (Committee member) / Joshi, Vineet V (Committee member) / Arizona State University (Publisher)
Created2022
Description
Corrosion is known to have severe infrastructure integrity implications in a broad range of industries including water and wastewater treatment and reclamation. In the U.S. alone, the total losses due to corrosion in drinking water and wastewater systems can account for economic losses as high as $80 billion dollars a year. Microbially induced corrosion is a complex phenomenon which involve various phases; 1) formation of biofilms on submerged surfaces, 2) creation of micro-environmental niches associated with biofilm growth, 3) altered availability nutrients, 4) changes in the pH and oxygen concentrations. Biofilms can harbor opportunistic or pathogenic bacteria for a long time increasing the risk of pathogen exposure for the end users. The focus of this thesis research was to study the kinetics of microbially induced corrosion of various materials in water and reclaimed water systems. The specific objective was to assess the biofilms formation potential on stainless steel 304, stainless steel 316, galvanized steel, copper, cPVC, glass, carbon steel, and cast iron in water and reclaimed water systems. Experiments were conducted using bioreactor containers, each bioreactor housed four sampling boxes with eight partitions, dedicated to each material type coupon. One bioreactor was stationed at ASU, and one at Vistancia Aquifer Storage and Recovery (ASR) well; while three bioreactors were stationed at Butler facility, at pre-disinfection, post-UV and post-chlorination. From each location, one submerged sampling box was retrieved after 1, 3, 6 and 12 months. Time series of biofilm samples recovered from various types of coupons from different locations were analyzed using physical and culture-based techniques for quantification of biofilms and detection of heterotrophic plate count (HPC) bacteria, Legionella, Mycobacterium, and sulfate reducing bacteria (SRB).
After one-year, galvanized steel had the highest concentration of HPC at 4.27 logs while copper had the lowest concentration of 3.08 logs of HPC. Bacterial growth data collected from the SRB tests was compiled to develop a numerical matrix using growth potential, biofilm formation potential and metal reduction potential of SRB isolates. This risk assessment matrix can be a useful tool for the water industry to evaluate the potential risk of MIC in their systems.
ContributorsNeal, Amber (Author) / Abbaszadegan, Morteza (Thesis advisor) / Fox, Peter (Committee member) / Alum, Absar (Committee member) / Arizona State University (Publisher)
Created2022
Description
Traditionally nanoporous gold is created by selective dissolution of silver or copper from a binary silver-gold or copper-gold alloy. These alloys serve as prototypical model systems for a phenomenon referred to as stress-corrosion cracking. Stress-corrosion cracking is the brittle failure of a normally ductile material occurring in a corrosive environment under a tensile stress. Silver-gold can experience this type of brittle fracture for a range of compositions. The corrosion process in this alloy results in a bicontinuous nanoscale morphology composed of gold-rich ligaments and voids often referred to as nanoporous gold. Experiments have shown that monolithic nanoporous gold can sustain high speed cracks which can then be injected into parent-phase alloy. This work compares nanoporous gold created from ordered and disordered copper-gold using digital image analysis and electron backscatter diffraction. Nanoporous gold from both disordered copper-gold and silver-gold, and ordered copper-gold show that grain orientation and shape remain largely unchanged by the dealloying process. Comparing the morphology of the nanoporous gold from ordered and disordered copper-gold with digital image analysis, minimal differences are found between the two and it is concluded that they are not statistically significant. This reveals the robust nature of nanoporous gold morphology against small variations in surface diffusion and parent-phase crystal structure.
Then the corrosion penetration down the grain boundary is compared to the depth of crack injections in polycrystal silver-gold. Based on statistical comparison, the crack-injections penetrate into the parent-phase grain boundary beyond the corrosion-induced porosity. To compare crack injections to stress-corrosion cracking, single crystal silver-gold samples are employed. Due to the cleavage-like nature of the fracture surfaces, electron backscatter diffraction is possible and employed to compare the crystallography of stress-corrosion crack surfaces and crack-injection surfaces. From the crystallographic similarities of these fracture surfaces, it is concluded that stress-corrosion can occur via a series of crack-injection events. This relationship between crack injections and stress corrosion cracking is further examined using electrochemical data from polycrystal silver-gold samples during stress-corrosion cracking. The results support the idea that crack injection is a mechanism for stress-corrosion cracking.
Then the corrosion penetration down the grain boundary is compared to the depth of crack injections in polycrystal silver-gold. Based on statistical comparison, the crack-injections penetrate into the parent-phase grain boundary beyond the corrosion-induced porosity. To compare crack injections to stress-corrosion cracking, single crystal silver-gold samples are employed. Due to the cleavage-like nature of the fracture surfaces, electron backscatter diffraction is possible and employed to compare the crystallography of stress-corrosion crack surfaces and crack-injection surfaces. From the crystallographic similarities of these fracture surfaces, it is concluded that stress-corrosion can occur via a series of crack-injection events. This relationship between crack injections and stress corrosion cracking is further examined using electrochemical data from polycrystal silver-gold samples during stress-corrosion cracking. The results support the idea that crack injection is a mechanism for stress-corrosion cracking.
ContributorsKarasz, Erin (Author) / Sieradzki, Karl (Thesis advisor) / Chawla, Nikhilesh (Committee member) / Peralta, Pedro (Committee member) / Rajagopalan, Jagannathan (Committee member) / Arizona State University (Publisher)
Created2020
Description
The passivity of metals is a phenomenon of vast importance as it prevents many materials in important applications from rapid deterioration by corrosion. Alloying with a sufficient quantity of passivating elements (Cr, Al, Si), typically in the range of 10% - 20%, is commonly employed to improve the corrosion resistance of elemental metals. However, the compositional criteria for enhanced corrosion resistance have been a long-standing unanswered question for alloys design. With the emerging interest in multi-principal element alloy design, a percolation model is developed herein for the initial stage of passive film formation, termed primary passivation. The successful validation of the assumptions and predictions of the model in three corrosion-resistant binary alloys, Fe-Cr, Ni-Cr, and Cu-Rh supports that the model which can be used to provide a quantitative design strategy for designing corrosion-resistant alloys. To date, this is the only model that can provide such criteria for alloy design.The model relates alloy passivation to site percolation of the passivating elements in the alloy matrix. In the initial passivation stage, Fe (Ni in Ni-Cr or Cu in Cu-Rh) is selectively dissolved, destroying the passive network built up by Cr (or Rh) oxides and undercutting isolated incipient Cr (Rh) oxide nuclei. The only way to prevent undercutting and form a stable protective passive film is if the concentration of Cr (Rh) is high enough to realize site percolation within the thickness of the passive film or the dissolution depth. This 2D-3D percolation cross-over transition explains the compositional dependent passivation of these alloys. The theoretical description of the transition and its assumptions is examined via experiments and kinetic Monte Carlo simulations. The initial passivation scenario of the dissolution selectivity is validated by the inductively coupled plasma mass spectrum (ICP-MS). The electronic effect not considered in the kinetic Monte Carlo simulations is addressed by density functional theory (DFT). Additionally, the impact of the atomic configuration parameter on alloy passivation is experimentally measured, which turns out to agree well with the model predictions developed using Monte Carlo renormalization group (MC-RNG) methods.
ContributorsXie, Yusi (Author) / Sieradzki, Karl KS (Thesis advisor) / Chan, Candace CC (Committee member) / Wang, Qing QHW (Committee member) / Buttry, Daniel DB (Committee member) / Arizona State University (Publisher)
Created2020
Description
AA 7XXX alloys are used extensively in aircraft and naval structures due to their excellent strength to weight ratio. These alloys are often exposed to harsh corrosive environments and mechanical stresses that can compromise their reliability in service. They are also coupled with fasteners that are composed of different materials such as Titanium alloys. Such dissimilar metal contact facilitates galvanic and crevice corrosion, which can further reduce their lifetimes. Despite decades of research in the area, the confluence of mechanical, microstructural, and electrochemical aspects of damage is still unclear. Traditionally, 2D and destructive methods have often been employed to study the corrosion and cracking behavior in these systems which can be severely limiting and lead to inaccurate conclusions. This dissertation is aimed at comprehensively studying the corrosion and cracking behavior of these systems using time-dependent 3D microstructural characterization, as well as correlative microscopy. The microstructural evolution of corrosion in AA 7075 was studied using a combination of potentiodynamic polarization, X-ray Computed Tomography (XCT) and Transmission X-ray Microscopy (TXM). In both experiments, a strong emphasis was placed on studying localized corrosion attack at constituent particles and intergranular corrosion. With an understanding of the alloy’s corrosion behavior, a dissimilar alloy couple comprising AA 7075 / Ti-6Al-4V was then investigated. Ex situ and in situ x-ray microtomography was used extensively to investigate the evolution of pitting corrosion and corrosion fatigue in AA 7075 plates fastened separately with Ti-6Al-4V screws and rivets. The 4D tomography combined with the extensive fractography yielded valuable information pertaining the preferred sites of pit initiation, crack initiation and growth in these complex geometries. The use of correlative microscopy-based methodologies yielded multimodal characterization results that provided a unique and seminal insight on corrosion mechanisms in these materials.
ContributorsNiverty, Sridhar (Author) / Chawla, Nikhilesh (Thesis advisor) / Liu, Yongming (Committee member) / Ankit, Kumar (Committee member) / Xiao, Xianghui (Committee member) / Arizona State University (Publisher)
Created2020