Theses and Dissertations at Montana State University (MSU)
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Item Reactive evaporation of chromium from stainless steel and the reactive condensation of chromium vapor species on ceramic surfaces(Montana State University - Bozeman, College of Engineering, 2018) Tatar, Gregory Steven; Chairperson, Graduate Committee: Paul E. GannonStainless steels are often used in high temperature (greater than or equal to 500°C) applications such as solid oxide fuel cells (SOFCs), combustion engine exhaust systems, and various power/chemical plant process equipment. At high temperatures and in oxidizing conditions, chromium containing oxides, such as chromia (Cr2O 3), form protective surface layers on the underlying stainless steel. Reactive evaporation of these surface layers, however, may form volatile chromium species such as CrO 2 (OH) 2 and CrO 3, compromise the protection of stainless steels, and cause deleterious downstream effects. Such effects include SOFC performance degradation and hazardous materials generation. This study focuses on both the reactive evaporation and reactive condensation processes and their dependencies on materials and environmental conditions. First, the corrosion behaviors of stainless steels were investigated in a variety of exposure conditions and then the nature of chromium vapor condensation was investigated on ceramic surfaces under various conditions. Ferritic stainless steel samples (T409) were examined after 700°C exposures (94 h) to dry or wet air or nitrogen, and with or without contacting aluminosilicate fibers. Surface compositions and structures were characterized using field emission scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction. The fibers had a substantial impact on corrosion behaviors; likely serving as a mass transport barrier for corrosive gas species. Observed corrosion behaviors under these different environments and their potential mechanisms are presented and discussed. Additionally, quantification of chromium content on fibers was performed using inductively coupled plasma mass spectroscopy. Fibers were observed to collect chromium in dry/moist air consistent with the formation of CrO 3 and CrO 2(OH) 2, respectively, and their subsequent reactive condensation. To better understand the reactive condensation of volatile chromium species onto various ceramic surfaces, volatile chromium species were generated from chromium containing sources at 500-900°C and flowed past samples of aluminosilicate fibers, alumina, mica, and quartz wool at temperatures ranging from 100-900°C for 24-150 hours. The ceramic surfaces were characterized using x-ray photoelectron spectroscopy. Analysis of Cr 2p 3/2 peak positions revealed the influence of temperature, material, and exposure time on the oxidation states of surface chromium compounds, and extent of chromium deposition. Potential mechanisms are proposed to help explain the observed trends.Item High temperature chlorosilane corrosion of iron and AISI 316L stainless steel(Montana State University - Bozeman, College of Engineering, 2016) Aller, Joshua Loren; Chairperson, Graduate Committee: Paul E. GannonChlorosilane gas streams are used at high temperatures (>500°C) throughout the semiconductor, polycrystalline silicon, and fumed silica industries, primarily as a way to refine, deposit, and produce silicon and silicon containing materials. The presence of both chlorine and silicon in chlorosilane species creates unique corrosion environments due to the ability of many metals to form both metal-chlorides and metal-silicides, and it is further complicated by the fact that many metal-chlorides are volatile at high-temperatures while metal-silicides are generally stable. To withstand the uniquely corrosive environments, expensive alloys are often utilized, which increases the cost of final products. This work focuses on the corrosion behavior of iron, the primary component of low-cost alloys, and AISI 316L, a common low-cost stainless steel, in environments representative of industrial processes. The experiments were conducted using a customized high temperature chlorosilane corrosion system that exposed samples to an atmospheric pressure, high temperature, chlorosilane environment with variable input amounts of hydrogen, silicon tetrachloride, and hydrogen chloride plus the option of embedding samples in silicon during the exposure. Pre and post exposure sample analysis including scanning electron microscopy, x-ray diffraction, energy dispersive x-ray spectroscopy, and gravimetric analysis showed the surface corrosion products varied depending on the time, temperature, and environment that the samples were exposed to. Most commonly, a volatile chloride product formed first, followed by a stratified metal silicide layer. The chlorine and silicon activities in the corrosion environment were changed independently and were found to significantly alter the corrosion behavior; a phenomenon supported by computational thermodynamic equilibrium simulations. It was found that in comparable environments, the stainless steel corroded significantly less than the pure iron. This is likely due to the alloying elements present in stainless steel that promote formation of other stable silicides. Mechanistic models were developed to describe the formation and evolution of metal silicide and/or metal chloride surface corrosion products in chlorosilane environments. These models will help inform materials selection and/or support process development for next-generation chlorosilane-based production and deposition systems. The implementation of low cost materials of construction in these systems could lower the cost of final products in these industries.Item Bacterial colonization and modification of grain boundaries on 316L stainless steel(Montana State University - Bozeman, College of Agriculture, 1993) Gillis, Richard JohnItem Microbially influenced corrosion of stainless steel 304 under halogenated fluids(Montana State University - Bozeman, College of Engineering, 1991) Agrawal, VivekItem Chemical effects of biofilm colonization on stainless steel(Montana State University - Bozeman, College of Letters & Science, 1996) Pendyala, JyostnaItem Electrochemistry of stainless steel colonized by manganese-oxidizing bacteria(Montana State University - Bozeman, College of Letters & Science, 1996) Dickinson, Wayne HaroldItem Characterization and optimization of direct drive friction welding parameters in small stainless steel tube welds(Montana State University - Bozeman, College of Engineering, 2013) Adams, Alex Jackson; Chairperson, Graduate Committee: David A. MillerRotational friction welding is a common joining process used to join cylindrical metal components. Typically, one piece is rotated and a secondary piece is held rigid. The two samples are then forced together in a controlled manner, and the resulting friction generates enough heat to weld the two pieces. This process was characterized and optimized for 304 Stainless Steel tubes with a .317 cm (.125 in) outer diameter and .14 cm (.055 in) inner diameter. The goal was to characterize and optimize parameters around a weld with no leak, strong ultimate tensile strength, and proper through-hole integrity. Also, solid bars were welded to some tubes to analyze a capped system. Key parameters to the process that were monitored and/or controlled include rotational speed, applied force, temperature, duration, and material upset. Often times the applied force is divided into two steps. A lower force is applied during heating (friction force), and a larger force is applied once rotation stops (forging force). The material upset, maximum temperature, and forging fore were the primary controlling variables in this study. Other parameters were held constant. A testing setup was built to analyze these factors. Modifications were made to a three axis mill to perform friction welding in a controlled environment. Then, tests were run to understand the effects each parameter had on weld quality. Welds with an upset greater than .1 cm held a pressure at a much higher success rate than welds with lower upsets. In general, the forging force was shown to have a large positive impact on ultimate tensile force. The integrity of the through-hole was compromised in many of the tube to tube tests. Several welds were post-drilled to recreate the through-hole. Tests with this done held a pressure 66.67% of the time. It was found that successful welding can be accomplished with this process, and different adjustments to testing procedures can maximize different qualities in the weld.Item Oxidation behavior of (Co,Mn) 3O 4 coatings on preoxidized stainless steel(Montana State University - Bozeman, College of Engineering, 2011) Hoyt, Kathryn Olivia; Chairperson, Graduate Committee: Paul E. GannonAs global energy challenges grow, alternative energy technologies like fuel cells are being investigated. Solid Oxide Fuel Cells (SOFCs) provide the advantages of high energy conversion efficiency, low emissions, fuel flexibility and both portable and stationary application. High material cost and need for longer material lifespan still impede the wider use of SOFCs. To produce substantial voltage, planar SOFCs are joined into stacks using interconnects. Interconnects both separate and connect each individual fuel, separating gas flow and conducting current. For SOFCs that operate at less than 800°C, metal alloys are being considered for the interconnect, particularly ferritic stainless steel. Ceramic coatings are being explored to improve the surface conductivity over time and significantly reduce Cr volatility from the steel. In addition, the coating must have a matching coefficient of thermal expansion (CTE) and be compatible with electrode and seal materials. One promising coating is (Co,Mn) 3O 4 spinel, which is deposited using various techniques, resulting in different thicknesses, compositions and microstructures. In this study, stainless steel 441HP samples were subjected to three levels of preoxidation prior to coating with 2 micron CoMn alloy using magnetron sputtering. Samples were subsequently annealed to Co 1.5Mn 1.5O 4 in 800°C air. Oxidation behaviors were evaluated as a function of exposure to laboratory air and dual atmospheres (3% H 2O and H 2 on one side, 3% H 2O and air on the other) and area specific resistance (ASR) measurements in lab air, all at 800°C. In addition, chemical and phase composition, mass gain, and adhesion were investigated using a complimentary suite of analytical techniques. Preoxidation was found to inhibit Fe transport from the stainless steel into the coating and exhibited a substantially thinner surface oxide layer after oxidation. Preoxidized samples also maintained slightly lower ASR values after 1650 hours in 800°C air compared to non-preoxidized samples. Oxidation behaviors, their possible mechanisms, and implications for SOFC interconnects will be presented and discussed.