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 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.