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 Corrosion of 316L stainless steel influenced by manganese oxidizing bacteria(Montana State University - Bozeman, College of Engineering, 2001) Geiser, Michael JosephItem Leptothrix discophora SP-6 : effects of biofilms on passive film chemistry of 316L stainless steel and modeling of growth(Montana State University - Bozeman, College of Engineering, 2002) Yurt, NurdanItem A fundamental study of hot corrosion and interdiffusion of chromium, aluminum, and silicon coatings on a nickel-201 substrate(Montana State University - Bozeman, College of Engineering, 2014) Gill, Zachery Edward; Co-chairpersons, Graduate Committee: Paul E. Gannon and Roberta AmendolaModern turbine engine systems require increased efficiency and durability. To achieve these goals, high-temperature materials with high-strength, low-cost and non-strategic compositions are needed. In advanced turbine applications, combustor liners, blades and vanes are exposed to corrosive combustion byproducts, such as alkali salts, at temperatures up to ~1700°C with high gas velocities, entrained particulates, and other foreign objects at pressures of up to 3 MPa (30 atm). These extreme conditions can drive a dangerous phenomenon known as "hot corrosion", an accelerated form of oxidation that occurs when metals and metal alloys are heated in the temperature range 700-900°C in the presence of alkali salts. An increased understanding of the fundamental behaviors of common high temperature alloys and their degradation mechanisms is therefore critical for the production of reliable components. In this study a model substrate, Nickel 201, was coated on one side with Cr, Al, or Si thin films (~1 micron) via magnetron sputtering physical vapor deposition (PVD). Uncoated and PVD coated samples were then exposed to laboratory air at 700°C and 900°C and to an environment similar in composition to atmospheres found in post combustion turbine systems, comprised of air/SO 2 gas mixture, at 700°C. The exposures were conducted over time intervals observing coating-substrate interactions and surface oxide development. Identical samples were subjected to the same exposures with addition of a deposit of sodium sulfate (Na 2SO 4), a model alkali salt. Sample mass gains were recorded and resulting oxide compositions assessed as a function of exposure time using microscopy techniques on sample surfaces and cross sections. The development of intermetallic species was determined by X-ray diffraction. At 700°C, coated and uncoated samples displayed different oxidation behaviors. Under laboratory air, no hot-corrosion occurred. While at 700°C in air/SO 2 exposures, evidence for hot corrosion on deposited samples was observed. When sodium sulfate was introduced at 900°C, coated and uncoated samples displayed rapid corrosion consistent with hot corrosion. The oxidation processes and coating/substrate inter-diffusion phenomena are presented and discussed in the context of establishing basic approaches to improve the fundamental understanding of hot corrosion, and the protection mechanisms of high temperature materials.