<p>Atmospheric corrosion, the most common form of metal corrosion, occurs within the interfacial region between a solid, and the surrounding atmosphere. In fact three phases and two interfaces are involved: the gas, a thin liquid layer, a solid, the gas/liquid and the liquid/solid interfaces. In this thesis, the vapor/liquid and liquid/metal interfaces have been studied by the in-situ techniques vibrational sum frequency spectroscopy (VSFS), and infrared reflection/absorption spectroscopy (IRAS). The main focus has been on characterization of the corrosive organic molecules formic acid, acetic acid, and acetaldehyde, at the two interfaces. Additionally, the headgroup of sodium dodecyl sulfate (SDS) has been examined at the air/water interface.</p><p>VSFS is an inherently surface sensitive laser spectroscopy technique, which provides vibrational spectra solely of the molecules residing at the surface of for example a liquid, despite the vast excess of the same molecules in the bulk. To obtain a comprehensive molecular picture of the organic compounds at the air/liquid interface, studies have been undertaken in several spectral regions, targeting the CH, C=O, C-O, OH, and SO3 stretching vibrations. Furthermore, the surrounding water molecules have been investigated in order to study hydration phenomena. Acetaldehyde has been determined to partly form a gem-diol (CH3CH(OH)2) at the air/water interface, whereas acetic acid forms various hydrogen-bonded species, with hydrated monomers at low concentrations and centrosymmetric cyclic dimers at high concentrations. Formic acid was found to form a different complex at very high concentrations, in addition to the species observed at low concentrations. Performing experiments with different polarizations of the laser beams has enabled the determination of the orientation of the interfacial molecules. The methyl group of acetic acid was concluded to be oriented close to the surface normal throughout the concentration range, whereas the tilt angle of the CH group of formic acid was determined to be ~35°. The SDS studies revealed that the headgroup orientation is constant in a wide range of concentrations, and also in the presence of sodium chloride.</p><p>IRAS has provided information regarding the composition and kinetics of the corrosion products formed upon exposure of a zinc oxide surface to the organic compounds. The importance of the water adlayer on metal surfaces has been confirmed by the faster kinetics observed at higher relative humidities. Exposure to formic acid resulted in the formation of zinc formate, whereas both acetic acid and acetaldehyde formed zinc acetate upon reaction with the zinc oxide surface. However, the kinetics were faster for acetic acid than acetaldehyde, which was explained in terms of an acetate-induced zinc dissolution process and a more complicated reaction path involved in the acetaldehyde case to form the zinc acetate surface species. Scanning electron microscopy indicated the formation of radially growing reaction products for acetic acid and filiform corrosion for acetaldehyde.</p>

Atmospheric corrosion, the most common form of metal corrosion, occurs within the interfacial region between a solid, and the surrounding atmosphere. In fact three phases and two interfaces are involved: the gas, a thin liquid layer, a solid, the gas/liquid and the liquid/solid interfaces. In this thesis, the vapor/liquid and liquid/metal interfaces have been studied by the in-situ techniques vibrational sum frequency spectroscopy (VSFS), and infrared reflection/absorption spectroscopy (IRAS). The main focus has been on characterization of the corrosive organic molecules formic acid, acetic acid, and acetaldehyde, at the two interfaces. Additionally, the headgroup of sodium dodecyl sulfate (SDS) has been examined at the air/water interface. VSFS is an inherently surface sensitive laser spectroscopy technique, which provides vibrational spectra solely of the molecules residing at the surface of for example a liquid, despite the vast excess of the same molecules in the bulk. To obtain a comprehensive molecular picture of the organic compounds at the air/liquid interface, studies have been undertaken in several spectral regions, targeting the CH, C=O, C-O, OH, and SO3 stretching vibrations. Furthermore, the surrounding water molecules have been investigated in order to study hydration phenomena. Acetaldehyde has been determined to partly form a gem-diol (CH3CH(OH)2) at the air/water interface, whereas acetic acid forms various hydrogen-bonded species, with hydrated monomers at low concentrations and centrosymmetric cyclic dimers at high concentrations. Formic acid was found to form a different complex at very high concentrations, in addition to the species observed at low concentrations. Performing experiments with different polarizations of the laser beams has enabled the determination of the orientation of the interfacial molecules. The methyl group of acetic acid was concluded to be oriented close to the surface normal throughout the concentration range, whereas the tilt angle of the CH group of formic acid was determined to be ~35°. The SDS studies revealed that the headgroup orientation is constant in a wide range of concentrations, and also in the presence of sodium chloride. IRAS has provided information regarding the composition and kinetics of the corrosion products formed upon exposure of a zinc oxide surface to the organic compounds. The importance of the water adlayer on metal surfaces has been confirmed by the faster kinetics observed at higher relative humidities. Exposure to formic acid resulted in the formation of zinc formate, whereas both acetic acid and acetaldehyde formed zinc acetate upon reaction with the zinc oxide surface. However, the kinetics were faster for acetic acid than acetaldehyde, which was explained in terms of an acetate-induced zinc dissolution process and a more complicated reaction path involved in the acetaldehyde case to form the zinc acetate surface species. Scanning electron microscopy indicated the formation of radially growing reaction products for acetic acid and filiform corrosion for acetaldehyde. / QC 20101029

Zr cladding alloys have been used for many years as the first safety barrier layer of a nuclear reactor. However, the recent Fukushima accidents and industrial demands to increase the burnup of fuels have led to increasing interest in a detailed mechanistic understanding of aqueous corrosion and hydrogen pickup and the performance at high temperatures. As part of an international MUZIC-2 programme (Mechanistic Understanding of Zr Corrosion and Hydrogen pickup), I have used a range of advanced microscopy techniques to study the microstructure, the nanoscale chemistry and the porosity in a series of zirconium alloys at different stages of corrosion and hydrogen pickup. Samples from both autoclave and in-reactor conditions were available to compare, I have focussed on RXA (recrystallised 580°C) Zr-1.0Nb and annealed (720°C) Zr-1.0Nb alloys. A set of samples from different exposures times were chosen to represent early, pre-transition and post-transition samples in order to compare the microstructure and microchemistry of the oxides, the metal-oxide interface and the metal. (Scanning) Transmission Electron Microscopy ((S)TEM), Transmission Kikuchi Diffraction (TKD) and automated crystal orientation mapping with TEM (ASTAR mapping) were used to study the grain structure and phase distribution. Significant differences in grain morphology were observed between samples oxidised in the autoclave with different corrosion rates, with more uneven metal-oxide interface, more parallel cracks and less organised oxide grains in the fast corroding samples. Comparing with autoclave samples, the in-reactor samples have shorter, less well-aligned monoclinic grains and more tetragonal grains. The rapidly oxidising annealed Zr-1.0Nb alloy also have much higher tetragonal grain fraction comparing with the slow corrosion rate RXA Zr-1.0Nb alloys. Porosity in the oxide is predicted to have a major influence on the overall rate of corrosion and hydrogen pickup, and there is much more porosity in the annealed Zr-1.0Nb alloy than found in either the RXA alloy or the similar alloy exposed to neutron irradiation. A combination of Energy Dispersion X-ray (EDX) mapping in STEM and Electron Energy Loss Spectroscopy (EELS) analysis of second phase particles can reveal the main and the minor element distributions respectively. The annealed Zr-1.0Nb alloys have Î2-Zr SPPs with nano crystalline structure and much larger size. Although they does not relate with the higher density of cracks in the oxide, the large SPP size can connect together all the small cracks that are generated by the huge amount of tetragonal to monoclinic phase transformation during corrosion and provides pathway for corrosion and hydrogen pickup. Two kinds of SPPs are found in the RXA Zr-1.0Nb alloys, one is Î2-Nb and another one is Zr-Nb-Fe Laves phase. Neutron irradiation seems to have little effect on promoting fast oxidation or dissolution of Î2-Nb precipitates, but encourages dissolution of Fe from Laves phase precipitates. Electron Energy Loss Spectroscopy (EELS) analysis of the oxidation state of Nb in Î2-Nb SPPs in the oxide revealed the fully oxidised Nb⁵⁺ state in the SPPs deep into the oxide, but Nb²⁺ in the crystalline SPPs near the metal-oxide interface. EELS, TKD and ASTAR mapping have also revealed the presence of suboxide layers with the hexagonal ZrO structure predicted by ab initio modelling. The combined thickness of the ZrO suboxide and oxygen-saturated layers at the metal-oxide interface correlates well to the estimated instantaneous oxidation rate, suggesting that the presence of this oxygen- rich zone combining with the part where porosity is not interconnected is the protective oxide that is rate limiting in the key in the transport processes involved in corrosion and hydrogen pickup.

No description available.

Magnesium alloys have been widely explored as potential biomaterials, but several limitations to using these materials have prevented their widespread use, such as uncontrollable degradation kinetics which alter their mechanical properties. In an attempt to further the applicability of magnesium and its alloys for biomedical purposes, two novel magnesium alloys Mg-Zn-Cu and Mg-Zn-Se were developed with the expectation of improving upon the unfavorable qualities shown by similar magnesium based materials that have previously been explored. The overall performance of these novel magnesium alloys has been assessesed in three distinct phases of research: 1) analysing the mechanical properties of the as-cast magnesium alloys, 2) evaluating the biocompatibility of the as-cast magnesium alloys through the use of in-vitro cellular studies, and 3) profiling the degradation kinetics of the as-cast magnesium alloys through the use of electrochemical potentiodynamic polarization techqnique as well as gravimetric weight-loss methods. As compared to currently available shape memory alloys and degradable as-cast alloys, these experimental alloys possess superior as-cast mechanical properties with elongation at failure values of 12% and 13% for the Mg-Zn-Se and Mg-Zn-Se alloys, respectively. This is substantially higher than other as-cast magnesium alloys that have elongation at failure values that range from 7-10%. Biocompatibility tests revealed that both the Mg-Zn-Se and Mg-Zn-Cu alloys exhibit low cytotoxicity levels which are suitable for biomaterial applications. Gravimetric and electrochemical testing was indicative of the weight loss and initial corrosion behavior of the alloys once immersed within a simulated body fluid. The development of these novel as-cast magnesium alloys provide an advancement to the field of degradable metallic materials, while experimental results indicate their potential as cost-effective medical devices.