The objective was to investigate if the established theory concerning corrosion calculations and electrochemical behavior of carbon is valid when steel is exposed to AC in an electrolyte consisting of 3,5 wt% NaCl and distilled water. The experimental work was divided in two main parts. The first part concerned corrosion testing, including weight loss measurements in stagnant conditions in combination with linear polarization resistance experiments. AC-current densities investigated was 0-, 50-, 75-, 100-, 150-, 220- and 500 A/m2. Polarization curves were produced on two weight loss samples, one exposed to 150 A/m2 and the other exposed to 220 A/m2, at the end of the weight loss experiments.From the corrosion testing it was found that the estimated corrosion current densities (icorr) from the weight loss experiments on samples exposed to AC-current densities below 500 A/m2, was in an approximate accordance with the icorr values estimated from LPR. At 500 A/m2 the measured corrosion rates and corresponding corrosion currents were slightly higher than the remainder. The higher corrosion rate measured from the samples exposed to 500 A/m2 is suggested related to a non-faradaic corrosion contribution. The suggested mechanism is a facilitation of grain boundary corrosion at high AC-current densities leading to the loss of grains. This being a non-faradaic corrosion contribution it is not possible to measure with LPR or polarization curves. The constant B [mV] from the Stern-Geary equation was estimated on basis of LPR measurements. The constant displayed a decreasing trend with increasing AC-current densities. This was explained by the lowering of anodic and cathodic tafel slopes with increasing AC-current densities. The polarization resistance (Rp’ - [Ω∙m2]) also showed a decreasing trend with increased AC-current densities. This was explained by the increased current response in the LPR measurements with increasing AC-current densities.The second part was intended to elucidate the effect of AC on electrochemical behavior of the carbon steel through producing polarization curves at different AC-current densities using a rotating electrode experimental setup. Alterations in corrosion current densities, cathodic tafel slopes, limiting current density of oxygen reduction and corrosion potentials on steel without AC and at current densities of 100-, 150-, 220- and 500 A/m2 are investigated during the electrochemical testing.  From the rotating electrode experiments no significant change in corrosion current was found with increasing AC-current densities. The potential range, in which limiting current of oxygen reduction (ilim) appeared, decreased with increasing AC-current densities. This decrease in ilim also affected the corrosion potentials (Ecorr). A decrease in the potential range of ilim leads to a more cathodic Ecorr. A particular observation was found at AC-current densities of 150 A/m2 were the effect of ilim seemed absent. The samples exposed to this AC-current density also attained the most cathodic corrosion potentials at approximately -850mVSCE which was between 200-400mV lower than the samples exposed to the remaining AC-current densities. Cathodic tafel slopes showed a decreasing trend with increasing AC-current density. Anodic rafel slopes were considered absent due to pitting above Ecorr.Pictures of the weight loss samples were captured in the SEM. It was found that the formation of pits occurred on all weight loss samples exposed to AC. On the samples without induced AC no sign of pitting was observed. Hence it was found that the corrosion attacks attained a more local character when AC-current was induced in the steel samples.

Stability and compatibility of the proton conducting electrolyte material La6WO12 with the potential cathode materials LaCoO3 and La2NiO4 were investigated by means of solid-solid diffusion couples. Reactivity studies were carried out at 1450 °C for various times. Reaction products were analysed by SEM and EDS. The study suggests a high reactivity between the electrolyte and the electrodes, which is detrimental for the fuel cell system.In the case of LaCoO3 as a cathode material a secondary phase of LaCo1-xWxO3 were formed at the interface and as precipitates in La6WO12. Theoretical models for both diffusion or interface controlled reactions failed to fit the experimental data. This failure is probably related to the reaction going towards equilibrium after a certain time, or poor connectivity between the materials. The solid- state reaction between La6WO12 and La2NiO4 showed formation of a composite layer of La2O3 and La6WO12 at the interface. The reaction kinetics of this product layer was diffusion controlled. In addition to the interface reaction, precipitates of La2O3 and La6WO12 were formed in the La2NiO4 phase.

This master’s thesis is based on experimental studies of the parameters influencing cold roll bonding (CRB) of the aluminum alloys AA1200 and AA3103,in the work hardened and annealed condition. The effect on the bond strength from the preparations parameters as degreasing agent, scratch brushing and exposure time for oxide growth is investigated in comparison to former studies. Further the effect of rolling speed and effect from contributing factors from the different testing methods is discussed. Three different methods for testing the bond strength are used. One of them was established during this study and was named Tensile Bond Strength Test (TBST). A final investigation of the fracture surfaces and bond interface in a scanning electron microscope (SEM) was carried out to analyze the bond mechanism and distribution of fractured oxides.The TBST is testing the direct bond strength with no peel or shear forces involved. It also only requires a fraction of the sample material for testing and any roll bonded sample is applicable for this test. These are the huge advantages with the test method. The test method is however still naive, and suffers from a series of challenges. The current test ranged is from 4MPa to 40MPa, but with potential for a large range expansion. Further are bond damaging effects, caused by the machining, reducing the accuracy of the measurements and compromising “grooving”; a measure taken for increasing the test range above 40MPa.The strain rate at which the samples were tested, showed to have strong influence on the measured bond strength. Much higher than the effect of any work hardening on either of the alloys. The preparation prior to roll bonding including an only 90s exposure time to air, ensures a very thin oxide layer and bonding at reductions down at 22.3%. Ductile “stretch lips” was found on the fracture surface, and run in direction normal to the rolling direction. The fraction of bonded surface area did not seem to follow the percent of reduction during roll bonding, which indicates a thinning of the oxide layer.

Zr70Ni16Cu6Al8 bulk metallic glass samples were cast and prepared for tensiletesting. Tensile tests were done at temperatures 77 K, 150 K and 295K, over a range of effective strain rates between 10−5 and 10−1/s in order toassess the dependence of plastic deformation through shear band sliding onthe aforementioned factors.A strong temperature dependence was found, with maximum shear bandsliding distance at 150 K. The sliding distance was also found to increasewith lowered strain rates. The temperature dependence is likely to stemfrom increased heat dissipation suppressing a decrease in viscosity, while thestrain rate dependence stems from a decrease in energy supply rate.Shear bands appeared at higher stresses at lower temperatures, resultingin larger elastic domains and higher ultimate tensile stresses. A mechanismwhere heat dissipation suppresses the formation of shear bands at low stresseswas proposed.v

<p>The solubility of divalent iron oxide in cryolite-based melts was studied. Both electrochemical and chemical techniques were employed. To ensure that only divalent iron was present in solution, the melt was contained in an iron crucible under an atmosphere of argon. The experimental work included investigation of the solubility as a function of alumina concentration, temperature and cryolite ratio (CR = NaF/AlF₃ molar ratio). The solubility at 1020 ºC was found to decrease from 4.17 wt% Fe in cryolite to 0.32 wt% Fe in cryolite saturated with alumina. FeO and FeAl₂O₄were found to coexist as solid phases in equilibrium with the melt at 5.03 wt% Al₂O₃; the former being the stable solid phase below this concentration and the latter at high alumina concentrations. The standard Gibbs energy of formation for FeAl₂O₄from its oxide components at 1020 ºC was determined to be -(17.6 ± 0.5) kJ mol^-1. The solubility of FeAl₂O₄ was found to increase from 0.25 wt% Fe at 981 ºC to 0.36 wt% at 1050 ºC in alumina-saturated melts. By assuming Henrian behaviour, the apparent partial molar enthalpy of dissolution of FeAl₂O₄ was found to be (64.8 2.5) kJ mol^-1. Experiments involving varying cryolite ratio in alumina-saturated melts at 1020 ºC showed a maximum solubility of 0.62 wt% Fe at a cryolite ratio of five. Modelling indicated that divalent iron species were present as NaFeF₃in acidic melts (CR < 3), while Na₃FeF₅and/or Na₄FeF₆ dominated in a basic environment (CR > 3).</p><p>The solubility of TiO_{2 i}n cryolite-alumina melts at 1020 ºC was measured. The analytical data showed that the titanium solubility decreased with increasing total oxide concentration, up to a concentration of ~3.5 wt% O, while it increased at higher concentrations. The solubility was found to be 3.1 wt% Ti and 2.7 wt% Ti, respectively, in cryolite and in alumina-saturated melts. Modelling indicated that the most probable titanium species are TiO²⁺ and TiO₃²-, which coexist in the solution; the former dominating at low alumina concentrations and the latter at high alumina concentrations. Unknown amounts of fluoride may also be associated with the titanium atoms. Determination of the solubility of TiO₂ in alumina-saturated melts as a function of temperature showed that the solubility increased from 1.9 wt% Ti at 975 ºC to 2.8 wt% Ti at 1035 ºC. The apparent partial molar enthalpy of dissolution of TiO₂was found to be (88.3 ± 4.1) kJ mol^-1, provided that Henry’s law holds.</p><p>The electrochemistry of divalent iron in cryolite-based melts was investigated by voltammetry, chronopotentiometry and chronoamperometry. A working electrode of copper was found to be best suited for the study of the reduction of Fe(II), while gold and platinum gave the best results under oxidising conditions. The reduction of Fe(II) ions was found to be diffusion controlled. The number of electrons involved was determined to be two. A discrepancy was observed between the diffusion coefficients obtained by the different techniques. The diffusion coefficient of Fe(II) in alumina-saturated melts at 1020 ºC was found to be D_Fe(II) = 3.0 x10^-5 cm² s^-1 by voltammetry. Experiments performed in an electrolyte with industrial composition at ~970 ºC gave a slightly higher value for the diffusion coefficient. The oxidation of Fe(II) on a gold or a platinum wire electrode showed that the process was diffusion controlled, involving one electron. The reversible potential for the redox couple Fe(III)/Fe(II) was found to be more cathodic than the reversible potential for the oxygen evolution by 350 to 400 mV, depending on the solvent composition and on the temperature.</p><p>The electrochemistry of TiO₂ in cryolite-alumina melts was studied by voltammetry. The deposition of titanium on tungsten was found to be a three-electron diffusion controlled process. The deposition peak increased with increasing titanium concentration. In alumina-saturated melts two waves were observed prior to the titanium deposition. The potential difference between the cathodic wave closest to the deposition peak and its corresponding oxidation peak indicated a diffusion controlled process that involved a one-electron charge transfer. However, in cryolite melts a single wave was observed prior to the titanium deposition. It is suggested that these cathodic waves might have been caused by underpotential deposition of titanium, and subsequent alloying with tungsten. It cannot be ruled out that redox reactions take place between tetravalent titanium and the titanium alloyed with tungsten, thereby forming trivalent titanium prior to the metal deposition.</p><p>In order to determine thermodynamic properties of FeAl₂O₄, a solid electrolyte galvanic cell was used. Cryolite was present in the half-cell containing FeAl₂O₄to ensure that alumina of the alpha modification was in equilibrium with FeAl₂O₄. An oxygen ion conducting yttria-stabilised zirconia tube served as the solid electrolyte. The EMF was measured in the rage 1245 to 1343 K. By using literature data at higher temperatures, thermodynamic properties for the reaction Fe(s) + ½O₂(g) + Al₂O₃(s,α) = FeAl₂O₄(s) could be calculated, i.e. ΔHº_1600K = –(270615 ± 1387) J mol^-1 and ΔSº_1600K= -(56.759 ± 0.856) J K^-1 mol^-1. New thermodynamic data for FeAl₂O₄ were also calculated, and a predominance area diagram for solid iron phases at 1293 K was constructed. The standard potential of the redox couple Fe(III)/Fe(II) as a function of the alumina content was derived from the solubility data of Fe(II) obtained in the present work and literature data for Fe(III). When the standard potentials are put into context of the Hall-Héroult process, the results indicate that neither the CO₂/CO anode gas nor the carbon anode itself can oxidise Fe(II) to Fe(III).</p><p>The mass transfer of the impurities Fe, Si and Ti between bath and aluminium in industrial Hall-Héroult cells was investigated. The experiments were performed in several types of cells with prebaked anodes. The impurities were added to the bath in the form of Fe₂O₃, SiO₂ and TiO₂. Bath and metal samples were collected periodically before and after the addition was made. With the criterion that the mass transport was diffusion controlled, a model involving first order reaction kinetics was used to calculate the mass transfer coefficients for transfer into the metal phase. Large scatter were observed in the obtained mass transfer coefficients, but the general trend seemed to be k_Fe > k_Si > k_Ti. By averaging the data obtained, it was found: k_Fe = (10 ± 3) x 10^-6 m s^-1, k_Si = (7 ± 3) x 10^-6m s^-1, and k_Ti = (5 ± 2) x 10-⁶ m s^-1.</p>

<p>Pitches are used on a large scale in the manufacture of carbon anodes for the production of primary aluminium. The role of the pitch is to act as a binder between the petroleum coke grains. The structure of the carbonized pitch binder (pitch coke) has an important impact on the overall performance of the anode. Even though the binder pitch is the minor constituent in an anode, it is impossible to make a good quality anode without a good quality binder pitch.</p><p>Pitch is an extremely complex mixture of numerous, essentially aromatic and heterocyclic compounds derived from pyrolysis of organic material or tar distillation. Upon heat treatment pitches form cokes in relatively high yields. Physical and chemical properties of the anode such as mechanical strength, electrical resistivity, thermal conductivity and resistance towards oxidation by air and CO2 are dependent on the structure of the aggregate material as well as the carbonized binder pitch. The properties of the pitch coke is in turn mainly dependent on the chemical characteristics of the parent pitch.</p><p>Coal-tar pitch is the preferred choice of binder material in anode manufacture today. However, the availability of high quality coal-tar is in decline and at least partial replacement by alternative binder sources will become increasingly important in the future. Due to environmental regulations, petroleum pitches are interesting as they generally have lower PAH emissions than coal-tar pitches during baking. Blends of coal-tar pitches and petroleum pitches are in use today on an industrial scale. The aluminium industry must be prepared to meet the challenges involved in adapting binder pitches from new sources which may be of inferior quality to the pitches available on the market today. An increased understanding of the processes involved in the transformation of a pitch into a coke and the link between raw material composition and properties and the final artifact is thus highly relevant.</p><p>Traditionally, the suitability of a binder pitch for use in anodes, has been defined from parameters like softening point, insolubility in toluene (TI) and quinoline (QI), coke yield, H/C atomic ratio, ash content and density. Although these parameters, which are mostly empirical in nature, give an indication of the pitch quality, more information on the chemical characteristics and carbonization behavior of pitches is certainly valuable. The present work aims to describe and explain the link between “classical” pitch properties, hydrogen transfer properties, information derived from NMR spectroscopy and the structure of the carbonized binder pitch.</p><p>Coal-tar and petroleum pitches pass through a fluid stage during carbonization. In the early stages of carbonization, free radicals are formed due to thermal rupture of C-C and C-H bonds in reactive components. Polymerization occurs mainly via a free radical mechanism leading to molecular size enlargement (aromatic growth)and the formation of oligomeric systems (mesogens). If the intermolecular reactivity of the pitch constituents is too high, extensive cross-linking and a rapid transformation of pitch molecules through polymerization will occur at a relatively low temperature. In this case, either mesophase will not be formed or the growth and coalescence of mesophase will take place under low fluidity/high viscosity conditions leading to a premature solidification of the pyrolysis system. An isotropic coke or a pitch coke of small optical domains will then be formed. On the other hand, if the pitch has a low thermal reactivity, aromatic growth is constrained and the mesogens will have sufficient mobility to stack parallel to each other and establish a liquid crystal system (mesophase). The growth and coalescence of mesophase take place at a higher temperature where the viscosity of the pyrolysis system is at a low level. Eventually, the system will solidify and an anisotropic coke of large well-developed optical domains is formed.</p><p>In particular, the presence of alkyl side chains and oxygenated functional groups are considered to lead to an increased thermal reactivity. If free radicals formed by thermal rupture of bonds in reactive pitch species can be stabilized by hydrogen transfer from within the system, extensive cross-linking at a too early stage is prevented. The initiation, growth and coalescence of mesophase are facilitated and consequently a coke of large well-developed optical domains is formed. Hydroaromatic rings and naphthenic rings in hydroaromatic species are considered to be principal hydrogen donor groups. Oxygen acceptor sites are believed to deplete the supply of donatable hydrogen and leave radicals free to recombine. The thermal reactivity of a pitch is thus dependent on both the amount of reactive species and the ability of the pitch to stabilize free radicals by hydrogen transfer.</p><p>In the present work, the subject of study was five coal-tar pitches and four petroleum pitches. In addition, a QI-free coal-tar pitch supplied by GrafTech International was studied. The pitches were characterized by 1H NMR and 13C NMR spectroscopy, hydrogen transfer properties, elemental analysis and the release of volatiles during carbonization. In addition, the pitches were characterized by more “traditional” pitch parameters like insolubility in quinoline (QI), insolubility in toluene (TI), softening point and coking value. The structure of the carbonized pitches was examined by optical microscopy and X-ray diffraction.</p><p>The hydrogen transfer properties of the pitches were evaluated from their ability to donate hydrogen to an acceptor compound, anthracene, or abstract hydrogen from a donor compound, 1,2,3,4-tetrahydronaphthalene (tetralin). A mixture of pitch and anthracene or tetralin was heat treated in sealed glass tubes filled with argon gas at 400 ºC. Two different heat treatment procedures were tested. In the first, the sample was kept at 400 ºC for 8 hours while in the second, the sample was heated at a rate of 5 ºC/min to 400 ºC with no soaking time. The major hydrogenated products from the reaction between anthracene and pitch were 9,10-dihydroanthracene (DHA) and 1,2,3,4-tetrahydroanthracene (THA). After the reaction, the semi-coke residue was dissolved in carbon disulphide and analyzed by gas chromatography. The hydrogen donor ability (HDa) was calculated from the amounts of DHA and THA formed and expressed as milligrams of hydrogen transferred to anthracene per gram of pitch. For the hydrogen donor ability test, the less severe heat treatment (5ºC/min to 400 ºC, no soaking time) was found to be the most appropriate. The reaction between tetralin and pitch gave one major dehydrogenated product, naphthalene. The hydrogen acceptor ability (HAa) was calculated from the ratio of naphthalene to tetralin as determined by gas chromatography and expressed as milligrams of hydrogen transferred per gram of pitch. For the acceptor ability test, the heat treatment at 400 ºC with 8 hours soaking time was found to be the most appropriate.</p><p>The release of volatiles during carbonization was studied by thermogravimetric analysis. The amount of volatiles released between 300 and 500 ºC (VM300-500)relative to the total amount of volatiles released at 1000 ºC was selected as a parameter reflecting the thermal behavior of pitches during the critical stages of carbonization.</p><p>Carbonization of pitches was performed under inert gas pressure (15 bar) and the green cokes obtained at 550 ºC were studied by optical microscopy. Computerized image analysis was performed to quantify the optical texture. The output parameters from the image analysis were the mosaic index, which is a measure of the optical domain size, and the fiber index, which is a measure of the parallel alignment of optical domains. The green cokes were further heat treated to 1150 ºC and the microstructure of the resulting calcined pitch cokes was characterized by X-ray diffraction.</p><p>The carbon disulphide soluble part of the pitches was investigated by 1H NMR and 13C NMR spectroscopy. Results from elemental analysis of the pitches were used in conjunction with the results obtained from the NMR spectroscopy. The main objective of the NMR analysis was to identify and quantify structures in the pitch which are considered either to increase or decrease the thermal reactivity. The coal-tar pitches were as expected found to be more aromatic than the pitches of petroleum origin. A relationship was found between the aromaticity of the pitches and the H/C atomic ratio as determined from elemental analysis. Elemental analysis is a rapid and convenient method to estimate the aromaticity of pitches. Due to a more hydroaromatic structure, the petroleum pitches were in general found to have a higher estimated concentration of donatable hydrogen which will suppress intermolecular reactivity. However, the petroleum pitches also had a high concentration of alkyl side chains which are generally believed to give increased thermal reactivity. Carbon connected to oxygen could not be distinguished in the NMR spectra. Pitch constituents containing heteroatoms are generally concentrated in the heavier pitch fractions which may not be soluble in carbon disulphide. This could be an explanation for the failure in the detection of aromatic carbon connected to heteroatoms. However,the oxygen content was determined by elemental analysis.</p><p>The pitches could be distinguished due to their ability to donate hydrogen to anthracene or abstract hydrogen from tetralin. The hydrogen donor ability was not found to correlate with the concentration of donatable hydrogen (NMR) which might have been expected. A likely explanation for this apparent inconsistency is that potential donatable hydrogen in reactive pitches will be preferentially consumed by free radicals and oxygenated acceptor sites instead of being transferred to anthracene.</p><p>A correlation between the hydrogen donor (HDa) and acceptor ability (HAa) was not found. This indicates that the two parameters represent two separate properties where both are linked to the thermal reactivity of the pitch. The ratio between the hydrogen donor and acceptor ability, HDa/HAa, was used as a parameter reflecting the thermal reactivity of pitches. Pitches which exhibit a high HDa/HAa ratio (low thermal reactivity) are expected to form an anisotropic coke of large optical domains. On the other hand, pitches with a relatively low HDa/HAa ratio are expected to have a high thermal reactivity and form a more isotropic (small optical domains) coke. Despite the higher concentration of donatable hydrogen, the petroleum pitches were not generally considered to have a lower thermal reactivity than the coal-tar pitches expressed by the HDa/HAa ratio.</p><p>The processes taking place during thermal treatment of pitches are reflected in the release of volatiles. A correlation was observed between the HDa/HAa ratio and the relative amount of volatiles released between 300 and 500 ºC (VM300-500). Thermally reactive pitches exhibiting a low HDa/HAa ratio will have a high activity at low temperatures and release low boiling point molecules and fragmentation species. If on the other hand the pitch has a low thermal reactivity, fragmentation species will be stabilized by hydrogen transfer and retained in the pyrolysis system.</p><p>The resulting thermally stable molecules of relatively low molecular weight may then act as solvating vehicles maintaining a low viscosity in the system and may also be important as hydrogen shuttling agents. When the system has reached a critical stage for mesophase growth and coalescence, these smaller thermally stable molecules (non-mesogens) are eventually released at higher temperatures.</p><p>The petroleum pitches developed cokes of relatively large optical domains (coarse mosaic). A correlation was observed between the HDa/HAa ratio and the mosaic index (size of optical texture) for the petroleum pitches. As expected, a high thermal reactivity (low HDa/HAa ratio) resulted in a pitch coke of small optical domains (high mosaic index). The HDa/HAa ratio was, however, not successful in predicting the size of optical texture in the cokes obtained from the coal-tar pitches. This was mainly due to the influence of QI material on the pitch coke structure. It is recognized that particulate matter (primary QI material) hinders the growth and coalescence of mesophase. This was found for the coal-tar pitches. Scanning electron (SEM) and polarized light microscopy images taken at a high magnification revealed how the QI particles were arranged and clustered around smaller anisotropic domains. The detrimental effect of QI material on the development of anisotropic texture in the resulting coke was demonstrated by comparing the structure of the coke obtained from a QI-free coal-tar pitch and a coal-tar pitch containing QI. The QI-free pitch developed a coke of large optical domains whereas the coke obtained from the pitch containing QI material had mainly a fine mosaic texture (small optical domains).</p><p>However, some large anisotropic domains were present in between the QI clusters. It is also not to be excluded that the QI fraction not only acts physically by obstructing the growth and coalescence of mesophase but may also be chemically active. Findings indicate that the oxygen is concentrated in the QI fraction. Solid QI particles with oxygenated functional groups or heteroatomic structures containing oxygen, which due to their large size are insoluble in quinoline, may act as acceptor sites for hydrogen thus increasing the thermal reactivity.</p><p>The average coherent stacks of the calcined (1150 ºC) pitch cokes was found to consist of between 7 and 8 graphene layers (Lc divided by d002). The average crystallite size (Lc) was fairly similar for all the calcined pitch cokes but significant differences were found. The coal-tar pitches generally developed cokes of slightly higher average crystallite sizes than the pitches of petroleum origin. The microstructure of the coal-tar pitch cokes is probably influenced by the amount and nature of the QI fraction. For the petroleum pitches there was a tendency that a high average crystallite size was connected to a more well-developed structure (larger domains) at the green coke stage.</p><p>The evaluation of hydrogen donor and acceptor abilities provides a rapid and relatively simple method to differentiate pitches which can be linked to the development of structure during carbonization. These properties thus reflect the thermal reactivity of pitches and can be connected to the release of volatiles during pyrolysis. However, for coal-tar pitches the QI content was found to be the most influential factor on the development of optical texture and must be considered in addition to the hydrogen transfer properties. Considerations on thermal reactivity from NMR spectroscopy and elemental analysis were found to generally support the results from the hydrogen donor and acceptor ability tests.</p>

<p>The main objectives of the thesis work were: (1), to study the oxidation of methanol and formic acid on platinum electrodes by employing conventional and advanced electrochemical methods, and (2), to develop membrane electrode assemblies based on polybenzimidazole membranes that can be used in fuel cells up to 200 °C.</p><p>D.c. voltammetry and a.c. voltammetry studies of methanol and formic acid on polycrystalline platinum in sulphuric acid electrolyte were performed to determine the mechanism and kinetics of the oxidation reactions.</p><p>A combined potential step and fast cyclic voltammetry experiment was employed to investigate the time dependence primarily of methanol oxidation on platinum. Charge measurements clearly demonstrated the existence of a parallel path at low potentials and short times without formation of adsorbed CO. Furthermore, experimental results showed that only the serial path, via adsorbed CO, exists during continuous cycling, with the first step being diffusion controlled dissociative adsorption of methanol directly from the bulk electrolyte. The saturation charge of adsorbed CO derived from methanol was found to be significantly lower than CO derived from formic acid or dissolved CO. This was attributed to the site requirements of the dehydrogenation steps, and possibly different compositions of linear, bridged or multiply bonded CO. The coverage of adsorbed CO from formic acid decreased significantly at potentials just outside of the hydrogen region (0.35 V vs. RHE), while it did not start to decrease significantly until about 0.6 V vs. RHE for methanol. Adsorbed CO from dissolved CO rapidly oxidized at potentials above about 0.75 V due to formation of platinum oxide.</p><p>Data from a.c. voltammograms from 0.5 Hz up to 30 kHz were assembled into electrochemical impedance spectra (EIS) and analyzed using equivalent circuits. The main advantages of collecting EIS spectra from a.c. voltammetry experiments are the ability to directly correlate the impedance spectra with features in the corresponding d.c. voltammograms, and the ability to investigate conditions with partially covered surfaces that are inaccessible in steady-state measurements.</p><p>A variety of spectral types were observed, and for methanol these showed only a single adsorption relaxation aside from the double-layer/charge-transfer relaxation, though some structure in the phase of the latter relaxation hints at another process. The charge-transfer resistance showed Tafel behaviour for potentials in the rising part of the oxidation peak consistent with a one-electron process in the rate-determining step. The rate limiting step was proposed to be the electrochemical reaction between adsorbed CO and OH at the edge of islands of OH, with competition between OH and CO adsorption for the released reaction sites. Only a single adsorption relaxation in methanol oxidation was observed, implying that only one single coverage is required to describe the state of the surface and the kinetics. It was assumed that this single coverage is that of OH, and all the surface not covered with OH is covered with CO so that the coverage of CO is not an independent variable. Inductive behaviour and negative relaxation times in the methanol oxidation were attributed to nucleation and growth behaviour. Linear voltammetry reversal and sweep-hold experiments also indicated nucleation-growth-collision behaviour in distinct potential regions, both in the forward and reverse potential scan for methanol oxidation on platinum.</p><p>In both methanol oxidation and formic acid oxidation, a negative differential resistance (NDR) was observed in the potential regions that possess a negative d.c. polarization slope, and was attributed to the formation of surface oxide which inhibited the oxidation of methanol or formic acid.</p><p>EIS spectra for formic acid clearly showed the presence of an additional low frequency relaxation at potentials where we expect adsorbed dissociated water or platinum oxide to be present, implying that more than one single coverage is required to describe the state of the surface and the kinetics. Two potential regions with hidden negative differential resistance (HNDR) behaviour were identified in the positive-going sweep, one prior to platinum oxide formation, assumed to involve adsorbed dissociated water, and one just negative of the main oxidation peak, assumed to involve platinum oxide. Oscillatory behaviour was found in the formic acid oxidation on platinum by adding a large external resistance to the working electrode circuit, which means that there is no longer true potentiostatic control at the interface. By revealing the system time constants, impedance measurements can be used to assist in explaining the origin of the oscillations. In the case of formic acid, these measurements showed that the oscillations do not arise from the chemical mechanism alone, but that the potential plays an essential role.</p><p>Preparation and optimization of gas-diffusion electrodes for high temperature polymer electrolyte fuel cells based on phosphoric acid doped polybenzimidazole (PBI) membranes was performed. This fuel cell allows for operating temperatures up to 200 °C with increased tolerance towards catalytic poisons, typical carbon monoxide. In this work we employed pure hydrogen and oxygen as the fuel cell feeds, and determined the optimum morphology of the support layer, and subsequently optimized the catalytic layer with respect to platinum content in the Pt/C catalyst and PBI loading. A smooth and compact support layer with small crevices and large islands was found to be beneficial with our spraying technique in respect to adhesion to the carbon backing and to the catalyst layer. We found that a high platinum content catalyst gave a significantly thinner catalyst layer (decreased porosity) on both anode and cathode with superior performance. The PBI loading was found to be crucial for the performance of the electrodes, and a relatively high loading gave the best performing electrodes.</p>

<p>Welding is the main joining method used in shipbuilding. A multi-electrode submerged arc welding is usually applied to long joints of massive components since the early 1950s.</p><p>The problem of welding stresses and deformations arose almost simultaneously with the introduction of welding as a joining method. In 1892 Slavianov, in his works, mentioned about "harmful stresses in metal" appearing during welding. Despite tremendous development in welding technology since that time, this problem is still present.</p><p>This dissertation presents the development of an experimental, numerical and analytical approach to the analysis of weldability. Focus is placed on the investigation of transient and residual welding stresses and distortions in thick-walled ship hull structures. The investigation mostly relies on the finite element analysis. Some recommendations about mitigation techniques for reducing their negative influence on welded structure efficiency are given.</p><p>A number of simulation systems for the sequentially coupled thermo-mechanical analysis of welding process, taking into account the welding conditions during the butt welding of thick-walled steel panels, are developed. Two welding techniques are investigated. The three-electrode one-pass welding process and the one-electrode multipass process are compared in terms of the joint characteristics and transient stresses and deformations.</p><p>The accuracy of each finite element models is evaluated based on experimental results and the results of the analytical solution.</p><p>Several techniques allowing to reduce residual stresses and deformations are investigated. Both, obtained results and governing phenomena, are studied and explained based on physical principles.</p><p>The first part of the thesis is a significant part of the work. Its purpose is a deep analysis of the thermo-mechanics of welding allowing better understanding of the complicated phenomena.</p>

The solubility of divalent iron oxide in cryolite-based melts was studied. Both electrochemical and chemical techniques were employed. To ensure that only divalent iron was present in solution, the melt was contained in an iron crucible under an atmosphere of argon. The experimental work included investigation of the solubility as a function of alumina concentration, temperature and cryolite ratio (CR = NaF/AlF3 molar ratio). The solubility at 1020 ºC was found to decrease from 4.17 wt% Fe in cryolite to 0.32 wt% Fe in cryolite saturated with alumina. FeO and FeAl2O4 were found to coexist as solid phases in equilibrium with the melt at 5.03 wt% Al2O3; the former being the stable solid phase below this concentration and the latter at high alumina concentrations. The standard Gibbs energy of formation for FeAl2O4 from its oxide components at 1020 ºC was determined to be -(17.6 ± 0.5) kJ mol-1. The solubility of FeAl2O4 was found to increase from 0.25 wt% Fe at 981 ºC to 0.36 wt% at 1050 ºC in alumina-saturated melts. By assuming Henrian behaviour, the apparent partial molar enthalpy of dissolution of FeAl2O4 was found to be (64.8 2.5) kJ mol-1. Experiments involving varying cryolite ratio in alumina-saturated melts at 1020 ºC showed a maximum solubility of 0.62 wt% Fe at a cryolite ratio of five. Modelling indicated that divalent iron species were present as NaFeF3 in acidic melts (CR &lt; 3), while Na3FeF5 and/or Na4FeF6 dominated in a basic environment (CR &gt; 3). The solubility of TiO2 in cryolite-alumina melts at 1020 ºC was measured. The analytical data showed that the titanium solubility decreased with increasing total oxide concentration, up to a concentration of ~3.5 wt% O, while it increased at higher concentrations. The solubility was found to be 3.1 wt% Ti and 2.7 wt% Ti, respectively, in cryolite and in alumina-saturated melts. Modelling indicated that the most probable titanium species are TiO2+ and TiO32-, which coexist in the solution; the former dominating at low alumina concentrations and the latter at high alumina concentrations. Unknown amounts of fluoride may also be associated with the titanium atoms. Determination of the solubility of TiO2 in alumina-saturated melts as a function of temperature showed that the solubility increased from 1.9 wt% Ti at 975 ºC to 2.8 wt% Ti at 1035 ºC. The apparent partial molar enthalpy of dissolution of TiO2 was found to be (88.3 ± 4.1) kJ mol-1, provided that Henry’s law holds. The electrochemistry of divalent iron in cryolite-based melts was investigated by voltammetry, chronopotentiometry and chronoamperometry. A working electrode of copper was found to be best suited for the study of the reduction of Fe(II), while gold and platinum gave the best results under oxidising conditions. The reduction of Fe(II) ions was found to be diffusion controlled. The number of electrons involved was determined to be two. A discrepancy was observed between the diffusion coefficients obtained by the different techniques. The diffusion coefficient of Fe(II) in alumina-saturated melts at 1020 ºC was found to be DFe(II) = 3.0 x10-5 cm2 s-1 by voltammetry. Experiments performed in an electrolyte with industrial composition at ~970 ºC gave a slightly higher value for the diffusion coefficient. The oxidation of Fe(II) on a gold or a platinum wire electrode showed that the process was diffusion controlled, involving one electron. The reversible potential for the redox couple Fe(III)/Fe(II) was found to be more cathodic than the reversible potential for the oxygen evolution by 350 to 400 mV, depending on the solvent composition and on the temperature. The electrochemistry of TiO2 in cryolite-alumina melts was studied by voltammetry. The deposition of titanium on tungsten was found to be a three-electron diffusion controlled process. The deposition peak increased with increasing titanium concentration. In alumina-saturated melts two waves were observed prior to the titanium deposition. The potential difference between the cathodic wave closest to the deposition peak and its corresponding oxidation peak indicated a diffusion controlled process that involved a one-electron charge transfer. However, in cryolite melts a single wave was observed prior to the titanium deposition. It is suggested that these cathodic waves might have been caused by underpotential deposition of titanium, and subsequent alloying with tungsten. It cannot be ruled out that redox reactions take place between tetravalent titanium and the titanium alloyed with tungsten, thereby forming trivalent titanium prior to the metal deposition. In order to determine thermodynamic properties of FeAl2O4, a solid electrolyte galvanic cell was used. Cryolite was present in the half-cell containing FeAl2O4 to ensure that alumina of the alpha modification was in equilibrium with FeAl2O4. An oxygen ion conducting yttria-stabilised zirconia tube served as the solid electrolyte. The EMF was measured in the rage 1245 to 1343 K. By using literature data at higher temperatures, thermodynamic properties for the reaction Fe(s) + ½O2(g) + Al2O3(s,α) = FeAl2O4(s) could be calculated, i.e. ΔHº1600K = –(270615 ± 1387) J mol-1 and ΔSº1600K = -(56.759 ± 0.856) J K-1 mol-1. New thermodynamic data for FeAl2O4 were also calculated, and a predominance area diagram for solid iron phases at 1293 K was constructed. The standard potential of the redox couple Fe(III)/Fe(II) as a function of the alumina content was derived from the solubility data of Fe(II) obtained in the present work and literature data for Fe(III). When the standard potentials are put into context of the Hall-Héroult process, the results indicate that neither the CO2/CO anode gas nor the carbon anode itself can oxidise Fe(II) to Fe(III). The mass transfer of the impurities Fe, Si and Ti between bath and aluminium in industrial Hall-Héroult cells was investigated. The experiments were performed in several types of cells with prebaked anodes. The impurities were added to the bath in the form of Fe2O3, SiO2 and TiO2. Bath and metal samples were collected periodically before and after the addition was made. With the criterion that the mass transport was diffusion controlled, a model involving first order reaction kinetics was used to calculate the mass transfer coefficients for transfer into the metal phase. Large scatter were observed in the obtained mass transfer coefficients, but the general trend seemed to be kFe &gt; kSi &gt; kTi. By averaging the data obtained, it was found: kFe = (10 ± 3) x 10-6 m s-1, kSi = (7 ± 3) x 10-6 m s-1, and kTi = (5 ± 2) x 10-6 m s-1.

Welding is the main joining method used in shipbuilding. A multi-electrode submerged arc welding is usually applied to long joints of massive components since the early 1950s. The problem of welding stresses and deformations arose almost simultaneously with the introduction of welding as a joining method. In 1892 Slavianov, in his works, mentioned about "harmful stresses in metal" appearing during welding. Despite tremendous development in welding technology since that time, this problem is still present. This dissertation presents the development of an experimental, numerical and analytical approach to the analysis of weldability. Focus is placed on the investigation of transient and residual welding stresses and distortions in thick-walled ship hull structures. The investigation mostly relies on the finite element analysis. Some recommendations about mitigation techniques for reducing their negative influence on welded structure efficiency are given. A number of simulation systems for the sequentially coupled thermo-mechanical analysis of welding process, taking into account the welding conditions during the butt welding of thick-walled steel panels, are developed. Two welding techniques are investigated. The three-electrode one-pass welding process and the one-electrode multipass process are compared in terms of the joint characteristics and transient stresses and deformations. The accuracy of each finite element models is evaluated based on experimental results and the results of the analytical solution. Several techniques allowing to reduce residual stresses and deformations are investigated. Both, obtained results and governing phenomena, are studied and explained based on physical principles. The first part of the thesis is a significant part of the work. Its purpose is a deep analysis of the thermo-mechanics of welding allowing better understanding of the complicated phenomena.