A stable transferred arc was produced with plasmagas containing up to 20 percent molar TiCl$ sb4$ in argon, helium and argon/hydrogen mixtures. This was achieved by replacing the commonly-used thoriated tungsten cathode tip with a tantalum carbide tip. Thus, corrosive reactions at the cathode surface, which were shown to be the cause of the observed instability, were prevented. This allowed the characteristics of stable titanium tetrachloride plasmas in a transferred arc reactor to be investigated. / Furthermore, an investigation was conducted into the feasibility of collecting titanium metal from the dissociated TiCl$ sb4$ molecule in the plasmagas. The titanium metal was collected in a molten bath, which also served as the anode in the transferred arc system. Three anode bath compositions were used in this study. Two of them, namely titanium and zirconium, were not able to reduce recombined titanium subchlorides in the bath. The third aluminum, was a reducing bath. When aluminum was used, about 60 percent of all titanium fed into the reactor was collected. / Finally, phenomena occurring on the surface of a thoriated tungsten cathode were studied in a transferred-arc reactor using argon or helium as the plasmagas. The effect of cathode geometry on the rate and mechanisms of cathode erosion were investigated. It was shown that the surface temperature of flat-tip cathodes operating in argon is near the melting point of tungsten. On the other hand, the surface temperature of flat-tip cathodes operating in helium and pointed-tip cathodes operating in either helium or argon are near the boiling point of tungsten. Some of the material vapourized from the cathode was redeposited on the cathode surface, forming crystals whose morphology and composition depended on their distance from the arc root and the plasmagas composition.

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A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. Johannesburg, 2016. / Metal dusting (MD) is a severe type of corrosion that occurs mainly in petrochemical industries. The occurrence of MD is mainly due to syngas attacking Fe-, Ni- and Co-based alloys at elevated temperatures. More recently, literature has shown that apart from syngas, liquid hydrocarbon sources have been causing problems on platformer units in refineries. In the first part of this study a highly corrosion resistant Ni-based alloy (Hastelloy C276), in its polished form, was subjected to MD conditions at 800 °C using a liquid hydrocarbon (toluene) and helium (carrier gas) for 1 h. Exposure to these conditions revealed the formation of carbon nanofilaments and graphite layers which were confirmed by laser Raman spectroscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Burning off the carbon nanofilaments and the graphite layers in laboratory air for 1 h at 800 °C revealed that pits were formed on the Hastelloy C276. These same pits were not evident when Hastelloy C276 was exposed to either the carrier gas (helium) or laboratory air alone. Besides MD being a continuous problem in industry, this mechanism has been shown to be beneficial in the synthesis of carbon nanofilaments viz., carbon nanofibers (CNTs) and nanotubes (CNFs). In the second part of this study, unpolished Hastelloy C276 blocks (as opposed to polished blocks) were used to synthesize carbon nanofilaments. This was done as prior studies had shown that carbon nanofilaments were produced with better quality and greater yields this way. Here the flow rate (80, 160 and 240 mL/min) and reaction duration (10, 15, 30, 45, 60, 120 and 240 min) were studied using toluene (a liquid hydrocarbon). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to assess the quality and quantity of the carbon nanofilaments synthesized. Besides the formation of carbon nanofilaments, a less important material known as graphite particle structures (GPSs) were also synthesized. These studies collectively showed that MD had taken place on the surface of Hastelloy C276 when exposed to toluene at 800 °C. / TG2016

The solute strengthening due to oxygen in Zircaloy-2 was investigated over the temperature range 1023 to 1873 K. The flow stress was determined in constant true strain rate hot compression over the strain rate 10('-4)-10('-1)s('-1). In order to provide data for LOCA modelling, the 0 concentration range 1260 to 12360 ppm was investigated. For the single phase (alpha) and (beta) materials, the flow curves were normal, whereas the ((alpha)+(beta)) materials exhibited significant yield drops and strain aging, especially when over 50 vol.% (beta) was present. These effects are attributed to the enrichment of the (beta) phase by the solutes Fe and Cr, which are considered to form ordered zones of the Zr(Fe,Cr)(,2) type in the vicinity of dislocations. The flow stress for both the (alpha) and (beta) Zircaloy-2 obeyed the relationship (sigma) = (sigma)(,0) exp(kc), where (sigma)(,0) and k are constants and c is the wt.% 0. The experimental values indicated that 0 strengthens the (beta) phase less effectively than the (alpha) phase. Rule of mixtures calculations were carried out to predict the flow stresses of the ((alpha)+(beta)) alloys; these agreed reasonably well with the experimental data, as long as due allowance was made for the 0 enrichment and 0 empoverishment of the (alpha) and (beta) phases, respectively. / Within the experimental range, the rate sensitivities increased with temperature from 0.13 to 0.20 and from 0.27 to 0.32 for the fully (alpha) and fully (beta) materials. By contrast, the m vs. T curves for the two-phase materials were marked by a rapid increase in m near the beginning and end of the ((alpha)+(beta)) domain, with a dip at large (beta) fractions, particularly in the high 0 alloys. The dip in rate sensitivity was associated with significant strain aging. The experimental activation energies decreased with stress from 440 to 330 kJ/mol (50 to 140 MPa) and from 150 to 125 kJ/mol (1 to 5 MPa) for the (alpha) and (beta) Zircaloy-2 respectively. The extrapolated zero-stress values of Q increased with 0 concentration from 520 to 570 kJ/mol and from 170 to 260 kJ/mol for the (alpha) and (beta) materials, respectively. These data indicate that high temperature dynamic recovery is easier to activate in the (beta) phase but is more sensitive to oxygen than is (alpha) Zircaloy-2.