3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Influence of temperature on the metal dusting of alloy 800(2018) Morudu, Kholo VeronicaMetal dusting (MD) is a severe form of corrosion in which iron, steels, and nickel (Ni) and cobalt (Co) based alloys disintegrate into a metal or carbide powder with a coke deposit when exposed to strongly carburising gases (carbon activity, ac>1) at elevated temperatures (400800°C). Temperature affects both the driving force and rate of the reaction, represented by gas phase supersaturation with carbon, and the rates of the various processes involved in converting that energy difference into the dusting process. Therefore, process streams such as reformer gas can be benign when hot, but becomes aggressive below critical temperatures. There are different views in literature about the effect of temperature on metal dusting of different materials and alloys. Alloy 800 experiences metal dusting (MD) at 525°C, which is the temperature of the tube sheet of reformers in petrochemical industry. This alloy is specifically used for tube ferrules in the reformers. The reformer trains can reach a critical (highest) internal temperature of 650°C. Therefore, these two temperatures were compared. The effect of temperature and exposure time on the metal dusting of Alloy 800 were investigated in terms of the form of attack and the degradation mechanism. From the results obtained, it was observed that the longer exposure periods result in more carbon deposition and the carbon filaments in the coke become finer as compared to the nanotubes obtained after shorter exposure periods. The alloy suffered metal dusting attack after a relatively short exposure period of three days (72 hours) at both temperatures of 525°C and 650°C, with very little coking.Item High Temperature Sulphidation and Naphthenic Acid Corrosion modeling in Crude oil refining(2018) Xaba, Nonkululeko P. NancyHigh temperature sulphidic (HTS) and naphthenic acid corrosion (NAC) pose not only reliability, but also health, safety and environmental threats to crude oil distillation unit operations. Based on an oil refinery’s crude oil diet properties, materials of construction and operational conditions in the distillation units, these high temperature corrosion phenomena occur in varying severity. In a bid to improve profit margins, oil refineries may choose to process opportunity crude oils at the risk of aggravating these corrosion mechanisms. In order to predict corrosion rates due to HTS and NAC, an industry corrosion model, Predict CrudeTM, was used on a refinery’s atmospheric and vacuum distillation units. The model was based largely on empirical data from a 4-year joint industry program that collected NAC and HTS corrosion rate data. The model provided qualitative corrosion rates for different distillation product streams for three model cases, based on total acid number (TAN) namely crude blend TAN 0.35, TAN 0.5 and TAN 0.8. The corrosion rates produced by the model for the 0.35 and 0.5 TAN cases were assessed against the measured corrosion rates, using conventional (industrial ultrasonic thickness and profile radiography) plant inspection techniques. Good agreement between the model predicted corrosion rates for the 0.35 and 0.5 TAN cases and the practically observed corrosion rates was found. The 0.8 TAN case was used to assess upset (unusual) conditions only, as the plant did not process 0.8 TAN crude blends. The 0.8 TAN case model data also explained some of the higher than modelled corrosion rates that were measured. Thus, the model provided a closely accurate prediction of corrosion rates to be expected in the atmospheric and vacuum distillation units.Item High temperature oxidation and corrosion behaviour of titanium aluminide alloy Ti-52.5AI-10.0Ni-0.2Ru (at.%)(2016) Mantyi, Hadio CapriceThe alloys Ti-52.5Al-10.0Ni (at.%) and Ti-52.5Al-10.0Ni-0.2Ru (at.%) were made by mixing, and melting their powders in a button arc furnace under an argon atmosphere. The high temperature oxidation and room temperature corrosion of behaviour of the alloys was investigated. Isothermal oxidation in air at 950°C for 120 hours and 720 hours was done. Cyclic oxidation behaviour of the alloys was also investigated in air and in a hot salt (Na2SO4) environment. The corrosion tests were conducted in 5 wt% and 25 wt% HCl. All the samples were characterised using scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray diffraction and hardness measurements. On solidification, the Ti-52.5Al-10.0Ni (at.%) alloy formed dendrites of γ-TiAl (~55 at.% Al) surrounded by a eutectic of γ-TiAl + Ti2NiAl3 (τ3) phases. Most of the nickel was found in the Ti2NiAl3 (τ3) phase (~12 at.%) with trace amounts in the dendrites (~0.5 at.%). The Ti-52.5Al-10.0Ni-0.2Ru (at.%) alloy formed dendrites of γ-TiAl (~53 at.% Al) surrounded by a eutectic of γ-TiAl + Ti2NiAl3 (τ3). Most of the nickel (~15 at.%) and ruthenium (~0.3 at.%) were in solid solution in the Ti2NiAl3 (τ3) phase, although small amounts of both metals were present in the dendrites (~1 at.% Ni and 0.1 at.% Ru). Under isothermal oxidation conditions, both alloys showed good oxidation resistance with a low mass gain (< 2%). The alloys formed a continuous scale of TiO2 and Al2O3 with good adherence to the substrate, but as exposure time increased, the scale was severely degraded and exfoliated from the surface. Cyclic oxidation conditions were more aggressive for both alloys. The Ti-52.5Al-10.0Ni-0.2Ru (at.%) alloy was more resistant and formed a nickel-rich sub-surface zone between the substrate and intermixed oxide layer. Both alloys had a fairly good corrosion resistance in HCl due to the presence of nickel. They formed a thin and non-continuous Al2O3 oxide scale on the surface of the γ-TiAl dendrites, with Ti3NiAl2O on the γ-TiAl + Ti2NiAl3 (τ3) eutectic regions. The acid mainly corroded the τ3 phase, thus attacking the eutectic and leaving the γ-TiAl dendrites exposed.Item The relationship between the metal dusting mechanism and the synthesis of carbon nanofilaments using toluene and a nickel based alloy(2016) Ramalall, Dawlall ShahilMetal 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.