3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Evaluation of selected repair methods for chloride-included corrosion damaged reinforced concrete railway bridges(2019) Jogiat, MohamedPremature deterioration of reinforced concrete railway bridges before and after repair is a serious concern as it is costly and poses a major risk on safety and performance. Reinforced concrete railway bridges in aggressive environments (near the sea) face the risk of ingress of corrosion agents (oxygen, moisture and chlorides) into the concrete to the reinforcing steel. Although, corrosion of the reinforcing steel is not the only cause of structural deficiencies in railway bridges, it is a significant contributor to deterioration and therefore of major concern. In order to guide the selection of a suitable repair option, one repair material from each category (patch repair mortars, barrier systems, electrochemical methods and corrosion inhibitors) was investigated. The effectiveness of selection was assessed by employing electrochemical techniques to quantify the performance of each selected repair material in stifling chlorideinduced corrosion in reinforced concrete. This study focuses on the evaluation of selected repair materials for chloride-induced corrosion in reinforced concrete using 100 x 100 x 500 mm long beam specimens. The four selected repairs were applied to the reinforced concrete beams after a period of 200 days after casting. The beams had a constant concrete cover to reinforcing steel of 20mm. The beam specimens were casted using admixed chlorides into the mix and were subjected to a cycle of 3 days wetting (with 5% NaCl solution) and 4 days drying. The beam specimens were monitored for half-cell potential (Cu/CuSO4), corrosion rate (coulostatic technique) and concrete resistivity (Wenner probe technique). Results indicate that the selected repair materials in this study had varied influences on the halfcell potential and corrosion rate values. The patch repair material replaced the concrete cover with a more durable material, confirmed from the Durability Index (DI) tests conducted. The resistivity of the repair mortar was measured to be higher than the concrete. Due to the replacement of the concrete cover, the corrosion rate values reduced when compared to the control reinforced beam specimens. However, the half-cell potential values indicated the probability of corrosion is still high after application. The barrier method, applied a silane-based sealer on the reinforced concrete beams. The resistivity of the concrete increased after application of the barrier method. The corrosion rates after application of the barrier method was lower than the corrosion rates of the control reinforced concrete beams. The half-cell potential results indicated the corrosion risk is still high after application of the barrier method. The electrochemical repair was the only repair material that showed more negative potentials than the control beams and corrosion rates were significantly higher than all the other repair methods after application. The reason for this can be attributed to the zinc anode dominating the potential and corrosion rate values. Therefore, no conclusion can be made on the corrosion condition of the reinforcing steel. Alternative methods should be employed in determining the effectiveness of sacrificial anode repairs.Item Atmospheric corrosion mapping of South Africa and the Greater Johannesburg Metropolitan Area (GJMA)(2019) Janse Van Rensburg, Darelle TaniaThe first corrosion map of South Africa (SA) was published in 1991. Since then only minor variations have been made to this map. However, due to the lack of differentiation of inland locations, overstatement of the corrosivity of environments, changes in international standards regarding the measurement of corrosive atmospheres, increased industrialisation of SA’s large metropolitan areas, global climate change effects and improved mapping techniques, this chart has become outdated. The present study focused on the development of a new corrosion map for SA, with the emphasis placed on the provision of more detail concerning the atmospheric corrosivity of the Greater Johannesburg Metropolitan Area (GJMA) – SA’s economic heartland. In the study, historical published and unpublished corrosion data for South Africa were assimilated and analysed. The atmospheric corrosivity of the GJMA was also measured using ASTM G116 wire-on-bolt and ISO 9226:1992 open aluminium, zinc and copper wire helix devices, including ISO 9226:2012 flat mild steel and hot-dip galvanised steel coupons. Coated mild steel specimens were also exposed to determine the corrosive effects of the GJMA’s atmosphere on organic materials. The morphologies and chemical compositions of the mild steel corrosion products, using SEM-EDS, FTIR and Raman spectroscopy, as well as pH and water-soluble salt measurement techniques were furthermore investigated, along with correlations between the measured corrosion data, and general meteorological and pollution parameters for the GJMA area. The study showed that the corrosivity of SA’s coastal environments decreases rapidly within the first 150 m from the ocean and that for most inland locations, very low (C1) to potentially high (C4) corrosive conditions may be expected, as per the ISO 9223 rating scheme. High correlations were also found between the first-year corrosion rates of mild steel, hot-dip galvanised steel, zinc, aluminium and copper. Moreover, it was confirmed that the corrosivity of the GJMA is influenced by precipitation, humidity, PM10 and PM2.5 particulate matter, as well as SO2, NO2, NO, CO and O3 levels in the atmosphere, including wind direction and daily temperatures (maximum and minimum). Other factors found to affect the corrosivity of the GJMA’s atmosphere are: elevations above sea and ground level; the presence of vegetation and large water bodies; topography; shielding and shading effects; the occurrence of an El Niño or La Nina event; and acid rain. Moreover, it was established that the GJMA is most corrosive during spring and summer and that at least 90% of the area can be rated Upper-C2 (low to medium) corrosive. The results furthermore revealed high correlations between the ISO 9223 (1992 and 2012) corrosion monitoring devices and that wire-on-bolt (CLIMAT) units are better indicators of the impact of atmospheric pollutants on the 12- month corrosion rate of hot-dip galvanised steel than uncoated steel. A strong linear correlation was also found regarding the average corrosion rate of mild steel with every 20 mg/m2 rise in the concentration of water-soluble salts in the corrosion product. The East Rand of the GJMA rated most corrosive, with corrosion trouble spots identified at Aeroport, Bonaero Park, Brakpan, Chloorkop, Dalpark, Dunnottar, and Nigel. The Kagiso-Randfontein area was the only other area, outside the East Rand, that rated more corrosive. Extrapolations of the long-term corrosion rates of mild steel and hot-dip galvanised steel in the GJMA were additionally made based on logarithmic regressions of the 6-, 12-, 18- and 24-month corrosion data. Finally, geoprocessed (metal specific) corrosion maps were developed for the GJMA, subsequently incorporated into several corrosion maps for SA (also metal specific), to provide better clarity regarding SA’s inland areas. Keywords: Corrosion, atmosphere, South Africa, Greater Johannesburg, inland Metropolitan, wire-on-bolt, CLIMAT, ISO 9223, mapping, pollution, mild steel, hot-dip galvanised steel, aluminium, zinc, copper, coatingsItem Investigating the corrosion resistance of copper-ruthenium coatings(2018) Zaba, WinnieCopper has been widely used, and its increased application in different industries resulted in high demands. With more applications, it has increased the need to improve the mechanical, and corrosion properties to improve its life span during operation. Pure copper has been used in many aqueous environments (e.g. pipe lines, storage tanks, and electro-winning) because of its good conductivity and corrosion resistivity. However, when it is exposed to harsh environmental conditions with high acidity it experiences corrosion. This research focuses on these corrosion characteristics. Appropriate surface chemistry is very important in many of the copper applications. Surface engineering techniques have been applied to improve the corrosion property of many materials like stainless steel, but only a few scientific investigations and developments have been done on copper. This development has been carried out by introducing ruthenium to the copper surface to improve the corrosion resistance and serviceability of copper in general. The ruthenium was added to the copper surface using thermal spray coating high velocity oxygen fuel spray coating, cold spray coating, spark plasma sintering, and electroplating. One should note that the nature of the surface coating and arrangement of the powder particles determines the overall protection that can be achieved/induced. For the different coatings and alloys, ruthenium was added in 0.5, 1, and 2 weight percent. This research found that in many cases, the corrosion resistance increased with increase in ruthenium content. The HVOF and SPS experienced the same trend. The CSC coating did not achieve any corrosion resistance improvement when compared to as-received copper. The electroplating of a copper substrate successfully decreased the corrosion rate in sulphuric acid at 65°C. The electroplated powders had contaminations that affected the corrosion characteristics of the coating and the alloy. Other properties of the material were also affected after the ruthenium addition. These also include the hardness of the materials which was increased with the increase in ruthenium content. The research limitations encountered was the shortage of equipment to perform other tests like the scratch adhesion test which would have validated the adhesion property of the coatings.Item Using ruthenium to modify surface properties of austenitic stainless steel for improved corrosion resistance(2017) Moyo, FortunateChromium oxide provides an inexpensive and practical means of increasing the corrosion resistance of austenitic stainless steel in most environments. However, the oxide is prone to dissolve in reducing acids and in chloride containing solutions, which compromises the durability and effective operation of structures made of austenitic stainless steel. This research project explored the use of thin ruthenium surface alloys produced by ion implantation, RF sputtering and pulsed electrodeposition (PED) to improve the corrosion resistance of AISI 304L austenitic stainless steel in reducing acids and chloride solutions via a technique known as cathodic modification. The properties of the alloyed 304L stainless steel were evaluated using a number of tools including X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), potentiodynamic polarisation, and electrochemical impedance spectroscopy (EIS). Preliminary tests in 1 M sulphuric acid showed that the ruthenium surface alloys sufficiently raised the corrosion potential of 304L stainless steel to ranges where the stability of chromium oxide is guaranteed. Surface alloys produced by RF sputtering and PED were associated with the best corrosion resistance, and protection efficiencies of at least 85%, but they spalled during corrosion exposure rendering them unsuitable for corrosion application. The corrosion of the ruthenium implanted surface alloys exhibited a strong dependence on the surface roughness of the stainless steel, with the least corrosion rates achieved on rough 304L stainless steel samples implanted with 1016 Ru/cm2 at 50 keV. Corrosion characterisation of these ruthenium implanted surface alloys was studied in various corrosive media including sulphuric acid, sodium chloride, magnesium chloride and simulated fuel cell solutions. Their corrosion rates in sulphuric acid decreased with increase in acid concentration, and exhibited non-Arrhenius behaviour in the acid solutions; corrosion rates were unaffected by increasing exposure temperature from 25 to 50°C. In 3.5 wt% sodium chloride, addition of ruthenium via ion implantation changed pit morphology from elongated to circular, indicating a diminished tendency for pits to initiate at manganese sulphide stringers. Corrosion rates of the ruthenium implanted stainless steels in the simulated fuel cell solutions were at least 69% lower than the target corrosion rate for use in polymer electrode membrane fuel cells (PEMFCs), thus presenting a possible practical application of ruthenium surface alloyed austenitic stainless steel.Item Development of diffusion carbide coatings(1993) Fazluddin, Shahed BhaggaThe formation of VC, NbC, and Cr-carbide coatings on steels immersed in molten borax baths containing carbide-forming constituents has been known for some time. A study was made of the formation of carbide coating on steel specimens treated in molten borax cont.ad.Lnr.q ferro-vanadium and V20S as bath additives. The prevalence of oxidizing or reducing conditions in the bath was found to playa decisive role in the formation of the VC layer. The influence of treatment factors such as time, temperature, and bath composition on the thickness of the coating was investigated. A detailed investigation into the behaviour of baths containing V205 and Al was carried out in this regard. Microstructural examination of coated specimens was performed using optical and electron microscopy. Microhardness tests, X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) analyses were carried out in order to characterise the layers produced. The corrosion behaviour of the vanadf.um carbide coating was evaluated by accelerated electrochemical means. Samples of mild steel and EN9(O.5% C) steel were treated for various lengths of time in order to obtain coatings of various thicknesses. The corrosion resLst.ance of the carbide coating was found to be superior to that of the untreated base alloys. Five diffet"ent tool steel materials were a.lso treated. Microscopy examination and hardness testing of the VC coated tool steels was performed in order to assess the effects of treatment by this process on the bas~ material. The austenitizing temperature and the nature of the carbides in the matrix of the tool steels was found to play an important role in this respect.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 Corrosion and other properties comparison of AISI 316L stainless steel surface alloyed with Ru/Ni mixtures with the parent metal and with Hastelloy© C-276(2016) Lekala, Makgale BarclaysThe surfaces of AISI 316L stainless steel plate were laser alloyed with ruthenium powder as well as a mixture of ruthenium and nickel powders using a Nd:YAG laser set at fixed operating parameters. The microstructure, elemental composition, and corrosion characteristics of the alloyed zone were analysed using optical and scanning electron microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and corrosion potential measurements. EDS analysis of the alloyed specimen showed that through the laser surface alloying, 2 mm surface layers with 12.5wt % Ru and 5.2wt% Ru were produced on an AISI 316L stainless steel. Similar microstructures which were dendritic and columnar grains, typical of weld beads under non-equilibrium cooling conditions were observed for all samples. Hardness profile measurements showed a significant increase from 160 HV for the substrate to a maximum of 247 HV for the alloyed layer. Using an Autolab potentiostat, the corrosion behaviour and resistance of the laser alloyed layers, substrate AISI 316L, and Hastelloy© C-276 were evaluated and compared in sulphuric acid solution of different concentration and temperatures. The Hastelloy© C-276, followed by the 12.5wt% Ru presented the most noble corrosion potential (Ecorr) and the lowest corrosion current density (icorr). However, in 60wt% H2SO4 and 40oC, the 5.22 wt% Ru alloys exhibited slightly better anticorrosive properties than 12.5wt% Ru. The observed corrosion potential, Ecorr, for untreated AISI 316L stainless steel sample in 40wt% sulphuric acid solution at 40oC was -277 mV. The 5.22 wt% Ru and 12.5wt% Ru alloyed stainless steel samples presented -240 mV, and 61 mV respectively in the same solution. Besides showing comparable performance to 5.2wt%Ru sample within specific short potential ranges, Hastelloy© C-276 was generally superior in all solutions. In addition it was found that the stability of the passive layer was improved with additions of Ru. Based on the developed costing equation the cost of 5 mm AISI 316L stainless steel plate with surface area (A = 1 m2) surface alloyed with 5.2wt% Ru to a depth of 2 mm using Nd: YAG laser is estimated at R15 989, and it is less than the cost of a Hastelloy© C-276 plate of similar size which is estimated at R19 900. As the material thickness increases, the cost benefit of laser surface treatment increases and vice versa. Reduction of the Ru additions to levels below 5.2wt% would improve cost competition without detracting from performance.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.