No abstract / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Crevice corrosion experiments have been conducted on Inconel 600 and Inconel 625 exposed to two principle test solutions of 1 M NaCl and 1 M NaCl + 0.01 M Na₂S₂0₃ (Sodium Thiosulphate) at three temperatures, 22°C, 55 °C and 80°C. The crevice corrosion tests were performed in a corrosion cell which was constructed from PTFE (Polytetrafluoroethylene, Teflon) and Pyrex glass. Features of the cell included the utilization of an artificial Teflon-metal crevice and provisions to monitor crevice corrosion current, active crevice corrosion potential and active crevice pH. Additional experiments included potentiodynamic anodic polarization tests on pure Ni, Alloy 600, and Alloy 625 in bulk solution environments and in simulated crevice solutions. Crevice corrosion morphology and compositional analysis of the corrosion products was studied using a scanning electron microscope equipped with an X-ray energy dispersive spectroscopy (EDS) system. Results show that crevice corrosion rates increase with increasing temperature for Alloy 600 in both principle test solutions. X-ray EDS analysis indicated that an insoluble nickel sulphide corrosion product formed on Alloy 600 in a solution of 1 M NaCl + 0.01 M Na₂S₂0₃. For the Alloy 600, in a solution of 1 M NaCl + 0.01 M Na₂S₂0₃, initiation times were significantly reduced and crevice corrosion propagation rates enhanced, as compared to Alloy 600 in 1 M NaCl. The decrease in initiation times has been attributed to the destabilizing nature of the S₂O₃⁻² species on the passive oxide film. Enhanced propagation rates have been attributed to the presence of H₂S in the crevice solution and the formation of an adsorbed species Ni(H₂S)ads which enhances the anodic dissolution reaction. The H₂S in the active crevice solution originated from the thermodynamically favoured electrochemical reduction of the S₂0₃⁻² species in the active crevice solution. Experiments on Alloy 625, which is alloyed with molybdenum, (Mo), show that it was virtually immune to crevice corrosion as compared to Alloy 600 which is not alloyed with Mo. The resistance of Alloy 625 to crevice corrosion initiation has been attributed to the stabilizing nature of MoO₂ in the passive oxide film. For an actively corroding system, the formation of the molybdate species MoO₄⁻² may act as an anodic inhibitor and effectively enhance the repassivation of the passive film. / Applied Science, Faculty of / Mining Engineering, Keevil Institute of / Graduate

A research report submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in partial fulfilment of the requirements for the degree of Master of Science in Engineering, Johannesburg, 2019 / Premature 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. / PH2020

A thesis submitted to the Faculty of Engineering and Built Environment, University of the Witwatersrand, in fulfilment of the requirements for the degree Doctor of Philosophy (Materials and Metallurgy) Johannesburg, February 2019 / The 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, coatings / E.K. 2020

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The corrosion chemistry of two active-metal systems has been studied primarily with X-ray Photoelectron Spectroscopy. First, the interaction of metallic lithium thin films with simple glass surfaces was investigated. Li was deposited on SiO₂, sodium silicate, potassium silicate, and B₂O₃ glasses in an ultra-high vacuum deposition and analysis chamber. The reaction of Li with SiO₂ results in substantial reduction of the glass matrix to form a thin product layer. A negatively-charged Si species was identified based on an unusually low XPS Si(2p) binding energy. The interaction of Li with alkali silicate glasses resulted in substantially less matrix breakdown than for SiO₂, but exchange of lithium for either sodium or potassium cations occurred at the Li/glass interface. The reaction between Li and B₂O₃ was limited to the top layers of the glass, as a passivating layer formed at the Li/B₂O₃ interface. Investigations into the oxidation of polycrystalline iron surfaces were initiated. Clean surfaces were exposed to controlled amounts of pure oxygen gas. The resulting oxide composition and thickness was determined using ultraviolet photoelectron spectroscopy (UPS), electron energy loss spectroscopy (EELS), and XPS. The results indicated the formation of a bilayer structure, with FeO near the oxide-metal interface, and Fe₃O₄ at the outer surface. Film growth was approximately logarithmic with time, and was strongly pressure dependent. Studies of the electronic properties of the characterized iron oxide surfaces were conducted by measuring the rate of electron transfer between the surface and redox-active species in an electrochemical cell. A strong dependence on film thickness was indicated. Photoemission of electrons from a solid is an inherently complex process; this is especially the case for XPS of clean and oxide-covered active metals. Improved theoretical models of XPS lineshapes were developed which provided new insight into the physical processes involved in photoemission. Additionally, these models provided improved qualitative and quantitative data interpretation through the use of least-squares fitting techniques.

The thermodynamic properties of chloride, nitrate and nitrite AFm hydrates have been determined. Investigations of solid solutions and thermodynamic calculations on the influence of these anions on mineralogical changes in cement paste were performed and compared with experiments. To calculate volume changes, densities of principal crystalline phases occurring in cement were critically assessed and tabulated, in some cases with addition of new data. Database was obtained by calculating densities from crystallographic data and unit cell contents. In hydrated cements, anion sites in AFm phase are potentially occupied by OH, SO₄ and CO₃ ions. C1, NO₃ or NO₂ ions readily displace hydroxide, sulfate and carbonate in the AFm structures. Nitrates and nitrites do not have ability to displace chloride from the Friedel’s salt (C1-AFm) though. The binding power of AFm for nitrite/nitrate/chloride was calculated and confirmed experimentally at 25°C. It was observed that presence of chloride, nitrate or nitrite alters the AFm/Aft balance and thereby affect the specific volume of paste solids. It was found that the success of nitrite as a corrosion inhibitor for protection of embedded steel arises from its “smart” behaviour. AFm normally stores and sequesters nitrite. If chloride ingress occurs in service, the AFm undergoes ion exchange, gaining chloride and forming Friedel’s salt, while releasing soluble nitrite ions to the pore fluid. As a result, the aqueous ratio of [NO₂^-]/[C1^-] increases and remains within the passivation range for steel.

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