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of SA’s large metropolitan areas, global climate change effects and improved mapping techniques, this chart has become outdated.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 In the study, historical published and unpublished corrosion data for South Africa were assimilated and analysed.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

No description available.

No description available.

No description available.

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.

No description available.

Thesis (Ph. D.)--Ohio State University, 2008. / Title from first page of PDF file. Includes bibliographical references (p. 228-234).

No description available.

No description available.