3. Electronic Theses and Dissertations (ETDs) - All submissions
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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 Effects of water on the stress corrosion cracking of carbon steel in ethanolic media(2013-07-23) Moyo, FortunateIn this study, the effect of water on the stress corrosion cracking (SCC) of ASTM 516 in ethanol was investigated. Ethanol is hygroscopic in nature and its water content can increase rapidly when exposed to humid conditions. The presence of water in ethanol is likely to increase ethanol’s oxygen content which is the major instigator of SCC. In order to have an insight into the SCC susceptibility of carbon steel in ethanol-water solutions, the corrosion behaviour of the steel in these solutions was first evaluated. Carbon steel specimens in ethanol solutions with water exhibited various extents of localised corrosion, which increased in severity with increase in water content from 1 to 5 vol%. The occurrence of localised attack suggested that the presence of water promoted the formation of surface films; a condition suitable for SCC. Carbon steel specimens subjected to slow strain rate tests, however, exhibited ductile fractures indicating that the presence of up to 5vol% water did not induce SCC of carbon steel in ethanol.