3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Effects of ozonation on cooling water systems(2010-07-26T12:14:44Z) Mosugelo, Keneetswe LilianCooling water systems are needed to dissipate heat. The mist from open system cooling towers has been implicated as a source of infections of Legionella pneumophila. As a result biocide is added to cooling water systems, but the addition of biocide worsens biofouling, scaling and corrosion. Increasing environmental pressure has resulted in a move away from biocides which are usually chlorine based chemicals, so that the use of ozone instead has recently been reported as a way of controlling microbiological growth. This study aims to compare the corrosion rates (using calculated and measured surface areas) of different metals exposed to chemically treated and ozone treated cooling water in an industrial cooling water system. The types of corrosion were also observed and recorded. The scales from different components of the cooling systems as well as scale from chemically and ozone treated D tower water were characterized qualitatively using X-ray diffraction (XRD), Thermo gravimetric Analysis (TGA) and Atomic Absorption Spectroscopy. The D tower is a cooling water circuit in which the cooling water is from the Vaal River. As expected, the stainless steel has the lowest corrosion rate of 0.000 milli inches/year followed by brass with 1.531 milli inches/year and lastly mild steel (2.098 milli inches/year). Water quality rather than the presence or absence of ozone determines the corrosion rate. This confirms the findings reported in the literature. Scale from chemically treated water contains many different compounds while scale from ozone treated contains only different polymorphs of CaCO3, which is present in the water source and magnesium calcite.Item Constitution of the Pt-Cr-Nb system(2009-11-06T12:48:27Z) Mulaudzi, Fulufhelo Marandela LloydSuperalloys based on platinum-group metals (PGMs) are being developed for high temperature applications. Currently, the optimum alloy is Pt84:Al11:Ru2:Cr3, and work is ongoing. Niobium is a possible addition to increase the melting point, but limited phase diagram data are available. Although work has been done on the Pt-Al-Nb system, there are no reported data for Pt-Cr-Nb. This is a study of the Pt-Cr-Nb system. As-cast samples of the Pt-Cr-Nb ternary system were investigated using scanning electron microscopy with energy-dispersive X-ray spectroscopy, and X-ray diffraction. The results were used to plot a solidification projection and all binary phases extended into the ternary, except for α´Pt, which was not identified in this investigation. The extensions of the binary phases were: (Cr) ~2 at.% Nb; (Pt) ~24 at.% Nb and 65 at.% Cr; (Nb): ~12 at.% Pt and ~17 at.% Cr; ~NbPt2: ~20 at.% Cr; ~NbCr2: ~18 at.% Pt; ~Cr3Pt: ~10 at.% Nb; ~βNbPt3: ~4 at.% Cr and ~Nb1-XPt1+X: ~13 at.% Cr. Five ternary phases were identified: τ1:~Nb17:Cr64:Pt19; τ2:~Nb28:Cr55:Pt17; τ3:~Nb30:Cr30:Pt40; τ4:~Nb45:Cr27:Pt28 and τ5:~Nb40:Cr18:Pt42. The liquidus surface was derived using the binary phase diagrams, identified primary phases, solidification sequences and the solidification projection, together with the eutectic compositions. Nineteen ternary invariant reactions were identified. Hardness measurements were made on all samples, and also toughness and fracture toughness were deduced. Alloys with (Pt) and ~NbPt2 were ductile with reasonable hardness. Alloys containing (Cr), (Nb), ~Cr3Pt, ~NbCr2, ~Nb3Pt and ~Nb2Pt were extremely brittle with cracks, whereas alloys containing ~βNbPt3 and ~Nb1-XPt1+X phases were slightly brittle.