This dissertation is a part of a collaborative work between the University of Texas, Austin-Texas, the University of Oulu, Oulu-Finland, and Outokumpu Stainless Oy, Tornio-Finland, , to develop commercial austenitic stainless steels with high strength and ductility. The idea behind this work involves cold-rolling a commercial metastable austenitic stainless steel -- AISI 301LN stainless steel to produce strain-induced martensite, followed by an annealing treatment to generate nano/sub-micron grained austenite. AISI 301LN stainless steel sheets are cold-rolled to 63% reduction and subsequently annealed at 600°C, 700°C, 800°C, 900°C and 1000°C for 1, 10 and 100 seconds. The samples are analyzed by X-Ray diffraction, SQUID, transmission electron microscopy, and tensile testing to fundamentally understand the microstructural evolution, the mechanism for the martensite [implies] austenite reversion, the formation of nano/sub-micron austenite grains, and the relationship between the microstructure and the strength obtained in this stainless steel. The results show that cold-rolled AISI 301LN stainless steel consist of dislocation-cell martensite, heavily deformed lath-martensite and austenite shear bands. Subsequent annealing at 600°C for short durations of 1 and 10 seconds leads to negligible martensite to austenite reversion. These 600°C samples exhibit a similar microstructure to the coldrolled sample. However, for samples annealed at 600°C for 100 seconds and those annealed at higher temperatures (700°C, 800°C, 900°C and 1000°C) exhibit equiaxed austenitic grains of sizes 0.2[mu]m-10[mu]m and secondary phase precipitates. The microstructural analysis also reveals that the martensite [implies] austenite reversion occurs via a diffusion-type reversion mechanism. In this regard, a generalized form of Avrami's equation is used to model the kinetics of martensite [implies] austenite phase reversion. The results from the model agree reasonably well with the experiments. Furthermore, the activation energy for grain growth in nano/sub-micron grained AISI 301LN stainless steel is found to be ~ 205kJ/mol, which is comparable to values observed in coarse grained commercial stainless steels (AISI 304, 316). However, the driving force for grain growth in nano/sub-micron grained AISI 301LN stainless steel is considerably higher when compared to other stainless steels. Finally, the average grain sizes in AISI 301LN stainless steels are related to the mechanical properties obtained, through the Hall-Petch relationship. / text

The development of biofilms on mild and stainless steel surfaces in pure and mixed batch cultures of the bacterial species Pseudomonas fluorescens and Desulfovibrio desulfuricans and the role of these biofilms in corrosion of steel has been investigated. Early events leading to the formation of biofilms have been elucidated by studying the attachment of bacterial cells to steel using epifluorescence microscopy. To identify the nature of the bacterial surface components involved in the initial adhesion to mild steel, lectins, their sugar inhibitors and saccharolytic and proteolytic enzymes have been employed. Polyclonal antibodies have been raised against bacteriallipopolysaccharides (LPS) and their influence on bacterial adhesion assessed. LPS have been analysed chemically by gas-chromatography (GC-FID) and gas chromatography-mass spectrometry (GC-MS) to determine their ~arbohydrate composition and fatty acid content. On the basis of the results obtained the ~nvolv~~~nt of glucose and N-acetylglucosamine, present in O-antigenic fractions of LPS, 10 the lOlnal attachment of the two bacterial species to mild steel is suggested. Both types of carbohydrates are likely to be involved in early attachment of Pseudomonas to mild steel, whereas only a polymeric fonn of N-acetylglucosamine seems to participate in adhesion of Desulfovibrio. The subsequent biofilm development on steel surfaces and their accompanying corrosion h~s been monitored by scanning electron microscopy (SEM). SEM studies reveal very different patterns of bacterial biofilms on mild and stainless steel and show varied degrees of corrosion occurring on these surfaces. Thin and patchy Pseudomonas biofilms are accompanied by little corrosion whilst thick. more continuous, Desul/ovibrio biofilms are associated with higher levels of corrosion. Energy dispersive X-ray analysis (BOAX) of corrosion products present on steel surfaces indicates ferrous sulphides as the major components in Desul/ovibrio biofilms. The corrosion of steel in bacterial cultures has also been investigated by kinetic polarisation measurements. The results obtained from cathodic and anodic polarisation curves, combined with SEM and EDAX analyses confmn the SEM observation. Stainless steel is not subjected to any great degree of fouling or corrosion under the chosen experimental conditions. The EPS associated with biofilms and released into the liquid phase of the culture media (free EPS) has been characterised. Proteins and carbohydrates in these polymers are detected colorimenically and by SDS-gel electrophoresis. Uronic acids, found in biofilmbound BPS. are not detected in free EPS. The GC-MS and GC-FIO analyses have aided in establishing types and quantities of neutral carbohydrates present in bacterial exopolymers and show that the neutral sugar composition of free and surface-associated BPS is not identical for a given bacterial culture. The biofilm-bound BPS are believed not to play a major role in corrosion of mild steel but to provide additional mechanisms in its facilitation. No correlation between levels of free BPS and corrosion of steel is found.

Abstract Purpose – The purpose of this paper is to supplement the scant previous investigations on the corrosion behaviour of 2205 and 2507 duplex stainless steels in selected organic acids containing chloride additions. Design/methodology/approach – Microstructural examination of the alloys was first carried out, after which the corrosion behaviour of the alloys in citric, oxalic, formic and acetic acids containing chloride additions at varying temperatures was studied using electrochemical techniques. Findings – The alloy 2507 material had a larger grain size than did the alloy 2205 sample. The corrosion resistances of the alloys generally are highest in acetic acids and lowest in citric acid. The addition of chloride had a pronounced effect on their corrosion resistance. Alloy 2507 generally exhibited higher corrosion resistance in all of the acids than alloy 2205, with the exception of acetic acid at room temperature. The 50:50 ratio of ferrite to austenite composition, as revealed by phase compositional analysis, indicated no significant possibility for galvanic corrosion between the phases. This suggests that the corrosion behaviour of the alloys is controlled by their grain sizes and chemical compositions. Originality/value – Although the corrosion behaviour of duplex stainless steels in some organic acid media has been reported, this investigation covers the major organic acids not previously reported. Since in real industrial systems a mixture of both organic and minerals acids/salts may typically exist, investigations of the combined effect of chloride ions with the organic acids reported in this paper typify real industrial operations. The paper thus provides a basis for material selection for the application of 2205 and 2507 in industrial systems where organic acids are mostly used.

published_or_final_version / Economics / Master / Master of Arts

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

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