No description available.

Pits have been observed to initiate at certain inclusions in stainless steel, which affect the composition and properties of the passive film in that region. In this thesis, the VG MA500 high resolution scanning Auger microscope associated with Link EDX is used to investigate the surface chemistry of inclusion groups in stainless steel 316 relative to the bulk composition, before and after exposure to sodium chloride solution. The work demonstrates that careful control of experimental conditions allows the progress of initiation of pits to be observed systematically. The role of inclusions as initiators of pitting corrosion has been studied from the very inception of attack. Detailed observation have been taken from separate manganese sulphide as well as mixture of manganese sulphide and multi-element oxide at various pH values of solutions to trace the behaviour of these inclusions during exposures. It is suggested from this investigation that corrosive attack starts on MnS inclusion proper. The dissolution of MnS takes place in sodium chloride solution followed by the acidification in the microarea and dissolution of metal in the region adjacent to the inclusion. The mechanism for the crevice corrosion is of importance after the microcrevice forms. The observations showed that sulphur precipitated on the area surrounding the inclusion group in acidic sodium chloride solution. This is in contrast to the finding in prior studies by electron probe or EDX analyses in which sulphur precipitation was only observed on the inclusion. Because pitting normally occurs on surfacesthat are initially exposed to air before being immersed in the corrosive solution, an understanding of the change of surface film associated with the change of medium is very important. XPS was used in this work to study the air formed and aqueous films, the transformation when the specimens were immersed in solutions. The study of the energy-loss structure of photoelectron peaks has lead to a better understanding of these various films. It has been used to study in greater detail the composition and properties of the films formed on Fe-Cr alloys during oxidation, vacuum annealing, and to find the relationship between the depth distribution of the components in the films and energy loss background of photoelectron peaks. Three features are of particular importance: the background loss tail height, the baseline slope and the post-peak loss structure. These features have been related quantitatively to the relative amounts of the element and the surrounding matrix in the layers which contribute the greatest signal at any given take-off angle. The present work illustrates that the additional depth information available from closer inspection of the inelastic loss background feature may aid an understanding of the change from the air-formed film to the aqueous film as well its effect on the initiation of pits in stainless steel 316. The combination of AES and EDX with high spatial resolution gave an opportunity to re-investigate the initiation of pits at the site of inclusion on stainless steel by offering both bulk and surface information. The use of AES and XPS associated with the energy loss structure of photoelectron peaks facilitates a comprehensive understanding for the whole surface of various films and the local areas which are involved in the formation of pits.

Thesis (M.Sc. in Applied Science(Metallurgical Engineering))--University of Pretoria, 2008. / Abstract in English. Includes bibliographical references (leaves 67-69).

<p> New electrophoretic deposition methods for the fabrication of advanced organic-inorganic composite coatings on metallic substrates for biomedical applications have been developed. In the proposed methods, chitosan was used as a matrix for the fabrication of multilayer and functional graded chitosan- hydroxyapatite (HA) coatings. The HA particles showed preferred orientation of c-axis parallel to the layer surface, which is similar to the bone structure. Electrochemical studies showed that the obtained coatings provided corrosion protection of the metallic substrates, such as stainless steel and Nitinol.</p> <p> The feasibility of co-deposition of chitosan and heparin has been demonstrated. Composite chitosan-heparin layers were used for the surface modification of chitosan-HA coatings. Obtained results paved the way for the electrophoretic fabrication of novel coatings for biomedical implants with improved blood compatibility.</p> <p> The feasibility of co-deposition of hyaluronic acid and HA has also been demonstrated. The co-deposition of hyaluronic acid and HA resulted in the fabrication of novel nanocomposite films by electrodeposition. The chemical composition, microstructure, corrosion protection, and other functional properties of the nanocomposites have been investigated. Co-deposition of hyaluronic acid and multiwalled carbon nanotubes has been studied by TGA/DT A and SEM studies.</p> <p> The feasibility of deposition of novel composites based on alginic acid has been demonstrated. New electrochemical strategies were used for the fabrication of alginic acid-HA, alginic acid-heparin and alginic acid -hyaluronic acid nanocomposites. The composition of these nanocomposite coatings can be varied by variation in bath composition for EPD.</p> <p> The electrochemical mechanisms for the fabrication of all these advanced organic-inorganic composite coatings have been developed.</p> / Thesis / Master of Applied Science (MASc)

Austenitic stainless steels are frequently used in the cooling circuits of nuclear reactors. It has been found that cold-worked 304 stainless steels can be particularly susceptible to stress corrosion cracking at the operating conditions of such reactors. Despite more than 130 years of research underlying mechanisms are still not properly understood. For this reason, the effects of cold-work and applied stress on the oxidation behaviour of 304SS have been studied in this thesis. A set of samples with/without prior cold-work, and with/without stress applied during oxidation, were oxidized in autoclaves under simulated pressurised water reactor primary circuit conditions. Atom-probe tomography and analytical transmission electron microscopy were used to investigate the local chemistry and microstructure in the different samples tested. Regions containing grain boundaries, deformation bands, and matrix material in contact with the environment, were extracted from the coupon specimens with a focused ion beam machine. Cross-sections of crack tips were studied with secondary ion mass spectrometry and electron backscatter diffraction. The compositions of oxides grown along the surface and the different microstructural features were analysed. Fe-rich spinels were found at the surface and Cr-rich spinels were observed along fast diffusion paths. Ni-enrichment was found at the metal/oxide interfaces and a Ni-rich phase was detected in precipitates ahead of grain boundary oxides. Li was observed in all oxidised regions and B segregation, originating from impurities in the alloy, was observed in grain boundaries and crack tip oxides. Cavities and hydrogen associated with Ni-rich regions were found ahead of the bulk Cr-rich oxide in some of the samples. The implications of these findings for the understanding of SCC mechanisms are discussed. It is suggested that Ni precipitation as well as the presence of deformation bands may play an important role in controlling SCC susceptibility in 304 stainless steel. A modification of the film-rupture model including internal oxidation and fast diffusion along H-stabilised vacancies in strain fields at the crack front is proposed.

This work was carried out aiming the study of the corrosion effect by the water produced, which is associate to the production of oil, mainly in wells in mature fields, such as the UN-SEAL. It is noteworthy that this water is known in the petroleum industry as water produced. The sample of produced water used was of the production field of Siririzinho I on-shore in Sergipe, which showed in its physicalchemical analysis, high levels of salinity among other compounds, which makes it an extremely aggressive corrosion for materials used in the petroleum industry. In this research the materials used in this work were the carbon steel ASTM A283 Gr C, the stainless steel ASTM A240 type 316L with and without coating of hard chrome and the duplex stainless steel ASTM A890 Gr 1B. The sample used had chemical compositions issued by manufacturers according to ASTM, in addition, microhardness tests and metallographic tests with the use of optical microscopy were used to characterize these materials. For the study of effect of the water produced attack in the materials, was designed and built a test circuit, which allowed the insertion of samples for monitoring and data collected of mass and thickness parameters , and the monitoring visual corrosion during the test. The samples were machined in two geometric shapes (rectangular plate and disk) to enable and facilitate the testing of weight and thickness measurement by applying the ultrasound technique and of the depth measurement. From the collected data was possible to calculate by established criteria using the standard NACE RP 0775, the rate of uniform corrosion by mass loss and by its development the rate of corrosion by thickness loss, that is the same form used for the calculation of corrosion rate by pit in agreement with the standard Petrobras N- 2364. By comparing the corrosion rates obtained in the study, can better evaluate the possible perspectives of application of the materials in contact with the corrosive fluid. / Este trabalho foi desenvolvido objetivando o estudo do efeito da corrosão por meio da água que está associada à produção do petróleo, principalmente em poços de campos maduros, como é o caso da UN-SEAL. Ressalta-se que esta água é conhecida na indústria do petróleo como água produzida. A amostra da água produzida utilizada foi a do campo de produção de Siririzinho I do ativo de produção de Sergipe Terra que apresentou em sua análise físico-química, altos teores de salinidade dentre outros compostos que a torna um meio extremamente agressivo à corrosão para os materiais utilizados na indústria do petróleo. Nesta pesquisa os materiais estudados foram os aços: carbono ASTM A283 Gr. C, o inox ASTM A240 tipo 316L sem e com revestimento de cromo duro e o inox duplex ASTM A890 Gr. 1B. As amostras utilizadas possuíam suas composições químicas emitidas pelos fabricantes de acordo com a norma ASTM, além disto, ensaios de microdureza e ensaios metalográficos com o uso da microscopia óptica foram utilizados na caracterização destes materiais. Para o estudo do efeito do ataque da água produzida nos materiais, foi projetado e construído um circuito de teste, o qual permitiu a inserção de corpos de prova para monitoramento e coleta de dados dos parâmetros de massa e de espessura, além do acompanhamento visual da corrosão ao longo do teste. Os corpos de prova foram usinados em duas formas geométricas (disco e placa retangular) de modo a permitir e facilitar a realização dos ensaios de medição de massa, medição de espessura através do ultra-som e da medição de profundidade. De posse dos dados coletados foi possível calcular através de critérios estabelecidos pela norma NACE RP 0775, a taxa de corrosão uniforme por perda de massa e por seu desenvolvimento a taxa de corrosão por perda de espessura, que é a mesma forma utilizada para o cálculo da taxa de corrosão por pites segundo a norma Petrobras N-2364. Através das taxas de corrosão obtidas no estudo, pode-se avaliar quais as possíveis perspectivas de aplicação dos materiais em contato com o fluido corrosivo.