Atualmente a maior parte da energia consumida no mundo provém de fontes como carvão, petróleo e gás natural. Nas últimas décadas, o aumento na demanda por petróleo e gás natural teve como resultado um grande aumento no uso de tubos de aço para transportar estes produtos por longas distâncias. Os aços de alta resistência e baixa liga (ARBL) produzidos de acordo com a norma API 5L são atrativos para estas aplicações por apresentarem boas propriedades mecânicas e soldabilidade aliadas a baixos custos. Entretanto, nestas aplicações os materiais são expostos a meios corrosivos com altos teores de H2S, tornando-os susceptíveis aos danos provocados pelo hidrogênio. Dentre estes um dos mais importantes é o trincamento induzido pelo hidrogênio (hydrogen induced cracking - HIC). Neste trabalho a resistência à corrosão e ao HIC de dois aços API 5L X65, cujas composições diferem principalmente com relação aos teores de Mn e de Nb, e de um aço API 5L X80 comercial foi investigada na solução A da norma NACE TM0284-2011. A avaliação da resistência à corrosão foi realizada em solução naturalmente aerada ou desaerada sem e com saturação com H2S por meio de ensaios de potencial de circuito aberto, curvas de polarização potenciodinâmica e espectroscopia de impedância eletroquímica. Análises microestruturais por microscopia óptica (MO) e eletrônica de varredura (MEV) foram realizadas para correlacionar a microestrutura com os dois parâmetros investigados. As análises por MO e MEV mostraram que os dois aços API 5L X65 possuem inclusões uniformemente distribuídas em uma matriz ferrítica com ilhas de perlita degenerada e microconstituintes M/A (martensita/ austenita) nos contornos de grão da ferrita. Por sua vez, o aço API 5L X80 apresentou matriz ferrítica com microconstituintes M/A, e inclusões de formas arredondadas e irregulares em maior número e distribuídas de forma irregular. Os resultados dos ensaios eletroquímicos mostraram que, para os três aços, a resistência à corrosão diminui sensivelmente na presença de H2S. Por outro lado, os ensaios de impedância evidenciaram aumento da resistência à corrosão com o tempo de imersão em todos os meios, provavelmente associado à formação de produtos de corrosão insolúveis e que precipitam sobre a superfície dos aços. Todos os ensaios mostraram que os dois aços API 5L X65 são mais resistentes à corrosão que o aço API 5L X80. Os ensaios de HIC mostraram que os dois aços API 5L X65 não são suscetíveis a este tipo de falha, contrariamente ao aço API 5L X80. Neste último houve formação de trincas da parte central e inferior (interna) da amostra fornecida em forma de tubo. A análise do caminho de propagação da trinca mostrou a presença de inclusões ricas em Mn e S, indicando que elas têm um papel relevante no mecanismo de trincamento. Os resultados de todos os ensaios de corrosão mostraram que o aço experimental API 5L X65 produzido pela CBMM com baixos teores de Mn e altos teores de Nb apresentou resistência à corrosão ligeiramente superior ao aço API 5L X65 comercial, indicando ser esta composição promissora para aplicações em meios sour. / Currently most of the energy consumed in the world comes from sources such as coal, oil and natural gas. In recent decades the increase in demand for oil and natural gas has resulted in a large increase in the use of steel tubes to transport these products over long distances. High strength low alloy (HSLA) steels produced according to the API 5L standard are attractive for these applications because they have good mechanical properties and weldability combined with low costs. However, in these applications the materials are exposed to corrosive media with high levels of H2S, making them susceptible to damage caused by hydrogen. Among them, one of the most important is hydrogen-induced cracking (HIC). In this work the resistance to corrosion and HIC of two API 5L X65 steels, whose compositions differ mainly with respect to their Mn and Nb contents, and a commercial API 5L X80 steel were investigated in solution A of the NACE standard TM0284-2011. The evaluation of the corrosion resistance was carried out in naturally aerated or in deaerated solution without and with saturation with H2S by means of open circuit potential tests, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). Microstructural analysis by optical microscopy (OM) and scanning electron microscopy (SEM) were performed to correlate the microstructure with the two investigated parameters. OM and SEM analyzes showed that the two API 5L X65 steels have inclusions evenly distributed in a ferritic matrix with degenerated perlite islands and M/A (martensite/austenite) microconstituents at the perlite grain boundaries. In turn, the API 5L X80 steel presented a ferritic matrix with M/A microconstituents and round-shaped and irregular-shaped inclusions in greater number and irregularly distributed. The results of the electrochemical tests showed that, for all three steels, the corrosion resistance decreases significantly in the presence of H2S. On the other hand, the EIS tests showed an increase in corrosion resistance with immersion time in all media, which is probably due to the formation of insoluble corrosion products that precipitate on the steels surfaces. All assays showed that the two API 5L X65 steels are more resistant to corrosion than the API 5L X80 steel. The HIC assays showed that the two API 5L X65 steels are not susceptible to this type of failure, unlike the API 5L X80 steel. In the latter there was cracks formation in the central and lower (inner) part of the sample supplied as a tube. The analysis of the crack propagation path showed the presence of Mn and S-rich inclusions, indicating that they play a key role in the cracking mechanism. The results of all corrosion tests showed that the experimental steel API 5L X65 produced by CBMM with low Mn contents and high levels of Nb exhibited slightly higher corrosion resistance than the API 5L X65 commercial steel, indicating that this composition is promising for sour media applications.

Na presente tese foi investigado o desempenho de camadas de organo-silanos sobre chapas de aço-carbono, similares às utilizadas na fabricação de veículos da indústria automobilística. As técnicas utilizadas neste estudo compreenderam a Espectroscopia de Impedância Eletroquímica (EIE) e a Resistência de Polarização Linear, por meio das quais foram determinadas respectivamente, as propriedades resistivas dos filmes e a redução das velocidades de corrosão do substrato. O estudo compreendeu a avaliação de diversos organo-silanos funcionais e não-funcionais, submetidos a diferentes condições experimentais. Os ensaios preliminares tiveram por finalidade selecionar aqueles com melhor desempenho como sistemas de proteção em monocamadas. A partir desta seleção e,, informações da literatura, novos ensaios foram conduzidos por meio de um Projeto Fatorial de Experimentos, combinando os silanos funcionais com melhor desempenho com outro silano não-funcional, designado como 1,2-bis(trietoxilsilil)etano (BTSE). Esta combinação teve por finalidade produzir camadas duplas de silanos para proteger o aço-carbono. As variáveis independentes estudadas foram a combinação entre as camadas de BTSE e do silano funcional, além das condições de hidrólise, temperatura e tempo de cura destes filmes. As variáveis dependentes ou resposta utilizadas foram: a impedância real a 0,03 Hz, obtida no ensaio de EIE, e a densidade de corrente de corrosão, originada das medidas de Resistência de Polarização Linear. Dentre as várias alternativas testadas, os melhores resultados foram apresentados pela combinação de camadas de BTSE com bis-(g-trimetoxisililpropil)amina (BTSPA) e BTSE com VS (viniltrietoxisilano) onde a primeira combinação determinou uma redução da ordem de 95% na velocidade de corrosão em relação a uma chapa sem tratamento e uma perda de espessura da ordem de 70% menor que um corpo-deprova revestido apenas com uma camada convencional de fosfatização. O BTSPA já na seleção inicial havia demonstrado excelentes resultados de proteção apenas como película monocamada, tendo seu desempenho melhorado consideravelmente em conjunto como o BTSE. Na segunda combinação, a velocidade de corrosão foi 90% menor do que num corpo-de-prova desprotegido e até 40% menor do que num corpo-de-prova revestido com uma camada de fosfato. / The aim of this thesis was the investigation performance of organo-silane layers on mild steel, similar to that used in car bodies in automotive assembly plants. The electrochemical impedance spectroscopy (EIS) and the polarization resistance (Rp) were used as tools to evaluate the resistive layers properties and the substrate corrosion rate, respectively. Several non-functional and functional silanes were evaluated under different experimental conditions. Preliminary testing was conducted in order to select the silane with best performance as monolayer protection. Based on this preliminary performance results and supplementary information from papers, new tests were conducted using a design of experiments (DOE), combining the best functional monolayers silanes with the non-functional silane 1,2- bis(triethoxylsilyl)ethane (BTSE). This combination had the intention to produce double-layers silane films to protect the mild steel. The independent variables tested in DOE were: combination between the BTSE and the functional silane layers, hydrolysis parameters, temperature and the time film curing. The real impedance values at 0,03 Hz, obtained in the EIE tests, and the polarization resistance were taken as response (dependent) variables to evaluate the design. Among the different alternatives, the best results were presented by combining the BTSE layer with the BTSPA (bis-(g trimetoxysilylpropyl)amine layer and BTSE layer with VS (vinyl triethoxy silane) layer. The first silanes combination had shown a 95% decrease in corrosion rate when compared to a non-treated sheet and a thickness loss 70% lower than on a specimen coated with ordinary phosphate layer. Already in the initial selection the BTSPA did show excellent protection results just as monolayer film, and its performance increased significantly when applied together with BTSE. The second silanes combination, had presented a 90% decrease in corrosion rate when compared to a non-treated sheet and a thickness loss 40% lower than a specimen coated with ordinary phosphate layer.

Made available in DSpace on 2016-08-19T12:56:45Z (GMT). No. of bitstreams: 1 Dissertacao Cristiane.pdf: 2090320 bytes, checksum: 517442cad2d717de912fdc61247ad7c2 (MD5) Previous issue date: 2013-06-07 / FUNDAÇÃO DE AMPARO À PESQUISA E AO DESENVOLVIMENTO CIENTIFICO E TECNOLÓGICO DO MARANHÃO / The use of biodiesel as fuel has presented a great potential in the world, both for its contribution to the environment as well as for being a fuel derived from renewable sources. However, this fuel is causing serious problems of corrosion metallic in materials used in diesel engines, transport systems and storage. In this sense, the present work aims, evaluating the corrosive effect of biodiesel and mixtures in aluminum by means of electrochemical techniques, as the open circuit potential and impedance electrochemical spectroscopy. For such, were used aluminum blades immersed in samples of B100 biodiesel and mixtures of B5 and B4, was also used microemulsion samples for obtaining better answers in the electrochemical trials. The surface of the metalic material was evaluated, after immersion trials, by visualization the optical microscope, in order to identify the effect caused by corrosion. The results obtained for trials showed that aluminum presented a good corrosion resistance in contact with the fuel samples, as well as in samples microemulsion. / A utilização do biodiesel como combustível tem apresentado um grande potencial no mundo inteiro, tanto pela sua contribuição ao meio ambiente como por ser um combustível derivado de fontes renováveis. Entretanto, este combustível vem ocasionando sérios problemas de corrosão metálica nos materiais usados em motores a diesel, sistemas de transporte e armazenamento. Nesse sentido, o presente trabalho tem por objetivo, avaliar o efeito corrosivo do biodiesel e misturas em alumínio por meio de técnicas eletroquímicas, como o potencial de circuito aberto e a espectroscopia de impedância eletroquímica. Para tal, foram utilizadas lâminas de alumínio imersas em amostras de biodiesel B100 e misturas B5 e B4, utilizou-se também amostras microemulsionadas para obtenção de melhores respostas nos ensaios eletroquímicos. A superfície do material metálico foi avaliada, após ensaios de imersão, por meio da visualização no microscópio ótico, como forma de identificar o efeito causado pela corrosão. Os resultados obtidos para os ensaios mostraram que o alumínio apresentou uma boa resistência à corrosão em contato com as amostras de combustível, bem como nas amostras microemulsionadas.

O presente trabalho estudou comparativamente, do ponto de vista fundamental, a reação de eletro-oxidação de monóxido de carbono (CO) em meio ácido, sobre Pt75Sn25/C, Pt65Ru35/C e Pt/C através da Espectroscopia de Impedância Eletroquímica. Os materiais foram preparados por redução com ácido fórmico e caracterizados fisicamente por EDX e DRX de alta intensidade e eletroquimicamente por voltametria. Previamente aos estudos de Impedância, foram realizados estudos potenciodinâmicos da reação de eletro-oxidação de CO adsorvido (?Stripping de CO?) e a oxidação de CO em uma solução saturada de CO. Para os materiais Pt65Ru35/C e Pt/C, estes estudos mostraram que há um deslocamento de potencial do inicio da oxidação para valores mais positivos quando CO está presente em solução se comparado a oxidação (stripping) de CO, devido a competição de sítios de adsorção entre moléculas de CO e H2O, responsáveis pela reação. Por outro lado, no material bimetálico Pt75Sn25/C a presença de CO em solução não influenciou de maneira significativa o potencial de inicio da oxidação, confirmando a ausência de adsorção competitiva neste material. A curva de polarização em estado estacionário revelou que, dentre os 3 materiais estudados, o material bimetálico Pt75Sn25/C apresentou a maior atividade eletrocatalítica, isto é, maiores densidades de correntes em menores sobrepotenciais. Este comportamento foi, em grande parte, esclarecido através dos experimentos de Espectroscopia de Impedância Eletroquímica. Com a Espectroscopia de Impedância Eletroquímica foi verificada a formação de espécies oxigenadas em Pt/C e em Pt65Ru35/C no intervalo de freqüências estudado, enquanto que em Pt75Sn25/C os diagramas de impedância revelaram um comportamento predominantemente capacitivo, indicando a ausência de reações de transferência de carga devido à oxidação da H2O para formar hidróxidos/óxidos na superfície do material. Também foi demonstrado que os processos relacionados a reação entre COads e espécies oxigenadas superficiais são mais rápidos em Pt75Sn25/C do que em Pt/C e Pt65Ru35/C, como ficou evidente no diagrama de Bode do ângulo de fase em função da freqüência. Mais especificamente, a menor resistência de transferência de carga para a reação em Pt75Sn25/C aparece em freqüências mais altas provavelmente devido à reação ocorrer com espécies oxigenadas já presentes na fase SnO2 presente no material. Por outro lado, os processos de adsorção são mais rápidos devido a menor energia de adsorção de CO sobre Pt, de acordo com os resultados encontrados por cálculos teóricos de DFT. / In this work, kinetic aspects of CO electro-oxidation on Pt/C, Pt75Sn25/C and Pt65Ru35/C prepared by chemical reduction with formic acid were studied by Electrochemical Impedance Spectroscopy (EIS). The bulk composition of the asprepared PtSn/C and PtRu/C bimetallic materials, analyzed by EDX, was 77:23 and 64:36, respectively. Using high intensity XDR measurements the bulk structure and phase determinations could be established. The analysis of the XRD profiles of Pt75Sn25/C revealed partial alloy formation and the presence of a SnO2 phase. For Pt65Ru35/C, no alloy formation was verified. Potentiodynamic oxidation of CO in a CO-saturated solution compared with CO stripping voltammetry pointed out the presence of competitive adsorption between CO and oxygen containing species on the surface of Pt/C and Pt65Ru35/C while for Pt75Sn25/C no competition for surface sites was verified. Impedance measurements were conducted in two different conditions: i) in the absence of CO in solution and ii) in a CO saturated solution. Provided by an equivalent circuit the charge transfer resistance (Rct), was obtained for the studied materials. The mean double layer capacitance (Cdl) was also estimated from circuit parameters and revealed its dependence on the carbon monoxide surface coverage (_CO). Furthermore, the relation between the _CO and the charge transfer resistance for the reaction is discussed. Using EIS it was demonstrated that the Rct for the carbon monoxide electro-oxidation reaction on Pt75Sn25/C is much lower than on Pt/C and Pt65Ru35/C, which was related to the presence of oxygen-containing species already present in the SnO2 phase as well as to the faster adsorption processes on this material compared to Pt/C and Pt65Ru35/C.

Orientador: Celia Marina de Alvarenga Freire / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia mecanica / Made available in DSpace on 2018-08-04T09:48:54Z (GMT). No. of bitstreams: 1 Santos_MarciaCristinaGoncalvesdos_M.pdf: 7174846 bytes, checksum: 71b57284d8926f02702b824e265c956a (MD5) Previous issue date: 2005 / Resumo: A excelente resistência do Zinco em diferentes ambientes corrosivos e sua habilidade em proteger o aço, fazem do aço galvanizado e eletrodepósitos, materiais comuns na indústria automotiva e de construção civil. Os substratos de aço galvanizado e eletrodepósitos de Zinco necessitam, no entanto, ser protegidos em ambientes altamente corrosivos por revestimentos orgânicos (pinturas).A má aderência do sistema Zinco/pintura faz com que a superfície do substrato de Zinco receba um pré-tratamento contendo Cromo relativamente barato e efetivo porém, tóxico ao homem e ao meio ambiente. Filmes poliméricos contendo silanos organofuncionais surgem como alternativa à substituição de pré-tratamento livre de Cromo hexavalente 'CR POT. +6¿, pois são moléculas que podem atuar como agente de acoplamento, promotores de adesão e como protetoras de superfície. A principal vantagem da sua utilização é a promoção da adesão entre materiais orgânicos (pinturas) e inorgânicos (metais). Os sistemas Galvanizado/Silano e Ligas eletrodepositadas/Silano, foram estudados nesta dissertação. Para tanto, técnicas como EIE (Espectroscopia de Impedância Eletroquímica), XPS (Espectrometria Fotoelétrica de Emissão de Raios-X), Ângulo de Contato e análises microscópicas foram utilizadas. A avaliação da resistência por barreira foi feita através da Espectroscopia de Impedância Eletroquímica em solução de 0,1 M de NaCI, comparando-se os desempenhos dos revestimentos. A cura induziu uma modificação importante nos diagramas de impedância, além de outras variáveis, como a concentração, o tempo de hidrólise, os tipos de silano e os substratos diferentes. Os resultados indicaram o Zn/Co revestido por VS e Zn/Fe revestido por 'gama¿-UPS como os sistemas mais promissores / Abstract: The Zinc' s excellent resistance, as used in differents corrosive environments, and its hability to protect steel, make galvanized steel (HDG) and Zinc electrodepositions very usual materiaIs in the automotive and construction industries. Galvanized steel and Zinc electrodepositions substrates need, however, to be protected from strongly corrosive environments by organic coatings (painting). Zinc/paint system weak adhesion results in a relatively cheap and effective zinc substrate pretreatment, however toxic to humam being, and also to the environment. Organofunctional silanes appear as an altemative to substitute this pretreatment, without making use of hexavalent chromium 'CR POT. +6¿), because they are bifuncional molecules that can act as adhesion promoters and surface protectors. The main benefit to employ them is to promote adhesion between organic (painting) and inorganic (metals) materiaIs. The GalvanizedlSilane and Electrodeposition Alloys/Silane systems had been studied in this essay. Techniques like EIS (Electrochemical Impedance Spectroscopy), XPS (X-Ray Emission Photoeletric Spectrometry), Contact Angle and microscopical analyses were used in this work. The barrier for resistance was evaluated through Electrochemical Impedance Spectroscopy in a 0,1M NaCI solution comparing coatings performances. Curing caused as importante modification in the impedance diagrams, as well as other variables like concentration, hydrolysis time, silane types and different substrates. The results indicate the Zn/Co alloy coated for VS and Zn/Fe alloy coated for 'gama¿-UPS as the systems most promising / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Mecânica

Magister Scientiae - MSc / Cancer, the second most common cause of death after heart disease, is a complex and multifactorial disease that up to date is still under extensive research. To achieve early detection of cancer disease the discovery of specific, sensitive and reliable biomarkers is required. Telomerase is a ribonucleo-protein complex that has been identified as an important target for cancer diagnostics and cancer therapy, because 85% of more than 950 primary tumours express telomerase activity. The standard method for the detection and quantification of telomerase activity is the polymerase chain reaction (PCR)-based assay known as the telomeric repeat amplification protocol (TRAP) assay. TRAP and other methods developed for telomerase detection have limitations for example its time consuming, requires complicated machinery, expensive equipment and reagents thus there is a need for a more sensitive, reliable and high-throughput method. Electrochemical biosensors are quickly emerging as an alternative for early detection of cancer because they can be designed to detect developing cancer biomarkers and to allow improved monitoring of cancer growth and patient therapy. This research study reported for the first time the successful fabrication and implementation of highly sensitive 3-mercaptopropionic acid indium telluride quantum dots (3MPA-In2Te3 QDs) based genosensor for detection of telomerase biomarker. The colloidal poly-dispersed 3MPA-In2Te3 QDs introduced into the genosensor system were successfully synthesized by a simple, inexpensive and reproducible aqueous method. The as prepared 3MPA-In2Te3 QDs was characterized by Ultraviolet Visible (UV-VIS) spectroscopy, Fluorescence (FL) spectroscopy, X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy and High Resolution Transmission/Scanning Electron Microscopy (HR TEM/SEM). Electro-analysis of 3MPA-In2Te3 QDs was done by Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). HR-TEM studies revealed formation of small sized QDs about 6 nm in diameter while UV-VIS studies showed presence of iv absorption peaks in the ultraviolet region (100-400 nm) which confirmed the formation of these small sized QDs. The good electrochemical, optical, physical and chemical properties of the 3MPA-In2Te3 QDs allowed them to be used as a mediating platform between deoxyribonucleic acid (DNA) and gold electrode (AuE). The successful detection of telomerase was achieved by hybridization process between the probe single stranded deoxyribonucleic acid (ssDNA) drop coated on the 3MPA-In2Te3 QDs/AuE surface and its complementary ssDNA in biological buffer solution (0.10 M tris-ethylenediamine tetraacetic acid (TE) buffer solution, pH 8.00). The response of the 3MPA-In2Te3 QDs based genosensor towards different concentration of complementary ssDNA was studied by CV, square wave voltammetry (SWV) and EIS. It was observed that all three analytical techniques exhibited good linearity since their linear correlation coefficients (R2) corresponded to 0.99. However, it was observed that EIS was the best technique for the detection of telomerase compared to both CV and SWV since it showed a higher sensitivity (2.44 Ω/nM) towards detecting telomerase with a detection limit as low as 0.00014 ng/mL. Control experiments were also carried out by monitoring the hybridization process in the presence and absence of complementary ssDNA and it was determined that the QDs based genosensor was highly selective towards complementary ssDNA. In view of the attractive analytical characteristics and advantages, the ultimate goal of the developed QDs based genosensor is to apply it in real clinical samples of cancer cells or bodily fluids of cancer patients for the detection of telomerase cancer biomarker.

The work presented on this thesis is focused on studies of the kinetics of electron transfer in bilayer lipid membranes (BLMs). Three different types of BLM were studied: i) tethered, ii) pore suspended (commonly known as ‘black’) and iii) based on the avidin – biotin interaction (these are part of the wider group of polymer cushioned BLMs). In order to produce tethered BLMs (tBLMs) of the best quality possible, self – assembled monolayers (SAMs) of a thiolipid (1,2 dipalmitoyl-sn-glycero-phosphothioethanol (DPPTE)) and of the same thiolipid mixed with L α phosphatidylcholine (EggPC) were characterised and their behaviour compared to that of SAMs of two alkanethiols (1 – heptanethiol and 1 – dodecanethiol). The SAMs that were formed by a mixture of lipids (DPPTE+EggPC) presented better kinetic parameters and were the chosen to produce tBLMs. Tethered BLMs were made by using the SAM described above as the lower leaflet; the second leaflet was deposited by vesicle fusion, the vesicles were made of EggPC. tBLMs are commonly used as model membranes, however in biophysical studies free-standing membranes or ‘black’ lipid membranes are more realistic models of cellular processes. The rates of electron transfer in both types of bilayer lipid membranes are compared. These BLMs were modified using two very important mitochondrial membrane associated molecules – ubiquinone-10 (UQ10) and α-tocopherol (VitE). The studies involved the use three redox couples, Fe(CN)_6^(3-/4-), Ru(NH_3 )_6^(3+/2+) and NAD+/NADH using cyclic voltammetry and electrochemical impedance spectroscopy. The NAD+/NADH couple is of particular interest as it is the key to several important biochemical processes. The last type of BLM that was studied was the BLMs based on the avidin – biotin interaction. Avidin was deposited on a platinum surface by electrodeposition and then vesicles composed of EggPC and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(biotinyl) (sodium salt) (DOPE(B)) are burst by applying +0.7V (vs. Ag/AgCl, KCl 3.5M), leading to the formation of a supported BLM. The vesicles used had methylene blue (MB) inside; its release, when the vesicles burst, was monitored by cyclic voltammetry and UV-Vis. The kinetic parameters were determined based on the EIS measurements using Fe(CN)_6^(3-/4-) and Ru(NH_3 )_6^(3+/2+) as redox couples.

The choice of corrosion protection system depends on the environment and needed lifetime for the product. The right corrosion protection should be selected in a sustainable point of view, since a well-selected coating system can reduce the environmental and economical impact, by using less and better material. The systems used for classifying corrosion protection often give a passed/not passed result for the number of years it is expected to last in a specific corrosive environment. In the last decades, Electrochemical Impedance Spectroscopy (EIS) has become a popular method for evaluating corrosion protection for organic coatings. EIS can collect quantitative data by monitoring the coatings electrochemical behavior over time, which can be used for optimizing the coating system. The purpose of this thesis was to try to predict how different combinations of coating layers and substrates will perform as a corrosion protection, which could provide information that can optimize the coating process. In this thesis, EIS has been used as a test method to evaluate organic coating systems for corrosion protection, by looking at barrier properties and adhesion for powder coatings on aluminum substrates. The main part of the coatings were applied in the coating plant at Fagerhult AB, but an external supplier has been used as a reference. The powders used in the coating process were based on polyester resins and the substrates were different aluminum alloys. The EIS measurements were performed in the chemistry lab at the School of Engineering at Jönköping University and depending on the sample setup was each sample evaluated for two or four weeks of testing. Two groups of samples had intact coatings and a third group had samples with an applied defect in the coating. The analysis of sample setups with intact coatings showed that the topcoat absorbed water faster than the primer. The samples showed no significant degradation in corrosion protection for the evaluated period and could thereby not provide enough information to be able to conclude which setup give the best corrosion protection over time. The samples with a defect in the coating indicated that two of the substrates provided similar adhesion in the coating-substrate interface. The coating from the external supplier was also included in the test and it showed the best adhesion of the tested samples. The main conclusion is that the coating system used at Fagerhult AB provides a very good corrosion protection. Longer testing time with EIS measurements on intact coatings is needed to be able to rank the different sample setups by failure of corrosion protection.

>Magister Scientiae - MSc / In the wide search for advanced materials for next generation lithium-ion batteries, lithium manganese orthosilicate, Li₂MnSiO₄ is increasingly gaining attention as a potential cathode material by virtue of its ability to facilitate the extraction of two lithium ions per formula unit, resulting in a two-electron redox process involving Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ redox couples. This property confers on it, a higher theoretical specific capacity of 333 mAhg⁻¹ which is superior to the conventional layered LiCoO₂ at 274 mAhg⁻¹ and the commercially available olivine LiFePO₄ at 170 mAhg⁻¹. Its iron analogue, Li₂FeSiO₄ has only 166 mAhg⁻¹ capacity as the Fe⁴⁺ oxidation state is difficult to access. However, the capacity of Li₂MnSiO₄ is not fully exploited in practical galvanostatic charge-discharge tests due to the instability of the delithiated material which causes excessive polarization during cycling and its low intrinsic electronic conductivity. By reducing the particle size, the electrochemical performance of this material can be enhanced since it increases the surface contact between the electrode and electrolyte and further reduces the diffusion pathway of lithium ions. In this study, a versatile hydrothermal synthetic pathway was employed to produce nanoparticles of Li₂MnSiO₄, by carefully tuning the reaction temperature and the concentration of the metal precursors. The nanostructured cathode material was further coated with a thin film of aluminium oxide in order to modify its structural and electronic properties. The synthesized materials were characterized by microscopic (HRSEM and HRTEM), spectroscopic (FTIR, XRD, SS-NMR, XPS) and electrochemical techniques (CV, SWV and EIS). Microscopic techniques revealed spherical morphologies with particle sizes in the range of 21-90 nm. Elemental distribution maps obtained from HRSEM for the novel cathode material showed an even distribution of elements which will facilitate the removal/insertion of Li-ions and electrons out/into the cathode material. Spectroscopic results (FTIR) revealed the vibration of the Si-Mn-O linkage, ascertaining the complete insertion of Mn ions into the SiO₄⁴⁻ tetrahedra. XRD and ⁷Li MAS NMR studies confirmed a Pmn21 orthorhombic crystal pattern for the pristine Li₂MnSiO₄ and novel Li₂MnSiO₄/Al₂O₃ which is reported to provide the simplest migratory pathway for Li-ions due to the high symmetrical equivalence of all Li sites in the unit cell, thus leading to high electrochemical reversibility and an enhancement in the overall performance of the cathode materials. The divalent state of manganese present in Li₂Mn²⁺SiO₄ was confirmed by XPS surface analysis. Scan rate studies performed on the novel cathode material showed a quasi-reversible electron transfer process. The novel cathode material demonstrated superior electrochemical performance over the pristine material. Charge/discharge capacity values calculated from the cyclic voltammograms of the novel and pristine cathode materials showed a higher charge and discharge capacity of 209 mAh/g and 107 mAh/g for the novel cathode material compared to 159 mAh/g and 68 mAh/g for the pristine material. The diffusion coefficient was one order of magnitude higher for the novel cathode material (3.06 x10⁻⁶ cm2s⁻¹) than that of the pristine material (6.79 x 10⁻⁷ cm2s⁻¹), with a charge transfer resistance of 1389 Ω and time constant (τ) of 1414.4 s rad⁻¹ for the novel cathode material compared to 1549 Ω and 1584.4 s rad-1 for the pristine material. The higher electrochemical performance of the novel Li₂MnSiO₄/All₂O₃ cathode material over the pristine Li₂MnSiO₄ material can be attributed to the alumina nanoparticle surface coating which considerably reduced the structural instability intrinsic to the pristine Li₂MnSiO₄ cathode material and improved the charge transfer kinetics.

Previous research indicates compositional variation of pentlandite [(Fe,Ni)9S8] and the effect this variation may have on the electrochemical behaviour of pentlandite is poorly understood. Pentlandite is the primary source of nickel and an important base metal sulfide (BMS) in the platinum industry. It hosts significant amounts of PGEs especially palladium and rhodium when compared to chalcopyrite and pyrrhotite. The aim of the project was to investigate the possible compositional variations of natural pentlandite and the effect of these variations on the electrochemical behavior thereof. To study possible compositional variations, single pentlandite particles - in the order of 100μm in size from flotation concentrates (PGM deposits) and massive samples (massive ore bodies) - from various sources were employed. Electron microprobe analysis indicated a compositional variation of the pentlandite particles hand-picked from the flotation concentrate samples. Variation was observed in the cobalt, iron and nickel content and this was independent of the deposit. A slight compositional variation was observed from the massive pentlandite samples. The effect the compositional variation may have on the electrochemical reactivity of pentlandite was investigated using electrochemical techniques, i.e. measurement of the polarisation resistance and mixed potential as well as performing linear anodic voltammetry, current density–transients and electrochemical impedance spectroscopy (i.e. capacitance). Poor electrochemical response of the pentlandite microelectrodes was observed. Pre–existing pores, deep pores, cracks and the brittle nature of pentlandite microelectrodes may have contributed to the poor electrochemical response of natural pentlandite particles hand-picked from the flotation concentrate. Slight compositional variations of the massive pentlandite sample influenced the electrochemical behaviour. In aerated solutions, iron enriched pentlandites were less reactive after progressive oxidation. The lower reactivity of the electrodes was a result of thick oxide films formed. This was illustrated by polarisation resistance and capacitance measurements. The lower reactivity of the electrodes was also related to the mechanism of the reduction of oxygen at oxidised passive electrode surfaces. It is however difficult to distinguish if the differences in the reactivity was a result of the Fe/Ni ratio or the influence of cobalt. Current density transients confirmed that the reactivity of a pentlandite electrode to be time dependent. The reactivity of the electrode decreased during oxidation. A variation in the electronic properties of the formed oxide film was observed. Slight compositional variation of pentlandite did not have a significant effect on the rest potential values as do changes in the type of sulfides (e.g. pyrite vs. pentlandite). This was confirmed by similar rest potential values of various pentlandite electrodes. The oxidation of synthetic pentlandite may be influenced by the chemical composition. In de-aerated solutions, anodic oxidation (as indicated by the linear anodic voltammogram) of synthetic pentlandite started at a potential lower than of the natural electrodes. In aerated solutions, the synthetic pentlandite was less reactive and formed thicker oxide films. Copyright / Dissertation (MEng)--University of Pretoria, 2010. / Materials Science and Metallurgical Engineering / unrestricted