Magnesium and its alloys have many interesting properties and thanks to them it can be used in many applications (transport industry, medicine etc.). Disadvantage of these materials is their high corrosion rate. For this reason, there is an effort to achieve high corrosion resistance through different modifications of magnesium and its alloys. In recent years the superhydrophobization of the surface seem to be an attractive solution for this question. This type surface modification minimalize contact between the surface and water. In this diploma thesis the superhydrophobic surface was created on the magnesium alloy AZ91. The first step included pretreatment of AZ91 surface by etching in solution of SnCl2 or ZnCl2. Next step was superhydrophobization in the ethanolic solution of stearic acid. The surface morphology and elemental analysis of the superhydrophobic coating were explored by use of scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The adhesion properties of the coating on the AZ91 were analysed by means of scratching test. Contact and sliding angles were measured for superhydrophobic coatings. Electrochemical characterization of the coatings was determined using potentiodynamic polarization (PD) and electrochemical impedance spectroscopy (EIS). Finally, the analysis of composition and the functional groups was made using Fourier-transform infrared spectroscopy (FTIR) and the phase composition analysis was performed using X-ray diffraction (XRD). The results show that the coatings prepared by etching did not lead to good corrosion properties, even though the value of contact angle was about 150 °. The reduction of corrosion resistance could be caused by not obtaining required surface morphology or insufficient binding of stearic acid in the form of stearate to the sample surface.

This thesis deals with the influence of grain refinement by ECAP on fatigue properties of magnesium alloy AZ 91. Tensile and fatigue tests were made on the as-cast state samples and samples after ECAP process. Metallographic analysis of the microstructure and fractographic analysis of the fracture surfaces was performed.

Orientador: Prof. Dr. Jorge Tomioka / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Engenharia Elétrica, 2014. / No contexto energético, as mudanças climáticas e as crescentes crises energéticas aumentaram as preocupações e as pesquisas na área de acumuladores de energia, visando alcançar melhorias no cenário energético e ambiental global. Ao mesmo tempo, a sociedade tornou-se dependente do consumo de energia elétrica, sendo que falhas no fornecimento não são permitidas. Portanto, observa-se a necessidade da energia ofertada ser de boa qualidade e, principalmente, não sofrer interrupções. Atrelado a esses fatores, a bateria de Magnésio ¿ Ar passou a ser considerada uma fonte promissora de energia elétrica, principalmente pela abundância de Magnésio (sétimo elemento mais presente no planeta), por não poluir o meio ambiente, ser barata e possuir energia especifica elevada. Essas características transformam esse acumulador em um potencial substituto para outras tecnologias. No entanto, a bateria demagnésio possui desvantagens que precisam ser vencidas por meio de pesquisas para alavancar o seu uso comercial massivo. Sob essa perspectiva, neste trabalho propõe-se o estudo da liga de magnésio AZ91 para aplicação em baterias de Magnésio ¿ Ar, por meio de ensaios de porosidade e de dureza, como também análise microestrutural e mensuração do potencial de circuito aberto e das curvas de polarização potenciodinâmica, utilizando-se uma célula eletroquímica de três eletrodos e um potenciostato em diferentes tempos de imersão da liga AZ91 na solução eletrolítica de NaCl 0,1 M. A porosidade da peça, observada no microscópio óptico com ampliação de 50 vezes, revelou que em cada fase de solidificação existem poros de tamanhos diferentes, com distribuição específica. Conforme o processo de resfriamento ocorre, nota-se a presença de poros maiores. Relacionado à porosidade, a dureza apresentou valores maiores na fase inicial de solidificação, onde a porosidade é menor. Por meio da análise microestrutural, foi possível verificar a presença de dendritas, com uma complexa dispersão de segunda fase na liga (alumínio), porosidades e inclusões. As medidas de potencial de circuito aberto indicam uma tendência de resistência à corrosão em temposmaiores de contato da amostra com a solução de NaCl. Não foi possível observar passivação aparente na amostra, nem pontos de corrosão localizada (pite) nas curvas de polarização potenciodinâmica. As variações no potencial de corrosão apontaram uma maior resistência à corrosão em tempos de imersão maiores; contudo, os valores estáveis de densidade de corrente de corrosão demonstram que a velocidade de corrosão permaneceu constante, sendo que a camada de óxido formada não contribuiu na proteção do material. Para a aplicação em baterias de magnésio, esses resultados conferem uma limitação da liga AZ91, pois a camada de óxido formada pode prejudicar o funcionamento do dispositivo em processos de descarga intermitente. As principais formas de controlar a eficiência da bateria de Magnésio ¿ Ar são por meio da composição da liga, da oxigenação, do pH, da temperatura e da concentração de sais NaCl no eletrólito. / In the energy sector, weather changes and rising energy crisis raised concerns and research in energy accumulators to achieve improvements in overall environmental and energy scenario. At the same time, society has become dependent on electricity consumption, so failures in supply are not tolerated. Therefore, the energy needs to have a good quality, and not suffer interruptions. Coupled to these factors, the battery Magnesium ¿ Air has been considered a promising source of electricity, mainly by the abundance of magnesium (seventh-most element present on the planet), not to pollute the environment, be cheap and have high specific energy. These characteristics transform this accumulator in a potential substitute for other technologies. However, the magnesium battery has disadvantages that need to be overcome through research to leverage their massive commercial use. From this point of view, this study proposes the characterization of magnesium alloy AZ91 for application in batteries Magnesium ¿ Air, testing the porosity and hardness, as well as microstructural analysis, and the measurement of the open potential circuit and the potentiodynamic polarization, in various immersion times using an electrochemical cell with three electrodes. The porosity of the part, observed in the optical microscope, revealed that at each stage of solidification, the pores have different sizes and distribution. As the cooling process occurs, could observe the presence of larger pores. Associated to the porosity, hardness values were higher in the initial phase of solidification, where the porosity was less. Through microstructural analysis was verified the presence of dendrites, with a complex dispersion of second phase in the alloy (aluminum), porosity and inclusions. The open potential circuit indicated a tendency for corrosion resistance in greater immersion time in the solution of NaCl. In all cases, the potentiodynamic polarization curves did not exhibit apparent passivity, or points of localized corrosion (pitting). The changes in corrosion potential showed greater resistance to corrosion in immersion times larger, however, the steady state values of corrosion current density demonstrated that the corrosion rate remained constant and the oxide layer formed did not protect the material. For use in magnesium batteries, these results provided a limitation of the AZ91 alloy, because the oxide layer can disturb the functioning of the device in cases of intermittent discharge. The main ways to control the battery efficiency Magnesium ¿ Air are through the alloy composition, oxygenation, pH, temperature and salt concentration of NaCl in the electrolyte.