Daniela C.L. Vasconcelos

Corrosion resistance of stainless steel coated with sol-gel silica

Abstract Sol–gel silica films were deposited on AISI 304 stainless steel using tetraethyl ortosilicate (TEOS) as chemical precursor. The coated steels were heat-treated from room temperature to 600°C. The structural evaluation of the coated steel samples was made by scanning electron microscopy, infrared absorption spectroscopy, and chemical deep profiles of Si, O, and Fe obtained by Rutherford backscattering spectroscopy. Corrosion experiments were carried out on samples with and without coatings, in 1 N H2SO4 and 3.5% NaCl solutions to measure corrosion potential, current density and pitting potential. The obtained sol–gel silica coatings were homogeneous, free of cracks and had thickness from 50 to 120 nm. Polarization curves of the coated samples indicated improvement of the corrosion resistance. For tests carried out in a 1 N H2SO4 solution, the corrosion potential increased from −341 mV (stainless steel) to +383 mV for coated sample heat-treated at 80°C, and up to +508 mV for coated sample heat-treated at 200°C. The corrosion current density decreased from 3.65 μA / cm 2 to 0.027 μA / cm 2 and 0.021 μA / cm 2 , for the 80°C and 200°C heat-treated silica coatings, respectively. For the corrosion experiment carried out in a 3.5% NaCl solution, the coated sample (heat-treated at 80°C) increased the corrosion potential c.a. 230 mV and increased by one order of magnitude the pitting potential.

Sol-Gel SiO2-CaO-P2O5 biofilm with surface engineered for medical application

Sol-gel film in the SiO2-CaO-P2O5 system was prepared from TEOS, TEP, alcohol and hydrated calcium nitrate in an acidic medium. The coatings were deposited on stainless steel using the dip-coating technique. After deposition, the composite was submitted to heat treatment, at different temperatures and exposure times to investigate the influence of such parameters on the surface morphology of the composite. The coated surfaces were characterized by AFM, SEM and FTIR. The present study showed that the formation of different textures (an important parameter in implant fixation) could be controlled by temperature and time of heat treatment.

Influence of process parameters on the morphological evolution and fractal dimension of sol–gel colloidal silica particles

Abstract Colloidal silica particles were synthesized from tetraethoxysilane (TEOS), ammonium hydroxide (NH 4 OH), ethanol (C 2 H 5 OH) and deionized water. Obtaining different structures was possible due to the variation of molar ratios of NH 4 OH, C 2 H 5 OH and H 2 O with respect to the precursor. The nature of the particles was evaluated using X-ray diffraction, energy dispersive spectroscopy (EDS) and Fourier transform infrared spectroscopy (FTIR). The morphology of the particles was described using electron microscopy (SEM). Pore structure was characterized by nitrogen gas sorption-desorption, showing colloidal silica particles with solid-pores surface areas varying from 0.48 m 2 g −1 up to 567 m 2 g −1 ; pore volume varying from 0.0013 up to 0.88 cm 3 g −1 and average pore radii varying between 1.4 and 36 nm. The fractal dimension of the silica particles and agglomerates varied from 2.0 to 2.12.

Propriedades mecânicas de materiais compósitos à base de cimento Portland e resina epoxi

The study of multi-functional materials of high performance, as the polymeric-cementitious composites, has been the focus of several researches in the industry of the civil engineering. This work investigates the effect of the combination of a thermorigid epoxy phase and the white Portland cement, followed by the evaluation of its compressive strength and modulus of elasticity. This composite, when the phases are individually compared, provides an increase of the compressive strength, a reduction of the density, and a significant change of the mechanical behaviour. The changes in mechanical behaviour are associated with the hydration of cement in the presence of resin, which was evident after infrared spectroscopy analysis.

Microstructural characterization of magnesias derived from different sources and their influence on the structure of ceramic films formed on a 3% silicon steel surface

A study on the influence of the type of magnesium oxide on the structure of the ceramic coating and consequently on the magnetic properties of a 3% Si steel was carried out. Decarburized samples coming from industrial equipment were coated with magnesia slurries and submitted to final annealing at 1200°C for 15 hours under H2 atmosphere. We used magnesia synthesized from magnesium chloride and dolomite. The magnesia samples were characterized using granulometry measurements, scanning electron microscopy (SEM), nitrogen adsorption-desorption analyses, helium pycnometry, thermal analyses (TG and DTA), reactivity tests and calcination mass loss. The magnesia sintered from brine has the highest density, surface area and porosity. The rheological behavior of magnesia slurries, the magnetic properties of the steel and the ceramic film structure were correlated with the type of magnesia used. The slurry made with MgO from dolomite has the highest viscosity and the slurry made with oxide from brine the smallest one. Brine magnesia films are darker, thicker and rougher and sea water magnesia films are light gray, thinner and smoother.

Spectroscopic studies of spessartine from Brazilian pegmatites

Abstract Mössbauer spectra (MS) at room temperature (RT) and Fourier-transform infrared (FTIR) spectra in the OH stretching region were acquired for natural spessartine-almandine garnet samples from different Brazilian pegmatites, including the complex, zoned Alto Mirador pegmatite, the simple, zoned Escondido pegmatite, and the simple Poaiá pegmatite. From MS, it is obvious that in the samples from the Alto Mirador pegmatite, ferrous iron is present at the dodecahedral site and ferric iron at the octahedral site of the garnet structure. The Fe2+/Fe3+ ratio is comparable for all samples from this pegmatite, i.e., ~10%, implying the same geological history, namely similar oxygen fugacities at the moment of garnet formation. In the simple pegmatites, however, almost no ferric iron was detected. On the basis of the multi-band FTIR spectra the nature of the point defects involved in the process of hydrogen uptake in spessartine-almandine garnet has been elucidated, if not completely clarified. The behavior of the absorption bands provides no evidence for multiple incorporation mechanisms, hence the hydrogarnet substitution is proposed to be the only mechanism for the incorporation of hydrogen in the samples studied. As geological setting is difficult to separate from chemical composition, FTIR spectra have the potential to provide useful information on some aspects of the geological history of the samples.

Preparação e caracterização estrutural de materiais cerâmicos obtidos via processo freeze-casting

Cerâmicas com porosidade controlada apresentam elevado potencial de aplicacao como membranas para separacao gasosa e processos de dessalinizacao. As propriedades e o desempenho desses materiais estao diretamente relacionados ao seu processo de fabricacao. Assim, e de vital importância que as variaveis do processo utilizado sejam conhecidas e controladas. Neste trabalho, cerâmicas porosas a base de alumina foram obtidas por meio do processo freeze-casting. Foram utilizados terc-butanol como solvente e acido citrico como dispersante. As amostras foram fabricadas a partir de suspensoes com concentracoes de alumina variando entre 15 e 30% em volume. Elas apresentaram reducao linear da porosidade total (entre 48 e 73%) e porosidade fechada inferior a 2%. A resistencia a flexao desses materiais variou entre 3,6 ± 0,7 e 21,9 ± 0,6 MPa. Ensaios de microscopia eletronica de varredura e microtomografia de raios X revelaram que as amostras apresentaram elevada conectividade de poros e orientacao preferencial de poros. As alteracoes nas condicoes de processamento resultaram em materiais com estruturas porosas distintas, demonstrando a versatilidade do processo freeze-casting.

Influence of MgO containing strontium on the structure of ceramic film formed on grain oriented silicon steel surface

The oxide layer formed on the surface of a grain oriented silicon steel was characterized by SEM and EDS. 3% Si steel substrates were coated by two types of slurries: one formed by MgO and water and other formed by MgO, water and SrSO4. The ceramic films were evaluated by SEM, EDS and X-ray diffraction. Depth profiles of Fe, Si and Mg were obtained by GDS. The magnetic core losses (at 1.7 Tesla, 60 Hz) of the coated steel samples were evaluated as well. The use of MgO containing strontium reduced the volume fraction of forsterite particles beneath the outermost ceramic layer. It was observed a reduced magnetic core loss with the use of the slurry with MgO containing strontium.

Evolução estrutural do filme cerâmico de forsterita obtido sobre o aço silício de grão orientado

In this work the formation on grain oriented silicon steel of a ceramic film obtained using MgO and TiO2 and SrSO4 additives is described . The material was heat treated at 900 0C, 1000 0C and 1200 0C and for 15 hours at 1200 0C. The coating was evaluated using scanning electron microscopy (SEM), electron microprobe (EDS), X-ray diffraction and glow discharge spectroscopy (GDS). The observed morphological evolution occurs with the formation of a continuous oxide layer on the outermost surface of the sheet. As the temperature is raised, the outermost surface layer becomes thicker while the particles formed underneath becomes richer in Mg. Beyond 1000 0C the formation of crystalline forsterite on the surface of the sheet was observed. The penetration depth of Mg in the material increases as the heat treatment temperature is raised, going from 75 nm at 900 0C to approximately 1.9 mm at 1200 0C.