Avaliação de indicadores de uso diverso como inibidores de corrosão

Very often hydrochloric acid is employed in acidification operations aiming to dissolve the mineral matrix in petroleum wheel operations, which always require intense use of corrosion inhibitors. This work presents an evaluation of common indicators, phenolfthaleine, fluorescein, methylene blue, alizarine S and methyl orange, as corrosion inhibitors for carbon steel in HCl 15% w/v at temperatures of 26, 40 and 60 ºC. Fluorescein and methyl orange show excelent corrosion inhibition efficiencies at 26 ºC; however at 60 ºC only fluorescein shows good corrosion inhibition when employed with alcohol and/or formaldehyde. For the fluorescein 1% w/v + formaldehyde 0.6% w/v mixture we present polarization and impedance curves and adsorption isotherms.

Estudo eletroquímico de um novo banho galvânico de zinco alcalino livre de cianetos

A systematic electrochemical study of a novel cyanide free galvanic plating solution of alkaline zinc is presented. Cell Hull tests and potentiodynamic linear polarization measurements were carried out to evaluate the influence of operational parameters, such as concentration, current efficiency and degradation of additives. The stability of the bath was assessed by a long term test during 40 h at 200 A m-2. The morphology of the coatings was characterized by scanning electron microscopy. The results showed good performance of the plating solution. The cost is similar to that of the commercial cyanide bath and lower than that of the commercial cyanide free bath.

Modelos contínuos do solvente: fundamentos

Continuum solvation models are nowadays widely used in the modeling of solvent effects and the range of applications goes from the calculation of partition coefficients to chemical reactions in solution. The present work presents a detailed explanation of the physical foundations of continuum models. We discuss the polarization of a dielectric and its representation through the volume and surface polarization charges. The Poisson equation for a dielectric was obtained and we have also derived and discuss the apparent surface charge method and its application for free energy of solvation calculations.

Estudo comparativo da eficiência da difenilcarbazida e do óleo de coco saponificado microemulsionados na inibição da corrosão de aço carbono

The effectiveness of microemulsions (ME) of saponified coconut oil (OCS-ME) and diphenylcarbazide (DC-ME) on a carbon steel corrosion inhibition process was evaluated using an electrochemical method of polarization resistance. The ME was prepared with OCS, butanol, kerosene and saline solutions. OCS-ME and DC-ME showed highest inhibitions effects (77% and 92%, respectively) at lower concentrations (0.5% and 0.48 - 0.50%, respectively). The surfactant OCS (in H2O) showed lower efficiency (63% at 0.20 - 0.25% concentration). The greatest inhibitory effect of DC-ME could be correlated with the chemical structure and the rich O/W ME system, which are very important for adsorption phenomena in interfacial ME systems.

Construccion y utilizacion de un electrodializador en serie para laboratorio

There are different concepts in electrochemistry that must be supported by a laboratory practice to better understand their importance in industrial applications. This work reports the procedure for manufacturing an electrodialyzer in series of two or more compartments with easy manipulation. Likewise, it reports the results obtained by using this system in the nitrate elimination from synthetic NaNO3 solutions at different concentrations. Among other results obtained in the electrodialysis of these solutions, the one that stands out is the demonstration of the linear relationship between the limiting current, obtained from the polarization curves, and nitrate concentration. Additionally, implementation of the separation treatment shows that the time of elimination of practically all ionic species depends on their concentration in solution and ranges between 50 and 80 min.

Tribocorrosão da liga TI6AL4V em solução salina tamponada com fosfato

The tribocorrosion behavior of Ti6Al4V alloy was investigated in a Phosphate Buffered Saline (PBS) solution by a reciprocating wear, using alumina ball as the counterface material, at different normal forces and sliding velocities. Dry wear experiments were performed in order to compare with the tribocorrosion experiments at open circuit potential and under anodic polarization. Dry wear induced a superior damage on the counterface, forming larger and shallower wear tracks compared with those experiments performed in PBS solution. The anodic current was increased by wear; however the volume of oxidized metal in tribocorrosion experiments correspond to a relative low percentage of the wear track volume.

Análise cristalográfica da solução sólida com estrutura tipo Tungstênio Bronze de niobato de potássio e estrôncio dopado com ferro

Solid solution of iron doped potassium strontium niobate with KSr2(FeNb4)O15-Δ stoichiometry was prepared by high efficiency ball milling method. Structural characterization was carried out by X-ray diffraction. Crystalline structure was analyzed by the Rietveld refinements using the FullProf software. The results showed a tetragonal system with the tetragonal tungsten bronze structure - TTB (a = 12.4631 (2) Å and c = 3.9322 (6) Å, V = 610.78 (2) ų). In this work, the sites occupancy by the K+, Sr2+ and Fe3+ cations on the TTB structure were determined. NbO6 polihedra distortion and its correlation with the theoretical polarization are discussed.

Utilização de um espectrógrafo de projeção como uma ferramenta para demonstrações sobre polarização da luz

This article describes the use of a projection spectrograph based on an overhead projector for use in classroom demonstrations on light polarization and optical activity. A simple adaptation on a previously developed apparatus allows illustrating several aspects of optical activity, such as circular and linear birefringence, including their wavelength dependence. Specifically, we use the projection spectrograph to demonstrate the optical activity of an aqueous solution of sugar (circular birefringence), of a quartz plate and of an overhead projector transparence film (linear birefringence). A historical survey about the optical activity discovery and the main principles involved is also presented.

Determinação da densidade de corrente de corrosão em meios de baixa condutividade: uso de microeletrodos para minimizar a queda ôhmica

The gravimetric and electrochemical tests are the most common techniques used in determining the corrosion rate. However, the use of electrochemical polarization is limited to electrolytes with sufficient conductivity for which Tafel curves are linear. In this study, we investigated a technique in which working microelectrodes of AISI 1020 steel were used to obtain the Tafel curves in diesel oil. The strategy was to reduce the electrode area and hence the ohmic drop. The diameter of the microelectrode was reduced to a value where the compensation of the Tafel curves became unnecessary. The results showed that for electrodes with diameters below 50 μm, the ohmic drop tends to a minimum and independent of the microelectrode diameter.

Construção de uma célula-autoclave para medidas eletroquímicas a altas temperaturas

Measurements at high temperature using liquid solutions require special cells and materials which are able to support the temperature and pressure developed inside. The constructed cell was designed to support pressures up to 20 bar, temperatures relatively high up to around 200 ºC, depending on the pressure developed inside the system. It also supports aggressive solutions since its inner wall is made of Teflon. The electrolyte has no contact with the metallic body of the cell. Then, it is supposed that this work represents a great contribution to the electrochemical studies of materials in solutions at high pressure and temperature

Avaliação da resistência à corrosão de implantes em aços inoxidáveis ISO 5832-9 e F138 removidos de pacientes

This work has compared the surfaces of two different steel samples used as orthopedical implants, classified as ASTM F138 and ISO5832-9, through optical emission spectroscopy, by means of SEM and EDS. The samples (implants) were also submitted to potentiodynamic cyclic polarization in Ringer lactate and NaCl 0.9 M L-1 solutions; ISO5832-9 sample did not show any kind of localized corrosion, but in the case of F138 steel was observed a pit localized corrosion in both solutions. In Ringer lactate solution it was observed a loss of about 63% for nickel and 26% for iron for F138 stell, compared to the initial composition.

Degradação fotoeletroquímica do herbicida bentazona sobre eletrodos de carbono modificados por TiO2

This work involved the study of degradation of the herbicide bentazone in aqueous solution by different routes, in order to search a method that generates safe products to the environment. It was tested electrochemical polarization methods involving positive and negative potential, irradiation with UV light and deposition of TiO2 on the electrode surface, seeking a catalytic effect. After different times of degradation, aliquots were removed and the scan of molecular absorption spectrum of UV-Vis was performed. From the spectra decay of bentazone, the kinetics of different processes was accompanied and the rate constants were determined.

SAPT: ligação de hidrogênio ou interação de van der Waals?

It is through the application of an electronic partition approach called Symmetry-Adapted Perturbation Theory (SAPT) that the nature of hydrogen bonds and van der Waals interactions can be unveiled according to the contribution of electrostatic, charge transfer, exchange repulsion, polarization, and dispersion terms. Among these, electrostatic partition governs the formation of the hydrogen bonds, whose energies are arguably high. However, the weakness of the interaction strength is caused by dispersion forces, whose contribution decisively lead to the stabilization of complexes formed via van der Waals interactions.

Theoretical investigations of the bulk modulus in the tetra-cubic transition of PbTiO3 material

Resulting from ion displacement in a solid under pressure, piezoelectricity is an electrical polarization that can be observed in perovskite-type electronic ceramics, such as PbTiO3, which present cubic and tetragonal symmetries at different pressures. The transition between these crystalline phases is determined theoretically through the bulk modulus from the relationship between material energy and volume. However, the change in the material molecular structure is responsible for the piezoelectric effect. In this study, density functional theory calculations using the Becke 3-Parameter-Lee-Yang-Parr hybrid functional were employed to investigate the structure and properties associated with the transition state of the tetragonal-cubic phase change in PbTiO3 material.

AVALIAÇÃO DE PROPRIEDADES FÍSICAS E QUÍMICAS DE ELETRODEPÓSITOS DE Ni-W-P OBTIDOS EM CÉLULA DE HULL

Ni–W–P electrodeposits were synthesized in a Hull cell in order to simulate the obtainment under industrial conditions. Complete coverage of panels was accomplished by applying total currents of 1.0 and 2.0 A. Panels obtained with a current of 1.0 A appeared brighter. The best compositional uniformities, as determined by Energy Dispersive Spectrometer (EDS) occurred in the current density ranges of 0.6 to 3.0 A dm−2 and 1.6 to 6.0 A dm−2 obtained with 1.0 and 2.0 A, respectively. However, the best morphological characteristics, as determined by Scanning Electro Microscope (SEM), were observed in those obtained with a total current of 1.0 A. Analysis of corrosion resistance by Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Linear Polarization (PLP) in NaCl have shown significant variations in the amount of corrosion potential, polarization resistance, and even total impedance. The alloys exhibited amorphous character (XRD) and crystallized above 400 °C to Ni and Ni3P phases, and possibly Ni–W, with a subsequent increase in hardness. The results suggest that under industrial conditions, current density variations due to the large and complex geometric shapes of substrates lead to formation of distinct alloys. Furthermore, these materials are potential substitutes for chromium deposits in many applications.