Simulación de procesos químicos mediante algoritmos evolutivos: aplicación al ajuste de parámetros de impedancia en sistemas electroquímicos

La Espectroscopía de Impedancia Electroquímica (EIS, desus siglas en inglés) es una potente herramienta para elanálisis de sistemas metal-recubrimiento orgánico ya queproporciona información acerca del comportamiento delsistema estudiado tanto a nivel cualitativo como cuantitativo.A través del ajuste de los datos de impedancia obtenidosexperimentalmente a un circuito eléctrico equivalenteadecuado, se puede obtener información directamente relacionadacon la resistencia y durabilidad del recubrimientoanalizado. Para el análisis paramétrico de estos datosse han venido empleando tradicionalmente algoritmos deregresión no lineal que presentan el inconveniente de quepara una correcta estimación se han de proporcionar parámetrosde partida que favorezcan una rápida y buenaconvergencia en este proceso de ajuste. Como métodoalternativo a estos algoritmos, en este artículo se planteael uso de algoritmos evolutivos (más concretamente algoritmosde evolución diferencial), como estrategia para labúsqueda de los parámetros del circuito que permitan realizareste ajuste. Para testear y validar el procedimiento quese propone, se emplea software comercial de uso habitual(ZSimpWin). Los resultados obtenidos muestran un mejorgrado de ajuste en la totalidad de los casos estudiados

Resolution of the apparent experimental discrepancies observed between SVET and SECM for the characterization of galvanic corrosion reactions

A new approach for studying the local distribution of anodic and cathodic sites in a corroding system using the scanning vibrating electrode technique (SVET) and scanning electrochemical microscopy (SECM) is presented. When zinc is coupled to iron, dissolution of the active metal occurs in a localized manner (e.g. pitting corrosion), allowing for cathodic sites to be developed on zinc. Local alkalization and oxygen consumption related to cathodic activity are detected above portions of the zinc surface using SECM. Anionic fluxes related to the generation of OH- ions could be measured on the zinc surface outside the corroding pit by rastering the SVET probe closer to the substrat

Evaluation of the corrosion protection of steel by anodic processing in metasilicate solution using the scanning vibrating electrode technique

Anodic processing in metasilicate solution was investigated for the improvement of the corrosion resistance of various steels, namely F111 low alloy carbon and 304 stainless steels, as well as on galvanized steel cut edges. The efficiency of the prior electrochemical treatment for each material was tested during their exposure to naturally-aerated aqueous chloride solutions of different aggressiveness. Analysis was performed using the scanning vibrating electrode technique (SVET) in order to detect local ionic current distributions over the samples under study associated to the corrosion reactions. The onset of corrosion processes were greatly inhibited after anodic processing with metasilicate on both the low alloy carbon steel and the galvanized steel cut edge. Conversely, SVET analysis of unbiased 304 steel samples tested in 0.1 M chloride-containing solution did not show differences between pristine and metasilicate-treated surfaces. Differences in the electrochemical reactivity between treated and non-treated 304 steel surfaces were only observed after partial removal of the corresponding passive layers under operator-controlled polarization

Rede geodésica para o monitoramento costeiro do litoral setentrional do estado do Rio Grande do Norte

Este trabalho apresenta os procedimentos técnicos envolvidos na implantação da Rede GPS do Litoral Setentrional do Rio Grande do Norte (RGLS), onde o objetivo é fornecer subsídios fundamentais aos levantamentos geodésicos destinados ao monitoramento de áreas costeiras, submetidas à intensa dinâmica e de grande importância socioeconômica e ecológica para o Estado do Rio Grande do Norte. A metodologia permitiu a determinação das coordenadas geodésicas e altitudes ortométricas das estações com precisão decimétrica em relação ao Sistema Geodésico Brasileiro (SGB), a partir do método de posicionamento relativo e da altimetria por GPS. Ainda, foi possível o estudo de aspectos geodésicos envolvidos na altimetria por GPS, tais como avaliação da situação física e da densidade das Referências de Nível (RRNN) disponíveis, avaliação absoluta e relativa do modelo geoidal, proposição de metodologia para a altimetria por GPS de precisão e desenvolvimento de software para essa finalidade, que contribuíram para o conhecimento geodésico na área de estudo

Inhibitive effect of sodium (E)-4-(4-nitrobenzylidenamino) benzoate on the corrosion of some metals in sodium chloride solution

The inhibition performance of a novel anionic carboxylic Schiff base, sodium (E)-4-(4-nitrobenzylideneamino)benzoate (SNBB), was investigated for various metals, namely low carbon steel F111, pure iron and copper, in neutral 10 mM NaCl solution. Potentiodynamic polarization, scanning vibrating electrode technique (SVET), quantum chemical (QC) calculation, and molecular dynamics (MD) simulation were employed. The potentiodynamic polarization data showed that SNBB acts as an effective corrosion inhibitor for both iron and F111 steel, but it is not effective for the copper. In situ spatially-resolved SVET maps evidenced a major change in surface reactivity for Fe and F111 steel immersed in 10 mM aqueous solution in the absence and in the presence of SNBB. Featureless ionic current density distributions were recorded in the presence of SNBB at both their spontaneous open circuit potential (OCP) and under mild anodic polarization conditions, while major ionic flows were monitored above the metals in the absence of SNBB. On the basis of computer simulations, it is proposed that SNBB produces a stable chelate film on iron and steel surfaces that accounts for the good corrosion inhibition efficiency observed. The different inhibition efficiencies of SNBB molecules on the iron and copper was attributed to the special chemical structure of SNBB molecule and its different chelation ability with the released metal ions on the metal surface. The QC calculations also confirmed the high corrosion inhibition efficiency of SNBB. The MD simulation indicated higher binding energy of SNBB on iron surface compared to that of copper surface. The interaction mode of SNBB on iron and F111 steel surfaces corresponds to a mixed chemical and physical adsorption, and it obeys the Langmuir isother

Corrosion resistance of ZrTi alloys with hydroxyapatite-zirconia silver layer in simulated physiological solution containing proteins for biomaterial applications

The degradation characteristics of hydroxyapatite-zirconia-silver films (HA-ZrO2-Ag) coatings on three ZrTi alloys were investigated in Ringer’s solution containing 10% human albumin protein at 37 °C. Samples were immersed for 7 days while monitored by electrochemical impedance spectroscopy (EIS) and linear potentiodynamic polarization (LPP). The electrochemical analysis in combination with surface analytical characterization by scanning electron microscopy (SEM/EDX) reveals the stability and corrosion resistance of the HA–ZrO2-Ag coated ZrTi alloys. The characteristic feature that describes the electrochemical behaviour of the coated alloys is the coexistence of large areas of the coating presenting pores in which the ZrTi alloy substrate is exposed to the simulated physiological environment. The EIS interpretation of results was thus performed using a two-layer model of the surface film. The blocking effect in the presence the human albumin protein produces an enhancement of the corrosion resistance. The results disclose that the Zr45Ti alloy is a promising material for biomedical devices, since electrochemical stability is directly associated to biocompatibilit

A fully validated microbiological assay for daptomycin injection and comparison to HPLC method

abstract Daptomycin (DPT) was the first lipopeptide antibiotic available for commercialization. It is active against gram-positive bacteria, including resistant strains. This work aimed to develop and validate a turbidimetric microbiologic assay to determine daptomycin in an injectable form. A 3x3 design was employed, at concentrations of 1, 2 and 4.0 µg/mL. The microorganism test used was Staphylococcus aureus ATCC 6538p, and Antibiotic Medium 3 was used as the culture medium. Method validation demonstrated that the bioassay was linear (r=0.9995), precise (RSD=2.58%), accurate (recovery 100.48± 2.11%), and robust. Degradation kinetics was also performed in an alkaline medium, indicating that daptomycin degradation follows first order kinetics under these conditions. The analyses of degraded solutions showed that daptomycin degradation products do not possess bactericidal activity. The bioassay was compared to HPLC method that was previously developed and no significant difference was found between them (p>0.05). The method proved to be appropriate for daptomycin injection quality control

Multiscale electrochemical analysis of the corrosion of titanium and nitinol for implant applications

Surface electrochemical activity of titanium and nitinol biomaterials in naturally aerated Ringer’s physiological solution was investigated using potentiodynamic polarization and scanning electrochemical microscopy (SECM) techniques. SECM was operated in feedback and redox competition modes as a function of potential applied to the substrate. The kinetics of the electron transfer rate on both materials was characterized by mathematical modelling of the Z-approach curves monitored under feedback conditions. The rate constant values greatly depended on the characteristics of the passive layers formed over the metals under potentiostatic control. A more insulating film was found on nitinol when biased at low polarizations, resulting in smaller tip current increments during tip approach to the investigated surface under positive feedback and competition operation modes. However, at higher anodic polarizations, nitinol passive layers experience breakdown, and therefore tip current values reflect the release of metal cations from the biomaterial surfac