Avaliação Eletroquímica do Efeito da Adição de Nanopartículas no Polimento Químico da Liga AA6062T5

The chemical surface preparation process aims to eliminate rolling marks or scratches providing a smoother surface. Eleven solutions for chemical polishing of aluminium alloy AA6062T5 were used aiming to get the best aesthetic condition without losing corrosion resistance. These solutions were combinations of NaOH, NaCl, Na2SO4, and HNO3 and different amount of SiO2 nanoparticles in these solutions. The samples submitted to these solutions were analysed by electrochemical measurements such as: potential monitoring in open circuit and electrochemical impedance spectroscopy using NaCl 0.1 mol L-1 as electrolyte. The sample subjected to treatment with a mixture of all reagents and with the addition of 1000 ppm SiO2, followed by cleaning with nitric acid solution showed the highest corrosion resistance. DOI: http://dx.doi.org/10.17807/orbital.v8i2.79

Effect of the outer layer of Al coatings deposited by cold gas spray on the microstructure, mechanical properties and corrosion resistance of the AA 7075-T6 aluminum alloy

The corrosion of AA 7075-T6 aluminum alloy is a critical issue for many industries. In this study, aluminum coatings were deposited onto AA 7075-T6 by cold gas spray and the effect of the porous outer layer on different properties of the coating, including corrosion-resistance was investigated. As-prepared and polished samples were used to study the microstructure, morphology, mechanical properties and corrosion resistance of the coating in 3.5 wt.% NaCl solution. Cross-sectional analysis showed a dense structure, low porosity (0.8%) and thickness up to 300 lm (* 100 lm for the porous outer layer and * 200 lm for the compact inner layer). The sliding wear test resulted in a volume loss of 3.2 9 10-4 mm3/Nm with an adhesive wear mechanism. The abrasive wear test showed a wear rate of 1.1 9 10-4 mm3/Nm for the asprepared coating and 0.8 9 10-4 mm3/Nm for the polished coating. The as-prepared coating pores and interparticle spacing in the outer layer were mostly responsible for the increase in wear rate. For the polished coating immersed in 3.5 wt.% NaCl solution during 900 h, the electrolyte reached some specific points of the substrate as revealed by the cross-sectional analysis. Inspection of the as-prepared coating demonstrated that the coating/substrate interface

Cold gas spray coatings: basic principles, corrosion protection and applications

In this review, the beginnings and evolution of the cold gas spray (CGS) technique are described, followed by the main fundamental aspects of the technique together with a description of the several spraying systems up to date. Sequentially, the main spray parameters and their influence on the properties of the coatings are reported. Afterwards, the most important methodologies for preparing the feedstock powders to be sprayed, the effect of the powder composition, microstructure, particle size and shape on the properties of the coatings are discussed. The nature of the spray gun and nozzle, and the substrate pre-treatments were also discussed. With regard to microstructure and properties, the chemical and physical characterization of the coatings and the performance in protecting the substrates against corrosion together with some mechanical properties are presented and compared. The lacking systematic studies about the great part of investigated systems is the main drawback to compare the published results. Closing this review, the main applications, and the potentialities of the CGS coatings are evidenced

Electrochemical Corrosion Characterization of Submicron WC-12Co Coatings Produced by CGS and HVAF Compared with Sintered Bulks

The electrochemical corrosion performance of WC-12 wt% Co in coating and bulk forms has been evaluated in a 3.56 wt% NaCl solution. The coatings were deposited by means of thermal spray techniques, i.e., cold gas spraying (CGS) and high-velocity air fuel (HVAF) spraying, while bulks with different WC sizes were manufactured by conventional pressing and sintering. Microstructural characterizations and phase composition determinations were carried out using scanning electron microscopy and X-ray diffraction. Differences in WC grain size and morphology, carbide dissolution, and cobalt binder phase transformation are discussed according to the inherent characteristics of each processing method. Together with surface roughness (polished/as-sprayed), these features have been observed to directly affect the electrochemical corrosion performance. Electrochemical measurements (open circuit potential, polarization resistance, electrochemical impedance spectroscopy, and polarization curves) showed that the as-sprayed CGS coating presented an electrochemical behavior similar to those of the bulk materials. This was attributed to the higher metallic character of this coating in comparison to that of the HVAF coating. The polished HVAF coating showed anodic activity lower than those of the bulk samples, most likely due to the presence of cobalt–tungsten carbide phases and eventually the lower amount of Co available for dissolution. Finally, the as-sprayed HVAF coating showed very high resistivity due to the presence of surface oxides generated during the deposition process.publishedVersionPeer reviewe

Electrochemical oxidations of organocobalt (III) derivatives with displaced electronic structure. Coenzyme B models 12

No presente trabalho estuda-se sistematicamente a influência orgânico axial R nos complexos (RCo(III)(SALEN)DMF) onde [Ver esquema no PDF] R = CH3, C2H5, n-C3H7, n-C4H9, s-C4H9, i-C4H9, CH2Cl, c-C6H11CH2, c-C6H11, C6H5CH2, p-CH3 C6H4CH2 e p-NO2 C6H4CH2, na oxidação eletroquímica destes compostos em dimetilformamida (DMF), 0,2M em perclorato de tetraetilamônio à temperatura ambiente e a -20°C. O método da voltametria cíclica foi utilizado, sendo os eletrodos de disco de platina, Ag/AgCl e fio de platina, empregados como eletrodos de trabalho, referência e auxiliar, respectivamente. Pode-se agrupar os compostos acima em três conjuntos, de acordo com o comportamento eletroquímico: A - Os compostos com R = CH3, C2H5, n-C3H7, n-C4H9, c-C6H11CH2 são oxidados monoeletrônica e reversivelmente em velocidades de varredura de potenciais de 10 a 50 V.s-1 . A velocidades de varredura mais baixas constata-se que uma reação química de pseudo-primeira ordem de decomposição da forma oxidada segue a etapa primária eletroquímica. A baixa temperatura, a velocidade da reação química é diminuída e observa-se essencialmente a etapa monoeletrônica e reversível. B - Os compostos com R = CH2Cl, C6H5CH2, p-CH3 C6H4CH2 e p-NO2 C6H4CH2 são oxidados monoeletrônica e quase-reversivelmente à temperatura ambiente. Abaixando-se a temperatura, a reação eletroquímica torna-se irreversível. São calculados os parâmetros cinéticos para as quatro reaçoes. c - Os compostos com R = i-C4H9, s-C4H9 e c-C6H11 sao oxidados monoeletrônica e reversivelmente a -20 °C. À temperatura ambiente a reação química que segue a etapa eletroquímica ée rápida demais para poder-se isolar a etapa primária. Analisando-se o efeito do ligante orgânico axial R sobre os potenciais de meia onda E1/2, idênticos aos potenciais padrão de eletrodo (para os compostos que sofrem oxidaçãoreversível) pois os coeficientes de difusão das formas oxidada e reduzida são iguais, verifica-se que os E1/2 são tanto menores quanto maior o poder de doação de elétrons do substituinte. Dentro do erro experimentar obtém-se uma correlaç~ão linear entre os E1/2 e os parâmetros polares de Taft, σ*, com ρ* = (0,25 ± 0,03) para os grupos R apropriados. O mesmo tipo de correlação foi obtido para as organocobaloximas em HClO4 1M com um valor de ρ* três vezes menor. Esta menor inclinação é parcialmente devida ao efeito do ligante equatorial e Sem dúvida à mudança do solvente. Enquanto no estudo das vinte e sete organocobaloximas todos os compostos eram oxidados monoeletrônica e reversivelmente,nos derivados de SALEN o efeito dos grupos R reflete-se também sobre a velocidade do processo de transferência de carga heterogênea. Para os derivados benzílicos e os dotados de baixa habilidade de doação de elétrons (CH2Cl) a etapa dominante da velocidade é a de transferência de carga heterogênea (a baixa temperatura). Propõe-se um paralelo entre os mecanismos de reações redox homogêneas de esfera externa e interna aos de eletrodo reversível e quase-reversível (a irreversível) respectivamente. Portanto, para os derivados de SALEN o efeito do ligante orgânico axial R reflete-se tanto na termodinâmica quanto na cinética da oxidação eletroquímica.The influence of the axial organic ligand R on the electrochemical oxidation of the complexes (RCo(III)(SALEN)DMF) where [Ver esquema no PDF] R = CH3, C2H5, n-C3H7, n-C4H9, s-C4H9, i-C4H9, CH2Cl, c-C6H11CH2, c-C6H11, C6H5CH2, p-CH3 C6H4CH2 e p-NO2 C6H4CH2, was studied by means of cyclic voltammetry in dimethylformamide (DMF) containing O,2M in tetraethylammonium perchlorate at room temperature and at -20°C. A platinum disk, Ag/AgCl, and a platinum wire were employed as the working, reference, and auxiliar electrodes, respectively. The above-mentioned compounds can be classified according to their electrochemical behavior: A) The complexes with R = CH3, C2H5, n-C3H7, n-C4H9, c-C6H11CH2 undergo a reversible one-electron oxidation in the 10-50 V.s-1 potential scan rate range. At slower scan rates, the oxidized product decomposes chemically, following the electrochemical step, via a solvent-assisted pseudo-first order reaction. At -20°C, this chemical is slow and reversible one-electron electrochemical oxidation is observed. B) The compounds with R = CH2Cl, C6H5CH2, p-CH3 C6H4CH2 and p-NO2 C6H4CH2 undergo a quasi-reversible one-electron electrochemical oxidation at room temperature. At -20°C the electrochemical process becomes totally irreversible. Kinetic parameters are calculated for the heterogeneous electron transfers. C) The compounds with R = i-C4H9, s-C4H9 and c-C6H11 undergo a reversible one-electron oxidation at -20°C. At room temperature, the irreversible chemical reaction following the electron transfer step is too fast to allow the isolation of the electrochemical step. The effect of the organic ligands R on the half-wave potentials, E1/2, was studied. For the compounds which exhibit reversible electrochemical oxidation, the E1/2 are identical to the standard electrode potentials, the diffusion coefficients of the reduced and oxidized species being identical. The E1/2 decreases with increasing electron-donating ability of the ligands R. Within the experimental error, a linear free-energy relationship between E1/2 and the Taft polar parameters σ* is obtained with ρ* = 0.25 ± 0.03. The same type of correlation was previously obtained for organocobaloximes in 1M HclO4, but with a σ* value three times smaller. The different slopes are attributable in part to the effect of the equatorial ligand and, of course, to the change of solvent. Whereas all 27 of the organocobaloximes previously studied exhibited reversible one-electron electrooxidation, the electrochemical behavior of the ALEN derivatives depends on the nature of the axial organic ligand R. At low temperatures, the rate determining step for the benzyl derivatives and the CH2Cl (electron withdrawing group) derivative is a heterogeneous charge transfer reaction. A parallel between the homogeneous outer- and inner-sphere redox mechanisms and reversible and quasi-reversible (to irreversible) electrochemical mechanisms is proposed. Therefore, for the SALEN derivatives, the axial organic ligand R exerts its influence on both the thermodynamic and kinetic aspects of the electrochemical oxidation

Electrochemical oxidations of organocobalt (III) derivatives with displaced electronic structure. Coenzyme B models 12

No presente trabalho estuda-se sistematicamente a influência orgânico axial R nos complexos (RCo(III)(SALEN)DMF) onde [Ver esquema no PDF] R = CH3, C2H5, n-C3H7, n-C4H9, s-C4H9, i-C4H9, CH2Cl, c-C6H11CH2, c-C6H11, C6H5CH2, p-CH3 C6H4CH2 e p-NO2 C6H4CH2, na oxidação eletroquímica destes compostos em dimetilformamida (DMF), 0,2M em perclorato de tetraetilamônio à temperatura ambiente e a -20°C. O método da voltametria cíclica foi utilizado, sendo os eletrodos de disco de platina, Ag/AgCl e fio de platina, empregados como eletrodos de trabalho, referência e auxiliar, respectivamente. Pode-se agrupar os compostos acima em três conjuntos, de acordo com o comportamento eletroquímico: A - Os compostos com R = CH3, C2H5, n-C3H7, n-C4H9, c-C6H11CH2 são oxidados monoeletrônica e reversivelmente em velocidades de varredura de potenciais de 10 a 50 V.s-1 . A velocidades de varredura mais baixas constata-se que uma reação química de pseudo-primeira ordem de decomposição da forma oxidada segue a etapa primária eletroquímica. A baixa temperatura, a velocidade da reação química é diminuída e observa-se essencialmente a etapa monoeletrônica e reversível. B - Os compostos com R = CH2Cl, C6H5CH2, p-CH3 C6H4CH2 e p-NO2 C6H4CH2 são oxidados monoeletrônica e quase-reversivelmente à temperatura ambiente. Abaixando-se a temperatura, a reação eletroquímica torna-se irreversível. São calculados os parâmetros cinéticos para as quatro reaçoes. c - Os compostos com R = i-C4H9, s-C4H9 e c-C6H11 sao oxidados monoeletrônica e reversivelmente a -20 °C. À temperatura ambiente a reação química que segue a etapa eletroquímica ée rápida demais para poder-se isolar a etapa primária. Analisando-se o efeito do ligante orgânico axial R sobre os potenciais de meia onda E1/2, idênticos aos potenciais padrão de eletrodo (para os compostos que sofrem oxidaçãoreversível) pois os coeficientes de difusão das formas oxidada e reduzida são iguais, verifica-se que os E1/2 são tanto menores quanto maior o poder de doação de elétrons do substituinte. Dentro do erro experimentar obtém-se uma correlaç~ão linear entre os E1/2 e os parâmetros polares de Taft, σ*, com ρ* = (0,25 ± 0,03) para os grupos R apropriados. O mesmo tipo de correlação foi obtido para as organocobaloximas em HClO4 1M com um valor de ρ* três vezes menor. Esta menor inclinação é parcialmente devida ao efeito do ligante equatorial e Sem dúvida à mudança do solvente. Enquanto no estudo das vinte e sete organocobaloximas todos os compostos eram oxidados monoeletrônica e reversivelmente,nos derivados de SALEN o efeito dos grupos R reflete-se também sobre a velocidade do processo de transferência de carga heterogênea. Para os derivados benzílicos e os dotados de baixa habilidade de doação de elétrons (CH2Cl) a etapa dominante da velocidade é a de transferência de carga heterogênea (a baixa temperatura). Propõe-se um paralelo entre os mecanismos de reações redox homogêneas de esfera externa e interna aos de eletrodo reversível e quase-reversível (a irreversível) respectivamente. Portanto, para os derivados de SALEN o efeito do ligante orgânico axial R reflete-se tanto na termodinâmica quanto na cinética da oxidação eletroquímica.The influence of the axial organic ligand R on the electrochemical oxidation of the complexes (RCo(III)(SALEN)DMF) where [Ver esquema no PDF] R = CH3, C2H5, n-C3H7, n-C4H9, s-C4H9, i-C4H9, CH2Cl, c-C6H11CH2, c-C6H11, C6H5CH2, p-CH3 C6H4CH2 e p-NO2 C6H4CH2, was studied by means of cyclic voltammetry in dimethylformamide (DMF) containing O,2M in tetraethylammonium perchlorate at room temperature and at -20°C. A platinum disk, Ag/AgCl, and a platinum wire were employed as the working, reference, and auxiliar electrodes, respectively. The above-mentioned compounds can be classified according to their electrochemical behavior: A) The complexes with R = CH3, C2H5, n-C3H7, n-C4H9, c-C6H11CH2 undergo a reversible one-electron oxidation in the 10-50 V.s-1 potential scan rate range. At slower scan rates, the oxidized product decomposes chemically, following the electrochemical step, via a solvent-assisted pseudo-first order reaction. At -20°C, this chemical is slow and reversible one-electron electrochemical oxidation is observed. B) The compounds with R = CH2Cl, C6H5CH2, p-CH3 C6H4CH2 and p-NO2 C6H4CH2 undergo a quasi-reversible one-electron electrochemical oxidation at room temperature. At -20°C the electrochemical process becomes totally irreversible. Kinetic parameters are calculated for the heterogeneous electron transfers. C) The compounds with R = i-C4H9, s-C4H9 and c-C6H11 undergo a reversible one-electron oxidation at -20°C. At room temperature, the irreversible chemical reaction following the electron transfer step is too fast to allow the isolation of the electrochemical step. The effect of the organic ligands R on the half-wave potentials, E1/2, was studied. For the compounds which exhibit reversible electrochemical oxidation, the E1/2 are identical to the standard electrode potentials, the diffusion coefficients of the reduced and oxidized species being identical. The E1/2 decreases with increasing electron-donating ability of the ligands R. Within the experimental error, a linear free-energy relationship between E1/2 and the Taft polar parameters σ* is obtained with ρ* = 0.25 ± 0.03. The same type of correlation was previously obtained for organocobaloximes in 1M HclO4, but with a σ* value three times smaller. The different slopes are attributable in part to the effect of the equatorial ligand and, of course, to the change of solvent. Whereas all 27 of the organocobaloximes previously studied exhibited reversible one-electron electrooxidation, the electrochemical behavior of the ALEN derivatives depends on the nature of the axial organic ligand R. At low temperatures, the rate determining step for the benzyl derivatives and the CH2Cl (electron withdrawing group) derivative is a heterogeneous charge transfer reaction. A parallel between the homogeneous outer- and inner-sphere redox mechanisms and reversible and quasi-reversible (to irreversible) electrochemical mechanisms is proposed. Therefore, for the SALEN derivatives, the axial organic ligand R exerts its influence on both the thermodynamic and kinetic aspects of the electrochemical oxidation

MATHEMATICAL APPROACH OF DIFFUSION IN THE ELECTROCHEMICAL CONTEXT

The aim of this study was to explain in detail the mathematical methods used to deal with diffusion equations, mainly for students and researchers interested in electrochemistry and related areas. Emphasis was placed on the deduction and resolution of diffusion equations, as well as addressing cartesian, spherical and cylindrical coordinates. Different aspects of mass transfer processes were discussed including the importance of the resolution of Fick's laws equations to understand and derive parameters of the electroactive species (e.g., diffusion coefficients, formal electrode potentials) from the electrochemical techniques. As an example, the resolution of diffusion equations for a reversible reduction process of soluble oxidized species was presented for the chronopotentiometry technique. This study is envisaged to broaden the understanding of these frequently used methods, in which mathematical deductions are not always completely understood