Electrochemical study of arsenopyrite weathering

Cyclic voltammograms and capacitance measurements are presented to characterize the mineral response at relatively moderate environmental conditions, pH 4.5 and T = 25degreesC. The experiments involve examining the rates of oxidation and the surface morphology of arsenopyrite, which is oxidized abiotically. The semiconducting properties of the mineral have been investigated in attempt to gain additional information of FeAsS dissolution behavior in acidic solutions at potentials close to the open circuit potential of the mineral. A mechanistic pathway for the anodic dissolution of arsenopyrite in open circuit conditions is also suggested. At high overpotentials, anodic reactions produce mainly sulfate and arsenate ions and may be described as hole limited. The reduction of orpiment-like compounds at potentials more negative than the open circuit potential is discussed.

Study of corrosion resistance of laser welded Au-Pd-Ag-In alloy using electrochemical.

The aim of this work was to evaluate the corrosion resistance of AuPdAgIn alloy, submitted to laser beam welding, in 0.9% NaCl solution, using electrochemical techniques. Measures of the open circuit potential (OCP) versus time were applied to electrochemical experiments, as well as potentiodynamic direct scanning (PDS) and electrochemical impedance spectroscopy (EIS) on AuPdAgIn alloy, submitted to laser beam welding in 0.9% NaCl solution. Some differences observed in the microstructure can explain the results obtained for corrosion potential, Ecorr, and corrosion resistance, Rp. EIS spectra have been characterized by distorted capacitive components, presenting linear impedance at low frequencies, including a non-uniform diffusion. The area of the laser weld presented corrosion potential slightly superior when compared to the one of the base metal. The impedance results suggest the best resistant corrosion behavior for laser weld than base metal region. This welding process is a promising alternative to dental prostheses casting.

Estudo da velocidade de despassivação de aços em estruturas de concreto armado, em meio de cloreto

The aim of the present study was to investigate the corrosion resistance in chloride medium of two cylindrical steel samples from civil construction (CA-50 and CA-60). For this purpose, electrochemical measurements were performed in NaCl solution, 4.0 g/L, naturally aerated. According to electrochemical responses, it was observed an active dissolution process at open circuit potential for both steels. In this comparative study, the differences between CA-50 and CA60 were not significant, since the values of polarization resistance estimated by EIS were the same order of magnitude for both types of steel (oscillating between 200 and 500 Ohm). For all evaluated thicknesses of concrete reinforcement, these values ranged between 2 and 9 kOhm, and the lower value was associated with lower thickness, particularly at 7, 21 and 35 days. When these results were compared with those determined for the CA-60, it was found that the layer of concrete reinforcement provide a protection against corrosion in chloride medium at least ten times larger

Filmes de 'CO' obtidos por deslocamento galvânico e suas propriedades magnéticas

Pós-graduação em Química - IQ

Espectroscopia de impedância eletroquímica da biodissolução da calcopirita na presença de íons ferrosos

Pós-graduação em Química - IQ

How do titanium and Ti6A14V corrode in fluoridated medium as found in the oral cavity? an in vitro study

The purpose of this work was to evaluate the corrosion of commercially pure (CP) titanium and Ti6Al4V in vitro at different F- concentrations regularly found in the oral cavity by using different electrochemical tests and surface analysis techniques. electrochemical impedance spectroscopy (EIS), open circuit potential (OCP) and potentio-dynamic polarization tests were associated to advanced characterization techniques such as SEM, EDS, AFM, ICP-MS and XPS. OCP tests revealed a higher reactivity of both CP titanium and Ti6Al4V at 12,300 ppm F- concentration than that recorded at 227 ppm F-. Also, a significant decrease of the corrosion resistance of both materials was noticed by EIS in fluoride solutions. Material loss caused by corrosion was noticed on titanium surfaces by SEM and AFM in the presence of high F- concentration. CP titanium degraded by pitting corrosion while Ti6Al4V suffered from general corrosion showing micro-cracks on surface. Furthermore, a high release of metallic ions from the test samples after immersion at high F concentrations was detected by ICP-MS, that can be potentially toxic to oral tissues. (C) 2014 Elsevier B.V. All rights reserved.

Electrochemical behavior and corrosion resistance of Ti-15Mo alloy in naturally-aerated solutions, containing chloride and fluoride ions

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Complementary study on the electrical and structural properties of poly(3-alkylthiophene) and its copolymers synthesized on ITO by electrochemical impedance and Raman spectroscopy

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Do oral biofilms influence the wear and corrosion behavior of titanium?

The main aim of this work was to study the simultaneous wear-corrosion of titanium (Ti) in the presence of biofilms composed of Streptococcus mutans and Candida albicans. Both organisms were separately grown in specific growth media, and then mixed in a medium supplemented with a high sucrose concentration. Corrosion and tribocorrosion tests were performed after 48 h and 216 h of biofilm growth. Electrochemical corrosion tests indicated a decrease in the corrosion resistance of Ti in the presence of the biofilms although the TiO2 film presented the characteristics of a compact oxide film. While the open circuit potential of Ti indicated a tendency to corrosion in the presence of the biofilms, tribocorrosion tests revealed a low friction on biofilm covered Ti. The properties of the biofilms were similar to those of the lubricant agents used to decrease the wear rate of materials. However, the pH-lowering promoted by microbial species, can lead to corrosion of Ti-based oral rehabilitation systems.

Effects of chemical composition on the corrosion of dental alloys

The aim of this study was to determine the effect of the oral environment on the corrosion of dental alloys with different compositions, using electrochemical methods. The corrosion rates were obtained from the current-potential curves and electrochemical impedance spectroscopy (EIS). The effect of artificial saliva on the corrosion of dental alloys was dependent on alloy composition. Dissolution of the ions occurred in all tested dental alloys and the results were strongly dependent on the general alloy composition. Regarding the alloys containing nickel, the Ni-Cr and Ni-Cr-Ti alloys released 0.62 mg/L of Ni on average, while the Co-Cr dental alloy released ions between 0.01 and 0.03 mg/L of Co and Cr, respectively.The open-circuit potential stabilized at a higher level with lower deviation (standard deviation: Ni-Cr-6Ti = 32 mV/SCE and Co-Cr = 54 mV/SCE). The potenciodynamic curves of the dental alloys showed that the Ni-based dental alloy with >70 wt% of Ni had a similar curve and the Co-Cr dental alloy showed a low current density and hence a high resistance to corrosion compared with the Ni-based dental alloys. Some changes in microstructure were observed and this fact influenced the corrosion behavior for the alloys. The lower corrosion resistance also led to greater release of nickel ions to the medium. The quantity of Co ions released from the Co-Cr-Mo alloy was relatively small in the solutions. In addition, the quantity of Cr ions released into the artificial saliva from the Co-Cr alloy was lower than Cr release from the Ni-based dental alloys.

Time-resolved surface enhanced resonance Raman spectro-electrochemistry of heme proteins

The membrane protein Cytochrome c Oxidase (CcO) is one of the most important functional bio-molecules. It appears in almost every eukaryotic cell and many bacteria. Although the different species differ in the number of subunits, the functional differences are merely marginal. CcO is the terminal link in the electron transfer pathway of the mitochondrial respiratory chain. Electrons transferred to the catalytic center of the enzyme conduce to the reduction of molecular oxygen to water. Oxygen reduction is coupled to the pumping of protons into the inter-membrane space and hence generates a difference in electrochemical potential of protons across the inner mitochondrial membrane. This potential difference drives the synthesis of adenosine triphosphate (ATP), which is the universal energy carrier within all biological cells. rnrnThe goal of the present work is to contribute to a better understanding of the functional mechanism of CcO by using time-resolved surface enhanced resonance Raman spectroscopy (TR-SERRS). Despite intensive research effort within the last decades, the functional mechanism of CcO is still subject to controversial discussions. It was the primary goal of this dissertation to initiate electron transfer to the redox centers CuA, heme a, heme a3 and CuB electrochemically and to observe the corresponding redox transitions in-situ with a focus on the two heme structures by using SERRS. A measuring cell was developed, which allowed combination of electrochemical excitation with Raman spectroscopy for the purpose of performing the accordant measurements. Cytochrome c was used as a benchmark system to test the new measuring cell and to prove the feasibility of appropriate Raman measurements. In contrast to CcO the heme protein cc contains only a single heme structure. Nevertheless, characteristic Raman bands of the hemes can be observed for both proteins.rnrnIn order to investigate CcO it was immobilized on top of a silver substrate and embedded into an artificial membrane. The catalytic activity of CcO and therefore the complete functional capability of the enzyme within the biomimetic membrane architecture was verified using cyclic voltammetry. Raman spectroscopy was performed using a special nano-structured silver surface, which was developed within the scope of the present work. This new substrate combined two fundamental properties. It facilitated the formation of a protein tethered bilayer lipid membrane (ptBLM) and it allowed obtaining Raman spectra with sufficient high signal-to-noise ratios.rnSpectro-electrochemical investigations showed that at open circuit potential the enzyme exists in a mixed-valence state, with heme a and and heme a3 in the reduced and oxidized state, respectively. This was considered as an intermediate state between the non-activated and the fully activated state of CcO. Time-resolved SERRS measurements revealed that a hampered electron transfer to the redox center heme a3 characterizes this intermediate state.rn

Estudo das propriedades inibidoras de corrosão das imidazolinas oleica e quaternária e seu encapsulamento em partículas inertes.

Entre os inibidores de corrosão clássicos que já são utilizados na indústria do petróleo, foram estudadas a imidazolina oleica e a quaternária através de técnicas eletroquímicas, gravimétrica e analíticas, para avaliar a eficiência de inibição e como esses inibidores atuam em meio ácido. O meio agressivo foi uma solução de NaCl 3,5% em massa acidificada com ácido clorídrico até atingir um pH=2 com o objetivo de simular o ambiente de extração petrolífera. O substrato empregado foi o aço carbono 1020. As técnicas eletroquímicas utilizadas foram: monitoramento do potencial de circuito aberto, medidas de resistência de polarização linear, espectroscopia de impedância eletroquímica (EIE ) e curvas de polarização. Os valores das componentes real e imaginária de impedância indicam uma resistência maior aos processos de transferência de carga com o aumento da concentração dos inibidores e os Diagramas de Bode de ângulo de fase, revelaram a presença de uma camada de inibidor adsorvida sobre o metal com uma constante de tempo em altas frequências observada para a imidazolina oleica e quaternária. Para a imidazolina quaternária, verificou-se que só para tempos maiores de imersão é que o filme se adsorve de forma eficiente demonstrando uma cinética mais lenta de adsorção. Nos ensaios gravimétricos, os resultados de taxa de corrosão em m/ano foram decrescentes com o tempo após período de imersão de 30 dias, para ambas as imidazolinas. O uso das técnicas analíticas foi necessário a fim de se compreender melhor o comportamento das imidazolinas sobre o aço no meio estudado. Os resultados da análise de íons férricos em solução, por emissão atômica, foram obtidos durante várias amostragens durante o período do ensaio de perda de massa, e foi possível verificar um processo de inibição da corrosão até doze dias de imersão do metal, depois disto ocorre um disparo na quantidade de ferro liberado em solução, sugerindo que pode estar ocorrendo uma degradação do inibidor após 12 dias de imersão. Para esclarecer esse ponto, análises por espectroscopia Raman dos produtos de fundo formados durante os ensaios de perda de massa indicaramm que a degradação pode realmente estar ocorrendo. Foi confirmado, também por espectroscopia Raman sobre a superfície do aço após imersão prévia em solução contendo a imidazolina oleica, que há uma película adsorvida que protege o metal do meio agressivo. Técnica de microscopia eletrônica de varredura foi utilizada para caracterizar os corpos de prova na ausência e presença do inibidor, depois dos ensaios eletroquímicos e foi possível caracterizar, através dessa técnica a maior eficiência inibidora do filme de imidazolina quaternária. Dois tipos de nanoconatiners foram avaliados para o encapsulamento das duas imidazolinas estudadas: nanocontainers a base do argilomineral haloiista e sílica mesoporosa tipo SBA 15. Resultados de impedância eletroquímica mostraram a liberação dos inibidores de corrosão encapsulados com o tempo de imersão. Análise na região do infravermelho por sonda de fibra ótica foi utilizada para comprovar química e qualitativamente a liberação do inibidor a partir dos nanorreservatórios, no meio agressivo.

Surface modifications of nanocrystalline diamond films and their functionalization with phthalocyanines

Boron-doped diamond is a promising electrode material for a number of applications providing efficient carrier transport, a high stability of the electrolytic performance with time, a possibility for dye-sensitizing with photosensitive molecules, etc. It can be functionalized with electron donor molecules, like phthalocyanines or porphyrins, for the development of light energy conversion systems. For effective attachment of such molecules, the diamond surface has to be modified by plasma- or photo-chemical processes in order to achieve a desired surface termination. In the present work, the surface modifications of undoped and boron-doped nanocrystalline diamond (NCD) films and their functionalization with various phthalocyanines (Pcs) were investigated. The NCD films have been prepared by hot filament chemical vapor deposition (HFCVD) on silicon substrates and were thereafter subjected to modifications with O2 or NH3 plasmas or UV/O3 treatments for exchange of the H-termination of the as-grown surface. The effectiveness of the modifications and their stability with time during storage under different ambients were studied by contact angle measurements and X-ray photoelectron spectroscopy (XPS). Furthermore, the surface roughness after the modifications was investigated with atomic force microscopy (AFM) and compared to that of as-grown samples in order to establish the appearance of etching of the surface during the treatment. The as-grown and the modified NCD surfaces were exposed to phthalocyanines with different metal centers (Ti, Cu, Mn) or with different side chains. The results of the Pc grafting were investigated by XPS and Raman spectroscopy. XPS revealed the presence of nitrogen stemming from the Pc molecules and traces of the respective metal atoms with ratios close to those in the applied Pc. In a next step Raman spectra of Ti-Pc, Cu-Pc and Mn-Pc were obtained with two different excitation wavelengths (488 and 785 nm) from droplet samples on Si after evaporation of the solvent in order to establish their Raman fingerprints. The major differences in the spectra were assigned to the effect of the size of the metal ion on the structure of the phthalocyanine ring. The spectra obtained were used as references for the Raman spectra of NCD surfaces grafted with Pc. Finally, selected boron doped NCD samples were used after their surface modification and functionalization with Pc for the preparation of electrodes which were tested in a photoelectrochemical cell with a Pt counter electrode and an Ag/AgCl reference electrode. The light sources and electrolytes were varied to establish their influence on the performance of the dye-sensitized diamond electrodes. Cyclic voltammetry measurements revealed broad electrochemical potential window and high stability of the electrodes after several cycles. The open circuit potential (OCP) measurements performed in dark and after illumination showed fast responses of the electrodes to the illumination resulting in photocurrent generation.

Aqueous colloidal stability evaluated by zeta potential measurement and resultant TiO2 for superior photovoltaic performance

Controlling the morphological structure of titanium dioxide (TiO 2) is crucial for obtaining superior power conversion efficiency for dye-sensitized solar cells. Although the sol-gel-based process has been developed for this purpose, there has been limited success in resisting the aggregation of nanostructured TiO2, which could act as an obstacle for mass production. Herein, we report a simple approach to improve the efficiency of dye-sensitized solar cells (DSSC) by controlling the degree of aggregation and particle surface charge through zeta potential analysis. We found that different aqueous colloidal conditions, i.e., potential of hydrogen (pH), water/titanium alkoxide (titanium isopropoxide) ratio, and surface charge, obviously led to different particle sizes in the range of 10-500 nm. We have also shown that particles prepared under acidic conditions are more effective for DSSC application regarding the modification of surface charges to improve dye loading and electron injection rate properties. Power conversion efficiency of 6.54%, open-circuit voltage of 0.73 V, short-circuit current density of 15.32 mA/cm2, and fill factor of 0.73 were obtained using anatase TiO 2 optimized to 10-20 nm in size, as well as by the use of a compact TiO2 blocking layer.

Sulphur potential measurements with a two phase sulphide oxide electrolyte

The open circuit potentials of the galvanic cell,Pt (or Au)¦(Ar + H2S + H2)primeparCaS + ZrO2(CaO)par (Ar + H2S+ H2)Prime£t (or Au) has been measured in the temperature range 1000 to 1660 K and PH2S:PH 2 ratios from 1.73×10–5 to 2.65×10–1. The solid electrolyte consists of a dispersion of calcium sulphide in a matrix of calcia-stabilized zirconia. The surface of the electrolyte is coated with a thin layer of calcium sulphide to prevent the formation of water vapour by reaction of hydrogen sulphide with calcium oxide or zirconia present in the electrolyte. The use of a lsquopoint electrodersquo with a catalytically active tip was necessary to obtain steady emfs. At low temperatures and high sulphur potentials the emfs agreed with the Nernst equation. Deviations were observed at high temperatures and low sulphur potentials, probably due to the onset of significant electronic conduction in the oxide matrix of the electrolyte. The values of oxygen and sulphur potentials at which the electronic conductivity is equal to ionic conductivity in the two-phase electrolyte have been evaluated from the emf response of the cell. The sulphide-oxide electrolyte is unsuitable for sulphur potential measurements in atmospheres with high oxygen potentials, where oxidation of calcium sulphide may be expected.