Portugaliae Electrochimica Acta

Professor Catedrático Jubilado César Augusto Nunes Viana: 06/06/1932 - 13/08/2004

Temperature Effects on Conductivities and Association Constants of Lithium and Tetra-Ethylammonium Salts in Six Aprotic Solvents of Medium to High Permittivities from - 30 ºC to + 10 ºC

A few non-aqueous dipolar aprotic solvents of medium to high permittivities, low viscosities and large temperature intervals between freezing and boiling points have been used to prepare electrolyte solutions applicable to primary and rechargeable high energy batteries. Lithium perchlorate is a salt often chosen to obtain those systems. In this work low temperature effects on conductivities and association constants of LiClO4 and Et4NBr in six different solvents have been analyzed, using the tetraethylammonium bromide salt for comparative reasons. The temperature interval was -30 ºC to 10 ºC. A few thermodynamic functions resulting from K A, T variations were determined being the results discussed on this basis. The accuracy and precision of results are high; whenever possible they are compared with others previously published.

Chronoamperometric Characterization of the Reduction of the Tin Film Electrogenerated in Sodium Hydrogenocarbonate

The observation of current hysteresis in the cathodic voltammetric profiles of the Sn/bicarbonate electrochemical system motivated chronoamperometric studies, since it is mentioned in literature the correlation between nucleation processes occurrence and current hysteresis presence. The nucleation occurrence was verified by chronoamperometric techniques when films electrogenerated within the passivation region are reduced. Mathematical adjustment between experimental data and calculated according theoretical modelling, was done by employing non-linear least squares fitting routines. This way, the electroreduction of the film formed at lower potentials can be explained in terms of diffusion and progressive nucleation and 3-D growth controlled by charge transfer. On the other hand, the reduction of the film formed at higher potentials follows the model of progressive nucleation and 3-D growth but with the inclusion of induction time. The analysis of the parametric dependence on potential step, enabled to evaluate the role of parallel and normal rate constants for each experimental case.

Os estudos cronoamperométricos foram motivados pela observação de histerese de corrente nos perfis de redução dos voltamogramas realizados no sistema Sn/bicarnonato, já que é sabido na literatura que a presença de histerese é considerada indicativo de ocorrência de processos de nucleação. A ocorrência de nucleação foi verificada mediante técnica cronoamperométrica quando filmes eletrogerados na região de passivação são reduzidos. Empregando rotinas de análise de mínimos quadrados não lineares, foi obtido o ajuste entre os resultados experimentais e os calculados usando modelos teóricos que descrevem a nucleação eletroquímica. Assim, a eletrorredução do filme formado no extremo inferior da região estudada, pode ser explicada em termos de difusão e nucleação progressiva e crescimento 3-D controlado por transferência de carga. Por outro lado, a redução do filme electrogerado no extremo superior segue o modelo de nucleação progressiva e crescimento 3-D controlado por transferência de carga, com a presença de tempo de indução. A análise da variação dos parâmetros modelísticos com o tamanho do degrau de potencial permitiu estabelecer a influência dos valores das constantes de velocidade paralela e perpendicular à superfície para cada caso experimental.

The Effect of Sour Gases and Some Anions on the Corrosion Behavior of Carbon Steel

The effect of the presence of CO2 and H2S in the well water used in the petroleum plant on corrosion of carbon steel has been tested using impedance measurements. Carbon dioxide leads to decrease in the resistivity of the film developed on the metallic surface, while the effect of hydrogen sulfide is less pronounced. Scanning electron micrographs have shown that corrosion products cover only small part of metallic surface in water containing CO2. Studies under polarization conditions will allow concluding that the dissolved gases in the well water reduce the ability of the film to protect the metal against corrosion. The influence of the oxoanions and halide ions on the corrosion rate of steel has also been analyzed.

Effect of Iodide Ions on Corrosion Inhibition of Mild Steel in 0.5 M H₂SO₄by Poly(4-Vinylpyridine)

The effect of iodide ions on the corrosion inhibition of mild steel in 0.5 M sulfuric acid in the presence of poly(4-vinylpyridine) (p4vp) was studied by weight loss and different electrochemical techniques such as electrochemical impedance spectroscopy, potentiodynamic polarization and polarization resistance measurements. The obtained results showed that poly(4-vinylpyridine) effectively reduces the corrosion rate of mild steel. The addition of potassium iodide (KI) enhanced the inhibition efficiency (P%). The synergism parameter (Sp) calculated from inhibition efficiency was found to be greater than unity, suggesting that the phenomenon of synergism exists between P4VP and iodide ions. The adsorption of p4vp alone and in combination with KI followed Frumkin adsorption isotherm. The effect of temperature indicated that the inhibition efficiency of p4vp decreases with increasing temperature. Study of temperature effect showed also that inhibition efficiency of (P4VP + KI) does not depend on temperature. The apparent activation energies in the absence and in the presence of P4Vp and ( P4VP + KI ) were determined.

Automatic Determination of Uric Acid in Urine in a FIA System with a Tubular Amperometric Detecto

This paper describes the use of a FIA system for the automatic determination of uric acid in urine, using a tubular amperometric detector. In this system, glassy carbon electrodes of tubular configuration were activated by successive cyclical scans from - 0.30 to 2.00 V versus Ag/AgCl reference electrode in 0.50 mol.L-1 sulphuric acid solution. Samples were diluted in supporting electrolyte before analysis and no other pre-treatment was employed. Such dilution enabled the matrix effects to be reduced and therefore improving the sensitivity provided by the activation step that allows the uric acid (UA) to be detected at lower concentration levels after dilution. Uric acid determination was performed in a single channel FIA manifold, which provided reproducibility of sample transport to the detector and enabled a sampling rate of 120 samples per hour to be achieved. The tubular configuration of the cell offered robustness to the system since it was rigidly fixed to the manifold, allowing the manifold to be generalised for routine analysis application. The results obtained with the FIA system for UA determination in urine were compared with those from the enzymatic method used in clinical analysis laboratories. No statistical difference between methods was found at the 95% confidence level. Relative deviations between both methods were within ± 4% and the proposed system showed good repeatability (about 3%, n = 10).

Influence of the Chemical Composition of Stainless Steels on the Electronic Structure of Passive Films Formed in Artificial Sea Water

The electronic structure of the passive films formed on stainless steels of different chemical composition in artificial sea water is examined by capacitance measurements (Mott-Schottky approach) and photo-electrochemistry. Analytical characterization is carried out by Auger Electron Spectroscopy. The influence of the main alloying elements (Cr, Ni) on the development of space charge regions, which can be described as a depletion or accumulation layers are discussed.

L'étude concerne la structure électronique des films passifs formés dans l'eau de mer artificielle, sur deux nuances d'acier inoxydable. Elle vise plus précisément à établir l'influence des principaux éléments d'alliage (Cr, Ni) sur les situations soit de déplétion soit d'accumulation dans les zones de charge d'espace formées par la polarisation aux interfaces métal/filme et film/électrolyte. Le travail expérimental repose sur les mesures de capacité (méthode de Mott-Schottky) et de photoélectrochimie. La caractérisation analytique des films passifs a été faite par spectroscopie des électrons Auger.

Degradation of a Textile Dye C. I. Direct Red 80 by Electrochemical Processes

The electrochemical removal of the textile dye C. I. Direct Red 80 (DR80) was carried out using three different materials as anodes: iron, polypyrrole (PPy) and boron doped diamond (BDD). Iron electrodes are consumed during the electrolysis, and promote flocculation/coagulation of the dye. Polypyrrole, a conducting polymer, was prepared by chemical/electrochemical precipitation over a cotton cloth; it enables the precipitation of the dissolved dye through a conversion reaction in a less soluble compound. Boron doped diamond electrode was used to achieve the complete mineralization of the dye. Bulk electrolysis were studied using Na2SO4 as electrolyte. In the case of the PPy electrode, NH4NO3 was also used as electrolyte. The variation of the dye concentration was followed by UV-visible absorbance measurements and chemical oxygen demand (COD) tests were also performed, to compare the rates of colour and COD removal in each case. From the COD measurements over the time of electrolysis, using the BDD anode, a mass transfer coefficient for the DR80 molecule was determined. The obtained results show an almost complete colour removal for all the electrodes tested and a COD removal between 50 and 90 %, being the best results obtained with the BDD electrode.

Semiconductor Electrochemistry and Localised Corrosion

In the present work, the electronic structure of the passive film formed on austenitic stainless steel of the 304 type and its implications on the initiation of localised corrosion are investigated taking into account concepts developed in semiconductor physics and semiconductor electrochemistry. Capacitance measurements (Mott-Schottky approach), show that the susceptibility of AISI 304 stainless steel to stress corrosion cracking (SCC) in boiling chloride containing aqueous solutions is closely linked to the formation of a chromium rich passive oxide film with p-type semiconductivity. A small polarisation is required to drastically change the electric field at the film-electrolyte interface, as a consequence of the high doping level of the passive film. Initiation of the SCC phenomenon is described as the consequence of localised changes in the semiconductive properties of the passive film.