Portugaliae
Electrochimica Acta

This paper describes results obtained with an AISI 304 stainless steel in 1 M HCl and 1 M KCl solutions, with the aim of contributing to the understanding of the processes occurring within growing pits. The kinetics of dissolution were studied at constant potential by the scratch method. In 1 M HCl a salt film formed below the passivation peak and its rate of formation increased as the applied potential was made more noble. In 1 M KCl the rate of film formation was not affected by potential. In this solution a passivating film developed even before the current maximum in the anodic polarization curve. Since the solutions found within growing pits usually have low pH and high chloride concentrations it is concluded that, as pits develop, their surface should become covered by salts layers of increasing thickness. Key Words: dissolution kinetics, pitting and stainless steel.

Electrode kinetic parameters, p and k, the charge transfer coefficient and the conditional charge transfer rate constant, respectively, for the anodic dissolution of copper, cobalt, zinc and bismuth have been calculated from voltammetric scans carried out in solutions containing a selected supporting electrolyte and the same supporting electrolyte plus EDTA, respectively. The values found for the parameters in both media showed that they are affected by the presence of the complexing agent  was found to be related to the conditional formation constant, of the metal-EDTA complex studied and the k0/k0EDTA ratios obtained evidenced adsorption of the metal-EDTA compound on the electrode surface for all the cases but bismuth; its extent being closely related to the metal transition character of the metal ion involved in the complex. Keywords: Electrode kinetic parameters; anodic dissolution of pure metals; arbitrary current method.

There is currently a great deal of interest in solid electrolytes and mixed conductors due to their potential use in high performance battery or fuel cell systems and also because they can be employed in solid electrochemical transducers for a variety of scientific and technological purposes. Due to the unusual range of behaviour of these materials and the special requirements related to their use, particular attention must be given to the problem of appropriate measurement and evaluation methods. One of the most useful characterization techniques for these "fast ionic conductors" is the dc measurement of their electrical conductivity. The concepts behind this approach and the dc measurement techniques of the total conductivity, the ionic conductivity and the electronic conductivity are given in this talk together with examples of its use employing solid electrolyte and mixed conductor samples which have a wider applicability in the area of materials research.

The System Cu/NaOH 0.1M, pH 12,8 was studied, in the presence and absence of sulphate anions. Different sweep-rates and anodic limits, were experimented and the analysis of such results was made in terms of charge, peak currents and peak potentials as a function of sweep-rate. Results have shown that the kinetics of the film formation, in the absence of sulphate are controlled by diffusion of ions through a solid phase, while in the presence of sulphate are under mixed control, by diffusion and adsorption. The observed effects have been interpreted in terms of a decrease in the barrier of the activation energy, due to the formation of intermediates having SO^ strongly adsorved. A more quantitative model requires a better theoretical description of the adsorption of sulphate ions at the Cu/alkaline solution, and Cu20/alkaline solution interfaces.

The corrosion of mild steel in de-aerated sulphuric acid sol at ions has been studied in the pH range 0.3-1.2 at two different levels of solution agitation. Potentiodynamic polarization in the pre-Tafel region was employed throughout at various exposure times and the resistance polarization, R^, ca1cu1ated. Rp varies with pH pointing towards a maximum at pH = 1 irrespective of agitation. Agitation did not interfere with Rp vs pH curve shape but displaced it towards lower values, with a more marked effect for pH < 1. Voltammograms run after 2 h hours exposure were found to be shifted towards lower current densities for rapid agitation at a 11 pH's indicating an "inhibiting effect, due to an increase in agitation. Two anodic Tafel regions were evident, the first showing slight dependency on pH and the second-with values in the range of 0.020-0.O4O V, often found in acid solutions. Calculated b^ values found by curve fitting of the plot E vs i, using different combinations of bg and bc, gave values of the order of 0.080 V which agree with experimental values. Calculated and experimental b values were found to be 0.110 V. The surface activity of the metal by degradation brought about by hydrogen evolution as well as by high density of crystal imperfection produced by cold work is also invoked (6,7). This work is concerned with the corrosion behaviour of mild steel in sulphuric acid solutions at low pH to different levels of agitation. The tendency of the system to corrode is evaluated using the polarization resistance method and unidirectional voltammetry.

The electrochemical behaviour of 1(o-nitrobenzylidene)amino-4,6-diphenyl-2-pyridone, in ethanolic Britton-Robinson buffer, has been studied using polarographic, voltammetric and controlled-potential electrolysis methods. Reduction of -NOQ group to -NHOH is first observed in acidic and neutral media, while reduction of tine aldiminic bond (similarly as it takes place in analogous compounds), accompanied by hydroxylamine reduction, occurs later. In basic medium, reduction of hydroxylamine is easier than that of the aldiminic bond. The hydroxylamine stability is pH dependent, if is stable in neutral medium and unstable in acidic and basic media. As a consequence, the appearance of an ECE mechanism — in the reduction of -N02 group — is observed in basic medium.

A comparative study of the electrodeposition of tin (II) sulphide on tin and tin amalgam, has been done in bicarbonate solutions using a potentiodynamic technique. The influence of sweep rate and sulphide concentration, on the peak -current has been studied. The results show, that in the early stages, the deposition takes place by a different mechanism in each substrate. However the thickning of the film seems to occur by diffusion through the solid phase, in both substrates, for high sulphide concentrations, while for low concentrations the rate-determining step is the diffusion in solution.

The formation of complexes [M0L2(dppe)^], [MoL^Cdppe)^] and [MoLL'(dppe^] (L=CNBut, L'=C0) (by imposing a cathodic clamp beyond the reduction peak of [MoC^idppe^] in the presence of L and L') was monitored by ramp-clamp reverse ramp voltammetry and, from the analysis of the product distribution, it was concluded that the unsaturated centres of the types {Moidppe^}, {MoLCdppe)^} and {MoL' (dppe^} discriminate (although equally) between the two substrates, the addition of CO at each site occurring 2.3 times as fast as the addition of CNBu*" to the corresponding site.

Electrochemical P ligand and E (electron-richness) and S (8) 3 (polarisability) metal site parameters, defined by other authors for octahedral 18-electron complexes with 16-electron square pyramidmetal centres, are extended to octahedral 18-electron compounds of the types [*1 L ] with 14-electron (n = 2) or facial 12-electron (n = 3) metal sites, by defining new overall P^^ ligand parameters as, e.g.,PnL-E1/2[Cr(CO)6-nLJ " E1/2[Cr, {cis-M(C0)4> (M = Cr, Mo, W) and {fac-M(CO)^ } (M = Cr, Mo) binding metal centres, and the corresponding ligand P^L and metal site (Es,B) parameters are estimated. For the c_is_- 14-electron metal sites (n = 2), bidentate chelating ligands (LL) are also considered.

A new on line technique, the packed bed wall-jet electrode, for the determination of total iron in hydroponic solutions has been developed. To avoid interference from 0- and to convert Fe(III) to Fe(II) a packed bed electrode is inserted upstream of a wall-jet electrode. The total iron is then determined by the oxidation of Fe(II) on the downstream electrode. The bed is shown to remove 99% of the 02 present in air saturated solutions, thereby removing the necessity for deoxygenating the test solution by bubbling N2. Linear calibration plots have been obtained for iron concentrations from 0.050 to 1.5 mmol dm and the technique has been tested in samples of nutrient film solution.

A model -is presented of the space charge layer formed •in semiconductors deposited as a thin layer on an insula tor substrate. The variations in charge transfer coefficients is stu died for tin oxide electrodes. The anodic current density arising from a tunnel effect is calculated by a simplified expression, which considers the variation in barrier width with applied potential.

Apresentam-se e discutem-se os resultados obtidos no estudo do comportamento de eléctrodos selectivos de suporte de resina condutora com sensor de sulfureto de prata (I) em resposta a baixas concentrações de catião prata(I). As calibrações foram realizadas com soluções padrões de nitrato de prata(I) (>10 M), com soluções tampões de complexaçao obtidos com o sistema catiao prata(I)-EDTA (10 a ~ 10 M em catião prata(I) livre), com soluções tampões de precipitação obtidas com halogenetos de prata(I) (10 a 10 M em catião livre) e com soluções de sulfureto com concentração de anião sulfureto fixada por variação de pC (10 a 10 M em catião prata(I) livre). Observou-se resposta linear em toda a gama de concentrações de 10 a -22 10 M em catiao prata(I).

The electron donor/acceptor character of a variety of organic groups (X),-CH(R) (NR') ,-CH(R) (OR') ,-CH(R) (OCOR' ) ,-CH(R)N and ^C=Y, was Z X investigated by studying the redox properties (by cyclic voltammetry), the electronic absorption and the ''"H n.m.r. spectra of substituted ferrocene complexes (FcX) with those groups as substituents. An ordering of these groups(X) according to their electron acceptor character was proposed on the basis of the electrochemical data, and the results were discussed in terms of the energy dependence of the involved molecular orbitals of FcX on the electronic properties of the substituent group(X).

The polarographic reduction of vitamin B1 was studied as a function of pH by using three different buffer solutions, namelly, acetate, phophate, and ammoniacal solutions. The results show the possibility of following electrochemical1y the transformations of thiamine in its yellow and thiole forms by measuring the height of the corresponding reduction waves for thiamine and its yellow form, and the height of the oxidation wave of the thiole form. A way of calculating the pK value of the transformation TH - YF is also suggested.

Existem muitos casos onde as respostas de frequência (curvas de impedância) nao podem ser predictas pela consideração de um circuito equivalente de Randles. As reacções nao são tão simples como as consideradas (oxidação ou redução num uníco passo) e um tratamento teórico mais elaborado é necessário para interpretar os resultados. Entre estes processos estão aqueles que envolvem intermediários adsorvidos. Nesta comunicação apresentam-se exemplos de reacções de corrosão onde esses intermediários se encontram presentes, as suas respostas no plano complexo de impedâncias e como a simulação de um circuito equivalente correspondente a esses processos mostra a presença desses mesmos intermediários. Estes reflectem-se no dito circuito equivalente pela presença de uma combinação em paralelo de uma resistência e de um condensador o qual, por sua vez, se encontra em série com a resistência de transferencia de carga e a resistência de transferência de massa (impedância de Warburg). Por sua vez, estes elementos estão em paralelo com a capacidade da dupla camada.