Portugaliae
Electrochimica Acta

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.