Portugaliae
Electrochimica Acta

The effect of both the type and thickness of three anticorrosive paints used as primers (chlorinated rubber, vynyl and alkyd binders) on the oxygen transport were studied. The permeability coefficient of oxygen was calculated measuring the evolution of DC current with time for coated steel samples immersed in 3 % NaCl solution whith their oxygen being varied by bubbling either oxygen, air or nitrogen. Before, during and after the immersion period a visual inspection of the coated system was also carried out. The results obtained make it possible to conclude that: a) the polarization of the coated steel and the current flux measurements provide useful quantitative information about the oxygen transport through each coated metal/electrolyte system; b) it was verified that the measured current is due to the oxygen reduction on the coated steel surface and its limiting value is characteristic of the system tested; this value was determined by the oxygen transport through the coating and c) the method used is simple and allowed the following order for the tested paints, as a function of their protective efficiency: chlorinated rubber > vynyl > alkyd.

The influence of surface preparation such as annealing, cooling, quenching of Pt(lll) and the way of contacting to the electrolyte on characteristics of Pt(111) in 0.1M h^SO^ and underpotential deposition (UPD) of copper on such treated surfaces have been investigated using cyclic voltammetry. Effects of Cu+ and CI" ions concentration on the UPD of copper have been monitored using coulometric data and the results are discussed in terms of coadsorptions of Cu+ and CI-.

Carbonation by atmospheric carbondioxide is a problem in affecting the performance of electrodes used in alkaline batteries. Zinc electrodes suffer loss of shelf-life, discharge capacity and cycle life when carbonates contaminate the electrolyte. Cyclic voltammetric studies on zinc in alkaline carbonate solutions revealed the formation of two types of passive films. Increase of carbonate and hydroxyI ion concentrations causes the dissolution of both these films. The enhanced dissolution is responsible for the loss of cycle life. Detailed mechanism of film-formation and dissolution is discussed.

The concept of ionic solvation and the way it has been presented in the literature is critically discussed.

A new corrosion inhibitor namely 2-salicylideneamino-6-methyl-benzothiazole (SAMEBT) has been synthesised and its inhibiting action on the corrosion of mild steel in 1 M HCI and 0.5 M H2SO4 has been investigated by weight loss measurements and potentiostatic polarisation studies. It behaves as a mixed type inhibitor in the above acidic solutions. It has been found to inhibit the dissolution of steel in both acid solutions by blocking the active sites of the mild steel surface. Hydrogen permeation studies and A.C. impedance measurements indicate the improved performance of the compound in HOI. The adsorption of SAMEBT on the steel surface from both acids is found to obey Temkin's adsorption isotherm.

Careful experiments were done with isotonic and hypertonic electrolytes in reference electrodes as well as with different forms of accurately making the liquid junction (open junction, dialysis membrane and frit restricted junctions) in cell assemblies involving ion selective electrodes. The ionic strength of test solutions was varied adding NaCI at concentrations which limit the physiological range. The experiments show that dialysis membrane restricted liquid junctions follow the same trend as open junction when NaCI concentration varies, contrary to what had been found in experiments made with modified commercial analysers by other authors. Solutions with and without added albumin were studied. Clear evidence that protein influences the liquid junction potential and thus measurements with ion selective electrodes is presented.

The pitting corrosion of mild steel was studied in 0.1 M NaCI + 0.015 M NaOH in water-glycerol, water-ethylene glycol, water-ethanol and water-dioxane solutions with various compositions (from 0.0 to 60 %v/v of the organic solvent component). The results obtained from the potentiodynamic and potentiostatic measurements show that the pitting corrosion is inhibited by the organic components in the medium. The inhibition percentage was increased with the increasing of the concentration of the organic solvent in the medium and reached about 80%. It is suggested that the organic solvents inhibited the pitting corrosion of mild steel by (i) increasing the viscosity of the medium, which leads to a decrease in the diffusion coefficient of the corrosion products. It also decreases the dielectric constant of the medium, which lowers the basicity of the solution, and (ii) adsorption of solvent molecules on steel surface.

Gasometric measurements and galvanostatic polarisation studies of different grades of magnesium in different electrolytes have been carried out at different current densities ranging from 0 - 100 mA cm-2. A relationship between current density and the rate of hydrogen evolution has been observed. The predominant role of anions in corrosion and other unusual behaviour of magnesium in aqueous solutions of electrolytes have been confirmed in this investigation. Cations are also found to play a decisive role in the anomalous behaviour of magnesium in these solutions. The present study has made it possible to arrive at a new parameter called 'Transition current density' denoted by the symbol *i^ _ * at which a transition from positive difference effect to negative difference effect or vice versa occurs. The dependence of this parameter on system variables has been explained suitably. Key words: magnesium, difference effect, transition current density, aqueous environments.