Portugaliae
Electrochimica Acta

The electrode processes of Neutral Red (3-amine- 7-dimethylamine-2-methyl-phenazine) have been studied voltammetrically on Glassy Carbon (GC) and on Indium Tin Oxide (ITO) electrodes in buffered aqueous solutions. Multicycle voltammetric treatment of neutral red yields a water insoluble surface film.

Magnesium, one of the lightest nonferrous metals assumes great importance due to many attractive structural, chemical and electrochemical features. The influence of the halide ions at concentrations ranging from 0.1 M to 2.0 M on the corrosion behaviour of primary magnesium has been investigated by galvanostatic polarisation and atomic absorption spectroscopy (AAS). The corrosion kinetic parameters namely open circuit potential (OCP), corrosion potential (Ecorr), corrosion current (Icorr) and Tafel constants (ba and bc) have been calculated from polarisation experiments. These data are interpreted to bring out the effects of the halide ions and their concentration on the corrosion of magnesium in these media. The amount of magnesium anode dissolved and estimated by the instrumental method has been compared with the electrochemical weight loss. The dissolution rate in the former is found to be more than in the latter. Suitable explanations have been forwarded for the excess dissolution of magnesium. The change in pH observed during the polarisation at high current densities has also been investigated and explained suitably in this paper.

The voltammetric behaviour of several ß-alkyloxy substituted meso-tetraphenylporphyrins has been investigated. The first oxidation potential of all compounds depends strongly upon the substituent constant G+. In contrast to this the second oxidation potential is insensitive to this constant. Furthermore, rate constants for the first anodic electron transfer process were determined. The estimated data confirm, that the distortion of the porphyrin chromophore induced by the substituents did not affect the electrochemical rate constant significantly.

Inhibition of corrosion of nickel and nickel base (Ni-Mo-Fe) alloy in 4 percent nitric acid has been studied in the presence of some azole compounds, using electrochemical polarization technique, at various concentrations of inhibitors and at different temperatures (25, 35, U5°C). All the inhibitors investigated were able to suppress both the anodic and cathodic processes appreciably for both the systems. However, in each case the inhibition afforded by these molecules towards the corrosion of nickel in nitric acid is always greater than those for nickel base alloy-nitric acid system. Among these inhibitors, 2-mercaptobenzo-thiazole was found to be the most effective inhibitor for both the systems. These inhibitor molecules, though structurally similar do not follow the same mechanism of inhibition. Either of the two mechanisms, viz., adsorption or surface complex formation or sometimes both of these have been found valid for individual inhibitors.

The construction of the EPR microspectroelectroelectrochemical cryostat equipped with non-aqueous reference half-cell with liquid junction is described. The electrochemically generated, chemically unstable, paramagnetic [0=Re(OEt)Cl2(py)2]+ complex cation was in situ detected and characterized by EPR spectroscopy.

Despite the high success rate with the use of stainless steel and titanium materials in orthopaedic and dentistry for hard tissue replacement, repair and augmentation procedures, it became apparent that biodegradation of these implants did occur. In order to investigate the systemic effects of corrosion products released from these metallic implants, in vitro and in vivo studies were undertaken. The results of these studies revels that significant levels of metal species are released into the biological tissues. The properties and advantages of microelectrodes makes them useful tools for quantification and characterisation of released metal species either by performing in vitro or in vivo studies. Microelectrodes are also used to evaluate the surface modification and mineralization processes that occurs at the implants surface. The goal of these studies is to develop a selective microelectrode' which should be able to monitor the species involved in the biodegradation and mineralization processes.

Corrosivity of natural water controls concentration of soluble oxygen and amount of mineral salts. In order to decrease the corrosivity of water are applied deoxygenation and demineralization methods. This article presents investigations on oxygen and mineral salts removal by ion exchangers. A special kind of a bed which contained two ionic forms: sulphite and hydroxyl was investigated for simultaneous removal of oxygen and anions from water. The natural water from a deep well supplied the bed after its initial decationization on strong acid cation exchanger. The model solutions were used with hydrochloric and sulphuric acids in amounts equal to decationization of water from the well, too. It was stated that the removal efficiency of oxygen and anions was very high. The bed may be used as a dysfunctional ionic column for the simultaneous removal of oxygen and anions if demineralization process is designed by ion exchanger's method. The corrosivity of water may be eliminated and technological installation should be simplified using such a bed.

An automatic acquisition and processing system for titration data based on a PC-compatible is described. The main advantages over the commercial available units are the possibility of using a uniform and objective criterion of potential stability, greater flexibility and much lower price. The titration system and its interfacing to the microcomputer is described as well as the controlling software written in GWBASIC. Examples of potentiometric acid-base titrations using the proposed system are discussed.

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.

Solutions of [CuL] (CI04)2, [CuLCI] (CI04), [CuLBr] (CI04), and [CuL(NCS)2], (L=Ci7H25N3S2; structure below) in acetonitrile give well-defined cyclic voltammograms with a gold disk as working electrode. One electron metal-centered, reversible or quasi-reversible oxidation-reduction processes were observed. Differences in the cyclovoltammetric behaviour of the four complexes correspond to differences in their structures. The half-wave potentials are not unusually high, and only that of [CuL] (CI04)2 falls within the range found for Typel copper proteins. These observations will be described and discussed.

Distributions of the induction times of silver potentiostatic electrodeposition coupled with hydrogen peroxide oxidation/reduction onto carbon microelectrodes have been obtained. The nucleation process has been shown to be affected by kinetic complexities due to the disproportionation of hydrogen peroxide and therefore neither the pure birth model nor the birth and death electronucleation model have shown to be applicable to this system. The double potential step and linear sweep voltammetry measurements point to the formation of silver peroxy species during the electrodeposition process.