Portugaliae
Electrochimica Acta

A study of the chemistry of chromium in the passive film on Aluminium substrates after ion implantation, electrochemically grown in aqueous solutions of carbonate/bicarbonate with added chloride ions, was done by X-Ray Photoelectron Spectroscopy (XPS). The results confirmed that Cr^+ is responsible for passivation and also that pitting of Al-Cr alloys occurs when the concentration of Cr*+ in the film is reduced due to oxidization to the a more soluble species, Cr^+. Clearly, enrichment of the passive film in the solute element of the alloy is beneficial, but the pitting potential is chromium concentration dependent, exhibiting a maximum. Results will be discussed in terms of the ability of Cr to produce a more effective hydrolysis when compared to Aluminium ions.

It is proposed that the passivation region for Aluminium could be extended, outside the 4 to 9 pH range, by surface alloying with W and Ta, using ion implantation as a non-equilibrium alloying method. Extension into the acid region was achieved by surface alloying with W. This is justified in virtue of the thermodynamic stability of the solute oxide. As Tantalum exhibits passivation all across the pH range, it is reasonable to believe that passivity of Aluminium could be induced or at least greatly improved by alloying with W and Ta [1]. A fluence of 1.3E16 (W +Ta).cm"2 at different ion beam energies, (40 KeV and 150 KeV was used for surface alloying 99.999% purity Aluminium from Good Fellows Metals. The passivation of the produced implanted alloys was studied by voltammetry and impedance spectroscopy. Pre-passivated samples, in acid sulphate solutions were used in neutral 0.1M NaCl solutions. The pitting potential values are more active when polarizing without pre-exposure to acid solutions; this seems to support the idea that some Ta is lost in acid media by dissolution through the film, which is probably a mixture of Ta205 and AI2O3 in proportions that would depend on potential amongst other factors. A comparison with the values obtained using the same experimental conditions, but without pre-passivation of the sample, is made. According to the dc and ac electrochemical data the protective character of the Al implanted alloys is presumably due to the formation of a passive film containing probably a mixed oxide of Ta205 and AI2O3. The passivation model seems to agree with the solute rich interfase model [2-3], which requires that the solute forms a more protective passivating species than the Aluminium under occluded cells conditions.

The effect of perchlorate ions on the electrochemical behaviour of copper in near neutral solutions was studied using cyclic voltammetry. Results showed that perchlorate ions do not interfere with the formation of the duplex oxide film on copper in the potential range before breakdown. In fact, results seem to be in agreement with the passivation model for Cu in mild alkaline solutions. There is a solubilization of the film that gives place to Cu (I ) soluble species, that are collected at -0.875 V (SCE). Solubilization of the film acidifies the interface and precedes breakdown with nucleation and growth of pits. Soluble species of Cu (I) and Cu (n), related to breakdown, were detected in the cathodic cycle.

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.