S. Kapusta

Anodic passivation of tin in slightly alkaline solutions

Abstract The anodic oxidation of tin in 0.2 M borate buffer solution has been studied by potentiodynamic, potentiostatic and galvanostatic techniques. It was found that although a large amount of charge is consumed during anodic oxidation, only a fraction of this charge is involved in film growth. The initial stages of metal passivation under dynamic conditions are best described by Muellers dissolution—precipitation model. Under potentiostatic conditions two different equations describe the kinetics of film formation. For anodic potentials lower than 1.6 V vs sce a logarithmic dependence holds between cathodic charge and time. At higher potentials, the growth rate is controlled by a nucleation process and the rate law is more complicated. Under galvanostatic conditions the logarithms of the growing time is proportional to the inverse of the cathodic charge. These results are discussed in terms of theories of film growth.

Carbon Dioxide Reduction at a Metal Phthalocyanine Catalyzed Carbon Electrode

Metal phthalocyanines (metal = Co, Ni) deposited on carbon electrodes have been found to catalyze the electroreduction of carbon dioxide to formic acid in acid solutions (pH = 3-7). The overpotential for CO/sub 2/ reduction on cobalt phthalocyanine is approximately 200 mV lower than on the best metal cathodes at currents up to 10 mA/cm/sup 2/. The main reaction product in long-term electrolysis is the formate ion at pH>5. Methanol is also produced at lower pH values. The phthalocyanine becomes inactive in the less acid solutions once a fairly negative potential is imposed, probably due to an irreversible reduction of the catalyst.

Capacitive studies of the semiconducting properties of passive tin electrodes

Abstract The semiconducting properties of passive films grown on tin in slightly alkaline solutions have been studied by means of capacitance measurements. The results confirm that different types of films grow at low and high potentials. Thin films follow a Mott-Schottky behavior; both flat band potentials and donor concentrations have been estimated; the location of the donor level has also been determined. The behavior of thick films is more complex, probably due to cracks in the film.

The anodic oxidation of sulfide on platinum electrodes

Abstract The anodic oxidation of sulfide species on a platinum electrode in alkaline solutions has been studied using a wide variety of techniques. Sulfide oxidation results in the formation of a surface layer containing platinum(IV) sulfide and sulfur, that passivates the electrode. Further oxidation is possible only after this layer is removed either by oxidation or reduction. Oxide formation is inhibited in sulfide solutions because of the competing adsorption of S2− and HO−. Similarly, sulfide oxidation to sulfur is practically eliminated on oxide covered surfaces. At high enough potentials, oxidation to sulfate is again possible. The oxide film can be chemically removed at high sulfide concentrations, resulting in further sulfur deposition. The electrocatalytical activity of sulfur deposits towards the oxidation of several compounds (e.g., methanol, CO, formic acid) is probably related to the inhibition of oxide formation. From the potential dependence of the electrode impedance some solid state properties (dielectric constant, resistivity) and the thickness of the surface film have been estimated.

Photoelectrochemical studies of passivating films on tin in slightly alkaline solutions

Abstract The photoresponse of passivating films grown on tin in slightly alkaline solutions has been studied under both galvanostatic and potentiostatic conditions. The response of films growing under galvanostatic conditions was used to determine the nature and magnitude of their deviation from stoichiometry, as a function of potential. The photoresponse of films formed at potentials between 0.5 and 1.6 V can be described by Gartners theory of depletion layer photoeffects. For thicker films the presence of localized levels acting as traps is postulated to explain the slow response and potential dependence of the photocurrents. The spectral response is independent of film thickness. Under sub-bandgap illumination indirect transitions with an absorption edge of 2.55eV are predominant. No evidence of impurity band transitions was found

Electron Transfer Reactions at Passive Tin Electrodes

The kinetics of charge transfer between passive tin electrodes and solutions of have been studied in the pH range 7.5–10.0 and the temperature range 1°–50°C. The results have been analyzed on the basis of the quantum theories of electron transfer at oxide covered metal electrodes, and semi‐quantitative agreement between theory and experiment was found. The electrochemical behavior of passive tin ranges from almost metallic, for very thin films, to that typical of n‐type semiconductors, for thick films. Exchange current densities are, in all cases, much smaller than those measured on bare metal electrodes.

Localized Corrosion of Iron in Alkaline Sulfide Solutions Iron Sulfide Formation and the Breakdown of Passivity

The breakdown of passivity of iron in alkaline sulfide solution is a two‐step process. The first step involves the nucleation of mackinawite patches at faults in the oxide film and their subsequent growth by dissolution of the oxide. The second step is associated with the growth of pits under these patches. At higher positive potentials, mackinawite may be oxidized to and sulfur with simultaneous formation of pyrite. The resistance of passive films to breakdown is affected not only by its thickness but also by its composition (expressed as deviation from stoichiometry). Both are determined by the potential and time of growth. Emphasis was given to the kinetics and mechanism of mackinawite deposition on passive iron, and its relationship with the passivity breakdown process.

A Study of Polymerization of Propargyl Alcohol on Steel

Photon correlation spectroscopy (PCS) and IR spectroscopy methods were used to try to verify polymerization of propargyl alcohol (PA) under homogeneous and heterogeneous conditions. Results of PCS measurements show that in systems where electrochemical measurements suggest strong inhibitive effect no PA polymerization takes place in the bulk of solution. The IR studies did not show any features that could be related to a 3-D polymer film and therefore an adsorption model is favored