S.B. Saidman

CORROSION OF AN AL-ZN-IN ALLOY IN CHLORIDE MEDIA

Abstract In order to explain the electrochemical behaviour of Al–Zn–In based alloys in chloride media, a commercial Al–Zn–In–Si anode and a ternary alloy, Al–5%Zn–0.02%In prepared in our laboratory, were investigated using potentiodynamic techniques, complemented by SEM, EDX and TEM. The influence of alloy composition, agitation and previous cathodisation on the electrochemical response of the alloys was analysed. Results of previous investigations with pure metals (Zn and In) and with binary alloys (Al–In, In–Al, Zn–In) are also considered in this paper for the sake of comparison. The attack initiation on the Al–Zn–In alloy is related to In–Zn rich zones, segregated at grain boundaries. The presence of In in true electric contact with Al and Zn promotes Cl − adsorption at potentials more positive than −1.1 V. Then, the presence of Zn facilitates a surface enrichment of indium by a displacement reaction. This in turn, assures an accumulation of adsorbed Cl − , which maintains an active state of the Al matrix.

Activation of aluminium by indium ions in chloride solutions

The effect of indium ions on the dissolution of aluminium has been investigated in acid Cl− solution by means of potentiostatic and potentiodynamic techniques, complemented by SEM. The activation process (dissolution at a potential more negative than pure Al) depends on the amount of In deposited at the bare Al surface, possibly forming an In-Al surface alloy where Cl− adsorbs. This In disposition reaction will depend on the potential, the actual In3+ concentration, the Cl− concentration and the local pH. At potentials more negative than −1.50 V, the main indium deposition reaction for low In3+ concentration is the galvanic Al/Al3+//InO2−/In reaction. At higher In3+ concentrations, electroreduction of InO2− on quasibare aluminium can occur. At potentials more positive than −1.50 V, the main In deposition reaction is the displacement reaction Al/Al3+//In3+/In and for higher In3+ concentrations, the electroreduction of In3+ can occur.

Electrochemical behaviour of Al-In alloys in chloride solutions

The present work is concerned with the study of the activation mechanism of Al in alloys produced by indium. The electrochemical behaviour of aluminium in NaCl solutions containing In 3+ ions and the dissolution of Al-In and In-Al alloys were studied using potentiostatic, galvanostatic and potentiodynamic techniques, complemented by SEM. It was concluded that the aluminium activation is obtained only when indium comes into a true metallic contact with aluminium within an active pit and in the presence of chloride ions. Polarization curves for dissolution of Al-In and In-Al alloys were compared. The results suggest that the initial step in the dissolution mechanism of the Al-In alloy can be interpreted through chloride ion adsorption on a surface In-Al alloy. This adsorption occurs at more electronegative potentials than that of pure aluminium, thus avoiding repassivation.

Cathodic polarization characteristics and activation of aluminium in chloride solutions containing indium and zinc ions

This work deals with the study of the cathodic and active behaviour of aluminium in acid chloride solutions containing indium and zinc ions. The cathodic behaviour was studied using potentiostatic and potentiodynamic techniques, complemented by SEM and EDX. During cathodization, preferential adsorption of zinc ions occurs, compared to that of H+ and In3+ ions. Once a critical amount of zinc is deposited, preferred indium deposition begins. The synergistic interaction between zinc and indium ions and aluminium leads to its activation in chloride media. This can be explained by displacement reactions that produce an indium accumulation and preferential zinc dissolution. This situation produces a new interface, quasi-free of Zn but rich in In, which favours Cl− ion adsorption at more negative potentials than aluminium, leading to its activation.

Stationary and non-stationary electrochemical response of polycrystalline indium in alkaline media

Stationary and non-stationary responses of polycrystalline indium electrodes in NaOH (0.05–4 M) solution at 25°C were studied. Anodically formed films, their kinetics and chemical stability were discussed as a function of potential domain, time diffusing process, and hydroxide concentration. These processes were investigated through potentiostatic polarisation conventional voltammetry and potentiostatic transients techniques. SEM observations were also applied. The overall anodic reaction can be interpreted through a model comprising a number of parallel reactions following the initial formation of In(OH)ad species at the electrode surface level. The relative contribution of each parallel reaction is determined by the concentration of OH− ion at the interface. The anodic behaviour of indium appears to be under a mixed control, a surface reaction plus a diffusion process. On increasing pH, the control process becomes the OH− ion diffusion through the anodic film.

Electrodeposition of indium and zinc on aluminium

Zinc and indium were deposited from sulphate and chloride electrolytes onto aluminium electrodes under potentiostatic conditions. The role of the anion, pH, cation concentration, cathodic potential and agitation were investigated. The deposit morphology and composition were studied by SEM and EDX. Potentiodynamic and galvanostatic techniques were also applied for product characterization. Once a critical amount of Zn was deposited preferred In deposition began without agitation. But under rotation or at low cathodic potentials Zn2+ discharge became the prevalent reaction. The results support the earlier hypothesis of the preferential adsorption of Zn ions [1].

Influence of In on the corrosion of Zn–In alloys

Abstract The anodic dissolution of Zn and Zn–5%In alloy was investigated in acid chloride and perchlorate solutions by means of polarisation curves and electrochemical impedance spectroscopy. The composition and morphology of surface attack were analysed by SEM/EDX. The presence of In, as a diluted solid solution in a rich Zn phase and as rich In phases segregated at grain boundaries, brings about a decrease of the rate of hydrogen evolution reaction. On the other hand, the strong interaction of In with OH − and Cl − at potentials more positive than −1.10 V vs SCE leads to an increase of the anodic dissolution rate of Zn near the corrosion potential. In perchlorate solutions, this is explained in terms of a reaction path involving the formation of the adsorbate ZnOH as an intermediate species. In chloride solutions, the more negative value of the potential of a zero charge of In allows Cl − to be adsorbed on Zn at more negative potentials increasing its oxidation rate.

Silver deposition on polypyrrole films electrosynthesised onto Nitinol alloy. Corrosion protection and antibacterial activity.

The electrosynthesis of polypyrrole films onto Nitinol from sodium salicylate solutions of different concentrations is reported. The morphology and corrosion protection properties of the resulting coatings were examined and they both depend on the sodium salicylate concentration. The immobilisation of silver species in PPy films constituted by hollow rectangular microtubes was studied as a function of the polymer oxidation degree. The highest amount of silver was deposited when the coated electrode was prepolarised at -1.00V (SCE) before silver deposition, suggesting an increase in the amount of non-oxidised segments in the polymer. Finally, the antibacterial activity of the coating against the Gram positive Staphylococcus aureus and Staphylococcus epidermidis bacteria was evaluated. Both strains resulted sensitive to the modified coatings, obtaining a slightly better result against S. aureus.

The influence of electrode material on NAD+ oxidation

Abstract The formation of oxidation products of NAD+, which catalyses the NADH oxidation, depends on the electrode material employed. Carbon paste electrodes with various pasting liquids and with various pasting liquid/graphite ratios are examined using cyclic voltammetry and amperometric measurements. A qualitative model of the carbon paste electrode is discussed that assumes the existence of surface sites in contact with aqueous media and pasting liquid. Vitreous carbon, reticulated vitreous carbon, pyrolitic graphite and platinum are also used as working electrodes. Anodic pretreatment increases the catalytic current for NADH oxidation associated with the oxidation products of NAD+. This is due to an increase in the surface area during pretreatment, which produces an increased number of surface sites in contact with aqueous media and pasting liquid. NAD+ incorporated into the paste itself results in adsorbed species being strongly attached to the carbon paste surface. The apparent rate constant of the electrochemical reaction is found to be 8.5 s−1.

Anodic behaviour of indium in sodium chloride solutions

Abstract Passive film growth, electrodissolution and pitting corrosion of indium electrodes in NaCl solutions in the pH range 2–12.6 were studied using potentiostatic and potentiodynamic techniques complemented with SEM. In the pH range between 10.5 and 12.6 the oxide formation should involve a mixed control, a surface reaction plus a diffusion process. On decreasing pH the process becomes more complex due to the formation of In(I). The effect of chloride ions upon the active and passive behaviour of indium has been determined. Chloride ions in neutral and alkaline solutions produce pitting at potential values more positive than those of the active—passive transition. Localized corrosion appears to be under a competitive surface mechanism involving the formation of the passive film and the nucleation and growth of an indium chloride salt layer.