L.M. Gassa

Magnesium and its alloys as degradable biomaterials: corrosion studies using potentiodynamic and EIS electrochemical techniques

Magnesium is potentially useful for orthopaedic and cardiovascular applications. However, the corrosion rate of this metal is so high that its degradation occurs before the end of the healing process. In industrial media the behaviour of several magnesium alloys have been probed to be better than magnesium performance. However, the information related to their corrosion behaviour in biological media is insufficient. The aim of this work is to study the influence of the components of organic fluids on the corrosion behaviour of Mg and AZ31 and LAE442 alloys using potentiodynamic, potentiostatic and EIS techniques. Results showed localized attack in chloride containing media. The breakdown potential decreased when chloride concentration increased. The potential range of the passivation region was extended in the presence of albumin. EIS measurements showed that the corrosion behaviour of the AZ31 was very different from that of LAE442 alloy in chloride solutions.

Kinetics and mechanism of the electroreduction of anodic layers formed on lead in 5 M H2SO4 at 25°C

Abstract The potentiostatic electroreduction of anodic layers containing Pb(II)-species formed on lead has been investigated in 5 M H 2 SO 4 at 25°C. By properly adjusting both the anodic layer formation and the electroreduction conditions three well-defined processes could be distinguished. (i) The electroreduction of the primary PbSO 4 layer which involves mainly an instantaneous nucleation and three-dimensional growth mechanism under diffusion control. (ii) The electroreduction of the PbO layer, which builds up progressively beneath the initially grown PbSO 4 porous layer, consists of two current contributions; one term can be associated with an instantaneous nucleation and 2D growth under diffusion control and the other one, which is the main current contribution, can be related to a progressive nucleation and 3D growth mechanism under charge transfer control. The latter includes a death correction term for either the growing or the disappearing right circular cones phase centres. (iii) The electroreduction of the composite PbSO 4 -PbO layer which can be successfully described in accordance with a complex mechanism by taking into account the simultaneous influence of the two single surface layer constituents.

A study of thin anodic WO3 films by electrochemical impedance spectroscopy

The properties of thin anodic passive films potentiostatically formed (1 V ≤ Ef ≤ 5 V vs sce) on polycrystalline tungsten in aqueous H3PO4, H2SO4, HNO3 and HClO4 solutions (pH ≈ 1.3) were studied using electrochemical impedance spectroscopy at room temperature. The data were analysed with a transfer function using a non-linear fitting routine, assuming that the resistance of the film is coupled in series with the faradaic impedance of the W(0) → W(VI) reaction, with these in parallel with the capacitance of the metal/passive film/electrolyte system. The relative permittivity (er) of the films was found to depend on the acid solution used. Except for the case of films grown in HNO3, the values of er, are smaller than those of thicker films grown galvanostatically. The film grown in H3PO4 solutions presented the lowest value of er, which could be due to the incorporation of phosphate ions into the film. The number concentration of donors in the films (ND) was found to decrease with Ef (ie increasing film thickness) and to be quite dependent on the electrolyte used: ND(H3PO4) < ND(HClO4) < ND(HNO3) < ND(H2SO4). The values for ND(H3PO4) are, nevertheless, 103 times larger than the ones for much thicker WO3 films and are of the same order of magnitude as the ones for annealed WO3 films. The flat band potential of the films grown in H3PO4 was found to be more than 0.1 V smaller than those for films grown in the other solutions.

An investigation by EIS of gramicidin channels in bilayer lipid membranes

Abstract The influence of Li + , Na + , K + , and Cs + ions on the characteristics and properties of Black Lipid Membranes (BLM) made of diphytanoylphosphatydilcholine (DPhPC), with thickness thinner than 10 nm, were investigated using electrochemical impedance spectroscopy (EIS). The impedance spectra of the base system including the film and its adjacent chloride ion containing aqueous solutions exhibited for all tested cations a single capacitive contribution. The corresponding time constant can be discussed in terms of a model which includes a dielectric dispersion in the membrane. BLM films doped with a low concentration of Gramicidin A have exhibited values of conductance in contact with electrolytes contain ing different cations at low aqueous concentration of Gramicidin A, that indicate ionic transference from one side to the other of the membrane. This phenomenon was detected by using either dc or ac voltage perturbations. Data obtained using EIS techniques show two time constants, which can be assigned to the membrane and to the channel/membrane contributions, respectively.

An electrochemical impedance spectroscopy study of electrodeposited manganese oxide films in borate buffers

The reduction-oxidation processes and conducting properties of manganese dioxide films prepared by electrochemical deposition on Pt substrate were studied in borate/boric acid solutions using electrochemical impedance spectroscopy (EIS). The characteristic parameters of the film can be determined as a function of the manganese oxide deposition routine as well as film thickness by application of transfer function analysis using non-linear fit routines. A model developed for thin layer intercalation electrodes, which is based on the insertion of protons and electrons, was used and the diffusion coefficients of protons were calculated for different manganese oxide thicknesses. The reversible behaviour of the manganese oxides during discharge-charge processes as a function of deposition routines was analyzed. Data obtained under a wide variety of experimental conditions show that the film generated on Pt substrate behaves as a n-type semiconductor.

Characterization of passive layers formed on lead by electrochemical impedance spectroscopy

Abstract The corrosion process and characteristics of the surface layer formed on lead in concentrated sulphuric acid solutions were studied using electrochemical impedance spectroscopy (EIS) and rotating-disc electrodes. Data obtained under a wide variety of experimental conditions show that the surface films generated in the passive potential range consist of various Pb(II)-containing species. The inner barrier PbO layer, which builds up progressively beneath the initially grown PbSO 4 porous layer, behaves as a p-type semiconductor. The influence of its formation potential on the semiconducting properties and diffusion processes is discussed from impedance spectra by application of transfer-function analysis using non-linear fitting routines.

Electrochemical impedance spectroscopy of thiourea electro-oxidation on copper electrodes in aqueous 0.5 M sulphuric acid

The anodisation of copper electrodes in aqueous thiourea (TU) containing 0.5 M sulphuric acid is studied by electrochemical impedance spectroscopy (EIS) combined with rotating disk electrode and ring-disk electrode, SEM and EDAX techniques. For E<−0.35 V (vs. MSE), Nyquist plots show two time constants. The first one, which appears at high frequencies, involves the contribution of the double layer capacity and the charge transfer resistance related to the electro-oxidation of TU to formamidine disulphide (FDS). The second time constant, which is observed at low frequencies, is related to complex electrochemical and chemical processes following TU electro-oxidation to FDS. Depending on the applied potential, this time constant can be assigned to either an electroadsorption process from TU electro-oxidation products or an electrochemical process under diffusion through an anodic film. At high TU concentration, the low frequency time constant corresponds to the typical response of a pure capacitor due to the formation of a thick anodic film under a diffusion controlled process. In this case, for E≥−0.35 V, Nyquist plots exhibit at least three time constants and an inductive loop. The inductive loop is due to surface relaxation associated with copper pitting. The formation of different types of anodic films is confirmed by SEM observations. EIS results are consistent with the complex reaction pathway previously proposed for the anodisation of copper in aqueous TU-containing acid solutions.

Comparative study of the passivity and the breakdown of passivity of polycrystalline iron in different alkaline solutions

The passivity and passivity breakdown of polycrystalline iron is investigated through electrochemical methods complemented with ellipsometry, by employing plain 0.04 M NaOH and saturated Ca(OH)2 solutions with the addition of chloride salt of concentrations up to 1 M. The comparative results indicate that the initial reactions of Fe(OH)2 formation at the inner part of the passive layer level (Fe3O4) are only slightly influenced by either the cation or the presence of Cl− ion in solution, in contrast to their large influence at the outer part of the passive layer (hydrous FeOOH species) where Fe2+/Fe3+ redox reactions take place. At high positive potential the breakdown of passivity is also influenced by both Cl− ion and cation present in solution.

A potentiodynamic study of anodic film formation on nickel in borate solutions

The potentiodynamic behaviour of Ni in buffered borate electrolytes (7.6 ⩽ pH ⩽ 9.3) is investigated. The reactions taking place in the potential region of the Ni/Ni(OH)2 and Ni(II)/Ni(III) redox couples can be formally interpreted with the same reaction patterns postulated for the electrochemical behaviour of nickel in strongly alkaline media including the multiplicity of phases related to the corresponding reactants and products. The main differences between the electrochemical characteristics of nickel over the whole pH range are in the growth rate of the anodic film and the change of its structure in relation to its degree of hydration, overall oxidation state and inner ionic composition.

The electrochemistry of nanostructured Ni–W alloys

This work reports on the features that Ni–W nanostructured alloys, electrodeposited on carbon steel by different current pulse programs, may present depending on their surface morphology and surface composition. The Ni–W nanostructured coating, with a cauliflower structure, lack of fragility, and high WO3/W surface composition ratio, is a stable electrode to catalyze hydrogen evolution reaction, exceeding bulk and electrodeposited Ni catalytic activity. Also, the nanostructured alloys must have a low WO3/W surface composition ratio for Ni and its oxides to provide protection and improve corrosion resistance in sulfate media.