Portugaliae
Electrochimica Acta

The corrosion and passivation behaviours of iron in ternary molten Li2CO3-Na2CO3-K2CO3 mixture at different temperatures (475- 550 oC) were studied. The techniques of measurements were: open- circuit potential, galvanostatic anodic polarization, and cyclic voltammetry. The amounts of iron dissolved in the melt were determined by atomic absorption spectroscopy after each experiment. Iron undergoes spontaneous passivation upon its immersion in the melt and the thickness of the oxide scales increases with the increase of temperature. During the passivation potential range different oxides and spinals are formed. These include a cubic solid solution of FeO and a-LiFeO2, a-LiFe5O8 and a-Fe2O3. At high anodic potentials the decomposition of carbonate takes place leading to passivity break down and CO2 and O2 gases evolution. Also, the spoliation of oxide scales in the carbonate melt occurs at the passivity breakdown. The values of corrosion parameters (Rp, io, icorr, ip) were calculated. The different values of activation energy of corrosion process were calculated. The results of cyclic voltammetric investigation indicate the formation of different oxides and spinals, and the cathodic and anodic decomposition of carbonate. The corrosion tests in 0.05 M HCl solution indicate that the oxide scales formed on iron are multilayered. The outer layer is less protective than the inner one.

The interfacial impedance (or shortly, the capacitance) of solid electrodes in the absence of faradaic reactions usually deviates from purely capacitive behaviour. On polycrystalline solid electrodes "frequency dispersion capacitance" is often observed. We present qualitative impedance experiments on pre-treated Ag-15%Cu alloy electrodes immersed in 0.1 M HClO4. The results suggest that models taking into account microscopic geometrical aspects only (i.e. roughness) are not suitable for interpreting the de facto link between roughness and capacitance dispersion. Instead, considering additional points, such as surface heterogeneities on an atomic scale (i.e. surface disorder) together with adsorption of certain ionic species, particularly the Cl-, on the surface of the metal can and do cause capacitance dispersion in the frequency range relevant in electrochemistry.

The inhibiting effect of N-cetyl N,N,N-trimethyl ammonium bromide (CTMAB) on the corrosion of mild steel in 1N H2SO4 solution has been studied by using techniques such as weight loss and electrochemical polarization, infrared (IR) and scanning electron microscopic (SEM) techniques. It is found that CTMAB is good inhibitor for corrosion of mild steel in 1N H2SO4. The maximum efficiency is about 96% for CTMAB at 10-1M concentration. The polarization curves indicate that the compound is mixed type inhibitor. The adsorption of inhibitor on mild steel surface is found to obey Langmuir adsorption isotherm. The activation energy Ea in the absence and presence of inhibitor is obtained by measuring the temperature dependence of corrosion method. The mechanisms of adsorption of CTMAB on mild steel have been investigated by IR spectra and SEM.

Copper nitroprusside (CuNP) was generated on the graphite paste electrode using a new methodology of the preparation. The electrochemical studies were carried out by cyclic voltammetry technique. The cyclic voltammogram of the modified electrode, CuNP showed two redox couples (E0’)1=0.22 and (E0’)2=0.88 V vs. SCE attributed to Cu(I)/Cu(II) and Fe(II)(CN)5NO/Fe(III)(CN)5NO, respectively. The nature of the cation affect the (E0’)1 and (E0’)2, as current intensity, shifting the E0’ for more positive potentials, for two redox processes. The voltammograms obtained with different KCl concentrations (0.1-3.0 mol L-1) exhibit a shift in the (E0’)1 to more positive potentials; this change was linear with the supporting electrolyte concentrations change. It was verified that the (E0’)1 remained practically constant at pH between 6 and 3. However, a new process with (E0’)3 (0.48 V) appears at pH<3 and it was ascribed to formation of intermediary species. The redox couple at (E0’)2=0.88 V presents an electrocatalytic response for N-acethylcysteine. The modified graphite paste electrode gives a linear response between 5.0´10-4 to 1.0´10-2 mol L-1 of N-acethylcysteine with a detection limit of 4.5´10-4 (±5%) mol L-1 (n=3) and an amperometric sensitivity of 4.9 mA/mmol L-1. The electrocatalytic oxidation of N-acethylcysteine compounds by the mediator has been used for the determination of N-acethylcysteine in a commercially pharmaceutical available product.

The electrodeposition of tin in presence of tartrate ions has been analysed by electrochemical techniques, mainly chronoamperometry, and by scanning electron microscopy (SEM). The obtained values of nucleus density with both techniques have been compared and discussed. The electrodeposition process follows an instantaneous nucleation with 3D growth under diffusion control at the initial times of the process, but a second nucleation process occurs at higher times. The influence of tartrate and of agitation conditions is also inferred from the crystal morphology.

The electro-oxidation of toluene in 0.1 M But4NPF6 + CH3CN solutions on glassy carbon electrodes was studied using electrochemical and spectroelectrochemical techniques. Toluene electro-oxidation yields an electrochemical inactive film on the electrode surface. In situ Fourier Transformed Infrared (FTIR) studies suggest the formation of a polymeric film, as the main product, on the surface. Depending on the experimental time scale toluene transformation is a complex reaction that could involve adsorption processes, pure charge transfer reaction and couple chemical reactions. Additions of small quantities of water to the electrolyte cause a cathodic displacement of the oxidation peak potential; in other words, the electro-oxidation reaction is favoured.