Portugaliae
Electrochimica Acta

In the chemical synthesis of polypyrrole (PPy), pyrrole oxidative polymerization was done. The use of alkali salts viz. FeCl3, Fe(NO3)3, as an oxidative polymerizer, creates impurities, affecting the purity of formed PPy. In this work, highly pristine PPy flexible electrodes (FE) were chemically synthesized using a 0.1 M solution of pyrrole acidified with 0.05 M HClO4 and 30 wt % H2O2, by the successive ionic layer adsorption and reaction (SILAR) method. FEs XRD patterns indicate amorphous PPy formation. A characteristic peak at 1559 cm-1 in the FTIR spectra confirms PPy formation. A gradual change in the surface morphology with SILAR immersion time was observed. Nyquist plots describe different impedance parameters, which vary with SILAR immersion times. In cyclic voltammetric analysis (CV), the maximum specific capacitance observed for the FEs prepared with an immersion time of 10 s was 520.48 Fg-1. Specific capacitance calculated using the galvanostatic charge discharge (GCD) analyses was 529.70 Fg-1, which was nearly the same as that observed by CV. FEs exhibited great cycling stability, even after 5000 cycles, with 74.43% of capacitance retention. Solid state symmetric device (SSD) produced specific capacitance as high as 312.28 Fg-1, with 76 % of retention, even after 5000 cycles.

In this paper, corrosion resistance was compared between molybdate conversion coatings containing SiO2 or TiO2 nanoparticles, on five magnesium alloys exposed to a 3.5 wt.% NaCl solution. The surface morphology of the molybdate conversion coatings containing SiO2 or TiO2 nanoparticles was examined by scanning electron microscopy (SEM). The corrosion resistance of the nanocoatings was investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and potentiodynamic cyclic anodic polarization measurements. The electrochemical methods showed that the addition of SiO2 or TiO2 nanoparticles to the conversion treatment has significantly improved the corrosion resistance property of molybdate conversion coatings for all tested Mg alloys, but revealed that TiO2 was more effective.

Cysteine’s action on 3003 aluminum alloy dissolution in a 2% NaCl solution, at different pH values, has been investigated. Weight-loss measurements, potentiodynamic polarization and electrochemical impedance at OCP were performed. The surface morphology was analyzed by SEM. The main results showed that the alloy underwent severe corrosion at 2 and 11 pH values, as compared to at 5 and 8 pH values. Furthermore, a duality effect of cysteine was noted, as it acted as a corrosion inhibitor or accelerator, depending on its concentration, and on the solution’s pH value. This particular behavior can be related to the instability of the formed film adsorbed onto the metal surface. All these findings show the high complexity of cysteine action on 3003 aluminum alloy, under the investigated conditions.

Three quaternary ammonium bromides were investigated as mild steel corrosion inhibitors in 1 M HCl. Two investigation methods were applied: electrochemical impedance spectroscopy (EIS) and polarization resistance (Rp method). The experimental impedance data showed capacitance frequency distribution, which imposed the introduction of a constant phase element, instead of a pure capacitance. Adequate structural models were advanced, to describe the interfacial processes. The models parameters values were determined. The resistance values found by impedance spectroscopy were compared to those of the polarization resistance provided by the Rp method. The inhibition efficiency of the investigated compounds was evaluated by both applied methods. The comparison of the investigated compounds’ protective properties outlined the best among them.

Catechol, which underwent electrochemical oxidation to produce o-benzoquinone, as Michael acceptor, taking part in a nucleophilic attack by dimethylamine, has been studied in an aqueous solution, with various pH values, different electrodes and different dimethylamine concentrations, using cyclic voltammetry, controlled potential coulometry and differential pulse voltammetry. The participation of o-benzoquinone reaction with dimethylamine, at higher nucleophiles concentrations, in the second potential’s scan, was observed. The products generated from the reaction were assumed to be 4-(dimethylamino)benzene-1,2-diol, which underwent electrons transfer at more negative potentials than those from catechol. Catechol pH effect, in dimethylamine presence, was studied by varying pH values from 3 to 9. The reaction was strongly influenced by pH, as well as by dimethylamine concentration. The reaction was mostly favorable in 150 mM of dimethylamine and 2 mM of catechol in a neutral medium. In both acidic and basic conditions, the reaction was not favored, due to amine protonation and hydroxylation. The reaction mechanism was of the ECE type, followed by the diffusion process.