Portugaliae
Electrochimica Acta

Guar gum was tested as corrosion inhibitor for carbon steel in 1 M H2SO4 solution using weight loss and Tafel polarization techniques. The results showed that the inhibition efficiency increases with the increasing of the guar gum concentration, which act as an inhibitor of the mixed type. The inhibition action of guar gum was discussed in terms of its horizontal adsorption on the metal surface. The adsorption follows Langmuir adsorption isotherm. The effect of the presence of chloride ion in pitting corrosion was analyzed by the potentiodynamic anodic polarization technique. The pitting corrosion potential changes with the concentration of Cl- ion according to a sigmoid S-shaped curve. This behaviour was explained on the basis of the formation of passivatable, active and continuously propagated pits.

The electrochemical data obtained by cyclic voltammetry on several camphor-type (A, B and C) ligands and derived complexes are revised. A direct positive correlation is established between the redox potentials (anodic or cathodic) of the camphor species and the σp-Hammett parameter of the R group on the camphor skeleton. A study of the effect of the ligand on the redox properties of some Pd, Pt or Cu camphor complexes shows that the ligands influence either the anodic or cathodic processes but the characteristics of the metal drive the potential values.

This work reports on cyclic voltammetry and spectroscopic UV-Vis investigations of some pyridazine derivatives 1-8 in dimethylformamide. In the electrochemical study, monochlorinated pyridazines 2-8 exhibit two reductions but in the case of dichlorinated derivative 1 an additional wave is seen for the reduction of the second carbon-chloride bond. The electronic absorption spectra display an intramolecular charge transfer band π-π* in the UV region of which depend substantially on the nature of both donor and acceptor moieties. These results indicate the π-electron delocalization in the conjugated system.

The bis(cyanoimide) complex trans-[Mo(NCN)2(dppe)2] (dppe = Ph2PCH2CH2PPh2) is susceptible of electrophilic attack to form the acylated and aroylated derivatives trans-[MO(NCN){NCNC(O)R}(dppe)2]Cl (R = Et 2a or Ph 2b). Herein, we report a preliminary study on the electrochemical behaviour of complexes 2 and the derivative trans-[Mo(NCN)Cl(dppe)2][BF4] 3, as investigated by cyclic voltammetry (CV) and controlled-potential electrolysis (CPE), what has allowed a comparison of the electron-donor ability of the ligands.

The electrochemical properties of a series of &[M(R,R’timdt)2&] dithiolenes (M = Ni, Pd, Pt; R,R’timdt = monoanion of disubstituted imidazolidine-2,4,5-trithione) have been studied by cyclic voltammetry, controlled potential coulometry, and EPR spectroscopy. All the compounds undergo two separate one-electron reductions and one two-electron oxidation, which proceed through a single step in the case of nickel complexes, and through two separate steps in the case of palladium and platinum complexes. Inductive effects played by the R and R’ substituents influence the formal electrode potentials of the electron transfer processes. EPR spectroscopy has proved that the unpaired electron in monoanions [M(R,R’timdt)2]- is located on a SOMO, which, even if mainly ligand centred, is significantly contributed by the metal.

The complexation of nitriloacetic acid (NTA) with copper and lead was studied by square wave stripping voltammetry (SWSV) using a mercury iridium microelectrode as the working electrode. The results show that NTA interfere with the measurements, but if the mercury iridium microelectrode is coated with different concentrations of agarose (0.75%, 1.5% and 15% w/v) this interference is greatly minimized. The best results were obtained with 1.5% agarose.

A flux cell with vitreous carbon tubular electrodes, as auxiliary and working electrodes, was tested. The following procedure was used: (1) study and optimization of the mercury deposition conditions at the working electrode, (2) study the electrodes position and distance in order to minimize dispersion of the sample, (3) study of the linearity and reproducibility using different concentrations of manganese (2.10-7 to 6.10-7 M), with differential pulse anodic stripping voltammetry (DPASV) and square wave stripping voltammetry (SWSV). Good linear correlation was obtained for concentration of manganese used.

The rate of mass transfer was studied at (i) a single horizontal cylinder cathode and (ii) a vertical array of closely spaced horizontal cylinders and an array of separated cylinders. The mass transfer coefficients were obtained by measuring the limiting current of the cathodic reduction of alkaline potassium ferricyanide. Variables studied were: cylinder diameter, physical properties of the solution, and nitrogen flow rate. The mass transfer coefficient was found to increase with increasing nitrogen superficial velocity. Decreasing the diameter was found to increase the mass transfer coefficient. The mass transfer data were correlated by the following equations: For single horizontal cylinder cathode: J = 2.138 (Re. Fr)-0.24 For an array of closely spaced cylinders: J = 50.118 (Re.Fr)-0.32 For an array of separated cylinders: J = 100 (Re.Fr)-0.28 Possible practical application of gas sparged array of horizontal cylinders in building electrochemical reactors are discussed.

Four commercial non-ionic surfactant compounds, namely tween 80, 60, 40 and 20, were tested as inhibitors for corrosion of nickel in 1.0 M H2SO4 solution. Weight loss measurements, potentiostatic polarization and cyclic voltammetry techniques were used in this study. It was found that all the four used compounds act as good inhibitors for acid corrosion of nickel. The inhibition efficiencies obtained by the three techniques were almost the same, and increase with increasing the hydrocarbon chain length, the presence of a double bond in the chemical structure of the surfactant and with increasing the surfactant concentration. The polarization studies show that tween compounds act as mixed inhibitors. The inhibition action of these surfactants is interpreted in view of their adsorption on the metal surface making a barrier to mass and charge transfer. It was found that the adsorption of only tween 20 and 40 follows Langmuir adsorption isotherm. The values of free energy of adsorption for them were calculated. It was found that the adsorption process is spontaneous and increases, for different surfactants, in the same direction as inhibition efficiency. The cyclic voltammetry shows that there is only one anodic peak corresponding to the dissolution reaction of nickel electrode. The current of this dissolution peak was used also for corrosion rate measurements and in evaluation of inhibition efficiencies of the used compounds.

The electrochemical oxidation of D-sorbitol and D-mannitol was studied on two sets of stepped platinum single-crystal surfaces vicinal to the basal plane (111), as well as on a set of surfaces with monoatomic steps vicinal to the surface Pt(100). The aim of this study, conducted by cyclic voltammetry, was to check the influence of the crystalline surface structure (regarding terrace and step orientations) on the electrochemical oxidation process of the two polyols in platinum. The electrochemical oxidation reactions of D-sorbitol and D-mannitol were shown to be markedly influenced by the symmetry of the bidimensional domains as well as by the distribution of the orientated defects (monoatomic steps) on the platinum surfaces. The introduction of orientated defects leads to a reduction on peak current densities along with an increase in activity at lower potentials. The terrace width is clearly decisive to the oxidation process on the set of surfaces belonging to the series Pt(S) [(n-1) (111) × (110)].

A rigorous analytical solution for linear diffusion corresponding to the double potential step problem for a reversible multistep process is derived. The expressions obtained are valid for any value of the formal potentials of each step and can be applied to any double pulse technique without restriction on the duration of both pulses. Differential pulse techniques are applied to the study of reversible processes with different number of steps.

The surface of platinum electrodes was modified by electrochemical polymerisation of tyramine to provide binding sites for covalent specific immobilisation of the nucleotide deoxyguanosine triphosphate (dGTP). The EQCM has been used to monitor the growth of polymeric films, which is clearly demonstrated by the decrease in the frequency, corresponding to a continuous mass increase. The carbodiimide coupling reaction was used to bind the terminal 5’ phosphate groups of the dGTP to the available primary amine functions on the polymer surface. The biomolecule immobilisation process was followed by measuring simultaneously the evolution of QC-frequency and open circuit potential. Intrinsic redox signal of guanine base residues provides evidence of the dGTP grafting.

The corrosion of electrogalvanized steel exposed to 0.1 M NaCl was studied using the SVET. Situations of localized corrosion, cathodic protection and corrosion protection due to surface pre-treatment were analyzed, putting in evidence the possibilities of the technique.

Oxidation of the alloy having nominal composition Ni-16Cr-8.5Co(wt.%) was studied in the presence of KCl and K2CO3 at 1173 K in air. Chemical composition of oxide scales and scale morphologies were determined by means of X-ray diffraction analysis and scanning electron microscopic studies. The oxide scales of KCl- and K2CO3- coated alloys showed complex microstructures and compositions. This behaviour was ascribed to the release of volatile metallic chlorides, evolution of CO/CO2 gas as well as formation of fluxing products. The high temperature oxidation resistance is not only related to the nature of the passive film but is also strongly dependent on the salt environments and the structure of alloy.

To evaluate the response of pyrite and arsenopyrite to oxidizing conditions, an electrochemical approach was employed, to set boundaries for the influence of the arsenopyrite content. The effects of galvanic interactions and local pH on the oxidation reaction of pyrite were also studied. With this purpose, artificial two-mineral electrodes were constructed ranging in concentration from 20-80 % arsenopyrite. The resulting cyclic voltammograms were analyzed and relative quantities of oxidation products were evaluated.

The aim of this paper is to evaluate the use of molibdate solutions for pre-treatments of zirconiun surfaces. Zirconium electrodes were held immersed in molibdate solutions for predetermined time intervals. With the subsequent determination of the pitting nucleation potential and the analysis of the surface morphology, it was possible to evaluate the effect of the molibdate concentration and of the solution pH on the pitting corrosion processes. Among the most important results, two are worthy of attention: (i) for all pH values, the immersion of zirconium electrodes in molibdate solutions hinders the pitting corrosion process of the metal, shifting the pitting nucleation potential to more positive values, (ii) the more pronounced shifts of the pitting nucleation potential were observed in alkaline solutions with high molibdate concentration and in acid solutions with low molibdate concentration; this effect was attributed to the adsorption of different species formed at different pH values.

The influence of potential-scan number on the Schiff base self-assembled monolayer was probed by the Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) techniques for the first time. The results showed that the charge transferring resistance (Rct) could be reduced with the potential-scan, suggesting that the structure of the self-assembled monolayer was altered. Also, the relationship between the content of C=N group, represented approximately by the reduction electric charge (Qr), and the monolayer’s packing degree which was represented by the value of Rct, was described. Providing a new way to change the interface characterization is the main contribution of this paper.