Portugaliae Electrochimica Acta

Prof. Doutor João Cabral

Effect of Gas Sparging on the Rate of Mass Transfer at Horizontal Cylinders

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.

Corrosion Inhibition of Nickel in Sulfuric Acid Using Tween Surfactants

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.

Electrochemical Oxidation of D-Sorbitol and D-Manitol on Platinum Monocrystalline Surfaces

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 oxidação electroquímica do D-sorbitol e D-manitol foi estudada para dois conjuntos de superfícies monocristalinas de platina, com degraus monoatómicos, vicinais ao plano basal (111) e ainda para um conjunto de superfícies com degraus monoatómicos vicinais à superfície Pt(100). Este estudo, por voltametria cíclica, teve por objectivo obter informação sobre a influência da geometria de sítios superfíciais (tanto da orientação dos degraus quanto da dos terraços) no processo de oxidação electroquímica dos dois polióis em platina. A reacção de oxidação electroquímica do D-sorbitol e D-manitol é sensível à simetria dos domínios bidimensionais, bem como à distribuição dos defeitos orientados (degraus monoatómicos) nas superfícies de platina. A introdução de defeitos ordenados leva a uma redução geral das densidades de corrente de pico a par de um aumento de actividade aos potenciais menos positivos. A largura dos terraços é claramente decisiva para o processo de oxidação nas superfícies pertencentes à série Pt(S) [(n-1) (111) × (110)].

Multistep Electrode Processes in Double Potential Step Techniques

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.

Oligonucleotide Immobilisation on Polytyramine-Modified Electrodes Suitable for Electrochemical DNA Biosensors

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.

Corrosion of Electrogalvanized Steel in 0.1 M NaCl Studied by SVET

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 Behaviour of IN-738 LC Superalloys in the Presence of Ionic Salts at 1173 K

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.