Portugaliae
Electrochimica Acta

The corrosion behavior of Zn-Al-Cu alloy was investigated in HCl solution in absence and in presence of different concentrations of inhibitors. The techniques of measurements were: weight loss, linear polarization and galvanic static polarization. It was found that the corrosion rate of this alloy was higher than that of zinc or aluminum. The inhibitors used were: heterocyclic piperidine (PP), 2- methyl piperidine (2mp), 3-methyl piperidine (3mp) and 4- methyl piperidine (4mp). These inhibitors are mixed type and their inhibition efficiency, I%, increases according to the order: 2mp < 3mp < pp < 4mp. The values of activation energy of corrosion were determined in absence and in presence of inhibitors. It was found that the presence of these inhibitors increases the values of activation energy. The adsorption of these inhibitors on the surface of the alloy follows Frumkin’s isotherm.

The inhibitive effect of the gum exudate from Acacia seyal var. seyal on the corrosion of mild steel in drinking water was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results obtained show that gum exudates could serve as effective inhibitors for the corrosion of steel in drinking water network. The percentage inhibition increases with increasing the concentration of the gum at 30 oC. The percentage inhibitor efficiency above 95 % was attained at gum concentration 400 ppm. The corrosion rates of steel and inhibition efficiencies of the gum exudates obtained from impedance and polarization measurements were in good agreement. Potentiodynamic polarization studies clearly reveal that the gum behaves predominantly as an anodic inhibitor. The study also shows that the inhibition efficiency was insignificantly affected by the temperature rise of the medium.

Exudates gums from Pachylobus edulis (PE) and Raphia hookeri (RH) were evaluated as corrosion inhibitors for aluminium in HCl using weight loss and thermometric measurements at 30 – 60 oC. The exudates were found to retard corrosion rate of aluminium. The inhibition efficiency (%1) increased with increase in concentration of the exudates. Increase in temperature increased the corrosion rate in the absence and presence of inhibitors but decreased the inhibition efficiency. Both PE and RH exudate gums were found to obey Temkin adsorption isotherm and Kinetic-Thermodynamic Model of El-Awady et al. at all the concentrations and temperatures studied. Phenomenon of physical adsorption is proposed from the activation parameters obtained. Thermodynamic parameters reveal that the adsorption process is spontaneous. Exudate gum from RH was found to be a better inhibitor than PE.

This work is a study of 2.5-difuryl-N-methylpyrrole (B1) compound on the corrosion of steel in HCl medium. The investigation was carried out using gravimetric and electrochemical polarisation measurements. The corrosion rate decreases continuously with concentration of the compound (B1) and the efficiency increases to reach 90 % at 10-3 M. Polarisation results indicate that B1 is a cathodic inhibitor. B1 is adsorbed on metal surface according to the Langmuir isotherm. Value of free enthalpy of adsorption obtained involves chemisorption between inhibitor molecules and metal surface.

The influence of heat treatment on the corrosion behavior of two aluminium matrix composites (AA2014/15 vol% Al2O3p - composite A and AA2009/20 vol% SiCw -composite B) was analyzed in 3.5% sodium chloride solution. The kinetic of the corrosion process was studied based on the gravimetric measurements. The corrosion damage and pit shape were analyzed by Image Analyzer. The corrosion damage in both composites was caused by the pitting attack on the surface. Pit shape of the composite A is different to the composite B. The corrosion rate of composite A is higher than composite B because the voids or gap between reinforcement particles and matrix are larger than in the composite B. The main attack of nucleation sites was at the interface region of the matrix and the reinforcement.

The inhibiting action of the calyx extract of Hibiscus sabdariffa on mild steel corrosion in 2 M HCl and 1 M H2SO4 solutions was assessed using a gasometric technique. The results demonstrate that Hibiscus sabdariffa extract suppressed the corrosion reaction in both acid media and inhibition efficiency increased with extract concentration with slightly higher values obtained in 1 M H2SO4. Synergistic effects increased the efficiency of the extract in the presence of halide additives. Adsorption characteristics of the extract were approximated by the Langmuir isotherm. The inhibition mechanisms, estimated from the temperature dependence of inhibition efficiency as well from kinetic and activation parameters show that the extract functioned via mixed-inhibition mechanism. It is suggested that molecular as well as protonated organic species in the extract contribute to the observed inhibiting action.

Organo-clay complex of ligand-hexadecyltrimethylammonium with montmorillonite was made for the purpose of application as a preconcentration agent in a chemically modified carbon paste electrode for determination of mercury (II) in aqueous solution. It was found out that the adsorption of Hg(II) by organo-clay complex is independent of the pH of the solution. It was also found out that the adsorption of the remaining metals Cd(II), Pb(II), Cu(II), Zn(II), and Ni(II) was dependent on the changes in pH solutions and increased when it varies from 1 to 8. The resultant material was characterized by cyclic and differential pulse anodic voltammetry using a modified graphite paste electrode in different supporting electrolytes. The mercury response was evaluated with respect to pH, electrode composition, preconcentration time, mercury concentration, possible interferences and other variables.

In the present paper, the electrochemical behaviour of four based iron FeCoC ternary alloys was studied in non deaerated solution of 10-3 M NaHCO3 + 10-3 M Na2SO4, at 25 °C, and compared to that of pure iron and cobalt. The inhibition effect of sodium molybdate has also been considered for different concentrations. It was noted that the oxygen oxidizes the FeCoC ternary alloys except that with the most important cobalt content. For this reason, a potentiodynamic polarisation was carried out at different temperatures (25, 35, 45 and 55°C) in which the current growth versus temperature indicates really that molybdate oxidizes this alloy.

The inhibitive effect of exudate gum from Dacroydes edulis in the corrosion of aluminium in HCl solutions was studied using weight loss and thermometric methods at 30-60 oC. The results reveal that the exudate gum acts as an inhibitor for corrosion of aluminium in HCl solution. The inhibition efficiency increases with an increase in the concentration of the exudate gum but decreases with increase in temperature. The Temkim adsorption isotherm was tested for its fit to the experimental data. The result confirms that the corrosion inhibition of the exudate gum from DE is attributed to the adsorption of molecules of phytochemicals present in the exudate gum on the surface of the metal. The free energies and equilibrium constant for the adsorption process were determined. A mechanism of physical adsorption is proposed.

The effect of 1-{[benzyl-(2-cyano-ethyl)-amino]-methyl}-5-methyl-1H-pyrazole-3-carboxylic acid methyl ester (P1) and 1-{[benzyl-(2-cyano-ethyl)-amino]-methyl}-5-methyl-1H-pyrazole-3-carboxylic acid ethyl ester (P2) was evaluated as corrosion inhibitors of steel in molar hydrochloric using weight loss measurements and electrochemical polarisation. The results obtained reveal that those compounds reduce the corrosion rate. The inhibiting action increases with the concentration of pyrazole compounds to attain 98.5 % at the 10-3 M of (P2). The increase in temperature leads to a decrease in the inhibition efficiency of the compounds in the temperature range 308  353 K. The adsorption isotherm of inhibitors on the steel has been determined. The thermodynamic data of activation and adsorption are determined.

The influence of 3-hydroxy-2-methylquinazoline -4-one on the corrosion inhibition of mild steel in 1 M HCl and 1 M H2SO4 has been studied using weight loss, and galvanostatic polarization studies. Result obtained reveals that this organic compound is a very good inhibitor and its inhibition efficiency increases with increasing its concentration to attain 92% at 1000 ppm at 25 ºC. The potentiodynamic polarization study indicates that this compound acts as a mixed type corrosion inhibitor. The rate of corrosion of mild steel rapidly increases with temperature over the temperature range of 25 ºC to 55 ºC, both in absence and presence of the inhibitor. Thermodynamic parameter for adsorption process has been calculated using the Langmuir’s adsorption isotherm.

The chemical and mineralogical compositions in Ordinary Portland Cement (OPC) were determined and cement which is low of C3A is more easily exposed to sulfate environment. The relation between physical strength of mortars sized 150  150  150 mm and the effects of different concentrations of sulfate solutions, SO42- (1%, 3% and 5%) for 3, 14 and 28 days was found to be that stronger the sulphate concentrations, the greater the sulfate attack, but in a weaker concentration situation, the attack was more efficient as the comparable damaged being achieved with smaller amount of sulphate. The morphological studies were observed under scanning electron microscope (SEM) and the distribution of the main component, including Mg2+ and SO42- ions, was analysed by Energy Dispersive X-Ray Analyzer (EDS) to observe the cracks and reactions.

This article is based on the plenary lecture that I was invited to present in the XIV Encontro da Sociedade Portuguesa de Electroquímica and X Iberic Meeting of Electrochemistry, a meeting which was “a tribute to the late Professor César Viana”. The best way I found to honour his memory was to review the highlights of a recent investigation on the computer simulation of solution/electrode interfaces carried out at our molecular simulation group, which would never happen without the enthusiasm and support of Prof. César Viana. In this paper we focus on the determination, from first principles, of the interaction potentials between ethanol and gold surfaces aiming at their use in the computer simulation of adsorption and self-assembly processes on noble metal electrodes. An analytical force field has been proposed and fitted to DFT results, by means of genetic algorithms. The force field has been tested against Monte Carlo simulations. Finally, the non-trivial, or even impossible, task of finding analytical functions that accurately match ab initio/DFT data, when many molecular degrees of freedom are involved, is briefly reviewed. The alternative of using neural networks to approach potential energy surfaces (PES) is presented and applied to the specific case of the ethanol-Au interactions. The article is signed by the persons who mainly contributed to this work. They all praised very much Prof. César Viana. It is our tribute to his memory.

This paper reports the mineralization of an aromatic pharmaceutical as clofibric acid and the dye indigo carmine in 0.05 M Na2SO4 of pH 3.0 by electrochemical advanced oxidation processes such as anodic oxidation without and with electrogenerated H2O2, electro-Fenton and photoelectro-Fenton using a boron-doped diamond (BDD) anode. These procedures produce the strong oxidant hydroxyl radical in the form of BDD(•OH) from water oxidation at BDD and/or •OH from Fenton’s reaction between added Fe2+ and H2O2 generated at the cathode by two-electron O2 reduction. Comparative degradation of both compounds is performed with an undivided electrolytic cell of 100 mL with an O2-diffusion cathode. The effect of current density and pollutant concentration on the degradation rate and current efficiency of the different methods is discussed. The decay kinetics of clofibric acid and indigo carmine and the evolution of their aromatic by-products and final carboxylic acids like oxalic and/or oxamic are described to clarify their reaction sequences. Anodic oxidation is able to completely mineralize both compounds mainly with BDD(•OH). Aromatic pollutants are more rapidly destroyed in electro-Fenton due to their faster reaction with •OH, but final Fe(III)-oxalate and Fe(III)-oxamate can only be oxidized with BDD(•OH). The most efficient method is photoelectro-Fenton due to the parallel photolysis of Fe(III)-oxalate complexes with UVA light, although Fe(III)-oxamate complexes are not photodecomposed. Under these conditions, the degradation rate can be enhanced using Cu2+ as co-catalyst because Cu(II)-oxalate and Cu(II)-oxamate complexes are quickly destroyed by •OH. Results on the fast mineralization of 2.5 L of cresols solutions of pH 3.0 by solar photoelectro-Fenton using a flow plant coupled to a solar photoreactor are also reported. The high efficiency and very low operational cost found for this procedure make it useful for the treatment of industrial wastewaters

The possibility of obtaining corrosion inhibition efficiency from the Scanning Vibrating Electrode Technique (SVET) is investigated. Results are compared with those obtained by Electrochemical Impedance Spectroscopy (EIS) for the inhibition of iron and zinc with cerium nitrate and 1H-benzotriazole. Examples of good and bad estimations are given and an interpretation for the discrepancies observed is advanced. Large deviations occur when corrosion products are formed since this leads to an underestimation of the currents measured by SVET. Visual inspection of the samples may identify the cases more susceptible to deviation. The approach discussed here can be used to obtain more quantitative information from SVET measurements but should not replace the traditional ways used to determine the inhibition efficiency.

Poly(3,4-ethylenedioxythiophene) (PEDOT) films were potentiostatically and potentiodynamically prepared on Pt electrodes from acetonitrile monomer solutions containing TBAPF6 or FcPF6 (or both). The polymer modified electrodes obtained with different growth charges were characterized electrochemically by cyclic voltammetry in a monomer free solution. The results point to the effective immobilization of the ferrocenium ion inside the PEDOT matrix by entrapment during the polymer synthesis in potentiostatic mode. It is discussed the influence of the presence of the organometallic cation in the polymerization solution in the growth process and the efectiveness of the incorporation of the metallocene is analysed on the basis of the electrochemical data collected for the films redox conversion.

A biosensor for the determination of pirimicarb was constructed by the co-immobilization of acetylcholinesterase and choline oxidase on a derivatised polyethersulfone membrane. The membrane was applied to an Universal Sensors electrode base system and the hydrogen peroxide formation was monitored upon addition of acetylcholine to an appropriately chosen buffer where the biosensor was dipped. The hydrogen peroxide is the product of the hydrolysis of choline, catalysed by cholineoxidase. Choline was formed by the action of acetylcholinesterase on acetylcholine. Pirimicarb inhibits these reactions and that fact can be used to determine its concentration. A calibration plot for pirimicarb was obtained.

The suitability of polytyramine (PTy) modified electrodes for the development of DNA biosensors is analysed in this work. PTy films were synthesised potentiodynamically on Pt electrode, from acidic aqueous solution. A cylindrospermopsin coding sequence (PKSM4 probe 5’-phosphate modified) was covalently bound on the polytyramine matrix using N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC) as coupling agents. Changes of the open circuit potential were registered as a first approach to monitorise the immobilization and hybridization processes. The modified electrodes have been characterised by cyclic voltammetry and square wave voltammetry, using methylene blue (MB), as electroactive indicator and the successful immobilization and hybridization steps are revealed by the increase of the peak currents. The hybridization has also been confirmed by measuring the fluorescence of the modified electrodes after staining with PicoGreen® DNA dye.

The textile dyes Reactive Black (RB5) and Basic Yellow 28 (BY28) were individually exposed to electrochemical treatment in 0.1 M KOH and in phosphate buffer solution pH 5.1, respectively. The cathodic decolorization of the azo dye RB5 was carried out using a Ni-polyvinylchloride composite. The methine dye BY28 was anodically treated on a Co-polyvinylchloride. The electrochemical study of the dye solution was carried out by repetitive cyclic voltammetry (RCV) under potentiostatic conditions in stationary and flow regime. The effect of various experimental parameters, such as potential and electrolysis time was investigated. The studies showed that for the RB5 (20 to 80 ppm) dye solutions, around 98 % of the color was removed after 60 min treatment at -3.0 V vs. SCE. It was also demonstrated that for the BY28 (15 ppm) dye solution, 6-10 cycles or low potentials (-300 mV) during 200 min electrolysis were sufficient for the decolorization of the solution. However, COD and TOC values did not decrease.