I. T. E. Fonseca

Studies on the stripping voltammetric determination and speciation of chromium at a rotating-disc bismuth film electrode

An adsorptive stripping voltammetric protocol coupled with a rotating-disc bismuth film electrode for the determination and speciation of chromium (III) and chromium (VI) in the presence of diethylenetriaminepentaacetic acid (DTPA) is presented. The developed methodology involves a mass-transport controlled preconcentration step, during which a Cr(III)-DTPA complex is adsorbed onto a pre-plated rotating-disc bismuth film electrode held at -0.4V, followed by a reductive square wave stripping scan. At -1.07 V vs. Ag/AgCl, a peak is recorded due to the catalytic reduction of Cr(III)-DTPA to Cr(II)-DTPA. As a result of different chemical behaviours of Cr(III) and Cr(VI) in the presence of DTPA, the corresponding voltammetric signals presented different stabilities in time. A univariate optimization study was performed with several experimental parameters as variables. For Cr(VI) and total chromium, Cr(III)+Cr(VI), an accumulation time of 60s at -0.4V vs. Ag/AgCl resulted in detection limits of 0.336 and 0.414 nM and quantification limits of 1.12 and 1.40 nM, respectively. The relative standard deviation for 10 measurements of 5.0 nM chromate was 2.4%. Interference of other electroactive trace metals and surfactants was considered. A simple speciation scheme was proposed and satisfactorily applied to Cr(III) and Cr(VI) determinations in river water samples.

The influence of the media on the corrosion of mild steel by Desulfovibrio desulfuricans bacteria: an electrochemical study

Abstract Electrochemical studies and weight loss measurements on the biocorrosion of mild steel in the presence and absence of sulphate reducing bacteria (SRB) grown either in lactate/sulphate and or in lactate/nitrate aqueous media, have been performed. Tafel analysis of the quasi-steady state polarisation curves gave the corrosion current densities,jcorr, of 20 and 30 μA cm−2 for mild steel samples in the SRB culture in lactate/sulphate and lactate/nitrate media, respectively. In the absence of SRB the corresponding values of jcorr were 19 and 9 μA cm−2, respectively. Corrosion current densities given by the linear polarisation resistance method were very close to the ones given by Tafel analysis. A comparison of the corrosion rates obtained either from electrochemical or from weight loss data is given for all the media under study. Open circuit potential and redox potentials measured over two days showed great differences for the lactate/sulphate and lactate/nitrate growth media. Scanning electron microscopy (SEM) observations resulted in good correlation with open circuit potential data.

Passivity breakdown of Al 2024-T3 alloy in chloride solutions: a test of the point defect model

Cyclic voltammograms of Al 2024-T3 alloy samples, in deaerated aqueous chloride solutions of various concentrations, have been recorded and analysed. A linear dependence between the breakdown potential, Eb, and the square root of the scan rate, m 1=2 , was found for all chloride concentrations and also for synthetic seawater leading to a values between 0.41 and 0.38. The breakdown potential at zero scan rate, Eb ðm ¼ 0Þ, showed also a good linear dependence on the logarithm of the activity of the chloride anions.From the slope of the straight line a value of 3. 29 was obtained for the ratio ( n=Jm) from which a n value less than or equal to 3 � 10 15 cation vacancies cm � 2 was calculated. Average critical concentration of cation vacancies on Al and c-Al2O3 computed from geometric arguments has showed excellent agreement with those computed from experimental data and analytical equations derived from the Point defect model (PDM). 2002 Published by Elsevier Science B.V.

VOLTAMMETRIC STUDIES OF THE TRANSPASSIVE DISSOLUTION OF MILD STEEL IN CARBONATE/BICARBONATE SOLUTIONS

The transpassive dissolution of mild steel in aqueous equimolar carbonate/bicarbonate normally aerated solutions has been investigated using cyclic voltammetry. For the first time a transpassive anodic peak preceding oxygen evolution at E>1.100 V (sce) has been detected together with the corresponding reduction peak. The formation of Fe (VI) species is proposed. Sweep reversal and sweep rate studies revealed that at least three processes are involved in the kinetics of peak V: charge transfer under resistive control, a coupled chemical reaction and diffusion of electroactive species. An increase in the charge of the peak corresponding to the reduction of Fe(III) to FE(II)(−0.800 V (sce)) is thought to be due to the chemical decomposition of Fe(VI) yielding Fe(III) species. An increase in the transpassive peak current with increasing temperature (12–73°C) was also indicative that a chemical step is involved. An apparent activation energy of ≈10 kJ mol−1 was obtained.

Some aspects of the electrochemical behaviour of mild steel in carbonate/bicarbonate solutions

The electrochemical behaviour of mild steel in aqueous solutions of sodium carbonate/sodium bicarbonate (600 ppm) has been investigated using potentiodynamic polarization. In the pre-passive region three well-defined peaks are observed associated to reduction peaks corresponding to Fe(II) and Fe(III) species. A transpassive anodic peak is also observed being attributed to Fe(VI) species showing, in sweep reversal experiments, an associated reduction peak and an increase in the peak associated to the reduction of Fe(III) species. The characterization of the transpassive peak will be subject of further publication.

The effect of SO2−4 on the electrochemical behaviour of Cu in neutral medium

Abstract The effect of sulphate ions on the anodic behaviour of copper in neutral aqueous solutions of various Na2SO4 concentrations was studied by cyclic voltammetry and chronocoulometry. The results show that SO2−4 ions increase film formation on Cu in the passive range but also its anodic dissolution in the transpassive region at potentials as low as 0 V sce. The breakdown of passivity is initiated by nucleation and growth of pits through the passivating film. The cathodic charge due to the reduction of soluble Cu(II) species formed during the transpassive process increases exponentially with holding time in the transpassive region and with SO2−4 concentration. SO2−4 ions increase the amount of soluble Cu species. Sweep rate studies indicate that film formation in the presence of SO2−4 can be reversible or controlled by a chemical step, depending on time scale, pH, and SO2−4 concentration.

Integrated electromodulated IR reflectance spectroscopy bands: Part 2: Methanol adsorbates at polycrystalline platinum and Pt(111) single-crystal electrodes in acid medium

Using electromodulated IR reflectance spectroscopy (EMIRS), absorption spectra of adsorbed species resulting from the chemisorption of HCHO, HCOOH and methyl formate, HCOOCH3, on both polycrystalline and (111) platinum surfaces, were first obtained in the range 1400–1900 cm−1, to cover completely the carbonyl stretching mode region. The same experimental conditions were used for all systems. EMIRS bands were integrated and analysed, using the procedure described in Part 1 (B. Beden, J. Electroanal. Chem., 345 (1993) 1). Characteristic frequencies were located precisely, allowing the spectral discrimination of several types of adsorbates. The same procedure was then used to follow the chemisorption of methanol on platinum in perchloric medium with time. As expected, formate species were found to dominate on the two types of platinum surface. However, during the initial stages of chemisorption, and particularly on the Pt(111) electrode, the formation of adsorbed methyl formate is likely. A mechanism is suggested, taking into consideration the possibility of surface reactions between adsorbates.

Electrocatalytic oxidation of d-sorbitol on platinum in acid medium: analysis of the reaction products

Abstract The electrocatalytic oxidation of d -sorbitol has been studied on polycrystalline platinum in 0.1 M HClO 4 . Prolonged electrolyses were carried out using a three potential plateau program with different values of the oxidation potentials. Chromatographic analyses showed that d -sorbitol was transformed into gluconic acid via the formation of glucose. Glucuronic acid and some degradation side products were also detected. When increasing the applied potential, the degradation of the initial molecule skeleton became more and more important. Moreover, a reaction mechanism for the electrooxidation of d -sorbitol was proposed and it was demonstrated that the consumption of d -sorbitol followed a first-order reaction law.

Electrosorption of sorbitol at platinum electrodes: Effect of the superficial structure

Abstract The electrosorption of sorbitol was investigated at polycrystalline and single-crystal platinum electrodes in an acidic medium. The coverage degree and the number of electrons per site involved in the electro-oxidation of the adsorbed species coming from the sorbitol were estimated from voltammetric measurements carried out at high sweep rates and for adsorption times varying from 0.1 to 500 s. The adsorption process depends on the superficial structure. The electrode surface was never saturated by the organic species coming from sorbitol adsorption and the number of electrons per site was found to be around 3, though this decreased to about 2 after long adsorption times. One of the main adsorbed species present at the electrode surface for long adsorption times is linearly adsorbed CO, as confirmed by in situ IR reflectance spectroscopy. The presence of other kinds of adsorbed species is also discussed.

A galvanic sensor for monitoring the corrosion condition of the concrete reinforcing steel: relationship between the galvanic and the corrosion currents.

This work reports a study carried out on the design and performance of galvanic and polarization resistance sensors to be embedded in concrete systems for permanent monitoring of the corrosion condition of reinforcing steel, aiming to establish a correlation between the galvanic currents, Igal, and the corrosion currents, Icorr, estimated from the polarization resistance, Rp. Sensors have been tested in saturated Ca(OH)2 aqueous solutions, under a variety of conditions, simulating the most important parameters that can accelerate the corrosion of concrete reinforcing steel, such as carbonation, ingress of chloride ions, presence or absence of O2. For all the conditions, the influence of temperature (20 to 55 °C) has also been considered. From this study, it could be concluded that the galvanic currents are sensitive to the various parameters following a trend similar to that of the Rp values. A relationship between the galvanic and the corrosion current densities was obtained and the limiting values of the Igal, indicative of the state condition of the reinforcing steel for the designed sensor, were established.