Portugaliae
Electrochimica Acta

This article highlights the fundamental role of mass-transport for interfacial reactions. First, the dissolution of particulate CaCO3 is discussed demonstrating how the dimensions of the dissolving particle can ‘switch’ the reaction mechanism from being diffusion to surface controlled. Second, the influence of mass-transoprt on electrochemical reactions is considered, specifically considering how electrode modification can alter the observed voltammetric response in the absence of changing the electrochemical mechanism or the rate of electron transfer. Finally, these observations on the chemically controlling role of mass-transport are concluded by considering nanoparticle toxicity and how ‘size effects’ may be exhibited even in the absence of altered thermodynamics or interfacial kinetics of the reactions involved.

This study describes the application of a Ti/Pt/PbO2 electrode as photoanode in the degradation of the dye Direct Red 80 (DR80). The electrode was prepared by platinization of a titanium substrate, followed by electrodeposition of a PbO2 layer. The presence of β-PbO2 phase was confirmed by X-ray diffraction. DR80 degradation tests were performed by different techniques, namely, photolysis, photocatalysis, electrocatalysis and photoelectrocatalysis. The best colour removals were obtained in the photoelectrocatalysis assays and were higher than 85% for 5.0 and 12.5 mg L-1 dye initial concentration and about 72% for 25.0 mg L-1 DR80 initial concentration, after applying a current intensity of 5 mA for 6 hours. At a DR80 initial concentration of 25.0 mg L-1, the best absorbance removals were obtained in the photoelectrocatalytic assays, with a current intensity of 50 mA, which led to a colour removal of 100% after 4 hours. Photolysis and photocatalysis presented similar colour removals that were 62, 26 and 18% for the initial concentrations of 5, 12.5 and 25 mg L-1, respectively.

The present work studies the behavior of hybrid sol-gel films based on tetraethyl orthosilicate (TEOS) and vinyltrimethylsiloxane (VTMS) precursors applied over commercial tinplate. In previous works VTMS films obtained by the sol-gel technology were tested to verify their good corrosion performance over short immersion times in a citric acid/sodium citrate buffer solution. However, at longer exposure times the high porosity inherent to organic sol-gel films allows the aggressive media to reach the metallic substrate and start the corrosion process. In order to overcome these limitations, hybrid organic-inorganic sol-gel coatings were obtained using the dip-coating method. This work also studies the influence of the ageing time of the hybrid sol-gel in the formation of a uniform and continuous film. The obtained layers were characterized using a Scanning Electron Microscopy (SEM) and mechanical profilometry. The corrosion performance in organic acid media was tested using polarization curves and Electrochemical Impedance Spectroscopy (EIS) with a 0.1M citric/citrate buffer solution as electrolyte. The results obtained suggest that a minimum ageing time of 10 days is needed to obtain uniform films. The electrochemical measurements indicate a clear improvement on the barrier properties on the hybrid sol-gel compared to the organic ones. A major decrease in the impedance values after 24 hours of immersion indicates that it is necessary to improve the long-term barrier properties to achieve an industrial application.

Corrosion of steel in concrete is one of the major causes of structure degradation, requiring expensive maintenance. The using of hot dip galvanized steel (HDGS) has been recognized as one effective measure to increase the service life of reinforced concrete structures in marine environmental. However, HDGS corrodes in contact with high alkaline environment of fresh concrete. Although this initial corrosion process allows the formation of a protecting layer barrier, the corrosion that occurs initially is harmful and chromate conversion layers are usually used to prevent it. Due to toxicity of Cr(VI), these kinds of pre-treatments have been forbidden and hybrid coatings have been proposed as alternatives [1-3]. To evaluate the performance of these coatings, beyond the laboratory characterization, in situ tests in real conditions should be performed. An electrochemical system to measure the macrocell current density (igal) was designed to evaluate the degradation of HDGS coated samples with different organic-inorganic hybrid films, embedded in mortar during 70 days, using an automatic data acquisition system. This system revealed to be feasible and highly sensitive to coatings degradation. Also, allow distinguishing different hybrid coatings with different thicknesses.