Portugaliae
Electrochimica Acta

The treatment of liquid organic phases to remove undesirable compounds is currently a priority in different industries. In oil refinery, it is necessary to reduce sulphur compounds, in gasolines, to below 50 ppm.The traditional form is catalytic hydrodesulphuration; however, this system is not effective for some compounds, e.g., alkyl substituted dibenzothiophenes. The processes for modulated complexation by electrochemistry are based on the capacity of a complexing agent, in a certain oxidation state, to form a bond with the molecule to be removed from a liquid phase. If the contaminant, the solute, is in an organic phase, the complexant must be soluble in aqueous phase. When the complexant is formed and in contact with both phases the contaminant is extracted. In a second stage, the complexing agent is oxidized (or reduced) to a state of less affinity to the contaminant, and the aqueous phase is placed in contact with a new organic phase where the contaminant will be discharged and concentrated. The complexant agent in aqueous phase is reduced (or oxidized) to its state of higher affinity for the contaminant and recirculated to the extraction stage. In this work, we present the conditions necessary for this process to be feasible, the criteria for the selection of the complexes, the methodology for matter balances, determination of energy consumption and thermodynamic calculations, and also three examples of an organic phase: dibenzotiophene, pyridine, and methyl thioether, using as complexing agent iron tetrakis-(p-sulphonatophenyl) porphyrin.

In the present work, the electrochemical behavior of molybdenum-silica-niobium alloy produced by arc cast technique is presented, keeping silica constant (at 24 wt %), using different niobium concentrations ( 5, 10, 15 & 20 wt %) as alloying elements. These samples were immersed in an electrochemical solution of 10 % by volume hydrochloric acid. The corrosion behavior was obtained using electrochemical techniques, namely: open circuit potential, polarization curves and electrochemical current noise. The potential behaviour as a function of time allows to determine the activity of the alloy as a function of its content, the polarization technique was used to obtain characteristic electrochemical parameters and the electrochemical current noise the type of attack while comparing them with the corrosion surface morphological features. Surface images showed different corrosion morphologies, being the most attacked the sample without niobium additions. It was found that niobium additions result in a better corrosion resistance when its concentration is increased in the alloy.

The corrosion inhibition of mild steel in one normal sulphuric acid solution by brij-30 has been studied in relation to the concentration of the inhibitor as well as the temperature using electrochemical polarization (galvanostatic and potentiostatic) techniques. The results were supplemented with scanning electron microscopy and infra-red spectroscopy. All the methods employed are in reasonable agreement. There is no particular relationship of inhibition with concentration and temperatures. The thermodynamic functions of dissolution and adsorption processes were calculated from experimental polarization data and the interpretation of the results is given. Adsorption of Brij-30 was found to follow the Langmuir’s adsorption isotherm. Brij-30 is a mixed type of inhibitor.

The corrosion inhibition of mild steel in one normal sulphuric acid solution by PEGME has been studied in relation to the concentration of the inhibitor as well as the temperature using electrochemical polarization (galvanostatic and potentiostatic) techniques. The results were supplemented with scanning electron microscopy and infra-red spectroscopy. All the methods employed are in reasonable agreement. There is no particular relationship of inhibition with concentration and temperatures. The thermodynamic functions of dissolution and adsorption processes were calculated from experimental polarization data and the interpretation of the results is given. Adsorption of PEGME was found to follow the Langmuir’s adsorption isotherm. PEGME is a mixed type of inhibitor.

The pitting corrosion of some stainless steel alloys (preoxidized at different conditions) in 3.5% NaCl solution was studied. The alloys are: one ferritic (15.05% Cr) (alloy1) and two austenitic stainless steel alloys (17.9% Cr,7.08% Ni) (alloy2) and (20.45% Cr, 8.3% Ni) (alloy3). Potentiodynamic anodic polarization and galvanic current-time measurements were used in these investigations. The susceptibility of the alloys to pitting corrosion decreases with the increase of chromium content of the alloy and with the presence of nickel in the alloy. The preoxidation of the alloys in different media improves their resistance to pitting corrosion in NaCl solution. The resistance to pitting corrosion for the investigated alloy increases according to the order: no oxidation < oxidation in air < oxidation in molten alkali nitrates < oxidation in molten alkali carbonates. This resistance to pitting corrosion may be due to the formation of a protective oxide film on the alloys’ surface. The composition of this film greatly depends on the chemical composition of the alloy, on the condition of the preoxidation process, and on the temperature.

The corrosion inhibition of mild steel in one normal sulphuric acid solution by CPB has been studied in relation to the concentration of the inhibitor as well as the temperature using electrochemical polarization (galvanostatic and potentiostatic) techniques. The results were supplemented with scanning electron microscopy and infra-red spectroscopy. All the methods employed are in reasonable agreement. There is no particular relationship of inhibition with concentration and temperatures. The thermodynamic functions of dissolution and adsorption processes were calculated from experimental polarization data and the interpretation of the results are given. Adsorption of CPB was found to follow the Langmuir’s adsorption isotherm. Brij-30 is a mixed type of inhibitor.

We present electrochemical impedance spectra made on gold alloy containing 30% silver electrodes of various roughnesses in aqueous perchlorate acid solution as supporting electrolyte in the absence and in the presence of mM of specifically adsorbed halide ions X (X = Br-, Cl-, I-), at potentials where the dominant electrode process is the adsorption of the above anions. Efforts were mainly concentrated on the importance of surface preparation technique of the electrode and its influence on impedance spectra. Atomic scale inhomogeneities are introduced by mechanical treatment and can be decreased by annealing. Due to the annealing the double layer behaves as (almost) an ideal capacitance in the absence of specific adsorption though the surface roughness remains the same. A study of the related impedance behaviour in the presence of adsorbates even at very low concentrations (10-4 M), revealed capacitance dispersion increasing with the extent of specific anion adsorption at the gold/silver surface.

Nucleation models have been tested using reported experimental nucleation data in several systems dealing with electrocrystallization from aqueous solutions, and with a wide range of supersaturations or overpotentials. The critical nucleus size has been calculated and the results obtained with the classical and atomistic models have been compared and discussed. In order to compare these values with those occurring in crystallization, the values of the critical nucleus size for several crystallization systems have also been calculated, and then compared and commented.