Portugaliae
Electrochimica Acta

Electrochemical response of glass carbon (GC) and edge plane pyrolytic graphite (EPPG) electrodes was investigated using electrochemical impedance spectroscopy (EIS) method in three kinds of electrolytes, i.e., 0.1 M Na2SO4 (aqueous solution), 0.1 M tetraethylammonium perchlorate (TEAP) (acetonitrile solution) and pure room temperature ionic liquids (RTILs) of 1-ethyl-3-methylimidazolium tetrafluorobroate (EMIBF4). The marked difference in the admittance plots was exhibited by the GC electrode, suggesting that the double layer occurring on GC electrode differed from each other when immersed in different electrolytes, indirectly proving the meso-porosity structure of GC electrode. While for EPPG electrode, similar results were obtained in these three selected electrolytes, which were interpreted by the “defects” in EPPG electrode.

In the past, mechanical properties have been the main criteria for selecting materials at the Miravalles Geothermal Field. Consideration of the corrosion resistance has been a secondary concern. The goal of this study was to determine the uniform corrosion rate of carbon steels currently used on pipelines (ASTM A 53, ASTM A515, ASTM A234 and API-5L) and compare this rate with the ASTM A-36 standard. The first step was the development of laboratory electrochemical and weight loss experiments. Ultrasonic thickness measurements of pipes for brine, steam and liquid-steam lines, in both straight and curve sections, showed negligible lost in material over the seven year period they have been on operation. The electrochemical study involved polarization curve analysis and corrosion rate measurements by Linear Polarization Resistance (LPR) and Electrochemical Noise Measurements (ENM). Electrochemical cells include triple and conventional testers (SCE as reference and platinum electrode as auxiliary) using brine degassed with nitrogen at 93 ± 2 ºC. According to the results, the ASTM-A36 has a similar general corrosion rate to those currently used on pipelines.

2-amino-1, 3, 4-thiadiazoles (AT), 2-amino-5-methyl-1, 3, 4-thiadiazoles (AMT), 2-amino-5-ethyl -1, 3, 4-thiadiazoles (AET) and 2-amino-5-propyl -1, 3, 4-thiadiazoles (APT) were synthesized. FT-IR and NMR studies were done in order to confirm the composition of the synthesized inhibitors. These compounds were evaluated as inhibitors for mild steel in 20% formic acid and 20% acetic acid by weight loss, potentiodynamic polarization and electrochemical impedance techniques. Scanning electron microscopic study (SEM) was also used to investigate the surface morphology of inhibited and uninhibited metal samples. The inhibition efficiency of these compounds was found to vary with the inhibitor concentration, immersion time, temperature and acid concentration. The adsorption of these compounds on the steel surface from both acids were found to obey Langmuir’s adsorption isotherm. These compounds are mixed type inhibitors in both acid solutions. Various thermodynamic parameters (Ea, ΔGads, ΔQ, ΔH, ΔS, t1/2) have also been calculated to investigate the mechanism of corrosion inhibition. Electrochemical impedance study was used to investigate the mechanism of corrosion inhibition.

Electrochemical oxidation of phenol, 4-aminophenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2-hydroxybenzoic acid, 2-tert-butylphenol and 2-tert-butyl-4-methoxyphenol has been carried out at sacrificial zinc anode. The products of these reactions have been identified by elemental analysis and infrared spectral data and are found to be zinc(II) phenoxides. Though these compounds do not form coordination compounds on refluxing with 1,10-phenanthroline or 2,2'-bipyridyl, yet the coordination compounds of these phenoxides with these ligands have been synthesized electrochemically. Current efficiencies of these system are quite high.

The inhibition effect of tetra-N-butylammonium iodide (TBAI) on mild steel in 1 N H2SO4 has been studied by using weight loss, electrochemical polarization, Fourier Transform Infrared (FTIR) and scanning electron microscopic (SEM) techniques. It has been concluded that the percentage inhibition increases with increasing the concentration of the inhibitor. The adsorption of TBAI on mild steel surface in 1 N H2SO4 obeys Langumir adsorption isotherm; surface analysis and IR studies are also carried out to establish the mechanism of corrosion inhibition.

Electrochemical methods were used to investigate the abilities of the homologous straight chain monocarboxylates (CnH2n+1COO-,n=1-13) to inhibit the pitting corrosion of two types of copper alloys in aerated, saline and near neutral aqueous solutions. The copper alloys were alloy (I) (Cu + 4.47% Fe) and alloy (II) (Cu + 10.67% Al + 5.02% Fe). It was found that these were susceptible to pitting corrosion in NaCl solutions, with increase of Cl- ions concentration. Alloy (II) was more susceptible to pitting corrosion than alloy (I). The performance of monocarboxylates was shown to be critically dependent upon their chain length. The range of chain lengths producing optimal inhibition was (6  n  10). These carboxylates showed abrupt decreases in inhibitor ability outside the optimal range. The dramatic variations in inhibition efficiencies probably resulted from competing reactions such as adsorption, solubility and micelle formation.

The comparative study of corrosion behaviour of brasses 70Cu30Zn and 60Cu40Zn in HNO3 solution in absence and in presence of 1-phenyl-5-mercapto-1,2,3,4-tetrazole (PMT) was studied by gravimetric and electrochemical methods. Results obtained are in good agreement and reveal that the corrosion rate depends on immersion time and zinc content in the alloy. Copper and zinc losses from each specimen studied, at various immersion times, were estimated by Atomic Adsorption Spectroscopic analysis. This shows that the inhibition efficiency of the inhibitor towards copper is more significant than zinc. PMT was adsorbed preferentially on the copper surface and inhibits the process of corrosion of brasses in the nitric acid medium.

Electrodeposition of Ni-Ce alloy on steel was studied and an electroplating bath was optimized for maximum cathodic current efficiency. Cerium sulphate content in the bath decreased the cathodic current efficiency. Hydrogen evolution reaction from NaOH solution was catalysed by cerium in the alloy and also the increase of cerium content increased the catalytic activity. Among the various Ni-Ce alloys, Ni77Ce23 alloy offered the maximum catalytic activity.

2,3,6,7,10,11-hexadecahexoxytriphenylene was synthesized by electrochemical oxidation of 1,2-didecahexoxybenzene in dichloromethane-tetrabutylammonium tetrafluoroborate (TBATFB) on platinum electrode. A green, adherent, insoluble deposit was grown on the Pt electrode surface. The deposit was isolated and characterized using Fourier Transform Infrared (FTIR) Spectroscopy and Nuclear Magnetic Resonance (NMR). The results suggest that the deposit shows a decahexoxy triphenylene structure, associated to the anion of the supporting electrolyte.

A new way for deriving the fundamental equation of the electrochemistry has been developed in the potential range in which Butler-Volmer behaviour is found. In this approach the frequency factor is considered to be a function of the potential instead of the activation energy. The frequency factor is calculated by means of a classical statistical mechanics treatment giving rise to a new definition for the symmetry factor. A new expression for the standard rate constant in adiabatic heterogeneous charge transfer reactions has been derived.

The effect of oil soluble commercial corrosion inhibitors (OSCI) on the growth of bacteria and its corrosion inhibition efficiency were investigated. Corrosion inhibition efficiency was studied by rotating cage test and the nature of biodegradation of corrosion inhibitor was also analyzed by using FTIR, NMR and GC-MS. This isolate has the capacity to degrade the aromatic and aliphatic hydrocarbon present in the corrosion inhibitors. The degraded products of corrosion inhibitors and bacterial activity determine the electrochemical behaviour of API 5LX steel. The influence of bacterial activity on degradation of corrosion inhibitor and its influence on corrosion of API 5LX have been evaluated by employing weight loss techniques and electrochemical studies. The present study also emphasizes the importance of evaluation of oil soluble corrosion inhibitor in stagnant model (flow loop test) and discusses the demerits of the oil soluble corrosion inhibitors in petroleum product pipeline.

In this preliminary work, for the first time, the electrochemical oxygen reduction reaction (ORR) was investigated using cyclic voltammetry (CV) on the electrodeposited manganese oxide (MnOx)-modified glass carbon electrode (MnOx-GC) at room temperature ionic liquids (RTILs) of EMIBF4, i.e., 1-ethyl-3-methylimidazolium tetrafluorobroate (EMIBF4). The results demonstrated that, after being modified by MnOx on GC, the reduction peak current of oxygen was increased to some extent, while the oxidation peak current, corresponding to the oxidation of superoxide anion, O2-, was attenuated in some degree, suggesting that MnOx catalyzed ORR in RTILs of EMIBF4, which is consistent with the results obtained in aqueous solution. To accelerate the electron transfer rate, multi-walled carbon nanotubes (MWCNTs) were modified on GC, and then MnOx was electrodeposited onto the MWCNTs-modified GC electrode to give rise to the MnOx /MWCNTs–modified GC electrode; consequently, the improved standard rate constant, κs, originated from the modified MWCNTs, along with the modification of electrodeposited MnOx, showed us a satisfactory electrocatalysis for ORR in RTILs of EMIBF4. In addition, not only for the MnOx-modified GC but also for the MnOx/ MWCNTs-modified GC, there is a novel small oxidation peak appearing at –0.2 V vs. solid Ag/AgCl, implying that the catalysis of MnOx for ORR in EMIBF4 is somewhat different from that observed in aqueous solution, though the exact interpretation is not achieved in this preliminary work. Initiating the catalysis of MnOx on ORR in RTILs is the main contribution of this work. Further discussions are in progress.

In order to investigate the electrochemical behaviour and morphology under different experimental conditions of poly-N-methylpyrrole doped with sodium p-toluensulphonate (NaTsO) in aqueous medium, the doped polymer was potentiostatically synthesized at four potential values: 0.6, 0.8, 0.9 and 1.0 V. The morphological study reveals a compact texture in these polymeric films; after being submitted to a positive potential those films were wrinkled. Cyclic voltammetry was used to characterize polymer films (pNMPy/TsO). The corresponding voltammograms revealed a reversible doping/de-doping process in those films obtained at ηp = 0.8 V. Different parameters such as: negative potential limit (Ei), positive potential limit (Ef), sweep rate (v) and continuous cycling were varied.

The effect of some mono azo dyes compounds on the dissolution of aluminium in 0.01 M NaOH solutions was studied using weight loss and galvanostatic polarization techniques. The percentage inhibition efficiency was found to increase with increasing concentration of inhibitor and with decreasing temperature. The inhibition mechanism was discussed on the basis of the formation of complex compound adsorbed on the metal surface. The adsorption process follows Frumkin isotherm. The effect of temperature on the rate of corrosion in the absence and presence of these compounds was also studied. Some activated thermodynamic parameters were computed.

A novel electrochemical sensor has been developed for the determination of nimesulide. The sensor is based on the NIM- molybdophosphoric acid (MPA) as the electroactive material in PVC matrix in presence of bis(2-ethyl hexyl) phthalate (BEP) as a plasticizer. The sensor showed a fast, stable, near Nernstian response for 1  10-2 –1  10-6 M NIM over the pH range 5 – 8 with a slope 55.6 0.5m V/decade and the response time is < 45 s. Selectivity coefficient data for some common ions show negligible interferences. The sensor was successfully applied for the determination of NIM in tablet and the results obtained are in good agreement with those obtained by the official method.

The glassy carbon electrode (GCE) modified with cobalt nitroprusside (CoNP) was prepared by electrochemical method. The CoNP modified electrode (ME) prepared electrochemically on a GCE in two-step. The electrochemical behavior of the modified electrode was studied by cyclic voltammetric technique. The cyclic voltammogram of CoNP showed a redox couple with formal potential [E0 =(Epa+Epc)/2] of 500 mV vs. SCE. The electrocatalytic oxidation of nitrite was studied on the CoNP modified glassy carbon electrode. The CoNP films on the glassy carbon electrode show an excellent electrocatalytic activity toward the oxidation of nitrite in 0.5 mol L-1 KNO3. Hydrodynamic amperometry was used to the detection of nitrite with CoNP. The linear detection limit of the CoNP elctrode for NO2- was from 20×10-6 to 220×10-4 mol L-1 and the detection limit was 10×10-6 mol L-1 (S/N=3).

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.