Portugaliae
Electrochimica Acta

Polyvinylpyrrolidone Oxime (PVPO) was synthesized and studied for mild steel (MS) corrosion inhibition in 1 M H2SO4, at different concentrations and temperatures. The corrosion inhibition efficiency was studied using weight loss method, polarization technique, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and quantum chemical calculations. The results from weight loss, potentiodynamic polarization and EIS showed that the inhibition efficiency (I. E.) increased with gradual increments in the inhibitor concentration, and decreased at higher temperatures. The polarization study also revealed that PVPO acted as a mixed type inhibitor, and Langmuir adsorption isotherm fitted well for the adsorption behavior. The highest corrosion efficiency was found to be 88.39%, with a concentration of 1000 ppm, and a temperature of 303 K. The corrosion inhibition mechanism has been further proposed, including the support from the theoretical study. SEM images also verified the MS surface smoothening in PVPO presence, and, hence, it has shown to be a good corrosion inhibitor.

Biomaterials are being used in implants as metallic supports, to restructure bone and tissue in biomedical applications, and so forth. In this study, the assessment of films from the Chitosan-Gelatin system in detached conditions was carried out, in order to evaluate the effect of the electrolyte over the membrane, to simulate a physiological solution. An equivalent circuit is proposed, so as to interpret the process of biopolymer degradation using electrochemical parameters, as well as a reaction mechanism for the membrane’s cathodic areas. The gathered information may be associated with the biodegradability ionic diffusion behavior, also in its application as a temporary coating for metal protection control during the corrosion process of biomaterials, such as magnesium, in bone regeneration.

The corrosion inhibition of aluminum, in 0.1 M Na2CO3, by Mentha pulegium essential oil, was studied using both polarization and impedance methods. The results show that Mentha pulegium is an effective inhibitor, providing an inhibition rate of 94.16 %, with a concentration of 800 ppm. The polarization study shows that the Mentha pulegium essential oil acted as a mixed inhibitor in 0.1 M Na2CO3. The activation energy calculation has proved that the inhibitor molecules adsorbed onto the aluminum surface, according to the physisorption mechanism.

Electrochemical oxidation of N-acetyl-p-aminophenol (AAP) and selected pharmaceutical products containing AAP is described in this study. Investigations were carried out at Pt electrode, with the application of cyclic (CV) and differential pulse (DPV) voltammetry. AAP was irreversibly oxidized in, at least, one electrode step, with potentials lower than the potential at which oxygen evolution started. Electrochemical methods based on voltammetric techniques (DPV and CV) have been developed for AAP determination in commercial pharmaceutical drugs. AAP content in pharmaceuticals was determined on the base of dependences of current intensity (electroanalytical measurements) and absorbance (UV-VIS spectra). The obtained results using both methods were in good agreement. The validation of CV, DPV and spectrophotometric methods is also presented. Spectrophotometric and voltammetric responses linearly increased with increasing AAP concentrations, in the range from 0.2 to 5 mmol L-1, with a correlation coefficient of 0.997 (UV-VIS), 0.999 (CV) and 0.998 (DPV). Both electrochemical methods are simple, reliable and sufficiently accurate and precise for AAP quantification in commercial drug samples (tablets). The analysis took only 5 minutes of manual operation, including Pt electrode pre-treatment.

The inhibitive action of Chromolaena odorata stems extract, in various concentrations, against mild steel corrosion in a 1 M NaCl solution, was studied using weight loss, potentiodynamic polarization methods and scanning electron microscopy. Maximum inhibition efficiency of 99.83 % was obtained, at 303 K, for an extract concentration of 3000 mgL-1. The activation and free energies for the inhibition reactions supported the physical adsorption mechanism. The extract adsorption onto the mild steel surface was found to be exothermic, spontaneous, and to obey the Langmuir adsorption model. FT-IR analysis showed the presence of hydroxyl (OH) and carbonyl(C=O) functional groups and aromatic rings in lignin, which are the binding groups that might be responsible for lignin’s inhibitive action against mild steel corrosion. Furthermore, SEM analysis revealed that the mild steel surface was affected by lignin’s adsorption, due to the formation of a protective film.

This paper has carried out the construction and evaluation of a selective electrode for silver, using 5-(4-dimethylaminobenzylidene)rhodanine as ionophore, and diocthyl phthalate (DOP) with plasticizer in PVC. The electrode shows fast potentiometric responses in the concentration range from 10-6 to 10-2 mol/dm3, with Nernstian slope (58.2 ± 0.8 mV/decade), and a response time of 20 s. Also, we presented a theoretical study of the molecule, optimized by DFT to a B3LYP/6-31G (d, p) level of theory. A good agreement was obtained between calculated geometrical parameters using DFT methods and experimental data. Analysis of the ionophore by the natural bond orbital (NBO) allowed explaining the possible centers of coordination with Ag+. Other characteristics of the electrodes were also studied.

The goal of this study was to propose an electrochemical sensor for Paracetamol determination. To reach this objective, a graphite electrode (GE) was modified with iron oxides, and it was tested in an alkaline aqueous solution, in order to appreciate its electrocatalytic properties towards paraacetylaminophenol anodic oxidation. The graphite electrode modification was performed, in a first step, by potentiostatic deposition of an iron film at -1 V/SCE, in an aqueous solution of 0.04 M Fe (NO3)3 + 0.15 M KNO3, followed, in a second step, by cyclic voltammetry in an aqueous solution of 0.1 M sodium hydroxide, at 25 ºC. The modified electrode was characterized by means of scanning electron microscope coupled to an EDAX X-Ray micro-analyser. Paraacetylaminophenol determination by means of this novel sensor was instantaneous in an alkaline solution. The performance of this sensor was obtained in the concentration range from 13 ppm to 320 ppm, with a sensitivity of 1.6×104 µA mol-1 L.

A carbon paste electrode has been modified with a red algal species of Porphyridium cruentum, and the characterization of its electro catalytic activity has been done with cyclic voltammetric studies of the potassium ferricyanide (K3[Fe(CN)6]) system. In order to highlight the electrode’s stable and fast response to the [Fe(CN)6]3-/[Fe(CN)6]4- redox couple, experiments were also conducted with a bare carbon paste electrode. FTIR spectra revealed the functional moieties of the bare and modified electrodes. The electrochemical surface area and surface coverage capacity were calculated for both electrodes. The effect of the scan rate helped to evaluate the nature of the electrode process, electron transfer mechanism, and kinetic parameters (rate constant and charge transfer coefficient).

Nickel and graphite were potentiostatically co-deposited using a nickel-graphite composite counter electrode (HCE) with tunable-friability. Graphite electrodes were produced at densities of 0.920, 1.026 and 1.188 g/cm3, and their suitability for constitution into HCE was assessed. The surface area of the nickel component was varied from 100% to about 60% and 30 %, and combined with the graphite electrode, to form HCE, designated as triplet, doublet and singlet, respectively. Deposition was done for about 8 hours in 1 M NiSO4, using the different HCE constitutions, an Ag/AgCl reference electrode and a custom deposition head which served as working electrode. The mechanism of graphite electrode unraveling was observed to be the formation of oxygen and CO2, due to oxidation reactions at HCE. The graphite electrode with a density of 0.920 g/cm3 was selected for HCE, due to its extensive surface porosity, a characteristic determined as favorable to the mechanism of electrode unraveling. Co-deposition of graphite with nickel was observed to increase as the nickel surface area was reduced from triplet to singlet. SEM micrographs show partially and fully embedded graphite particles in the nickel matrix, while the presence of nickel and graphite was affirmed.

The corrosion inhibition of zinc coated steel sheets immersed in 3.5% NaCl with sulfisoxazole (SSZ) has been reported by weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy, hydrogen permeation studies and quantum mechanical studies. All these techniques indicate that SSZ significantly inhibits zinc coated steel corrosion in a 3.5% NaCl medium. Polarization studies confirmed that the inhibition mode follows a mixed type. The adsorption of the compounds onto the zinc coated steel surface obeyed Langmuir adsorption isotherm.

The aim of our studies was to show the effect of the addition of tin to aluminum in a solution of sodium chloride (NaCl 3%), by weight, as well as the influence of the alloy immersion time on its corrosion resistance. To do this, we have used, as electrochemical techniques, potentiodynamic polarization and electrochemical impedance spectroscopy and, as metallurgical techniques, hardness parameters and optical microscopy performed on the alloys made of pure aluminum (99.99%) and pure tin (99.99%). The obtained results show that the addition of tin enhances aluminum electrochemical activation, as well as the spontaneous formation of an oxide layer containing Al2O3, which then protects the metal from further corrosion. During all that process, the alloy immersion in 3% NaCl acts by promoting tin’s attack and the alloy’s corrosion.

The aim of this study was to produce an inexpensive and easy to implement electrochemical sensor. The synthesis of a composite from poly (vinyl alcohol) and natural clay enabled to obtain a material with interesting properties, in view of the results achieved from thermal analysis and transmission electron microscopy. The layer film of natural poly (vinyl alcohol) clays was used to modify the glassy carbon electrode of which electrochemical response was good in the presence of gallic acid in a phosphate buffer solution. The mechanism of electro-oxidation reaction was proposed, and the pH effect has been approached in order to confirm the reaction mechanism. The concentration linearly increased with the oxidation current peak, with a regression coefficient of 0.989 and a detection limit of 1.005 μM (S/N=3). The method thus proposed is satisfactory for the determination of gallic acid in a solution.

In the present study, vicine was extracted from sunflower seeds, and has been evaluated as a corrosion inhibitor for carbon steel alloy, at different concentrations and temperatures. Vicine showed maximum inhibiting efficiency of 97%, at 25 ºC. The inhibitor’s efficiency was reduced when the temperature increased. Kinetic parameters (Ea, ΔH*, ΔG* and ΔS*) were calculated. The corrosion reaction was suppressed by vicine, because the energy barrier of the corrosion reaction was increased, and it became non-spontaneous, by an endothermic process. Furthermore, ΔHads, ΔGads and ΔSads were also calculated, and showed that the inhibitor was physically adsorbed by a spontaneous and exothermic process. The corrosion was inhibited by simple blocking the reaction sites. The adsorption process obeyed the Langmuir adsorption isotherm. The theoretical and experimental studies depicted that the inhibitor worked by a mixture of physical-chemical adsorption modes.

Three substituted imidazoles – 2-(3-methoxyphenyl)-4,5-diphenyl-1H-imidazole (IM-1), 2,4,5-triphenyl-1H-imidazole (IM-2), and 2-(3-nitrophenyl)-4,5-diphenyl-1H-imidazole (IM-3) – were synthesized, and their inhibiting action was tested using mass loss, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods. The results show that methoxy substituted imidazole performed better as a corrosion inhibitor than NO2 substituted imidazole. These findings were corroborated by density functional theory (DFT) and molecular dynamics (MD) simulations methods. IM-1 was found to exhibit maximum IE of 97.5%, at 100 mgL-1, among the studied IMs. PDP study revealed that all the three IMs inhibitors predominantly acted as cathodic inhibitors, and the adsorption study showed that they followed Langmuir adsorption isotherm. The formation of an inhibitor film on the MS surface was confirmed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). MD study revealed that binding energy and interaction energy of the inhibitors molecules on the MS surface followed the order IM-1> IM-2> IM-3. All the three IMs molecules adsorbed onto the mild steel surface by flat orientation. DFT and MD study results corroborated the experimental results.

The inhibiting effect of benzaldehydethiosemicarbazone (BTSC) on the mild steel alloy corrosion in a 1 M sulfuric acidic solution was potentiostatically investigated at four temperatures, in the range of 298.15 to 328.15 K. Three BTSC concentrations, ranging from 100 to 300 mg/L, were tested. Mild steel corrosion feasibility decreases with increasing inhibitor concentrations, and also with the rise in temperature. A protection efficiency of 96% was obtained at 300 mg/L, and 328.15 K. Potentiostatic polarization studies showed that BTSC acted as a mixed type inhibitor. The main kinetic effect of the BTSC inhibitor added to the sulfuric acid solution was to considerably enhance activation energy values, pre-exponential factor and activation entropy of the alloy corrosion. This was because BTSC shifted the corrosion reaction on the mild steel surface to reaction sites where energy was relatively higher than that on which the corrosion occurred in the inhibitor absence. The inhibitor adsorption followed the Langmuir adsorption isotherm. The activation thermodynamic functions (Ea, Kads, ΔGads., ∆Hads. and ∆Sads) were evaluated. The obtained activated parameters revealed that BTSC adsorption took place through chemisorption. Scanning electron microscopy (SEM) technique was used to provide insight into the formation of a protective film on the alloy surface. To provide a relationship between the BTSC’s molecular structure and its corrosion inhibition capability, quantum chemical studies were achieved using density functional theory (DFT) at the B3LYP/6-311G level.

In the present work, we report the development of a polyphenol oxidase (PPO) sensor for the selective and sensitive detection of catechol, using biosynthesized silver nanoparticles (AgNPs). For the sensor development, AgNPs biosynthesized using the leaf extract of Convolvulus pluricaulis, were successfully deposited onto a polypyrrole modified graphite electrode (Gr/PPy). The resulting Gr/PPy/AgNPs electrode was further used as a matrix for the immobilization of the PPO enzyme extracted from Manilkara Zapota (sapota). The morphological characteristics of the developed Gr/PPy/AgNPs/PPO sensor were studied using a scanning electron microscope (SEM). The sensor performance was evaluated and optimized using cyclic voltammetry (CV), differential pulse voltammetry (DPV), chrono amperometry (CA) and electrochemical impedance spectroscopy (EIS) methods. Under neutral pH conditions, the developed sensor showed excellent electro catalytic activity towards catechol detection. The sensor performed well in the concentration range of 0.001 to 0.015 mM, with the detection limit of 0.47 µM, and sensitivity of 13.66 µM-1cm-2. The biosensor response was found to be uninfluenced by some common interferents, and it also showed good storage stability and repeatability. The practical applicability of the PPO biosensor for catechol detection in real samples was assessed by examining the catechol content in a green tea sample. The sensor could detect catechol content in a green tea sample, to an accuracy of about 98%, thereby establishing its efficiency in real sample analysis.

The electrochemical behaviour of 4-tertbutylcyclohexanone semicarbazone (TBCHSC) and its Co(II) complex was investigated by a glassy carbon electrode, cyclic voltammetry technique. Semicarbazone synthesized from the condensation of 4-tertbutylcyclohexanone and semicarbazide hydrochloride (1:1 molar ratio) was further treated with cobalt(II) acetate (2:1 molar ratio), to form the Co(II)-semicarbazone complex. The semicarbazone was studied in buffer solutions (Britton-Robinson universal buffer and phosphate buffer), with a pH in the range of 3 to 11. The reduction process was found to be irreversible and diffusion controlled, for both semicarbazone and its Co(II) complex. The effects of change on the sweep rate, concentration, pH and solvent were evaluated. The semicarbazone reduction mechanism was suggested on the basis of the obtained results. Kinetic parameters, such as charge transfer coefficient (αn), diffusion coefficient (D01/2), and rate constant (k°f,h), were calculated from cyclic voltammetric measurements. Semicarbazone and its Co(II) complex were tested against bacterial and fungal species. The metal complex had higher activity than the free ligand.

The corrosion inhibition effect of 1-octyl-2-(octylthio)-1H-benzimidazole (T3) on mild steel in a 1 M hydrochloric acid solution was studied using weight loss measurement, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques, at 308 K. This compound has exhibited a corrosion inhibition efficiency of 93% at 10-3 M concentration. The adsorption of this molecule onto the mild steel surface obeyed Langmuir adsorption isotherm. Potentiodynamic polarization measurements indicated that the studied compound acted as a mixed type corrosion inhibitor. EIS results showed that an increased inhibitor concentration led to an increase in the polarization resistance and decrease in the double layer capacitance.

The present study aimed to evaluate the inhibition effect of an organic compound, namely, (E)-N'-(2-hydroxybenzylidene)isonicotinohydrazide (BIH), for carbon steel corrosion in a 1.0 M HCl solution, by using weight loss (WL), potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). Results show that BIH is a good inhibitor, and the percentage of inhibition efficiency increases on its higher concentrations. The maximum inhibition efficiency of 94% was obtained at 5×10-3 M. Polarization studies revealed that the BIH compound acts as a mixed type inhibitor. EIS showed that increasing the concentration of the inhibitor led to an increase in the charge transfer resistance and a decrease in the double layer capacitance. It was found that the adsorption of this compound obeyed the Langmuir adsorption isotherm. The associated activation energies and thermodynamic parameters of the adsorption process were evaluated and discussed. The temperature effect was studied in the range from 303 to 333 K. In addition, quantum chemical calculations based on the density function theory (DFT) and Monte Carlo simulations were done to support the experimental results.

This paper reports the fabrication of a lithium ion supercapacitor from vertically-aligned carbon nanotubes (VACNTs) directly grown on a conductive substrate (SUS 310S alloy), using alcohol catalytic chemical vapour deposition technique. CNTs direct growth technique on an electrically conducting foil simplifies the electrode assembly, thus reducing the fabrication process, because the foil can directly act as a current collector. With the VACNT direct growth technique, the supercapacitor electrode was easily prepared and assembled with a non-aqueous 1 M LiPF6 electrolyte. Experimental results show that CNTs (multi-walled type structures of good quality) were perpendicularly grown to the substrate. This device demonstrates a specific capacitance of up to 101 F g-1 (at a scan rate of 1 mVs-1), and a high-rate capability, up to a scan rate of 1000 mVs-1. The VACNT electrode electrochemical performance was also measured by galvanostatic charge-discharge and electrochemical impedance spectroscopy. The effect of free standing CNTs direct growth on the current collector makes insulating binder material unnecessary, thus producing better ion accessibilities to its surface. This also contributes to the good and reliable electrochemical supercapacitor performance.