Portugaliae
Electrochimica Acta

The non-toxic anticorrosion properties of nicotinamide have been studied on aluminum in an acidic medium. The techniques included weight loss method, electrochemical measurements, quantum chemical calculations, Monte Carlo simulation, infra-red spectroscopy and scanning electron microscopy. This study gathers information about the inhibitor molecule adsorption onto the surface of metal specimens. Thermodynamic parameters were utilized to predict the adsorption mechanism. The dependence of corrosion inhibition efficiency on various parameters was examined, and it was seen that the increase in the inhibitor concentration and in temperature led to an increase in the inhibition efficiency. The dipole moment and EHOMO-ELUMO influenced the inhibition efficiency, which was observed by quantum chemical studies.

In this work we made a synthesis of two molecules of the same family, the pyrazole 4-(4,5-dihydro-1H-pyrazol-5-yl)-N,N-dimethylaniline D and N,N-dimethyl-4-(3-methyl-4,5-dihydro-1H-pyrazol-5-yl)aniline D10. These two molecules have a good inhibiting activity against the corrosion of mild steel in 1 M HCl. This activity has been confirmed by gravimetric and electrochemical studies; we use a potentiodynamic polarization and the impedance spectroscopic technique. From this investigation, we observe that the integration of a methyl group in the pyrazole D allows decreasing slightly the corrosion of steel. For more information about the action mode of our inhibitors, we launched theoretical calculation by DFT method. We used these calculations to discuss the stability, the reactivity, and the adsorption of our pyrazolic inhibitors with iron in acid medium.

Mild steel corrosion inhibition in 1.0 M HCl, by two stereoisomers of 2-phenyl-benzothiazin-3-one (BHT1) and 3-phenyl-benzothiazin-2-one (BHT2), was researched using experimental and theoretical studies. The experimental results showed that the inhibition efficiency depends on the concentration and molecular structure of the investigated compounds, and it reached a maximum of 95 % and 96 %, at 10-4 M and 10-5 M of BHT1 and BTH2, respectively. This difference can be explained by the formation of hydrogen bonds, in BHT1 case. In addition, the polarization studies indicated that the above mentioned products acted as mixed type inhibitors. The molecular structure influence on the corrosion inhibition efficiency was theoretically investigated using DFT calculations. The structural and electronic parameters were determined, and showed good agreement with the experimental results.

This work presents the thermometric and gravimetric analyses of aluminum corrosion control in a HCl medium, using Ricinus communis (castor oil) extract. The obtained plant extract was subjected to phytochemical analyses for the identification of various sample constituents. Applying the thermometric technique, reaction values for the aluminum dissolution in free and inhibited HCl media were used to determine the extract inhibition efficiency. The gravimetric method was carried out using one factor at a time, and response surface methodology. Thermodynamic parameters of activation energy, heat of adsorption and free Gibbs energy of the corrosion inhibition process were determined. Central composite design of Design Expert Software was employed in the response surface methodology. The results analysis revealed that Ricinus communis extract contained phytochemicals of alkaloids, cardiac glycosides, flavonoids, phenolics, phytates, saponins and tanins. The extract adsorption onto the aluminum surface obeyed the physical adsorption mechanism; the activation energy was lower than the threshold value of 80 kJ/mol required for chemisorption. A quadratic model adequately described the relationship between the inhibition efficiency and the inhibitor concentration, temperature and time factors. The thermometric and gravimetric techniques recorded high inhibition efficiencies of 83.93% and 82.61%, respectively, showing that the Ricinus communis extract can be employed to control aluminum corrosion in an HCl medium.

A sensitive, accurate electrochemical sensor based on reduced graphene oxide modified carbon paste electrode (RGO/CPE) was developed to measure curcumin levels in human blood serum. The as-prepared electrode (RGO/CPE) was verified to outperform bare carbon paste electrode (CPE), with increased oxidation and reduction peaks at +0.505 V and +0.408 V, respectively. Curcumin measurement was performed using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. RGO/CPE showed a desirable linear response towards curcumin, within the concentration range of 10-6000 µM; the detection limit (S/N = 3) was 3.183 µM. Moreover, electrochemical impedance spectroscopy (EIS) and field emission scanning electron microscope (FESEM) were applied to gain further insight into the electrochemical behavior of the proposed electrode. The results revealed that the usage of RGO caused increased sensitivity of the sensor response to curcumin; therefore, RGO/CPE can be considered a promising electrochemical sensor for curcumin determination in human blood serum.

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.