Portugaliae
Electrochimica Acta

This work reports the development of a glassy carbon electrode (GCE), modified with an oxidized poly diphenylamine/multi-walled carbon nanotubes-β-cyclodextrin (OPDPA /MWCNT-β-CD) composite film, for ceftazidime determination. The OPDPA/MWCNT-β-CD film surface morphology was investigated using field emission scanning electron microscopy (FESEM) and cyclic voltammetry (CV). Ceftazidime was adsorbed onto the OPDPA/MWCNT-β-CD, at open circuit, for 240 seconds, and then oxidized in acidic media. The electrochemistry of ceftazidime oxidation was investigated using cyclic voltammetry and differential pulse voltammetry (DPV). Under optimum conditions, the peak current linearly increased with ceftazidime concentration, in the range of 5×10-8 M to 1×10-5 M. The obtained detection limit was 1.0×10-9 M. This sensor was employed for ceftazidime determination in the biological and pharmaceutical samples.

A comparative study of the corrosion behavior of pure magnesium and AZ91D anodes in reinforced cement concrete was undertaken in the present work. The steel reinforcements were electrochemically kept in contact with these anodes in a chloride atmosphere, and the half-cell potential drop was observed. Bare steel reinforcements were tied to the anodes and were also kept in a high chloride atmosphere to test the mechanical properties. The yield stress and ultimate tensile stress were found to decrease by approximately 50 MPa, while the reduction in percentage elongation was approximately 25% for reinforcements tied to AZ91D and pure Mg, at the end of 80 days, compared to the fresh steel reinforcement. The rate of corrosion of pure Mg was reportedly slightly higher compared to AZ91D, due to the presence of inter-metallics, as inferred through micro-graphs.

New corrosion inhibitors of benzimidazole derivatives, namely: 6-methoxy-2-(((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)sulfinyl)-1-vinyl-1H-benzo[d]imidazole (EMSB), 6-Methoxy-2-(((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)) sulfinyl)-1-(prop-2-yn-1-yl)-1H benzimidazole (MSVB) and 6-methoxy-2-(((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)sulfinyl)-1-(phenacyl)-1-H benzimidazole (MSBP), have been synthesized and their inhibiting action on the corrosion of carbon steel in acidic bath (1 M HCl) has been investigated by various corrosion monitoring techniques, such as weight loss measurement, potentiodynamic polarization, adsorption, electrochemical impedance spectroscopy (EIS) and basic computational calculations. The results of the investigation show that the inhibition efficiency of all the three inhibitors increases with increase in concentration of inhibitors and decreases with increase in temperature. The inhibitors, MSBP, MSVP, and EMSB show corrosion inhibition efficiency of 98, 97 and 93% respectively, at 10-3 M and 303 K. EIS measurements showed an increase of the transfer resistance with the inhibitors concentration. Polarization studies showed that the studied inhibitors are mixed type in nature and the adsorption of benzimidazole is described by the Langmuir isotherm. In addition, density functional theory (DFT) calculations and molecular dynamics simulations (MDS) were undertaken to describe the electronic and adsorption properties of the synthesized inhibitors constituents, including the synergistic/dispersive interactive effects of the multiple adsorptions of the various active constituents in the inhibitor film on the iron surface. Also DFT and Molecular dynamic (MD) simulations were employed to correlate the experimental findings.

The electrochemical behaviour of nicotine extracted from commercial cigarette tobacco were studied in a blood media, at a glassy carbon electrode (GCE), using cyclic voltammetry technique. The nicotine compound oxidized the blood component in neutral media, at pH 7.4. The response was evaluated with respect to different pH values, scan rates, concentrations and temperatures. From the voltammogram of nicotine in a blood medium, it was found that the oxidation current peak of nicotine was +999 mV in acidic and alkaline media, which enhanced about twice in an acidic medium and disappeared in an alkaline medium. In its turn, at highly acidic media, three reduction current peaks appeared in the nicotine voltammogram in blood media. The electrochemical behavior of nicotine was investigated using cyclic voltammetry technique; the method was successfully applied for the determination of the nicotine compound which had a reversible redox couple in a blood medium from the peak potential separation of 100 mV. Also, it was found that the average value of the diffusion coefficient at the cathodic electrode was 4.075x1011 cm-2 s-1, and 22.625x1011 cm-2 s-1 at the anodic electrode.

The present work highlights the results of the application of a green inhibitor for material conservation. The anticorrosive performance of the pectin bio-polymer was established for the corrosion control of mild steel in a 1 M phosphoric acid medium. Electrochemical measurements such as potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) studies were adopted for the corrosion inhibition studies. The studies were carried out by varying the concentrations of pectin in the temperature range from 303 K to 318 K. Conditions were optimized to get maximum inhibition efficiency. The surface morphology study was done by scanning electron microscope (SEM), and elemental mapping was done using energy dispersive X-ray (EDX) studies to confirm the adsorption and interaction of the inhibitor with the material. Studies showed an increase in the inhibition efficiency with an increase in the concentration of pectin and also in the temperature. Maximum inhibition efficiency of 70% was achieved by the addition of 800 ppm of inhibitor. Pectin acted as a mixed type inhibitor by bringing down both cathodic and anodic reactions. Kinetic and thermodynamic parameters revealed chemical adsorption of pectin onto the mild steel surface. A suitable mechanism was proposed for the adsorption of pectin which was reaffirmed by the surface morphology studies. Pectin emerged as a potential eco-friendly green inhibitor for the corrosion control of mild steel, with economic benefits.

An irreversible electrode reaction influenced by the reversible potential dependent inhibition is theoretically analysed. The consequence of reactivation of the electrode surface is the continuation of electro-oxidation and the appearance of the second anodic peak in cyclic voltammetry. An indicator of the change of the electrode kinetics caused by the inhibitor is proposed.

In the present paper, the methanol extract from the leaves of Pistacia terebinthus L. (PT) was characterized by GC/MS analysis. It was tested as a corrosion inhibitor for iron in a 3% NaCl solution, using gravimetric measurements and electrochemical methods (potentiodynamic polarizations and electrochemical impedance spectroscopy). Fourteen compounds were identified; the major compounds are palmitic acid (31.5%), stearic acid (25.7%) and oleic acid (11.3%). The data showed that the tested extract offers good corrosion resistance. The constituents of the PT extract affected polarization resistance (Rp) and corrosion current density (icorr), demonstrating that the proposed inhibitor hinders corrosion reactions. The results obtained at 200 ppm are very interesting, and reach maximum values of 96.96%, 89% and 64.82%, respectively, which were confirmed by scanning electron microscopy (SEM), coupled with energy scattering of X-ray spectroscopy (EDX).

Electrochemical methods for the recycling of tungsten carbide (WC-10Co) resources suffer from passivation in the acidic medium caused by WO3 and also in NaOH electrolytes, due to hydroxide formation. We found that an ammoniacal solution is a promising electrolyte for sustainable electrochemical dissolution of both tungsten (W) and cobalt (Co). The ammoniacal medium performs greatly when supported with Cl-, SO42- and CO32- ions. Poor dissolution/corrosion tendency of WC-10Co in a diluted NH4OH solution enhanced many folds in the presence of Cl-, SO42- and CO32- ions. Among these supporting ions, Cl- emerged as the most suitable for the electrochemical leaching of W and Co from the WC-10Co, accompanying the least noble behavior of WC-10Co. An electrolyte composed of 150 g/L of ammonia and 5% (w/v) of NH4Cl yielded the maximum anodic current density. Microscopic examination of the electrochemically treated samples shows scattered active sites responsible for the oxidative dissolution of WC-10Co. The usefulness of W and Co dissolution in ammonia-additive salt followed the order NH4OH-NH4Cl>NH4OH-(NH4)2SO4>NH4OH-(NH4)2CO3.

Carbonaceous materials are used to generate composites for numerous purposes, due to their extraordinary properties. In this context, we have carried out a study on the modified properties, viz. thermal stability and specific capacitance of the composites fabricated by the introduction of a carbonaceous material (graphite and graphene oxide) into the Polyvinyl Butyral matrix. Since it has excellent adhesibility and dielectric permittivity, and it can be employed as an electrode in sensors, polyvinyl butyral is used in the current investigation, along with carbonaceous materials, for electrochemical and bandgap properties (HOMO-LUMO) studies. Spectroscopic FT-IR, XRD and SEM analyses remark the comprehensive entanglement of the two components. Various fabricated composites show an increased thermal stability, with a percentage of carbonaceous filler, as suggested by TGA. Cyclic voltammetry studies reveal that PVB acts as an excellent binder, and as a good matrix for the charge movement through it, because it has a good level of dielectric permittivity, thus enabling fabricated materials to be developed as good candidates for supercapacitive batteries

The electrochemical degradability of Al-20% Mg and hypereutectic Al-22% Si industrial alloys was evaluated in an aggressive acidic environment, namely 1 M H2SO4, using potentiodynamic polarization, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) techniques. The microstructure and constituting phases of the surface alloys were characterized by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM), coupled with Energy Dispersive X-ray Spectroscopy (EDX). It was found that the two alloys’ corrosion behavior mainly depends on their crystalline phases. The presence of the active intermetallic β- Al3Mg2 phase in the Al alloy with high Mg content induced a preferential Mg dissolution, which caused a severe intergranular attack on this alloy by the corrosive solution. Meanwhile, the Al alloy containing high Si content, which presented the eutectic Al-Si phase, showed a uniform and weaker dissolution. It was also observed that a rise in temperature reduced the corrosion performance of the two studied alloys, as these corroded faster than pure aluminum.

Use of Amaranthus extract, as mild steel corrosion inhibitor, in a pickling paste with H2SO4, has been investigated. The electrochemical experiments were conducted to bring forth results regarding various parameters, viz., corrosion current, anodic polarization and cathodic polarization. The corrosion current gradually decreased with time, for both uninhibited and inhibited systems. Polarization curves values indicate that Amaranthus extract adsorption modified anodic dissolution mechanism, as well as cathodic hydrogen evolution. From the results, it is clear that both cathodic and anodic reactions were inhibited, and that inhibition increased as the inhibitor concentration increased in acidic media, but the cathode was more polarized. This result indicated that Amaranthus extract can be classified as a cathodic inhibitor, in a 4 N H2SO4 solution. Surface analysis via scanning electron microscope (SEM) showed a significant improvement on the mild steel plate surface morphology, in the presence of optimum inhibitor concentrations. Thus, by conducting these experiments, our aim is to use inhibited pickling acid in the paste form, so that it can be conveniently applied on large structures, as well as on small tools, to be pickled/cleaned. As a contribution to the current interest on environmentally friendly green corrosion inhibitors, the present study investigates the inhibiting effect of Amaranthus extract, a green inhibitor which is commonly known as Pigweed.

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.