Portugaliae
Electrochimica Acta

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.

We developed and applied in our work a new polyepoxide polymer, decaglycidyl phosphorus penta methylene dianiline, to study its coating behavior in a marine environment. Then, we tested the new crosslinked and formulated macromolecular binder (DGPMDAP), which is used as an anticorrosive coating on E24 carbon steel, in 3.5% NaCl. So as to evaluate and examine the DGPMDAP coating performance in the field of E24 carbon steel corrosion, we applied the different E1 (DGPMDAP/MDA) and E2 (DGPMDAP/MDA/PN) formulations on the metal substrate. The stationary and transient electrochemical studies are very interesting; and, in order to confirm the obtained results, we dispersed the charge of the natural phosphate that was incorporated in the E2 (DGPMDAP/MDA/PN) macromolecular matrix, through the use of a polarizing optical microscope.

Quaternary Ni-B-W-Mo coatings were deposited on AISI 1040 steel by the electroless method. Some of the specimens were heat-treated at 300, 350, 400, 450 and 500 °C, for 1 hour. The deposited coatings contained a high weight percentage of W, and a low B content was detected by energy dispersive X-ray analysis. The coatings in as-deposited state were amorphous, and heat treatment resulted in crystallization with the precipitation of borides. W and Mo were present in the coatings, mainly in a solid solution form. The surface morphology study revealed densely nodulated structures, which are pertinent for sodium borohydride reduced electroless nickel coatings. Electrochemical impedance spectroscopy studies were conducted to determine the corrosion resistance of the as-deposited and heat-treated coatings in a 3.5% NaCl electrolyte. The corroded surfaces were observed under scanning electron microscope, to study the corrosion mechanism.

Plant extracts containing heteroatoms can be used as corrosion inhibitors as they are non-polluting, cheap and eco-friendly. The present work focuses on Asparagus Racemosus (Shatavari) as a corrosion inhibitor of aluminium in acidic medium. The techniques that have been used include weight loss method, quantum chemical analysis and scanning electron microscopy. It was seen that at 4000 ppm concentration of inhibitor the corrosion inhibition efficiency was 72.28%. The inhibition efficiency increased with increase in concentration. The best description of adsorption isotherm was seen to follow Langmuir adsorption isotherm. In order to elucidate the reactivity and molecular structure of inhibitor, quantum chemical parameters were utilized. The surface properties of the metal specimen were determined by SEM.

In this work, activated carbon was produced from residue of fruit of Sapindus and used for the application of adsorption removal of malachite green dye from simulated aqueous solution. Adsorption kinetics of malachite green onto actived carbon was studied in a batch system. The effects of pH and contact time were examined. The malachite green maximum adsorption occurred at pH 6 (4.5 mg/g) and the lowest adsorption occurred at pH 2 (4.1 mg/g). The apparent equilibrium was reached after 120 min. Optimal experimental conditions were determined. In order to determine the best-fit-adsorption Kinetics, the experimental data were analyzed using pseudo-first-order, pseudo-second-order, pseudo-third-order, Esquivel, and Elovich models. Linear regressive and non-linear regressive methods were used to obtain the relative parameters. The statistical functions were estimated to find the suitable method which fit better the experimental data. Both methods were suitable to obtain the parameters. The non-linear pseudo-first-order model was the best to fit the equilibrium data. The present work showed that activated carbon can be used as a low cost adsorbent for the malachite green removal from water.

Benzaldehyde oxime can be readily converted to the corresponding benzaldehyde in biphasic medium. The reaction was carried out with platinum electrodes at room temperature in an undivided cell within constant biphasic medium consisting of chloroform containing benzaldehyde oxime and an aqueous solution of potassium perchlorate as a mediator with a catalytic amount of H2SO4. The electrochemical oxidation was optimized with various experimental parameters to result in the high yield of benzaldehyde (80%).

Bitter leaves extract as inhibitive agent in HCl medium for the treatment of mild steel through pickling was studied. Thermometric, gravimetric and potentiodynamic polarization methods were employed in the corrosion inhibition study. The bitter leaves extract was analyzed using gas chromatography-mass spectrometer. The analysis of the extract revealed the presence of C6H8O (96 g/mole: 2,4-Hexadienal); C7H12 (96 g/mole: 3,4-Heptadiene; 1,3-Diethylallene) and C10H18O2 (170 g/mole: 2-Decenoic acid) as the predominant chemical constituents. The activation energy for the corrosion inhibition process ranged from 39.831 to 77.533 kJ/mol, while the heat of adsorption ranged from -16.093 to -30.224 kJ/mol. These values showed that exothermic and spontaneous adsorption of the extract on the mild steel followed the mechanism of physical adsorption. Maximum inhibition efficiency of 85.4% was obtained. The extract was highly efficient in the corrosion inhibition function. The plant-based inhibitor of bitter leaves extract is a suitable additive for pickling, cleaning and descaling operations.

We made a nano-porous alumina membrane by anodization under 40 V by two steps (10 minutes and 120 minutes) in oxalic acid, followed by chemical treatment in phosphoric acid assisted by an ultrasonic bath. Thus, we achieved a total relatively short time (3 h 30 min) for the synthesis, having all physicochemical processes been done at room temperature (25 °C). We verified a hexagonal arrangement structure with 56 nm inter-pore diameter, and density of 1010 pores/cm2, characteristic of the self-ordered porous alumina. Therefore, our membranes can be used as a template for fast synthesis of nanostructures as nanowires, nano-tubes.

Aluminum alloy corrosion behavior (AA6063) in a 0.5 M hydrochloric acid (HCl) solution was studied. It has been observed that the ethanolic extract of Cordia dichotoma seeds (EECdS) acts as an excellent inhibitor for AA6063 corrosion in a 0.5 M HCl solution. In this study, conventional weight loss, Fourier transformed infra-red spectroscopy (FTIR), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) analysis techniques were used. It was found that the percentage inhibition efficiency increased with higher CdS concentrations. The thermodynamic and kinetic parameters, along with the equilibrium constant for the adsorption processes, showed that CdS performed well as an inhibitor. CdS adsorption on the alloy followed the Langmuir adsorption isotherm, at all the studied concentrations. FTIR, EIS and SEM investigations also indicated that CdS presence effectively lowered the dissolution currents.

N1,N2-Bis(1-Phenylethylidene)ethane-1,2-diamine (PEED) was tested as a corrosion inhibitor for C-steel in a 1.0 M HCl solution, by using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The results showed that PEED is a very good inhibitor, as its inhibition efficiency reached 93.8 %, with a concentration of 1.0x10-3 M, at 298 K. Tafel polarization study revealed that PEED acted as a mixed type inhibitor that obeyed Langmuir adsorption isotherm. The thermodynamic activation parameters for the corrosion reaction were calculated and discussed. Quantum chemical parameters and Fukui function were obtained by DMol3/GGA/PW91/DNP+ level of theory, which was performed using Materials Studiov 8.0 software from Biovia-Accelrys. Monte Carlo simulation was implemented to search for the equilibrium configurations of the PEED/Fe(111) adsorption system, in a 1.0 M hydrochloric acid solution.

Conducting polypyrrole was synthesized by the chemical oxidative method, and characterized using FTIR spectroscopy. An epoxy based paint containing conducting polypyrrole was prepared and applied on a low carbon steel sample, using a film applicator. The uncoated and painted steel samples were kept immersed in the simulated concrete pore solution. The Mott-Schottky technique was used to study the passive film formation on an uncoated low carbon steel, and the conducting polypyrrole coated low carbon steel was immersed in a simulated pore solution. The peaks at 1447 cm-1 and 1149 cm-1 are the responsible peaks of protonated nitrogen atoms in polypyrrole. The peak at 1538 cm-1 refers to the C-C stretching vibration in the pyrrole ring, and the peak at 1042 cm-1 denotes the C-H stretching vibration in the pyrrole ring. The M-S slope indicates that the passive film on uncoated low carbon steel exhibited n-type semi-conductive behaviour, and that the polypyrrole coating exhibited p- n type behaviour. However, with the increasing chloride contents and immersion time, the polypyrrole coating exhibited only p type behaviour.