Portugaliae
Electrochimica Acta

The corrosion inhibition characteristics of nitrogen containing amino acid L-tryptophan on mild steel in 0.1 M HCl solution, in the temperature range of 30-50 oC, was studied by weight loss and potentiodynamic polarization measurements. L-tryptophan significantly reduces the corrosion rates of mild steel; the maximum inhibition efficiency being 83% at 50 oC in presence of inhibitor concentration of 500 ppm. The effect of the addition of very small concentrations of anionic surfactant, sodium dodecyl sulfate (SDS), and of cationic surfactant, cetyl trimethyl ammonium bromide (CTAB), on the corrosion inhibition behavior of L-tryptophan was also studied. The inhibition efficiency (IE) of L-tryptophan significantly improved in presence of both surfactants. The effect of SDS and CTAB on the corrosion inhibition behavior of L-tryptophan appears to be synergistic in nature, the values of synergism parameter being greater than unity. The adsorption of inhibitors on mild steel surface obeyed Langmuir’s adsorption isotherm. The calculated thermodynamic parameters for adsorption reveal a strong interaction between the inhibitors and the mild steel surface. The results obtained by electrochemical studies are consistent with the results of the weight loss measurements. L-tryptophan acts more anodic than cathodic inhibitor.

The effect of extract of Musa paradisiaca on corrosion inhibition of mild steel in aqueous 0.5 M sulphuric acid was investigated by weight loss method, potentiodynamics polarisation technique and electrochemical impedance spectroscopy (EIS). The inhibition efficiency is found to increase with increase in concentration of the extract. Polarization measurement indicates that Musa paradisiaca acts as a mixed-type inhibitor and the inhibition efficiency decreases with rise in temperature. The inhibition is assumed to occur via adsorption of inhibitor molecules on metal surface, which obeys Langmuir adsorption isotherm. The activation energy (Ea) and other thermodynamic parameters for inhibition process were calculated. These thermodynamic parameters indicate a strong interaction between the inhibitor and the mild steel surface. SEM and AFM studies confirmed the adsorption of inhibitor molecules on mild steel surface.

The inhibitive effect of ocimum gratissimum by seed extract on the corrosion of zinc – aluminium (ZA) alloy in 2 M hydrochloric acid (HCl) solution has been studied using gravimetric methods. Inhibition increases with concentration of extract but decreases with temperature. This observation implies that ocimum gratissimum seed extract is an effective and non toxic inhibitor of the corrosion of zinc – aluminium alloy. Adsorption of the extract on the surface obeyed the Freundlich adsorption isotherm. The calculated rate constant (k) shows a first order kinetics in the absence and presence of the inhibitor. The kinetic parameter B, measured for the reaction has a high negative value which implies that ocimum gratissimum becomes more effective as the temperature increases. Synergistic effect of halide additives shows an increase in the efficiency of the extract. However synergism parameter shows that synergism of halide additives decreases with increased concentration of inhibitor.

The inhibition efficiency (IE) of phyllanthus amarus extract (PAE)-Zn2+system, in controlling corrosion of carbon steel in an aqueous solution containing 60 ppm of Cl-, has been evaluated by weight loss method. Weight loss study reveals that the formulation consisting of 2 mL of PAE and 25 ppm of Zn2+ has 98% inhibition efficiency in controlling corrosion of carbon steel immersed in an aqueous solution containing 60 ppm of Cl-. Synergistic parameters suggest that a synergistic effect exists between PAE and Zn2+. Polarization study reveals that this system functions as mixed type of inhibitor controlling the cathodic reaction and anodic reaction to an equal extend. AC impedance spectra reveal that a protective film is formed on the metal surface. The FTIR spectra reveal that the protective film consists of Fe2+-phyllanthus complex.

The inhibition action of 3-ethyl-4-amino-5-mercapto-1,2,4-triazole (EAMT) on the corrosion of 6061-Al alloy in different concentrations of aqueous sodium hydroxide solution has been investigated at different temperatures, using potentiodynamic polarization and electrochemical impedance spectroscopic techniques. The surface morphology of the metal surface was investigated by scanning electron microscopy (SEM). The experimental results showed that the presence of EAMT in sodium hydroxide solution decreases the corrosion rates and the corrosion current densities (icorr), and increases the charge transfer resistance (Rp). It was found that the inhibitor efficiency depends on the concentration of the inhibitor, concentration of the corrosive media and temperature. The inhibition was assumed to occur through adsorption of the inhibitor molecule on the metal surface. The adsorption of the inhibitor on the metal surface is found to obey Langmuir adsorption isotherm. EAMT acts as a mixed inhibitor. Thermodynamic parameters for the adsorption processes were determined from the experimental data. The results obtained from both the techniques are in good agreement.

Special Issue Dedicated to the XII Iberian Meeting of Electrochemistry & XVI Meeting of the Portuguese Electrochemical Society

Metal carbonyl clusters are molecules or molecular ions perfectly defined in size, composition and structural details, which belong by size to the field of nanomaterials. Their molecular structures result from subtle balances between the metal-metal and metal-carbonyl interactions and usually adopt close-packed structures in which a chunk of cubic or hexagonal metal lattice is surrounded by a shell of CO ligands. Very often, such derivatives display extended redox activity affording reversible electron cascades. In many cases such activity increases if interstitial or semi-interstitial atoms of the main group elements (C, N, P, etc.) are inserted in their frames. This in fact triggers establishment of further metal-to-interstitial atom(s) interactions which not only contribute to the number of cluster valence electrons, but also modifies the bonding character of the frontier molecular orbitals.

The electrochemical properties of the one-dimensional coordination polymers [{CuX}2(YNC10H14O)]n (X=Cl: Y=NMe2 1a, Y=NHMe 1b, Y=NH2 1c; X=Br: Y=NH2 2c) and dimers [{Cu(YNC10H14O)}2(μ-X)2] (X=Cl: Y=NMe2 3a; Y=NHMe 3b; X=Br: Y=NMe2 4a; Y=NHMe 4b) were studied by cyclic voltammetry (CV) and controlled potential electrolysis (CPE). All complexes display anodic and cathodic processes. The anodic processes involve the oxidation of the metal site (Cu(I)Cu(II)), while the cathodic processes are based on the ligand. The substituent (Y) at the camphor hydrazones (YNC10H14O) plays a relevant role in the electrochemical properties and reactivity of Cu(I) coordination polymers and Cu(I) dimmers, as corroborated by CPE.

In this work LaNiO3 oxide was prepared by a self-combustion method using citric acid. The electrodes were obtained by coating a nickel foam support with the oxide suspension. Optical microscopy and cyclic voltammetry were used on the electrodes characterization. The evaluation of the electrodes electrocatalytic activity, towards the oxygen evolution reaction in alkaline medium, was performed by means of steady state measurements. The reaction follows a first order kinetics, with respect to OH- concentration, with Tafel slopes close to 40 mV, for low overpotentials. Based on the apparent and real current densities it was possible to conclude that the increase on the electrode activity, when compared with the published data, is mostly related to geometric factors. This fact has been associated with the high electrode/electrolyte contact area provided by the foam nickel substrate. Synergetic effects between the Ni foam and the perovskite oxide cannot be discarded.

The influence of the hydrodynamics of the system on the extent of the electrochemical degradation of phenol, using a boron doped diamond (BDD) anode was investigated. Two different electrochemical cells were used: a batch stirred cell (A), with a volume of 200 mL and a BDD anode of 17.5 cm2, and a batch with recirculation cell (B), with a BDD anode of 70 cm2 and connected to a tank of 30 L. Assays were performed at different stirring speeds and applied current density of 300 A m-2, for cell A, and various flow rates, with an imposed current density of 100 A m-2, for cell B. Chemical oxygen demand (COD) and total organic carbon (TOC) tests were performed to the samples collected throughout the assays, as well as UV-Vis spectrophotometric measurements. For cell A, after 2 h assay, COD removals between 84 and 94% and TOC removals ranging from 54 and 86 % were attained. For the assays run with cell B, during 10 h, COD and TOC removals varied from 27 to 51% and from 23 to 46%, respectively. The influence of the turbulence near the electrode’s surface in the combustion efficiency was also analysed.

Metal recovery by reduction of metal ions present in model solutions, containing one or more heavy metals, was performed. To prepare the model solutions, sulfates and/or chlorides of Cu(II), Cd(II), Pb(II) and Zn(II) were used, at pH 3.5. Assays were run in a one or two compartments cell, at different applied potentials, using a steel plate as cathode, a platinum plate as anode and an Ag/AgCl, KClsat as reference electrode. The metal recovery yield was determined by atomic absorption spectroscopy. The phases corresponding to the metallic deposits were identified by X-ray diffraction. For the solutions containing just one metal ion, the best metal recoveries and the corresponding applied potentials, obtained in a one cell compartment, were the following: Cu(II) 99% at E = - 100 mV; Pb(II) 99% at E = - 800 mV; Cd(II) 93% at E = - 900 mV; and Zn(II) 38% at E = - 1300 mV. The metal removals from the mixed solution of four heavy metals, after 4 consecutive chronoamperometries, performed in a one compartment cell, at the best applied potentials determined with the solutions containing only one metal ion, were the following: Cu(II) 99%; Pb(II) 68%; Cd(II) 92%; and Zn(II) 10%.

The present work reports the results concerning the development and implementation of the first electrochemical biosensor for acrylamide determination, based on a direct biochemical interaction between the analyte and intact bacterial cells, with intracellular enzymatic activity. The biological recognition element consisted of whole cells of Pseudomonas aeruginosa containing intracellular amidase activity, which catalyses the hydrolysis of acrylamide producing ammonium ion (NH4+) and acrylic acid. The transduction process was accomplished by means of an ammonium ion selective electrode. Whole cells were firstly immobilized on single discs of polymeric membranes, such as polyethersulphone, nylon and polycarbonate, which were, then, attached to the surface of the selective electrode. However, it was observed a significant loss of cells each time the biosensor was used, namely at the beginning of the assay, when the membranes were attached to the ammonium electrode, and after the assay, when removed for storage purposes. This evidence determined a premature decrease in the biosensor’s stability. Instead of using single membrane discs, a “sandwich” design, with two membrane discs was considered. This way the cells remain contained between the membranes, never contacting the electrode’s surface, preventing their premature loss. Consequently, the activity of the biosensor could be maintained for longer periods of time. The analytical performance of the biosensor was evaluated. The best results were obtained when polyethersulphone double membranes were used. A typical response of 120 mV (after 6 min reaction time), a Nernstian slope of 48 mV/decade, a limit of detection of 6.31×10-4 M and a half-life time of 27 days, are examples of some figures of merit observed for this biosensor.

Electroless nickel coatings possess several advantages over electroplating such as ability to coat any material and with uniform thickness. Besides, these coatings are used mainly for their wear resistance and corrosion resistance applications. The present study addresses the corrosion behavior of the coating based on electrical impedance spectroscopy. The effect of the four parameters viz. bath temperature, reducing agent concentration, nickel source concentration and annealing temperature, on the electrochemical characteristics (charge transfer resistance and double layer capacitance) are studied with the help of Taguchi method and grey relational analysis. It is found that the bath temperature has the most significant influence on the corrosion behavior of the coating followed by nickel source concentration. The microstructural characterization of the coating is done with the help of scanning electron microscope, X-ray diffraction analysis and energy dispersive X-ray analysis.

The structure and properties of the passive film formed on the surface of AISI 316 stainless steel in aqueous ethanoic acid have been investigated using EIS. Experiments were carried out at 30 ºC in different concentrations of ethanoic acid. Effects on the film properties due to the change of electrode potential, exposure durations and addition of chloride ions to the electrolyte were also studied. Impedance parameters were determined using simple model and equivalent electrical circuit. Results suggest the formation of multilayered passive film on the steel surface. The film possessed dual structure. Inner layers were thin and compact, whereas the outer layer was porous and defective. The measured capacitive behavior was of non ideal nature and hence replaced by constant phase element or CPE. Formation of the passive film and the change in its structure has been explained using impedance parameters.

The inhibition of copper corrosion in 1 M HNO3 by (E)-(4-(4-methoxybenzylideneamino)-4H-1, 2, 4-triazole-3, 5-diyl) dimethanol (MBATD) was investigated by polarization, ac impedance, adsorption studies, quantum chemical calculations, molecular dynamics methods and surface morphological studies. Polarization studies showed that MBATD acts as a mixed type inhibitor. Adsorption followed the Langmuir isotherm with negative value of free energy. Various thermodynamic and kinetic parameters were calculated to understand the energy changes associated during the inhibition process. The correlation between inhibitive effect and molecular structure is ascertained by quantum chemical calculations using density functional theory and Fukui functions. The molecular dynamics method has also been used to simulate the adsorption of inhibitor molecule on the metal surface.

Expired ranitidine was tested as a corrosion inhibitor for mild steel in 1 M HCl using different techniques: weight loss, potentiodynamic polarization, open circuit potential and electrochemical impedance spectroscopy. The polarization resistance (Rp) value increased with increase in the concentration of the inhibitor. Results obtained revealed that ranitidine performed excellently as a corrosion inhibitor for mild steel in this medium at 303 K. The protection efficiency increased with increasing inhibitor concentration. The maximum protection efficiency of 90% has been obtained at 400 ppm. On the other hand, the efficiency decreases with increasing temperature. The adsorption of the inhibitor on the mild steel surface followed Langmuir adsorption isotherm model. The activation and thermodynamic parameters of dissolution and adsorption were calculated and discussed. The negative value of ∆Gads (-40 kJ mol-1) indicates spontaneous chemical adsorption. Results obtained from polarization, EIS and weight loss measurements are in good agreement with each other.

The effect of five compounds of ethoxylated fatty esters with different number of ethylene oxide units on the corrosion of copper in 1 M HNO3 solution has been studied using weight loss and galvanostatic polarization techniques. The inhibition efficiency was found to increase with increasing concentration and number of ethylene oxide units per molecule. Inhibition effect was explained on the basis of adsorption of ethoxylated fatty esters on the metal surface through their ethoxy groups. The adsorption process was found to follow Temkin adsorption isotherm. Polarization data suggested that the used additives act as mixed type inhibitors. It was found also that the ethoxylated fatty esters provide a good protection to copper against pitting corrosion in chloride containing solutions.

The corrosion resistance of three stainless steel materials, namely, stainless steel (SS), stainless steel electroplated with zinc (SS-Zn) and stainless steel electroplated with zinc followed by blue passivation (BP), has been evaluated in an aqueous solution containing 3.5% NaCl. A potentiodynamic polarization study and AC impedance spectra have been used to investigate the corrosion behaviour of these metals. The corrosion resistance of these materials in 3.5% NaCl increased in the following order: SS>SS+Zn+BP >SS+Zn.

The emeraldine base (EB) was synthesized by chemically oxidative polymerization using ammonium persulphate as an oxidant in hydrochloride aqueous medium. The polymer was chemically deposited on mild steel specimens using tetra methyl urea (TMU) as solvent through solvent evaporation method. The coating of polypyrrole (PPy) on carbon steel was deposited by chemical polymerization. A bi-layered polymer coating comprising of inner coat of PPy with top coat of EB (PPy/EB) was also deposited on mild steel following identical procedure. The deposited EB, PPy and PPy/EB coatings were characterized by Fourier Transform Infrared (FTIR) Spectroscopy and Scanning Electron Microscopy (SEM). The anticorrosive properties of single and bi-layered coatings was investigated in major corrosive environments such as 0.1 M HCl, 5% NaCl solution, artificial seawater, distilled water, tap water and open atmosphere by conducting various corrosion tests which include: immersion test, open circuit potential measurements, potentiodynamic polarization measurements, and atmospheric exposure test. The results of immersion tests showed that the PPy/EB coating gave best protection in all media under investigation, the protection efficiency being in the range of 72 to 79% after 30 days of immersion. The result of OCP measurements showed significant positive shift in the corrosion potential for single as well as bi-layered coatings in all corrosive medium under investigation; the bi-layered coating showing more positive corrosion potential. The potentiodynamic polarization studies also confirmed lower corrosion rates for PPy/EB coating than the single polymer coatings.

Sediments can sink or deliver elements to water column. Iron is considered an essential element for the development of cyanobacteria and a limitant to phytoplankton growth. Studies on the chemical speciation of iron in seawater have been conducted, but there is no information about its chemical speciation in pore water, as reported here. This paper presents a voltammetric method to analyze dissolved iron and its chemical speciation in sediment pore water, using adsorptive cathodic stripping voltammetry (AdCSV) and the technique of competitive ligand exchange (CLE-AdCSV), respectively. The limit of detection (LOD = 0.30E−6 M), the quantification (LOQ = 0.90E−6 M), precision (RSD = 4.9%) and accuracy (98%), were calculated from experiments using certified reference material SLRS-4 (National Research Council Canada). The chemical speciation of pore water from sediment samples collected during April 2009, in the Patos Lagoon Estuary (RS, Brazil), was analyzed for the first time, revealing that the ratio (labile iron to dissolved iron) is significantly lower in pore water extracted from sediments of the upper layer (0 to −5 cm), than from the overlaying water or of the pore water extracted from sediments of the lower layer (−15 to −20 cm).