Portugaliae
Electrochimica Acta

The possibility of obtaining corrosion inhibition efficiency from the Scanning Vibrating Electrode Technique (SVET) is investigated. Results are compared with those obtained by Electrochemical Impedance Spectroscopy (EIS) for the inhibition of iron and zinc with cerium nitrate and 1H-benzotriazole. Examples of good and bad estimations are given and an interpretation for the discrepancies observed is advanced. Large deviations occur when corrosion products are formed since this leads to an underestimation of the currents measured by SVET. Visual inspection of the samples may identify the cases more susceptible to deviation. The approach discussed here can be used to obtain more quantitative information from SVET measurements but should not replace the traditional ways used to determine the inhibition efficiency.

Poly(3,4-ethylenedioxythiophene) (PEDOT) films were potentiostatically and potentiodynamically prepared on Pt electrodes from acetonitrile monomer solutions containing TBAPF6 or FcPF6 (or both). The polymer modified electrodes obtained with different growth charges were characterized electrochemically by cyclic voltammetry in a monomer free solution. The results point to the effective immobilization of the ferrocenium ion inside the PEDOT matrix by entrapment during the polymer synthesis in potentiostatic mode. It is discussed the influence of the presence of the organometallic cation in the polymerization solution in the growth process and the efectiveness of the incorporation of the metallocene is analysed on the basis of the electrochemical data collected for the films redox conversion.

A biosensor for the determination of pirimicarb was constructed by the co-immobilization of acetylcholinesterase and choline oxidase on a derivatised polyethersulfone membrane. The membrane was applied to an Universal Sensors electrode base system and the hydrogen peroxide formation was monitored upon addition of acetylcholine to an appropriately chosen buffer where the biosensor was dipped. The hydrogen peroxide is the product of the hydrolysis of choline, catalysed by cholineoxidase. Choline was formed by the action of acetylcholinesterase on acetylcholine. Pirimicarb inhibits these reactions and that fact can be used to determine its concentration. A calibration plot for pirimicarb was obtained.

The suitability of polytyramine (PTy) modified electrodes for the development of DNA biosensors is analysed in this work. PTy films were synthesised potentiodynamically on Pt electrode, from acidic aqueous solution. A cylindrospermopsin coding sequence (PKSM4 probe 5’-phosphate modified) was covalently bound on the polytyramine matrix using N-hydroxysuccinimide (NHS) and 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC) as coupling agents. Changes of the open circuit potential were registered as a first approach to monitorise the immobilization and hybridization processes. The modified electrodes have been characterised by cyclic voltammetry and square wave voltammetry, using methylene blue (MB), as electroactive indicator and the successful immobilization and hybridization steps are revealed by the increase of the peak currents. The hybridization has also been confirmed by measuring the fluorescence of the modified electrodes after staining with PicoGreen® DNA dye.

The textile dyes Reactive Black (RB5) and Basic Yellow 28 (BY28) were individually exposed to electrochemical treatment in 0.1 M KOH and in phosphate buffer solution pH 5.1, respectively. The cathodic decolorization of the azo dye RB5 was carried out using a Ni-polyvinylchloride composite. The methine dye BY28 was anodically treated on a Co-polyvinylchloride. The electrochemical study of the dye solution was carried out by repetitive cyclic voltammetry (RCV) under potentiostatic conditions in stationary and flow regime. The effect of various experimental parameters, such as potential and electrolysis time was investigated. The studies showed that for the RB5 (20 to 80 ppm) dye solutions, around 98 % of the color was removed after 60 min treatment at -3.0 V vs. SCE. It was also demonstrated that for the BY28 (15 ppm) dye solution, 6-10 cycles or low potentials (-300 mV) during 200 min electrolysis were sufficient for the decolorization of the solution. However, COD and TOC values did not decrease.

The goal of this study was to evaluate the internal corrosion in a liquid-steam pipeline (type A515 low carbon steel) of the Geothermal Field of Miravalles (CGM) in Costa Rica, and to determine the corrosion thickness for future geothermal fields. The monitoring was carried out using the following techniques: linear polarization resistance (LPR), electrical resistance (ER), and weight loss. The results show for neutral or alkaline pH conditions and slightly free of oxygen, that the corrosion thickness should be 1.8 mm, in order to guarantee the integrity of the pipe over the field lifetime.

Laccase of different lots was used in the construction of biosensors for the determination of polyphenolic content of wine. Laccase was immobilized on modified polyethersulfone membranes and applied to a Universal Sensor electrode base system. Parameters of the biosensors thus prepared were studied with respect to caffeic acid as the model substrate. Sensitivity of 0.102 mA × M1, RSD of 1.9%, linear range of 5 to 35 × 10-6 M, limit of detection of 8.8 × 107M, were determined. Different values of the applied potential near 0 V were tested in order to choose that one which gives the best performance of the present biosensors. Citrate and tartarate buffer solutions at pH 3.5 were used. Polyphenolic content of different red wines was determined at +100 mV vs. Ag/AgCl.

Simulation of the effects of natural atmospheres on metals using electrochemical techniques together with accelerated tests presents certain characteristics that make it highly interesting from the point of view of the study and prediction of corrosion. It will enable us to forecast the behavior of a metal exposed to a particular environment without the need to perform field test, with a consequent saving in terms of time and financial expenses. The aim of this study is to establish the correlation between the atmospheric corrosion rate of copper (obtained by atmospheric exposure at stations located in the province of Las Palmas, Spain) with the different variables that intervene in the corrosion process. In the same way, for the determination of the mathematical model that best fits the results obtained experimentally and which takes into account the most influential parameters in the corrosion process, the power Law model has been used which generically reproduces the electrochemical process which occurs in atmospheric corrosion.

Simple, rapid and precise amperometric methods for quantification of ascorbic acid (AA) are presented. Glassy carbon (GC), carbon paste (CP) and modified carbon paste (MCP) electrodes are used for this purpose. MCP electrodes are of CP with 10 % vanadate (V). All electrodes are inserted in a wall-jet device with an Ag/AgCl reference electrode and a platinum auxiliary electrode. This device is coupled to a flow injection analysis (FIA) set-up. Hydrodynamic and amperometric parameters are optimized unvaryingly. GC electrodes present slopes of 4.75  105 nA L mol-1 ( 6.4 %) under optimum conditions. CP and MCP electrodes show higher sensitivity, with slopes of 6.37  105 nA L mol-1 ( 6.6 %) and 7.32  105 nA L mol-1 ( 4.4 %). Linear responses range (1.0-2.0)  10-6 to (0.8-1.0)  10-5 mol L-1. Correlation coefficients were > 0.994 and about 48 samples are analysed per hour. Application of the proposed method to the analysis of soft drinks is presented.

Compared to pure zinc plating, zinc alloys provide several advantages. A zinc alloy generally offers superior sacrificial protection to steel as since it corrodes more slowly than pure zinc. A search for a non-cyanide zinc plating bath resulted on the development of a zinc nickel sulphamate bath. To obtain better quality zinc-nickel deposits triethanolamine was added to the bath. In this paper the influence of triethanolamine in zinc-nickel plating from sulphamate bath, has been investigated through current efficiency measurements, cyclic voltammetry, scanning electron microscope, and X-ray diffraction techniques. In the presence of triethanolamine in the zinc-nickel sulphamate bath the current efficiency of alloy deposition increased to a maximum of 99% at the current density of 1.5Adm-2 and thereafter decreased. TEA adsorbed on the electrode surface obeying Henry’s isotherm. On mild steel surfaces, uniform smooth slate gray deposit with smaller crystallites was produced, in presence of TEA in solution, and the electrodeposits exhibited additional Zn(OH)2 and gamma-Ni3Zn22 phases. Hydrogen evolution was suppressed by TEA and thereby current efficiency is increased for alloy deposition.

Electrochemical response of glass carbon (GC) and edge plane pyrolytic graphite (EPPG) electrodes was investigated using electrochemical impedance spectroscopy (EIS) method in three kinds of electrolytes, i.e., 0.1 M Na2SO4 (aqueous solution), 0.1 M tetraethylammonium perchlorate (TEAP) (acetonitrile solution) and pure room temperature ionic liquids (RTILs) of 1-ethyl-3-methylimidazolium tetrafluorobroate (EMIBF4). The marked difference in the admittance plots was exhibited by the GC electrode, suggesting that the double layer occurring on GC electrode differed from each other when immersed in different electrolytes, indirectly proving the meso-porosity structure of GC electrode. While for EPPG electrode, similar results were obtained in these three selected electrolytes, which were interpreted by the “defects” in EPPG electrode.

In the past, mechanical properties have been the main criteria for selecting materials at the Miravalles Geothermal Field. Consideration of the corrosion resistance has been a secondary concern. The goal of this study was to determine the uniform corrosion rate of carbon steels currently used on pipelines (ASTM A 53, ASTM A515, ASTM A234 and API-5L) and compare this rate with the ASTM A-36 standard. The first step was the development of laboratory electrochemical and weight loss experiments. Ultrasonic thickness measurements of pipes for brine, steam and liquid-steam lines, in both straight and curve sections, showed negligible lost in material over the seven year period they have been on operation. The electrochemical study involved polarization curve analysis and corrosion rate measurements by Linear Polarization Resistance (LPR) and Electrochemical Noise Measurements (ENM). Electrochemical cells include triple and conventional testers (SCE as reference and platinum electrode as auxiliary) using brine degassed with nitrogen at 93 ± 2 ºC. According to the results, the ASTM-A36 has a similar general corrosion rate to those currently used on pipelines.

2-amino-1, 3, 4-thiadiazoles (AT), 2-amino-5-methyl-1, 3, 4-thiadiazoles (AMT), 2-amino-5-ethyl -1, 3, 4-thiadiazoles (AET) and 2-amino-5-propyl -1, 3, 4-thiadiazoles (APT) were synthesized. FT-IR and NMR studies were done in order to confirm the composition of the synthesized inhibitors. These compounds were evaluated as inhibitors for mild steel in 20% formic acid and 20% acetic acid by weight loss, potentiodynamic polarization and electrochemical impedance techniques. Scanning electron microscopic study (SEM) was also used to investigate the surface morphology of inhibited and uninhibited metal samples. The inhibition efficiency of these compounds was found to vary with the inhibitor concentration, immersion time, temperature and acid concentration. The adsorption of these compounds on the steel surface from both acids were found to obey Langmuir’s adsorption isotherm. These compounds are mixed type inhibitors in both acid solutions. Various thermodynamic parameters (Ea, ΔGads, ΔQ, ΔH, ΔS, t1/2) have also been calculated to investigate the mechanism of corrosion inhibition. Electrochemical impedance study was used to investigate the mechanism of corrosion inhibition.

Electrochemical oxidation of phenol, 4-aminophenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2-hydroxybenzoic acid, 2-tert-butylphenol and 2-tert-butyl-4-methoxyphenol has been carried out at sacrificial zinc anode. The products of these reactions have been identified by elemental analysis and infrared spectral data and are found to be zinc(II) phenoxides. Though these compounds do not form coordination compounds on refluxing with 1,10-phenanthroline or 2,2'-bipyridyl, yet the coordination compounds of these phenoxides with these ligands have been synthesized electrochemically. Current efficiencies of these system are quite high.

The inhibition effect of tetra-N-butylammonium iodide (TBAI) on mild steel in 1 N H2SO4 has been studied by using weight loss, electrochemical polarization, Fourier Transform Infrared (FTIR) and scanning electron microscopic (SEM) techniques. It has been concluded that the percentage inhibition increases with increasing the concentration of the inhibitor. The adsorption of TBAI on mild steel surface in 1 N H2SO4 obeys Langumir adsorption isotherm; surface analysis and IR studies are also carried out to establish the mechanism of corrosion inhibition.

Electrochemical methods were used to investigate the abilities of the homologous straight chain monocarboxylates (CnH2n+1COO-,n=1-13) to inhibit the pitting corrosion of two types of copper alloys in aerated, saline and near neutral aqueous solutions. The copper alloys were alloy (I) (Cu + 4.47% Fe) and alloy (II) (Cu + 10.67% Al + 5.02% Fe). It was found that these were susceptible to pitting corrosion in NaCl solutions, with increase of Cl- ions concentration. Alloy (II) was more susceptible to pitting corrosion than alloy (I). The performance of monocarboxylates was shown to be critically dependent upon their chain length. The range of chain lengths producing optimal inhibition was (6  n  10). These carboxylates showed abrupt decreases in inhibitor ability outside the optimal range. The dramatic variations in inhibition efficiencies probably resulted from competing reactions such as adsorption, solubility and micelle formation.

The comparative study of corrosion behaviour of brasses 70Cu30Zn and 60Cu40Zn in HNO3 solution in absence and in presence of 1-phenyl-5-mercapto-1,2,3,4-tetrazole (PMT) was studied by gravimetric and electrochemical methods. Results obtained are in good agreement and reveal that the corrosion rate depends on immersion time and zinc content in the alloy. Copper and zinc losses from each specimen studied, at various immersion times, were estimated by Atomic Adsorption Spectroscopic analysis. This shows that the inhibition efficiency of the inhibitor towards copper is more significant than zinc. PMT was adsorbed preferentially on the copper surface and inhibits the process of corrosion of brasses in the nitric acid medium.

Electrodeposition of Ni-Ce alloy on steel was studied and an electroplating bath was optimized for maximum cathodic current efficiency. Cerium sulphate content in the bath decreased the cathodic current efficiency. Hydrogen evolution reaction from NaOH solution was catalysed by cerium in the alloy and also the increase of cerium content increased the catalytic activity. Among the various Ni-Ce alloys, Ni77Ce23 alloy offered the maximum catalytic activity.