Portugaliae
Electrochimica Acta

The inhibition of copper corrosion in 3.5% NaCl solution has been studied at 25 oC using three inhibitors: 1-phenyl-2,4-dithiobiuret(Inh I), 1-p-methoxyphenyl-2,4-dithiobiuret(Inh II) and 1-p-chlorophenyl-2,4-dithiobiuret(Inh III). The inhibition efficiencies of these compounds have been evaluated by weight loss and electrochemical methods (impedance spectroscopy and polarisation curves). The surface study was done by using SEM and ESCA techniques. The inhibition efficiencies of the inhibitors follow the sequence Inh II > Inh I > Inh III. The inhibitors Inh I, Inh II and Inh III appear to inhibit corrosion process through the formation of a protective film which was found to consist of Cu(I)-inhibitor complex, cuprous chloride CuCl or CuCl2- complex ions or both on the surface.

Optical, electrochemical and spectroelectrochemical features of a family of triisopropylsilyl (TIPS) end-capped oligothienoacenes from the tetramer to the octamer are studied. The oxidation potentials and the formation and stability of the different oxidized species are influenced by the oligothienoacene chain length. The effect of the steric hindrance caused by the substituents on the -dimerization of the radical cations is also investigated by comparing a trimethylsilyl (TMS) substituted pentathienoacene with a TIPS substituted one.

Square-wave voltammetry was used to explore the adsorption property of cefadroxil complex with copper ions on the hanging mercury drop electrode (HMDE). By employing the adsorptive stripping voltammetric approach, a sensitive electroanalytical method for the quantitative analysis of cefadroxil antibiotic was achieved. A well-developed voltammetric peak was obtained in pH 10 Britton-Robinson buffer (B-R buffer) at -650 mV. The cyclic voltammetric studies indicated that the reduction process was irreversible and primarily controlled by adsorption. An investigation of the variation of adsorptive voltammetric peak current with supporting electrolyte, pH, accumulation time, accumulation potential, scan rate, pulse amplitude, SW frequency, working electrode area and convection rate has resulted in the recognition of optimal experimental conditions for cefadroxil analysis. The studied electroanalytical signal showed a linear response for cefadroxil in the concentration range 6E-7 - 2E-6 mol L-1 (r = 0.999). A limit of detection of 2E-9 mol L-1 with relative standard deviation of 2.783 RSD% and mean recovery of 99% was obtained. Possible interferences by several substances usually present in pharmaceutical formulation were also evaluated. The analytical quantification of cefadroxil in commercially available pharmaceutical formulation was performed and compared with data from HPLC technique.

Corrosion of the embedded steel reinforcements (rebars) in concrete is the major reason for the deterioration of concrete structures. One of the effective methods of overcoming this serious problem is by applying a barrier coating to the steel rebars. The barrier coating on steel rebars isolates the steel from the immediate environment and also acts as an insulator to prevent the corrosion. Four different coatings have been formulated using resins such as epoxy silicone-polyamide, polyester polyol-aromatic isocyanate, and acrylic polyol-aromatic isocyanate. These formulations consist of either ordinary portland cement (OPC) or fly ash as extender (pozzolanic pigments) and titanium dioxide (TiO2) and zinc phosphate as the main pigment. All these coatings have been studied for their barrier protection and other mechanical properties. Evaluation study of these coatings include Electrochemical Impedance Spectroscopy (EIS), chemical resistance tests and mechanical tests such as flexibility, impact resistance, adhesion, hardness and abrasion resistance, etc., as per ASTM standards and the results are discussed elaborately. It has been concluded from this study that the epoxy silicone-polyamide resin based coating formulation shows good mechanical properties in addition to the barrier protection to the steel rebars from the corrosive environments.

Adsorption of film forming corrosion inhibitors on metallic surfaces led to a decrease in corrosion rates, since the adsorbed organic molecules behave as chemical and physical barriers for cathodic and anodic reactions. A novel and simple methodology to characterize commercial corrosion inhibitors is proposed. Fundamental studies of film forming corrosion inhibitors adsorption on platinum and its correlation with corrosion inhibition efficiencies of steel provide useful information for quality control of film forming inhibitors.

In the present work, a study of the photoelectrochemical characteristics and chemical structure of anodic films formed onto different Sn1-xInx (0 ≤ x < 0.3) alloys in 0.2 M NaOH is presented. The electrochemical behavior of the alloys was studied by means of cyclic voltammetry. The optical band gaps of the passive films were determined by photocurrent spectroscopy, while its chemical structure was studied by Fourier transform infrared reflection-absorption spectroscopy. The experimental results indicate that the anodic films consist of hydroxylated species of the type InySn(1-y)(OH)4-y. The films are enriched in In and, for x > 0.25, In(OH)3 is the dominant component of the surface layers. The existing theoretical background was used to estimate the relative concentration of Sn and In in the films from the band gap measurements.

This work aims to investigate the electro reduction of nitrate at copper elecrode in HClO4 medium by the linear sweep voltammetry and convuctivity measurement methods. Two charge transfer steps were detected, the first one corresponding to adsorption and reduction of nitrate to nitrite and the second step consists of reducing nitrate to monoxide of nitrogen.

The effect of phenylthiourea as a corrosion inhibitor for low carbon steel at different hydrochloric acid concentrations, different temperatures and fixed speed of electrode rotation, were addressed in this work. Polarization technique was used to evaluate the corrosion rates parameters. The corrosion rate of low carbon steel increases with temperature and follows Arrhenius equation in all acid concentrations in presence and absence of the inhibitor. Detailed thermodynamic parameters of activation (ΔHact and ΔSact) for the corrosion reaction were obtained using nonlinear estimation method, while adsorption parameters (ΔGads, ΔHads and ΔSads) were obtained using graphical method. Maximum inhibitor efficiency was (96.44%) obtained at 1 M HCl at 333 K and 1 g/L inhibitor concentration.

The corrosion of zinc in 0.01 to 0.04 M H2SO4 was studied using gravimetric and gasometric methods of monitoring corrosion. The results obtained indicate that clarithromycin is a good adsorption inhibitor for the corrosion of zinc in H2SO4. The inhibition efficiency of clarithromycin increases with increasing concentration but decreases with increasing temperature. There was no significant difference between the inhibition efficiencies of clarithromycin obtained at various concentrations of H2SO4 (P>0.5), but values of inhibition efficiency tend to decrease with increasing concentration of the acid. The adsorption of clarithromycin on zinc surface is endothermic, spontaneous and is best described by Langmuir adsorption isotherm. Calculated values of activation energies and free energies of adsorption indicate that the adsorption of clarithromycin on zinc surface supports the mechanism of physical adsorption.

Influence of various carrier additives and brightener additives on the voltammetric behavior of zinc electro deposition from an alkaline non-cyanide bath was studied. Two different cathodic peaks were observed. Peak I has been attributed to hydrogen reduction / UPD of zinc and peak II to reduction of the metal. The extent of polarization of the zinc deposition to more negative potentials and the corresponding peak current decide the nature of deposits. PVA was found to be the best carrier additive. PVA chains retain zinc hydroxyl anions and control the speed of the rate determining step. Unsaturated aldehydes bring about 3-dimensional nucleation and further increase the polarization when added with PVA.

DNA and guanine are efficient fluorescence quenchers of the cationic conjugated polymer, poly {9,9-bis[N,N-(trimethylammonium)hexyl] fluorene-co-l,4-phenylene} (CCP). Studies with CCPs, of average chain length ~6, 12 and 100 repeat units, with single strand (ss) DNA, double strand (ds) DNA, and guanine, in 25/75 acetonitrile/water (v/v) mixtures result in Stern-Volmer quenching plots that show upward curvature. Initial Stern-Volmer constants, kSV, are in the range ≈ 3-20 x 107 M-1 which is much higher than possible by diffusional encounter quenching. Aggregation studies in acetonitrile/water mixtures show that aggregation is also an effective quencher of CCP fluorescence, and we note that both aggregation and quenching by DNA or guanine is accompanied by a reduction in solution absorbance at 380 nm. Comparison of the relationship between changes in absorbance and changes in emission intensity suggest that both solvent and chemical induced fluorescence quenching are due to aggregation. We interpret the correlated changes in absorption and emission, high quenching constants, and upward curving Stern-Volmer plots as evidence that the dominant mechanism for fluorescence quenching by DNA or guanine is via induced aggregation of the polymer. The upward curvature of Stern- Volmer plots and high kSV values for DNA and guanine are indicative of “aggregate energy migration quenching” in which CCP aggregates around a DNA or guanine molecule to form an aggregate complex in which excitation energy migrates between and along the polymer chains until it is quenched at an aggregate trap.

A comprehensive photophysical and spectroscopic Raman study guided by Density Functional Theory and Hartree-Fock calculations has been done in a series of fully oxidized oligothiophenes with variable chain length, up to four rings. A comparison with the properties of oligoenes and oligothiophenes is proposed.

The corrosion rates in the presence of nizoral (NZR), an antifungal drug, as mild steel corrosion inhibitor in 0.1 M H2SO4 was measured by the weight loss method in the temperature range from 30 – 50 ºC. Results obtained revealed that NZR acts as an inhibitor for mild steel in sulphuric acid solution. Inhibition efficiency increased with increase in concentration of the inhibitor but decreased with increase in temperature. The addition of KI increased the inhibition of NZR to a considerable extent. The experimental results suggest that the presence of iodide ions in the solution stabilized the adsorption of Nizoral molecules on the mild steel surface thereby improving the inhibition efficiency of Nizoral. The adsorption characteristics of the inhibitor were approximated by Langmuir adsorption isotherm and kinetic/thermodynamic adsorption model of El-Awady et al. Mechanism of physical adsorption is proposed from the activation and thermodynamic parameters calculated.

The use of stainless steel 304(SS) tanks to deposit thiourea in industry has been evaluated by studying the corrosion behaviour in 1% thiourea solution, before and after zinc plating. Weight loss method, 1% copper sulphate test, polarization study and AC impedance spectra have been used. The corrosion resistance of SS decreased when zinc was used for electrodeposition. Hence SS vessels electroplated with zinc should not be used to store thiourea solution, in industry.

The effect of iodide ion on the corrosion inhibition of aluminium in NaOH in the presence of Gum Arabic (GA) was studied using weight loss and hydrogen evolution techniques at 303 and 313 K. The results obtained showed that inhibition efficiency increased with increasing GA concentration. The inhibiting action of GA was considerably enhanced by the addition of KI. The adsorption of KI, GA alone and in combination with iodide ion was found to obey Temkin adsorption isotherm. The inhibiting effect of GA and (GA+KI) increased with increase in temperature of the corrosion medium, indicating chemical adsorption mechanism. Synergism parameter evaluated was found to be greater than unity for the different concentrations of GA, which shows that the enhanced inhibition of GA caused by the addition of iodide ion is due to synergistic effect. It could be deduced from the thermodynamic parameters obtained that adsorption of GA alone and in combination with KI onto the metal surface is spontaneous.

Inhibitive and adsorption properties of ethanol extract of Phyllanthus amarus for the corrosion of mild steel in H2SO4 were investigated using gravimetric, thermometric and gasometric methods. Ethanol extract of Phyllanthus amarus leaves is a good adsorption inhibitor for the corrosion of mild steel in H2SO4. Thermodynamic consideration indicates that the adsorption of the extract is exothermic and spontaneous. Also, the adsorption characteristic of the inhibitor is consistent with the assumptions of Langmuir adsorption isotherm. From the results and findings of the study, a physical adsorption mechanism is proposed for the adsorption of ethanol extract of Phyllanthus amarus on mild steel surface. The inhibition potentials of ethanol extract of Phyllanthus amarus leaves are enhanced by its phytochemical constituents.

The adsorption of [(4-hydroxy-6-methyl-2-oxo-2H-pyran-3-yl)-(4-methoxy-phenyl)-methyl]-urea on iron in phosphoric media was studied by using electrochemical technique. The adsorption mechanism was investigated using adsorption isotherms. The experimental data fit localized adsorption models. The values for free energy of adsorption, ΔG0ads, were also calculated at each surface coverage, θ, of the studied compound by applying the mathematical model of Bockris – Swinkels adsorption isotherm. The variation of free energy of adsorption, ΔG0ads with surface coverage, θ, was interpreted in terms of deviation from ideal condition assumed in Langmuir model. A possible adsorption model of HPU2 molecules on to the metal surface was suggested.

1-(3-methyl-4H-1,4-benzothiazin-2-yl)ethanone (1,4-BT) was synthesised via a new mechanism. 1,4-BT was thereafter tested as corrosion inhibitor of mild steel in hydrochloric solution at 293 K by weight loss measurements and electrochemical techniques (potentiodynamic polarisation, polarisation resistance, impedance spectroscopy (EIS)). Inhibiting efficiency (E%) increased with concentration and reached highest values up to 98% even at low concentration with changing the mechanism of the corrosion process. Results obtained showed that the inhibitor studied was an efficient inhibitor, especially in cathodic domain. The temperature effect on the corrosion behaviour of mild steel in 1 M HCl with and without 1,4-BT at 5 10-3 M was studied in the temperature range from 293 to 333 K. E% remains the same even at high temperature. The adsorption free energy and activation parameters for the mild steel dissolution reaction, in the presence of 1,4-BT, were determined. 1,4-BT is adsorbed on the mild steel surface according to a Langmuir isotherm adsorption model.