Portugaliae
Electrochimica Acta

Abstract Copper (Cu) powder production, in a sulfuric acid (H2SO4) solution with titanium (Ti) ions, was studied by electrolysis, using Cu anodes. It was empirically proven that this process occurs by three different stage-based mechanisms that depend on the electrolyte composition and electrolysis conditions. The first mechanism occurs in a H2SO4 solution with Cu2+ ions and Ti4+ (tetravalent ions). Cu2+ are cathodically reduced, forming Cu powder (CP), since the process occurs at current densities (J) lower than the limiting one. So, part of the current that would be consumed by the hydrogen ions (H+) reduction reaction is spent to reduce Ti4+, which results in Ti3+ (trivalent ions). These, in the cathode space, reduce Cu2+, which, simultaneously, regenerates Ti4+. Then, these diffuse and are again reduced at the cathode, and Ti3+are formed, reducing Cu2+, in a cyclic process that further increases CP production current efficiency (CE) at the near-cathode space, forming more dispersed particles. The second mechanism occurs in a H2SO4 solution with only Ti4+. During the electrochemical circuit current flow, the Cu anode is oxidized to form Cu2+, and Ti4+ are reduced to Ti3+, on the cathode surface. Then, Cu2+ and Ti3+, by diffusing the solution volume and meeting in the inter-electrode space, chemically interact with each other, due to the red-ox (reduction-oxidation) potential difference, forming a dispersed CP and Ti4+. These diffuse one more time, and are again reduced to Ti3+. These processes are cyclically repeated, i.e., Ti4+ work as catalyst. The third mechanism takes place in H2SO4 with Ti3+. When the current flows through the electrolyte, the Cu anode is oxidized, forming Cu2+, which are immediately reduced in the anode by Ti3+, producing CP. Ti4+ are formed due to the red-ox reaction diffusion to the cathode, being reduced to Ti3+on its surface, which again interact with Cu, producing CP on the anode. Since, in all mechanisms (except the first), CP is not directly produced on the cathode surface, but in the inter-electrode space, further growth of Cu particles does not occur. Thus, CP particles of spherical shape, with sizes from 0.01 to 0.1 μm, are formed, with a CE from 95.2 to 99.1%, under optimal conditions. Keywords: ultrafine CP production mechanisms, H2SO4 solution and Ti+ electrolytes.

Abstract Novel sulphur (S)-containing Schiff’s bases, N-((1H-indol-3-yl)methylene)thiazol-2-amine (I3A2AT) and(13E)-N1,N2-bis((thiophene-2-yl)methylene) cyclohexane-1,2-diamine (T2CDACH), were synthesized. The structures were deep-rooted by mass, UV-visible, Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) analyses. The ligands inhibition efficiency (w%) on carbon steel (CS) corrosion in 1 M hydrochloric acid (HCl) was studied using gravimetric analysis, electrochemical impedance spectroscopy (EIS), potentiodynamic polarisation (PDP), electrochemical noise (ECN), quantum chemical and surface studies. Both Schiff’s bases acted as excellent inhibitors on CS corrosion in 1 M HCl. The excellent inhibitors performance was confirmed by the formation of protective adsorption films onto the CS surface. The inhibitors adsorption onto the CS surface followed Langmuir’s adsorption isotherm. Energies of the highest and lowest occupied molecular orbitals (EHOMO and ELUMO), number of electrons transferred (∆N), electronegativity, chemical hardness, and so forth, were evaluated by quantum chemical studies. An acceptable correlation was observed between the results of quantum chemical calculations and other corrosion monitoring techniques. Keywords: adsorption, w%, PDP, quantum chemical approach and S-containing corrosion inhibitors.

Abstract Metals and alloys corrosion is one of the most challenging and damaging occurrence that is linked with enormous economic and safety losses, and it becomes more severe during some manufacturing processes by which metallic surfaces are treated for other industrial application processes. The use of corrosion inhibitors is one of the best way to protect metals and alloys against damage. The problem of the environmental toxicity of some of the employed inhibitors and the high cost involved in controlling and preventing corrosion have prompted this review. Computational chemistry methods are mostly significant in reducing the cost of protecting metals and alloys against corrosion. This review article begins with the summary of the most used computational methods, parameters and, finally, summarizes the results of some studies made by different authors in the field of corrosion science and engineering. Keywords: density functional theory (DFT), Hartree-Fock (H-F), Ab-initio, semi-empirical, acid corrosion, inhibition and alloys.

Abstract The high toxicity of industrial metal corrosion inhibitors raises various environmental and health problems. Thus, the study of metals and alloys corrosion inhibition, in acidic media, by eco-compatible organic compounds, has become a very attractive research field. In this paper, garlic (Allium Sativum) extract inhibition efficiency (IE) against API X60 carbon steel (CS) and 316L stainless steel (SS) corrosion, in a 1 M sulphuric acid (H2SO4) solution, has been investigated using electrochemical techniques, including potentiodynamic polarization (PPD) and electrochemical impedance spectroscopy (EIS). The experimental results showed the remarkable corrosion inhibitive performance of garlic extract (GE). The corrosion IE, which depends on the inhibitor concentration, increased up to 90%, for SS, and 67% for CS, as shown from the PPD tests. EIS analysis showed that the corrosion resistance (CRST) was increased in the medium with GE, indicating the properties improvement of the passive films formed on the steels surfaces. Keywords: corrosion, steel, H2SO4, green inhibitor and GE.

Abstract Molecularly imprinted polymers (MIPs) are an important class of compounds with wider sensing applications for the determination of substances ranging from small molecular masses to macro size. The hyphenation of MIP principle with other likewise conducting polymers yields the devices for sensing purposes. MIPs are robust against environmental conditions, more economical than natural receptors, and their preparation is also adequate for substances without natural receptors. Organic mediated MIPs compounds are of current interest, due to their applicability as quantification tools to determine electroactive substances in a variety of real samples. MIPs are highly selective for target molecules, mechanically strong, resistant to temperature and pressure, inert towards acids, bases, metal ions and organic solvents, highly stable for longer periods, and operative at room-temperature. Therefore, during past years, MIPs have been used as electrochemical and optical sensors, sorbents, solid phase media, and so on. Herein, there is a focus on the use of Pyrrole (Py) as a monomeric molecule to fabricate MIPs. Py or poly-Py (p-Py) based MIPs are synthesized and used in various capacities as chemo electrochemical sensors. A detailed discussion on the application of Py-mediated MIPs for the electrochemical determination of some organic compounds of therapeutic and environmental interest is herein presented as a review. Keywords: fabrication, Py, MIPs, electrochemical sensors and determination.

Abstract The purpose of this study was to fabricate lead ion selective electrode (Pb-ISE) sensors based on a 2:8 ratio of methyl-methacrylate-co-butyl acrylate thin film copolymers (MB28). These sensors were prepared by using a photo-polymerization technique on a pencil graphite electrode (PGE), with a poly-pyrrole-Cl (p-Py-Cl) thin film as a sensor transducer. During the Pb-ISE sensor fabrication process, the membrane composition optimization process has been carried out. The best sensor composition contained 1 mg potassium tetrakis (4-chlorophenyl) borate (KTpClPB) and 4.3 mg Pb ionophore IV. The fabricated Pb-ISE had a Nernstian number of 28.2 ± 0.5 mV/dec, a broad linear range from 10-3 to 10-10 M and a limit of detection (LOD) of (6.6 ± 1.6) x 10-11 M, providing an excellent performance. In addition, the sensor showed a fairly good coefficient of effectiveness (CE) (Log Kpota,b) against K+ (potassium), Na+ (sodium), NH4+ (ammonium), Mg2+ (magnesium), Cu2+ (copper) and Cd2+ (cadmium) cations, which were -6.6 ± 0.2, -6.1 ± 0.2, -6.7 ± 0.2, -12.4 ± 0.3, -4.4 ± 0.2 and -6.1 ± 0.1, respectively. The Pb-ISE sensor worked best in the pH range from 3 to 8. The validation process has been carried out by comparing the measurements results of the artificial samples, at the concentrations from 25 to 100 ppm, with those of the real sample from Angke river water. The outcome was outstanding and comparable to that of the standard UV-Vis spectrophotometry measurement methods. Keywords: MB28 copolymer, photo-polymer, Pb-ISE, PGE and p-Py-Cl.

Abstract A new corrosion inhibitor, namely, 1H-1,2,4-triazol-4-amine, 3, 5-diphenyl-N-(phenyl methylene) (HTADP), has been synthesised, and its inhibiting action on ordinary steel corrosion in an acidic media has been investigated by weight loss and various electrochemical techniques. The obtained results revealed that this organic compound is a very good inhibitor. Its inhibition efficiency (IE) exceeded 90%, even at very low concentrations. HTADP was able to reduce steel corrosion more effectively in 1 M HCl. Potentiodynamic polarization studies showed that HTADP is a mixed type inhibitor, predominantly influenced by the cathodic process. The adsorption of this inhibitor onto the ordinary steel surface in 1 M HCl was found to follow the Langmuir’s adsorption isotherm. The adsorption and activation thermodynamic data were determined and discussed. A protective film was formed on the steel surface, which changed the processes at the metal-solution interface. Keywords: corrosion, steel, inhibition, adsorption and acidic media.

Abstract Mild steel (MS) corrosion inhibition by Terminalia mantaly (TM), in 1.0 M HCl, was studied using weight loss and linear polarization methods, at different reaction conditions. Gas chromatography-mass spectrometry (GC-MS) was used to determine the possible extract components. Fourier transformed infrared spectroscopy (FTIR) and optical microscopy were used to investigate the interaction of the plant extract phytochemicals with the MS surface. The obtained results revealed that the inhibition efficiency percentage (IE%) increased with a decrease in the TM concentration, and decreased with a rise in temperature (T). The IE% also decreased as the hydrochloric acid (HCl) concentration increased. Inhibition occurred through the TM phyto-molecules adsorption onto the MS surface. The corrosion inhibition kinetics followed a zero order reaction, and it was uni-molecular. The linear polarization showed that TM is a mixed-type inhibitor. Gibb’s free energies values confirmed a spontaneous process, while TM adsorption onto the MS surface was found to obey physisorption, aligning with the Langmuir’s and El-Awady’s adsorption isotherm models. Keywords: Terminalia mantaly, activation energy, corrosion, Gibb’s free energy, linear polarization and MS.

Abstract Ethylenediamine (EDA), diethylenetriamine (DETA) and N,N,N',N'-tetrakis (2-hydroxypropyl) ethylenediamine (THPrED) are used relatively often in galvanic processes. Tetra-substituted derivatives, such as N, N, N ', N'-tetrakis (2-hydroxyethyl) ethylenediamine, (THEtED) are quite biologically stable and hardly degradable. In recent years, much attention has been devoted to electrochemical oxidation, using anodes with a high over-potential of O2 evolution, such as the boron-doped diamond (BDD). DETA and THPrED electrochemical treatment using a BDD anode was herein studied. The degradation efficiency of the amines was evaluated under different current intensities and reaction times. To determine the products formed in the oxidation process, ion chromatography (IC) was used. A high decrease in the current between the first and the second CV scan indicated the polymer film formation on the BDD electrode surface. Hydroxyl radicals formed at a potential of about 2 V and higher caused further oxidation of the electrode reaction products. It was found that NH4+, CH3-COOH, N2, EDA, CO2 and NO3- were formed in a short reaction time, and at low current intensity. The mineralization occurred during substrates electrolysis, due to rapid DETA and THPrED decomposition. After 180 min of reaction, αTOC and αN values for DETA were 94% and 18%, respectively. For THPrED, αTOC was 98.6% and αN was 43.6%. Therefore, the electrochemical approach was considered a very promising method in practical application for the treatment of wastewater containing amines. Keywords: EDA, DETA, THPrED, hydroxyl radicals, BDD anode, direct and mediated oxidation and mineralization.

Abstract A graphene oxide-triazole hybrid anti-corrosive coating was done by fabricating a triazole derivative – 2-(5-mercapto-4-((3-nitrophenyl)amino)-4H-1,2,4-triazol-3-yl)isoindoline-1,3-dione (4-NBT) on a graphene oxide (GO) coated Cu electrode. The GO-4-NBT hybrid coating effect on the Cu surface corrosion behavior was electrochemically monitored through cyclic voltammetry (CV), potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). The fabrication of a protective coating was done in two steps. Firstly, GO was electrochemically deposited on the Cu electrode in two different aggressive media (1 M HCl and 0.1 M Na2SO4), separately. Secondly, different 4-NBT concentrations were employed to reinforce GO corrosion resistant properties. CV studies revealed that GO-4-NBT effectively suppressed the metal oxidation and oxygen reduction. EIS studies suggested that the electrochemical process on the Cu surface with GO and GO-4-NBT was charge transfer controlled. The corrosion inhibition efficiency (IE) measured by PDP and EIS was enhanced with a related raise in 4-NBT concentration. Electrochemical studies revealed that the GO-4-NBT was a mixed type of inhibitor that predominantly inhibited the anodic reaction, especially in the case of 0.1 M Na2SO4. Adsorption studies further indicated the involvement of a stable and spontaneous adsorption mechanism, most probably by chemisorption. GO-4-NBT has shown significant corrosion protection activity in 0.1 M Na2SO4. Keywords: GO-4-NBT, Cu electrode, anti-corrosion coating, electrochemical studies by, PDP and EIS, and adsorption studies.

Abstract The use of pesticides has been increased in recent years, to enhance crops productivity, which may lead to a serious global concern of environmental pesticides monitoring. Isoproturon (IPU: 3-(4-isopropylphenyl)-1,1-dimethylurea) is an herbicide widely used in wheat crops. However, when it enters in the ecosystem, it is heavily toxic to humans. Thus, there is an urgent need to develop sensitive and selective IPU detection methods. In the present work, a novel polymethylmethacrylate/ferrite (PMMA/M(FexOy)) polymer nanocomposite (PNC) modified glassy carbon electrode (GCE) was developed for IPU detection. The PNC/GCE surface morphology was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). PNC electrochemical characterization (EC) was performed by cyclic voltammetry (CV) that showed a quasi-reversible redox behavior. PNC/GCE demonstrated an excellent square wave voltammetric (SWV) response towards IPU, with a limit of quantification (LOQ) of 1.98 × 10-7 M, and a limit of detection (LOD) of 6.5 × 10-8 M, in 1 M HClO4 (perchloric acid), at pH 2.0. EC investigations reflected a peak current that was linearly related to IPU concentrations, with a high detection sensitivity. It also showed much better CV and SWV IPU responses than those of a bare GCE, and further environmental stability, without a high influence of common interfering ions. Keywords: IPU, PPMA/M(FexOy), EC, CV and SWV.

Abstract Pinus oocarpa seed extract corrosion inhibition effect on galvanized steel has been studied in 2 M hydrochloric acid, at 303 K and 333 K, by gravimetric methods. The inhibitor efficiency decreased with higher temperatures, which suggests physisorption. Potassium iodide (KI) synergistic action produced an increase in the extract inhibition efficiency, but its parameter decreased with higher inhibitor concentrations. The galvanized steel optical microscopy shows that the metal surface was coated with P. oocarpa seed extract, and that the cracks observed in the inhibitor absence were filled. This observation suggests that the extract can be used as a coating to prevent galvanized steel corrosion. Keywords: corrosion inhibitor, optical microscopy, gravimetric methods, P. oocarpa and synergistic effect.

Abstract The formation of thick anodic oxide films on antimony in diluted solutions of oxalic acid (CO(OH)2) was studied under galvanostatic and isothermal conditions. The film formation was always accompanied by a dissolution process which strongly depended on the growth conditions. The formation efficiency, as determined by the dissolved metal amount, was affected by the CO(OH)2 concentration, the current density and the anodization time. The dissolved antimony amount increased with higher CO(OH)2 concentrations and anodization time, and with lower current densities. The analysis of the total current density suggests the occurrence of a formed film and a dissolving component. According to the calculations, the anodic oxides composition is close to Sb2O3. The growth of anodic Sb2O3 took place at high electric fields within the oxide film. The thicknesses of the formed films were calculated by taking into account their dissolution. The film formation efficiency was determined at various current densities. Keywords: antimony anodic dissolution, film formation efficiency, galvanostatic anodization and electric field intensity.

Abstract Green corrosion inhibitors find various applications in the field of corrosion engineering and technology. In the present work, powder forms of Pimenta Dioica leaves were added into corrosion media to study their effect. Mild steel coupons corrosion studies showed an increase in the inhibitor efficiency, at higher concentrations, in corrosive media. The coupons electrochemical behavior was studied by OCP measurements. P. Dioica inhibition efficiency, adsorbed layers mechanism and mild steel corrosion rate were analyzed using weight loss measurements. Keywords: Pimenta Dioica, green inhibitor, mild steel, potable water and corrosion study.

Abstract Aniline corrosion inhibitor effect on zinc in a H2SO4 solution has been evaluated by weight loss (WL), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and Scanning Electron Microscope (SEM) techniques. The corrosion rate increased with higher acid concentrations. At constant inhibitor content, with higher acid concentrations, the corrosion rate increased. With higher inhibitor concentrations the corrosion rate decreased, while inhibition efficiency (IE) percentage increased. The maximum IE of 90.18% was obtained at 60 mM of aniline in a 0.5 M H2SO4 solution. Polarization studies revealed that aniline acts as a mixed type inhibitor. EIS spectra are semicircular, which indicates that zinc corrosion was mainly controlled by a charge transfer process. SEM reveals the appearance of a smooth surface on zinc in aniline presence, probably due to the formation of an adsorptive film of electrostatic character. It was found that there is a good agreement between the different tested techniques. Keywords: Zinc, H2SO4, aniline effect, polarization and EIS.

Abstract The adsorption capability of the series of some Macrocyclic Polyether compounds containing 1, 3, 4-thiadiazole entity n-MCTH (n = 1-5), especially 3- MCTH, 4-MCTH and 5 MCTH, and their protonated forms, was studied in the light of DFT quantum modeling and Monte Carlo dynamics calculations. Sensitivity to corrosion has been quantified using the degree of planarity, global and local electronic proprieties, as well as the inhibitor strength of interaction, in neutral and protonated forms, with the (111) iron surface in the metallic complex. The results of both approaches showed the supremacy of the interactions of neutral and proton variants of the 5-MCTH-Fe complexes, compared to their homologues of 3-MCTH-Fe and 4-MCTH-Fe, due to the significant involvement of aryl rings, in addition to the thiadiazole ring, in the process of electron donation and acceptance. Keywords: DFT calculations; Monte Carlo dynamics calculations; global electronic proprieties; local electronic proprieties; and complexation strength.

The inhibition effect of Thymus willdenowii Boiss & Reut essential oil (TW) on the mild steel corrosion in 1 M HCl has been investigated using weight loss measurements, surface analysis (SEM-EDX, three-dimensional profilometry and FT-IR), potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The TW oil anticorrosion effect was evaluated using polarization potentiodynamic curves obtained after 30, 60 and 90 min of immersion in a 1 M HCl medium. Gravimetric results have shown that TW oil has a significant inhibition efficiency value of 81.42%, which was attained at 3 g/L. The polarization measurements have shown that TW is a mixed type inhibitor, with a significant reduction in cathodic and anodic current densities. Measurements by electrochemical impedance spectroscopy revealed that resistance to the charge transfer has increased with higher TW oil concentrations. From the use of SEM-EDX and three-dimensional profilometry, it is clear that the metal surface has remarkably improved in the TW oil presence, compared to the one exposed to the acid medium without essential oil. From the TW proprieties and the obtained results, it can be concluded that this oil is a new natural substance that can be used against material corrosion in aggressive solutions.

Stearalkonium chloride (SAC) ultrasonic velocity has been measured in methanol, at 301 K temperature. Jacobson’s model has been used to evaluate adiabatic and molar compressibility, molar sound velocity, solvation number, relative association, relaxation strength and other acoustical constants. The results of ultrasonic measurements of different SAC solutions in methanol indicate that there is a signification interaction between SAC and methanol molecules in diluted solutions. The conductometric study indicates that SAC behaves as a weak electrolyte in methanol. The thermodynamic constants calculated from conductance measurements for SAC solutions in methanol depict that micellization is favored over dissociation processes. The refractive index variation with SAC solutions concentrations shows a marked change in the refractive index value at critical micelle concentrations (CMC). Data treatment of obtained ultrasonic velocity, conductance measurement and refractive index shows that there is significant interaction between SAC and methanol molecules in diluted solutions, and that SAC molecules do not aggregate appreciably below the CMC.

Ethyl 3-hydroxy-2-(p-tolyl)-3,4-dihydro-2H-benzo[b][1,4]thiazine-3-carboxylate (EHBT) inhibition effect and its adsorption onto a mild steel surface in phosphoric acid (2 M H3PO4) was investigated, at temperatures varying between 298 and 328 K, by weight loss, EIS and potentiodynamic polarization techniques. The tested compound showed an inhibition efficiency that was superior to 88 % for a concentration equal to 5 × 10-3 M. Polarization measurements indicated that the examined EHBT acted as a mixed inhibitor. The protection efficiency increased with higher inhibitor concentrations and decreased with an increase in temperature. EHBT adsorption onto the mild steel surface obeyed Langmuir adsorption isotherm. EHBT inhibition action was also evaluated by surface SEM images.