Portugaliae
Electrochimica Acta

The inhibition of corrosion of 304 SS in 2 N HCl solutions by N – furfuryl – N' phenyl thiourea (FPTU) has been investigated using potentiodynamic polarization techniques. The results obtained reveal that FPTU performs excellently as anodic inhibitor (IE> 93%) for 304 SS in HCl solution. The inhibitor functions through adsorption following Temkin’s adsorption isotherm, and the inhibition was governed by physisorption mechanism. The thermodynamic parameters deduced for the adsorption process reveal spontaneous adsorption of the compound on the alloy steel surface.

Two microsized graphite-design sensors based on ionophore technique, polyvinyl chloride carboxylated (PVC-COOH) and β-cyclodextrin (β-CD), are used for fabrication of two membrane sensors for the two studied drugs, metronidazole (MZ), sensor 1, and spiramycin (SP), sensor 2. Fast and stable Nernstian responses near 1E-5 - 1E-3 M for MZ and 1E-5-1E-2 M for SP over pH range 5.5-7.5 for the two electrodes reveal the performance characteristics of these electrodes which have been evaluated according to IUPAC recommendations. The aim of this work is to develop a new, simple, accurate and precise method for the determination of MZ and SP in their binary mixtures, which can be applied in routine quality control. The method is successively applied for the determination of the two drugs in their pharmaceutical formulations. Validation of the method according to the quality assurance standards shows suitability of the proposed electrodes for the use in the quality control assessment of these drugs. The recovery percentages for the determination of the two drugs by the two proposed selective electrodes are 99.86 ± 0.249 % and 99.69 ± 0.856% for sensors 1 and 2, respectively. Statistical comparison between the results obtained by this method and the reported one is done and no significant difference is found.

Bright nickel deposits were electrolytically applied on steel in the nickel Watts bath. The effect of some operational parameters on metal deposition in bright nickel plating was investigated. The investigation indicated that the weight of bright nickel deposited on metal during the process of electroplating was affected by plating temperature, voltage, current density, plating bath pH and plating time. The study established that the deposition of best bright nickel was obtained at a plating temperature of 56 ºC, current density of 6 A/dm2 and plating time of 18 minutes. Brightener is used in applications requiring outstanding appearance with minimum thickness of applied nickel plating. It can also be used for heavy deposit applications because it exhibits unparalleled ductility and low stress. Brightener was used in this study to determine the best nickel plating in the process. Boric acid was added for fixing the bath pH. The compositions of the brightener and nickel solution used are included in the text.

The performance of triethylenetetramine-tribenzylidene (TTTB) and triethylenetetramine-trisalicylidene (TTTS) as corrosion inhibitors for zinc in hydrochloric acid is investigated. At lower concentrations, both inhibitors accelerate the attack but inhibit corrosion at higher concentrations, e.g., 96–100% with 1.0% concentration in 0.5 M and 1.0 M HCl. The efficiency of TTTB decreases while that of TTTS remains almost constant (≥ 99.7%) up to 120 minutes and in the temperature range 35 – 65 ºC. The activation energies are higher in inhibited than in plain acid with both inhibitors. The free energy of adsorption (ΔGads) and heat of adsorption (Qads) are negative, which suggests that there is spontaneous adsorption on metal surface, and from the values of (ΔGads) and (Qads), the values of entropy of adsorption (ΔSads) were calculated. Galvanostatic polarization shows that corrosion is under mixed control with predominance of the cathodic part. In uninhibited 1.0 M HCl, complete cathodic protection is achieved at a current density of 4.2224 Adm-2, but in presence of these inhibitors, much lower current densities are required. Plot of log (θ/1-θ) versus log Cinh gives a straight line, suggesting that inhibitors cover both the anodic and cathodic regions through general adsorption following Langmuir isotherm. The mechanism of inhibition has been proposed.

Two sensitive voltammetric techniques (adsorptive and catalytic cathodic stripping) are described for the determination of timonacic, in the presence of copper or nickel ions, respectively. The measured peaks which correspond to the reduction or copper (I) or nickel – timonacic complexes are measured at -773 and -782 mV, respectively. The different experimental parameters have been carefully studied. The methods have been fully validated. The detection limits were 1.5 and 13.6 ng mL-1, respectively. The methods have been applied for the determination of timonacic in pharmaceutical tablets. The obtained results have been compared statistically with those obtained from a published method.

Essential oil from fennel (Foeniculum vulgare) (FM) was tested as corrosion inhibitor of carbon steel in 1 M HCl using electrochemical impedance spectroscopy (EIS), Tafel polarisation methods and weight loss measurements. The results show that the increase of the charge-transfer resistance (Rct) with the oil concentration supports the molecules of oil adsorption on the metallic surface. The polarization plots reveal that the addition of natural oil shifts the cathodic and anodic branches towards lower currents. Such shifts indicate that FM oil acts as a mixed-type inhibitor. The global rate of corrosion estimated by weight loss measurements confirms the above results. The inhibition efficiency attains a maximum of 76 % at 3 mL/L, but decreases with the rise of temperature. The analysis of FM oil, obtained by hydro-distillation, using Gas Chromatography (GC) and Gas Chromatography/Mass Spectrometry (GC/MS) showed that the major components were limonene (20.8 %) and β-pinene (17.8%). The adsorption of FM on the steel surface has been discussed according to the chemical composition of the oil, giving an explanation to the obtained results.