Portugaliae
Electrochimica Acta

The influence of 2,6-bis-(hydroxy)-pyridine (P1), 2,6-bis-(chloro)-pyridine (P2) and diethyl 1,1’-(pyridine-2,6-diyl)bis(5-methyl-1H-pyrazol2-3-carboxylate ( P3) on the corrosion of steel in 1 N HCl solution has been studied by weight loss measurements, potentiodynamic and impedance spectroscopy methods. The inhibiting action increases with the concentration of these compounds to attain 91,5% at 10-3 M for P3. We note good agreement between gravimetric and electrochemical methods. The polarisation measurements show also that the pyridines act essentially as mixed inhibitors and the cathodic curves indicate that the reduction of proton at the steel surface happens with an activating mechanism. The temperature effect on the corrosion behaviour of iron in 1 M HCl without and with these inhibitors at different concentrations was studied in the temperature range from 313 to 353 K, and allows deducing the apparent activation energy, enthalpy and entropy of the dissolution process and the free energy were determined and discussed. The inhibitors were adsorbed on the iron surface according to the Langmuir adsorption isotherm model at different temperatures and some thermodynamic data for the adsorption process are calculated. The experimental study has been finished by quantum theoretical study; the quantum chemical calculations, based on DFT methods at B3LYP/6-31G** level of theory, were performed, by means of the GAUSSIAN 03 set of programs. Structural parameters, such as the frontier molecular orbital energies (EHOMO and ELUMO), gap of energy E, charge distribution, absolute hardness η and softens , fraction of electrons N transferred from pyridine molecules to steel, as well as electronic parameters such as Mulliken atomic populations have been determined. The objective of this quantum theoretical treatment is to attempt to find relationships between their molecular and electronic structures and inhibition efficiency.

The inhibition action of the Pyrazolone Derivative (PYR) on the corrosion of mild steel in 0.5 M sulphuric acid was investigated by weight loss, polarization, impedance and SEM. Results obtained revealed that PYR performed excellently as corrosion inhibitor with efficiency of 91% at 11 ppm at 298 K. Its adsorption on mild steel obeys Langmuir and Temkin isotherm. Polarization curves indicate that PYR behaves as mixed type. The value of ∆G°ads indicates the spontaneous physisorption of PYR. The SEM results confirm the presence of a protective surface layer over the mild steel surface. The reactivity of the compound was analysed through theoretical calculation.

In this study, we investigated the use of ion exchange processes using a chelating resin, Diaion CRB02 for the removal of boron from the bittern solution left after the extraction of sodium sulfate and sodium chloride from the water of Lake Qarun, located in Egypt. The effects of parameters such as the initial boron concentration and the pH value on the breakthrough volume were studied using boric acid as the synthetic simulant of the bittern solution. The breakthrough capacity was shown to be directly proportional to the height of the resin bed and inversely proportional to the initial boron concentration and the feed flow rate. In addition, the optimum pH for boron removal was found to be 10. An electrically assisted process, which had been found to be effective for a strongly acidic cation exchange resin, was also applied to the ion exchange by taking the electric current as a parameter. However, no remarkable effect was observed, which may result from the difference in the function group between an ion exchange resin using electrostatic attractive force and a chelating resin using complex formation.

Electroless Ni–P based composite coatings are more popular due to their excellent hardness, yield strength, wear resistance, frictional resistance, corrosion resistance, and good lubricity. The present study deals with significance of various coating process parameters on the corrosion behavior of the Ni–P–Al2O3 composite coatings on mild steel substrate. Corrosion behavior of the composite coatings after heat treatment at various annealing temperatures (300 °C, 400 °C, and 500 °C) are investigated by potentiodynamic polarization test using 3.5% NaCl solution. For maximization of corrosion resistance, the electrochemical parameters, corrosion potential (Ecorr) and corrosion current density (Icorr), are optimized using Taguchi based grey relational analysis. For optimization four coating process parameters are considered, namely, concentration of nickel source, concentration of reducing agent, concentration of second phase particles (alumina particles), and annealing temperature, as main design factors. The optimum combinations of the said design factors are obtained from the analysis. Analysis of variance (ANOVA) reveals that the concentration of alumina particles and annealing temperature has the significant influence on the corrosion resistance of the composite coatings. The microstructure of the surface is studied by scanning electron microscopy (SEM) and chemical composition is studied by energy dispersive X–ray analysis (EDX). The X–ray diffraction analysis (XRD) is used to identify the phase transformation behavior of the composite coatings.

Cashew nut testa tannin [CASTAN] has been found to inhibit the corrosion of aluminium in hydrochloric acid solutions using gravimetric, thermometric and UV/visible spectrophotometric techniques. CASTAN inhibition was by adsorption on aluminium following Temkin isotherm in 0.1 M HCl and Langmuir isotherm in 0.5 M and 2.0 M HCl at 303 Kelvin. Physical adsorption on aluminium has been proposed in studied HCl solutions; therefore, CASTAN is a cathodic inhibitor. Earlier reports [1] showed CASTAN to contain quercetin, azaleatin, catechin, epicatechin, cyanidin and delphinidin. However, UV/visible spectrophotometric analysis of CASTAN in ethanol reveals quercetin as its major component. This work therefore investigated the correlation between computed molecular parameters and inhibitive properties of CASTAN and adsorption sites on its components. Calculated quantum chemical parameters namely: EHOMO (highest occupied molecular orbital energy), ELUMO (lowest unoccupied molecular orbital energy), energy gap (ΔE) and dipole moment ( ) suggest that CASTAN is a soft inhibitor and it’s components inhibited aluminium corrosion in protonated forms. Calculated Mulliken charges implicated some electron rich sites, namely: the aromatic and conjugated C=C, C=O and O-H as adsorption sites on the inhibitor molecules. Proposed kinetic model reveals complex reaction mechanism, parallel reactions, for aluminium corrosion inhibition by CASTAN.