Portugaliae
Electrochimica Acta

In this work the characteristics of corrosion and inhibition of 1008 stainless steel in 0.5 mol.L-1 H2SO4 were investigated by physical and electrochemical methods. Organic compounds containing the heteroatom of N as benzotriazole (BTAH), tolytriazole (TTAH) and mixtures of BTAH + TTAH were used as corrosion inhibitors. The techniques used were: open circuit potential, anodic potentiodynamic polarization measurements, chronoamperometry, electrochemical impedance spectroscopy, weight loss and optical microscopy. The anodic polarization showed that the increase of both concentrations BTAH or TTAH diminishes the current density in all the potential anodic range studied; these results together with the weight loss measurements suggest that the inhibitors act by blocking the surface and that the interaction among them is only of destructive nature when inhibiting efficiency is approximated 100%, revealing that at low concentration both inhibitors act without affecting the mechanisms of the cathodic processes, but the anodic polarization curves in presence of the TTAH showed a strong inhibitive effect in all the potential range studied. However, both gravimetric and electrochemical results suggest a synergic effect for the inhibitive efficiency of the mixture 1x10-3 mol.L-1 BTAH + 1x10-3 mol.L-1 TTAH.

This work is centered on the study of the corrosion inhibition of copper in 1 M HNO3 by N-phenyl oxalic dihydrazide (PODH) and oxalic N-phenylhydrazide N’-phenylthiosemicarbazide (OPHPT) synthesized in our laboratory. The inhibition efficiency obtained by chemical (weight loss) and electrochemical (potentiodynamic polarization) techniques shows that these compounds are very good inhibitors. Polarization curves indicate that OPHPT acts as a mixed type inhibitor while PODH acts on the cathodic reaction. The adsorption of these inhibitors is found to obey Langmuir adsorption isotherm. The thermodynamic functions of adsorption and dissolution processes were calculated.

The inhibitive action of the extract of Ficus nitida leaves toward general and pitting corrosion of C-steel, nickel and zinc in different aqueous media was investigated. Weight loss measurements, potentiostatic and potentiodynamic polarization techniques were used in this study. It was found that the presence of ficus extract in the corrosive media (acidic, neutral or alkaline) decreases the corrosion rates of the three tested metals. The inhibition efficiency increases as the extract concentration is increased. The inhibition efficiency depends on the type of corroded metal and on the corrosive solution. It was also found that the presence of ficus extract in the chloride containing solution shifts the pitting potentials of the tested metals toward the noble direction. The inhibitive action of ficus extract is discussed in view of adsorption of its components, the poly aromatic compounds, friedelin, epifriedelanol and nitidol, on the metal surface. It was found that such adsorption follows Langmuir adsorption isotherm. The calculated values of the free energy of adsorption indicated that the adsorption process is spontaneous.

Theoretical analyses show that in a sampled dc polarography on the static mercury drop electrode the response may consist of a pre-wave and a main wave that are separated by a minimum. This is a general characteristic of electrode reactions complicated by strong adsorption of the product. A steep pre-wave and the minimum are the indications of Frumkin isotherm. A flat plateau of the pre-wave is a special case of the steep plateau which appears at lower reactant concentrations. If the adsorption is weak, the pre-wave and the main wave are usually merged into a single wave.

A modified electrodialytic cell integrates electrodialysis and reduction of metal ions. The cell is able to recover metallic copper from wastewater containing 1000 ppm cupric ions and bring the concentration down to about 1 ppm. The kinetic data of decreasing copper ion concentration fit well in first order kinetics and allow calculation of the over all reaction rate constant. Effect of several parameters, namely, width of wastewater compartment, applied potential and concentration of anolyte and catholyte solution; on the over all reaction rate constant and specific energy consumption were studied. The best combination of parameters results in an overall rate constant of 7.84 × 10-4 sec-1 and specific energy consumption of 48.18 kW-h/kg copper.

By cyclic voltammetry the active/passive transition of Nb electrode has been investigated in concentration range from 0.1 M to 10 M aqueous solutions of H2SO4 and KOH. Results indicate the strong influence of the concentration and electrolyte nature to the active/passive transitions and stability of passive films. Depending on electrolyte concentrations, at potential of 1 V the calculated thickness of passive films varied from 2.2 nm to 3.2 nm. For the same concentrations of H2SO4 and KOH the formed passive films in KOH are thicker than in H2SO4. By multiple cycle sequences in which the final anodic potential is gradually enlarged, the barrier properties of passive films on Nb electrode were confirmed. In the first positive scan after the active/passive transition, no cathodic or reactivation peaks for both 1 M H2SO4 and KOH were observed. Only for higher concentrations of KOH (> 2 M) the small reactivation process was recorded. In concentration of 5 M and 10 M KOH, if after the passive film formation the Nb electrodes were maintained for 15 min at cathodic potential of -1.55 (SHE), the cyclic voltammograms indicated the complete dissolution of passive films. The voltammetric data have confirmed that the Nb electrode is more resistant in concentrated H2SO4 than in concentrated KOH solutions.