Rahela Gašparac

Investigations of the Mechanism of Corrosion Inhibition by Polyaniline. Polyaniline-Coated Stainless Steel in Sulfuric Acid Solution

The objective of this work was to elucidate the mechanism by which polyaniline (PANI) films passivate stainless steel surfaces in highly corrosive H 2 SO 4 solution. A variety of experimental methods, including measurements of the open-circuit potential, Auger depth profiling, and the scanning reference electrode technique (SRET), was used. These studies have shown that passivation is achieved because the oxidized and protically doped emeraldine-salt form of PANI holds the potential of the underlying stainless steel electrode in the passive region. Because of this electrostatic mechanism of corrosion inhibition, the entire stainless steel surface does not have to be coated with PANI in order to achieve passivation. Auger depth profiling experiments show that, in analogy to the case of a bare stainless steel surface whose potential is potentiostatically maintained in the passive region, the stainless steel surface beneath the PANI film is enriched in Cr after exposure to the H 2 SO 4 solution. Finally, SRET was used to explore passivation of the stainless steel in regions exposed to solution by pinholes intentionally cut through the PANI film. SRET shows that the PANI film surrounding the pinhole can heal the stainless steel in the pinhole region.

In Situ Studies of Imidazole and Its Derivatives as Copper Corrosion Inhibitors. I. Activation Energies and Thermodynamics of Adsorption

The objective of this work was to investigate the efficiency of imidazole and its derivatives 4-methylimidazole, 4-methyl-5-hydroxy-methylimidazole, 1-phenyl-4-methylimidazole, and 1-(p-tolyl)-4-methylimidazole for corrosion inhibition of copper in 0.5 M hydrochloric acid. Corrosion inhibition was studied using potentiodynamic methods. These studies have shown that 1-(p-tolyl)-4-methylimidazole has the best inhibitory efficiency. Activation energies were obtained by measuring the temperature dependence of the corrosion current. The activation energies in the presence of the various inhibitors are low (3-5 kJ/mol), with the best inhibitor showing the highest value. The adsorptive behavior of the imidazole derivatives on the copper electrode surface follows a Freund-lich-type isotherm. The standard free energies of adsorption are also low (14-16 kJ/mol), indicating that imidazole and its derivatives physisorb on the copper surface. Possible mechanisms of corrosion inhibition for these molecules are discussed.

The Effect of Protic Doping Level on the Anticorrosion Characteristics of Polyaniline in Sulfuric Acid Solutions

In a previous paper we investigated the mechanism by which fully protically doped (emeraldine-salt form) polyaniline (PANI) protects stainless steel surfaces from corrosion in I M sulfuric acid solution. A mechanism was proposed, based on prior work by Deng et al., whereby passivation is achieved because the emeraldine-salt form holds the potential of the underlying stainless steel electrode in the passive region. This paper describes investigations of PANI-coated stainless steel surfaces exposed to more dilute sulfuric acid solutions. This is an interesting issue because while the solution becomes less corrosive as the concentration of the H 2 SO 4 decreases, the concomitant change in pH also changes the protic doping level of the PANI film. UV-visible spectroscopy indicated that partially protic films are obtained upon exposure of PANI to solutions with H 2 SO 4 concentrations in the range from 500 to 1.0 mM. Exposure to 0.1 mM H 2 SO 4 yielded the protically undoped emeraldine-base material. We have found that both the partially protically duped PANI films and the totally undoped emeraldine-base films also hold the potential of the substrate stainless steel electrode in the passive region. Hence, the mechanism by which PANI protects the underlying metal surface from corrosion is independent of doping level.

In Situ and Ex Situ Studies of Imidazole and Its Derivatives as Copper Corrosion Inhibitors. II. AC Impedance, XPS, and SIMS Studies

The objective of this work was to investigate the efficiency of imidazole derivatives for corrosion inhibition of copper in 0.5 M hydrochloric acid. Corrosion inhibition was studied using impedance spectroscopy. Imidazole and its derivatives 4-methylimidazole, 4-methyl-5-hydroxymethylimidazole, 1.-phenyl-4-methylimidazole, 1-(p-tolyl)-4-methylimidazole were investigated. These studies have shown that 1-(p-tolyl)-4-methylimidazole is the best inhibitor in this series and that it acts as mixed inhibitor. The nature of the chemical interaction between these molecules and the copper surface was investigated for Cu exposed to solutions having two very different pH values: 0.5 M HCI and unbuffered purified water. X-ray photoelectron spectroscopy and secondary ion mass spectrometry were used to explore the nature of the interaction. Possible mechanisms of corrosion inhibition for these molecules are discussed.

Electrochemical and Transport Properties of Templated Gold/Polypyrrole-Composite Microtube Membranes

We have been investigating the transport properties of nanotube membranes prepared via the template method. Membranes containing gold (Au) nanotubes prepared by electroless deposition of Au within the pores of commercially available filters have been studied most extensively. The advantage of these membranes is that the inside diameter of the Au nanotubes can be precisely controlled down to molecular dimensions. Currently, we have no other materials synthesis strategy that allows for such precise control over the diameter of the nanotubes obtained. We are interested in developing such chemistries so that nanotubes having any desired inside diameter that are composed of materials other than metals can be prepared. We describe here the electrochemical polymerization of polypyrrole (PPy) microtubes within templated prepared Au microtubes. We demonstrate that the effective inside diameter of the PPy microtubes can be controlled by varying the number of voltammetric scans used during the PPy electropolymerization. The electrochemical characteristics and transport properties of these composite Au/PPy microtube membranes are described here.