Patrick J. Kinlen

A Mechanistic Investigation of Polyaniline Corrosion Protection Using the Scanning Reference Electrode Technique

Growing environmental concerns regarding the use of heavy metals in coating formulations have lead to a new coating strategy employing inherently conducting polymers (ICPs), such as polyaniline (PANI), as a key component. The principal potential advantage offered by the ICP coating technology is toleration of pinholes and minor scratches. This paper describes the application of the scanning reference electrode technique (SRET) to the study of PANI coatings on carbon steel. SRET results demonstrate that conductive PANI passivates pinhole defects in coatings on carbon steel. In addition, it is shown that phosphonic acid salts of PANI are more effective for corrosion protection than sulfonic acid salts. A model is proposed which entails passivation of the metal surface through anodization of the metal by PANI and formation of an insoluble iron-dopant salt at the metal surface.

Corrosion protection using polyanujne coating formulations

Abstract Growing environmental concerns regarding the use of heavy metals in coating formulations has led to a new coating strategy employing inherently conducting polymers (ICP) as a key component. ICPs (such as polyaniline, polypyrrole and polythiophene) are electrically conductive owing to a system of conjugated double bonds. Observations of metal passivation complement this conductive nature and offer a viable alternative to traditional corrosion protection (1–8). A key potential advantage that the ICP coating technology offers is toleration of pin holes and minor scratches. The basis for this argument is that, since the ICP coating is conductive, the entire coating acts to passivate any areas of exposed metal. This paper describes a model for polyaniline (PANI) corrosion protection and presents data which clearly demonstrate significant corrosion protection in a salt fog environment. ESCA and electrochemica data are presented which show that an Fe-PANI complex is formed in the process of coating steel with PANI. The Fe-PANI complex is shown to catalytically reduce oxygen. Preliminary electrochemical impedance results are also presented which show an additional time constant at 20 kHz, which appears to correlate with the effectiveness of PANI toward corrosion protection.

A solid-state pH sensor based on a Nafion-coated iridium oxide indicator electrode and a polymer-based silver chloride reference electrode

Abstract As an alternative to the glass pH electrode, an entirely solid-state pH sensor (pH-sensing and reference electrodes) has been developed based on an annealed permselective polymer(Nafion tm TM)-coated iridium oxide pH indicator electrode and a polymer-modified silver-silver chloride reference electrode. When a solution of Nafion is coated onto the iridium oxide surface and annealed at 210°C, it becomes permselective to cations. The membrane thus transports protons, but attenuates the effects of anionic oxidizing or reducing (redox) species that interfere with the response of an uncoated electrode. The reference-electrode design involves coating a silver-silver chloride surface with a chloride-ion-containing polymer (e.g., triethylamine quaternized polychloromethylstyrene). The chloride ion is trapped within this polymer layer by encapsulating it with a Nafion outer layer. After annealing, the Nafion membrane effectively blocks chloride-ion diffusion to the test solution and maintains a constant chloride-ion activity on the silver chloride surface; thus a constant electrode potential is maintained. Several sensor designs based on coated wires, cermets and alumina ceramics have been evaluated for pH response and stability. Distinctive features of the solid-state technology include glass-free construction, chemical resistance and high impact strength.

Synthesis and Characterization of Organically Soluble Polyaniline and Polyaniline Block Copolymers

Abstract An emulsion process has been developed for the direct synthesis of the emeraldine salt of polyaniline (PANI) that is soluble in organic solvents. The process entails forming an emulsion composed of water, a water soluble organic solvent (e.g., 2-butoxyethanol), a water insoluble organic acid (e.g., dinonylnaphthalene sulfonic acid) and aniline. The resulting product is truly soluble in organic solvents such as xylene and toluene, of high molecular weight (Mw >22,000) and film forming. The emulsion polymerization process was also employed to synthesize ABA triblock polymers of polyaniline where the B block is a diamine terminated polymer and the A block is polyaniline. As cast, PANI films were only moderately conductive (10 −5 S/cm). However, PANI conductivity could be enhanced by up to five orders of magnitude by treatment of the films with surfactants or low molecular weight alcohols and ketones. Blends of PANI and plasticizers such as toluenesulfonamide were found to exhibit conductivities up to 100 S/cm. X-ray diffraction studies of cast PANI films help to explain the conductivity enhancement by showing that surfactant and solvent treatments significantly increase the crystallinity and long range ordering of the polymer.

Determination of the relative kinetic acidities of weak acids using electrogenerated bases

Abstract The rates of reaction of two electrogenerated bases (2,2′-di-t-butylazobenzene radical anion and 2,2′,4,4′,6,6′-hexaisopropylazobenzene radical anion) with a series of substituted phenylacetonitriles and benzylsulfones have been measured in acetonitrile using a chronoamperometric method. Cyclic voltammetry and chronoamperometry were used to characterize the reaction sequence as DISPI rather than pure ECE. Good Bronsted correlations were found between the rate constants measured and Bordwells DMSO p K a scale. Thus, the electrochemical measurement provides a convenient, rapid method for estimating p K a values for families of weak acids.

An Emulsion Polymerization Process for Soluble and Electrically Conductive Polyaniline

A new emulsion process has been developed for the direct synthesis of the emeraldine salt of polyaniline (PANI) that is soluble in organic solvents. The process entails forming an emulsion composed of water, a water soluble organic solvent (e.g., 2-butoxyethanol), a water insoluble organic acid (e.g., dinonylnaphthalene sulfonic acid) and aniline. Aniline is protonated by the organic acid to form a salt which partitions into the organic phase. As oxidant (ammonium peroxydisulfate) is added, PANI salt forms in the organic phase and remains soluble. As the reaction proceeds, the reaction mixture changes from an emulsion to a two phase system, the soluble PANI remaining in the organic phase. With dinonylnaphthalene sulfonic acid (DNNSA) as the organic acid, the resulting product is truly soluble in organic solvents such as xylene and toluene (not a dispersion), of high molecular weight (M{sub w} > 22,000), film forming and miscible with many polymers such as polyurethanes, epoxies and phenoxy resins. As cast, the polyaniline film is only moderately conductive, (10{sup {minus}5} S/cm), however treatment of the film with surfactants such as benzyltriethylammonium chloride (BTEAC) or low molecular weight alcohols and ketones such as methanol and acetone increases the conductivity 2--3 orders of magnitude.

Annealed Perfluorinated Cation Exchange Polymers for Corrosion Resistant Coatings

Casting and annealing a perfluorinated cation exchange (Nafion) polymer onto aluminum or 316SS results in a system which inhibits pitting and crevice corrosion that normally occurs in the presence of chloride ions and dissolved oxygen. This inhibition is believed to results from the unique chloride ion rejection properties of the annealed Nafion coating. Under neutral and basic conditions (pH > 5), localized corrosion is suppressed. Potentiodynamic polarization scans of the annealed Nafion coated aluminum and 316SS positive to the potential of the bare electrode does not produce pits. Electrochemical impedance results show that corrosion is unchanged after the coated alloy has been polarized above the pitting potential. In addition, the electrochemical impedance results indicate that the electrical resistance of the coating is low, hardly increasing the uncompensated resistance above that found for the bare metal. Hence, the movement of water and positive ions through the coating is unhindered Long-term stability and long-term corrosion inhibition afforded by the polymer must still be assessed to determine the practical limits of applicability of this coating system.

Mechanistic studies of the electrohydrodimerization of acrylonitrile

Abstract The electrohydrodimerization (EHD) of acrylonitrile (AN) to adiponitrile (ADN) has been examined using the electroanalytical techniques of cyclic voltammetry (CV), rotating disk electrode voltammetry (RDE) as well as differential capacitance measurements. The results confirm the formation of a hydrophobic reaction layer adjacent to the electrode surface. The evidence derives from three sources: (i) hydrogen evolution suppression with quaternary ammonium salt (QAS) addition, (ii) double layer capacitance data which verifies co-adsorption of QAS and ADN and (iii) RDE voltammetry data which shows AN diffusion through the compact layer limits the current. Differential capacitance measurements are seen as a means for selection of candidate QASs for the EHD process.

Nanostructure of electrically conducting polyaniline prepared by a novel emulsion polymerization process

A soluble polyaniline (PANI) salt with moderate conductivity was synthesized by a novel emulsion polymerization process. The conductivity of the processed PANI films can be substantially increased by treating the polymer films with surfactants or with low molecular weight alcohols. Transmission electron microscopy (TEM) images of thin polymer films revealed the existence of small islands of conducting PANI embedded in a non-conducting, dopant matrix. The conductivity of the PANI films is affected by the spatial distribution and the connectivity of these small islands. The conductivity enhancement observed upon treatment with surfactants is due to self-assembly of conducting PANI molecules into an interconnected network morphology. In the case of alcohol treatment the film conductivity is enhanced due to extraction of excess dopant phase and the subsequent densification of PANI islands to form highly conducting pathways.