Enrichment of chromium-content in passive layers on stainless steel coated with polyaniline

Enrichment of chromium-content in passive layers on a 304 stainless steel (SS) by coating polyaniline (PANI) has been confirmed by means of electrochemical and X-ray photoelectron spectroscope (XPS) techniques and thus prepared oxide films demonstrated to be highly stable in aggressive environment. PANI films are deposited on SS from a sulfuric acid solution with aniline monomer. The PANI-coated SS is found to maintain the passive state in a deaerated 1 M H₂SO₄ at 45 °C for several weeks. The passive state of SS remains in the same solution for several days even after the PANI films were completely removed. The passive layers underlying the PANI are certified to have higher chromium-content compared with the air-formed and anodically-formed oxide films on SS. It is suggested that PANI films enhance the passivation of SS by increasing the Cr₂O₃ content in the passive layer and by preventing the chromic oxide from directly interacting with aggressive solution.     

Electrosynthesis and protection role of polyaniline–polvinylalcohol composite on stainless steel

Polyaniline–polyvinyl alcohol (PANI–PVA) composite has been electrodeposited on stainless steel surface from aqueous sulfuric acid solution of aniline monomer in presence of soluble PVA at different concentrations. The PVA increased the rate of electropolymerization where 4 g/L PVA formed a composite of 37 wt% PANI and 63 wt% PVA composition. The composite layer exhibited more adhesion to the steel surface in comparison with PANI layer but with less thermal stability. It has higher protection role for the stainless steel (SS) against general and pitting corrosion. It enhanced the passivation of the SS surface by increasing the thickness of oxide film and improving the composition.     

Effects of polymerization potential on the permselectivity of poly(o-phenylenediamine) coatings deposited on Pt-Ir electrodes for biosensor applications

In attempts to improve the permselectivity of poly-o-phenylenediamine (PoPD) for biosensor applications, we investigated the influence of applied electropolymerization potential on the permeability properties of the non-conducting, non-ionic form of PoPD deposited on Pt-Ir wire electrodes in neutral media, using fixed potential amperometry and cyclic voltammetry. The analytes chosen were hydrogen peroxide (the signal transduction molecule in many oxidase-based biosensors) and ascorbic acid (AA, the archetypal interference species in biological applications of biosensors). The permselectivity (S%) of Pt/PoPD, expressed as the percentage interference by AA in hydrogen peroxide calibrations carried out at +0.7 V versus SCE, showed three distinct ranges: very poor (∼70%) for mild anodic applied polymerization potentials (<200 mV), indicating little or no PoPD coating formed on the electrode surface; moderate (∼2%) when the PoPD was generated using intermediate potentials (250-300 mV); and excellent (<0.3%) for polymerization potentials >400 mV. There was also a trend in this last S% range for further improvement with increasing polymerization potential, which reached an optimum value of 0.10 ± 0.02% (n = 19 sensors) when the PoPD electrodeposition was carried out at 700 mV versus SCE. This enhancement in S% was due to a surprising combination of increases in permeability for hydrogen peroxide and decreases in AA permeability for the higher values of applied polymerization potential. In addition, the data indicate that, for the conditions used here, electropolymerization at the fixed applied potential of 0.7 V was superior to CV in terms of permselectivity for hydrogen peroxide detection by PoPD-modified electrodes in media containing AA.     

Electropolymerization of phenol, o-nitrophenol and o-methoxyphenol on gold and carbon steel materials and their corrosion protection effects

This paper reports the results of a comparative study of the electropolymerization of phenol, o-methoxyphenol and o-nitrophenol by cyclic voltammetry on gold and carbon steel electrodes. The aim of this work is to synthesize homogeneous and adherent polyphenols film to protect carbon steel material against corrosion. Gold electrodes were used to optimize the experimental parameters such as the initial concentration of the monomer, the pH, the potential scan rate and the anodic potential limit value. Results showed that poly-o-metoxyphenol synthesized on carbon steel using optimized parameters obtained from gold electrode leads to more effective protection. This is probably due to the electron-donating mesomeric effet (+M) of the methoxy group which stabilized the phenoxy radicals obtained during the monoelectronic discharge of o-methoxyphenol. The best electropolymerization conditions involved an aqueous solution of 0.04 M o-methoxyphenol at pH 10.7, 5 mV s⁻¹ potential scan rate and an anodic potential limit (1.64 V/SCE) avoiding the degradation of the polymer film. The application of these conditions on steel electrodes leads to the formation of a stable, adherent and inhomogeneous film of polymer. A polymerization mechanism was proposed with consideration of results from literature.     

Electropolymerization of 4-nitro-1,2-phenylenediamine and electrochemical studies of poly(4-nitro-1,2-phenylenediamine) films

In this work, we report the anodic electropolymerization of 4-nitro-1,2-phenylenediamine (4NoPD) in different supporting electrolytes at different pH. The feasibilities of forming the polymer poly(4-nitro-1,2-phenylenediamine) (P4NoPD) on gold and glassy carbon electrodes (GCE) were shown. The P4NoPD films were generated by continuous potential cycling between −0.15 V and +1.10 V (versus Ag|AgCl, saturated KCl). The pH of the electropolymerization medium, the nature of the background electrolyte and the number of cycles used were found to influence strongly the amount of polymer deposited. The reduction of nitro-groups of the P4NoPD films was also found to be dependent on solution conditions, especially pH.     

Anion exchange influence on the electrochemomechanical properties of polyaniline

The anion influence on the volume changes of polyaniline (PANI) thick films (∼200 μm) during redox switching was studied using voltammetric experiments with simultaneous microscopical observation. The area occupied by the polymer film in the microscope images was measured and used to compute the film volume and its variations. It is found that the electrolyte anion has a definite influence on the film volume changes. When a film equilibrated with perchloric acid is transferred to sulphuric acid, fully reduced and relaxed and afterwards subject to voltammetric cycling, a net volume increase is observed, spanning several potential cycles until a stationary profile is reached. This change is not reversed when the film is transferred back to perchloric medium, and in this electrolyte the response shows a faster evolution. Exchanges with hydrochloric acid show behaviour more similar to HClO₄. IR measurements show that anion exchange is complete before the voltammetric cycling is started, indicating that anion ingress/exchange alone is not the only cause for PANI volume changes. The results are interpreted in terms of polymer backbone conformational changes and polymer/anion interactions.     

A passivation mechanism of doped polyaniline on 410 stainless steel in deaerated H2SO4 solution

To investigate the role of polyaniline (PANI) in the corrosion protection of stainless steel (SS) in oxygen-deficient acidic solution, a separate doped PANI film electrode on a glass substrate was prepared and the test solution (1 M H₂SO₄) was purged with high-purity N₂ until dissolved oxygen level decreased more than two orders of magnitude. In this deaerated 1 M H₂SO₄ solution, the galvanic coupling interaction between the separate PANI film electrode and 410 SS was studied. Results reveal that the separate PANI film can passivate the 410 SS steadily for a long period of time. A variety of experimental methods including potentiodynamic measurement, potentiostatic (current-time) examination and X-ray photoelectron spectroscopy (XPS) are used to explore the mechanism by which the separate PANI film passivated the galvanic coupling SS in the deaerated sulfuric solution. These studies show that passivation is achieved because PANI film provides a large critical current at the early stage of coupling and a persistent passive current by its electrochemical dedoping/re-doping equilibrium activity with the acidic environment at the subsequent stage of coupling.     

Corrosion studies on electrochemically deposited PANI and PANI/epoxy coatings on mild steel in acid sulfate solution

The corrosion behavior of mild steel and mild steel covered by electrochemically deposited (a) polyaniline (PANI) film, (b) epoxy coating and (c) PANI/epoxy coating system in 0.1 M sulfuric acid solution were investigated by electrochemical impedance spectroscopy (EIS). Electrochemical deposition of PANI film was performed from aqueous solution of 0.5 M sodium benzoate and 0.1 M aniline at constant current density of 1.5 mA cm⁻². Epoxy coatings on mild steel and mild steel modified by PANI film were deposited at constant voltage. It was shown that thin PANI film had provided good corrosion protection of mild steel in 0.1 M sulfuric acid solution, and could be used for modification of mild steel prior to epoxy coating deposition. The increased corrosion protection of mild steel by PANI/epoxy coating system in the same solution was obtained.     

Atomic emission spectroelectrochemistry applied to dealloying phenomena II. Selective dissolution of iron and chromium during active-passive cycles of an austenitic stainless steel

Atomic emission spectroelectrochemistry was used to investigate selective dissolution of a 304 austenitic stainless steel sample in 2 M H₂SO₄. The partial dissolution rates of Fe, Cr, Ni, Mn, Mo, and Cu were measured as function of time during a series of potentiostatic triggered activation/passivation cycles. When first exposed to sulfuric acid solution, the steel sample was in a passive state with a total steady state ionic dissolution rate expressed as an equivalent current density of 10 μA cm⁻². A transition into the active and passive state could be triggered by cathodic (−700 mV vs. Ag/AgCl) and anodic (+400 to +700 mV vs. Ag/AgCl) potentiostatic pulses respectively of variable time. Excess Cr dissolution was observed during the activation cycle as compared to Fe and a depletion of Cr dissolution was observed during the passivation cycle. These results are interpreted in terms of the dissolution of a Cr rich passive layer during activation and selective dissolution of Fe, Mn, Ni and other elements to form a Cr rich passive layer during passivation. Quantitative analysis of the excess Cr showed that the residual film contained approximately 0.38 μg Cr/cm². Fe does not appear to be incorporated into the film at this early stage of passive film growth. Residual films of metallic nickel and copper were formed on the surface during the active period that subsequently dissolved during passivation.     

Characterization of permselective coatings electrosynthesized on Pt-Ir from the three phenylenediamine isomers for biosensor applications

The permeability of polymers, electrosynthesized at neutral pH on Pt-Ir cylinders from each of the three isomers of phenylenediamine (oPD, mPD and pPD), to H₂O₂ (signal transduction molecule in many oxidase-based biosensors) and ascorbic acid (AA, archetypal interference species in biological applications of biosensors) was measured, and used to determine the permselectivity of the three polymers. PmPD was the coating with the greatest permselectivity for H₂O₂ over AA, for low concentrations of AA. For AA levels greater than 200 μM, however, poly-ortho-phenylenediamine (PoPD) was superior. Furthermore, stability studies indicated that the permselectivity of PmPD degraded rapidly, even after 1 day, supporting the choice of PoPD as the permselective membrane for biosensor implantation where AA levels are high, such as in brain monitoring. A variety of techniques were used to gain further insight into the PPD layers, specifically electrochemical quartz crystal microbalance, mass spectrometry and scanning electron microscopy. Together these studies indicate that PmPD forms the thickest layer (∼15 nm) and is the least soluble of the polymers, that the PoPD layer is ∼4 nm thick and may consist mostly of tetramers, while PpPD is the thinnest (∼3 nm) and appears to consist of trimers.     

Electrochemical synthesis and corrosion performance of poly(pyrrole-co-o-anisidine)

The electrochemical synthesis of poly(o-anisidine) homopolymer and its copolymerization with pyrrole have been investigated on mild steel. The copolymer films have been synthesized from aqueous oxalic acid solutions containing different ratios of monomer concentrations: pyrrole:o-anisidine, 9:1, 8:2, 6:4, 1:1. The characterization of polymer films were achieved with FT-IR, UV–visible spectroscopy and cyclic voltammetry techniques. The electrochemical synthesis of homogeneous-stable poly(o-anisidine) film with desired thickness was very difficult on steel surface. Therefore its copolymer with pyrrole has been studied to obtain a polymer film, which could be synthesized easily and posses the good physical–chemical properties of anisidine. The kinetics and rate of copolymer film growth were strongly related to monomer feed ratio. The introduction of pyrrole unit into synthesis solution increased the rate of polymerization and the substrate surface became covered with polymer film soon, while this process required longer periods in single o-anisidine containing solution. The protective behavior of coatings has been investigated against steel corrosion in 3.5% NaCl solution. For this aim electrochemical impedance spectroscopy (EIS) and anodic polarization curves were utilized. The synthesized poly(o-anisidine) coating exhibited significant protection efficiency against mild steel corrosion. It was shown that 6:4 ratio of pyrrole and anisidine solution gave the most stable and corrosion protective copolymer coating.     

Chemical and electrochemical synthesis of polyaniline/platinum composites

The synthesis of polyaniline/platinum composites (PANI/Pt) has been achieved using both chemical and electrochemical methods. The direct chemical synthesis of PANI/Pt proceeds through the oxidation of aniline by PtCl₆²⁻ in the absence of a secondary oxidant. SEM images of these samples indicate that the Pt particles are on the order of ∼1 μm for the chemically prepared composite. Electrochemical PANI/Pt synthesis is initiated by the uptake and reduction of PtCl₆²⁻ into an a priori electrochemically deposited PANI film. This method produces a uniform dispersion of Pt particles with smaller particles with diameters ranging between 200 nm and 1 μm. The results indicate that electrochemical methods may be more suitable for controlling particle dimension. Both materials show reduced proton doping relative to PANI without Pt, indicating the metal particles directly influence proton doping and the oxidation state of the polymer. The electrochemical data indicate that the conductivity in solution is sufficient such that the normal acid doping is attainable for PANI/Pt produced using either synthetic method.     

Prussian Blue-modified microelectrodes for selective transduction in enzyme-based amperometric microbiosensors for in vivo neurochemical monitoring

Prussian Blue-modified carbon fiber microelectrodes (CFE/PBs) are proposed as an alternative to the more conventional metal transducers used for H₂O₂-detecting biosensors in brain extracellular fluid (ECF). The main advantages of this approach are the very small dimensions (∼10 μm diameter) and the low applied potentials needed (0.0 V versus SCE). Electrocatalytic and physiochemical properties of PB deposits were studied using cyclic voltammetric (CV), amperometric and spectroscopic methods (FTIR and VIS). Optimized CFE/PB microsensors displayed a H₂O₂ current density of 1.00 ± 0.04 A M⁻¹ cm⁻² with a detection limit of 10⁻⁸ M. Furthermore, to improve stability and selectivity properties, several polymeric films were investigated: Nafion, poly(o-phenylenediamine) (PoPD), and a hybrid configuration of these two polymers. Finally, the PoPD film was selected due to its excellent anti-interference properties. The use of this permselective film also enhanced the stability of PB against solubilization at high pH, albeit at the expense of slightly lower H₂O₂ sensitivity (0.48 ± 0.02 A M⁻¹ cm⁻²) and higher detection limit (∼10⁻⁷ M). However, the use of the PoPD film significantly enhanced the selectivity against the main endogenous brain interference species (ascorbic acid, uric acid, dopamine and 3,4-dihydroxyphenylacetic acid), expressed as the ratio of the sensitivity slopes (SH₂O₂/Sinterference), which was close to 600 for all interference molecules studied. A prototype of a CFE/PB-based glucose microbiosensor design is presented, together with preliminary studies of its characteristics in vitro and its functionality in brain ECF in vivo.     

In situ electrochemical preparation of multi-walled carbon nanotubes/polyaniline composite on the stainless steel

Polyaniline–carbon nanotubes (PANI–CNTs) composites have been deposited via in situ electropolymerization on stainless steel (SS) surface. The presence of the oxidized multi-walled carbon nanotubes (mCNTs) in the composite was confirmed by thermal gravimetric analysis (TGA) and scanning electron microscope. Introducing 28 and 70 mg L⁻¹ mCNT in the electrolyte increased the growth rate of PANI from 38 to 67 and 83 mC/cycle, respectively. The mCNT decreases the porosity of the PANI, forming networks which held the polymer. Influences of the composite layer on the passivation and corrosion of the stainless steel were studied and compared with pure PANI layer. It was confirmed that a higher resistant passive film was formed on the steel under the composite layer compared to that formed under the pure PANI.     

Electrochemical copolymerization of m-toluidine and o-phenylenediamine

Electroactive copolymers of m-toluidine (MT) and o-phenylenediamine (OPD) were prepared electrochemically in aqueous sulfuric acid by potential cycling and characterized with cyclic voltametry, in situ conductivity measurements and FT-IR spectroscopy. The voltammograms of the copolymers exhibit different behavior for different concentrations of OPD in the comonomer feed. At optimum conditions the resulting poly(OPD-co-MT) shows an extended useful potential range of the redox activity as compared to the corresponding homopolymers. The effect of scan rate and pH on the electrochemical activity was studied. The copolymer was electrochemically active even at pH 8.0. The stability of the copolymer film was also tested. The copolymer has a potential region of maximum conductivity different from that of PMT and POPD. The conductivity of the copolymer is between the conductivity of the homopolymers. The vibrational bands at 3122/3450 and 2922/875 cm⁻¹ in the FT-IR spectra of the copolymer indicate the presence of both OPD and MT units, respectively, in the copolymer backbone.     

Silver electrocrystallization at polyaniline-coated electrodes

The initial stage of silver electrocrystallization is studied at polyaniline (PANI)-coated platinum electrodes by means of potentiostatic current transients and electron microscopic observations. Data for the nucleation frequency and the number of active sites for nucleation are obtained by interpreting of current transients according to the theory for nucleation and 3D growth under diffusion limitations. It is found that depending on the PANI layers thickness, d, two different regimes for silver nucleation and growth exist. For thin PANI coatings (d<0.3 μm), the crystallization occurs with high nucleation frequency at active sites located at the polymer layer/metal electrode interface, the number of active sites decreasing sharply with increasing PANI coverage. For thick PANI layers (d>0.3 μm), silver nucleation occurs with a two orders of magnitude lower nucleation frequency at active sites located most probably at the polymer surface, their number remaining constant for thicknesses up to 1.4 μm. It is established that reduction of the PANI layer occurring in parallel with the silver electrodeposition does not influence the number of active sites for nucleation. The results obtained by interpretation of current transients are in good agreement with results for the number of crystals obtained by microscopic observation.     

Application of poly(<Emphasis Type="Italic">o</Emphasis>-phenylenediamine) in rechargeable cells

The electrochemical synthesis of poly(o -phenylenediamine) (PoPD) from an aqueous medium was suitably modified by controlling the switching potential to enhance the growth of the polymer. The charge–discharge data for the cell Zn/1 M ZnSO₄ (pH 4)/PoPD are presented. The polymer was modified by incorporating Pt microparticles into its matrix during electropolymerization. The PoPD-Pt composite electrode was also characterized as a cathode active material in aqueous cells.     

Electrochemical synthesis of WO3/PANI composite for electrocatalytic reduction of iodate

Composite film of polyaniline (PANI) and tungsten oxide (WO₃) was electrodeposited by cyclic voltammetric technique from a solution of aniline and tungstic acid. The obtained WO₃/PANI film displayed a significant enhancement of electrocatalytic activity for iodate reduction and a better stability than that of pure WO₃ and PANI films. Result of amperometric experiment revealed a good linear relationship with concentration of IO₃⁻ from 20 to 500 μM, with a high sensitivity of 0.54 μA/μM and a detection limit of 2.7 μM for the determination of iodate. This composite film was also successfully applied in determination of iodate in commercial table salt.     

Phase separation and dewetting in polystyrene/poly(vinyl methyl ether) blend thin films in a wide thickness range

Phase separation and dewetting processes of blend thin films of polystyrene (PS) and poly(vinyl methyl ether) (PVME) in two phase region have been studied in a wide film thickness range from 65 μm to 42 nm (∼2.5R_g, R_g being radius of gyration of a polymer) using optical microscope (OM), atomic force microscope (AFM) and small-angle light scattering (LS). It was found that both phase separation and dewetting processes depend on the film thickness and were classified into four thickness regions. In the first region above ∼15 μm the spinodal decomposition (SD) type phase separation occurs in a similar manner to bulk and no dewetting is observed. This region can be regarded as bulk. In the second region between ∼15 and ∼1 μm, the SD type phase separation proceeds in the early stage while the characteristic wavelength of the SD decreases with the film thickness. In the late stage dewetting is induced by the phase separation. In the third region between ∼1 μm and ∼200 nm the dewetting is observed even in the early stage. The dewetting morphology is very irregular and no definite characteristic wavelength is observed. It is expected that the irregular morphology is induced by mixing up the characteristic wavelengths of the phase separation and the dewetting. In the fourth region below ∼200 nm the dewetting occurs after a long incubation time with a characteristic wavelength, which decreases with the film thickness. It is considered that the layered structure is formed in the thin film during the incubation period and triggers the dewetting through the capillary fluctuation mechanism or the composition fluctuation one.     

Potentiodynamic deposition of poly (o-anisidine-co-metanilic acid) on mild steel and its application as corrosion inhibitor

For the first time, poly (o-anisidine-co-metanilic acid) (PASM) was deposited on mild steel substrate by electrochemical polymerization of o-anisidine and metanilic acid monomers in aqueous solution of 0.1 M H₂SO₄. The electrochemical polymerization of o-anisidine takes place in the presence of metanilic acid monomer and uniform, strongly adherent coating was obtained on the substrate. The electroactivity of copolymer was studied by cyclic voltammetry and AC impedance techniques. There is an increasing anodic current due to oxidation of metanilic acid monomer at the surface of the electrode when the applied potential is cycled from −0.2 V to 0.8 V. These deposits were characterized by Fourier transform infrared (FTIR) spectroscopy, UV-vis and TG/DTA techniques. The effect of various concentrations of PASM copolymer solution in acid rain corrosive media has been studied through potentiodynamic polarization, AC impedance and I-E curve methods. The soluble form of polymeric solution provided better anti-corrosive behavior in artificial acid rain solution.