Passivated Iron Electrode Research Articles

Determination of the zero-charge ph value of a passive iron surface in aqueous solution

Two techniques of measurement of pH0, which represents the pH of solution at which the total charge of passive metal electrode surface is zero have been developed. The procedures are based on the use of acid-base equilibria between oxide on electrode surface and solution. In the former technique, the pH of the drop of solution that is applied onto the electrode surface is measured. At pH0, the pH of the drop does not change in time. In the latter procedure, the potential of a passive electrode that is immersed into solutions with various pH values is measured. pH0 has been measured on a passive iron electrode. The pH0 values measured by the two procedures are similar. Comparison of pH0 of passive electrode and various oxides one to determine the composition of a passive film. The introduction of surface-active ions Ba2+ and Cl− to the solution leads to a shift of pH0 values and electrode potential in opposite directions.

Improvement of healing treatment with NaNO 3 for a passivated iron electrode covered with an ultrathin polymer coating to prevent iron corrosion in some solutions containing aggressive anions

An ultrathin and ordered polymer coating was prepared on a passivated iron electrode by modification of a 16-hydroxyhexadecanoate ion HO ( CH 2 ) 15 CO 2 - self-assembled monolayer with 1,2-bis(triethoxysilyl)ethane (C 2H 5O) 3Si(CH 2) 2Si(OC 2H 5) 3 and octyltriethoxysilane C 8H 17Si(OC 2H 5) 3. Further, the passivated and polymer-coated electrode was healed by treatment in 1.0 M NaNO 3 for 4 h. Prevention of passive film breakdown and iron corrosion for the passivated, polymer-coated and healed electrode was examined by monitoring of the open-circuit potential and repeated polarization measurements in oxygenated 0.1 M KClO 4, 0.1 M Na 2SO 4 and 0.1 M NaCl for many hours. The values of the time for passive film breakdown, t bd were >240, 22.2 and 9.5 h in these solutions, respectively. The protective efficiencies for the electrode were extremely high, more than 99.9% before t bd, indicating complete protection of substrate iron against corrosion in these solutions, unless passive film breakdown occurred. The presence of NO 3 - on the passive surface by treatment in 1.0 M NaNO 3 was detected by X-ray photoelectron and FTIR reflection spectroscopies. The self-healing activity of adsorbed NO 3 - to suppress passive film breakdown was discussed.

Prevention of passive film breakdown and corrosion of iron in 0.1 M KClO 4 with and without Cl − by covering with an ultrathin two-dimensional polymer coating and healing treatment in 0.1 M NaNO 3

A two-dimensional polymer coating, the self-assembled monolayer of 16-hydroxy hexadecanoate ion HO(CH 2) 15 CO 2 - modified with 1,2-bis(triethoxysilyl)ethane (C 2H 5O) 3Si(CH 2) 2Si(OC 2H 5) 3 and octadecyltriethoxysilane C 18H 37Si(OC 2H 5) 3 was prepared on the passivated iron electrode and further, the passive film was healed by additional treatment in 0.1 M NaNO 3. This electrode was immersed in oxygenated 0.1 M KClO 4 solutions with and without 1 × 10 −4 to 1 × 10 −2 M of Cl −. Protection of passive film against breakdown by covering the electrode with the polymer coating was examined by monitoring the open-circuit potential during immersion in the solutions for many hours to determine the time for passive film breakdown, t bd. Repeated polarization measurements were carried out during immersion in these solutions for obtaining the protective efficiency, P. The t bd value of the passivated, polymer-coated and healed electrode in 0.1 M KClO 4 solutions with and without Cl − increased with a decrease in the concentration of Cl −. No breakdown occurred on the electrode during immersion in 0.1 M KClO 4 solutions with and without 1 × 10 −4 of Cl − for 360 h. The P values were extremely high, more than 99.9% before t bd, indicating complete protection of iron from corrosion. The effect of healing treatment in 0.1 M NaNO 3 on passive film breakdown was investigated by electron-probe microanalysis.

10.1007/s11175-008-2015-4

Using the method of phase modeling, the pH values of solutions corresponding to the uncharged surface of passive iron and ferric oxide γ-Fe2O3 (pH0) are compared. According to the theory of connected places, the charge of metal oxide surface is determined by the adsorption or desorption of hydrogen ions leading to a change in the potential drop at the oxide/solution interface. Preliminarily passivated iron electrode was washed with twice-distilled water and placed into 0.5 M NaNO3 solution with various pH values; the variation in the potential (ΔE) with time was studied. The pH0 value for passive electrode under the open-circuit conditions was determined by the dependence of ΔE on the pH value (pH0 6.2 ± 0.1). The pH0 value was close to that for γ-Fe2O3 (pH0 6.2), which was determined by the method of potentiometrical titration of oxide suspension in the nitrate solution. The introduction of surface-active ions Ba2+ and Cl− changes the charge of passive iron surface: Ba2+ ions increase the electrode potential, while Cl− ions decrease it. Comparing the pH0 values for passive electrode and metal oxides, one can identify the composition of passive electrode surface.

Corrosion prevention of passivated iron in 0.1 M NaCl by coverage with an ultrathin polymer coating and healing treatment in 0.1 M NaNO 3

An ultrathin, ordered and two-dimensional polymer coating was prepared on a passivated iron electrode by modification of 16-hydroxyhexadecanoate ion HO(CH 2) 15CO 2 − self-assembled monolayer with 1,2-bis(triethoxysilyl)ethane (C 2H 5O) 3Si(CH 2) 2Si(OC 2H 5) 3 and octadecyltriethoxysilane C 18H 37Si(OC 2H 5) 3. Subsequently, the electrode was healed in 0.1 M NaNO 3. Protection of passivated iron against passive film breakdown and corrosion of iron was examined by monitoring of the open-circuit potential and repeated polarization measurements of the polymer-coated and healed electrode in an aerated 0.1 M NaCl solution during immersion for many hours. Localized corrosion was markedly prevented by coverage with the polymer coating and the healing treatment in 0.1 M NaNO 3. Prominent protection of iron from corrosion in 0.1 M NaCl was observed before the breakdown occurred. The electrode surface covered with the healed passive film and polymer coating was analyzed by contact angle measurement, X-ray photoelectron spectroscopy and electron-probe microanalysis.

Determination of pH value of isoelectric solution and identification of passive iron surface phases using immersion method

Using the method of phase modeling, the pH values of solutions corresponding to the uncharged surface of passive iron and ferric oxide γ-Fe2O3 (pH0) are compared. According to the theory of connected places, the charge of metal oxide surface is determined by the adsorption or desorption of hydrogen ions leading to a change in the potential drop at the oxide/solution interface. Preliminarily passivated iron electrode was washed with twice-distilled water and placed into 0.5 M NaNO3 solution with various pH values; the variation in the potential (ΔE) with time was studied. The pH0 value for passive electrode under the open-circuit conditions was determined by the dependence of ΔE on the pH value (pH0 6.2 ± 0.1). The pH0 value was close to that for γ-Fe2O3 (pH0 6.2), which was determined by the method of potentiometrical titration of oxide suspension in the nitrate solution. The introduction of surface-active ions Ba2+ and Cl− changes the charge of passive iron surface: Ba2+ ions increase the electrode potential, while Cl− ions decrease it. Comparing the pH0 values for passive electrode and metal oxides, one can identify the composition of passive electrode surface.

Complete protection of a passive film on iron from breakdown in a borate buffer containing 0.1 M of Cl − by coverage with an ultrathin film of two-dimensional polymer

An ultrathin film of two-dimensional polymer was prepared on a passivated iron electrode by modification of a 16-hydroxyhexadecanoate ion HO ( CH 2 ) 15 CO 2 - self-assembled monolayer with 1,2-bis(triethoxysilyl)ethane (C 2H 5O) 3Si(CH 2) 2Si(OC 2H 5) 3 and octadecyltriethoxysilane C 18H 37Si(OC 2H 5) 3. This film prevented passive film breakdown examined by potentiodynamic anodic polarization of the coated electrode in the borate buffer solution containing 0.1 M of Cl −. Neither current spikes nor the pitting potential was observed in the passive and transpassive regions of polarization curve. The anodic current density was decreased in these regions markedly, implying hindrance to permeation of Cl − and water through the film. Structure of the film was clarified by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement with a drop of water. Electron-probe microanalysis of the passivated surface coated with the film after anodic polarization scanning up to the transpassive region revealed that the polymer film prevents pit initiation by an attack on the passive film with Cl −.

Direct modification of a passivated iron electrode with alkyltriethoxysilanes for preventing passive film breakdown in a borate buffer containing 0.1 M of chloride ion

A passive film on an iron electrode was modified with alkyltriethoxysilanes directly. In order to examine the protective ability of the modified passive film against breakdown, the pitting potential, E pit was measured by anodic polarization of the modified electrode in a borate buffer solution (pH 8.49) containing 0.1 M of Cl −. The value of E pit for the modified electrode shifted in the positive direction from that of the unmodified electrode, indicating prevention of passive film breakdown. The modified passive film was not broken down in the passive and transpassive regions of polarization curve in some cases. However, many current spikes appeared in the all curves of the modified electrodes. The modified surface of passivated electrode was characterized by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement. There were defects and clusters of associated water within the modified film and hence, Cl − could permeate through the defects, leading to appearance of current spikes and occurrence of breakdown.

Ultrathin protective films of two-dimensional polymers on passivated iron against corrosion in 0.1 M NaCl

Prevention of iron corrosion in an aerated 0.1 M NaCl solution was investigated by polarization and mass-loss measurements of a passivated iron electrode covered with ultrathin and ordered films of two-dimensional polymers. The films were prepared on the passivated electrode by modification of a 16-hydroxyhexadecanoate ion HO ( CH 2 ) 15 CO 2 - self-assembled monolayer with 1,2-bis(triethoxysilyl)ethane (C 2H 5O) 3Si(CH 2) 2Si(OC 2H 5) 3 and alkyltriethoxysilane C n H 2 n + 1 Si(OC 2H 5) 3 ( n = 8 or 18). Because crevice corrosion occurred at the initial stage of immersion in the solution preferentially, the edge of electrode covered with the polymer film was coated with epoxy resin. The open-circuit potentials of the covered electrodes in the solution were maintained high, more than −0.2 V/SCE for several hours, indicating that no breakdown of the passive film occurred on the surface. The protective efficiencies of the films were extremely high, more than 99.9% unless the passive film was broken down. The efficiencies after immersion for 24 h almost agreed with those obtained by mass-loss measurements. X-ray photoelectron spectroscopy and electron-probe microanalysis of the passivated surface covered with the polymer film after immersion in the solution for 4 h revealed that pit initiation on the passive film was suppressed by coverage with the polymer film completely.

Current transients of passive iron observed during micro-indentation in pH 8.4 borate buffer solution

Micro-indentation test of passive iron electrode in deaerated pH 8.4 borate buffer solution was carried out to investigate the rupture and repair of passive film. During driving a conical diamond micro-indenter with a load of 0.1 N order downward to the electrode and driving upward from the electrode, a couple of anodic current peaks were observed. The first current peak spiked during loading was responsible for partial exposure of iron substrate to the solution due to rupture of the passive film when the indenter tip contacted. The second peak emerged during unloading when elastic deformation recovered, which was ascribed to repair at the ruptured sites. The second peak current was larger than the first one. Both peaks were sensitively influenced by electrode potential or concentration of sulfate ions containing in solution. The model for a series of rupture and repair processes of the passive film by micro-indentation was proposed to discuss the current transients.

Impedance of a passive iron electrode in a solution containing a reducing agent

Frequency dependences of impedance of a passive iron electrode in 0.5 M Na2SO4 solutions with and without 0.01 M K4[Fe(CN)6] are obtained by a pulsed method and the Fourier transform. At 1–1000 Hz, the results in 0.5 M Na2SO4 satisfactorily agree with our previous results obtained by a lock-in method. With the [Fe(CN)6]4- ions present, the impedance decreases faster at lower frequencies. The impedance of the oxide film/solution interface relaxes longer the film impedance. These conclusions are similar to those obtained earlier.

Preparation of a two-dimensional polymer film on passivated iron by modification of a carboxylate ion self-assembled monolayer with alkyltriethoxysilanes for preventing passive film breakdown

The effect of an ultrathin, regularly arranged polymer film on prevention of passive film breakdown on iron in the presence of chloride ion was investigated. The film of two-dimensional polymer was prepared by modification of a 16-hydroxyhexadecanoate ion HO ( CH 2 ) 15 CO 2 - self-assembled monolayer adsorbed on a passivated iron electrode with 1,2-bis(triethoxysilyl)ethane(C 2H 5O) 3Si(CH 2) 2Si(OC 2H 5) 3 and octyltriethoxysilane C 8H 17Si(OC 2H 5) 3. The pitting potentials of the passivated electrodes bare and covered with the polymer film were determined by anodic polarization measurements in a borate buffer solution containing 0.1 M of Cl −. The pitting potential of the coated electrode was higher than that of the uncoated one, indicating prevention of passive film breakdown. No breakdown was observed over the potential range in the passive and transpassive regions by covering the passive film with the well-arranged two-dimensional polymer film. The film was characterized by X-ray photoelectron and FTIR reflection spectroscopies and measurement of the contact angle with a drop of water.

Prevention of passive film breakdown on iron by coverage with one-dimensional polymer films of a carboxylate ion self-assembled monolayer modified with alkyltriethoxysilanes

A film composed of a one-dimensional polymer was fabricated by modification of a 16-hydroxyhexadecanoate ion HO(CH 2) 15CO 2 − self-assembled monolayer (SAM) adsorbed on a passivated iron electrode with octadecyltriethoxysilane C 18H 37Si(OC 2H 5) 3. The pitting potential, E pit of the passivated electrode coated with this film was measured by anodic polarization in a borate buffer solution containing 0.1 M of Cl −. The E pit value of the electrode coated with the film was markedly shifted from the value of the bare electrode in the positive direction, indicating prevention of passive film breakdown. No breakdown occurred over the potential range of passive region in some cases. Structure of the modified SAM was discussed by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement of the electrode surface covered with the film. Suppression of Cl − accumulation at a defect of the passive film was revealed by electron-probe microanalyses of the surfaces uncoated and coated with the SAM modified with octyltriethoxysilane C 8H 17Si(OC 2H 5) 3 after anodic polarization in the borate buffer containing Cl −.

Prevention of passive film breakdown on iron by coverage with one-dimensional polymer films of a carboxylate ion self-assembled monolayer modified with alkyltriethoxysilanes

A film composed of a one-dimensional polymer was fabricated by modification of a 16-hydroxyhexadecanoate ion HO(CH2)15CO2− self-assembled monolayer (SAM) adsorbed on a passivated iron electrode with octadecyltriethoxysilane C18H37Si(OC2H5)3. The pitting potential, Epit of the passivated electrode coated with this film was measured by anodic polarization in a borate buffer solution containing 0.1 M of Cl−. The Epit value of the electrode coated with the film was markedly shifted from the value of the bare electrode in the positive direction, indicating prevention of passive film breakdown. No breakdown occurred over the potential range of passive region in some cases. Structure of the modified SAM was discussed by X-ray photoelectron and FTIR reflection spectroscopies and contact angle measurement of the electrode surface covered with the film. Suppression of Cl− accumulation at a defect of the passive film was revealed by electron-probe microanalyses of the surfaces uncoated and coated with the SAM modified with octyltriethoxysilane C8H17Si(OC2H5)3 after anodic polarization in the borate buffer containing Cl−.

Impedance and Photoadmittance of Passive Iron Electrodes in Different Solutions

Electric characteristics of the oxide film at the surface of an anodized iron electrode in 0.5 M Na2SO4, NaNO3, and Na2MoO4 solutions are studied using the impedance and photoadmittance measurements and taking polarization curves. The impedance frequency spectra correspond to an equivalent circuit comprising two parallel (RC) chains connected in a series. The relaxation time of the high-frequency circuit approaches that of the photopotential; hence, it can be related to the impedance of the oxide film. The low-frequency component of the impedance describes the impedance of the film/solution interface. In the Na2SO4 solution, the oxide film resistance is significantly higher and the capacitance is lower than in two other solutions, which can be explained by the film thickening. The marked concurrent decrease in the resistance and increase in the capacitance of the film/solution interface at 1.2 V in all solutions is caused by oxygen adsorption, which precedes oxygen evolution.

Mechanism for protection of iron corrosion by an intrinsically electronic conducting polymer

The protection mechanism of an iron substrate, coated with intrinsically electronic conducting polymer (ICP) was devised on the basis of experimental data collected by various local measurement techniques around an artificial pinhole area; namely, potential, current, impedance spectra, and in situ Raman spectroscopy. The local potential and the current between ICP and the defect area allowed verification that the polymer coating is able to make passive a small area of bare iron. The local impedance spectra exhibited the characteristics of a passive iron electrode by capacitance behavior dominating the whole impedance spectrum. On the contrary, classical impedance spectra, defined by the overall potential over the overall current revealed merely the impedance spectra of ICP involving doping and undoping processes. The protection by ICP coating vanishes before exhaustion of the whole charge stored initially. Furthermore, with a polyaniline coating with an inner poly(diamino-naphthalene) layer, it was shown that even when active corrosion is taking place at the iron, the film remained in the oxidized state. Besides, the oxidation state near the defect area was higher compared to the overall oxidation state of the ICP coating, though the interface potential is the same on the whole surface area, including the defect area.

Analysis of the impedance frequency spectrum of a passive iron electrode by combined impedance and photoadmittance measurements

The frequency spectra of electrochemical impedance Z, photocurrent Δi, and photopotential ΔE are obtained for a passive Fe electrode in 0.2 M KOH in a frequency range 1.3 to 8300 Hz. The validity of relationship ΔE/Δi = –Z is proved experimentally. The oxide film impedance ZF is calculated from the frequency spectrum of ΔE with expression ZF = –ΔE/Δg. The generation current Δg is found from the photocurrent frequency spectrum. The frequency impedance spectrum ZF/S of the oxide/solution interface is determined with equation ZF/S = Z – ZF – Rel, where Rel is the solution resistance.

Impedance and photoadmittance of a passive iron electrode in alkaline sodium sulfide solutions

The intensity-modulated photocurrents (photoadmittance) and the electrochemical impedance of an anodically oxidized iron electrode are studied in alkaline solutions containing the sulfide ion. The photooxidation of S2- proceeds via a stage of their adsorption. The concentration of S2- adsorbed on the surface decreases at a potential shift in the positive direction

Reactivation of Passivated Iron Electrodes in a Magnetic Field

In the presence of a magnetic field, the reactivation of a passivated iron electrode in solution was studied experimentally with cyclic voltammetry, electrochemical impedance spectroscopy, and holographic microphotography. In the presence of the magnetic field, symmetric solution flow of the electrolyte phase was observed when the reactivation current increased abruptly. This phenomenon is caused by the Lorentz force on ions moving in both electric and magnetic fields. The thick film formed on the iron electrode surface during the reactivation is also discussed, and is believed to be for the short formation time and the big difference in the conductivity of the ions involved.

The inhibition of pit nucleation and growth on the passive surface of iron in a borate buffer solution containing Cl − by oxidizing inhibitors

The inhibiting effects of oxidizing oxyanion inhibitors on pit nucleation and growth were investigated for a passivated iron electrode in an aerated borate buffer solution containing Cl − at pH 8.49 by polarization measurements. The pitting potential E pit and the repassivation potential E rep were shifted towards positive potentials by the addition of MoO 4 2−, WO 4 2− and NO 2 −, indicating suppression of pit nucleation and propagation. CrO 4 2− and VO 4 − suppressed pit nucleation but stimulated pit growth. The value of E rep was increased in the presence of NO 3 −, whereas little effect on Epit, was obtained. The inhibition of pit nucleation and propagation with the oxidizing inhibitors is understandable from consideration of a hard acid-hard base interaction based on the hard and soft acids and bases principle.