Magnetic field effects on anodic polarisation behaviour of iron in neutral aqueous solutions

Abstract

The presence of a magnetic induction field with a flux density of 0·4 T caused the anodic active-passive transition potential of iron in neutral aqueous solutions of sodium sulphate to move in the noble direction and resulted in a reduction of the passive potential range. Anodic current densities at the active-passive transition potential and in the passive potential range increased in the presence of the magnetic field. There was a synergistic effect between the magnetic field and the presence of chloride in solution on the breakdown of iron passivity.     

Repassivation behaviour of stressed aluminium electrodes in aqueous chloride solutions

The combined influence of mechanical stress and chemical environment on the pitting susceptibility of aluminium electrodes was studied by integrating open circuit potential measurements and electrochemical repassivation modelling. The transient degradation of aluminium oxide owing to a tensile stress field and its subsequent repassivation are observed as an abrupt negative potential excursion followed by a slow decay of the corrosion potential back towards its initial value prior to the mechanical loading. Experiments were performed and an electrochemical repassivation model was established to quantitatively describe the dependence of oxide dissolution rate and the reciprocal of cathodic Tafel slope on chloride concentration.     

The influence of different aggressive anions on the electrochemical behaviour of aluminium in sodium nitrate aqueous solutions

The effects of different experimental parameters influencing the determination of critical pitting and protection potentials of aluminium and its alloys have been studied by potentiostatic and potentiodynamic methods. The resistance of aluminium against corrosion in aqueous media can be attributed to a rapidly formed surface oxide film. The addition of the aggressive anions like: chloride, thiocyanide, hydroxyl, sulphide, formate, and acetate (Cl⁻, SCN⁻, OH⁻, S²⁻, HCOO⁻ and CH₃‐ COO⁻) lead to extensive localized attack in all of the cases. The breakdown of the passive film takes into account the migration of aggressive anions through the film. Breakdown occurs when aggressive anions reach the metal‐film interface. E_π is the critical pitting potential, E_p ist the protection potential and the pitting can be formed only in the E_π–E_p polarization range as it was proved in many experiments [1–3]. The most likely action mechanism of aggressive anions is not a complete dissolution of the film, nor penetration of aggressive anions through solid oxide as suggested for nickel and iron [4]. It is more likely to be somewhere in between the two i.e. action of aggressive anions is that of complexing aluminium ions and pulling in water to hydrate the layer in a way similar to that occurring at cathodic hydrogen evolution, where such dramatic increase of hydrogen evolution rate is observed after a certain cathodic potential is reached. Localized corrosion can be prevented by the action of adsorptive inhibitors which prevent the adsorption of the aggressive anions or by the formation of a more resistant oxide film on the metal surface. The corrosion mechanism is not modified by the addition of ammonium rhodanide but only slowed down.     

The effects of added impurities upon the electrochemical behaviour of iron in liquid ammonia solutions

Polarization data are presented for an iron electrode in liquid ammonia solutions containing ammonium salts and small amounts of the impurities: water, oxygen, nitrogen, ammonium carbamate, urea and boron trifluoride at 40°C. Additions of water up to a limiting concentration increase the iron dissolution currents in the active region. Additions of oxygen have a similar effect. The presence of either oxygen or water assists the onset of passivation and gives the phenomenon a more irreversible character.In ammonia, one mole of oxygen with two moles of Fe(II) forms a complex which can be cathodically reduced on vitreous carbon (1.0F per mol of Fe(II) or on iron (2.0F per mol of Fe(II)). On an iron cathode molecular oxygen is reduced at a rate determined by the mass transfer of oxygen, except at values of electrode potential more positive than the iron passivation potential.Nitrogen and urea do not affect the polarization behaviour of iron. Ammonium carbamate increases the iron dissolution currents obtained with nitrate or fluoroborate electrolytes but considerably inhibits the process in chloride electrolytes. Boron trifluoride profoundly increases the activity of iron towards dissolution.     

The effects of added impurities upon the electrochemical behaviour of iron in liquid ammonia solutions

Polarization data are presented for an iron electrode in liquid ammonia solutions containing ammonium salts and small amounts of the impurities: water, oxygen, nitrogen, ammonium carbamate, urea and boron trifluoride at — 40°C. Additions of water up to a limiting concentration increase the iron dissolution currents in the active region. Additions of oxygen have a similar effect. The presence of either oxygen or water assists the onset of passivation and gives the phenomenon a more irreversible character.In ammonia, one mole of oxygen with two moles of Fe(II) forms a complex which can be cathodically reduced on vitreous carbon (1.0F per mol of Fe(II)) or on iron (2.0F per mol of Fe(II)). On an iron cathode molecular oxygen is reduced at a rate determined by the mass transfer of oxygen, except at values of electrode potential more positive than the iron passivation potential.Nitrogen and urea do not affect the polarization behaviour of iron. Ammonium carbamate increases the iron dissolution currents obtained with nitrate or fluoroborate electrolytes but considerably inhibits the process in chloride electrolytes. Boron trifluoride profoundly increases the activity of iron towards dissolution.     

Doping competition of anions during the electropolymerization of pyrrole in aqueous solutions

The doping competition of anions in the electropolymerization of pyrrole was studied in aqueous solutions containing two kinds of anions with the same concentration of 0.1 mol dm^-3. The composition analysis of the polypyrrole (PPy) films prepared shows that the doping amount of p-toluene sulfonate (TsO^-) prevails over that of perchlorate, chloride and nitrate anions. The conductivity of the PPy film is close to the higher value of the two PPy films doped with a single anion. When polymerization is performed in the solution containing TsO^- and NO₃^-, TsO^-is mainly doped at Structure I, while NO₃^- is mainly at Structure II of the PPy film.     

Action of chelators on solid iron in phosphate-containing aqueous solutions

To study the effect of strong iron-ligands on steel corrosion, mild steel electrodes were immersed in solutions containing 20 mM phosphate buffer (pH=7.2) and between 0.01 mM and 1 M of either the iron(II)-chelators 2,2^′-bipyridine or FerroZine, or the iron(III)-chelators citrate or acetylacetonate. Resulting surface reactions were investigated by quantifying the electrochemical potential (E), the electrochemical polarization resistance (R_p), the corrosion current (I_corr) and the release of iron into solution. The surface was further analyzed by scanning electron microscopy (SEM/SEM-EDAX) and atomic force microscopy. Concentrations of 0.1 mM of any of the chelators led to slight, temporary changes in E, I_corr and R_p. Concentrations of 10 mM resulted in characteristic changes of E, which were the same for all chelators and in the precipitation of FePO₄ in the case of citrate and acetylacetonate, or vivianite [Fe₃(PO₄)₂ · 8H₂O] in the case of bipyridine and FerroZine. Concentrations of 1 mM of both iron(III)-chelators led to a temporary drop of E similar to that found with 0.1 mM chelator. With iron(II)-chelators, E dropped to about −500 mV before oscillating for several days. The amplitudes of the oscillations were up to 200 mV with periods of 30 and 20-25 min for bipyridine and FerroZine, respectively.     

Electrochemical behaviour of iron in alkaline sulphide solutions

Abstract

Cyclic voltammograms and galvanostatic polarisation curves were traced for the iron electrode in alkaline solutions containing sulphide species. In sulphide free NaOH solution, the galvanostatic anodic polarisation curves are characterised by two well defined steps before tending finally to values characteristic for the oxygen evolution reaction. However, using cyclic voltammetry technique, three anodic peaks are distinct. On the other hand, the cathodic half cycles using both techniques are characterised by two cathodic reduction steps before hydrogen evolution commences. In the presence of increasing sulphide content the polarisation curves are characterised by the presence of several anodic and cathodic regions. The anodic regions correspond to the simultaneous formation and oxidation of different Fe sulphide and oxide species. A mechanism is thus proposed for the formation and reduction of iron sulphide species.     

Influence of some anions on the stress corrosion of low alloy steels in aqueous solutions

Polarization and stress corrosion behaviour of two types of low alloy steels, AISI 4130X and AISI 4145, were studied in agitated 0.05 M sodium acetate and 0.05 M trisodium phosphate solutions saturated with natural gas at room temperature. The magnitude of currents, shown in potentiokinetic polarization curves in the range of —0.8 V(SCE) to +0.8 V(SCE) in these media, is very small compared with earlier results obtained in sulphate and chloride solutions.Stress corrosion experiments conducted using a slow strain rate technique under cathodic conditions indicate the onset of hydrogen embrittlement on both types of steel in sulphate, chloride, acetate and phosphate solutions. Under anodic conditions, the results in 0.05 M sodium acetate solutions indicate strong susceptibility to stress corrosion compared with no susceptibility in 0.05 M trisodium phosphate solutions if there is no onset of pitting. These results are in contrast with results showing strong anodic dissolution and consequent absence of stress corrosion under anodic conditions in sulphate and chloride media.Results of Auger and IR analyses of anodic surface films formed in acetate and phosphate solutions are presented and discussed in relation to their observed mechanical behaviour.     

Studies of copper impedance in alkaline aqueous solutions in the presence of inorganic anions

The behaviour of copper in alkaline aqueous solutions (pH 12) containing inorganic anions such as hydroxides, fluorides, chlorides, bromides, iodides, sulphates, perchlorates, carbonates and phosphates has been investigated by the a.c. impedance technique over a wide frequency range (100 kHz - 10 mHz). The experimental data were approximated by five equivalent circuits and the parameters of the two models that the results fitted best are presented. The impedance of copper was measured at three constant potentials and as a function of the exposure time. The results give valuable information about the properties of copper surface layers and their corrosion resistance.     

Electrochemical and corrosion behaviour of iron in acetonitrile solutions

The behaviour of the iron-acetonitrile system has been examined and data important from a practical point of view have been obtained. The simultaneous presence of oxygen and acidity is completely unacceptable even at level of 10^?3M in H⁺. The influence of stirring is also potentially dangerous in the presence of dissolved oxygen and free acidity. The conditions for the onset of localized corrosion (e.g. crevice corrosion) are outlined.     

Electrochemical behaviour of iron in methanol and dimethylformamide solutions

The electrochemical behaviour of pure iron in methanol solutions has been investigated. The build-up and stability of a passive state has been evaluated by varying the water content and the concentration of Cl^? ions. The water content turns out to be the determining factor for passivity; the presence of Cl^? does not allow the build-up and maintenance of the passive state. Iron is always found in solution as Fe²⁺. Further experiments have been carried out in dimethylformamide, in order to determine the influence of variations in dielectric constant and in dipole moment of the medium.     

On intensification of iron passivation by benzotriazole in aqueous solutions

The passivation of iron and mild steel by benzotriazole (BTA) and its equimolar composition with sodium phenyl undecanoate (SFU) in a borate buffer solution with pH 7.36 is studied. BTA is shown to be an inefficient passivator compared to SFU. However, the passivating effect of BTA can be increased by means of the preliminary formation of its adsorption layer at a cathodic potential E = −0.65 V and its combined use with SFU. The adsorption of the inhibitors studied can be described formally by the Frumkin isotherm. An analysis of the attraction interactions between the adsorbing particles of the inhibitors enabled us to suppose that, at the adsorption of the mixture on the iron free of oxides (E = −0.65 V), a layer adjacent to the electrode surface consists mainly of SFU anions, while the outer layer is formed by BTA. The adsorption of BTA on the iron electrode surface, where the SFU monolayer was preliminarily adsorbed, is much stronger compared to that on the reduced surface of the same electrode. Based on this observation, a two-stage treatment of the surface of a mild steel with SFU followed by BTA passivating solutions is proposed. Corrosion tests under severe conditions of everyday abundant moisture condensation on the steel surface confirmed the high efficiency of the method.     

Influence of cavitation on the passive behaviour of duplex stainless steels in aqueous LiBr solutions

The objective of this work is to study the influence of cavitation on the passive behaviour of EN 1.4462, its filler metal (EN 1.4462F), and the welded metal (EN 1.4462W) obtained by Gas Tungsten Arc Welding using electrochemical techniques. The hydrodynamic conditions of the medium were modified using an ultrasonic-induced cavitation facility.Potentiostatic experiments were used to study the effects of cavitation on the passive behaviour of the alloys. The experiments were carried out in 850 g/L LiBr solutions with and without an inhibitor (Lithium Chromate). The solution with Li₂CrO₄ (commercial solution) contains LiOH as the pH regulator. The potentiodynamic cyclic curves of the stainless steels under the static condition were used to set the values of the imposed potentials.In this work, the electrochemical behaviour of the alloys described by the potentiodynamic curves has been related to their passive behaviour under potentiostatic conditions when the pulses of cavitation were applied. The results demonstrate that cavitation affects the passive behaviour of the alloys; the influence depends on the potential applied and on the presence or absence of chromates in the medium. Only under certain circumstances the hydrodynamic conditions can suppose a breakdown of passive film formed under static conditions.     

Corrosion behaviour of stainless steels in aqueous solutions of methanesulfonic acid

The corrosion behaviour of different grades of stainless steel in five commercial products labeled as 70% aqueous methanesulfonic acid was investigated. Chemical analysis showed that these products contain different amounts of impurities, most probably the consequence of the production procedure. Three austenitic stainless steel grades, 304, 316 and 316Ti, are widely used for storage and transportation vessels for methanesulfonic acid (MSA). Corrosion behavior has been studied by electrochemical techniques, immersion tests and by surface analysis. The selected stainless steels are highly stable in the most purified acid, but the presence of impurities in MSA disrupts the metal’s surface, leading to corrosive attack.     

The mechanism of formation of iron oxide and oxyhydroxides in aqueous solutions at room temperature

For a better understanding of the atmospheric rusting of iron and steels, the present work is aimed to explore the mechanism of formation of green rusts, Fe₃O₄, α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH and amorphous ferric oxyhydroxide in aqueous solution at room temperature. The formation processes on which end products are determined are strongly affected by the oxidation rate, pH and the structure and composition of initial and intermediate species of iron. The systematic diagram of formation processes of iron oxide and oxyhydroxides has been presented, in which both dissolved and solid species of iron are included.     

Inhibition of iron corrosion from scratched lacquered tinplate by stannous ions

Abstract

The inhibition of iron corrosion from damaged lacquered tinplate in oxygen free pH 4 citrate buffer solution by the addition of stannous tin has been demonstrated using an accelerated simulated can corrosion test. The inhibition was total for tinplate of nominal coating mass 2·8 g m^?2 for additions of 5–15 ppm of stannous ion, but was only partial for tinplate of nominal coating masses 1·4, 1·0, and 0·5 g m^? up to a 60 ppm addition. The extent of undermining of the lacquer from around a standard scratch was shown to be dependent on the tin coating mass remaining on the tinplate, rather than the presence of stannous ions in solution.     

Cyclic voltammetry at the iron electrode in high temperature aqueous sodium hydroxide solutions

The cyclic sweep polarization behaviour of iron in sodium hydroxide solutions in the temp. range 200–250°C is investigated using a new pressure-balanced electrochemical cell which is fully described. It is shown that the corrosion of iron in this environment takes place at a potential which is controlled by the hydrogen electrode reaction. Active dissolution of the metal is not observed, confirming the existence of a solid-state barrier layer between the metal and the solution in the porous oxide.     

On the passivation of iron in aqueous solutions of zinc 1-hydroxyethane-1,1-diphosphonate

This work is devoted to studying the passivating ability of the zinc complex of the 1-hydroxyethane-1,1-diphosphonic acid (HEDP) in a borate buffer solution. For the first time, we used the in situ ellipsometric method to study the mechanism of formation of a protective film on iron in the presence of HEDP, HEDPZn, and ZnSO₄ in the course of the cathodic polarization of the electrode. The investigations of adsorption of HEDPZn on iron (at E = −0.65 V) in combination with X-ray photoelectron spectroscopy (XPS) have shown that on the metal surface there is formed a multilayer protective film consisting of an internal layer of Zn(OH)₂ and an outer layer consisting of HEDP complexes with Fe²⁺ and/or Zn²⁺. It has been found that the thickness of the passivating film does not exceed 60 ?, of which 7–10 ? correspond to the low-soluble zinc hydroxide. Original Russian Text ? Yu.I. Kuznetsov, G.V. Zinchenko, L.P. Kazanskii, N.P. Andreeva, Yu.B. Makarychev, 2007, published in Korroziya: Materialy, Zashchita, 2006, No. 9, pp. 19–26.