The dissolution of iron under cathodic polarization

It has been shown that the dissolution rate for iron at cathodic potential is higher than expected on the grounds of the Wagner-Traud mixed potential theory. This observation cannot be explained by a parallel chemical process as suggested by Kolotyrkin et al. The dissolution appeared to be unaffected by potential, pH, cations (Li⁺, Na⁺, K⁺), anions (SO^2?₄, Cl^?, ClO^?₄) and solvent (water-dioxan mixtures). However, the dissolution rate was decreased by an order of magnitude when the electrode was connected to a strong magnet, suggesting that the mechanical degradation of the surface under the influence of the cathodically evolved hydrogen is the main cause of the measured effects.     

The dissolution of iron under cathodic polarization

It has been shown that the dissolution rate for iron at cathodic potential is higher than expected on the grounds of the Wagner-Traud mixed potential theory. This observation cannot be explained by a parallel chemical process as suggested by Kolotyrkin et al. The dissolution appeared to be unaffected by potential, pH, cations (Li⁺, Na⁺, K⁺), anions (SO^2-₄, Cl^?, ClO^-₄) and solvent (water-dioxan mixtures). However, the dissolution rate was decreased by an order of magnitude when the electrode was connected to a strong magnet, suggesting that the mechanical degradation of the surface under the influence of the cathodically evolved hydrogen is the main cause of the measured effects.     

Anodic dissolution of iron silicides in alkaline electrolyte

Anodic dissolution of iron and its silicides (FeSi, FeSi₂, as well as the eutectic alloy FeSi₂-Si) and pure Si, in 0.1 to 5.0 N NaOH solutions is studied by cyclic voltammetry and x-ray photoelectron spectroscopy. Principal characteristic features of the silicide anodic dissolution are revealed and the composition of surface films investigated. It is shown that, despite an increase in Si solubility at higher pHs, the iron silicides are highly resistant to anodic dissolution due to especial protective properties of the complex oxide surface film. Original Russian Text ? A.B. Shein, I.L. Rakityanskaya, S.F. Lomaeva, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 1, pp. 59–63.     

Reproducibility of corrosion parameters for the acidic dissolution of pure iron: potentiostatic polarisation

Polarisation curves were determined potentiostatically for pure polycrystalline iron corroding in oxygen-free 0.5 M H₂SO₄. Four different working electrode pre-treatments (abrasion/polishing, pre-polarisation and time to establish E_corr) were employed and the reproducibility of E_corr and calculated corrosion parameters (i_corr, Tafel slopes and i₀) for each treatment was determined. Electrode pre-treatment effects changes in working electrode catalytic activity with subsequent variation in the reproducibility of polarisation curves and measured and calculated corrosion parameters. A method incorporating abrasion/polishing followed by anodic/cathodic pre-polarisation resulted in general in improved parameter reproducibility and cathodic and anodic Tafel slopes close to those predicted by the reaction mechanisms.     

Anodic dissolution of iron in acidic chloride solutions

The dissolution of iron in acidic chloride solutions was studied by both rotating disc and impedance techniques. Two regions of different slope were observed in the Tafel graphs after the effects of diffusion were eliminated. The Tafel slope was seen to change from ca. 44 to ca. 85 mV/decade at more positive potentials. The lower slope was confirmed by impedance measurements. An electrode reaction sequence is proposed for the dissolution reaction in the region where the Tafel slope is ca. 44 mV/decade.The addition of decylamine reduced the rate of the dissolution reaction, but the form of the polarization curves and the mechanism of dissolution remained unchanged. It was also established that decylamine is not involved in the electrode reaction of the dissolving polycrystalline iron.     

Impedance investigation of the anodic iron dissolution in perchloric acid solution

The anodic iron dissolution in 0.5 mol dm⁻³ perchloric acid (HClO₄) was investigated by the electrochemical impedance spectroscopy, the potentiodynamic sweep and the scanning electron microscopy measurements. The anodic polarization behavior of iron in HClO₄ solution showed that the strong current oscillations occurred in a narrow potential region, particularly the pitting corrosion was observed in the active dissolution region. These characteristics were quite different from those of iron in the sulfuric acid (H₂SO₄). At the potentials 82 and 132 mV more positive than the corrosion potential (−482 mV vs. SCE), the impedance spectra for the iron in HClO₄ displayed two inductive arcs; however, by gradually increasing potential the lower frequency inductive arc disappeared at −300 mV at first, and then the higher frequency inductive arc changed into a capacitive arc at −250 mV. Based on the impedance display of iron at various potentials, a reaction model involving two adsorbed intermediate species was proposed, in terms of which the impedance behavior at different potentials were described. Occurrence of the pitting corrosion in active dissolution region was explained.     

To the Tafel slopes at the anodic dissolution of iron in sulfuric electrolytes

Based on the hypothesis of the reversibility of an iron electrode, one can relatively easily explain the experimentally obtained Tafel slopes without multistage complex schemes involving hypothetic intermediate iron hydroxo compounds. On the other hand, the coincidence of the experimental slope with the slope following from the relation derived may be considered as a proof of the reversibility of the iron electrode. The Tafel slope is reproducible both on the clean and partially passivated iron surface. Original Russian Text ? Yu.P. Vishnevskaya, D.A. Tkalenko, M.V. Byk, V.A. Rupp, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 5, pp. 540–542.     

Steady-state anodic dissolution of iron in neutral and close-to-neutral media

In neutral and close-to-neutral solutions, which contain no surface-active substances, iron dissolves via a two-step scheme. The transfer of the first electron across the interface involves water molecules that dissociate during the adsorption; the transfer of the second electron limits the process under steady-state conditions. In parallel, a passivator, namely, adsorbed oxygen is formed via a similar scheme. The passivator is removed from the surface due to its chemical reaction with hydroxonium ions, water molecules, or hydroxide ions. The process is adequately described by a mathematical model based on an assumption that the metal dissolves from an energy-uniform surface free from passivating species. Original Russian Text ? A.Yu. Aleksanyan, A.N. Podobaev, I.I. Reformatskaya, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 1, pp. 71–74.     

Electrochemical dissolution behaviour of iron in dilute sodium hydroxide solution

The anodic dissolution and passivation behaviour of pure iron in 0.1N sodium hydroxide solution has been investigated using a galvanostatic pulse technique. Analysis of the resulting chronopotentiograms has yielded information on the transfer overvoltage/dissolution current relationship and the passivation behaviour of iron in this environment. These results have been used to identify a possible reaction mechanism for the dissolution and have been related to current theories of stress corrosion cracking in this system.     

The effect of sorbed hydrogen on the kinetics of active dissolution of iron

The effect of hydrogen adsorbed or absorbed by iron (0.009% C) on the iron dissolution is studied on a bipolar electrode-membrane in 0.5 M SO ₄ ²⁻ solutions (pH 1.30) by cyclic potential pulses. Expressions that allow one to calculate the hydrogen coverage on the iron surface (θ) as a function of the potential variation in a cyclic stepwise manner and also the hydrogen concentration in the near-surface metal layer (C) as a function of variations in the intensity of the diffusion flow of hydrogen atoms in the membrane are given. The method of cyclic potential pulses together with the analysis of solutions for metal ions shows that the iron dissolution rate substantially decreases as θ increases. A bipolar electrode-membrane allowed the determination of the C intervals corresponding to the inhibition of iron dissolution (at C < C _c ≈ 3 × 10⁻⁸ g-at/cm³), the activating effect of hydrogen absorbed by the metal on the anodic process (for C > C _c), and the metal destruction (for C ≫ C _c). The absorbed hydrogen is assumed to accelerate the ionization of iron due to the formation of new dissolution sites as a result of plastic deformations of the metal. Thus, the effects of two forms of sorbed hydrogen on the iron dissolution are separated. Original Russian Text ? A.I. Marshakov, A.A. Rybkina, T.A. Nenasheva, 2007, published in Korroiya: Materialy, Zashchita, 2006, No. 5, pp. 2–14.     

The inhibition of the dissolution of iron in sulfuric acid by halide ions

The dissolution rate of mild steel in 1 N H₂SO₄ without and with different additions of Cl^?, Br^? and I^? has been measured. Analysis of the results indicates that halide ions adsorbed on the metal surface inhibit the dissolution reaction. Adsorption occurs according to the Frumkin isotherm.     

Anodic dissolution of iron and cobalt germanides in an alkaline electrolyte

Anodic dissolution of iron and cobalt germanides and their eutectic alloys with germanium in 1 M NaOH was studied by electrochemical methods and microprobe analysis. In anodically etching the FeGe₂−Ge and CoGe₂−Ge eutectic alloys, it was the Ge phase that predominantly dissolved in the alkaline solution, while the FeGe₂ intermetallic phase was found to be more stable than both Ge and a pure iron germanide. The anodic dissolution of FeGe₂ revealed a passive range (which is absent in acidic solutions) attributed to the oxidation of iron, probably, to γ-Fe₂O₃.     

Localised dissolution of iron in buffered and non-buffered chloride containing solutions

Pitting corrosion of pure iron was studied by using conventional samples, and artificial pit electrodes. Experiments were conducted in solutions of 0.01, 0.1 and 1 mol dm⁻³ NaCl at pH values of 7, 10, 11 and 12; and in borate buffer solutions with the same chloride concentrations and pH 8.7 and 9.2. Four times higher concentration of borate salt was required to reach inhibiting capacity of the hydroxyl anions, as determined via pitting potentials. From measurements of solution resistance, the increase in local conductivity due to dissolved corrosion products exuded from the pit was calculated for each bulk solutions. For the artificial pit electrodes, contribution of the borate species to the internal pit solution conductivity in low chloride solution was associated with the difference between transition potential, E_T, values for buffered and non-buffered solutions, and this contribution was also used to explain the non-linear dependence of E_T on [Cl⁻]. Further analysis was conducted using the concept of the critical i.x parameter for stable pit propagation.     

Inhibition of the anodic dissolution of iron in alkaline solution by metal complex ions

The possible inhibiting effects of GaO₃^3?, GeO₃^2?, CrO₃^2?, and MoO₄^2? ions on anodic and cathodic reactions on iron in 5 M KOH were studied. It was shown that a concentration of these ions of 10^?3 M inhibits the anodic iron dissolution reaction 2–3 times, while no effect on the hydrogen evolution reaction was observed. The effect was explained by the underpotential deposition of adatoms of the metal complexing ions on the iron electrode.     

On inhibiting the iron anodic dissolution in acid sulfate electrolyte by tetrabutylammonium cations

The surface coverage with atomic hydrogen (θ_H) is calculated for iron electrode as a function of the number of adsorbed tetrabutylammonium cations and hydrogen concentration in the metal phase (C _H). It is shown that the effect of the organic inhibitor of acid corrosion on the rate of iron potentiostatic dissolution is due to the increase in the θ_H/C _H ratio. Original Russian Text ? A.I. Marshakov, T.A. Nenasheva, A.A. Rybkina, M.A. Maleeva, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 1, pp. 83–89.     

Anomalous surface morphology of iron generated after anodic dissolution under magnetic fields

The effects of applied magnetic fields on the anodic dissolution of iron in a sulfuric acid solution were studied by potentiodynamic polarization measurements and electrode morphology observations. Uneven anodic dissolution occurs in the presence of magnetic field and the extent of the electrode surface inhomogeneity increases with magnetic flux density. Severely local dissolution at two edge areas of the iron electrode in the presence of magnetic fields is caused by the inhomogeneous distribution of the magnetic flux density at the ferromagnetic iron electrode and the resultant enhancement of the mass transport rate of interfacial film at local areas.     

Polyaniline-coated iron: studies on the dissolution and electrochemical activity as a function of pH

Polyaniline coatings were electrodeposited from an oxalic acid solution onto iron and their electrochemical activity and corrosion protection properties studied as a function of pH. It was found that the coating (emeraldine salt) had a limited effect on the corrosion protection of iron in acidic solutions. However, in an alkaline borate solution, where the conducting polyaniline was converted to the emeraldine base, the coating had a clear beneficial effect on the local breakdown of iron by chloride anions; much higher pitting potentials were recorded following a 2 h immersion period for the polyaniline-coated substrate relative to the uncoated electrode. Relatively small anions, such as acetates, nitrates and borates, were transported readily across the polymer interface. However, the emeraldine base inhibited the transport of the much larger ethylenediamine tetraacetate (EDTA) species to the iron interface, preventing complexation of the iron by EDTA.     

Peculiar effect of chloride ions on the anodic dissolution of iron in solutions of various acidity

Anodic behavior of iron in chloride solutions is studied in the pH range from 0 to 6 and at the concentration of chloride ions from 0.2 to 3 mol/l. Electrochemical measurements were supplemented with the raster electron microscopy studies, of electrode surfaces. The observed independence of the limiting dissolution current of pH is explained by a hypothesis that water does not participate in the anodic process at sufficiently large anodic overpotentials. Depending on the concentration of chloride ions, the continuity of the logi _a-pH dependence is first interrupted and the, restored; the reaction order in hydroxide ions changes from a positive value to zero. This is probably caused by the “dynamic passivation” of iron with the adsorbed oxygen atoms and their competitive replacement by chloride ions.     

To the co-effect of impurity atoms and structure on the dissolution of iron and its low alloys

With the use of tracers, the separate effect of small nickel additions, as well as phosphorus and manganese impurities (including their combinations with sulfur) on the corrosion electrochemical behavior of active iron is studied. Nickel (2.2%) and phosphorus (only 0.07%), which are substantially more corrosion resistant than iron, are found to noticeably suppress the active dissolution of iron from its alloys, whereas the unstable manganese (0.43%) and its sulfides (in an alloy containing 0.43% Mn and 0.06% S) accelerate the dissolution. An effect similar to that of manganese is produced by a simple increase in the defectiveness of the iron crystalline structure. The largest deceleration is observed at a small surface coverage of the dissolving alloy with nickel or phosphorus, whereas the activating effect of manganese and its sulfides is accompanied by their selective transfer to the solution. With an increase in the potential, both effects decrease in magnitude. Generalizing these and other data on the effects of impurities and structural defects on the active dissolution of iron made us reveal the substantial effect of the metal purity and its surface defectiveness (including that induced by a potential increase) on the steady-state kinetics of the process. All the regularities of the effect theoretically follow from the crystal-chemistry concept of dissolution.Translated from Zashchita Metallov, Vol. 41, No. 2, 2005, pp. 141–148.Original Russian Text Copyright © 2005 by Plaskeev.     

Effect of atomic hydrogen on the anodic dissolution of iron in a weakly acidic sulfate electrolyte

Using a membrane electrode, atomic hydrogen is shown to decelerate the dissolution of iron in sulfate and sulfate-citrate electrolytes (pH 5.5) in a potential range of the active metal dissolution and accelerates the process at the prepassivation potentials. Impedance spectra of iron at a controlled degree of surface coverage with hydrogen atoms are recorded. Rate constants of elementary stages of the anodic process are calculated and the reaction scheme of the iron dissolution in sulfate environments is made more accurate.