The Effect of Sorbed Hydrogen on the Dissolution of Iron in a Thiocyanate-Containing Sulfuric Acid

With the cycling of potential step, the dissolution rate of iron is shown to decrease with an increase in the degree of its surface coverage with hydrogen atoms. With a bipolar electrode (a membrane), an increase in the concentration of hydrogen in the metal phase was found to decelerate the anodic dissolution of iron in a base sulfate electrolyte and accelerate it in the presence of thiocyanate ions. The latter phenomenon indicates the possibility of activating the ionization of iron with the hydrogen dissolved in it. The rate constants of individual stages of the cathodic evolution of hydrogen are calculated.     

Determining the partial currents of silver ionization, its oxide formation, and chemical dissolution by multicycle chronoammetry with an RRDE

A multicycle chronoammetry with a rotating disc electrode with a ring (RRDE) enables one to experimentally discriminate between the partial currents of the substrate metal ionization, anodic formation of the oxide, and chemical dissolution of the oxide in the summary polarization current of the disc. The technique is approved by an example of Ag|Ag₂O|OH⁻(H₂O) system. In a range of relatively small anodic potentials of the Ag disc (0.48 to 0.51 V), the active dissolution of silver at the open surface sites and via pores in the surface film dominates; the phase formation current and, accordingly, the current efficiency of the process rapidly drop. At the potentials of the voltammogram maximum (0.52 to 0.53 V) when the silver active dissolution current is suppressed, the phase formation currents prevail and substantially exceed the chemical dissolution rate of the oxide. The thickness of an Ag₂O film rapidly increases under these conditions, and the current efficiency of the oxide formation is close to 100% for the whole polarization period. The rate constant of the chemical dissolution of an Ag(I) oxide is practically independent of the anodic phase-formation potential, but slightly depends on the oxide film thickness, reflecting changes in the film structure and, possibly, in its composition, from AgOH to Ag₂O. Original Russian Text ? D.A. Kudryashov, S.N. Grushevskaya, A.V. Vvedenskii, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 3, pp. 321–330.     

The corrosion behaviour of Al-cast iron in sulfuric acid

The polarisation behaviour of Al-cast iron has been determined in sulfuric acid at 25°C and has been compared to the behaviour of classical cast iron and commercial steel in the same environment. It was concluded from indirect electrochemical measurements (δE_corr/δpH and δlog i_corr/δpH) that the metal dissolution reaction was independent of pH under the given experimental conditions, and that b_A = 0,05 V for the three alloys. b_C changes from 0,116 V on steel to 0,305 V on classical cast iron and ∞ on Al-cast iron. The B values are respectively 0,015 V; 0,019 V and 0,024 V 0,024 V.     

Inhibiting the anodic dissolution of a steel with polydiallyldimethylammonium chloride and acrylamide copolymers with trimethylammoniumethylacrylate chloride and sodium acrylate

Polydiallyldimethylammonium chloride, as well as acrylamide copolymers with trimethylammoniumethylacrylate, which are industrial flocculants, are tested as corrosion inhibitors on St45 steel. The inhibitor properties are manifested by polydiallyldimethylammonium chloride and acrylamide copolymer with trimethylammoniumethylacrylate chloride. These are determined by the adsorption of polymer molecules on the metal surface. The protective effect of inhibitors is estimated. A mechanism of the adsorption of the surface-active substances on an iron electrode surface is proposed. Original Russian Text ? M.V. Boiko, E.E, Akimova, S.B. Bulgarevich, V.A. Fevraleva, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 6, pp. 669–672.     

Phase nanotomography in the superficial layers

A method of phase nanotomography and spectral multiplexing for a nondestructive monitoring of the nanoscale disturbance of refractive index distribution across the nonuniform near-electrode layer at a metal/solution interface is analyzed. It is shown that the amplitude Fourier-spectroscopy of reflection from the nonuniform near-electrode layer with a weak (nanoscale) disturbance of the complex refractive index profile, and weak optical parameters dispersion makes one realize the multiplex principle of measuring and reconstructing the refractive index (and hence, the concentration) distribution across the near-electrode solution layer by using either interference pattern or additional Fourier-transform. Combining the multiplex reconstruction of a three-dimensional profile of the dissolved-products layer and the standard Fourier-spectroscopy allows in situ controlling both the metal dissolution intensity at any point of its surface and the metal dissolution product composition over each section of the near-electrode layer. Original Russian Text ? V.A. Kotenev, D.N. Tyrin, A.Yu. Tsivadze, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 5, pp. 490–497.     

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.     

Korrosionsuntersuchungen zur Entwicklung von Dekontaminationslösungen für Nuklearanlagen. Teil I: Das Korrosionsverhalten verschiedener Werkstoffe der Kerntechnik in sauren und alkalischen Redoxlösungen

Investigation of corrosion in the development of decontamination solutions for nuclear systems. Part I: The corrosion behaviour of several nuclear materials in acid and alkaline redox solutions The corrosion behaviour of some nuclear materials has been investigated in two types of decontamination solutions – an acid and an alkaline redox system. In nitric cer(IV) solutions the metal dissolution of the low-alloy steel 21CrMo V 5 11 depends only on the concentration of the nitric acid. For the molybdenum alloy TZM as well as the stainless steel X10CrNiTi 18 9 and the nickel chromium alloy Nimonic 80 A, the metal dissolution depends on both the concentration of the nitric acid and the concentration of cer(IV). For 21CrMo V 5 11, the corrosion rates are two to four, and for TZM two to three orders of magnitude higher than for X10CrNiTi 18 9 and Nimonic 80 A. There is no danger of contact corrosion in galvanic couples between these materials. In alkaline permanganate solutions, the metal dissolution of the materials investigated are independent of the sodium hydroxide concentration. For 21 CrMo V 5 11, X10CrNiTi 18 9 and Nimonic 80 A the metal dissolution is not influenced by permanganate concentrations higher than 3 g/L. The corrosion behaviours of these materials are the same, and their corrosion rates are low. For the molybdenum alloy TZM, the metal dissolution increases with increasing permangate concentration; the metal loss is two orders of magnitude higher than for steels and Nimonic 80 A. In contact with the other more noble materials, the metal loss of TZM is increased by a factor of three to four by galvanic corrosion. In both types of solutions the dissolution of the metal is regular for the steels as well as for Nimonc 80 A, while it is uneven in the case of TZM.     

Dissolution of magnetite coupled galvanically with iron in environmentally friendly chelant solutions

The galvanic coupling between magnetite and iron in ethylenediaminedisuccinic acid (EDDS) solutions both with and without added iron(II) ions has been studied using electrochemical methods. The galvanic coupling accelerates the corrosion of iron due to the small shift in its potential in the anodic direction. At the same time, the potential of magnetite is cathodically polarized away from the potential range where the only faradaic process—the reductive dissolution of magnetite—takes place and results in a considerable decrease in its dissolution. Magnetite dissolves faster at the galvanic potential when [EDDS] ? [Fe²⁺] whereas iron is affected to a much lesser extent. The ratio between the rates of dissolution of magnetite at the galvanic potential to that at its steady state potential tends to decrease at higher temperatures and at higher EDDS and Fe²⁺ concentrations. This study shows that temperature plays a decisive role in the dissolution of magnetite coupled to iron. To completely remove it from the iron surface, high temperatures should be used. Corrosion inhibitors have to be considered when high losses of the base metal cannot be tolerated.     

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.     

The Anomalous Dissolution of Iron As a Result of the Chemical Conjugation between Iron Ionization and Hydrogen Evolution

Based on an idea about common intermediate particles, a physicochemical model of iron dissolution and hydrogen evolution is proposed. According to the model, the independence of the dissolution rate of a cathodically polarized metal of the potential is a particular case of chemically conjugate anodic–cathodic reactions. Parameters of the model calculated with the use of kinetic diagrams are compared with the experimental values. Assumptions which provide a satisfactory agreement between the calculated and experimental data are formulated. Equations describing the dissolution of iron both in the anodic and cathodic ranges are obtained.     

Critical potential of the surface development of Ag-Au alloys

Based on the model taking into account the appearance of a morphological instability and the evolution of a surface with time during the selective anodic dissolution of a homogeneous A-B alloy with the predominant content of the electrochemically negative metal A, equations for the critical potential E _cr and overpotential η_cr of the surface development are derived. Criteria of the nature of chemical hindrances of the selective dissolution in the over-critical range are formulated. The E _cr and η_cr values corresponding to the breakage of the morphological stability of the A-B alloy surface (with the atomic silver part X _Ag = 0.65 to 0.95) in an acidic nitrate solution containing diverse amounts of silver ions (10⁻⁴ to 10⁻² M) are experimentally found. The character of the effect of the atomic gold content in the alloy and the concentration of silver ions in the solution on η_cr and E _cr values of the alloys is determined. The nature of the kinetic limitations of the selective dissolution in the over-critical potential range is clarified. Original Russian Text ? A.V. Vvedenskii, O.V. Koroleva, O.A. Kozaderov, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 1, pp. 28–37.     

The Effect of Adsorbed Atomic Hydrogen on the Electrochemical Dissolution Kinetics of Iron

A functional dependence of the potentiostatic dissolution rate of iron on the metal surface coverage with hydrogen () is found. An increase in the anodic current with time after the potential jump is related to the decrease in . A potential dependence of the dissolution rate of iron at = const is obtained. The high Tafel slope of this polarization curve shows that the first one-electron stage of iron ionization is substantially irreversible.     

Microbial oxidation of refractory gold sulfide concentrate by a native consortium

A defined mesophilic consortium including an iron oxidizing bacterium and a sulfur oxidizing bacterium was constructed to evaluate its ability for bioleaching a flotation concentrate from Andacollo mine in Neuquén, Argentina. Experiments were performed in shake flasks with a pulp density of 10% (w/v), using a basal salt medium containing ferrous iron at pH 1.8. The leaching solutions were analyzed for pH, redox potential (using specifics electrodes), ferrous iron (by UV-Vis spectrophotometry) and metal concentrations (by atomic absorption spectroscopy). The results showed that the consortium was able to reduce the refractory behavior of the concentrate, allowing 91.6% of gold recovery; at the same time, high dissolution of copper and zinc was reached. These dissolutions followed a shrinking core kinetic model. According to this model, the copper solubilization was controlled by diffusion through a product layer (mainly jarosite), while zinc dissolution did not show a defined control step. This designed consortium, composed of bacterial strains with specific physiological abilities, could be useful not only to optimize gold recovery but also to decrease the leachates metallic charge, which would be an environmental advantage.     

Mutual influence of portions of simple and binary inhibitors in the acid corrosion of iron

The effects of inhibitors as power functions of their concentrations were examined analytically and graphically. The mutual influence of their portions of the was estimated. The behavior of the inhibitors in solutions of H₂SO₄ and HClO₄ was compared. The mechanism of the corrosion inhibition was proposed. It was found that the additives influence each other in the inhibition of iron dissolution and hydrogen depolarization. Original Russian Text ? V.V. Ekilik, G.N. Ekilik, A.G. Berezhnaya, E.N. Balakshina, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 6, pp. 645–649.     

Einfluß des Metallsubgefüges auf den anodischen Auflösungsmechanismus von Eisen

Influence of the metal substructure on the anodic dissolution mecanism of iron The electrochemical behaviour of iron shows a pronounced dependence on the metal substructure. The surfaces formed at the rupture of the specimens are dissolved according to a non-catalytic mecanism while a heat treatment of these rupture surfaces produces a change in the mechanism so that the dissolution occurs according to a catalytic mechanism. The two mechanisms are characterized by a rather different dissolution potentials; these differences can be attributed to the state of deformation of the crystal latice.     

Passivation and local activation of α-Fe + Fe3C-based nanocomposites in neutral media

Regularities of the anodic dissolution and processes of local activation of α-Fe + Fe₃C nanocomposites containing 9 to 92 mol % of cementite are studied. It is shown that the sequence of anodic processes in borate solutions is the same for pH 6.3 and 7.4, namely, the dissolution and passivation of the ferrite component, dissolution and passivation of the cementite component, and anodic oxygen evolution at the nanocomposites’ passive surface. With an increase in the pH the passivation potentials decrease, which agrees with a thermodynamic model of the formation of two-layer passive film Fe₃O₄/γ-Fe₂O₃. It is found that the chloride concentration range, in which the local activation of α-Fe + Fe₃C is possible, narrows with an increase in the cementite content. Original Russian Text ? A.V. Syugaev, S.F. Lomaeva, S.M. Reshetnikov, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 1, pp. 58–64.     

The effect of atomic hydrogen on the kinetics of iron passivation in neutral solutions

Current-time dependences are recorded on iron in borate buffer (pH 7.4 and 6.7) and its mixture with NS4 solution (pH 6.7) at the potentials of passivity, active-passive transition, and prepassivation of iron. Hydrogenation of the metal is found to accelerate the dissolution of iron in steady passive state, produce no effect on the growth of a barrier layer, and prevent the formation of a primary passivating film. Atomic hydrogen decelerates the active dissolution of iron, which determines the anodic current at the initial stage of the metal passivation.     

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.     

Anodic dissolution of the copper-nickel alloy under transient conditions

Anodic dissolution of an electrolytic copper-nickel alloy in 1 M HCl was studied by nonequilibrium electrochemical methods. The dissolution of the alloy began with selective ionization of nickel. Then the rate of the anodic dissolution was limited by nonsteady-state bulk diffusion. Original Russian Text ? V.N. Tseluikin, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 5, pp. 556–558.     

SIZE DISTRIBUTION AND BEHAVIOUR OF VC PARTICLES IN FERRITIC NODULAR CAST IRON

Size distribution and precipitation of VC particles in ferritic nodular cast iron havebeen examined by techniques of ultrasonic screening,small angle X-ray scattering andchemical dissolution.The VC particles are sized over a wide range.The superfine ones,dispersed in the ferritic matrix,are the majority.A saturated dispersion with ultrafineVC particles may be approached as the V content increasing in the iron.Thi5 seems toplay an important role in the precipitation strengthening for ferritic nodular cast iron.