Atomic-topographic model of the electrochemical dissolution of a solid metal taking into account the effect of point lattice defects

A topokinetic model of a dissolving solid metal containing point defects, that is, either admixture atoms of a substantially more lowly or highly corrosion-resistant metal or the crystal lattice vacancies, is proposed. The model takes into account the mutual consistency of the atomic relief and the dissolution rate. Similarly to a defect-free crystal, the steady-state dissolution rate is determined by the elementary acts of transferring the base atoms to solution from step kinks whose amount substantially depends on the concentration of the point defects. Original Russian Text ? Yu. V. Alekseev, G.Yu. Alekseev, I.Yu. Alekseev, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 5, pp. 479–487.     

Passivating film as a structured solid solution of the alloy oxide particles. The structuring effects (Ordering and Stratifying)

The investigations deal with the passivating films that can be modeled as a solid solution of the alloy oxides. Models of unstructured (regular) solutions enable one to explain the dependence of the dissolution rate of a Fe-Cr alloy at a chromium concentration higher than 0.2 atomic parts. Irregular solution models, which take into account the alloy structuring due to the interactions between its components is applicable to the compositions below a chromium content of 0.2. The physical effects predicted by the model are studied, namely the higher dissolution rate of the alloy compared to the dissolution rate of the most active component, the different alloying mechanisms at the different kinds of interactions between the components, as well as the kinetic metastability and its role in the initiation of pitting by some admixtures. Original Russian Text ? Yu.V. Alekseev, I.Yu. Alekseev, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 4, pp. 346–359.     

Essential shortcomings of Tafel equations in describing electrochemical dissolution kinetics of a solid electrode

A simplified topokinetic dissolution model of a solid taking into account the correlation between the dissolution rate and the atomic surface topography enables one to interpret a linear (in semilogarithmic coordinates) segment of the steady-state curves of the active anodic dissolution of metals. Its slope corresponds to the product of a concentration of kinks and the elementary reaction rate, both of which exponentially depend on the potential, rather than a simple (Tafel) elementary rate of the atom dissolution at a semi-crystal kink. Original Russian Text ? Yu.V. Alekseev, G.Yu. Alekseev, V.A. Bityurin, A.V. Plaskeev, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 6, pp. 568–573.     

2D Atomic-Topological Model of the Dissolution Kinetics of a Metallic Crystal: Typical Atomic Relief Positions and Fragments

The atomic-topological kinetics problem of the self-consistency between the dissolution kinetics and the atomic relief of the dissolving boundary is solved for an imaginary two-dimensional crystal in two ways: (i) in terms of the model of atomic positions determined by the number of the nearest neighbors and (ii) in terms of the typical atomic relief structures (steps, kinks, etc.). To a sufficient accuracy, the results provided by these two models coincide. The approaches developed are aimed at solving the problem of the dissolution of an actual three-dimensional crystal.__________Translated from Zashchita Metallov, Vol. 41, No. 3, 2005, pp. 244–257.Original Russian Text Copyright © 2005 by G. Alekseev, Yu. Alekseev, Bityurin.     

Development or Stagnation of the Theory of Metal Passivity

Methods and approaches of physicochemical modeling, as well as the adequacy of the mathematical apparatus used for constructing two variants of the theory of a passively dissolved metal state are compared. Their essential differences are noticed only as a background, while their contents are discussed chiefly in view of their applicability to solving the problem formulated. The first approach presented in the works of Yu.A. Popov and coworkers declares the adherence to the theory of K. Fetter based on the hypothesis about the equilibrium oxygen exchange between the oxide and solution. The second approach developed in the kinetic-electrostatic model of Ya.M. Kolotyrkin and Yu.V. Alekseev rejects this hypothesis and assumes that of all the processes in the domain between the metal and solution are equally important. For disproving our concept, the opponents suggest a “developed Fetter's model.” The results obtained in this approach and their adequacy are the main object of this investigation. __________ Translated from Zashchita Metallov, Vol. 41, No. 5, 2005, pp. 528–543. Original Russian Text Copyright ? 2005 by Alekseev.     

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.     

Dissolution mechanism and partial electrode processes at infralow frequency AC polarization of brasses in chloride media. I. α-brass Cu20Zn

Being typical of a steady-state anodic polarization, the uniform dissolution of α-brass Cu20Zn is retained for the anodic half-period in the pulsed mode (f = 0.17 Hz). At such an ac polarization, the dissolution is alternated with the copper deposition in the cathodic half-periods to form a loose fine-grain deposit at the electrode. Under severe hydrodynamic conditions, no copper deposit is formed. With an increase in the current in the cathodic half-periods, the pH of a near-electrode solution layer increases, thus changing the anodic processes. Namely, the oxidation of brass components to soluble chloride complexes gives place to the formation of poorly soluble products. Original Russian Text ? I.K. Marshakov, O.Yu. Kuksina, V.Yu. Kondrashin, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 2, pp. 128–134.     

The effect of chloride and sulfate anions on the iron dissolution rate in neutral and nearly neutral media

Analysis of results of steady-state potentiostatic measurements on iron in the neutral and nearly neutral borate solutions is complicated by the fact that the metal is in the state of active-passive transition near the free-corrosion potential. This difficulty is overcome using the method of neural network simulation. The study of the behavior of neural-network model shows that chloride and sulfate ions can promote or inhibit iron dissolution depending on their concentration and the pH value of solution. The ambiguous effect of anions is explained by the fact that, when they are adsorbed at the metal, they prevent its cations from passing to the solution, and, entering into the metal-water adsorption complexes, they assist the decomposition of complexes thus promoting the dissolution. Sulfate ions, compared with chloride ions, are, probably more adsorption-active with respect both to iron and to water adsorbed on it. Original Russian Text ? A.Yu. Aleksanyan, I.I. Reformatskaya, A.N. Podobaev, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 2, pp. 135–138.     

Character of the dissolution and partial electrode processes at the alternating-current infralow-frequency polarization of brasses in chloride media. II. Cu44Zn β-brass

The dissolution of β-brasses in a chloride medium at a pulse anodic polarization proceeds at first (up to ∼1 min) nearly uniformly, but when the charge of anodic pulse is carried out partly on the ionization of zinc and partly on the ionization of copper, it becomes selective. In this case the most part of the copper component does not oxidize, but rearranges to form the individual phase, which causes a substantial destruction of the brass electrode. When the cathodic alternating-current component is also switched on and hydrogen evolution begins, pH of the layer near electrode increases preventing the formation of soluble oxidation products in the subsequent anodic half-period. Thus, on a β-brass electrode, the partial cathodic processes affect the anodic oxidation of copper and zinc. Original Russian Text ? I.K. Marshakov, O.Yu. Kuksina, V.Yu. Kondrashin, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 4, pp. 360–366.     

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.     

Statistical aspect of the independence principle of heterogeneous reactions taking into account the surface state

The independence principle of electrode reactions is shown to be no more than a particular example of the theoretical-probabilistic regularities determined by the statistical nature of a large scope of systems studied by the chemical kinetics. It does not exclude but explains the possible mutual effect of different reactions on each other. In the case of a mass exchange between a structured solid and a solution or a gas, the independence principle should be formulated in terms of the superficial atom positions determined by the number of close neighbors. Original Russian Text ? Yu. V. Alekseev, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 4, pp. 339–347.     

Scanner-assisted quantitative reflectometry in the checking of corrosion-electrochemical systems

The described method for the study of the metal-electrolyte interface combines videoreflectometric and electrochemical measurements. Scanned digital optical images of a surface area in an electrolyte solution made it possible to examine a 3D structure of spatially heterogeneous surface layers composed of dissolution products of metals and alloys in liquids. Parallel electrochemical measurements allowed one to monitor the anodic and cathodic processes during the dissolution of specimens. The combined method was found to be highly sensitive to thin heterogeneous near-electrode layers, provide highly efficient corrosion checking of long specimens, and allow parallel studies of a large number of specimens. The method was tested in the in situ checking of the microtopography of a layer of products formed in the dissolution of a Cu-Ni alloy in a solution of NaCl. Original Russian Text ? V.A. Kotenev, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 5, pp. 534–539.     

Structural characteristics and catalytic activities of nanocrystalline Ni-Mo-B coatings obtained by catalytic electroless reduction

The structures, chemical states of elements, and catalytic activities of Ni-Mo-B alloys with different molybdenum contents, which were obtained by catalytic electroless reduction of metal ions, were studied. The rates of the partial reactions (heterogeneous hydrolysis of dimethylamine borane, reduction of nickel ions, and evolution of molecular hydrogen) were found to make a bell-shaped curve when plotted versus the concentration of molybdate ions in solution. Original Russian Text ? V.M. Krutskikh, M.V. Ivanov, A.B. Drovosekov, E.N. Lubnin, B.F. Lyakhov, Yu.M. Polukarov, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 6, pp. 619–625.     

The corrosion of welded joints of 16GS + 08Kh13 bimetal, made of the specimens cut out from a column after the long-term operation

Corrosion-electrochemical investigations into bimetals are conventionally carried out on samples with the carrying layer preliminarily removed. In this work, the liability of welded joints to HSC without removal of the carrying layer is investigated, which has considerably enhanced the information content of the results. A possibility is demonstrated of the application of ANV-70B electrodes and Sv-07Kh25N13 welding wire, developed at the Paton Electric Welding Institute, in the process of welding in an Ar + O₂ gas mixture for repairing equipment manufactured from bimetal equipped with a cladding layer of 08Kh13 steel. It is established that the preliminary heating of the bimetal up to 350°C in the welding process with the application of both ANV-70B electrodes and Sv-07Kh25N13 welding wire enhances the resistance of the metal to hydrogen sulfide corrosion cracking. Original Russian Text ? V.A. Kachanov, Yu.B. Danilov, T.E. Shepil, E.K. Gvozdikova, V.Yu. Kozin, A.I. Kabashnyi, S.M. Ivanuna, K.A. Yushchenko, L.V. Chekotilo, A.V. Bulat, 2007, published in Korroziya: Materialy, Zashchita, 2007, No. 1, pp. 14–19.     

Multicycle chronoammetry of RRDE in the investigation of the anodic oxide formation

The method of multicycle chronoammetry of RRDE makes it possible to obtain separately the partial currents of metal electrode ionization, anodic oxide formation and chemical oxide dissolution. The method is tested for Ag∣Ag₂O∣OH⁻(H₂O) system. In the range of low anodic potentials (0.48 ÷ 0.51 V) the process of active silver dissolution prevails; the phase formation current rapidly drops. At higher potentials (0.52 ÷ 0.53 V) the phase formation current prevails and noticeably exceeds the rate of the chemical oxide dissolution. The thickness of Ag₂O film rapidly increases; and the net phase formation current is close to 100%.     

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.     

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.     

Determining the integral dissolution rate of two-phase alloys in cryolite-alumina melts

Electron microscopy and x-ray diffraction analysis are used in studying Cu-Fe and Ni-Fe-Al alloys upon their anodic polarization in a cryolite-alumina melt, thus, clarifying the regularities of their dissolution and the formation of oxide coatings. Based on the results obtained, a technique of determining the integral corrosion rate of the alloys is proposed. Original Russian Text ? A.Yu. Filatov, E.V. Antipov, M.I. Borzenko, S.Yu. Vasil’ev, V.M. Denisov, V.V. Ivanov, S.M. Kazakov, Z.V. Kuz’minova, V.K. Laurinavichyute, V.V. Lunin, D.A. Simakov, V.I. Shtanov, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 6, pp. 664–668.     

Thermodynamic Theory of “Irregular” Solutions and Kinetic Model of the Passive Alloy Dissolution

Thermodynamic model of an irregular solution developed earlier for small mixing parameter values (nearly regular solution) is generalized for arbitrary (real) values of the parameter. In this case, an interaction between components results in a chemically nonuniform solution, i.e., in the formation of micrograins of diverse composition, which increase in the size with an increase in the mixing parameter and a decrease in the concentration of one of the components. The passivating film on the alloy surface is modeled as a solid solution of oxide “molecules” of the components, which enables us to proceed from thermodynamics to the reaction rates theory and bring the theory in agreement with the data on the passive alloy dissolution. The chemical non-uniformity forces us to interpret the values referred to in the theory (component concentrations and dissolution rates) as statistically average, i.e., averaged over the surface of the specimen dissolved and over the time of experiment. However, this approach may be insufficient (when it is necessary to take into account fluctuations or deviations from the mean values) for example, under conditions when the surface nonuniformity may result in pit nucleation. __________ Translated from Zashchita Metallov, Vol. 41, No. 6, 2005, pp. 562–572. Original Russian Text Copyright ? 2005 by Alekseev.     

On the effectiveness of coatings obtained from film-forming polymers containing inhibitive functional groups

Experimental data on the influence of the film-forming polymer on the effectiveness of polymeric coatings on metals are discussed. Polystyrene, poly(butyl methacrylate), and copolymers of butyl methacrylate and styrene with 2-vinylpyridine (BMAVP and SVP) were used as film-forming polymers for the coatings. The correlation between the composition of the film-forming polymers, the properties of the coatings (adhesion and permeability), the coating functions (metal corrosion under the coatings), and the coating inhibition factor was studied. A criterion for the latter was proposed. According to the data obtained, the inhibited coatings prepared from the copolymers BMAVP-5, SVP-5, and SVP-10 (containing 5 and 10% VP) were most effective. Original Russian Text ? V.I. Zavrazhina, Yu.P. Gladkikh, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 1, pp. 108–112.