Corrosion-mechanical characteristics of the materials of nonmagnetic retaining rings of turbogenerators. III. Crack formation in the course of service

We analyze reasons for failures of rotors and retaining rings of powerful turbogenerators made of traditional materials and establish that corrosion cracking in working media and alternating stresses are the main reasons for failures. Holding specimens made of new 18Mn−18Cr steel under stress in hightemperature water and a 22% NaCl solution leads to transcrystalline corrosion cracking at a rate of 0.5–40·10⁻¹¹ m/sec. In the course of dissolution of high-nitrogen chromium-manganese steels in the electrochemically active state, their corrosion ratei _cor is 0.1–4 mA/cm². Alternating stresses in chloride-containing acidic media most strongly intensify selective dissolution of manganese and then iron and chromium, which is in agreement with results of an analysis of products of corrosion on actual retaining rings. We established that a saturated solution of copper dichloride is the most aggressive medium for high-nitrogen chromium-manganese steels. Even short-term contact of the steel with this solution at room temperature leads to intense pitting, while long-term contact results in catastrophic intercrystalline cracking at a rate of 1.56·10⁻⁹ m/sec. In subsequent stages, this contact causes selective dissolution of the steel and pitting corrosion. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 2, pp. 115–122, March–April, 1998     

A study on electrodeposited Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> alloys

Zn_1−x Fe _x alloys were electrochemically deposited on AISI 4140 steel substrates from sulfate bath. The bath was consisted of 40 g dm⁻³ ZnSO₄·7H₂O, 20–40 g dm⁻³ FeSO₄·7H₂O_, 25 g dm⁻³ Na₃C₆H₅O₇, and 16 g dm⁻³ H₃BO_3. The effect of bath composition on the electrical resistivity, the phase structure, and the corrosion behavior were investigated by the current–voltage measurements versus temperature, the X-ray diffraction (XRD) analysis, the atomic absorption spectrometry analysis, and the polarization measurements, respectively. Iron content was shown to strongly affect the structure, the electrical resistivity and the corrosion stability of Zn–Fe alloys.     

Corrosion behavior in SBF for titania coatings on Mg–Ca alloy

TiO₂ coating was obtained by sol–gel method to improve the corrosion resistance of Mg–Ca alloy in human body environment. The corrosion behavior of Mg–1.0 Ca alloy with TiO₂ coating was investigated by electrochemical tests and immersion tests in simulated body fluid (SBF). Bare Mg–1.0 Ca alloy suffered serious attack after immersed in simulated body fluid only for 48 h. While for the Mg–1.0 Ca alloy with TiO₂ coating, the surface almost maintained intact with only several collapses after immersed in SBF for 168 h. The electrochemical test results showed that the free corrosion current (i _corr) of Mg–1.0 Ca alloy substrate was 3.3275e−2A/cm², while the i _corr of TiO₂ coating was only 1.58549e−5A/cm². Therefore, TiO₂ coating significantly improved the corrosion resistance of Mg–1.0 Ca alloy in SBF. This enhances the potential of Mg–Ca alloy used as biodegradable orthopedic material.     

Studies on electrodeposition of corrosion resistant Ni–Fe–Mo alloy

A study to optimize the process parameters for electrodeposition of a Ni–Fe–Mo alloy is reported. A 2²full factorial design was successfully employed for the experimental design analysis of the results. The optimum experimental conditions for producing the corrosion resistant alloy were 120 mA/cm² current density, 20 rpm cathode rotation, 9.0 pH at 30 °C. The alloy was deposited at 61% current efficiency, with an average composition of 62 wt% Ni, 17wt% Fe, 21wt% Mo and traces of boron, and with E _corr −0.506 V, R _p 8.883 × 10³ Ohm cm² and I _corr 6.468 × 10⁻⁷ A/cm². The deposit obtained under these conditions had an amorphous character, good adherence, high corrosion resistance and a nodular morphology. Electrochemical corrosion tests verified that the electrodeposited Ni–Fe–Mo alloy had better corrosion resistance than the Fe–Mo alloy.     

An electrochemical evaluation on the crevice corrosion of 430 stainless steel by micro capillary tubing method

The aim of this study was to investigate the initiation of crevice corrosion for ferritic 430 stainless steel in artificial crevice electrode cells using the IR drop mechanism. The 430 stainless steel artificial crevice electrodes were potentiodynamically polarized in solutions of sodium chloride with different concentrations. The potentiostatic polarization was measured for various artificial crevice sizes by measuring the potentials in the crevice by the depth profile technique using a micro capillary tube which was inserted into the crevice. The criterion for IR>Δ Φ^*, where Δ Φ^* is the difference between the applied potential, E_SURF, and the electrode potential of the active/passive transition, E_A/P, was also measured during the process of crevice corrosion. The potentials in the crevice were successfully measured from −220 mV versus SCE to −360 mV versus SCE, which is lower than that of the external surface potential of −200 mV versus SCE. Thus these results show that evaluation of corrosion using the IR drop mechanism in the crevice was more objective and easier to reproduce than the existing methods.     

Modeling Thermal Conductivity of Electrolyte Mixtures in Wide Temperature and Pressure Ranges: Seawater and Its Main Components

A model has been established for calculating the thermal conductivity of aqueous electrolyte solutions containing the Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, SO₄²⁻, CO₃²⁻, HCO₃⁻, and Br⁻ ions. The model is based on a previously developed computational framework for the thermal conductivity of mixed-solvent electrolyte systems, which has been expanded by explicitly accounting for pressure effects in addition to temperature and electrolyte composition effects. The model consists of a contribution of the solvent, a contribution of individual species expressed using modified Riedel coefficients, and an ionic strength-dependent term that is due to interactions between species. The model accurately represents the thermal conductivity of solutions containing single and multiple salts at temperatures ranging from 273 K to 573 K, pressures up to at least 1400 bar, and concentrations up to the limit of solid saturation. Further, the model has been applied to seawater and used to elucidate the discrepancies between the experimental data for seawater and those for Na–K–Mg–Ca–Cl–SO₄ salt solutions. With parameters evaluated on the basis of data for binary and multicomponent salt solutions, the model provides reliable predictions of the thermal conductivity of seawater.     

Heterogeneous chemiluminescent assay (generalizing paper)

The results of applying heterogeneous chemiluminescent analysis for determination of micro-components are generalized, and the possibility of using this method for determination of toxic anions in waters is demonstrated. Anions undergo conversion beforehand, which causes formation of new analytical forms, i.e., derivates being active in chemiluminescent reactions with luminol. If volatile products are generated as analytical forms, they undergo gas extraction from the matrix with detection using a chemiluminescent indicator in the carrier gas stream at the interface gas-liquid (aqueous solution of luminol). Thus, the techniques for determination of Cl⁻, Br⁻, I⁻, ClO₃⁻, BrO₃⁻, IO₃⁻, NO₃⁻, NO₂⁻, S², CrO₄²⁻, AsO₂⁻, Cl₂ and chloramines are developed. The detection limit (DL) is 0.5–5.0 μg/l, and the analysis time is 2–9 min. In another case, products of conversion are sorbed from the solution on a solid sorbent, and detection is carried out by a luminol solution on the solid surface. A series of heteropoly acids (phosphorus, arsenic, silicon, and germanium) and their ion associates with cation surfactants, along with other large anions, are determined in the form of sorbable ion associates. The DL is 0.02–0.7 μg/l, and the analysis time is 30 min. The advantages of application of heterogeneous chemiluminescent analysis over analysis in homogeneous media are indicated, and the prospects for development of the theoretical bases and field of application are demonstrated.     

Effect of temperature on the interaction of ÉP823 steel with lead melts saturated with oxygen

We study the corrosion behavior of ferritic-martensitic éP823 steel in a static lead melt, saturated with oxygen, at 550 and 650°C. At these temperatures, a complex magnetite-base scale is formed on the surface of steel, but the mechanisms of its growth are different. At 550°C, corrosion has a cyclic character. On the surface of steel, a Fe_1+x Pb_2−x O₄−Fe_1+x Cr_2−x O₄ two-layer scale is formed periodically. Reaching the critical thickness (18 μm), it exfoliates along the interface with the matrix, to which oxygen-containing lead penetrates, whereupon this process is repeated. The corrosion rate is ∼0.08 mm/year. At 650°C, the intensification of reactions of formation of chromium spinel and plumboferrite induces the growth of a porous scale, where lead is accumulated. This scale has good adherence to the matrix and is formed as a compact conglomerate owing to the efficient mass transfer at all interfaces, which leads to a catastrophic rate of thinning of the specimen (3.82 mm/yr) in a lead melt. On the basis of experimental data, we propose schemes of the oxidation of chromium steels in a lead melt with a high oxygen activity at different temperatures. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 2, pp. 77–84, March–April, 2007.     

Phosphonium iodine as nickel corrosion inhibitor in 1 M sulfuric acid medium

The inhibiting action of alkyltriphenylphosphonium iodine salt ((C₈H₁₇)Ph₃P⁺,I⁻) towards the corrosion behaviour of nickel in 1 M H₂SO₄ solution has been studied. This compound was found to retard both anodic and cathodic reactions of nickel corrosion. At constant temperature, the corrosion rate decreases with increasing inhibitor concentration. On the other hand, the increase in temperature leads to an increase in the corrosion rate. The activation energy, ΔE _a, were calculated. They were found 19.3 kJ mol⁻¹ and 71.1 kJ mol⁻¹, respectively for the uninhibited solution and in the presence of 10⁻³ M of phosphonium salt. The inhibitor adsorption was identified to occur according to Langmuir isotherm. The equilibrium constant, k, as well as the free energy of adsorption, Δ_ads G°, for inhibitor process were then calculated. Phosphonium iodine exhibited a singular behaviour for T ≥ 318 K where inhibitor desorption increases.     

Use of high-purity AlxGa1−x as layers in epitaxial structures for high-power microwave field-effect transistors

The fabrication of high-purity layers of Al_xGa_1−x As solid solutions in the range 0⩽x⩽0.38 by molecular beam epitaxy is reported. The low-temperature photoluminescence spectra of these layers reveal predominantly the free exciton recombination line (X). The narrow width of the X line, the high intensity ratio of this line to that of the band-acceptor transition line, and the linear dependence of the X line intensity on the excitation power density in the range between 1×10⁻⁴ and 100V·cm⁻² indicate a low concentration of background impurities in these layers. Using this material in pseudomorphic AlGaAs/InGaAs/GaAs heterostructures for high-power microwave transistors produced devices with a specific saturated output power of 0.9 W/mm at 18 GHz. Pis’ma Zh. Tekh. Fiz. 25, 8–15 (August 12, 1999)     

Dynamics of Drop Coalescence on a Surface: The Role of Initial Conditions and Surface Properties

An investigation of the coalescence of two water drops on a surface is presented and compared with drop spreading. The associated capillary numbers are very low (< 10⁻⁵). The drops relax exponentially towards equilibrium. The typical relaxation time t_c decreases with contact angle. t_c is proportional to the drop size R, thus defining a characteristic velocity U^* = R/t_c. The corresponding U* values are smaller by many orders of magnitude than the bulk hydrodynamic velocity (U = σ /η, with σ the gas–liquid surface tension and η the viscosity). The dynamics of receding (coalescence) and spreading motion is found to be of the same order when coalescence or spreading is induced by a syringe. The dynamics of coalescence induced with the syringe deposition is systematically faster by an order of magnitude than condensation-induced coalescence. This disparity is explained by the coupling of the contact line motion with the oscillation of the drop observed for syringe deposition but absent for condensation-induced coalescence. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Atmospheric effects on the thermally induced sintering of nanoparticulate gold films

Gold (Au) films were formed by sintering of Au nanoparticles (NPs) under gas flows of air, oxygen (O₂), nitrogen (N₂), or N₂ bubbled through formic acid (FA/N₂). The microstructure changes of the Au nanoparticulate films were studied when different atmospheres were applied. The Au film sintered under FA/N₂ showed the progressive agglomeration and grain growth with porosity in the film, while the film sintered under N₂ had NPs without participating grain growth. A necking between NPs was observed in the film, however, unnecked NPs were still found. The Au film sintered under O₂ atmosphere showed the NPs agglomeration with various sizes up to 50 nm. X-ray characteristic peaks of the (111)-preferred orientation were observed in all samples. All samples showed N–H stretching at 3200–3300 cm⁻¹ regardless of sintering atmosphere. Hydrocarbon chains (C–H) at 2850–3000 cm⁻¹ were detected in the film sintered under N₂. For the Au film sintered under O₂, C–H stretching at 2850–3000 cm⁻¹, C–H deformation at 1350–1470 cm⁻¹, and C–O stretching at 1200–1300 cm⁻¹ were observed. C–O stretching at 1600–1700 cm⁻¹ was observed for the film sintered under FA/N₂ atmosphere. The electrical resistance of the film was related with microstructures and organic residual materials left in the film. Even though either porosity or carbon residues were observed in the film, the sintering of NPs in FA/N₂ or N₂ showed the sheet resistance comparable to that of electroplated one.     

Nanotechnology-associated coatings for aircrafts

Polymeric epoxy-based composites are modified with nanopowders of silicon oxide (∼ 100 nm). By the method of spraying, these composites are applied to specimens of 2024-T3 aluminum alloy preliminary treated with molybdate solutions to get conversion layers. Three types of polymeric coatings are considered: reference, treated by silica, and with additional polyurethane coatings. The aim of modification of polymeric coatings is to absorb and/or block unwanted ions/molecules (Cl⁻, O₂, OH⁻, H₂O, etc.) and improve the protective properties of the films. The tests carried out by the method of electrochemical impedance spectroscopy, in a salt-fog chamber, and by immersion in a 0.5 M NaCl solution reveal high anticorrosion characteristics of the coating. New coatings are promising for the corrosion protection in the aircraft industry. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 3, pp. 103–108, May–June, 2007.     

Surface reactivity and electrophoretic deposition of ZrO<Subscript>2</Subscript>–MgO mechanical mixture

Electrophoretic deposition (EPD) is a precision technique useful for obtaining high quality ceramic bodies with controlled dimensions and smooth coatings. The electrophoretic deposition rate is highly dependent on the surface chemistry of the powders, especially when dealing with multi-component systems. The objective of this work is to study the surface reactivity of both ZrO₂ and MgO in ethanol suspension to provide experimental benchmarks to control EPD of a ZrO₂–3 wt% MgO mechanical mixture (Z3M) in ethanol. Infrared spectroscopy (FTIR) showed that ZrO₂ surface spontaneously reacts with ethanol, generating negative electrophoretic mobility of the particles (−0.07 × 10⁻⁸ V⁻¹ s⁻¹) measured by Electroacoustic Sonic Amplitude (ESA). MgO surface also spontaneously reacted with ethanol, but a positive electrophoretic mobility was observed in this case (0.26 × 10⁻⁸ V⁻¹ s⁻¹). Scanning Electron Microscopy of Z3M dried from ethanol suspension showed that MgO particles were located around the ZrO₂ particles, forming composite agglomerates, probably due to the electrostatic attraction between MgO and ZrO₂ particles. Homogeneous deposits could be obtained from EPD of Z3M ethanol suspensions. Mercury intrusion porosimetry showed that the ZrO₂–MgO green deposited bodies using different voltages had similar pores diameters distributions, indicating that the ZrO₂–MgO agglomerates are not affected by the increasing deposition rates.     

Composition effect on the catalytic activity and electrical conductivity of cobalt oxide compounds

This work examines the catalytic activity and electrical conductivity of electrodeposited nonstoichiometric cobalt oxide compounds. Their composition is Co, 47–53; H₂O, 10–18; and OH⁻ groups, 8–12%. The content of OH⁻ groups is shown to determine their catalytic activity and ionic conductivity. These compounds are potential electrode materials for the reduction of oxygen.     

Gibbs’ energy of formation of YBa2Cu3O7-x (tetragonal)

The high temperature ceramic oxide superconductor YBa₂Cu₃O_7-x (1–2–3 compound) is generally synthesized in an oxygen-rich environment. Hence any method for determining its thermodynamic stability should operate at a high oxygen partial pressure. A solid-state cell incorporating CaF₂ as the electrolyte and functioning under pure oxygen at a pressure of 1·01 × 10⁵ Pa has been employed for the determination of the Gibbs’ energy of formation of the 1–2–3 compound. The configuration of the galvanic cell can be represented by: Pt, O₂, YBa₂Cu₃O_7−x , Y₂BaCuO₅, CuO, BaF₂/CaF₂/BaF₂, BaZrO₃, ZrO₂, O₂, Pt. Using the values of the standard Gibbs’ energy of formation of the compounds BaZrO₃ and Y₂BaCuO₅ from the literature, the Gibbs’ energy of formation of the 1–2–3 compound from the constituent binary oxides has been computed at different temperatures. The value ofx at each temperature is determined by the oxygen partial pressure. At 1023 K for O content of 6·5 the Gibbs’ energy of formation of the 1–2–3 compound is −261·7 kJ mol⁻¹.     

On the thermal mechanism of microwave breakdown of high-temperature superconducting films

Theoretical arguments supporting the thermal nature of the microwave breakdown of high-temperature superconducting films are compared with experimental data. A comparison of the theoretical and experimental values of the threshold field for breakdown of a uniform film, B _f, and the threshold field for breakdown at nonsuperconducting defects, B _d, confirms the dependence corresponding to a thermal mechanism: B _f, B _d∝ (T _c-T ₀)^1/2. It is shown that the space-time picture of the observed breakdown is apparently due to overheating of the film near defects with a size of 10⁻⁵–10⁻⁶ m. The amplitude of the breakdown field may ultimately be limited by the abrupt decrease in the energy of critical disturbances for the initiation of breakdown. Pis’ma Zh. Tekh. Fiz. 24, 12–17 (June 12, 1998)     

Corrosion study of single crystal Ni–Mn–Ga alloy and Tb<Subscript>0.27</Subscript>Dy<Subscript>0.73</Subscript>Fe<Subscript>1.95</Subscript> alloy for the design of new medical microdevices

Once placed in a magnetic field, smart magnetic materials (SMM) change their shape, which could be use for the development of smaller minimally invasive surgery devices activated by magnetic field. However, the potential degradation and release of cytotoxic ions by SMM corrosion has to be determined. This paper evaluates the corrosion resistance of two SMM: a single crystal Ni–Mn–Ga alloy and Tb_0.27Dy_0.73Fe_1.95 alloy. Ni–Mn–Ga alloy displayed a corrosion potential (E _corr) of −0.58 V/SCE and a corrosion current density (i _corr) of 0.43 μA/cm². During the corrosion assay, Ni–Mn–Ga sample surface was partially protected; local pits were formed on 20% of the surface and nickel ions were mainly found in the electrolyte. Tb_0.27Dy_0.73Fe_1.95 alloy exhibited poor corrosion properties such as E _corr of −0.87 V/SCE and i _corr of 5.90 μA/cm². During the corrosion test, this alloy was continuously degraded, its surface was impaired by pits and cracks extensively and a high amount of iron ions was measured in the electrolyte. These alloys exhibited low corrosion parameters and a selective degradation in the electrolyte. They could only be used for medical applications if they are coated with high strain biocompatible materials or embedded in composites to prevent direct contact with physiological fluids.     

Corrosion of armco iron and model Fe−Cr-Al alloys in oxygen-containing lead melts

We studied the influence of oxygen-containing (6·10⁻³ wt.% O) lead on the corrosion of Armco iron and Fe−16Cr, Fe−16Cr−1Al alloys at a temperature of 650°C under stationary conditions. The front of corrosion propagates according to a linear law and this process is periodically repeated. In each period, an oxide film based on Fe₃O₄ magnetite is formed on the surface of the metal and lead penetrates into the suboxide zone. This leads to the exfoliation the external oxide film and then the process is repeated. Under the indicated testing conditions, alloying with chromium and aluminum intensifies the process of corrosion in iron. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 2, pp. 84–88, March–April, 1997.     

Silicon metal-dielectric-semiconductor varicaps with an yttrium oxide dielectric

This paper reports an investigation of the electrophysical properties of metal-dielectric-semiconductor varicaps with an yttrium oxide dielectric, prepared by resistive vacuum evaporation of the rare-earth metal with subsequent thermal oxidation of the film in air at 500–550 °C. It is found that the electrical conductivity of the samples follows the Poole-Frenkel law. High-frequency capacitance-voltage characteristics are used to determine the specific capacitance of the dielectric, C ₀=0.027–0.03 μF/cm², the slope of the capacitance-voltage characteristic, dC/dV=35–40 pF/V, the fixed charge in the dielectric, Q _f=(1.7−2.7)×10⁻⁸ C/cm², and the density of surface states at the flat-band potential, N _ss=(1−2)×10¹¹ cm⁻²·eV⁻¹. The capacitance tuning range factor for the metal-dielectric-semiconductor varicaps is 2.5–3. These structures are shown to be applicable as metal-dielectric-semiconductor varicaps with a low control voltage and a high quality factor. Pis’ma Zh. Tekh. Fiz. 23, 50–55 (June 26, 1997)