Application of Electrochemical Impedance Method to Studying Corrosion of Porous Powder Steels

Electrochemical impedance spectroscopy (10⁴–10^–3 Hz) and weighing methods are applied to studying the corrosion behavior of powder austenitic 316L stainless steel with a porosity = 15, 20, and 38% and compact steel of the same composition in a 3% HCl + 5% NaCl solution and oil pool water (pH 6.0; total mineralization 184.8 g/l, preferentially chlorides of Na, Ca, and Mg). Hodographs and phase angles of the electrode impedance are analyzed for high ac frequencies. The procedure of corrosion rate determination for powder steel from impedance data is described. The corrosion rate of powder steel calculated to the geometric surface area of a sample is shown to increase in acid and neutral chloride solutions with an increase in the porosity of samples from 15 to 38%, and, at 20%, with the exposure of samples to the electrolytes (from 2 to 120 h). Advantages of the impedance method over the weighing method are demonstrated for determination of the corrosion rate of porous powder steels. Based on impedance measurements, the specific surface areas of powder steel samples are assessed.     

General laws governing variation in properties of cast alloys of the Al-Zn-Mg system

Among existing high-strength corrosion-resistant aluminum alloys, those of the Al-Mn-Zn system are most promising. They have a different phase composition, depending on the content of magnesium and Zinc: + (Al₃Mg₂), + + T(Al₂Mg₃Zn₃), + T, + T + (MgZn2) and + The majority of industrial Al-Mg-Zn alloys correspond to the phase regions + T and + T + with respect to composition. A high level of strength and satisfactory overall corrosion resistance are characteristic for these alloys. Al-Mg-Zn alloys may, however, tend to the most dangerous form of corrosion - stress-induced corrosion cracking. Using methods of experiment planning in the study, we investigated Al-Zn-Mg alloys of various compositions for the purpose of selecting alloy compositions with a high level of mechanical properties and stress-induced corrosion cracking.All-Union Scientific-Research Institute of Aviation Materials. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 24–28, December, 1994.     

Control of the phase composition and nitrogen content in the nitride layer in the nitriding of the steel 38Kh2MYuA

It is known that in order to obtain a high-hardness, wear- and corrosion-resistant article a nitrided layer of nitride (+)-phases should be formed on its surface. However, in some cases, for example, in nitriding high-speed die steels and steel 38Kh2MYuA, the formation of brittle nitride surface layers should be eliminated and only a zone of internal nitriding (a+ + MN) should be formed in order to provide the requisite hardness and wear resistance. The article concerns preparation of nitrided layers with different compositions on the widely used 38Kh2MYuA nitralloy.     

Dynamic recrystallization of beta-phase in titanium alloy

Conclusion Dynamic recrystallization of cast -grain in alloys type VT-6S, which proceeds most actively at high temperatues (1200°C) and high rates of deformation (10 sec^–1), causes refinement of cast grain and reorientation of crystallites.DeceasedA. A. Baikov Institute of Metallurgy. TsNIIKM Prometei. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 34–36, September, 1991.     

Formation of a Rhenium-Base Coating on a Nb-Base Alloy

To protect Nb–5Mo–15W alloy against high-temperature oxidation a novel coating was developed involving electroplating of a Re–Ni film, followed by pack cementation with Cr and Al. The coating consisted of a duplex-layer structure, an inner (Re–Cr) or Re(Cr) layer and an outer Cr(Al) or NiAl layer. The Re–Ni film containing more than 70at.%Re, developed in the present investigation, is more useful than the conventional low Re–Ni film. The inner (Re–Cr) and Re(Cr) layer acts as a diffusion barrier between the Nb–5Mo–15W alloy substrate and the outer -Cr(Al) or -NiAl layer, which forms a protective -Al₂O₃ scale. The coated Nb–5Mo–15W alloy was oxidized in air at 1373K for up to 360ks, showing very good oxidation resistance.     

Kinetics and mechanism of low-pressure,high-temperature oxidation of silicon-II

The reaction of oxygen at low pressures with silicon layers on tungsten ribbons was studied. An abrupt transition was observed between a condition of passivation, in which a thin film of SiO₂ formed at low temperatures, and a steady-state combustion condition at high temperatures. The latter state is characterized by the formation of volatile SiO. The boundary between these two states has been defined in terms of the pressure-temperature relation. Oxygen consumption in the combustion state is represented by first-order reaction kinetics with an activation energy of 13 ± 1 kcal/mole. The stability of the two states has been defined by a thermodynamic analysis of the SiO₂ layer stability. The oxygen consumption dependence on temperature has been described by a kinetic model which involves a consideration of the various elementary steps in the reaction.     

International Standards in Corrosion of Metals and Alloys. The Activities of ISO/TC 156 “Corrosion of Metals and Alloys”

We continue to inform readers about the activities of the Technical Committee Corrosion of metals and alloys of the International Organization for Standardization (ISO/TC 156). The work program of the Committee for 2000 is given. The scheme of the participation of specialists in corrosion in the activity of the Committee, as well as the procedure of submitting draft standards for the development as International standards, is shown.     

Structure of carburized Fe-Ti and Fe-Ti-Cr alloys

One method for the production of material of a new class is the use of the ferrite austenite + carbide transformation. The transformation occurs under specific temperature-concentration parameters of carburization of low-carbon ferritic iron-carbide-forming element alloys. Ferrite decomposes with formation of austenite-carbide colonies similar to eutectoid ones. The colonies are preferentially oriented in the direction of carbon flow, i.e., normal to the carburized surface. Fe-Ti and Fe-Ti-AE alloys, where AE is the alloying element, are of special interest in this connection. Titanium carbide is extremely hard and, when used as an agent reinforcing the surface layer, increases the hardness and wear resistance of the alloy. In addition, TiC dissolves impurities poorly, and therefore in the alloying of Fe-Ti alloys by elements improving their thermal stability or corrosion resistance, its formation will not deplete the matrix. However, the addition of a third component to the Fe-Ti alloy may cause degeneration of the structures of the colonies. The present work is devoted to the effect of different concentrations of chromium on the structure of carburized Fe-Ti alloys.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 4–7, May, 1995.     

Peculiarities of the Effect of Perchlorate Ions on Electrodeposition of Cadmium in Aqueous–Acetonitrile Electrolytes

It is shown that, depending on the composition of aqueous–acetonitrile solvent, either acetonitrile (AN) molecules or perchlorate anions predominantly adsorb on the electrode. The highest surface concentration of AN and the lowest rate of cadmium electroreduction are observed in the mixture with the most loosened structure (x ₂ 0.4). The salting-out action of mixed solvent in the domains of its structure stabilization primarily manifests itself in an increase in the adsorption activity of ClO^– ₄ anions and a corresponding promotion of Cd²⁺ discharge as a result of " effect. The maximum process rate is observed in the domain, where the AN structure is ordered by monomeric water molecules.     

Effect of lamellar structure parameters on the properties of titanium alloy VT3-1

Conclusion An increase in the ultimate breaking strength, stress-rupture strength, and fatigue limit of alloy VT3-1 with a lamellar structure may be achieved as a result of refining any parameter of the structure, particularly -phase platelet thickness, and increasing the volume fraction of secondary -phase. An increase in ductility characteristics, toughness, and creep resistance may be provided by increasing the dimensions of -colonies and primary -phase particles (up to 2.5–3.5 m) and reducing the volume fraction and dispersivity of secondary -phase lamellar precipitates. Coarsening of -grains leads to an increase in a_c, k_Q, and refinement leads to an increase in and a_n.Qualitative dependences for mechanical properties of alloy VT3-1 on lamellar structure parameters made it possible to isolate those structural parameters which have the most marked effect on properties.The properties of alloys with a finely lamellar structure (d25 m, b_{I, II}<2 m) are most sensitive to structure. In this case a change in -colony size by 10 m and -platelet thickness by 1 m affects the properties 3–20 times more strongly than a change in -grain size by 100 m. The effect of finely dispersed secondary -phase precipitates is greater, the coarser the primary -phase structure. Refinement of primary -phase structure with an increase in secondary phase platelet thickness to 1 m or more reduces the sensitivity of alloy mechanical properties to the effect of secondary -phase.With coarsening of the intragranular structure (d>25 m, b_{I, II}2 m) the effect of structural parameters d and b on properties is markedly weakened: on strength properties (_f, ₁₀₀ ⁴⁵⁰ ) by a factor of 100, on ductility (, ), by a factor of 10 to 20, and on impact strength and fracture toughness (a_n, a_c, K_Q) by a factor of five.The qualitative relationships obtained between structure and mechanical properties of alloy VT3-1 are fundamental for controlling the structure of semifinished titanium alloy products.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 52–55, July, 1986.     

Protection of Sintered Steels against Corrosion in Neutral Media with an N-M-1 Inhibitor

The effect of an N-M-1 inhibitor (a salt of cyclohexylamine and C₁₀ to C₁₆ aliphatic acids) on the corrosion of sintered powder steels 13 and 141 (with the 14 to 17% porosity) in distilled or tap water at 20 to 80°C, as well as in 0.05 M Na₂SO₄ solution, is studied by gravimetric and electrochemical methods. The protective concentration of the inhibitor is lower in distilled, than in tap water or sodium sulfate solution. The protective action decreased with an increase in temperature. In tap water or sodium sulfate solution, the corrosion-inhibiting effect is weaker for 13 than for 141.     

Observations on the effect of low sulfur activity on the oxidation of chromium-depleted zones in a stainless steel

Coupon specimens of 20% Cr-25% Ni-Nb-stabilized steel have been oxidized in an atmosphere of CO ₂-2%CO-670 vppb COS at 0.1 MPa pressure for periods up to 500 hr at 1123 K. Standard specimens, annealed at 1203 K prior to testing, showed an enhancement of iron in the surface scale and a much increased propensity to spall compared with control tests in a sulfur-free atmosphere. The main purpose of the work was to examine the effect of sulfur on the ease of formation of a healing layer under duplex attack produced by depleting specimens in chromium by prior vacuum annealing. It is shown that although a chromium-rich layer had formed, extensive breakdown occurred in the sulphidizing atmosphere leading to continued internal oxidation. Sulfur was found to partition at the base of this attack and to be associated with a large concentration of nickel. Spaliation was also enhanced in the depleted specimens, the favored site being the interface between the spinel and outer iron-rich oxide of the duplex structure. Partitioning of both sulfur and carbon was observed at this interface in those regions of the specimen showing healing layer breakdown.     

Improvement of cyclic high-temperature corrosion resistance of pack-aluminized heat-resistant stainless steels

Modification of pack aluminizing for heat-resistant stainless steels was studied to improve corrosion resistance by controlling the microstructure of the coating layer. The major process parameters examined include the pack powder composition, coating time, and temperature. Depending on the combination of these parameters, the microstructure of the coating layer can be controlled to form either a continuous layer of internal-diffusion barrier (IDB) or an interdiffusion zone (IZ). At the coating-process temperatures, the IDB forms as a mixture of - and -aluminide, whereas the IZ forms as a mixture of -ferrite and -aluminide. But the phase shown in the IZ at room temperature is formed by transformation from the phase during cooling. Even though the hardness of the IDB is higher than the other phases present in the coating layer, the aluminide coating layer with the IDB shows outstanding cyclic high-temperature corrosion resistance. As long as the stable IDB forms, the corrosion resistance increases with the thickness of the aluminide-coating layer.     

Oxidation-Rate Excursions During the Oxidation of Copper in Gaseous Environments at Moderate Temperatures

The kinetics of oxidation of copper powders in oxygen and in dry and humid air was investigated using thermogravimetric analysis (TGA). The extent of oxidation grew linearly with time until the weight-based thickness of the oxide film reached 0.13–1.22 nm, depending on the temperature. Between 30 and 90°C there was little difference between the kinetic curves observed in air and in oxygen, respectively. Higher humidity of the air resulted in an increased oxidation rate. Following the initial linear segment, the oxidation kinetics could be best described in terms of a logarithmic rate law between 30 and 45°C and in terms of a power law between 60 and 90°C. The activation energy for the initial linear stage was (44±2) kJ and for the subsequent oxidation (102±12) kJ. Delayed increases in oxidation rate were observed with a ca. 0.1-m powder around 100°C, with a ca. 1-m powder around 320°C, and with a < 10m powder around 360°C. A three-stage model consisting of an initial linear stage, parabolic growth culminating in cracking of the oxide film, and subsequent re-start of the parabolic growth, gave good agreement with the experimental data. Whenever the powder is relatively uniform and the distribution of film-cracking times among the powder grains is narrow, e.g., within 23% of the median cracking time, an increase in the oxidation rate of the entire sample can be observed.     

Electrochemical Behavior of Nickel Hydride in Sodium Hydroxide Solutions

Electrochemical behavior of nickel hydride Ni₂H (-phase) is studied in 0.01–1 N NaOH by using common (VA) and cyclic (CVA) voltammetry, chronocoulometry, amperometry, and potentiometry. The limiting anodic and cathodic currents in VA and CVA curves are caused by the hydride decomposition via the following scheme Ni₂H -phase Ni + H_abs, where the intermediate -phase contains 0.003 at. % H, that is, one tenth that in the saturated -phase (0.03 at. %). At open circuit, the hydride maintains the equilibrium hydrogen potential. In the first 30 min, the hydrogen ionization from hydride is limited by solid-state diffusion and, later, the hydride decomposition. The anodic process involves ionization of sorbed hydrogen, while the cathodic process represents its electrochemical desorption: H₂O + H_ads + e H₂ + OH^–. The hysteresis observed in the cathodic CVA and open-circuit chronograms of the hydride potential in the beginning of anodic dissolution reflect the changes in the surface coverage of hydride with adsorbed hydrogen. The rate constant of hydride decomposition k, the rate Vitself, and the equilibrium constant K are as follows: k = k = 8 × 10^–5 s^–3, V = 3 × 10^–5 C/cm², and K = 10. The kinetic parameters of hydrogen electrochemical ionization from the hydride are b _a = 0.12 V and = 0.5.     

Special features of the α → γ transformation in powder carbon steels

The structural state of powder steels after sintering depends on the technological regimes of their production and can markedly affect the transformations in subsequent heat treatment. The effect of the technological regimes on the nature of the transformation is investigated.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 14–16, March, 1996.     

Effect of rapid cyclic electrothermal treatment on the structure of nickel steel

Conclusions The high dislocation density of austenite undergoing the transformation is due to the influence of fresh dislocations that occur during the transformation in virtue of its martensitic character and to dislocations inherited from the original phases.Since the increase of the dislocation density in austenite causes an increase in the number of martensite crystals, it can be assumed that the increase in the number of phase nuclei is due to an increase in the density of fresh dislocations. In this case the refining of martensite is due to an increase in the number of nuclei and to the barrier effect of elements of the substructure. The formation of atmospheres of impurity atoms at dislocations in alloys with carbon stabilizes the austenite and intensifies recrystallization processes. Cementite particles in phase are not inherited by austenite with heating to the transformation temperature.Institute of Metal Physics, Academy of Sciences Ukrainian SSR. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 20–23, January, 1975.     

Mechanisms of formation of diffusion aluminide coatings on nickel-base superalloys

Studies on microstructures and compositions of model diffusion aluminide coatings coupled with existing information on diffusion in nickel-aluminum intermetallic compounds indicate that only two basic types of these coatings can be formed on nickel-base superalloys. The first type is formed by inward diffusion of aluminum from coating media of sufficiently high aluminum activity to cause formation of the (Ni₂Al₃) phase. Subsequent stabilizing heat treatments required for practical use of this type of coating cause some outward diffusion of nickel to form a three-zone (NiAl) coating. The second type is formed by outward diffusion of nickel from the substrate alloy to react with aluminum of suitable activity to stabilize that (NiAl) phase of composition through which only nickel motion can occur. This process produces a two-zone (NiAl) coating. It follows that the structural type of coating to be expected from any given coating process can, in principle, be predicted from knowledge of the aluminum activity present during formation of the coating. The initial stages of diffusional degradation of these coatings involve continued outward diffusion of nickel from the substrate through the (NiAl) coating with the resultant dilution of aluminum content of the coating; little inward diffusion of aluminum occurs.     

Effect of nitrogen and chromium on the mechanical properties of austenitic steels at −196°C

Conclusion Nitrogen lowers and chromium raises (with >3% N) the strength (_b0.2), ductility (, ), and fracture toughness of austenitic stainless steel of the Kh(18-24)N20 type in the aged condition. The effect of chromium on the properties is due to a change in the solubility of nitrides.I. P. Bardin Central Scientific-Research Institute of Ferrous Metallurgy. Translated from Metallovedenie Termicheskaya Obrabotka Metallov, No. 8, pp. 6–8, August, 1979.     

Recrystallization of maraging alloys

Conclusions In the maraging alloys investigated the transformation is not accompanied by refining of the original grains. The coarse-grained structure is eliminated by recrystallization of austenite at 900–1000°C, which occurs mainly by means of grain boundary migration.Physics of Metals Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 10–12, April, 1969.