Corrosion-electrochemical properties of α-Fe + Fe<Subscript>3</Subscript>C + TiC nanocomposites in acidic sulfate solutions

Corrosion-electrochemical properties of model α-Fe + Fe₃C + TiC three-phase nanocrystalline composites in acidic sulfate solutions are studied. Anodic processes on α-Fe + Fe₃C + TiC composites are determined chiefly by the oxidation of ferrite and cementite components. The passivation of nanocomposites takes place due to the formation of γ-Fe₂O₃ · nH₂O surface hydroxides with a high water content n = 0.7–2.2 and low protective properties. The high activity of α-Fe + Fe₃C + TiC composites with respect to hydrogen evolution is determined by the cementite component and increases with an increase in the dispersion of Fe₃C inclusions. Under the effect of cathodic hydrogen, titanium carbide decomposes to produce free carbon.     

Deformation-induced dissolution of the Fe2B boride in nanocrystalline α-Fe

Structural-phase transformations in the α-Fe+Fe₂B composite with a total boron content of 15 at. % during mechanical grinding in a planetary ball mill have been studied using X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. It has been established that dissolution of the Fe₂B boride with the formation of an amorphous phase and segregations of boron atoms at grain boundaries occur after the grain size reaches 3 nm. In this work a comparative analysis of the deformation-induced dissolution of Fe₂B and Fe₃C in the Fe-B and Fe-C systems has been made and the possible mechanisms of the structural-phase transformations have been discussed.     

Complete and Incomplete Wetting of Ferrite Grain Boundaries by Austenite in the Low-Alloyed Ferritic Steel

Low-carbon low-alloyed ferritic steels are the main material for the production of high-strength pipes for the transportation of oil and gas. The formation of brittle carbide network during the lifetime of a pipeline could be a reason for a catastrophic failure. Among other reasons, it can be controlled by the morphology of grain boundary (GB) carbides. The microstructure of a low-alloyed ferritic steel containing 0.09 at.% C and small amounts of Si, Mn, Nb, Cu, Al, Ni, and Cr was studied between 300 and 900 °C. The samples were annealed very long time (700 to 4000 h) in order to produce the equilibrium morphology of phases. The (α-Fe)/(α-Fe) GBs can be either completely or incompletely wetted (covered) by the γ-Fe (austenite) above the temperature of eutectoid transition. The portion of (α-Fe)/(α-Fe) GBs completely wetted by γ-Fe is around 90% and does not change much between 750 and 900 °C. The (α-Fe)/(α-Fe) GBs can be either completely or incompletely wetted (covered) by the Fe₃C (cementite) below the temperature of eutectoid transition. The portion of (α-Fe)/(α-Fe) GBs completely wetted by Fe₃C changes below 680 °C between 67 and 77%. The formation of the network of brittle cementite layers between ductile ferrite grains can explain the catastrophic failure of gas- and oil-pipelines after a certain lifetime.     

Effect of silicon on the phase formation in mechanically activated systems based on Fe(75)C(25): Temperature-induced transformations in mechanosynthesized composites

Transformations realized in mechanosynthesized amorphous-nanocrystalline Fe(75)C(25 − x)Si(x) (0 ≤ x ≤ 10 at %) alloys during heating have been studied using dynamic magnetic susceptibility measurements, X-ray diffraction, and metallography. In contrast to mechanosynthesized alloys consisting of α-Fe, Fe₃C, and amorphous phases, the annealed alloys with x > 5 at % were found to exhibit the formation of an additional phase such as Fe₅SiC. After heating to 700 and 800°C, the powder particles of alloys contain a large amount of uniformly distributed graphite particles of ∼0.5 μm in size. The formation of particles results from the cementite decomposition, which is accelerated at the expense of partial silicon dissolution in cementite and in the presence of α-Fe nanograins as well.     

A comparative study of mechanical alloying and mechanical milling of Nd8Fe88B4

A comparative study was made of structure and magnetic properties of Nd₈Fe₈₈B₄ prepared by mechanical alloying (MA) using elemental powders as starting materials and by mechanical milling (MM) of the alloy. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) combined with transmission electron microscopic (TEM) studies revealed that both milling procedures resulted in a mixture of α-Fe and an amorphous phase. The thermal stability of the as-milled powders produced by MA was comparable to that of the as-milled powders produced by MM. Heat treatment of the milled powders above the crystallization temperature resulted in the formation of a nanocrystalline mixture of Nd₂Fe₁₄B and α-Fe, but annealed MA powders demonstrated a somewhat coarser structure in comparison with annealed MM powders. Therefore, higher remanences and coercivities were obtained by MM.     

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.     

Iron layer formation during cementite decomposition in carburising atmospheres

In the following report cementite (Fe₃C) formation and subsequent decomposition is investigated on pure iron samples at 700 °C in CO-H₂-H₂O gas mixtures. The carbon activities of the atmospheres are a_C=15.9 and 20, values higher than the value of the equilibrium α-Fe+Fe₃C. During the carburisation process cementite forms at the surface. Graphite deposition at the surface initiates cementite decomposition. An iron layer of 1-3 μm thickness forms between cementite and graphite as a result of cementite decomposition. In previous studies of metal dusting on iron it was found that at lower temperatures T?650°C the decomposition product iron is found in the coke as small particles.     

Thermoelectric properties of mechanically alloyed iron disilicides consolidated by various processes

Mn doped p-type iron disilicide powders have been produced by a mechanical alloying process. As-milled powders were of metastable state and mostly transformed to β-FeSi₂ phase by subsequent isothermal annealing. As-milled powders were consolidated by various processes such as sintering of the cold compact in vacuum, vacuum hot pressing (VHP), and spray drying/atmospheric plasma thermal spraying. Phase transitions during the processes were investigated using XRD, EDS, and SEM. As-consolidated specimens consisted of a mixture of α-Fe₂Si₅ and ε-FeSi phases, which were gradually transformed into a thermoelectric semiconducting α-FeSi₂ phase by subsequent isothermal annealing in the vicinity of 845°C in vacuum. However, some residual α and ε phases remained even after prolonged annealing. Thermoelectric properties were measured as a function of temperature and correlated with phase transformation. They showed optimum values in the vacuum hot pressed specimen due to a higher fraction of β phase and/or higher density.     

Synthesis of γ-TiAl based alloy by mechanical alloying and reactive hot isostatic pressing

Elemental Ti-48Al-2Cr-2Nb powders were mechanically alloyed (MA) for 8 h and 45 h. The MA powders were then consolidated by reactive hot isostatic pressing (HIP). The microstructure of the HIPed materials consisted of equiaxed γ-TiAl and α₂-Ti₃Al phases. During the high-temperature annealing of the HIPed materials, the α₂-Ti₃-Al phase transformed into a lamellar structure consisting of alternating laths of α₂-Ti₃Al and γ-TiAl. It is suggested that a high content of interstitial elements together with the microalloying elements of niobium and/or chromium in MA powders raises α/(α + γ) transus to a higher temperature.     

Corrosion of Highly Dispersed Systems Based on Iron and Fe-Si Alloys in Neutral Electrolytes. II. Iron-Based Systems Obtained by Grinding in Heptane with an Organosilicon Additive

The corrosion behavior of highly dispersed iron powders obtained by mechanical grinding in a solution of triethoxy(vinyl)silane (0.3 wt %) in heptane was studied in 0.85% NaCl at 3°C. The protective properties of the layer covering the powder particles were found to be determined by oxide and organosilicon compounds. This layer is significantly less effective than the layer formed in the presence of oleic acid.__________Translated from Zashchita Metallov, Vol. 41, No. 3, 2005, pp. 289–294.Original Russian Text Copyright © 2005 by Syugaev, Lomayeva, Reshetnikov.     

Structure-phase transformations in electrospark iron-carbon surfaces at different heating temperatures

The results of investigation of the surface structure-base transformation of iron specimens subjected to carbonization by the electrospark method and heating in the temperature range of 20–800°C are given. It is known that, after electrospark alloying with graphite, a surface layer consisting of mainly austenite and cementite forms with a carbon content in γ-Fe. The quantity of the austenite reduces with heating, and, at 300°C, it fully turns into a ferrite-cementite mixture (perlite). The heating of the specimens reduces the value of the microstresses, and the diffraction lines narrow down coming nearer to the annealed armco-iron values.     

Bulk amorphous FC20 (Fe–C–Si) alloys with small amounts of B and their crystallized structure and mechanical properties

The addition of a small amount (0.4 mass%) of B to a commercial FC20 cast iron was found to cause the formation of an amorphous phase in melt-spun ribbon and cast cylinders with a diameter of up to 0.5 mm. The structure of a melt-spun B-free FC20 alloy consisted of α-Fe, γ-Fe and Fe₃C. The effectiveness of additional B is presumably due to the generation of attractive bonding nature among the constituent elements. The amorphous alloy ribbon exhibits a high tensile strength of 3480 MPa and good bending ductility. The annealing causes the formation of an amorphous phase containing α-Fe particles with a size of about 30 nm. The mixed phase alloy exhibits an improved tensile strength of 3800 MPa without detriment to good ductility. With further increasing temperature, the mixed amorphous and α-Fe structure changes to α-Fe+Fe₃C+graphite through the metastable structure of α-Fe+Fe₃C. The structure after annealing for 900 s at 1200 K has fine grain sizes of about 0.5 μm for α-Fe, 0.3 μm for Fe₃C and 1 μm for graphite. The graphite-containing alloy exhibits high tensile strength of 1200–2000 MPa and large elongation of 5–13%. The high tensile strength and good ductility were also obtained for the 0.5 mm cylinder annealed at 1200 K. The good mechanical properties are due to the combination of fine subdivision of crack initiation sites by the homogeneous dispersion of small graphite particles and the dispersion strengthening of Fe₃C particles against the deformation of the α-Fe phase. The synthesis of the finely mixed α-Fe+Fe₃C+graphite alloys having good mechanical properties by crystallization of the new amorphous alloy in the melt-spun ribbon and cast cylinder forms is encouraging for the future development of a new Fe-based high-strength and high-ductility material.     

Corrosion of iron and iron-silicon finely dispersed systems in neutral media. Part III. Fe-Si systems prepared by milling in pure heptane or with oleic-acid

Corrosion behavior and the surface layer structure are studied for Fe₈₀Si₂₀ powders mechanically milled in heptane, in particular, with 0.3 wt % of oleic acid. In heptane, the powders become enriched with amorphous phase as time passes, which improves the corrosion resistance significantly. In the presence of oleic acid, some small amount of the amoprhous phase is formed indeed in the bulk of the particles; however, the powders’ surfaces become covered by an oxide-organic protective film. With an increase in the milling time, in the presence of oleic acid, the Fe-Si alloy is depleted of silicon because of the SiO₂ phase segregation, which deteriorates the powders’ corrosion resistance. Original Russian Text ? A.V. Syugaev, S.F. Lomaeva, S.M. Reshetnikov, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 4, pp. 348–355.     

Effect of pore size on the high temperature oxidation of Ni-Fe-Cr-Al porous metal

This study investigated the effect of the pore size of Ni-22.4%Fe-22%Cr-6%Al porous metal on its hightemperature oxidation. Two types of open porous metals with pore sizes of 800 μm and 580 μm were used. A 24-hour isothermal oxidation test was conducted at three different temperatures of 900 °C, 1000 °C, and 1100 °C under a 79% N₂ + 21% O₂ atmosphere. The results of the BET analysis revealed that the specific surface area increased as the pore size decreased from 800 μm to 580 μm. The high-temperature oxidation results showed that porous metals exhibited far lower levels of oxidation resistance compared with bulk metals, and that the oxidation resistance of porous metals decreased with a decreasing pore size. According to the microstructural observations of the oxide layers, the 900 °C and 1000 °C oxidation layer contained Ni, Cr, and Al oxides mainly on the strut. The 800 μm porous metal strut exhibited similar oxidation behavior at 1100 °C to that found at lower temperatures. In contrast, the 580 μm porous metal strut was found to consist of Ni and Fe oxides in the upper layer and Ni, Cr, and Al oxides in the lower layer, representing a low oxidation resistance. For powders affixed to the strut inside the porous metal, a different oxide-forming behavior from that of the strut was observed. In addition, the Ni-Fe-Cr-Al porous metal high-temperature oxidation microscopic mechanism is also discussed.     

Carbonization of α-Fe upon mechanical alloying

Methods of thermomagnetic analysis (TMA) and Mössbauer spectrometry (⁵⁷Fe) have been used to study the processes of the carburizing of α-Fe under the conditions of mechanical milling in a medium of liquid hydrocarbons. It has been established that, under the chosen conditions of the mechanical synthesis of carbides, the process of carbonization at T < 375 K occurs through the decomposition of the deformation-induced martensite, i.e., the supersaturated bct solid solution α″-Fe(C) with the formation of transitional hcp ε and ε′ phases that precede the formation of cementite. The milling of the metallic iron in the toluene medium substantially enhances the catalytic capability of disperse powders of α-Fe in the process of conversion of cyclic structures of hydrocarbons into other chemical forms. The increase in the dispersity of the iron powder to a nanocrystalline state leads to an increase in the chemical activity of carbon and an increase in the rate of diffusion sufficient for the formation in the Fe-C mixture of both primary cementite (θ′) with an anomalously low Curie temperature T _C(θ′)(first stage) and secondary cementite (θ″) at the second stage of mechanosynthesis. The parameters of hyperfine interactions have been calculated for a number of synthesized carbides. It has been shown that the change in the carbon concentration in iron carbides is determined by the following inequality: c _C(θ′) > c _C(ε) > c _C(ε′). The boundary of the temperature stability of cementite has been established. The effect of the decomposition of the θ phase (Fe₃C) upon thermal cycling θ ? γ in the temperature range of 300 < T < 1075 K has been revealed. Based on the results obtained, a scheme of the sequence of phase transformations that occur in the Fe-C system under the conditions of low-temperature mechanosynthesis has been derived.     

Effect of mechanical alloying on the formation of Sm2Fe17Nx compound

Elemental powders of iron and samarium were mechanically alloyed in the concentration range of Sm_xFe_100−x (x=11, 13, 15, 17). A two-phase mixture of amorphous Sm−Fe and α-Fe phases was formed in all compositions studied. The effect of the starting composition on the formation of Sm₂Fe₁₇ intermetallic compound was investigated by annealing mechanically alloyed powders. When the Sm content was 15 at.%, the annealed powders consisted of nearly a Sm₂Fe₁₇ single phase. For the preparation of a hard magnetic Sm₂Fe₁₇N_x compound, additional nitriding treatment was performed under an N₂ gas flow at 450°C for various time intervals. It was found that nitrogenation for 3 hours was just enough to allow the formation of the Sm₂Fe₁₇N_x compound. The coercivity increased when the nitrogenation time increased up to 3 hours and then tended to decrease with further nitrogenation.     

Precipitation behavior of B2-ordered aluminide

Fine dispersion of disordered phases is obtained in Ni−Al−Cr and Fe−Al−Co temary systems. A transmission electron microscope investigation has been performed on the precipitation of α-Cr in B2-ordered β-NiAl with different stoichiometry and α-Fe in B2-FeAl(Co) compound. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of NiAl and FeAl increases appreciably with the fine precipitation of α-Cr and α-Fe, and over-age softening occurs after prolonged aging. In the case of B2-NiAl(Cr), perfect lattice coherency is maintained at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, a loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface, followed by climbing around the particles. On the other hand, in the case of B2-FeAl(Co), the disordered α-Fe phase is present as a precipitate in the B2-FeAl(Co) matrix and has a cubic-cubic orientation with the matrix. At the early aging periods, prismatic dislocation loops formed in the B2-FeAl(Co) matrix. B2-FeAl(Co) matrix is typically hardened by the precipitation of α-Fe.     

Influence of Surface Roughness on Corrosion and Tribological Behavior of CP-Ti After Thermal Oxidation Treatment

In this study, tribological and corrosion behavior of commercially pure titanium (CP-Ti) with different surface roughness values after thermal oxidation was investigated. The CP-Ti specimens were prepared with three different roughness values from silicon carbide paper, Ra = 0.1, 0.3, and 0.6 μm, and the thermal oxidization process was conducted at a temperature of 850 °C for 8 h in an O₂ atmosphere. Structural, mechanical, corrosion, and tribological properties of untreated and thermally oxidized CP-Ti with different surface roughness values were investigated through x-ray diffraction, scanning electron microscopy, microhardness, potensiostat, and pin-on-disk techniques. The corrosion and tribological behavior of CP-Ti improved as an oxide layer was formed by thermal oxidation. It was observed that the surface roughness had an effect on these characteristics. It was established that the decreased roughness improves the tribological and corrosion properties.     

Oxidation of Fe-C alloys at 700°C

The oxidation of Fe-C alloys containing 0.5 and 1.0% C was studied in 1 atm O₂ at 700° C. The oxidation rate is considerably slower than for pure Fe. The oxide scale formed is detached, multilayered, and overoxidized, containing little or no FeO. A thin film of graphite was identified at the metal-oxide interface by electron diffraction. It is proposed that the slow oxidation and abnormal scale are caused by a residue of graphite left at the metal surface from the oxidation of Fe₃C. This inhibition of the oxidation of Fe by carbon at 700°C is in contrast to the stimulation observed at 500°C.     

Corrosion-electrochemical properties of nanocomposites of α-Fe+Fe<Subscript>3</Subscript>C+VC in acidic and alkaline sulfate solutions

This work studies corrosion-electrochemical properties of three-phase composites of α-Fe+ Fe₃C + VC in acidic and alkaline sulfate solutions. Nanosize carbide inclusions are characterized by a high level of activity in the reaction of cathodic hydrogen evolution and result in a high rate of composite dissolution in acidic solutions. The presence of carbides and carbon has a negative effect on passivation of the iron matrix. The resistance of vanadium carbide inclusions to anodic oxidation increases upon a decrease in the amount of carbon in the carbide.