Anwendung der Mößbauer-Spektroskopie auf die Untersuchung von Deckschichten in Naturumlaufkesseln

Application of Mößbauer spectroscopy to the study of surface layers in natural convection boilers Qualitative and quantitative analysis of oxide layers on tubes in steam generators have been performed by ⁵⁷Fe Mössbauer Spectroscopy in transmission and backscattering geometry. The tubes were exposed to the following conditions for ca. 127000 hours: 120–174 atm., 320–353 °C and 9.5–10.5 pH; the water contained < 10 and < 4 mg/l of P₂O₅ and SiO₂, respectively (The results of the chemical analysis of the tube materials were: 15 Mo 3 : 0.12% C, 0.15% Si, 0.5% Mn, 0.04% P, 0.04% S, ? 0.3% Cr, 0.25% Mo, rest Fe; St. 45.8. III: < 0.22% C, 0.1% Si, ? 0.45% Mn, 0.05% P, 0.05% S, ? 0.3% Cr, rest Fe). The Mössbauer investigations showed that the protective oxide layers contained mainly non-stoichiometric magnetite (Fe_3?xO₄; x ? 0.03), partly in microcristalline form (<500 Å). In addition to magnetite, hematite (α-Fe₂ O₃) was also detected in one of the samples. The composition of the top surface layers (～5000 Å) was studied by Conversion Electron Mössbauer Spectroscopy (CEMS). The high phosphate content in two of the four investigated samples can be probably attributed to apatite or hydroxylapatite.     

Iron oxide hollow spheres: Microwave–hydrothermal ionic liquid preparation,formation mechanism,crystal phase and morphology control and properties

This paper reports on the microwave–hydrothermal ionic liquid method for the synthesis of a variety of iron oxide nanostructures such as α-FeOOH hollow spheres, β-FeOOH architectures and α-Fe₂O₃ nanoparticles. The formation mechanism for α-FeOOH hollow spheres is discussed. The effects of the reaction parameters on the morphology and crystal phase of the final product are studied. The relationship between the morphology and crystal phase of the product is discussed. A general thermal transformation strategy is designed to prepare α-Fe₂O₃ hollow spheres using α-FeOOH hollow spheres as the precursor and template. By thermal treatment of the as-prepared α-FeOOH hollow spheres, α-Fe₂O₃ hollow spheres showing good photocatalytic activity are obtained. And by autocatalysis of the adsorbed available Fe(II) on the α-FeOOH surfaces, Fe₃O₄ hollow spheres are also obtained.     

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media

The influence of Zn-dopant on the precipitation of α-FeOOH in highly alkaline media was monitored by X-ray diffraction (XRD), ⁵⁷Fe Mössbauer and Fourier transform infrared (FT-IR) spectroscopies and field emission scanning electron microscopy (FE SEM). Acicular and monodisperse α-FeOOH particles were precipitated at a very high pH by adding a tetramethylammonium hydroxide solution to an aqueous solution of FeCl₃. The XRD analysis of the samples precipitated in the presence of Zn²⁺ ions showed the formation of solid solutions of α-(Fe, Zn)OOH up to a concentration ratio r = [Zn]/([Zn] + [Fe]) = 0.0909. ZnFe₂O₄ was additionally formed in the precipitate for r = 0.1111, whereas the three phases α-FeOOH, α-Fe₂O₃ and ZnFe₂O₄ were formed for r = 0.1304. In the corresponding FT-IR spectra, the FeOH and FeO stretching bands were sensitive to the Zn²⁺ substitution, whereas the FeOH bending bands of α-FeOOH at 892 and 796 cm⁻¹ were almost insensitive. The Mössbauer spectra showed a high sensitivity to the formation of α-(Fe, Zn)OOH solid solutions which were monitored on the basis of a decrease in B_hf values in dependence on Zn-doping. A strictly linear decrease in B_hf for α-FeOOH doped with Zn²⁺ ions was measured up to r = 0.0291, whereas for r = 0.0476 and higher there was a deviation from linearity. The presence of α-(Fe, Zn)OOH, α-Fe₂O₃ and ZnFe₂O₄ phases in the samples was determined quantitatively by Mössbauer spectroscopy. Likewise, Mössbauer spectroscopy did not show any formation of the solid solutions of α-Fe₂O₃ with Zn²⁺ ions. FE SEM showed a strong effect of Zn-doping on the elongation of acicular α-FeOOH particles (500–700 nm in length) up to r = 0.1111. For r = 0.1304 the sizes of ZnFe₂O₄ particles were around 30–50 nm, and those of α-Fe₂O₃ particles were around 500 nm, whereas a relatively small number of very elongated α-(Fe, Zn)OOH particles was observed. A possible mechanism of the formation of α-(Fe, Zn)OOH, α-Fe₂O₃ and ZnFe₂O₄ particles was suggested.     

Mössbauer spectroscopy study of MgAl2O4-matrix nanocomposite powders containing carbon nanotubes and iron-based nanoparticles

Materials involved in the catalytic formation of carbon nanotubes are for the first time systematically studied by Mössbauer spectroscopy between 11 K and room temperature. Mg_1−xFe_xAl₂O₄ (x=0.1, 0.2, 0.3, 0.4) solid solutions are transformed into carbon nanotubes–Fe/Fe₃C–MgAl₂O₄ composite powders by reduction in a H₂–CH₄ gas mixture. The oxides are defective spinels of general formulae (Mg_1−x²⁺Fe_x−3α²⁺Fe_2α³⁺□_αAl₂³⁺)O₄²⁻. Ferromagnetic α-Fe, ferromagnetic Fe₃C and a γ-Fe form, the latter possibly corresponding to a γ-Fe–C alloy, are detected in the composite powders. An attempt is made to correlate these results with the microstructure of the powder. It seems that the nanoparticles, which catalyze the formation of the carbon nanotubes, are detected as Fe₃C in the post-reaction Mössbauer spectroscopy analysis.     

Effects of DBU, morpholine, and DMA on corrosion of low carbon steel exposed to steam

Effects of morpholine, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and dimethylamine (DMA) on oxidation kinetics and oxide phase formation/transformation of AISI 1018 steel at 120 °C were evaluated. Low carbon steel samples were exposed to steam in an autoclave containing amine added aqueous solution at pH of 9.5 for 1, 2, 4, 6, 8, and 12 h. Control samples exposed to plain steam and amines showed the highest and lowest weight loss respectively. Fourier Transform Infrared Spectrophotometry (FTIR) showed that DBU containing steam favored formation of magnetite (Fe₃O₄) while steam with DMA formed more α and γ-FeOOH. Transformation of magnetite to hematite (α-Fe₂O₃) was fastest for morpholine. Analysis of oxides morphology was done utilizing Scanning Electron Microscopy (SEM). Oxides formed in plain or DMA containing steam exhibited acicular particles of goethite/hematite (α-FeOOH/α-Fe₂O₃) compared to DBU containing steam that showed equiaxed particles of magnetite/maghemite (Fe₃O₄/γ-Fe₂O₃). Morpholine containing steam promoted agglomeration of thin sharp platelets into coarse flakes of hematite.     

Field sludge characterization obtained from inner of pipelines

Physicochemical characterization of sludge obtained from refined hydrocarbons transmission pipeline was carried out through Mössbauer spectroscopy and X-ray diffraction. The Mössbauer and X-ray patterns indicate the presence of corrosion products composed of different iron oxide and sulfide phases. Hematite (α-Fe₂O₃), magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃), magnetic and superparamagnetic goethite (α-FeOOH), pyrrhotite (Fe_1−xS), akaganeite (β-FeOOH), and lepidocrocite (γ-FeOOH) were identified as corrosion products in samples obtained from pipeline transporting Magna and Premium gasoline. For diesel transmission pipeline, hematite, magnetite, and magnetic goethite were identified. Corrosion products follow a simple reaction mechanism of steel dissolution in aerated aqueous media at a near-neutral pH. Chemical composition of the corrosion products depends on H₂O and sulfur inherent in fluids (traces). These results can be useful for decision-making with regard to pipeline corrosion control.     

Oxidation behavior of NixFe1−x(OH)2 in Cl-containing solution

The addition of Ni leads to the formation of protective rust layer on steel and subsequently high corrosion resistance of steel in Cl⁻-containing environment. α-FeOOH, β-FeOOH, γ-FeOOH and Fe₃O₄ are formed mainly on steels exposed to Cl⁻-containing atmosphere. It is expected that systematic investigation of the effect of Ni(II) on the formation process of each oxide in solution should lead to elucidation of the role of Ni in the formation of anticorrosive oxide layer. This study reports the oxidation behavior of Ni_xFe_1−x(OH)₂ in Cl⁻-containing solution at two different pH regions (condition I under which solution pH is allowed to decrease and condition II under which solution pH is maintained at 8) where γ-FeOOH and Fe₃O₄ are predominantly formed, respectively, upon the oxidation of Fe(OH)₂. In the presence of Ni(II) in the starting solution, the formation of Ni(II) doped β-FeOOH with very low crystalline was facilitated and the formation of γ-FeOOH was suppressed with increasing Ni(II) content and with increasing oxidation rate of Fe(II). Ni(II) was found to have Fe₃O₄-suppressing effect under condition II.     

The detection of genomic DNA from bacterial cells using iron oxide nanoparticles synthesized by a hydrothermal process

We used iron oxide nanoparticles in order to extract purified DNA from bacterial cells. Magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) are synthesized with FeSO₄·7H₂O via a hydrothermal process and used as a medium to detect DNA. Various characterizations were performed including X-ray powder diffraction (XRD), high resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy, vibrating sample magnetometry, and Mössbauer spectroscopy. According to the XRD results, the XRD peaks of the synthesized magnetite and maghemite nanoparticles corresponded well with JCPDS standard data, respectively. The particle size of the iron oxide nanoparticles was about 20 nm, and the particle shape was almost spherical, which was confirmed by observation of the HRTEM image. The magnetite nanoparticles have a face-centeredcubic inverse spinel structure with a space group Fd \(\bar 3\) m, as confirmed by HRTEM and Mössbauer spectroscopy analyses. An agarose gel eletrophoresis analysis was performed to confirm the extraction ability of DNA using these iron oxide nanoparticles, revealing stronger reaction of the maghemite nanoparticles than the magnetite nanoparticles.     

Microstructure,mechanical properties and corrosion performance of a few TMT rebars

The present work aims to study the evaluation of the microstructure, tensile properties, hardness, corrosion behaviour of the various grades of rebars. The microstructures of all rebar samples comprise an outer tempered martensite ring with an inner core of ferrite-pearlite in between a narrow bainitic transition zone. Maximum hardness is achieved at the periphery which gradually decreases towards the centre. Chinese grade has a similarity with the Fe 600 rebar in terms of strength and % elongation, whereas Fe 500D and Fe 500 have lower strength but higher ductility. The EDS analyses of corrosion products obtained after immersion test in 5% NaCl solution for a period of 7 days apparently indicate the occurrences of iron oxy-hydroxides and iron oxides. X-ray diffraction and FTIR studies of the corrosion products formed on all thermo-mechanically treated (TMT) rebar surfaces after the aforesaid immersion test primarily indicate the presence of Lepidocrocite (γ-FeOOH), Goethite (α-FeOOH), Magnetite (Fe₃O₄) and Maghemite (γ-Fe₂O₃). Fe 500 (12?mm) and Chinese rebars have lower corrosion resistance as compared to the other rebars in 5% NaCl solution. Nevertheless, all the TMT rebars are corrosion resistant and can be satisfactorily used for construction purposes.     

Kinetics of Oxidation of Fe–6Si

Surface oxidation of Fe–6Si during annealing in low-pressure air (～10⁴ Pa) in the temperature range 500–550 °C was investigated using resistivity measurements, Mössbauer spectroscopy, X-ray diffraction and scanning-electron microscopy (SEM). The time dependence of the resistivity exhibits an increase in two steps, which indicates changes in the structure and/or phase composition of the alloy. Structure and phase investigations show that the first step can be explained as formation of hematite (α-Fe₂O₃) and the second step is due to transformation of the hematite to magnetite (Fe₃O₄). The kinetics of the transformations were derived from the resistivity data. The activation energies (estimated from Arrhenius plots) of 194 kJ/mol and 165 kJ/mol were obtained for the formation of hematite and transformation of hematite to magnetite, respectively.     

Phasenumwandlungen im Rost

Phase conversions in rust The principal components of rust are lepidocrocite (γ-FeOOH), göthite (α-FeOOH) and magnetite (Fe₃O₄). The first crystal phase, possibly via (FeOH)₂, is lepidocrocite. This may later be transformed into göthite and magnetite. The paper describes pseudo-morphoses of gothite and magnetite after lepidocrocite. Magnetite, which at higher temperatures is formed directly on the iron, grows epitactically and forms a protective layer which prevents further corrosion. In contrast, magnetite transformed from lepidocrocite cannot be expected to have such an effect. The necessary adhesion and freedom from pores is conditioned by the epitaxy of a protective layer.     

Characteristics of iron corrosion scales established under blending of ground, surface, and saline waters and their impacts on iron release in the pipe distribution system

Interior scales on PVC, lined ductile iron (LDI), unlined cast iron (UCI) and galvanized steel (G) were analyzed by XRD, RMS, and XPS after contact with varying water quality for 1 year. FeCO₃, α-FeOOH, β-FeOOH, γ-Fe₂O₃, Fe₃O₄ were identified as primary UCI corrosion products. No FeCO₃ was found on G. The order of Fe release was UCI > G ? LDI > PVC. For UCI, Fe release decreased as % Fe₃O₄ increased and as % Fe₂O₃ decreased in scale. Soluble Fe and FeCO₃ transformation indicated FeCO₃ solid was controlling Fe release. FeCO₃ model and pilot data showed Fe increased as alkalinity and pH decreased.     

A mössbauer investigation of the surface of α-iron-I. Corrosion and passivation in H2O2 solution

α-iron foils previously exposed to air were studied by DCEMS (Depth Selective Conversion Electron Mössbauer Spectroscopy). The foils were found to be covered by a thin (～ 4 nm) passive layer not giving rise to any Mössbauer signal, but detectable by DCEMS. ESCA studies showed that it contained ferric ions and -OOH but no α-iron. The first stages of corrosion of α-iron in water and the passivating influence of H₂O₂ were studied by DCEMS. Oxide thicknesses and distributions in the range of 5–100 nm were determined. At room temperature, corrosion is inhibited at H₂O₂ concentrations of 0.006 and 16.8%, while intermediate concentrations lead to the formation of γ-FeOOH. At 85°C the corrosion is not fully inhibited.     

The X-ray photo-electron spectra ofseveral oxides of iron and chromium

X-ray photo-electron spectra of Fe₂O₃, Fe₃O₄, α-FeOOH, γ-FeOOH, Cr₂O₃,Cr(OH)₃· 0·4H₂O and CrO₃ were measured. The peak binding energies of 2p, 3s and 3p electrons of Fe and Cr in the above substances were determined. The largest valency dependence was observed in 2p electrons. Binding energies of O 1s electrons were also measured for those oxides and hydroxides. For quantitative analysis the ratios of photo-electron cross-sections of Fe 2p_3/2 and Cr 2p3/2 to O 1s electron levels were estimated as 1.45 and 1.71, respectively, for excitation by A1 Kα_1,2 radiation.     

An electrochemical study of phase-transitions in rust layers

Isolated rust layers have been investigated by electrochemical methods to find out whether their reduction and re-oxidation can affect the atmospheric corrosion of iron. At potentials below 0 mV, first a thin Fe²⁺-containing surface layer is formed on top of the γ-FeOOH. This reduced surface layer can dissolve into the cell electrolyte at acid pH, or at lower potentials the Fe²⁺-ions can react with γ-FeOOH to Fe₃O₄. The formation of magnetite could be followed by in-situ magnetic measurements. The reduced surface layer can easily be oxidized back to γ-FeOOH, magnetite can partly be oxidized to γ-Fe₂O₃.     

Corrosion resistance and mechanical properties of low-alloy steels under atmospheric conditions

The corrosion resistance and mechanical strength of four newly developed low-alloy steels (LAS) were compared with a carbon steel (SS400) and a weathering steel (Acr-Ten A) using a laboratory-accelerated test that involved cyclic wet/dry conditions in a chloride environment (5 wt.% NaCl). The new LAS were designated 1605A, 1605B, 1604A, and 1604B. After 72 cycles of cyclic corrosion tests, the susceptibility of the steels to corrosion could be listed in the following order based on their weight loss (from high to low): SS400 > Acr-Ten A > 1604B ? 1604A > 1605B ? 1605A. The change in mechanical properties by corrosion was the least for SS400, Acr-Ten A was second, and effects of corrosion on the mechanical properties of the other four low-alloy steels were similar. Finally, the characteristics of the rust layers on each LAS sample were observed by SEM, and analyzed by FTIR and EPMA. The results indicated that most of the rust layers on the test steels were composed of a loose outer rust layer and a dense inner rust layer. The outer rust layer of each steel was composed of α-FeOOH, γ-FeOOH, magnetite (Fe₃O₄), H₂O, and amorphous ferric oxyhydroxide (FeO_x(OH)_3−2x, x=0-1), while the inner rust layer was composed mainly of Fe₃O₄ with a little α-FeOOH. In addition, it was apparent that the copper and chromium alloying additions were enriched, respectively, at the rust-layer/substrate interface and in the rust layers. Finally, combining the results of the accelerated tests and the rust layer analysis showed that low-alloy steels, such as 1605A and 1605B, have better weathering steel properties than Acr-Ten A for use in the humid and salty weather.     

New method of mechanical alloying of ODS steels using iron oxides

Processes of mechanical alloying of oxide-dispersion-strengthened reactor pressure-vessel steels by cold high-pressure torsion of a powder mixture of low-stable Fe₂O₃ (Fe₃O₄) iron oxides and the bcc matrix alloyed with Y and Ti have been investigated using Mössbauer spectroscopy, X-ray diffraction analysis, and electron microscopy. Some features of decomposition of iron oxides and phase transformations in the matrices synthesized by mechanical alloying with formation of solid solutions supersaturated with oxygen and various compounds of oxygen with iron and alloying elements, in particular, special nanooxides of yttrium and titanium have been established.     

Corrosion resistance of the Dhar iron pillar

The corrosion resistance of the 950-year old Dhar iron pillar has been addressed. The microstructure of a Dhar pillar iron sample exhibited characteristics typical of ancient Indian iron. Intergranular cracking indicated P segregation to the grain boundaries. The potentiodynamic polarization behaviour of the Dhar pillar iron and mild steel, evaluated in solutions of pH 1 and 7.6, indicate that the pillar iron is inferior to mild steel under complete immersion conditions. However, the excellent atmospheric corrosion resistance of the phosphoric Dhar pillar iron is due to the formation of a protective passive film on the surface. Rust analysis revealed the presence of crystalline magnetite (Fe_3−xO₄), α-Fe₂O₃ (hematite), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), akaganeite (β-FeOOH) and phosphates, and amorphous δ-FeOOH phases. The rust cross-section revealed a layered structure at some locations.     

Mössbauer spectroscopy of the nanocrystalline materials

X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements were used to study the magnetic properties and the parameters of hyperfine interactions in the nanocrystalline (with a grain size less than 10 nm) and microcrystalline alloys Fe₉₀Ge₁₀, Fe₇₇Al₂₃ and pure α-Fe. It has been established that the nanocrystalline state does not affect the formation of the saturation magnetization, Curie temperature, isomeric shift, and hyperfine magnetic field. No additional sextets in the Mössbauer spectra and no additional features in the temperature dependences of dynamic magnetic susceptibility of the alloys investigated have been revealed. In the Mössbauer spectrum of pure nanocrystalline iron, a slight line broadening (～20%) is observed.     

A Mössbauer spectroscopic study of rust formed during simulated atmospheric corrosion

Steel coupons were subjected to 100% relative humidity and were inoculated every day with 100 μl of 0.01 N solutions of NaCl, Na₂SO₄, LiCl or CsCl. The first solid rust constituent that formed contained significant amounts of both γ-FeOOH and ferrihydrite. In contrast, only γ-FeOOH was observed in the rust formed during atmospheric corrosion and during wet-dry cycling with distilled water in the laboratory. The ferrihydrite in time was converted to γ-FeOOH, α-FeOOH, and γ-Fe₂O₃. The fractions of ferrihydrite + γ-FeOOH in the rust formed as a function of time during atmospheric exposure and during rusting in the laboratory environment were the same in the two cases.