Aufklärung von Korrosionsprozessen bei der Erdölverarbeitung mittels Mössbauer- und Röntgenuntersuchung – II. Wasserkondensator und Separatoren

Study of the corrosion processes at the primary oil refining by Mössbauer and X-ray investigation – II. Water condensator und separators Samples from the tube bundle surface of a water condensator and from benzene-water separators in an operating installation for primary oil refining are investigated. The compositions of the samples and the types of corrosion on the technological route of the refined oil were established on the basis of Mössbauer spectroscopy and X-ray diffraction analysis.     

Korrosionsprozesse in Separatorabwässern der atmosphärischen Erdöldestillation

Corrosion processes in separator effluents from atmospheric oil distillation Using Mössbauer spectroscopy the composition of corrosion products has been studied which had formed in hydrogen sulphide containing effluents from an installation for the atmospheric oil distillation. Flat steel specimens had been placed in the installation. The compositions found are correlated with gravimetric corrosion rates. On this basis, fundamental mechanism for such corrosion processes is suggested.     

Phasenumwandlungen in Korrosionsschichten auf Eisen: Mößbauerspektroskopische Untersuchungen

Phase transformations in corrosion layers on iron: Mössbauer spectroscopic studies The corrosion of steel surfaces was studied by Mössbauer spectroscopic phase analysis in scattering and transmission geometry. In atmospheres of low corrosivity, long exposure times result mainly in the relatively stable β-phases among the possible iron corrosion products. An increase in corrosivity by addition of hydrochloric acid leads to a pronounced formation of β-FeOOH, whereas the α-phases are absent. The influence of different coatings on the phase composition was also studied. The formation and transformation of the different iron containing phases in rust can be understood in terms of a stability diagram. Furthermore, the effect of so-called “rust transformers” was studied. None of these products showed the desired effect of phase transformations towards the formation of protective compounds such as Fe₃O⁴. In some cases it was found that the rust transformers even caused a destruction of an already existing protective layer.     

Der Einfluß eines Magnetfeldes auf Korrosionsprozesse

Magnetic field effect on corrosion processes The corrosion products of low carbon steel at static conditions were studied by means of Mössbauer spectroscopy. The corrosion media are sea water and model solution in the presence or absence of an external magnetic field. The magnetic field acts on the dispersity of the appearing phases and increases the paramagnetic components in the rust. The corrosion media and the orientation of the coupons have been of importance for the corrosion processes.     

Mössbauer and x-ray diffraction phase analysis of rusts from atmospheric test sites with different environments in Sweden

Rust samples from mild steel panels exposed at eight different atmospheric test sites in Sweden have been studied by transmission ⁵⁷Fe-Mössbauer spectroscopy and X-ray powder diffraction. All samples exhibited superparamagnetic effects in Mössbauer spectra recorded at room temperature, due to fine particles. The corrosion products were mainly identified in low temperature (∼10 K) Mössbauer spectra and with X-ray powder diffraction. In all samples, α-FeOOH and γ-FeOOH were the main constituents (>70%). Only in the marine samples were β-FeOOH and Fe_3-σO₄ found. The α-FeOOH/γ-FeOOH ratio obtained was found to increase with the SO₂ concentration at the test sites.     

Simultaneous Acquisition of Mössbauer Spectra and Microbalance Data in Situ: A Possible New Technique for the Study of Iron Corrosion and Oxidation

Abstract

Mössbauer spectra were obtained from an iron foil suspended from the arm of a microbalance. The collection of simultaneous Mössbauer and microbalance information appears to provide a powerful in situ technique for the study of iron corrosion and oxidation.     

Mößbauer- und ESCA-Untersuchungen zur Bildung oxidischer Eisenphasen in wäßrigen Medien unter der Einwirkung organischer Korrosionsinhibitoren

Mössbauer and ESCA investigations on the formation of oxidic iron phases in aqueous solution under the influence of organic corrosion inhibitors Corrosion layers on steel grown in water of well defined hardness and chloride concentration were studied by Mossbauer and ESCA spectroscopy with particular emphasis on the influence of added organic inhibitors. Relatively thick layers were found with an unexpectedly small iron content (as FeOOH). The layers contain a remarkable amount of constituent ions from the solution and fragments of the inhibitors. The latter seem to be decomposed by the corrosive medium: It is assumed that the whole organic molecule determines the kind of transportation of the inhibitor to the iron metal, but that the inhibition itself is due to functional groups only.     

Characterisation and comparison of corrosion products originated in steel pipelines transporting sour gas and crude oil

In this work, X-ray diffraction and Mössbauer spectroscopy techniques were used to characterise and carry out a comparison of the corrosion products obtained during the pigging activities in two sour-hydrocarbon-transporting pipelines located in the Gulf of Mexico: the first one being a gas pipeline and the other one a crude oil pipeline. The results indicate that for these specific conditions, there are differences between the corrosion products formed in each pipeline. Iron sulphides and oxides were the main corrosion products and their presence is directly related to the operating conditions prevailing in the pipelines as well as to the hydrocarbon quality. As for the sour gas pipeline, higher concentrations of greigite and pyrite were observed, whereas more proportions of magnetite, mackinawite and marcasite were found in the sour crude oil pipeline. The most important parameters in the formation of the different types of corrosion products are the water content, hydrogen sulphide concentration and oxygen presence in the system.     

Characterization of oxides of stainless steel UNS S30400 formed in offshore environment

Thick corrosion products of austenitic stainless steel obtained from a steam turbine structure exposed in offshore environment were studied by optical stereomicroscopy, X-ray diffraction, Mössbauer and energy-dispersive X-ray spectroscopy. The external surface of mechanical structure was exposed to a marine atmosphere and the internal one to a combustion gas for two decades. Characterization of corrosion products showed high content of akaganeite in the outer region and a mixture of akaganeite and hematite in the inner surface. Besides these characteristics, a distribution of nanosized superparamagnetic particles of akaganeite was found in the rusts.     

The formation of structure and phase composition and magnetic properties of Fe(Fe<Subscript>3</Subscript>C,Fe<Subscript>5</Subscript>SiC)-SiO<Subscript>2</Subscript> nanocomposites upon mechanical alloying

Structural and phase transformations that occur in Fe(Fe₃C, Fe₃SiC)-SiO₂ (amorphous quartz) systems during mechanical alloying in an Ar atmosphere and in air have been studied by X-ray diffraction, Mössbauer spectroscopy, IR spectroscopy, electron microscopy, and magnetometry. It has been shown that the mechanoactivation treatment leads to the formation of isolated particles 2–20 nm in size with a complex phase composition (Fe, FeSi alloy, oxides, silicates, and carbides), which depends on the milling atmosphere. It has been found out that the magnetic properties of such systems strongly depend on the oxygen and carbon compounds existing in the system, which cannot be detected by X-ray diffraction, but their presence is testified by the data of Mössbauer spectroscopy.     

Mössbauer spectroscopy of interfaces in metals

Publications on the studies of interfaces in metals by the methods of Mössbauer (nuclear gammaresonance) spectroscopy are reviewed. Physical principles of the Mössbauer effect, various methods of nuclear gamma-resonance (NGR) spectroscopy and the Mössbauer spectra parameters are considered. The available results on grain boundary studies in coarse-grained and nanostructured materials and on interlayer interfaces in multilayers are analyzed. Capabilities of application of absorption and emission Mössbauer spectroscopy as well as of conversion electrons Mössbauer spectroscopy (CEMS) are discussed.     

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.     

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.     

X-ray and intrinsic magnetic properties of nanocrystalline Sm2(Fe,M)17 (M = Si,Ga, Co,Cr, Zr or Mo)

The structure and magnetic properties of ball-milled and subsequently annealed Sm₂Fe_17–xM_x compounds (M = Si, Ga, Co, Cr, Zr, Mo, for x = 2) were investigated by X-ray diffraction coupled with magnetic measurements and Mössbauer spectroscopy. Rietveld analysis have shown that all these samples crystallize in the Th₂Zn₁₇ type rhombohedral structure. The substituting elements Si, Ga, Co occupy the 18h atom site, while Cr, Zr, Mo prefer the 6c site. Mössbauer studies corroborate these results. The X-ray analysis cannot solve the possible M atom distribution over Fe atom sites when atomic factors of Fe and M are close. Mössbauer spectroscopy appears as the efficient tool to raise this ambiguity. The Curie temperature increases upon Zr and Cr substitution reaching a maximum for x = 1, then decreases because of magnetic dilution.     

Mößbauerspektroskopische Untersuchungen der Eisenkorrosion

Mößbauerspectroscopic studies of the corrosion of iron The mößbauerspectroscopic phase analysis is applied for studying the surface corrosion of pure iron. The results concern the rate, the layer structure, and phase transformations during the corrosion under mild and humid conditions, and the sub-rusting below protecting coatings. The study of exposed samples yields informations about the influence of aggressive atmospheres and of climate factors. Some conclusions are given concerning the formation of the lowest corrosion layer and the subsequent phase transformations.     

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.     

The application of conversion electron Mössbauer spectroscopy (C.E.M.S.) to the study of corrosion inhibition in brass condenser tubes

The construction is described of a simple counter for measuring the conversion electron Mössbauer spectrum of the inside surface of a tube. The counter is used to investigate the effects of adding ferrous sulphate to the cooling water of condensers with brass tubes to inhibit corrosion. It is concluded that the treatment leads to the formation on the inside of the tube of a layer of FeOOH, the exact nature of which depends slightly on the local conditions.     

Substitutional effects in RFe3 intermetallics (R=Dy,Sm, Y)

Samples of RFe₃ (R=Dy, Sm, Y) were doped with Si, Al and Ti and studied by Mössbauer spectroscopy and X-ray diffraction. Most Mössbauer spectra were analyzed with three sextets, corresponding to the 6c, 3b and 18 h inequivalent lattice sites for iron. A quadrupole split doublet was also present in some spectra, which became dominant for the case of Al substitution. The collapse of the hyperfine magnetic structure is a magnetic effect due to substitutions: the X-ray diffraction patterns yielded a particle size of about 100 nm, thus precluding the occurrence of superparamagnetism in these systems.     

N–N interaction and nitrogen activity in the iron base austenite

The Monte Carlo computer simulation of the f.c.c. Fe–N alloy is performed using the data of abundances of different iron sites in the austenite lattice as obtained by means of Mössbauer spectroscopy. The validity of the interpretations of available Mössbauer spectra was tested based on the data of calculation of N–N interaction energies in the two first coordination spheres on the interstitial sublattice, which could satisfy the experimental data of nitrogen distribution in austenite derived from Mössbauer spectra. It is shown that no values of N–N interaction energies exist that could be consistent with the data of conversion electron Mössbauer spectroscopy (CEMS), where a thin surface layer only contributes to the Mössbauer spectra. A strong N–N repulsion (>0.14 eV) in the first coordination sphere and a soft N–N repulsion (<0.07 eV) in the second one was found to be consistent with the studies performed by means of transmission Mössbauer spectroscopy (TMS), according to which single nitrogen atoms and 180° N–N pairs exist in the nitrogen austenite. A possibility of the long-range order-like Fe₄N is shown at the energy values determined for two coordination spheres, provided a small N–N repulsion in the third coordination sphere occurs. It is also observed that the available data of nitrogen activity in Fe–N austenite are not consistent with the values of N–N interaction energies determined from the analysis of Mössbauer spectra, i.e. the available activity data do not correspond to the nitrogen distribution in the iron austenite as it follows from Mössbauer data. The concentration dependence of nitrogen activity and an effect of the N₂ gas pressure on the nitrogen solubility in Fe–N austenite are calculated using the values of N–N interaction energies in two coordination spheres.     

Micromagnetic and Mössbauer spectroscopic investigation of strain-induced martensite in austenitic stainless steel

Strain-induced martensite in 18/8 austenitic stainless steel was studied. Magnetic measurements and Mössbauer spectroscopic investigations were performed to characterize the amount of α’-martensite due to room-temperature plastic tensile loading. The effects of cold work and annealing heat treatment were explored using magnetic Barkhausen noise, saturation polarization, coercive force, hardness, and conversion electron Mössbauer spectra measurements. The results of the magnetic measurements were compared to results obtained by Mössbauer spectroscopy. The suggested Barkhausen noise measurement technique proved to be a useful quantitative and nondestructive method for determining the ferromagnetic phase ratio of the studied alloy.