Role of hydrogen in stress corrosion cracking of austenitic stainless steels

Effects of hydrogen charging on the stress corrosion cracking (SCC) behavior of 304 and 310 stainless steels under sustained load were investigated in boiling 42% MgCl₂ solution. The cracking was accelerated by the incorporation of hydrogen into the steel without altering the crack growth mechanism. The fact that the active dissolution is almost unaffected by the hydrogen charging and tensile stress indicates that the phenomenon of hydrogen-promoted SCC is unlikely a result of hydrogen-facilitated active dissolution. In contrast, hydrogen significantly promotes anodic dissolution in the potential range where the active-to-passive transition occurs. The electrochemical noise detected in the SCC process implies that the crack propagation process is discontinuous and hydrogen charging can raise the frequency of film breakdown at the crack tip. These observations suggest that the hydrogen-promoted SCC may result from the hydrogen-induced passivity degradation.     

A Mössbauer study of the silica-like structures of FePO4 and (Al0.67Fe0.33)PO4

Summary Samples of FePO₄ and (Al_0.67Fe_0.33)PO₄ have been investigated by M?ssbauer spectroscopy. The data obtained on quadruple splittings are in agreement with neutron scattering and XRD data on (Al, Fe)O₄ tetrahedra structures.     

New in-beam Mössbauer spectroscopy station at the Budapest Research Reactor

We report about a new in-beam Mössbauer station which is intended to utilize the cold neutron source and guide system installed on the 10th beam line of the Budapest Research Reactor. This new in-beam facility enables us to broaden the number of nuclides accessible for Mössbauer studies in various materials. In this article we describe our new system and summarize the possibilities of its application to Mössbauer spectroscopy.     

Stacking fault energy of cryogenic austenitic steels

Stacking fault energy and stacking fault nucleation energy are defined in terms of the physical nature of stacking faults and stacking fault energy, and the measuring basis for stacking fault energy. Large quantities of experimental results are processed with the aid of a computer and an expression for calculating stacking fault energy has been obtained as γ³⁰⁰_SF(mJ·m^-2)=γ⁰_SF+1.59Ni-1.34Mn+0.06Mn²-1.75Cr+0.01Cr²+15.21Mo-5.59Si-60.69(C+1.2N)^1/2 + 26.27(C+1.2N)(Cr+Mn+Mo)^1/2+0.61[Ni·(Cr+Mn)]^1/2.     

Generation of long-lived isomeric states via bremsstrahlung irradiation

A method to generate long-lived isomeric states effectively for Mössbauer applications is reported. We demonstrate that this method is better and easier to provide highly sensitive Mössbauer effect of long-lived isomers (>1 ms) such as ¹⁰³Rh. Excitation of (γ,γ) process by synchrotron radiation is painful due mainly to their limited linewidth. Instead, (γ,γ’) process of bremsstrahlung excitation is applied to create these long-lived isomers. Isomers of ⁴⁵Sc, ¹⁰⁷Ag, ¹⁰⁹Ag, and ¹⁰³Rh have been generated from this method. Among them, ¹⁰³Rh is the only one that we have obtained the gravitational effect at room temperature.     

Emission Mössbauer studies of 57Co2+ and 119Sb5+ adsorbed on γ-Fe2O3 and NiFe2O4 surfaces

Summary Surface chemical states of⁵⁷Co²⁺ and¹¹⁹Sb⁵⁺ ions adsorbed on γ-Fe₂O₃ and NiFe₂O₄ were studied in the presence of the aqueous phase using the magnetic interaction between the substrates and⁵⁷Fe²⁺ or¹¹⁹Sn⁴⁺ arising from the adsorbed species. Two chemical forms were observed for the⁵⁷Co²⁺ adsorbed on γ-Fe₂O₃; one giving a peak in the high-field region and another giving a broad distribution in the lower fields, which were attributed to⁵⁷Co²⁺ at surfaceB sites of the spinel structure of γ-Fe₂O₃ and⁵⁷Co²⁺ species weakly bonded to the substrate, respectively. In case of the NiFe₂O₄ substrate, the latter species was dominant and was converted to the former by heating. Most of the¹¹⁹Sb⁵⁺ ions adsorbed on the γ-Fe₂O₃ and NiFe₂O₄ particles were found to have a certain magnetic interaction with the magnetically ordered ions of the substrates. Paper presented at the ICAME-95, Rimini, 10–16, September 1995     

Dynamic Jahn-teller effect in a Fe-N6 cage molecule

Mössbauer spectroscopic studies of [Fe(di(amH)-sar)](NO₃)_4·H₂O where di(amH)-sar represents 1,8 diamino 3,6,10,13,16,19-hexaza bicyclo [6,6,6] icosane in the temperature range of 4.2 to 300 K suggest that it undergoes a dynamic Jahn-Teller effect as revealed from the plots of temperature dependent quadrupole coupling constant, chemical shift and line width values. The spectrum down to 4.2 K, shows a quadrupole doublet with no magnetic hyperfine splitting.     

Mössbauer In Situ Studies of the Surface of Mars

For the first time in history, a Mössbauer spectrometer was placed on the surface of another planet. Our miniaturized Mössbauer spectrometer MIMOS II [1–4] (Figure 2) is part of the instrument payload of NASA’s twin Mars Exploration Rovers (MER) “Spirit” and “Opportunity” (see Figure 1), which in January 2004 successfully landed at the Gusev crater and the Meridiani Planum landing sites, respectively. MIMOS II determines the Fe-bearing mineralogy of Martian soils and rocks at the Rovers’ respective landing sites [5]. The main goals of this planetary twin mission are to: (1) identify hydrologic, hydrothermal, and other processes that have operated and affected materials at the landing sites; (2) identify and investigate the rocks and soils at both landing sites, as there is a possible chance that they may preserve evidence of ancient environmental conditions and possible prebiotic or biotic activities. With MIMOS II, besides other minerals the Fe silicate olivine has been identified in both soil and rocks at both landing sites. At the Meridiani site the Fe sulfate jarosite has been identified by MIMOS II which is definitive mineralogical proof of the presence of water at this site in the past.     

Mössbauer study of iron oxide modified montmorillonite

Montmorillonite particles were modified by iron oxides using the precipitation process with the aim to monitor the differences in the structural and magnetic properties of intercalated and adsorbed Fe³⁺. The Mössbauer spectra recorded at 5 K in zero and 6 T external fields, IR spectra and TG curves measured in zero and 32 mT fields identified the ferrihydrite pillars in an interlayer space of the montmorillonite structure and γ-Fe₂O₃ nanoparticles adsorbed on the mineral surface. The temperature dependent Mössbauer spectra (25–300 K) reflect the superparamagnetic behaviour of maghemite nanoparticles and ferrihydrite pillars with the blocking temperatures of about 80 and 25 K, respectively.     

Evolution of dislocation loops in austenitic stainless steels implanted with high concentration of hydrogen

Abstract

It has been found that under certain conditions, hydrogen retention would be strongly enhanced in irradiated austenitic stainless steels. To investigate the effect of the retained hydrogen on the defect microstructure, AL-6XN stainless steel specimens were irradiated with low energy (100 keV) H₂⁺ so that high concentration of hydrogen was injected into the specimens while considerable displacement damage dose (up to 7 dpa) was also achieved. Irradiation induced dislocation loops and voids were characterised by transmission electron microscopy. For specimens irradiated to 7 dpa at 290 °C, dislocation loops with high number density were found and the void swelling was observed. At 380 °C, most of dislocation loops were unfaulted and tangled at 7 dpa, and the void swellings were observed at 5 dpa and above. Combining the data from low dose in previous work to high dose, four stages of dislocation loops evolution with hydrogen retention were suggested. Finally, molecular dynamics simulation was made to elucidate the division of large dislocation loops under irradiation.     

The Mössbauer community in the USA

Scientists in the United States assumed major roles in developing the Mössbauer community during its early years. However, since the termination of the Mössbauer Effect Methodology meetings in 1976, there has been little in the way of regular Mössbauer meetings in the United States. Nevertheless, there is an active United States Mössbauer community, as noted by the number of annual publications – 156 in 2004. In recent decades, attendance of Mössbauer researchers from the United States at the International Conferences on the Applications of the Mössbauer Effect (ICAME) has been far below what would be expected from the number of contributions in the Mössbauer literature. Attempts have been made, unsuccessfully, to arrange for regular Mössbauer meetings. Models for possible future Mössbauer meetings of US scientists are discussed, including a regular biannual meeting, and another being a virtual Mössbauer conference. Also discussed are other models to maintaining an active Mössbauer community in the United States, making use of information technologies that are available to us along with other resources we can use.     

La–Zn Substituted Hexaferrites Prepared by Chemical Method

La–Zn substituted M-type Ba hexaferrite powders were prepared by sol-gel (Mx) and organometallic precursor (Sk) methods with Fe/Ba ratio of 11.6 and 10.8, respectively. The compositions (LaZn) _x Ba_{1 ? x} Fe_{12 ? x} O₁₉ with 0.0 ≤ x ≤ 0.6 were annealed at 975°C/2 h. The cationic site preferences of nonmagnetic La³⁺ instead of Ba²⁺ ions and Zn²⁺ instead of Fe³⁺ ions were determined by Mössbauer spectroscopy. The La³⁺ ions substitute the large Ba²⁺ ions at 2a site and for x ≥ 0.4 also at 4f₂ site. The nearly all Zn²⁺ ions are placed at the 4f₁ sites. The thermomagnetic analysis of χ(?) confirms that only the small substitutions for x ≤ 0.4 can be taken as a single-phase hexaferrites. The coercivity H _c almost does not change at x = 0.2 for (Mx) samples and further decrease up to x = 0.6. For (Sk) samples at substitution x = 0.2 the values of H _c are decreasing and at higher x the values nearly do not change. The Curie points, T _c, slowly decrease with x for both (Mx) and (Sk) samples.     

Effect of the Zn2+ distribution on the magnetic properties of Ba2Zn2Fe12O22(Zn2-Y) hexaferrite investigated by thermal-scan Mössbauer spectroscopy

Summary The role of the cation distribution on the magnetic characteristics of Ba₂Zn₂Fe₁₂O₂₂(Zn₂-Y) hexaferrite has been analysed by using M?ssbauer techniques. For this purpose we developed a low-cost microcomputer-controlled thermal-scan M?ssbauer apparatus that allows accurate measurements of the Curie temperature. The measuring times are much shorter than those attained with previously described similar apparata. The variation of the Curie temperature with the sintering temperature of Zn₂-Y has been correlated with different cation distributions of Zn²⁺ among the two tetrahedral sublattices present in the Y-structure.

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Riassunto Utilizzando tecniche M?ssbauer si è analizzato il ruolo della distribuzione cationica sulle proprietà magnetiche dell’esaferrito Ba₂Zn₂Fe₁₂O₂₂ (Zn₂-Y). A questo scopo è stato messo a punto un apparato a basso costo controllato da microcomputer per analisi termiche che utilizza l’effetto M?ssbauer. L’apparecchiatura consente misure di temperatura di Curie con tempi di misura inferiori a quelli di apparecchiature simili descritte in precedenza. La variazione della temperatura di Curie al variare della temperatura di sinterizzazione osservata nell’esaferrito Zn₂-Y è stata correlata con la distribuzione del catione Zn²⁺ tra i due siti tetraedrici presenti nella struttura Y.

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Резюме Используя технику эффекта Мессбауэра, анализируется роль распределения катионов на магнитные свойства гексаферрита Ba₂Zn₂Fe₂O₂₂(Zn₂-Y). Для этой цели предлагается недорогостоящая установка, контролируемая компьютером для тырмического анализа, которая исрользует зффект Мессбауэра. Предложенная установка позволяет точно измерять температуру Кюри, причем времена измерений оказываются существенно меныше, чем у родобных установок, орисанных в литературе. Изменение температуры Кюри в завицимости от температурм спекания Zn₂-Y коррелирует с различными распределениями катионов Zn²⁺ между двумя тетраздрическими подрешетками, присутстствующими в Y-структуре.

     

Phase composition and distribution of corrosion products grown on galvanised steel in contact with hot water

The composition of localised corrosion products formed on galvanised steel in contact with hot water was determined using X-ray diffraction, electron probe microanalysis, Raman and Mössbauer spectroscopies. Iron carbonate, a Zn–Fe mixed carbonate, goethite, lepidocrocite and akaganeite were identified together with some Fe-free components. The necessity of using different, complementary techniques of analysis is emphasised and some features of the corrosion mechanism are discussed.     

Effect of preparation procedure on the magnetic and transport properties of double perovskite Sr2FeMoO6

Ordered and disordered double perovskite Sr2FeMoO6 ceramics have been investigated by powder x-ray diffraction,magnetic and transport measurements, as well as Moessbauer spectroscopy. The heavily disordered sample can be acquired by annealing the ordered samples in argon. The annealing procedure affects not only the nature of grain boundaries but also the grain itself. The evidence of Moessbauer spectra performed at 77 and 300K indicates that there exist small oxygen deficient clusters of SrFeO3-y in the disordered sample. The paramagnetic Fe^4 and Fe^3 ions in the compound subsist down to 77K and the ratio of Fe^4 /Fe^3 increases with decreasing temperature.     

57Fe Mössbauer and magnetic studies of DyFe12−x Ta x compounds

Refinements of the X-ray diffraction patterns show that DyFe_12?x Ta _x compounds with x = 0.5–0.7 crystallise in the ThMn₁₂-type structure and that the Ta atoms occupy the 8i sites. Spin reorientations have been detected by ac magnetic susceptibility for all compounds below room temperature. First the moments shift direction from easy axis to easy cone at T _sr1, then to easy plane at T _sr2. Both T _sr1 and T _sr2 increase with increasing Ta content up to x = 0.65 before decreasing with further increase in Ta content. Analyses of the Mössbauer spectra indicate that the individual Fe site hyperfine fields derived at 4.5 K for DyFe_11.35Ta_0.65 are B _hf = 37.4 T, 32.2 T and 27.6 T for the 8i, 8j and 8f sites, respectively.     

Effect of Precipitation Temperature and Number of Iron Atoms per Molecule on the Structure of Hydrated Iron(III) Oxyhydroxide Ferritin Cores Synthesised in vitro

Hyperfine Interactions - Horse spleen apoferritin was reconstituted with varying numbers of iron atoms per protein shell at 25 and 50°C. Samples with mean particle sizes ranging from...     

Structural Behavior of Pt–Sn Supported on MgO

Pt–Sn supported on magnesia and alumina were characterized, before and after treatment with hydrogen, by Mössbauer spectroscopy and X-ray diffraction. For the calcined samples on both supports tin is present as SnO₂ and platinum as metal. After reduction with hydrogen, platinum and tin diffuse into the magnesia lattice to form a solid solution. On alumina Sn(IV), Sn(II), Sn(0), Pt, Pt₃Sn, PtSn and PtSn₂ alloys are formed. The SnO interacts strongly with the alumina support. The catalytic activity of both Pt–Sn catalysts is strongly affected by the support. On alumina the dehydrogenation of cyclohexane is very high, whereas that on magnesia is almost non-active.     

Isotope shift measurement at 244fAm *

Strips of Fe_62.5Co₆Ni_7.5Zr₆Cu₁Nb₂B₁₅, prepared by rapid quenching from the melt, were annealed at different temperatures and studied performing Mössbauer and magnetoelastic measurements. By increasing annealing temperature, the long range order increases strengthening the exchange interactions. The maximum of magnetoelastic response corresponds to the maximum of the out of plane magnetization. A possible explanation for the onset of the perpendicular magnetic anisotropy is proposed.     

Effect of FeCl3 and acetone on the structure of Na–montmorillonite studied by Mössbauer and XRD measurements

⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, X-ray fluorescence spectroscopy and infrared spectroscopy were used to study the effect of FeCl₃ and acetone on the structure of a Na–bentonite. XRD indicated the incorporation of Fe³⁺ ions into the interlayer space since the basal lattice spacing of montmorillonite increased to 1.6 from 1.24 nm after treatment with FeCl₃ dissolved in acetone. Interlayer Na⁺ ions could be exchanged to Fe³⁺. Magnetically split Mössbauer subspectra with internal magnetic fields 41 and 46 T at 74 K, were associated with two main Fe³⁺ microenvironments within the interlayer regions. The resultant Fe–montmorillonite was successfully applied as a catalyst in the preparation of 1,1-diacetates from aromatic aldehydes and acetic acid anhydride.