The atmospheric corrosion of iron as studied by Mössbauer spectroscopy

The composition of the rust on the surface of steel panels was determined after atmospheric exposure times of 2 weeks, 2 months and 6 months. The initial product is γ-FeOOH which converts in time to a mixture of α-FeOOH and γ-Fe₂O₃. Steel exposed for times of the order of 25 years is covered with corrosion product consisting largely of γ-Fe₂O₃. The similarity between the composition of the corrosion products and precipitates formed from FeSO₄ solution under mildly acidic conditions at 90°C suggests that the dominant anion in these atmospheric corrosion experiments is sulfate.     

Sn Mössbauer spectroscopy of the intermetallic compound HoRhSn

The powder X-ray diffraction data revealed that the structure of HoRhSn is isotypic with the hexagonal ZrNiAl type (space group ). Bulk magnetic AC and DC measurements have shown that this compound is ferromagnetic with a Curie temperature of T_C = 6.3(2) K. The thermal variation of the hyperfine parameters has been studied in detail by ¹¹⁹Sn Mössbauer spectroscopy. Especially, large distribution of magnetic hyperfine fields observed at the tin sites points to a rather complicated magnetic structure of probably non-collinear character but with the magnetic holmium moments arranged in directions close to the c-axis.     

Characterization of Delhi iron pillar rust by X-ray diffraction, Fourier transform infrared spectroscopy and Mössbauer spectroscopy

Rust samples obtained from the region just below the decorative bell capital of the Delhi iron pillar (DIP) have been analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Mössbauer spectroscopy. The identification of iron hydrogen phosphate hydrate in the crystalline form by XRD was unambiguous. Very weak diffraction from the oxyhydroxides/oxides of iron was observed indicating that these phases are most likely to be present in the amorphous form in the rust. The present XRD analysis of rust obtained from an inaccessible area of the DIP has also been compared with earlier analyses of DIP rust obtained from regions accessible to the public. FTIR indicated that the constituents of the scale were γ-, α-, δ-FeOOH, Fe_3−xO₄ and phosphate, and that the scale was hydrated. The unambiguous identification of the iron oxides/oxyhydroxides in the FTIR spectrum implied that they are present in the amorphous state, as XRD did not reveal these phases. The FTIR results have also been compared with earlier FTIR spectroscopic results of atmospheric rust formation. Mössbauer spectroscopy indicated that the rusts contained γ-FeOOH, superparamagnetic α-FeOOH, δ-FeOOH and magnetite, all in the amorphous form. The Mössbauer spectrum also confirmed that iron in the crystalline iron hydrogen phosphate hydrate, whose presence was confirmed by XRD, was in the ferric state indicating that it was a stable end corrosion product.     

Mössbauer spectroscopy used for testing of reactor steels

The investigation was focused on the application of Mössbauer spectroscopy (MS) in the evaluation of the microstructure parameters of materials used in nuclear industry. The usefulness of this method is documented on the evaluation of degradation processes going on in the nuclear power plant reactor pressure vessel (RPV) steels. Experimental MS results of different commercially used RPV-steels as well as results from the original irradiated Russian 15Kh2MFA RPV surveillance specimens are presented and discussed in the paper. The systematic changes in the relative areas of Mössbauer spectra components due to irradiation with high energy neutrons (E_n>0.5 MeV) were observed mainly during the first period (1-year stay in irradiation containers in operating conditions by a ‘speed factor’ of about 10). It could be explained due to changes caused by precipitation of elements like Cu or Cr mainly in carbides to the surface. These results confirm that the close environment of Fe atoms in bcc lattice of RPV-steels stay after initial changes almost stable and perhaps could be correlated with the ductile–brittle transition temperature curve from mechanical tests or with the defects density curve (obtained from the transmission electron microscopy studies) in dependence to the increased neutron fluency.     

Oxidation study of iron by Mössbauer conversion electron and γ-ray scattering

⁵⁷Fe Mössbauer conversion electron and 14.4 keV γ-ray back-scattering have been applied to study the surface of oxidized iron samples. While γ-ray scattering explores a surface depth of about 10–20 μm, conversion electron spectra are indicative for the state of the Fe ion in the outermost surface layer up to about 3000 Å thick. Several oxide phases, like haematite, magnetite and wüstite, could be identified in the various oxide layers by their Mössbauer parameters. It is shown that the electron scattering technique is sensitive enough to allow a phase analysis and a kinetic study of oxide film growth at 500°C in an early stage, i.e. for oxidation times of the order of minutes, even if natural iron samples are used. Some examples demonstrate that due to the different depth-selectiveness of electron and γ-ray scattering spectra the combination of both techniques can give information about the type of oxide phases which are present in different surface depths.     

Spin and charge density on iron nuclei in the BCC Fe–Mo alloys studied by Fe Mössbauer spectroscopy

Iron molybdenum alloys were prepared for the molybdenum concentration range 0–50 at.% by the arc melting method. X-ray diffraction patterns show single BCC phase for the Mo concentration up to 18 at.% with the lattice constant increasing upon addition of Mo. Sample with 21 at.% of Mo looks like composed of many BCC phases differing by the lattice constant. Finally, sample with 50 at.% of Mo looks like being composed of two BCC phases with various lattice constants and some amount of the non-stoichiometric λ-phase having symmetry P6₃/mmc. Mössbauer data indicate random solutions up to 12 at.% of Mo with magnetic order at room temperature. Room temperature paramagnetic phase appears for the sample with 18 at.% of Mo and its content increases with the increasing concentration of Mo. Traces of magnetically ordered phase (at room temperature) are seen for the sample with 40 at.% of Mo. Sample with 50 at.% of Mo is paramagnetic at room temperature. Contributions to the hyperfine field and isomer shift on the iron nuclei have been determined as the function of the distance between iron nucleus and Mo impurity up to the third co-ordination shell within the single-phase random solution range. Mo atom as the nearest iron neighbor changes iron hyperfine field by −4.18 T, as the second neighbor makes change by −2.30 T and finally as the third neighbor changes the field by +0.51 T. Corresponding changes in the isomer shift are as follows: −0.033 mm/s, −0.005 mm/s and +0.003 mm/s. The average hyperfine field and the isomer shift decrease linearly versus Mo concentration at rates −0.383 T/at.% and −5.06 × 10⁻⁴ mm/(s at.%), respectively. Hence, addition of Mo increases the electron density on the iron nucleus at the rate +1.7 × 10⁻³ electron a.u.⁻³(at.%)⁻¹.     

Hyperfine interactions studied by Sn Mössbauer spectroscopy in TbAuSn and TmAuSn compounds

X-ray diffraction, magnetic susceptibility measurements and ¹¹⁹Sn Mössbauer spectroscopy were used to study structure and hyperfine interactions in intermetallic TbAuSn and TmAuSn compounds. It was shown that the first compound orders antiferromagnetically at low temperatures with Tb magnetic moments inclined at an angle of about 25° to the crystallographic c-axis. The second compound stays non-magnetic in the whole measured region. The occurrence of a magnetic hyperfine field distribution in TbAuSn suggests that this compound crystallizes in the disordered CaIn₂-type of structure.     

Magnetic, transport and Mössbauer effect study of UFeAl

We present the results of magnetic susceptibility, electrical resistivity and Mössbauer effect measurements performed on two crystallographic structures of UFeAl: high-temperature (HT) and low-temperature (LT) phases, i.e. MgZn₂ and Fe₂P-type structures respectively.

The exchange-enhanced Pauli susceptibility and spin-fluctuation characteristics of the temperature dependence of the electrical resistivity of UFeAl are almost independent of the type of crystal structure.

⁵⁷Fe Mössbauer spectra of both crystallographic phases of UFeAl, recorded in the temperature range 10–295 K, consist of two symmetric quadrupole doublets for each crystal structure associated with the non-equivalent crystallographic positions of Fe. The Debye temperatures for the HT and LT structures of UFeAl were also estimated.     

A Mössbauer effect investigation of the formation of MnZn nanoferrite phase

In this paper, Mn_0.5Zn_0.5Fe₂O₄ nanopowders were prepared by mechanochemical processing of the mixture of two single phase ferrites, MnFe₂O₄ and ZnFe₂O₄. Room-temperature ⁵⁷Fe Mössbauer spectroscopy was used to study the mechanically induced evolution of the ZnFe₂O₄/MnFe₂O₄ mixture submitted to the high-energy milling process. The Mössbauer spectrum of the ZnFe₂O₄/MnFe₂O₄ sample milled for 30 h revealed the presence of Mn_0.5Zn_0.5Fe₂O₄. The mean crystallite size of the mechanosynthesized mixed ferrite, estimated using Scherrer's formula, was found to be 14 nm.     

Sn Mössbauer studies of the DyAgSn and DyAuSn compounds

X-ray diffraction, magnetic susceptibility measurements and ¹¹⁹Sn Mössbauer spectroscopy were used to study the intermetallic DyAuSn and DyAgSn compounds. It was shown that both compounds order antiferromagnetically at low temperatures. The Dy magnetic moments are normal to the hexagonal c-axis in DyAuSn and are inclined at an angle of 45° to this axis in DyAgSn. The occurrence of a magnetic hyperfine field distribution in DyAgSn suggests that this compound crystallizes in the disordered CaIn₂-type of structure.     

Spin- and charge density oscillations around Ir impurity in α-Fe studied by Fe Mössbauer spectroscopy

Random solution of iridium in ferromagnetic α-Fe of the BCC structure has been investigated by means of the 14.4 keV Mössbauer transition in ⁵⁷Fe at room temperature. Iridium has been randomly substituted on the iron sites with the concentration up to 11 at.%. Contributions to the hyperfine field and isomer shift on the iron nuclei have been determined as the function of the distance between iron nucleus and iridium impurity up to the third co-ordination shell. Iridium atom as the nearest iron neighbor changes iron hyperfine field by +0.66 T, as the second neighbor makes change by −1.88 T and finally as the third neighbor changes the field by +0.70 T. Corresponding changes in the isomer shift are as follows: −0.031, +0.003 and +0.033 mm/s. Hence, it was found that iridium atoms lead to the very strong oscillations in the spin- and charge density around themselves in the α-Fe BCC structure.     

Gd Mössbauer effect and magnetic properties of GdMn6Ge6

The magnetic properties of GdMn₆Ge₆ have been studied by magnetic measurements and ¹⁵⁵Gd Mössbauer spectroscopy. The Mn sublattice orders ferromagnetically in high magnetic fields and at high temperatures but in low fields and low temperatures there is a tendency to antiferromagnetic ordering. Antiferromagnetic order was also found in YMn₆Ge₆. The electric field gradient derived from the quadrupolar splitting of the ¹⁵⁵Gd Mössbauer spectra is substantially larger than in the isotypic compound GdMn₆Sn₆.     

Positron annihilation and Mössbauer spectroscopy applied to WWER-1000 RPV steels in the frame of IAEA High Ni Co-ordinated Research Programme

Positron annihilation spectroscopy (PAS) and Mössbauer spectroscopy (MS) were applied in the evaluation of the microstructure parameters and degradation processes of nuclear reactor pressure vessel (RPV) steel surveillance specimens. Study was oriented to the material investigation of Russian WWER-1000 steels (15Kh2MNFAA and 12Kh2N2MAA) with high Ni content (1.26 wt% in base metal and 1.7 wt% in weld). For comparison, the WWER-440 weld metal (Sv10KhMFT) without Ni was also measured. Specimens were studied in as-received form, after irradiation in LVR-15 experimental reactor to the neutron fluence F_(E>0.5MeV)=1.47×10²⁴ m⁻² s⁻¹ and after annealing process in vacuum at 475 °C per 2 h. Changes due to different chemical composition and irradiation were registered using MS. Post-irradiation thermal treatment and annealing of defects was well detected by different PAS techniques. Results confirm the hypothesis that Ni affects size (decrease) and distribution (more homogeneous) of the Cu- and P-rich clusters and M_xC_x carbides.     

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.     

Structural and Mössbauer spectroscopic investigation of Fe substituted Ti–Ni shape memory alloys

In the present investigation the effect of Fe substitution in Ti₅₁Ni₄₉ alloy has been studied. The alloys were synthesized through radio frequency induction melting. The alloy was characterized through X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Mössbauer spectroscopy and positron annihilation techniques. It was found that the Fe substitution stabilized the TiNi type cubic (a = 2.998 Å) phase. The microstructure and presence of the oxide phase in Ti₅₁Ni₄₅Fe₄ alloy have been investigated by scanning electron microscopy. The positron annihilation measurements indicated a similar bulk electron density in both the as-cast and annealed (1000 °C for 30 h) alloys, typically like that of bulk Ti. Mössbauer spectroscopy studies of as-cast and annealed iron substituted samples showed regions in the samples where nuclear Zeeman splitting of Fe levels occurred and an oxide phase was found to be present in as cast Ti₅₁Ni₄₅Fe₄ alloy, while annealed sample indicated the presence of bcc iron phase.     

Mössbauer spectroscopic studies of chromium depletion in a 310 stainless steel during air oxidation

The application of conversion electron Mössbauer spectroscopy (CEMS) to the oxidation of an ultra-thin 310 stainless steel foil is presented. It is shown that during oxidation the foil deviates from the expected oxidation behaviour, derived from thick film kinetics, due to the effects of chromium depletion. Evidence is presented for the observation, in the spectra, of chromium depletion. The experimentally derived time-to-failure is compared with that predicted by theory.     

Gd Mössbauer investigation of GdTSi compounds (T = Co, Fe, Mn)

We have studied the ¹⁵⁵Gd Mössbauer effect in the compounds GdMnSi, GdFeSi and GdCoSi (tetragonal, CeFeSi-type structure). From the quadrupic splitting of the spectra, we determined the electric field gradient at the nuclear site. We also present values for the effective hyperfine field and the isomer shift.     

Thickness determination of oxide films on iron by electron back-scattering Mössbauer spectroscopy

Magnetite films in the range 265–4520 Å have been grown on natural iron substrates and subsequently investigated by electron back-scattering Mössbauer spectroscopy. In particular, the percentage, P, of the total spectrum area contributed by the oxide has been determined as a function of oxide thickness, d. It was found that d up to 3000Å may be expressed (to an accuracy of 5%) by d (Å)=–1.95 × 10³ ln (1–0.01 P). The experimental data have been compared with the theoretical predictions of both Huffman and Bainbridge. Good agreement between experiment and Huffman's predictions of P is obtained using values of the electron attenuation coefficient, , of 1.10 × 10⁴ cm² g^–1 for the 7.3 keV electrons and 1.73 × 10⁴ cm² g^–1 for the 5.4 keV electrons. A good fit of our data to Bainbridge's expression requires a somewhat lower effective , value of 0.8 × 10⁴ cm² g^–1. The experimental P value for the thickest oxide (4520 Å) is lower than the theoretical predictions, probably as a result of a neglected mechanism recently identified by Tricker, Ash, and Cranshaw.     

A Mössbauer spectroscopy study of Fe based cemented carbides

Mössbauer spectroscopy has been used as a novel characterization technique to investigate Fe charge states, Fe complexes and hyperfine interaction parameters of different phases in WC-10Fe and WC-10(FeNi) materials sintered at three different temperatures (1350, 1430 and 1510 °C). The materials were characterized using standard cemented carbide quality control, and spectroscopic techniques to evaluate the structural changes and magnetic effects of the binder. The WC-10Fe grade had the highest Vickers hardness ranging from 1282 to 1320 HV₃₀, for the different sintering temperatures. X-ray diffraction data showed the presence of WC and the metal binder phase, α-Fe and γ-FeNi. Transmission Mӧssbauer spectroscopy spectra obtained for the milled powders revealed only the α-Fe phase with a hyperfine magnetic field, B_hf ~33 T. Conversion electron Mössbauer spectroscopy on the sintered compacts revealed the presence of multiple fields, suggesting the possibility of minor phases present in the binder which were not detected using X-ray diffraction. The corresponding spectra for the sintered WC-Fe grades exhibited two magnetic fields with hyperfine parameters of ~33 T and ~17 T, respectively. These fields were assigned to α-Fe with some W atoms in solution and a W-rich Fe phase, respectively. The Mӧssbauer spectrum for the FeNi binder sample at the lowest sintering temperature of 1340 °C showed a paramagnetic doublet with an isomer shift, δ = −0.08 mm/s and electric quadrupole splitting, ∆E_Q = 0.00 mm/s and a weak hyperfine magnetic field, B_hf = 15.7 T. The doublet has been assigned to γ-FeNi and the magnetic component to a W-rich γ-FeNi phase. For the higher sintering temperature, a distribution of magnetic fields (~33 T, 25 T and 9 T) was evident in the Mössbauer spectrum. These magnetic fields were tentatively assigned to multiple W-rich γ-FeNi phases.