C. A. Barrero

New analysis of the Mössbauer spectra of akaganeite

The Mossbauer spectra of akaganeite have always been interpreted considering both the tetragonal structure and the chlorine content. However, very recently it has been suggested that the crystallographic structure is not tetragonal but monoclinic, thus another interpretation for the Mossbauer spectra is required. For this purpose, we have prepared and characterized by several techniques synthetic akaganeite. Our results suggest that the two crystallographic sites required by the monoclinic symmetry are not distinguishable in the paramagnetic state as previously assumed, but they are only discernible in the low temperature magnetic region. At room temperature the spectrum is fitted with two doublets whose origin is related to the chlorine content, i.e. one Fe site assigned to Fe3+ ions located close to chloride ions and the other Fe site to those located close to chloride vacancy sites. The low temperature spectra can be adequately fitted with four sextets, whose hyperfine parameters must be subjected to some constraints. The origin of these components is related to the two different crystallographic sites and to the chlorine content. In-field Mossbauer spectrometry at low temperature suggests that the magnetic structure behaves as a system which consists of two asperimagnetic-like structures antiferromagnetically coupled, and not as a collinear antiferromagnet as usually assumed.

Thermally Induced Magnetite–Haematite Transformation

The products of thermal treatments of pure and copper doped magnetites have been investigated using Mössbauer spectrometry, XRD and thermal analysis techniques. The samples were heated in air between RT and 800°C at several heating rates. Samples treated at 520°C during 12 and 24 hours consist only of well-crystallized haematite. On the other hand, magnetites treated at 350°C consisted of mixtures of haematite, maghemite and magnetite, with relative amount of each phase depending on the presence of copper as well as on the heating time. Results show that the transformation of magnetite to haematite goes through the formation of maghemite, and that the presence of copper delays this transformation.

Mossbauer characterization of rust obtained in an accelerated corrosion test

We have performed drying-humectation cyclical processes (CEBELCOR) on eight A36 low carbon steel coupons in NaCl solutions containing 1×10−2 M and 1×10−1 M concentrations. The main purpose of these experiments is to contribute to the understanding of the conditions for akaganeite formation. Additionally, and with the idea to perform a complete characterization of the rust, this work also considers the formation of other iron oxide phases. The corrosion products were characterized by Mössbauer spectroscopy and X-ray diffraction techniques. Gravimetric analysis demonstrates that the coupons presented high corrosion rates. Magnetite/maghemite was common in the rust stuck to the steel surface, whereas akaganeite was present only in traces. In the rust collected from the solutions, i.e., the rust that goes away from the metal surface easily, a magnetite/maghemite was not present and akaganeite showed up in larger quantities. These results support the idea that high concentrations of Cl− ions are required for the akaganeite formation. We concluded that akaganeite is not easily bonded to the rust layer; this may lead to the formation of a less protective rust layer and to higher corrosion rates.

Properties of goethite grown under the presence of Cr3+, Cu2+ and Mn2+ ions

This study is focused on properties of goethite related to the inhibition of the corrosion process in low alloy, weathering steels. These steels are characterized by the presence of small amounts of Cr3+, Cu2+ and Mn2+ ions, which drive their protecting behavior in mild atmospheres. Several goethite samples with 5 mole % content of the alloying elements in nominal composition are synthesized by a hydrolysis route. Additionally, Cl−, SO42− are used as precursor ions in order to simulate the presence of atmospheric pollutants. All samples are analyzed by X-ray diffraction and 57Fe transmission Mössbauer spectroscopy (MS). The presence of the alloying elements and pollutant ions reduces the overall magnetic interactions in goethite, which is reflected in the lowering of the hyperfine field of maximum probability at 77 K, in comparison to that of pure goethites. However, the mechanisms of the magnetic interaction reductions are different for each combination of alloying element and pollutant ions. Finally the combined effect of anions and cations produces a wider distribution of particle sizes.

Fe-doped TiO2 prepared by mechanical alloying

The effect of different milling conditions on the formation of Fe-doped TiO2 powders by mechanical alloying was investigated by Mössbauer spectrometry. The milling conditions investigated were ball to powder weight ratio, milling time, rotation velocity of supporting disc, and the type of starting reactive iron and its concentration. X-ray diffraction shows that high energy mechanical milling of undoped anatase TiO2 induce the anatase to rutile phase transformation via high pressure srilankite. Mössbauer spectra for the majority of the doped samples were decomposed into one sextet and one or two doublets. The sextets was attributed to the presence of α-Fe or hematite impurities. The doublets were assigned to Fe3+ incorporated in the TiO2 structure, and to the Fe2+ located either at the surface or the interstitial sites of TiO2. A greater incorporation of Fe in the TiO2 structure was observed when samples were prepared from hematite instead of α-Fe.

Variable Temperature Mössbauer Study of Some Rust Converters

The conversion properties of 15 different combinations of rust converters have been studied. The results indicate that the rust layer without conversion is composed of spinel phase, akaganeite, magnetically ordered goethite, and lepidocrocite. On the other hand, besides the mentioned iron oxides and oxyhydroxides, the converted-rust layer consists of iron phosphate and iron tannate. Mössbauer spectrometry has allowed us to classify the degree of conversion of the different formulations according to both the tannic acid content and the mixture of the isopropyl and terbutanol alcohol. It was found that there is a specific mixture of alcohol for each tannic concentration for which the power of conversion is greatly enhanced. It is also concluded that non-stoichiometric spinel phase underwent the greater transformation of about 90%, followed by magnetically blocked goethite, in which 30% of it was transformed. In contrast, akaganeite seems to be not noticeably affected by the converters. It was not possible to assess the effect on lepidocrocite.

Magnetic properties of the mechanically alloyed (Fe0.85Mn0.15)0.3Cu0.7 system

Samples of nominal composition (Fe0.85Mn0.15)0.3Cu0.7 were prepared by mechanical milling starting from pure element powders. In order to elucidate the effect of the alloying time upon the magnetic properties of the system, milling times ranging from 1 hour up to 72 hours were considered. The phase distribution present on the as-milled materials was identified from the analysis of X-ray diffraction data. The room temperature magnetic properties of the samples were studied by means of 57Fe Mössbauer spectroscopy and vibrating sample magnetometry, whereas their low temperature magnetic behavior was characterized through magnetic susceptibility measurements. The results evidenced a strong dependence of the magnetic properties on the milling time and, concretely, the occurrence of a superparamagnetic behavior in the long-time-milled samples for which an extended solid solution was obtained. This fact is attributable both to the obtained crystallite sizes, which resulted to be of the order of a few nanometers, and to a milling-driven increase of the lattice parameter.

ON MAGNETITE FORMATION AS A CORROSION PRODUCT OF STEEL

A Mossbauer study on magnetites formed under the influence of differing concentrations of sulfate and chloride ions and also under the presence Cu, Mn and Cr ions is presented. It is found that different concentrations of sulfate and chloride ions in the synthesis procedure does not influence the physical and chemical properties of the magnetite. On the other hand, the concentrations of these ions in accelerated corrosion tests influence mainly the amount of magnetite formed. Mossbauer results suggest that the physical and crystallographic properties and amounts of the final products can be differently affected by the combined effect of sulfate and chloride ions plus the presence of alloying elements.