Wiistite

At pressures below 30 kbar FeO is not in equilibrium with respect to nonstoichiometric wustite. However, in many experimental works and thermodynamic calculations the nonstoichiometry of wustite is not taken into account, which leads to confusion and incompatibility of the results. As a result, values may differ by 2 to 3 kcal, as we showed earlier (Mel nik, 1972b). Such large discrepancies preclude precise calculation of the parameters of wustite stability. 

Thus it is obvious that at atmospheric pressure the value of the reaction  

On the basis of a generalization of experimental data on the activity of oxygen in wustite, Kurepin (1975) calculated for the reaction  

Calculation of AGjj- from this equation for lower temperatures leads to an error of up to 0.2 kcal/mol, therefore from 1000 to 298°K a nonlinear extrapolation was made independently for the enthalpic = —62420 

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When mild steel is heated in air at between 575 and 1 370°C an oxide or scale forms on the steel surface. This scale consists of three well-defined layers, whose thickness and composition depend on the duration and temperature of heating. In general, the layers, from the steel base outwards, comprise a thick layer of wiistite, the composition of which approximates to the formula FeO, a layer of magnetite (FejOJ, and a thin layer of haematite (FejO,). 

When the steel is rapidly cooled, the thickness and composition of these layers remain more or less unchanged, but when it is slowly cooled through 575°C the scale becomes enriched in oxygen and the remaining wiistite layer breaks down to some extent into an intimate mixture of finely divided iron and magnetite. ... 

Holding of the temperature between 400 and 575°C causes the iron particles to coagulate and the scale becomes further enriched in oxygen. Since wiistite is unstable below 575°C, scales produced at temperatures lower than this contain magnetite and haematite only. In addition, the scales are often cracked and porous. This is due to the difference in contraction... 

Sulphuric acid is used to a very large extent for pickling low-alloy steels. The rate at which it removes the scale depends on (q) the porosity and number of cracks in the scale, (b) the relative amounts of wiistite, decomposed wiistite, magnetite and haematite in the scale, and (c) factors affecting the activity of the pickle. 

As in the case of wiistite, one of the earliest applications of atomistic simulations was to explore the likely stability of this defect cluster. It was found that not only the 2 2 2 arrangement but also other cluster geometries were preferred over isolated point defects. 

Figure 2.13 Heat capacity of wiistite around the Neel temperature [19]. O Feo.990 I c0 947O V Feo.93sO + Feo.9250. Reproduced by permission of the Mineralogical Society of America.

A small amount of Cr could be incorporated in wiistite at 1350 °C (Bogdandy Engell, 1971) and MgO and MnO were completely miscible with FeO the mixed phases are important in the reduction of iron ores. Wiistite can be doped with small amounts of Mn, Mg, Ca and <10 g kg Si or Al to promote reduction (Moukassi et al., 1984). In green rust Fe has been replaced by Ni" (Refait Genin, 1997) and by Mg (Refait et al. 2001). 

Fe(OH)2 exists as hexagonal plates as do the green rusts (Feitknecht Keller, 1950 Bernal et ak, 1959). The basic morphology of wiistite is cubic, but this compound is frequently obtained as very irregular particles. It is formed as irregular rounded crystals 20-100 (xm across by reduction of hematite with H2/H2O at 800 °C (Moukassi et al., 1984). 

Hematite, wiistite, maghemite and magnetite are semiconductors magnetite displays almost metallic properties. For a compound to be a semiconductor, the essential characteristic is that the separation between the valence band of orbitals and the conduction band is less than 5 eV this condition is met for the above oxides. In a semiconductor the Fermi level (i. e. the level below which all electron energy levels are filled) lies somewhere between the valence band and the conduction band. 

To the naked eye, goethite and akaganeite appear yellow-brown, lepidocrocite orange and hematite usually red (Plate 6.1). Feroxyhyte and ferrihydrite are dark reddish brown, maghemite brown to brownish red and magnetite and wiistite are black. 

Like X-ray diffraction patterns, neutron and electron diffraction patterns provide averaged information about the structure of a compound. Details of these techniques are given in works by Hirsch et al. (1965) and West (1988). Neutron diffraction involves interaction of neutrons with the nuclei of the atoms. As the neutrons are scattered relatively evenly by all the atoms in the compound, they serve to indicate the positions of the protons in an oxide hydroxide. This technique has been applied to elucidation of the structure and/or magnetic properties of goethite (Szytula et al., 1968 Forsyth et al., 1968), akaganeite (Szytula et al., 1970), lepidocrocite (Oles et al., 1970 Christensen Norlund-Christensen, 1978), hematite (Samuelson Shirane, 1970 Fernet et al., 1984) and wiistite (Roth, 1960 Cheetham et al., 1971 Battle Cheetham, 1979). A neutron diffractogram of a 6-line ferrihydrite was recently produced by Jansen et al. (2002) and has helped to refine its structure (see chap. 2). 

Gt goethite Lp lepidocrocite Ak akaganeite Fh ferrihydrite Hm hematite Mt magnetite Mh maghemite Wu wiistite... 

For hematite and wiistite in the same media, the kinetic curves were deceleratory. 

The reduction of hematite with H2 at 387-610 °C has been followed in situ using TEM and an environmental cell (Rau et al., 1987). The reduction reaction started at nudeation sites on the edge of the sample and as the reaction proceeded, a particle showed four reaction zones consisting of umeacted hematite, lamellar magnetite, porous magnetite and finally porous iron (the temperature was too low for wiistite). 

Wiistite is reduced to iron at temperatures greater than 700 °C in both CO/CO2 and H2/H2O mixtures. SEM examination of partly reduced crystals showed that the product could be porous iron, dense iron overlying porous wiistite or dense iron and wiistite together depending on the reaction conditions and their effect on the relative rates of the chemical and the diffusion processes (St. John et al., 1984,1984a). 

For oxidation of iron to occur at high temperatures, the oxygen partial pressure must be above that of the dissociation pressure of the appropriate corrosion products. For example, at ca. 700 °C, an oxygen partial pressure of greater than 10 Pa is required for wiistite to form. In air, of course, this condition is readily satisfied, at least initially. As oxidation continues and the film thickens and becomes coherent, an oxygen gradient across the film is established and the composition of the corrosion layer changes. 

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